KR100983148B1 - Steel material having excellent haz toughness and manufacturing method of the same - Google Patents

Abstract

An object of the present invention is to provide a steel material and a method of manufacturing the same, which improves the toughness of the HAZ (welding heat affected zone) and reduces the variation in the HAZ toughness.

C: 0.01% to 0.2% (meaning of "mass%", the same as below), Si: 0.5% or less (0% not included), Mn = 2.5% or less (0% not included), Ti: 0.03% Or less (not including 0%), and N: 0.01% or less (does not contain 0%), P: 0.02% or less (does not contain 0%), and S: 0.015% or less (0%) ), And Al: 0.01% or less (including 0%), and also contains REM: 0.0010 to 0.1% and Zr: 0.001 to 0.05%, respectively, and the balance is iron and inevitable impurities. The steel material consisting of the steel may be made to contain REM and Zr in the inclusions, and to satisfy the solid solution REM: 0.0010% or less (including 0%) and the solid solution Zr: 0.0010% or less (including 0%). .

Weld heat affected zone, REM, inclusions, membrane filter, martensite

Description

Steel material excellent in toughness of weld heat affected zone and its manufacturing method {STEEL MATERIAL HAVING EXCELLENT HAZ TOUGHNESS AND MANUFACTURING METHOD OF THE SAME}

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to steel materials used in structures such as bridges, high-rise buildings, ships, and the like, and more particularly, a site affected by heat in welding (hereinafter referred to as "welding heat influencer" or "HAZ"). The steel material which improved the toughness of the case, and its manufacturing method.

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 joined by welding, in particular, HAZ has a problem of being easily affected by toughness due to heat influence during welding. This toughness deterioration is remarkable as the amount of heat input during welding increases, and the cause is that as the amount of heat input during welding increases, the cooling rate of HAZ decreases, and the quenching property decreases to produce coarse island-like martensite. It is thought that it is. Therefore, in order to improve the toughness of the HAZ, it may be considered that heat input during welding may be suppressed as much as possible. On the other hand, however, in addition to improving the welding work efficiency, it is required to employ a high heat input welding method in which the heat input amount of welding such as electro gas welding, electro slag welding, submerging welding and the like is 40 kJ / mm or more.

Several steel materials which suppress the deterioration of HAZ toughness when the high heat input welding method is adopted are already proposed. For example, Patent Document 1 proposes a steel material in which fine TiN is dispersed and reprecipitated in steel, thereby suppressing coarsening of austenite grains generated in HAZ when high heat input welding is performed to suppress deterioration of HAZ toughness. . However, as a result of the investigation by the present inventors, when the weld metal becomes a high temperature of 1400 degreeC or more, the said TiN is received by the heat | fever received at the time of welding in the site | part (henceforth, a "bond part") among HAZ especially. It was found that the solid solution disappeared and the deterioration of the HAZ toughness could not be suppressed.

In addition, Patent Document 2 discloses controlling the presence mode of oxides and nitrides contained in steel as a technique for improving the toughness of the base metal and HAZ. This patent document 2 describes that by using Ti and Zr in combination, fine oxides and nitrides are formed to improve the toughness of the base material and HAZ. Moreover, in order to produce such a fine oxide and nitride, what is necessary is just to add Ti and Zr in this order in a manufacturing process. However, as a result of the present inventors' investigation, in order to further increase the toughness of the HAZ, the amount of oxide may be increased. However, when Ti or Zr is added in a large amount in order to increase the amount of oxide in the technique disclosed in Patent Document 2, the amount of Ti or Zr is increased. It turned out that carbides, such as these, are formed and the toughness of steel materials (base material) falls rather.

By the way, the present applicant has previously proposed the steel material which the toughness of HAZ does not deteriorate even when it receives the high temperature heat effect at the time of welding. This steel is obtained by dispersing complex oxides such as La ₂ O ₃ -SiO ₂ based oxides, Ce ₂ O ₃ -SiO ₂ based oxides, and La ₂ O ₃ -Ce ₂ O ₃ -SiO ₂ based oxides in steel materials. Since the composite oxide is in a liquid phase in molten steel, it is finely dispersed in steel, and furthermore, since it does not lose solid solution even under heat influence during welding, it contributes to the improvement of toughness of HAZ. In the said patent document 3, in order to produce the said composite oxide, La and Ce are added to molten steel which adjusted the amount of dissolved oxygen, and what is necessary is just to add Si after that. In addition, Patent Document 3 discloses that the toughness of the HAZ is further increased by containing Ti in the steel and depositing TiN in the steel structure, and in order to generate such TiN, it is necessary to add Ti to the molten steel in which the composite oxide is formed. It is starting.

In addition, although it is not a technique aimed at improving HAZ toughness, Patent Document 4 contains elements such as REM and Zr in steel materials and actively contains solid solution REM and solid solution Zr to prevent hydrogenous ultrasonic flaw defects. The technique which improves the internal quality of a steel plate and maintains the soundness of an internal quality is proposed. In this technique, in order to ensure stable high capacity, Al, Ca, Ti, etc. are combined and added.

[Patent Document 1] Japanese Patent Publication No. 55-26164

[Patent Document 2] Japanese Patent Application Laid-Open No. 2003-213366

[Patent Document 3] Japanese Unexamined Patent Publication No. 2005-48265

[Patent Document 4] Japanese Patent Application Laid-open No. Hei 8-120401

MEANS TO SOLVE THE PROBLEM With respect to the technique disclosed previously as patent document 3, this inventor pays attention to inclusions of compositions other than the complex oxide of the said composition, and can improve the toughness of HAZ by disperse | distributing inclusions of a composition different from the said patent document 3 in steel materials. We reviewed whether there was any.

An object of the present invention is to improve the toughness of the HAZ (welding heat affected zone) and reduce the variation of the HAZ toughness by dispersing inclusions having a composition different from that of the steel proposed in Patent Document 3 in the steel, and the It is to provide a manufacturing method.

The steel material which concerns on this invention which could solve the said subject is C: 0.01 to 0.2% (meaning "mass%", the same below.), Si: 0.5% or less (not containing 0%), Mn: 2.5 P: 0.02% or less (0% or less), Ti: 0.03% or less (0% not included), and N: 0.01% or less (0% not included), and P: 0.02% or less (0%) %), S: 0.015% or less (0%), Al: 0.01% or less (including 0%), and REM: 0.0010 to 0.1% and Zr : It is 0.001 to 0.05%, respectively, and remainder is a steel material which consists of iron and an unavoidable impurity, contains REM and Zr in an inclusion, solid solution REM: 0.0010% or less (including 0%), and solid solution Zr: 0.0010 It has a gist at the point of satisfying% or less (including 0%).

When the said steel material measures the composition of the inclusions contained in the said steel material, when the molar ratio of elements other than O, C, N, and S among the elements contained in the said interference is converted into moles, when the whole amount of elements after conversion is made into 1 mol, It is preferable that the mole fraction of REM is 0.050 or more, and the mole fraction of Zr is 0.04 or more.

When manufacturing the said steel, REM and Zr were added to the molten steel which adjusted the total oxygen amount [O] ₁ to 0.0020 to 0.015%, and the dissolved oxygen amount [O] ₂ of molten steel was adjusted to 0.0010 to 0.0035%, What is necessary is just to cast afterwards.

In order to adjust the dissolved oxygen amount [O] ₂ , for example, the total oxygen amount [O] ₁ may be measured, and REM and Zr may be added to satisfy the following formula (1) according to the total oxygen amount [O] ₁ . . However, in the formula (1), [REM] and [Zr] are the addition amount (mass%) of REM or Zr, respectively, and [O] ₁ is the total amount of oxygen (mass%) of molten steel before adding REM and Zr. to be.

[REM] + [Zr] ≤ 15 × [O] ₁ ... (One)

According to the present invention, by adding REM and Zr to the steel composite, inclusions having a composition different from that of the steel proposed in the Patent Document 3 can be dispersed in the steel, and even if the inclusion reaches a high temperature of 1400 ° C. in the steel Since the solid solution does not disappear, the deterioration of the toughness of the weld heat affected zone HAZ can be prevented even when the high heat input welding is performed, not only the small heat input welding or the middle input heat welding. Moreover, according to this invention, the fluctuation | variation of HAZ toughness can be suppressed by reducing the quantity of the solid solution REM and the solid solution Zr contained in steel materials as much as possible.

MEANS TO SOLVE THE PROBLEM The present inventors examined whether the improvement of HAZ toughness could not be achieved by disperse | distributing inclusions of the composition different from the said patent document 3 in steel materials. As a result, it was found that the HAZ toughness can be improved by adding REM and Zr to the steel composite and adjusting the inclusions to include REM and Zr. In addition, if the amount of solid solution REM and solid solution Zr contained in the steel is reduced as much as possible, the phenomenon of locally deteriorating toughness can be prevented and the variation of the HAZ toughness can be suppressed. .

First, the steel materials of the present invention contain inclusions containing REM and Zr in inclusions. The inclusion of REM and Zr in inclusions means that the steel contains single inclusions or composite inclusions of REM and Zr. In addition, hereinafter, for the sake of convenience of explanation, a single inclusion and a complex oxide may be collectively referred to as an "inclusion".

Since the inclusions of REM and Zr are heat-dissipated at the time of welding and are not dissolved even at a high temperature of 1400 ° C., inclusion of these inclusions can prevent the austenite grains from coarsening in the HAZ during welding. . As a result, the toughness of the HAZ can be further improved.

In addition, by adding REM and Zr in combination and containing them as inclusions in the steel, the formation of coarse Zr single carbide and coarse REM sulfide caused by toughness of the steel (base material) can be prevented. The toughness of HAZ can be improved while suppressing toughness deterioration. That is, in the case of adding REM or Zr alone, in order to increase the number of inclusions, the amount of addition of REM or Zr must be increased. However, if the addition amount of REM or Zr is excessively increased, the size of the single inclusion of REM or the only inclusion of Zr is increased. Increases, rather deteriorates HAZ toughness. Therefore, when REM or Zr is added alone, there is a limit to the amount of addition. Therefore, the amount of addition of REM or Zr cannot be increased, and the amount of fine inclusions cannot be increased beyond a certain level. Therefore, HAZ toughness could not be improved.

On the other hand, when inclusions containing REM and Zr are contained in steel, the absolute amount of inclusions contained in steel can be increased more than when containing REM alone or Zr alone, thereby improving the toughness of HAZ. You can.

Thus, the toughness of HAZ can be improved by containing the inclusion of REM and Zr in steel materials. Therefore, in order to improve the toughness of HAZ, it is considered that it is desirable to actively add REM and Zr to generate a large amount of inclusions in the steel. However, as a result of welding steels with a high content of REM and Zr, and measuring the toughness of the HAZ at a plurality of sites, it was found that the toughness locally decreased and the measurement was not constant, particularly in the vicinity of the bond part having a large thermal influence. It became. As a result, observation of the tissues of locally deteriorated toughness revealed that REM and Zr were segregated at the grain boundaries. As a result of repeated studies to reduce segregation of REM and Zr, it was found that the amount of solid solution REM and the amount of solid solution Zr in steel materials should be reduced.

That is, it is important for the steel material of this invention to satisfy solid solution REM: 0.0010% or less (including 0%), and solid solution Zr: 0.0010% or less (including 0%). If the amount of solid solution REM in steel exceeds 0.0010% or the amount of solid solution Zr exceeds 0.0010%, when the heat is affected during welding, REM and Zr segregate at grain boundaries and locally reduce toughness. Therefore, the amount of solid solution REM is made 0.0010% or less, preferably 0.0008% or less, and more preferably 0.0005% or less. The amount of solid solution Zr is made 0.0010% or less, preferably 0.0008% or less, and more preferably 0.0005% or less. It is preferable that the amount of solid solution REM and the amount of solid solution Zr are reduced as much as possible, and most preferably 0%.

It is preferable that the sum total of solid solution REM and solid solution Zr is 0.0015% or less, More preferably, it is 0.0010% or less.

The amount of solid solution REM contained in the steel is ICP [Inductively Coupled Plasma; What is necessary is just to calculate by subtracting the amount of REM contained in the inclusions contained in steel materials computed by electrolytic extraction and ICP-MS from REM content (total REM content) calculated by the analysis by the inductively coupled plasma] -MS method.

In addition, also about solid-solution Zr amount, what is necessary is just to calculate by subtracting the amount of Zr contained in the inclusion contained in steel materials from Zr content (total Zr content).

For the steel materials of the present invention, "contains REM and Zr in inclusions" means a composite inclusion containing (a) a single inclusion of REM and a single inclusion of Zr, or (b) a REM and Zr. Or (c) a single inclusion of REM and a single inclusion of Zr, and a composite inclusion containing REM and Zr.

When the said steel material measures the composition of the interference | inclusion contained in the said steel, and molar ratio of elements other than O, C, N, and S among the elements which comprise the said interference | inclusion, when the whole element amount after conversion was made into 1 mol, It is preferable that the mole fraction of REM is 0.050 or more, and the mole fraction of Zr satisfies 0.04 or more. It is preferable that molar fraction of REM is 0.10 or more, More preferably, it is 0.15 or more, More preferably, it is 0.20 or more. On the other hand, it is preferable that the mole fraction of Zr is 0.08 or more, More preferably, it is 0.10 or more, More preferably, it is 0.15 or more.

The steel material of this invention measures the composition of the inclusions contained in the said steel material, converts the abundance of elements other than O, C, N, and S among the elements which comprise the said inclusion, and converts the whole element amount after conversion into 1 mol. In doing so, the sum of the mole fractions of REM and Zr is preferably 0.10 or more. If the sum is less than 0.10, the amount of inclusions contributing to the toughness improvement of HAZ is insufficient and the toughness of HAZ cannot be sufficiently improved. The total is more preferably 0.15 or more, and still more preferably 0.20 or more.

The composition of the remaining inclusions other than the inclusion of REM and the inclusion of Zr is not particularly limited, but may be CaO, SiO ₂ , Al ₂ O ₃ , MnO, TiN, or TiC.

The composition of the inclusions included in the steel can be measured by observing the cross section of the steel, for example, with an Electron Probe X-ray Micro Analyzer (EPMA) and quantitatively analyzing the inclusions identified within the observation field. have. For observation of EPMA, for example, the acceleration voltage is 7 mA, the sample current is 0.003 mA, the viewing field area is 1 cm 2, and the composition at the central portion of the inclusion is quantitatively analyzed by characteristic X-ray wavelength dispersion spectroscopy. The size of the inclusions to be analyzed is assumed to be 0.2 µm or more in maximum diameter, and the number of analyzes is 100 randomly selected.

The element to be analyzed is an element other than O, C, N, S, and considering the composition of the steel of the present invention, the element to be analyzed is Al, Mn, Si, Ti, Zr, Ca, REM (for example, La and Ce). When the molar ratio of Al, Mn, Si, Ti, Zr, Ca, and REM included in the inclusions is converted into moles, and the total amount of elements after conversion is 1 mol, the mole fraction of each element included in the inclusions to be analyzed is calculated. do.

Next, the component composition in the steel material (base material) of this invention is demonstrated. The steel materials of the present invention are characterized by containing REM: 0.0010 to 0.1% and Zr: 0.001 to 0.05%. The reason for determining this range is as follows.

REM and Zr are elements which contribute to the toughness improvement of HAZ by forming single inclusions or composite inclusions of REM and Zr in steel materials. Examples of the inclusions of REM include oxides of REM and sulfides of REM. Examples of the inclusions of Zr include oxides of Zr, carbides of Zr, nitrides of Zr, and the like. Examples of the composite inclusions of REM and Zr include forms such as oxides, sulfides, or oxysulfides containing REM and Zr. In addition, these inclusions may also be in the form of coexisting with nitride (for example, TiN or the like) or other sulfide (for example, CaS or MnS).

REM should be 0.0010% or more, Preferably it is 0.003% or more, More preferably, it is 0.006% or more, More preferably, it is 0.010% or more. However, when excessively added, coarse inclusions (for example, oxides, etc.) are generated and the toughness of the base material is degraded, so it must be suppressed to 0.1% or less. Preferably it is 0.09% or less, More preferably, you may be 0.08% 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, La, Ce, and Y are used. It is preferable to contain at least 1 sort (s) of element chosen from the group which consists of, and it is preferable to contain La and / or Ce more preferably.

Zr should be 0.001% or more, Preferably it is 0.003% or more, More preferably, it is 0.005% or more. However, when excessively added, coarse Zr carbides are formed and the toughness of the base material is deteriorated, so it must be suppressed to 0.05% or less. Preferably it is 0.04% or less, More preferably, you may be 0.03% or less.

The steel material of this invention contains REM and Zr, and as a basic element, it is C: 0.01-0.2%, Si: 0.5% or less (not containing 0%), Mn: 2.5% or less (containing 0%) Ti: 0.03% or less (not including 0%), and N: 0.01% or less (not including 0%). The reason for determining such a range is as follows.

C is an indispensable element in order to secure the strength of the steel (base material), and it is necessary to contain C at least 0.01%. It is preferable to contain C 0.02% or more, More preferably, you may be 0.03% or more. However, when it exceeds 0.2%, a lot of island-like martensite is produced | generated in HAZ at the time of welding, not only causing toughness of HAZ but also adversely affecting weldability. Therefore, C needs to be suppressed to 0.2% or less, Preferably it is 0.18% or less, More preferably, it is 0.15% or less.

Si is an element which has a deoxidation effect and contributes to the strength improvement of steel materials (base materials). In order to exhibit such an effect effectively, it is preferable to contain 0.02% or more, More preferably, it is 0.05% or more, More preferably, it is good to contain 0.1% or more. However, when it exceeds 0.5%, since the weldability of a steel material (base material) and base material toughness deteriorate, it is necessary to suppress it to 0.5% or less. Preferably it is 0.45% or less, More preferably, you may suppress to 0.4% or less. Moreover, when further high toughness is calculated | required by HAZ, it is good to suppress Si to 0.3% or less. More preferably, it is 0.05% or less, More preferably, it is 0.01% or less. However, when the Si content is suppressed in this way, the toughness of the HAZ is improved, but the strength tends to be lowered.

Mn is an element which contributes to the strength improvement of a steel material (base material), and in order to exhibit such an effect effectively, it is preferable to contain Mn 0.5% or more. More preferably, it is 0.7% or more, More preferably, it is 0.8% or more. However, when it contains excessively more than 2.5%, HAZ toughness will deteriorate and weldability of steel materials (base material) will deteriorate. Therefore, it is necessary to suppress Mn amount to 2.5% or less. Preferably it is 2.3% or less, More preferably, it is 2% or less.

Ti is an element which forms nitrides, such as TiN, and Ti oxide in steel materials, and contributes to the toughness improvement of HAZ. In order to effectively exhibit such an effect, it is preferable to contain Ti 0.005% or more, More preferably, it is 0.007% or more, More preferably, you may be 0.010% or more. However, excessive addition deteriorates the toughness of the steel (base metal), so it must be suppressed to 0.03% or less. Preferably it is 0.025% or less, More preferably, you may be 0.020% or less.

N is an element that precipitates nitride (for example, ZrN, TiN, etc.), which prevents coarsening of austenite grains formed in HAZ during welding and promotes ferrite transformation, thereby improving HAZ toughness. Contribute to. In order to exhibit these effects effectively, it is contained 0.002% or more. More preferably, it is 0.003% or more. As N increases, the micronization of the austenite grains is promoted, and thus it is effective in improving the toughness of the HAZ. However, when it exceeds 0.01%, the amount of solid solution N will increase and the toughness of a base material will deteriorate. Therefore, N needs to be suppressed to 0.01% or less, Preferably it is 0.009% or less, More preferably, you may be 0.008% or less.

In addition to the above-mentioned elements, the steel of the present invention contains P: 0.02% or less (does not contain 0%), S: 0.015% or less (does not contain 0%), and Al: 0.01% or less (0% It will be satisfied). The reason for determining such a range is as follows.

P is an element which tends to segregate, especially segregates at grain boundaries in steel materials and deteriorates toughness. Therefore, P needs to be suppressed to 0.02% or less, Preferably it is 0.018% or less, More preferably, you may be 0.015% or less.

S is a harmful element that combines with Mn to form sulfide (MnS), which degrades the toughness of the base material and the ductility in the sheet thickness direction. Therefore, S should be suppressed to 0.015% or less, Preferably it is 0.012% or less, More preferably, it is 0.008% or less, Especially 0.006% or less.

Al is an element with strong deoxidation power, and when added excessively, it becomes difficult to reduce the oxide to produce a desired oxide. Therefore, Al needs to be suppressed to 0.01% or less, Preferably it is 0.0090% or less, More preferably, you may be 0.0080% or less. In addition, Al may be 0%.

The containing element prescribed | regulated by this invention is as above, and remainder is iron and an unavoidable impurity. As the inevitable impurities, incorporation of elements (for example, Mg, As, Se, etc.) carried in according to the situation of raw materials, materials, manufacturing facilities, and the like may be allowed.

Next, the manufacturing method which can be suitably employ | adopted in manufacturing the steel material of this invention is demonstrated. In order to secure a predetermined amount of solid solution REM and solid solution Zr, the steel of this invention adds REM and Zr to the molten steel which adjusted the total oxygen amount [O] ₁ to 0.0020 to 0.015%, and dissolved oxygen amount [O] ₂ After adjusting to 0.0010 to 0.0035% of range, it can manufacture by casting.

That is, when REM and Zr are combined and added to molten steel which appropriately controlled the total oxygen amount [O] ₁ , REM and Zr can be formed in the steel as an oxide of one inclusion. At this time, by adjusting the amount of REM and Zr compositely added to the molten steel, the dissolved oxygen amount [O] ₂ of the molten steel is appropriately controlled and the molten steel can be cast to reduce the amount of solid solution REM and solid solution Zr in the steel.

Usually, the total oxygen amount [O] ₁ in molten steel refine | purified primarily by the converter and the electric furnace exceeds 0.015%. When REM or Zr is added to this molten steel, since the amount of oxygen in molten steel is too large, reaction of REM, Zr, and oxygen will become severe and it is unpreferable in solvent operation. In addition, coarse REM oxide and coarse ZrO ₂ are generated, and the base metal toughness itself deteriorates.

Therefore, in the present invention, REM oxide is added as a inclusion of REM, Zr oxide as an inclusion of Zr, or REM and Zr as a composite inclusion of REM and Zr by adding REM and Zr to molten steel with a total oxygen amount [O] ₁ adjusted less than before. It is possible to produce oxides comprising Zr.

On the other hand, among the inclusions of REM and Zr, in particular, from the viewpoint of increasing the amount of oxides, a large amount of REM and Zr may be added to the molten steel in which the total oxygen amount [O] ₁ is adjusted. Zr hires in steel. However, as the employment REM and the employment Zr increase, the HAZ toughness changes as described above.

Therefore, in the present invention, by adjusting the amount of REM and Zr added to molten steel, the amount of dissolved oxygen [O] ₂ after adding REM and Zr is adjusted more than before, so that REM and Zr are not dissolved in casting. It was.

The total amount of oxygen [O] ₁ before adding REM and Zr is less than the amount of total oxygen contained in ordinary molten steel, and should be suppressed to 0.015% or less, preferably 0.01% or less, and more preferably 0.008% or less. do. However, if the total oxygen amount [O] ₁ is too small and less than 0.0020%, the amount of oxygen will be insufficient. Therefore, even if REM and Zr are combined, the amount of oxides contributing to the improvement of toughness of HAZ cannot be secured. REM and Zr, which could not be solved, are dissolved in steel, or Zr forms carbides and the like, which degrades the toughness of the base metal. Therefore, it is preferable to adjust the total amount of oxygen [O] ₁ before adding REM and Zr to 0.0020% or more, More preferably, it is 0.0025% or more.

The total amount of oxygen [O] _{1 means} the total amount of oxygen contained in molten steel (total amount of O), and the sum of the amount of oxygen contained in the molten steel as a dissolved atom (so-called free oxygen) and the amount of oxygen existing as an oxide-based inclusion. It means total amount of oxygen. The amount of oxygen contained in the molten steel as the dissolved atom can be measured by using an oxygen sensor using a solid electrolyte. The total amount of oxygen can be measured by a general inert gas fusion-infrared absorption method or the like.

In order to adjust the total oxygen amount [O] ₁ in the molten steel to the above range, for example, a method of deoxidation using an RH degassing refinery, a ladle heated refining apparatus, a simple molten steel treatment plant, or the like is used. The method of deoxidation, the method of deoxidation by adding deoxidation elements, such as Si, Mn, Ti, Al, to molten steel, etc. are mentioned. Of course, you may adjust total oxygen amount [O] ₁ by combining these methods suitably. When employ | adopting the method of adding a deoxidation element, you may add a deoxidation element when going out to a ladle from a converter.

The order of complex addition of REM and Zr to the molten steel in which the total oxygen amount [O] ₁ is adjusted is not particularly limited. For example, (a) Zr may be added after REM is added, and (b) Zr is added. REM may be added later, and (c) REM and Zr may be added simultaneously. When adding two or more types of REM, you may add simultaneously or separately. For example, you may add in order of Ce->Zr-> La.

The form of REM or Zr added to molten steel is not particularly limited. For example, as REM, pure La, pure Ce, pure Y, or the like, pure Zr, or Fe-Si-La alloy or Fe-Si-Ce alloy And Fe-Si-La-Ce alloy may be added. In addition, you may add Misch Metal to molten steel. A misch metal is a mixture of a cerium group rare earth element, Specifically, it contains about 40 to 50% of Ce, and about 20 to 40% of La.

After composite addition of said REM and Zr, as long as it does not affect the said dissolved oxygen amount [O] ₂ immediately before casting, you may add an alloying element and adjust the component of steel materials.

The said dissolved oxygen amount [O] ₂ just before casting is made into 0.0010% or more. If the amount is less than 0.0010%, the amount of oxygen is insufficient, so that REM and Zr are solid-dissolved in the steel during casting, causing a change in the HAZ toughness. Therefore, dissolved oxygen amount [O] ₂ is made into 0.0010% or more, Preferably it is 0.0015% or more. However, when the dissolved oxygen amount [O] ₂ becomes excessive, a large amount of coarse oxide is formed during casting, thereby lowering the toughness of the base material itself. Therefore, dissolved oxygen amount [O] ₂ should be suppressed to 0.0035% or less, Preferably it is 0.0030% or less, More preferably, you may be 0.0025% or less.

In order to control the said dissolved oxygen amount [O] ₂ in 0.0010 to 0.0035% of range, what is necessary is just to adjust the addition amount of REM and Zr according to the total oxygen amount [O] ₁ , Specifically, according to the total oxygen amount [O] ₁ What is necessary is just to determine the addition amount of REM and Zr so that Formula (1) may be satisfied, and to add an element in the range of the addition amount of REM and Zr determined. In formula, [REM] and [Zr] are addition amounts (mass%) of REM or Zr, respectively, and [O] ₁ is the total oxygen amount (mass%) of molten steel before adding REM and Zr. The coefficient 15 on the right side is a value determined as a result of repeating the experiment.

[REM] + [Zr] ≤ 15 × [O] ₁ ... (One)

However, the amount of REM (total REM) and Zr (total Zr) contained in the steel material must satisfy the range specified by the above-mentioned component composition.

When the amount of dissolved oxygen [O] ₂ is less than 0.0010% by adding a large amount of REM or Zr in contrast to the total amount of oxygen [O] ₁ , an oxide [for example, MnO or iron oxide (for example, FeO)] may be added.

The molten steel obtained by preparing such a component may be continuously cast in accordance with a conventional method to form a slab, and then hot rolled in accordance with a conventional method.

The steel according to the present invention can be used, for example, as a material for structures such as bridges, high-rise buildings, ships, etc., and can prevent the deterioration of the toughness of the weld heat-affected zone even in large heat input welding to medium heat welding. Can be.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are not intended to limit the present invention, and the present invention may be modified as appropriate within a range suitable for the purpose of the preceding and the following. These are all included in the technical scope of this invention.

[Example 1]

After the molten iron was first refined in a 240 ton converter, the molten iron was pulled out from the converter in a ladle and subjected to secondary refining while adjusting the components and adjusting the temperature.

In the ladle, the chemical component composition was adjusted by adjusting the total oxygen amount [O] ₁ shown in Table 1 by the deoxidation method shown in Table 1 below. Total oxygen amount [O] ₁ means the total amount of oxygen in which the amount of oxygen contained in the molten steel as dissolved atoms and the amount of oxygen existing as oxide-based inclusions is used. The amount of oxygen contained in the molten steel as dissolved atoms is determined using an oxygen sensor using a solid electrolyte. The total amount of oxygen was measured by a general inert gas fusion-infrared absorption method. In addition, in addition to the total oxygen amount [O] ₁ , the following Table 1 also shows the dissolved oxygen amount of molten steel before adding REM and Zr.

The addition amount of REM and Zr was computed according to said total oxygen amount [O] ₁ so that Formula (1) was satisfied, REM and Zr were added, and it adjusted to the dissolved oxygen amount [O] ₂ shown in following Table 1. In Table 1 below, the sum of the addition amount [REM] of REM, the addition amount [Zr] of Zr, and the addition amount [REM] + [Zr] of REM and Zr is shown. Incidentally, the ratio of the total REM and the addition amount of Zr and the total amount of oxygen _{[O] 1 ([REM]} + [Zr]) / [O] 1 are shown together.

After adjusting to the dissolved oxygen amount [O] ₂ , it cast after adjusting a chemical component so that it does not affect the said [O] ₂ amount.

In the secondary refining, dehydration and desorption were performed using an RH type degassing refiner or the like.

In Table 1, La is in the form of Fe-La alloy, Ce is in the form of Fe-Ce alloy, REM is in the form of a mismetal containing about 50% La and about 25% Ce, Zr is Zr Each was added individually. Blanks in Table 1 indicate that no elements were added.

In Fig. 1, the relationship between the total amount of oxygen [O] ₁ before adding REM and Zr and the sum of the amounts of addition of REM and Zr is shown in a graph. In Fig. 1,? Indicates the results of numbers 1 to 12 of Table 1 below, and x indicates the results of numbers 17 to 20 of Table 1 below.

In addition, following Table 2 shows the component composition of the steel material after component adjustment. However, the blanks in Table 2 below indicate that no elements were detected.

The molten steel after component adjustment was cast into the slab by the continuous casting machine, and the sample was cut out from the cross section in the D / 4 position of the slab (where D is the thickness of the slab). The cut-out sample surface was observed 10,000 times using EPMA "JXA-8500F (device name) made from Nippon Denshi, and the component composition was quantitatively analyzed about the inclusion of 0.2 micrometer or more in maximum diameter. Observation conditions made the acceleration voltage 7 kW, the sample current 0.003 kPa, the observation field area 1 cm <2>, and the number of analyzes as 100 randomly selected, and quantitatively analyzed the composition of components in the center part by wavelength dispersion spectroscopy of characteristic X-rays. . The element to be analyzed is Al, Mn, Si, Ti, Zr, Ca, La, Ce, and the inclusions to be analyzed when molar conversion of the abundance of the elements to be analyzed and the total amount of elements after conversion are 1 mole. The mole fraction of each element contained in was computed. The calculation result of the mole fraction is shown in Table 3 below. Blanks in Table 3 below indicate that no detection was made.

As a result of observing the sample surface with EPMA, most of the observed inclusions were composite inclusions containing REM and Zr, but also inclusions of REM and inclusions of Zr were produced as single inclusions.

In addition, the amount of solid solution REM and the amount of solid solution Zr contained in steel materials were computed in the following procedure. First, the amount of REM and Zr contained in steel as inclusions was measured by the electrolytic extraction method. In electrolytic extraction, the sample was extracted (electrolysis) under a current of 500 A / m 2 or less using a solution containing triethanolamine 2 kPa and 1 g of tetramethylammonium chloride in 100 kPa of methanol as an electrolytic solution. As a result, the matrix was dissolved and solid solution REM and Zr were also extracted in the electrolyte solution. The size of the sample was 15 mm long x 15 mm wide x 5 mm long.

Subsequently, the extracted electrolyte solution was filtered using a membrane filter (filter diameter: 47 mm, pore size: 0.1 µm), the filter residue was transferred to a platinum crucible, heated by a gas burner, and incinerated. Subsequently, an alkali flux (a mixture of sodium carbonate and sodium tetraborate) was added, and the residue was further heated by a gas burner to melt the residue. Next, after adding 18 volume% hydrochloric acid to liquefy the melt, it moved to the flask, the pure water was further added, it massed up, and the analysis liquid was obtained. REM and Zr concentrations in the assay were measured by ICP-MS method.

The amount of REM and amount of solid solution Zr is determined by subtracting the amount of REM and Zr contained in the inclusions thus obtained from the amount of REM (total REM) or Zr (total Zr), which is analyzed by a separate conventional ICP-MS method. Obtained. The calculated results are shown in Table 2 together.

2 shows the relationship between the dissolved oxygen amount [O] ₂ contained in the molten steel before casting, and the amount of solid solution REM or solid solution Zr contained in the steel.

Next, in order to evaluate the toughness of HAZ affected by heat during welding, a welding regeneration test was performed to simulate high heat input welding. The welding reproduction test was performed by heating so that the whole sample cut out from the slab might be 1400 ° C, held at this temperature for 5 seconds, and then cooled. Cooling was adjusted so that cooling time from 800 degreeC to 500 degreeC might be 300 second.

The impact characteristic of the sample after cooling was evaluated by performing the V notch Charpy test and measuring the absorption energy (vE- ₄₀ ) at _-40 degreeC.

Samples are taken three from the same steel grade in accordance with JIS Z2242 "Charpy Impact Test Method of Metallic Materials", and the results of measuring vE- ₄₀ for each sample and their average values are shown in Table 4 below. The average value of vE- ₄₀ is 150 J or more as a pass (good HAZ toughness).

In addition, for each sample, the variation in toughness was evaluated based on the following maximums and minimums of the vE _{- 40} values. The evaluation results are shown in Table 4 below.

[Evaluation Criteria for Maximum Value and Minimum Value]

(Circle): The maximum value or minimum value of HAZ toughness is 150 J or more.

X: The maximum value or minimum value of HAZ toughness is less than 150J.

[Integrated evaluation criteria]

(Circle): The minimum value is 150 J or more in three measurement results, and high HAZ toughness is ensured stably.

(Triangle | delta): Although at least 1 is 150 J or more in three measurement results, the variation of HAZ toughness is large and the minimum value is less than 150 J.

X: Out of three measurements, all are less than 150J.

In Fig. 3, for each sample shown in Table 4 below, the average value of the HAZ toughness (circle in the figure), and the widths of the maximum and minimum values of the HAZ toughness are shown graphically.

From the above result, it can consider as follows. As is apparent from FIG. 1, REM and Zr are added to molten steel obtained by adjusting the total oxygen amount [O] ₁ to 0.0020 to 0.015% (20 to 150 ppm) before adding REM and Zr. When added, it turns out that HAZ toughness becomes favorable and the variation of HAZ toughness also becomes small. Moreover, the formula of the straight line shown in FIG. 1 is ([REM] + [Zr]) = 15x [O] ₁ .

As is apparent from Table 4 and Fig. 2, when the dissolved oxygen amount [O] ₂ before casting is adjusted in the range of 0.0010 to 0.0035% (10 to 35 ppm), the amount of solid solution REM and solid solution Zr contained in the steel is reduced. It turns out that it can reduce below stationary.

As is apparent from Tables 4 and 3, the numbers 1 to 12 are examples satisfying the requirements defined in the present invention, in particular, the amount of REM and the amount of solid solution Zr in the chemical composition of steel are appropriately adjusted, Since the amount of solid solution Zr is controlled appropriately, the average value of HAZ toughness is 150 J or more, and HAZ toughness is excellent. In addition, fluctuations in HAZ toughness are also reduced.

On the other hand, Nos. 13 to 21 are examples which deviate from the requirements stipulated in the present invention, in particular, the amount of REM or Zr in the chemical composition of the steel is out of the range specified in the present invention, or the amount of solid solution REM and the amount of solid solution Zr is present. Since it deviates from the range prescribed | regulated by this invention, the average value of HAZ toughness becomes less than 150 J, and HAZ toughness deteriorates. In addition, a large variation in the HAZ toughness also increases.

Table 1

[Table 2]

[Table 3]

Table 4

[Example 2]

The slabs of the number 3, the number 4, the number 8, the number 14, and the number 20 obtained by the said 1st experiment example were hot-rolled, and the steel plate of thickness 30mm was obtained.

About the cross section perpendicular | vertical to the rolling direction of the obtained steel plate, the component composition of an interference | inclusion was quantitatively analyzed on the conditions similar to the said 1st experiment example, and the mole fraction of each element contained in the interference | inclusion made into the analysis object was computed. The observation position was set to the D / 4 (where D is the thickness of the steel sheet) position of the steel sheet, and the observation conditions were the same as those of the first experimental example except that the viewing field area was 1 cm 2. The calculation result of the mole fraction is shown in Table 5 below. Blanks in Table 5 below indicate that no detection was made.

Next, in order to evaluate the toughness of HAZ affected by heat during welding, a welding regeneration test was performed to simulate high heat input welding. The welding reproduction test was performed by heating the test piece cut out from the steel sheet so that the whole was 1400 ° C., holding it at this temperature for 5 seconds, and then cooling it. The cooling rate was adjusted so that the cooling time from 800 degreeC to 500 degreeC might be 300 second.

The impact characteristic of the test piece after cooling was evaluated by performing the V notch Charpy test and measuring the absorption energy (vE- ₄₀ ) in _-40 degreeC. The results of measuring vE- ₄₀ for each sample and their averages are shown in Table 6 below. In addition, the results of evaluating the variation of the HAZ toughness as in the first experimental example is shown in Table 6 below.

In Fig. 4, for each sample shown in Table 6 below, the average value of the HAZ toughness (circle in the figure), and the width of the maximum and minimum values of the HAZ toughness are shown in a graph.

As apparent from the following Table 5 and Table 6, it can be seen that the impact characteristics of the test piece cut out from the steel sheet obtained by hot rolling are almost the same as the impact characteristics of the sample cut out from the slab, so that the HAZ toughness does not change even when hot rolling. . In addition, the ease of variation of the HAZ toughness also shows the same tendency in the samples cut out from the steel sheet and slab obtained by hot rolling.

[Table 5]

Table 6

1 is a graph showing the relationship between the total amount of oxygen [O] ₁ before adding REM and Zr, and the sum of the amounts of addition of REM and Zr.

2 is a graph showing the relationship between the dissolved oxygen amount [O] ₂ contained in molten steel before casting, and the amount of solid solution REM or solid solution Zr contained in the steel.

3 is a graph showing the average value of the HAZ toughness and the width of the maximum and minimum values of the HAZ toughness.

4 is a graph showing the average value of the HAZ toughness and the width of the maximum and minimum values of the HAZ toughness.

Claims (4)

C: 0.01 to 0.2% (meaning of "mass%", the same below), Si: 0.5% or less (not including 0%), Mn: 2.5% or less (not including 0%), Ti: 0.03% or less (not including 0%), and N: 0.01% or less (not including 0%), P: 0.02% or less (not including 0%), S: 0.015% or less (not including 0%), and Al: 0.01% or less (including 0%) is satisfied, Also, REM: 0.0010 to 0.1% and Zr: 0.001 to 0.05%, respectively, The remainder is a steel material consisting of iron and inevitable impurities, Contains REM and Zr in the inclusions, Solid solution REM: 0.0010% or less (including 0%), Solid solution Zr: satisfies 0.0010% or less (including 0%), The mole fraction of REM was measured when the composition of the inclusions contained in the steel was measured, and the molar ratio of elements other than O, C, N, and S among the elements contained in the inclusions was converted into moles, and the total amount of elements after conversion was 1 mole. It is 0.050 or more and Zr mole fraction satisfy | fills 0.04 or more, The steel material excellent in the toughness of the weld heat influence part. delete It is a method of manufacturing the steel material of Claim ₁ , REM and Zr are added to molten steel which adjusted total oxygen amount [O] ₁ to 0.0020 to 0.015% of range, and dissolved oxygen amount [O] ₂ of 0.0010 to 0.0035% of range The steel sheet manufacturing method which is excellent in the toughness of the weld heat affected part characterized by casting after adjusting with the. The total amount of oxygen [O] ₁ is measured, and according to the total amount of oxygen [O] ₁ , REM and Zr are added to adjust the dissolved oxygen amount [O] ₂ so as to satisfy the following formula (1). Manufacturing method of steel with excellent toughness of weld heat affected zone. [REM] + [Zr] ≤ 15 × [O] ₁ ... (One) However, in formula (1), [REM] and [Zr] are the addition amount (mass%) of REM or Zr, respectively, and [O] ₁ is the total oxygen amount (mass%) of molten steel before adding REM and Zr.

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