JP2000119797A - High tensile strength steel material for welding, excellent in toughness in weld heat-affected zone, and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material having high safety, which is excellent in toughness at low temperature in a weld heat-affected zone (HAZ) at large heat input welding of (2. 5 to 100) kJ/mm and used for structures for shipbuilding, building, pressure vessel, line pipe, or the like and a stable manufacturing method of this steel material by mass production. SOLUTION: This high tensile strength steel material for welding, excellent in toughness in HAZ, has a composition containing, as principal components, 0.01-0.15% C, <=0.6% Si, 0.5-2.5% Mn, 0.005-0.025% Ti 0.0001-0.0050% Mg, and 0.0003-0.0020% B and also containing inevitable impurities. Further this steel material has a duplex phase matter structure where Mg oxides function as nucleation sites for sulfides or Ti nitrides.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a welding heat affected zone (H
The present invention relates to a steel material excellent in low-temperature toughness in AZ) and a method for manufacturing the same, and particularly to a large heat input welded steel material and an ultra-large heat input welded steel material optimal for performing arc welding, electron beam welding, laser welding, etc. Things.

[0002]

2. Description of the Related Art When thick steel plates are used for steel column members accompanying the recent rise in height of building structures, ultra-large heat input exceeding 50 kJ / mm or more, such as submerged arc welding, is required for manufacturing four-sided boxes. Welding has been applied. In particular, it has recently been pointed out that the necessity of improving the toughness levels of the base metal and the HAZ for steel sheets for building is pointed out from the viewpoint of safety of building structures. On the other hand, as for offshore structures, high HAZ toughness having a strength of YP360 to 460 MPa has been developed as steel for offshore structures. Further, in the long-distance pipeline for transporting natural gas, increasing the strength of the line pipe is being studied from the viewpoint of increasing the pressure for improving transport efficiency and reducing the amount of steel pipe used. One of the important properties required for steel materials used in these applications is HAZ toughness.

[0003] In recent years, heat treatment techniques or controlled rolling and thermomechanical processing (TMCP) have advanced to a high degree, and it has become easy to improve the low-temperature toughness of the steel itself.
Since AZ is reheated to a high temperature at the time of welding, the microstructure of the steel material is completely lost, and the microstructure becomes extremely coarse and H
The AZ toughness is significantly deteriorated. Conventionally, various kinds of knowledge and techniques have been developed to improve the high heat input welding HAZ toughness. However, since the heat history applied to the HAZ is very different between ultra-high heat input welding and large heat input welding, large heat input welding is required. HAZ of heat welding HAZ toughness improvement technology is super large heat input welding
In many cases, it cannot be applied to the improvement of Z toughness. When the large heat input welding HAZ toughness improving technology described above is classified, it can be roughly classified into two technologies. One is a technology for preventing austenite grain coarsening using the pinning effect of particles in steel, and the other is an effective grain refining technology using ferrite transformation in austenite grains. Representative proposals that disclose those technologies are shown below.

First, iron and steel, 61st year (1975) first
No. 1, page 68, examines the effect of suppressing austenite grain growth on various types of nitrides and carbides in steel. In the case of Ti-added steel, fine particles of TiN are formed in the steel. A technique for effectively suppressing austenite grain growth has been disclosed. JP-A-60-184663
In the gazette, Al: 0.04 to 0.10%, T
i: 0.002 to 0.02%, REM: 0.003 to
Includes 0.05% to prevent coarsening of the HAZ structure during large heat input welding by utilizing the formation of sulfides and oxides in REM, and heat input: HAZ toughness even with large heat input welding of 150 kJ / cm Techniques for improvement are disclosed. Also, Japanese Patent Application Laid-Open No. Sho 60-2
No. 45768 discloses a particle diameter of 0.1 to 3.0 μm,
Number of particles: 5 × 10 ³ to 1 × 10 ⁷ / mm ^{3 For} a steel containing either a Ti oxide or a composite with Ti oxide / Ti nitride, heat input: large input of 150 kJ / cm Heat welding H
A technique for improving the HAZ toughness by dissolving the HAZ structure by making these particles act as ferrite transformation nuclei in AZ is disclosed. Japanese Patent Application Laid-Open No. Hei 2-254118 discloses that a high heat input welding HA is used for steel containing appropriate amounts of Ti and S.
A technique for improving HAZ toughness by dissolving intragranular ferrite with a composite precipitate of TiN and MnS as a nucleus in the Z structure and refining the HAZ structure is disclosed. JP 61
No. 253344, Al: 0.005 to 0.0
8%, B: 0.0003-0.0050%, T
i, Ca and REM containing 0.03% or less of at least one of REM. Ca
BN in the cooling process starting from oxidation / sulfide or TiN
Is formed, and a ferrite is formed from the material to improve the HAZ toughness of large heat input. Further, CAMP-ISIJ Vol. 3 (1990) 80
On page 8, the effect of N on the intragranular ferrite transformation in Ti oxide steel is described.
3) No. 10 reports the effect of B on intragranular ferrite transformation in steels containing Ti oxide.
Also, Japanese Patent Application Laid-Open No. Hei 9-157787 discloses that Ti, Mg
And a particle diameter of M of 0.01 to 0.20 μm.
40,000 to 100,000 g-containing oxides / mm
⁽²⁾ a composite comprising MnS and a Ti-containing oxide having a particle diameter of 0.20 to 5.0 μm and containing 20 to 400 / mm
²⁾ Super large heat input welding HAZ of 500 kJ / cm or more by suppressing γ grain growth and promoting intragranular ferrite transformation.
A high-tensile steel excellent in toughness is disclosed.

[0005]

However, the following problems have been pointed out in each of the above-mentioned technologies. First, iron and steel, 61st year (1975) eleventh
No., page 68, aims at austenite grain growth using nitrides such as TiN. However, the effect is exhibited in large heat input welding, but 1350 in super large heat input welding. Most of the T
There is a drawback that iN dissolves and the grain growth effect is lost. According to the technique disclosed in Japanese Patent Application Laid-Open No. 60-184663, sulfur / oxide has a higher stability at a high temperature of 1350 ° C. or higher than nitride, so that the effect of suppressing grain growth is maintained. It is difficult to finely disperse oxides. Since this sulfur / oxide has a low density, even if the pinning effect of individual particles is maintained, the super large heat input welding H
There is a limit to reducing the austenite grain size of AZ, and this alone cannot improve toughness. In the technique disclosed in Japanese Patent Application Laid-Open No. 60-245768, T
Considering the high-temperature stability of i-oxide, its effect is maintained even in ultra-high heat input welding, but in ultra-high heat input welding HAZ, when austenite grains become coarse, the HAZ structure is refined only by intragranular transformation alone. There are limits. JP 2
The technique disclosed in Japanese Patent No. -254118 exhibits an effect when the residence time at 1350 ° C. or more is relatively short as in large heat input welding. When the residence time is long, there is a problem that the ferrite transformation nucleus disappears due to the solid solution of TiN during this period, and the effect cannot be exhibited. JP-A-61-2
In the technique disclosed in JP-A-53344, REM, Ca
Even if BN is formed on oxidation / sulfide of TiN or TiN,
It is difficult to increase the number of oxidations and sulfides of REM and Ca, and furthermore, there is a problem that TiN does not function as a ferrite formation nucleus as a solid solution and the effect cannot be exhibited.
Further, CAMP-ISIJ Vol. 3 (1990) 8
Also in the technique disclosed on page 08 and in Iron and Steel 79 (1993) No. 10, the level of HAZ toughness was not always sufficient.

Further, the technique disclosed in Japanese Patent Application Laid-Open No. 9-157787 is applied only to the case of ultra-high heat input welding where the heat input is 500 kJ / cm or more.
There is a problem that the HAZ toughness in the case of welding heat input of less than / cm is not mentioned.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a steel material having excellent low-temperature toughness in a heat affected zone (HAZ) and a method for producing the same. In particular, the present invention relates to arc welding, electron beam welding, laser welding and the like. An object of the present invention is to provide an optimal large heat input welding steel and an ultra-high heat input welding steel material and a method for producing the same. Here, the above-mentioned steel material is a steel plate, a hot-rolled steel plate, a shaped steel,
Refers to those including steel pipes.

[0008] The present inventors conducted research (oxide metallurgy) on chemical components (compositions) and their microstructures in order to improve the HAZ toughness of steel materials, and found a new high HA.
Z toughness steel was developed. In this study of oxide metallurgy, by controlling the composition and distribution of oxides and acting as heterogeneous nucleation sites such as sulfides and nitrides,
Technology that not only enables crystal grain growth control, intragranular ferrite transformation, matrix cleaning, etc., but also changes the composition of the oxide itself and controls its transformation ability according to the target steel properties. It is. However, this practical application has been a pioneering role in this field in the fields of thick plates, steel bars, and steel pipes, and its main technology has been disclosed in the prior art described above. Technology for improving low-temperature toughness in HAZ using intragranular transformed ferrite generated as nuclei (Ti deoxidized steel and Ti-Al
Composite deoxidized steel) and 2) composite precipitates: These are merely techniques for improving toughness during heat treatment using intragranular transformed ferrite generated with MnS + VN as nuclei (non-heat treated steel for hot forging). Also, despite the long time that these technologies have been put into practical use, research on oxide metallurgy has been stagnant, and the superior concept has not been fully utilized.

The present inventors have further studied to overcome the above problems, disperse more and more effective oxides more finely than before, and select an oxide species suitable for the above-mentioned purpose and a dispersion technique thereof. As a result of studying, it can be expected that in addition to the increase in intragranular transformed ferrite density and the formation ability, the effect of suppressing the growth of austenite grains during reheating (refinement) can be expected. , P,
If an oxide capable of trapping S or hydrogen can be found, it can be used to clean the matrix and prevent the surface swarf of the slab, etc. Furthermore, if an appropriate oxide can be dispersed at a high density, high-temperature creep can be achieved. It has been found that the strength can be expected to be improved. When this oxide metallurgy is completed, in the steel material manufacturing process, de-P, de-S and dehydrogenation processes in the hot metal pre-treatment and steel making process are simplified, low-temperature heating in the rolling process, reduction of TCMP and forming process It is possible to easily omit preheating and heat treatment at the time of welding. In terms of material development, ultra-high heat input welding steel, HAZ
We have obtained the knowledge that the development of new steel materials such as high-strength linepipe with excellent toughness and high-tensile steel with reduced preheating can be expected.

The present inventors conducted an exploratory study on oxide species having the above-mentioned effects, and found that
Having found that oxides are the most promising, we continued our study of Mg oxide metallurgy. As a result, Mg oxide (composite oxide) has a strong ability to generate intragranular ferrite transformation, suppresses the growth of austenite grains during reheating (miniaturization), and fixes various impurity elements P and S. It was also clarified that it had an effect.

The present invention is a result obtained as a result of the above-mentioned research, and invented a new oxide metallurgy technology which has not been clarified at all. The feature is that after adding Ti to low carbon boron-containing steel, Mg is added, and O is added.
By controlling the amount, a multi-phase material containing Ti and Mg (including sulfides such as MnS and CuS) is finely dispersed in the steel so that the Mg oxide (composite oxide) is strong. A high strength steel material for welding excellent in HAZ toughness which has an ability to generate intragranular ferrite transformation, and is characterized by suppressing growth of austenite grains during reheating (refinement) and fixing impurity elements P and S. It is a manufacturing method. The specific gist is as follows.

(1) C: 0.01-0.15%, Si:
0.6% or less, Mn: 0.5 to 2.5%, Ti: 0.0
05 to 0.025%, Mg: 0.0001 to 0.005
0%, B: 0.0003 to 0.0020% as a main component, and other unavoidable impurities, Mg oxide has a double phase structure functioning as a nucleation site of sulfide or Ti nitride A high-strength steel material for welding excellent in HAZ toughness, characterized in that:

(2) C: 0.01 to 0.15%, Si:
0.6% or less, Mn: 0.5 to 2.5%, Ti: 0.0
05 to 0.025%, Mg: 0.0001 to 0.005
0%, B: 0.0003 to 0.0020% as a main component, and other unavoidable impurities. A multi-phase material containing Mg and Ti is composed of a central part mainly containing Mg oxide, and a Ti nitride. High strength steel material for welding excellent in HAZ toughness, characterized by having a double phase structure consisting of a surface layer mainly containing an oxide and Mn sulfide.

(3) The welding excellent in HAZ toughness according to the above (1) or (2), wherein the oxynitride structure comprises a Mn-deficient layer of (Mg, Mn) S around MgO nuclei. For high tensile steel. (4) C: 0.01 to 0.15%, Si: 0.6% or less, Mn: 0.5 to 2.5%, P: 0.030% or less,
S: 0.005% or less, Al: 0.010% or less, T
i: 0.005 to 0.025%, Mg: 0.0001 to
0.0050%, O: 0.001 to 0.004%, N:
0.001 to 0.006%, B: 0.0003 to 0.0
020% as a main component, other unavoidable impurities, and having a structure in which 40 / mm ² or more of a multiphase material containing 0.0001 to several tens μm of Ti and Mg is dispersed. High strength steel material for welding with excellent HAZ toughness.

(5) C: 0.01-0.15%, Si:
0.6% or less, Mn: 0.5 to 2.5%, Ti: 0.0
05 to 0.025%, Mg: 0.0001 to 0.005
0%, B: 0.0003-0.0020% as a main component
And other unavoidable impurities, Mg and Ti
The central part mainly containing Mg oxide
And a surface layer mainly containing Ti nitride and Mn sulfide
Having a crystal structure consisting of
Having a particle size of 0.0001 μm to tens of μm
And Mg containing Mg, and 1 μm of the
250 m / mm particle size^TwoAs described above,
The particle size of the phase material of about 0.1 μm is 1 particle / μm ^TwoDispersion
Excellent HAZ toughness characterized by having a texture
High strength steel for welding.

(6) Nb: 0.005 to 5%
0.10%, V: 0.01 to 0.10%, Ni: 0.0
5 to 2.0%, Cu: 0.05 to 1.2%, Cr: 0.
(1) to (1) to (10), wherein one or more of Mo: 0.05 to 0.8% are contained.
The high tensile strength steel material for welding according to any one of (5) and having excellent HAZ toughness.

(7) C: 0.01 to 0.15%, Si:
0.6% or less, Mn: 0.5 to 2.5%, Ti: 0.0
05 to 0.025%, Mg: 0.0001 to 0.005
0%, B: 0.0003 to 0.0020% as a main component, at the time of smelting molten steel consisting of other unavoidable impurities, first add Ti as a deoxidizing agent to the molten steel, and then add Mg. A method for producing a high-strength steel material for welding excellent in HAZ toughness, characterized by the following.

(8) C: 0.01-0.15%, Si:
0.6% or less, Mn: 0.5 to 2.5%, Ti: 0.0
05 to 0.025%, Mg: 0.0001 to 0.005
0%, B: 0.0003 to 0.0020% as a main component, and at the time of melting molten steel composed of other unavoidable impurities, first add Ti as a deoxidizing agent to the molten steel, and then leave for 2 to 30 minutes. A method for producing a high-strength steel material for welding having excellent HAZ toughness, characterized by subsequently adding Mg and leaving the mixture to stand for 2 to 30 minutes before starting casting.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, the present invention focuses on Mg oxide as an oxide, and if this can be finely dispersed in steel, 1) MgO can be used in the same manner as TiN and VN. It has a high value for use as an intragranular transformation nucleus due to its good compatibility with Fe (ferrite), and 2) a very large particle size from the viewpoint that the thermally austenite grain size can be made fine by the thermally stable MgO pinning effect. It has become possible to significantly improve the HAZ toughness in heat input welding. In particular, this is achieved by adding Mg after adding Ti to low-carbon boron-containing steel and controlling the amount of O, The purpose of the present invention is to finely disperse a multi-phase material containing Ti and Mg (including sulfides such as MnS and CuS) in steel. Here, Ti and M
The oxide containing g (including sulfides such as MnS and CuS) is mainly composed of Ti oxide, M
compounds such as g-oxide or composite oxide of Ti and Mg,
Others, for example, oxides or complex oxides such as Mn, Si, Al, and Zr, nitrides such as TiN, Mn, Cu, C
a, a sulfide such as Mg, or a composite sulfide.

In the present invention, the Ti—Mg composite oxide finely dispersed in the low-carbon boron-containing steel is obtained by: 1) formation of fine intragranular ferrite in coarse austenite grains, and / or 2) austenite. It has been clarified that the coarsening of the grains is suppressed, the HAZ structure is refined, and the HAZ characteristics are significantly improved. And 3) hardness is 2
The effect is exhibited when the heat input is 50 Hv or less, the heat input is 5 kJ / mm or less, and the Mg content is 0.0010% or less. 4) The hardness is 250 Hv or more and the heat input is 5 kJ / mm or 5 or less.
It has been found that the effect is exhibited when kJ / mm or more and the amount of Mg exceeds 0.0010%. The present inventors consider these reasons as follows. That is,
HAZ hardness is 250Hv or less and heat input is 5kJ / m
When it is as low as m or less, the austenite grain size is constant at 50 to 200 μm because the cooling rate after welding is high.
Therefore, the oxide has the function of promoting the formation of intragranular ferrite. Here, when the amount of Mg exceeds 0.0010%, M
Since the generation of nS is less likely to occur, the generation of MnS is suppressed, and the effect of intragranular transformation is weakened. Therefore, the amount of Mg is 0.1.
It needs to be 0010% or less. On the other hand, when the HAZ hardness is 250 Hv or more and the heat input is 5 kJ / mm or less, the austenite grain size is almost constant. The HAZ toughness is improved due to the bainite structure. At 5 kJ / mm or more, Mg oxide is used as pinning particles because the austenite particle size becomes coarse. At this time, in order to finely disperse the Mg oxide, M
g must be 0.0010% or more.

In the case described above, Ti, Mg
The size and density of the composite oxide are key. However, when the amount of Mg is large, there is a case in which Mg alone exists in addition to the composite oxide of Ti and Mg, and
When the amount of Mg is small, there is a case where an oxide of Ti alone exists in addition to the composite oxide of Ti and Mg. But,
The size of the single and composite oxides of Ti and Mg is 0.00
When the thickness is 1 to 5 μm, there is no problem in HAZ toughness because these composite oxides are finely dispersed.

This composite oxide was dispersed in a larger amount and finer than the Ti oxide generated when Ti alone was added, and it was found that the effect on the above 1) and 2) was greater. However, in order to obtain such an effect, Mg
A crystal structure in which the oxide functions as a nucleation site of a sulfide or a Ti nitride, that is, a multiphase substance containing Mg and Ti has a central part mainly containing Mg oxide,
A crystal structure consisting of a surface layer mainly containing nitrides and Mn sulfides, and the crystal structure is formed around the MgO nucleus by (M
(g, Mn) It must be a composite oxide composed of a Mn-deficient layer of S. That is, the form of the oxide in the present invention is:
This is an oxide containing Mg and Ti in the center and Ti nitride and Mn sulfide in the surface layer. The particle diameter is 0.0001 μm to several tens μm, and their density is about 1 μm containing Mg and 250 particles / mm.
² or more, and the particle size of about 0.1μm containing Mg needs is one / [mu] m ² or more.

For that purpose, Ti, M contained in steel
g amount becomes very important, and the amounts of Ti and Mg are each set to 0.1.
005-0.025%, 0.0001-0.0050%
It is necessary to limit to the range. These lower limits are the minimum amounts for dispersing the composite oxide in a large amount and finely, and T
Although i depends on the amounts of O and N, since the low-temperature toughness due to TiC generation in HAZ is deteriorated, the upper limit must be set to 0.025%. Requires extremely difficult steelmaking, so the upper limit must be made 0.0050%. A preferable range of the Mg content is 0.0001 to 0.0030%.

The size of the composite Mg oxide of Ti and Mg is
If it is less than 0.001 μm, the oxide is too small to have the effect of suppressing austenite grain coarsening or the effect of forming intragranular ferrite. If the size exceeds 5.0 μm, the oxide is too large, and similarly, austenite grain coarsening occurs. The suppression effect or the effect of forming intragranular ferrite is lost. The oxide having the effect of suppressing the austenite grain coarsening is 0.1%.
A Ti oxide of 0.1 μm exists around a Mg oxide of about 01 μm. Therefore, 0.1 μm
It is preferable that the Mg oxide and the Ti nitride are finely dispersed in this size. Also, the Mg oxide at the center is
Since the melting point is extremely high, the particles do not disappear even at the welding temperature, and the distribution of the particles is not impaired.

The formation of inclusions having a double-phase structure as described above is closely related to deoxidation conditions. As for the order of the deoxidizing elements, it is necessary to perform the deoxidizing in the order of weak to strong deoxidation. That is, the first is the state of deoxidation of Si and Mn,
Next, Ti deoxidation is performed. Next, Mg deoxidation is performed in a state in which Ti oxide is considerably present as an oxide. At this time, M
Depending on the amount of g, the Ti oxide becomes Mg oxide, and the Ti oxide is reduced to TiN. When the amount of Mg is small, the amount of Ti oxide is large, and the amount of Mg oxide is small.
When the amount of g is large, it becomes Mg oxide or an oxide of Mg and Ti. What is important here is that Al is not added. This is because the addition of Al causes coagulation and coarsening of the oxide, which makes it difficult to disperse a fine oxide. In the present invention, basically, it is an essential requirement that no Al is added.

Next, a manufacturing method for obtaining a high-tensile steel material for welding according to the present invention will be described. Particularly, as a production method for obtaining a high-tensile steel material for welding according to the present invention, the component composition of the high-tensile steel material for welding is defined as follows: C: 0.01 to 0.1
5%, Si: 0.6% or less, Mn: 0.5 to 2.5%,
Ti: 0.005 to 0.025%, Mg: 0.0001
~ 0.0050%, B: 0.0003 ~ 0.0020%
One of the features is to use a molten steel composed of a main component and other unavoidable impurities as a starting material.The second important feature is that when the molten steel having the above composition is produced, a deoxidizing agent is first used as a deoxidizing agent in the molten steel. After adding Ti, the mixture is left for 2 to 30 minutes, preferably 3 to 10 minutes, and then Mg is further added, and the mixture is further left for 2 to 30 minutes, preferably 3 to 10 minutes.
And then start casting. It is considered that the casting starting temperature is preferably around 1570 ° C.

By adopting such casting conditions, the oxide of Si and Mn was formed before adding Ti, but after adding Ti, it is allowed to stand for 2 to 30 minutes, and preferably 3 to 30 minutes. Si, Mn and Ti
Oxide (Si and Mn amount is reduced and reduced)
And an oxide of Ti, and after adding Mg, 2-30
For 3 minutes, preferably 3 to 10 minutes.
An oxide of i, Mn, Ti, Mg (the amount of Si, Mn, Ti is reduced and reduced) is formed, and Ti and M
g of oxide (the amount of Ti is reduced by reduction) and M
g of oxide are formed. On the other hand, when there is no leaving time as described above, the reaction is started in a state where there is no time for forming an oxide of Ti or Mg.
It is considered that i or Mg does not function effectively and exists alone, and the significance of addition is diminished.

To further explain pinning, fine TiN usually precipitates on dislocations, so that it hardly precipitates when the cast slab is solidified. During rolling, or during cooling, fine TiN precipitates. On the other hand, TiN + MgO is thermally stable, and MgO and TiN have good lattice constants. Therefore, it is considered that TiN is preferentially deposited on MgO.

On the other hand, oxides that are effective in forming intragranular ferrite are those in which Ti nitride or Mn sulfide exists around Mg and Ti oxides. The size is about 0.3 to 3.0 μm, and it is preferable that particles of these sizes are finely dispersed. The form of the particles is Ti nitride mainly composed of TiN in the surface layer, and its lattice constant (0.4242 nm) is very close to the length of [110] with ferrite of 0.4054 nm. Is high. In addition, sulfide mainly composed of MnS is also present in the surface layer, and the presence of the Mn-deficient layer facilitates ferrite formation.

The density of the Ti / Mg composite oxide is necessary in the case of intragranular transformation. The number is 40 pieces / mm
^If it is less than ² , the number of oxide dispersions is so small that it has no effect on austenite grain coarsening suppression or intragranular transformation.
mm ² or more is required. The density of Mg and Ti oxides effective for the formation of intragranular transformation is 250 particles / m by CMA measurement.
It is preferably at least m ² . CMA of density in this case
The measuring method is a method of measuring an area of 0.5 mm × 0.5 mm by CMA using a beam diameter of 1 μm.

Further, Mg oxide + Ti nitride, which is effective in suppressing austenite grain coarsening, is very fine, about 0.1 μm, and cannot be measured by CMA. Also Zen
Since pinning is determined by the radius and volume fraction of the oxide from the relationship of er, it is difficult to introduce the concept of density.
Therefore, here, it is acceptable if the composition of the oxide is Mg oxide and Ti nitride and the Mg content is at least 0.0005% or more.

Further, in order to obtain a large amount of fine Ti-Mg oxide, it is important to limit the amount of O. If the O content is too small, a large amount of the composite oxide cannot be obtained, and if it is too large, the cleanliness of the steel deteriorates. Therefore, the amount of O is 0.001 to
It was limited to 0.004%. The reasons for limiting the component elements will be described below. C content is limited to 0.01 to 0.15%. Carbon is an extremely effective element for improving the strength of steel,
At least 0.01% is necessary for the manifestation of the crystal grain refining effect. However, if the amount of C is too large, the low-temperature toughness of the base material and HAZ is remarkably deteriorated. Therefore, the upper limit is set to 0.15%.

Si is an element added for deoxidation and for improving the strength. However, if added in a large amount, the HAZ toughness is remarkably deteriorated, so the upper limit was made 0.6%. Steel can be sufficiently deoxidized with Ti or Mg, and Si need not always be added. Mn is an element indispensable for securing a balance between strength and low-temperature toughness, and its lower limit is 0.5%. However, if the amount of Mn is too large, the hardenability of steel increases and H
Not only deteriorates AZ toughness, but also continuous cast pieces (cast pieces)
Is promoted at the center, and the low-temperature toughness of the base material is also deteriorated.

The addition of Ti forms fine TiN, suppresses coarsening of austenite grains in the reheated slab and in the welded HAZ, refines the microstructure, and improves the base material and HAZ.
To improve the low temperature toughness of steel. When the amount of Al is small (for example, 0.010% or less), Ti forms an oxide and Ti
As a preferential nucleus of N, it acts as an intragranular ferrite forming nucleus of HAZ, and also has the effect of making the HAZ structure finer. In order to exhibit such Ti addition effect, at least 0.005
% Ti addition is required. However, if the amount of Ti is too large, coarsening of TiN and precipitation hardening due to TiC occur, deteriorating low-temperature toughness. Therefore, the upper limit was set to 0.025%.

Mg is a strong deoxidizing element and combines with oxygen to form a fine oxide (composite oxide containing a small amount of Ti or the like).
To form Mg oxide finely dispersed in steel is stable even at a high temperature as compared with TiN, and suppresses coarsening of γ grains throughout the HAZ or generates fine intragranular ferrite in coarsened austenite grains, resulting in HAZ. Improve toughness. For this purpose, at least 0.0001% of Mg is required. However, since it is very difficult for steelmaking to put a large amount of Mg into steel, the upper limit was made 0.0050%. The preferred Mg content is 0.0001% to 0.0030.
%.

As for the amount of O, it is effective to reduce the amount of the strongly deoxidized element Al as much as possible and to control it to 0.001 to 0.004% in order to sufficiently obtain a fine oxide when Ti and Mg are added. It is. N forms TiN and suppresses coarsening of austenite grains in the slab during reheating and in the welded HAZ, thereby improving the low-temperature toughness of the base material and the HAZ. The minimum required for this is 0.001%. However, if the amount of N is too large, it causes deterioration of HAZ toughness due to slab surface flaws and solid solution N, so the upper limit must be suppressed to 0.006%.

B is a very effective element for dramatically improving the hardenability of steel in a very small amount, suppressing the formation of upper bainite, and obtaining a structure mainly composed of lower bainite. 1
% Mn. Further, B enhances the effect of improving the hardenability of Mo, and also synergistically increases the hardenability together with Nb. To obtain such an effect, B
Should be at least 0.0003%. On the other hand, excessive addition not only deteriorates the low-temperature toughness, but also increases
In some cases, the effect of improving the hardenability of the steel may be lost, so the upper limit was made 0.0020%.

Further, according to the present invention, the impurity elements P,
The S content is set to 0.030% or less and 0.005% or less, respectively. The main reason for this is to further improve the low-temperature toughness of the base material and HAZ. The reduction of the P content reduces the center segregation of the slab, prevents grain boundary fracture, and improves low temperature toughness. Also, the reduction of the S content has the effect of reducing MnS stretched by controlled rolling and improving ductility.

Next, the purpose of adding Nb, V, Ni, Cu, Cr and Mo will be described. The main purpose of adding these elements to the basic components is to maintain strength, low-temperature toughness, and HA without deteriorating the excellent characteristics of the steel of the present invention.
This is for further improving properties such as Z toughness and expanding the size of a steel material that can be manufactured. Therefore, the amount of addition is of a nature that should be restricted.

Nb coexists with Mo to suppress the recrystallization of austenite during controlled rolling, thereby making the crystal grains finer, but also contributing to precipitation hardening and hardenability, and the effect of toughening the steel. Having. Nb must be at least 0.005% or more. However, if the Nb content is too large, the HAZ toughness is adversely affected, so the upper limit was made 0.10%. V
Has almost the same effect as Nb, but the effect is Nb
Was considered weaker than V of at least 0.01%
The addition of V is indispensable, and the upper limit of V is set to 0.1 from the viewpoint of HAZ toughness.
Up to 10% is acceptable. The purpose of adding Ni is to improve strength and low-temperature toughness. Ni is added by Mn or C
Compared to the addition of r and Mo, not only is hard formation of a hardened structure harmful to low-temperature toughness in the rolled structure (especially the center segregation zone of the slab), but the addition of a small amount of Ni is effective in improving the HAZ toughness (In terms of HAZ toughness, the particularly effective addition amount of Ni is 0.3% or more). However, if the addition amount is too large, not only deteriorates the HAZ toughness,
Since the economy is impaired, the upper limit is set to 2.0%. In addition, Ni addition is performed during continuous casting and hot rolling.
It is also effective in preventing u-cracks. In this case, Ni is C
It is necessary to add 1/3 or more of the u amount.

Cu has almost the same effect as Ni, and also has an effect on improving corrosion resistance and resistance to hydrogen-induced cracking. Further, the addition of about 0.5% or more of Cu greatly increases the strength by precipitation hardening. However, if added excessively, the toughness of the base material and HAZ is reduced due to precipitation hardening, and C is reduced during hot rolling.
Since u cracks occur, the upper limit is set to 1.2%.

[0042] Cr increases the strength of the base material and the welded portion, but if too much, significantly deteriorates the HAZ toughness. For this reason, the upper limit of the amount of Cr is 1.0%. Mo coexists with Nb and strongly suppresses recrystallization of austenite during controlled rolling, and is also effective in refining the austenite structure. However, excessive Mo addition degrades HAZ toughness, so the upper limit was made 0.80%.

Lower limit of Ni, Cu, Cr and Mo contents
05% is the minimum amount at which the effect on the material due to the addition of each element becomes significant. Next, the order of deoxidation will be described. The order of the deoxidizing elements must be added in the order of weak to strong deoxidizing. That is, first, deoxidation of Ti is performed in a state of deoxidation of Si. At this time, the oxide contains a considerable amount of Ti oxide. Next, Mg deoxidation is performed. At this time, although depending on the amount of Mg, the Ti oxide becomes an Mg oxide and the Ti oxide is reduced to replace TiN. When the amount of Mg is small, the amount of Ti oxide is large, and the amount of Mg oxide is small. Conversely, when the amount of Mg is large, it becomes an Mg oxide or an oxide of Mg and Ti.

It should be noted here that Al
Is not added. The addition of Al causes the oxides to become coarse and coarse, making it difficult for fine oxides to be dispersed. Therefore, this system is basically a steel to which Al is not added. However, Al is inevitably mixed, but 0.0
If it is 15% or less, there is no problem of the oxide coarsening and coarsening.

Next, the size and the number of the composite oxide of Ti and Mg will be described. When the size of the composite Mg oxide of Ti and Mg is less than 0.001 μm, the oxide is too small to have the effect of suppressing austenite grain coarsening or the effect of forming intragranular ferrite. Is too large, the effect of suppressing austenite grain coarsening or the effect of forming intragranular ferrite is also lost.

The oxide effective for suppressing the austenite grain coarsening is around 0.1 μm of Mg oxide around 0.1 μm.
1 μm of Ti nitride is present. Therefore, Mg oxide and Ti nitride of about 0.1 μm are preferable, and the oxide of this size is finely dispersed. Since the central portion of the Mg oxide has an extremely high melting point, the particles do not disappear even at the welding temperature, and the distribution of the particles is not impaired.

On the other hand, oxides which are effective for the formation of intragranular ferrite are those in which Ti nitride or Mn sulfide exists around Mg and Ti oxides. The size is about 0.3 to 3.0 μm, and it is preferable that particles of these sizes are finely dispersed. As for the form of the particles, Ti nitride mainly composed of TiN is present in the surface layer, and its lattice constant is very close to that of ferrite. In addition, sulfide mainly composed of MnS is also present in the surface layer, and the presence of the Mn-deficient layer facilitates ferrite formation.

The density of the composite oxide of Ti and Mg is necessary in the case of intragranular transformation. The number is 40 pieces / mm
^If it is less than ² , the number of oxide dispersions is so small that it has no effect on austenite grain coarsening suppression or intragranular transformation.
mm ² or more is required. The density of Mg and Ti oxides effective for the formation of intragranular transformation is 250 particles / m by CMA measurement.
It is preferably at least m ² . CMA of density in this case
The measuring method is a method of measuring an area of 0.5 mm × 0.5 mm by CMA using a beam diameter of 1 μm.

Further, Mg oxide + Ti nitride, which is effective in suppressing austenite grain coarsening, is very fine, about 0.1 μm, and cannot be measured by CMA. Also Zen
Since pinning is determined by the radius and volume fraction of the oxide from the relationship of er, it is difficult to introduce the concept of density.
Therefore, here, it is acceptable if the composition of the oxide is Mg oxide and Ti nitride and the Mg content is at least 0.0005% or more.

As the Mg-added material, pure metal Mg or Mg alloy may be used.

[0051]

Next, embodiments of the present invention will be described. <Example 1> Steel ingots of various steel components were produced by laboratory melting (50 kg, 120 mm thick steel ingot). These ingots were rolled under various conditions into steel plates having a thickness of 13 to 30 mm, and various mechanical properties were investigated. Mechanical properties of steel sheet (yield strength:
YS, tensile strength: TS, -20 ° C in Charpy impact test
Absorbed energy at: vE _-20 and 50% fracture surface transition temperature:
vTrs) was investigated in the direction perpendicular to the rolling. HAZ toughness (absorbed energy at −20 ° C. in Charpy impact test:
vE _-20 ) was evaluated by HAZ reproduced with a reproduction heat cycler (maximum heating temperature: 1400 ° C., 800 to 500 ° C.).
Of cooling time [Δt _{80 0-500]:} 28 seconds). The size and number of the Ti and Mg composite oxides were determined by CMA analysis.

Examples are shown in Table 1. The steel sheet manufactured according to the present invention has a HAZ Charpy absorbed energy at −20 ° C. of more than 150 J and has excellent HAZ toughness. On the other hand, in the comparative steel, since the chemical composition or the size and density of the Ti and Mg composite oxides are inappropriate, the Charpy absorbed energy of HAZ at -20 ° C is remarkably inferior. Steel 1
Sample No. 5 has a low O content and hence a low density of Mg and Ti composite oxides, and therefore has a low Charpy absorption energy of HAZ.
Since the steel 16 has too much Al, the Mg and Ti composite oxides have little density and the HAZ has low Charpy absorbed energy. Since steel 17 has too little Ti content, HA 17
Z has low Charpy absorbed energy. Since steel 18 has a large amount of Ti, the Charpy absorbed energy of HAZ is slightly low. Steel 19 has a large O content, so the Mg and Ti composite oxides have a large particle size, the density of Ti and Mg oxides at 0.001 to 5 μm is low, and the HAZ has low Charpy absorbed energy. Since the steel 20 does not contain Mg, the Charpy absorbed energy of the HAZ is slightly lower. <Example 2> In a converter, alloy elements other than Ti and Mg are added. The addition of Ti and Mg is performed by secondary refining. Fifteen minutes after the addition of Ti, metallic Mg or Mg alloy is blown into the molten steel. After 20 minutes, continuous casting is performed to form a slab. The slab is heated, for example, to 1150 ° C.
The slab is extracted 60 minutes after the temperature reaches 50 ° C., and is immediately subjected to rough rolling to 100 mm and finish rolling to 20 mm, for example. After that, water cooling is performed, and the water cooling stop temperature is set to, for example, 400 ° C., thereby completing the manufacture of the thick steel plate. This thick steel plate is formed, and the inner and outer surfaces are welded. The heat input at this time is, for example, 3.5 kJ / cm in the case of 20 mm. After that, the tube was expanded and UOE
It was a steel pipe.

[0053]

As described above, the present invention relates to Ti and M
g in an appropriate amount to form oxides of Ti and Mg,
Moreover, the intragranular transformation is promoted with a particle diameter of about 1 μm containing Mg, and the HAZ toughness is improved by refining the crystal grains with a particle diameter of about 0.1 μm containing Mg.
Highly safe steel materials used for structures such as shipbuilding, construction, pressure vessels, line pipes, etc., which have excellent low-temperature toughness in the HAZ in large heat input welding of 100 kJ / mm, are produced stably in large quantities. It became possible.

[0054]

[Table 1]

[0055]

[Table 2]

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Naoki Saito 20-1 Shintomi, Futtsu City, Chiba Prefecture Nippon Steel Corporation Technology Development Division (72) Inventor Hiroshi Tamehiro 20-1 Shintomi, Futtsu City, Chiba Prefecture New Japan 4K013 BA14 DA03 DA08 DA09 DAEA EA18

Claims (8)

[Claims] 1. C: 0.01 to 0.15%, Si: 0.1%
6% or less, Mn: 0.5 to 2.5%, Ti: 0.005
0.025%, Mg: 0.0001 to 0.0050
%, B: 0.0003 to 0.0020% as a main component,
A high-strength steel material for welding having excellent HAZ toughness, characterized by having a double-phase structure in which Mg oxide functions as a nucleation site of sulfide or Ti nitride, which is composed of other unavoidable impurities. 2. C: 0.01 to 0.15%, Si: 0.1%.
6% or less, Mn: 0.5 to 2.5%, Ti: 0.005
0.025%, Mg: 0.0001 to 0.0050
%, B: 0.0003 to 0.0020% as a main component,
Other unavoidable impurities, the two-phase material containing Mg and Ti contains a central part mainly containing Mg oxide,
A high-strength steel material for welding excellent in HAZ toughness, having a double-phase structure consisting of a surface layer mainly containing i-nitride and Mn sulfide. 3. The multi-phase structure has (M) around the MgO nucleus.
3. A high-tensile steel material for welding having excellent HAZ toughness according to claim 1 or 2, comprising a Mn-deficient layer of (g, Mn) S. 4. C: 0.01 to 0.15%, Si: 0.1%
6% or less, Mn: 0.5 to 2.5%, P: 0.030%
Hereinafter, S: 0.005% or less, Al: 0.010% or less, Ti: 0.005 to 0.025%, Mg: 0.00
01-0.0050%, O: 0.001-0.004
%, N: 0.001 to 0.006%, B: 0.0003
A structure in which 40% / mm ² or more of a multi-phase substance containing Ti and Mg having a particle size of 0.0001 to several tens of μm and having a main component of about 0.0020% and other unavoidable impurities is dispersed. A high-strength steel material for welding excellent in HAZ toughness characterized by having. 5. C: 0.01 to 0.15%, Si: 0.
6% or less, Mn: 0.5 to 2.5%, Ti: 0.005
0.025%, Mg: 0.0001 to 0.0050
%, B: 0.0003 to 0.0020% as a main component,
Other unavoidable impurities, the two-phase material containing Mg and Ti contains a central part mainly containing Mg oxide,
i has a double phase structure consisting of a surface layer mainly containing nitride and Mn sulfide, and the particle diameter of the double phase containing Ti and Mg is 0.0001 μm to several tens μm. The multi-phased material contains, and the particle diameter of about 1 μm of the multi-phased substance is 250 particles / mm ² or more, and the particle diameter of about 0.1 μm of the multi-phased substance is 1 particle / μm ² or more. A high strength steel material for welding excellent in HAZ toughness, characterized by having a fine structure. 6. Nb: 0.005 to the main component.
-0.10%, V: 0.01-0.10%, Ni: 0.
05-2.0%, Cu: 0.05-1.2%, Cr:
The composition according to claim 1, wherein one or more of 0.05 to 1.0% and Mo: 0.05 to 0.8% are contained.
5. A high tensile strength steel material for welding according to any one of the above items 5, which is excellent in HAZ toughness. 7. C: 0.01-0.15%, Si: 0.
6% or less, Mn: 0.5 to 2.5%, Ti: 0.005
0.025%, Mg: 0.0001 to 0.0050
%, B: 0.0003 to 0.0020% as a main component,
A method for producing a high-strength steel material for welding excellent in HAZ toughness, wherein Ti is first added as a deoxidizing agent to molten steel at the time of melting molten steel including unavoidable impurities, and then Mg is added. 8. C: 0.01 to 0.15%, Si: 0.1 to 0.1%.
6% or less, Mn: 0.5 to 2.5%, Ti: 0.005
0.025%, Mg: 0.0001 to 0.0050
%, B: 0.0003 to 0.0020% as a main component,
When smelting molten steel consisting of other unavoidable impurities, first add Ti as a deoxidizing agent in the molten steel, leave it for 2 to 30 minutes, then add Mg, and leave it for another 2 to 30 minutes. A method for producing a high-strength steel material for welding excellent in HAZ toughness, characterized by starting casting.