JP2003221643A - Steel product showing excellent toughness of heat- affected zone in ultra-high heat input welding - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel product showing an excellent toughness at the heat-affected zone in ultra-high heat input welding employing a heat input of >300 kJ/cm, and its manufacturing process. <P>SOLUTION: Molten steel is deoxidized by adding Si and/or Mn to adjust the dissolved oxygen to 0.0030-0.0120 mass%. Next, REM in added to obtain molten steel containing 0.01-0.18% C, 0.05-0.40% Si, 0.5-3.0% Mn, ≤0.03% P, 0.0005-0.0060% S, 0.0030-0.0200% REM, ≤0.0070% O and 0.0040-0.0070% N wherein the amounts of Al and Ti are each limited to ≤0.004%. The obtained molten steel is cast to obtain a steel raw material which is subsequently hot-rolled to obtain a steel product. This provides a structure wherein particles having an average particle size of ≤10 μm of at least one chosen from REM sulfide, REM oxide and REM oxysulfide, and particles having an average particle size of ≤1 μm comprising Mn oxide, Mn sulfide and Mn oxysulfide or a composite of two or more chosen from these, and dispersed. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material suitable for welded structures such as shipbuilding, construction, and bridges, and more particularly to a steel material suitable for use in ultra-high heat input welding. The "super-high heat input welding" referred to in the present invention means that the heat input for welding is 300 kJ /
It means a weldment exceeding cm. Also, the steel material is
It includes thick steel plate and shaped steel. The thick steel plate is the plate thickness.
A steel plate of 25 mm or more.

[0002]

2. Description of the Related Art In recent years, with the increase in the size of welded structures such as ships, buildings and bridges, there has been a demand for higher strength and thicker steel materials. Along with this, from the viewpoint of improving the construction efficiency of structures and reducing construction costs, improvement of welding efficiency is required,
High-efficiency welding with large heat input has been aimed. For example, in a large container ship, the welding heat input of submerged arc welding, electrogas welding, electroslag welding, etc. is 300k.
Large heat input welding that exceeds J / cm is applied.

Generally, the heat-affected zone of welding (hereinafter also referred to as HAZ) is exposed to a high temperature during welding, crystal grains are apt to coarsen, and the cooling rate becomes slow and fragile as the welding heat input increases. It is known that the upper bainite structure is likely to be formed, and that the brittle structure such as island martensite is likely to be formed and the HAZ toughness is likely to be deteriorated.

In order to solve the problem of deterioration of the toughness of the high heat input welding HAZ, for example, in Japanese Patent Application Laid-Open Nos. 2-250917, 2-254118, and 3-53367, Ti is disclosed.
N is finely dispersed in steel and combined with MnS or REM oxysulfide to suppress coarsening of austenite grains,
Techniques for improving the toughness of high heat input welding HAZ have been proposed.

Further, Japanese Patent Application Laid-Open No. 57-51243 proposes a technique for finely dispersing Ti oxide to increase the toughness of a high heat input welding HAZ. Further, in JP-A-62-170459, a combination of fine dispersion of Ti nitride and precipitation of BN having a ferrite nucleation ability while reducing the amount of solid solution B is combined,
A technology for increasing the toughness of large heat input welding HAZ has been proposed.

Further, Japanese Patent Application Laid-Open No. 60-204863 proposes a technique for improving the toughness of a high heat input welding HAZ by controlling the sulfide morphology by adding Ca.
Japanese Patent Publication No. 14180/1992 proposes a technique for improving the toughness of a high heat input welding HAZ by adding REM to control the morphology of sulfides. In addition, Japanese Examinations 4-54734
The official gazette contains B: 0.0003 to 0.0030% and S: 0.01
A high toughness welding steel has been proposed in which the total content of Al is 0.003% or less, the content of Ti is less than 5%, the content of Ti, REM, and Ca is 0.003 to 0.04% in total. This technique eliminates the formation of Al ₂ O ₃ and MnS and reduces Ti, REM and Ca
Oxides, sulfides, and oxysulfides are formed, and BN, which is the precipitation nucleus of intragranular ferrite, becomes significantly precipitated.
It is said that HAZ toughness is improved.

Further, in Japanese Unexamined Patent Publication No. 5-78740, Ce:
Including 0.0001 to 0.030%, S: 0.005% or less,
A method for producing a steel excellent in low-temperature toughness in a weld heat-affected zone is proposed, in which a steel having a composition substantially containing no Al is reheated in a temperature range of 1000 to 1250 ° C and then hot-worked. According to this technique, finely dispersed Ce oxide is used as a nucleus to form radially fine acicular ferrite to improve the HAZ toughness.

[0008]

However, in the steel material manufactured by the conventional technique mainly using TiN described above,
When the high heat input welding method of over 300 kJ / cm is applied, HAZ
However, since it is exposed to the high temperature region where TiN melts for a long time, TiN
Dissolves and the effect of refining the crystal grains disappears.
An embrittlement structure is generated due to an increase in Ti and solute N,
There is a problem that the HAZ toughness may be significantly reduced.

Further, in the prior art using the above-mentioned Ti oxide, it is considerably difficult to disperse the oxide uniformly and finely, and various studies have been made to improve the dispersibility of the oxide by compounding it. However, the heat input is 300 kJ / cm
It has been difficult to suppress the growth of austenite grains sufficiently in ultra-high heat input welding exceeding 10 ° C, and it has been difficult to make the ultra-high heat input welding HAZ stable and tough.

Also, according to the technique disclosed in Japanese Examined Patent Publication No. 4-54734, the superheat input welding in which the heat input exceeds 300 kJ / cm
There was a problem that the growth of austenite grains in the HAZ could not be sufficiently suppressed, and it was still difficult to make the ultra-high heat input welding HAZ stable and tough.
Moreover, in the technique described in Japanese Patent Laid-Open No. 5-78740, Ce
It is difficult to stably and finely disperse oxides, and the heat input is 30
There was a problem that it was difficult to make the super-high heat input welding HAZ exceeding 0 kJ / cm stable and tough.

The present invention advantageously solves the above-mentioned problems of the prior art, and proposes a steel material having excellent weld heat-affected zone toughness in super-heat-input welding with a heat input exceeding 300 kJ / cm, and a method for producing the steel material. And

[0012]

[Means for Solving the Problems] In order to achieve the above-mentioned problems, the present inventors have made extensive studies on various factors affecting the HAZ toughness of ultra-high heat input welding. As a result, there is a limit to making dispersed particles effective particles for improving HAZ toughness in ultra-high heat input welding only by adjusting the composition of oxides and sulfides in molten steel as in the past. I thought up. In addition to adjusting the particle composition of oxides, sulfides, etc. in molten steel, the present inventors further control the morphology of dendrites formed in the solidification process, and It has been found that it can be dispersed stably and remarkably uniformly and finely. The finely dispersed particles thus formed can effectively contribute to the refinement of austenite grains even in the HAZ of ultra-high heat input welding with a heat input of 300 kJ / cm or more, and can significantly improve the HAZ toughness.

The present inventors have made it possible to control the dendritic morphology by adding REM after deoxidizing with Si and Mn and adjusting the dissolved oxygen content of the molten steel before solidification to 0.0030 to 0.0120 mass%. I found that there is. After adjusting the amount of dissolved oxygen in the molten steel to a predetermined range, by adding REM, REM oxysulfide crystallized at the solid-liquid interface, and as a result, unidirectional growth of dendrite was suppressed and dendrite became equiaxed. , It was found that the secondary arm of dendrite was miniaturized. Furthermore, the inventors of the present invention, as a secondary deoxidation product, have one or more Mn oxides, sulfides, and oxysulfides compounded between the refined dendrite secondary arms. , A large amount of fine dispersed particles are formed uniformly, and these fine dispersed particles effectively contribute to the prevention of coarsening of austenite grains even in the welding heat affected zone of super large heat input welding with a heat input of 300 kJ / cm or more. Confirmed to do.

Furthermore, the present inventors have found that when the N content is increased to 0.0040 mass% or more, which is higher than usual, in addition to REM and S, REM oxysulfide or
It has been found that CeN 3 in which Ce of REM (rare earth element) is bonded to N easily precipitates using REM sulfide as a nucleus and becomes a heat affected zone where a large amount of fine ferrite is generated.

The present invention was completed by further studies based on the above findings. That is, the gist of the present invention is as follows. (1)% by mass, C: 0.01 to 0.18%, Si: 0.05 to 0.40
%, Mn: 0.5 to 3.0%, P: 0.03% or less, S: 0.0005 to
0.0060%, Al: 0.004% or less, Ti: 0.004% or less, REM
: 0.0030 to 0.0200%, O: 0.0070% or less, N: 0.0040
REM sulfide particles, REM oxide particles, and REM oxysulfide particles having an average particle size of 10 μm or less and having a composition of ˜0.0070% and the balance Fe and unavoidable impurities. Above, Mn oxide with an average particle size of 1 μm or less,
A steel material excellent in toughness in the heat-affected zone of a super-heat-input weld, having a structure in which particles of one or more of Mn sulfide and Mn oxysulfide are compounded and dispersed. (2) In (1), in addition to the above composition, in mass%, Nb: 0.1% or less, V: 0.2% or less, Cu: 1.5% or less, Ni: 3.0% or less, Cr: 1.0% or less, Mo. : 0.8% or less, B: 0.0003 to 0.0040%, one or more selected from the group consisting of two or more, a super-high heat input welding heat-affected zone steel plate having excellent toughness. (3) Deoxidizing by adding Si and / or Mn to molten steel,
After adjusting the dissolved oxygen amount to 0.0030 to 0.0120 mass%, RE
M was added to adjust the amount of dissolved oxygen to 0.0010 to 0.0050 mass% and the composition was adjusted so that the mass% was C: 0.01 to 0.
18%, Si: 0.05 to 0.40%, Mn: 0.5 to 3.0%, P: 0.03
% Or less, S: 0.0005 to 0.0060%, REM: 0.0030 to 0.0200
%, N: 0.0040 to 0.0070%, and Al and Ti are each limited to 0.004% or less to obtain molten steel, and then the molten steel is cast into a steel material, and the steel material is hot-rolled to a predetermined size. A method for producing a steel material having excellent toughness in the heat-affected zone of a super-heat-input welding, which is characterized in that (4) In (3), prior to the deoxidation, preliminary deoxidation by adding Al is performed, and the amount of dissolved oxygen before the deoxidation is 0.0080 to 0.
A method for producing a steel material excellent in toughness in a heat-affected zone of a super-high heat input weld, which is characterized by adjusting to 170% by mass.

The term "excellent toughness in the heat-affected zone of super-high heat input welding" as used in the present invention means that in the heat-affected zone of welding (in the following, also referred to as HAZ) in high heat input welding exceeding 300 kJ / cm 40
Charpy absorbed energy _V E _{-40 at} ℃ 100J
The case having the above is meant.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the composition of the steel material of the present invention will be explained. In the following, mass% is simply expressed as%. C: 0.01 to 0.18% C is an element that increases the strength of steel, and at least 0.01% is necessary to obtain the strength required for a structural steel material (base material yield strength: 300 MPa or more). However, if it is contained excessively, the toughness of the welded portion and the weld crack resistance are deteriorated. Therefore, in the present invention, C is limited to the range of 0.01 to 0.18%. The content is preferably 0.03 to 0.12%.

Si: 0.05 to 0.40% Si acts as a deoxidizing agent, and in the present invention, 0.05% or more is required for proper deoxidation. As the material toughness deteriorates, island-shaped martensite is generated in the ultra-high heat input welding HAZ,
The toughness is significantly reduced. Therefore, Si is limited to the range of 0.05 to 0.40%. The content is preferably 0.10 to 0.30%.

Mn: 0.5 to 3.0% Mn acts as a deoxidizing agent, and as a secondary deoxidizing product, one or two of fine oxides, sulfides and oxysulfides.
It is an element that forms a composite particle of more than one kind, suppresses coarsening of austenite grains of HAZ, and improves HAZ toughness. Mn also has the function of increasing the strength of steel by solid solution strengthening. In order to obtain such effects, the present invention requires the content of 0.5% or more. On the other hand, 3.0
An excessive content of more than 100% significantly deteriorates the toughness of the weld. Therefore, in the present invention, Mn is limited to the range of 0.5 to 3.0%. The content is preferably 0.8 to 1.7%.

P: 0.03% or less P is an element which is unavoidably contained in steel as an impurity and deteriorates the toughness of the steel, so P is preferably reduced as much as possible. In particular, if the content exceeds 0.03%, HAZ
Deterioration of toughness becomes remarkable. Therefore, P is limited to 0.03% or less. It should be noted that excessive reduction of P increases the refining cost and is economically disadvantageous, so 0.005% or more is preferable.

S: 0.0005 to 0.0060% S is combined with REM in the present invention containing REM, and
Sulfides (sulfides) or REM oxysulfides (oxysulfides) that crystallize at the solid-liquid interface during the solidification stage, suppressing unidirectional growth of dendrites and making dendrites equiaxed. It has the effect of making the arms smaller. Further, S also has an effect of binding to Mn as a secondary deoxidation product, finely crystallizing as a sulfide or oxysulfide of Mn, and preventing austenite grain coarsening of HAZ.

When S is less than 0.0005%, REM crystallizes as an oxide and the above-mentioned effects cannot be achieved. On the other hand, 0.0060
%, Coarse MnS is formed and the toughness is significantly reduced. Therefore, in the present invention, S is limited to the range of 0.0005 to 0.0060%. Al: 0.004% or less Al is a strong deoxidizing element, which forms alumina (Al ₂ O ₃ ) by combining with oxygen in molten steel and reduces dissolved oxygen. Therefore, REM oxysulfide (oxysulfide) It inhibits the formation of Mn oxide or oxysulfide (oxysulfide) as a secondary deoxidation product, and adversely affects the morphology control of dendrite and fine dispersion of the secondary deoxidation product. Therefore, in the present invention, without performing Al deoxidation, Si, Mn deoxidation,
The Al content was limited to 0.004% or less.

Ti: 0.004% or less Like Al, Ti is an element having a stronger deoxidizing power than Si and Mn, and it is necessary to reduce it as much as possible in order to finely disperse the secondary deoxidizing product. is there. Therefore, in the present invention,
Like Al, it was limited to 0.004% or less. REM: 0.0030 to 0.0200% REM is combined with S and / or O in the solidification process of molten steel to form REM sulfides (sulfides), REM oxides (oxides), and REM oxysulfides (oxysulfides). One or more of them are crystallized at the solid-liquid interface to suppress unidirectional growth of dendrites and have the effect of making dendrites equiaxed. Then, by making the dendrites equiaxed, the interval between the secondary dendrite arms is made finer. Such an effect is recognized when the content of REM is 0.0030% or more. However, when the content exceeds 0.0200%, coarse REM compounds increase and the base material toughness deteriorates. Therefore, REM is 0.00
It was limited to the range of 30-0.0200%. Incidentally, preferably, 0.
0050 to 0.0100%.

O: 0.0070% or less O contributes to equiaxed crystallization of dendrite as oxysulfide together with REM and S, and is finely dispersed as oxide or oxysulfide together with Mn and / or S. It suppresses the coarsening of austenite grains in the super-high heat input welding HAZ
Z has the effect of improving toughness. However, the content of more than 0.0070% increases the amount of oxides in the steel and deteriorates the cleanliness of the steel. It is more preferable that the content of REM oxide, oxysulfide, Mn oxide, and oxysulfide be 0.0015% or more in order to disperse the required amount.

N: 0.0040 to 0.0070% N bonds with Ce which is one of REM (rare earth elements) and precipitates as CeN during cooling during welding to serve as a nucleation site for ferrite. By finely dispersing and depositing CeN, a large amount of fine ferrite is generated, and the toughness of the weld heat affected zone is further improved. If N is less than 0.0040%, Ce
The precipitation amount of N 2 is small, the generation of fine ferrite is small, and further improvement of the toughness of the weld heat affected zone cannot be achieved. On the other hand, 0.
When it is contained in excess of 0070%, the amount of solute N increases and the toughness of the weld heat affected zone decreases.

In addition to the above basic composition, Nb: 0.1% or less, V: 0.2% or less, Cu: 1.
5% or less, Ni: 3.0% or less, Cr: 1.0% or less, Mo: 0.8
% Or less, B: 0.0003 to 0.0040%, and one or more kinds selected from the group can be contained. Nb, V, Cu,
Ni, Cr, Mo, and B are all elements that increase the strength of steel, and are preferably selected and contained as necessary in order to secure the base metal strength and the weld joint strength.

Nb has the functions of improving the strength and toughness of the base material and increasing the joint strength. Such an effect becomes remarkable when the content is 0.005% or more.
If the content exceeds 1%, the HAZ toughness decreases. Therefore, in the present invention, Nb is preferably limited to 0.1% or less. V improves the strength and toughness of the base material, precipitates as VN, and acts as a nucleus of ferrite transformation. Such an effect is remarkable when the content is 0.010% or more, but the content exceeding 0.2% rather deteriorates the toughness. Therefore, it is preferable to limit V to 0.2% or less.

Ni is an element that increases strength while maintaining high toughness of the base material. Such an effect is effective when the content is 0.10% or more, but even if the content exceeds 3.0%, the effect is saturated and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, in the present invention, Ni is preferably limited to 3.0% or less. Cu, like Ni, is an element that increases strength. Such effects are remarkable when the content is 0.10% or more, but the content exceeding 1.5% causes hot brittleness and deteriorates the surface properties of the steel sheet. Therefore, Cu is 1.5%
It is preferable to limit to the following.

Further, both Cr and Mo are elements which effectively act to increase the strength of the steel material (base material). Such an effect becomes remarkable when Cr: 0.10% or more and Mo: 0.05% or more are contained. On the other hand, if they are contained in excess, they adversely affect the toughness, so it is preferable to limit the content to Cr: 1.0% or less and Mo: 0.8% or less. B has the effect of increasing the strength of steel through the improvement of hardenability, and HA
Z forms BN, reduces the solute N and acts as a ferrite transformation nucleus. If such an effect is less than 0.0003%, the effect is not sufficient. On the other hand, if the content exceeds 0.0040%, the hardenability is remarkably increased and the base material toughness may be deteriorated. Therefore, B is preferably limited to the range of 0.0003 to 0.0040%.

The balance other than the above components is Fe and inevitable impurities. Further, in addition to the above-mentioned composition, the steel material of the present invention has REM sulfide particles with an average particle diameter of 10 μm or less, RE
One or two of M oxide particles and REM oxysulfide particles
It has a structure in which at least one kind and particles having an average particle size of 1 μm or less and a composite of one kind or two or more kinds of Mn oxide, Mn sulfide, and Mn oxysulfide are dispersed.

REM sulfide particles, REM oxide particles, and REM oxysulfide particles crystallize at the solid-liquid interface during the solidification process and have the effect of suppressing unidirectional growth of dendrites, but the average particle size is 10 μm. If it becomes too coarse, such an effect cannot be expected. In order to suppress unidirectional growth of dendrites, REM sulfide particles, REM oxide particles, RE
The average particle size of the M 2 oxysulfide particles is preferably 1 μm or more. REM of 10 μm or less, preferably 1 μm or more
In order to crystallize one or more of sulfide particles, REM oxide particles, and REM oxysulfide particles at the solid-liquid interface, the amount of dissolved oxygen before the addition of REM is 0.0030 to 0.0120% by mass.
It is preferable to adjust

In order to suppress unidirectional growth of dendrites, one or more of such REM sulfide particles, REM oxide particles, and REM oxysulfide particles are
It is preferable to disperse 70 particles / mm ² or more in terms of particle number density.
If it is less than 70 pieces / mm ² , the above effect cannot be expected and the solidified structure cannot be equiaxed. During welding of steel materials, REM particles act as precipitation nuclei for CeN, and the weld heat affected zone has a fine ferrite structure.

Finely dispersed with an average particle size of 1 μm or less, Mn
Particles composed of one or more of oxides, Mn sulfides, and Mn oxysulfides have the effect of suppressing the growth of austenite grains in the superheat-heat welding HAZ, but have an average grain size of 1
If it is coarsened to exceed μm, such an effect cannot be expected. These finely dispersed Mn-based composite particles are secondary deoxidation products, the secondary dendrite arm spacing in the solidification stage is made fine, and the dissolved oxygen amount after REM addition is 0.0010 to 0.0
It can be produced by adjusting the amount to 050% by mass. Such Mn-based composite particles are preferably dispersed at a particle number density of 1 × 10 ⁶ particles / mm ² or more. 1 x 10 ⁶ pieces / mm
^When it is less than ² , the pinning effect of the austenite grains in the high temperature retention region of the HAZ becomes small, the HAZ becomes coarse grains, and the HAZ toughness deteriorates.

The average particle size and the particle number density per unit area of the dispersed particles were determined by polishing the C cross section of the test piece taken from the steel material at right angles to the rolling direction, and electrolytically corroding the polished surface to obtain the dispersed particles. After appearing, observation is performed using a scanning electron microscope, 10 fields of view are picked up at a magnification of 5000 times, and the obtained image is processed and calculated using an image analyzer.

Next, a method for manufacturing a steel material according to the present invention will be described. Molten steel having the above composition is smelted by a commonly known method such as a converter, an electric furnace, and a vacuum melting furnace, and then, by a deoxidation treatment or a degassing process, first, the dissolved oxygen content is adjusted to 0.0030 to 0.0120 mass%. , REM is added to adjust the amount of dissolved oxygen to 0.0010 ~
Adjust to 0.0050% by mass. In the present invention, the deoxidation treatment is Al or
Instead of deoxidation by Ti, deoxidation by adding Si and / or Mn. As the preliminary deoxidation, prior to the deoxidation by adding Si and / or Mn, the preliminary deoxidation by adding Al may be performed. When performing preliminary deoxidation by adding Al, the amount of dissolved oxygen before deoxidation by adding Si and / or Mn is 0.0080 to
It is preferably adjusted to 0.0170% by mass. When performing preliminary deoxidation by adding Al, Al remaining in molten steel is
It must be 0.004% or less. If Al remains in excess of 0.004%, it becomes difficult to form the desired REM oxysulfide.

In the present invention, the amount of dissolved oxygen before the addition of REM is set to 0.
Adjust from 0030 to 0.0120%. As a result, one or more of the REM sulfide particles, REM oxides, and REM oxysulfide particles crystallize at the solid-liquid interface during the solidification process, suppressing unidirectional growth of dendrites, etc. Axial crystallization can be achieved, the secondary dendrite arm spacing becomes smaller, and Mn-based inclusions (dispersed particles) are generated by subsequent secondary deoxidation.
Is miniaturized. If the amount of dissolved oxygen is less than 0.0030%, it becomes difficult to form the desired REM oxysulfide, and the above effects cannot be expected. On the other hand, the amount of dissolved oxygen before adding REM is 0.01
If it exceeds 20%, REM becomes an oxide and it becomes difficult to form a desired REM sulfide or REM oxysulfide. For this reason, the ability to suppress unidirectional growth of dendrites decreases, and the secondary dendrite arm interval cannot be made fine.

When REM is added, sulfides, oxides, and oxysulfides are formed, and the amount of dissolved oxygen after addition is 0.0010
It is preferable to add S at the same time so that the concentration becomes 0.0050%. As a result, REM sulfide particles, RE
Either M oxide particles or REM oxysulfide particles can be easily crystallized at the solid-liquid interface, suppressing unidirectional growth of dendrites.

When the amount of dissolved oxygen after the addition of REM is less than 0.0010%, the secondary dendrite arm interval becomes large, and coarsening of austenite grains can be prevented as a secondary deoxidation product.
It becomes impossible to finely disperse the Mn-based composite particles, and the austenite grain coarsening suppressing ability decreases. On the other hand, when the amount of dissolved oxygen after REM addition exceeds 0.0050% and increases, Mn oxides become coarse, and it is difficult to form fine Mn-based composite particles that are effective in preventing coarsening of austenite grains, and coarsening of austenite grains occurs. Inhibitory ability is reduced.

In the present invention, the amount of dissolved oxygen is adjusted to 0 by adding REM.
0010 ~ 0.0050% by mass, while adjusting the molten steel composition to the above composition, then cast and steel material (slab)
And The casting method is not particularly limited, but in order to control the size and morphology of the dispersed particles within the above range,
In the solidification stage, it is preferable to use a continuous casting method capable of controlling the casting speed and cooling rate. The factors that determine the size of the dispersed particles are mainly the dissolved oxygen amount, Mn, and S amount, but since the cooling rate during casting also affects, the casting method is continuous casting rather than ingot casting. It is preferable that

Next, these steel materials, preferably 1000
Reheat to ~ 1300 ° C. If the reheating temperature is less than 1000 ° C,
Since the deformation resistance in hot rolling becomes high and the amount of reduction per pass cannot be made large, the number of rolling passes increases, leading to rolling efficiency, and crimping of casting defects in the steel material (slab). May not be possible. On the other hand, when the reheating temperature exceeds 1300 ° C., the crystal grains are remarkably coarsened, and the scale loss due to heating increases and the yield decreases. For this reason, the reheating temperature of steel materials is 1000 to 13
It is preferably in the range of 00 ° C. The temperature is more preferably 1050 to 1200 ° C.

The reheated steel material is then hot-rolled into a steel material having a predetermined size and shape. The rolling end temperature is preferably 650 ° C. or higher in order to secure the toughness of the steel material. If the steel material is shaped steel, the rolling end temperature shall be 750 ° C or higher. After completion of hot rolling, from the viewpoint of improving strength and toughness, accelerated cooling with an average cooling rate of 1.0 ° C / s or more may be performed up to 250 to 600 ° C (cooling stop temperature). The cooling rate for accelerated cooling is 1.0
If it is less than ° C / s, the structure becomes coarse and the toughness of the base material decreases.
Further, the cooling stop temperature for accelerated cooling is preferably 600 ° C. or lower and 250 ° C. or higher from the viewpoint of base material toughness. After the accelerated cooling, it is cooled to room temperature by air. Further, in the present invention, there is no problem even if tempering is performed from the viewpoint of reducing the residual stress of the steel material.

[0042]

[Example] Molten steel having the composition shown in Table 1 was melted in a converter to obtain R
After degassing H, the steel material (260m
m thick slab). During the melting process, the amount of dissolved oxygen immediately before the addition of REM was adjusted by deoxidation treatment. In addition, in some cases, preliminary deoxidation was performed by adding Al. Also, REM, S
The amount of dissolved oxygen was adjusted after the addition of REM by changing the amount added. Then, the content of the other components was adjusted to obtain molten steel having the composition shown in Table 1.

Then, the obtained steel material was reheated under the conditions shown in Table 2, hot-rolled under the conditions shown in Table 2, and then air-cooled to obtain thick steel plates having the plate thicknesses shown in Table 2. With respect to the obtained thick steel sheet, the base material structure, base material tensile properties, and base material toughness were investigated. (1) Base material structure A test piece was sampled from the obtained thick steel plate, and the type of dispersed particles, the average particle size, and the particle number density were examined. The type of dispersed particles, the average particle size, and the particle number density per unit area were determined by polishing the C cross section of the test piece and then electrolytically corroding the polished surface to reveal the dispersed particles, and then using a scanning electron microscope. Observe and image each 10 fields of view at a magnification of 5000 times, calculate the obtained image using an image analyzer, calculate the average value of each field of view, and further calculate the average value of each field of view. The value of The type of dispersed particles was determined using an EDX apparatus equipped with a scanning electron microscope. (2) Tensile properties of base metal JIS No. 4 tensile test pieces are taken from the C direction of 1 / 4t part of the thickness of the obtained thick steel plate, and the tensile test is carried out according to JIS Z 2204, and the yield is obtained. The point YP and tensile strength TS were determined. (3) Base metal toughness A JIS No. 4 impact test piece is taken from the C direction of the 1 / 4t part of the thickness of the obtained thick steel plate, and a Charpy impact test is carried out in accordance with the provisions of JIS Z 2242. The absorption energy vE _-40 (J) at the surface transition temperature vTrs and -40 ℃ was obtained.

Further, with respect to the obtained thick steel plate, the super-high heat input welding HAZ toughness was investigated. (4) Ultra-high heat input welding HAZ Toughness With respect to the thick steel plate obtained, a welded joint was produced by using electroslag welding or electrogas welding.
Weld bond, weld heat affected zone (HAZ
Collect the Charpy impact test piece from the center) and JIS Z
According to the regulations of 2242, a Charpy impact test was carried out at −40 ° C., the absorbed energy vE ₋₄₀ (J) was obtained, and the toughness of the super large heat input welded portion was evaluated.

The results obtained are shown in Table 2.

[0046]

[Table 1]

[0047]

[Table 2]

In all of the examples of the present invention, TS has a high strength of 500 MPa or more and a good base metal property having high toughness, a bond portion of ultra-high heat input welding exceeding 300 kJ / cm, and − in HAZ.
It is a thick steel plate with extremely good super-high heat input welding HAZ toughness, with absorbed energy at 40 ° C of 150 J or more. On the other hand, in the comparative example outside the scope of the present invention, the bond portion of the super large heat input welding, the crystal grains of HAZ are coarsened, and −40
The absorbed energy at ℃ is as low as 70 J or less, and it is a thick steel sheet with extremely high heat input welding HAZ toughness.

Although the steel material of the present invention is intended for ultra-high heat input welding, high heat input welding exceeding 100 kJ / cm,
Or small heat input welding such as carbon dioxide welding (heat input 20 kJ / cm
Needless to say, even if multi-layer welding is performed, sufficiently high HAZ toughness can be obtained and it can be sufficiently applied for large heat input welding or small heat input welding.

[0050]

INDUSTRIAL APPLICABILITY As described above, according to the present invention, the super-high heat input welding HAZ, which is a steel material for welded structure excellent in toughness, can be manufactured inexpensively and stably, and has a remarkable industrial effect.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C21C 7/06 C21C 7/06 C22C 38/14 C22C 38/14 38/58 38/58 (72) Inventor Toshiyuki Hoshino 1-chome, Mizushima Kawasaki-dori, Kurashiki City, Okayama Prefecture (without street number) F-term inside Mizushima Works, Kawasaki Steel Corporation (reference) 4K013 AA09 BA08 BA14 CB00 DA17 EA19 EA20 EA26 EA28 FA02

Claims (4)

[Claims] 1. In mass%, C: 0.01 to 0.18%, Si: 0.05 to 0.40%, Mn: 0.5 to 3.0%, P: 0.03% or less, S: 0.0005 to 0.0060%, Al: 0.004% or less, Ti : 0.004% or less, REM: 0.0030 to 0.0200%, O: 0.0070% or less, N: 0.0040 to 0.0070%, REM sulfide having a composition of balance Fe and unavoidable impurities and having an average particle size of 10 μm or less Particle, REM
One or more of oxide particles and REM oxysulfide particles, and Mn oxide and Mn sulfide having an average particle size of 1 μm or less,
A steel material having excellent super-heat-input welding heat-affected zone toughness, characterized by having a structure in which particles in which one or more of Mn oxysulfides are compounded are dispersed. 2. In addition to the above composition, N% by mass is further included.
b: 0.1% or less, V: 0.2% or less, Cu: 1.5% or less, N
i: 3.0% or less, Cr: 1.0% or less, Mo: 0.8% or less,
B: One or two selected from 0.0003 to 0.0040%
The thick steel sheet excellent in toughness of the heat-affected zone of the super-high heat input welding according to claim 1, characterized by containing at least one kind. 3. The molten steel is deoxidized by adding Si and / or Mn to adjust the dissolved oxygen amount to 0.0030 to 0.0120 mass%, and then REM is added to the dissolved oxygen amount to 0.0010 to 0.0050 mass%.
In addition to the above, the composition is adjusted so that C: 0.01 to 0.18%, Si: 0.05 to 0.40%, Mn: 0.5 to 3.0%, P: 0.03% or less, S: 0.0005 to 0.0060%, REM : 0.0030 to 0.0200%, N: 0.0040 to 0.0070%, and Al and Ti respectively
Ultra-high heat input welding heat-affected zone characterized by making molten steel limited to 0.004% or less, then casting the molten steel into a steel material, and then hot rolling the steel material into a steel material of a prescribed shape A method of manufacturing a steel material having excellent toughness. 4. The pre-deoxidation in which Al is added is performed before the deoxidation, and the amount of dissolved oxygen before the deoxidation is adjusted to 0.0080 to 0.0170% by mass. Ultra-high heat input welding Heat-affected zone Steel manufacturing method with excellent toughness.