KR20150015506A - Steel material having excellent toughness in weld-heat-affected zone - Google Patents

Abstract

The object of the present invention is to provide a steel material excellent in HAZ toughness even when heat input heat of 60 kJ / mm or more is subjected to large-scale heat welding. The steel material of the present invention comprises (a) an oxide containing Zr, REM and Ca, (b) an inclusion having a circle equivalent diameter of 0.1 to 2 탆 among all the inclusions contained in the steel, , And (c) the composition of the inclusions contained in the steel material satisfies the relationship represented by the following formula (1): " (1) " will be.
[Equation 1]

Description

STEEL MATERIAL HAVING EXCELLENT TOUGHNESS IN WELD-HEAT-AFFECTED ZONE}

The present invention relates to a steel material used for bridges, high-rise buildings, vessels and the like, and more particularly relates to a steel material used in bridges, high-rise buildings, ships and the like, This is an excellent steel material.

Properties required for a steel used for bridges, high-rise buildings, vessels, and the like are becoming increasingly strict in recent years, and particularly good toughness is required. These steel materials are generally welded to each other in many cases. However, there is a problem that toughness of the welded joint portion, particularly HAZ, is easily affected by heat during welding. This toughness deterioration becomes more conspicuous as the heat input at the time of welding becomes larger. The reason for this is that when the heat input at the time of welding is large, the cooling rate of the HAZ is slowed, and the quenching property is lowered, which is considered to be the reason why coarse island-shaped martensite is produced. Therefore, in order to improve the toughness of the HAZ, it is considered that the amount of heat input at the time of welding should be suppressed as much as possible. On the other hand, in order to increase the efficiency of welding work, it is desired to employ a substitution heat welding method in which the heat input amount of welding is 50 kJ / mm or more, for example, electrogas welding, electroslag welding, submerged arc welding or the like.

Therefore, the present applicant has proposed a steel material for suppressing the deterioration of HAZ toughness in the case of adopting the heat-welding method in the Patent Documents 1 to 3. These steels are characterized in that they contain at least one of an oxide of REM and CaO and ZrO ₂ as oxides which become nuclei of ferrite transformation in the grains. The above oxides are present in a liquid phase in molten steel, and thus are finely dispersed in the steel. Moreover, since the oxide is thermally stable, it is not dissolved even when exposed to a high temperature of, for example, 1400 DEG C for a long time, and thus contributes greatly to the improvement of the HAZ toughness.

In addition, the present applicant has conducted researches to improve the technique of using oxide serving as nuclei of ferrite transformation in the grains disclosed in Patent Document 1, and to provide a steel material in which HAZ toughness is not deteriorated even when welding is performed with a larger heat input amount The technology of Patent Document 4 has been repeatedly proposed. In Patent Document 4, the size and the number of the whole oxides (not limited to the oxides which are the nuclei of the ferrite transformation in the grains) and the number of the oxides are deeply involved in the improvement of the HAZ toughness, Discloses that a steel material excellent in HAZ toughness can be obtained even if large heat input of about 50 kJ / mm is performed by reducing the number of coarse oxides exceeding 5.0 占 퐉 in diameter to 5 or less.

Japanese Patent Application Laid-Open No. 2007-100213 Japanese Patent Application Laid-Open No. 2007-247004 Japanese Patent Application Laid-Open No. 2007-247005 Japanese Patent Application Laid-Open No. 2009-197267

According to Patent Document 4, since the number of coarse oxides is remarkably suppressed, HAZ toughness can be increased even when welding is performed at a larger heat input than the HAZ toughness evaluation method disclosed in the embodiment of Patent Document 1. That is, in Patent Document 1, a heat cycle (heat input condition: 1400 DEG C x 5 seconds, cooling time Tc = 300 seconds) for cooling the temperature from 800 DEG C to 500 DEG C to 300 seconds after holding at a heating temperature of 1400 DEG C for 5 seconds, And the absorption energy (vE- ₄₀ ) at -40 ° C was measured. On the other hand, in Patent Document 4, the absorption energy when a heat cycle (heat input condition: 1400 占 폚 for 30 seconds, cooling time Tc = 300 seconds) in which the holding time at 1400 占 폚 is prolonged for 30 seconds is applied in the same manner as described above And in this case, it was confirmed that good HAZ toughness was obtained. However, since the amount of heat of welding has recently become larger and larger, it is required to improve the HAZ toughness in the case of performing welding of a large heat input.

The object of the present invention is to provide a steel material excellent in HAZ toughness even when heat input welding with a heat input of 60 kJ / mm or more is performed.

A steel material excellent in toughness of the weld heat affected zone according to the present invention which can solve the above problems means a steel having a composition of C: 0.02 to 0.15% (equivalent to a mass% 0.001 to 0.10%, REM: 0.0003 to 0.015%, Ca: 0.0003 to 0.010%, Zr: 0.0010 to 0.050%, N: 0.010% or less, O: 0.0005 to 0.010%, and the balance being iron and inevitable impurities. And,

(a) the steel comprises an oxide containing Zr, REM and Ca,

(b) Of inclusions contained in the steel material, inclusions having a circle-equivalent diameter of 0.1 to 2 占 퐉 have an observation area of not less than 120 per mm 2 of the film and an oxide having a circle-equivalent diameter of more than 3 占 퐉. Or less, and

(c) the composition of the inclusions having a circle equivalent diameter of 0.1 to 2 탆 contained in the steel material satisfies the following formula (1).

[Equation 1]

The number density of the inclusions defined by (b) above is a value obtained by observing with an electron probe X-ray micro analyzer (EPMA).

In the above formula (1), when the element X is Ti, N and Al, Insol.X is obtained by electrolytically extracting the electrolytic solution of the steel material by using a filter having a scale of 0.1 mu m or a scale of 2.0 mu m, (ICP emission spectrometry) and the amount of element N were determined by the indian phenol blue absorption spectrophotometry, and the amounts of Ti and Al in the extracted residues remaining in the residue of X The value obtained by subtracting the element X amount Insol.X _{2 .0} from the extraction residue left on the filter having the scale of 2.0 탆 from the amount Insol.X _{0 .1} .

The steel material, as another element,

At least one element selected from the group consisting of Cu: not more than 2%, Ni: not more than 3.5%, Cr: not more than 3%, and Mo: not more than 1%

[2] A non-aqueous electrolyte secondary battery comprising at least one of Nb: 0.25% or less and V: 0.1%

[3] B: not more than 0.005%

And the like.

According to the present invention, oxides (Zr, oxides containing REM and Ca) which become nuclei of the? Transformation in the grains (? Means a mixed structure of ferrite or ferrite and bainite, hereinafter the same) In addition, the size and number (i.e., particle size distribution) of the inclusions and oxides present in the steel are also controlled appropriately. Therefore, it is possible to provide a steel material excellent in HAZ toughness at the time of heat input heat of 60 kJ / mm or more in heat input. In other words, the steel material of the present invention is particularly useful for improving the toughness of a HAZ having not only a predetermined amount of fine inclusions having a circle-equivalent diameter of 0.1 to 2 탆, but also an effect of improving HAZ toughness, Since the number of oxides is significantly suppressed, HAZ toughness is excellent. Further, according to the present invention, since the composition ratio of the Ti oxide and Al oxide contained in the fine inclusions is appropriately controlled, even if welding is performed at a larger heat input amount than the HAZ toughness evaluation method disclosed in the embodiment of Patent Document 4, the HAZ toughness .

The inventors of the present invention have conducted studies to provide a steel material excellent in HAZ toughness at the time of welding with a higher level of heat even after the above-mentioned Patent Document 4 is proposed. As a result, it is found that a heat cycle "in which the temperature from 800 ° C. to 500 ° C. is cooled to 450 seconds after holding at a heating temperature of 1400 ° C. for 60 seconds" (heat input condition: 1400 ° C. × 60 Sec and a cooling time Tc = 450 sec), it is not sufficient to reduce the oxide of 5.0 占 퐉 or more to 5 or less in circle equivalent diameter as in the case of Patent Document 4 in order to provide a steel having excellent HAZ toughness , It is very important to reduce the number of oxides having a circle equivalent diameter (hereinafter, simply abbreviated as particle diameter) exceeding 3 占 퐉, which has not been fully taken into consideration in the above-mentioned Patent Document 4, It is found that the composition ratio of Ti oxide and Al oxide contained in the fine inclusions of 0.1 to 2 占 퐉 is important, and the present invention has been completed.

As described above, according to the present invention,

(A) Increasing the number of fine inclusions having a diameter of 0.1 to 2 탆 (120 pieces / mm 2 or more) equivalent to a circle useful for improving the HAZ toughness,

(B) The number of oxides having a diameter of more than 3 mu m (5.0 pieces / mm < 2 > or less) which adversely affects the improvement of HAZ toughness is reduced,

(C) The composition ratio of the Ti oxide and the Al oxide contained in the fine inclusions having a circle equivalent diameter of 0.1 to 2 占 퐉 is set to a predetermined range (as a concrete measuring means, the value calculated by the electrolytic extraction method is expressed by the above- As a result, HAZ toughness can be improved even when welding is performed at a larger heat input amount than in Patent Document 4.

That is, in relation to the Patent Document 4, there is a characteristic portion of the present invention in addition to (A) above and (B) and (C) above. If the composition ratio of the Ti oxide and the Al oxide is appropriately controlled with respect to the fine inclusions as defined by the above (C), the inclusion is lowered in melting point, so that the inclusions are liquefied at the time of large heat welding, It is easy to form inclusions which are nuclei of transformation, and HAZ toughness is improved.

Further, it is difficult to measure the composition ratio of Ti oxide and Al oxide in the fine inclusions with high precision by EPMA as described later. Therefore, in the present invention, the electrolytic extraction method, ICP emission spectrometry, . Therefore, in the above (C), the composition ratio of the Ti oxide and Al oxide contained in the extraction residue remaining after passing through the filter having the scale of 2.0 탆 but not passing through the filter having the scale of 0.1 탆 is specified. Therefore, the characteristic part of the present invention defines the number density of the inclusions having a circle-equivalent diameter of 0.1 to 2 탆 [the above-mentioned (A)] and the composition ratio of the Ti oxide and the Al oxide included in the inclusion [the above (C)] There is.

In Patent Document 4, the number of oxides having a circle-equivalent diameter of more than 5.0 占 퐉 is controlled. In contrast, in the present invention, as described in (B) above, The HAZ toughness can be further improved. According to the examination results of the present inventors, in order to realize good HAZ toughness, if the composition ratio of the Ti oxide and Al oxide included in the fine inclusions is controlled, as in Patent Document 4, And it is apparent that it is only necessary to control the number of oxides having a circle equivalent diameter exceeding 3 占 퐉.

In the present specification, in order to distinguish oxides (i.e., oxides containing Zr, REM, and Ca) which become nuclei of alpha transformation in the grains from all the oxides contained in the steel, for convenience of explanation, Quot ;, " Ca-based oxide ", and the latter is sometimes referred to as " total oxide ". Further, the oxide includes a composite oxide in which an oxide and an inclusion other than an oxide (for example, a sulfide, a nitride, a carbide, or a complex compound thereof) are combined, in addition to a single oxide composed only of an oxide.

In addition, the essential components (Zr, REM and Ca) constituting the Zr.REM.Ce-based oxide are sometimes referred to as " α-transformation nucleation elements in the grains "in some cases.

In addition, in the present invention, the steel material of the present invention includes oxides, sulfides, nitrides, carbides, or complex compounds thereof contained in the steel material, Are referred to collectively as " all inclusions ". In the present specification, an inclusion having a circle-equivalent diameter of 0.1 to 2 占 퐉 among all the inclusions contained in the steel is referred to as "microscopic inclusions".

In this specification, an oxide having a circle-equivalent diameter of 0.1 to 2 탆 is referred to as a "fine oxide" and an oxide having a circle-equivalent diameter of more than 3 탆 is referred to as a "coarse oxide" May be distinguished. In Patent Document 4, oxides having a circle equivalent diameter of more than 5 占 퐉 are defined as "coarse oxides". In the present specification, oxides having a circle equivalent diameter of more than 3 占 퐉 are referred to as "coarse oxides" .

In the present specification, " a steel material excellent in HAZ toughness of large heat welding " means that a steel material is maintained at 1400 캜 for 60 seconds, and a thermal cycle (thermal history) for cooling the temperature from 800 캜 to 500 캜 to 450 캜 Means that the absorption energy (vE- ₄₀ ) at -40 캜 satisfies 100 J or more. This thermal history corresponds to the thermal history to be obtained when the heat input amount is 60 kJ / mm or more and the heat input welding is performed. The thermal history may be sometimes referred to as " thermal history of large heat input ". The amount of heat input by the heat cycle is higher than the heat input amount (about 50 kJ / mm) by the heat cycle described in the above-mentioned Patent Document 4. In that sense, the "heat of entry heat"Quot; heat of substitution heat " described above is different. The vE _-40 may be the greater, preferably at least the vE _-40 130J.

Hereinafter, the requirements of (a) to (c) constituting the present invention will be described in detail.

(a) For Zr, REM, Ca-based oxides

First, the Zr.REM.Ca based oxide which is a starting point of? Transformation in the grain will be described. The Zr.REM.Ce-based oxide means that the oxide includes Zr oxide, REM oxide and Ca oxide.

A part of the Zr.REM.Me.Ce-based oxide may be present as a single oxide containing the? -Conjugated nucleating element in the particle alone or as a composite oxide containing? -Conjugated nucleating elements in two or more kinds of particles There may be. Examples of the single oxide include ZrO _{2 in} Zr, CaO in Ca, and M ₂ O ₃ , M ₃ O ₅ and MO ₂ when REM is represented by the symbol "M". These oxides may be present in the form of agglomerated or co-precipitated with other compounds such as sulfides or nitrides in the oxides.

The Zr-REM-Ca-based oxide needs to contain Ti oxide and Al oxide. By containing Ti oxide and Al oxide in the fine ZrREM.Ca-based oxide having a circle equivalent diameter of 0.1 to 2 占 퐉,? Transformation is promoted in the particle, and the improvement in HAZ toughness can be further enhanced. Details of the composition ratios of the Ti oxide and Al oxide contained in the fine Zr.REM.Ce-based oxide will be described later.

A part of the Ti oxide may exist as a single oxide (for example, Ti ₂ O ₃ , Ti ₃ O ₅ , TiO ₂ ). A part of the Al oxide may be present as a single oxide (for example, Al ₂ O ₃ ).

(b) About the particle size distribution of all inclusions

Next, the number and size of all inclusions that characterize the present invention will be described. The steel material of the present invention, when observed by EPMA,

(i) a fine inclusion having a circle-equivalent diameter of 0.1 to 2 占 퐉 is 120 or more per 1 mm2 of observation field area,

(ii) coarse oxide having a circle-equivalent diameter exceeding 3 占 퐉 is 5.0 or less per 1 mm2 of observation field area.

In the steel material of the present invention, the number of oxides having a circle-equivalent diameter exceeding 3 占 퐉 is controlled, and as described in Patent Document 4, an oxide having a circle-equivalent diameter of more than 3 占 퐉 and a circle- It is not necessary to separately control oxides exceeding 5 占 퐉. This is because, in the steel material of the present invention, the composition ratio of the Ti oxide and Al oxide contained in the fine inclusions is appropriately controlled.

In the steel material of the present invention, as specified by (ii) above, the number of coarse oxides having a circle equivalent diameter of more than 3 탆 is required to be 5.0 or less per 1 mm 2 of the observation field area. The smaller the number, the better the number is, and preferably 3 or less per 1 mm 2, more preferably 1 or less per 1 mm 2, and most preferably 0 per 1 mm 2.

The number of coarse oxides exceeding 3 占 퐉 in the circle equivalent diameter can be determined by observing the cross section of the steel material with, for example, EPMA, quantitatively analyzing the composition of the inclusions recognized in the observation field, The oxide equivalent of at least 5 mass% may be determined by observing the circle equivalent diameter of the oxide by, for example, a transmission electron microscope (TEM).

On the other hand, in the steel material of the present invention, as defined by (i) above, the number of fine inclusions having a circle-equivalent diameter of 0.1 to 2 탆 needs to be 120 or more per 1 mm 2 of the observation field area. By producing the fine inclusions at a predetermined amount or more, the oxide which becomes the nucleus of the? Transformation in the grain is increased, so that the HAZ toughness can be improved. The number of the fine inclusions is preferably not less than 200 per mm 2, more preferably not less than 500 per mm 2, more preferably not less than 1000 per mm 2.

The number of fine inclusions having a circle equivalent diameter of 0.1 to 2 占 퐉 may be obtained by measuring the cross section of the steel material by, for example, TEM observation. Further, in the steel material of the present invention, inclusions having a circle equivalent diameter of less than 0.1 占 퐉 do not contribute to the HAZ toughness improving action by inclusion dispersion, and thus are not included in the number of inclusions.

The " circle-equivalent diameter " is the diameter of the circle assumed to be equal to the area of the inclusion (meaning oxide in the case of oxide), and is recognized on the TEM observation surface.

(c) the composition ratio of Ti oxide and Al oxide in the fine inclusions

The steel material of the present invention has the greatest feature that the composition ratio of Ti oxide and Al oxide satisfies a predetermined range to fine inclusions having a circle equivalent diameter of 0.1 to 2 탆 contributing to improvement of HAZ toughness have. That is, when the Ti oxide and Al oxide contained in the fine Zr.REM.Ca based oxide serving as the nucleus of? Transformation in the particle are controlled to fall within a predetermined range, the Zr.REM.multidot.Ca A part of the system oxide is liquefied, and this liquid material crystallizes into a crystal structure which effectively acts as a nucleus of alpha transformation in the grain in the subsequent cooling process. As a result, the interfacial energies of the α in the particles and the austenite (γ) in the parent phase are reduced, and the interfacial energies of the α and Zr REM  Ca-based oxides in the particles are further lowered, . As a result, the HAZ toughness of the steel is improved.

The composition ratio of the Ti oxide and Al oxide as fine inclusions having a circle equivalent diameter of 0.1 to 2 占 퐉 is measured by a combination of an electrolytic extraction method, an ICP emission spectrometry method and an indian phenol blue light absorption method. Specifically, the steel material of the present invention must satisfy the relationship of the following expression (1).

[Equation 1]

Insol.Ti, Insol.N, and Insol.Al represent concentrations of Ti, N, and Al of the compound type contained in the steel material, and are values calculated in the following order. That is, the steel material is electrolytically extracted, and the electrolytic solution after extraction is filtered using a filter having a scale of 0.1 占 퐉 or a scale of 2.0 占 퐉, respectively, and the extracted residues remaining on the filter are respectively recovered. Next, the amounts of Ti and Al in the amounts of Ti, N and Al contained in the extracted residue (hereinafter, these elements are represented by X) were measured by ICP emission spectrometry and the amount of element N was quantified by the indocyanine blue absorption spectrophotometry and the amount of the element X contained in the amount of the element X Insol.X _{0 .1,} extraction residue remaining on the filter grid 2.0㎛ contained in the extraction residue remaining on the filter grid 0.1㎛ the Insol.X _{2 .0} by, and subtracting from Insol.X Insol.X _{2 .0 0.1,} and calculates the Insol.X (see the following formula).

That is, Insol.Ti, Insol.N and Insol.Al in the above-mentioned formula (1) indicate the amounts of Ti, N and Al contained in inclusions passing through a 2.0 μm filter and not passing through a scale of 0.1 μm. In the present invention, the thus measured values are regarded as the amounts of Ti, N and Al included in the fine inclusions having the circle equivalent diameters of 0.1 to 2 탆.

In the present invention, it is important to define the relationship between the amounts of Ti, N and Al contained in the fine inclusions having a circle equivalent diameter of 0.1 to 2 탆 in the steel material, and such fine inclusions are useful for improving the HAZ toughness. On the other hand, inclusions having a circle-equivalent diameter exceeding 2 占 퐉 (particularly, circle-equivalent diameter exceeding 3 占 퐉) become a starting point of brittle fracture and deteriorate HAZ toughness inversely.

The term "Insol.Ti-3.4 × Insol.N" means the amount of Ti contained as the Ti oxide in the electrolytic extracted residue.

Ti represents Ti oxide (for example, TiO ₂ ), Ti nitride (TiN), or a composite compound thereof (for example, TiO ₂ ) For example, oxynitride, etc.). As Ti present as a compound, carbide and the like can be mentioned in addition to the above. However, since there is almost no Ti carbide having a particle size exceeding 0.1 占 퐉 remaining on a filter with a scale of 0.1 占 퐉, Insol.Ti has a Ti amount derived from Ti carbide Are not included.

On the other hand, the above-mentioned Insol.N means the N amount of the compound type existing as a compound in the steel, and N exists as a nitride. As the nitride, TiN, ZrN, BN, AlN and the like can be mentioned, but the above-mentioned Insol.N means the amount of N constituting substantially TiN. ZrN, BN, and AlN do not grow at a size remaining on the filter with a scale of 0.1 mu m, and therefore Insol.N does not contain the amount of N derived from ZrN, BN or AlN.

Since the atomic weight of Ti is 47.88 and the atomic weight of N is 14.01, the ratio of atomic weight of Ti to atomic weight of N is about 3.4. Therefore, by calculating 3.4 × Insol.N, the amount of Ti forming TiN can be obtained. Further, by subtracting the amount of Ti (3.4 x Insol.N) forming TiN from Insol.Ti, the amount of Ti present as Ti oxide in the steel can be calculated.

The term Insol.Al means the amount of Al present as a compound in a steel material and substantially means the amount of Al constituting the Al oxide (Al compound represented by Al ₂ O ₃ ). As described above, since there is almost no Al nitride that grows in a size remaining on the filter with a scale of 0.1 탆, the amount of Al derived from Al nitride is included in Insol.Al. It does not.

The above formula (1) represents the composition ratio of the Ti oxide and the Al oxide contained in the extraction residue (that is, the inclusion having a circle equivalent diameter of 0.1 to 2 탆) passing through the scale of 2.0 탆 filter and not passing through the scale of 0.1 탆 (On a mass basis), and shows only the inclusions effective for improving the HAZ toughness.

Significance defining Equation (1) is demonstrated in the following embodiments. In other words, although Nos. 32 and 33 shown in Tables 1 and 2 are steel materials having substantially the same composition, No. 32 is controlled to have a value in the range of 1.0 to 8, Good. On the other hand, No. 33 did not improve the HAZ toughness because the value of the formula (1) was less than 1.0.

In addition, the same consideration can be obtained by comparing Nos. 4, 16, and 29 shown in Tables 1 and 2 below. Namely, these are steel materials having almost the same composition, but Nos. 4 and 16 have good HAZ toughness because the value of Equation 1 is controlled in the range of 1.0 to 8. On the other hand, in No. 29, since the value of the formula (1) is less than 1.0, the HAZ toughness could not be improved.

As the electrolytic solution, a solution capable of dissolving the matrix (matrix) of the steel material by electrolysis can be used. For example, a methanol solution of 10% acetylacetone-1% tetramethylammonium chloride or the like can be used.

The electrolytic condition may be a condition capable of dissolving the parent phase of the steel material. For example, the current density is preferably 100 to 200 A / m < 2 >.

In the present invention, as described above, inclusions contained in a steel material are recovered by an electrolytic extraction method, and the recovered inclusions are separated by using filters different in scale, and the composition of fine inclusions having a circle equivalent diameter of 0.1 to 2 탆 ICP emission spectrometry and Indian phenol blue spectrophotometry. As a result, the amount of Ti constituting the Ti oxide contained in the fine inclusions can be accurately determined. In other words, for the analysis of the composition of inclusions contained in the steel, it is general to identify inclusions using conventional EPMA and quantitatively analyze the composition of inclusions. However, the composition of fine inclusions having a circle equivalent diameter of about 0.1 to 2 탆 Even when analyzed by EPMA, it is difficult to accurately quantify, for example, the amount of Ti constituting the Ti oxide and the amount of Ti constituting the Ti nitride. Not only the circle equivalent diameter of the inclusions effective for improving the HAZ toughness is as small as 0.1 to 2 탆 but also Ti oxides and Ti nitrides are each present in the steel material singly and are usually present as complex compounds Because. Therefore, even when analyzed by EPMA, it is not possible to accurately quantify only the amount of Ti constituting the Ti oxide from the composite compound of Ti oxide and Ti nitride. On the other hand, in the present invention, since the composition of the inclusions is measured by a combination of the electrolytic extraction method, the ICP emission spectrometry method and the Indian phenol blue absorption method, the composition ratio of the Ti oxide and Al oxide in the fine inclusions can be accurately determined.

If the value of the left side of Equation (1) is less than 1.0, the Al oxide is excessively added to the Ti oxide, so that the? Transformation capacity in the grain is lowered and the HAZ toughness is deteriorated. Therefore, the value of the left side of Equation 1 is 1.0 or more, preferably 1.5 or more, and more preferably 2.0 or more.

However, if the value of the left side of Equation (1) exceeds 8, the Ti oxide becomes excessive with respect to the Al oxide, so that the melting point of the oxide increases and it becomes difficult for the oxide to liquefy in the HAZ at the time of welding. HAZ toughness can not be improved thereby. Therefore, the value of the left side of Equation (1) is 8 or less, preferably 7.5 or less, more preferably 7.0 or less.

(d) Preferred form

The steel material of the present invention is characterized in that when the composition of the total oxides contained in the steel material is measured and converted into mass as a single oxide (total amount is 100%), ZrO ₂ is contained in an amount of 5 to 50%, an oxide of REM , It is preferable that it satisfies 5 to 50% of M ₂ O ₃ ) and 50% or less of CaO. By satisfying this composition, the oxide effectively functions as the nucleus of the ferrite transformation in the grain. If the lower limit of the respective oxides is less than the lower limit of the respective oxides, the amount of oxides that form ferrite in the grain during welding becomes insufficient and the function of improving the HAZ toughness is difficult to be exerted. On the other hand, when the upper limit of the respective oxides is exceeded, the oxides are coarsened, and the number of fine oxides effectively acting as the generation nuclei of the ferrite in the grains is decreased, so that the HAZ toughness improving function is hardly exhibited effectively.

More preferably, the ZrO ₂ is at least 8%, and more preferably at least 10%. On the other hand, a more preferable upper limit is 45%, and a more preferable upper limit is 40%.

The oxide of the REM is more preferably at least 10%, more preferably at least 13%. On the other hand, a more preferable upper limit is 45%, and a more preferable upper limit is 40%. When the REM is represented by the symbol M, the oxide of the REM is present in the form of M ₂ O ₃ , M ₃ O ₅ , MO ₂ , and the like in the steel material. However, the amount of the oxide when all of the REM oxides are converted into M ₂ O ₃ .

The CaO effectively functions as the nucleus of the ferrite transformation in the grain, but if it is contained in excess, the ferrite transformation ability in the grain may deteriorate in some cases. In addition, if CaO is contained excessively, there is a case that the nozzle used for casting may be damaged. Therefore, the upper limit of CaO is preferably 50%, more preferably 45% or less, still more preferably 40% or less, particularly preferably 30% or less. In order to effectively exhibit this action, CaO is preferably contained in an amount of 3% or more, more preferably 5% or more, and further preferably 10% or more.

The remaining components of the total oxide composition are not particularly limited, and oxides of oxide forming elements (for example, SiO ₂ , Al ₂ O ₃ , MnO, and the like) contained in the steel material of the present invention can be mentioned.

The composition of the total oxides contained in the steel is measured by observing the surface of the steel with, for example, EPMA and quantitatively analyzing the oxide recognized in the observation field. The details of the measurement conditions are described in the column of the later-described embodiments.

Next, the composition of the steel material (base material) of the present invention will be described. The steel of the present invention is characterized in that the steel contains as main components 0.02 to 0.15% of C, 0.5% or less of Si, 2.5% or less of Mn, 0.03% or less of P, 0.02% or less of S, 0.10%, REM: 0.0003-0.015%, Ca: 0.0003-0.010%, Zr: 0.0010-0.050%, and N: 0.010% or less. The reason for setting this range is as follows.

C is an element that can not be missed in order to secure the strength of the steel material (base material), and it is necessary to contain 0.02% or more. The amount of C is preferably 0.04% or more, and more preferably 0.05% or more. However, when the amount of C exceeds 0.15%, a large amount of island-shaped martensite (MA) is generated in the HAZ at the time of welding, resulting in deterioration of toughness of the HAZ, and also adversely affecting weldability. Therefore, the C content is 0.15% or less, preferably 0.10% or less, and more preferably 0.08% or less.

Si has an effect of deoxidizing and contributes to the improvement of the strength of the steel material (base material) by solid solution strengthening. In order to exhibit such an effect effectively, Si is preferably contained in an amount of 0.01% or more. The amount of Si is more preferably 0.02% or more, more preferably 0.05% or more, particularly preferably 0.10% or more. However, if the amount of Si exceeds 0.5%, the weldability and toughness of the steel material deteriorate. Therefore, the amount of Si is 0.5% or less, preferably 0.45% or less, and more preferably 0.40% or less.

In order to increase HAZ toughness in particular, it is recommended that Si is 0.30% or less, preferably 0.05% or less, and more preferably 0.01% or less. However, the HAZ toughness is improved as the amount of Si is suppressed, but the strength of the steel material may be lowered.

Mn is an element contributing to the improvement of the strength of the steel material (base material). In order to exhibit such an effect effectively, it is preferable to contain 0.4% or more. The Mn content is more preferably 0.50% or more, still more preferably 0.7% or more, particularly preferably 0.8% or more. However, if the Mn content exceeds 2.5%, the weldability of the steel material (base material) is deteriorated. Therefore, it is necessary to suppress the Mn content to 2.5% or less. The amount of Mn is preferably 2.3% or less, and more preferably 2.0% or less.

P is an element that is liable to segregation, and is particularly segregated in grain boundaries in steel to deteriorate HAZ toughness. Therefore, it is necessary to suppress the amount of P to 0.03% or less. The P content is preferably 0.020% or less, and more preferably 0.015% or less. In addition, P usually contains about 0.001% inevitably.

S is a harmful element that combines with Mn to form sulfide (MnS), deteriorating toughness of the base material and ductility in the thickness direction. Further, when S forms a sulfide (for example, LaS or CeS) of REM by bonding with REM such as La or Ce, formation of REM oxide is inhibited and HAZ toughness is deteriorated. Therefore, it is necessary to suppress the S content to 0.02% or less. The amount of S is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.006% or less. In addition, S usually contains about 0.0005% inevitably.

Al is an element acting as a deoxidizer. However, if it is added in excess, the oxide is reduced to form a coarse Al oxide, and the HAZ toughness is deteriorated. Therefore, the amount of Al needs to be suppressed to 0.050% or less. The amount of Al is preferably 0.04% or less, more preferably 0.03% or less, still more preferably 0.025% or less, particularly preferably 0.010% or less. In addition, Al usually contains about 0.0005% inevitably.

Ti is an element that contributes to the improvement of HAZ toughness by producing nitride such as TiN or Ti containing oxide in the steel material. In order to exhibit such an effect, Ti should be contained in an amount of 0.005% or more. The amount of Ti is preferably 0.007% or more, and more preferably 0.010% or more. However, if it is added in excess, the base material itself is hardened by the solid solution strengthening of Ti and is connected to the deterioration of the HAZ toughness, so that Ti should be suppressed to 0.10% or less. The amount of Ti is preferably 0.07% or less, and more preferably 0.06% or less.

REM (rare-earth element) and Ca are the elements required to produce each oxide. When the oxide is contained, the oxide tends to be finely dispersed, and the finely dispersed oxide becomes the nucleus of? Transformation in the particle, thereby contributing to the improvement of the HAZ toughness.

The REM should be contained in an amount of 0.0003% or more, preferably 0.001% or more, and more preferably 0.0020% or more. However, if REM is added excessively, the coarse oxide is excessively produced, so that the HAZ toughness is deteriorated. In addition, when REM is excessively added, a solid solution REM is generated, and this tends to deteriorate the toughness of the base material. Therefore, the amount of REM should be suppressed to 0.015% or less. The amount of REM is preferably 0.010% or less, and more preferably 0.007% or less.

In the present invention, REM means a lanthanoid element (15 elements from La to Lu) and Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, more preferably at least one of La and Ce.

Ca should be contained in an amount of 0.0003% or more, preferably 0.0005% or more, more preferably 0.0008% or more, and still more preferably 0.001% or more. However, when Ca is excessively added, CaO is excessively produced and inclusions having a high CaO concentration are produced, so that the effect of acting as a transformation nucleus in the particle of the inclusions is weakened and the HAZ toughness is deteriorated inversely. Therefore, it is necessary to suppress the amount of Ca to 0.010% or less. The amount of Ca is preferably 0.009% or less, and more preferably 0.008% or less.

Zr is an element contributing to improvement of HAZ toughness by producing a complex oxide containing Zr. In order to exhibit such an effect effectively, it is necessary to contain 0.0010% or more. The amount of Zr is preferably 0.002% or more, and more preferably 0.0023% or more. However, when Zr is excessively added, a large amount of ZrO ₂ is produced, so that the effect of acting as a transformation nucleus in the particle of the inclusions is weakened. Further, when Zr is excessively added, fine nitride (ZrN) or carbide (ZrC) which causes precipitation strengthening is formed, and toughness of the base material itself is lowered. Therefore, the amount of Zr is suppressed to 0.050% or less. The amount of Zr is preferably 0.04% or less, more preferably 0.03% or less, further preferably 0.01% or less.

N is an element for depositing nitride (for example, ZrN or TiN). The nitride fixes the ferrite transformation by preventing the coarsening of the austenite grains generated in the HAZ during welding due to the pinning effect, Contributes to improvement of HAZ toughness. In order to effectively exhibit such effects, N is preferably contained in an amount of 0.003% or more. The N content is more preferably 0.004% or more, and still more preferably 0.005% or more. The more N is, the more nitride is formed to promote the fineness of the austenite grains, so that it is effective for improving the toughness of HAZ. However, when the amount of N exceeds 0.010%, the amount of solid solution N is increased, the toughness of the base material itself deteriorates, and the HAZ toughness also deteriorates. Therefore, it is necessary to suppress the N content to 0.010% or less. The N content is preferably 0.009% or less, and more preferably 0.008% or less.

The steel material of the present invention contains the above element as an essential component, and the amount of O (oxygen) is 0.0005 to 0.010%. Here, the O (oxygen) content of 0.0005 to 0.010% represents the total oxygen amount, which means the total amount of O (oxygen) forming the oxide and free O (oxygen) contained in the steel.

The residual component of the steel is iron and unavoidable impurities (for example, Mg, As, Se and the like).

The steel material of the present invention, as another element,

At least one element selected from the group consisting of Cu: not more than 2%, Ni: not more than 3.5%, Cr: not more than 3%, and Mo: not more than 1%

[2] A non-aqueous electrolyte secondary battery comprising at least one of Nb: 0.25% or less and V: 0.1%

[3] B: not more than 0.005%

And the like. The reason for setting this range is as follows.

[1] A method of manufacturing a magnetic recording medium, comprising at least one element selected from the group consisting of Cu, Ni, Cr, and Mo

Cu, Ni, Cr, and Mo are all elements contributing to enhance the strength of the steel, and they can be added singly or in combination.

If the amount of Cu exceeds 2%, the strength of the base material is excessively increased so that the toughness of the base material is deteriorated inversely, so that the HAZ toughness may be lowered. Therefore, the amount of Cu is preferably 2% or less. The amount of Cu is more preferably 1.8% or less, and still more preferably 1.5% or less. In order to effectively exhibit the action by the addition of Cu, it is preferable to be contained in an amount of 0.05% or more. The amount of Cu is more preferably 0.1% or more, and still more preferably 0.2% or more.

If the amount of Ni exceeds 3.5%, the strength of the base material is excessively increased so that the toughness of the base material is deteriorated inversely, so that the HAZ toughness may be lowered. Therefore, the amount of Ni is preferably 3.5% or less. The amount of Ni is more preferably 3.0% or less, and still more preferably 2.5% or less. In order to effectively exhibit the action by the addition of Ni, it is preferable that the content is 0.05% or more. The amount of Ni is more preferably 0.1% or more, and still more preferably 0.2% or more.

If the amount of Cr exceeds 3%, the strength of the base material is excessively increased so that the toughness of the base material is deteriorated to the contrary, so that the HAZ toughness may be lowered. Therefore, the amount of Cr is preferably 3% or less. The amount of Cr is more preferably 2% or less, further preferably 1% or less. In order to effectively exhibit the effect of Cr addition, it is preferable that the content is 0.05% or more. The amount of Cr is more preferably 0.1% or more, and still more preferably 0.15% or more.

If the amount of Mo exceeds 1%, the strength of the base material is excessively increased so that the toughness of the base material is deteriorated inversely, so that the HAZ toughness may be lowered. Therefore, the amount of Mo is preferably 1% or less. The amount of Mo is more preferably 0.9% or less, and still more preferably 0.80% or less. In order to effectively exert the action by Mo addition, it is preferable that the Mo content is contained by 0.05% or more. The amount of Mo is more preferably 0.1% or more, and still more preferably 0.15% or more.

[2] At least one of Nb and V

Nb and V are elements which have the function of preventing coarsening of austenite grains during welding and improving HAZ toughness due to the precipitation of carbonitride in all of them and the finishing effect of the carbonitride. Nb and V can be added singly or in combination.

However, if the amount of Nb exceeds 0.25%, the precipitated carbonitrides are coarsened and the HAZ toughness may be deteriorated inversely. Therefore, the amount of Nb is preferably 0.25% or less. The amount of Nb is more preferably 0.2% or less, further preferably 0.15% or less. Further, in order to effectively exhibit the action by the addition of Nb, it is preferable to contain 0.002% or more. The amount of Nb is more preferably 0.01% or more, and still more preferably 0.02% or more.

When the amount of V exceeds 0.1%, the precipitated carbonitride is coarsened like Nb, and the HAZ toughness may be deteriorated inversely. Therefore, the V content is preferably 0.1% or less. The V content is more preferably 0.09% or less, and still more preferably 0.08% or less. In order to effectively exhibit the action by the addition of V, it is preferable that the content is 0.002% or more. The amount of V is more preferably 0.005% or more, and still more preferably 0.01% or more.

[3] B (boron)

B is an element which suppresses generation of intergranular ferrite and improves HAZ toughness. However, if the amount of B exceeds 0.005%, BN is precipitated at the austenite grain boundary, and conversely, the toughness may be lowered. Therefore, the amount of B is preferably 0.005% or less. The amount of B is more preferably 0.0040% or less. Further, in order to effectively exhibit the action by the addition of B, it is preferable to contain 0.0010% or more. The amount of B is more preferably 0.0015% or more.

Even when the steel material of the present invention is kept at 1450 캜 for 60 seconds and then subjected to cooling for cooling at a cooling time of 800 캜 to 500 캜 for 450 seconds, the steel material exhibits an absorption energy (vE _-40 ) of 100 J (Particularly, 130 J or more) can be ensured. Therefore, the steel material according to the present invention can be used, for example, as a material for a structure such as a bridge, a high-rise building material, a ship, etc., and can be used not only for small to medium heat input welding but also for large heat input welding with an input heat amount of 60 kJ / Deterioration of the toughness of the heat affected zone can be prevented. The steel material of the present invention is intended to be a thick steel sheet having a thickness of about 3.0 mm or more.

Next, a manufacturing method that can be suitably employed in manufacturing the steel material of the present invention will be described. The steel material of the present invention may be obtained by deoxidizing molten steel, then adding Ti, and then adding Al. It is possible to appropriately control the composition ratio of the Ti oxide and the Al oxide contained in the fine inclusions having the circle equivalent diameter of about 0.1 to 2 占 퐉 by adding Ti to the deoxidized molten steel and then adding Al (Ti? Al) A steel material satisfying the formula (1) can be produced. That is, the Ti oxide has a lower interface energy with the molten steel than the Al oxide or the Zr.REM.Ca-based oxide, so Ti is added before adding Al, Zr, REM and Ca to the molten steel to form a fine Ti oxide And as a result, a predetermined amount of fine inclusions having a circle equivalent diameter of 0.1 to 2 탆 contributing to HAZ toughness can be produced. Further, by adding Al after Ti is added, a composite oxide containing Ti and Al can be produced, and the activity as Ti oxide can be lowered to less than 1. [ After the composite oxide is formed, Zr, REM, and Ca based oxides are formed by adding Zr, REM, and Ca, which are stronger and more acid elements than Ti and Al, and reduction of Ti oxide or Al oxide is suppressed at this time. A predetermined amount of Ti oxide and Al oxide can be contained in the Zr-REM-Ca-based oxide. By adding Al after Ti is added in this way, even if the addition amounts of Ti, Al, Zr, REM and Ca are the same, it is possible to produce a large amount of oxides which become nuclei of alpha transformation in the grains.

On the other hand, even if Ti is added (Al? Ti) after Al is added, the composition of the inclusions can not be adjusted to satisfy the above-mentioned formula (1). Since the deoxidation power of Ti is weaker than that of Al, it is impossible to reduce the Al oxide formed before the addition of Ti after adding Al to the molten steel. Therefore, the amount of Ti oxide is reduced and the amount of Zr.REM.Ce- Ti oxide can not be contained. In addition, the Ti oxide formed at this time is present as a single oxide, and the activity as Ti oxide is close to 1. Therefore, when Zr, REM and Ca, which are stronger in deoxidization than Ti, are added in this state, the Ti oxide is reduced to reduce the amount of Ti oxide produced, and to contain a predetermined amount of Ti oxide in the Zr.REM.Ca-based oxide I can not. Therefore, when producing the steel material of the present invention, it is recommended not to use Al for deoxidation of molten steel. Al deoxidation may cause Al oxide to remain in the molten steel, so that it is difficult to form a ZrREM.Ca-based oxide containing a predetermined amount of Ti oxide.

The molten steel may be deoxidized by a known method. For example, after adjusting the components for elements other than Al, Ti, REM, Ca and Zr, at least one element selected from C, Si and Mn is used Then deoxidized, and thereafter Ti may be added and then Al added.

In the addition of Al, REM, Ca and Zr after Ti is added, for example,

(1) after addition of Ti, REM, Ca and Zr may be added in any order after Al is added,

(2) after addition of Ti, REM, Ca and Zr may be added simultaneously after Al is added,

(3) After adding Ti, Al, REM, Ca and Zr may be added simultaneously.

The form of REM, Ca, Zr and Ti to be added to the molten steel is not particularly limited. For example, pure R, pure Ce, pure Y, pure Ca, pure Zr, pure Ti, or Fe- Si-La alloy, Fe-Si-Ce alloy, Fe-Si-Ca alloy, Fe-Si-La-Ce alloy, Fe-Ca alloy, Fe-Zr alloy, Fe-Ti alloy and Ni-Ca alloy . In addition, mischmetal may be added to molten steel. The misch metal is a mixture of rare earth elements, specifically containing about 40 to 50% of Ce and about 20 to 40% of La. However, misch metal often contains Ca as an impurity. Therefore, when the mischmetal contains Ca, it is necessary that the total Ca amount including the Ca amount satisfies the range specified by the present invention.

The molten steel obtained by adjusting the components in this manner can be continuously cast into slabs according to a conventional method, and then subjected to hot rolling in accordance with a conventional method to produce the steel material of the present invention.

This application claims the benefit of priority based on Japanese Patent Application No. 2012-138047 filed on June 19, The entire contents of the specification of Japanese Patent Application No. 2012-138047 filed on June 19, 2012 are hereby incorporated by reference herein.

<Examples>

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited by the following Examples from the ground, but may be carried out by appropriately modifying them in a range that is suitable for the purpose Of course, all of which are within the technical scope of the present invention.

A vacuum melting furnace (capacity: 150 kg) was used to dissolve the disclosed steel containing the chemical components shown in the following Table 1 (iron and inevitable impurities in the remaining part). In dissolving the disclosed steel, components are adjusted for elements other than Al, Ti, REM, Ca, and Zr, and deoxidized by using at least one element selected from C, Si, and Mn, Respectively. Thereafter, Al and Ti were added to molten steel whose dissolved oxygen amount was adjusted, and then REM, Ca and Zr were added. Table 1 below shows the order of addition of Al and Ti. The steel disclosed in the following Table 1 was produced by the same method except that the order of addition of Ti and Al was changed. Ti is a Fe-Ti alloy, Zr is a Fe-Zr alloy, REM is a misch metal containing about 25% of La and about 50% of Ce, Ca is a Ni-Ca alloy Respectively. It was also confirmed that the total O content (oxygen content) of the steel steels satisfying the requirements specified by the present invention among the steel steels shown in Table 1 was in the range of 0.0005 to 0.010%.

After the above elements were added, they were cast by a 150 kg ingot and cooled. The obtained ingot was heated and hot-rolled to produce a thick steel sheet having a thickness of 30 to 80 mm. The hot rolling was performed at a heating temperature of 1100 캜 and a rolling end temperature of 880 캜.

With respect to the obtained thick steel sheets, the composition of the total oxides and the number density of inclusions and oxides were measured by the following procedure. That is, a sample was cut from the cross section at the position of t / 4 (where t is the thickness of the steel sheet) of the obtained thick steel sheet, and the cut sample surface was used with Japan Electronics TE-EPMA "JXA-8500F And the composition of the inclusions having a circle-equivalent diameter of 0.1 탆 or more was quantitatively analyzed. The observation conditions were as follows: the acceleration voltage was 20 kV, the sample current was 0.01,, the observation field area was 1 to 5 cm 2, the number of analyzes was 100 or more, and the composition of the inclusions was quantitatively analyzed by wavelength dispersive spectroscopy did. The relationship between the X-ray intensity of each element and the element concentration is previously determined using a calibration curve using Si, Mn, S, Al, Ti, La, Ce, Ca, Zr and O The X-ray intensity obtained from the inclusion to be analyzed and the amount of the element contained in the inclusion were determined from the calibration curve.

Among the obtained quantitative results, inclusions having an oxygen content of 5% or more were regarded as oxides. At this time, when a plurality of elements were observed from one inclusion, the composition of the oxides was calculated in terms of a single oxide of each element from the ratio of X-ray intensity indicating the presence of the elements. In the present invention, the average composition of the oxides was used as the oxide in the form of a single oxide. The average composition of oxides of REM, ZrO ₂ and CaO in the oxides is shown in Table 2 below. When the metal element is represented by M, the oxide of REM exists in the form of M ₂ O ₃ , M ₃ O ₅ and MO _{2 in} the steel, but the composition is calculated by converting all the oxides into M ₂ O ₃ . The "others" shown in the following Table 2 are oxides of REM, oxides other than ZrO ₂ and CaO (for example, Al ₂ O ₃ , MnO, SiO _2, etc.).

Next, the quantified inclusions were subjected to TEM observation (observation magnification: 30,000 times) to measure the circle-equivalent diameter, and the number of inclusions having a circle-equivalent diameter (particle diameter) of 0.1 to 2 μm was measured. Table 2 shows values obtained by converting the number of inclusions per 1 mm 2 of the observation field area.

In addition, the oxide-based inclusions having an oxygen content of 5 mass% or more in the obtained quantitative results were measured by TEM observation (observation magnification: 30,000 times), and the circle equivalent diameter (particle diameter) The number of oxides was measured. The values of the number of oxides converted in terms of the observation field area of 1 mm &lt; 2 &gt; are shown in Table 2 below.

Next, a sample of 10 mm x 20 mm x 20 mm was cut out from the cross section at the position of t / 4 (t is the thickness of the steel sheet) of the obtained thick steel sheet, and the electrolytic solution after electrolytic extraction was divided into 0.1 mu m Mu m filter, and the extracted residue remaining on the filter was recovered. As the electrolytic solution, a methanol solution of 10% acetylacetone-1% tetramethylammonium chloride was used. The electrolytic extraction was carried out at a current density of 100 to 200 A / m 2.

The amount of Ti and Al contained in the recovered extracted residue was quantified by ICP emission spectrometry and the amount of N was measured by an indian phenol blue spectrophotometry using an ultraviolet visible spectrophotometer "UVmini-1240 (Shimazu Seisakusho Co., Ltd.)" , And the value of the left side of the equation (1) was calculated by the above procedure. The results are shown in Table 2 below.

Next, in order to evaluate the toughness of the HAZ subjected to heat at the time of welding, substitution heat welding was simulated and the following welding reproducibility test was carried out. In the welding reproduction test, the sample cut from the t / 4 position (t, plate thickness) of the thick steel sheet was heated so as to be 1400 占 폚, held at this temperature for 60 seconds, and then subjected to a thermal cycle for cooling. The cooling rate was adjusted so that the cooling time from 800 ° C to 500 ° C was 450 seconds.

The impact characteristics of the sample after cooling were obtained by taking three V-notch Charpy test pieces from the sample after the thermal cycle was applied in the rolling direction, and performing an impact test according to JIS Z2242. In the impact test, the absorbed energy at -40 DEG C (vE- ₄₀ ) was measured, and the average value was calculated three times. In the present invention, an average value of vE- ₄₀ of 100 J or more is acceptable (good HAZ toughness). The measurement results are shown in Table 2 below.

The following can be considered from the following Tables 1 and 2. Nos. 1 to 18 and 32 are examples satisfying the conditions defined by the present invention, and a large number of minute inclusions having a circle equivalent diameter of 0.1 to 2 탆 are generated so that oxides having a circle equivalent diameter of more than 3 탆 are not produced Moreover, since the composition of the fine inclusions is appropriately controlled, a steel material having good HAZ toughness is obtained.

Nos. 19 to 31 and 33 are examples deviating from any one of the requirements specified by the present invention. In No. 19 of these, since the amount of Al contained in the steel is excessively large, a large number of coarse oxides having a circle equivalent diameter exceeding 3 탆 are produced, and HAZ toughness is deteriorated. No. 20 is considered to be an excessively large amount of N contained in the steel material, and the amount of solute N contained in the steel material is excessive and the HAZ toughness is deteriorated.

In No. 21, since the amount of Ti contained in the steel material is excessively large, the base material is solid-solubilized by the employment of Ti, and as a result, the HAZ toughness is deteriorated. In No. 22, since the amount of Ti contained in the steel is too small, HAZ toughness is deteriorated. In No. 23, since the amount of Zr contained in the steel material is excessively large, the amount of ZrO ₂ is increased and the action of the Zr · REM · Ca-based oxide which becomes the nucleus of the α-transformation in the grain weakens and no microstructure is obtained. HAZ toughness It is thought that it is deteriorated. In No. 24, since the amount of Zr contained in the steel material is too small, the amount of ZrO ₂ is reduced, and the amount of Zr · REM · Ca-based oxide which becomes the nucleus of α transformation in the grain is thought to be reduced. It is considered that the HAZ toughness is deteriorated.

In No. 25, since the amount of REM contained in the steel material is large, the amount of oxide of REM is increased, and the oxide of REM is coarsened, and coarse oxide having a circle equivalent diameter exceeding 3 탆 is excessively produced. It is considered that the improvement effect is not exerted. In No. 26, since the amount of REM contained in the steel is too small, the amount of oxide of REM is reduced, and the amount of Zr · REM · Ca-based oxide which is the nucleus of α-transformation in the grain is reduced. It is considered that the HAZ toughness is deteriorated. In No. 27, since the amount of Ca contained in the steel material is excessively large, the amount of CaO is increased and the action of the Zr · REM · Ca-based oxide which becomes the nucleus of the α transformation in the grain is weakened and the microstructure is not obtained, . In No. 28, since the amount of Ca contained in the steel material is too small, CaO is not produced, and the amount of Zr · REM · Ca-based oxide which becomes the nucleus of α-transformation in the grain is thought to be reduced. It is considered that the HAZ toughness is deteriorated.

Nos. 29, 30 and 33 are different from the conditions recommended by the present invention for the addition order of Ti and Al in the solvent, so that the value of the above formula (1) It is an example that is off. HAZ toughness is deteriorated thereby. No. 31 has a poor balance of the amounts of Ti, N and Al, and the composition of the inclusions contained in the steel material does not satisfy the relation of the above-mentioned formula (1) and exceeds the range specified by the present invention. , The inclusions are not liquefied at the time of welding with large heat, and inclusions that are the nuclei of alpha transformation in the grains are hardly formed, and the HAZ toughness is not improved.

Claims (5)

C: 0.02 to 0.15% (meaning% by mass, the same applies to the following components),
Si: 0.5% or less,
Mn: 2.5% or less,
P: 0.03% or less,
S: 0.02% or less,
Al: 0.050% or less,
Ti: 0.005 to 0.10%
REM: 0.0003 to 0.015%,
Ca: 0.0003 to 0.010%
Zr: 0.0010 to 0.050%
N: 0.010% or less,
O: 0.0005 to 0.010%
The balance being iron and inevitable impurities,
(a) the steel material comprises an oxide containing Zr, REM and Ca,
(b) of all the inclusions contained in the steel,
An inclusion having a circle-equivalent diameter of 0.1 to 2 탆 is 120 or more per 1 mm 2 of the observation field area,
An oxide having a circular equivalent diameter of more than 3 占 퐉 is 5.0 or less per 1 mm2 of the observation field area,
(c) A steel material excellent in toughness of a weld heat-affected zone, wherein the composition of the inclusions having a circle equivalent diameter of 0.1 to 2 占 퐉 contained in the steel material satisfies the following formula (1).
[Equation 1]

The method according to claim 1,
Wherein the steel material is, as another element,
Cu: 2% or less,
Ni: 3.5% or less,
Cr: 3% or less and
Mo: 1% or less, and having excellent toughness at the weld heat affected zone. 3. The method according to claim 1 or 2,
Wherein the steel material is, as another element,
Nb: 0.25% or less,
And V: 0.1% or less, and having excellent toughness at the weld heat affected zone. 3. The method according to claim 1 or 2,
Wherein the steel material is, as another element,
B: 0.005% or less, excellent in toughness of weld heat affected zone. The method of claim 3,
Wherein the steel material is, as another element,
B: 0.005% or less, excellent in toughness of weld heat affected zone.