JP6627536B2 - Steel with excellent toughness in weld heat affected zone and method of manufacturing steel with excellent toughness in weld heat affected zone - Google Patents

Description

  The present invention relates to a steel material having excellent toughness of a heat affected zone and a method for producing a steel material having excellent toughness of a heat affected zone.

In recent years, it has been desired to employ large heat input welding such as electrogas arc welding. In such large heat input welding, the heat affected zone is heated to a high temperature of 1400 ° C. or higher. When heated to a high temperature, austenite grains (γ grains) become coarse and the toughness of the weld heat affected zone decreases.
Conventionally, in the heat affected zone of large heat input welding, in order to prevent austenite grains from coarsening, grain boundaries are pinned (also referred to as pinning) with fine oxides, carbides, nitrides, etc., and IGFs are also used. (Intragranular ferrite) A technique for dispersing an oxide serving as a transformation nucleus has been proposed (for example, see Patent Documents 1 to 3).

Japanese Patent No. 3699630 Japanese Patent No. 3728130 Japanese Patent No. 3752076

By the way, recently, the required quality for steel materials has become strict, and further improvement in toughness of the heat affected zone has been demanded.
Here, in the steel materials described in Patent Literatures 1 to 3, the oxide serving as the IGF transformation nucleus was not sufficiently formed, and there was a possibility that the crystal grain size in the weld affected zone could not be sufficiently reduced. In addition, when Ca is contained, precipitation of ferrite may not be promoted in an oxide serving as an IGF transformation nucleus.

  The present invention has been made in view of the above-described circumstances, and a steel material having excellent toughness of a weld heat-affected zone further improving the toughness of a weld heat-affected zone by promoting intragranular ferrite transformation, and An object of the present invention is to provide a method for producing a steel material having excellent toughness of a weld heat affected zone.

  In order to solve the above-described problems, the present inventors have conducted intensive studies. As a result, while increasing the Mg content in oxide particles serving as IGF transformation nuclei and increasing the number density of the oxide particles, the IGF transformation was performed. It has been found that the nucleus can be made to act effectively, and the crystal grain size in the weld heat affected zone can be refined to greatly improve the toughness.

The present invention has been made based on the above findings, the steel material excellent in the toughness of the weld heat affected zone according to the present invention, in mass%,
C: 0.03% or more and 0.2% or less,
Si: 0.4% or less,
Mn: 0.5% or more and 2.0% or less,
P: 0.015% or less,
S: 0.006% or less,
Al: 0.001% or more and 0.01% or less,
Ti: 0.007% or more and 0.02% or less,
Mg: more than 0.001% and 0.006% or less
O: 0.001% or more and 0.004% or less,
N: 0.0025% or more and 0.006% or less,
And the balance has a chemical component consisting of Fe and unavoidable impurities, and the number of TiN particles having a particle size of 0.01 μm or more and less than 0.5 μm and containing an oxide made of Mg and Al is 10,000 particles / mm ² or more. Characterized in that Mg-containing oxide particles having a particle diameter of 0.5 μm or more and 5 μm or less containing 50% by mass or more of Mg are present at a number density of 50 particles / mm ² or more. .

According to the steel material having excellent toughness of the weld heat-affected zone having this configuration, the number of TiN particles having an average particle diameter of 0.01 μm or more and less than 0.5 μm including oxides composed of Mg and Al is 10,000 particles / mm ² or more. Since it exists at a density, the coarsening of austenite grains can be reliably suppressed by the pinning effect of the grain boundaries.
Since the Mg-containing oxide particles having a particle size of 0.5 μm or more and 5 μm or less serving as IGF transformation nuclei contain 50% by mass or more of Mg, the precipitation of ferrite using the Mg-containing oxide particles as nuclei is promoted. Is done. In addition, since the Mg-containing oxide particles are present at a number density of 50 particles / mm ² or more, precipitation of intragranular ferrite is promoted, and the crystal grain size in the heat affected zone can be further refined. .
Therefore, it is possible to greatly improve the toughness of the heat affected zone.

Here, in the steel material having excellent toughness of the heat affected zone of the present invention, when [O], [Mg], [Al], [Ti], and [N] are each represented by mass% of each element. In addition, the effective Ti amount calculated by the following equation (1) or (2) is preferably in the range of −0.01% or more and + 0.005% or less.
[O] −0.66 × [Mg] −0.89 × [Al] ≧ 0,
Effective Ti amount = [Ti] −2 × ([O] −0.66 × [Mg] −0.89 × [Al]) − 3.4 × [N] (1)
[O] −0.66 × [Mg] −0.89 × [Al] <0,
Effective Ti amount = [Ti] −3.4 × [N] (2)

  In this case, since the “effective Ti amount” defined by the above-described expressions (1) and (2) is set within a range of −0.01% or more and + 0.005% or less, the “effective Ti amount” depends on the N amount. Thus, the amount of Ti can be optimized, and embrittlement of the weld heat affected zone by solid solution N or TiC can be suppressed.

Further, in the steel material excellent in the toughness of the weld heat affected zone of the present invention, in mass%,
Cu: 1.5% or less,
Ni: 1.5% or less,
Mo: 1% or less,
Cr: 1% or less,
Nb: 0.05% or less,
V: 0.05% or less,
B: 0.002% or less,
May be contained alone or in combination.

  These elements are elements having an effect of improving the properties of the base material, the heat-affected zone, and the like, and may be appropriately selected and added. However, if the addition amount is too large, the toughness of the weld heat affected zone deteriorates, so that each element is preferably regulated within the above range.

  The method for manufacturing a steel material having excellent toughness of the weld heat affected zone of the present invention is a method for manufacturing a steel material having excellent toughness of the weld heat affected zone for manufacturing the steel material having excellent toughness of the weld heat affected zone described above, A step of adding Mg to molten steel to increase the Mg concentration by more than 0.002% by mass to 0.007% or less, and an oxygen source for oxidizing 0.001% or more of Mg by mass And adding an oxygen source to molten steel.

According to the method for manufacturing a steel material having excellent toughness of the weld heat-affected zone having this configuration, Mg is added to molten steel, and the Mg concentration is set to be in a range of more than 0.002% to 0.007% or less by mass%. Since the method includes the step of adding Mg and the step of adding an oxygen source for oxidizing 0.001% or more by mass of Mg into molten steel, Mg-containing oxide particles having a high Mg content can be reliably obtained. Can be formed. Therefore, Mg-containing oxide particles having a particle size of 0.5 μm or more and 5 μm or less containing 50% or more by mass of Mg can be present at a number density of 50 / mm ² or more.
Note that the Mg addition amount in the Mg addition step and the oxygen source addition amount in the oxygen source addition step are each adjusted so that the Mg amount after the oxygen source addition step becomes more than 0.001% and 0.006% or less. become.

Further, in the method of the present invention for producing a steel material having excellent toughness in a weld heat-affected zone, in the oxygen source adding step, Mg of 0.001% or more by mass% is added by adding SiO ₂ to the slag. Preferably, it is oxidized.
In this case, by adding SiO ₂ to the slag and increasing the abundance ratio of SiO _{2 in} the slag, Mg oxide is formed gently by the reaction between the slag and the molten steel, and the particle size is reduced to 0. Relatively fine Mg-containing oxide particles of not less than 0.5 μm and not more than 5 μm can be reliably formed.

  As described above, according to the present invention, a steel material excellent in the toughness of a welding heat affected zone, in which the toughness of a welding heat affected zone is further improved by promoting intragranular ferrite transformation, and the toughness of a welding heat affected zone, It is possible to provide a method for producing a steel material excellent in quality.

It is a flow figure of the manufacturing method of the steel material excellent in the toughness of the welding heat affected zone which is the embodiment of the present invention.

  Hereinafter, a steel material having excellent toughness of a weld heat affected zone and a method of manufacturing a steel material having excellent toughness of a weld heat affected zone, which are embodiments of the present invention, will be described with reference to the accompanying drawings. Note that the present invention is not limited to the following embodiments.

First, a steel material having excellent toughness in the weld heat affected zone manufactured in the present embodiment will be described.
The steel material excellent in the toughness of the welding heat affected zone in the present embodiment is represented by mass%,
C: 0.03% or more and 0.2% or less,
Si: 0.4% or less,
Mn: 0.5% or more and 2.0% or less,
P: 0.015% or less,
S: 0.006% or less,
Al: 0.001% or more and 0.01% or less,
Ti: 0.007% or more and 0.02% or less,
Mg: more than 0.001% and 0.006% or less
O: 0.001% or more and 0.004% or less,
N: 0.0025% or more and 0.006% or less,
And the balance has a chemical component consisting of Fe and inevitable impurities.

Further, in the steel material having excellent toughness of the weld heat-affected zone according to the present embodiment, when [O], [Mg], [Al], [Ti], and [N] are each represented by mass% of each element. The effective Ti amount calculated by the following equation (1) or (2) is in the range of -0.01% or more and + 0.005% or less.
[O] −0.66 × [Mg] −0.89 × [Al] ≧ 0,
Effective Ti amount = [Ti] −2 × ([O] −0.66 × [Mg] −0.89 × [Al]) − 3.4 × [N] (1)
[O] −0.66 × [Mg] −0.89 × [Al] <0,
Effective Ti amount = [Ti] −3.4 × [N] (2)

Further, in the steel material having excellent toughness of the weld heat-affected zone according to the present embodiment, in mass%,
Cu: 1.5% or less,
Ni: 1.5% or less,
Mo: 1% or less,
Cr: 1% or less,
Nb: 0.05% or less,
V: 0.05% or less,
B: 0.002% or less,
May be contained alone or in combination.

And, in the steel material having excellent toughness of the weld heat-affected zone according to the present embodiment, 10000 particles / mm ² of TiN particles having a particle diameter of 0.01 μm or more and less than 0.5 μm including oxides composed of Mg and Al are included. In addition to the above number density, Mg-containing oxide particles having a particle size of 0.5 μm or more and 5 μm or less containing 50% or more by mass of Mg are present at a number density of 50 particles / mm ² or more.

  Hereinafter, the chemical component and the reason for defining the TiN particles and the Mg-containing oxide particles as described above will be described.

(C: carbon)
C is an element having an effect of improving the strength and toughness of the base material and the welded portion. Here, if the content of C is less than 0.03%, the above-described effects cannot be achieved. On the other hand, when the content of C exceeds 0.2%, the toughness of the heat affected zone is deteriorated, and the weldability may be deteriorated.
From the above, in the present embodiment, the content of C is limited to the range of 0.03% to 0.2%.

(Si: silicon)
Si is added for deoxidation of molten steel, but if its content exceeds 0.4%, the toughness of the heat affected zone is deteriorated and the weldability may be deteriorated.
From the above, in the present embodiment, the content of Si is limited to a range of 0.4% or less.

(Mn: manganese)
Mn is an element having an effect of improving the strength and toughness of the base material and the welded portion. Here, if the content of Mn is less than 0.5%, the above-described effects cannot be achieved. On the other hand, if the Mn content exceeds 2.0%, the toughness of the weld heat affected zone deteriorates, and the weldability also deteriorates.
From the above, in the present embodiment, the content of Mn is limited to the range of 0.5% to 2.0%.

(P: phosphorus)
P is an element that affects the toughness of the base material, but is inevitably contained. Therefore, in the present embodiment, the upper limit is set to 0.015%.

(S: sulfur)
S is an element that affects the toughness of the base material, but is inevitably contained. Therefore, in the present embodiment, the upper limit is set to 0.006%.

(Al: aluminum)
Al is an element added to promote deoxidation of molten steel. Further, it is an element contained in an oxide serving as a precipitation nucleus of TiN particles that pin the grain boundary. Here, when the Al content is less than 0.001%, deoxidation cannot be sufficiently performed, and an oxide serving as a precipitation nucleus of TiN particles may not be sufficiently generated. On the other hand, if the content of Al exceeds 0.01%, the toughness of the heat affected zone may deteriorate.
From the above, in the present embodiment, the Al content is limited to the range of 0.001% or more and 0.01% or less.

(Ti: titanium)
Ti is an element that forms pinning particles and has an effect of suppressing coarsening of the austenite phase. Here, when the content of Ti is less than 0.007%, the above-described effects may not be obtained. On the other hand, when the content of Ti exceeds 0.02%, carbides and the like are excessively generated, and the toughness of the weld heat affected zone may be deteriorated.
From the above, in the present embodiment, the content of Ti is limited to the range of 0.007% or more and 0.02% or less.

(Mg: magnesium)
Mg is an element that forms an oxide serving as a precipitation nucleus of the above-mentioned pinning particles and an Mg-containing oxide particle serving as an IGF transformation nucleus. Here, when the content of Mg is 0.001% or less, the above-described effects may not be obtained. On the other hand, when the content of Mg exceeds 0.006%, a large amount of oxide is generated, and there is a possibility that product defects due to inclusions may occur frequently.
From the above, in the present embodiment, the content of Mg is limited to the range of more than 0.001% and 0.006% or less.

(O: oxygen)
O is an element that forms an oxide serving as a precipitation nucleus of the above-described pinning particles and an Mg-containing oxide particle serving as an IGF transformation nucleus. Here, when the content of O is less than 0.001%, the above-described effects may not be obtained. On the other hand, if the O content exceeds 0.004%, the cleanliness of the molten steel is reduced, the mechanical properties are reduced, and product defects due to inclusions may occur frequently.
From the above, in the present embodiment, the O content is limited to the range of 0.001% to 0.004%.

(N: nitrogen)
N is an element that forms pinning particles together with Ti and has an effect of suppressing coarsening of the austenite phase. Here, when the content of N is less than 0.0025%, the above-described effects may not be obtained. On the other hand, if the N content exceeds 0.006%, carbides and the like are excessively generated, and the toughness of the weld heat affected zone may be deteriorated.
From the above, in the present embodiment, the content of N is limited to the range of 0.0025% or more and 0.006% or less.

(Cu, Ni)
Since Cu and Ni are elements having an effect of improving the strength and toughness of the base material, they may be added as appropriate. However, when the content of Cu and the content of Ni each exceed 1.5%, the weldability and the toughness of the weld heat affected zone may be deteriorated. For this reason, when adding Cu and Ni, it is preferable to limit each content to 1.5% or less.

(Mo, Cr)
Since Mo and Cr are elements having an effect of improving the strength and toughness of the base material, Mo and Cr may be appropriately added. However, if the Mo content and the Cr content each exceed 1.0%, the toughness of the base metal, the weldability, and the toughness of the weld heat affected zone may be degraded. For this reason, when adding Mo and Cr, it is preferable to limit each content to 1.0% or less.

(Nb)
Nb is an element having an effect of making crystal grains of the base material finer, and thus may be appropriately added. However, if the Nb content exceeds 0.05%, the toughness of the heat affected zone may deteriorate. For this reason, when adding Nb, it is preferable to limit the content of Nb to 0.05% or less.

(V)
V is an element having an effect of improving the strength of the base material, and may be appropriately added. However, if the V content exceeds 0.05%, the weldability and the toughness of the heat affected zone may deteriorate. For this reason, when adding V, it is preferable to limit the content of V to 0.05% or less.

(B)
B is an element having an effect of improving the hardenability and improving the mechanical properties of the base metal and the heat affected zone by welding, and therefore may be appropriately added. However, if the B content exceeds 0.002%, the weldability and the toughness of the heat affected zone may deteriorate. Therefore, when B is added, the content of B is preferably limited to 0.002% or less.

(Effective Ti amount)
Ti first combines with oxygen to form a Ti oxide, and then forms TiN (Ti nitride) which becomes pinning particles. If excessive Ti is present after the formation of the Ti oxide and the Ti nitride, there is a possibility that TiC is formed and precipitation embrittlement occurs. When unreacted N is present with a small amount of Ti, it becomes solid solution N and causes embrittlement. Therefore, in the present embodiment, the “effective Ti amount” is defined as the Ti amount after forming the oxide and the nitride by the above-described equations (1) and (2). The coefficients of the respective elements in the equations (1) and (2) are values stoichiometrically determined from assumed oxides and nitrides.
Here, when the effective Ti amount is less than -0.01%, the amount of solute N becomes excessive, and when the effective Ti amount exceeds + 0.005%, the TiC precipitation amount becomes excessive. The toughness of the part may be deteriorated. For this reason, in the present embodiment, the effective Ti amount is defined within the range of −0.01% or more and + 0.005% or less.

(TiN particles)
TiN particles are pinning particles that pin the grain boundaries and suppress the austenite phase from coarsening.
Here, the oxide composed of Mg and Al serves as a precipitation nucleus of TiN particles. The TiN particles formed by being deposited on the oxide composed of Mg and Al exhibit a very strong pinning force in a high temperature region of 1400 ° C. or higher. Further, since the oxide composed of Mg and Al is very fine, the TiN particles formed with the oxide composed of Mg and Al as precipitation nuclei have a fine particle diameter of 0.01 μm or more and less than 0.5 μm. Thus, the pinning effect can be sufficiently exhibited.
From the above, in the present embodiment, a configuration in which TiN particles having an average particle diameter of 0.01 μm or more and less than 0.5 μm and containing an oxide composed of Mg and Al are present at a number density of 10000 particles / mm ² or more. I have.

(Mg-containing oxide particles)
Mg-containing oxide particles are particles that act as IGF transformation nuclei (precipitation nuclei of intragranular ferrite).
Here, when Mg is contained by 50% by mass or more, the precipitation of ferrite is promoted. Further, Mg-containing oxide particles having a particle size of 0.5 μm or more and 5 μm or less are effective in promoting intragranular transformation.
From the above, in the present embodiment, the Mg-containing oxide particles having a particle diameter of 0.5 μm or more and 5 μm or less containing 50% or more by mass of Mg are present at a number density of 50 particles / mm ² or more. I have.

Hereinafter, a method for producing a steel material having excellent toughness of the weld heat-affected zone according to the present embodiment will be described with reference to FIG.
First, converter blowing is performed to melt molten steel having a predetermined composition (melting step S01).
Next, the molten steel is transferred from the converter to the ladle to perform vacuum degassing (vacuum degassing step S02).

Next, Mg is added to the molten steel so that the Mg concentration is in the range of more than 0.002% and 0.007% or less by mass% (Mg addition step S03).
In the Mg addition step S03, it is preferable to add Mg as an Mg-containing alloy such as a Si-Mg alloy or an Fe-Si-Mg alloy.

Next, an oxygen source for oxidizing 0.001% or more of Mg by mass% is added to the molten steel (oxygen source adding step S04).
In the oxygen source adding step S04, by adding SiO ₂ in the slag, thereby oxidizing the Mg. The specific composition of the slag before and after the oxygen source adding step S04 is, for example, the slag composition before addition of SiO ₂ is 10 to 20% by mass%, and the slag composition after addition of SiO ₂ is 20 to 25%. It becomes.
Here, in the present embodiment, in the oxygen source adding step S04, an oxygen source that oxidizes 0.001% to 0.003% by mass of Mg is added to the molten steel.

  By adjusting the amount of Mg added in the Mg adding step S03 and the amount of the oxidizing source added in the oxygen source adding step S04, the content of Mg in the molten steel is set within a range of more than 0.001% and 0.006% or less. Will be in control.

The molten steel whose composition has been adjusted in this way is poured into a mold of a continuous casting apparatus via a tundish to perform continuous casting (casting step S05).
Through the steps as described above, a steel material having excellent toughness of the weld heat affected zone according to the present embodiment is manufactured.

According to the steel material having excellent toughness of the weld heat-affected zone according to the present embodiment having the above-described configuration, TiN particles having a particle diameter of 0.01 μm or more and less than 0.5 μm including an oxide composed of Mg and Al are included. However, since they are present at a number density of 10000 / mm ² or more, coarsening of austenite grains can be reliably suppressed by the pinning effect of the grain boundaries.
Further, in the present embodiment, Mg-containing oxide particles having a particle diameter of 0.5 μm or more and 5 μm or less containing 50% or more by mass of Mg are present at a number density of 50 particles / mm ² or more. By using the Mg-containing oxide particles as nuclei, precipitation of intragranular ferrite can be promoted, and the crystal grain size in the heat affected zone can be further refined.
Therefore, in the steel material having excellent toughness of the heat affected zone of the present embodiment, the toughness of the heat affected zone can be greatly improved.

  Further, in the steel material having excellent toughness of the weld heat-affected zone according to the present embodiment, the “effective Ti amount” defined by the above formulas (1) and (2) is −0.01% or more and + 0.1%. Since it is set within the range of 005% or less, the amount of Ti can be optimized according to the amount of N, and deterioration of the toughness of the weld heat affected zone due to precipitation of solid solution N or TiC can be suppressed. .

  In the steel material having excellent toughness of the weld heat-affected zone according to the present embodiment, Cu, Ni, Mo, Cr, Nb, V, and B are added as necessary, and the content is defined within the above range. Therefore, various characteristics can be improved without deteriorating the toughness of the heat affected zone.

  According to the method for manufacturing a steel material having excellent toughness of the weld heat-affected zone according to the present embodiment, Mg is added to molten steel, and the Mg concentration is in a range of more than 0.002% to 0.007% or less by mass%. And an oxygen source adding step S04 of adding an oxygen source for oxidizing 0.001% or more by mass of Mg into molten steel, so that 50% or more of Mg is added by mass%. Mg-containing oxide particles can be reliably formed.

Further, according to the method for producing a steel material having excellent toughness of the weld heat-affected zone of the present embodiment, since SiO ₂ is added to the slag in the oxygen source addition step S04, the reaction between the slag and the molten steel Since the Mg oxide is formed slowly, relatively fine Mg-containing oxide particles having a particle size of 0.5 μm to 5 μm can be reliably formed.

  As mentioned above, although the steel material excellent in the toughness of the weld heat affected zone and the method for producing the steel material excellent in the toughness of the weld heat affected zone according to the embodiment of the present invention have been specifically described, the present invention is not limited to this. It can be changed as appropriate without departing from the technical idea of the invention.

(Example 1)
Table 1 shows the chemical composition of the steel material and the state of dispersion of the inclusions, and Table 2 shows the manufacturing conditions and mechanical properties of the steel material.
The number of pinning particles in Table 1 was measured by preparing an extracted replica sample from the center of the thickness of the steel base material and observing it with a TEM at a magnification of 30,000 times over an area of 2000 μm ² .
In Table 1, 0. The measurement of the number of oxides having a size of 5 to 5 μm was performed similarly by cutting out a small piece from the center of the thickness of the steel base material, keeping the material at 1400 ° C. for 20 minutes, cooling with water, and making the mirror-polished surface 1000 times larger. This was performed by observing with an optical microscope over an area of 4 mm ² .
In Table 1, 0. The Mg content of the oxide having a size of 5 to 5 μm is determined by analyzing the composition of 20 oxides of 0.5 to 5 μm by EPMA-WDS, and subtracting the analysis value of ground iron (Fe) to obtain the average composition. I asked.

  The steel of the present invention has unprecedented good HAZ toughness in a fusion line of an electrogas weld or an electroslag weld having a welding heat input of 20 to 100 kJ / mm. The steel of the present invention strictly controls the amounts of Al, Ti, Mg, O, and N, optimizes the existence form of Ti and N in the HAZ by using the concept of the effective Ti amount, and is effective in suppressing γ grain growth. The excellent HAZ toughness is achieved even in large heat input welding by having a suitable oxide dispersion state. On the other hand, the comparative steel is inferior in the mechanical properties of the base material and the HAZ because the dispersion state of the chemical components and oxides is not proper.

  Steel 12 has a too low C content, and steel 13 has a too high C content, so that the toughness of the base metal and HAZ is inferior. Steel 14 is inferior in HAZ toughness because the amount of Si is too high. Steel 15 has too low Mn content, and steel 16 has too high Mn content, so that the toughness of the base metal and HAZ is inferior. Steel 17 has inferior toughness of the base metal and HAZ because the P content is too high. Steel 18 has poor toughness of the base metal and HAZ because S is too high. In steel 19, the number of pinning particles is small because the amount of Al is too low, the γ grains are coarsened, and the HAZ toughness is poor. Steel 20 has an Al content of too high. Since the Mg in the oxide of 5 to 5 μm is low and the number of pinning particles is small, the HAZ toughness is poor.

  Since the amount of Ti in the steel 21 is too low, the number of the pinning particles TiN is small, and the HAZ structure is significantly coarsened and the HAZ toughness is poor. Since the amount of Ti in the steel 22 is too high, the HAZ toughness is inferior due to TiC precipitation embrittlement. Since the amount of Mg in the steel 23 is too low, the number of Mg-containing oxide particles, which are the precipitation nuclei of TiN, is small, the γ grains are coarsened, and the HAZ toughness is poor.

  In steel 24, since the amount of O is too low, the number of Mg-containing oxide particles is small, γ grains are coarsened, and HAZ toughness is inferior. Since the steel 25 has an excessively high O content, the cleanliness of the steel deteriorates, the number of fracture starting points increases, and the HAZ toughness deteriorates. In steel 26, the N content is too low, so that the number of pinning particles, TiN, is small, and the HAZ structure is remarkably coarsened to deteriorate the HAZ toughness. Since the amount of N in the steel 27 is too high, the effective Ti amount is out of the appropriate range, so that the amount of dissolved N becomes excessive and the HAZ toughness deteriorates. Steel 28 has an excessively high Ti content, and steel 29 has an excessively low Ti content, so that the effective Ti amount is inappropriate. Also, the number of Mg-containing oxide particles is small. For this reason, HAZ toughness is inferior.

(Example 2)
In melting the steel having the composition of the steel 1 of the present invention shown in Table 1, Mg was added to make the Mg concentration 0.006% by mass%, and then a SiO ₂ source was added to the ladle slag, and the slag in the slag was added. The SiO ₂ concentration was increased from 17% to 22% by weight. Thereafter, the continuously cast slab was rolled to produce a steel plate having a thickness of 70 mm.
As a comparison, when smelting a steel having the composition of the present invention steel 1 shown in Table 1, Mg was added to make the Mg concentration 0.005% by mass%, and then no SiO ₂ source was added to the ladle slag. With the SiO ₂ concentration in the slag kept at 17% by mass, the continuously cast slab was rolled to produce a steel plate having a thickness of 70 mm. At this time, 0. The number and average composition of the oxides having a size of 5 to 5 μm were measured.

In these measurements, a small piece was cut out from the center of the thickness of the steel base material, kept at 1400 ° C. for 20 minutes, then cooled with water, and the mirror-polished surface was observed with an optical microscope at a magnification of 1000 × over an area of 4 mm ^2. Asked by. Furthermore, the composition of 20 oxides of 0.5 to 5 μm was analyzed by EPMA-WDS, and the average composition was obtained by subtracting the analysis value of ground iron (Fe).

In the present invention in which the concentration of SiO ₂ in the slag is increased, the number of oxides having a size of 0.5 to 5 μm is 60 / mm ² , and the Mg concentration is 60%. _{2 When the} concentration was not increased, the number of oxides having a size of 0.5 to 5 μm was 50 / mm ² , and the Mg concentration was 56%. By increasing the concentration of SiO ₂ in the slag, it was possible to increase the number of oxides effectively acting as ferrite generation nuclei.

S03 Mg addition process
S04 Oxygen source addition step

Claims (5)

In mass%,
C: 0.03% or more and 0.2% or less,
Si: 0.4% or less,
Mn: 0.5% or more and 2.0% or less,
P: 0.015% or less,
S: 0.006% or less,
Al: 0.001% or more and 0.01% or less,
Ti: 0.007% or more and 0.02% or less,
Mg: more than 0.001% and 0.006% or less
O: 0.001% or more and 0.004% or less,
N: 0.0025% or more and 0.006% or less,
Containing a chemical component consisting of Fe and inevitable impurities,
TiN particles having a particle diameter of 0.01 μm or more and less than 0.5 μm including an oxide composed of Mg and Al are present at a number density of 10,000 particles / mm ² or more,
Excellent in the toughness of the weld heat-affected zone, wherein Mg-containing oxide particles having a particle diameter of 0.5 μm or more and 5 μm or less containing 50% or more by mass of Mg are present at a number density of 50 particles / mm ² or more. Steel material. Further, when [O], [Mg], [Al], [Ti], and [N] are mass% of each element, the effective Ti calculated by the following equation (1) or (2) is used. The steel material having excellent toughness of the weld heat-affected zone according to claim 1, wherein the amount is in the range of -0.01% or more and + 0.005% or less.
[O] −0.66 × [Mg] −0.89 × [Al] ≧ 0,
Effective Ti amount = [Ti] −2 × ([O] −0.66 × [Mg] −0.89 × [Al]) − 3.4 × [N] (1)
[O] −0.66 × [Mg] −0.89 × [Al] <0,
Effective Ti amount = [Ti] −3.4 × [N] (2) Mass%,
Cu: 1.5% or less,
Ni: 1.5% or less,
Mo: 1% or less,
Cr: 1% or less,
Nb: 0.05% or less,
V: 0.05% or less,
B: 0.002% or less,
The steel material having excellent toughness of the weld heat affected zone according to claim 1 or 2, wherein the steel material comprises one or more of the following. A method for producing a steel material having excellent toughness of a weld heat-affected zone, wherein the steel material has excellent toughness of a welded heat-affected zone according to any one of claims 1 to 3.
A step of adding Mg to the molten steel to make the Mg concentration in the range of more than 0.002% by mass% and not more than 0.007%,
An oxygen source adding step of adding an oxygen source for oxidizing 0.001% or more of Mg by mass% to molten steel;
A method for producing a steel material having excellent toughness in a heat-affected zone of a weld, comprising: 5. The toughness of the weld heat-affected zone according to claim 4, wherein in the oxygen source adding step, Mg of 0.001% or more by mass% is oxidized by adding SiO ₂ to the slag. Method of manufacturing steel.

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