JP3287125B2 - High tensile steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel used for welding structures such as pressure vessels, ships, bridges, buildings, marine structures and line pipes, and particularly to a welded portion (welded metal portion). And a high-strength steel which has excellent toughness of the weld heat-affected zone) and resistance to galvanization cracking of the weld heat-affected zone and can be used at low temperatures.

[0002]

2. Description of the Related Art In recent years, high-strength steel for welding used for offshore structures installed in ice-sea areas, line pipes for cold regions, or large steel structures such as ships and LNG tanks, has high quality characteristics. Improvement requests are becoming more stringent. Steel materials for these applications are required to have sufficient strength according to the purpose of use, of course, especially the toughness of the weld metal part,
There is a high demand for improvement in toughness in a heat affected zone (hereinafter, referred to as HAZ) of a base material in contact with a weld metal and improvement in hot-dip galvanizing crack resistance of HAZ.

Conventionally, the HAZ toughness of a high-strength steel has a large effect on the austenite grain size of the weld,
It is known that the transformation structure, the precipitation state of the fine hardened phase, and the amount of solute N in the steel sheet are main factors, and various measures have been proposed based on this knowledge.

[0004] For example, regarding (1) and (2), a method in which a small amount of Ti is added and TiN is finely precipitated in steel to suppress coarsening of austenite crystal grains ("Iron and Steel", Vol. No., page 1232), added a small amount of Ca and added CaS
A method of forming fine grains by refining austenite crystal grains and depositing intragranular ferrite with CaS and CaO as nuclei ("Journal of the Japan Welding Society", February 52, 1983, Vol. 52, No. 2) No. 49), a method of similarly refining crystal grains using an oxide of a rare earth element (hereinafter referred to as REM) (JP-A-64-15320), and further using Ti oxide particles as nucleation sites. Method for Refining Microstructure by Generating Ferrite Inside (JP-A-57-51243 and JP-A-61-797)
No. 45) has been proposed.

[0005] As for the low carbon equivalent, Si and Al
There has been proposed a method of suppressing precipitation of a hardened phase by reducing (aluminum) (Japanese Patent Application Laid-Open No. 2-190423). Regarding the method, a method of reducing the amount of N (nitrogen) contained in steel, a method of fixing N as AlN by adding an excessive amount of Al, and the like have been proposed.

However, in the above countermeasures, TiN
Is considered to be largely dissolved in the base material in the portion heated to 1400 ° C. or higher, and in particular, it is inevitable that the austenite crystal grains in the HAZ near the melting line become coarse when large heat input welding is performed . In addition, TiN dissolved in the heating process
Does not reprecipitate during the cooling process. That is, in the portion where TiN is dissolved, ferrite transformation in the grains during the cooling process does not occur, and further, the amount of dissolved nitrogen is increased.
There is a disadvantage that deterioration of AZ toughness cannot be avoided.

On the other hand, regarding the use of oxide particles of Ca, REM, and Ti, it is very difficult to uniformly disperse these in a finely stable state in steel during melting.
Actual production problems remain. Even if these oxide particles can be dispersed, the particle size is relatively large, and it is possible to exhibit the function as a ferrite formation nucleus. It is considered that it does not reach the point where the stop effect is exhibited. In other words, the technology to suppress the coarsening of austenite crystal grains in the part heated to a high temperature of 1400 ° C or higher has not yet been developed, and methods such as low carbon equivalent, low Si, and low Al have been developed. HAZ even when using techniques such as controlling the precipitation morphology of the fine hardened phase and reducing the amount of solute N
There is naturally a limit to improving toughness.

[0008] Al, which is a deoxidized metal, has high reactivity with oxygen in molten steel and produces alumina (Al ₂ O ₃ ). This Al ₂ O
_{3 Since} (solid) has a high interfacial energy with molten steel (liquid Fe), the generated oxide particles are agglomerated in the molten steel and coarsen, and either float or float as a few μm-sized aggregated inclusions in the steel. It is well known that it is difficult to keep finely dispersed in steel because it stays.

The Al-containing steel has a bainite-based structure. For this reason, from the viewpoint of increasing the toughness of the HAZ, it is considered that a low Al-based composition is preferable. Furthermore, if possible, reducing the Al to an extremely low level so that Al does not remain in the steel suppresses the formation of island-like martensite, which degrades toughness in the HAZ, and improves the HAZ toughness as a structure mainly composed of fine ferrite grains. (The Journal of the Japan Welding Society, Vol. 10, No. 3, 409 issued in March 1992)
Page, see).

[0010] Plating treatment is widely used in welding structures as a means of enhancing rust resistance and improving appearance, and hot-dip galvanizing exhibits an excellent paintability improving effect. It is in. However, hot-dip galvanizing is often applied to steel after welding, and depending on the composition of the steel, the shape of the assembled structure, the method of immersion in the plating bath, etc., "liquid metal embrittlement (crystal grain boundary It has been pointed out that cracking of the weld heat-affected zone due to embrittlement caused by the intrusion of zinc into the weld heat-affected zone "often occurs.

In particular, in recent years, there has been an increase in the tendency of steel materials to increase in strength in response to demands for larger and lighter steel structures, and the number of occurrences of cracks in the heat affected zone of welding has increased (more than 55 kgf / mm ²⁾ . The cracking of the HAZ due to liquid metal embrittlement has attracted great attention. For this reason, studies on liquid metal embrittlement cracking countermeasures have been actively conducted, and various reports or proposals have been made.

For example, “Iron and Steel, 70th Year (1984), 10th
No., pp. 131-137 "and" Journal of the Japan Welding Society, Vol. 4 (198
6), No. 4, pp. 93-99, reports that "It is effective to reduce the hardness of HAZ to prevent liquid metal embrittlement cracking of HAZ by molten zinc." . But,
Reducing the hardness of the HAZ means reducing the amount of alloying components in the steel material, lowering the strength level, and contradicting the trend toward higher strength.

As a measure for preventing the liquid metal embrittlement cracking of the HAZ by the molten zinc, for example, Japanese Patent Publication No. 2-5814 discloses that the contents of C, Mn, Si, Nb, V, Ti and Al are limited. And 93−8.8 × 10 ³ C (C−0.1) −63Si−38Mn + 340V
It is disclosed that by satisfying ≧ 42, a high-strength low-alloy steel excellent in resistance to plating cracking of HAZ can be obtained.

Japanese Patent Application Laid-Open No. 2-57669 discloses B
(Boron) is limited to 0.0002% or less, and C + Mn / 10 + Si / 30 + Cr / 10 + Mo / 20 + V / 3 + Ti / 5−1 /
It is disclosed that by satisfying 40000B ≦ 0.19, a high-strength steel excellent in plating crack resistance can be obtained.

However, even with these methods, when the size of the structure becomes large, the residual stress in welding and the thermal stress during plating increase, so that the occurrence of hot-dip galvanizing cracks is not eliminated, and complete prevention measures are established. It has not been done.

Next, regarding the improvement of the toughness of the weld metal part,
In order to ensure the toughness of the weld metal after arc welding of carbon steel and low alloy steel, it was considered necessary to make the structure of the weld metal a fine acicular ferrite-based structure. To obtain a fine acicular ferrite structure, it is important to add Ti and B to the weld metal and to adjust the amount of oxygen. When the amount of oxygen is adjusted to 200 to 400 ppm, a fine acicular ferrite structure is obtained, but the amount of oxygen is reduced.
If the content is less than 100 ppm, the hardenability is increased to form an upper bainite structure, and conversely, the toughness is reduced. (Toughnes
s Improvementin Weld Metal of Carbon and HSLA Stee
ls in Japan, First United States-Japan Sympo. Advan
ces in Welding Metallugy, (1991) p.227-250).

Current electron beam welding and laser beam welding using a high energy density heat source are highly efficient low heat input welding methods capable of obtaining deep penetration with a narrower bead width than arc welding, and are performed by welding. The greater the thickness, the more the cost advantage is exhibited. However, the toughness of the as-welded weld metal is not always good (Journal of the Japan Welding Society, No. 54
Vol. (1985), No. 2, pp. 46-50), which hinders the expansion of the application of welding methods using high energy density heat sources.

In order to improve the toughness of the weld metal part, there is a method of performing heat treatment after welding. However, the number of working steps is increased, and the above-mentioned cost merit cannot be obtained.

The cause of the decrease in the toughness of the weld metal by the welding method using the high energy density heat source is that a fine acicular ferrite structure cannot be obtained due to a low oxygen content (amount of oxide). That is, in the welding method using a high energy density heat source, since the steel material itself is melted and joined without using an oxygen source such as slag (flux) as in the arc welding method, the composition of the obtained weld metal portion is basically the same. Has the same oxygen content as the base material.
It is often 0 ppm or less. Therefore, the amount of oxygen required for obtaining a fine acicular ferrite structure exhibiting good toughness by arc welding is far less than the amount of oxygen required to be 200 to 400 ppm.

As a method of solving this, for example,
No. 62-64486 proposes a method in which a steel in which Ti oxide is uniformly dispersed is welded by a low oxygen welding method such as an electron beam welding method to increase the toughness of a weld metal. JP-A-63-126638 discloses a steel substantially free of Al (Al ≦
0.007%) by a high energy density welding method to adjust the Sol.Al content of the weld metal to an appropriate range, without intentionally introducing non-metallic inclusions (Ti oxide),
A method for improving the toughness of a weld metal has been proposed.

However, the method of uniformly dispersing the Ti oxide is considerably difficult by the smelting method, the size of the particles is relatively large, and it is not sufficient by the welding method using a high energy density heat source having a high cooling rate. It does not act as a ferrite transformation nucleus.

Further, as an effect of the steel substantially not containing Al, the rejection of C from the transformed ferrite to the untransformed austenite is suppressed, the concentration of C in the austenite is reduced, and the ferrite of the untransformed austenite is reduced. It is said that the transformation is not hindered and a fine needle-like ferrite structure is obtained as a result. However, in the welding method using a high energy density heat source with a low heat input and an extremely fast cooling rate,
Untransformed austenite, which has a large degree of supercooling, is more energetically advantageous to precipitate as a second phase (carbide or martensite) than to nucleate ferrite from a place other than nonmetallic inclusions. It is clear that the use of non-metallic inclusions (oxides) is significantly inferior.

[0023]

As described above, the improvement of various performances of the steel sheet for welding has been studied separately for the base metal, the weld metal and the HAZ, and the respective influencing factors have been clarified.

TiN for improving HAZ toughness is 1400
In a welded portion heated to a temperature of not less than ° C., it is not possible to suppress the coarsening of austenite crystal grains by dissolving in the base metal, and no improvement effect is obtained. Further, it is difficult to uniformly disperse Ca, REM, and Ti oxides at the time of smelting, and a pinning effect for refining austenite crystal grains cannot be obtained.

Various types of hot-dip galvanizing cracking resistance have been proposed. However, as the size of the structure increases, welding residual stress and thermal stress at the time of plating increase. However, complete preventive measures have not yet been established.

It has the effect of improving the toughness of the weld metal.
As described above, it is difficult to uniformly disperse the Ti oxide at the time of smelting, the particles become large, and the Ti oxide does not act as a ferrite transformation nucleus in a welding method using a high energy density heat source. In addition, a steel having substantially reduced Al has a problem that a needle-like ferrite structure cannot be obtained by a welding method using a high energy density heat source.

An object of the present invention is to provide a steel material which can be mass-produced, has an appropriate size and an appropriate number of inclusion particles dispersed in the steel, and has a vicinity of a melting line heated to a high temperature of 1400 ° C. or more. Austenite crystal grains are suppressed from coarsening even in the region of
It is an object of the present invention to provide a welding steel material in which the structure is refined over the entire welded portion and which has good low-temperature toughness and excellent hot-dip galvanizing cracking resistance.

[0028]

Heretofore, the improvement of various properties of a steel sheet for welding has been studied separately for a base metal, a weld metal and a HAZ, and the influence factors of each have been clarified.

The present inventors have studied the above influential factors in more detail, and as a result, have obtained the following findings.

The improvement of the low-temperature toughness of HAZ is largely attained by suppressing austenite grain coarsening.

In order to improve the hot-dip galvanizing cracking resistance,
"Materials, Vol. 30 (1981), No. 329, pp. 83-89, A paper entitled" Weld joint strength of steel in molten zinc "" In molten zinc, grain boundary fracture is dominant, and cracks are likely to occur in the coarse grain area because the grain boundary area is smaller than in the fine grain area.

Therefore, it is considered that a crack was generated from the coarse-grained region near the bond portion and the fracture occurred. As described, the cracks in the weld heat-affected zone due to hot-dip galvanizing are caused by the structure due to welding, that is, due to the coarsening of austenite grains. It is not a solution.

The low-temperature toughness of the weld metal portion is such that the microstructure of the weld metal is refined by promoting precipitation of intragranular ferrite due to the effect of reducing hardenability due to the refinement of austenite grains,
Low temperature toughness can be improved.

Based on this finding, a method for refining austenite grains was examined.

In the production of steel, Al is a basic element added for the purpose of deoxidation and particle size adjustment. On the other hand, in the case of Ti-added steel, it has been reported that when the Al content is reduced, the HAZ toughness is improved through effects such as a decrease in hardenability and a refinement of the structure (Journal of the Japan Welding Society, Vol.
992), No. 3, pp. 93-99).

Therefore, the present inventors have proposed that Ti-free C-Si-
A study focused on Al using steel with a simple composition of Mn system. As a result, "in a steel in which fine inclusions having a specific Mn-Al-Si-O (oxygen) balance are dispersed, 1400
Even if it is heated to above ℃, the austenite grain coarsening is suppressed. ”

The present invention has been made based on the above-mentioned new findings, and the gist of the invention lies in the following high-strength steels (1) to (5). Unless otherwise specified,% means mass%.

(1) In the ternary phase diagram of MnO—Al ₂ O ₃ —SiO ₂ , MnO is 23 to 56% and Al ₂ O ₃ is 4 to ₂ % by mass%.
A ternary composite oxide particle having a composition in a region where each range of 7% and SiO ₂ is 30 to 54% overlaps, and the diameter is
A high-strength steel, wherein fine particles having a particle size of less than 0.2 μm are dispersed at least 6 particles / mm ² .

(2) The high-tensile steel according to (1), wherein 5% or less of O (oxygen) of the oxide constituting the fine particles of the composite oxide is replaced with S (sulfur).

(3) The high-tensile steel according to the above (1) or (2), wherein the average composition of the steel is as follows.

The average composition of the steel is expressed in mass%, C: 0.01 to 0.2.
2%, Si: 0.01-0.5%, Mn: 0.3-3.0%, Sol.Al:
0.008% or less, Insol.Al: 0.0002-0.005%, O: 0.0
01-0.005%, P: 0.02% or less, S: 0.02% or less,
And N: 0.01% or less, and the first group contained Nb: 0.
005-0.1%, V: One or two of 0.005-0.1%
In the second group, Ni: 0.05-3.0%, Cu: 0.05-0.
It contains one or two of the 7%, and the third group contains Cr: 0.
03-1.5%, Mo: 0.03-0.7% 1 or 2 types
The fourth group contains B: 0.0003-0.002%.
Including one or two or more of the first to fourth groups,
The part is Fe and inevitable impurities.

(4) In addition to the components described in (3) above,
The high-strength steel according to the above (1) or (2), containing one or two of Ti: 0.006 to 0.02% and Zr: 0.005 to 0.025%.

(5) In addition to the components described in (3) and / or (4) above, further contains 0.0005 to 0.005% of Ca: and / or one or more rare earth elements (REM): 0.005 to 0.05%. The high-strength steel according to (1) or (2) above.

(5) -1 C: 0.01-0.2%, Si: 0.01-0.5%, Mn: 0.3-3.0
%, Sol.Al: 0.008% or less, Insol.Al: 0.0002-0.00
5%, O: 0.001 to 0.005%, P: 0.02% or less, S: 0.0
2% or less, and N: 0.01% or less.
The group was Nb: 0.005 to 0.1% and V: 0.005 to 0.1%.
The second group contains 0.05% to 3.0% Ni,
Cu: contains one or two of 0.05-0.7%,
Group: Cr: 0.03 to 1.5%, Mo: 0.03 to 0.7%
Or the second group, and group 4 contains B: 0.0003-0.
One of the first to fourth groups containing 002%
(1) containing two or more groups, further containing 0.0005 to 0.005% of Ca: and / or one or more rare earth elements (REM): 0.005 to 0.05%, and the balance consisting of Fe and inevitable impurities
Or (2) high tensile steel.

(5) -2 C: 0.01-0.2%, Si: 0.01-0.5%, Mn: 0.3-3.0
%, Sol.Al: 0.008% or less, Insol.Al: 0.0002-0.00
5%, O: 0.001 to 0.005%, P: 0.02% or less, S: 0.0
2% or less, and N: 0.01% or less.
The group was Nb: 0.005 to 0.1% and V: 0.005 to 0.1%.
The second group contains 0.05% to 3.0% Ni,
Cu: contains one or two of 0.05-0.7%,
Group: Cr: 0.03 to 1.5%, Mo: 0.03 to 0.7%
Or the second group, and group 4 contains B: 0.0003-0.
One of the first to fourth groups containing 002%
Includes 2 or more groups, with Ti: 0.006-0.02% and Zr: 0.005
One or two of 0.025% and Ca: 0.0005 to 0.00
5% or / and one or more rare earth elements (REMs): 0.005
The high-strength steel according to the above (1) or (2), containing up to 0.05%, with the balance being Fe and unavoidable impurities.

[0046]

The basic principle of the present invention is that the Mn-Al-Si-based oxide, which usually agglomerates in molten steel and separates by flotation or becomes a large inclusion with a particle size of several μm, contains fine inclusions of less than 0.2 μm. HAZ toughness, HA
Z is to improve the galvanizing crack resistance and the toughness of the weld metal part, particularly the toughness of the weld metal part in welding using a high energy density heat source. Hereinafter, the basic principle will be described first.

As described above, it has been considered that Al ₂ O ₃ having a high interfacial energy with molten steel is difficult to finely disperse in steel. However, the melting point of the composite oxide to be produced can be lowered by minimizing the amount of Al added and keeping the production of Al ₂ O ₃ alone to a very small amount.

FIG. 2 is a ternary phase diagram of MnO—Al ₂ O ₃ —SiO ₂ . As is clear from the figure, the oxide has a low melting point due to complexation. Generally, when comparing the solid / liquid interface and the liquid / liquid interface, the latter has lower interfacial energy,
A small amount of liquid is dispersed in a large amount of liquid, and the whole liquid is easily suspended uniformly. Therefore, if the composite oxide is liquid at the time of smelting steel, the molten steel and the composite oxide come into contact at the liquid / liquid interface, so that the interfacial energy is significantly reduced, and the composite oxide does not agglomerate and has a diameter of 0.2. It will be suspended in the molten steel as fine inclusions of μm or less. Then, even after the molten steel is solidified, the composite oxide remains finely dispersed in the steel.

The complex oxide referred to here is MnO—Al ₂ O
_3- SiO _{2 is} a ternary composite oxide . That is, MnO, Si
In addition to O ₂ and Al ₂ O ₃ , an acid composed of these three components
Other oxides in an amount that will not substantially affect the melting point of the product _{(e.g., CaO, MgO, TiO 2,} ZrO 2, (REM) O
Etc.) or sulfides (eg, MnS, CuS, TiS, Ca
S, etc.), or even contain these compound inclusions <br/> have good.

In particular, in the composite oxide in which a part of O (oxygen) is replaced with S (sulfur), in other words, in the case where a part of MnO is replaced with MnS, the MnS has a difference between molten steel and particles. Since it has the effect of lowering the interfacial energy, it is preferable as finely dispersed particles. However, when the substitution ratio of O in the oxide by S increases, the generated MnS aggregates and the inclusions become coarse, so that the substitution ratio is desirably 5% or less.

MnO—Al ₂ O ₃ —SiO ₂ 3
The source composite oxide is hereinafter referred to as a Mn-Al-Si composite oxide.

[0052] MnO is the major oxide constituting the Mn-Al-Si-based composite oxide, the content of the oxides of Al ₂ O ₃ and SiO ₂ are, MnO-Al ₂ O ₃ -SiO shown in FIG. 1 _{2 In the} ternary system diagram, Mn
_{O: 23~56%, Al 2 O} 3: 4~27%, SiO 2: each range of 30 to 54% overlap area, i.e., selected in the region indicated by hatching in FIG. If the composition is within this range, the melting point of the composite oxide becomes 1400 ° C or lower, which is lower than the melting point (melting temperature) of the molten steel having a normal high-tensile steel composition of about 1550 ° C, as is clear from FIG. It is.

The inclusions in the high-tensile steel of the present invention are Al—Si—
Made of Mn-based composite oxide, the main inclusions have a particle size of 0.2μ
The fine inclusions of less than m are uniformly dispersed at a rate of 6 / mm ² or more. Here, the reason that the particle size of the inclusions must be less than 0.2 μm is that the particle size of the main inclusions is less than 0.2 μm.
When the particle size is not less than μm, such inclusions are likely to aggregate to form a large cluster, which is not effective from the viewpoint of austenite grain refinement. In addition, the amount of inclusions generated is 6
If the number is less than the number of pieces / mm ² , the austenite grains grow to 300 μm or more.

Although the above description has been given of the phenomenon in which the steel material is melted, the same phenomenon occurs in the weld metal, and such a steel sheet is welded using a high energy density heat source (the oxygen in the molten pool is low). 0.2μ in diameter in the weld metal
m or less can be dispersed to improve the toughness of the weld metal portion. Such a material can also be used as a welding material.

Next, a method of manufacturing a steel material according to the present invention will be described.

As a method for finely dispersing the composite oxide, there is a method described below.

(A) A melt at 1550 ° C. or higher obtained by heating and melting a mixed oxide previously blended so as to have the composition shown by the hatched portion in FIG. 1 is poured into molten steel.

(B) A mixed powder in which the fine powder of each oxide is blended so as to have the composition shown by the hatched portion in FIG. 1 is added to molten steel at 1550 ° C. or higher.

(C) In the stage of preparing the composition of the molten steel, the composite oxide produced by the metal-slag reaction is adjusted so as to have the composition indicated by the hatched portion in FIG.

In the above methods (a) and (b), it is appropriate that the mixed oxide is added in an amount of 0.3 to 10 g per kg of molten steel.
As for the added mixture, those floating on the molten steel are separated and removed as slag, but those in the molten steel exist in the liquid phase until the last stage of solidification, so those remaining in the steel at the solidification stage are agglomerated. Finely dispersed without coarsening.

In the method (c), since the element having the highest bonding (reactivity) with oxygen is Al, the element is prepared in the early stage of melting.
It is not preferable to generate a large amount of Al ₂ O ₃ , and in this regard, preparation of the amounts of Al and O and reforming of the slag are very important.

Next, the reason why the HAZ toughness of the steel of the present invention, the hot-dip galvanizing crack resistance of the HAZ, and the toughness of the weld metal portion are extremely high will be described.

Normally, Al is an acid-soluble Al containing solid solution Al or Al nitride (hereinafter referred to as Sol. Al) and a non-acid-soluble Al mainly present as a Mn-Al-Si-based oxide in a steel material. Al
(Here, Insol.Al). Suppression of coarsening of austenite crystal grains in the welded portion in the steel of the present invention,
Mainly finely dispersed inclusions (Mn-Al-Si based composite oxide)
Based on the pinning effect.

The size of the Mn—Al—Si based composite oxide is as follows:
As described above, the thickness is desirably less than 0.2 μm, and as many particles as possible are dispersed from the viewpoint of improving the toughness due to the refinement of the structure of the weld metal part and the HAZ, or from the viewpoint of improving the galvanizing cracking resistance. It is desirable. However, the number of dispersed particles is more than 3 million particles / cm ³ ,
In a planar observation using an optical microscope or the like, if the number is 6 / mm ² or more, the HAZ toughness level currently required can be satisfied. However, the number of dispersed particles is preferably 30 / mm ² or more.

The high-tensile steel of the present invention is characterized by the above-mentioned dispersed state of inclusions, and there is no particular limitation on the alloy components. The alloy components and their contents may be selected according to the purpose of use of the high-strength steel and the mechanical and chemical properties required therefor. However, as a high-strength steel that is used particularly at low temperatures and requires high HAZ toughness,
Those having the chemical composition described in (5) are preferable. Less than,
The components of the steel will be described. In this specification, the “average composition of steel” is a composition obtained by chemical analysis including the above-mentioned inclusions, and% of the component content means mass%.

C: C is for securing the strength of the steel material and Nb, V
Is added in order to bring about the effect of making the structure finer at the time of addition. Below 0.01%, these effects are not sufficient. However, if there is too much C, martensite (α ')
And similar pearlite (α / Fe ₃ C) to deteriorate the HAZ toughness, increase the hot-dip galvanizing cracking susceptibility in the HAZ and adversely affect the toughness and weldability of the base metal and weld metal. Therefore, C is desirably set to 0.2% or less.

Si: Si is an element effective for preliminary deoxidation of molten steel, but does not form a solid solution in cementite. Therefore, when added in a large amount, untransformed austenite grains hinder the decomposition of ferrite grains and cementite. Promotes the formation of island-like martensite, and generates a coarse oxide by being combined with MnO. For these reasons, the addition of Si is performed in a range where the content in steel is 0.5% or less. In addition, 0.01% or more is required to secure the minimum amount of Si to be contained in the inclusions.

[0068] Mn: Mn with is an element necessary for securing strength, in element effective as a deoxidizer, and the above-described Al ₂ O ₃
It is also necessary to produce MnO and MnS that cause a decrease in the melting point of the MnO. Therefore, the content of Mn needs to be 0.3% or more. However, the excessive addition of Mn causes the coarsening of the composite oxide similarly to Si, increases the hardenability and deteriorates the weldability and the HAZ toughness, so that its content should not exceed 3.0%.

P: P is an impurity element unavoidably contained in steel, and is a grain boundary segregation element, and thus causes grain boundary cracking in HAZ. Therefore, the lower the content, the lower the content, the better. Further, in order to improve the base material toughness, the toughness of the weld metal part and the HAZ, and to reduce the segregation at the center of the slab, the content is desirably 0.01% or less.

S: When S is present in a large amount, a precipitate of MnS alone is formed as a starting point of a weld crack.

Therefore, the content is set to 0.02% or less. However, in order to further improve the toughness of the base metal, the toughness of the weld metal portion and the HAZ, and to reduce slab center segregation, the content is preferably 0.005% or less. On the other hand, S in which MnS is substituted for a part of oxygen of the Mn-Al-Si-based oxide has an effect of lowering the interfacial energy of the composite oxide as described above. Therefore, the presence of a certain amount in the range below the upper limit is rather preferable. In order to utilize the effect of S, it is desirable that S is contained at 0.0002% or more.

Sol.Al: An increase in Sol.Al is a negative factor for the fine dispersion of the Mn—Al—Si based composite oxide. That is, when Sol.Al increases and the Al amount increases as a whole, Mn-Al-
The Si-based oxide aggregates and coarsens, and the desired effect of suppressing austenite grain growth cannot be obtained. Also, the reduction of Sol.Al is effective for suppressing the formation of island-like martensite, which is a fine hardened phase that adversely affects toughness. From the above viewpoints, the lower the content of Sol. Al, the more preferable the upper limit is 0.008%.

Insol.Al: Al forms coarse Al ₂ O ₃ when it is present in a large amount as described above. Therefore, the content of Insol.Al, including the amount added as an oxide, must be 0.005% or less so as not to lower the cleanliness of the steel. But,
As described above, 0.0002% or more is required to generate a composite oxide necessary for refining the HAZ structure and obtain a sufficient austenite crystal grain growth inhibitory effect.

O (oxygen): The dispersed particles of the present invention are mainly composed of a composite oxide. If O is less than 0.001%, a desired amount of inclusions cannot be obtained. However, if present in a large amount, the cleanliness deteriorates remarkably, so that it is difficult to secure practical toughness for both the base metal, the weld metal part and the HAZ. Therefore, the content is set to 0.005% or less.

N (nitrogen): When N is present in a large amount, similarly to oxygen, it deteriorates the toughness of the base metal, the weld metal and the HAZ. Normally, N is made harmless in the form of TiN precipitation by the addition of Ti. However, in the present invention, even when the addition of Ti is considered, TiN may form a solid solution at the time of heating and may cause the hardening of the weld metal portion and the HAZ. In addition, the upper limit is set to 0.01% because the nitride may be coarsened by aggregation. If no nitride is formed, the toughness of the weld metal and the HAZ is deteriorated due to the increase of the solute N.
% Is preferable.

The main components of the high-strength steel according to the present invention are as described above. In addition to these, Nb, V, Ni, Cu, C
At least one element selected from the group consisting of r, Mo and B, the group consisting of Ti and Zr, and the group of elements consisting of Ca and REM can be added.

V and Nb: V and Nb are effective in ensuring the strength of steel materials. However, if added in excessive amounts, the hardenability of the steel is excessively increased, the weld metal portion and the HAZ toughness are deteriorated, and hot-dip galvanizing is performed. It tends to increase crack sensitivity. Therefore, if added, the content should be 0.1% or less. The lower limit is 0.005% to ensure strength.
Is desirable.

Cu, Ni: All of these elements increase the strength and toughness of the steel material, have little adverse effect on the weld metal part and the HAZ toughness, and do not increase the hot-dip galvanizing cracking susceptibility. However, when the Cu content exceeds 0.7%, slab surface cracks occur frequently during continuous casting and rolling, and stable operation becomes difficult. In addition, an increase in the amount of Ni causes a significant increase in manufacturing cost. Therefore, Cu and
The Ni content should be 0.7% and 3.0% or less, respectively. Desirable lower limits are each 0.05%.

Cr, Mo: Cr and Mo increase the hardenability of the steel material and are effective for securing the strength.
In order to prevent hardening of the steel and to suppress low-temperature cracking of the weld, the content of Cr is 1.5% or less and the content of Mo is 0.7% or less. Desirable lower limits are each 0.03%.

B: B enhances the hardenability of the steel material and is effective in ensuring the strength. However, if added excessively, it causes problems such as low-temperature cracking of the weld because it is a grain boundary segregation element. Therefore, B
Should not exceed 0.002%. In addition,
If the effect of B is to be used positively, the content is desirably 0.0003% or more.

Ti: Ti combines with N in steel to form a nitride, suppresses coarsening of austenite crystal grains in a region heated to less than 1400 ° C., and acts as precipitation nuclei for ferrite grains. It has the effect of making the tissue finer. To use such an effect, Ti is added. But 0.006
%, The above effect is not exhibited, and if the content exceeds 0.02%, the base metal toughness and the weld metal part and HAZ
Affects toughness. In the steel of the present invention, since most of O (oxygen) is present as an Al-Si-Mn-based oxide, oxides such as TiO ₂ and Ti ₂ O ₃ are not generated, and Al-Si-
Si exists in a form in which Ti is partially substituted for Si and Mn of the Mn-based oxide.

Zr: Zr is an element having almost the same action as Ti. When Zr is added for the same reason as Ti, its content is preferably 0.005 to 0.025%.

Ca: Ca is an element that forms oxides and sulfides that serve as precipitation nuclei for intragranular ferrite. It also controls the form of sulfides, improves low-temperature toughness, and is also effective in improving hydrogen-induced cracking resistance, which is particularly important in steel for line pipes and the like. To obtain such an effect of Ca, 0.005%
The above content is required. On the other hand, if the content exceeds 0.01%, Ca-based large inclusions and clusters are formed to deteriorate the cleanliness of the steel. REM: REM produces oxides and sulfides that become ferrite precipitation nuclei like Ca. In addition, the sulfide form is controlled to improve the low-temperature toughness, and is also effective in improving the resistance to hydrogen-induced cracking. However, when the content is less than 0.005%, the above effect cannot be exerted. When the content exceeds 0.05%, coarse inclusions are formed, toughness and weldability are deteriorated, and further, the cleanliness is also deteriorated. The high tensile strength steel can increase the toughness of the weld metal part, particularly when welding is performed using a high energy density heat source.

The end face of the high-strength steel of the present invention is machined into an I-shaped groove and butt-welded using a high energy density heat source.

The high-strength steel of the present invention is inserted into a joint end face of a steel material of another material, and butt-welded using a high energy density heat source.

The high-tensile steel of the present invention is used as a welding material (filler wire).

Although the high-tensile steel of the present invention improves the toughness of the welded metal portion as it is, it may be heat-treated after welding.

[0088]

EXAMPLES Steel slabs of the steels of the present invention and comparative steels having the chemical compositions shown in Table 1 were prepared. Nos. 16 to 19 of the present invention steel and comparative steel
After adding 0.5 to 8.0 g of molten oxide of 1600 ° C. having the composition shown in Table 2 to 1 kg of molten steel,
It was cast into a 150kg steel ingot and made into a billet by the ingot method. Only those that remained in the steel due to this oxide addition became dispersed particles. Comparative steels Nos. 20 to 24 were produced by a usual melting method without adding a molten oxide.

[0089]

[Table 1]

[0090]

[Table 2]

The above slabs were made into thick plates 50, 30 and 15 mm under the conditions of rolling and heat treatment as described in (1) below. Various sample pieces were collected from the central part of the thickness of the obtained steel sheet, and the number of dispersed particles, the average composition of the particles, and the mechanical properties of the base material were investigated.

The number and average composition of the dispersed particles were determined by the following test methods (2) and (3).

Table 2 shows the results of the examination of the dispersed particles, and Table 3 shows the mechanical properties of the base material.

[0094]

[Table 3]

A 50 mm steel plate was welded using a high energy density heat source to investigate the toughness of the weld metal, and 30 mm was used to investigate the HAS toughness by a weld reproduction heat cycle test.
mm was used for investigating hot-dip galvanizing cracks by a restraint joint test.

A. HAZ toughness investigation 11mm in length, 11mm in width and length from the center of 30mm thick steel plate
A 60 mm test material was sampled, subjected to a welding reproduction heat cycle shown in (4) below, and subjected to a Charpy impact test shown in (5) below. Table 4 shows the results.

[0097]

[Table 4]

B. Test of hot-dip galvanizing cracking A restraint joint test was performed as a test for reproducing cracking of the welded heat-affected zone. As shown in FIG. 4, this constrained joint test piece is composed of three 15 mm-thick steel plates (1) combined in a cross, and one pass of fillet welding at two intersecting corners (heat input: 17 kJ / cm
, Cooling time: about 8 seconds), a test bead (6) was prepared, and a restraining bead (7) was provided at two other intersecting corners by multi-layer welding (20 embossments). Is manufactured by applying a residual stress corresponding to the yield stress of the steel sheet to the surface of the steel sheet. This restraint joint test piece is hot-dip galvanized bath (480 ° C)
The test bead is immersed for 30 minutes and inspected for cracks on the test bead surface (6). After observing the cracks, the hardness of the bond was measured by a Vickers hardness meter, and the austenite particle size of the bond was measured by a linear analysis method. Table 4 shows the results.

C. Investigation of Toughness of Weld Metal Using High Energy Density Heat Source A 50 mm thick steel plate (1) was subjected to I-groove processing, and butt welding of the same steel type was performed without a root gap. The welding conditions are shown in (6) below. In the as-welded state, a Charpy impact test specimen (3, JIS No. 4 specimen) was taken from the position shown in FIG.
The test was performed at the same temperature in the range of 0 ° C. to 0 ° C. with the number of tests set to 3. Table 5 shows the results.

[0100]

[Table 5]

(1) Rolling and heat treatment conditions (described in the second column “Treatment method” in Table 3) N: After normal rolling, 850-950 ° C. × 1 h → air-cooled normalization QT: Normal rolling After that, 850 ~ 950 ℃ × 1h → water cooling → 600
DQT: Rolling finish temperature is 850-950 ° C, direct quenching (water cooling) → 630 ° C × 1h → Air-cooled tempering Accelerated cooling: Rolling finish temperature is 750-800 ° C Air cooling after water cooling from 400 to 500 ° C (2) Investigation of the number of dispersed particles less than 0.2 μm in diameter Energy dispersive X-ray detector (EDS) for each steel type
Photographs were taken using a scanning electron microscope equipped with a sample, and the number of dispersed particles having a diameter of less than 0.2 μm in about 3 mm ² was counted in terms of the sample surface.

(3) Investigation of average composition of dispersed particles having a diameter of less than 0.2 μm Further, the composition of each oxide was analyzed by EDS. However, the sensitivity of EDS in the composition analysis is not sufficiently accurate for O (oxygen), and because it is a minute inclusion, the characteristic X-ray from Fe of the base material is detected. For this reason, from the quantitative values for the three elements Mn, Al, and Si,
The amounts of MnO, Al ₂ O ₃ , and SiO ₂ were converted, and the composition of the Mn—Al—Si-based inclusions was determined from this. The obtained composition was averaged for each steel type to obtain an average composition of fine inclusions. Also, Mn, A
l, Obtain the total amount of O when the amount of Si is converted to oxide,
The quantitative value of S obtained by the S analysis was divided by the total amount of O to obtain the replacement ratio of S to O.

(4) Welding heat cycle conditions Condition 1: After holding at the maximum heating temperature of 1400 ° C. for 5 seconds, cooling from 800 to 500 ° C. in 60 seconds. Condition 2: Holding at the maximum heating temperature of 1400 ° C. for 5 seconds. After that, it cools from 800 to 500 ° C in 180 seconds (equivalent to the heat cycle near the weld line when 30mm steel plate is welded with welding heat input of 70kJ / cm and 200kJ / cm, respectively). (5) Impact test Three welding heat cycle tests were performed for each steel type, and each was processed into a JIS No. 4 Charpy test piece. With respect to this sample, the average value of the absorbed energy (vE _-60 ) when performing the Charpy impact test at −60 ° C. was determined.

(6) Electron beam welding conditions Acceleration voltage: 60 kV Beam current: 50 mA Welding speed: 500 mm / min Work distance: 325 mm a / b: 0.9 (a is the distance from the center of the electron beam converging lens to the remarkable focus of the electron beam. (The distance b is the distance from the center of the converging lens to the surface of the workpiece.) The average composition of the inclusions in Table 2 is shown in FIG. ● shown in the figure
Is the composition of inclusions of the Mn-Al-Si-based oxide in the steels of the present invention (Nos. 1 to 15), and ○ is the composition of inclusions in the comparative steels (Nos. 16 to 18, 20 to 24). And ◎ indicates that the S content was as high as 10.5% in the inclusion composition of the comparative steel (No. 19).

As is clear from the number of dispersed particles in Table 2,
The steel having the inclusion composition in the range of the present invention indicated by
m or less fine inclusions per 1 mm ² are dispersed, whereas the comparative steel indicated by ○ has few fine inclusions of 0.2 μm or less and many coarse inclusions exceeding 0.2 μm. Exists. This is presumably because the inclusions in the range of the present invention have a low melting point and a low interfacial energy between the molten steel and the inclusions, so that they were finely dispersed, as described in the section of action. In addition,
It is considered that those indicated by で indicate that the Sn content of the base material was large, so that the amount of MnS in the inclusions was increased and the particles were agglomerated and coarse.

Table 3 shows that both the steel of the present invention and the comparative steel are excellent in tensile strength and toughness (fracture transition temperature).

From Table 4, it can be seen that the inclusions of the Mn-Al-Si-based oxide have the composition in the hatched portion of FIG. 1 and the inclusions are 6 particles / mm ² or more of dispersed particles having a diameter of less than 0.2 μm. The present invention steel (Nos. 1 to 15) which exists has a HAZ equivalent toughness of 45 vJ- ₆₀ / kg when the heat input is 45 kJ / cm by the welding reproducible heat cycle test.
m and at a heat input of 100 kJ / cm, 11 vE _-60 / kgm or more was obtained, indicating that the HAZ toughness is remarkably excellent. In addition, as seen in Nos. 6, 7 and 10 of the steel of the present invention,
When the number of finely dispersed particles is 30 / mm ² or more, even when welding with a large heat input (reproduction heat cycle test), 20 vE
Absorbed energy of _-60 / kgm or more is obtained, and excellent toughness is obtained. In addition, as a result of investigating the hot-dip galvanizing cracking property of the restraint joint test piece, no cracking was observed in the steels of the present invention (Nos. 1 to 15). Furthermore, a weld metal part and a HAZ obtained by welding using a high energy density heat source
It can be also seen that this also shows excellent toughness.

On the other hand, the comparative steel No. 16 had a high Sol. Al content of 0.010%, so the content of Al ₂ O _{3 in the} inclusion composition was as high as 70%, and the number of particles of 0.2 μm or less was five. The austenitic crystal grain size was as large as 143 μm, the impact value of the HAZ and the weld metal was low, and cracks were observed upon immersion in the hot dip galvanizing solution.

No. 17 is a steel not containing Al, in which the content of MnO in the inclusion composition is as high as 60%, the number of particles of 0.2 μm or less is as small as 3, and the austenite crystal grain size is as large as 132 μm. Low impact value of HAZ and weld metal,
Micro-cracks were observed upon immersion in the hot-dip galvanizing solution.

In No. 18, the content of Sol. Al was as high as 0.030%, so that the Al ₂ O ₃ content of the inclusion composition was 40% and the MnO content was 20%, out of the range of the invention, and the number of particles of 0.2 μm or less was included. And the austenite grain size is as large as 148 μm,
The impact values of the HAZ and the weld metal were low, and cracks were observed when immersed in the hot dip galvanizing solution.

No. 19 has a high S content of 0.030%,
The S content of the inclusion composition is as high as 10.5%, the shape of the particles is large, the number of particles less than 0.2 μm is as small as two, the austenite crystal particle size is as large as 137 μm, and the impact value of HAZ and weld metal is large And cracking was observed upon immersion in a hot dip galvanizing solution.

[0112] Nos. 20 to 24 are steels produced by ordinary smelting without the addition of mixed oxides, so that the inclusion compositions are all out of the range of the invention, and the number of particles of 0.2 μm or less is 4%.
Austenite grain size 175 ~ 199
μm, the impact value of the HAZ and the weld metal part was low, and micro-cracks or cracks were observed upon immersion in the hot-dip galvanizing solution.

[0113]

The steel of the present invention has a high base metal toughness,
Excellent in AZ toughness, hot-dip galvanizing crack resistance of HAZ and toughness of weld metal, and excellent low-temperature toughness even in welded structures using large heat input welding method or welding method using high energy density heat source Can be secured.

[Brief description of the drawings]

FIG. 1 shows the composition range (shaded area) of fine inclusions in the steel of the present invention.
It is a diagram showing on a MnO-Al ₂ O ₃ -SiO ₂ system equilibrium phase diagram.

FIG. 2 is an MnO—Al ₂ O ₃ —SiO ₂ system equilibrium diagram, in which thin lines and numerical values indicate melting points (° C.).

FIG. 3 is a diagram showing a position at which an impact test piece is collected from an electron beam weld.

FIG. 4 is a diagram showing a restraint joint test piece.

[Explanation of symbols]

1. Steel plate 2. Weld metal 3.
3. Charpy impact test specimen Weld metal notch position 5. 5. HAZ notch position
Test beads 7. Restraint bead

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yuichi Komizo 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. (56) References JP-A-5-271864 (JP, A) JP-A-1-152239 (JP, A) JP-A-63-195217 (JP, A) JP-A-6-41686 (JP, A) JP-A-64-15321 (JP, A) (58) Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (5)

(57) [Claims] In ternary phase diagram of the claims 1] _{MnO-Al 2 O 3 -SiO 2} , MnO is 23 to 56% by mass%, Al ₂ O ₃ is 4-27%
And a ternary composite oxide particle having a composition in a region where each range of 30 to 54% of SiO ₂ overlaps, and the diameter thereof is
A high-strength steel, wherein fine particles having a particle size of less than 0.2 μm are dispersed at least 6 particles / mm ² . 2. The high tensile strength steel according to claim 1, wherein 5% or less of O (oxygen) of the oxide constituting the fine particles of the composite oxide is replaced with S (sulfur). 3. The steel has an average composition in mass%, C: 0.01-0.2%, Si: 0.01-0.5%, Mn: 0.3-3.0
%, Sol.Al: 0.008% or less, Insol.Al: 0.0002-0.00
5%, O: 0.001 to 0.005%, P: 0.02% or less, S: 0.0
2% or less, and N: 0.01% or less, In the first group, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%
The second group contains Ni or 0.05 to 3.0.
%, Cu: One or two of 0.05-0.7%,
In the third group, Cr: 0.03 to 1.5%, Mo: 0.03 to 0.7%
One or two, and Group 4 contains B: 0.0003
Group 1 to group 4 containing ~ 0.002%
Or two or more groups, with the balance being Fe and unavoidable impurities
The high-strength steel according to claim 1 or 2, wherein 4. The composition according to claim 3, further comprising Ti: 0.0
06 to 0.02% and Zr: 0.005 to 0.025% of one or <br/> high tensile steel according to claim 1 or 2 containing. 5. The composition according to claim 3, further comprising Ca in an amount of 0.0005 to 0.005% or / and at least one rare earth element in an amount of 0.005 to 0.05%, in addition to the components described in claim 3 or / and 4. 4 high tensile steel.