JP5202031B2 - Steel material excellent in toughness of weld heat-affected zone and method for producing the same - Google Patents

Description

  The present invention relates to a steel material used for structures such as bridges, high-rise buildings, and ships. More specifically, the present invention relates to a portion that is affected by heat (hereinafter referred to as “welding heat affected zone” or “ The present invention relates to a steel material having improved toughness (sometimes referred to as “HAZ”) and a manufacturing method thereof.

  The properties required for steel materials used in bridges, high-rise buildings, ships and the like have become increasingly severe in recent years, and particularly good toughness is required. In general, these steel materials are often joined by welding. In particular, HAZ has a problem that the toughness is easily deteriorated due to thermal influence during welding. This toughness deterioration becomes more prominent as the heat input during welding increases, and the cause is that the larger the heat input during welding, the slower the cooling rate of the HAZ, and the lower the hardenability and the generation of coarse island martensite. It is thought that there is to do. Therefore, in order to improve the toughness of the HAZ, it is considered that the heat input during welding should be suppressed as much as possible. However, on the other hand, in order to increase the welding work efficiency, it is desired to employ a high heat input welding method in which the heat input of welding is 50 kJ / mm or more, such as electrogas welding, electroslag welding, submerged welding, and the like.

  Several steel materials that suppress the HAZ toughness deterioration when the high heat input welding method is adopted have already been proposed. For example, Patent Document 1 discloses a steel material in which fine TiN is dispersed and reprecipitated in the steel material to suppress coarsening of austenite grains generated in the HAZ when high heat input welding is performed, and deterioration in HAZ toughness is suppressed. Has been proposed. However, as a result of investigations by the present inventors, when the weld metal reaches a high temperature of 1400 ° C. or higher, the above-described heat is received by the heat received during welding at a portion of the HAZ that is particularly close to the weld metal (hereinafter sometimes referred to as “bond portion”). It was found that TiN disappeared in solid solution, and the deterioration of HAZ toughness could not be suppressed.

  Patent Document 2 discloses controlling the form of oxides and nitrides contained in steel as a technique for improving the toughness of the base material and the HAZ. Patent Document 2 describes that by using Ti and Zr in combination, fine oxides and nitrides are generated to improve the toughness of the base material and the HAZ. Further, it is disclosed that Ti and Zr should be added in this order in the manufacturing process in order to generate such fine oxides and nitrides. However, as a result of studies by the present inventors, it is sufficient to increase the amount of oxide in order to further increase the toughness of HAZ. However, in the technique disclosed in Patent Document 2, a large amount of Ti or Zr is used to increase the amount of oxide. It has been found that when added, carbides such as Ti and Zr are formed, and the toughness of the steel (base material) is decreased.

By the way, the present applicant has previously proposed a steel material in which the HAZ toughness does not deteriorate even if it is affected by high temperature heat during welding. This steel material is composed of complex oxides such as La ₂ O ₃ —SiO ₂ oxide, Ce ₂ O ₃ —SiO ₂ oxide, and La ₂ O ₃ —Ce ₂ O ₃ —SiO ₂ oxide in the steel material. This composite oxide is in a liquid state in molten steel, so it is finely dispersed in the steel, and it does not disappear in solid solution even if it is affected by heat at the time of welding. This contributes to improving the toughness of HAZ. . Patent Document 3 discloses that in order to generate the composite oxide, La and Ce are added to the molten steel whose dissolved oxygen content is adjusted, and then Si is added. Further, Patent Document 3 describes that the toughness of HAZ can be further enhanced by adding Ti to the steel material and precipitating TiN in the steel material structure, and in order to generate such TiN, the composite oxide is generated. It discloses that Ti should be added to molten steel.

In addition, although it is not a technique aiming at the improvement of HAZ toughness, Patent Document 4 contains elements such as REM and Zr in the steel material, and by positively containing solid solution REM and solid solution Zr, There has been proposed a technique for preventing the hydrogen-based ultrasonic flaw detection defect to improve the internal quality of the thick steel plate and maintaining the soundness of the internal quality. In this technique, Al, Ca, Ti and the like are added in combination in order to secure a stable solid solution amount.
Japanese Patent Publication No.55-26164 JP 2003-213366 A JP 2005-48265 A JP-A-8-120401

  The present inventors pay attention to inclusions having a composition other than the composite oxide having the above composition with respect to the technique previously disclosed as Patent Document 3, and include inclusions having a composition different from that of Patent Document 3 in the steel material. Investigations were made as to whether the toughness of the HAZ could be increased by dispersing.

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

  The steel materials according to the present invention that have solved the above problems are: C: 0.01 to 0.2% (meaning “mass%”; the same shall apply hereinafter), Si: 0.5% or less (including 0%) Not including), Mn: not more than 2.5% (not including 0%), Ti: not exceeding 0.03% (not including 0%), and N: not exceeding 0.01% (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%), Further, REM: 0.0010 to 0.1% and Zr: 0.001 to 0.05% respectively, and the balance is a steel material composed of iron and inevitable impurities, and REM and Zr are contained in the inclusions. In addition, solid solution REM: 0.0010% or less (including 0%) and solid solution Zr: 0.0010% or less (including 0%) It includes the features in that satisfactory.

  The above steel material measures the composition of inclusions contained in the steel material, converts the abundance ratio of elements other than O, C, N, and S among the elements contained in the inclusions, and converts the element amount after conversion. When the total is 1 mol, it is preferable that the molar fraction of REM is 0.050 or more and the molar fraction of Zr is 0.04 or more.

The steel material, as another element,
(1) Ca: 0.01% or less (excluding 0%),
(2) Cu: 2% or less (not including 0%), Ni: 3.5% or less (not including 0%), Cr: 3% or less (not including 0%), Mo: 1% or less ( Nb: not more than 0.25% (not including 0%), V: not more than 0.1% (not including 0%), and B: not more than 0.005% (including 0%) One or more elements selected from the group consisting of:
Etc. may be included.

When manufacturing the steel material, REM and Zr are added to the molten steel in which the total oxygen amount [O] ₁ is adjusted to a range of 0.0020 to 0.015%, and the dissolved oxygen amount [O] ₂ of the molten steel is changed. After adjusting to the range of 0.0010 to 0.0035%, casting may be performed.

In order to adjust the dissolved oxygen amount [O] ₂ , for example, the total oxygen amount [O] ₁ is measured, and REM is satisfied so as to satisfy the following formula (1) according to the total oxygen amount [O] _1. And Zr may be added. However, in (1), [REM] and [Zr] are the addition amounts (mass%) of REM or Zr, respectively, and [O] ₁ is the total oxygen amount of the molten steel before adding REM and Zr. (Mass%).
[REM] + [Zr] ≦ 15 × [O] ₁ (1)

  According to the present invention, by adding REM and Zr to the steel material in combination, inclusions having a composition different from the steel material proposed in Patent Document 3 can be dispersed in the steel material. Even if a high temperature is reached, solid solution does not disappear in the steel material, so that it is possible to prevent toughness deterioration of the weld heat affected zone (HAZ) even if high heat input welding is performed as well as small to medium heat input welding. Moreover, according to this invention, the variation in HAZ toughness can be suppressed by reducing the amount of solid solution REM and the amount of solid solution Zr contained in steel materials as much as possible.

  The present inventors have repeatedly studied whether or not the improvement of HAZ toughness can be achieved by dispersing inclusions having a composition different from that of Patent Document 3 in the steel material. As a result, it has been found that HAZ toughness can be improved by adding REM and Zr to a steel material and adjusting the inclusion to contain REM and Zr. Furthermore, the inventors have found that if the solid solution REM amount and the solid solution Zr amount contained in the steel material are reduced as much as possible, the phenomenon of local deterioration of toughness can be prevented, and variation in HAZ toughness can be suppressed, and the present invention has been completed. .

  First, the steel material of the present invention contains REM and Zr in inclusions. The inclusion of REM and Zr in the inclusion means that the steel material contains a single inclusion or a composite inclusion of REM and Zr. Hereinafter, for convenience of explanation, the single inclusion and the composite oxide may be collectively referred to as “inclusion”.

  Since inclusions of REM and Zr are affected by heat during welding and do not disappear as a solid solution even at a high temperature of 1400 ° C., if these inclusions are included, the coarse austenite grains in the HAZ during welding Since it is possible to suppress crystallization and promote intragranular transformation during cooling, the HAZ structure can be refined. As a result, the toughness of HAZ can be further improved.

  Moreover, by adding REM and Zr together and including them as inclusions in the steel material, it is possible to prevent the formation of coarse Zr single carbides and coarse REM sulfides that cause toughness deterioration of the steel (base material). As a result, the toughness of the HAZ can be improved while suppressing toughness deterioration of the base material. That is, when adding REM or Zr alone, in order to increase the number of inclusions, it is necessary to increase the amount of REM or Zr added. The size of the inclusions and single inclusions of Zr is increased, and the HAZ toughness is deteriorated. Therefore, when REM or Zr is added alone, there is a limit to the amount of addition, so that the amount of REM or Zr added cannot be increased, and the amount of fine inclusions cannot be increased beyond a certain level. Therefore, the HAZ toughness could not be improved.

  On the other hand, if inclusions containing REM and Zr are contained in the steel material, the absolute amount of inclusions contained in the steel material is increased as compared with the case where REM is contained alone or Zr is contained alone. Therefore, the toughness of HAZ can be further improved.

  Thus, by including inclusions of REM and Zr in the steel material, the toughness of the HAZ can be improved. Therefore, in order to improve the toughness of HAZ, it is desirable that REM and Zr are positively added to generate a lot of inclusions in the steel material. However, when steel materials with increased contents of REM and Zr are welded and the toughness of HAZ is measured at multiple locations, the toughness decreases locally and the measured values vary, especially in the vicinity of the bond where heat effects are large. There was found. Then, when the structure of the part where the toughness fell locally was observed, it became clear that REM and Zr were segregating at the grain boundary. As a result of repeated studies to reduce the segregation of REM and Zr, it was found that the amount of solute REM and the amount of solute Zr in the steel material should be reduced.

  That is, it is important for the steel material of the present invention to satisfy a solid solution REM: 0.0010% or less (including 0%) and a solid solution Zr: 0.0010% or less (including 0%). If the amount of solute REM in the steel material exceeds 0.0010% or the amount of solute Zr exceeds 0.0010%, REM and Zr segregate at the grain boundaries when subjected to thermal effects during welding, and toughness Is reduced locally. Therefore, the solid solution REM content is 0.0010% or less, preferably 0.0008% or less, more preferably 0.0005% or less. The amount of solid solution Zr is 0.0010% or less, preferably 0.0008% or less, more preferably 0.0005% or less. The amount of solid solution REM and the amount of solid solution Zr should be reduced as much as possible, and most preferably 0%.

  The total of the solid solution REM and the solid solution Zr is preferably 0.0015% or less, and more preferably 0.0010% or less.

  From the REM content (total REM content) calculated by analyzing by ICP [Inductively Coupled Plasma] -MS method, as shown in the examples described later, What is necessary is just to calculate by subtracting the amount of REM contained in the inclusion contained in the steel material calculated by electrolytic extraction and ICP-MS.

  Similarly, the solid solution Zr amount may be calculated by subtracting the Zr amount contained in the inclusions contained in the steel material from the Zr content (total Zr content).

  Regarding the steel material of the present invention, “contains REM and Zr in inclusions” means (a) a single inclusion of REM and a single inclusion of Zr, or (b) a composite containing REM and Zr. It means that it contains inclusions, or (c) contains a single inclusion of REM and a single inclusion of Zr, and a composite inclusion containing REM and Zr.

  Examples of REM single inclusions include REM oxides and REM sulfides. Examples of Zr single inclusions include Zr oxides, Zr carbides, and Zr nitrides. It is done. Examples of the composite inclusion of REM and Zr include oxides, sulfides, and oxysulfides containing REM and Zr. In addition, these inclusions may be in the form of coexistence with nitride (for example, TiN) or other sulfide (for example, CaS, MnS).

  The steel material measures the composition of inclusions contained in the steel material, converts the abundance ratio of elements other than O, C, N, and S among the elements constituting the inclusions in molar terms, and the element amount after conversion It is preferable that the molar fraction of REM is 0.050 or more and the molar fraction of Zr is 0.04 or more when the whole is 1 mol. The molar fraction of REM is preferably 0.10 or more, more preferably 0.15 or more, and still more preferably 0.20 or more. On the other hand, the molar fraction of Zr is preferably 0.08 or more, more preferably 0.10 or more, and still more preferably 0.15 or more.

  The steel material of the present invention measures the composition of inclusions contained in the steel material, converts the abundance ratio of elements other than O, C, N, and S among the elements constituting the inclusions, When the total amount of elements is 1 mol, the total of the molar fractions of REM and Zr is preferably 0.10 or more. If the total is less than 0.10, the amount of inclusions contributing to the improvement of HAZ toughness is insufficient, and the HAZ toughness 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 REM inclusion and the Zr inclusion is not particularly limited, but may be, for example, CaO, SiO ₂ , Al ₂ O ₃ , MnO, TiN, or TiC.

The composition of inclusions contained in the steel material is determined by observing the cross section of the steel material with, for example, an electron probe X-ray micro analyzer (EPMA) and quantitatively analyzing the inclusions observed in the observation field. Can be measured. In the EPMA observation, for example, the acceleration voltage is 7 kV, the sample current is 0.003 μA, the observation visual field area is 1 cm ^2, and the composition at the center of the inclusion is quantitatively analyzed by wavelength dispersion spectroscopy of characteristic X-rays. The size of inclusions to be analyzed is a maximum diameter of 0.2 μm or more, and the number of analyzes is 100 selected at random.

  The analysis target element is an element other than O, C, N, and S, and considering the composition of the steel material of the present invention, the analysis target element is Al, Mn, Si, Ti, Zr, Ca, REM (for example, La And Ce). When the abundance ratio of Al, Mn, Si, Ti, Zr, Ca and REM contained in inclusions is converted into moles, and the total amount of elements after conversion is set to 1 mole, each inclusion contained in the inclusions to be analyzed What is necessary is just to calculate the mole fraction of an element.

  Next, the component composition in the steel material (base material) of the present invention will be described. The steel material of the present invention is characterized in that it contains REM: 0.0010 to 0.1% and Zr: 0.001 to 0.05%. The reasons for setting these ranges are as follows.

  REM and Zr are elements that contribute to improving the toughness of HAZ by forming single inclusions or composite inclusions of REM and Zr in the steel material.

  The REM should be 0.0010% or more, preferably 0.003% or more, more preferably 0.006% or more, and still more preferably 0.010% or more. However, if added excessively, coarse inclusions (for example, oxides) are generated and the toughness of the base material deteriorates, so it should be suppressed to 0.1% or less. Preferably it is 0.09% or less, More preferably, it is 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 It is preferable to contain at least one element selected from the group consisting of, and more preferably La and / or Ce.

  Zr should be 0.001% or more, preferably 0.003% or more, and more preferably 0.005% or more. However, if added excessively, coarse Zr carbide is generated and the toughness of the base material deteriorates, so it should be suppressed to 0.05% or less. Preferably it is 0.04% or less, More preferably, it is 0.03% or less.

  The steel material of the present invention includes REM and Zr, and as basic elements, C: 0.01 to 0.2%, Si: 0.5% or less (not including 0%), Mn: 2.5% or less (Not including 0%), Ti: not more than 0.03% (not including 0%), and N: not more than 0.01% (not including 0%). The reason for setting such a range is as follows.

  C is an element indispensable for securing the strength of the steel material (base material), and needs to be contained in an amount of 0.01% or more. C is preferably contained in an amount of 0.02% or more, more preferably 0.03% or more. However, if it exceeds 0.2%, a lot of island martensite is generated in the HAZ at the time of welding and not only causes deterioration of the toughness of the HAZ, but also adversely affects the weldability. Therefore, C must be suppressed to 0.2% or less, preferably 0.18% or less, more preferably 0.15% or less.

  Si is an element that has a deoxidizing action and contributes to improving the strength of the steel (base material). In order to exhibit such an effect effectively, it is preferable to contain 0.02% or more, more preferably 0.05% or more, and still more preferably 0.1% or more. However, if it exceeds 0.5%, the weldability and base material toughness of the steel (base material) deteriorate, so it is necessary to keep it to 0.5% or less. It is preferably 0.45% or less, and more preferably 0.4% or less. In addition, when the 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 decrease.

  Mn is an element that contributes to improving the strength of the steel material (base material), and it is preferable to contain 0.5% or more in order to effectively exhibit these effects. More preferably, it is 0.7% or more, More preferably, it is 0.8% or more. However, if the content exceeds 2.5%, the HAZ toughness deteriorates and the weldability of the steel (base material) deteriorates. Therefore, the amount of Mn needs to be suppressed to 2.5% or less. Preferably it is 2.3% or less, More preferably, it is 2% or less.

  Ti is an element that contributes to improving the toughness of the HAZ by generating a nitride such as TiN or Ti oxide in the steel material. In order to exert such an effect effectively, Ti is preferably contained in an amount of 0.005% or more, more preferably 0.007% or more, and further preferably 0.010% or more. However, if added excessively, the toughness of the steel (base material) is deteriorated, so it should 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 nitrides (for example, ZrN and TiN). The nitrides prevent austenite grains formed in the HAZ during welding and promote ferrite transformation, thereby improving HAZ toughness. Contributes to In order to exhibit such an effect effectively, 0.002% or more is contained. More preferably, it is 0.003% or more. The more N, the more refined austenite grains are promoted, which effectively works to improve the toughness of HAZ. However, if it exceeds 0.01%, the amount of solute N increases and the toughness of the base material deteriorates. Therefore, N must be suppressed to 0.01% or less, preferably 0.009% or less, more preferably 0.008% or less.

  The steel material of the present invention contains the above elements, P: 0.02% or less (not including 0%), S: 0.015% or less (not including 0%), and Al: 0.01% or less ( 0% is included). The reason for setting such a range is as follows.

  P is an element that easily segregates, and particularly segregates at a grain boundary in the steel material to deteriorate toughness. Therefore, P must be suppressed to 0.02% or less, preferably 0.018% or less, more preferably 0.015% or less.

  S is a harmful element that combines with Mn to produce sulfide (MnS) and degrades the toughness of the base material and the ductility in the thickness direction. Therefore, S should be suppressed to 0.015% or less, preferably 0.012% or less, more preferably 0.008% or less, and particularly 0.006% or less.

  Al is an element having a strong deoxidizing power, and when added in excess, the oxide is reduced and it becomes difficult to produce a desired oxide. Therefore, Al must be suppressed to 0.01% or less, preferably 0.0090% or less, more preferably 0.0080% or less. Al may be 0%.

  The contained elements specified in the present invention are as described above, and the balance is iron and inevitable impurities. As the inevitable impurities, mixing of elements (for example, Mg, As, Se, etc.) brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. can be allowed.

The steel material of the present invention is
(1) In order to improve HAZ toughness, Ca: 0.01% or less (not including 0%),
(2) Cu: 2% or less (not including 0%), Ni: 3.5% or less (not including 0%), Cr: 3% or less (not including 0%) in order to increase the strength of the steel material ), Mo: 1% or less (not including 0%), Nb: 0.25% or less (not including 0%), V: 0.1% or less (not including 0%), and B: 0.005 Containing one or more elements selected from the group consisting of% or less (not including 0%),
Etc. are also effective. The reasons for setting these ranges are as follows.

[(1) Ca]
Ca is an element having an action of improving the HAZ toughness of the steel material. More specifically, Ca has an action of controlling the form of inclusions (specifically, spheroidizing MnS) to reduce the anisotropy of the steel material, and the anisotropy of the steel material is reduced. As a result, the HAZ toughness is improved. In order to exhibit such an effect effectively, it is preferable to make it contain 0.0003% or more. More preferably, it is 0.0005% or more, More preferably, it is 0.001% or more. However, if added excessively, a coarse oxide is formed, and the HAZ toughness deteriorates. Therefore, Ca is preferably 0.01% or less. More preferably, it is 0.008% or less, More preferably, it is 0.005% or less.

[(2) Cu, Ni, Cr, Mo, Nb, V and B]
Cu is an element for solid solution strengthening of the steel material, and in order to exhibit such an effect effectively, it is preferable to contain 0.05% or more. More preferably, it is 0.1% or more, More preferably, it is 0.2% or more. In particular, when 0.6% or more is contained, in addition to solid solution strengthening, aging precipitation strengthening is also exhibited, and a significant improvement in strength becomes possible. However, if the content exceeds 2%, the toughness of the steel material (base material) is lowered, so Cu should be suppressed to 2% or less. Preferably it is 1.8% or less, More preferably, you may be 1.6% or less.

  Ni is an element that effectively acts to increase the strength of the steel material and improve the toughness of the steel material. In order to exert such an effect, Ni is preferably contained in an amount of 0.05% or more. More preferably, it is 0.1% or more, More preferably, it is 0.2% or more. The more Ni, the better. However, since it is an expensive element, it is preferable to suppress it to 3.5% or less from an economical viewpoint. More preferably, it is 3.3% or less, More preferably, it is 3% or less.

  In order to increase the strength by adding Cr, the content is preferably 0.01% or more. More preferably it is 0.02% or more, and still more preferably 0.03% or more. However, if it exceeds 3%, weldability deteriorates, so Cr is preferably suppressed to 3% or less. More preferably, it is 1.5% or less, More preferably, it is 1% or less.

  In order to increase the strength by adding Mo, it is desirable to contain 0.01% or more. More preferably, the content is 0.02% or more, and further preferably 0.03% or more is recommended. However, if it exceeds 1%, the weldability deteriorates, so Mo is preferably 1% or less. More preferably, it is 0.9% or less, and more preferably 0.8% or less.

  In order to increase the strength by adding Nb, it is preferable to contain 0.005% or more. More preferably, it is 0.01% or more, More preferably, it is 0.03% or more. However, if it exceeds 0.25%, the toughness of the base material is deteriorated, so Nb is preferably suppressed to 0.25% or less. More preferably, it is 0.23% or less, and still more preferably 0.2% or less.

  In order to increase the strength by adding V, it is desirable to contain 0.005% or more. More preferably 0.01% or more, still more preferably 0.03% or more. However, if it exceeds 0.1%, the weldability deteriorates and the toughness of the base material deteriorates, so V is preferably 0.1% or less. More preferably, it is 0.08% or less, and more preferably 0.06% or less.

  B increases the strength of the steel material and, in the process of cooling the HAZ heated during welding, combines with N in the steel to precipitate BN and promote ferrite transformation from within the austenite grains. In order to exhibit such an effect effectively, it is preferable to contain 0.0003% or more. More preferably it is 0.0005% or more, and still more preferably 0.0008% or more. However, if it exceeds 0.005%, the toughness of the steel (base material) is deteriorated, so B is preferably 0.005% or less. More preferably, it is 0.004% or less, and further preferably 0.003% or less.

Next, a production method that can be suitably employed in producing the steel material of the present invention will be described. In order to reduce the solid solution REM and the solid solution Zr to a predetermined amount or less, the steel material of the present invention is converted into a molten steel in which the total oxygen amount [O] ₁ is adjusted to a range of 0.0020 to 0.015%. Is added to adjust the amount of dissolved oxygen [O] ₂ to a range of 0.0010 to 0.0035%, and then cast.

That is, if REM and Zr are combined and added to molten steel in which the total oxygen amount [O] ₁ is appropriately controlled, REM and Zr can be produced in the steel as oxides that are one form of inclusions. By adjusting the amount of REM and Zr added to the molten steel at this time, the dissolved oxygen amount [O] ₂ of the molten steel is appropriately controlled, and if this molten steel is cast, the amount of solid solution REM and solid solution in the steel material The amount of Zr can be reduced.

Usually, the total oxygen amount [O] _{1 in} molten steel primarily refined in a converter or electric furnace exceeds 0.015%. If REM or Zr is added to this molten steel, the amount of oxygen in the molten steel is too large, and the reaction between REM, Zr and oxygen becomes violent, which is not preferable for melting work. In addition, coarse REM oxide and coarse ZrO ₂ are generated, and the base metal toughness itself deteriorates.

Therefore, in the present invention, by adding REM and Zr to molten steel in which the total oxygen amount [O] ₁ is adjusted to be smaller than before, REM oxide as REM inclusions, Zr oxide as Zr inclusions, or An oxide containing REM and Zr can be formed as a composite inclusion of REM and Zr.

On the other hand, among the inclusions of REM and Zr, in particular, from the viewpoint of increasing the amount of oxide, a large amount of REM and Zr may be added to the molten steel in which the total oxygen amount [O] ₁ is adjusted. Excess REM and Zr that do not form are dissolved in the steel. However, when the solid solution REM and the solid solution Zr increase, as described above, the HAZ toughness varies.

Therefore, in the present invention, by adjusting the amount of REM and Zr added to the molten steel, the amount of dissolved oxygen [O] ₂ after adding REM and Zr is adjusted to be larger than before, and REM and Zr are being cast. It was decided to prevent solid solution.

The total oxygen amount [O] ₁ before adding REM and Zr is less than the normal total oxygen amount contained in the molten steel after primary smelting, and should be suppressed to 0.015% or less, preferably 0. 0.01% or less, more preferably 0.008% or less. However, if the total oxygen content [O] ₁ is made too small and less than 0.0020%, the oxygen content becomes insufficient. Therefore, even if REM and Zr are added in combination, the oxide content that contributes to improving the toughness of HAZ is secured. REM and Zr, which cannot be formed and oxides cannot be formed, dissolve in the steel material, or Zr forms carbide or the like to deteriorate the toughness of the base material. Therefore, the total oxygen amount [O] ₁ before adding REM and Zr in combination is preferably adjusted to 0.0020% or more, more preferably 0.0025% or more.

The total oxygen amount [O] ₁ means the total oxygen amount (total O amount) contained in the molten steel, and is present as an oxide inclusion in the molten steel as the dissolved atoms (so-called free oxygen). This means the total amount of oxygen combined with the amount of oxygen being used. The amount of oxygen contained in the molten steel as dissolved atoms can be measured by using an oxygen sensor using a solid electrolyte. The total oxygen amount can be measured by a general inert gas melting-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 deoxidizing using an RH type degassing refining apparatus, a ladle heating type refining apparatus, a simple type molten steel processing facility, etc. Examples of the deoxidizing method include a method of adding a deoxidizing element such as Si, Mn, Ti, and Al to the molten steel to perform deoxidation. Of course, the total oxygen amount [O] ₁ may be adjusted by appropriately combining these methods. When employing a method of adding a deoxidizing element, the deoxidizing element may be added when steel is removed from the converter to the ladle.

The procedure for adding REM and Zr in combination to the molten steel with the total oxygen content [O] ₁ adjusted is not particularly limited. For example, (a) REM may be added, then Zr may be added, (b) REM may be added after adding Zr, or (c) REM and Zr may be added simultaneously. When a plurality of types of REM are added, they may be added simultaneously or separately. For example, Ce and La may be used as REM and added in the order of Ce → Zr → La.

  The form of REM or Zr added to the molten steel is not particularly limited. For example, as REM, pure La, pure Ce, pure Y, or pure Zr, and further Fe-Si-La alloy, Fe-Si-Ce alloy, An Fe—Si—La—Ce alloy or the like may be added. Moreover, you may add misch metal to molten steel. Misch metal is a mixture of cerium group rare earth elements, and specifically contains about 40 to 50% of Ce and about 20 to 40% of La.

After the REM and Zr are added together, an alloy element may be added to adjust the components of the steel material so long as the dissolved oxygen amount [O] ₂ immediately before casting is not affected.

The amount of dissolved oxygen [O] ₂ immediately before casting is set to 0.0010% or more. If it is less than 0.0010%, the amount of oxygen becomes insufficient, so REM and Zr are dissolved in the steel during casting, which causes variation in HAZ toughness. Accordingly, the dissolved oxygen amount [O] ₂ is set to 0.0010% or more, preferably 0.0015% or more. However, when the amount of dissolved oxygen [O] ₂ becomes excessive, a large amount of coarse oxide is generated during casting, and the toughness of the base metal itself is lowered. Therefore, the amount of dissolved oxygen [O] ₂ should be suppressed to 0.0035% or less, preferably 0.0030% or less, more preferably 0.0025% or less.

In order to control the amount of dissolved oxygen [O] ₂ in the range of 0.0010 to 0.0035%, the amount of REM and Zr added may be adjusted according to the total amount of oxygen [O] _1. Determines the addition amount of REM and Zr so as to satisfy the following formula (1) according to the total oxygen amount [O] ₁ , and adds the element within the range of the determined addition amount of REM and Zr. . In the 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 the molten steel before adding REM and Zr). %). The coefficient 15 on the right side is a value determined as a result of repeated experiments.
[REM] + [Zr] ≦ 15 × [O] ₁ (1)
However, the amount of REM (total REM) and the amount of Zr (total Zr) contained in the steel material must satisfy the range defined by the above component composition.

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

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

  The steel material according to the present invention can be used as a material for structures such as bridges, high-rise buildings, ships, etc., and prevents toughness deterioration of the weld heat affected zone not only in small to medium heat input welding but also in large heat input welding. Can do.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

[Experimental Example 1]
After the hot metal was first refined in a 240-ton converter, the steel was discharged from the converter to 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 the deoxidation method shown in Table 1 below while adjusting the total oxygen amount [O] ₁ shown in Table 1 below. The total oxygen amount [O] ₁ means the total oxygen amount that is the sum of the oxygen amount contained as dissolved atoms in the molten steel and the oxygen amount present as oxide inclusions, and the oxygen amount contained as dissolved atoms in the molten steel Was measured using an oxygen sensor using a solid electrolyte, and the total oxygen content was measured by a general inert gas melting-infrared absorption method. In addition to the total oxygen amount [O] ₁ , the following Table 1 also shows the dissolved oxygen amount of the molten steel before adding REM and Zr.

The amount of REM and Zr added is calculated so as to satisfy the above formula (1) according to the total amount of oxygen [O] ₁ , and the amount of dissolved oxygen [O] ₂ shown in Table 1 below is added by adding REM and Zr. Adjusted. Table 1 below shows the REM addition amount [REM], the Zr addition amount [Zr], and the total addition amount of REM and Zr ([REM] + [Zr]). In addition, a ratio ([REM] + [Zr]) / [O] ₁ of the total amount of REM and Zr added and the total oxygen amount [O] ₁ is also shown.

After adjusting the amount of dissolved oxygen [O] ₂ , the chemical components were adjusted to such an extent that the amount of [O] ₂ was not affected, and then casting was performed.

  In the secondary refining, dehydrogenation and desulfurization were performed using an RH type degassing refining apparatus.

  In Table 1 below, La is in the form of an Fe-La alloy, Ce is in the form of an Fe-Ce alloy, REM is in the form of a misch metal containing about 50% La and about 25% Ce, and Zr is Zr alone. And each was added. The blank in Table 1 below indicates that no element is added.

FIG. 1 is a graph showing the relationship between the total amount of oxygen [O] ₁ before adding REM and Zr and the total amount of addition of REM and Zr. In FIG. 1 to 3, 5 to 8, 10, and 12, x indicates No. in Table 1 below. The results of 17 to 20 are shown respectively.

  Table 2 below shows the component composition of the steel after component adjustment (the balance is iron and inevitable impurities). However, the blank in Table 2 below indicates that no element was detected.

The molten steel after component adjustment was cast into a slab with a continuous casting machine, and a sample was cut out from a cross section at a D / 4 (where D is the thickness of the slab) position of the slab. The cut sample surface was observed at 10,000 times using EPMA “JXA-8500F (device name)” manufactured by JEOL, and the component composition was quantitatively analyzed for inclusions having a maximum diameter of 0.2 μm or more. The observation conditions were an acceleration voltage of 7 kV, a sample current of 0.003 μA, an observation field area of 1 cm ² , and an analysis number of 100 randomly selected, and components at the center of the inclusion by wavelength dispersion spectroscopy of characteristic X-rays. The composition was quantitatively analyzed. The analysis target elements are Al, Mn, Si, Ti, Zr, Ca, La, and Ce, and the analysis is performed when the abundance ratio of the elements to be analyzed is converted into moles, and the converted element amount is 1 mole. The molar fraction of each element contained in the inclusions to be targeted was calculated. The calculation results of the molar fraction are shown in Table 3 below. The blank in Table 3 below indicates 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 REM inclusions and Zr inclusions were also generated as single inclusions. .

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

  Next, the extracted electrolyte was filtered using a membrane filter (filter diameter was 47 mm, pore size was 0.1 μm), and the residue with the filter was transferred to a platinum crucible and heated with a gas burner for ashing. Next, an alkali flux (a mixture of sodium carbonate and sodium tetraborate) was added and heated again with a gas burner to melt the residue. Next, 18 vol% hydrochloric acid was added to make the melt into a solution, and then the solution was transferred to a volumetric flask, and further diluted with pure water to obtain an analysis solution. The REM and Zr concentrations in the analysis solution were measured by ICP-MS method.

  By subtracting the REM amount and Zr amount contained in the inclusions thus obtained from the REM amount (total REM amount) or Zr amount (total Zr amount) separately analyzed by the usual ICP-MS method, The REM amount and the solid solution Zr amount were determined. The calculated results are also shown in Table 2 below.

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

  Next, in order to evaluate the toughness of HAZ which is affected by heat during welding, a welding reproduction test shown below was performed by simulating high heat input welding. The welding reproduction test was performed by heating the sample cut from the slab to 1400 ° C., holding at this temperature for 5 seconds, and then cooling. The cooling was adjusted so that the cooling time from 800 ° C. to 500 ° C. was 300 seconds.

The impact characteristics of the sample after cooling were evaluated by conducting a V-notch Charpy test and measuring the absorbed energy (vE _-40 ) at -40 ° C.

Three samples were collected from the same steel type in accordance with JIS Z2242 “Charpy impact test method for metal materials”, and the results of measuring vE- ₄₀ for each sample and their average values are shown in Table 4 below. An average value of vE- ₄₀ of 150 J or more is regarded as acceptable (haz toughness is good).

For each sample, the toughness variation was evaluated based on the following criteria based on the maximum and minimum values of the vE- ₄₀ value. The evaluation results are shown in Table 4 below.

[Evaluation criteria for maximum and minimum values]
(Circle): The maximum value or minimum value of HAZ toughness is 150J or more.
X: The maximum value or minimum value of HAZ toughness is less than 150J.

[Comprehensive evaluation criteria]
○: The minimum value is 150 J or more among the results of the three measurements, and high HAZ toughness is stably secured.
Δ: Of the three measured results, at least one is 150 J or more, but the variation in HAZ toughness is large, and the minimum value is less than 150 J.
X: All of the results of the three measurements are less than 150 J.

  FIG. 3 is a graph showing the average value of HAZ toughness (circles in the figure) and the maximum and minimum widths of HAZ toughness for each sample shown in Table 4 below.

From the above results, it can be considered as follows. As apparent from FIG. 1 above, the above formula (1) is applied to a molten steel in which the total oxygen amount [O] ₁ before adding REM and Zr is adjusted to 0.0020 to 0.015% (20 to 150 ppm). It can be seen that if REM and Zr are added so as to satisfy, the HAZ toughness is improved and the variation in the HAZ toughness is reduced. The equation of the straight line shown in FIG. 1 is ([REM] + [Zr]) = 15 × [O] ₁ .

As apparent from Table 4 and FIG. 2, if the amount of dissolved oxygen [O] ₂ before casting is adjusted to a range of 0.0010 to 0.0035% (10 to 35 ppm) and cast, it is included in the steel material. It turns out that the amount of solid solution REM and the amount of solid solution Zr can be reduced below a predetermined value.

  As apparent from Table 4 and FIG. 1-3, 5-8, 10, 12 are examples that satisfy the requirements defined in the present invention, and the REM amount and the Zr amount are particularly appropriately adjusted among the chemical components of the steel material, and the solid solution REM Since the amount and the solid solution Zr amount are appropriately controlled, the average value of the HAZ toughness is 150 J or more, and the HAZ toughness is excellent. Moreover, the variation in HAZ toughness is also reduced.

  On the other hand, no. 13 to 21 are examples deviating from the requirements defined in the present invention, and among the chemical components of the steel material, the REM amount or the Zr amount is particularly out of the range defined in the present invention, or the solid solution REM amount and the solid solution Zr. Since the amount is outside the range defined in the present invention, the average value of HAZ toughness is less than 150 J, and the HAZ toughness is inferior. In addition, the number of HAZ toughness variations is increasing.

[Experiment 2]
No. obtained in Experimental Example 1 above. 3, No. 8, no. 14, no. 20 slabs were hot-rolled to obtain a steel plate having a thickness of 30 mm.

About the cross section perpendicular | vertical with respect to the rolling direction of the obtained steel plate, the component composition of an inclusion is quantitatively analyzed on the same conditions as the said Experimental example 1, and the molar fraction of each element contained in the inclusion made into analysis object is calculated. did. The observation position was the same as that of Experimental Example 1 except that the position of the steel sheet was D / 4 (where D is the thickness of the steel sheet), and the observation condition was that the observation visual field area was 1 cm ² . The calculation results of the molar fraction are shown in Table 5 below. The blank in Table 5 below indicates that no detection was made.

  Next, in order to evaluate the toughness of HAZ which is affected by heat during welding, a welding reproduction test shown below was performed by simulating high heat input welding. The welding reproduction test was performed by heating a test piece cut out from the steel plate so that the whole was 1400 ° C., holding at this temperature for 5 seconds, and then cooling. The cooling rate was adjusted so that the cooling time from 800 ° C. to 500 ° C. was 300 seconds.

The impact characteristics of the test piece after cooling were evaluated by measuring the absorbed energy (vE- ₄₀ ) at -40 ° C by conducting a V-notch Charpy test. The results of measuring vE- ₄₀ for each sample and the average values are shown in Table 6 below. Table 6 below shows the results of evaluating the variation in HAZ toughness in the same manner as in Experimental Example 1.

  FIG. 4 is a graph showing the average value of HAZ toughness (circles in the figure) and the maximum and minimum widths of HAZ toughness for each sample shown in Table 6 below.

  As is clear from Table 5 and Table 6 below, 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, and hot rolling. It can be seen that the HAZ toughness does not change. Moreover, the ease of variation in HAZ toughness also shows the same tendency in the steel sheet obtained by hot rolling and the sample cut out from the slab.

FIG. 1 is a graph showing the relationship between the total amount of oxygen [O] ₁ before adding REM and Zr and the total amount of addition of REM and Zr. FIG. 2 is a graph showing the relationship between the amount of dissolved oxygen [O] ₂ contained in the molten steel before casting and the amount of solute REM or solute Zr contained in the steel material. FIG. 3 is a graph showing the average value of HAZ toughness and the width of the maximum and minimum values of HAZ toughness. FIG. 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 (6)

C: 0.01 to 0.2% (meaning “mass%”; the same shall apply hereinafter),
Si: 0.5% or less (excluding 0%),
Mn: 2.5% or less (excluding 0%),
Ti: 0.03% or less (not including 0%), and N: 0.01% or less (not including 0%),
P: 0.02% or less (excluding 0%),
S: 0.015% or less (excluding 0%) and Al: 0.01% or less (including 0%),
Furthermore,
REM: 0.0010 to 0.1% and Zr: 0.001 to 0.05%, respectively,
The balance is steel consisting of iron and inevitable impurities,
Containing REM and Zr in the inclusions,
Solid solution REM: 0.0010% or less (including 0%);
Solid solution Zr: 0.0010% or less (including 0%) is satisfied ,
A total of solid solution REM and solid solution Zr: a steel material excellent in toughness of a weld heat affected zone characterized by satisfying 0.0010% or less .   The composition of inclusions contained in the steel material is measured, and the abundance ratio of elements other than O, C, N, and S among the elements contained in the inclusions is converted into moles, and the total amount of elements after conversion is 1 mole. The steel material according to claim 1, wherein the molar fraction of REM is 0.050 or more and the molar fraction of Zr is 0.04 or more. The steel material is still another element,
The steel material according to claim 1 or 2, comprising Ca: 0.01% or less (not including 0%). The steel material is still another element,
Cu: 2% or less (excluding 0%),
Ni: 3.5% or less (excluding 0%),
Cr: 3% or less (excluding 0%),
Mo: 1% or less (excluding 0%),
Nb: 0.25% or less (excluding 0%),
4. One or more elements selected from the group consisting of V: 0.1% or less (excluding 0%) and B: 0.005% or less (excluding 0%) A steel material according to any one of the above. A method for producing the steel material according to any one of claims 1 to 4,
REM and Zr are added to the molten steel in which the total oxygen amount [O] ₁ is adjusted in the range of 0.0020 to 0.015%, so that the dissolved oxygen amount [O] ₂ is in the range of 0.0010 to 0.0035%. A method for producing a steel material having excellent toughness of a weld heat-affected zone, characterized by casting after adjustment. The total oxygen content [O] ₁ was measured, the total oxygen content [O] the amount of dissolved oxygen by the addition of REM and Zr so as to satisfy the following formula (1) in accordance with ₁ [O] ₂ adjusted The manufacturing method according to claim 5.
[REM] + [Zr] ≦ 15 × [O] ₁ (1)
However, in (1), [REM] and [Zr] are the addition amounts (mass%) of REM or Zr, respectively, and [O] ₁ is the total oxygen amount of the molten steel before adding REM and Zr. (Mass%).

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