WO2007020683A1

WO2007020683A1 - Thick steel plate excelling in toughness of large heat input welded joint

Info

Publication number: WO2007020683A1
Application number: PCT/JP2005/014917
Authority: WO
Inventors: Hitoshi Hatano; Yoshitomi Okazaki
Original assignee: Kabushiki Kaisha Kobe Seiko Sho
Priority date: 2005-08-15
Filing date: 2005-08-15
Publication date: 2007-02-22
Also published as: CN101213316A

Abstract

A thick steel plate that even in the application of large heat input welding, excels in toughness of welded joint at -40˚C level low temperatures and that satisfies the requirement of strength demanded for welded structures, such as those of ships, marine structures, bridges and building structures. There is provided a thick steel plate excelling in toughness of large heat input welded joint, which consists of a steel composed of 0.01 to 0.15% (in terms of “mass%”, applicable hereinafter) C, 0.80% or less (not including 0%) Si, 1.2 to 2.40% Mn, 0.013 to 0.10% Ti, 0.0015 to 0.005% B, 0.0040 to 0.0100% N, 0.0010 to 0.005% O, less than 0.010% (not including 0%) Al and the balance essentially of iron and unavoidable impurities, and wherein 60 area% or more of the metal structure thereof consists of bainite.

Description

Specification

Thick steel plate with high heat input welded joint toughness

Technical field

[0001] The present invention relates to a thick steel plate having excellent weld joint toughness. In particular, even a welded joint obtained by high heat input welding exhibits excellent toughness at a low temperature of _40 ° C level. It relates to a thick steel plate. In the present invention, high strength refers to 490 MPa or more.

Background art

[0002] Thick steel plates have been used as materials for welded structures such as ships, marine structures, bridges, and building structures. Conventionally, the toughness of heat-affected zones in welded joints (hereinafter referred to as “HAZ toughness”). In order to ensure that the TiZ-containing oxide is dispersed in the base material, it is attempted to refine the structure by generating ferrite from within the grains when the HAZ part is cooled. I came.

[0003] For example, Patent Document 1 discloses that the HAZ part is coarsened by precipitating either four or two types of Ti oxide, a certain layer, or a composite of Ti oxide and Ti nitride. It is described that the γ → strain transformation at the time of cooling in the region is controlled to produce intragranular ferrite and toughness is improved.

[0004] Patent Document 2 also discloses that a steel having an appropriate alloy design is cooled after welding by allowing both a Ti oxide and a Ti nitride + MnS composite having a predetermined size and distribution to coexist. It describes that the low temperature toughness of HAZ is improved by promoting the formation of intragranular ferrite.

[0005] In recent years, however, it has been required to further increase the heat input during welding in order to increase production efficiency. However, the above technology still does not provide sufficient HAZ toughness when high heat input welding is applied. is there.

[0006] In addition, the above-mentioned welded structures are often exposed to low temperatures because of their use, and there is a demand for a thick steel plate having good HAZ toughness even at a low temperature of -40 ° C.

Patent Document 1: Japanese Patent Publication No. 7-824 (see “Claims”, column 5) Patent Document 2: Japanese Patent Publication No. 5-77740 ([Claims], column 7) Disclosure of Invention

Problems to be solved by the invention

[0007] The present invention has been made in view of such circumstances, and the object thereof is excellent in weld joint toughness at a low temperature of -40 ° C even when subjected to large heat input welding, and for ships. The purpose of the present invention is to provide a thick steel plate that meets the strength requirements for welded structures such as marine structures, bridges, and building structures.

Means for solving the problem

[0008] The thick steel plate excellent in toughness of the high heat input welded joint according to the present invention that has solved the above-mentioned problems is C: 0.01-0.15% (meaning "mass%", the same shall apply hereinafter), Si : 0.80% or less (0% not included), Mn: 1-2.40%, Ti: 0.013-0.10%, B: 0.0015-0.005%, N: 0.0040-0.0100%, 0: 0.0010-0.005%, A1: It is less than 0.010% (not including 0%), with the balance being steel consisting essentially of iron and inevitable impurities, with the gist being that 60% by area or more of the metal structure is bainite.

[0009] The amount of solute B in the steel is preferably 5 ppm or more.

[0010] As another element,

(1) Ni: 3.0% or less (excluding 0%), Cu: 3.0% or less (excluding 0%), Cr: 2% or less (excluding 0%), and Mo: 1.5% or less (Not including 0%), one or more selected from the group consisting of,

(2) Nb: 0.10% or less (excluding 0%) and / or V: 0.10% or less (excluding 0%),

(3) Ca: 0.005% or less (excluding 0%), Mg: 0.005% or less (excluding 0%), Zr: 0.05% or less (excluding 0%), and REM: 0.02% or less (0% One or more selected from the group consisting of:

And the like are preferred.

[0011] Among the inclusions contained in the steel, Ti inclusions having an average particle diameter of 0.05 to 1 μm are 10000 / cm ² or more when observed at a magnification of 1 000 times, and the average grains Inclusions having a diameter of 2 / im or more are preferably 2000 / cm ² or less when observed at a magnification of 200 times. The invention's effect

[0012] According to the present invention, the strength is 490 MPa or more, and even when high heat input welding is performed.

A steel plate with good weld joint toughness can be provided at a low temperature of -40 ° C.

[0013] A welded joint obtained by high heat input welding of a thick steel plate according to the present invention exhibits excellent toughness even at low temperatures. For example, a welded structure such as a ship, an offshore structure, a bridge, or a building structure. It can use suitably as materials, such as.

Brief Description of Drawings

FIG. 1A is a photomicrograph showing an example of a steel sheet structure.

FIG. 1B is a micrograph showing another example of a steel sheet structure.

BEST MODE FOR CARRYING OUT THE INVENTION

[0015] The inventors of the present invention have intensively studied to obtain a thick steel plate excellent in weld joint toughness at low temperatures even when high heat input welding is performed. As a result, (1) it is possible to form intragranular bainite (preferably full grain intragranular bainite) together with intragranular ferrite rather than allowing only intragranular ferrite to grow during cooling after welding as in the prior art. (2) To that end, it is only necessary to increase the concentration of B and N contained in the thick steel plate. (3) If the metal structure of the thick steel plate is mainly bainite, it is generated during the bainite transformation. The present invention was completed by finding that the matrix toughness can be further enhanced because N can be dissolved in the MA structure (island martensite structure).

In addition to the above, (4) while suppressing the formation of coarse inclusions in the steel sheet and generating a large number of fine Ti-based inclusions, a greater number of intragranular bainites can be obtained more reliably. It has also been found that the toughness of high heat input welded joints at low temperatures can be further increased. Hereinafter, the function and effect of the present invention will be described in detail.

[0017] In the thick steel sheet of the present invention, it is important to suppress the formation of intergranular ferrite and to form intragranular bainite together with intragranular ferrite when the HAZ part after welding is cooled. Therefore, it is necessary to increase the concentration of B and N contained in the steel plate.

[0018] That is, conventionally, an attempt has been made to produce a fine metal structure by producing a ferrite from within the grains during cooling after welding by producing a Ti-containing oxide in a steel sheet. Shi Strength With these technologies, the low temperature toughness of HAZ obtained by high heat input welding at a level required in recent years (for example, heat input: 50 kj / mm or more) was not satisfactory. With high heat input welding, the cooling rate in the HAZ is reduced, so even if Ti-containing oxides are formed in the steel sheet, the formation of intergranular ferrite is given priority and nucleation of intragranular ferrite is suppressed. Conceivable.

[0019] Therefore, the present inventors have repeatedly studied a method for suppressing the formation of intergranular ferrite during cooling after welding and promoting the formation of intragranular ferrite. As a result, we learned that if intragranular bainite is produced together with intragranular ferrite, a thick steel plate with excellent weld joint toughness can be obtained even at high temperatures even when high heat input welding is performed. . In order to produce intragranular bainite together with intragranular ferrite, it was found that the concentration of B and N contained in the steel plate should be relatively increased. It was also found that controlling the morphology of the Ti oxide increases the intragranular bainite fraction and improves HAZ toughness.

[0020] It is preferable that the steel sheet of the present invention generates intragranular bainite in the HAZ part during cooling after welding, but it is slightly from the grain boundary as shown in the micrographs shown in Figs. 1A and 1B. If so, bainite and ferrite may be generated.

First, the chemical components contained in the thick steel plate according to the present invention will be described including the above contents.

[0022] The thick steel plate of the present invention has, as a basic component, mass%, C: 0.01 to 0.15%, Si: 0.80% or less

(Excluding 0%), Μη: 1 to 2 to 40%, Ti: 0 to 013 to 0 to 10%, Β: 0.0015 to 0.005%, Ν: 0.0040 to 0.0100%, Ο: 0.0010 to 0.005%, Α1: Less than 0.010% (excluding 0%). Hereinafter, the reason for defining the amount of each element will be described in detail.

[0023] C: 0.01 to 0.15%

C is an element necessary for ensuring the strength of the base material, and must be contained at 0.01% or more. Preferably 0.03. / ₀ or more, and if it is 0.03% or more, the effect of promoting the formation of intragranular bainite during cooling after welding is enhanced. More preferably 0.05. / ₀ or more. However, if the C content is excessive, the weld crack resistance and HAZ toughness deteriorate, so the C content must be kept to 0.15% or less. To increase HAZ toughness, it is preferable to keep the C content below 0.13%. More preferably, it is 0.11% or less. [0024] Si: 0.80% or less (excluding 0%)

Si is a useful element as a preliminary deoxidizer, but if included excessively, both the base metal toughness and HAZ toughness will decrease. Therefore, the upper limit of Si content is 0.80%. Preferably it is 0.50% or less, more preferably 0.30% or less.

[0025] Μη: 1.2-2.40%

Μη has the effect of improving hardenability, and also has the effect of promoting the formation of intragranular bainite at the heel part and improving toughness during cooling after welding. In order to exhibit such an effect effectively, it is necessary to contain Μη 1.2% or more, preferably 1.3% or more, more preferably 1.5% or more. However, if it is contained in excess, the toughness deteriorates instead, so the amount of Μη must be kept below 2.40%. Preferably it is 2.0% or less, more preferably 1.8% or less.

[0026] Ti: 0.013-0.10%

Ti forms oxides and nitrides (hereinafter collectively referred to as “Ti inclusions”) in the steel sheet, and generates cracks during cooling after welding, producing intragranular ferrite and intragranular bainite in the HAZ part. It is an important element that has the effect of promoting HA and greatly improving HA Z toughness. In order to exert such an effect effectively, it is necessary to contain at least 0.013%. High Ti content increases the Ti content in the Ti oxide, and also increases the amount of Ti nitride produced. It should be contained at 0.015% or more. More preferably, it is 0.018% or more. However, if the amount of Ti is excessive, both HAZ toughness and base metal toughness deteriorate, so it is necessary to keep it to 0.10% or less. Preferably it is 0.050% or less, More preferably, it is 0.030% or less.

[0027] B: 0.0015 to 0.005%

B is a useful element in solid solution to improve hardenability and secure strength. In the HAZ zone during cooling after welding, solute B (free B) has the effect of suppressing the formation of ferrite from grain boundaries and improving HAZ toughness. Further, when the HAZ part is cooled, B combines with free N present in the steel to form a nitride, which precipitates together with the Ti oxide, thereby causing intragranular ferrite and intragranular grains to form. It greatly promotes the formation of innite. In order to exert these effects effectively, it is preferable to contain 0.0015% or more. Preferably it is 0.0020% or more, more preferably 0.025% or more. However, if the amount of B is excessive and the amount of solute B increases, the hardenability decreases, and the base metal toughness and HAZ toughness also deteriorate. Therefore, the B content must be kept below 0.005%. Preferably it is 0.0050% or less, More preferably, it is 0.0040% or less, More preferably, it is 0.0035. / ₀ or less.

[0028] Ν: 0.0040-0.0100%

Ν is an element effective in improving the HAZ toughness by combining with Ti and Β to form nitrides, promoting the formation of intragranular ferrite and intragranular bainite in the HAZ during cooling after welding. is there. In order to exhibit such an effect effectively, it is necessary to contain 0.0040% or more. Preferably, it is contained more than 0.0040%, more preferably 0.0045% or more, still more preferably 0.0050% or more. However, if the N content is excessive, both the base metal toughness and HAZ toughness deteriorate, so the N content must be kept below 0.0100%. Preferably it is 0.0040% or less, More preferably, it is 0.0035% or less.

[0029] ○: 0.0010 to 0.005%

○ (Oxygen) is an element effective in promoting the formation of intragranular bainite in the HAZ part during the cooling after welding by forming Ti oxide. In order to exert such an effect effectively, it is necessary to contain 0.0010% or more of oxygen, preferably 0.0015% or more, more preferably 0.0020% or more. If the oxygen content is excessive, however, coarse oxides are likely to be formed, and the HAZ toughness is deteriorated on the contrary. Therefore, it must be suppressed to 0.005% or less. Preferably it is 0.0038% or less, More preferably, it is 0.0030% or less.

[0030] A1: Less than 0.010% (excluding 0%)

A1 is a strong deoxidizing element, and if A1 is contained excessively, the proportion of A1 in the oxide increases, inhibiting the formation of intragranular bainite in the HAZ part during cooling after welding. Therefore, in the thick steel plate of the present invention, the A1 amount should be reduced as much as possible. Therefore, in the present invention, the A1 content is suppressed to less than 0.010%. Preferably it is 0.007% or less, more preferably 0.004% or less. The above “0% free” means that it is inevitably mixed as an impurity, and does not mean that A1 is positively added. If it is less than 0.010%, it is acceptable. Meaning.

[0031] The thick steel plate according to the present invention contains the above chemical components, and the balance is substantially iron and iron. Steel that consists of inevitable impurities P and S included as inevitable impurities are P: 0.02% or less (less than 0%) and Z or S: 0.01% or less (less than 0%) It is preferable that

[0032] P: 0.02% or less (less than 0%) and Z or S: 0.01% or less (less than 0%)

P (phosphorus) and S (sulfur) are elements that are present as inevitable impurities in the steel sheet, and it is preferable to reduce them as much as possible because they have adverse effects such as reducing weldability and base metal toughness. So P is 0.02. It is more preferable to suppress the ratio to ₀ or less, more preferably 0.020% or less, and still more preferably 0.010% or less. Further, S is preferably suppressed to 0.01% or less, more preferably 0.010% or less, and further preferably 0.005% or less. The above “not containing 0%” means that it is inevitably mixed in as an impurity, and does not mean that P or S is positively added. Means you can.

[0033] The thick steel plate according to the present invention, in addition to the above elements, as other elements,

(a) Ni: 3.0% or less (excluding 0%), Cu: 3.0% or less (excluding 0%), Cr: 2% or less (excluding 0%), and Mo: 1.5% or less (excluding 0%) One or more selected from the group consisting of:

(b) Nb: 0.10% or less (less than 0%) and / or V: 0.10% or less (less than 0%)

(c) Ca: 0.005% or less (0% not included), Mg: 0.005% or less (0% not included), Zr: 0.05% or less (0% not included), and REM: 0.02 1 or more selected from the group consisting of less than% (excluding 0%)

Etc. may be included. The reason for defining such a range is as follows.

[0034] Ni: 3.0% or less (excluding 0%), Cu: 3.0% or less (not including 0%), Cr: 2% or less (less including 0%), and Mo: 1.5. 1 or more selected from the group consisting of / ₀ or less (0, / not including ₀ )

Ni, Cu, Cr and Mo are all useful elements that improve the hardenability.

[0035] Ni is an element that enhances the hardenability and improves the strength of the base material, and strengthens the matrix to contribute to the improvement of the base material toughness and HAZ toughness. When the Ni content is excessive, On the other hand, in order to deteriorate the HAZ toughness, the Ni content is preferably suppressed to 3.0% or less, more preferably 2.0% or less, and further preferably 1.0% or less. Ni exhibits its effect when added in a small amount, but it is desirable to contain 0.2% or more in order to exhibit its effect more effectively.

[0036] Cu is an element that improves hardenability and improves the strength of the base metal by solid solution strengthening and precipitation strengthening. However, if Cu is excessively contained, the HAZ toughness will be reduced, so it should be kept below 3.0%. More preferably, it is 2.0% or less, and further preferably 1.0% or less.

[0037] Cu exhibits its effect when added in a small amount, but it is desirable to contain 0.2% or more in order to exhibit its effect more effectively. In addition, when adding more than 0.5% Cu, it is preferable to add Ni together with Cu to prevent hot cracking during rolling. At this time, the Ni content (mass%) The content is preferably about half or more of the content (% by mass). More preferably, it is recommended to add more than the chemical equivalent of Ni.

[0038] Cr is an element having an action of improving hardenability and improving the strength of the base material. However, if it is contained in excess, the amount of MA (island martensite) produced will increase and the HAZ toughness will deteriorate. Therefore, the amount of Cr is preferably added within a range of 2% or less. More preferably, it is 2.0% or less, more preferably 1.5% or less, and particularly preferably 1.0% or less. Although Cr exhibits its effect when added in a small amount, it is preferably contained in an amount of 0.2% or more in order to exhibit its effect more effectively.

[0039] Mo is an element that contributes to the improvement of the strength of the base metal by increasing the hardenability. However, if the Mo amount becomes excessive, the HAZ toughness deteriorates significantly, so it is preferable to add it within a range of 1.5% or less. More preferably, it is 1.0% or less, and further preferably 0.6% or less. Mo exhibits its effect when added in a small amount, but in order to exhibit its effect more effectively, 0.05% or more is preferably contained.

[0040] Nb: 0.10% or less (0% not included) and / or V: 0.10% or less (0% not included)

Nb is an element that effectively enhances hardenability and temper soft resistance. When the Nb content is excessive, both the base metal toughness and HAZ toughness decrease. In particular, Nb tends to segregate around the Ti oxide, so in the HAZ part during cooling after welding, grains with Ti oxide as the core are used. Suppresses the formation of inner bainite and causes HAZ toughness to deteriorate. Therefore, Nb is preferably added in a range of 0.10% or less. More preferably, it is 0.050% or less, More preferably, it is 0.030% or less, Especially preferably, it is 0.010. / ₀ or less. Nb exerts its effect when added in a small amount, but 0.003% or more is desirable in order to exhibit its effect more effectively.

[0041] V, like Nb, is an element that effectively increases hardenability and temper softening resistance. However, when V is excessive, both the base metal toughness and HAZ toughness decrease, as in the case of Nb above. V also tends to segregate around the Ti oxide, and therefore, in the HAZ part during cooling after welding, the formation of intragranular bainite with the Ti oxide as the core is suppressed and the HAZ toughness is deteriorated. Become. Therefore, V is preferably contained within a range of 0.10% or less. More preferably, it is 0.050% or less, and further preferably 0.030% or less. V also exerts its effect when added in a small amount, but 0.003% or more should be contained in order to achieve its effect more effectively.

[0042] Ca: 0.005% or less (excluding 0%), Mg: 0.005% or less (excluding 0%), Zr: 0.05% or less (excluding 0%), and REM: 0.02% or less 1 or more selected from the group consisting of (excluding 0%)

Ca: 0.005% or less (less than 0%)

Ca has the effect of reducing the anisotropy of inclusions by spheroidizing sulfides such as MnS and improving HAZ toughness. If excessive Ca is added, the toughness of the base metal and the HAZ toughness will decrease, so the upper limit should be 0.005%. More preferably, it is 0.0050% or less, More preferably, it is 0.003. / ₀ or less. Ca is preferably contained in an amount of 0.0005% or more in order to exert its effect more effectively than its ability to exert its effect when added in a small amount.

[0043] Mg: 0.005% or less (excluding 0%)

Mg has the effect of improving the HAZ toughness by refining inclusions and improving the toughness of the base metal. However, if added excessively, the toughness of the base metal deteriorates instead of the HAZ toughness, so it is preferable to add within 0.005% or less. More preferably, it is 0.0050% or less, More preferably, it is 0.003% or less. Mg exhibits its effect when added in a small amount, but it is preferable to contain 0.0002% or more in order to exhibit its effect more effectively. [0044] Zr: 0.05% or less (excluding 0%)

Zr has an effect of improving the HAZ toughness, but if added in excess, the base metal toughness deteriorates instead of the HAZ toughness, so it is preferable to add it within a range of 0.05% or less. More preferably, it is 0.050% or less, and further preferably 0.03% or less. Zr exhibits its effect when added in a small amount, but it is preferably contained in an amount of 0.005% or more in order to exhibit its effect more effectively.

[0045] REM: 0.02% or less (excluding 0%)

REM (rare earth element) has an effect of improving HAZ toughness, but if added excessively, the base metal toughness deteriorates instead of HAZ toughness, so it is preferable to add it within a range of 0.02% or less. More preferably, it is 0.020% or less, and further preferably 0.03% or less. In addition, REM is preferably contained in an amount of 0.001% or more in order to exert its effect more effectively than its ability to exert its effect when added in a small amount.

[0046] Ca, Mg, Zr and REM exhibit their effects when added alone, but when two or more of them are added in combination, the total amount is preferably 0.06% or less.

[0047] Next, the metal structure of the thick steel plate according to the present invention will be described.

[0048] The metal structure of the thick steel plate in the present invention is mainly composed of bainite, and specifically, the ratio of area ratio of bainite to the metal structure is 60% or more. The MA structure (island martensite structure) formed during bainite transformation dissolves more N than ferrite, so when the bainite fraction is less than 60%, the ferrite fraction increases, and the N solid solution in the MA structure As the amount decreases, the base material toughness deteriorates. On the other hand, if the bainite fraction is 60% by area or more, N can be dissolved in the MA structure formed during bainite transformation, and the base metal toughness can be increased. The bainite fraction in the metal structure is preferably 70 area% or more, more preferably 75 area% or more, further preferably 80 area% or more, particularly preferably 85 area% or more, and most preferably 90 area%. That's it. Most preferred is a thick steel plate substantially composed of bainite.

[0049] The remaining structure other than bainite in the metal structure may be formed of a structure such as pearlite or martensite as long as it is basically a force amount that is ferrite.

[0050] The bainite fraction (area ratio) in the metal structure is the pressure at the 1/4 position in the plate direction. Cross-sectional force parallel to the direction of elongation A test piece was collected, the surface of this test piece was mirror-polished, and then etched with a 2% solution of nitral. The range of 200 X 150 μm was 10 times 400 times using an optical microscope. The places were photographed, and the bainite fraction was measured with an image analyzer.

[0051] The steel plate according to the present invention preferably has a solid solution B content of 5 ppm or more. The higher the amount of B dissolved in the steel sheet, the higher the base material bainite fraction and the higher the base material toughness. The amount of solute B in the steel sheet exhibits its effect even with a very small amount, but it is preferably 5 ppm or more in order to exhibit its effect more effectively. More preferably, it is 8 ppm or more, and more preferably 10 ppm or more.

[0052] The solid solution B present in the steel sheet was measured by the following procedure. In other words, 1/4 position force in the plate thickness direction Measure the B amount using the curcumin absorbance method for the residue obtained by electric field extraction of the collected specimen, and calculate the difference value obtained by subtracting this B amount from the total B amount. The amount of solute B is used. The electrolytic extraction was performed using a methanol solution containing 10% acetylacetone and 1% tetramethylammonium chloride as the electrolysis solution under a current of 200 A / m ² or less. A filter with a roughness of 1 μm was used for filtration after extraction.

[0053] In order to increase the amount of solute B present in the steel sheet to 5 ppm or more, as described later, for example, the heating temperature, the rolling reduction at 850 to 950 ° C, It is effective to adjust the cooling rate.

[0054] The thick steel plate according to the present invention has 10,000 inclusions / cm ² when Ti-based inclusions having an average particle size of 0.05 to 1 µm are observed at a magnification of 1000 times among the inclusions contained in the steel. In addition, the number of inclusions having an average particle diameter of 2 μm or more is preferably 2000 / cm ² or less when observed at a magnification of 200 times.

[0055] The presence of many Ti-based inclusions (hereinafter sometimes referred to as “fine Ti-based inclusions”) having an average particle size of 0.05 to 1 μm in the steel allows cooling after welding. In the HAZ part at the time, the formation of bainite from the grains can be promoted. Furthermore, inclusions having an average particle size of 2 zm or more (hereinafter sometimes referred to as “coarse inclusions”) act as ferrite nuclei. Can be promoted. In addition, coarse inclusions are likely to be the starting point of fracture in a fine bainite structure. Therefore, the effect of fine bainite structure can be fully exhibited by reducing coarse inclusions. it can.

[0056] Ti inclusions with an average grain size of 0.05 to 1 μm present in the steel sheet are more desirable than 20000 / cm ² because the number of Ti inclusions promotes the formation of intragranular bainite. More preferably, it is 40,000 pieces / cm ² or more. From the viewpoint of the effect, there is no upper limit to the number of fine Ti inclusions, but the upper limit is about 1 X 108 Zcm ² . In the present invention, the average particle size of Ti inclusions refers to a value obtained by converting the particle size of Ti inclusions into a circle equivalent particle size.

[0057] Ti-containing oxides are preferred as fine Ti-based inclusions. Ti-containing oxides are likely to precipitate. However, since Ti-containing nitride has the same effect as Ti-containing oxide, Ti-containing nitride may be deposited. As the Ti-containing oxide, Si, Ca, Mg or the like may be contained as an alloy element other than Ti. Mn is particularly suitable as an element contained together with Ti. More preferably, the ratio force of Ti + Mn in all alloy elements constituting the oxide is 60 mass% or more (more preferably 70 mass% or more).

[0058] The observation magnification when observing the Ti-based inclusions is 1000 times, for example, by using a field emission scanning electron microscope (FE-SEM).

[0059] On the other hand, inclusions having an average particle diameter of 2 μm or more are more preferably 1000 pieces / cm ² or less, more preferably 500 pieces / cm ² or less when observed at a magnification of 200 times. is there. In the present invention, the average particle diameter of inclusions refers to a value obtained by converting the particle diameter of inclusions as an equivalent circle diameter.

[0060] Coarse inclusions include oxides, sulfides, nitrides and the like made of various alloy elements.

[0061] The observation magnification when observing inclusions is 200 times. For example, a field emission scanning electron microscope (FE-SEM), a scanning electron microscope (SEM), an EPMA (electron probe microanalyzer) apparatus If you want to observe it,

[0062] As described above, if a large number of fine Ti inclusions (particularly Ti-containing oxides) are generated while suppressing the formation of coarse inclusions, in the HAZ part during cooling after welding. Intragranular bainite is easily formed, and HAZ toughness can be greatly improved.

[0063] In order to control the Ti inclusions and the number of inclusions within the above range, Ti is added as described below. It is effective to adjust the amount of oxygen dissolved in the molten steel and the retention time from the addition of Ti to the penetration.

[0064] The method of manufacturing the thick steel plate according to the present invention is not particularly limited as long as it satisfies the above requirements, but can be reliably manufactured by employing the following manufacturing method.

[0065] In order to reduce the coarse inclusions while increasing the number of fine Ti inclusions, the amount of oxygen dissolved in the molten steel before the addition of Ti and the forging after the addition of Ti in the melting stage It is effective to strictly manage the retention time until.

[0066] Specifically, Ti is added to the molten steel in which the dissolved oxygen content is adjusted within the range of 20 to 100 ppm in the melting stage. By adjusting the amount of oxygen dissolved in the molten steel immediately before the addition of Ti, it is a force that can generate a large number of the above-mentioned fine Ti inclusions (especially fine Ti oxides). . In order to produce more fine Ti oxide, it is preferable to adjust the amount of dissolved oxygen in molten steel to 20 ppm or more, more preferably 25 ppm or more. However, if the amount of oxygen dissolved in the molten steel prior to the addition of Ti is excessive, Ti oxides become coarser and oxides other than Ti are likely to form, which is preferable. Therefore, it is preferable to keep the amount of dissolved oxygen below lOOppm in the molten steel before adding Ti. More preferably, the amount of dissolved oxygen in the molten steel is suppressed to 70 ppm or less, and Ti is added.

[0067] In the melting stage, to control the amount of oxygen dissolved in the molten steel immediately before adding Ti, for example, deoxidation by adding Mn, vacuum C (carbon) deoxidation, addition of Si Thus, deoxidation, etc. may be performed alone or in any combination.

[0068] Next, after adding Ti, in order to adjust to the final component of the steel sheet, C, Si, Mn, etc. are added and then swallowed. In order to reliably obtain the thick steel sheet of the present invention, Ti is added. It is effective to hold the molten steel in a stationary state for a certain period of time until it is poured after the addition.

[0069] Specifically, after addition of Ti, it is kept stationary for about 10 to 50 minutes. This is because, if Ti is added and held in a stationary state for 10 minutes or more, the coarse inclusions float and separate during that time, and the number of inclusions having an average particle size of 2 μm or more can be reduced. More preferably, it is 15 minutes or more, and further preferably 20 minutes or more. However, if the holding time after the addition of Ti is too long, the fine Ti-based inclusions that disperse in the molten steel agglomerate and coarsen together, making it suitable for effectively acting as a nucleus for the formation of intragranular binders. The amount of Ti inclusions cannot be secured The holding time is 50 minutes or less. More preferably, it is 40 minutes or less.

[0070] It should be noted that the retention after addition of Ti may be performed at a temperature between about 1550 ° C and 1650 ° C, as is done in normal melting. In actual operation, it is common to add Si, Mn, and C together with Ti to achieve the final component amount. The operation time may be adjusted to the above range.

[0071] On the other hand, in order to make 60% by area or more of the metal structure of the steel sheet bainitic, it is effective to dissolve B in the steel sheet. To that end, the heating temperature should be about 1000-1250 ° C, and 850

It is recommended that the rolling reduction at ~ 950 ° C be 40% or less and the cooling rate at 500 ~ 700 ° C after rolling is 5 ° C / sec or more.

[0072] When the heating temperature is less than 1000 ° C, B does not dissolve sufficiently in the steel. On the other hand, when it exceeds 1250 ° C, heating is excessive and the γ grain size becomes too large and TiN decomposes and dissolves. Increase heels. As a result, since the solute B is reduced, the bainite fraction is lowered and the toughness is deteriorated. More preferably, the heating temperature is set to 1050 ° C or higher and 1200 ° C or lower.

[0073] In the rolling after heating, since B precipitates in the range of 850 to 950 ° C, it is preferable to operate so as not to reduce the pressure as much as possible in this temperature range. In other words, if the rolling reduction in this temperature range exceeds 40%, a large amount of B precipitates, so that it hardly dissolves in the steel and bainite is difficult to form. More preferably, the rolling reduction should be suppressed to 30% or less.

[0074] After the rolling, the cooling rate in the range of 500 to 700 ° C is preferably 5 ° C / sec or more. If the cooling rate in this temperature range is less than 5 ° C / sec, it is difficult to form bainite.

. More preferably, it is 10 ° C Zsec or more.

[0075] Conditions other than those described above in the production process are not particularly limited, and usual production conditions may be employed. Further, as shown in the following examples, heat treatment may be performed as necessary to adjust the strength and toughness of the base material.

[0076] The plate thickness of the thick steel plate according to the present invention is not particularly limited, but even a thickness of about 50 to 100 mm provides high strength, strength, and excellent weld joint toughness at low temperatures. . Example

[0077] Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples as well as the present invention. It is also possible to carry out with modification, and they are all included in the technical scope of the present invention.

[0078] Steel having the chemical composition shown in Table 1 or 2 was melted and forged by the method shown in Table 3 to obtain a slab. The obtained slab was heated to the heating temperature shown in Table 3, and then rolled under the rolling conditions shown in Table 3 to obtain a steel plate having a thickness of 50 mm. In order to adjust the strength and toughness of the base material, tempering treatment was performed to 500 to 650 ° C as necessary.

[0079] A sample was taken from the obtained steel sheet, and the amount of solute B, metal structure (bainite fraction), size and number of inclusions present in the base material, base material characteristics, and HAZ toughness were measured. The measurement procedure for each measurement item is as follows.

[0080] [Solubility B amount]

Using the sample collected from the obtained steel plate, the amount of solute B was calculated according to the procedure described above. The calculation results are shown in Table 3 below.

[0081] [Metal structure (Bainite fraction)]

Using the sample collected from the obtained steel plate, the bainite fraction was measured by the procedure described above.

. The measurement results are shown in Table 4 below.

[0082] The remaining metal structures were ferrite, pearlite, and martensite.

[0083] [Measurement of size and number of inclusions]

The size and number of inclusions present in the base material were measured by the following method.

[0084] <Measurement position (sample collection position)>

Samples were taken so that a cross section parallel to the rolling direction could be observed from 1/4 of the plate thickness. Using the obtained samples, the number of inclusions having an average particle size of 2 × m or more and Ti-based inclusions having an average particle size force of S0.05 to lzm was measured as follows. Here, the average particle size means a value obtained by converting the particle size into a circular equivalent particle size.

[0085] <Counting the number of inclusions with an average particle size of 2 μm or more>

Using an EPMA apparatus, an area of 100 mm ² (that is, 10 mm × 10 mm) was observed at a magnification of 200 times, and the number of inclusions having an average particle diameter of 2 zm or more was measured. For the size of inclusions, the equivalent particle diameter was obtained as the average particle diameter (the same applies hereinafter).

[0086] <Measurement of the number of Ti inclusions having an average particle size of 0.05 to 1 μm> Using FE-SEM / EDX equipment, composition analysis was performed on 20 inclusions with an average particle size of 0.05 to 1 μm, and the proportion of inclusions with a Ti content of 10% by mass or more out of 20 inclusions. Asked. Next, in the area of 0.1 mm ² , 10 arbitrary fields of 0.01 mm ² were photographed using a 1000 times backscattered electron image, and the number of inclusions with an average particle size of 0.05 to lxm was measured using an image analyzer. Then, the number of Ti inclusions having an average particle size of 0.05 to 1 μm per 1 cm ² was obtained by multiplying the total number of the 10 fields of view by the ratio of the Ti-containing inclusions and further multiplying by 1000. The results are shown in Table 4 below.

[0087] [Base material properties]

JIS4 test specimens were collected from the steel plates and the tensile strength and vE-40 of the steel plates were measured. A material with a tensile strength of 490 MPa or more and a vE-40 force of 7 J or more is evaluated as having excellent base material properties, and such excellent base material properties are ensured. As described below, the welded joint toughness was evaluated. The results are shown in Table 4 below.

[0088] [Toughness of welded joint]

A test piece (size: 12.5mm x 32mm x 55mm) cut out from the steel plate force is heated to 1400 ° C, held at this temperature for 5 seconds, and then cooled from 800 ° C to 500 ° C in 500 seconds. (Corresponding to the thermal history of HAZ when SAW welding was performed at a heat input of 60 kj / mm), Charpy specimens were taken from each specimen and vE-40 was measured. The case of vE-40 force S47J or higher was evaluated as having excellent weld joint toughness. The results are shown in Table 4 below.

[0089] [Table 1]

Four]

[0093] From Table:! ~ 4, it can be considered as follows.

[0094] Νο.1 to 17 satisfy the requirements stipulated in the present invention, and a high-strength thick steel plate excellent in weld joint toughness at low temperature is obtained even in high heat input welding.

[0095] On the other hand, Νο.21 to 32 and Νο.38 to 41 do not satisfy any of the requirements stipulated in the present invention, so that the base material characteristics are insufficient. The result was inferior to joint toughness (toughness). That is, .ο.21 is inferior in the base metal toughness because of its low bainite fraction. In No. 22, the heating temperature is high, so the bainite fraction is low and the base material toughness is poor. 23ο · 23 has a low cooling rate at 500 to 700 ° C, resulting in a low bainite fraction and poor base material toughness. . No.24 is inferior in HAZ toughness due to a large amount of C. No.25 has a large amount of Si, so both the base metal toughness and HAZ toughness are inferior. No. 26 cannot secure the strength of the base metal due to insufficient Mn content. No. 27 could not secure HAZ toughness due to the large amount of Mn. No.28 was inferior in HAZ toughness due to lack of oxygen. No. 29 was unable to secure sufficient HAZ toughness due to the large amount of oxygen. No. 30 is inferior in HAZ toughness because the amount of nitrogen is too small. No.31 is inferior in HAZ toughness due to the large amount of nitrogen. No.32 has a high A1 content, resulting in inferior HAZ toughness. No.38 is inferior in HAZ toughness due to the large amount of B. No.39 has insufficient B amount

The result was inferior to HAZ toughness. No. 40 resulted in inferior HAZ toughness due to insufficient Ti content. No. 41 is inferior in HAZ toughness due to the large amount of Ti.

In addition, No.33-37 are reference examples, and it is clear from No.33-35 that it is better to add Ni, Cu, Cr, Mo within the specified range to ensure HAZ toughness. . From No.36 to 37, it is clear that Nb and V should be added within the specified range in order to ensure HAZ toughness.

Claims

The scope of the claims

C: 0.01 to 0.15% (meaning “mass%”, the same shall apply hereinafter),

Si: 0.80% or less (excluding 0%),

Mn: l.2-2.40%,

Ti: 0.013-0.10%,

B: 0.0015-0.005%,

N: 0.0040-0.0100%,

〇: 0.0010-0.005%

A1: Less than 0.010% (excluding 0%),

A thick steel plate excellent in toughness of high heat input welded joints, characterized in that the balance is steel consisting essentially of iron and inevitable impurities, and 60% by area or more of the metal structure is bainite.

The thick steel plate according to claim 1, wherein the amount of solute B is 5 _PP m or more.

As other elements,

Ni: 3.0% or less (excluding 0%),

Cu: 3.0% or less (excluding 0%),

Cr: 2% or less (excluding 0%), and

Mo: l. 5% or less (excluding 0%),

The thick steel plate according to claim 1 or 2, comprising at least one selected from the group consisting of:

Furthermore, it contains Nb: 0.10% or less (excluding 0%) and / or V: 0.10% or less (not including 0%) as other elements. Thick steel plate. As other elements,

Ca: 0.005% or less (excluding 0%),

Mg: 0.005% or less (excluding 0%),

Zr: 0.05% or less (excluding 0%), and

REM: 0.02% or less (excluding 0%),

The thick steel plate according to any one of claims 1 to 4, comprising one or more selected from the group consisting of: Of the inclusions contained in steel, When the average particle size is 0.05-: Ti-based inclusion force with a magnification of 1000 times, it is 10000 pieces Zcm ² or more,

The thick steel plate according to any one of claims 1 to 5, wherein the number of inclusions is 2,000 pieces / cm ² or less when observed at an inclusion force magnification of 200 times with an average particle size of 2 µm or more.