KR101512257B1 - Steel material having superior toughness of welded heat-affected zone, and method for manufacturing same - Google Patents

Abstract

The steel material according to the present invention is characterized in that it comprises: (a) 5 to 50% of ZrO ₂ , 5 to 50% of REM oxide, and 50% or less of CaO (0% (B) not less than 120 pieces / mm 2 of inclusions having a circle equivalent diameter of 0.1 to 2 탆, not more than 5.0 pieces / mm 2 of oxides of more than 3 탆, not more than 5 탆 of all the inclusions (C-1) the ratio of the number of REM and Zr-containing inclusions I satisfying the molar ratio of REM and Zr of 0.6 to 1.4 satisfying the relationship of (c-1) the number of all inclusions when the number of oxides was 5.0 / Zr, Al, Ca, and Ti-containing inclusions satisfying a ratio of the total number of moles of REM and Zr to the total number of moles of Al, Ca and Ti of from 0.5 to 1.2, and / or (c-2) II ratio of not less than 40%. As a result, the HAZ toughness is improved even when the heat input amount is 50 kJ / mm or more and the heat input welding is performed.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having excellent toughness of a weld heat affected zone and a manufacturing method thereof,

TECHNICAL FIELD The present invention relates to a steel used for bridges, high-rise buildings, ships and the like, and more particularly to a steel material used for bridges, high-rise buildings, ships and the like, particularly toughness (hereinafter referred to as " weld heat affected zone " Toughness) and a manufacturing method thereof.

Properties required for steels used for bridges, high-rise buildings, ships 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 is conspicuous as the heat input during welding becomes larger. The reason for this is that the cooling rate of the HAZ is slowed down when the heat input at the time of welding is lowered and the quenching property is lowered to generate coarse island-shaped martensite have. Therefore, in order to improve the toughness of the HAZ, it is considered that the amount of heat input at the time of welding is preferably suppressed as much as possible. On the other hand, in order to increase the efficiency of the welding work, it is required to employ a large heat input 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 and 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 by containing an oxide of REM and / or CaO and ZrO ₂ as an oxide serving as nuclei of ferrite transformation in the ingot. The oxides are present in a liquid state in molten steel, and thus are finely dispersed in the steel. Further, the oxide is thermally stable and does not disappear even when it is exposed to a high temperature of, for example, 1400 deg. C for a long period of time, thereby contributing greatly to the improvement of HAZ toughness.

The applicant of the present invention has further studied to provide a steel material excellent in HAZ toughness at the time of welding at a higher level even after the above-mentioned Patent Document 1 is disclosed. As a result, the invention described in Patent Document 4 was first proposed . Patent Document 4 discloses that the size and the number of all oxide inclusions in the steel (not limited to the oxide serving as the nucleus of the ferrite transformation in the ingot) and the number thereof are deeply involved in the improvement of the HAZ toughness, Discloses that a steel material excellent in HAZ toughness can be obtained even when the heat input amount of about 50 kJ / mm or so is subjected to large heat treatment by reducing the number of coarse oxides exceeding 5.0 占 퐉 in diameter to 5 or less. 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. Specifically, in Patent Document 1, a heat cycle (heat input condition: 1400 占 폚 占 5 seconds, a cooling time Tc = 300 seconds), and the absorbed energy (vE- ₄₀ ) at -40 ° C was measured. However, in Patent Document 4, a heat cycle at 1400 ° C for 30 seconds 30 seconds, and cooling time Tc = 300 seconds), the absorption energy was measured in the same manner as described above, and it was confirmed that good HAZ toughness was obtained even in this case.

On the other hand, Patent Documents 5 to 7 do not describe a technique of using REM oxide and ZrO ₂ in combination as in Patent Documents 1 to 4. However, when REM is added to molten steel whose dissolved oxygen amount is adjusted, about 300 kJ / cm (about 30 kJ / It is possible to improve the HAZ toughness at the time of carrying out the substitutional heat welding in excess of the above-mentioned HAZ.

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 Japanese Patent Application Laid-Open No. 2003-221643 Japanese Patent Application Laid-Open No. 2003-286540 Japanese Patent Application Laid-Open No. 2002-363687

A demand for further improvement of the welding operation efficiency is inevitable, and the amount of heat of welding is also increased with this. Up to now, a steel material which exhibits excellent performance even under the conditions of a large heat input of 50 kJ / mm or more, which has not been reviewed so far, is desired. The object of the present invention is to provide a steel material excellent in HAZ toughness and a method of manufacturing the same, even when large heat input heat of 50 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, has a composition of C: 0.02 to 0.15% (meaning the mass% (Not including 0%), P: not more than 0.03% (not including 0%), S: not more than 0.02% (not including 0%), Al: 0.050 , Ti: 0.005 to 0.10%, Zr: 0.0005 to 0.050%, REM: 0.0003 to 0.015%, Ca: 0.0003 to 0.10% (inclusive of 0% 0.010% and O: 0.0005 to 0.010%, with the balance being iron and inevitable impurities. (A) When the composition of all the oxide inclusions contained in the steel is measured and converted into a single oxide by mass conversion, an average composition of ZrO ₂ : 5 to 50%, an oxide of REM ₂ O ₃ : 5 to 50%, CaO: not more than 50% (excluding 0%), and (b) all the inclusions contained in the steel, The observation area is at least 120 per mm 2, and the oxide having a circular equivalent diameter of more than 3 탆 is 5.0 or less per observation field area of 1 mm 2, and the oxide having a circular equivalent diameter of more than 5 탆 is observed at an observation field area of 5.0 And (c-1) the composition of all inclusions contained in the steel material is measured, the REM and Zr content (REM / Zr) satisfying the molar ratio (REM / Zr) The number ratio of the inclusions I is not less than 30%, and / or (c-2) (REM + Zr) / (Al + Ca + Ti) ratio of the total moles of REM and Zr to the total molar amount of Al, Ca and Ti is 0.5 to 1.2 for all the inclusions when the composition of all inclusions is measured. , And the number ratio of Zr, Al, Ca and Ti-containing inclusions II is 40% or more.

The steel material, as another element,

[1] Cu: not more than 2% (not including 0%) and / or Ni: not more than 3.5% (not including 0%),

[2] Cr: not more than 3% (not including 0%) and / or Mo: not more than 1% (not including 0%),

[3] Nb: not more than 0.25% (not including 0%) and / or V: not more than 0.1% (not including 0%),

[4] B: 0.005% or less (not including 0%)

And the like.

In the steel material of the present invention, when REM is added to molten steel in which the dissolved oxygen amount Q _Of is adjusted to the range of 0.0003 to 0.01% by mass, the dissolved oxygen amount Q _Of of the molten steel and the addition amount Q _REM of _{REM satisfy} the following formula (1) At the same time the addition of the amount of REM, in the molten steel was adjusted to the amount of dissolved oxygen Q _of the above-described range, to the addition of REM, Zr, Ti, Ca, and Al, REM and Zr to a group a elements, that satisfy Ti, (2) and / or (3) below, when the elements are Ca, Al, and Al, respectively.

(2) REM and Zr are simultaneously added to the a group element, or the other element is added within 5 minutes after addition of one of REM and Zr.

(3) In the case where the b-group element is added before and / or after the addition of the a-group element and the b-group element is added before the a-group element is added, The time from the start of element addition to the start of the addition of the first element among the a group elements is t1 (minute), and when the group b element is added after the addition of the a group element, The time from the start of addition of the last element to the start of the addition of the first element among the group b elements is t2 (minutes), and the total of t1 and t2 is 3 minutes or more (0? T1, 0? t2, where t1 and t2 are not zero).

According to the present invention, an oxide (Zr, an oxide containing REM and Ca) serving as a nucleus of an? -Granular transformation (? Means ferrite or a mixed structure of ferrite and bainite, hereinafter the same) And the ratio of the number of inclusions containing a predetermined element in a specific relation to the number and the number of inclusions and oxides existing in the steel (that is, the particle size distribution) and all the inclusions is appropriately controlled, It is possible to provide a steel material excellent in HAZ toughness at the time of welding. Particularly, in the steel material of the present invention, it is apparent that not only a predetermined amount of fine inclusions having a circle-equivalent diameter of 0.1 to 2 탆, which is useful for improving HAZ toughness, exists but also adversely affects the improvement of HAZ toughness. It was found that the number of both oxides of the crude oxide and the oxide of the ferrite having a circle equivalent diameter exceeding 5 占 퐉 was significantly suppressed and the REM and Zr content The ratio of the number of REM, Zr, Al, Ca, and Ti-containing inclusions II in which the ratio of the number of inclusions I and / or the total number of moles of REM and Zr and the total number of moles of Al, Ca and Ti satisfies a specific relationship is set to a predetermined amount It is possible to increase the toughness of the HAZ even if welding is performed at a larger heat input than the HAZ toughness evaluation method disclosed in the embodiment of Patent Document 4. [

1 is a diagram showing an example of the order of addition of elements when a group-B element is added before and after the addition of the group-B element.
Fig. 2 is a graph showing the relationship between the value (Z value) of the left side of the equation (1) defined in the present invention and the number of oxides having a circle-equivalent diameter of more than 3 占 퐉 per 1 mm2 of the observation field area.
Fig. 3 is a graph showing the relationship between the number per square millimeter of observation area and the absorbed energy (vE- ₄₀ ) at -40 DEG C of oxides having a circle-equivalent diameter exceeding 3 mu m.

The present invention relates to a technique for obtaining a steel material having improved HAZ toughness even when welding is performed with a larger heat input amount by improving the technique using the oxides as the core of the in-line? Transformation described in the above Patent Documents 1 to 4.

In other words, the inventors of the present invention have conducted research to provide a steel material excellent in HAZ toughness at the time of high-level heat welding even after the above-mentioned Patent Document 4 is proposed. As a result, it was found that a thermal cycle for cooling the temperature from 800 DEG C to 500 DEG C for 400 seconds after holding for 5 seconds at a heating temperature of 1450 DEG C (heat input condition: 1450 DEG C x 5 Sec and a cooling time Tc = 400 sec), it is insufficient to reduce the oxide having a circular equivalent diameter of more than 5.0 m to not more than 5, as in Patent Document 4, in order to provide a steel having excellent HAZ toughness, When the number of oxides exceeding 3.0 占 퐉, which has not been considered at all in the past including the Patent Document 4, is reduced and the composition of all inclusions contained in the steel is measured, the number of all inclusions contained in the steel is determined by REM The ratio of the total number of moles of REM and Zr to the total number of moles of Al, Ca and Ti, and the ratio of the total number of moles of REM and Zr, (REM + Zr) / (Al + Ca + T it is extremely important that the ratio of the number of REM, Zr, Al, Ca, and Ti-containing inclusions II satisfying 0.5 to 1.2 is 40% or more. Thus, the present invention has been completed.

Thus, a feature of the present invention is that,

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

(B) The number of oxides having a diameter of more than 5 mu m (5.0 pieces / mm < 2 > or less) which adversely affects HAZ toughness improvement is reduced

(C) In the present invention, the number of oxides having a circle-equivalent diameter of more than 3 占 퐉, which has become apparent for the first time to adversely affect HAZ toughness improvement, is also reduced (5.0 pieces /

(D) When the composition of all the inclusions contained in the steel is measured, it is preferable that the ratio of REM / Zr (REM / Zr) to REM / Zr containing inclusions I (REM + Zr) / (Al + Ca + Ti) ratio of the total moles of REM and Zr to the total moles of Al, Ca and Ti is 0.5 to 1.2 Of the inclusions II containing REM, Zr, Al, Ca, and Ti is 40% or more.

By providing such feature portions, even when welding is performed with a larger heat input amount than that of the above Patent Document 4, the HAZ toughness can be improved. That is, in relation to the Patent Document 4, there are the feature portions of the present invention in addition to the above (a) and (b), in addition to the above (c) and (d).

Strictly speaking, the above-mentioned (A) regulation differs from the above-mentioned Patent Document 4. In Patent Document 4, a fine number of the oxides is controlled with respect to oxides, but in the present invention, not only oxides but also And the number of the inclusions in the inclusions is controlled. According to the examination results of the present inventors, in order to realize a satisfactory HAZ toughness, it is preferable to use a large oxide (hereinafter, simply referred to as " particle diameter "Lt; / RTI > oxide) are very large. If a large inclusion of 0.1 to 2 占 퐉 in particle diameter is controlled so as not to generate this large oxide, it is possible to secure desired characteristics even if it is expanded to all inclusions without limiting to oxide.

In order to satisfy the above requirement (c), it is not sufficient to control the amount of dissolved oxygen in the molten steel before the REM addition, as in the case of Patent Document 4 and Patent Documents 5 to 7 described above, _of the thus been found that it is extremely important to properly control the amount Q of _REM REM. Specifically, the amount of REM (Q _REM ) satisfying the following formula (1) is added according to the dissolved oxygen amount Q _Of of molten steel before REM addition. This makes it possible to suppress the generation of the REM oxide having a large grain size adversely affecting the realization of the desired HAZ toughness. Details of the technical meaning of the following equation (1) will be described later.

Further, in order of the above (d) requirements, to provided the requirements for the number ratio of the inclusions I, REM and order of addition of Zr, and it is necessary to note the addition interval of time of these elements, the amount of dissolved oxygen to adjust the Q _Of In adding REM to a molten steel, it is important to control the time from the addition of REM and Zr at the same time, or the addition of one of REM and Zr to the addition of the other element within 5 minutes It became clear.

It has also become apparent that it is important to appropriately control the addition conditions of REM, Zr, Ti, Ca and Al in order to satisfy the requirement for the number ratio of the inclusions II out of the requirements of the above (D). Specifically, the order of addition of each group and the addition interval time between groups when REM, Zr, Ti, Ca, and Al are divided into a group (REM and Zr) and b group (Ti, Ca, .

The technical significance of the requirements in (D) above will also be described in detail later.

In this specification, for the sake of convenience of explanation, in order to distinguish an oxide serving as a nucleus of the in-vivo? Transformation, that is, an oxide containing Zr, REM, and Ca from all the oxides contained in the steel, Ca-based oxide ", and the latter is sometimes referred to as " all oxide-based inclusions ". The oxides also include complex oxides in which inclusions other than oxides (for example, sulfides, nitrides, carbides, or complex compounds thereof) are combined in addition to the single oxides. The essential components (Zr, REM, and Ca) constituting the above-mentioned Zr.REM.Ce-based oxide are sometimes referred to as " in-the-a-transformation nucleation element ".

Here, the Zr.REM.Ca based oxide which is a starting point of the in-vivo? Transformation will be described. The Zr-REM-Ca-based oxide means that the oxide of Zr, the oxide of REM and the oxide of Ca are necessarily included. Zr, REM, and Ca constitute the elements constituting the Zr.REM.Ca based oxide (the? -Form nucleation elements in the grain) are Zr, REM and Ca. However, in addition to these, oxides such as Ti, Mn, Si and Al, Or may contain a middle component.

The form of the Zr.REM.Ce-based oxide is not particularly limited and may be present as a single oxide containing the? -Conjugated nucleating element in the grain alone, or may be a composite containing two or more of? Or may be present as an oxide. Examples of the oxide alone, the Zr ZrO _2; Ca is CaO; In the REM, when REM is represented by the symbol "M", M ₂ O ₃ , M ₃ O ₅ , MO ₂ and the like are exemplified. These oxides may be aggregated with each other or may exist in the form of complex precipitates of other compounds such as sulfides or nitrides in the oxides.

It is preferable that the Zr 占 REM 占 Ca based oxide further contains an oxide of Ti. The presence of Ti oxide further promotes in-situ alpha transformation and further enhances the HAZ toughness. The oxide of Ti may be present as a single oxide (e.g., Ti ₂ O ₃ , Ti ₃ O ₅ , TiO ₂ ), or may be a composite oxide containing at least one of Zr · REM · Ca- Or may be present in the form of.

In addition, the steel material of the present invention includes sulfides, nitrides, carbides, or complex compounds thereof in addition to the above-mentioned oxides. In the present specification, however, the oxides, sulfides, nitrides, carbides, Collectively referred to as "all inclusions".

In the present specification, among all the oxide inclusions included in the steel, oxides having a circle equivalent diameter of 0.1 to 2 占 퐉 are referred to as "fine oxides", oxides having a circle equivalent diameter of more than 3 占 퐉 are referred to as "coarse oxides" Oxides having diameters larger than 5 占 퐉 are referred to as " nodular oxides ", respectively, and they are sometimes distinguished from each other. In Patent Document 4, oxides having a circle equivalent diameter exceeding 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, the term " a steel material excellent in HAZ toughness of large heat welding " means a steel material which is held at 1450 캜 for 5 seconds and then subjected to a thermal cycle Means that the absorbed energy (vE- ₄₀ ) at -40 ° C satisfies 130 J or more when the temperature (heat input condition: 1450 ° C. × 5 seconds, cooling time Tc = 400 seconds) is given. The vE _-40 may be the greater, preferably at least the vE _-40 150J. The above-mentioned heat cycle is sometimes referred to as " large heat history of large heat input. &Quot; The heat input by the heat cycle is higher than the heat input by the heat cycle described in Patent Document 1 and Patent Document 4. In this sense, The heat input level of " substitution heat welding " described in Document 4 is different.

In the present invention, the reason why the temperature of the heat cycle is set to 1450 占 폚 is that the heat temperature of a portion of the HAZ, particularly near the weld metal (sometimes referred to as a bond portion), exceeds 1400 占 폚 to about 1450 占 폚 .

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

[(a) About the average composition of the oxide]

When the composition of all the oxide inclusions contained in the steel material is measured and converted into mass as a single oxide (total amount is 100%), the steel material of the present invention has an average composition of ZrO ₂ : 5 to 50%, REM oxide (M ₂ O ₃ ): 5 to 50% and CaO: 50% or less (excluding 0%) as represented by the symbol M, thereby effectively acting as the nucleus of α-transformation in the mouth. When the lower limit of the respective oxides is less than the lower limit of the respective oxides, the amount of the oxide which becomes the nucleus of the? Transformation at the time of welding is insufficient and the function of improving the HAZ toughness is not exhibited. 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 nuclei of the in-gauge alpha transformation is decreased, and the HAZ toughness improving action is not effectively exerted.

The ZrO _{2 content} is 5% or more, preferably 8% or more, and more preferably 10% or more. On the other hand, the upper limit is 50%, the preferable upper limit is 45%, and the more preferable upper limit is 40%.

The oxide of the REM is at least 5%, preferably at least 10%, and more preferably at least 13%. On the other hand, the upper limit is 50%, the preferable upper limit is 45%, and the more preferable upper limit is 40%. In addition, an oxide of REM are, represents the REM into symbols M, M ₂ O _3, M ₃ O _5, in the form of MO ₂ and so on in the steel, however, in the present invention, in terms of both an oxide of REM as M ₂ O ₃ Of the total amount of water.

The above-mentioned CaO effectively acts as the nucleus of the? -Form transformation in the mouth, but if it is contained in excess, the? -Faturation of the mouth is deteriorated. In addition, if CaO is contained excessively, it causes a loss of the nozzle used in casting. Therefore, the upper limit is set to 50%, preferably 45% or less, more preferably 40% or less, particularly preferably 30% or less. In order to effectively exhibit the above-mentioned action, CaO is preferably contained in an amount of 3% or more. CaO is more preferably 5% or more, and further preferably 10% or more.

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

The composition of all the oxide inclusions contained in the steel material can be determined by observing the surface of the steel material with, for example, an electron probe micro-probe micro-analyzer (EPMA) Analysis can be performed. The details of the measurement conditions are described in the column of the later-described embodiments.

[(b) About particle size distribution of all inclusions]

Next, the number and size of all the inclusions that characterize the present invention will be described. In the steel material of the present invention,

(i) fine inclusions having a circle-equivalent diameter of 0.1 to 2 占 퐉 are 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,

(iii) An oxide having a pore equivalent diameter exceeding 5 占 퐉 is observed at an observation field area of not more than 5.0

Of the present invention. Particularly, the present invention is characterized in that both of (ii) and (iii) are specified for an oxide having a large circle-equivalent diameter (particle diameter).

Here, when all of the requirements (ii) and (iii) are satisfied, it means that the number of oxides having a particle diameter of more than 3 m but not more than 5 m is 5.0 or less. That is, in order to secure a very high HAZ toughness of vE _-40 & ge; 130J even when receiving the history of the large-heat heat according to the present invention, the oxide having an " Is extremely important and it is not possible to control the number of oxides within the range, it has been found for the first time that the oxide becomes a starting point of brittle fracture and the HAZ toughness is lowered by the inventors of the present invention.

Hereinafter, the technical significance of (ii) and (iii) above will be described in detail with reference to Tables 5 and 6 of the embodiment.

Nos. 1 to 32 of Table 5 below are examples satisfying all the requirements specified in the present invention. Considering the above (ii) and (iii), the number of oxides in excess of 3 占 퐉 was suppressed to 4.64 even in No. 5 (1.440 pieces), which had the largest number of oxides in excess of 5 占 퐉 among the Nos. 1 to 32, As a result, good HAZ toughness can be secured.

On the other hand, Nos. 35 to 38, 49, 53, 54 and 61 in the following Table 6 satisfy the above requirement (iii) but do not satisfy the requirement (ii). In detail, the number of oxides in excess of 5 占 퐉 is 0.440 to 2.250, which is suppressed to 5.0 or less, but the number of oxides in excess of 3 占 퐉 is increased from 5.0 to 5.71 to 10.65, Was not obtained.

The above Nos. 35 to 38, 49, 53, 54, and 61 are included in the scope of Patent Document 4 in that they satisfy the above requirement (iii) , It can be understood that the desired HAZ toughness specified in the present invention can not be achieved if the requirement (ii) is not satisfied. Therefore, in the present invention, (ii) is further defined as a requirement for ensuring desired HAZ toughness in addition to (iii).

It is also understood from the requirements of (ii) and (iii) above that the desired HAZ toughness is particularly deeply involved in the number of oxides of more than 3 [mu] m and not more than 5 [mu] m. That is, depending on the production conditions, there may be more than 5.0 oxides in an extremely narrow range exceeding 3 占 퐉 and 5 占 퐉 or less. For example, the number of the fine regions of (i) It was unexpectedly found by the present inventors that the desired HAZ toughness could not be obtained merely by the presence of more than 5.0 oxides in the coarse region exceeding 3 탆 and 5 탆 or less .

The detailed mechanism of why the desired HAZ toughness can be ensured by satisfying both (ii) and (iii) above is unclear, but when the temperature exceeds 1400 ° C and reaches 1450 ° C, the disappearance of TiN accelerates Toughness is lowered. However, it is considered that such decrease in toughness can be suppressed by reducing oxides of 3 m or more and 5 m or less.

As described above, in the present invention, it is necessary to satisfy the requirements of (ii) and (iii) simultaneously. That is, the number of coarse oxides having a grain size of more than 3 占 퐉 is 5.0 or less, and the number of grains having a grain size exceeding 5 占 퐉 is 5.0 or less. The number of these groups is preferably as small as possible, and in all cases, preferably 3.0 or less, more preferably 2.0 or less, particularly preferably 1.0 or less, and most preferably 0. In detail, it is preferable to appropriately control both of them, and in the range of the present invention (5.0 or less in all), it is preferable that the grain size is larger than 5 占 퐉 It is preferable to reduce the number of oxides. Concretely, the number of rough oxide is preferably as close as possible to the lower limit (0), more preferably not more than about 1.0, and most preferably not more than zero, while the number of coarse oxides is not more than 5.0 ), Or about 4.0 or less may be preferably used.

The number of oxides having a circle equivalent diameter exceeding 3 占 퐉 and the number of oxides exceeding 5 占 퐉 can be measured by observing the cross section of the steel material with, for example, EPMA and quantitatively analyzing the composition of the inclusions identified in the observation field of view , And the inclusion having an oxygen content of 5% or more is used as an oxide, and the circle equivalent diameter of the oxide is measured and observed by, for example, a scanning electron microscope (SEM).

Above, (ii) and (iii), which characterize the present invention, have been described in detail.

In the steel material of the present invention, as described in (i) above, it is necessary to set the number of fine inclusions having a circle-equivalent diameter of 0.1 to 2 탆 to 120 or more per observation field area of 1 mm 2. The number of fine inclusions is not less than 120 per 1 mm 2 of observation field area, preferably not less than 200 per mm 2, more preferably not less than 500 per mm 2, more preferably not less than 700 per mm 2.

The number of fine inclusions having a circle equivalent diameter of 0.1 to 2 占 퐉 may be determined by observing the cross section of the steel material by, for example, SEM.

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 a circle assumed to make the areas of the inclusions (including the oxide) equal to each other, and is confirmed on the SEM observation surface.

[(c) REM, Zr, Al, Ca, and Ti-containing inclusions satisfying the number ratio of REM and Zr-containing inclusions I satisfying the REM / Zr ratio of 0.6 to 1.4 and the ratio (REM + Zr) / (Al + Ca + About the number ratio of II]

In the steel material of the present invention, the number and the size of all the inclusions are appropriately adjusted, and when the composition of all the inclusions contained in the steel is measured,

(c-1) the ratio of the number of REM and Zr-containing inclusions I (hereinafter simply referred to as inclusions I) satisfying the molar ratio (REM / Zr) of REM / Zr of 0.6 to 1.4 is 30%

Zr, Al, Ca, and Ti containing inclusions satisfying the following formula (c-2): the total molar number of REM and Zr and the total molar ratio of Al, Ca and Ti [ II (hereinafter, sometimes simply referred to as " inclusion II ") is 40% or more, HAZ toughness is further increased.

At least one of the requirements (c-1) and (c-2) may be satisfied, and both of them may be satisfied.

The requirement (c-1) is that the inclusion ratio of REM / Zr to the inclusions containing REM and Zr among the α-transformation nucleation elements (REM, Zr and Ca) I is the number ratio. On the other hand, the requirement of the above (c-2) is to realize the desired HAZ toughness for the inclusions containing the? -Conjugated nucleating elements (Zr, REM and Ca) in the grain and the other elements (Ti and Al) constituting the inclusions (REM + Zr) / (Al + Ca + Ti) and the ratio of the number of the inclusions II.

That is, as apparent from the later examples, it has been found that even if the requirements of (a) and (b) are almost the same, a variation occurs in the toughness of the steel. That is, (a), (b), by controlling the composition and the average size and particle size distribution of the inclusions of the oxide, the absorption energy in Fig -40 ℃ subjected to heat welding by substituting (vE _-40, as defined in ) Can achieve 100J or more. However, in addition to the above (a) and (b), the ratio of the number of inclusions I specified in (c-1) and / By controlling the ratio, vE- ₄₀ can achieve 130 J or more.

With respect to (c-1), for example, No. 2 shown in Table 5 below and No. 33 shown in Table 6 below, although the particle size distribution of all inclusions in the above-mentioned (b) A difference of 42 J was generated in the absorption energy (vE- ₄₀ ) at 40 캜. Therefore, as a result of further investigations by the present inventors, it has been found that the number ratio of all the inclusions of the inclusions I satisfying the molar ratio (REM / Zr) of REM to Zr of 0.6 to 1.4 with respect to REM and Zr constituting the inclusions is (No. 2 above) was excellent in the in-line a transformation ability and HAZ toughness, but the number ratio of the inclusions I satisfying the above ratio was less than 30% (the above No. 33) It was found that the desired HAZ toughness could not be secured. REM and Zr are elements that generate oxides which are nuclei of the in-phase a transformation. It is found that the relationship between the number ratio of the inclusions I to all inclusions and the HAZ toughness has a good correlation, and (c -1). That is, the inclusions I having a REM / Zr ratio of 0.6 to 1.4 or a ratio of REM / Zr ratio of 1.4 or more to the inclusions satisfying the REM / Zr ratio of 0.6 to 1.4 are excellent in the in- And further contributes to improving the HAZ toughness by making the structure finer.

By setting the ratio of the number of inclusions I to the total number of inclusions to 30% or more, the absorption energy (vE- ₄₀ ) at -40 캜 can be 130 J or more even if welding is carried out with a large heat quantity. The number ratio of the inclusions I to the total number of inclusions is preferably as high as possible, preferably at least 40%, more preferably at least 50%. The number ratio of the inclusions I is preferably as large as possible, and most preferably 100%.

As in the case of (c-1) above, for Example (c-2), for example, No. 17 shown in Table 5 below and No. 51 shown in Table 6 below have a particle size distribution (VE- ₄₀ ) at -40 DEG C, a difference of 36J was generated. Therefore, the inventors of the present invention have found that the ratio [REM + Zr) / (REM + Zr) of the total moles of REM and Zr to the total moles of Al, Ca, and Ti with respect to REM, Zr, Ti, (Al + Ca + Ti)] of 0.5 to 1.2 was controlled to be 40% or more for all inclusions of the inclusions II (No. 17 above), the in-line a transformation was promoted and the HAZ toughness was improved, It was found that the desired HAZ toughness could not be secured in the case where the number ratio of the inclusions II satisfying the above ratio was less than 40% (the above No. 51).

(REM + Zr) / (Al + Ca + Ti) ratio satisfies the above-mentioned range, REM and Zr among the elements serving as nuclei of the in-gauge alpha transformation are suitably selected in relation to other elements (Al, Ca, Ti) constituting the inclusions So that the in-plane alpha transformation is promoted, and the metal structure in the HAZ is further refined, so that the HAZ toughness is improved.

That is, as a result of examining the relationship between the composition of the inclusions dispersed in the steel and the HAZ toughness, in order to make the metal structure finer by generating the in-gaseous α in the HAZ, And good coherence. It has been clarified by experiments of the present inventors that inclusions containing Ti in addition to REM and Zr are effective as inclusions having good compatibility with the? phase. However, in order to grow the ingot? Generated from the inclusions containing REM, Zr and Ti as starting points in the subsequent austenite phase, it is desired that the inclusions containing REM, Zr and Ti and the austenite phase consistency are also good . Therefore, the present inventors have further studied to improve the consistency of the austenite phase with inclusions containing REM, Zr and Ti, and it was learned that controlling the melting point of the inclusions can control the? -Formation in the grain. That is, when the inclusions are once melted in the austenite phase at the time of welding, the affinity between the melted inclusions and the austenite phase becomes good, and in the cooling process, the inclusions are crystallized while maintaining consistency with the surrounding austenite phase. When the temperature is further lowered, the alpha phase starts to be generated, but it is preferentially produced from the inclusions having good compatibility with alpha, and the alpha produced from the inclusions is also good with the austenite so that the alpha transformation is promoted, The effect of improving the toughness of the HAZ is improved.

Therefore, the present inventors investigated the melting point behavior of inclusions using a high-temperature laser microscope in order to find a component composition region where the melting point is lowered for inclusions containing REM, Zr and Ti. As a result, when the ratio of (REM + Zr) / (Al + Ca + Ti) based on moles conversion of these elements is in the range of 0.5 to 1.2, the melting point of the inclusions containing REM, Zr and Ti is influenced by the content of Ca and Al , It has been found that the melting point of the inclusions is locally lowered and the intramolecular α-transformation ability is increased.

That is, the inclusion II, which satisfies (REM + Zr) / (Al + Ca + Ti) ratio of 0.5 or less compared to inclusions having a ratio of (REM + Zr) / (Al + Ca + Ti) of less than 0.5 or a ratio of (REM + Zr) / It is found that the metal structure in the HAZ is made finer and the HAZ toughness is improved.

The composition of the inclusions contained in the steel can be obtained by quantitatively analyzing the composition of the inclusions identified in the observation field by observing the cross section of the steel with, for example, EPMA and measuring the composition of all the inclusions contained in the steel , The ratio of the number of inclusions I to the total number of inclusions, and the ratio of the number of inclusions II. Further, in the steel material of the present invention, the composition of the inclusions having a circle equivalent diameter of 0.1 탆 or more is quantitatively analyzed. Inclusions having a circle-equivalent diameter of less than 0.1 mu m are too small and can not be quantitatively analyzed with high accuracy.

Next, the composition of the steel material (base material) of the present invention will be described. The steel material of the present invention contains as main components C: 0.02 to 0.15%, Si: 0.5% or less (not including 0%), Mn: not more than 2.5% (not including 0%), P: (Not including 0%), S: not more than 0.02% (not including 0%), Al: not more than 0.050% (not including 0%), N: not more than 0.010% (not including 0% 0.005 to 0.10% of Ti, 0.0005 to 0.050% of Zr, 0.0003 to 0.015% of REM, and 0.0003 to 0.010% of Ca. The reason for setting this range is as follows.

C is an indispensable element 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 is an element having a deoxidizing action and contributing 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 Si content is more preferably 0.05% or more, and more preferably 0.1% or more. However, if the amount of Si exceeds 0.5%, the weldability and toughness of the steel become poor, so the Si content needs to be suppressed to 0.5% or less. The amount of Si is preferably 0.3% or less, more preferably 0.25% or less, further preferably 0.21% or less.

Mn is an element contributing to improvement of the strength of the steel material (base material). However, if the Mn content exceeds 2.5%, the weldability of the steel material (base material) is lowered. Therefore, it is necessary to suppress the Mn content to 2.5% or less. The amount of Mn is preferably 2.30% or less, and more preferably 2.0% or less. In order to effectively exhibit the above-mentioned effect, Mn is preferably contained in an amount of not less than 0.2%. The Mn content is more preferably 0.40% or more, further preferably 0.60% or more, particularly preferably 0.8% or more.

P is an element that is susceptible to segregation, and is segregated in grain boundaries, particularly in the steel, thereby reducing HAZ toughness. Therefore, it is necessary to suppress the P content to 0.03% or less. The P content is preferably 0.02% or less, more preferably 0.015% or less. In addition, P usually contains about 0.001% inevitably.

S is a harmful element that binds with Mn to generate sulfides (MnS), thereby deteriorating the toughness and ductility of the base material. 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, the amount of S needs to be suppressed 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 HAZ toughness is lowered. Therefore, it is necessary to suppress the amount of Al to 0.050% or less. The amount of Al is preferably 0.04% or less, more preferably 0.03% or less, further preferably 0.025% or less, particularly preferably 0.010% or less. In addition, Al usually contains about 0.0005% inevitably.

N is an element for precipitating nitride (for example, ZrN or TiN). The nitride prevents the coarsening of austenite grains generated in the HAZ during welding by the pinning effect, Thereby contributing to improvement of HAZ toughness. The more N is, the more nitride is formed and the fineness of the austenite grains is promoted, so that it is effective for improving the toughness of HAZ. However, when the N content exceeds 0.010%, the amount of solid solution N increases, the toughness of the base material itself decreases, and the HAZ toughness also deteriorates. Therefore, it is necessary to suppress the N content to 0.010% or less. The amount of N is preferably 0.0090% or less, more preferably 0.008% or less. Further, in order to effectively exhibit the above-mentioned effect, N is preferably contained in an amount of 0.003% or more. The amount of N is more preferably 0.004% or more, and still more preferably 0.005% or more.

Ti is an element contributing to improvement of HAZ toughness by producing nitride such as TiN or an oxide including Ti in the steel. 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 hardening of the base material itself due to strengthening of the solid solution of Ti leads to deterioration of 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.

Zr is an element that contributes to the improvement of HAZ toughness by producing a composite oxide containing Zr. In order to exhibit such an action, it is necessary to contain 0.0005% or more. The amount of Zr is preferably 0.0015% or more, and more preferably 0.0020% or more. However, when Zr is added excessively, a large amount of coarse Zr oxide (for example, ZrO ₂ ) is generated and HAZ toughness is deteriorated. 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, and still more preferably 0.01% or less.

REM (rare-earth element) and Ca are the elements required to produce each oxide. By containing these oxides, the oxides are apt to be finely dispersed, and the finely dispersed oxides serve as nuclei of the? -Form transformation in the grain, thereby contributing to the improvement of the HAZ toughness.

The REM content should be 0.0003% or more, preferably 0.001% or more, and more preferably 0.0020% or more. However, when REM is added excessively, a solid-dissolved REM is generated, and this tends to segregate, thereby deteriorating 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, 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, and it is more preferable to contain La and / or 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, a coarse Ca sulfide is formed and the toughness of the base material is deteriorated. Further, when Ca is excessively added, CaO is excessively generated, inclusions having a high CaO concentration are produced, deviating from the optimum inclusion composition range, so that the effect of the inclusion acting as the transformation nucleus of the inclusions is weakened and HAZ toughness is rather deteriorated. Therefore, the amount of Ca is suppressed to 0.010% or less. The content of Ca is preferably 0.009% or less, more preferably 0.008% or less, still more preferably 0.005% 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 oxygen amount represents the total amount of oxygen, which means the total amount of oxygen forming the oxide and free oxygen dissolved in the steel. The residual component of the steel may be iron and inevitable impurities (for example, Mg, As, Se, etc.).

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

[1] Cu: not more than 2% (not including 0%) and / or Ni: not more than 3.5% (not including 0%),

[2] Cr: not more than 3% (not including 0%) and / or Mo: not more than 1% (not including 0%),

[3] Nb: not more than 0.25% (not including 0%) and / or V: not more than 0.1% (not including 0%),

[4] B: 0.005% or less (not including 0%),

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

"[1] Cu and / or Ni"

Both Cu and Ni are elements contributing to increase the strength of the steel, and they can be added singly or in combination.

However, when the amount of Cu exceeds 2%, the strength of the base material becomes excessively high, and the toughness of the base material is rather lowered, so that the HAZ toughness is lowered. Therefore, the amount of Cu is preferably 2% or less. The amount of Cu is more preferably 1.8% or less, and further 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 0.20% or more.

If the amount of Ni exceeds 3.5%, the strength of the base material becomes excessively high as in the case of Cu, and the toughness of the base material is lowered, so that the HAZ toughness is lowered. Therefore, the amount of Ni is preferably 3.5% or less. The amount of Ni is more preferably 3.0% or less, further 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.

"[2] Cr and / or Mo"

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

However, if the Cr content exceeds 3%, the strength of the base material becomes excessively high, thereby deteriorating the toughness of the base material, thereby deteriorating the HAZ toughness. Therefore, the amount of Cr is preferably 3% or less. The amount of Cr is more preferably 2% or less, further preferably 1.0% 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 more preferably 0.15% or more.

Similarly to Cr, when Mo is more than 1%, the strength of the base material is excessively increased so that the toughness of the base material is lowered, thereby lowering the HAZ toughness. 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 0.8% 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 more preferably 0.15% or more.

"[3] Nb and / or V"

Nb and V are precipitated as carbonitrides and are an element having an effect of preventing coarsening of austenite grains during welding due to the pinning effect of the carbonitride and improving HAZ toughness. Nb and V may be added singly or in combination.

However, if the amount of Nb exceeds 0.25%, the precipitated carbonitride is coarsened and the HAZ toughness is rather lowered. 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 0.010% or more, and still more preferably 0.02% or more.

Similarly to Nb, when V is more than 0.1%, precipitated carbonitride is coarsened and HAZ toughness is rather lowered. Therefore, the V content is preferably 0.1% or less. The amount of V is more preferably 0.09% or less, and 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.

"[4] B (boron)"

B is an element which inhibits the generation of grain boundary ferrite and improves toughness. However, if the amount of B exceeds 0.005%, BN is precipitated at the austenite grain boundaries, resulting in deterioration of toughness. Therefore, the amount of B is preferably 0.005% or less. The amount of B is more preferably 0.004% or less, further preferably 0.0030% or less. Further, in order to effectively exhibit the action by the addition of B, it is preferable to contain 0.001% or more. The amount of B is more preferably 0.0015% or more.

Next, a manufacturing method that can be suitably employed in manufacturing the steel material of the present invention will be described.

In order to produce the steel material of the present invention,

(1) an amount to satisfy the amount of dissolved oxygen according to the addition of REM in the molten steel was adjusted to the range of the Q _Of 0.0003~0.01% by mass, to the amount of dissolved oxygen Q Q _REM and REM in the molten steel _Of Formula (1) Of REM should be added.

When REM, Zr, Ti, Ca and Al were added to molten steel in which the dissolved oxygen amount Q _Of was adjusted to the above range, REM and Zr were regarded as a group element, Ti, Ca, It is also important that the addition conditions of the respective elements satisfy the following (2) and / or (3).

(2) REM and Zr are simultaneously added to the a group element, or the other element is added within 5 minutes after addition of one of REM and Zr.

(3) In the case where the b-group element is added before and / or after the addition of the a-group element and the b-group element is added before the a-group element is added, The time from the start of element addition to the start of the addition of the first element among the a group elements is t1 (minute), and when the group b element is added after the addition of the a group element, The time from the start of addition of the last element to the start of the addition of the first element among the group b elements is t2 (minutes), and the total of t1 and t2 is set to 3 minutes or more (0? T1, 0? t2, where t1 and t2 are not zero).

This will be described in detail below.

[(1) Relation between the amount of dissolved oxygen in molten steel and the amount of REM added]

The above formula (1) is set in order to secure the desired HAZ toughness specified in the present invention. By appropriately adding the amount of _REM added Q _REM in accordance with the dissolved oxygen amount Q _Of of the molten steel based on the formula (1) HAZ toughness can be ensured (see later examples).

The coefficient of the left side of the above formula (1) is a value based on the reaction formula of the formation of the oxide of REM in the molten steel expressed by the following formula (2).

The fact that the dissolved oxygen amount Q _Of of the molten steel and the addition amount Q _REM of the _REM satisfy the above formula (1) means that the addition amount Q _REM of the REM involved in the formation of the oxide of the REM is set to be small. As a result, the number of oxides of the REM to be produced is reduced, and as a result, the number of coarse / fine roughed oxides is reduced to within the range of the present invention, thereby securing the desired HAZ toughness.

If the Z value exceeds -12.00, the balance between the dissolved oxygen amount Q _Of of the molten steel and the addition amount Q _REM of the _REM is deteriorated, and the addition amount Q _REM of the _REM is increased, so that a coarse REM oxide is produced. As a result, the HAZ toughness is lowered. Therefore, the Z value is set to -12.00 or less. The Z value is preferably -12.25 or less, more preferably -12.50 or less, and further preferably -12.75 or less. The lower limit of the Z value is not particularly limited, but is approximately -15 in consideration of the amount of REM in the steel and the like.

Further, in the Patent Document 4, no consideration is given to the expression (1). Therefore, there is a case where the addition amount Q _REM of the _REM is increased so that the value (Z value) of the left side of the equation (1) exceeds -12.00 without satisfying the relation of the formula (1). Although the above-described Patent Documents 5 to 7 disclose that REM is added to molten steel in which the dissolved oxygen amount Q _Of is adjusted, it is not considered at all that the addition amount of _REM is determined depending on the dissolved oxygen amount Q _Of . . In the above Patent Documents 5 to 7, there is no mention of using REM in combination with Zr and Ca, and therefore, as defined in the present invention, oxides containing Zr, REM and Ca having a HAZ toughness improving action (Zr ㆍ REM ㆍ Ca-based oxide) was not obtained at first.

Next, the addition amount Q _REM of the REM constituting the formula (1) and the dissolved oxygen amount Q _Of will be described.

First, the addition amount Q _REM of the _REM may be suitably added in accordance with the dissolved oxygen amount Q _Of as described above. Further, the addition amount Q _REM of the _REM is set to be larger than that of the REM contained in the steel material of the present invention. This is because the amount of REM added before casting is volatilized in the casting process or dispersed in slag, and the amount of REM contained in the steel becomes smaller.

The dissolved oxygen amount Q _Of of the molten steel is set in the range of 0.0003 to 0.01% by mass. Dissolved oxygen means oxygen in a free state existing in molten steel without forming an oxide. That is, in order to produce the steel material of the present invention, firstly, as a precondition, the dissolved oxygen amount Q _Of of the molten steel is adjusted in the range of 0.0003 to 0.01% by mass. When the dissolved oxygen amount Q _Of of the molten steel is less than 0.0003 mass%, the dissolved oxygen amount Q _Of of the molten steel is insufficient, so that it is not possible to secure a predetermined amount of the Zr · REM · Ca based oxide serving as the nucleus of the in- I can not. If the dissolved oxygen amount Q _Of is insufficient, Zr, which could not form an oxide, forms a carbide, or REM or Ca forms a sulfide, which is a cause of deteriorating the toughness of the base material itself. Therefore, the dissolved oxygen amount Q _Of is 0.0003 mass% or more. The dissolved oxygen amount Q _Of is preferably 0.001 mass% or more, and more preferably 0.0020 mass% or more.

On the other hand, if the dissolved oxygen amount Q _Of exceeds 0.01 mass%, the amount of dissolved oxygen in the molten steel is excessively large, so that the reaction between oxygen in the molten steel and the element becomes vigorous, Oxide is formed and HAZ toughness is lowered. Therefore, the dissolved oxygen amount Q _Of should be suppressed to 0.01 mass% or less. The dissolved oxygen amount Q _Of is preferably 0.008 mass% or less, more preferably 0.007 mass% or less.

However, the dissolved oxygen amount Q _Of in the primary refined molten steel in an electric furnace or an electric furnace usually exceeds 0.01 mass%. Therefore, in the manufacturing method of the present invention, it is necessary to adjust the dissolved oxygen amount Q _Of of the molten steel to the above range by some method.

Examples of the method for adjusting the dissolved oxygen amount Q _Of of the molten steel include a method of vacuum deoxidation using a RH type degassing apparatus and a method of adding deoxidizing elements such as Si, Mn, Ti and Al And the dissolved oxygen amount Q _Of may be adjusted by appropriately combining these methods. Alternatively, the dissolved oxygen amount _QOf may be adjusted by using a ladle heating type refining apparatus or a liver transplanting steel processing facility instead of the RH type degassing refining apparatus. In this case, the dissolved oxygen adjusted _Of Q by vacuum deoxidation is not able to, adjusting the dissolved oxygen amount Q _Of when there is employed a method of adding a de-acid elements, such as Si. When a method of adding a deoxidizing element such as Si is adopted, a deoxidizing element may be added to the ladle from the converter.

[(2, 3) REM, Zr, Ti, Ca, and Al)

After adjusting the dissolved oxygen amount Q _Of of the molten steel to the above range, it is preferable that the ratio of the number of inclusions I is 30% or more as described in the above (c-1) It is important that the conditions satisfy the above requirement (2). In order to make the number ratio of the inclusions II to be 40% or more as described in the above-mentioned (c-2), REM, Zr, Ti, Ca, It is important that the addition conditions of Al satisfy the requirement of (3). Therefore, in order to set the number ratio of the inclusions I to 30% or more and the number ratio of the inclusions II to be 40% or more, It is important to satisfy all the requirements.

[(2) About the order of addition of REM and Zr]

(2) defines only the addition order of the a group elements (REM, Zr), whereby the number ratio of the inclusions I can be adjusted.

In order to increase the number ratio of the inclusions I to the total number of inclusions, it is necessary to add REM and Zr simultaneously or almost simultaneously (within 5 minutes) to the molten steel in which the dissolved oxygen amount Q _Of is adjusted.

When REM and Zr are separately added, Zr may be added after adding REM, or REM may be added after addition of Zr. In either case, Zr (or REM) may be added after adding REM (or Zr) It is necessary to set the interval up to the addition of 5 minutes or less. This interval is preferably within 4 minutes, and more preferably within 3 minutes.

In addition, it is preferable to pay attention to the order of addition of the group b elements (Ti, Ca, Al) for the purpose of further improving HAZ toughness by the Zr.REM.Ca based oxide. For example, it is recommended that Ca be added after REM and Zr.

In addition, for the purpose of improving the HAZ toughness by improving the toughness of the HAZ by the refinement of the Ti oxide, it is preferable to add Ti, for example, to the molten steel before adding the REM. The Ti oxide has a smaller interface energy with the molten steel than the Zr.REM.Ce-based oxide. Therefore, by adding Ti before adding Zr, REM and Ca to the molten steel, the Ti oxide can be made finer, It is possible to produce a fine oxide which contributes to HAZ toughness. Then, after adding Ti, Zr, REM and Ca are added as described above to obtain a Zr · REM · Ca based oxide serving as a nucleus of a desired in-plane α-transformation.

Even when REM is added after the addition of Ti to the molten steel in which the dissolved oxygen amount _QOf is adjusted, as described later, the amount of addition of _REM Q _REM is changed in accordance with the dissolved oxygen amount Q _Of of molten steel to REM , The size and density of the oxide can be appropriately controlled. When Ti is added before REM, the dissolved oxygen of the molten steel is reduced due to the formation of oxides by binding with Ti. However, Ti is harder to bond with oxygen than REM, and Ti oxide has a small interfacial energy with molten steel. It is difficult to form a coarse oxide having an equivalent diameter exceeding 3 mu m. Further, since REM and Zr are easier to bond with oxygen than Ti, it is possible to generate the inclusions by adding Ti first rather than REM and Zr.

[(3) About order of addition of a group element (REM, Zr) and b group element (Ti, Ca and Al)] [

(3) defines the addition conditions of the a group element and the b group element, whereby the number ratio of the inclusions II can be adjusted.

In order to increase the number ratio of the inclusions II to the total number of inclusions, it is necessary to suitably control the addition conditions of REM, Zr, Ti, Ca, and Al added to molten steel in which the dissolved oxygen amount Q _Of is adjusted. Specifically, when REM and Zr are group a elements, Ti, Ca and Al are group b elements, it is necessary to add group b elements before and / or after addition of group a elements. That is, it is necessary to add the a group element and the b group element at the same time without adding them at the same time.

In addition, it is necessary to appropriately control the addition time interval of the a group element and the b group element. That is, when the group-b element is added before the addition of the group-a element, the time from the start of addition of the first element among the group-b elements to the start of addition of the first element among the group-a elements is t1 the addition of the first element of the group b element (the group element added first after the addition of the a group element) from the start of addition of the last element among the a group elements, in the case of adding the group b element after the addition of the a group element When the time until the start time is t2 (minute), the total of t1 and t2 needs to be 3 minutes or more.

In the calculation of t1, the time up to the start of the addition of the first element among the a group elements means the time to the time when the first a group element is added. For example, when REM and Zr are added at the same time, it is time to the simultaneous addition. When Zr is added after REM is added, REM (the first element added in the group a element) is added Means time to the point of time.

Further, in calculating t2, the time until the start of addition of the last element among the a group elements means the last time when all the a group elements are added. For example, when REM and Zr are added at the same time, they are added at the same time. When Zr is added after REM is added, it is preferable that Zr (an element finally added to the group a element) It means time.

Here, the addition interval time of the a group element and the group b element and the addition sequence of the a group element and the b group element will be described with reference to the drawings. Fig. 1 shows an example of the order of addition of elements when a group-B element is added before and after the addition of the group-B element. In Fig. 1, a ₁ and a ₂ denote a group elements, and & cir & indicates the starting point of addition of each element. In addition, b ₁ to b ₄ represent b group elements, and ● represents the starting point of addition of each element.

In FIG. 1, elements are added in the order of b ₁ ? B ₂ ? A ₁ ? A ₂ ? B ₃ ? B ₄ , and the first element added to the a group element is added to the end of the a ₁ and a group elements B ₂ is an element to be added first, and b ₃ is an element to be added _first when a group element is added. As the t1 also have, in the b because the time of group A elements of starting the addition of the first addition of the start first element of a group A elements from the viewpoint of the element point, Figure 1, the addition of a ₁ from the addition of the starting point of time of b ₁ discloses The time to the start of the operation is t1. Also referred to as the t2 is, in the so a group element of the b group element of the time required in starting the addition of the first element time from the addition of the start of the last element, Figure 1, the addition of b ₃ from the addition of the start of a ₂ The time until the start time becomes t2.

In Fig. 1, when a group element is added at the same time, a ₁ and a ₂ are added at the same time. In this case, the start point of addition of the first element among the a group elements and the addition of the last element among the a group elements The starting point becomes the same.

The addition interval time of the a group element and the group b element will be described in more detail with specific examples.

First, as a first example, the case where Al → Ti → REM → Zr → Ca is added in that order will be described. In this case, the above-mentioned t1 is the time from the start of addition of Al (b group element to be added first) to the start of addition of REM (a group element to be added first) Refers to the time from the start of addition of Zr (a group element added last) to the start of addition of Ca (b group element added first to the remaining group b elements).

As a second example, the case where Al → Ti → REM and Zr are added simultaneously in the order of Ca → Ca will be described. In this case, the term t1 refers to the time from the start of addition of Al (the first group element to be added) to the start of the addition of REM and Zr, and the term t2 refers to the start time of addition of REM and Zr To the start of addition of Ca (b group element to be added to the remaining group b element at the beginning).

The total of t1 and t2 should be 3 minutes or more. By setting the total of t1 and t2 to 3 minutes or more, inclusions containing Al, Ca, and Ti in an appropriate amount including REM and Zr can be produced. The total of t1 and t2 is preferably 5 minutes or more, more preferably 7 minutes or more. The upper limit of the sum of t1 and t2 is not particularly limited, but if the time is too long, the productivity is lowered, so the upper limit is about 20 minutes.

In the case where the a group element is not added before adding the b group element, or when the a group element is not added after the group b element is added, t1 or t2 may be calculated as 0 minute. However, t1 = t2 = 0 is excluded.

For the order of addition of the a-group element and the b-group element, the group-b element may be added after the group-a element is added, and the a-group element may be added after the group-b element is added. It is also possible to add the group-B element, then the group-A element, and then the Group-B element. When the group b element is added both before and after the group a element is added, the kind and content of all the group b elements need to be controlled before and after the addition of the a group element. For example, a part of the group b element may be added, then the group a element may be added, then the remainder of the group b element may be added, or the same element may be added before and after the addition of the a group element.

The a-group element and the b-group element may be added simultaneously within the range satisfying the above requirement (b-2), or may be added separately.

When the addition conditions of REM and Zr are added so as to satisfy the requirement of (b-1) in adding the b-group element, the number ratio of the inclusions I can be controlled to 30% or more, HAZ toughness can be improved.

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

The molten steel obtained by adjusting the components in this manner may be subjected to a continuous casting by a conventional method to obtain a slab, followed by hot rolling according to a conventional method.

When the steel material of the present invention was kept at 1450 占 폚 for 5 seconds and then cooled for 800 seconds to 500 占 폚 for 400 seconds (heat input condition: 1450 占 폚 占 5 seconds, cooling time Tc = 400 seconds ), It is possible to secure an absorption energy (vE- ₄₀ ) of _-30 J or more at -40 ° C. Therefore, the steel material according to the present invention can be used as a material for structures such as bridges, high-rise buildings, ships, etc., and can be used not only for small to medium heat input welding but also for large heat input welding with heat input amount of 50 kJ / Deterioration of the toughness of the heat affected zone can be prevented. The steel material of the present invention is intended for a steel sheet or the like having a thickness of about 3.0 mm or more.

The present invention will now be described in more detail with reference to the following examples. However, it is needless to say that the present invention is not limited by the following examples, and it is of course possible to carry out the present invention by appropriately changing the above- All of which are included in the technical scope of the present invention.

Example

Using a vacuum melting furnace (capacity: 150 kg), the steel having the composition (mass%) shown in the following Tables 3 and 4 (iron and inevitable impurities in the remaining part) was dissolved under the conditions shown in Tables 1 and 2 below , And cast into a 150 kg ingot and cooled. Thereafter, the steel sheet was heated and rolled to produce a steel sheet. It is also confirmed that the total O content of the disclosed steel satisfying the requirements specified in the present invention among the steel shown in Tables 3 and 4 is in the range of 0.0005 to 0.010%.

In the case of dissolving the above described steel in the vacuum melting furnace, components are adjusted for elements other than Ti, Zr, REM and Ca, and at the same time, deoxidation is performed using at least one element selected from C, Si, The dissolved oxygen amount _QOf was adjusted. Table 1 shows the dissolved oxygen amount Q _Of after the adjustment.

After adding Ti to the molten steel in which the dissolved oxygen amount Q _Of was adjusted, Zr and REM were added, and then Ca or Ca and Al were added. The order of addition of Zr and REM is shown in Tables 1 and 2 below. In this case, when Zr is added after REM is added, or when REM is added after addition of Zr, the necessary time (addition interval time) from the addition of one element to the addition of the other element is Are shown in Table 1 and Table 2 below. The sum (t1 + t2) of addition time intervals of the a group elements (REM and Zr) and the b group elements (Ti, Ca and Al) are shown in Tables 1 and 2 below.

The addition amount of _REM is represented by Q _REM , and these values are shown in Tables 1 and 2 below. The Z values calculated by substituting the values of the dissolved oxygen amount Q _Of and REM addition amount Q _REM into the following equation (1) are also shown in Tables 1 and 2 below.

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 Al were added in the form of pure Al, respectively. However, in No. 12 of Table 3 and No. 39 and 41 of Table 4 below, only Ce, not the form of mischmetal, was added.

After the above elements were added, they were cast into an ingot and cooled. The obtained ingot was hot-rolled to prepare a steel sheet having a thickness of 30 to 80 mm. A sample was cut from a cross section at a position of t / 4 (where t is the thickness of the steel sheet) of the steel sheet obtained, and the composition of all the oxide inclusions contained in the sample was measured. The average composition was calculated.

The composition of all the oxide inclusions was measured in the following order. The cut surface of the sample was observed using an electron beam microprobe X-ray spectrometer (EPMA: "JXA-8500F (apparatus name)") manufactured by Nippon Denshi Datum Inc. and the composition of the inclusions having a circle equivalent diameter of 0.1 μm or more was quantitatively analyzed . The observation conditions were quantitative analysis of the composition of the inclusions at the center of the inclusions by the wavelength dispersive spectroscopy of the characteristic X-rays, with an acceleration voltage of 20 kV, a sample current of 0.01 μA and an analysis number of 100 or more. 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, Zr, La, Ce, Ca and O , And the amount of the element contained in the inclusion was determined from the X-ray intensity obtained from the inclusion to be analyzed and the calibration curve.

Among the obtained quantitative results, inclusions having an oxygen content of 5 mass% or more were regarded as oxides. At this time, when a plurality of elements were observed from one inclusion, the composition of the oxide was calculated on the basis of the ratio of the X-ray intensity indicating the presence of the elements to a single oxide of each element in terms of mass. In the present invention, the average composition in terms of mass of the oxide as the sole oxide was taken as the average composition of the oxide. The average compositions of the oxides, ZrO ₂ , REM oxides and CaO are shown in Tables 5 and 6 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 , or MO _{2 in} the steel, but all oxides are converted into M ₂ O ₃ to calculate the composition. The "others" shown in Tables 5 and 6 are oxides of ZrO ₂ , REM, and oxides other than CaO (for example, Al ₂ O ₃ , MnO, SiO _2, etc.).

Next, the quantified inclusions were measured by SEM observation, and the number of inclusions having a circle equivalent diameter (particle diameter) of 0.1 to 2.0 占 퐉 was measured. Table 5 and Table 6 below show the number of the measurement results converted into the observed field area per 1 mm 2.

The circle-equivalent diameter of the oxide having an oxygen content of 5 mass% or more in the obtained quantitative results was measured by SEM observation, and the number of oxides with a circle-equivalent diameter (particle diameter) exceeding 3 占 퐉 and the circle- The number of oxides exceeding 5 탆 was measured. In Table 5 and Table 6, the number of oxides is converted into 1 square millimeter of observation field area.

Fig. 2 shows the relationship between the Z value and the number of oxides having a circle equivalent diameter of more than 3 占 퐉 per 1 mm 2 of the observation field area. 2 shows the results of the results of Nos. 1 to 32 shown in Tables 5 and 6 (shown in Fig. 2) and the results of Nos. 35 to 40, 53, 54, and 61 , The Z value was plotted in the range of -12.50 to -11.50.

As is clear from Fig. 2, it can be seen that, when REM is added so as to satisfy the above formula (1) according to the dissolved oxygen amount Q _Of of molten steel, generation of oxides having a circle equivalent diameter exceeding 3 탆 is suppressed.

Next, for the inclusions containing REM and Zr among the quantified inclusions, the molar ratio of REM and Zr was calculated, and for all the inclusions, REM and Zr-containing inclusions satisfying the range of REM / Zr ratio of 0.6 to 1.4 (Number ratio of the inclusions I) was calculated, and the results are shown in Tables 5 and 6. < tb > < TABLE > (REM + Zr) / (Al + Ca + Ti) of the total number of moles of REM and Zr and the total number of moles of Al, Ca and Ti is calculated for inclusions containing REM, Zr, Ti, Ca and Al among the quantified inclusions (Number ratio of inclusions II) of the inclusions II containing REM, Zr, Al, Ca and Ti satisfying the range of (REM + Zr) / (Al + Ca + Ti) ratio of 0.5 to 1.2 is calculated for each number of inclusions, The results are shown in Tables 5 and 6 below.

Next, in order to evaluate the toughness of the HAZ subjected to the heat at the time of welding, the weld recreation test shown below was carried out by simulating the substitution heat welding. In the welding reproducibility test, a sample cut from the t / 4 position (t, plate thickness) of the steel sheet was heated so as to be 1450 DEG C, held at this temperature for 5 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 400 seconds (heat input condition: 1450 ° C × 5 seconds, cooling time Tc = 400 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, those having an average value of vE- ₄₀ of 130 J or more are regarded as acceptable (good HAZ toughness). The measurement results are shown in Tables 5 and 6 below.

From the following Tables 1 to 6, it can be considered as follows. Nos. 1 to 32 satisfy the requirements specified in the present invention. When the composition of all the oxide inclusions contained in the steel is measured and converted into a single oxide, the oxide of ZrO ₂ , REM and CaO are mixed in a predetermined amount , It is possible to suppress generation of oxides having a circle-equivalent diameter of more than 3 占 퐉 and oxides having a circle-equivalent diameter of more than 5 占 퐉 and to produce many inclusions having a circle-equivalent diameter of 0.1 to 2 占 퐉, The number of the inclusions I is 30% or more, and / or the ratio of the number of the inclusions II is 40% or more with respect to the number of inclusions. Thus, a steel having good HAZ toughness is obtained. Further, it is understood that the higher the Si content, the better the HAZ toughness.

On the other hand, Nos. 33 to 64 are examples deviating from any one of the requirements specified in the present invention. Of these, No. 33, No. 51, No. 52, No. 55 to No. 58, No. 62, and No. 64, the time until addition of Zr is added after REM is added, and No. 34 and No. 60 are added after adding Zr and then adding REM , The number ratio of the inclusions I is less than 30%, since the time until the inclusion I does not satisfy the requirements specified in the present invention. Therefore, the HAZ toughness is deteriorated.

Nos. 35 to 40, 53, 54, and 61, the balance of the dissolved oxygen amount Q _Of of the molten steel and the addition amount Q _REM of the _REM does not satisfy the above formula (1) In particular, an oxide having a circle-equivalent diameter of more than 3 mu m and not more than 5 mu m). Therefore, the HAZ toughness is deteriorated. No.41 indicates that the oxide content of REM when the composition of all the oxide inclusions included in the steel is measured and converted into mass by the single oxide is less than the range specified in the present invention, The amount of the oxide is insufficient, and the HAZ toughness is deteriorated.

Nos. 42 and 59, when the amount of REM contained in the steel is large and the composition of all the oxide inclusions contained in the steel is measured, and the amount of oxide of REM when the mass of the oxide is mass-converted into the single oxide exceeds the range specified by the present invention Therefore, the oxide is coarsened, and the number of fine oxides that act as nuclei of the? -Form transformation in the grain is decreased, so that the HAZ toughness improving action is not exerted. In No. 43, since the amount of Zr contained in the steel material is excessively small, the amount of ZrO ₂ in the composition of all the oxide inclusions is reduced, and the amount of the Zr · REM · Ca-based oxide which becomes the nucleus of the in-gauge α-transformation is considered to be small. HAZ toughness is deteriorated thereby. In Nos. 44 and 63, since the amount of Zr contained in the steel material was excessively large, the amount of ZrO ₂ in the composition of all the oxides was increased. As a result, the amount of oxide which becomes the nucleus of the? Transformation at the time of welding is insufficient, the microstructure is not obtained, and the HAZ toughness is deteriorated.

In No. 45, since the amount of Ca contained in the steel is excessively large, the amount of CaO in the composition of all oxide inclusions is large. As a result, the amount of oxide which becomes the nucleus of the? Transformation at the time of welding is insufficient, the microstructure is not obtained, and the HAZ toughness is deteriorated. In No. 46, since the amount of Ca contained in the steel material is too small, the amount of CaO is not generated. As a result, the amount of Zr, REM, Ca-based oxide which is the nucleus of the? -Form transformation in the grain decreases and HAZ toughness is deteriorated. In No. 47, since the amount of Ti contained in the steel material is excessively large, the base material is solid-solved by the employment of Ti, resulting in deterioration of HAZ toughness. In No. 48, since the amount of Ti contained in the steel is too small, the amount of inclusions having a circle equivalent diameter of 0.1 to 2 탆 serving as nuclei of the in-gauge alpha transformation can not be ensured. Therefore, the HAZ toughness is deteriorated. In No. 49, 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. In No. 50, it is considered that the amount of N contained in the steel is too many, and the amount of solute N contained in the steel is excessive, and the HAZ toughness is considered to be low.

Next, Fig. 3 shows the relationship between the number of oxide per square meter of observation area and the absorbed energy (vE- ₄₀ ) at -40 DEG C of oxides having a circle-equivalent diameter exceeding 3 mu m. 3, the results of Nos. 1 to 32 shown in the following Tables 5 and 6 are denoted by o, and the results of Nos. 35 to 40, 49, 53, 54 and 61 (exceeding 5.0 of the comparative example) ●.

As is apparent from Fig. 3, when the number of oxide per square centimeters of the observation field area per square millimeter of the oxide having a circle-equivalent diameter exceeding 3 占 퐉 is 5.0 or less, it can be seen that even when heated and held at 1450 占 폚 for 5 seconds, good HAZ toughness is exhibited .

Next, consideration will be made by employing No. 2 shown in Table 5 and No. 33 shown in Table 6 below. These steel materials are almost the same except that the number ratio of the inclusions I to the number of all inclusions is different. (The number ratio of the inclusions II to all the inclusions is less than 40%.) When the number ratio of the inclusions I is less than 30% with respect to the number of all inclusions (No. 33), vE- ₄₀ is less than 130 J, while when the number of inclusions is 30% .2), it is found that the vE- ₄₀ becomes 130 J or more, and HAZ toughness can be improved. Similar results were obtained for No. 23 shown in Table 5 and No. 62 shown in Table 6 below. That is, in the case where the number ratio of the inclusions I was 30% or more with respect to the number of all inclusions (No. 23), vE- ₄₀ was 130 J or more, and HAZ toughness could be improved.

Next, Nos. 17 and 51 shown in Table 5 and Table 6 below will be employed. These steel materials are almost the same except that the number ratio of the inclusions II to the number of all the inclusions is different (all the inclusions I to the total number of inclusions are less than 30%). In the case where the number ratio of the inclusions II is less than 40% with respect to the number of all inclusions (No. 51), vE- ₄₀ is less than 130 J, while when the number of inclusions is 40% .17), it can be seen that the vE- ₄₀ becomes 130 J or more, and the HAZ toughness can be improved. The same results are also obtained for No. 26 shown in Table 5 and No. 64 shown in Table 6 below. That is, in the case where the number ratio of the inclusions II was 40% or more with respect to the number of all inclusions (No. 26), vE- ₄₀ was 130 J or more and HAZ toughness could be improved.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention.

The present application is based on Japanese Patent Application (Japanese Patent Application No. 2011-008258) filed on January 18, 2011, and Japanese Patent Application (Japanese Patent Application No. 2011-206542) filed on September 21, 2011 , The contents of which are incorporated herein by reference.

The steel material of the present invention is excellent in HAZ toughness even when it is subjected to large heat welding, and can be used for bridges, high-rise buildings, ships and the like.

Claims (6)

C: 0.02 to 0.15% (meaning% by mass, the same applies to the following components),
Si: not more than 0.5% (not including 0%),
Mn: not more than 2.5% (not including 0%),
P: not more than 0.03% (not including 0%),
S: not more than 0.02% (not including 0%),
Al: not more than 0.050% (not including 0%),
N: 0.010% or less (not including 0%),
Ti: 0.005 to 0.10%
Zr: 0.0005 to 0.050%
REM: 0.0003 to 0.015%
Ca: 0.0003 to 0.010%, and
O: 0.0005 to 0.010%
The balance being iron and inevitable impurities,
(a) when the composition of all the oxide inclusions contained in the steel is measured and mass-converted into a single oxide,
ZrO ₂ : 5 to 50%
Oxide of REM (M ₂ O ₃ if REM is represented by the symbol of M): 5 to 50%, and
CaO: not more than 50% (not including 0%),
(b) of all the inclusions contained in the steel,
The number of inclusions having a circle equivalent diameter of 0.1 to 2 占 퐉 is 120 or more per 1 mm2 of observation field area,
An oxide having a circle-equivalent diameter of more than 3 占 퐉 and not more than 5 占 퐉 is not more than 5.0 per 1 mm2 of observation field area,
An oxide having a circle-equivalent diameter exceeding 5 占 퐉 is 5.0 or less per 1 mm2 of observation field area,
(c-1) the number of REM and Zr-containing inclusions I satisfying the molar ratio (REM / Zr) of REM / Zr of 0.6 to 1.4 with respect to the number of all inclusions when the composition of all inclusions contained in the above- A ratio of 30% or more,
(REM + Zr) / (Al + Ca + Ti) / (Ca + Ti) / (Ca + Ti) ratio of the total molar number of REM and Zr and the total molar number of Al, Ca and Ti with respect to the number of all inclusions, when the composition of all the inclusions contained in the above- ) Satisfies at least one of (c-1) and (c-2) when the ratio of the numbers of REM, Zr, Al, Ca and Ti containing inclusions II satisfying 0.5 to 1.2 is 40% Wherein the welded heat affected zone has an excellent toughness. The steel material according to claim 1, wherein the steel material includes at least one of the following groups (a) to (d) as other elements.
(a) at least one selected from the group consisting of not more than 2% of Cu (not including 0%) and not more than 3.5% of Ni (not including 0%),
(b) at least one selected from the group consisting of Cr: not more than 3% (excluding 0%) and Mo: not more than 1% (not including 0%),
(c) at least one selected from the group consisting of not more than 0.25% of Nb (not including 0%) and not more than 0.1% of V (not including 0%),
(d) B: 0.005% or less (not including 0%). A method for manufacturing the steel material according to any one of claims 1 to 3,
According to the addition of REM to molten steel in a dissolved oxygen amount Q _Of adjusted to a range of 0.0003~0.01% by weight and the amount of REM that to the amount of dissolved oxygen _Of Q and Q REM addition amount of _REM in the molten steel satisfy the following expressions (1) At the same time,
When REM, Zr, Ti, Ca, and Al are added to molten steel in which the dissolved oxygen amount Q _Of is adjusted to the above range, when REM and Zr are taken as a group element, Ti, Ca, , And the addition condition of each element satisfies at least one of the following (2) and (3): (1) A method for producing a steel material excellent in toughness of a weld heat affected zone.

(Where Q _REM is a value in mass% of REM)
(2) REM and Zr are simultaneously added to the a group element, or the other element is added within 5 minutes after addition of one element of REM and Zr.
(3) When the group b element is added before or after the addition of the a group element, or both before and after the addition of the group a element, and when the group b element is added before the addition of the a group element, The time from the start of the addition of the first element to the start of the addition of the first element among the elements of the group a is t1 (minutes), and when the element b is added after the addition of the element a, the time from the start of addition of the last element among the group a elements to the start of the addition of the first element among the group b elements is t 2 (min), and the total of t 1 and t 2 is set to 3 minutes or more t1, 0? t2, where t1 and t2 are not 0). delete delete delete

Images