JP2010121199A - Steel having excellent weld heat-affected zone toughness and base metal low temperature toughness, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel in which variation in HAZ toughness is reduced, and further, the low temperature toughness of the base metal itself is increased as well, and to provide a method for producing the same. <P>SOLUTION: The invention is characterized in that: (A) the steel comprises inclusions including REM and Zr; (B) solid solution REM and solid solution Zr in the steel satisfy solid solution REM: ≤0.0010% (including 0%) and solid solution Zr: ≤0.0010% (including 0%); and (C), when the metallic structure of the steel is observed by an EBSP (Electron Diffraction Image) method, the steel satisfies inequality; D≤30 (wherein, D denotes the average equivalent circle diameter (μm) of the crystal grains surrounded by large angle boundaries with a crystal orientation difference of ≥15° upon measurement of the orientation difference of the adjoining two crystals by an EBSP method), and inequality; 50≤M (wherein, M denotes the ratio (area%) of the crystal grains surrounded by large angle boundaries with a crystal orientation difference of ≥55° occupied in the whole of the steel). <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a steel material used for structures such as bridges, high-rise buildings, and ships, and more specifically, a part that is affected by heat when welding (hereinafter referred to as “welding heat affected zone” or “HAZ”). It is related with the steel material which improved the toughness of, and its manufacturing method.

  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. However, among the welded joint portions, particularly 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 lower the cooling rate of the HAZ and the lower the hardenability, producing 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.

Therefore, the present applicant has proposed a steel material that suppresses HAZ toughness deterioration in the case of employing a high heat input welding method in Patent Document 1. This steel material is characterized in that it contains REM oxide and / or CaO and ZrO ₂ as oxides, and since these oxides exist in liquid form in the molten steel, they are finely dispersed in the steel. In addition, the solid solution does not disappear even if it is affected by heat during welding, which contributes to improved HAZ toughness.

  Although it is not a technique aimed at improving the HAZ toughness, Patent Document 2 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.

On the other hand, steel materials used in ships and the like are also required to have high strength. However, if the strength of the steel materials is increased, the yield strength exceeds the brittle fracture strength, and brittle fracture is likely to occur during elastic deformation. Therefore, according to the unified rules of the International Classification Society (IACS), the toughness grade is set for each structural member from the fracture mechanics method (K concept) in order to prevent brittle fracture, which is required as the strength class increases. This is achieved by improving the base material toughness. Therefore, in order to ensure the safety of the structure under severe use environment, as described above, the HAZ toughness in the welded joint portion is good, and the base material toughness (particularly, the base material toughness in the low temperature region) is It is important to be good.
Japanese Patent Laid-Open No. 2007-1001000 JP-A-8-120401

  An object of the present invention is to provide a steel material in which variation in HAZ toughness is reduced and the low-temperature toughness of the base material itself is enhanced. Moreover, the other objective of this invention is to provide the manufacturing method of the said steel material.

The steel material excellent in the toughness of the weld heat affected zone and the base material low temperature toughness according to the present invention, which has solved the above-mentioned problems, is C: 0.04 to 0.13% (meaning “mass%”; the same applies hereinafter). Si: 0.5% or less (not including 0%), Mn: 2% or less (not including 0%), Ti: 0.02% or less (not including 0%), and N: 0.01 %: Not more than 0% (not including 0%), Cu: not more than 0.3% (not including 0%), Ni: not more than 0.4% (not including 0%), and Nb: 0.25 %: At least one selected from the group consisting of 0% 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%), REM: 0.0010-0.1%, and Zr: 0 The 0,010 to 0.05% respectively contain a steel balance of iron and inevitable impurities,
(A) The steel material includes inclusions containing REM and Zr,
(B) Solid solution REM and solid solution Zr in steel
Solid solution REM: 0.0010% or less (including 0%),
Solid solution Zr: 0.0010% or less (including 0%) is satisfied,
(C) When the metal structure of a steel material is observed by a backscattered electron diffraction image method (EBSP method), it has a gist in that the following formulas (1) and (2) are satisfied. However, in the following formula (1), D is an EBSP method for measuring an orientation difference between two adjacent crystals, and an average equivalent circle diameter of a crystal grain surrounded by a large-angle grain boundary having a crystal orientation difference of 15 ° or more ( μm). In the following formula (2), M means the ratio (area%) of the crystal grains surrounded by the large-angle grain boundaries having a crystal orientation difference of 55 ° or more to the entire steel material.
D ≦ 30 (1)
50 ≦ M (2)

  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. It is recommended that the molar fraction of REM be 0.05 or more and the molar fraction of Zr be 0.04 or more.

  The steel material may further contain (i) Ca: 0.01% or less (not including 0%), (ii) B: 0.005% or less (not including 0%), etc. as other elements. Good.

In the steel material of the present invention, REM and Zr are added to a molten steel in which the total oxygen amount [O] ₁ is adjusted in the range of 0.0020 to 0.015%, and the dissolved oxygen amount [O] ₂ is 0.0010 to 0. After adjusting to a range of .0035%, casting, and heating the obtained steel slab to a temperature range of Ac ₃ point or more and 1200 ° C. or less, the average temperature of the steel slab is Ar ₃ point + 10 ° C. or more and 900 ° C. or less. In the temperature range, hot rolling is performed by controlling the maximum rolling reduction per pass to 12% or less and the cumulative rolling reduction to 40% or more, and the average temperature of the obtained hot rolled material is Ar ₃ or more. It can be produced by cooling from the temperature range to a temperature range where the surface temperature of the hot-rolled material is 500 ° C. or less at an average cooling rate of 5 ° C./second or more.

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 (3) in response to ₁ [O] ₂ adjusted It is recommended to do. However, in the following formula (3), [REM] and [Zr] are the addition amount (mass%) of REM or Zr, respectively, [O] ₁ is the total oxygen of the molten steel before adding REM and Zr. Amount (% by mass).
[REM] + [Zr] ≦ 15 × [O] ₁ (3)
The steel material may be tempered by cooling to a temperature range where the surface temperature of the steel material is 500 ° C. or less at a cooling rate of 5 ° C./second or more and then heating to a temperature range of 500 ° C. or more and less than Ac ₁ point. .

  According to the present invention, variation in the HAZ toughness can be suppressed by reducing the amount of the solid solution REM and the amount of the solid solution Zr contained in the steel as much as possible. In addition, according to the present invention, when the metal structure is observed, the average equivalent circle diameter of the crystal grains surrounded by the large-angle grain boundaries having a crystal orientation difference of 15 ° or more is set to 30 μm or less. The low temperature toughness of the base material itself can be improved by setting the ratio of the crystal grains surrounded by the large-angle boundaries of 55 ° or more to the entire steel material to 50 area% or more.

  The present inventors have repeatedly studied in order to suppress the HAZ toughness variation and increase the toughness of the base metal itself, with respect to the steel material in which HAZ toughness of the welded joint portion is improved by adding REM and Zr to the steel material. . As a result, it is assumed that (A) REM and Zr are combined and added to the steel, and the inclusions are adjusted to contain REM and Zr to increase the HAZ toughness. Furthermore, (B) the solids contained in the steel If the amount of dissolved REM and the amount of solid solution Zr 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 (C) the crystal orientation of the metal structure of the steel material. By appropriately controlling the size of the crystal grains surrounded by the large-angle grain boundaries having a difference of 15 ° or more and the fraction of crystal grains surrounded by the large-angle grain boundaries having a crystal orientation difference of 55 ° or more, The present inventors have found that low temperature toughness can be improved and completed the present invention. Hereinafter, (A) to (C) will be described in detail.

[(A) HAZ toughness of welded joint]
The steel material of the present invention includes inclusions containing REM and Zr. "Contains REM and Zr in the inclusion" means (a) contains a single inclusion of REM and a single inclusion of Zr or (b) contains a composite inclusion containing REM and Zr (C) means that it 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). 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 Therefore, the HAZ structure can be refined and the toughness of the 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.

  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, it is preferable that the molar fraction of REM is 0.05 or more and the molar fraction of Zr is 0.04 or more. 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 sum of the molar fraction of the REM and the molar fraction of the 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.

[(B) Variation in HAZ toughness of welded joints]
Welded steel with increased REM and Zr contents, and measured the toughness of HAZ at multiple locations. Especially in the vicinity of the bond part (part of HAZ close to the weld metal) that has a large thermal effect, It was found that the toughness decreased and the measured values varied. 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, 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 content may be calculated by subtracting the Zr content contained in the inclusions contained in the steel material from the Zr content (total Zr content).

[(C) Low temperature toughness of base metal itself]
The steel material of the present invention needs to satisfy the following formulas (1) and (2) when the metal structure is observed by a backscattered electron diffraction image method (EBSP method). Satisfying both equations improves the low temperature toughness of the base metal itself.
D ≦ 30 (1)
50 ≦ M (2)

  In the above formula (1), D measures the orientation difference between two adjacent crystals by the EBSP method, and the average equivalent circle diameter (μm) of the crystal grains surrounded by a large-angle grain boundary with a crystal orientation difference of 15 ° or more. Means. In the present invention, the value of D is set to 30 μm or less. It is generally known that a brittle crack is bent, detoured, or retained at a large-angle grain boundary having a crystal orientation difference of 15 ° or more. Therefore, by refining the crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 15 ° or more, the position where the brittle crack is bent, detoured and retained increases, so the impact characteristics are increased and the base metal itself The low temperature toughness is increased. The smaller the value of D, the better, preferably 28 μm or less, and more preferably 25 μm or less.

  In the above formula (2), M means the ratio (area%) of crystal grains surrounded by large-angle grain boundaries having a crystal orientation difference of 55 ° or more to the entire steel material. In the present invention, the value of M is 50 area% or more. This is because brittle crack bending, detouring, and retention due to large-angle grain boundaries with a crystal misorientation of 15 ° or more are further exhibited by large-angle grain boundaries with a crystal misorientation of 55 ° or more. Therefore, also in the present invention, the ratio of crystal grains surrounded by large-angle grain boundaries having a crystal orientation difference of 55 ° or more to the entire steel material is set to 50 area% or more. The value of M is preferably 55 area% or more, more preferably 60 area% or more.

  The observation of the metal structure is performed at a t / 4 position in the thickness direction when the thickness of the steel material is t (mm). A specific observation procedure will be described in the section of the example below.

[Metal structure of steel]
The steel material of the present invention is composed of a bainite-based structure. By using bainite as a main component, the strength of the steel material can be secured. By bainite-based is meant that the area ratio of bainite is 80% or more when the metal structure is observed. The steel material of this invention may be comprised only from bainite, and martensite, a ferrite, etc. may produce | generate as structure | tissues other than bainite. In order to prevent a decrease in strength, the ferrite structure is preferably as small as possible, and is preferably less than 4% by area.

[About component composition]
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: 0.0010 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 0.0015% or more, and more preferably 0.002% 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 0.0010% or more, preferably 0.0015% or more, and more preferably 0.002% 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.

  In addition to containing REM and Zr, the steel material of the present invention includes, as basic elements, C: 0.04 to 0.13%, Si: 0.5% or less (not including 0%), Mn: 2% or less (0 %), Ti: 0.02% or less (not including 0%), and N: 0.01% or less (not including 0%), and Cu: 0.3% or less (not including At least one element selected from the group consisting of Ni: 0.4% or less (not including 0%), and Nb: 0.25% or less (not including 0%). It is a waste. 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 by 0.04% or more. C is preferably contained at 0.05% or more, more preferably 0.06% or more. However, if it exceeds 0.13%, 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 0.13% or less, preferably 0.12% or less, more preferably 0.11% 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 it is contained in excess of 2%, the HAZ toughness deteriorates and the weldability of the steel (base material) deteriorates. Therefore, the amount of Mn needs to be suppressed to 2% or less. Preferably it is 1.8% or less, More preferably, it is 1.6% 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 0.02% or less. Preferably it is 0.018% or less, More preferably, you may be 0.016% 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, it is preferable to contain 0.002% or more. 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.

  Cu, Ni, and Nb are all elements that increase the strength of the steel material. In particular, steel materials used in ships and the like that require low-temperature toughness require strength in addition to good base material toughness and HAZ toughness. Therefore, the steel material of the present invention contains at least one element as an essential element. There is a need to. Preferably, both Cu and Ni may be contained, or only Nb may be contained.

  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. However, if the content exceeds 0.3%, the toughness of the steel (base material) is lowered, so Cu is preferably suppressed to 0.3% or less. Preferably it is 0.28% or less, More preferably, you may be 0.25% 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 0.4% or less from an economical viewpoint. More preferably, it is 0.38% or less, More preferably, it is 0.35% or less.

  Nb is an element having a recrystallization inhibitory effect and is an element that contributes effectively to refinement of the structure. In order to exhibit such an action effectively, 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.

  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
(I) In order to improve HAZ toughness, Ca: 0.01% or less (not including 0%),
(Ii) In order to increase the strength of the steel material, B: 0.005% or less (not including 0%),
Etc. are also effective. The reasons for setting these ranges are as follows.

  (I) 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.

  (Ii) B enhances 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.

[About manufacturing method]
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. The cast slab obtained by casting (for example, slab) is obtained from Ac ₃ point or more and 1200 ° C. or less so that the metal structure satisfies the requirements of the above formulas (1) and (2). (Hereinafter, the temperature in this temperature range may be referred to as “heating temperature” or “T1”), followed by hot rolling. In the hot rolling, in the temperature range where the average temperature of the steel slab is Ar ₃ point + 10 ° C. or more and 900 ° C. or less (hereinafter, the temperature in this temperature range may be referred to as “T2”), The maximum rolling reduction is 12% or less, and the cumulative rolling reduction is 40% or more. Next, the surface temperature of the hot rolled material is 500 ° C. from the temperature range where the average temperature of the obtained hot rolled material is Ar ₃ or higher (hereinafter, the temperature in this temperature range may be referred to as “T3”). Cooling is performed at a cooling rate of 5 ° C./second or more to the following temperature range (hereinafter, the temperature in this temperature range may be referred to as “T4”). The reason why this range is specified will be described below.

First, when 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 generated 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 and deoxidizing the molten steel. 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 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 (3) 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 (3), [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 (3)
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 steel slab obtained by casting is heated at a heating temperature (T1) of Ac ₃ point or higher and 1200 ° C. or lower. The heating temperature (T1) needs to be heated to Ac ₃ point or higher in order to make the metal structure of the steel slab austenite. However, if the heating temperature exceeds 1200 ° C., the initial austenite grains become coarse, so that the transformation structure cannot be sufficiently refined. Therefore, heating temperature (T1) shall be 1200 degrees C or less.

The temperature at the Ac ₃ point can be calculated from the following equation. In the formula, [] indicates the content (% by mass) of each element.
Ac ₃ (° C.) = 908-223.7 × [C] + 438.5 × [P] + 30.49 × [Si] −34.43 × [Mn] −23 × [Ni] (a)

The steel slab heated to the heating temperature (T1) is hot-rolled, but in the hot rolling, the maximum temperature per pass in the temperature range where the average temperature of the steel slab is Ar ₃ point + 10 ° C. or more and 900 ° C. or less It is necessary that the rolling reduction is 12% or less and the cumulative rolling reduction is 40% or more. By controlling the rolling conditions in the temperature range of Ar ₃ point + 10 ° C. or more and 900 ° C. or less, the growth of austenite grains can be suppressed, and strain can be efficiently introduced into the austenite grains before transformation. Crystal grains surrounded by large-angle grain boundaries of 15 ° or more can be refined, and the low temperature toughness of the base material itself can be improved.

When the maximum rolling reduction per pass in the temperature range of Ar ₃ point + 10 ° C. or higher and 900 ° C. or lower exceeds 12%, strain is excessively accumulated in the austenite grains, and the strain recovery phenomenon occurs, and the structure after transformation ( Since the crystal grains surrounded by the large-angle grain boundaries are coarsened, the low temperature toughness of the base material itself is deteriorated. Therefore, in order to suppress the recovery of strain, the maximum rolling reduction per pass in the temperature range of Ar ₃ point + 10 ° C. to 900 ° C. is set to 12% or less. Preferably it is 11% or less, More preferably, it is 10% or less. Decreasing the maximum rolling reduction per pass increases the effect of suppressing the coarsening of crystal grains surrounded by large-angle grain boundaries. However, if the maximum rolling reduction is made too small, the manufacturing time becomes longer and the productivity becomes worse. . Accordingly, the lower limit of the maximum rolling reduction per pass is preferably 6%.

The cumulative rolling reduction in the temperature range of Ar ₃ point + 10 ° C. or more and 900 ° C. or less is 40% or more. When the cumulative rolling reduction is less than 40%, the amount of strain introduced into the austenite grains is reduced, and the number of nucleation sites after transformation is reduced, so that the crystal grains surrounded by the large-angle grain boundaries are coarsened, and the base metal itself Low temperature toughness deteriorates. Therefore, the cumulative rolling reduction in the temperature range of Ar ₃ point + 10 ° C. or higher and 900 ° C. or lower is set to 40% or higher. Preferably it is 45% or more, more preferably 50% or more. The upper limit of the cumulative rolling reduction in the temperature range of Ar ₃ point + 10 ° C. or higher and 900 ° C. or lower is not particularly limited, but is usually about 60%.

The temperature at the Ar ₃ point can be calculated from the following equation. In the formula, [] represents the content (% by mass) of each element, and t represents the finished thickness (mm) of the product.
Ar ₃ (° C.) = 910−310 × [C] −80 × [Mn] −20 × [Cu] −55 × [Ni] + 0.35 × (t−8) (b)

The cumulative rolling reduction can be calculated by the following formula. t ₀ is start rolling thickness of the steel strip in the temperature range average temperature of 900 ° C. or less of the billet (mm), t ₁ is the average temperature of the steel strip is a steel strip in the temperature range of not lower than Ar ₃ point + 10 ° C. The rolling end thickness (mm) is meant.
Cumulative rolling reduction = [(t ₀ −t ₁ ) / t ₀ ] × 100 (c)

  The average temperature of the steel slab is managed by the temperature at the t / 4 position calculated by the procedure described in the section of an example described later. t means the thickness (mm) of the slab.

  In addition, the maximum rolling reduction per one pass and cumulative rolling reduction in the temperature range (austenite recrystallization area | region) where the average temperature of a steel slab exceeds 900 degreeC are not specifically limited.

Next, the hot-rolled material obtained by hot rolling is from a temperature range (T3) where the average temperature is Ar ₃ or higher to a temperature range (T4) where the surface temperature of the hot-rolled material is 500 ° C. or lower. By cooling at an average cooling rate of 5 ° C./second or more, the fraction of crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 55 ° or more can be increased, and the toughness of the base material itself can be improved. it can.

That is, the steel material of the present invention is mainly composed of a bainite structure. In general, bainite has a K-S in which the close-packed surface of the crystal lattice of austenite and bainite and the close-packed direction corresponding thereto are substantially parallel. It is known to generate with a (Kurdjumov-Scahs) relationship. In this relationship, bainite is generated by selecting any one of a maximum of 24 orientations with respect to austenite, but this selected tendency changes as the temperature of the bainite transformation changes. It is said that the bainite form changes (Kawada et al .: CAMP-ISJvol16, No. 3 (2003), PS30). This is because as the transformation temperature decreases, the bainite changes from the diffusion transformation represented by ferrite transformation to the shear transformation represented by martensite, or the nucleation ability of transformation by the transformation temperature reduction, and the growth of the produced structure. This is thought to be due to the fact that the structure after transformation changes greatly due to changes in speed and the like. From the above, by enlarging the average cooling rate from the temperature range (T3) of the Ar ₃ point or higher to the temperature range (T4) of 500 ° C. or lower, it is surrounded by a large-angle grain boundary having a crystal orientation difference of 55 ° or higher. The fraction of crystal grains produced can be increased.

After cooling to a temperature range of 500 ° C. or lower (T4) at an average cooling rate of 5 ° C./second or higher, tempering may be performed as necessary. By tempering, strain introduced by hot rolling or transformation disappears, so that the low temperature toughness of the base material can be further enhanced. Tempering may be performed by heating to a temperature of 500 ° C. or higher and lower than Ac ₁ point, for example.

The temperature of the Ac ₁ point can be calculated from the following equation. In the formula, [] indicates the content (% by mass) of each element.
Ac ₁ (° C.) = 723-14 × [Mn] + 22 × [Si] −14.4 × [Ni] (d)

  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.

  In Experimental Examples 1 and 2 below, the same steel type was used, and the HAZ toughness and variation of the steel material (Experimental Example 1) and the low temperature toughness of the steel material itself (Experimental Example 2) were examined. Overall, the characteristics of the steel materials were evaluated.

[Experimental Example 1 (Evaluation of HAZ toughness and its variation)]
After the hot metal was first refined in a 240-ton converter, the steel was removed from the converter into a ladle and subjected to secondary refining while adjusting the components and adjusting the temperature.

In the ladle, deoxidation was performed using Si and Mn, and the chemical component composition was adjusted 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 according to the total oxygen amount [O] ₁ so as to satisfy the above formula (3), 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, REM was in the form of a misch metal containing about 50% La and about 25% Ce, and Zr was added alone as Zr alone.

FIG. 1 is a graph showing the relationship between the total oxygen amount [O] ₁ before adding REM and Zr and the total amount of addition of REM and Zr ([REM] + [Zr]). In FIG. As a result of Nos. 1 to 4, × indicates No. in Table 1 below. The results of 9 to 12 are shown respectively. In FIG. 1, the unit of the total oxygen amount [O] ₁ is expressed in ppm.

  Table 2 below shows the component composition of the steel material after component adjustment (the balance is iron and inevitable impurities).

The molten steel after the component adjustment was cast into a slab with a continuous casting machine, and a sample was cut out from the cross section at the t / 4 (where t 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.

  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 along with the filter was transferred to a platinum crucible and heated with a gas burner to be ashed. 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 3 below. In Table 3, “<0.0001” means that no element was detected.

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. In FIG. 2, the unit of dissolved oxygen amount [O] ₂ is expressed in ppm. In FIG. 2, only data in which solute REM or solute Zr is detected are plotted.

  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 and FIG. 3, the above-mentioned (3) is added to the 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 formula, 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 × 10 ⁻⁴ × [O] ₁ .

As is apparent from Tables 1 and 3 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), casting is performed. It can be seen that the amount of solid solution REM and the amount of solid solution Zr contained in the steel material can be reduced to a predetermined value or less.

  As is apparent from Tables 2 to 4 and FIG. 1-4 is an example which satisfies the requirements prescribed | regulated by this invention, and while especially REM amount and Zr amount are appropriately adjusted among the chemical components of steel materials, solid solution REM amount and solid solution Zr amount are appropriate. Therefore, 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. 5 to 13 are examples that deviate from the requirements defined in the present invention, and in particular, the amount of REM or Zr among the chemical components of the steel material is out of the range defined in the present invention (Nos. 5 to 8, 13), Or since the amount of solid solution REM and the amount of solid solution Zr are outside the range specified in the present invention (Nos. 9 to 12), 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 (Evaluation of low temperature toughness of base material)]
A slab (steel types a to m) obtained by casting under the conditions described in Experimental Example 1 was heated to the heating temperature (T1) shown in Table 5 below, and then hot rolled to obtain a hot rolled material. It was. The hot rolling was performed under the conditions shown in Table 5 below for the maximum rolling reduction and the cumulative rolling reduction per pass in the temperature range (T2) where the average temperature of the slab is Ar ₃ point + 10 ° C. or higher and 900 ° C. or lower. The cumulative rolling reduction was calculated using the above formula (c).

Next, the hot-rolled material obtained by hot rolling is a temperature at which the surface temperature of the hot-rolled material is 500 ° C. or less from the temperature range (T3) in which the average temperature of the hot-rolled material is Ar ₃ or higher. Cooled to zone (T4). Table 5 below shows the cooling start temperature (T3) and the average cooling rate during cooling.

  No. in Table 5 below. About 23, after the surface temperature of the hot-rolled material was cooled to a temperature range (T4) of 500 ° C. or lower, it was tempered by heating to 580 ° C.

  The average temperature of the slab or the hot rolled material was controlled by the temperature at the t / 4 position, where t is the thickness of the slab or the hot rolled material. The temperature at the t / 4 position was calculated according to the following procedure.

<Calculation method of average temperature>
(1) Using a process computer, the heating temperature at an arbitrary position in the thickness direction from the front surface to the back surface of the steel slab is calculated based on the ambient temperature from the start of heating to extraction and the in-furnace time.
(2) Using the calculated heating temperature, calculate the rolling temperature at any position in the plate thickness direction based on the rolling pass schedule during rolling and the cooling method between the passes (water cooling or air cooling). Roll while calculating using a suitable method.
(3) The steel sheet surface temperature is measured using a radiation thermometer installed on the rolling line (however, it is also calculated on a process computer).
(4) The steel plate surface temperature actually measured at the start of rough rolling, at the end of rough rolling, and at the start of finish rolling is collated with the calculated surface temperature on the process computer.
(5) When the difference between the calculated surface temperature and the measured steel plate surface temperature is ± 30 ° C. or more, the measured steel plate surface temperature is replaced with the calculated surface temperature to obtain the calculated surface temperature on the process computer.
(6) Using the corrected calculated surface temperature, obtain the temperature at the t / 4 position.

  On the other hand, the surface temperature of the hot-rolled material was measured using a radial thermometer installed on the rolling line.

Table 5 below also shows the product thickness (mm) of the rolled material obtained by cooling. Table 5 below shows Ac ₃ points, Ar ₃ points calculated using the above formulas (a), (b), and (d) based on the chemical composition shown in Table 2 above. The value of Ac ₁ point is also shown.

  Next, after mirror polishing from a t / 4 position (t is the plate thickness) of the obtained rolled material, a test piece was collected, etched with a 2% nitric acid-ethanol solution (a nital solution), and optically viewed in five fields of view. Observation was performed at 400 times using a microscope, and the bainite fraction (area%) in the steel structure was measured by image analysis. At this time, all the structures other than ferrite and martensite were regarded as bainite. The bainite fraction (area%) is shown in Table 6 below.

  Further, the metal structure of the rolled material is observed according to the following procedure, and the average equivalent circle diameter D of the crystal grains surrounded by the large-angle grain boundaries having a crystal orientation difference of 15 ° or more and the large-angle grains having a crystal orientation difference of 55 ° or more. The ratio M of the crystal grains surrounded by the boundary in the entire steel material was determined. The values of D (μm) and M (area%) are shown in Table 6 below.

<< Calculation method of D >>
(1) Prepare a sample cut parallel to the rolling direction (longitudinal direction) so as to include both the front and back surfaces of the rolled material.
(2) Polishing with a wet emery polishing paper of # 150 to # 1000 or a polishing method having the same function as that, and a mirror finish using a polishing agent such as diamond slurry.
(3) The mirror polished surface is an EBSP (Electron Back Scattering Pattern) device manufactured by TexSEM Laboratories, and the orientation of two crystals is set at a measurement range of 200 μm × 200 μm and a pitch of 0.5 μm at the t / 4 position in the plate thickness direction. The difference was measured, and the boundary where the crystal orientation difference was 15 ° or more was defined as a large-angle grain boundary. Note that measurement points with a confidence index indicating the reliability of the measurement direction smaller than 0.1 were excluded from the analysis target.
(4) In the grain distribution map, the maximum width (usually along the plate thickness direction) and the maximum length (usually along the rolling direction) of the crystal grain surrounded by the large-angle grain boundary having a crystal orientation difference of 15 ° or more. Length), the area of the crystal grains was calculated, the equivalent circle diameter of the crystal grains was calculated, and the average value was obtained.

<< Calculation method of M >>
The ratio M of the crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 55 ° or more to the whole steel material is calculated by analyzing the text data of the crystal orientation difference in the step (3) in the calculation method of D above. did. In the analysis of the text data, those with a crystal orientation difference of 5 ° or less were deleted as noise, and the area fraction of crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 55 ° or more in the entire metal structure was calculated. .

  FIG. 4 shows the relationship between the average equivalent circle diameter D and the cumulative rolling reduction in the non-recrystallized region. As apparent from FIG. 4, when the cumulative reduction ratio in the non-recrystallized region is 40% or more, the average equivalent circle diameter D of the crystal grains surrounded by the large-angle grain boundaries having a crystal orientation difference of 15 ° or more is 30 μm or less. Can be.

Average cooling from the ratio M of crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 55 ° or more to the entire steel material and the temperature range (T3) of Ar ₃ or higher to a temperature range (T4) of 500 ° C. or lower The relationship with speed is shown in FIG. As is apparent from FIG. 5, if the average cooling rate from the temperature range (T3) of Ar ₃ or higher to the temperature range (T4) of 500 ° C. or lower is controlled to 5 ° C./second or higher, the crystal orientation difference is 55 °. The ratio M of the crystal grains surrounded by the large-angle grain boundaries to the entire steel material can be 50 area% or more.

  Next, the low temperature toughness of the obtained rolled material was evaluated by the following procedure.

<Evaluation method of low temperature toughness>
The low temperature toughness of the rolled material was evaluated by performing a V-notch Charpy test and measuring the impact characteristics of the rolled material by measuring the absorbed energy (vE- ₆₀ ) at -60 ° C. The measurement of vE- ₆₀ was carried out according to JIS Z2242 by collecting U4 test piece defined by NK (Japan Maritime Association) classification from t / 4 position. The measurement results are shown in Table 6 below.

In the shipbuilding E grade in the NK class, the impact characteristics of the base material are evaluated at a test temperature of −40 ° C. In this experimental example, the conditions are more stringent and the test temperature is −60 ° C., and the absorbed energy (vE _-60 ) The average value of 100 J or more was determined to be acceptable (the low temperature toughness of the base material was good).

FIG. 6 shows the relationship between the average equivalent circle diameter D and the rolling material vE- ₆₀ . As apparent from FIG. 6, when the average equivalent circle diameter D of the crystal grains surrounded by the large-angle grain boundaries having a crystal orientation difference of 15 ° or more is 30 μm or less, vE- ₆₀ can be 100 J or more. It can be seen that the low temperature toughness of the base metal itself can be improved.

FIG. 7 shows the relationship between the ratio M of crystal grains surrounded by large-angle grain boundaries having a crystal orientation difference of 55 ° or more and vE- ₆₀ of the rolled material. As is clear from FIG. 7, if the ratio M of the crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 55 ° or more to the entire steel material is 50 area% or more, vE- ₆₀ should be 100 J or more. It can be seen that the low temperature toughness of the base metal itself can be improved.

  When the results of Experimental Example 1 and Experimental Example 2 are combined, it can be considered from Table 4 and Table 6 as follows.

  No. 21 to 25 and 27 to 33 are examples using steel types a to d that satisfy the requirements defined in the present invention. As is apparent from Table 4, HAZ toughness is good and there is little variation in HAZ toughness. As is clear from Table 6, the low temperature toughness of the base material itself is also good.

  No. No. 26 uses steel type b that satisfies the requirements specified in the present invention, and as is clear from Table 4, HAZ toughness is good and there is little variation in HAZ toughness, but per pass in the non-recrystallized region. Since the maximum rolling reduction of 12% exceeds 12%, the average equivalent circle diameter D of crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 15 ° or more exceeds 30 μm, and the low-temperature toughness of the base material is poor. It has become.

  No. 34 and no. 35, as is clear from Table 6, the low temperature toughness of the base metal is good, but since the steel type e and the steel type f that do not satisfy the requirements specified in the present invention are used, as is clear from Table 4, The HAZ toughness is poor and the HAZ toughness variation is also increasing.

  No. 36 to 42 use steel type g to steel type m that do not satisfy the requirements defined in the present invention, and as is apparent from Table 4, the HAZ toughness is poor and the HAZ toughness variation is also large. Further, as apparent from Table 6, since the metal structure is not properly controlled, the low temperature toughness of the base material itself is also deteriorated.

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 relationship between the average equivalent circle diameter D and the cumulative rolling reduction in the non-recrystallized region. FIG. 5 shows the ratio M of the crystal grains surrounded by large-angle grain boundaries having a crystal orientation difference of 55 ° or more to the entire steel material, and the temperature range (T3) of Ar ₃ or higher to a temperature range of 500 ° C. or lower (T4). It is a graph which shows the relationship with the average cooling rate to. FIG. 6 is a graph showing the relationship between the average equivalent circle diameter D and the vE- ₆₀ of the rolled material. FIG. 7 is a graph showing the relationship between the ratio M of the crystal grains surrounded by large-angle grain boundaries having a crystal orientation difference of 55 ° or more to the whole steel material and the vE- ₆₀ of the rolled material.

Claims (7)

C: 0.04 to 0.13% (meaning “mass%”; the same applies hereinafter)
Si: 0.5% or less (excluding 0%),
Mn: 2% or less (excluding 0%),
Ti: 0.02% or less (not including 0%), and N: 0.01% or less (not including 0%),
Furthermore,
Cu: 0.3% or less (excluding 0%),
Including at least one selected from the group consisting of Ni: 0.4% or less (excluding 0%) and Nb: 0.25% 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: 0.0010 to 0.05%, respectively,
The balance is steel consisting of iron and inevitable impurities,
(A) The steel material includes inclusions containing REM and Zr,
(B) Solid solution REM and solid solution Zr in steel
Solid solution REM: 0.0010% or less (including 0%),
Solid solution Zr: 0.0010% or less (including 0%) is satisfied,
(C) Toughness and base of weld heat affected zone characterized by satisfying the following equations (1) and (2) when the metallographic structure of the steel material is observed by backscattered electron diffraction imaging (EBSP method): Steel material with excellent low-temperature toughness.
D ≦ 30 (1)
50 ≦ M (2)
[However, in the formula (1), D is a measurement of the orientation difference between two adjacent crystals by the EBSP method, and the average equivalent circle diameter of a crystal grain surrounded by a large-angle grain boundary with a crystal orientation difference of 15 ° or more ( μm). In the formula (2), M means the ratio (area%) of the crystal grains surrounded by the large-angle grain boundaries having a crystal orientation difference of 55 ° or more to the entire steel material. ] 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 satisfies 0.05 or more and the molar fraction of Zr satisfies 0.04 or more. The steel material according to claim 1 or 2, wherein the steel material further contains Ca: 0.01% or less (not including 0%) as another element. The steel material according to any one of claims 1 to 3, wherein the steel material further contains B: 0.005% or less (not including 0%) as another element. 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%. After adjusting, casting,
After heating the obtained slab to a temperature range of Ac ₃ points or more and 1200 ° C. or less,
In the temperature range where the average temperature of the steel slab is Ar ₃ point + 10 ° C. or more and 900 ° C. or less, the maximum rolling reduction per pass is controlled to 12% or less, the cumulative rolling reduction is controlled to 40% or more, and hot rolling is performed.
The hot-rolled material obtained is cooled at an average cooling rate of 5 ° C / second or more from a temperature range where the average temperature of the hot-rolled material is Ar ₃ or higher to a temperature range where the surface temperature of the hot-rolled material is 500 ° C or lower. A method for producing a steel material having excellent weld heat affected zone toughness and base metal low temperature toughness. 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 (3) in response to ₁ [O] ₂ adjusted The manufacturing method according to claim 5.
[REM] + [Zr] ≦ 15 × [O] ₁ (3)
[However, in the formula (3), [REM] and [Zr] are the addition amount (mass%) of REM or Zr, respectively, and [O] ₁ is the total oxygen of the molten steel before adding REM and Zr. Amount (% by mass). ] The steel material is cooled at a cooling rate of 5 ° C / second or more to a temperature range of 500 ° C or lower, and then tempered by heating to a temperature range of 500 ° C or higher and less than Ac ₁ point. The manufacturing method as described.

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