KR20110007050A - Steel having excellent toughness in welding heat affected zone, and manufacturing method thereof - Google Patents

Abstract

PURPOSE: A steel material with a superior toughness of a weld heat-affected zone and a manufacturing method thereof are provided to improve HAZ toughness, even if the welding is performed with the heat input amount of 50kj/mm or greater. CONSTITUTION: A steel material with a superior toughness of a weld heat-affected zone comprises C 0.02~0.15 weight%, O 0.0005~0.010 weight%, Ti 0.005~0.10 weight%, REM 0.0003~0.015 weight%, Ca 0.0003~0.010 weight%, Zr 0.0010~0.050 weight%, N less than 0.010 weight%, Al less than 0.05 weight%, S less than 0.02 weight%, P less than 0.03 weight%, Mn less than 2.5 weight%, Si less than 0.5 weight%, and the impurity. The steel material is oxide including Zr, REM, and Ca.

Description

STEEL HAVING EXCELLENT TOUGHNESS IN WELDING HEAT AFFECTED ZONE, AND MANUFACTURING METHOD THEREOF}

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to steel materials used in bridges, high-rise buildings, ships, and the like. In particular, the present invention relates to excellent toughness of a portion (hereinafter, referred to as a "welding heat-affecting zone" or "HAZ") that is affected by heat when welded. A steel material and a manufacturing method thereof.

The characteristics required for steel materials used in bridges, high-rise buildings, ships, and the like have become increasingly strict in recent years, and particularly good toughness is required. Although these steel materials are generally welded and joined, there is a problem that, among the weld joints, in particular, HAZ is affected by heat during welding and tends to deteriorate toughness. This toughness deteriorates remarkably as the amount of heat input during welding increases, and the cause is that the cooling rate of HAZ decreases as the amount of heat input during welding increases, resulting in a decrease in hardenability and generation of coarse island-like martensite. It is becoming. Therefore, in order to improve the toughness of HAZ, it is thought that what is necessary is just to suppress the heat input at the time of welding. On the other hand, in order to improve the welding work efficiency, it is desired to employ a large heat input welding method having a welding heat input amount of 50 kJ / mm or more, for example, electro gas welding, electro slug welding, and submerging welding.

Accordingly, the present applicant proposes to Patent Documents 1 to 3 a steel material that suppresses the deterioration of the HAZ toughness when the high heat input welding method is adopted. These steels are characterized by containing oxides and / or CaO of REM and ZrO ₂ as oxides which become nuclei of ferrite transformation in particles. Since the said oxide exists in a liquid phase in molten steel, it is microdispersed in steel. In addition, the oxide is thermally stable and, for example, does not lose solid solution even after long exposure to high temperature of 1400 ° C. level, thus greatly contributing to the improvement of HAZ toughness.

On the other hand, Patent Documents 4 to 6 do not use a combination of REM oxide and ZrO ₂ in combination with Patent Documents 1 to 3, but when REM is added to molten steel in which dissolved oxygen is adjusted, about 300 kJ / cm (about 30 kJ / It is disclosed that the HAZ toughness when the high heat input welding exceeding mm) can be improved.

Japanese Patent Application Publication No. 2007-100213 Japanese Patent Application Publication No. 2007-247004 Japanese Patent Application Publication No. 2007-247005 Japanese Patent Application Publication No. 2003-221643 Japanese Patent Application Publication No. 2003-286540 Japanese Patent Application Publication No. 2002-363687

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to provide a steel material having excellent HAZ toughness and a method for producing the same, especially when a large heat input welding with a heat input amount of 50 kJ / mm or more is performed.

The steel material excellent in the toughness of the weld heat affected zone according to the present invention, which was able to solve the above problems, is C: 0.02 to 0.15% (mean of mass%. The same for the following components.), Si: 0.5% or less (0 %), Mn: 2.5% or less (does not contain 0%), P: 0.03% or less (does not contain 0%), S: 0.02% or less (does not contain 0%), Al : 0.05% or less (0% not included), Ti: 0.005 to 0.10%, REM: 0.0003 to 0.015%, Ca: 0.0003 to 0.010%, Zr: 0.0010 to 0.050%, N: 0.010% or less (0% Not included), O (oxygen): 0.0005 to 0.010%, the remainder being a steel material consisting of iron and unavoidable impurities,

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

(b) ZrO ₂ : 5 to 50%, an oxide of REM (meaning M in the symbol M ₂ O ₃ ) as the average composition when the composition of all the oxides contained in the steel is measured and converted into a single oxide. 5 to 50%, CaO: 50% or less (not including 0%), and further

(c) Of the total inclusions included in the steel, at least 120 inclusions having an equivalent diameter of 0.1 to 2 µm per observation field area and 120 or more oxides having a diameter of more than 3 µm having a circular equivalent diameter of 5.0 per 1 mm 2 observation area. It has the point that the oxide of more than 5 micrometers in a circular equivalent diameter is 5.0 or less per 1 mm <2> observation field area.

Inclusions contained in the steel may further contain Al ₂ O ₃ , and when the composition of the inclusions is measured and converted into a single oxide, the total inclusions are

(d) for the Al ₂ O _3, or the number ratio of inclusions of a ratio of Al ₂ O ₃ satisfies the less than 20% by mass is more than 90%, or,

(e) it is preferred that for the Al ₂ O ₃ and CaO, the number ratio of the inclusions satisfying the mass ratio (CaO / Al ₂ O ₃₎ is 0.35 excess of CaO to Al ₂ O ₃ exceeds 80%.

Incidentally, the inclusions contained in the steel may further contain Al ₂ O ₃ , and when the composition of the inclusions is measured and converted into single oxides,

(d) for the Al ₂ O _3, and the number ratio of inclusions of a ratio of Al ₂ O ₃ satisfies the less than 20% by weight exceeds 90%, and,

(e) it is preferred that for the Al ₂ O ₃ and CaO, the number ratio of the inclusions satisfying the mass ratio (CaO / Al ₂ O ₃₎ is 0.35 excess of CaO to Al ₂ O ₃ exceeds 80%.

The steel is another element,

(1) Cu: 2% or less (does not contain 0%) and / or Ni: 3.5% or less (does not contain 0%),

(2) Cr: 3% or less (without 0%) and / or Mo: 1% or less (without 0%),

(3) Nb: 0.25% or less (without 0%) and / or V: 0.1% or less (without 0%),

(4) B: 0.005% or less (does not contain 0%)

You may further contain elements, such as these.

In the steel of the present invention, in the case of adding REM to molten steel in which the dissolved oxygen amount Q _Of of molten steel is adjusted to a range of 0.001 to 0.01 mass%, the dissolved oxygen amount Q _{O f} of the molten steel and the amount of addition of REM _REM ) can be manufactured by adding the amount of REM which satisfy | _fills following formula (1).

[Equation 1]

2logQ _REM + 3logQ _Of ≤ -12.00

According to the present invention, while a predetermined amount of oxides (oxides containing Zr, REM and Ca), which are nuclei of ferrite transformation in particles, is generated, the size and number of inclusions and oxides present in steel materials (ie, particle size distribution) Since it is also appropriately controlled, it is possible to provide a steel having excellent HAZ toughness at the time of high heat input welding. Particularly, in the steel of the present invention, a crude material having a circular equivalent diameter of more than 3 µm that is apparently not only present a predetermined amount of fine inclusions having a circular equivalent diameter of 0.1 to 2 µm useful for improving the HAZ toughness but also adversely affects the improvement of HAZ toughness. Since the number of both the oxide and the coarse oxide having a circular equivalent diameter of more than 5 µm is significantly suppressed, even when welding is performed with a heat input larger than that of the HAZ toughness evaluation method disclosed in the Example of Patent Document 1, HAZ toughness Can increase.

In addition, according to the present invention, since the number ratio of the predetermined inclusions is preferably controlled appropriately, the HAZ toughness can be further improved.

BRIEF DESCRIPTION OF THE DRAWINGS The graph which shows the relationship between the value (Z value) of the left side of Formula 1 prescribed | regulated by this invention, and the number per square meter viewing area of an oxide whose circular equivalent diameter exceeds 3 micrometers.
Fig. 2 is a graph showing the relationship between the number of oxides per observation field area of 1 mm 2 and the absorption energy (vE- ₄₀ ) at −40 ° C. of an oxide having a circular equivalent diameter exceeding 3 μm.
3 is a graph showing the composition of components of individual inclusions contained in steel materials (number 1 in Table 1) used in Examples.
FIG. 4 is a graph showing the relationship between the number of oxides having an equivalent diameter of more than 3 µm per observation field area 1 mm 2 and absorbed energy (vE- ₄₀ ) at −40 ° C., and part of the data shown in FIG. 2. Excerpt from the graph.

The present invention relates to a technique for obtaining a steel material in which the HAZ toughness does not deteriorate even when welding is performed at a larger heat input amount by improving the technique using an oxide that is the nucleus of the ferrite transformation in the particles disclosed in Patent Document 1.

In other words, the inventors of the present invention have continued to provide a steel material excellent in the HAZ toughness at the time of high heat input welding even after the disclosure of Patent Document 1, and as a result, the Japanese Patent Application No. 2008-39335 The invention described was first proposed (hereinafter referred to as a sailor invention). In the source invention, the size and number of all oxides in the steel material (not limited to oxides that become the nucleus of ferrite transformation in the particles, but all oxides) are deeply involved in the improvement of HAZ toughness. When the coarse oxide exceeding 5 micrometers is reduced to 5 or less, it is disclosed that the steel material excellent in HAZ toughness is obtained even if the heat input welding of about 50 kJ / mm is performed. Thus, according to the source invention, since the number of coarse oxides is remarkably suppressed, HAZ toughness can be improved even if welding is performed with a heat input larger than HAZ toughness evaluation method disclosed in the Example of the said patent document 1. That is, in the said patent document 1, after maintaining for 5 second at the heating temperature of 1400 degreeC, the heat cycle which cools the temperature from 800 degreeC to 500 degreeC to 300 second (heating conditions: 1400 degreeC x 5 second, cooling time Tc = 300 second) ), And the absorbed energy (vE- ₄₀ ) at -40 ° C was measured, but in the source invention, a heat cycle (heating condition: 1400 ° C x 30 seconds, cooling) in which the holding time of 1400 ° C was extended to 30 seconds was used. The absorption energy at the time Tc = 300 seconds) was measured in the same manner as described above, and it was confirmed that good HAZ toughness was obtained even in this case.

The inventors of the present invention have conducted research to provide a steel material excellent in HAZ toughness at a higher level of high heat input welding even after the above-described source invention has been proposed. As a result, "heat cycle which cools the temperature from 800 degreeC to 500 degreeC to 400 second after hold | maintaining for 5 seconds at the heating temperature of 1450 degreeC which is a condition of a large heat input more than a source invention" (heating condition: 1450 degreeC x 5 second, Even if the cooling time Tc = 400 seconds), in order to provide a steel having excellent HAZ toughness, it is not enough to reduce the oxides of more than 5.0 µm to 5 or less in the original equivalent diameter as in the source invention, and thus, the source invention The inventors found that it was extremely important to reduce the number of oxides exceeding 3.0 µm, which had not been previously conceived at all, and completed the present invention.

Thus, the feature part of the present invention,

(A) At the same time increasing the number of fine inclusions with a diameter of 0.1 to 2 μm (120 pieces / mm 2 or more) useful for improving HAZ toughness,

(B) reducing the number of oxides having a diameter larger than 5 µm equivalent to the HAZ toughness improvement (5.0 / mm 2 or less),

(C) By reducing the number of oxides having a diameter larger than 3 µm in diameter (5.0 / mm 2 or less) for the first time, it is apparent that the adverse effect on the HAZ toughness improvement in the present invention is reduced (5.0 / mm 2 or less), so that welding is performed at a larger heat input amount than the source invention. Even if it does, HAZ toughness was improved. That is, in the relationship with the source invention, the feature part of this invention exists in the point which defined said (C) in addition to said (A) and said (B).

Strictly speaking, the provisions of (A) are different from those of the source invention, and in the invention of the source, the minute number of the oxide is controlled in the present invention, whereas in the present invention, not only the oxide but also all of the steel present in the steel material. It differs in that it controls the fine number in the said interference | inclusion with the inclusion object. According to the results of the present inventors, in order to realize good HAZ toughness, in particular, an oxide having a large equivalent diameter (hereinafter sometimes simply abbreviated as "particle diameter") having a large oxide (more than 3 µm and more than 5 µm in the present invention). It is evident that the contribution of both of the oxides is very large. If the large oxide is controlled so as not to be produced, the inclusions having a particle size of 0.1 to 2 µm, which are to be controlled, are not limited to oxides, and the desired characteristics can be secured even if they are expanded to all the inclusions.

Moreover, in order to provide the requirement of said (C), it is inadequate only to control the amount of dissolved oxygen in molten steel before REM addition, like source invention or the above-mentioned patent documents 4-6, and the amount of dissolved oxygen in the molten steel (Q _Of It was also found that it is extremely important to properly control the amount of REM added (Q _REM ). In detail, according to the dissolved oxygen amount Q _Of of molten steel before REM addition, REM (Q _REM ) of the quantity which satisfy | fills following formula 1 is added. Thereby, generation | occurrence | production of REM type oxide with a large particle diameter which is indispensable for realization of desired HAZ toughness can be suppressed. Details, such as the technical significance of the following formula 1 will be described later.

[Equation 1]

2logQ _REM + 3logQ _Of ≤ -12.00

In the present specification, in order to distinguish between oxides which become nuclei of ferrite transformation in particles, that is, oxides containing Zr, REM and Ca, and all oxides contained in steel, the former is particularly referred to as “Zr, REM, Ca”. System oxide ”, and the latter may be referred to as“ all oxides ”in particular. In addition, the oxide also includes a complex oxide in which inclusions other than oxides (for example, sulfides, nitrides, carbides, or complex compounds thereof) are complexed in addition to the oxides alone.

In addition, the essential components (Zr, REM, and Ca) constituting the above-described Zr-REM-Ca-based oxide are sometimes referred to as "in-particle ferrite transformation nucleation elements".

In addition, the steel of the present invention includes sulfides, nitrides, carbides, composite compounds thereof, and the like, in addition to the above-described oxides, but in this specification, oxides, sulfides, nitrides, carbides, composite compounds, and the like contained in the steels are collectively referred to. It is called "the whole inclusion."

In addition, in this specification, among all the oxides contained in steel materials, an oxide having a circular equivalent diameter of 0.1 to 2 µm is a "fine oxide", an oxide having a circular equivalent diameter of more than 3 µm is "coarse oxide", and a circular equivalent diameter is 5 Oxides larger than µm are referred to as "super coarse oxides", and these may be distinguished from each other. Further, in the source invention, an oxide having a larger equivalent diameter than 5 µm is defined as "coarse oxide", but in this specification, an oxide having a larger equivalent diameter than 3 µm is referred to as "coarse oxide".

In this specification, the term "steel material excellent in HAZ toughness of high heat input welding" refers to a heat cycle (thermal history) in which the temperature from 800 ° C to 500 ° C is cooled to 400 seconds after holding at 1450 ° C for 5 seconds with respect to the steel material. ), It means that the absorption energy (vE- ₄₀ ) at -40 ° C satisfies 100 J or more. This vE- ₄₀ is so large that it is good, Preferably vE- ₄₀ is 130J or more. The heat cycle described above may be referred to as "high heat input heat history" in particular. The heat input amount by this heat cycle is high compared with the heat input amount by the heat cycle of a source invention or patent document 1, In that sense, "large heat input welding" of this invention and " Heat input welding ”is different.

In the present invention, the temperature of the heat cycle is set to 1450 ° C. In the heat cycle temperature (1400 ° C) described in the source invention, the heat temperature of a part (sometimes called a bond part) in the HAZ is particularly close to the weld metal. Is considered to be about 1450 ° C. above 1400 ° C.

EMBODIMENT OF THE INVENTION Hereinafter, the requirements of said (a)-(c) which comprise this invention are demonstrated in detail.

[(a) About Zr, REM, and Ca-based Oxides]

First, the Zr.REM.Ca-based oxide serving as a starting point of ferrite transformation in particles will be described. The Zr-REM-Ca-based oxide means that the oxide of Zr, the oxide of REM, and the oxide of Ca must be included. The elements constituting the Zr-REM-Ca-based oxides (ferrite-transformed nucleation elements in the particles) are Zr, REM, and Ca. Other than these, oxide-forming elements such as Ti, Mn, Si, and Al, and others It may contain the component in steel.

The presence form of the Zr-REM-Ca-based oxide is not particularly limited, and may be present as a single oxide containing ferrite-transformed nucleation elements in the particles alone, and includes two or more kinds of ferrite-transformed nucleation elements in the particles. It may exist as a composite oxide to make. Examples of the oxide alone, the Zr ZrO _2; Ca is CaO; When the REM REM nd that the symbols of "M", and the like include _{M 2 O 3, M 3 O} 5, MO 2. These oxides may be present after being aggregated with each other, and may be present in the form of complex precipitates of other compounds such as sulfides and nitrides in the oxides.

It is preferable that said Zr-REM-Ca type | system | group oxide further contains the oxide of Ti. The presence of further Ti oxide promotes ferrite transformation in the particles, further improving the HAZ toughness. Oxides of Ti is solely oxide may be present as (for _{example, Ti 2 O 3, Ti 3} O 5, TiO 2), may be present in the form of composite oxide with the Zr and REM and Ca-based oxide.

[(b) About Average Composition of Oxide]

The steel of the present invention has an average composition when the composition of all oxides contained in the steel is measured and mass converted as a single oxide (total 100%),

ZrO ₂ in 5 to 50%,

Oxide of REM (M ₂ O _{3 when} REM is represented by the symbol of M): 5 to 50%,

CaO: 50% or less (does not contain 0%)

It satisfies this, and it acts effectively as a nucleus of ferrite transformation in a particle | grain. If the lower limit of each oxide is lower, the amount of oxides that form nuclei of ferrite in the particles at the time of welding is insufficient, and the effect of improving HAZ toughness is not exhibited. On the other hand, when the upper limit of each oxide is exceeded, the oxide is coarsened, and the number of fine oxides that effectively act as a production nucleus of ferrite in the particles decreases, so that the HAZ toughness improving effect is not effectively exhibited.

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

The oxide of REM is 5% or more, preferably 10% or more, and more preferably 13% or more. On the other hand, an upper limit is 50%, a preferable upper limit is 45%, and a more preferable upper limit is 40%. In addition, the amount of time an oxide of REM are expressed to REM as symbol M, the steel material in the form of, such as _{M 2 O 3, M 3 O} 5, MO 2 , but all of the oxides of REM have in terms of M ₂ O ₃ it means.

The CaO acts effectively as a nucleus of the ferrite transformation in the particles, but when contained in excess, the ferrite transformation ability in the particles is deteriorated. Moreover, when CaO is contained excessively, the melt of a nozzle used at the time of casting will be caused, and therefore an upper limit shall be 50%. Preferably it is 45% or less, More preferably, it is 40% or less, Especially preferably, it is 30% or less. In order to exhibit the said effect effectively, it is preferable to contain CaO 3% or more. More preferably, it is 5% or more, More preferably, it is 10% or more.

In addition, there may be mentioned, without the other components of the composition of the total oxide is not particularly limited, and oxides of the oxide-forming element contained in the steel material of the present invention (for example, SiO ₂ or Al ₂ O _3, MnO, etc.).

The composition of all oxides contained in the steel is measured by observing the surface of the steel with, for example, an Electron Probe X-ray Micro Analyzer (EPMA) and quantitatively analyzing the oxides recognized within the observation field. do. The detail of a measurement condition is demonstrated in the column of the Example mentioned later.

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

Next, the number and size of the total inclusions which characterize the present invention will be described.

Steel of the present invention,

(i) 120 or more fine inclusions having a diameter equivalent to 0.1 to 2 탆 per 1 mm 2 of the viewing field area,

(ii) coarse oxides having a diameter larger than 3 μm in a circular equivalent diameter of 5.0 or less per 1 mm 2 of observation field area;

(iii) Coarse oxides exceeding 5 micrometers in circular equivalent diameter satisfy all 5.0 or less per 1 mm <2> observation field area.

In particular, in the present invention, the greatest feature lies in the point that both of (ii) and (iii) are defined for an oxide having a large equivalent diameter (particle diameter).

Here, satisfying both the requirements of (ii) and (iii) means that the number of oxides having a particle diameter of more than 3 µm and 5 µm or less is less than 5.0. That is, even in the case of receiving a high heat input history according to the present invention, in order to ensure very high HAZ toughness with vE- ₄₀ ≥ 100J, reduction of oxide having a particle size of more than 3 µm and 5 µm or less, which was not conceived in the source invention at all. It is extremely important, and when the number of oxides in this range cannot be controlled, it has been clarified for the first time by the present inventors that the oxides become a starting point of brittle fracture and the HAZ toughness deteriorates.

Hereinafter, the technical meaning of said (ii) and said (iii) is demonstrated in detail, referring an Example.

First, see Table 4. Numbers 1 to 16 are examples that satisfy all of the requirements defined in the present invention. In consideration of the above (ii) and (iii), all of the numbers 1 to 16 in Table 4 are substantially 0 (up to 0.95) of oxides larger than 5 µm, and 0.85 of oxides larger than 3 µm. 4 to 4.93, and both of (ii) and (iii) are satisfied. As a result, good HAZ toughness can be secured.

On the other hand, the number 17-20 of Table 4 is an example which satisfy | fills the requirements of said (iii), but does not satisfy the requirements of said (ii). Specifically, oxides larger than 5 μm are suppressed to about 0.03 to 1.2 and 5.0 or less, while oxides larger than 3 μm are increased to about 5.2 to 8.4 by more than 5.0, resulting in HAZ toughness. This is falling.

Here, the numbers 17 to 20 are included in the scope of the source invention in that they satisfy the requirements of (iii) above. However, the numbers 17 to 20 do not satisfy the requirements of (ii) above even if they fall within the scope of the source invention. It can be seen that the desired HAZ toughness specified in the present invention cannot be achieved.

Therefore, in this invention, in addition to said (iii), said (ii) is further prescribed | required as a requirement for ensuring desired HAZ toughness.

In addition, from (ii) and (iii), it can be seen that the number of oxides more than 3 µm and 5 µm or less is particularly involved in achieving desired HAZ toughness. That is, depending on the production conditions, more than 5.0 oxides may be present in an extremely narrow range of more than 3 µm and less than or equal to 5 µm, but for example, the number of fine regions of the above (i) is increased to a large number. Even if the number of ultra-light zones was reduced, more than 5.0 oxides were present in the coarse areas of more than 3 µm and not more than 5 µm, and it was unexpected finding for the present inventors that desired HAZ toughness was not obtained.

Although the detailed mechanism is not clear about how the desired HAZ toughness can be secured by satisfying both the above (ii) and (iii), the loss of TiN proceeds rapidly when the temperature exceeds 1400 ° C to 1450 ° C. Toughness falls. However, it is thought that such a fall in toughness can be reduced by reducing the oxide of more than 3 micrometers and 5 micrometers or less.

As described above, in the present invention, it is necessary to simultaneously satisfy the requirements of (ii) and (iii). That is, the number of coarse oxides whose particle diameter is more than 3 micrometers is 5.0 or less, and the number of coarse oxides whose particle diameter is more than 5 micrometers is 5.0 or less. The fewer these numbers are, the better they are, and in any case, it is preferably 3 or less, more preferably 1 or less, and most preferably substantially 0. Specifically, it is preferable to include the balance of both, and to control them appropriately, and within the scope of the present invention (all 5.0 or less), the super coarse particles having a particle diameter of more than 5 μm are larger than coarse oxide having a particle size of more than 3 μm. It is desirable to reduce the number of oxides. Specifically, the number of coarse oxides is better to be closer to the lower limit (0), and generally 1 or less is preferable, and the number of coarse oxides is the upper limit (5.0) ) May be used, and about 4 or less may be preferably used.

In addition, the number of oxides exceeding 3 micrometers and the number of oxides exceeding 5 micrometers by a circular equivalent diameter can be made into the cross section of steel materials, for example with the Electron Probe X-ray Micro Analyzer (EPMA). Observing and quantitatively analyzing the composition of the inclusions recognized in the observation field, and using the inclusions having an oxygen content of 5% or more as oxides, and observing and measuring the original equivalent diameter of the oxides by, for example, a transmission electron microscope (TEM) Get it.

In the above, the above (ii) and said (iii) which characterize this invention were described in detail.

In the steel materials of the present invention, as defined in the above (i), it is necessary to set the fine inclusions having a diameter of 0.1 to 2 µm to 120 or more per square meter of observation field area. The number of fine inclusions is 120 or more per 1 mm <2> observation field area, Preferably it is 200 or more per 1mm <2>, More preferably, it is 500 or more per 1mm <2>, More preferably, it is 1000 or more per 1mm <2>.

In addition, the number of the fine inclusions of 0.1-2 micrometers in circular equivalent diameter may be calculated | required by measuring the cross section of steel materials, for example by observing with a transmission electron microscope (TEM).

In the steel of the present invention, inclusions having a diameter of less than 0.1 μm in a circular equivalent diameter hardly contribute to the HAZ toughness improving effect due to inclusion dispersion, and are not included in the number of inclusions.

Said "circle equivalent diameter" is a diameter of the circle assumed so that the area of an inclusion (including an oxide) may become equal, and is recognized on a transmission electron microscope (TEM) observation surface.

In the above, the requirements of said (a)-(c) among the characteristic part of this invention were demonstrated.

It is preferable that the interference | inclusion contained in the steel materials of this invention satisfy | fills the requirements of following (d) and / or following (e). That is, when the composition of the inclusion is measured and converted into a single oxide, the total number of inclusions is

(d) for the Al ₂ O _3, or the number ratio of inclusions (hereinafter, referred to as inclusions I) that the ratio of Al ₂ O ₃ satisfies the less than 20% by mass is more than 90%, and / or ,

(e) the number ratio of about Al ₂ O ₃ and CaO, inclusions (in the case called hereinafter inclusions II) satisfying the mass ratio (CaO / Al ₂ O ₃₎ is 0.35 excess of CaO to Al ₂ O ₃ It is preferable to exceed this 80%, and HAZ toughness will become higher by this. The requirements of (d) and (e) above specify the inclusion composition and the number ratio for realizing higher HAZ toughness based on the results of the examples described later.

That is, as will be clear from the later examples, it is found that even if coarse oxides having a diameter of more than 3 µm in circular equivalents exist in approximately the same number per 1 mm 2 of observation field area, variations occur in the toughness of steel materials. It became. For example, as shown in following Table 5 and FIG. 4, numbers 1 and 3 are steel materials in which about 1.8 coarse oxides with a circular equivalent diameter of more than 3 micrometers exist per 1 mm <2> observation field area. However, the difference of 14 J was produced in the absorption energy (vE- ₄₀ ) in _-40 degreeC of these steel materials. Therefore, as a result of further study by the present inventors, it became clear that the component composition of each interference | inclusion influences toughness value.

According to the control of the size and particle size distribution of the inclusions and the oxides in (a) to (c) above, the absorbed energy (vE- ₄₀ ) at -40 ° C can achieve 100J or more even when welding is performed with a large heat input. Also, by controlling the number of inclusions in (d) and (e) above, vE- ₄₀ can achieve about 130J or more (see later examples).

Specifically, in the above (d), attention is paid to Al ₂ O ₃ included in the inclusions, and the number ratio of inclusions I (the ratio with respect to all inclusions) having a small Al ₂ O ₃ ratio is controlled to be 90% or more. It is. Al ₂ O ₃ is an oxide that is less likely to act as a nucleus of intracellular ferrite transformation than CaO or the like. And according to the result of the inventors' examination, it turned out that the relationship of the number ratio of the said inclusions I with respect to all the inclusions, and HAZ toughness which is a more preferable HAZ toughness condition has a good correlation, and prescribed | regulated said (d). It was. The larger the ratio of the number of the inclusions I to the total number of inclusions is, the more preferable it is 93% or more, and still more preferably 95% or more.

On the other hand, the inclusion II of the (e) in the can, and Al ₂ O paying attention to both the ₃ and CaO contained in the inclusions, the mass ratio of CaO to Al ₂ O ₃ (CaO / Al ₂ O ₃₎ satisfies 0.35 exceeded The number ratio to total inclusions is controlled to greater than 80%. Inclusion I based on (d) is defined based only on the ratio of Al ₂ O ₃ , while inclusion II based on (e) produces oxides which become nuclei of ferrite transformation in the particles. The ratio of Al ₂ O ₃ is different in relation to CaO which is an oxide to be mentioned. Considering the effect on the HAZ toughness, CaO has a positive effect, whereas Al ₂ O ₃ has a negative effect. And, according to the examination result by the inventors, "Al ₂ O ₃ weight ratio of CaO to (CaO / Al ₂ O ₃₎ have inclusions II satisfying 0.35 exceeds" the number ratio and a more preferable HAZ toughness of the entire inclusions of It turned out that the relationship of HAZ toughness which is a condition has a good correlation, and prescribed | regulated said (e).

The larger the ratio of the number of inclusions II to the total number of inclusions, the more preferable it is 83% or more, and even more preferably 85% or more.

Regarding the requirements of (d) and (e), only one of the present inventions may be satisfied, and both of them may be satisfied, and therefore both are preferred embodiments of the present invention. That is, some inclusions satisfy the requirements of inclusions I specified in (d) above, some satisfy the requirements of inclusions II described in (e) above, and both inclusions I and inclusions II requirements. In some cases, a desirable level of HAZ toughness can be achieved as long as at least one of (d) and (e) is satisfied. For example, numbers 3, 8, and 11 in Table 5 described later are examples satisfying both of (d) and (e), numbers 1 and 14 of Table 5 are examples satisfying only (d), No. 13 in Table 5 is an example satisfying only the above (e).

The composition of the inclusions included in the steel may be obtained by observing the cross section of the steel, for example, with EPMA, and quantitatively analyzing the composition of the inclusions recognized within the observation field, and after measuring the composition of all the inclusions included in the steel, the total inclusions. Al ₂ O ₃ are good ask the number ratio of inclusions of II and the number ratio of the inclusions is less than 20% by mass of I, CaO / Al ₂ O ₃ ratio of greater than 0.35 to satisfy the number of occupied. In addition, in the steel of the present invention, the composition is quantitatively analyzed for inclusions having a circular equivalent diameter of 0.1 µm or more. Inclusions with a circular equivalent diameter of less than 0.1 µm are too small to be quantitatively analyzed.

Next, the component composition in the steel material (base material) of this invention is demonstrated. The steel materials of the present invention are C: 0.02 to 0.15%, Si: 0.5% or less (not containing 0%), Mn: 2.5% or less (not containing 0%), and P: 0.03% or less as a basic component. (0% not included), S: 0.02% or less (0% not included), Al: 0.05% or less (0% not included), Ti: 0.005 to 0.10%, REM: 0.0003 to 0.015% , Ca: 0.0003 to 0.010%, Zr: 0.0010 to 0.050%, and N: 0.010% or less (not including 0%). The reason for determining this range is as follows.

C is an element which cannot be removed in order to secure the strength of the steel (base metal), and it is necessary to contain C at least 0.02%. C amount is preferably 0.04% or more, and more preferably 0.05% or more. However, if the amount of C exceeds 0.15%, a large amount of island-like martensite (MA) is generated in the HAZ during welding, which not only causes deterioration of the toughness of the HAZ but also adversely affects the weldability. Therefore, the amount of C is 0.15% or less, Preferably it is 0.1% or less, More preferably, it is 0.08% or less.

Si is an element which has a deoxidation effect and contributes to the strength improvement of steel materials (base materials) by solid solution strengthening. In order to exhibit such an effect effectively, it is preferable to contain Si 0.01% or more. Si is more preferably 0.02% or more, still more preferably 0.05% or more, particularly preferably 0.1% or more. However, when Si amount exceeds 0.5%, the weldability and toughness of steel materials will deteriorate. Therefore, Si amount is 0.5% or less, Preferably it is 0.45% or less, More preferably, it is 0.4% or less.

In addition, in order to especially increase HAZ toughness, it is recommended that Si be 0.3% or less, Preferably it is 0.05% or less, More preferably, it is 0.01% or less. However, while reducing the amount of Si improves the HAZ toughness, the strength of the steel may decrease.

Mn is an element which contributes to the strength improvement of steel materials (base materials). In order to exhibit such an effect effectively, it is preferable to contain 0.4% or more. Mn amount is more preferably 0.5% or more, still more preferably 0.7% or more, and particularly preferably 0.8% or more. However, when Mn amount exceeds 2.5%, the weldability of steel materials (base material) will deteriorate. Therefore, it is necessary to suppress Mn amount to 2.5% or less. Mn amount becomes like this. Preferably it is 2.3% or less, More preferably, it is 2% or less.

P is an element susceptible to segregation, and in particular, segregates at grain boundaries in steel materials and degrades HAZ toughness. Therefore, P amount needs to be suppressed to 0.03% or less. P amount is preferably 0.02% or less, and more preferably 0.015% or less. In addition, P usually contains about 0.001% inevitably.

S is a harmful element that combines with Mn to form sulfide (MnS) and deteriorates the toughness of the base material and the ductility in the sheet thickness direction. In addition, when S combines with REM such as La or Ce to form sulfides of REM (for example, LaS or CeS), the formation of oxides of REM is inhibited, thus deteriorating HAZ toughness. Therefore, the amount of S needs to be suppressed to 0.02% or less. S amount is preferably 0.015% or less, more preferably 0.01% or less, and still more preferably 0.006% or less. S is usually inevitably contained in about 0.0005%.

Al is an element which acts as a carbonate. However, excessive addition reduces the oxide to form coarse Al oxide, which degrades the HAZ toughness. Therefore, Al amount needs to be suppressed to 0.05% or less. Al amount is preferably 0.04% or less, more preferably 0.03% or less, still more preferably 0.025% or less, and particularly preferably 0.01% or less. In addition, Al usually inevitably contains about 0.0005%.

Ti is an element that contributes to the improvement of HAZ toughness by forming nitrides such as TiN and oxides containing Ti in steel materials. In order to exhibit such an effect, it is necessary to contain Ti 0.005% or more. Ti amount becomes like this. Preferably it is 0.007% or more, More preferably, it is 0.01% or more. However, when excessively added, the base metal itself hardens due to solid solution strengthening of Ti, which leads to a decrease in HAZ toughness. Therefore, Ti must be suppressed to 0.10% or less. Ti amount becomes like this. Preferably it is 0.07% or less, More preferably, it is 0.06% or less.

REM (rare earth element) and Ca are elements necessary to produce each oxide. By containing these oxides, oxides tend to be finely dispersed, and the finely dispersed oxides become the nuclei of ferrite in the particles, thereby contributing to the improvement of the HAZ toughness.

REM should be contained 0.0003% or more, Preferably it is 0.001% or more, More preferably, it is 0.002% or more. However, when REM is excessively added, solid solution REM is produced, which causes segregation to deteriorate the toughness of the base metal. Therefore, REM amount should be suppressed to 0.015% or less. REM amount becomes like this. Preferably it is 0.01% or less, More preferably, it is 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 more preferably La and / or Ce.

Ca should be contained 0.0003% or more, preferably 0.0005% or more, more preferably 0.0008% or more, and still more preferably 0.001% or more. However, when Ca is excessively added, CaO is excessively generated to produce inclusions having a high CaO concentration, which deviates from the optimum inclusion composition range, thereby weakening the effect of acting as a metamorphic nucleus in the particles of inclusions and deteriorating HAZ toughness. do. Therefore, Ca amount is suppressed to 0.010% or less. Ca is preferably 0.009% or less, and more preferably 0.008% or less.

Zr is an element which contributes to the improvement of HAZ toughness by producing a composite oxide containing Zr. In order to exhibit such an effect effectively, it is necessary to contain 0.0010% or more. Zr amount is preferably 0.002% or more, and more preferably 0.0023% or more. However, when Zr is added in excess, a large amount of ZrO ₂ is produced, so that the effect of acting as a transformation nucleus in particles of inclusions is weakened. In addition, when Zr is excessively added, fine nitrides (ZrN) and carbides (ZrC), which cause precipitation strengthening, are formed, leading to a decrease in toughness of the base material itself. Therefore, Zr amount is suppressed to 0.050% or less. The amount of Zr becomes like this. Preferably it is 0.04% or less, More preferably, it is 0.03% or less, More preferably, it is 0.01% or less.

N is an element that precipitates nitride (eg, ZrN, TiN, etc.). The nitride has a peening effect, which prevents coarsening of the austenite particles produced in the HAZ during welding, thereby promoting ferrite transformation. Contribute to the improvement of toughness. In order to exhibit such an effect effectively, it is preferable to contain N 0.003% or more. N amount is more preferably 0.004% or more, and still more preferably 0.005% or more. As the N content increases, the nitride is formed to promote the miniaturization of the austenite particles, and thus it is effective in improving the toughness of the HAZ. However, when N amount exceeds 0.010%, the solid solution N amount will increase, the toughness of the base material itself will deteriorate, and HAZ toughness will also fall. Therefore, N amount needs to be suppressed to 0.010% or less. N amount is preferably 0.009% or less, and more preferably 0.008% or less.

The steel material of this invention contains the said element as an essential component, and 0 (oxygen) amount is 0.0005 to 0.010%. Here, the amount of O (oxygen) 0.0005 to 0.010% represents the total amount of oxygen and means the total amount of O (oxygen) forming an oxide and free O (oxygen) dissolved in steel. Remaining part components of steel materials may be iron and unavoidable impurities (for example, Mg, As, Se, etc.).

The steel of the present invention is another element,

[1] Cu: 2% or less (without 0%) and / or Ni: 3.5% or less (without 0%),

[2] Cr: 3% or less (without 0%) and / or Mo: 1% or less (without 0%),

[3] Nb: 0.25% or less (without 0%) and / or V: 0.1% or less (without 0%),

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

It is also effective to contain elements, such as these. The reason for determining this range is as follows.

[1] Cu and / or Ni.

Cu and Ni are both elements which contribute to increasing the strength of the steel, and can be added alone or in combination. However, when the amount of Cu exceeds 2%, the strength of the base material is raised too much to deteriorate the toughness of the base material, so that the HAZ toughness also decreases. Therefore, it is preferable to make Cu amount into 2% or less. Cu amount is more preferably 1.8% or less, and still more preferably 1.5% or less. Moreover, in order to exhibit the effect by Cu addition effectively, it is preferable to contain 0.05% or more. Cu amount is more preferably 0.1% or more, and still more preferably 0.2% or more.

When the amount of Ni exceeds 3.5%, similarly to Cu, the strength of the base material is raised too much to deteriorate the toughness of the base material, so that the HAZ toughness also decreases. Therefore, it is preferable to make Ni amount into 3.5% or less. Ni amount is more preferably 3% or less, and still more preferably 2.5% or less. Moreover, in order to exhibit the effect | action by Ni addition effectively, it is preferable to contain 0.05% or more. Ni amount is more preferably 0.1% or more, and still more preferably 0.2% or more.

[2] Cr and / or Mo

Both Cr and Mo are elements that contribute to increasing the strength of the steel, and can be added alone or in combination. However, when Cr exceeds 3%, the strength of the base material is raised too much to deteriorate the toughness of the base material, and hence the HAZ toughness is lowered. Therefore, 3% or less of Cr amount is preferable. Cr amount is more preferably 2% or less, and still more preferably 1% or less. Moreover, in order to exhibit the effect by Cr addition effectively, it is preferable to contain 0.05% or more. Cr amount is more preferably 0.1% or more, and still more preferably 0.15% or more.

Similarly to Cr, when Mo exceeds 1%, the strength of the base material is excessively increased to deteriorate the toughness of the base material, and thus the HAZ toughness is lowered. Therefore, it is preferable to make Mo amount into 1% or less. Mo amount is more preferably 0.9% or less, and still more preferably 0.8% or less. Moreover, in order to exhibit the effect | action by Mo addition effectively, it is preferable to contain 0.05% or more. Mo amount is more preferably 0.1% or more, and still more preferably 0.15% or more.

[3] Nb and / or V

Both Nb and V are precipitated as carbonitrides, and the pinning effect of the carbonitrides prevents coarsening of the austenite particles during welding, thereby improving the HAZ toughness. Nb and V can be added individually or in combination, respectively. However, when the amount of Nb exceeds 0.25%, the deposited carbonitride is coarsened, which deteriorates the HAZ toughness. Therefore, it is preferable to make Nb amount into 0.25% or less. Nb amount is more preferably 0.2% or less, and still more preferably 0.15% or less. In addition, in order to exhibit the effect | action by Nb addition effectively, it is preferable to contain 0.002% or more. Nb amount is more preferably 0.01% or more, and still more preferably 0.02% or more.

Similarly to Nb, when V exceeds 0.1%, the deposited carbonitride is coarsened, which deteriorates HAZ toughness. Therefore, V amount is preferably 0.1% or less. V amount is more preferably 0.09% or less, and still more preferably 0.08% or less. Moreover, in order to exhibit the effect | action by V addition effectively, it is preferable to contain 0.002% or more. V amount is more preferably 0.005% or more, and still more preferably 0.01% or more.

《[4] B (boron)》

B is an element which suppresses formation of grain boundary ferrite and improves toughness. However, when B amount exceeds 0.005%, it will precipitate as BN in an austenite grain boundary, and will cause a fall of toughness. Therefore, the amount of B is preferably 0.005% or less. B amount is more preferably 0.004% or less. Moreover, in order to exhibit the effect by B addition effectively, it is preferable to contain 0.0010% or more. B amount is more preferably 0.0015% or more.

Next, the manufacturing method which can be suitably employ | adopted in manufacturing the steel material of this invention is demonstrated. Steel material of the present invention according to the addition of REM in which is adjusted to a range of 0.001 to 0.01 mass%, the amount of dissolved oxygen (Q _Of) in molten steel, the molten steel, the amount of dissolved oxygen (Q _Of) (% by weight) and the addition amount of REM in the molten steel It can manufacture by adding REM so that (Q _REM ) (mass%) may satisfy following formula (1).

[Equation 1]

2logQ _REM + 3logQ _Of ≤ -12.00

Here, Equation 1 is set to ensure the desired HAZ toughness specified in the present invention, based on Equation 1, if the addition amount of _REM (Q _REM ) is appropriately added according to the dissolved oxygen amount (Q _Of ) of the molten steel Desired HAZ toughness can be obtained (see later example).

In addition, the coefficient of the left side of the said Formula 1 is a value based on REM oxide production | generation reaction formula in the molten steel shown by the said Formula 2.

[Equation 2]

2REM + 3O = REM ₂ O ₃

If the dissolved oxygen amount Q _Of of the molten steel and the addition amount Q _{REM of REM} satisfy the above formula 1, it means that the addition amount Q _{REM of} REM involved in the production of REM oxide is set to be small. As a result, the number of generated REM oxides is also reduced, and as a result, the number of coarse and supercoarse oxides is reduced within the scope of the present invention, and it is considered that desired HAZ toughness is secured.

When the Z value exceeds -12.00, the balance between the dissolved oxygen amount Q _Of of molten steel and the addition amount Q _REM of _REM becomes poor, and the addition amount Q _{REM of REM} increases to produce coarse REM oxide. As a result, the HAZ toughness is lowered. Therefore, the above Z value is -12.00 or more. Z value becomes like this. Preferably it is -13 or less, More preferably, it is -13.1 or less, More preferably, it is -13.3 or less. The lower limit of the Z value is not particularly limited, but considering the amount of REM in steel or the like, it is approximately -15.

In addition, in said source invention, it does not pay attention to said Formula 1 at all. Therefore, there was a case where the addition amount Q _{REM of REM} was increased so that the value (Z value) of the left side of Formula 1 exceeded -12.00, without satisfying the relationship of Formula 1. In addition, although Patent Documents 4 to 6 describe adding REM to molten steel in which dissolved oxygen amount (Q _Of ) is adjusted, the amount of _REM added (Q _REM ) is determined and added according to dissolved oxygen amount (Q _Of ). Is not considered at all. Further, Patent Documents 4 to 6 do not describe the use of REM, Zr, and Ca in combination, and therefore, oxides containing Zr, REM, and Ca having HAZ toughness-improving action as initially defined in the present invention are initially obtained. Not lost

Next, the addition amount Q _REM and dissolved oxygen amount Q _{Of of} REM constituting Equation 1 will be described.

First, the added amount (Q _REM) of the REM is, and it may be added as dissolved oxygen _(Of Q) as described above. In addition, the addition amount Q _{REM of REM} is set in large quantity compared with the amount of REM contained in the steel materials of this invention. This is because the amount of REM added before casting is volatilized in the casting process or the like and dispersed in slag, so that the amount of REM contained in the steel is reduced.

In addition, dissolved oxygen amount (Q _Of ) of molten steel shall be in the range of 0.001-0.01 mass%. Dissolved oxygen means oxygen in the free state which does not form oxide and exists in molten steel. That is, in order to manufacture the steel material of this invention, as a precondition, first, the dissolved oxygen amount Q _Of of molten steel is adjusted to 0.001-0.01 mass%. If the dissolved oxygen amount (Q _Of ) of the molten steel is less than 0.001 mass%, the dissolved oxygen amount (Q _Of ) in the molten steel is insufficient, so that a predetermined amount of Zr, REM, and Ca-based oxides, which become nuclei of the ferrite transformation in the particles, cannot be secured. HAZ toughness cannot be improved. In addition, 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 causes deterioration of the toughness of the base material itself. Therefore, the said dissolved oxygen amount Q _Of is made into 0.001 mass% or more. The dissolved oxygen amount Q _Of is preferably 0.0015% by mass or more, and more preferably 0.0020% by mass or more.

On the other hand, if the amount of dissolved oxygen (Q _Of ) exceeds 0.01% by mass, the amount of oxygen in the molten steel is too large, so that the reaction of oxygen in the molten steel with the element becomes severe, which is undesirable in solvent operation and produces coarse oxide. Rather, it degrades the HAZ toughness. Therefore, the said dissolved oxygen amount Q _Of should be suppressed to 0.01 mass% or less. The dissolved oxygen amount Q _Of is preferably 0.008% by mass or less, and more preferably 0.007% by mass or less.

By the way, the dissolved oxygen amount Q _Of in the molten steel refine | purified primarily by the converter and the electric furnace is normally exceeding 0.01 mass%. Therefore, in the manufacturing method of this invention, it is necessary to adjust the dissolved oxygen amount Q _Of in molten steel to the said range by some method.

As a method of adjusting the dissolved oxygen amount Q _Of in molten steel, the method of vacuum deoxidation using an RH type degassing | purification refiner, the method of adding deoxidizing elements, such as Si, Mn, Ti, Al, etc. are mentioned, for example. These methods may be appropriately combined to adjust the dissolved oxygen amount Q _Of . In addition, instead of the RH type degassing apparatus, the dissolved oxygen amount Q _Of may be adjusted using a ladle heating type refinery apparatus, a simple molten steel treatment plant, or the like. In this case, since it is impossible to adjust the dissolved oxygen amount Q _Of by vacuum deoxidation, a method of adding a deoxidizing element such as Si may be employed to adjust the dissolved oxygen amount Q _Of . When employ | adopting the method of adding carbonic elements, such as Si, you may add a deoxidation element when going out to a ladle from a converter.

As described above, after adjusting the dissolved oxygen amount (Q _Of ) in the molten steel to the above range, the REM is added and then cast, but in the present invention, the relationship between the dissolved oxygen amount (Q _Of ) and the addition amount (Q _REM ) of _REM , It is important to satisfy the above formula 1, and the addition order of the component elements other than REM is not particularly limited. This is because REM has a stronger bond with oxygen than other constituent elements and is highly involved in the formation of coarse and supercoarse oxides that adversely affect HAZ toughness. Therefore, it is necessary to pay special attention to the amount of _REM added (Q _REM ).

According to the addition of the component elements other than REM, by, reducing the Al ₂ O ₃ amount contained in each of the inclusions, as shown in the above (d), for the total number of inclusions, the content of Al ₂ O ₃ 20 In order to make the number ratio of the inclusions I less than mass% more than 90%, when manufacturing steel materials, it is recommended to suppress Al amount of steel materials to 0.03% or less. In addition, in order to control Al amount of steel materials in this way, Al may be added in consideration of loss | disappearance of Al by reoxidation, etc. suitably. Moreover, the more preferable amount of Al in steel materials is 0.025% or less.

In addition, as shown in the above (e), in order to make the number ratio of inclusions II which satisfy the CaO / Al ₂ O ₃ ratio exceeding 0.35 with respect to the total number of inclusions more than 80%, when manufacturing steel materials, molten steel It is recommended to increase the ratio of the amount of Ca added to the amount of Al to the amount of Al added (the amount of Ca added / amount of Al) to exceed 0.30. As for Ca addition amount / Al addition amount ratio, it is more preferable to be 0.4 or more, More preferably, you may be 0.5 or more.

Moreover, it is a preferable aspect of this invention to pay attention to the addition order of Ti for the purpose of further improving HAZ toughness by refinement | miniaturization of Ti oxide. That is, it is preferable to add Ti before adding REM. Since Ti oxide has a smaller interfacial energy with molten steel than Zr-RFM-Ca-based oxides, Ti oxide can be refined by adding Ti before adding Zr, REM and Ca to molten steel, resulting in HAZ toughness. It can produce fine oxides that contribute to. And after adding Ti, Zr, REM, and Ca are added, and the Zr-REM-Ca type | system | group oxide used as the nucleus of ferrite transformation in a desired particle | grain is obtained.

Even in the case where REM is added after the addition of Ti to the molten steel in which the dissolved oxygen amount Q _Of is adjusted, the addition amount Q _{REM of REM} is represented by Equation 1 according to the dissolved oxygen amount Q _Of of molten steel as described below. The addition of REMs to satisfy allows proper control of the size and density of the oxides. If Ti is added before REM, the dissolved oxygen of molten steel decreases because it combines with Ti to form oxides, but Ti is difficult to bond with oxygen compared to REM, and since Ti oxide has a small interfacial energy with molten steel, This is because it is difficult to form coarse oxides having a diameter of more than 3 µm.

The forms of REM, Ca, Zr, and Ti added to molten steel are not particularly limited. For example, as REM, pure La, pure Ce, pure Y, or pure Ca, pure Zr, pure Ti, or Fe-Si- La alloy, Fe-Si-Ce alloy, Fe-Si-Ca alloy, Fe-Si-La-Ce alloy, Fe-Ca alloy, Fe-Zr alloy, Fe-Ti alloy, Ni-Ca alloy, etc. may be added. . Moreover, you may add misch metal to molten steel. The misch metal is a mixture of rare earth elements, and specifically, contains about 40 to 50% of Ce and about 20 to 40% of La. However, since misch metal often contains Ca as an impurity, when misch metal contains Ca, it is necessary to satisfy the range prescribed | regulated by this invention.

The molten steel obtained by the component adjustment in this manner may be continuously cast in accordance with a conventional method to form a slab, and then hot rolled according to the conventional method.

The steel material of the present invention is maintained at 1450 ° C. for 5 seconds, and then, even when a heat history of cooling is performed with the cooling time from 800 ° C. to 500 ° C. as 400 seconds, 100 J as absorbed energy at 40 ° C. (vE- ₄₀ ). The above (especially 130J or more) can be secured. Therefore, the steel material which concerns on this invention can be used as a material of structures, such as a bridge, a high-rise building, and a ship, for example, and also in large heat input welding whose heat input amount is 50 kJ / mm or more, as well as medium heat input welding from quench heat. It is possible to prevent the deterioration of the toughness of the weld heat affected zone. The steel material of this invention is intended for the thick steel plate etc. whose plate | board thickness is about 3.0 mm or more.

(Example)

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not restrict | limited by the following example, of course, It is a matter of course that it changes and implements suitably in the range which may be suitable for the meaning of the previous and the later. Possible, and they are all included in the technical scope of the present invention.

In the first experimental example, the relationship between the requirements defined in the above (a) to (c) and the HAZ toughness are examined. In the second experimental example, for some steel grades used in the first experimental example, (d) and The relationship between the requirements of (e) and the HAZ toughness was examined.

[Example 1]

Using the vacuum melting furnace (150 kg of capacity), the test-providing steel (remaining part is iron and inevitable impurities) of the component composition (mass%) shown in following Table 2 and Table 3 is carried out on the conditions shown in following Table 1, and is 150 Cooled by casting to kg ingot. Thereafter, heating and rolling were performed to produce a thick steel sheet. Moreover, it is confirming that the total amount of O of the test provision steel which satisfy | fills the requirements prescribed | regulated by this invention among the test provision steel shown in following Table 2, Table 3 is 0.0005 to 0.010% of range.

In solventing the test-provided steel in a vacuum melting furnace, the component is adjusted for elements other than Ti, Zr, REM and Ca, and deoxidized using at least one element selected from C, Si, Mn and Al. The dissolved oxygen content (Q _Of ) of the molten steel was adjusted. The dissolved oxygen amount (Q _Of ) after the adjustment is shown in Table 1 below.

Ti was added to the molten steel in which the dissolved oxygen amount (Q _Of ) was adjusted, followed by Zr, REM, and Ca. The addition amount of _REM is made into Q _REM , and this value is shown in following Table 1. In addition, the Z value computed by substituting the value of the said dissolved oxygen amount Q _Of and the addition amount Q _REM of _REM in following Formula 1 'is shown in Table 1 below.

[Equation 1 ']

Z = 2logQ _REM + 3logQ _Of

Ti is in the form of a Fe-Ti alloy, Zr is in the form of a Fe-Zr alloy, REM is in the form of a misch metal containing about 25% La and about 50% Ce, and Ca is a Ni-Ca alloy. Each was added in the form. However, No. 12 of Table 2 added only Ce, not the form of misch metal.

After the element was added, it was cast into an ingot and cooled. The obtained ingot was hot-rolled to produce a thick steel sheet having a thickness of 30 to 80 mm. The sample is cut out from the cross section at the position t / 4 (where t is the thickness of the steel sheet) of the obtained thick steel sheet, the component composition of all the oxides contained in the sample is measured, and the mass composition is converted to the single oxide to calculate the average composition of the oxide. It was.

The component composition of all the oxides was measured in the following order. The cut surface of the sample was observed using an Electron Probe X-ray Micro Analyzer (EPMA; `` JXA-8500F (device name)) manufactured by Japan Electron Datum, and the equivalent diameter was measured. The component composition was quantitatively analyzed for inclusions of 0.1 µm or more. Observation conditions make acceleration voltage 20 mA, sample current 0.01 mA, observation field area 1-5 cm <2>, and analysis number 100 or more, and quantitatively analyze the component composition in the center part of an interference | inclusion with the characteristic X-ray wavelength dispersion spectroscopy. It was. The element to be analyzed is Al, Mn, Si, Ti, Zr, Ca, La, Ce, O (oxygen) and S. Using a known substance, the relationship between the X-ray intensity of each element and the element concentration is previously determined as a calibration curve. The X-ray intensity obtained from the inclusions to be analyzed and the amount of elements contained in the inclusions were determined from the calibration curve.

In the obtained quantitative result, an inclusion having an oxygen content of 5% or more was used as an oxide. At this time, when a plurality of elements were observed from one inclusion, the composition of the oxide was calculated by converting them into a single oxide of each element from the ratio of X-ray intensities indicating the presence of those elements. In this invention, what averaged the mass conversion as a single oxide in this way was made into the average composition of oxide. Among the oxides, average compositions of oxides of REM, ZrO ₂ and CaO are shown in Table 4 below. In addition, an oxide of REM are expressed as the metal element M, the steel material in the form of M ₂ O ₃ and M ₃ O _5, MO _2, but the composition was calculated in terms of all the oxides M ₂ O _3. Also referred to as "other" as shown in, the following Table 4, an oxide (e.g., Al ₂ O _3, MnO, SiO ₂ and so on) of the oxides, other than ZrO ₂ and CaO in the REM.

Next, the equivalent circular diameter was measured about the quantitative inclusions, and the number of inclusions whose diameter (particle diameter) is 0.1-2.0 micrometers was measured. The number which converted the measurement result into 1 mm <2> of observation field areas is shown in Table 4 below.

In addition, in the obtained quantitative results, the equivalent circular diameter of the oxide having an oxygen content of 5% or more was measured, and the number of oxides having a circular equivalent diameter (particle diameter) exceeding 3 μm and the original equivalent diameter (particle diameter) exceeding 5 μm. The number of oxides was measured. Table 4 below shows the value of the number of oxides converted to 1 mm 2 of the viewing field area.

1 shows the relationship between the Z value and the number per 1 mm 2 observed viewing area of the oxide whose circular equivalent diameter exceeds 3 µm. In FIG. 1, in order to show the critical significance of Z value, the Z value is shown among the results of the numbers 1-16 ((circle of FIG. 1)) and the results of the numbers 17-22 (● of FIG. 1) shown in Table 4 below. Plots in the range of -12.5 to -11.5 are plotted.

As apparent from FIG. 1, 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, the production of an oxide having a diameter of more than 3 µm in original equivalent is suppressed.

Next, in order to evaluate the toughness of the HAZ subjected to the heat effect at the time of welding, a welding reheat test shown below was conducted by simulating high heat input welding. The welding reproduction test was heated so that the sample cut out from the t / 4 position (however, t is the plate | board thickness) of a thick steel plate may be set to 1450 degreeC, hold | maintained at this temperature for 5 second, and gave the heat cycle to cool. The cooling rate was adjusted so that the cooling time from 800 ° C to 500 ° C was 400 seconds.

The impact characteristic of the sample after cooling evaluated three V notch Charpy test pieces in the rolling direction from the sample after giving the said heat cycle, and performed the impact test according to JISZ2242. In the impact test, the absorption energy (vE- ₄₀ ) at _-40 degreeC was measured, and the average value of 3 times was computed. In this invention, it is set as the pass (good HAZ toughness) that the average value of vE- ₄₀ is 100J or more. The measurement results are shown in Table 4 below.

From Table 1 to Table 4, it can be considered as follows. Nos. 1 to 16 are examples satisfying the conditions specified in the present invention, and after adjusting the composition of all oxides contained in the steel material and converting them into a single oxide, they are adjusted to contain a predetermined amount of ZrO ₂ , oxides of REM and CaO. In order to prevent the formation of an oxide having a circular equivalent diameter of more than 3 µm and an oxide having a circular equivalent diameter of more than 5 µm, many inclusions having a diameter of 0.1 to 2 µm are produced. Thus, steel having good HAZ toughness has been obtained. .

On the other hand, the numbers 17 to 32 are examples that deviate from any of the requirements defined in the present invention. Nos. 17 to 22 indicate that the balance of dissolved oxygen amount Q _Of in molten steel and addition amount Q _{REM in REM} does not satisfy Equation 1, and therefore, an oxide having a circular equivalent diameter of more than 3 μm (particularly, a circular equivalent diameter Many oxides exceeding 3 micrometers and 5 micrometers or less) are produced | generated. Therefore, the HAZ toughness is deteriorated.

No. 21 and No. 23 are oxides that form nuclei of ferrite in particles during welding because the oxide content of REM when measuring the composition of all oxides contained in steel and converted to a single oxide is less than the range defined by the present invention. Amount is lacking and HAZ toughness deteriorates. Numerals 22 and 24 indicate that the amount of REM contained in the steel is large, and the amount of REM in the conversion of a single oxide by measuring the composition of all the oxides contained in the steel exceeds the range specified in the present invention. As a result, the number of fine oxides acting as a production nucleus of the ferrite in the particles decreases, and the effect of improving the HAZ toughness is not exhibited.

Numeral 25 indicates that the amount of Zr contained in the steel material is too small, so that the amount of ZrO ₂ in the composition of all the oxides decreases, and the amount of Zr.REM.Ca-based oxides that become the nucleus of the ferrite transformation in the particles is considered to be low. Therefore, HAZ toughness deteriorates. No. 26 represents an excessively large amount of Zr contained in the steel, so that the amount of ZrO ₂ in the composition of all the oxides is large. Therefore, the effect | action which acts as a particle | grain transformation nucleus of an interference | inclusion becomes weak, a microstructure is not obtained and HAZ toughness is deteriorated.

Since No. 27 contains too much Ca in steel materials, the amount of CaO which occupies for the composition of all oxides increases. Therefore, the effect | action which acts as a particle | grain transformation nucleus of an interference | inclusion becomes weak, a microstructure is not obtained and HAZ toughness is deteriorated. 28 is too small a quantity of Ca contained in steel materials, and CaO amount is not produced | generated. Therefore, the amount of Zr.REM.Ca-based oxides that become nuclei of the ferrite transformation in the particles is not generated and the HAZ toughness is deteriorated.

In the case of No. 29, since the amount of Ti contained in steel materials is too big | large, since a base material solidified by solid solution of Ti, HAZ toughness deteriorates as a result. No. 30 indicates that the amount of Ti contained in the steel material is too small, so that the amount of inclusions having a diameter of 0.1 to 2 μm as the equivalent of the core of the ferrite transformation in the particles cannot be ensured. Therefore, the HAZ toughness is deteriorated.

Since the number 31 contains too much Al in steel materials, many coarse oxides whose original equivalent diameter exceeds 3 micrometers generate | occur | produce, and HAZ toughness deteriorates. Numeral 32 is an example in which the amount of N contained in the steel is too large. The amount of solid solution N contained in the steel is excessive, and it is considered that the HAZ toughness is deteriorated.

Next, Fig. 2 shows the relationship between the number of oxides having an original equivalent diameter of more than 3 µm per observation field area 1 mm 2 and the absorption energy (vE- ₄₀ ) at -40 ° C. In FIG. 2, the result of the numbers 1-16 shown in following Table 4 was shown by (circle), and the result of the numbers 17-22, 31 (an example exceeding 5.0 in a comparative example) was shown by (circle).

As apparent from FIG. 2, it can be seen that when the number of oxides having an equivalent diameter of more than 3 μm per observation viewing area 1 mm is 5.0 or less, good HAZ toughness is exhibited even when heated and maintained at 1450 ° C. for 5 seconds. .

[Example 2]

The numbers 1, 3, 8, 11, 13 to 16 shown in Table 4 above were examined for the relationship between the composition of the individual inclusions contained in the steel and the HAZ toughness.

The composition of each inclusion contained in steel materials was measured by the following procedure. That is, similarly to the first experimental example, the cut sample surface was observed using EPMA ("JXA-8500F (device name)") manufactured by Nippon Electron Datum, and the composition of the composition was determined for inclusions having a diameter of 0.1 µm or more. Quantitative analysis.

As an example of quantitative analysis, FIG. 3 shows the results of analyzing the composition of individual inclusions included in the steel of No. 1 shown in Table 4 above. The X axis represents the number of observed inclusions, and the Y axis represents the composition of each inclusion separately. In the observation field area of 1.56 cm 2, 254 inclusions having a circular equivalent diameter of 0.1 μm or more were observed.

Next, the equivalent circular diameter was measured about the inclusions quantitatively analyzed, the number of the inclusions whose original equivalent diameter is 0.1 micrometer or more was measured, and this was made into the number of all inclusions. On the other hand, among the inclusions having a diameter equivalent to 0.1 μm or more, the proportion of Al ₂ O ₃ contained in the inclusions was less than 20% by mass, and the number of CaO and Al ₂ O ₃ contained in the inclusions. The number ratio of inclusions (inclusion II) whose mass ratio (CaO / Al ₂ O ₃ ) satisfies more than 0.35 was measured, respectively, and the number ratio with respect to the number of all inclusions was computed. The calculation results are shown in Table 5 below.

In addition, Table 5 shows the addition amount of Ca and the addition amount of Al (input amount) when these steel materials are manufactured. In addition, the addition ratio (Ca addition amount / Al addition amount) of Ca addition amount and Al addition amount is computed, and a calculation result is shown in following Table 5. In addition, the absorption energy (vE- ₄₀ ) in _-40 degreeC of each steel material shown in the said Table 4 is shown in Table 5 below.

Here, with respect to the steel materials shown in Table 5 below, the relationship between the number of particles per observation field area 1 mm 2 and the vE- ₄₀ of an oxide whose circular equivalent diameter exceeds 3 µm is shown in FIG. 4. FIG. 4 shows an extract of a part of the data shown in FIG. 2.

From FIG. 4, Table 4, and Table 5, it can consider as follows. As for the steel materials of No. 1, 3, 8, 11, 13-16, all satisfy | fill the requirements prescribed | regulated by this invention as shown in the said Table 4, vE- ₄₀ was 100J or more. By the way, as is apparent from Fig. 4, the numbers 1 and 3, the numbers 11, 14 and 15, the numbers 8, 13 and 16 are approximately equal in number per unit area of the oxides whose diameters are larger than 3 µm, respectively. It was found that a deviation occurred in the value of vE- ₄₀ . The steel of Nos. 1, 3, 8, 11, 13, 14 of steel, so the number ratio of the entire inclusions of inclusions I and / or II inclusions, to further meet the preferred requirements specified in the present invention, vE _-40 It can be seen that the value of is larger than 130J.

Claims (6)

C: 0.02 to 0.15% (mean of mass%. The same for the following components),
Si: 0.5% or less (not including 0%),
Mn: 2.5% or less (not including 0%),
P: 0.03% or less (not including 0%),
S: 0.02% or less (not including 0%),
Al: 0.05% or less (not including 0%),
Ti: 0.005 to 0.10%,
REM: 0.0003 to 0.015%,
Ca: 0.0003 to 0.010%,
Zr: 0.0010 to 0.050%,
N: 0.010% or less (not including 0%),
O: 0.0005 to 0.010%,
Remaining part is steel material which consists of iron and inevitable impurities,
(a) the steel comprises an oxide containing Zr, REM and Ca,
(b) When the composition of all the oxides contained in the steel is measured and converted into a single oxide, the average composition,
ZrO ₂ : 5-50%,
Oxide of REM (M ₂ O _{3 when} REM is represented by the symbol of M): 5 to 50%,
CaO: satisfy | fills 50% or less (not containing 0%), and also
(c) of all inclusions contained in the steel,
120 or more inclusions having a diameter of 0.1 to 2 μm in a circular equivalent diameter per 1 mm 2 of observation field area,
Less than 5.0 oxides per 1 mm 2 observed in the observation area,
A steel having excellent toughness of a weld heat affected zone, wherein an oxide having a diameter of more than 5 µm in a circular equivalent diameter is 5.0 or less per 1 mm 2 of the viewing field area. The inclusions included in the steel material may further contain Al ₂ O ₃ .
When the composition of the inclusion is measured and converted into a single oxide, the total number of inclusions is
(d) for the Al ₂ O _3, or the number ratio of inclusions of a ratio of Al ₂ O ₃ satisfies the less than 20% by mass is more than 90%, or,
(e) Al ₂ O ₃ and for the CaO, Al ₂ O ₃ weight ratio of CaO to (CaO / Al ₂ O ₃₎ is that the number ratio of inclusions exceeds 80%, the weld heat affected satisfying 0.35 exceeded Steel with excellent negative toughness. The inclusions included in the steel material may further contain Al ₂ O ₃ .
When the composition of the inclusion is measured and converted into a single oxide, the total number of inclusions is
(d) for the Al ₂ O _3, and the number ratio of inclusions of a ratio of Al ₂ O ₃ satisfies the less than 20% by weight exceeds 90%, and,
(e) Al ₂ O ₃ and for the CaO, Al ₂ O ₃ weight ratio of CaO to (CaO / Al ₂ O ₃₎ is that the number ratio of inclusions exceeds 80%, the weld heat affected satisfying 0.35 exceeded Steel with excellent negative toughness. The steel according to any one of claims 1 to 3, wherein the steel further contains any one of the following as another element.
Cu: 2% or less (does not contain 0%)
Ni: 3.5% or less (does not contain 0%)
Cr: 3% or less (does not contain 0%)
Mo: 1% or less (does not contain 0%)
Nb: 0.25% or less (not including 0%)
V: 0.1% or less (does not include 0%)
B: 0.005% or less (does not include 0%) The method of manufacturing the steel according to any one of claims 1 to 3, wherein the molten steel is added to molten steel in which the dissolved oxygen amount (Q _Of ) of molten steel is adjusted to a range of 0.001 to 0.01 mass%. The dissolved oxygen amount (Q _Of ) and the REM addition amount (Q _REM ) of the amount of the REM satisfies the following formula 1, characterized in that the manufacturing method of the steel excellent in the toughness of the weld heat affected zone.
[Equation 1]
2logQ _REM + 3logQ _Of ≤ -12.00 Claim A method for producing a steel product according to 4, according to the addition of REM in the molten steel is adjusted to a range of the amount of dissolved oxygen (Q _Of) in molten steel from 0.001 to 0.01 mass%, the amount of dissolved oxygen in the molten steel (Q _Of) and The addition amount (Q _REM ) of REM adds REM of the quantity which satisfy | _fills following Formula 1, The manufacturing method of the steel excellent in the toughness of the weld heat affected zone.
[Equation 1]
2logQ _REM + 3logQ _Of ≤ -12.00

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