KR20100057504A - Steel material having excellent toughness in welding heat-affected zone, and method for producing the same - Google Patents

Abstract

The steel material which reduced the variation of the HAZ toughness of the present invention includes inclusions containing REM and Zr, and in addition, solid solution REM and solid solution Zr in the solid solution have a solid solution REM: 0.0010% or less (including 0%) and solid solution Zr. : 0.0010% or less (including 0%) is satisfied. The first rolled material obtained from this steel, whose yield ratio is reduced to 80% or less, has a structure containing bainite and / or martensite and ferrite, and the ferrite fraction of the entire structure is 4 to 24 area%, When the metal structure of the steel material is observed by the backscattered electron diffraction image method (EBSP method), the following equation (1) is satisfied. In Equation 1 below, D means an average circle equivalent diameter (µm) of crystal grains surrounded by diagonal grain boundaries whose crystal orientation difference is 15 ° or more by measuring the orientation difference between two adjacent crystals by the EBSP method.

35≤D

In addition, the second low rolling material having improved low-temperature toughness obtained from this steel material satisfies the following expressions (2) and (3) when the metal structure is observed by the backscattered electron diffraction image method (EBSP method). In Equation 3 below, M means a ratio (area%) of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 55 ° or more.

D≤30

50≤M

Description

STEEL MATERIAL HAVING EXCELLENT TOUGHNESS IN WELDING HEAT-AFFECTED ZONE, AND METHOD FOR PRODUCING THE SAME}

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to steel materials used in structures such as bridges, high-rise buildings, ships, and the like, and specifically, a portion subjected to heat influence when welded (hereinafter referred to as a "welding heat affected zone" or "HAZ"). It is related with the steel materials which improved toughness, and its manufacturing method.

The characteristics required for steel materials used in bridges, high-rise buildings, ships, and the like have become increasingly strict in recent years, and particularly good toughness is required. In general, these steels are often joined by welding, but there is a problem that, among the welded joints, in particular, HAZ is affected by heat during welding, and thus the toughness tends to deteriorate. This toughness deteriorates remarkably as the heat input at the time of welding increases, and it is thought that the cause of the heat generation at the time of welding decreases the cooling rate of HAZ, decreases hardenability, and produces coarse island martensite. Therefore, in order to improve the toughness of the HAZ, it is considered that the amount of heat input during welding may be suppressed as much as possible. On the other hand, however, in order to improve the welding work efficiency, it is desirable to employ a high heat input welding method having a weld heat input amount of 50 kJ / mm or more, for example, electroslag welding and submerged welding.

Therefore, the present applicant proposes a steel material which suppresses the deterioration of the HAZ toughness when the high heat input welding method is adopted in Japanese Patent Laid-Open No. 2007-100213. This steel is characterized by containing oxides and / or CaO of REM and ZrO ₂ as oxides. These oxides are finely dispersed in the steel because they exist in the liquid phase in molten steel. Because it does not lose employment even when received, it contributes to improving the toughness of HAZ.

On the other hand, although it is not a technique aimed at improving the HAZ toughness, Japanese Patent Application Laid-Open No. 8-120401 includes hydrogen elements such as REM and Zr in steel, and actively contains solid solution REM and Zr solid solution. The technique which prevents flaw detection and improves the internal quality of a thick steel plate, and maintains the integrity of an internal quality is proposed. In this technique, Al, Ca, Ti, and the like are added in combination to secure a stable high capacity.

However, in recent years, buildings and structures (such as offshore structures) have become higher and larger in size, and movements using high-strength 590 MPa-class high-strength steels, instead of 490 MPa-class steels, which have been conventionally used, have become stronger. However, the technique described in Japanese Patent Application Laid-Open No. 8-120401 addresses the improvement of the HAZ toughness, but for example, the steel having the resistance yield ratio (YR of 80% or less) required for high tensile strength steel used in buildings and structures. It is not being reviewed.

Applicant discloses Japanese Patent Application Laid-open No. Hei 8-209294 as a steel having both high tensile strength and resistance ratio. In this case, by dispersing fine carbonitride and securing a certain amount of ferrite, a resistive ratio is achieved while achieving a tensile strength of 590 MPa or more. However, the improvement of the HAZ toughness in the case of welding more than 50 kJ / mm of heat input is not fully examined, and the realization of the high tensile strength steel which is excellent in both the characteristics of resistance yield ratio and HAZ toughness is urgently desired.

On the other hand, steel used in ships and the like also requires high strength. However, when the steel is made high, the yield strength exceeds the brittle fracture strength, and brittle fracture easily occurs during elastic deformation. For this reason, the IAA Uniform Regulations set toughness grades for each structural member from the fracture mechanics method (K concept) to prevent brittle fractures. It responds by improving the toughness of the base metal required. Therefore, in order to ensure the safety of the structure under the strict use environment, as described above, it is important that the HAZ toughness at the weld joint is good and the base metal toughness (particularly, the base metal toughness in the low temperature region) is good.

An object of the present invention is to provide a steel material in which variation in HAZ toughness is reduced. Furthermore, it is providing the steel rolling material which yield ratio reduced from 80% or less obtained from this steel material, or the steel rolling material which improved low-temperature toughness. In addition, another object of the present invention is to provide a method for producing the steel and each rolled material.

The steel material of this invention which could solve the said subject is C: 0.03-0.2% (The meaning of "mass%. Below.), Si: 0.5% or less, Mn: 2% or less, Ti: 0.03% or less, and N : 0.01% or less, P: 0.02% or less, S: 0.015% or less, and Al: 0.01% or less, and further, REM: 0.0010 to 0.1% and Zr: 0.0010 to 0.05%, respectively. The balance is made of iron and inevitable impurities,

(A) In addition to the inclusions containing the REM and Zr steel,

(B) The employment REM and the employment Zr in steel satisfy the employment REM: 0.0010% or less and the employment Zr: 0.0010% or less.

In the said steel materials, when the composition of the inclusions contained in the said steel materials is measured, the abundance ratio of elements other than O, C, N, and S among the elements contained in this inclusion is converted into moles, and when the whole element amount after conversion is made into 1 mol, It is recommended that the mole fraction of REM be 0.05 or more and the Zr mole fraction of 0.04 or more.

The first rolled material of the present invention, which has been able to solve the above problems, has a Mn of 1.0 to 2% in the steel of the present invention, and further includes Cu: 2% or less, Ni: 2% or less as another element. Pressure reduction obtained by rolling steel materials containing at least one element selected from the group consisting of Cr: 3% or less, Mo: 1% or less, Nb: 0.05% or less, V: 0.1% or less, and B: 0.005% or less As a series,

(C) The tissue contains bainite and / or martensite and ferrite, and the fraction of ferrite in the whole tissue is 4 to 24 area%, and the total fraction of bainite and martensite is 74 area% or more and less than 96 area%. ego,

(D) When the metal structure of steel materials is observed by the backscattered electron diffraction method (EBSP method), the following formula (1) is satisfied.

Equation 1

35≤D

(In Equation 1, D denotes an average circle equivalent diameter (mu m) of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 15 ° or more by measuring the azimuth difference between two adjacent crystals by the EBSP method.]

In the steel of the present invention, the second steel cold roll of the present invention, which has solved the above problems, is made C: 0.04 to 0.13%, Ti: 0.02% or less, and further Cu: 0.3% or less and Ni: 0.4%. Or a steel rolling material obtained by rolling a steel material containing at least one member selected from the group consisting of Nb: 0.25% or less,

(E) The following Equations 2 and 3 are satisfied when the metal structure of the steel is observed by the backscattered electron diffraction method (EBSP method).

Equation 2

D≤30

Equation 3

50≤M

[Equation 2] In Equation 2, D means an average circle equivalent diameter (mu m) of crystal grains surrounded by diagonal grain boundaries whose crystal orientation difference is 15 ° or more by measuring the orientation difference between two adjacent crystals by the EBSP method. In addition, in Formula (3), M means the ratio (area%) of the crystal grain enclosed by the diagonal grain boundary whose crystal orientation difference is 55 degrees or more in the whole steel material.]

In the steel of the present invention, after adding REM and Zr to the molten steel in which the total oxygen amount [O] ₁ is adjusted in the range of 0.0020 to 0.015%, the dissolved oxygen amount [O] ₂ is adjusted in the range of 0.0010 to 0.0035%, and then casting It can manufacture by). Here, the [O] ₁ wherein the total amount of oxygen is measured, it is recommended to adjust the dissolved oxygen content [O] ₂ by the addition of Zr and REM to satisfy the total amount of oxygen [O] according to the _first to equation (4).

[REM] + [Zr] ≤15 × [O] ₁

[Where, in formula (4), [REM] and [Zr] are the addition amount (mass%) of REM or Zr, respectively, and [O] ₁ is the total amount of oxygen (mass%) of molten steel before addition of REM and Zr. ]

The 1st steel rolling material of this invention hot-rolls the steel material of this invention obtained by the said method so that rolling end temperature may be 870 degreeC or more, and is quenched from the temperature range of Ar ₃ or more, Ac _1- Ac ₃ points quenching from the temperature region, by carrying out each step of tempering in sequence in the temperature range of less than Ac ₁ point can be prepared.

In the second steel sheet of the present invention, the steel of the present invention obtained by the above method is heated at a temperature range of Ac ₃ or more and 1200 ° C or lower, and then the average temperature of the steel pieces is Ar ₃ points + 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 rolling material is Ar ₃ or more, and the surface temperature of the hot rolling material. It can manufacture by cooling to an average cooling rate of 5 degrees C / sec or more to the temperature range of 500 degrees C or less. Here, after cooling at a cooling rate of 5 degrees C / sec or more to the temperature range where the surface temperature of the said hot rolled material is 500 degrees C or less, it is recommended to heat by tempering to the temperature range of 500 degrees C or more and less than Ac ₁ point.

According to the steel of this invention, the variation of HAZ toughness can be suppressed by reducing the solid solution REM amount and solid solution Zr amount contained in steel materials as much as possible.

In addition, according to the first rolled material of the present invention obtained from the steel of the present invention, the bainite and / or martensite structure is mainly composed of a structure containing 4 to 24% of ferrite, and the structure is observed. The average circle equivalent diameter of the crystal grains enclosed by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more is made 35 micrometers or more, and the yield ratio of a base material can be reduced to 80% or less, ensuring the strength of 590 Mpa or more.

Moreover, according to the 2nd steel rolling material of this invention obtained from the steel of this invention, when the metal structure is observed, the average circle equivalent diameter of the crystal grain enclosed by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more shall be 30 micrometers or less, In particular, the low-temperature toughness of the base material itself can be improved by making the ratio of the crystal grains enclosed by the diagonal grain boundary whose crystal orientation difference is 55 degrees or more to the whole steel material 50 area% or more.

MEANS TO SOLVE THE PROBLEM The present inventors repeated examination in order to reduce the yield ratio of a base material while suppressing the variation of HAZ toughness with respect to the steel material which combined REM and Zr with steel materials and improved the HAZ toughness of a weld joint. As a result, (I) the addition of REM and Zr to the steel is adjusted to contain REM and Zr in the inclusions to increase the HAZ toughness, and (II) the amount of solid solution REM and Zr contained in the steel. Reducing as much as possible, it was found that the phenomenon of locally deteriorating toughness can be prevented and the variation in HAZ toughness can be suppressed, thereby completing the steel material of the present invention.

Furthermore, using such steels, the metal structure of (III) steel is a bainite and / or martensite structure main body, and is a structure containing 4 to 24% of ferrite, and (IV) the metal structure of steel By properly controlling the size of the crystal grains surrounded by the diagonal grain boundary having an orientation difference of 15 ° or more, it was found that the yield ratio of the base material can be reduced, thereby completing the first steel rolling material of the present invention.

Further, by using the steel, if the metal structure of the steel material (V) is properly controlled, the size of crystal grains surrounded by diagonal grain boundaries with a crystal orientation difference of 15 ° or more and the fraction of crystal grains surrounded by diagonal grain boundaries with a crystal orientation difference of 55 ° or more are appropriately controlled. It was found out that the low-temperature toughness of the base material itself could be improved, and the second strong rolling material of the present invention was completed.

Hereinafter, (I)-(IV) is demonstrated in detail.

[(I) HAZ Toughness of Weld Joints]

The steel material of this invention contains the inclusion containing REM and Zr. "Containing REM and Zr in inclusion" means (a) containing a single inclusion of REM and a single inclusion of Zr, or (b) containing a composite inclusion comprising REM and Zr, or (c) a single inclusion of REM. It is meant to contain a inclusion and a single inclusion of Zr, and at the same time contain a composite inclusion containing REM and Zr.

Examples of the inclusions of REM include oxides of REM and sulfides of REM, and examples of the inclusions of Zr include oxides of Zr, carbides of Zr, nitrides of Zr, and the like. Examples of the composite inclusions of REM and Zr include forms such as oxides, sulfides, or acid sulfides containing REM and Zr. On the other hand, these inclusions may further be in the form of coexisting with nitrides (for example, TiN, etc.) or other sulfides (for example, CaS, MnS, etc.). In addition, in the following, for convenience of explanation, a single inclusion and a complex oxide may be referred to as "inclusions".

The inclusions of REM and Zr are affected by heat during welding and are not dissolved even at high temperatures of 1400 ° C. Containing these inclusions suppresses the coarsening of austenite grains in the HAZ during welding or during cooling. Since the transformation in the particles can be promoted, the HAZ structure can be refined and the toughness of the HAZ can be further improved.

Furthermore, by adding REM and Zr in combination and containing them as inclusions in steel, it is possible to prevent the formation of coarse Zr single carbide or coarse REM sulfide which causes the toughness of steel (base material) to be prevented. The toughness of HAZ can be improved while suppressing toughness deterioration. That is, when REM or Zr is added alone, the amount of REM or Zr must be increased in order to increase the number of inclusions. However, when the amount of addition of REM or Zr is excessively increased, the size of the single inclusion of REM or Zr alone is increased. Rather, it degrades the HAZ toughness. Therefore, when REM or Zr is added alone, the amount of addition is limited. Therefore, the amount of addition of REM or Zr cannot be increased and the amount of fine inclusions cannot be increased beyond a certain level. Therefore, HAZ toughness could not be improved.

On the other hand, if the inclusions containing REM and Zr are contained in the steel, the toughness of the HAZ can be further increased because the absolute amount of the inclusions contained in the steel can be increased than when the REM is contained alone or when Zr is included alone. Can be improved. Thus, the toughness of HAZ can be improved by containing the inclusion of REM and Zr in steel materials. Therefore, in order to improve the toughness of HAZ, it is considered that it is desirable to actively add REM and Zr to generate a large amount of inclusions in the steel.

The steel material of this invention measured the composition of the inclusion contained in this steel, converted the abundance ratio of elements other than O, C, N, and S among the elements which comprise this inclusion, and made the whole element amount after conversion into 1 mol. In this case, the mole fraction of REM is preferably 0.05 or more and the mole fraction of Zr is 0.04 or more. It is preferable that molar fraction of REM is 0.10 or more, More preferably, it is 0.15 or more, More preferably, it is 0.20 or more. On the other hand, it is preferable that the mole fraction of Zr is 0.08 or more, More preferably, it is 0.10 or more, More preferably, it is 0.15 or more.

The sum of the mole fraction of REM and the mole fraction of Zr is preferably 0.10 or more. If the sum is less than 0.10, the amount of inclusions contributing to the toughness improvement of the HAZ is insufficient, and the toughness of the HAZ cannot be sufficiently improved. The sum is more preferably 0.15 or more, and more preferably 0.20 or more.

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

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

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

[(II) Deviation of HAZ Toughness of Weld Joints]

The steels with high REM and Zr contents were welded, and the toughness of the HAZ was measured at a plurality of points. It turned out that the measured value was uneven. Therefore, when the tissue in the part where the toughness was locally degraded was observed, REM and Zr were segregated in the grain boundary. In order to reduce segregation of REM and Zr, studies have been repeated, and it has been found that it is preferable to reduce the amount of solid solution REM and the amount of solid solution Zr in steel materials.

That is, it is important that the steel material of the present invention satisfies the solid solution REM: 0.0010% or less (including 0%) and the solid solution Zr: 0.0010% or less (including 0%). When the amount of solid solution REM in steel exceeds 0.0010% or the amount of solid solution Zr exceeds 0.0010%, when thermally affected by welding, REM or Zr segregates at grain boundaries and locally reduces toughness. Therefore, the amount of solid solution REM is made 0.0010% or less, preferably 0.0008% or less, and more preferably 0.0005% or less. The solid solution Zr amount is 0.0010% or less, preferably 0.0008% or less, and more preferably 0.0005% or less. It is preferable to reduce the amount of solid solution REM and the amount of solid solution Zr as much as possible, and most preferably 0%.

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

The amount of solid solution REM contained in the steel is, as shown in Examples described later, Inductively Coupled Plasma (ICP); What is necessary is just to calculate by subtracting the amount of REM contained in the inclusions contained in steel materials computed by electrolytic extraction and ICP-MS from the REM content (total REM content) calculated by the analysis by the inductively coupled plasma] -MS method. Similarly, the amount of solid solution Zr may be calculated by subtracting the amount of Zr contained in inclusions included in the steel from the Zr content (total Zr content).

[(III) Metal Structure of Base Metal]

The metal structure of the first rough-rolled material of the present invention includes bainite and / or martensite and ferrite, and the ferrite fraction occupies 4 to 24 area% in the total structure, and the total fraction of bainite and martensite is 74 area%. It is less than 96 area%.

8 is a graph showing the relationship between the ferrite fraction and the yield ratio, which summarizes the results of the examples described later. It can be seen from FIG. 8 that the ferrite fraction needs to be 4% or more in order to achieve a yield ratio of 80% or less. In order to further reduce the yield ratio, the ferrite fraction is preferably 7% or more, and more preferably 10% or more.

On the other hand, Figure 7 is a graph showing the relationship between the ferrite fraction and the tensile strength (TS), summarized the results of the examples described later, from Figure 7, to increase the tensile strength to 590MPa or more to ensure that the ferrite fraction to 24% or less You can see what you need to do. In order to further increase the tensile strength, the ferrite fraction is preferably 22% or less, more preferably 20% or less.

The metal structure may be composed only of bainite and / or martensite and ferrite, but is not limited to this and other structures (cementite or island martensite (MA)) which may be inevitably formed in the manufacturing process. ) Is also included.

[(IV) Yield Ratio of Base Material]

When the 1st strong rolling material of this invention observes a metal structure by a backscattering electron diffraction image method (EBSP method), it is necessary to satisfy | fill following formula (1). By satisfying formula (1), the yield ratio of the base material can be made 80% or less.

Equation 1

35≤D

In Equation 1 above, D means an average circle equivalent diameter (µm) of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 15 ° or more by measuring the orientation difference between two adjacent crystals by the EBSP method. In this invention, this value of D shall be 35 micrometers or more.

9 is a graph showing the relationship between the average circle equivalent diameter D of the grains surrounded by the diagonal grain boundaries and the yield ratio, and summarizes the results of the examples described later. 9 shows that in order to achieve the yield ratio of 80% or less, in addition to adjusting the said ferrite fraction, it is necessary to make the said average circle equivalent diameter D into 35 micrometers or more. The yield strength of the metal material is proportional to the half power of the reciprocal of the particle diameter, which is known as the law of the Hall patch, and the yield point increases as the grain size becomes finer. In order to make yield ratio further small, Preferably it is 37 micrometers or more, More preferably, it is 39 micrometers or more.

The metal structure is observed at the t / 4 position in the plate thickness direction when the plate thickness of the steel is t (mm). The specific observation order is demonstrated in the term of the Example mentioned later.

[(V) Low Temperature Toughness of Base Material itself]

The second strong rolling material of the present invention needs to satisfy the following expressions (2) and (3) when the metal structure is observed by the backscattered electron diffraction image method (EBSP method). By satisfying both equations, the low temperature toughness of the base material itself is improved.

Equation 2

D≤30

Equation 3

50≤M

In Equation 2, D means an average circle equivalent diameter (µm) of crystal grains surrounded by diagonal grain boundaries whose crystal orientation difference is 15 ° or more by measuring the orientation difference between two adjacent crystals by the EBSP method. In this invention, this value of D shall be 30 micrometers or less. Brittle cracks are generally known to bend, bypass or rectify at diagonal grain boundaries with a crystal orientation difference of 15 ° or more. Therefore, by miniaturizing the crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 15 ° or more, the position at which the brittle cracks are bent, bypassed, and rectified increases, so that the impact characteristics are increased and the low temperature toughness of the base material itself is increased. The smaller the value of D, the better. Preferably it is 28 micrometers or less, More preferably, it is 25 micrometers or less.

In the above Equation 3, M means the ratio (area%) of the crystal grains surrounded by the diagonal grain boundaries having a crystal orientation difference of 55 ° or more in the whole steel. In this invention, this value of M is made into 50 area% or more. This is because the bending, bypassing, and rectifying action of the brittle crack due to the diagonal grain boundary having a crystal orientation difference of 15 ° or more is further exhibited by the diagonal grain boundary having a crystal orientation difference of 55 ° or more among the diagonal grain boundaries. Therefore, also in this invention, the ratio which the crystal grain enclosed by the diagonal grain boundary whose crystal orientation difference is 55 degrees or more occupies in the whole steel material shall be 50 area% or more. The value of M is preferably 55 area% or more, more preferably 60 area% or more.

The metal structure is observed at the t / 4 position in the plate thickness direction when the plate thickness of the steel is t (mm). The specific observation order is demonstrated in the term of the Example mentioned later.

The second steel sheet of the present invention is composed of the structure of the bainite subject. By using the bainite main body, the strength of the steel sheet can be ensured. The bainite principal means that the area ratio of bainite is 80% or more when the metal structure is observed. The second strong rolling material of the present invention may be composed of only bainite, and martensite, ferrite, or the like may be formed as a structure other than bainite. On the other hand, the smaller the ferrite structure is, the better it is to prevent the decrease in strength, and it is preferable that it is generally less than 4 area%.

[Component composition]

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

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

REM should be 0.0010% or more, preferably 0.0015% or more, more preferably 0.002% or more. However, when excessively added, coarse inclusions (for example, oxides, etc.) are generated and the toughness of the base metal is degraded, so it must be suppressed to 0.1% or less. Preferably it is 0.09% or less, More preferably, you may be 0.08% or less.

In the present invention, REM means a lanthanoid element (15 elements from La to Lu) and Sc (scandium) and Y (yttrium), and among these elements, it is selected from the group consisting of La, Ce, and Y. It is preferable to contain at least 1 type of element, More preferably, it is good to contain La and / or Ce.

Zr should be 0.0010% or more, Preferably it is 0.0015% or more, More preferably, it is 0.002% or more. However, when excessively added, coarse Zr carbides are produced and the toughness of the base material is degraded. Therefore, it should be suppressed to 0.05% or less. Preferably it is 0.04% or less, More preferably, you may be 0.03% or less.

The steel of the present invention contains REM and Zr as well as C: 0.03-0.2%, Si: 0.5% or less, Mn: 2% or less, Ti: 0.03% or less, and N: 0.01% or less as basic elements. will be.

Within the composition range of the said steel material, it is Mn: 1.0-2% especially in the 1st steel rolling material of this invention.

Within the composition range of the steel, in the second rolled steel of the present invention, C: 0.04 to 0.13%, Ti: 0.02% or less, Cu: 0.3% or less, Ni: 0.4% or less, and Nb: 0.25 It contains at least one element selected from the group consisting of% or less.

The reason for determining this range is as follows.

C is an indispensable element in order to secure the strength of the steel (base material), and it is necessary to contain C at least 0.03%. It is preferable to contain C 0.04% or more, More preferably, you may be 0.05% or more. However, if the content exceeds 0.2%, a large amount of island-like martensite is generated in the HAZ during welding, which not only causes the toughness of the HAZ but also adversely affects the weldability. Therefore, it is necessary to suppress C to 0.18% or less, preferably 0.16% or less, more preferably 0.14% or less. Especially in the second rolling material of the present invention, it is necessary to contain 0.04% or more, preferably 0.05% or more, more preferably 0.06% or more, 0.13% or less, preferably 0.12% or less, More preferably, it is necessary to suppress it to 0.11% or less.

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

Mn is an element which contributes to the strength improvement of steel materials (base material), and in order to exhibit such an effect effectively, it is preferable to contain 0.5% or more. More preferably, it is 0.7% or more, More preferably, it is 0.8% or more. Especially in the 1st rolled material of this invention, it is necessary to contain 1.0% or more, Preferably it is 1.2% or more, More preferably, it is 1.4% or more. However, when it contains excessively more than 2%, HAZ toughness will deteriorate and weldability of steel materials (base material) will deteriorate. 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 the improvement of toughness of HAZ by forming nitrides or Ti oxides such as TiN in steel materials. In order to exhibit such an effect effectively, Ti is preferably contained 0.005% or more, more preferably 0.007% or more, and still more preferably 0.010% or more. However, excessive addition deteriorates the toughness of the steel (base metal), so it should be suppressed to 0.03% or less. Preferably it is 0.028% or less, More preferably, you may be 0.026% or less. In particular, in the second rolled material of the present invention, it should be suppressed to 0.02% or less, preferably 0.018% or less, and more preferably 0.016% or less.

N is an element that precipitates nitride (eg, ZrN, TiN, etc.), and this nitride prevents coarsening of austenite grains formed in HAZ during welding and promotes ferrite transformation, thereby contributing to improving HAZ toughness. do. 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. As N increases, the micronization of austenite grains is promoted, and therefore, it is effective in improving the toughness of HAZ. However, if it exceeds 0.01%, the amount of solid solution N increases and the toughness of the base material deteriorates. Therefore, N needs to be suppressed to 0.01% or less, Preferably it is 0.009% or less, More preferably, you may be 0.008% or less.

In addition to containing the above elements, the steel of the present invention includes P: 0.02% or less (not including 0%), S: 0.015% or less (not including 0%), and Al: 0.01% or less (including 0%). Is satisfied. The reason for determining this range is as follows.

P is an element which tends to segregate, and in particular, it segregates at grain boundaries in steel materials and deteriorates toughness. Therefore, P should be controlled to 0.02% or less, preferably 0.018% or less, and more preferably 0.015% or less.

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

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

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

Steel of the present invention,

(i) Ca: 0.01% or less to improve HAZ toughness,

(ii) Cu: 2% or less, Ni: 2% or less, Cr: 3% or less, Mo: 1% or less, Nb: 0.05% or less, V: 0.1% or less, and B: 0.005% to increase the strength of the steel It is also effective to contain at least one element selected from the group consisting of the following. The reason for determining this range is as follows.

(i) Ca is an element having the effect of improving the HAZ toughness of steel materials. More specifically, Ca has the effect of reducing the anisotropy of the steel by controlling the shape of the inclusions (specifically, by spheroidizing MnS), and improving the HAZ toughness by reducing the anisotropy of the steel. 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, More preferably, it is 0.001% or more. Excessive addition, however, forms coarse oxides that deteriorate the HAZ toughness. 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) Cu, Ni, Cr, Mo, Nb, V, and B are all elements which act to raise the strength of steel materials.

Particularly, since steel used for ships and the like requiring low-temperature toughness requires strength in addition to good base metal toughness and HAZ toughness, the second high rolling material of the present invention needs to contain at least one element as an essential element. Preferably, both Cu and Ni may be contained or only Nb may be contained.

Cu is an element which solid-state strengthens steel materials, and in order to exhibit such an effect effectively, it is preferable to contain Cu or more. More preferably, it is 0.1% or more, More preferably, it is 0.2% or more. However, when it contains more than 2%, since the toughness of steel materials (base material) will fall, it is good to suppress Cu to 2% or less. Preferably it is 1.8% or less, More preferably, you may be 1.6% or less. In the second strong rolling material of the present invention, 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 and at the same time improve the toughness of the steel. In order to exhibit such an action, it is preferable to contain Ni at least 0.05%. More preferably, it is 0.1% or more, More preferably, you may be 0.2% or more. Although more Ni is preferable, since it is an expensive element, it is preferable to suppress it to 2% or less from an economic viewpoint. More preferably, it is 1.5% or less, More preferably, you may be 1% or less. In the second high rolling material of the present invention, it is preferable to suppress it to 0.4% or less. More preferably, it is 0.38% or less, More preferably, you may be 0.35% or less.

In order to increase Cr by adding Cr, it is preferable to contain 0.01% or more. More preferably, it is 0.02% or more, More preferably, it is 0.03% or more. However, since the weldability deteriorates when it exceeds 3%, it is preferable to suppress Cr to 3% or less. More preferably, it is 1.5% or less, More preferably, you may be 1% or less.

In order to add Mo and to raise strength, it is preferable to contain 0.01% or more. More preferably, it is 0.02% or more, More preferably, it is recommended to contain 0.03% or more. However, since it will worsen weldability when it exceeds 1%, it is preferable to make Mo 1% or less. More preferably 0.9% or less, and more preferably 0.8% or less is recommended.

Nb is an element having a recrystallization inhibiting effect, which effectively contributes to the refinement of the structure and at the same time, is an element that increases the strength of steel by effectively depositing carbides and nitrides. In order to exhibit such an effect effectively, it is preferable to contain 0.005% or more. More preferably, it is 0.008% or more, More preferably, it is 0.01% or more, More preferably, it is 0.03% or more. However, in the first steel rolling material of the present invention, when it exceeds 0.05%, the structure becomes excessively fine and the yield ratio becomes high. Therefore, it is preferable to suppress Nb to 0.05% or less. More preferably, it is 0.04% or less, More preferably, you may be 0.03% or less. In the second high rolling material of the present invention, since the toughness of the base material is deteriorated when it exceeds 0.25%, it is preferable to suppress Nb to 0.25% or less. More preferably, it is 0.23% or less, More preferably, you may be 0.2% or less.

In order to increase the strength by adding V, the content is preferably 0.005% or more. More preferably, it is 0.01% or more, More preferably, it is good to contain 0.03% or more. However, since the weldability deteriorates and the toughness of a base material deteriorates when exceeding 0.1%, V is desirable to be 0.1% or less. More preferably, it is good to suppress it to 0.08% or less, More preferably, it is 0.06% or less.

B increases the strength of the steel and simultaneously binds with N in the steel to precipitate BN in the process of cooling the heated HAZ during welding to promote ferrite transformation from 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, More preferably, you may be 0.0008% or more. However, since exceeding 0.005% will degrade the toughness of steel materials (base material), it is preferable to make B into 0.005% or less. More preferably, it is 0.004% or less, More preferably, you may be 0.003% or less.

[Production method]

Next, the manufacturing method which can be suitably employ | adopted in manufacturing the steel material of this invention is demonstrated. Steel material of the present invention is the employment REM and to reduce the employment Zr less than a predetermined amount, the total amount of oxygen [O] dissolved oxygen [O] by the addition of REM and Zr in the molten steel is adjusted to the range of the _12% 0.0020~0.015 After adjusting to 0.0010 to 0.0035% of range, it casts and obtains a steel material (casting). The reason for defining such a range is explained below.

First, when REM and Zr are compositely added to molten steel in which the total oxygen amount [O] ₁ is properly controlled, REM and Zr can be generated in the steel as an oxide as a form of inclusions. At this time, the dissolved oxygen amount [O] ₂ of molten steel is appropriately controlled by adjusting the amount of REM and Zr compounded in the molten steel, and the amount of solid solution REM and solid solution Zr in the steel can be reduced.

Usually, the total amount of oxygen [O] ₁ in the molten steel firstly refined in a converter or an electric furnace exceeds 0.015%. When REM or Zr is added to this molten steel, since the amount of oxygen in molten steel is too large, reaction of REM, Zr, and oxygen becomes severe, and it is unpreferable in solvent operation. In addition, coarse REM oxide and coarse ZrO ₂ are generated to degrade the base material toughness itself.

Therefore, in the present invention, REM oxide is added as a inclusion of REM, Zr oxide as an inclusion of Zr, or REM as a composite inclusion of REM and Zr by adding REM and Zr to molten steel in which the total amount of oxygen [O] ₁ is adjusted slightly less than before. And an oxide containing Zr.

On the other hand, from the viewpoint of increasing the amount of oxides among the inclusions of REM and Zr, in particular, a large amount of REM and Zr may be added to molten steel in which the total oxygen amount [O] ₁ is adjusted. Employed in steel. However, when the amount of solid solution REM or Zr increases, the HAZ toughness will be varied as described above.

Therefore, in the present invention, by adjusting the amount of REM and Zr added to the molten steel, the dissolved oxygen amount [O] ₂ after adding REM and Zr is adjusted to be a little larger than before, so that REM and Zr are not dissolved in casting. I did it.

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 the primary smelting, and should be suppressed to 0.015% or less, preferably 0.01% or less, and more preferably 0.008 It should be less than%. However, when the total amount of oxygen [O] ₁ is too small and less than 0.0020%, the amount of oxygen becomes insufficient. Therefore, even when a combination of REM and Zr is added, the amount of oxide that contributes to the improvement of toughness of HAZ cannot be secured, and further, an oxide is formed. REM or Zr, which could not be done, is dissolved in steel, or Zr forms carbides or the like to deteriorate the toughness of the base material. Therefore, it is preferable to adjust the total amount of oxygen [O] ₁ before adding REM and Zr to 0.0020% or more, More preferably, it is 0.0025% or more.

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

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

The order in which REM and Zr are added to the molten steel in which the total oxygen amount [O] ₁ is adjusted is not particularly limited. For example, (a) Zr may be added after REM is added, and (b) REM after Zr is added. May be added and (c) REM and Zr may be added simultaneously. When adding two or more types of REM, you may add simultaneously or separately. For example, Ce and La may be used as the REM and may be 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, Fe What is necessary is just to add a -Si-La-Ce alloy. In addition, you may add micro metal to molten steel. A micrometal is a mixture of rare earth elements, specifically, it contains about 40 to 50% of Ce and about 20 to 40% of La.

After composite addition of said REM and Zr, if the dissolved oxygen amount [O] ₂ just before casting does not have an influence, you may add an alloying element and adjust the component of steel materials.

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

In order to control the dissolved oxygen content [O] ₂ in the range of 0.0010~0.0035%, may be adjusted by the addition amount of REM and Zr based on the total amount of oxygen [O] _1, specifically, to follow the [O] ₁ The total amount of oxygen mathematics The addition amount of REM and Zr may be determined so as to satisfy Equation 4, and an element may be added within the determined addition amount range of REM and Zr. In formula (4), [REM] and [Zr] are the addition amount (mass%) of REM or Zr, respectively, and [O] ₁ is the total oxygen amount (mass%) of molten steel before adding REM and Zr. The coefficient 15 on the right side is a value determined as a result of repeated experiments.

Equation 4

[REM] + [Zr] ≤15 × [O] ₁

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

On the other hand, when a little more REM or Zr is added with respect to the total oxygen amount [O] ₁ and the dissolved oxygen amount [O] ₂ is less than 0.0010%, an oxide [for example, MnO or iron oxide (for example, FeO) is used as an oxygen source. )] May be added.

Next, the manufacturing method of the 1st steel rolling material of this invention obtained from the steel material (steel piece) of the said invention is demonstrated. The steel strip obtained by casting as mentioned above is annealed from the temperature range of ₃ points or more of Ar after the hot rolling by making rolling end temperature 870 degreeC or more, and then the temperature range (Austenite ferrite two-phase region) of Ac _1- Ac ₃ points. Hereinafter, it is quenched from only "two phase zone"), and is then tempered in the temperature range below Ac ₁ point.

As described above, the steel of the present invention is characterized by dispersing inclusions containing REM and Zr in steel in order to improve HAZ toughness after welding. Since such inclusions are dispersed in steel, hot rolling In the later quenching process, the transformation inside the particles is promoted, and it is recognized that the metamorphic tissue after the quenching is easy to become fine. The refinement of the tissue effectively acts to improve the toughness of the base material itself, but when the structure becomes finer, the yield point increases because the yield point increases from the law of the hole patch. Therefore, in order to realize a yield ratio (resistance ratio) of 80% or less, the structure after quenching becomes excessively finer than necessary, so that crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 15 ° or more do not become smaller than necessary. It is necessary to perform rolling at a comparatively high temperature.

Specifically, in the present invention, it is important to perform hot rolling so that the rolling end temperature is 870 ° C or higher. Fig. 4 is a graph showing the relationship between the rolling end temperature and the average circle equivalent diameter D of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 15 ° or more, which summarizes the experimental results of the examples described later. In order to achieve an yield ratio of 80% or less, in order to achieve an average circle equivalent diameter D of crystal grains surrounded by diagonal grain boundaries with a crystal orientation difference of 15 ° or more to 35 µm or more, the rolling end temperature is 870 ° C or more, as is apparent from FIG. You need to.

FIG. 5 is a graph showing the relationship between the quenching start temperature (meaning quenching start temperature in the case of performing direct quenching (DQ) in FIG. 5) and the ferrite fraction, and summarizes the experimental results of the examples described later. In order to suppress the ferrite fraction to 24% or less in order to achieve a tensile strength of 590 MPa or more, it is necessary to make the quenching start temperature above the ferrite transformation start temperature (Ar ₃ point) from FIG. 5.

As the quenching method, in addition to direct quenching (DQ) of quenching the hot rolled material immediately after hot rolling, the hot rolled material may be quenched (RQ) offline. On the other hand, in the said DQ process, since retry is not possible in a process, strict temperature control of the said quenching start temperature is calculated | required more than the case of the said RQ process.

In addition, in order to mix a prescribed amount of ferrite phase in the hard bainite structure and / or martensite structure, it is necessary to perform the second quenching from the two phase region. Fig. 6 is a graph showing the relationship between the temperature (hereinafter sometimes referred to as the heating temperature) and the ferrite fraction when quenched from this temperature range while being maintained at a temperature range near the two-phase zone. It is summarized. In order to achieve a ferrite fraction of 4% or more in order to achieve a yield ratio of 80% or less, as shown in FIG. 6, it is necessary to maintain at a temperature of Ac ₁ or more and Ac ₃ or less (two phase station temperature). The holding time at the two-phase station temperature may be, for example, 5 minutes or more.

After heating to the biphasic region, quenching (for example, RQ) is performed, followed by tempering at a temperature below the ferrite transformation start temperature (temperature below Ac ₁ point). Thereby, the intensity | strength of steel materials can be adjusted to about 590 Mpa or more.

The temperature of the steel slab is controlled by the temperature at the t / 4 position calculated in the order described in the section of Examples described later. t means the thickness of the steel piece (mm). Also, the Ar ₃ point and the Ac ₃ point, the temperature of the Ac ₁ point may be measured by the procedure shown in the Examples reviews.

The first rolled material of the present invention can be used, for example, as a material for structures such as bridges, high-rise buildings, ships, etc., and can prevent the deterioration of the toughness of the weld heat affected zones in small to medium heat welding as well as large heat input welding.

Next, the manufacturing method of the 2nd steel rolling material of this invention obtained from the steel material (steel piece) of the said invention is demonstrated. The cast steel obtained by casting as described above (for example, slab) is a steel sheet obtained so that the metal structure satisfies the requirements of the formulas (2) and ( ₃ ) above the Ac ₃ point, the temperature range of 1200 ℃ or less (hereinafter, the temperature The reverse temperature is heated to "heating temperature" or "T1"), and then hot rolling is performed. In hot rolling, in the temperature range (hereinafter, the temperature of this temperature range may be called "T2") where the average temperature of the steel slab is Ar ₃ points + 10 ° C or more and 900 ° C or less, the maximum reduction ratio per pass is 12. The cumulative reduction ratio is made 40% or less. From the temperature range (hereinafter, the temperature of this temperature range may be called "T3") which the average temperature of the hot rolling material obtained is Ar ₃ or more (hereinafter, it is called "T3"), the temperature range (hereinafter, Cooling is performed at a cooling rate of 5 ° C./sec or more until the temperature in this temperature range may be referred to as “T4”. The reason for defining such a range is explained below.

Casting the thus obtained billet is heated to the heating temperature (T1) to the Ac ₃ point or higher, less than 1200 ℃. The heating temperature T1 needs to be heated to Ac ₃ or more in order to make the metal structure of the steel piece austenite. However, when heating temperature exceeds 1200 degreeC, since austenite grain becomes coarse, transformation tissue cannot be refined enough. Therefore, heating temperature T1 shall be 1200 degrees C or less.

The temperature of the Ac ₃ point can be calculated from the following equation (5). In formula, [] shows content (mass%) of each element.

Ac ₃ (° C.) = 908-223.7 × [C] + 438.5 × [P] + 30.49 × [Si] -34.43 × [Mn] -23 × [Ni]

The steel sheet heated at the heating temperature (T1) is hot rolled. In hot rolling, the maximum rolling reduction per pass is 12% or less in the temperature range where the average temperature of the steel slab is Ar ₃ points + 10 ° C or more and 900 ° C or less. It is necessary to make the reduction ratio 40% or more. By controlling the rolling conditions at an Ar ₃ point + 10 ° C or more and a temperature range of 900 ° C or less, growth of austenite grains can be suppressed and deformation can be efficiently introduced into the austenite grains before transformation. The grains surrounded by the diagonal grain boundary having an orientation difference of 15 ° or more can be refined, and the low temperature toughness of the base material itself can be enhanced.

When the maximum reduction ratio per pass in the temperature range of Ar ₃ points + 10 ° C or more and 900 ° C or more exceeds 12%, deformation is excessively accumulated in the austenite grains, and recovery of the deformation occurs, and after transformation Because the tissue (grains surrounded by diagonal grain boundaries) is coarsened, the low temperature toughness of the base material itself worsens. Therefore, in order to suppress a recovery of a deformation | transformation, the maximum reduction ratio per 1 pass in the temperature range of Ar <_3> +10 degreeC or more and 900 degrees C or less shall be 12% or less. Preferably it is 11% or less, More preferably, it is 10% or less. The smaller the maximum reduction ratio per pass increases the effect of suppressing the coarsening of the grains surrounded by the diagonal grain boundary, but if the maximum reduction ratio is too small, the production time becomes longer and the productivity worsens. Therefore, the lower limit of the maximum reduction ratio per one pass is preferably set to 6%.

The cumulative reduction in the Ar ₃ point + 10 ° C or higher and 900 ° C or lower is 40% or more. If the cumulative reduction ratio is less than 40%, the amount of deformation introduced into the austenite grain is small, and the nucleation site after transformation is reduced, so that the grains surrounded by the diagonal grain boundary are coarsened and the low temperature toughness of the base material itself is coarse. Worse Therefore, the cumulative reduction rate in the temperature range of Ar ₃ point + 10 degreeC or more and 900 degrees C or less shall be 40% or more. Preferably it is 45% or more, More preferably, it is 50% or more. Although the upper limit of the cumulative reduction ratio in the temperature range of Ar <_3> +10 degreeC or more and 900 degrees C or less is not specifically limited, Usually, it is about 60%.

The temperature of the Ar ₃ point can be calculated from the following equation (6). In formula, [] represents content (mass%) of each element, and t means the finishing thickness (mm) of a product.

Ar ₃ (° C.) = 910-310 × [C] -80 × [Mn] -20 × [Cu] -55 × [Ni] + 0.35 × (t-8)

The cumulative reduction ratio may be calculated by the following Equation 7. t ₀ is the rolling start thickness (mm) of the steel slab in the temperature range where the average temperature of the steel slab is 900 ° C. or less, and t ₁ is the rolling end thickness of the steel slab in the temperature range where the average temperature of the steel slab is Ar ₃ points + 10 ° C. or more. (mm) means.

Cumulative reduction ratio = [(t ₀ -t ₁ ) / t ₀ ] × 100

The average temperature of the said steel piece is managed by the temperature in the t / 4 position calculated by the procedure demonstrated in the term of the Example mentioned later. t means the thickness of the slab (mm).

On the other hand, the maximum reduction rate and cumulative reduction rate per pass in the temperature range (the austenite recrystallization area) in which the average temperature of a steel piece exceeds 900 degreeC is not specifically limited.

Next, the hot-rolled material obtained by the hot rolling, the average temperature of the Ar ₃ point or higher temperature range (T3) less than the surface temperature of the hot-rolled material 500 ℃ from the temperature range (T4) an average cooling rate of 5 ℃ / sec or more up to By cooling, the fraction of the crystal grain enclosed by the diagonal grain boundary whose crystal orientation difference is 55 degrees or more can be raised, and the toughness of the base material itself can be improved.

That is, although the second rough rolling material of the present invention is mainly composed of bainite structure, in general, bainite is KS which is said that the closest face of the crystal lattice of austenite and bainite and the closest direction thereof are almost parallel to each other. It is known to create with (Kurdjumov-Scahs) relationship. In this relationship, bainite is said to generate any one of a maximum of 24 orientations with respect to austenite, but it is said that the tendency to select is changed by changing the temperature of bainite transformation and the bainite form is changed. (Kawada et al .: CAMP-ISJ vol 16, No. 3 (2003), PS30). As the transformation temperature decreases, bainite changes from diffusion transformation represented by ferrite transformation to shear transformation represented by martensite, or change in nucleation capacity of the transformation and growth rate of generated tissue due to transformation temperature decrease. It is thought that this is because the organization after metamorphosis is greatly changed. In view of the above, by increasing the average cooling rate from the temperature range T3 above the Ar ₃ point to the temperature range T4 below 500 ° C., the fraction of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 55 ° or more can be increased. have.

After cooling to the temperature range T4 of 500 degrees C or less at the average cooling rate of 5 degrees C / sec or more, you may temper as needed. Since the deformation introduced by hot rolling or transformation is lost by tempering, the low-temperature toughness of a base material can further be improved. Tempering may be performed, for example, by heating at a temperature of 500 ° C or higher and less than Ac ₁ point.

The temperature of the Ac ₁ point can be calculated from the following Equation 8. In formula, [] shows content (mass%) of each element.

Ac ₁ (° C.) = 723-14 × [Mn] + 22 × [Si] -14.4 × [Ni]

The second rolled material of the present invention can be used, for example, as a material for structures such as bridges, high-rise buildings, ships, etc., and can prevent the deterioration of the toughness of the weld heat affected zones in small to medium heat welding as well as large heat input welding.

Hereinafter, although an Example demonstrates this invention further in detail, the following Example is not a property which limits this invention, It is also possible to change suitably and to implement in the range which may be suitable for the purpose of the previous and the latter, All are included in the technical scope of the present invention.

Example 1

In the following Experimental Examples 1-1 and 1-2, the HAZ toughness and its deviation (Experimental Example 1-1), and the strength and yield ratio of the rolled steel (Experimental Example 1-2) were examined using the same steel grade. Then, Experimental Example 1-1 and Experimental Example 1-2 were combined to evaluate the properties of the steel (steel rolled material).

Experimental Example 1-1 (Evaluation of HAZ Toughness and Its Deviation)

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

In the fruit blowing, the chemical component composition was adjusted while deoxidizing using Si and Mn, and adjusting to the total amount of oxygen [O] ₁ shown in Table 1 below. The total oxygen amount [O] ₁ means the total amount of oxygen that is the sum of the amount of oxygen contained in the molten steel as dissolved atoms and the amount of oxygen existing as oxide inclusions. The amount of oxygen contained in the molten steel as dissolved atoms is determined using an oxygen sensor using a solid electrolyte. The total amount of oxygen was measured by a general inert gas fusion-infrared absorption method. In addition, in addition to the total oxygen amount [O] ₁ , the following Table 1 also showed the dissolved oxygen amount of molten steel before adding REM and Zr.

The addition amount of REM and Zr was computed in order to satisfy said Formula (2) according to the said total oxygen amount [O] ₁ , and it adjusted to the dissolved oxygen amount [O] ₂ shown in following Table 1 by adding REM and Zr. In following Table 1, the addition amount [REM] of REM, the addition amount [Zr] of Zr, and the sum ([REM] + [Zr]) of REM and Zr addition amount are shown. Moreover, the ratio ([REM] + [Zr]) / [O] ₁ of the sum total of REM and Zr addition amount, and the total oxygen amount [O] ₁ is also shown.

After adjusting to the dissolved oxygen amount [O] ₂ , it cast after adjusting a chemical component to such an extent that it does not affect this [O] ₂ amount.

On the other hand, in the secondary refining, dehumidification, desorption, etc. were performed using an RH type degassing | purification refiner etc.

In Table 1 below, REM was in the form of a micrometal containing about 50% La and about 25% Ce, and Zr was added as Zr alone.

In FIG. 1, the relationship between the total amount of oxygen [O] ₁ before adding REM and Zr, and the sum total of the addition amount of REM and Zr ([REM] + [Zr]) is shown graphically. In FIG. 1, (circle) is No. of Table 1 shown below. As a result of 1-5, x is No. of following Table 1. The results of 11-15 are shown, respectively. In addition, in FIG. 1, the unit of total amount of oxygen [O] ₁ was described in ppm.

In addition, the following Table 2 shows the component composition (residues of iron and unavoidable impurities) of the steel material after component adjustment.

The molten steel after component adjustment was cast into the slab by the continuous casting machine, and the sample was cut out from the cross section in the t / 4 position of a slab (t is the thickness of slab). The cut sample surface was observed 10,000 times using EPMA "JXA-8500F (device name) made from Japan Electronics, and the component composition was quantitatively analyzed about the inclusion of 0.2 micrometer or more in maximum diameter. Observation conditions made acceleration voltage 7kV, sample current 0.003 microamperes, observation field area 1cm <^2> , and the number of analyses into 100 randomly selected, and quantitatively analyzed the component composition in the center part of the inclusion by wavelength dispersion spectroscopy of characteristic X-rays. The element to be analyzed is Al, Mn, Si, Ti, Zr, Ca, La, Ce. When the molar ratio of the element to be analyzed is molar, and the total amount of elements after conversion is 1 mole, The mole fraction of each element included in the inclusion was calculated. The calculation result of the mole fraction is shown in Table 3 below.

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

In addition, the amount of solid solution REM and the amount of solid solution Zr contained in steel materials were calculated in the following order. First, the amount of REM and Zr contained in steel materials as inclusions were measured by the electrolytic extraction method. Electrolytic extraction used the solution containing 2 cc of triethanolamine and 1 g of tetramethylammonium chlorides in 100 cc of methanol as electrolyte solution, and the said sample was extracted (electrolysis) under the current of 500 A / m <^2> or less. As a result, the matrix was dissolved and solid solution REM and Zr were also extracted in the electrolyte solution. The size of the sample was 15 mm long x 15 mm wide x 5 mm long.

Subsequently, the electrolytic solution after extraction was filtered using a membrane filter (47 mm in diameter, 0.1 mu m in pore size), and the residue was transferred to a platinum crucible for each filter and heated by a gas burner to be ashed. Subsequently, an alkali flux (a mixture of sodium carbonate and sodium tetraborate) was added, and the residue was further melted by heating with a gas burner. Next, 18% by volume of hydrochloric acid was added to liquefy the melt, which was then transferred to a volumetric flask, and pure water was further added to raise the scale to obtain an analyte. REM and Zr concentrations in the analyte were measured by the ICP-MS method.

The amount of REM and Zr contained in the inclusions thus obtained were subtracted from the amount of REM (total REM) or Zr (total Zr) analyzed separately by the usual ICP-MS to determine the amount of solid solution REM and amount of Zr. . The calculated result was combined with Table 3 below. In Table 3, "<0.0001" means that an element was not detected.

In FIG. 2, the relationship between the dissolved oxygen amount [O] ₂ contained in molten steel before casting, and the amount of solid solution REM or solid solution Zr contained in steel materials is shown graphically. In addition, in FIG. 2, the unit of dissolved oxygen amount [O] ₂ was described in ppm. 2, only the data in which the solid solution REM or the solid solution Zr was detected was plotted.

Next, in order to evaluate the toughness of HAZ subjected to the heat effect at the time of welding, the welding regeneration test shown below was performed by simulating high heat input welding. The welding reproduction test was performed by heating the sample cut out of the slab to 1400 ° C., holding it at this temperature for 5 seconds, and then cooling the sample. Cooling was adjusted so that cooling time from 800 degreeC to 500 degreeC might be 300 second.

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

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

In addition, the variation of toughness was evaluated based on the maximum value and minimum value of vE- ₄₀ value about each sample with the following reference | standard. The evaluation results are shown in Table 4 below.

[Evaluation Criteria for Maximum Value and Minimum Value]

○: The maximum or minimum HAZ toughness is 150 J or more.

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

[General Evaluation Criteria]

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

(Triangle | delta): Although at least 1 out of 3 measurements is 150 J or more, the variation of HAZ toughness is large and the minimum value is less than 150 J.

X: All of the three measurement results are less than 150J.

3, the average value (circle mark in a figure) of HAZ toughness, and the width | variety of the maximum value and minimum value of HAZ toughness are shown graphically about each sample shown in Table 4 below.

From the above result, it can consider as follows. As is clear from Figs. 1 and 3, REM and Zr are satisfied in 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 the addition of HSA improves the HAZ toughness and reduces the variation in the HAZ toughness. In addition, the formula of the straight line shown in FIG. 1 is ([REM] + [Zr]) = 15x10 <^-4> [[O] ₁ .

As is clear from Table 1, Table 3, and Fig. 2, when the dissolved oxygen amount [O] ₂ before casting is adjusted in the range of 0.0010 to 0.0035% (10 to 35 ppm), casting is performed. It turns out that the amount of solid solution Zr can be reduced below a predetermined value.

As is apparent from Tables 2 to 4 and FIG. 1 to 5 are examples satisfying the requirements specified in the present invention. In particular, the amount of REM and Zr in the chemical composition of the steel are appropriately adjusted, and the amount of solid solution REM and Zr is appropriately controlled. The average value is 150J or more, and HAZ toughness is excellent. In addition, the variation of the HAZ toughness is also reduced.

Meanwhile, No. 6-15 is an example which deviates from the requirements prescribed | regulated by this invention, Especially REM amount or Zr amount among the chemical components of steel materials is out of the range prescribed | regulated by this invention (No. 6-10, 15), or solid solution REM amount Since the amount of solid solution Zr deviates from the range prescribed | regulated by this invention (No. 11-14), the average value of HAZ toughness is less than 150J, and HAZ toughness is inferior. In addition, a large variation in the HAZ toughness also increases.

Experimental Example 1-2 (Evaluation of Tensile Strength and Yield Ratio of the Base Material)

The slab (steel grades a1 to o1) obtained by casting under the conditions described in Experimental Example 1-1 is subjected to hot rolling so that the finish rolling end temperature becomes the temperature shown in Table 5 below, and the obtained hot rolled material is Ar ₃ or more. Quenched from the temperature range. Quenching is performed by directly quenching from the quenching start temperature shown in Table 5 below after hot rolling (denoted by DQ in Table 5 below) or by heating the hot rolled material obtained by hot rolling offline to the quenching start temperature shown in Table 5 below. It was then quenched (indicated by RQ in Table 5 below).

After quenching, heating was maintained at a temperature range of Ac _{1 to} Ac ₃ , and quenching was performed from this temperature range. The holding time was 5 minutes. Table 5 shows the heating temperatures.

Subsequently, tempering was performed in a temperature range of less than Ac ₁ point. Table 5 shows the tempering temperatures.

The Ar ₃ point, the Ac ₁ point, and the Ac ₃ point of each slab were measured by the following method. The measurement results are shown in Table 5 below.

<< measurement method of Ar ₃ points (ferrite transformation start temperature at the time of cooling) >>

After processing the φ8mm × 12mm length processed formaster specimen taken from the slab and heating it at 1100 ° C. for 10 seconds in a processed formaster tester, processing is performed at 1000 ° C. with a cumulative reduction ratio of 25%, further 900 The cumulative reduction ratio was made 25% at 0 ° C, and then cooled from 800 ° C to an average cooling rate of 1 ° C / sec. The temperature at which the volume starts to expand during cooling was measured as the Ar ₃ point temperature.

<< measurement of Ac ₁ point (ferrite transformation start temperature at heating) and Ac ₃ point (ferrite transformation end temperature at heating) >>

The temperature at which the volume starts to decrease when the processed formaster test piece is heated from room temperature to 1000 ° C. with an average heating rate of 10 ° C./sec as Ac ₁ point temperature, and the temperature at which the volume starts to expand and continues to expand. Was measured as Ac ₃ point temperature.

The rolling end temperature, the quenching start temperature, the heating temperature, and the tempering temperature were managed at the average temperature at the t / 4 position when the thickness of the hot rolled material was t. The temperature in the t / 4 position was computed in the following procedure.

<< calculation method of rolling end temperature >>

(1) Using a process computer, the heating temperature of an arbitrary position in the plate thickness direction from the front surface to the back surface of a steel piece is computed based on the atmospheric temperature and furnace stay time from a heating start to extraction.

(2) Using the calculated heating temperature, the rolling temperature at any position in the plate thickness direction is suitable for the calculation of the differential method or the like based on the rolling pass schedule during rolling or the data of the cooling method (water cooling or air cooling) between the passes. It rolls, calculating using a method.

(3) Steel plate surface temperature is measured using the radial thermometer provided on the rolling line (however, it also calculates 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 compared with the calculated surface temperature on the process computer.

(5) When the difference between the calculated surface temperature and the measured steel sheet surface temperature is ± 30 ° C. or more, the measured steel sheet surface temperature is replaced with the calculated surface temperature to be the calculated surface temperature on the process computer.

(6) The rolling finish temperature at the t / 4 position is obtained using the corrected calculated surface temperature.

<< calculation method of quenching start temperature, heating temperature, tempering temperature >>

(1) Using a process computer, the heating temperature of an arbitrary position in the plate thickness direction from the surface of the steel piece to the back surface is calculated based on the atmosphere temperature and the furnace stay time from the start of heating to the end of heating.

(2) The temperature at the t / 4 position is obtained from the calculated heating temperature.

In following Table 5, the product thickness (mm) of the rolling material obtained by cooling was also shown.

Next, the metal structure of the obtained rolling material was observed in the following procedure, and the ferrite fraction was measured.

<< observation of metal structure >>

(1) The sample cut | disconnected in parallel to a rolling direction (long direction) is prepared so that both the surface and the back surface of a rolling material may be included.

(2) It grind | polishing by the wet emery polishing paper of # 150- # 1000, or the grinding | polishing method which has a function equivalent to it, and mirror-finished by using abrasive | polishing agents, such as a diamond slurry.

(3) The polished sample is etched with 3% nitric acid-ethanol solution (nital solution) to reveal grain boundaries of the ferrite structure.

(4) The tissue of the t / 4 position (t is a sample thickness) was photographed by the magnification of 100 times or 400 times. Ferrite tissue is colored black. In this experiment example, it was taken as a photograph of 6 cm x 8 cm.

(5) Next, the photographed picture is input to an image analysis device (the area of the picture corresponds to 600 μm × 800 μm when the magnification is 100 times, and 150 μm × 200 μm when the magnification is 400 times). The input to the image analysis device is input so that the sum of the areas is 1 mm x 1 mm or more at any magnification (that is, at least 6 photos at 100 times and at least 35 photos at 400 times). ).

(6) In the image analysis device, the area ratio of black is calculated for each picture, and the average value of all pictures is taken as the ferrite fraction.

In addition, in the said microscopic observation, in every Example, the remainder confirmed that it was bainite structure and / or martensite structure.

Next, the metal structure of the said rolled material was observed in the following procedure, and the average circle equivalent diameter D of the crystal grain enclosed by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more was calculated | required. The value of D (µm) is shown in Table 6 below.

<< calculation method of D >>

(1) The sample cut | disconnected in the direction parallel to a rolling direction (long direction) is prepared so that both the surface and the back surface of a rolling material may be included.

(2) It grind | polishing by the wet emery polishing paper of # 150- # 1000, or the grinding | polishing method which has a function equivalent to it, and mirror-finished by using abrasive | polishing agents, such as a diamond slurry.

(3) The mirror polished surface is an EBSP (Electron Back Scattering Pattern) device manufactured by TexSEM Laboratories Co., Ltd., and the measurement range is 200 μm × 200 μm and the pitch is 0.5 μm at the t / 4 position in the plate thickness direction. The orientation difference of the crystal was measured, and the boundary where the crystal orientation difference was 15 degrees or more was made into diagonal boundary. In addition, the measuring point whose confidence index which shows the reliability of a measuring direction is smaller than 0.1 was excluded from the analysis object.

(4) In the grain distribution map, the maximum width (normally along the sheet thickness direction) and the maximum length (normally along the rolling direction) of the grains surrounded by diagonal grain boundaries with a crystal orientation difference of 15 ° or more are measured and measured. , The area of the crystal grains was calculated, and the equivalent circle diameter of the crystal grains was calculated to obtain an average value.

Next, the yield ratio and the tensile strength of the obtained rolled material were measured in the following procedure, and the yield ratio was computed.

Measurement of yield strength and tensile strength

From the t / 4 position of the rolled material (t is the thickness of the rolled material), a specimen No. 4 of JIS Z2201 is taken so as to be perpendicular to the rolling direction (long direction), and a tensile test is performed under the conditions specified in JIS Z2241. Yield strength (YS) and tensile strength (TS) were measured. The yield ratio was calculated from YS and TS. YS, TS and yield ratio are shown in Table 6 below. In the present invention, when the TS is 590 MPa or more and the yield ratio is 80% or less, the tensile properties were evaluated as excellent (passed).

Based on the above result, FIGS. 4-9 were created.

4 is a graph showing the relationship between the rolling end temperature and the average circle equivalent diameter D of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 15 ° or more. 4, No. 5 shown in Tables 5 and 6. Only the results of 33-38 were shown.

5 is a graph showing the relationship between the quenching start temperature and the ferrite fraction. On the other hand, in FIG. 5, No. shown in Table 5 and Table 6 among the examples which performed direct quenching (DQ) after hot rolling. Only the results of 28-32 were shown.

6 is a graph showing a relationship between a heating temperature and a ferrite fraction when the heating is maintained at a temperature near the two phase region. On the other hand, in Fig. 6, Nos. Only the results of 21-25 were shown.

7 is a graph showing the relationship between the ferrite fraction and the tensile strength (TS). On the other hand, in Fig. 7, Nos. Only the results of 28-32 were shown.

8 is a graph showing the relationship between the ferrite fraction and the yield ratio. 8, No. 5 shown in Tables 5 and 6. All the results of 21-48 were shown.

9 is a graph showing the relationship between the average circle equivalent diameter D and the yield ratio of grains surrounded by diagonal grain boundaries. 9, No. 5 shown in Tables 5 and 6. Only the results of 33-38 were shown.

In summary, the results of Experimental Example 1-1 and Experimental Example 1-2 can be considered as follows from Table 4 and Table 6.

No. 21-23, 26-30, 35-38 are the examples using the steel grades a1-e1 which satisfy | fill the requirements prescribed | regulated by this invention. As is clear from Table 4, HAZ toughness is favorable, there are few variations of HAZ toughness, As is apparent from Table 6, a tensile strength of 590 MPa or more and a yield ratio of 80% or less are realized.

No. Since 24 and 25 use the steel grade a1 which satisfy | fills the requirements prescribed | regulated by this invention, as is clear from Table 4, since HAZ toughness is favorable and there is little variation in HAZ toughness, yield does not occur because ferrite is not produced | generated. The ratio exceeds 80%.

No. Since 31 and 32 use the steel grade d1 which satisfies the requirements specified in the present invention, as is clear from Table 4, the HAZ toughness is good and the HAZ toughness is small, but the ferrite exceeds 24%. As a result, the tensile strength is less than 590 MPa.

No. Since 33 and 34 use steel grade e1 which satisfies the requirements specified in the present invention, as shown in Table 4, diagonal grains having good HAZ toughness and small HAZ toughness but small crystal orientation difference of 15 ° or more are used. Since the average circle equivalent diameter D of the crystal grains enclosed by the system is less than 35 µm, the yield ratio of the base material is higher than 80%.

No. Since 39-48 uses the steel grades f1-steel type o1 which do not satisfy the requirements prescribed | regulated by this invention, as is clear from Table 4, HAZ toughness is bad and the variation of HAZ toughness is also large. In particular, No. The ferrite fractions 43, 45 and 46 are not properly controlled, so the tensile strength is low or the yield ratio is large.

[Example 2]

In Experimental Examples 2-1 and 2-2 described below, the HAZ toughness and variation thereof (Experimental Example 2-1) and low-temperature toughness (Experimental Example 2-2) of the steel material were examined using the same steel grade, Experimental Example 2-1 and Experimental Example 2-2 were combined to evaluate the properties of the steel (steel rolled material).

Experimental Example 2-1 (Evaluation of HAZ Toughness and Its Deviation)

Steel materials were obtained by the same method as Experimental Example 1-1, and the steel materials were evaluated by the same measurement and test methods as Experimental Example 1-1. Each steel grade of the composition shown in Table 8 obtained the steel materials on the conditions similar to Table 7, and evaluated. The measurement and evaluation results are shown in Tables 9 and 10.

In FIG. 10, the relationship of the total amount of oxygen [O] ₁ before adding REM and Zr, and the sum total of the addition amount of REM and Zr ([REM] + [Zr]) is shown graphically. In FIG. 10, (circle) is No. of Table 7 shown below. As a result of 101-104, x is No. The results of 109 to 112 are shown respectively. In addition, in FIG. 7, the unit of total amount of oxygen [O] ₁ was described in ppm.

In FIG. 11, the relationship between the dissolved oxygen amount [O] ₂ contained in molten steel before casting, and the amount of solid solution REM or solid solution Zr contained in steel materials is shown graphically. In FIG. 11, the unit of dissolved oxygen amount [O] ₂ was described in ppm. 2, only the data in which the solid solution REM or the solid solution Zr was detected was plotted.

From the results shown in Tables 7, 9 and Figs. 10 to 12, the same things as those mentioned in the discussion of Example 1-1 can be said.

As is clear from Tables 8 to 10 and FIG. 101 to 104 are examples satisfying the requirements specified in the present invention. In particular, the amount of REM and Zr in the chemical composition of the steel are appropriately adjusted, and the amount of solid solution REM and Zr is appropriately controlled. The average value is 150J or more, and HAZ toughness is excellent. In addition, the variation of the HAZ toughness is also reduced.

Meanwhile, No. 105 to 113 are examples that deviate from the requirements stipulated in the present invention, and especially the amount of REM or Zr in the chemical composition of the steel is out of the range defined in the present invention (No. 105 to 108, 113), or the amount of solid solution REM Since the amount of solid solution Zr deviates from the range prescribed | regulated by this invention (No. 109-112), the average value of HAZ toughness is less than 150J, and HAZ toughness is inferior. In addition, a large variation in the HAZ toughness also increases.

Experimental Example 2-2 (Evaluation of Low Temperature Toughness of the Base Material)

The slab (steel grades a2 to m2) obtained by casting under the conditions described in Experimental Example 2-1 was heated to a heating temperature (T1) shown in Table 5 below, followed by hot rolling to obtain a hot rolled material. Hot rolling was performed under the conditions shown in Table 5 below for the maximum reduction rate and cumulative reduction rate per pass in the temperature range T2 where the average temperature of the slab was at least Ar ₃ points + 10 ° C and 900 ° C or lower. The cumulative reduction ratio was calculated using the above equation (7).

Next, by cooling the hot-rolled material obtained by the hot rolling, until the average temperature of the hot rolled plate Ar ₃ point or higher temperature than the surface temperature is 500 ℃ temperature of hot-rolled material from the station (T3) inverse (T4). The cooling start temperature T3 and the average cooling rate at the time of cooling are shown in Table 5 below.

No. in Table 11 below. Regarding 123, after cooling to the temperature range T4 whose surface temperature of a hot rolling material is 500 degrees C or less, it heated at 580 degreeC and performed tempering.

In addition, the average temperature of the said slab or the said hot rolled material was managed by the temperature in the t / 4 position, when thickness of the slab or the hot rolled material was t. The temperature at the t / 4 position was calculated in the following order.

<< calculation method of average temperature >>

(1) Using a process computer, the heating temperature of an arbitrary position in the plate thickness direction from the front surface to the back surface of a steel piece is computed based on the atmospheric temperature and furnace stay time from a heating start to extraction.

(2) Using the calculated heating temperature, the rolling temperature at any position in the sheet thickness direction is suitable for the differential method or the like based on the rolling pass schedule during rolling or the data of the cooling method (ice cooling or air cooling) between the passes. It rolls, calculating using a method.

(3) Steel plate surface temperature is measured using the radial thermometer provided on the rolling line (however, it also calculates 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 compared with the calculated surface temperature on the process computer.

(5) When the difference between the calculated surface temperature and the measured steel sheet surface temperature is ± 30 ° C. or more, the measured steel sheet surface temperature is replaced with the calculated surface temperature to be the calculated surface temperature on the process computer.

(6) The temperature at the t / 4 position is obtained using the corrected calculated surface temperature.

In addition, the surface temperature of the hot rolling material was measured using the radial thermometer provided on the rolling line.

In following Table 11, the product thickness (mm) of the rolling material obtained by cooling was also shown. Table 11 also shows the values of Ac ₃ point, Ar ₃ point, and Ac ₁ point calculated using the equations (5), (6), and (8) based on the chemical composition shown in Table 8. Indicates.

Next, after the mirror polishing, the test piece is taken from the t / 4 position (t is a plate thickness) of the obtained rolled material, and this is etched with a 2% nitric acid-ethanol solution (nital solution), followed by an optical microscope at 5 views. It observed at 400 times using, and measured the bainite fraction (area%) in steel structure by image analysis. At this time, all structures other than ferrite and martensite were considered to be bainite. The bainite fraction (area%) is shown in Table 12 below.

Further, the metal structure of the rolled material was observed in the following order, and the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundaries having a crystal orientation difference of 15 ° or more, and the crystal grains surrounded by the diagonal grain boundaries having a crystal orientation difference of 55 ° or more occupy the whole steel. Obtained the ratio M. The values of D (μm) and M (area%) are shown in Table 12 below.

<< calculation method of D >>

It is the same as the method in Example 1-2.

<< calculation method of M >>

The ratio M which crystal grains enclosed by the diagonal grain boundary whose crystal orientation difference is 55 degrees or more in the whole steel material analyzed and computed the text data of the crystal orientation difference in the process of (3) in the said D calculation method. Analysis of the text data eliminated noise having a crystal orientation difference of 5 ° or less, and calculated an area fraction of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 55 ° or more in the entire metal structure.

13 shows the relationship between the average circle equivalent diameter D and the cumulative reduction ratio in the unrecrystallized region. As is apparent from FIG. 13, when the cumulative reduction ratio in the non-recrystallized region is 40% or more, the average circle equivalent diameter D of the crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 15 ° or more can be 30 μm or less.

The relationship between the ratio M which the crystal grains enclosed by the diagonal grain boundary with a crystal orientation difference of 55 degrees or more occupies in the whole steel, and the average cooling rate from the temperature range T3 of Ar ₃ or more point to the temperature range T4 of 500 degrees C or less is shown in FIG. Indicates. As is apparent from FIG. 5, when the average cooling rate from the temperature range T3 at the point of Ar ₃ or higher to the temperature range T4 at the temperature of 500 ° C. or less is controlled at 5 ° C./sec or more, a diagonal grain boundary having a crystal orientation difference of 55 ° or more is obtained. The ratio M which the enclosed crystal grains occupy in the whole steel can be 50 area% or more.

Next, the low temperature toughness of the obtained rolling material was evaluated in the following order.

<< evaluation method of low temperature toughness >>

The low-temperature toughness of the rolled material was evaluated by performing a V notch Charpy test to measure the impact properties of the rolled material by measuring absorbed energy (vE- ₆₀ ) at -60 ° C. The measurement of vE- ₆₀ was carried out according to JIS Z2242 by taking a U4 test piece determined by the NK (Japanese Maritime Association) classification from the t / 4 position. The measurement results are shown in Table 6 below.

On the other hand, in shipbuilding grade E in the NK classification, the impact characteristics of the base metal are evaluated at the test temperature of -40 ° C. Thus, in this Experimental Example, the conditions were more strictly the test temperature of -60 ° C, and the absorbed energy (vE _-60 ). Was measured, and this average value was 100J or more as the pass (good low-temperature toughness of a base material).

15 shows the relationship between the average equivalent circle diameter D and the vE- ₆₀ of the rolled material. As is apparent from Fig. 15, when the average circle-equivalent diameter D of the crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 15 ° or more is 30 µm or less, vE _-60 can be 100 J or more, thereby improving the low temperature toughness of the base material itself. I can see.

The relationship between the ratio M which the crystal grain enclosed by the diagonal grain boundary whose crystal orientation difference is 55 degrees or more in the whole, and vE- ₆₀ of a rolled material is shown in FIG. As is apparent from Fig. 16, when the ratio M occupied by diagonal grain boundaries with a crystal orientation difference of 55 ° or more in the whole steel is 50 area% or more, vE _-60 can be made 100J or more, resulting in low temperature toughness of the base material itself. It can be seen that it can be improved.

In summary, the results of Experimental Example 2-1 and Experimental Example 2-2 can be considered as follows from Table 10 and Table 12.

No. 121-125 and 127-133 are the examples using the steel grades a2-d2 which satisfy | fill the requirements prescribed | regulated by this invention. As is clear from Table 4, HAZ toughness is favorable, there are few deviations of HAZ toughness, and it is clear from Table 12. As described above, the low temperature toughness of the base material itself is also good.

No. Since 126 uses the steel grade b2 which satisfies the requirements specified in the present invention, as is clear from Table 10, the HAZ toughness is good and the HAZ toughness is small, but the maximum per one pass in the unrecrystallized area is used. Since the reduction ratio exceeds 12%, the average circle equivalent diameter D of the crystal grains enclosed by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more exceeds 30 micrometers, and the low-temperature toughness of a base material worsens.

No. 134 and No. As apparent from Table 12, 135 has good low-temperature toughness, but uses steel grade e2 and steel grade f2 that do not satisfy the requirements specified in the present invention, and as shown from Table 10, HAZ toughness is poor. The variation in toughness of HAZ is also increasing.

No. Since 136-142 uses the steel grade g2-the steel grade m2 which do not satisfy the requirements prescribed | regulated by this invention, as is clear from Table 10, HAZ toughness is bad and the variation of HAZ toughness is also large. In addition, as apparent from Table 12, since the metal structure is not properly controlled, the low temperature toughness of the base material itself is also deteriorated.

BRIEF DESCRIPTION OF THE DRAWINGS In Example 1, it is a graph which shows the relationship of the total amount of oxygen [O] ₁ before adding REM and Zr, and the sum total of REM and Zr addition amount.

FIG. 2 is a graph showing the relationship between the dissolved oxygen amount [O] ₂ contained in molten steel before casting, and the amount of solid solution REM or solid solution Zr contained in the steel material in Example 1. FIG.

3 is a graph showing the average value of the HAZ toughness, and the width of the maximum value and the minimum value of the HAZ toughness in Example 1. FIG.

4 is a graph showing the relationship between the rolling end temperature and the average circle equivalent diameter D of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 15 ° or more in Example 1. FIG.

5 is a graph showing the relationship between the quenching start temperature and the ferrite fraction in Example 1. FIG.

FIG. 6 is a graph showing the relationship between the heating temperature and the ferrite fraction when the heating is maintained at a temperature near the two-phase station in Example 1. FIG.

FIG. 7 is a graph showing the relationship between the ferrite fraction and the tensile strength TS in Example 1. FIG.

8 is a graph showing the relationship between the ferrite fraction and the yield ratio in Example 1. FIG.

9 is a graph showing the relationship between the average circle equivalent diameter D and the yield ratio of crystal grains surrounded by diagonal grain boundaries in Example 1. FIG.

FIG. 10 is a graph showing the relationship between the total amount of oxygen [O] ₁ before adding REM and Zr and the sum of the amount of REM and Zr added in Example 2. FIG.

FIG. 11 is a graph showing the relationship between the dissolved oxygen amount [O] ₂ contained in molten steel before casting, and the amount of solid solution REM or solid solution Zr contained in the steel in Example 2. FIG.

FIG. 12 is a graph showing an average value of HAZ toughness, and a width of maximum and minimum values of HAZ toughness in Example 2. FIG.

FIG. 13 is a graph showing the relationship between the average equivalent circle diameter D and the cumulative reduction ratio in the non-recrystallized region in Example 2. FIG.

14 shows the ratio M of crystal grains enclosed by diagonal grain boundaries having a crystal orientation difference of 55 ° or more in the whole steel in Example 2, and a temperature range T4 of 500 ° C. or less from a temperature range T3 of Ar ₃ or more points. It is a graph which shows the relationship of average cooling rate.

15 is a graph showing a relationship between an average circle equivalent diameter D and vE- ₆₀ of a rolled material in Example 2. FIG.

FIG. 16 is a graph showing a relationship between a ratio M of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 55 ° or more in the whole steel material and vE _-60 of the rolled material in Example 2. FIG.

Claims (14)

As steel, C: 0.03-0.2% (meaning "mass%", the same below), Si: 0.5% or less, Mn: 2% or less, Ti: 0.03% or less, and N: 0.01% or less, P: 0.02% or less, S: 0.015% or less, and Al: satisfy 0.01% or less, Add to REM: 0.0010 to 0.1% and Zr: 0.0010 to 0.05%, respectively. The balance consists of iron and inevitable impurities, (A) In addition to the inclusions containing the REM and Zr steel, (B) hire REM and hire Zr in steel    Employment REM: 0.0010% or less,    Employment Zr: satisfying less than 0.0010% Steel. The method of claim 1, The mole fraction of REM is measured when the composition of the inclusions contained in the steel is measured and the molar 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. Steel material which satisfy | fills 0.05 or more and the mole fraction of Zr is 0.04 or more. The method of claim 1, Mn: steel of 1.0 to 2%. The method of claim 3, wherein As another element, Cu: 2% or less, Ni: 2% or less, Cr: 3% or less, Mo: 1% or less, Nb: 0.05% or less, V: 0.1% or less, and B: 0.005% or less Steel comprising at least one element selected from the group consisting of: The method of claim 1, C: 0.04-0.13%, Ti: 0.02% or less, Add to Cu: 0.3% or less, Ni: 0.4% or less, and Nb: Steel including at least one member selected from the group consisting of 0.25% or less. The method of claim 5, Further steel, which contains B: 0.005% or less as another element. The method according to claim 3 or 5, Furthermore, as another element, steel materials containing Ca: 0.01% or less. As a steel rolling material obtained by rolling the steel material of Claim 3, (C) The tissue contains bainite and / or martensite and ferrite, and the fraction of ferrite in the whole tissue is 4 to 24 area%, and the total fraction of bainite and martensite is 74 area% or more and less than 96 area%. ego, (D) The strong rolling material which satisfy | fills following formula (1) when the metal structure of steel materials is observed by back-scattering electron diffraction image method (EBSP method). Equation 1 35≤D (In Equation 1, D denotes an average circle equivalent diameter (mu m) of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 15 ° or more by measuring the azimuth difference between two adjacent crystals by the EBSP method.] As a steel rolling material obtained by rolling the steel material of Claim 5, (E) The strong rolling material which satisfy | fills following formula (2) and (3) when the metal structure of steel is observed by back-scattering electron diffraction image method (EBSP method). Equation 2 D≤30 Equation 3 50≤M [Equation 2] In Equation 2, D means an average circle equivalent diameter (mu m) of crystal grains surrounded by diagonal grain boundaries whose crystal orientation difference is 15 ° or more by measuring the orientation difference between two adjacent crystals by the EBSP method. In addition, in Formula (3), M means the ratio (area%) of the crystal grain enclosed by the diagonal grain boundary whose crystal orientation difference is 55 degrees or more in the whole steel material.] As a manufacturing method of the steel material of Claim 1, Manufacture of steel to be cast after adding REM and Zr to the molten steel in which the total oxygen amount [O] ₁ is adjusted to the range of 0.0020 to 0.015%, and adjusting the dissolved oxygen amount [O] ₂ to the range of 0.0010 to 0.0035%. Way. The method of claim 10, The total amount of oxygen [O] ₁ is measured, and REM and Zr are added to adjust the dissolved oxygen amount [O] ₂ so as to satisfy the following equation (4) according to the total amount of oxygen [O] ₁ . Equation 4 [REM] + [Zr] ≤15 × [O] ₁ [Where, in formula (4), [REM] and [Zr] are the addition amount (mass%) of REM or Zr, respectively, and [O] ₁ is the total amount of oxygen (mass%) of molten steel before addition of REM and Zr. ] As a manufacturing method of the high rolling material of Claim 8, After hot-rolling the steel material obtained by the method of Claim 10 so that rolling end temperature may be 870 degreeC or more, Quenching from a temperature range above Ar ₃ Quenching from the temperature range of Ac ₁ point-Ac ₃ point, The manufacturing method of the rolling material which performs each process of tempering sequentially in the temperature range below Ac ₁ point. As a manufacturing method of the high rolling material of Claim 9, After heating the steel obtained by the method of Claim 10 to the temperature range of Ac ₃ or more and 1200 degrees C or less, the maximum temperature per one pass in the temperature range whose average temperature of a steel piece is Ar ₃ points + 10 degrees C or more and 900 degrees C or less. Hot rolling by controlling the reduction rate to 12% or less and the cumulative reduction rate to 40% or more, Method of producing a step-down series, the average temperature of the resulting hot-rolled material, the surface temperature of the hot rolled material to cool to an average cooling rate of 5 ℃ / sec or more to less than 500 ℃ temperature range from a temperature region not less than Ar ₃ point. The method of claim 13, A method for producing a cold rolled material, wherein the hot rolled material is cooled to a temperature range of 500 ° C. or less at a cooling rate of 5 ° C./sec or more, and then heated to a temperature range of 500 ° C. or more and less than Ac ₁ point to temper.

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