JP7205619B2 - steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel material having excellent weld heat affected zone (HAZ) toughness.

High-strength steel sheets with a yield strength of about 300 to 700 MPa are used for various welded steel structures such as buildings, bridges, shipbuilding, line pipes, construction machinery, offshore structures, and tanks. These structures are required to have good HAZ toughness under a wide range of welding conditions, from low heat input welding with a welding heat input of about 5 kJ/mm to ultra-high heat input welding with a welding heat input exceeding 130 kJ/mm. be done.

In the HAZ, the closer to the fusion line, the higher the heating temperature during welding, especially in the region heated to 1400 ° C. or more near the fusion line, the austenite (γ) is remarkably coarsened, and the HAZ structure after cooling is coarsened. toughness deteriorates. This tendency becomes more conspicuous as the welding heat input increases.

Conventional techniques for improving toughness of HAZ are mainly based on two basic techniques when broadly classified. One of them is a technique to prevent coarsening of austenite by utilizing the pinning effect of particles in steel. Fine particles that contribute to refining the crystal grains of the HAZ are called pinning particles. Another technique is to refine the effective grain size by utilizing intragranular ferrite transformation of austenite.

WO 2014/091604 (Patent Document 1), JP 2013-204118 (Patent Document 2), JP 2002-3986 (Patent Document 3), sulfides containing fine Mg and Mn A steel material is described in which particles are dispersed in the steel and the pinning effect of the sulfide particles suppresses the growth of γ grains during welding, thereby improving the HAZ toughness.

In addition, International Publication No. 2001/027342 (Patent Document 4), JP-A-2000-80437 (Patent Document 5), JP-A-2000-80436 (Patent Document 6), JP-A-11-236645 (Patent Document Document 7) describes a steel material that can suppress the growth of γ grains during welding and improve HAZ toughness by dispersing fine oxide particles containing Ti and Mg in the steel. there is

Furthermore, in JP-A-2001-342537 (Patent Document 8), JP-A-2001-226739 (Patent Document 9), and JP-A-2001-288509 (Patent Document 10), fine Ti, Ca and Al A steel material is described in which coarsening of the HAZ structure is suppressed and toughness is improved by dispersing oxide particles containing in the steel and using these particles as ferrite transformation nuclei.

Furthermore, in International Publication No. 2011/148754 (Patent Document 11) and Japanese Patent Application Laid-Open No. 2009-174059 (Patent Document 12), fine TiN particles are dispersed in steel, and welding is performed by the pinning effect of TiN particles. It describes a steel material capable of suppressing the growth of γ grains during aging and improving the HAZ toughness.

In addition, in JP 2015-7264 (Patent Document 13) and JP 2012-52224 (Patent Document 14), by dispersing fine AlMn-based oxide particles in steel, A steel material capable of suppressing γ grain growth and improving HAZ toughness is described.

Furthermore, in International Publication No. 2015/075771 (Patent Document 15), Japanese Patent Application Laid-Open No. 2015-98642 (Patent Document 16), and International Publication No. 2014/199488 (Patent Document 17), TiN particles, MnS particles and A steel material is disclosed in which coarsening of the HAZ structure is suppressed and toughness is improved by dispersing these composite particles and Ti oxide particles in steel and using these particles as ferrite transformation nuclei.

Japanese Patent Application Laid-Open No. 2001-89825 (Patent Document 18) describes a steel material containing Bi as an optional component by utilizing refinement of oxides containing Mg in order to increase HAZ toughness. Japanese Patent Application Laid-Open No. 2007-100203 (Patent Document 19) describes a steel material containing Mg and Ag, or further containing Bi, in order to suppress the growth of γ grains. Japanese Patent Application Laid-Open No. 2011-218370 (Patent Document 20) describes a steel material containing Bi for refining the solidified structure.

WO2014/091604 Japanese Patent Application Laid-Open No. 2013-204118 Japanese Patent Application Laid-Open No. 2002-3986 WO 2001/027342 Japanese Patent Application Laid-Open No. 2000-80437 Japanese Patent Application Laid-Open No. 2000-80436 Japanese Patent Laid-Open No. 11-236645 Japanese Patent Application Laid-Open No. 2001-342537 Japanese Patent Application Laid-Open No. 2001-226739 Japanese Patent Application Laid-Open No. 2001-288509 WO2011/148754 Japanese Patent Application Laid-Open No. 2009-174059 Japanese Patent Application Laid-Open No. 2015-7264 Japanese Patent Application Laid-Open No. 2012-52224 WO2015/075771 Japanese Patent Application Laid-Open No. 2015-98642 WO2014/199488 Japanese Patent Application Laid-Open No. 2001-89825 Japanese Patent Application Laid-Open No. 2007-100203 Japanese Patent Application Laid-Open No. 2011-218370

Even with steel materials manufactured by these techniques, the HAZ refinement effect can be obtained. However, there is a demand for a steel material that provides excellent weldability even under severer welding conditions.
An object of the present invention is to provide a steel material having good HAZ toughness even after welding.

As a result of extensive studies by the inventors, it was found that by including elements such as Pb, Bi, Se, or Te (hereinafter sometimes referred to as X elements) in steel and optimizing the manufacturing conditions in the steelmaking process, It was found that a large amount of fine particles were generated, and the HAZ toughness of the steel material was improved. The gist of the present invention is as follows.

[1] % by mass,
C: 0.01 to 0.20%,
Si: 1.00% or less,
Mn: 0.1-2.5%,
Mg: 0.0005-0.0100%,
Al: 0.015 to 0.500%,
P: 0.020% or less,
S: 0.020% or less,
N: 0.0100% or less,
O: less than 0.0030%,
Cu: 0-2.0%,
Ni: 0 to 2.0%,
Cr: 0 to 2.0%,
Mo: 0-1.0%,
Nb: 0 to 0.10%,
W: 0 to 2.0%,
V: 0 to 0.20%,
B: 0 to 0.010%,
Ti: 0 to 0.100%,
Zr: 0 to 0.10%,
Ta: 0 to 0.10%,
Ag: 0-0.10%,
Hf: 0-0.10%,
Ca: 0 to 0.0100%,
REM: 0-0.010%,
Sn: 0-0.50%,
Sb: 0-0.50%
and further
[X _total ], which is the total content of one or more of the X elements Pb, Bi, Se, and Te, is 0.0001 to 0.0100%, and the balance is Fe and impurities,
[X _total ^* ] obtained by any one of the following formulas (1), (2), and (3) composed of the above [X _total ] and the contents of Ca, O, and S is 0.0. 0001 to 0.0050% of the steel material, and at a position 1/4 the thickness of the steel material from the surface of the steel material,
The number density of particles containing 5 atomic% or more of Mg with respect to the total of Ca, Mg, Mn and S and having an equivalent circle diameter of 0.01 to 0.10 μm is 1.0 × 10 ⁷ /mm A steel material characterized by being ³ or more.
The [X _total ^* ] is represented by formula (1) when {[Ca] ≤ (40/16) x [O]},
When {[Ca]>(40/16)×[O]} and {([Ca]−(40/16)×[O])≦(40/32)×[S]}, formula (2 ) and when {[Ca]>(40/16)×[O]} and {([Ca]−(40/16)×[O])>(40/32)×[S]} is represented by the formula (3).
[X _total ^* ]=[X _total ]... formula (1),
[X _total ^* ]=[X _total ]−([Ca]−( _40/16 )×[O])×M X total /400 Formula (2),
[X _total ^* ]=[X _total ]−([S]×M X total / ₂₈₈ ) Equation (3),
In the above formulas (2) and (3),
[Ca], [O], [S], [Pb], [Bi], [Se], and [Te] are Ca, O, S, Pb, Bi, Se, and Te content in % by mass, respectively. If not contained, 0 is substituted, M _Xtotal = ([Pb] + [Bi] + [Se] + [Te]) / ([Pb] / 207 + [Bi] / 209 + [Se] / 79 + [Te]/128).

[2] % by mass,
Cu: 0.02-2.0%,
Ni: 0.02 to 2.0%,
Cr: 0.02 to 2.0%,
Mo: 0.02-1.0%,
Nb: 0.01 to 0.10%,
W: 0.01 to 2.0%,
V: 0.01 to 0.20%,
B: 0.0003 to 0.010%,
Ti: 0.005 to 0.100%,
Zr: 0.01 to 0.10%,
Ta: 0.01 to 0.10%,
Ag: 0.01-0.10 % ,
Hf: 0.01-0.10 %
The steel material according to the above [1], characterized by containing one or more of

[3] in % by mass,
Ca: 0.0001 to 0.0100%,
REM: 0.001-0.010%
The steel material according to the above [1] or [2], characterized by containing one or both of

[4] % by mass,
Sn: 0.01 to 0.50%,
Sb: 0.01-0.50%
The steel material according to any one of [1] to [3] above, which contains one or both of

[5] Among the particles, the number ratio of particles containing 0.5 atomic % or more of the X element with respect to the total of Ca, Mg, Mn, S and the X element is 30% or more. The steel material according to any one of [1] to [4] above.

[6] Among the particles, the number ratio of particles in which at least a part of the outer periphery contains a region containing 0.5 atomic % or more of the X element as a ratio to the total of Ca, Mg, Mn, S and the X element is 30% or more, the steel material according to any one of the above [1] to [5].

[7] Among the particles, the number ratio of particles containing 0.5 atomic% or more of the X element in a region of 50% or more of the outer circumference with respect to the total of Ca, Mg, Mn, S and the X element is 30 % or more, the steel material according to the above [6]

[8] The number density of the particles containing 5 atomic% or more of Mg with respect to the total of Ca, Mg, Mn and S and having an equivalent circle diameter of 0.01 to 0.10 μm is 1.0 × 10 ⁷ to 1.0×10 ¹¹ pieces/mm ³ , the steel material according to any one of the above [1] to [7].

According to the steel material of the present invention, good HAZ toughness can be exhibited even after welding.

FIG. 4 is a diagram showing the relationship between [X _total ^* ] and toughness after a simulated thermal cycle test in the first embodiment; FIG. 10 is a diagram showing the relationship between [X _total ^* ] and toughness after a simulated thermal cycle test in the second embodiment. Ca, Mg, Mn, S and 0.5 atomic % or more of the X element relative to the sum of the X elements at a position 1/4 the thickness of the steel material from the surface of the steel material in the second embodiment FIG. 4 is a diagram showing the relationship between the number ratio of particles (X-containing particles 1) and the toughness after a simulated thermal cycle test. FIG. 10 is a diagram showing the relationship between [X _total ^* ] and toughness after a simulated thermal cycle test in the third embodiment. In the third embodiment, Ca, Mg, Mn, S, and the X element are added to at least a part of the outer periphery at a position 1/4 the thickness of the steel from the surface of the steel, at a rate of 0.5 atomic % with respect to the total of the elements 4 is a graph showing the relationship between the number ratio of particles (X-containing particles 2) in which regions containing the element X are present and the toughness after the simulated thermal cycle test. FIG. 10 is a diagram showing the relationship between [X _total ^* ] and toughness after a simulated thermal cycle test in the fourth embodiment. Ca, Mg, Mn, S, and the X element in a ratio of 0.5 atomic % or more to the total of the X elements are added to the outer circumference at a position 1/4 the thickness of the steel material from the surface of the steel material in the fourth embodiment. 2 is a diagram showing the relationship between the number ratio of particles (X-containing particles 3) contained in a region of 50% or more of the , and the toughness after a simulated thermal cycle test. FIG.

The steel material according to the first embodiment is premised on being a steel material manufactured by a manufacturing method including deoxidation with Al and Mg. The present inventors conducted detailed investigations and studies on the relationship between HAZ structure and toughness. As a result, it was found that the austenite grains in the HAZ must be remarkably refined (grain refinement) in order to improve the HAZ toughness. Utilizing the pinning effect of particles in steel is effective for refining austenite grains. However, depending on the welding heat input and the temperature used, the effect of improving toughness by refining austenite grains in the HAZ using the pinning effect was limited.

In view of the above circumstances, the present inventors have found that the steel contains one or more X elements selected from the group consisting of Pb, Bi, Se, or Te, and the X element improves the toughness of the HAZ. A study was performed on the contributing effective X _total [X _total ^* ]. Furthermore, by optimizing the manufacturing conditions in the steelmaking process, the conditions under which particles of a predetermined size containing a predetermined amount or more of Mg are generated in steel so as to have a number density within a predetermined range were also investigated. In addition to the formation conditions, the conditions under which the X element contributes to the improvement of HAZ toughness were also investigated. The HAZ toughness was evaluated by conducting a simulated thermal cycle test in which a sample taken from a steel material was given a heat history that simulates welding (equivalent to a welding heat input of 450 kJ/cm). Specifically, after the simulated thermal cycle test, the Charpy absorbed energy was measured at −40° C. to −30° C. with 3 tests in accordance with JIS Z 2242:2005, and the HAZ toughness was evaluated at the lowest value.

[X _total ^* ] is an index that correlates with the formation of oxides, sulfides, and acids/sulfides of Ca, and ranges from 0.0001 to 0.0001 as shown in FIGS. 0050% (1 to 50 ppm) was found to significantly improve the toughness of the HAZ. Here, [X _total ^* ] is represented by formula (1) when {[Ca] ≤ (40/16) x [O]}, {[Ca] > (40/16) x [O ]} and {([Ca]-(40/16) × [O]) ≤ (40/32) × [S]} is represented by formula (2), {[Ca]>(40/ 16)×[O]} and {([Ca]−(40/16)×[O])>(40/32)×[S]} is represented by equation (3).
[X _total ^* ]=[X _total ]... formula (1),
[X _total ^* ]=[X _total ]−([Ca]−( _40/16 )×[O])×M X total /400 Formula (2),
[X _total ^* ]=[X _total ]−([S]×M X total / ₂₈₈ ) Equation (3).
[Ca], [O], [S], [Pb], [Bi], [Se], and [Te] are Ca, O, S, Pb, Bi, Se, and Te content in % by mass, respectively. is the amount, and if it does not contain, 0 is substituted, and in the above formulas (2) and (3),
M _Xtotal = ([Pb]+[Bi]+[Se]+[Te])/([Pb]/207+[Bi]/209+[Se]/79+[Te]/128). In the above equations (2) and (3), the value of [X _total ^* ] is set to 0 when the value of [X _total ^* ] is negative.

Furthermore, fine particles that contribute to improving the toughness of the HAZ were examined using an electron microscope, as will be described later. As a result, at a position 1/4 the thickness of the steel material from the surface of the steel material, the ratio of Mg to the total of Ca, Mg, Mn and S is 5 atomic% or more, and the equivalent circle diameter is 0.01. It was newly found that the HAZ toughness can be further improved by generating particles having a particle size of up to 0.10 μm such that the number density is 1.0×10 ⁷ particles/mm ³ or more. Furthermore, at a position 1/4 the thickness of the steel material from the surface of the steel material, it contains 5 atomic% or more of Mg in terms of the total of Ca, Mg, Mn and S, and the equivalent circle diameter is 0.01 to The number density of 0.10 μm particles is 1.0×10 ⁷ /mm ³ or more, and among those particles, the proportion of Ca, Mg, Mn, S and the X element to the total is 0.5 atomic % or more When the number ratio of X-containing particles containing the X element is 30% or more, as shown in FIG.
Then, at a position 1/4 the thickness of the steel material from the surface of the steel material, it contains 5 atomic% or more of Mg in terms of the total of Ca, Mg, Mn and S, and has an equivalent circle diameter of 0.01 to 0. The number density of 10 μm particles is 1.0×10 ⁷ /mm ³ or more, and among those particles, Ca, Mg, Mn, S and the total of the X elements are present in at least a part of the outer periphery of the particles When the number ratio of X-containing particles having a region containing 0.5 atomic % or more of the X element is 30% or more, as shown in FIG. It was found that the HAZ toughness is further improved than when the number ratio of particles containing 0.5 atomic % or more of the X element to the total of is 30% or more.
In addition, at a position 1/4 the thickness of the steel material from the surface of the steel material, it contains 5 atomic% or more of Mg as a proportion of the total of Ca, Mg, Mn and S, and the equivalent circle diameter is 0.01 to 0. The number density of 10 μm particles is 1.0×10 ⁷ /mm ³ or more, and among those particles, 50% or more of the outer circumference of the particles contains Ca, Mg, Mn, S and the X element. When the number ratio of the X-containing particles containing 0.5 atomic % or more of the X element to the total is 30% or more, as shown in FIG. 7, Ca, Mg, Mn, S And it was found that the HAZ toughness is further improved as compared with the case where the number ratio of particles in which a region containing 0.5 atomic % or more of the X element relative to the total of the X elements exists is 30% or more. In this specification, the equivalent circle diameter refers to the diameter of a circle having an area equal to the projected area of the measured particle, and is specifically derived by the following formula.
Equivalent circle diameter = √ {4 × (area of the particle) / π}

(First embodiment)
The steel material according to the first embodiment will be described in detail below.

First, the chemical composition of the steel material according to the first embodiment will be explained.
The steel material according to the first embodiment has, in mass %, C: 0.01 to 0.20%, Si: 1.00% or less, Mn: 0.1 to 2.5%, Mg: 0.0005 to 0.0100%, Al: 0.015 to 0.500%, P: 0.020% or less, S: 0.020% or less, N: 0.0100% or less, O: less than 0.0030%, Cu: 0-2.0%, Ni: 0-2.0%, Cr: 0-2.0%, Mo: 0-1.0%, Nb: 0-0.10%, W: 0-2.0 %, V: 0-0.20%, B: 0-0.010%, Ti: 0-0.100%, Zr: 0-0.10%, Ta: 0-0.10%, Ag: 0 ~0.10%, Hf: 0-0.10%, Ca: 0-0.0100%, REM: 0-0.010%, Sn: 0-0.50%, Sb: 0-0.50% Furthermore, the total content of one or more of the X elements Pb, Bi, Se, and Te is 0.0001 to 0.0100%, and the balance is Fe and impurities.

In addition, in the following description of chemical components, % by mass is expressed as %. In the following description, when the upper limit and the lower limit of the element content are connected by "-" to indicate a range, unless otherwise noted, the range includes the upper limit and the lower limit. Therefore, when expressed as 0.01 to 0.20% by mass, the range means the range of 0.01% by mass or more and 0.20% by mass or less.

C: 0.01-0.20%
C is an element that increases the strength of the base material. If the C content is less than 0.01%, the effect of improving the strength of the base material is small, so the lower limit was made 0.01% or more. A more preferable lower limit of the C content is 0.06% or more. On the other hand, if the C content exceeds 0.20%, cementite or a mixture of martensite and austenite (called Martensite-Austenite Constituent: MA), which is the starting point of brittle fracture, increases, so the HAZ toughness decreases. do. Therefore, the upper limit of the C content is made 0.20% or less. Especially for the HAZ toughness and low temperature toughness of high heat input welding, even a relatively small amount of small cementite or MA can easily become the starting point of brittle fracture and lower the HAZ toughness, so the upper limit of the C content is strict. It is preferable to regulate The upper limit of the C content is preferably 0.15% or less, more preferably 0.13% or less, even more preferably 0.10% or less, still more preferably 0.08% or less.

Si: 1.00% or less Si functions as a deoxidizer and contributes to an increase in strength, but if it is contained excessively, MA, which is a hard embrittlement structure, tends to form in the microstructure of the HAZ. . Since this MA degrades the toughness of the HAZ, it is desirable to limit the Si content, but Si may be intentionally included as long as it is 1.00% or less. The upper limit of the Si content is preferably 0.50% or less, more preferably 0.30% or less. Since a smaller Si content is desirable for improving HAZ toughness, there is no particular need to limit the lower limit, and the lower limit is 0% or more. However, reducing the Si content to less than 0.03% may lead to an increase in cost, and in that case it is desirable to set the lower limit to 0.03% or more.

Mn: 0.1-2.5%
Mn must be contained in an amount of 0.1% or more as an effective component for ensuring the strength and toughness of the base material. In order to ensure strength, the lower limit of the Mn content is more preferably 0.3% or more, more preferably 0.4% or more, and still more preferably 0.5% or more. The inclusion of a large amount of Mn leads to segregation and formation of hard phases, and lowers HAZ toughness. The upper limit is set to 2.5% or less within the allowable range. A more preferable upper limit of the Mn content is 2.3% or less, more preferably 2.0% or less.

P: 0.020% or less P is an element that causes intergranular embrittlement and is harmful to toughness. Therefore, the smaller the P content, the better. If the P content exceeds 0.020%, the HAZ toughness is lowered even if the austenite grains of the HAZ are refined, so the upper limit of the P content is limited to 0.020% or less. It is preferably 0.010% or less, more preferably 0.008% or less. Although it is not necessary to limit the lower limit of the P content, it is technically not easy to make the P content 0%, so the lower limit may be more than 0%. The lower limit of the P content may be 0.001% or more.

S: 0.020% or less S is an element that forms pinning particles containing Mg and contributes to the improvement of HAZ toughness. If the S content exceeds 0.020%, the stability of the pinning particles at high temperatures is lowered, and the effect of improving the HAZ toughness may not be sufficiently obtained. Therefore, the upper limit of the S content is made 0.020% or less. A preferable upper limit of the S content is 0.015% or less. In order to improve the HAZ toughness, the upper limit of the S content may be 0.010% or less and 0.008% or less. Although it is not necessary to limit the lower limit of the S content, it is technically not easy to make the S content 0%, so the lower limit may be more than 0%. On the other hand, in order to increase the amount of particles that contribute to pinning, the lower limit of the S content is preferably 0.0020% or more. In order to generate more particles, the lower limit of the S content may be 0.0025% or more, or 0.0030% or more.

Mg: 0.0005-0.0100%
Mg is an important element that forms pinning particles and contributes to improving HAZ toughness. If the Mg content is less than 0.0005%, a sufficient number of particles may not be obtained, so the lower limit is made 0.0005% or more. In order to generate a larger amount of particles, the lower limit of the Mg content is preferably 0.0007% or more, more preferably 0.0008% or more, and still more preferably 0.0010% or more. On the other hand, even if the Mg content exceeds 0.0100%, the effect of improving HAZ toughness is saturated, impairing economy. Therefore, the upper limit of the Mg content is made 0.0100% or less. The upper limit of the Mg content may be 0.0080% or less or 0.0050% or less.

Al: 0.015-0.500%
Al is an element that functions as a deoxidizing agent and reduces the amount of dissolved oxygen in molten steel. The lower limit of the Al content is set to 0.015% or more in order to promote the formation of pinning particles. The lower limit of Al content is preferably 0.020% or more, more preferably 0.030% or more. However, if Al is contained excessively, the HAZ toughness deteriorates, so the upper limit of the Al content is made 0.500% or less. A preferable upper limit of the Al content is 0.300% or less. In order to improve the HAZ toughness, the upper limit of the Al content may be 0.150% or less, 0.100% or less, or 0.080% or less.

N: 0.0100% or less N is an element that forms nitrides, and when the N content is high, coarse nitrides such as AlN and TiN tend to form. These coarse grains become starting points for brittle fracture and may lead to a decrease in HAZ toughness. Therefore, the upper limit of the N content is made 0.0100% or less. A preferable upper limit of the N content is 0.0070% or less, more preferably 0.0050% or less. A smaller N content is desirable, but reducing the N content to less than 0.0020% may lead to an increase in cost, so 0.0020% or more may be the lower limit. The lower limit of the N content may be 0.0030% or more.

O: less than 0.0030% O is an element that forms oxides, and when the content is large, coarse oxides tend to form. Coarse oxides serve as starting points for fracture and lower the HAZ toughness, so the upper limit of the O content is made less than 0.0030%. The upper limit of the O content is preferably 0.0028%, more preferably 0.0025% or less, and still more preferably 0.0022%. On the other hand, reducing the O content to less than 0.0001% leads to an increase in cost, which is not preferable. In order to generate particles containing 5 atomic % or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm so that the number density is 1.0×10 ⁷ particles/mm ³ or more, It is preferable to contain 0.0001% or more of O. In order to generate more fine particles, the lower limit of the O content may be 0.0005% or more, or 0.0010% or more.

Total content of X elements Pb, Bi, Se, Te [Xtotal]: 0.0001 to 0.0100%
The steel material according to the first embodiment contains 5 atomic % or more of Mg as a proportion of the total of Ca, Mg, Mn and S at a position ¼ of the thickness of the steel material from the surface of the steel material, and The number density of particles having an equivalent circle diameter of 0.01 to 0.10 μm is 1.0×10 ⁷ /mm ³ or more. When the X elements Pb, Bi, Se, and Te are contained, the number of particles containing Mg increases, grain growth of austenite grains in the HAZ is suppressed, and HAZ toughness is improved. In order to increase the number density of the particles containing Mg, one or more of the X elements Pb, Bi, Se, and Te are contained, and the total content of the X elements (Pb, Bi, Se, Te The lower limit of the total content of [X _total ]) must be 0.0001% or more. Preferably, the lower limit of the total content of X elements is 0.0005% or more, more preferably 0.0010% or more, and still more preferably 0.0020% or more. The lower limit of the content of each X element may be 0%, preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more. Although the effect of the X element is not necessarily clear, it is possible that the X element contributes to the refinement and increase of the pinning grains. On the other hand, if these elements are contained excessively, the HAZ toughness is lowered. Therefore, the upper limit of the total content of the X elements is made 0.0100% or less. The upper limit of the total content of the X elements is more preferably 0.0080% or less, still more preferably 0.0050% or less, and most preferably 0.0030% or less. The upper limit of the content of each X element is preferably 0.0100% or more. It is more preferably 0.0080% or less, still more preferably 0.0050% or less.

The rest of the chemical components of the steel material according to the first embodiment are iron (Fe) and impurities. Impurities are components that are mixed in from raw materials such as ores, scraps, and other factors when industrially producing steel materials, and are allowed within a range that does not adversely affect the steel materials according to the first embodiment. means However, among the impurities, P, S, O and N must be limited to upper limits as described above.

In addition, the steel material according to the first embodiment basically contains the above chemical components, but in order to improve the mechanical properties and HAZ toughness of the steel material (base material), if necessary, Cu: 0 to 2.0%, Ni: 0 to 2.0%, Cr: 0 to 2.0%, Mo: 0 to 1.0%, Nb: 0 to 0.10% , W: 0-2.0%, V: 0-0.20%, B: 0-0.010%, Ti: 0-0.100%, Zr: 0-0.10%, Ta: 0- One or more of 0.10%, Ag: 0 to 0.10%, Hf: 0 to 0.10% may be contained. However, since these elements are not essential components, the lower limit of the total amount of these elements is 0%.

Cu: 0-2.0%
Cu is an element effective in increasing the strength of the base material, and may contain Cu. is limited to 2.0% or less. Preferably, the upper limit of the Cu content is 1.0% or less, more preferably 0.8% or less, and even more preferably 0.5% or less. Cu may be mixed as an impurity from scrap or the like during the production of molten steel, but there is no particular need to limit its lower limit, and it may be 0% or more. In order to improve the strength of the base metal, the lower limit of the Cu content is preferably 0.02% or more. More preferably, the lower limit of the Cu content is 0.1% or more.

Ni: 0-2.0%
Ni is an element effective in improving toughness and strength, and may be contained. However, even if the Ni content exceeds 2.0%, the effect is saturated, so the upper limit of the Ni content is limited to 2.0% or less from the viewpoint of economy. The upper limit of the Ni content is preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.7% or less. Ni may be mixed as an impurity from scraps or the like during the production of molten steel, but the lower limit thereof is not particularly limited, and may be 0% or more. In order to improve the strength of the base metal, the lower limit of the Ni content is preferably 0.02% or more. More preferably, the lower limit of the Ni content is 0.1% or more.

Cr: 0-2.0%
Cr is an element that increases the strength of the base material by improving hardenability and precipitation strengthening, and may be contained. However, if the Cr content exceeds 2.0%, MA tends to form in the HAZ, and the HAZ toughness decreases. Therefore, the Cr content is limited to 2.0% or less. The upper limit of the Cr content is preferably 1.0% or less, more preferably 0.5% or less. Cr may be mixed as an impurity from scrap or the like during the production of molten steel, but there is no need to limit its lower limit, and it may be 0% or more. In order to improve the strength of the base metal, the lower limit of the Cr content is preferably 0.02% or more. More preferably, the lower limit of Cr content is 0.1% or more.

Mo: 0-1.0%
Mo is an element that improves the hardenability and increases the strength of the base metal, and may be contained. However, if the Mo content exceeds 1.0%, a hard structure may be generated in the HAZ and the HAZ toughness may decrease, so the upper limit of the Mo content is limited to 1.0% or less. The upper limit of the Mo content is preferably 0.5% or less, more preferably 0.3% or less. Although Mo may be mixed as an impurity from scrap or the like during the production of molten steel, there is no particular need to limit its lower limit, and it may be 0% or more. In order to improve the strength of the base material, the lower limit of the Mo content is preferably 0.02% or more. More preferably, the lower limit of Mo content is 0.1% or more.

Nb: 0-0.10%
Nb is an element that improves hardenability, and also contributes to refinement of the structure by suppressing the formation of precipitates and recrystallization. Nb may be contained in order to increase the strength of the base material and improve the toughness and productivity of the base material. However, if the Nb content exceeds 0.10%, hard structures and inclusions are formed in the HAZ, and the HAZ toughness may decrease, so the Nb content is limited to 0.10% or less. The Nb content is preferably 0.05% or less, more preferably 0.04% or less. Nb may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof is not particularly limited, and may be 0% or more. The Nb content is preferably 0.01% or more in order to improve the strength and toughness of the base metal and to be economical.

W: 0-2.0%
W is an element that contributes to improvement of hardenability and precipitation strengthening, and W may be contained in order to increase the strength of the base material and improve the toughness. However, if the W content exceeds 2.0%, a hard structure may be generated in the HAZ and the HAZ toughness may decrease, so the W content is limited to 2.0% or less. The W content is preferably 1.0% or less, more preferably 0.5% or less. W may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof is not particularly limited, and may be 0% or more. The W content is preferably 0.01% or more in order to improve the strength and toughness of the base material.

V: 0-0.20%
V is an element that improves hardenability, forms carbides and nitrides, and is an element that is effective in increasing the strength of the base material. Therefore, V may be contained. However, if the V content exceeds 0.20%, the precipitation of carbonitrides in the HAZ becomes significant, and the HAZ toughness may deteriorate, so the V content is limited to 0.20% or less. Preferably, the V content is 0.10% or less. Although V may be mixed as an impurity from scrap or the like during the production of molten steel, there is no particular need to limit its lower limit, and it may be 0% or more. In order to improve the strength of the base metal, the V content is preferably 0.01% or more.

B: 0-0.010%
B is an element that remarkably increases the hardenability and improves the strength and toughness of the base material and HAZ, and B may be contained. However, if the B content exceeds 0.010%, the HAZ toughness and weldability may deteriorate, so the upper limit of the B content is limited to 0.010% or less. The upper limit of the B content is preferably 0.007% or less, more preferably 0.005% or less. Although the lower limit of the B content may be 0% or more, the lower limit of the B content is preferably 0.0003% or more in order to obtain the effect of increasing the strength. More preferably, the lower limit of the B content is 0.0005% or more, and still more preferably 0.001% or more.

Ti: 0-0.100%
Ti is an element that forms TiN and contributes to refinement of crystal grains, and Ti may be contained in order to improve strength and toughness. However, if the Ti content exceeds 0.100%, excessive TiC may be generated and the HAZ toughness may deteriorate, so the upper limit of the Ti content is limited to 0.100% or less. Preferably, the upper limit of the Ti content is 0.050% or less, more preferably 0.030% or less. Although Ti may be mixed as an impurity from scrap or the like during the production of molten steel, there is no particular lower limit to the Ti content, and the content may be 0% or more. Preferably, the lower limit of the Ti content is 0.005% or more, more preferably 0.010% or more.

Zr: 0-0.10%
Zr is an element that forms carbides and nitrides and is effective in increasing the strength of the base material and refining the structure, so Zr may be contained. However, if the Zr content exceeds 0.10%, coarse nitrides may be formed and the toughness may decrease, so the upper limit of the Zr content is limited to 0.10% or less. Preferably, the upper limit of the Zr content is 0.05% or less. The lower limit of the Zr content is not particularly limited and may be 0% or more, but the lower limit of the Zr content is preferably 0.01% or more in order to improve the strength of the base material.

Ta: 0-0.10%
Ta is an effective element for ensuring the strength and toughness of the base metal, and may be contained. However, if the Ta content exceeds 0.10%, the HAZ toughness may deteriorate, so the upper limit of the Ta content is limited to 0.10% or less. Preferably, the upper limit of the Ta content is 0.05% or less. Ta may be mixed as an impurity from scrap or the like during the production of molten steel, but there is no particular need to limit its lower limit, and it may be 0% or more. The lower limit of Ta content may be 0.01% or more.

Ag: 0-0.10%
Ag is an element effective in increasing the strength of the base material and refining the structure, and Ag may be contained. However, if the Ag content exceeds 0.10%, the HAZ toughness may deteriorate, so the upper limit of the Ag content is limited to 0.10% or less. Preferably, the upper limit of Ag content is 0.05% or less. Ag may be mixed as an impurity from scrap or the like during the production of molten steel, but there is no particular need to limit its lower limit, and it may be 0% or more. The lower limit of Ag content may be 0.01% or more.

Hf: 0-0.10%
Hf is an element that contributes to the generation of pinning particles, and may be contained. However, if the Hf content exceeds 0.10%, coarse nitrides are formed in the HAZ, and the HAZ toughness may decrease, so the upper limit of the Hf content is limited to 0.10% or less. Preferably, the upper limit of the Hf content is 0.05% or less. The lower limit of the Hf content is not particularly limited, and may be 0% or more. The lower limit of Hf content may be 0.01% or more.

In addition, in order to control the form of inclusions, the steel material according to the first embodiment further contains Ca: 0 to 0.0100%, REM: 0 to 0, as necessary instead of part of Fe One or both of .010% or less may be contained.

Ca: 0-0.0100%
Ca is an element that forms oxides and sulfides, and may be contained in order to control the form of inclusions. However, if the Ca content exceeds 0.0100%, coarse oxides are likely to be generated, so the upper limit of the Ca content is limited to 0.0100% or less. The upper limit of the Ca content is preferably 0.0060% or less, more preferably 0.0050% or less, still more preferably 0.0040% or less, still more preferably 0.0030% or less. In order to promote the formation of pinning particles, it is preferable to limit the upper limit of the Ca content to 0.0005% or less. The lower limit of the Ca content is not particularly limited, and may be 0% or more. The lower limit of Ca content is preferably 0.0001% or more.

REM: 0-0.010%
REM is an element that forms oxides and sulfides, and may be contained in order to control the form of inclusions. However, if the REM content is high, coarse oxides are likely to be formed, and the HAZ toughness may be lowered, so the upper limit of the REM content is limited to 0.010% or less. Preferably, the upper limit of the REM content is 0.005% or less, more preferably 0.004% or less. In order to generate pinning particles, it is preferable to limit the upper limit of the REM content to 0.0005% or less. The lower limit of the REM content is not particularly limited, and may be 0% or more. The lower limit of the REM content may be 0.001% or more. REM in the present invention is a general term for a total of 17 elements including lanthanoid elements such as La and Ce, and Sc and Y. When these elements are added, even if a misch metal containing these elements is used, the effect does not change at all.

Further, in order to improve the corrosion resistance of the steel material according to the first embodiment, Sn: 0 to 0.50% and Sb: 0 to 0.50% are added as necessary instead of part of Fe. You may contain one or both of.

Sn: 0-0.50%, Sb: 0-0.50%
Sn and Sb may be contained from the viewpoint of corrosion resistance and the like, but excessive inclusion may impair HAZ toughness. Therefore, the contents of Sn and Sb are set to 0.50% or less, preferably 0.20% or less, and even more preferably 0.10% or less. There is no particular need to limit the lower limits of these elements, and they may be 0% or more. The contents of Sn and Sb may each be 0.01% or more.

From the viewpoint of HAZ toughness, the chemical composition of the steel material according to the first embodiment preferably has a carbon equivalent Ceq represented by the following formula in the range of 0.25 to 0.50. When the Ceq is 0.30 or more, the steel material has more excellent HAZ toughness. Moreover, when the Ceq is 0.45 or less, the generation of MA is suppressed and the HAZ toughness is improved, which is more preferable. More preferably, Ceq is 0.40 or less.

Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15
[C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] in the formula are mass % of C, Mn, Cr, Mo, V, Cu, and Ni, respectively. , and 0 is substituted if it does not contain.

In the chemical composition of the steel material according to the first embodiment, the following formula composed of the total [X _total ] of the contents of Pb, Bi, Se, and Te and the contents of Ca, O, and S [X _total ^* ] obtained by any one of (1), formula (2), and formula (3) is 0.0001 to 0.0050%.
[X _total ^* ] is represented by formula (1) when {[Ca] ≤ (40/16) x [O]}, {[Ca] > (40/16) x [O]} and When {([Ca]−(40/16)×[O])≦(40/32)×[S]}, it is represented by formula (2), {[Ca]>(40/16)× [O]} and {([Ca]−(40/16)×[O])>(40/32)×[S]} is represented by equation (3).
[X _total ^* ]=[X _total ]... formula (1),
[X _total ^* ]=[X _total ]−([Ca]−( _40/16 )×[O])×M X total /400 Formula (2),
[X _total ^* ]=[X _total ]−([S]×M X total / ₂₈₈ ) Equation (3).
[Ca], [O], [S], [Pb], [Bi], [Se], and [Te] are Ca, O, S, Pb, Bi, Se, and Te content in % by mass, respectively. is the amount, and if it does not contain, 0 is substituted, and in the above formulas (2) and (3),
M _Xtotal = ([Pb]+[Bi]+[Se]+[Te])/([Pb]/207+[Bi]/209+[Se]/79+[Te]/128).

When [X _total ^* ] is 0.0001% or more, the number density of particles containing 5 atomic % or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm is sufficiently high. [X _total ^* ] is preferably 0.0005% or more. Further, when [X _total ^* ] is 0.0050% or less, the steel material has excellent HAZ toughness. [X _total ^* ] is preferably 0.0035% or less, more preferably 0.0020% or less.
[X _total ^* ] is in the above range, if particles containing 5 atomic% or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm are present at a predetermined number density, the steel material Contributes to improvement of HAZ toughness. [X _total ^* ], which is the amount of X element (% by mass) that contributes to the generation of the particles, is considered as follows.

In the chemical composition of the steel material according to the first embodiment, oxygen combines with Ca to form Ca oxides. Here, oxygen used for forming Ca oxide is defined as effective O. As shown in FIG. When the Ca content in the steel material is less than the O content, all Ca becomes Ca oxide and CaS is not generated (Case 1). When the Ca content in the steel material is large relative to the O content, Ca (available Ca) that has not been used to form Ca oxides combines with S and X elements to form CaS containing X elements. Then, when the S content in the steel material is larger than the effective Ca, the formation amount of CaS containing the X element is determined by the effective Ca (Case 2). On the other hand, when the S content in the steel material is less than the effective Ca, the formation amount of CaS containing the X element is determined by the S content (Case 3).

In general, CaS-based inclusions (inclusions containing CaS as a main component) are large in size. Specifically, most of the CaS-based inclusions are particles having an equivalent circle diameter of 1.0 μm or more. Therefore, the X element used to form CaS containing the X element is the particle that contributes to the improvement of HAZ toughness (the equivalent circle diameter containing 5 atomic% or more Mg is 0.01 to 0.10 μm). Not used for generation. Therefore, [X _total ^* ] (% by mass) is the amount of the X element excluding the X element used to form CaS containing the X element from the X element contained in the steel material. [X _total ^* ] is expressed by the following formulas (1), (2), and (3), respectively, for case 1, case 2, and case 3 above. Further, since the atomic weights of Ca, O and S are 40, 16 and 32, respectively, Case 1, Case 2 and Case 3 are expressed by the following conditional expressions.
Case 1: [Ca]≤(40/16)*[O].
Case 2: {[Ca]>(40/16)*[O]} and {([Ca]-(40/16)*[O])≤(40/32)*[S]}.
Case 3: If {[Ca]>(40/16)*[O]} and {([Ca]-(40/16)*[O])>(40/32)*[S]}.

The process of deriving the formula [X _total ^* ] is shown below. In the following formula, the total content of Pb, Bi, Se, and Te is represented by [X _total ]. When an element symbol is written behind "%", it indicates the content in mass % of the element in the steel material. For example, "% X _total " is the total X _total of the contents of Pb, Bi, Se and Te in the steel. Furthermore, when "(as CaS)" is attached behind it, it is the content in mass% of the element contained in CaS containing the X element. That is, [% X (as CaS)] indicates the content in mass% of the X element contained in CaS containing the X element, and [% Ca (as CaS)] is the content of Ca contained in CaS containing the X element. The content in mass % is shown, and [% S (as CaS)] shows the content in mass % of S contained in CaS containing the X element. [% Ca (as CaO)] indicates the content in mass% of Ca contained in Ca oxide, and [% Ca ^* ] is the mass of Ca (effective Ca) used to form CaS containing the X element. %, and [% O] indicates the content in mass % of O (available O (oxygen)) used for forming Ca oxide. "M" indicates atomic weight, followed by an element symbol as a subscript. For example, "M _Ca " is the atomic weight of Ca.

[X _total ^* ] (% by mass) is the amount of X element [% X _total ] contained in the steel material, excluding X element [% X (as CaS)] used to form CaS containing X element. is the amount of X element,
[X _total ^* ] = [% X _total ] - [% X (as CaS)]
is indicated by
In Case 1 where the Ca content is less than the O content, all Ca becomes Ca oxide, and CaS is not produced, [X _total ^* ] is given by the following formula (1).
[ _Xtotal ^* ]=[ _Xtotal ]... Formula (1)

Next, when the Ca content in the steel material is large relative to the O content, Ca (available Ca) that was not used to form Ca oxides combines with S and X elements to form CaS containing X elements. Generate. The present inventors measured the content ratio (atomic number ratio) of each element in CaS including the X element in the composition system of the steel material of the present disclosure. As a result, it was found that the atomic number ratio in CaS containing the X element was approximately Ca:S:X element=50:45:5. Based on this knowledge, [% X (as CaS)] is based on the premise that the atomic ratio in CaS containing the X element is Ca: S: X element = 50: 45: 5, and the X element is It is necessary to consider whether the amount of CaS formed is limited by the Ca content or the S content. When the amount of CaS formed is limited by the Ca content, Ca: X elements = 50: 5 = 10: 1,
[%X(as CaS)]=[%Ca ^* ]M _Xtotal /10M _Ca
becomes. On the other hand, when the amount of CaS formed is limited by the S content, S: X elements = 45: 5 = 9: 1,
[% X (as CaS)] = [% S] M _Xtotal /9M _S
becomes.

The atomic number ratio in CaS containing the X element is obtained by analyzing inclusions using steel materials having various chemical compositions containing the Ca, S, and X elements. Specifically, elemental analysis of inclusions in steel is performed using an Energy Dispersive X-ray Spectrometry (EDS) attached to a Scanning Electron Microscope (SEM). The atomic number ratio was determined based on the measurement results of CaS.

In Case 2, where the S content in the steel material is greater than the effective Ca, and the amount of CaS containing the X element is determined by the effective Ca, [%X (as CaS)] is [%Ca ^* ] M _Xtotal /10M _Ca. Therefore, [X _total ^* ] is represented by the following formula.
[X _total ^* ] = [% X _total ] - [% X (as CaS)]
= [% _Xtotal ]-[%Ca ^* ] _MXtotal /10M _Ca
= [% X _total ]-([% Ca]-[% Ca (as _CaO )]) M X total /10 M _Ca

On the other hand, in case 3 where the S content is less than the effective Ca and the formation amount of CaS containing the X element is determined by the S content, the following relationship holds.
[% X (as CaS)] = [% S] M _Xtotal /9M _S
In Case 3, where the S content is less than the effective Ca and the amount of CaS containing the X element is determined by the S content in the steel, [%X (as CaS)] is [%S]M _Xtotal / 9M _S. Therefore, the effective X is given by the following equation.
[X _total ^* ] = [% X _total ] - [% X (as CaS)]
= [% X _total ]-[% S] M _X total /9 M _S

M _Xtotal is the equivalent atomic weight of the X element calculated from the mass% content of Pb, Bi, Se, and Te, and the atomic weight (Pb: 207, Bi: 209, Se: 79, Te: 128) is calculated by the formula
M _Xtotal = ([Pb]+[Bi]+[Se]+[Te])/([Pb]/207+[Bi]/209+[Se]/79+[Te]/128)

To summarize the above, when {[Ca] ≤ (40/16) x [O]}, the effective X _total is
[X _total ^* ]=[X _total ]... formula (1),
When {[Ca]>(40/16)×[O]} and {([Ca]−(40/16)×[O])≦(40/32)×[S]},
[X _total ^* ]=[X _total ]−([Ca]−( _40/16 )×[O])×M X total /400 Formula (2),
When {[Ca]>(40/16)×[O]} and {([Ca]−(40/16)×[O])>(40/32)×[S]},
[X _total ^* ]=[X _total ]−([S]×M X total / ₂₈₈ ) Equation (3)
becomes.
[Ca], [O], [S], [Pb], [Bi], [Se], and [Te] are Ca, O, S, Pb, Bi, Se, and Te content in % by mass, respectively. is the amount, and if it does not contain, 0 is substituted, and in the above formulas (2) and (3),
M _Xtotal = ([Pb]+[Bi]+[Se]+[Te])/([Pb]/207+[Bi]/209+[Se]/79+[Te]/128). However, the atomic weights are O: 16, Ca: 40, S: 32, Pb: 207, Bi: 209, Se: 79, and Te: 128.

Next, the particles contained in steel will be explained.
In the steel material according to the first embodiment, a position 1/4 of the thickness of the steel material from the surface of the steel material (when the steel material is a steel plate, a position of 1/4 depth of the thickness in the thickness direction from the surface , when the steel material has a circular cross-section, the position of 1/4 of the diameter from the surface toward the center) contains 5 atomic% or more Mg as a proportion of the total of Ca, Mg, Mn and S, and is equivalent to a circle Particles having a diameter of 0.01 μm or more and 0.10 μm or less are present at a number density of 1.0×10 ⁷ particles/mm ³ or more. These particles include MgS-based, (Mg, Mn)S-based, etc. particles containing 5 atomic % or more of Mg. Particles containing Mg, in particular, are highly stable at high temperatures and tend to be finely dispersed. big. By setting [X _total ^* ] to 0.0001 to 0.0050%, it is presumed that the effect of grains containing Mg to suppress grain growth of austenite grains is enhanced.

Particles with an equivalent circle diameter of less than 0.01 μm are less effective in suppressing grain growth of austenite, and particles with an equivalent circle diameter of more than 0.10 μm hinder the generation of fine particles as a result. Therefore, in the present invention, particles containing 5 atomic % or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm, which are highly effective in suppressing grain growth of austenite, are targeted. Particles less than 01 μm and greater than 0.10 μm may be present.

In addition, at a position 1/4 the thickness of the steel material from the surface of the steel material, the ratio of Mg to the total of Ca, Mg, Mn and S is 5 atomic% or more, and the equivalent circle diameter is 0.01 to 0.10 μm. If the number density of the particles is 1.0 × 10 ⁷ /mm ³ or more, the effect of suppressing the grain growth of austenite becomes remarkable, so the lower limit is 1.0 × 10 ⁷ /mm ³ or more. . On the other hand, there is no upper limit to the number density of particles containing 5 atomic % or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm. According to the findings of the present inventors, the effect of suppressing the grain growth of austenite saturates when it exceeds 1.0×10 ¹¹ /mm ³ , so it is preferably 1.0×10 ¹¹ /mm ³ or less. . [X _total ^* ] is 0.0001% or more and 0.0050% or less, and the number density of particles containing 5 atomic% or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm is 1.0 × 10 If it is ⁷ pieces/mm ³ or more, the effect of improving the HAZ toughness becomes remarkable. The number density of particles with an equivalent circle diameter of 0.01 to 0.10 μm containing Mg in an amount of 5 atomic% or more in the ratio to the total of Ca, Mg, Mn and S is It is obtained by multiplying the number density of particles by the number ratio of particles with an equivalent circle diameter of 0.01 to 0.10 μm containing 5 atomic % or more of Mg with respect to the total of Ca, Mg, Mn and S.

Among the particles contained in the steel material according to the first embodiment having an equivalent circle diameter, a number density, and an equivalent circle diameter of 0.01 to 0.10 μm, 5 atomic % as a ratio to the total of Ca, Mg, Mn and S The above number ratio of Mg-containing particles is determined by elemental analysis and image analysis using an electron microscope. The content of Mg contained in the particles is measured for the entire particles by elemental mapping with an energy dispersive X-ray spectrometry (EDS), and can be obtained as the average value. .

Specifically, among particles observable with a transmission electron microscope (TEM), first, the number density of particles having an equivalent circle diameter of 0.01 to 0.10 μm is measured. Next, among particles having an equivalent circle diameter of 0.01 to 0.10 μm, the number ratio of particles containing 5 atomic % or more of Mg to the total of Ca, Mg, Mn and S is measured. Other types of electron microscopes such as Field-Emission Transmission Electron Microscopy (FE-TEM) may be used as the electron microscope.

The number of particles may be measured by heating a steel material to 1400° C., holding the steel material for about 3 seconds, and then rapidly cooling the steel material by using a focused ion beam (FIB) to prepare a thin film sample. This is because, for example, when cementite or alloy carbonitrides are formed, it is difficult to measure the number of particles having a size of 0.01 to 0.10 μm, which is the object of observation. It is possible to prepare a sample with little cementite or carbonitride by heating to a high temperature to dissolve precipitates other than those to be observed, followed by rapid cooling, or by applying a thermal cycle in which ferrite is generated during rapid cooling. can. Particles containing Mg are stable even when heated to a high temperature, and their morphology does not change substantially during cooling. Therefore, even if such a heat cycle is applied, the measurement result of the number of particles hardly changes. In addition, since the particles containing Mg do not substantially change their morphology during rolling, the morphology of the particles hardly changes before and after hot rolling or cold rolling, for example.

A thin film sample is observed with a TEM, and the number of particles with a size of 0.01 to 0.10 μm is measured in a visual field having an area of at least 100 μm ² and converted to the number per cubic mm. The thickness of the measurement sample can be measured by electron energy-loss spectroscopy (EELS method). In this embodiment, a thin film sample having a thickness of 0.05 to 0.20 μm is prepared so that the observation position is a quarter of the thickness of the steel material from the surface of the steel material, and the sample has a thickness of 10 μm×10 μm. The number of particles in a 10 μm field of view was counted. When 50 or more particles can be detected in one field of view, the number of particles detected in that field of view is divided by the volume of the sample in that field of view (vertical 10 µm x horizontal 10 µm x thickness (µm) x 10 ^-9 ). , the number of particles having an equivalent circle diameter of 0.01 to 0.10 μm per 1 cubic mm was calculated. If the number of particles detected in one field of view is less than 50, another field of view is observed until the total number of detected particles reaches 50 or more, and when the number of detected particles reaches 50 or more. The number per cubic mm was calculated by dividing the total number of particles detected by the total volume observed. Conversely, when 1,000 or more particles were expected in one field of view, a region (for example, a region of 2 μm×3 μm) where the number of particles was to be measured was extracted from the field of view to calculate the number density. If a thin film sample with a thickness of 0.10 μm is counted by extracting an area of 2 μm × 3 μm from the field of view, and the number of particles is 60, the number density of particles is 1 per cubic mm. .0×10 ¹¹ pieces.

In the first embodiment, particles having a Mg concentration of 5 atomic % or more are defined as Mg-containing particles. Such particles are highly effective in refining austenite, and elements other than Mg may be detected. The concentration of Mg contained in the particles is quantified by EDS. An electron beam diameter of 0.0001 to 0.01 μm and a TEM observation magnification of 10,000 to 100,000 times are used for this quantification, and an arbitrary position within the particles is quantified.

Next, the number ratio of particles containing 5 atomic % or more of Mg among the particles having an equivalent circle diameter of 0.01 to 0.10 μm whose number was measured is measured. When the number of particles in one field of view is large, it is a difficult task to identify all the particles one by one. Therefore, at least 20 or more particles with an equivalent circle diameter of 0.01 to 0.10 μm are identified as to whether they contain 5 atomic % or more of Mg, and the number density is obtained from the abundance ratio.

By including the X element in the steel, the number density of Mg-containing particles with an equivalent circle diameter in the range of 0.01 to 0.10 μm is remarkably increased, and the effect of suppressing the grain growth of austenite grains is enhanced. guessed. The reason why the number density of Mg-containing particles increases significantly due to the inclusion of the X element is not necessarily clear, but for example, the X element may suppress the Ostwald growth of the Mg-containing particles.

Next, a method for manufacturing a steel material according to the first embodiment will be described.
When controlling the state of existence of the X element in steel, it is effective to control the smelting process. Specifically, as a steel melting method, for example, the molten steel temperature is set to 1650 ° C. or less, and the O concentration of the molten steel is controlled to 0.0100% or less. Mg is added at the same time, or after the X element is added, Mg is added without any other steps in between. In order to control the O concentration of molten steel to 0.0100% or less, preliminary deoxidation with Si, Mn, Al, or the like may be performed. After adding Mg, the content of other elements may be adjusted to a predetermined range to control [X _total ^* ] within the range of 0.0001 to 0.0050%. Casting preferably employs continuous casting. Thereby, a cast slab having particles containing 5 atomic % or more of Mg, having an equivalent circle diameter of 0.01 to 0.10 μm, and having a number density of 1.0×10 ⁷ /mm ³ or more is obtained. can be done.

In a state where the O concentration of the molten steel is controlled to 0.0100% or less, a deoxidizing element such as Al is added, the X element is added, and then Mg is added (or the X element and Mg are added simultaneously. ), Mg-containing particles are formed in the molten steel and finely dispersed in the steel after casting. Since these fine particles are stable at high temperatures, it is possible to suppress coarsening of the γ grains heated by welding. On the other hand, when a deoxidizing element such as Al is added in a state where the O concentration of molten steel exceeds 0.0100%, inclusions such as oxysulfides take in the X element and aggregate and float, so the X element is also discharged. be done. Therefore, it can be inferred that by controlling the amount of O in molten steel before adding a deoxidizing element such as Al to 0.0100% or less, the emission of the X element can be suppressed and effectively utilized.

More preferably, as a steel smelting method, for example, after adding a deoxidizing element such as Al while controlling the O concentration of the molten steel to 0.0100% or less and the S concentration of the molten steel to 0.0200% or less , Mg is added after adding the X element. By adding the deoxidizing element, the X element, and Mg while controlling the O concentration and the S concentration of the molten steel in this way, the formation of coarse inclusions and the discharge of the X element can be suppressed.

If deoxidation by Al or the like is insufficient, Ca, Mg, or REM may be added as an element that promotes deoxidation. In order to promote the formation of particles containing Mg, it is preferable to limit the amount of Ca and the amount of REM in the molten steel before adding Mg to 0.0005% or less. By limiting the content of Ca, which forms sulfides, and REM, S can be used to form pinning particles. Even if Ca and REM are not intentionally added, they may be mixed into molten steel from refractories used in molten steel ladle, flux or slag added for purposes such as desulfurization, alloy raw materials, and the like. In order to suppress the content of Ca and REM to 0.0005% or less, the amount of Ca and REM contained in the refractory, flux, slag, alloy raw material, etc. should be controlled. The forms and shapes of Ca and REM in the molten steel may be controlled so as to be stable oxides that are difficult to mix into the molten steel.

The reasons for specifying the O concentration of the molten steel and the order of addition of the X element, the deoxidizing element such as Al, and Mg as described above will be explained. Simply adding the X element to the steel hardly produces particles containing the X element. Although the reason for this is not clear, it is presumed that the X element has a high vapor pressure in molten steel and does not easily remain in the molten steel even if it is added in a large amount. In addition to the control of [X _total ^* ], by controlling the deoxidation product that becomes the nucleus of the particles containing the X element, the X element is captured by the inclusions and the particles that contribute to the improvement of the toughness of the HAZ are produced. presumed to generate

The heating, rolling, and heat treatment conditions after casting are appropriately selected according to the target mechanical properties of the steel material, such as controlled rolling/controlled cooling, direct quenching/tempering after rolling, and quenching/tempering after cooling once after rolling. You can choose.

(Second embodiment)
Next, the steel material according to the second embodiment will be described in detail.

The second embodiment has the following features in addition to the features of the first embodiment.

The steel material according to the second embodiment contains Mg of 5 atomic% or more in a ratio to the total of Ca, Mg, Mn and S at a position 1/4 of the thickness of the steel material from the surface of the steel material, and is equivalent to a circle It has particles with a diameter of 0.01 to 0.10 μm and a number density of 1.0×10 ⁷ /mm ³ or more, among which the total of Ca, Mg, Mn, S and the X element The number ratio of the X-containing particles 1 containing 0.5 atomic % or more of the X element is 30% or more. When such X-containing particles are increased, grain growth of austenite grains in the HAZ is suppressed, and HAZ toughness is improved.

In the steel material according to the second embodiment, the steel material contains 5 atomic% or more of Mg as a proportion of the total of Ca, Mg, Mn and S, has an equivalent circle diameter of 0.01 to 0.10 μm, and has a number density of 1 0×10 ⁷ particles/mm ³ or more (particles containing Mg), 30% or more of the particles are 0.5 atomic % in proportion to the total of Ca, Mg, Mn, S and the X element X-containing particles 1 containing the above X element. Although the reason is not necessarily clear, it is speculated that when the Mg-containing particles contain an X element (at least one of Pb, Bi, Se, and Te), grain growth is suppressed and stability at high temperatures is enhanced. In order to obtain such an effect, the number ratio of the X-containing particles 1 among the Mg-containing particles is set to 30% or more. The number ratio of the X-containing particles 1 is preferably 50% or more, more preferably 70% or more.

Among the particles contained in the steel material according to the second embodiment and having an equivalent circle diameter, a number density, and an equivalent circle diameter of 0.01 to 0.10 μm, 5 atoms in the proportion to the total of Ca, Mg, Mn and S % or more of Mg, and further, among particles containing 5 atomic % or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm, the number ratio of X-containing particles 1 is determined by an electron microscope. Determined by elemental analysis and image analysis using The content of Mg and X elements contained in the particles is measured for the entire particle by elemental mapping using an energy dispersive X-ray spectrometry (EDS), and the average value is obtained. be able to.

Specifically, among the particles observable with a transmission electron microscope (TEM), first, at a position 1/4 the thickness of the steel material from the surface of the steel material, the equivalent circle diameter is 0.01. A particle number density of ˜0.10 μm is measured. Next, the number ratio of particles containing 5 atomic % or more of Mg among particles having an equivalent circle diameter of 0.01 to 0.10 μm is measured. Furthermore, among particles having an equivalent circle diameter of 0.01 to 0.10 μm containing Mg in an amount of 5 atomic% or more in proportion to the total of Ca, Mg, Mn and S, the sum of Ca, Mg, Mn, S and the X element The number ratio of particles containing 0.5 atomic % or more of the X element is measured. Other types of electron microscopes such as Field-Emission Transmission Electron Microscopy (FE-TEM) may be used as the electron microscope. If the concentration of the element contained in the particles is 0.5 atomic % or more, it can be reliably detected by an analytical instrument, so particles containing 0.5 atomic % or more of the X element can be measured.

Furthermore, among the particles with an equivalent circle diameter of 0.01 to 0.10 μm containing Mg at a ratio of 5 atomic% or more to the total of Ca, Mg, Mn and S, whose number density was measured, Ca, Mg, Mn, The abundance of the X-containing particles 1 containing 0.5 atomic % or more of the X element relative to the sum of the S and the X element can be obtained by determining by EDS. Also in this case, in the same manner as the measurement of the number density of particles containing 5 atomic % or more of Mg as a ratio to the total of Ca, Mg, Mn and S, Ca , Mg, Mn, S, and the X element in a proportion of 0.5 atomic % or more to the total of the X element is identified, and the existence ratio thereof is determined. Here, "containing 0.5 atomic % or more of X element" means that the proportion of X atoms is 0.5 atomic % or more with respect to the total of Ca, Mg, Mn, S, and X elements. .

Since the X-containing particles 1 are particularly stable at high temperatures and tend to be finely dispersed, they have the effect of suppressing grain growth of austenite grains in the process of heating the steel by the heat input during welding and then cooling it. is assumed to be large. Among the particles containing 5 atomic% or more of Mg in the proportion to the total of Ca, Mg, Mn and S and having an equivalent circle diameter of 0.01 to 0.10 μm, the higher the number ratio of X-containing particles 1, the more austenite It is presumed that the effect of suppressing the grain growth of grains is increased. The reason why the X-containing particles 1 containing the X element are excellent in high temperature stability and easily finely dispersed is unknown at present, but it is possible that the X element suppresses the Ostwald growth of the particles containing Mg, for example. have a nature.

Next, a method for manufacturing a steel material according to the second embodiment will be described.
The manufacturing method according to the second embodiment further controls the timing of adding Mg in the manufacturing method according to the first embodiment. Specifically, a deoxidizing element such as Al is added, and the X element and Mg are added at the same time, or Mg is added within 40 minutes after adding the X element. As a result, particles containing 5 atomic % or more of Mg, having an equivalent circle diameter of 0.01 to 0.10 μm, and having a number density of 1.0×10 ⁷ particles/mm ³ or more, of which Ca , Mg, Mn, S, and the above-mentioned X element, the number ratio of particles containing 0.5 atomic % or more of the X element is 30% or more.

By limiting the time from the addition of the X element to the addition of Mg to within 40 minutes (including 0 minutes), the yield and uniformity of the X element can be maintained, and the state of the X element dissolved in the molten steel can be maintained. Mg can be added by Thus, by dissolving the X element in the molten steel and adding Mg, the generation of the X-containing particles 1 is promoted.

(Third embodiment)
Next, the steel material according to the third embodiment will be described in detail.

The third embodiment has the following features in addition to the features of the first or second embodiment.
The steel material according to the third embodiment contains Mg of 5 atomic% or more as a ratio to the total of Ca, Mg, Mn and S at a position 1/4 of the thickness of the steel material from the surface of the steel material, and is equivalent to a circle Particles having a diameter of 0.01 to 0.10 μm and a number density of 1.0×10 ⁷ particles/mm ³ or more, of which Ca, Mg, Mn, S and The ratio of the number of the X-containing particles 2 having a region containing 0.5 atomic % or more of the X element to the total of the X elements is 30% or more. Here, the perimeter refers to a 2 nm thick region along the grain-steel interface. When such X-containing particles 2 are increased, grain growth of austenite grains in the HAZ is suppressed, and HAZ toughness is improved. Although the reason is not necessarily clear, when the X element (one or more of Pb, Bi, Se, and Te) is contained in at least part of the outer circumference of the Mg-containing particles, grain growth is suppressed and stability at high temperatures is also improved. presumed to be higher. In order to obtain such an effect, the number ratio of the X-containing particles 2 among the particles containing Mg is set to 30% or more. The number ratio of the X-containing particles 2 is preferably 50% or more, more preferably 70% or more.

The X-containing particles 2 can be observed by a method similar to the method described in the second embodiment. Identification of X-containing particles in which at least a part of the outer periphery contains Ca, Mg, Mn, S and a region containing 0.5 atomic% or more of the X element in a proportion to the total of the X elements is performed by a TEM with an aberration correction function. using Specifically, the content of the X element on the periphery of the particle is measured by EDS at intervals of 1 nm, and the content of the X element at one or more measurement points is the total of Ca, Mg, Mn, S and the X element X-containing particles 2 containing 0.5 atomic % or more of the X element in at least part of the outer periphery. The electron beam diameter used for EDS is 0.0001 to 0.001 nm. Also in this case, in the same manner as the measurement of the number density of particles containing 5 atomic % or more of Mg as a ratio to the total of Ca, Mg, Mn and S, at least 20 particles among the particles whose number was measured, It is sufficient to identify whether or not 0.5 atomic % or more of the X element is contained in at least a part of the outer circumference, and to obtain the abundance ratio. In the third embodiment, the measurement points containing the X element may be less than 50% of the circumference.

X-containing particles are particularly stable at high temperatures and tend to be finely dispersed, so they have the effect of suppressing the grain growth of austenite grains in the process of heating the steel by the heat input during welding and then cooling it. presumed to be large. Among the particles containing 5 atomic% or more of Mg in the proportion to the total of Ca, Mg, Mn and S and having an equivalent circle diameter of 0.01 to 0.10 μm, the higher the number ratio of X-containing particles 2, the more austenite It is presumed that the effect of suppressing the grain growth of grains is enhanced. The reason why the X-containing particles 2 containing the X element in at least a part of the outer circumference are excellent in high-temperature stability and easily finely dispersed is unknown at present. may have been suppressed.

Next, a method for manufacturing a steel material according to the third embodiment will be described.
The manufacturing method according to the third embodiment further controls the timing of starting continuous casting in the manufacturing method according to the second embodiment. Specifically, continuous casting starts within 30 minutes after the addition of Mg. As a result, the ratio of Ca, Mg, Mn and S to the total contains Mg of 5 atomic% or more, the equivalent circle diameter is 0.01 to 0.10 μm, and the number density is 1.0 × 10 ⁷ / mm ³ or more particles, of which the number ratio of particles containing 0.5 atomic % or more of the X element in at least a part of the outer periphery with respect to the total of Ca, Mg, Mn, S and the X element is 30 % or more can be obtained.

By starting continuous casting within 30 minutes after the addition of Mg, steel slabs can be produced without giving time for Mg to form coarse inclusions. As a result, X-containing particles in which a region containing 0.5 atomic % or more of the X element as a proportion of the total of Ca, Mg, Mn, S and the X element exists in at least a part of the outer periphery of the fine particle containing Mg. The production of 2 is promoted.

(Fourth embodiment)
Next, the steel material according to the fourth embodiment will be described in detail.

The fourth embodiment has the following features in addition to the features of the third embodiment.
The steel material according to the fourth embodiment contains Mg of 5 atomic% or more in proportion to the total of Ca, Mg, Mn and S at a position 1/4 of the thickness of the steel material from the surface of the steel material, and is equivalent to a circle It has particles with a diameter of 0.01 to 0.10 μm and a number density of 1.0×10 ⁷ /mm ³ or more, among which the total of Ca, Mg, Mn, S and the X element The number ratio of the X-containing particles 3 containing 0.5 atomic % or more of the X element in 50% or more of the outer peripheral region is 30% or more. When such X-containing particles 3 are increased, grain growth of austenite grains in the HAZ is suppressed, and HAZ toughness is improved. Although the reason is not necessarily clear, when the X element (one or more of Pb, Bi, Se, and Te) is contained in 50% or more of the outer peripheral region of the Mg-containing particles, grain growth is suppressed and stability at high temperatures is improved. presumed to be more likely. In order to obtain such an effect, the number ratio of the X-containing particles 3 among the Mg-containing particles is set to 30% or more. The number ratio of the X-containing particles 3 is preferably 50% or more, more preferably 70% or more.

X-containing particles 3 can be observed by a method similar to the method described in the third embodiment. Identification of particles containing 0.5 atomic % or more of the X element as a ratio to the total of Ca, Mg, Mn, S and the X element in a region of 50% or more of the outer circumference is performed using a TEM with an aberration correction function, The content of the X element in the outer periphery of the particle was measured by EDS at intervals of 1 nm, and the content of the X element at more than half of the measurement points was 0.00 as a ratio to the total of Ca, Mg, Mn, S and the X element. Particles containing 5 atomic % or more are defined as X-containing particles 3 containing 0.5 atomic % or more of the X element in 50% or more of the outer peripheral region. Also in this case, in the same manner as the measurement of the number density of particles containing 5 atomic % or more of Mg as a ratio to the total of Ca, Mg, Mn and S, at least 20 particles among the particles whose number was measured, It is sufficient to identify whether or not 50% or more of the outer circumference contains 0.5 atomic % or more of the X element in terms of the total ratio of Ca, Mg, Mn, S and the X element, and determine the abundance ratio.

The reason why the X-containing particles 3 containing the X element in 50% or more of the outer peripheral region are excellent in high-temperature stability and easily finely dispersed is unknown at present. It may inhibit growth.

Next, a method for manufacturing a steel material according to the fourth embodiment will be described. The manufacturing method according to the fourth embodiment further controls the timing of adding Mg in the manufacturing method according to the third embodiment. Specifically, Mg is added within 40 minutes after adding the X element and within 90 minutes after adjusting the O concentration. As a result, particles containing 5 atomic % or more of Mg, having an equivalent circle diameter of 0.01 to 0.10 μm, and having a number density of 1.0×10 ⁷ /mm ³ or more, of which the outer circumference In the region of 50% or more of the above, a cast slab having a number ratio of 30% or more of particles containing 0.5 atomic% or more of the X element as a ratio to the total of Ca, Mg, Mn, S and the X element can be obtained. can.

By adding Mg within 40 minutes after adding the X element and within 90 minutes after adjusting the O concentration, the yield and uniformity of the X element can be maintained, and the state in which the X element is dissolved in the molten steel can be maintained. Mg can be added while maintaining the By dissolving the X element in the molten steel and adding Mg in this way, the generation of the X-containing particles 3 of 0.5 atomic % or more in 50% or more of the outer peripheral area is promoted.

(First embodiment)
Hereinafter, the steel material according to the first embodiment will be specifically described with reference to examples. However, the conditions in the following examples are examples of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to the following examples.

A slab obtained by melting and casting steel was hot-rolled into a steel plate (thickness: 25 mm).
In the melting process, the molten steel temperature is set to 1650° C. or lower, and the molten steel O concentration is set to 0.0100% or lower. The content was adjusted to a predetermined range, [X _total ^* ] was adjusted, and then cast by continuous casting to obtain a slab.

A sample is collected from the obtained steel plate, and is analyzed using X-ray fluorescence analysis, combustion-infrared absorption, inert gas fusion, inductively coupled plasma mass spectrometry (ICP mass spectrometry), or the like. An analysis of the components of the steel sheet was carried out. The content of X elements (Pb, Bi, Se, Te) contained in the steel sheet was determined by ICP mass spectrometry. Tables 1 to 4 show the analysis results of the steel sheet components. Blanks in the table mean that they are not intentionally added, and underlined parts indicate that they are out of the scope of the invention.

The carbon equivalent Ceq shown in Tables 2 and 4 was determined by the following formula.
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15
[C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] in the formula are mass % of C, Mn, Cr, Mo, V, Cu, and Ni, respectively. , and 0 is substituted if it does not contain.

Further, [X _total ^* ] shown in Tables 2 and 4 was determined by any one of the following formulas (1) to (3).
[X _total ^* ] is the formula (1) when {[Ca] ≤ (40/16) x [O]},
When {[Ca]>(40/16)×[O]} and {([Ca]−(40/16)×[O])≦(40/32)×[S]}, formula (2 ), and when {[Ca]>(40/16)×[O]} and {([Ca]-(40/16)×[O])>(40/32)×[S]} , Equation (3).
[X _total ^* ]=[X _total ]... formula (1),
[X _total ^* ]=[X _total ]−([Ca]−( _40/16 )×[O])×M X total /400 Formula (2),
[X _total ^* ]=[X _total ]−([S]×M X total / ₂₈₈ ) Equation (3).
[Ca], [O], [S], [Pb], [Bi], [Se], and [Te] are Ca, O, S, Pb, Bi, Se, and Te content in % by mass, respectively. is the amount, and if it does not contain, 0 is substituted, and in the above formulas (2) and (3),
M _Xtotal = ([Pb]+[Bi]+[Se]+[Te])/([Pb]/207+[Bi]/209+[Se]/79+[Te]/128).

A sample was taken from the surface of the obtained steel plate (thickness: 25 mm) at a position 1/4 the thickness of the plate, heated and held at 1400° C. for 3 seconds, then rapidly cooled to prepare a thin film sample by FIB. This thin film sample was observed with a TEM, and the number of particles having an equivalent circle diameter of 0.01 to 0.10 μm was measured per area of 100 μm ² or more.
In this measurement, a thin film sample having a thickness of 0.05 to 0.20 μm was prepared, and the number of particles in the sample was counted in a field of view of 10 μm×10 μm. When 50 or more particles can be detected in one field of view, the number of particles detected in that field of view is divided by the volume of the sample in that field of view (vertical 10 µm x horizontal 10 µm x thickness (µm) x 10 ^-9 ). , the number of particles with a size of 0.01 to 0.10 μm per cubic mm was calculated. If the number of particles detected in one field of view is less than 50, another field of view is observed until the total number of detected particles reaches 50 or more, and when the number of detected particles reaches 50 or more. The number per cubic mm was calculated by dividing the total number of particles detected by the total volume observed. Conversely, when 1000 or more particles were expected in one field of view, a region (for example, a region of 2 μm×2 μm) where the number of particles was to be measured was extracted from the field of view to calculate the number density.
The thickness of the thin film samples was measured by the EELS method. Then, using the measurement result of the thickness of the thin film sample, the number of particles of the thin film sample was converted to the number per cubic mm, and the particle number density (particles/mm ³ ) was obtained.

Next, 20 or more particles out of the particles having an equivalent circle diameter of 0.01 to 0.10 μm were counted, and the whole particles were mapped by EDS to obtain the average concentration of Mg. Then, the number ratio of particles having a Mg concentration of 5 atomic % or more among the above particles was determined. Here, the concentration of Mg refers to the ratio of Mg to the sum of Ca, Mg, Mn and S. Multiplying this number ratio by the particle number density (particles/mm ³ ) obtained by the above method, the number density of particles containing 5 atomic % or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm. (number/mm ³ ) was determined and evaluated according to the criteria shown below. The results are shown in Tables 5 and 6.

(Particle number density standard)
Good: The number density of particles containing 5 atomic % or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm is 1.0×10 ⁷ particles/mm ³ or more.
Bad: The number density of particles containing 5 atomic % or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm is less than 1.0×10 ⁷ /mm ³ .

Also, a sample was taken from the surface of the obtained steel plate (thickness: 25 mm) at a position 1/4 of the plate thickness, and a simulated heat cycle test was performed to give a small piece a heat history that reproduces welding. Specifically, the simulated heat cycle test was performed under the conditions of holding at 1400° C. for 23 seconds and cooling from 800° C. to 500° C. in 300 seconds (corresponding to a welding heat input of 450 kJ/cm).
Then, a V-notch test piece was produced from the sample after the simulated thermal cycle test, and a Charpy test was performed in accordance with JIS Z 2242:2005. The Charpy test was performed at a test temperature of −30° C. with 3 tests, and the minimum value of the measured Charpy absorbed energy (vE ₋₃₀ ) was evaluated. If the minimum value of the Charpy absorbed energy of the three test pieces was 100 J or more, it was determined that the HAZ toughness was excellent. The results are shown in Tables 5 and 6. Note that the underlined parts in the table indicate that they are out of the scope of the invention.

As shown in Table 5, the steel components and [X _total ^* ] (effective X amount) are within the scope of the present invention, and the steel materials (No. A1 to A38) manufactured under appropriate manufacturing conditions were subjected to the simulated heat cycle test. It can be seen that the Charpy absorption energy at −30° C. afterward is high. On the other hand, as shown in Table 6, the steel materials (No. A101 to A110) whose steel composition or [X _total ^* ] (effective X amount) is outside the range of the present invention are It can be seen that the Charpy absorbed energy is lower than that of the invention examples.

No. In No. 101, the X element was not contained, the number ratio of particles containing the X element was less than 30%, and the Charpy absorbed energy was lowered. No. A102-No. A105 had large amounts of Pb, Bi, Se, and Te, respectively, and the Charpy absorbed energy decreased.

No. 106 does not contain Mg, and No. Since A107 has a small amount of Al, the Charpy absorbed energy decreased. No. A108 had an excessive amount of O, so the Charpy absorbed energy was lowered. No. A109 lacks the effective amount of X, and No. A110 had an excessive amount of effective X, and the Charpy absorbed energy decreased.

(Second embodiment)
Hereinafter, the steel material according to the second embodiment will be specifically described with reference to examples. However, the conditions in the following examples are examples of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to the following examples.

A slab obtained by melting and casting steel is hot-rolled into a steel plate.
In the smelting process, Al, X element, and Mg were added in the order shown in Table 9 with the molten steel temperature set to 1650° C. or lower and the molten steel O concentration set to 0.0100% or lower. After the addition of the X element, the time until the addition of Mg was adjusted as shown in Table 9, the content of the other elements was adjusted to a predetermined range, [X _total ^* ] was adjusted, and then continuous casting was performed. It was cast to obtain a slab.
The steel sheet components of the second embodiment were analyzed in the same manner as in the first example. The analysis results are shown in Tables 7 and 8. A blank column in the table means not intentionally added.

(Number ratio of X-containing particles 1)
By EDS, for 20 or more particles containing 5 atomic% or more of Mg with respect to the total of Ca, Mg, Mn and S and having an equivalent circle diameter of 0.01 to 0.10 μm, the content of element X Mean values were measured. Among the particles containing 5 atomic % or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm, 0.5 atomic % or more in terms of the total of Ca, Mg, Mn, S and X elements The number ratio of the X-containing particles 1 containing the X element was determined and evaluated according to the criteria shown below. Table 9 shows the results.

Good: The number ratio of X-containing particles is 30% or more.
Bad: The number ratio of X-containing particles is less than 30%.

(Charpy test)
The Charpy test was measured under the same conditions as in the first example, and evaluated by the lowest value of the measured Charpy absorbed energy (vE ₋₃₀ ). If the minimum value of the Charpy absorbed energy of the three test pieces was 150 J or more, it was determined that the HAZ toughness was excellent. Table 9 shows the results.

As shown in Table 9, the steel components and [X _total ^* ] (effective X amount) are within the scope of the present invention, and the steel materials (No. B1 to B38) manufactured under appropriate manufacturing conditions were subjected to the simulated heat cycle test. It can be seen that the Charpy absorption energy at −30° C. afterward is high. On the other hand, No. in which the time from the addition of X element to the addition of Mg exceeds 40 minutes. Since B39 did not satisfy the conditions for X-containing particles, the minimum value of Charpy absorbed energy was not 150 J or more. In addition, No. Also in B39, the minimum value of Charpy absorbed energy at -30°C was 100 J or more.

(Third embodiment)
Hereinafter, the steel material according to the third embodiment will be specifically described with reference to examples. However, the conditions in the following examples are examples of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to the following examples.

A slab obtained by melting and casting steel is hot-rolled into a steel plate.
In the smelting process, Al, X element, and Mg were added in the order shown in Table 12 with the molten steel temperature set to 1650° C. or lower and the molten steel O concentration set to 0.0100% or lower. Furthermore, the content of other elements was adjusted to a predetermined range, [X _total ^* ] was adjusted, and then continuous casting was performed to obtain a slab. Table 12 shows the time from the addition of the X element to the addition of Mg and the time from the addition of Mg to the start of continuous casting.
The steel sheet components of the third embodiment were analyzed in the same manner as in the first example. The analysis results are shown in Tables 10 and 11. A blank column in the table means not intentionally added.

(Number ratio of X-containing particles 2)
By EDS, for 20 or more particles containing Mg at a ratio of 5 atomic% or more to the total of Ca, Mg, Mn and S and having an equivalent circle diameter of 0.01 to 0.10 μm, X element in the outer periphery of the particle content was measured at intervals of 1 nm (spot size: 0.0001 to 0.001 nm). Then, the content of the X element at at least one measurement point is 0.5 atomic % or more in proportion to the total of Ca, Mg, Mn, S and the X element, and 0.5 atoms are added to at least a part of the outer circumference % or more of the X element. From the results, among the particles containing 5 atomic % or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm, at least a part of the outer periphery has a region containing 0.5 atomic % or more of the element X. The number ratio of the X-containing particles 2 was determined and evaluated according to the criteria shown below. The results are shown in Table 12.

Good: The number ratio of the X-containing particles 2 is 30% or more.
Bad: The number ratio of X-containing particles 2 is less than 30%.

(Charpy test)
The Charpy test was performed under the same conditions as in the first example except that the test temperature was -40°C, and the minimum value of the measured Charpy absorbed energy (vE _-40 ) was evaluated. If the minimum value of the Charpy absorbed energy of the three test pieces was 100 J or more, it was determined that the HAZ toughness was excellent. The results are shown in Table 12.

As shown in Table 12, the steel components and [X _total ^* ] (effective X amount) are within the range of the present invention, and the steel materials (No. C1 to C38) manufactured under appropriate manufacturing conditions were subjected to the simulated heat cycle test. It can be seen that the Charpy absorption energy at −40° C. afterward is high. No. in which the time from the addition of the X additive element to the addition of Mg exceeds 40 minutes. No. 3 in which the time from the addition of C39 and Mg to the start of continuous casting exceeds 30 minutes. Since C40 did not satisfy the conditions for X-containing particles 2, the minimum value of Charpy absorbed energy at -40°C did not satisfy 100 J or more. In addition, No. 1, whose minimum value of Charpy absorbed energy at -40°C did not satisfy 100J. C39 and No. Including C40, No. C1-No. C40 had a minimum Charpy absorbed energy of 100 J or more at -30°C.

(Fourth embodiment)
Hereinafter, the steel material according to the fourth embodiment will be specifically described with reference to examples. However, the conditions in the following examples are examples of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to the following examples.

A slab obtained by melting and casting steel is hot-rolled into a steel plate.
In the melting process, Al, X element, and Mg were added in the order shown in Table 15 with the molten steel temperature set to 1650°C or lower and the molten steel O concentration set to 0.0100% or lower. Furthermore, the content of other elements was adjusted to a predetermined range, [X _total ^* ] was adjusted, and then continuous casting was performed to obtain a slab. Table 15 shows the time from the addition of the X element to the addition of Mg, the time from the adjustment of the O concentration of the molten steel to the addition of Mg, and the time from the addition of Mg to the start of continuous casting.
The steel sheet composition of the fourth embodiment was analyzed in the same manner as in the first example. The analysis results are shown in Tables 13 and 14. A blank column in the table means not intentionally added.

(Number ratio of X-containing particles 3)
In EDS, for 20 or more particles with an equivalent circle diameter of 0.01 to 0.10 μm containing Mg of 5 atomic% or more in the ratio to the total of Ca, Mg, Mn and S, the content of X element in the outer periphery of the particle Quantities were measured at 1 nm intervals. Particles in which the content of the X element in 50% or more of the measurement points is 0.5 atomic % or more with respect to the total of Ca, Mg, Mn and S were defined as X element-containing particles 3 . From the results, of the particles containing 5 atomic% or more of Mg and having an equivalent circle diameter of 0.01 to 0.10 μm, 50% or more of the outer peripheral region contains 0.5 atomic% or more of the X element. The number ratio of contained particles 3 was determined and evaluated according to the criteria shown below. The results are shown in Table 15.

Good: The number ratio of the X-containing particles 3 is 30% or more.
Bad: The number ratio of X-containing particles 3 is less than 30%.

(Charpy test)
The Charpy test was conducted under the same conditions as in the third example, and it was determined that the HAZ toughness was excellent if the minimum value of the Charpy absorbed energy of the three test pieces was 150 J or more. The results are shown in Table 15.

As shown in Table 15, the steel components and [X _total ^* ] (effective X amount) are within the range of the present invention, and the steel materials (No. D1 to D38) manufactured under appropriate manufacturing conditions were subjected to the simulated heat cycle test. It can be seen that the Charpy absorption energy at −40° C. afterward is high. No. in which the time from the addition of the X element to the addition of Mg exceeds 40 minutes. D39, No. where the time from the addition of Mg to the start of continuous casting exceeds 30 minutes. No. D40 and the time from adjustment of O concentration of molten steel to addition of Mg exceeds 80 minutes. Since D41 did not satisfy the conditions of X-containing particles 3, the minimum value of Charpy absorbed energy at -40°C did not satisfy 150J. In addition, No. 1 did not satisfy 150 J in the lowest value of Charpy absorbed energy at -40°C. D39, No. D40, No. No. including D41. D1 to D41 had a minimum Charpy absorbed energy of 100 J or more at -30°C.

ADVANTAGE OF THE INVENTION According to this invention, the steel material which has favorable HAZ toughness after welding can be provided. In particular, the steel material of the present invention is suitable for various welded steel structures such as buildings, bridges, shipbuilding, line pipes, construction machinery, offshore structures, tanks, etc., which require high tensile strength with a yield strength of about 300 to 700 MPa. can be used for

Claims (8)

in % by mass,
C: 0.01 to 0.20%,
Si: 1.00% or less,
Mn: 0.1-2.5%,
Mg: 0.0005-0.0100%,
Al: 0.015 to 0.500%,
P: 0.020% or less,
S: 0.020% or less,
N: 0.0100% or less,
O: less than 0.0030%,
Cu: 0-2.0%,
Ni: 0 to 2.0%,
Cr: 0 to 2.0%,
Mo: 0-1.0%,
Nb: 0 to 0.10%,
W: 0 to 2.0%,
V: 0 to 0.20%,
B: 0 to 0.010%,
Ti: 0 to 0.100%,
Zr: 0 to 0.10%,
Ta: 0 to 0.10%,
Ag: 0-0.10%,
Hf: 0-0.10%,
Ca: 0 to 0.0100%,
REM: 0-0.010%,
Sn: 0-0.50%,
Sb: 0-0.50%
contains
[X _total ], which is the total content of the X elements Pb, Bi, Se, and Te, is 0.0001 to 0.0100%, the balance being Fe and impurities,
[X _total ^* ] obtained by any one of the following formulas (1), (2), and (3) composed of the [X _total ] and the content of Ca, O, and S is 0.0001 0.0050% of the steel material, and at a position 1/4 the thickness of the steel material from the surface of the steel material,
The number density of particles containing 5 atomic% or more of Mg with respect to the total of Ca, Mg, Mn and S and having an equivalent circle diameter of 0.01 to 0.10 μm is 1.0 × 10 ⁷ / A steel material characterized by being ³ mm or more.
The [X _total ^* ] is
When {[Ca] ≤ (40/16) × [O]}, it is represented by formula (1),
When {[Ca]>(40/16)×[O]} and {([Ca]−(40/16)×[O])≦(40/32)×[S]}, formula (2 ),
When {[Ca]>(40/16)×[O]} and {([Ca]-(40/16)×[O])>(40/32)×[S]}, formula (3 ).
[X _total ^* ]=[X _total ]... formula (1),
[X _total ^* ]=[X _total ]−([Ca]−( _40/16 )×[O])×M X total /400 Formula (2),
[X _total ^* ]=[X _total ]−([S]×M X total / ₂₈₈ ) Equation (3),
[Ca], [O], [S], [Pb], [Bi], [Se], and [Te] are the percentages by mass of Ca, O, S, Pb, Bi, Se, and Te, respectively. is the amount, and if it does not contain, 0 is substituted,
In the above formulas (2) and (3),
M _Xtotal = ([Pb]+[Bi]+[Se]+[Te])/([Pb]/207+[Bi]/209+[Se]/79+[Te]/128). in % by mass,
Cu: 0.02-2.0%,
Ni: 0.02 to 2.0%,
Cr: 0.02 to 2.0%,
Mo: 0.02-1.0%,
Nb: 0.01 to 0.10%,
W: 0.01 to 2.0%,
V: 0.01 to 0.20%,
B: 0.0003 to 0.010%,
Ti: 0.005 to 0.100%,
Zr: 0.01 to 0.10%,
Ta: 0.01 to 0.10%,
Ag: 0.01-0.10 % ,
Hf: 0.01-0.10 %
The steel material according to claim 1, characterized by containing one or more of in % by mass,
Ca: 0.0001 to 0.0100%,
REM: 0.001-0.010%
The steel material according to claim 1 or 2, characterized by containing one or both of. in % by mass,
Sn: 0.01 to 0.50%,
Sb: 0.01-0.50%
The steel material according to any one of claims 1 to 3, characterized in that it contains one or both of. 3. The number ratio of particles containing 0.5 atomic % or more of the X element as a ratio to the total of Ca, Mg, Mn, S and the X element among the particles is 30% or more. The steel material according to any one of 1 to 4. Among the particles, the number ratio of particles in which at least a part of the outer circumference has a region containing 0.5 atomic% or more of the X element as a ratio to the total of Ca, Mg, Mn, S and the X element is 30%. The steel material according to any one of claims 1 to 5, characterized by the above. Among the particles, the number ratio of particles containing 0.5 atomic% or more of the X element in a region of 50% or more of the outer periphery as a ratio to the total of Ca, Mg, Mn, S and the X element is 30% or more. The steel material according to claim 6, characterized in that there is The number density of particles is 1.0×10 ⁷ to 1, which contains Mg in an amount of 5 atomic % or more relative to the total of Ca, Mg, Mn, and S, and has an equivalent circle diameter of 0.01 to 0.10 μm. The steel material according to any one of claims 1 to 7, characterized in that it is 0×10 ¹¹ pieces/mm ³ .

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