JPH1025536A - Steel material excellent in toughness of welded heat-affected zone and method for producing the same - Google Patents

Abstract

(57) [Problem] To provide a steel material excellent in toughness of a weld joint HAZ having a small to large heat input and a method of manufacturing the same. SOLUTION: (1) By weight%, C: 0.03 to 0.2%, Si: 0.02 to 0.
5%, Mn: 0.6-2%, Al: 0.00005-0.02%, Ti: 0.0005-0.02%, O
(Oxygen): A steel material having excellent toughness of the weld heat affected zone in which at least 4 oxides satisfying the following conditions are dispersed per 1 mm ² , including 0.001 to 0.01%. : Mn: 5-50at%, Al + Ti: 50-95at%: Spinel-type or inverted-spinel-type crystal structure with lattice constant 0.75-0.9nm: Long diameter 1-10μm (2) For slabs with short side 350mm or less The method for producing a steel material according to (1), wherein accelerated cooling or direct quenching and tempering is performed after performing controlled rolling during hot rolling, or heat treatment is performed after performing hot rolling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having excellent toughness in a heat affected zone of a weld used for a welded structure such as an offshore structure, a high-rise building, a pipeline, a ship and a tank, and a method for producing the same.

[0002]

2. Description of the Related Art There is a tendency that demands for improving the properties of welding high-tensile steel materials used for large structures such as marine structures for ice-sea areas, line pipes for cold regions, ships or LNG tanks, etc., are becoming increasingly severe. Needless to say, sufficient strength is required in accordance with the purpose of use, and in particular, there is a high demand for improving the toughness of the weld heat affected zone (hereinafter, referred to as HAZ) of the base metal in contact with the weld metal.

[0003] In order to meet such demands, many methods for improving HAZ toughness have been proposed. For example, it is a method of including a trace amount of Ti in steel or adding Ca. However, these methods are effective, but they are not enough to satisfy the recent severe strength and toughness levels and construction efficiency, and until welding can be realized with increased heat input until satisfactory in construction. Has not been reached.

Recently, much attention has been paid to a method of improving HAZ toughness using an oxide as a method superior to the above method (Japanese Patent Application Laid-Open Nos. 57-51243 and 61-79745). Such).

[0005] This is because TiO or Ti dispersed uniformly is used.
_The present invention proposes a method of forming intragranular ferrite using ₂ O ₃ particles as nuclei to refine the structure. Ti oxide is TiN
Since it does not form a solid solution with the base material as described above, it is excellent in maintaining its effect even in a region heated to a high temperature near the melting line.

[0006] However, it is difficult to obtain a sufficient effect only by the toughness improving effect only by the dispersion of TiO or Ti ₂ O ₃ , and improvement of the toughness improving effect by combined use with other precipitated particles is being sought. .

For example, Japanese Patent Application Laid-Open No. Sho 62-1842 discloses a method for improving toughness by combining an oxide with MnS and Ti nitride.

According to Japanese Patent Application Laid-Open No. 5-78740, steel having finely dispersed Ti oxide has a HAZ structure in a region in which austenite grains are coarse near the melting line (coarse-grain region HAZ: a region heated to 400 ° C. or higher). The effect of refining is large, but the area where the austenite grain size is slightly large (subcoarse grain area:
(A region heated to 1200 to 1350 ° C.), it is pointed out that the effect is small, and it has been proposed to promote the generation of intragranular ferrite by using Ce oxide.

[0009] However, it is difficult to finely and uniformly disperse the Ce oxide particles, and a problem remains in practical operation. C
As for Ce and REM, similarly to Ce, it is very difficult to finely disperse these oxides in steel, and there are many problems in actual operation.

[0010] Japanese Patent Application Laid-Open No. 7-278736 discloses Al-
A method for improving HAZ toughness by dispersing Mn oxide particles is disclosed. The Al-Mn oxide disclosed here can be finely dispersed in steel relatively easily, but there is a problem that the allowable range of the composition range and the refining conditions is narrow at the time of melting.

Techniques such as suppression of the formation of island martensite and reduction of the amount of solute N by methods such as low carbon equivalent, low Si, and low Al are the same as the above-mentioned techniques for improving toughness using nitride or oxide. The aim was to achieve a synergistic effect, and the effect of improving the HAZ toughness alone was naturally limited.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a relatively wide range of heat input from small heat input to large heat input welding, which can be manufactured relatively easily without narrow refining conditions. , HAZ, and a method of manufacturing the same.

[0013]

Means for Solving the Problems In order to investigate in detail the mechanism of improving the toughness of HAZ by promoting the formation of intragranular ferrite by using oxides, the present inventors have prepared steels having oxides of various compositions in a laboratory. The following items were confirmed by conducting a basic study on the state of ferrite formation in the melted and dispersed particles and in the grains. (A) For steel that generates a large amount of intragranular ferrite during cooling in the welding heat cycle. Contains oxides containing Mn, Al, and Ti as main components. This oxide has a lattice constant of 0.75
An oxide having a spinel-type or reverse spinel-type crystal structure of about 0.9 nm.

Here, an oxide having a spinel-type or reverse spinel-type crystal structure is represented by the chemical formula XY ₂ O ₄ (X, Y
Is an oxide belonging to a cubic system in a form of a crystal structure represented by Mn, Ti, Al, or the like. The lattice constant refers to the length of one side of the cubic crystal.

In the following description, an oxide having a spinel-type or reverse spinel-type crystal structure may be referred to as “XY ₂ O ₄ ”.

(B) An oxide having a spinel-type or reverse spinel-type crystal structure containing Mn, Ti, and Al is
MnAl ₂ O ₄ (lattice constant: 0.820 nm), MnTi
₂ O ₄ (0.863 nm), Mn ₂ AlO ₄ (0.837 n
m), Mn ₂ TiO ₄ (0.867 nm) and the like are known. The composition ratio of Al, Mn, and Ti in the actual oxide formed in the steel is not always an integral multiple and MnAl
_In Al ₂ O ₄ , MnTi ₂ O ₄ , Mn ₂ AlO ₄ , Mn ₂ TiO _4, etc., Al, Mn, and Ti are substituted with each other, and the lattice constant changes according to the concentration of the element to be substituted, and has a constant width. It is thought that it will have something.

(C) The oxide promotes the formation of intragranular ferrite because the crystal structure of the spinel-type or inverse spinel-type oxide belongs to the same cubic system as ferrite, and the lattice constant is around 0.81 nm. This is presumed to be due to the extremely excellent crystal matching with ferrite.

(D) The above-mentioned oxide can be easily dispersed in steel as long as it satisfies the composition of steel, without limiting the refining conditions to a very narrow range.

The present invention has been accomplished by setting the range of Al, Ti, Mn, O, etc., suitable for dispersing the above-mentioned oxide during solidification.

The gist of the invention is to have the following composition in which oxides having a lattice constant of 0.75 to 0.9 nm containing Al, Mn, and Ti in a predetermined range are uniformly dispersed in a spinel or inverse spinel crystal structure. It is steel.

(1) C: 0.03-0.2% by weight
%, Si: 0.02 to 0.5%, Mn: 0.6 to 2%,
P: 0.03% or less, S: 0.03% or less, Al: 0.
02% or less, Ti: 0.02% or less, O (oxygen): 0.
From 001 to 0.01% and N: it contains 0.01% or less, below, and Mn-Al-Ti-based oxides satisfying has excellent HAZ toughness dispersing 1 mm ² per average four or more steel .

Mn: 5 to 50 at%, Al + T
i: 50 to 95 at%: Spinel type or inverted spinel type crystal structure with lattice constant of 0.75 to 0.9 nm: Long diameter 1 to 10 μm (2) Control rolling at the time of hot rolling on a slab having a short side of 350 mm or less The method for producing a steel material according to the above (1), wherein the steel sheet is subjected to accelerated cooling or direct quenching and tempering after heat treatment, or heat treatment is performed after hot rolling.

The weight% in the above (1) indicates the weight% of each element in the whole steel including the oxide. In addition, at% in the above refers to at of Mn, Al, Ti, and the like when the total atomic ratio of metal elements such as Mn, Al, and Ti constituting oxides other than oxygen is 100 at%. %
Point out. Elements other than Mn, Al, and Ti include Si and the like.

The "longer diameter of the oxide" refers to the passing length in the direction in which the passing length is maximum for each oxide within the field of view of the optical microscope as described later. “The average diameter of 4 or more oxides having a major axis of 1 to 10 μm is dispersed per 1 mm ² is 4 or more.”
Means that the oxide in the 10μm exists average of 4 or more per 1 mm ^2.

The term "steel material" mainly refers to a steel plate, that is, a thick steel plate, a hot-rolled steel plate (hot coil) or the like, but is not limited to a steel plate.

In the above (2), the “short side” corresponds to, for example, in the case of continuous casting, the thickness of a continuously cast slab.
In the case of an ingot ingot, the dimension in the thickness direction, which is the thinnest when the ingot is viewed as a rectangular parallelepiped, corresponds to the ingot. If the thickness is tapered from the top to the bottom of the ingot, the maximum thickness shall be used.

The hot rolling in the above (2) means, in the case of continuous casting, hot rolling of a continuously cast slab to a steel material. Direct rolling, that is, hot rolling without cooling to a cold ingot after continuous casting. Includes the case. In the case of ingot ingot making, it refers to hot rolling to a billet (slab) by slab rolling and then rolling to steel. However, the controlled rolling is not performed in the step of rolling into a “slab” and is limited to the step of rolling into “steel”.

In the above (2), the heat treatment performed after hot rolling corresponds to quenching and tempering, normalizing, and the like.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below together with the operation thereof. First, the reasons for limiting oxides will be described.

1. Oxide Oxide contains Mn, Al and Ti as main components.
n: 5 to 50 at%, (Al + Ti): 50 to 95 at
% Is essential. The oxide must be an oxide having a spinel or inverse spinel crystal structure with a lattice constant of 0.75 to 0.9 nm.

The reason why Mn and (Al + Ti) are limited to the above ranges is that oxides having a composition outside this range may be used as ferrite nucleation sites even if they have a spinel type or inverse spinel type crystal structure. It does not function strongly, and the desired improvement in HAZ toughness cannot be obtained. If this oxide does not have a spinel-type or inverse spinel-type crystal structure, the function as a nucleation site of ferrite is weak, and the intended effect cannot be obtained.

The fact that the oxide is within the above-mentioned chemical composition range can be confirmed by EDX (energy dispersive X-ray microanalyzer) or the like. In addition, it is possible to confirm that the oxide has a spinel or inverse spinel type crystal structure having a predetermined lattice constant by X-ray diffraction of the electrolytic extraction residue.

It is ideal if the composition, crystal structure, and lattice constant of each oxide can be measured while checking the oxides in the steel. However, the composition, crystal structure,
At present, there is no device capable of accurately detecting the lattice constant. Therefore, the (a) composition and (b) crystal structure and lattice constant of the oxide were separately measured as described above. Further, the measurement of the density of the oxide was performed in the visual field of the optical microscope as described later, but the oxide having the composition range and the crystal structure of the present invention could be identified by the color tone and the shape in the visual field of the optical microscope. .

As long as the composition is within the scope of the present invention by EDX measurement and such an oxide is recognized as the main phase of the oxide, the main phase observed in the X-ray diffraction image of the electrolytic extraction residue is Lattice constant 0.75-0.9 nm
Of XY ₂ O ₄ .

Further, in a steel material having an oxide satisfying the above composition, crystal structure, and lattice constant, the major axis is 1 to 10 μm.
It is necessary that at least 4 particles of m are dispersed per 1 mm ² . In the case of an oxide having a major axis of less than 1 μm, the probability of ferrite nucleation is low, and the structure of the ferrite cannot be sufficiently refined. On the other hand, the major axis of the oxide is 10 μm
If it exceeds, not only the toughness of the base material but also the ductility such as elongation and drawing decrease, so the range of the major axis of the oxide is 1 to 10 μm.
And

The density of the oxide number and organization improving effect is sufficiently appear not HAZ since toughness is not improved 1 mm ² per 4 or more in the case of less than 4 per 1 mm ^2. This can be measured with an optical microscope of about 500 times, as described in Examples described later. Density of the oxide is preferably as high as possible as long as there is a composition within the scope of the present invention becomes higher than 50 per 1 mm ^2, so impairs the toughness and ductility of the base material, 50 pieces / mm ² or less It is desirable that

2. Chemical Composition Next, the reasons for limiting the alloy components in the present invention will be described. The expression “%” for the alloy elements of the entire steel indicates “% by weight”.

C: C is necessary for securing strength.
If it is less than 0.03%, the strength is not sufficient, but on the other hand,
If it is contained in excess of 0.2%, martensite or pseudo-pearlite is generated in the HAZ to degrade the HAZ toughness and adversely affect the toughness and weldability of the base material. I do.

Si: Si is an effective element from the viewpoint of securing strength, and therefore, is required to be contained in steel at 0.02% or more. On the other hand, since Si does not form a solid solution in cementite, 0.5%
If contained in a large amount exceeding 0.5%, untransformed austenite inhibits decomposition into ferrite and cementite, promotes the formation of island-like martensite, which is a fine hardened structure, and deteriorates HAZ toughness. .

Mn: Mn is an element effective for deoxidizing molten steel, and is an essential element in the present invention also as a constituent element of an oxide serving as a ferrite formation nucleus. It is also an element effective in ensuring the strength and toughness of the base material. Therefore, 0.6
% Or more is required. However, if the content exceeds 2%, the hardenability is unnecessarily increased and the weldability and the HAZ toughness are deteriorated.

P: Although P is an unavoidable impurity element, it is often a cause of HAZ grain boundary cracking because it is a grain boundary segregating element. For this reason, P is preferably as low as possible, but from an economic viewpoint, the upper limit is set to 0.03%. Further, in order to improve the base material toughness and the HAZ toughness and to reduce the slab center segregation, the content is desirably 0.01% or less.

S: Like P, S is an impurity element inevitably contained in steel. When S is present in a large amount, Mn
S and the like are formed and become a starting point of welding cracks and the like. For this reason, S is preferably as low as possible, but the upper limit is made 0.03% in consideration of economy. Further, in order to improve the base material toughness and the HAZ toughness and reduce the slab center segregation, the content is desirably 0.01% or less.

Al: Al is an essential element also as a constituent element of an oxide serving as a ferrite generation nucleus. Al required for oxide formation is 0.000 in calculation from oxide density.
About 1% is sufficient, but this value is below the limit of analysis accuracy. Therefore, Al must be added without any particular limitation. If the lower limit is forcibly determined, there is no detection specification below the analysis accuracy, but 0.0001
% Is desirable. However, as for the upper limit, if Al exceeds 0.02%, the formation of oxides that generate ferrite becomes rather difficult, and H
The content is set to 0.02% or less because an increase in island martensite in AZ is caused.

Ti: Like Al, Ti is also essential as a constituent element of the oxide. However, like Al, the amount of Ti required for oxide formation in calculation is below the analytical limit. If the lower limit is determined in the sense of an essential element, there is no specification for detection, but it is desirable that the lower limit be 0.001% or more. On the other hand, the upper limit is set to 0.02% or less because excessive Ti exceeding 0.02% causes single precipitation of coarse TiC and is harmful to the toughness of the HAZ and the base material.

N: N is an unavoidable impurity element contained in steel, and if present excessively, it lowers toughness. In the steel of the present invention, if the content is 0.01% or less, the effect is relatively small, so the upper limit is made 0.01%. However, from the viewpoint of the HAZ toughness of the steel according to the present invention, the lower the N, the better, for example, it is desirable to be able to be 0.004% or less.

O (oxygen): O is required to be at least 0.001% in order to form an oxide serving as a ferrite formation nucleus. However, if O is excessively present in the steel, the upper limit is set to 0.01% because the base metal toughness and ductility deteriorate.

In order to improve the strength or toughness other than the above elements, the following elements can be included in a predetermined range, and other elements other than the following elements can be included.

Cu: Cu is effective in improving the strength of the base material, and is therefore added when a high-strength steel is used. If it is included, if it is less than 0.2%, a sufficient increase in strength cannot be obtained, so it is desirable to make it 0.2% or more. On the other hand, 1.5%
Exceeding 1.5 degrades the surface properties of the slab and raises the cost of surface care. Depending on the degree of deterioration, the yield may be significantly reduced.

Ni: Ni is dissolved in a steel matrix to increase toughness and hardenability to improve strength. Therefore, Ni is used for steel used in a low-temperature environment.
If it is less than 0.2%, the improvement in toughness cannot be expected to be large. On the other hand, if the content exceeds 3%, the hardenability becomes excessive and the HAZ toughness is rather reduced.
It is better to do the following.

Cr: Cr is added when high strength steel is used because it enhances hardenability. If it is less than 0.05%, there is no effect of improving hardenability, so it is desirable to make it 0.05% or more. On the other hand, if it exceeds 1%, an oxide scale that is difficult to peel off is formed on the steel surface and causes surface flaws.

Mo: Mo is added in the case of high-strength steel because it increases hardenability and temper softening resistance and improves strength. If it is less than 0.05%, sufficient tempering softening resistance cannot be expected.
It is desirable to set it to 5% or more. On the other hand, if it exceeds 1%, the toughness of the base material and HAZ is greatly reduced.

V: V is added in the case of high-strength steel because V precipitates carbonitride to increase temper softening resistance and improve strength. If it is less than 0.02%, a clear improvement in strength cannot be obtained, so it is desirable to make it 0.02% or more. On the other hand, if it exceeds 0.2%, the strength is excessively increased and the toughness is reduced.

Nb: Nb is effective in expanding the non-recrystallization temperature range during rolling, facilitating controlled rolling, and improving strength and toughness. If it is less than 0.01%, the effect of suppressing recrystallization is not sufficiently exhibited, so it is desirable that the content be 0.01% or more. On the other hand, if it exceeds 0.2%, the HAZ toughness is deteriorated.

3. Manufacturing method Steel having the above composition range is melted in a converter or an electric furnace or the like so that a predetermined oxide is dispersed after solidification. In order to obtain an oxide within the scope of the present invention, in refining, it is necessary to avoid advancing Al deoxidation for the first time, and to adjust alloying elements other than deoxidizing elements together with Mn and Si. It is desirable to cast a molten steel obtained by adding Al immediately before tapping in a state where deoxidation has progressed due to the above. By adopting such relatively simple refining conditions, the target oxide can be uniformly dispersed as long as the steel is within the range of the composition of the present invention.

The control of the ratio of Al and the like in the oxide can be performed by controlling the ratio of aluminum in the flux or slag as described in detail in Examples.
The ratio of Si and Mn in the oxide is controlled by the composition of Si and Mn in the molten steel at the time of initial deoxidation.

In casting, a slab is manufactured by continuous casting or ingot ingot making. In the case of continuous casting or ingot ingot making, it is preferable that the cooling rate at the time of solidification is faster from the viewpoint of uniform dispersion of the oxide.

Therefore, the short side of the steel at the time of solidification is 350 mm
The following is assumed. When the short side exceeds 350 mm, the solidification rate becomes slow, and the major axis of the oxide often exceeds 10 μm, and the toughness and ductility of the base material decrease. In addition, major axis 1
Since oxides of 10 μm to 10 μm are not uniformly dispersed, a variation in toughness occurs in the HAZ. The shorter side is preferably smaller if it is 350 mm or less, but is preferably 50 mm or more in consideration of production efficiency and the like.

Thereafter, hot rolling is performed to produce a steel material having a predetermined thickness. Regarding the manufacturing conditions after heating before hot rolling, even if various techniques such as controlled rolling and accelerated cooling, which are currently known, are applied, the properties of the HAZ are not affected at all. Further, even if an appropriate heat treatment is performed after hot rolling to improve the mechanical properties of the base material, the properties of the HAZ are not adversely affected at all.

The above hot rolling may be thick plate rolling or hot rolling of a hot coil.

[0060]

EXAMPLES Next, the effects of the present invention will be described with reference to examples.

[Example 1] Table 1 is a list of chemical compositions of steels used in the examples. Steel having a base material composition as shown in the table was produced using a laboratory-scale 150 kg vacuum melting furnace. At that time, the following method was employed to change the oxide composition.

In melting the steel of the example of the present invention, first,
After preliminary deoxidation by Si and Mn, Ti addition, then Al
Deoxidation was performed in the order of addition.

It is difficult to control the Al composition in the oxide by adding metal Al, and the Al composition is controlled by adding a flux to molten steel. In addition, Si and M in the oxide
The ratio of n was adjusted by controlling the composition balance of Si and Mn in the molten steel in the initial deoxidation and controlling the amount of active oxygen before introducing Ti and Al.

On the other hand, in Comparative Example B2, Al was added from the stage of initial deoxidation, and in Comparative Example B3, Ti was similarly added. In Comparative Example B4, as a result of sufficient deoxidation with Mn, the activity oxygen when Ti and Al were charged was insufficient. In Comparative Example B5, Mn was insufficient in the initial deoxidation. It is.

By making a difference in the treatment method in the molten steel stage as described above, the steels of the present invention and the comparative example could be manufactured.

[0066]

[Table 1]

A1, A2, B1
Obtained the same short side 150 mm slabs for steel other than
After hot forging (corresponding to slab rolling), hot rolling was performed to obtain a steel sheet. With respect to these steel sheets, the oxide dispersion density and the oxide composition of the major axis of 1 to 10 μm were investigated. Oxide dispersion density is 50
The micro sample was observed and measured with a 0 × optical microscope.

The chemical composition of the oxide phase is as follows:
Observed with an EDX device (scanning electron microscope with EDX),
It was confirmed by measuring the composition ratio for each oxide. The crystal structure was confirmed by performing X-ray diffraction on the extraction residue obtained by the galvanostatic method.

Table 2 is a table summarizing these measurement results.

[0070]

[Table 2]

Further, a tensile test of a steel sheet base material (test piece: JIS Z
22014) and Charpy impact test (specimen: JIS Z2202)
No. 4), the strength and toughness were investigated, and a reproduction HAZ toughness test was performed to evaluate the toughness of HAZ.

The reproducible HAZ toughness test was performed at a quarter of the thickness of the steel sheet.
11mm, thickness 11mm, length 60mm taken from
The test piece was heated at a maximum heating temperature of 1400 ° C., and then cooled from 800 ° C. to 500 ° C. (see Table 3 below).
In gave reproducible weld heat cycle of t ⁸⁰⁰ ₅₀₀ and described) and 20, 60 and 180 seconds. These are thermal cycles corresponding to welding heat input of 30, 100 and 300 kJ / cm, respectively. After the reproduction welding heat cycle was applied, the test piece was processed into a JIS Z2202 No. 4 Charpy test piece and subjected to an impact test.

Table 3 shows the results of these tests.

[0074]

[Table 3]

Steels A1 to A4 and B1 to B5 were steels whose base materials had substantially the same composition and oxide compositions were varied to investigate base material performance and reproduced thermal cycle toughness.

Steels A1, A2, and B1 are steels in which oxide dispersion density is changed by changing a mold size when solidifying molten steel refined under the same refining conditions.
The length of the short side of each of these steels is 100 m for A1.
m and A2 were 150 mm, and B2 was 400 mm.

As a result of making the refining conditions the same, steel A1, A
2. The composition of the oxide contained in B1 was almost the same.

These facts are clearly shown in the EDX analysis results and X-ray diffraction results shown in Table 2. That is, steel A
The oxides contained in 1, A2, and B1 fall within the composition range of the present invention, have an XY ₂ O ₄ type crystal structure, and have a lattice constant within the range of the present invention. In addition, steel A1 and A2
Satisfies the oxide density within the scope of the present invention. For this reason, the steels A1 and A2 have excellent base metal toughness and show excellent toughness under any of the reproducible welding heat cycle conditions.

On the other hand, the steel B1 having a smaller oxide density than the steels A1 and A2 shows the same performance as the steels A1 and A2 with respect to the strength and toughness of the base metal, but it is apparent that the reproduced HAZ toughness is significantly deteriorated. It is.

Steels A3 and A4 are examples in which the effect of MnS was studied by reducing the amount of S in the steel. Almost no MnS precipitation was observed in these steels, but the reproduced HA
The Z toughness showed a good value.

The steel B2 is a steel which has been deoxidized mainly by Al by performing Al deoxidation in the early stage of refining. The oxides are mainly Al oxides.
It is clear that no oxides of the type Y ₂ O ₄ are present. For this reason, the reproduced HAZ toughness is significantly deteriorated as compared with the steel of the present invention.

Steel B3 is the result when Ti oxide was formed. In steel B3, similarly to steel B2, a decrease in reproducible heat cycle toughness is observed.

Steel B4 shows the results when a large amount of Mn oxide is formed. In this case, a decrease in the reproducible heat cycle toughness and a decrease in the base metal toughness are also observed.

Steel B5 is an example in which the amount of Mn in the oxide is significantly reduced. In this case, an oxide phase having a desired crystal structure cannot be obtained, and the reproduced thermal cycle toughness decreases.

Steels A5 and B6 are examples in which the amount of nitrogen is increased. In steel B6 in which nitrogen is present in excess of the range of the present invention, a cooling time (t ⁸⁰⁰ ₅₀₀ ) of 800 to 500 ° C. simulating large heat input welding, despite the fact that the particles limited by the present invention are dispersed. However, the reproduced HAZ toughness for 180 seconds deteriorates.

On the other hand, steel A5 in which N is lower than steel B6
Shows still good toughness even at t ⁸⁰⁰ ₅₀₀ = 180 sec.

The steels A6 to A14 are examples containing reinforcing elements such as Cu and Ni. Each steel contains an oxide within the range of the present invention, and shows stable and high reproducible HAZ toughness under any condition of small heat input to large heat input.

Example 2 Table 4 shows the chemical composition of the steel according to the present invention manufactured at an actual manufacturing site.

[0089]

[Table 4]

The specific manufacturing method is as follows.

After adjusting the carbon concentration in the converter, preliminary deoxidation was performed in the ladle using Si and Mn, and slag containing Al and capable of controlling the oxygen potential by slag reforming was produced. And the Al ₂ O ₃ concentration in the slag is 18%
Controlled to a degree. Further, the total oxygen concentration was adjusted by a ladle refining facility. After refining, short side 3 of molten steel by continuous casting
It was cast into a 00 mm slab, and was rolled into a 40 mm thick steel plate by controlled rolling and accelerated cooling.

The density, composition, and crystal structure of the oxide were measured in the same manner as in Example 1.

Table 5 shows the density, composition and lattice constant of the oxide particles.

[0094]

[Table 5]

In the study of the toughness of the HAZ, the toughness of the welded heat-affected zone of the actual joint was examined in addition to the reproduction HAZ toughness test performed in Example 1. For investigating the toughness of the actual joint, submerged arc welded joints in which the heat input was changed to 30, 90, 120 and 250 kJ / cm were produced. The Charpy impact test was used for the toughness investigation, and the notch position was the position where the weld metal and the weld heat affected zone (base metal) near the melting line were 1: 1 (referred to as bond) and the position 1 mm closer to the base material than the bond. (Referred to as HAZ 1 mm). The Charpy test result of the actual joint was evaluated at the fracture surface transition temperature.

Tables 6 and 7 show the base material performance and reproduction H, respectively.
It is a list | wrist which shows AZ toughness and the performance of a real joint.

[0097]

[Table 6]

[0098]

[Table 7]

As shown in the present embodiment, the characteristics of the steel of the present invention clearly appear even when the steel is manufactured at an actual manufacturing site.
It is evident that it has excellent base metal properties and weld heat affected zone toughness.

[0100]

According to the present invention, good toughness can be obtained in all areas of the HAZ after welding from small heat input to large heat input, without employing narrow refining conditions, which are rare in actual steady operation. An obtainable steel material and a method for producing the same can be provided, and the effect on the related industry is very large.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor: Yuichi Komizo 4-33, Kitahama, Chuo-ku, Osaka-shi, Osaka Prefecture Inside Sumitomo Metal Industries, Ltd. (72) Toru Takayama 4-chome, Kitahama, Chuo-ku, Osaka-shi, Osaka 5-33 No. Sumitomo Metal Industries, Ltd.

Claims (2)

[Claims] (1) C: 0.03 to 0.2% by weight, S
i: 0.02 to 0.5%, Mn: 0.6 to 2%, P:
0.03% or less, S: 0.03% or less, Al: 0.02
%, Ti: 0.02% or less, O (oxygen): 0.00
Mn-Al-Ti-based oxides containing 1 to 0.01% and N: 0.01% or less and satisfying the following and 1
A steel material with excellent toughness in the heat affected zone of the weld, characterized in that 4 or more particles are dispersed on average per mm ² . : Mn: 5 to 50 at%, Al + Ti: 50 to 95
at%: spinel-type or inverse spinel-type crystal structure having a lattice constant of 0.75 to 0.9 nm: major axis 1 to 10 μm 2. A slab having a short side of 350 mm or less is subjected to accelerated cooling or direct quenching and tempering after performing controlled rolling during hot rolling, or is subjected to heat treatment after performing hot rolling. The method for producing a steel material according to claim 1.