KR20160025545A - Steel material having excellent toughness in welding heat-affected zone, and method for producing the same - Google Patents

Abstract

A steel material for reducing HAZ toughness of the present invention includes an inclusion containing REM and Zr. Also, a solid solution REM and a solid solution Zr of the steel material satisfying the solid solution Zr with 0.0010% or less (including 0%) and the solid solution Zr with 0.0010% or less (including 0%). A first rolled material with a yield ratio of 80% or less obtained from the steel material includes a bainite and/or a martensite and a ferrite; and has a tissue having a ferrite fraction of 4-24 area% occupied in the whole tissue. When a metal tissue of the steel rolled material is observed by a back scattered electron diffraction phase (EBSP) method, a mathematical equation 1 35<=D is satisfied. In the mathematical equation 1, D indicates an average diameter (μm) corresponding to a circle of a crystal grain surrounded by an opposite grain boundary where a mis-orientation is 15° or greater by measuring the mis-orientation of two adjacent crystals by the EBSP method. Also, a second steel rolled material having excellent low temperature toughness obtained by the same steel material satisfies a mathematical equation 2 of D<=30 and a mathematical equation 3 of 50<=M when a metal tissue thereof is observed by the back scattered electron diffraction phase (EBSP) method. In the mathematical equation 3, M indicates a ratio (area%) of a crystal grain surrounded by an opposite grain boundary having crystal mis-orientation of 55° or more occupied in the whole steel material.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having excellent toughness of a weld heat affected zone,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material used for a structure such as a bridge, a high-rise building, a ship and the like. More specifically, the present invention relates to a steel material ), And a method for producing the same.

The properties required for steels used in bridges, high-rise buildings, ships and the like are becoming increasingly strict in recent years, and particularly good toughness is being demanded. These steels are often joined by welding. HAZ, among the welded joints, has a problem that the toughness tends to deteriorate due to thermal influence during welding. This toughness deterioration is conspicuous as the heat input at the time of welding becomes larger. The reason for this is that the cooling rate of the HAZ is lowered when the heat input at the time of welding is lowered, and the hardenability is lowered to produce coarse island martensite. Therefore, in order to improve the toughness of the HAZ, the amount of heat input at the time of welding should be suppressed as much as possible. On the other hand, in order to improve the efficiency of the welding operation, it is desired to employ a large heat input welding method in which the heat input amount of welding is 50 kJ / mm or more, for example, electroslag welding or submerged welding.

Therefore, the present applicant has proposed in Japanese Patent Application Laid-Open No. 2007-100213 a steel material for suppressing the deterioration of HAZ toughness when employing the heat welding method. This steel material is characterized by containing oxides of REM and / or CaO and ZrO ₂ as oxides. Since these oxides are present in a liquid state in the molten steel, they are finely dispersed in the steel and, furthermore, Even if it does not lose employment, it contributes to improvement of personality of HAZ.

On the other hand, Japanese Patent Application Laid-Open No. Hei 8-120401 does not aim at improving the HAZ toughness. However, Japanese Unexamined Patent Publication (Kokai) No. Hei 8-120401 discloses a steel material containing elements such as REM and Zr and positively containing solid- There has been proposed a technique of preventing the fusion bonding and improving the internal quality of the steel sheet and maintaining the integrity of the internal quality. In this technique, Al, Ca, Ti, and the like are added in combination in order to secure a stable high capacity.

However, in recent years, buildings and structures (for example, offshore structures) have become higher and larger, and the use of high-strength steels having a strength of 590 MPa, which is high in strength instead of 490 MPa, has become stronger. However, the technique of Japanese Patent Application Laid-Open No. Hei 8-120401 copes with the improvement of the HAZ toughness. However, it is difficult to improve the toughness of the HAZ by using a steel material having a low resistance ratio (YR of 80% or less) required for a high- Is not being reviewed.

The present applicant has disclosed Japanese Patent Application Laid-Open No. 8-209294 as a steel material having high tensile strength and low resistance. Here, the fine carbonitride is dispersed, and at the same time, a certain amount or more of ferrite is secured, thereby realizing a tensile strength of 590 MPa or more while achieving a low resistance. However, the improvement of the HAZ toughness in the case of welding with a heat input of 50 kJ / mm or more has not been thoroughly investigated, and it is urgently required to realize a high tensile steel excellent in both resistance and HAZ toughness.

On the other hand, high strength is also required for steels used in vessels and the like, but when the strength of the steel is increased, the yield strength exceeds the brittle fracture strength, and brittle fracture tends to occur during the elastic deformation. For this reason, the IACS unification rules set a toughness grade for each structural member from fracture mechanics (K concept) to prevent brittle fracture, and as the intensity class increases, Thereby improving the toughness of the base material. Therefore, in order to secure the safety of the structure under a strict use environment, it is important that the toughness of the base material (in particular, the toughness of the base material at low temperature) is good as well as the HAZ toughness at the welded joint as described above.

It is an object of the present invention to provide a steel material with reduced deviation of HAZ toughness. Further, the present invention is to provide a rolled steel sheet obtained by reducing the yield ratio to 80% or less, or a rolled steel sheet having improved low temperature toughness. It is another object of the present invention to provide a method for manufacturing the above-mentioned steel material and the respective rolled steel sheets.

The steel material according to the present invention which can solve the above problems has a composition of C: 0.03 to 0.2% (meaning "mass%", the same shall apply hereinafter), Si: not more than 0.5%, Mn: not more than 2%, Ti: not more than 0.03% : 0.01% or less, P: not more than 0.02%, S: not more than 0.015% and Al: not more than 0.01%, and further contains 0.0010 to 0.1% of REM and 0.0010 to 0.05% The balance being iron and inevitable impurities,

(A) The steel material includes an inclusion containing REM and Zr,

(B) the employed REM and the employed Zr in the steel meet the employment REM: not more than 0.0010% and the employment Zr: not more than 0.0010%.

In the steel material, the composition of the inclusions contained in the steel material is measured, and the molar ratio of the elements other than O, C, N and S among the elements contained in the inclusions is converted into molar amount, , A mole fraction of REM of 0.05 or more, and a mole fraction of Zr of 0.04 or more.

The first rolled steel sheet of the present invention which can solve the above problems is characterized in that the steel of the present invention has Mn of 1.0 to 2% and further contains 2% or less of Cu, 2% or less of Ni, A steel material comprising at least one element selected from the group consisting of Cr: at most 3%, Mo: at most 1%, Nb: at most 0.05%, V: at most 0.1%, and B: at most 0.005% As a serial,

(C) the structure includes bainite and / or martensite and ferrite, the ferrite fraction of the whole structure is 4 to 24% by area, the total fraction of bainite and martensite is 74 to less than 96% ego,

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

[Equation 1]

35 D

(In the formula (1), D means the average circle equivalent diameter (占 퐉) of the crystal grains surrounded by the diagonal grain boundary having the crystal orientation difference of 15 占 or more by measuring the azimuth difference between two adjacent crystals by the EBSP method.

The second rolled steel sheet of the present invention which can solve the above problems is characterized in that the steel of the present invention contains 0.04 to 0.13% of C and 0.02% or less of Ti, 0.3% or less of Cu, 0.4% or less of Ni, Or less, and 0.25% or less of Nb, and is a rolled steel sheet obtained by rolling a steel material containing at least one selected from the group consisting of:

(E) When the metal structure of the steel material is observed by the back scattering electron diffraction method (EBSP method), the following equations (2) and (3) are satisfied.

&Quot; (2) &quot;

D? 30

&Quot; (3) &quot;

50? M

(In the formula (2), D means the mean circle equivalent diameter (占 퐉) of the crystal grains surrounded by the diagonal grain boundary having the crystal orientation difference of 15 degrees or more by measuring the azimuth difference between two adjacent crystals by the EBSP method. In the formula (3), M means a ratio (area%) of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 55 degrees or more in the entire steel material.

In the steel material of the present invention, REM and Zr are added to molten steel in which the total amount of oxygen [O] ₁ is adjusted in the range of 0.0020 to 0.015%, and the amount of dissolved oxygen [O] ₂ is adjusted in the range of 0.0010 to 0.0035% ). 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).

&Quot; (4) &quot;

[REM] + [Zr]? 15 x [O] ₁

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

A first step-down series of the present invention, the steel material of the present invention obtained by the above method, the rolling end temperature after hot rolling is at least 870 ℃, Ar ₃ quenching, from a point over the temperature range of Ac ₁ point ₃ points ~Ac 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.

The second rolled steel strip according to the present invention is obtained by heating the steel material of the present invention obtained by the above method to a temperature in the range of Ac ₃ point to 1200 ° C and then measuring the average temperature of the steel strip at Ar ₃ point + In the temperature range, the maximum reduction rate per pass is 12% or less, the cumulative reduction rate is controlled to 40% or more, and hot rolling is performed. From the temperature range in which the average temperature of the obtained hot rolled material is equal to or higher than Ar ₃ points, By cooling to an average cooling rate of 5 deg. C / sec or more to a temperature range of 500 DEG C or lower. Here, it is recommended that after cooling to a temperature range where the surface temperature of the hot rolled material is 500 캜 or lower, cooling at a cooling rate of 5 캜 / second or higher and then heating at a temperature of 500 캜 or higher and less than Ac ₁ point.

According to the steel material of the present invention, it is possible to suppress the deviation of the HAZ toughness by reducing the amount of the solidified REM and solid Zr contained in the steel material as much as possible.

Further, according to the first rolled steel sheet of the present invention obtained from the steel material of the present invention, the bainite and / or martensite structure is the main body and the ferrite is in the range of 4 to 24% , The average equivalent circle diameter of the crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 15 degrees or more is 35 mu m or more. Thus, the yield ratio of the base material can be reduced to 80% or less while securing the strength of 590 MPa or more.

According to the second rolled steel sheet of the present invention obtained from the steel material of the present invention, the average circle-equivalent diameter of crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 15 degrees or more when the metal structure is observed is set to 30 μm or less, Particularly, by setting the ratio of the crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 55 degrees or more to the entire steel material to 50 percent by area or more, the low temperature toughness of the base material itself can be improved.

1 is a graph showing the relationship between the total amount of oxygen [O] ₁ before addition of REM and Zr and the sum of the amounts of REM and Zr added in Example 1. Fig.
2 is a graph showing the relationship between dissolved oxygen amount [O] ₂ contained in molten steel before casting and amount of solidified REM or solid Zr contained in the steel 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 finish temperature and the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 15 degrees 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 in heating and holding at a temperature near the bimodal in Example 1. 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 the diagonal grain boundary in Example 1. Fig.
10 is a graph showing the relationship between the total amount of oxygen [O] ₁ before the addition of REM and Zr and the sum of the amounts of REM and Zr added in Example 2. FIG.
11 is a graph showing the relationship between the amount of dissolved oxygen [O] ₂ contained in molten steel before casting and the amount of solidified REM or solid Zr contained in the steel material in Example 2. FIG.
12 is a graph showing the average values of HAZ toughness and the widths of maximum and minimum values of HAZ toughness in Example 2;
13 is a graph showing the relationship between the average circle-equivalent diameter D and the cumulative rolling reduction in the non-recrystallized region in Example 2. Fig.
14 is a graph showing the relationship between the ratio M in which the crystal grains surrounded by the diagonal grain boundary having the crystal orientation difference of 55 degrees or more occupies the entire steel material and the temperature range T3 in the temperature range from Ar ₃ point to 500 deg. And the average cooling rate.
15 is a graph showing the relationship between the average circle-equivalent diameter D and vE- ₆₀ of the rolled material in Example 2. Fig.
16 is a graph showing the relationship between the ratio M of the crystal grains surrounded by the diagonal grain boundaries in the diagonal grain boundary having a crystal orientation difference of 55 degrees or more in Example 2 and vE- ₆₀ of the rolled material.

The present inventors have repeatedly studied to reduce the deviation of HAZ toughness and to reduce the yield ratio of the base material with respect to a steel material in which RAZ and Zr are added to a steel material to improve the HAZ toughness of the welded joint. As a result, it is assumed that (I) the HAZ toughness is increased by adjusting REM and Zr to be mixed with the steel to contain REM and Zr in the inclusion, and furthermore, (II) It is possible to prevent the phenomenon that the toughness is locally deteriorated if it is reduced as much as possible so that the deviation of the HAZ toughness can be suppressed and the steel material of the present invention is completed.

(III) a structure in which the metal structure of the steel material is bainite and / or martensitic structure and contains ferrite in an amount of 4 to 24%; (IV) It was found out that the yield ratio of the base material can be reduced by appropriately controlling the size of the crystal grains surrounded by the diagonal grain boundary having the azimuth difference of 15 degrees or more to complete the first rolled steel sheet of the present invention.

When the steel material is used to appropriately control the size of the crystal grains surrounded by the diagonal grain boundary having the crystal orientation difference of 15 degrees or more and the grain fractions surrounded by the diagonal grain boundary having the crystal orientation difference of 55 degrees or more among the metal structures of the steel (V) It was found that the low temperature toughness of the base material itself could be improved, and the second pressure-tightening material of the present invention was completed.

Hereinafter, (I) to (IV) will be described in detail.

[(I) HAZ toughness of weld joint]

The steel material of the present invention includes inclusions containing REM and Zr. (A) contains a single inclusion of REM and a single inclusion of Zr, or (b) contains a composite inclusion containing REM and Zr, or (c) contains a single inclusion of REM alone Quot; means that the inclusion contains a single inclusion of Zr and also contains a composite inclusion including REM and Zr.

Examples of the inclusion of REM include oxides of REM and sulfides of REM, and the sole inclusions of Zr include oxides of Zr, carbides of Zr, and nitrides of Zr. The complex inclusions of REM and Zr include oxides, sulfides, oxysulfides and the like including REM and Zr. On the other hand, these inclusions may be in the form of coexisting with a nitride (for example, TiN or the like) or another sulfide (for example, CaS or MnS). On the other hand, for convenience of explanation, a single inclusion and a composite oxide may be collectively referred to as &quot; inclusions &quot; hereinafter.

Since inclusions of REM and Zr are thermally affected by welding, they do not disappear even when heated to a high temperature of 1400 DEG C, so that inclusion of these inclusions can suppress coarsening of austenite grains in HAZ during welding, Since the transformation in the grains can be promoted, the HAZ structure can be miniaturized and the toughness of the HAZ can be further improved.

In addition, by adding REM and Zr in combination and containing them as inclusions in the steel, it is possible to prevent the formation of a single carbide of coarse Zr or a sulfide of coarse REM, which causes deterioration of toughness of the steel material (base material) The toughness of the HAZ can be improved while inhibiting toughness deterioration. That is, when REM or Zr is added singly, in order to increase the number of inclusions, the addition amount of REM or Zr must be increased. However, if the addition amount of REM or Zr is excessively increased, the size of single inclusions of REM or single inclusions of Zr becomes large Rather, it deteriorates the HAZ toughness. Therefore, when REM or Zr is added singly, there is a limit to the amount of addition, and therefore the amount of REM or Zr can not be increased, and the amount of fine intervening material can not be increased beyond a certain level. Therefore, the HAZ toughness could not be improved.

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

In the steel material of the present invention, the composition of the inclusions contained in the steel material is measured, and the molar ratio of the elements other than O, C, N, and S among the elements constituting the inclusions is converted into moles, , It is preferable that the mole fraction of REM is 0.05 or more and the mole fraction of Zr is 0.04 or more. The mole fraction of REM is preferably 0.10 or more, more preferably 0.15 or more, and further preferably 0.20 or more. On the other hand, the mole fraction of Zr is preferably 0.08 or more, more preferably 0.10 or more, and still more preferably 0.15 or more.

The sum of the molar fraction of the REM and the molar fraction of Zr is preferably 0.10 or more. If the total is less than 0.10, the amount of intervening material contributing to the toughness improvement of the HAZ is insufficient and the toughness of the HAZ can not be sufficiently improved. The total is more preferably at least 0.15, and still more preferably at least 0.20.

On the other hand, the composition of inclusions other than the rest of the REM inclusions and inclusions of Zr is not particularly limited, for example, CaO and SiO _2, Al ₂ O _3, may if MnO, TiN, TiC.

The composition of the inclusions contained in the steel can be measured by observing the cross section of the steel with, for example, an electron probe micro-probe micro-analyzer (EPMA) and quantitatively analyzing the inclusions recognized in the observation field. EPMA was observed by quantitatively analyzing the composition at the central portion of inclusions by wavelength dispersive spectroscopy of characteristic X-rays, for example, with an acceleration voltage of 7 kV, a sample current of 0.003, and an observation field area of 1 cm ² . 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.

Mn, Si, Ti, Zr, Ca, and REM (for example, La and Ce) in consideration of the composition of the steel material of the present invention, the element to be analyzed is an element other than O, C, N, It is good. The molar fraction of each element contained in the inclusions to be analyzed when the abundance ratio of Al, Mn, Si, Ti, Zr, Ca, and REM contained in inclusions is converted into moles and the total amount of the elements after conversion is 1 mole good.

[(II) Variation of HAZ toughness of welded joint]

It was found that the toughness of the HAZ was measured at a plurality of places, and in particular, in the vicinity of a bonding part (particularly a part close to the weld metal in the HAZ) having a large heat effect, And the measurement value was found to be uneven. Therefore, when the texture of the portion where the toughness is locally observed was observed, it was found that REM or Zr was segregated at grain boundaries. As a result of repeated studies to reduce the segregation of REM and Zr, it has been found that it is desirable to reduce the amount of solidified REM and solid Zr in steel.

That is, it is important that the steel material of the present invention satisfies 0.0010% or less (including 0%) of solid solution REM and 0.0010% or less (including 0%) of solid solution Zr. When the amount of the solidified REM in the steel exceeds 0.0010% or the amount of solid Zr exceeds 0.0010%, REM or Zr is segregated at the grain boundaries when subjected to heat during welding, and the toughness is locally lowered. Therefore, the amount of solid solution REM is set to 0.0010% or less, preferably 0.0008% or less, more preferably 0.0005% or less. The solid content of Zr is preferably 0.0010% or less, preferably 0.0008% or less, more preferably 0.0005% or less. The amount of the employed REM and the amount of the employed Zr are preferably reduced as much as possible, and most preferably 0%.

The total amount of the solid solution solid solution and the solid solution solid solution is preferably not more than 0.0015%, more preferably not more than 0.0010%.

The amount of solidified REM contained in the steel material is determined by ICP (Inductively Coupled Plasma; Inductively Coupled Plasma] From the REM content (total REM content) calculated by analysis by the MS method, the amount of REM contained in the inclusions contained in the steel material calculated by electrolytic extraction and ICP-MS may be calculated. Similarly, the amount of Zr contained in the inclusions contained in the steel may be calculated by subtracting the amount of Zr contained in the steel from the Zr content (total Zr content).

[(III) On the metal structure of the base metal]

The metal structure of the first pressure-sensitive expandable member of the present invention includes bainite and / or martensite and ferrite, and has a ferrite fraction of 4 to 24% by area, a total fraction of bainite and martensite of 74% Or more and less than 96 area%.

Fig. 8 is a graph showing the relationship between the ferrite fraction and the yield ratio, and summarizes the results of the examples described later. From Fig. 8, it can be seen that it is necessary to set the ferrite fraction to 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, FIG. 7 is a graph showing the relationship between the ferrite fraction and the tensile strength (TS), and summarizes the results of Examples to be described later. From FIG. 7, it is understood that, in order to securely increase the tensile strength to 590 MPa or more, It can be seen that it is necessary to do. In order to further increase the tensile strength, the ferrite fraction is preferably not more than 22%, more preferably not more than 20%.

On the other hand, the metal structure may be composed of bainite and / or martensite and only ferrite. However, the present invention is not limited thereto, and other structures (such as cementite and martensite (MA)), which can inevitably be formed in the manufacturing process, ).

[(IV) On the yield ratio of the base material]

The first rolled steel sheet of the present invention must satisfy the following formula (1) when the metal structure is observed by the back scattering electron diffraction method (EBSP method). By satisfying the expression (1), the yield ratio of the base material can be made 80% or less.

Equation 1

35 D

In the above formula (1), D means the average circle equivalent diameter (占 퐉) of the crystal grains surrounded by the diagonal grain boundary having the crystal orientation difference of 15 占 or more by measuring the azimuth difference between two adjacent crystals by the EBSP method. In the present invention, the value of D is 35 mu m or more.

9 is a graph showing the relationship between the average circle-equivalent diameter D and the yield ratio of crystal grains surrounded by the diagonal grain boundary, and summarizes the results of the examples described later. It can be seen from Fig. 9 that in order to achieve a yield ratio of 80% or less, it is necessary to adjust the average circle equivalent diameter D to 35 mu m or more in addition to adjusting the ferrite fraction. The reason why the yield strength of a metal material is proportional to 1/2 of the reciprocal of the grain size is known as the law of Hall Patch, and the yield point increases as the grain size becomes finer. In order to further reduce the yield ratio, it is preferably at least 37 mu m, and more preferably at least 39 mu m.

Observation of the metal structure is performed at t / 4 position in the thickness direction of the steel sheet when the thickness of the steel sheet is t (mm). The specific observation sequence is described in the section of the later-mentioned embodiments.

[(V) Low Temperature Toughness of Base Material]

The second rolled steel sheet of the present invention must satisfy the following equations (2) and (3) when the metal structure is observed by the back scattering electron diffraction method (EBSP method). The low temperature toughness of the base material itself is improved by satisfying both formulas.

Equation 2

D? 30

Equation 3

50? M

In the above formula (2), D means the average circle equivalent diameter (占 퐉) of the crystal grains surrounded by the diagonal grain boundary having the crystal orientation difference of 15 占 or more by measuring the azimuth difference between two adjacent crystals by the EBSP method. In the present invention, the value of D is set to 30 占 퐉 or less. It is generally known that brittle cracks are bent, diverted, or rectified at a diagonal grain boundary having a crystal orientation difference of 15 degrees or more. Therefore, the crystal grains surrounded by the diagonal grain boundary having the crystal orientation difference of 15 degrees or more are made finer and the position where the brittle cracks are bent, bypassed, and rectified increases, so that the impact property increases and the low temperature toughness of the base material itself increases. The smaller the value of D is, the better the value is 28 탆 or less, and more preferably 25 탆 or less.

In the above formula (3), M means the ratio (area%) of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 55 degrees or more in the entire steel material. In the present invention, the value of M is set to 50 area% or more. This is because the bending, detouring, and rectifying actions of the brittle crack due to the diagonal grain boundaries having a crystal orientation difference of 15 degrees or more are further exerted by the diagonal grain boundaries having a crystal orientation difference of 55 degrees or more particularly in the diagonal grain boundaries. In the present invention, therefore, the ratio of the crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 55 degrees or more to the entire steel is set to 50% or more. The value of M is preferably at least 55% by area, more preferably at least 60% by area.

Observation of the metal structure is performed at t / 4 position in the thickness direction of the steel sheet when the thickness of the steel sheet is t (mm). The specific observation sequence is described in the section of the later-mentioned embodiments.

The second rolled steel sheet of the present invention is composed of a bainite-based structure. By using a bainite-based material, strength of the pressure-sensitive laminated material can be secured. Bainite-based means that the area ratio of bainite is 80% or more when a metal structure is observed. The second rolled steel sheet of the present invention may be composed only of bainite, and martensite, ferrite, or the like may be produced as a structure other than bainite. On the other hand, in order to prevent the strength from lowering, the smaller the ferrite structure is, the better, it is preferable that the ferrite structure is less than 4% by area.

[About composition of ingredients]

Next, the composition of the steel material (base material) of the present invention will be described. The steel material of the present invention is characterized by containing 0.0010 to 0.1% of REM and 0.0010 to 0.05% of Zr. The reason for setting this range is as follows.

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

The REM should be 0.0010% or more, preferably 0.0015% or more, and more preferably 0.002% or more. However, when it is excessively added, coarse inclusions (e.g., oxides, etc.) are produced and the toughness of the base material deteriorates, so that it should be suppressed to 0.1% or less. It is preferably 0.09% or less, more preferably 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, REM is selected from the group consisting of La, Ce and Y , And more preferably at least one element selected from the group consisting of La and / or Ce.

Zr should be 0.0010% or more, preferably 0.0015% or more, and more preferably 0.002% or more. However, if it is added in excess, the coarse Zr carbide is produced and the toughness of the base material deteriorates. Therefore, it should be suppressed to 0.05% or less. Preferably 0.04% or less, and more preferably 0.03% or less.

The steel material of the present invention contains REM and Zr, and contains 0.03 to 0.2% of C, 0.5% or less of Si, 2% or less of Mn, 0.03% or less of Ti, and 0.01% will be.

Within the composition range of the steel, in the first rolled steel sheet of the present invention, Mn is preferably set to 1.0 to 2%.

In particular, in the second pressurized steel sheet according to the present invention, C: 0.04 to 0.13% and Ti: 0.02% or less, Cu: not more than 0.3%, Ni: not more than 0.4%, and Nb: 0.25 % Or less of the total number of elements.

The reason for setting this range is as follows.

C is an indispensable element in order to secure the strength of the steel material (base material), and it is necessary to contain 0.03% or more. The content of C is preferably 0.04% or more, more preferably 0.05% or more. However, when the content exceeds 0.2%, a lot of scarlet martensite is generated in the HAZ at the time of welding, resulting in deterioration of the toughness of the HAZ, and also adversely affecting weldability. Therefore, it is necessary to control C to 0.18% or less, preferably 0.16% or less, more preferably 0.14% or less. In particular, in the second rolled 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, and 0.13% or less, preferably 0.12% More preferably 0.11% or less.

Si is an element having a deoxidizing action and contributing to improvement of strength of a steel material (base material). In order to effectively exhibit such effects, the content is preferably 0.02% or more, more preferably 0.05% or more, and still more preferably 0.1% or more. However, if it exceeds 0.5%, the weldability of the steel material (base material) and the toughness of the base material deteriorate, and therefore, it is necessary to suppress it to 0.5% or less. , Preferably not more than 0.45%, and more preferably not more than 0.4%. On the other hand, when the HAZ is required to have a higher toughness, it is preferable to suppress Si to 0.3% or less. More preferably not more than 0.05%, and still more preferably not more than 0.01%. However, if the Si content is suppressed in this manner, the toughness of the HAZ improves, but the strength tends to decrease.

Mn is an element contributing to the improvement of the strength of the steel material (base material). In order to effectively exhibit such effect, it is preferable that Mn is contained by 0.5% or more. , More preferably not less than 0.7%, still more preferably not less than 0.8%. In particular, in the first rolled material of the present invention, the content should be 1.0% or more, preferably 1.2% or more, and more preferably 1.4% or more. However, if it exceeds 2%, the HAZ toughness deteriorates and the weldability of the steel material (base material) deteriorates. Therefore, it is necessary to suppress the Mn content to 2% or less. Preferably 1.8% or less, and more preferably 1.6% or less.

Ti is an element contributing to the improvement of toughness of HAZ by producing nitride such as TiN or Ti oxide in steel. In order to effectively exhibit such effects, Ti is preferably contained in an amount of 0.005% or more, more preferably 0.007% or more, and still more preferably 0.010% or more. However, if it is added excessively, the toughness of the steel material (base material) deteriorates, so it should be suppressed to 0.03% or less. Preferably 0.028% or less, and more preferably 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, more preferably 0.016% or less.

N is an element for depositing a nitride (for example, ZrN or TiN). This nitride prevents coarsening of the austenite grains generated in the HAZ during welding and promotes ferrite transformation, thereby contributing to enhancement of 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. The greater the number N, the finer the austenite grains are promoted, and thus the HAZ toughness improves effectively. However, when it exceeds 0.01%, the amount of solid solution N increases and the toughness of the base material deteriorates. Therefore, N should be suppressed to 0.01% or less, preferably 0.009% or less, more preferably 0.008% or less.

The steel of the present invention contains not only the above elements but also P: not more than 0.02% (not including 0%), S: not more than 0.015% (not including 0%) and Al: not more than 0.01% ). The reason for setting this range is as follows.

P is an element which is liable to segregate, and is particularly segregated in grain boundaries in steel to deteriorate toughness. Therefore, P should be suppressed to 0.02% or less, preferably 0.018% or less, more preferably 0.015% or less.

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

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

The contained elements defined in the present invention are as described above, and the remainder are iron and inevitable impurities. As the inevitable impurities, incorporation of the elements (for example, Mg, As, Se, etc.) to be brought in can be permitted depending on the conditions of the raw materials, materials, and manufacturing facilities.

In the steel material of the present invention,

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

(ii) at most 2% Cu, at most 2% Ni, at most 3% Cr, at most 1% Mo, at most 0.05% Nb, at most 0.1% , And the like are also effective. The reason for setting this range is as follows.

(i) Ca is an element having an effect of improving the HAZ toughness of a steel material. More specifically, Ca has an effect of reducing the anisotropy of the steel material by controlling the shape of the inclusions (specifically, spheroidizing MnS), and the HAZ toughness is improved by reducing the anisotropy of the steel material. In order to exhibit such an effect effectively, it is preferable to contain 0.0003% or more. , More preferably 0.0005% or more, and even more preferably 0.001% or more. However, if it is added in excess, a coarse oxide is formed and HAZ toughness is rather deteriorated. Therefore, Ca is preferably 0.01% or less. More preferably not more than 0.008%, and still more preferably not more than 0.005%.

(ii) Cu, Ni, Cr, Mo, Nb, V, and B are elements that act to increase the strength of the steel.

In particular, since the steel used for ships requiring low-temperature toughness is required to have strength in addition to good base material toughness and HAZ toughness, the second rolled steel sheet of the present invention is required to contain at least one element as an essential element. Preferably, it may contain both Cu and Ni, or may contain only Nb.

Cu is an element that strengthens the steel material. In order to effectively exhibit such effects, Cu is preferably contained in an amount of 0.05% or more. More preferably, it is 0.1% or more, and more preferably 0.2% or more. However, if the content exceeds 2%, the toughness of the steel material (base material) is lowered. Therefore, Cu is preferably suppressed to 2% or less. Or less, preferably 1.8% or less, and more preferably 1.6% or less. In the second rolled steel sheet of the present invention, Cu is preferably suppressed to 0.3% or less. Preferably 0.28% or less, and more preferably 0.25% or less.

Ni is an element which effectively acts to increase the strength of the steel material and improve the toughness of the steel material. In order to exhibit such action, it is preferable that Ni is contained in an amount of 0.05% or more. More preferably 0.1% or more, and more preferably 0.2% or more. Ni is more preferable, but since it is an expensive element, it is preferable to suppress the Ni content to 2% or less from the viewpoint of economy. More preferably 1.5% or less, and still more preferably 1% or less. In the second pressure-sensitive expandable member of the present invention, it is preferable to suppress it to 0.4% or less. More preferably, it is 0.38% or less, and more preferably 0.35% or less.

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

In order to increase the strength by adding Mo, it is preferable that the Mo content is 0.01% or more. It is more preferably 0.02% or more, and more preferably 0.03% or more. However, when it exceeds 1%, the weldability is deteriorated, so that Mo is preferably 1% or less. Or less, more preferably 0.9% or less, and still more preferably 0.8% or less.

Nb is an element having an effect of inhibiting recrystallization and effectively contributes to the miniaturization of the structure and effectively precipitates carbides and nitrides, thereby strengthening the steel material. 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 0.01% or more, and still more preferably 0.03% or more. However, in the first rolled steel sheet of the present invention, if it exceeds 0.05%, the structure becomes excessively fine and the yield ratio becomes high. Therefore, Nb is preferably suppressed to 0.05% or less. More preferably, it is 0.04% or less, and more preferably 0.03% or less. In the second pressure-sensitive expandable member of the present invention, when it exceeds 0.25%, the toughness of the base material is deteriorated. Therefore, Nb is preferably suppressed to 0.25% or less. More preferably, it is 0.23% or less, and more preferably 0.2% or less.

In order to increase the strength by adding V, it is preferable that the content is 0.005% or more. More preferably 0.01% or more, and more preferably 0.03% or more. However, when it exceeds 0.1%, the weldability deteriorates and the toughness of the base material deteriorates. Therefore, V is preferably 0.1% or less. , More preferably 0.08% or less, and still more preferably 0.06% or less.

B enhances the strength of the steel and, at the same time as the HAZ heated during welding, cools with N in the steel during precipitation to precipitate BN, thereby promoting ferrite transformation from within the austenitic grains. In order to exhibit such an effect effectively, it is preferable to contain 0.0003% or more. More preferably 0.0005% or more, and still more preferably 0.0008% or more. However, when it exceeds 0.005%, the toughness of the steel material (base material) is deteriorated. Therefore, B is preferably 0.005% or less. More preferably, it is 0.004% or less, and more preferably 0.003% or less.

[Manufacturing method]

Next, a production process that can be suitably employed in producing the steel material of the present invention will be described. 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 0.0010 to 0.0035%, and then cast to obtain a steel material (cast steel). Hereinafter, reasons for defining such a range will be described.

First, if REM and Zr are mixedly added to molten steel in which the total amount of oxygen [O] ₁ is appropriately controlled, REM and Zr can be produced in the form of oxides as a form of inclusions in the steel. At this time, the dissolved oxygen amount [O] ₂ of the molten steel is appropriately controlled by adjusting the amounts of REM and Zr added to the molten steel, and casting the molten steel can reduce the amount of solidified REM and solid Zr in the steel.

Generally, the total amount of oxygen [O] ₁ in the molten steel subjected to primary refining in an electric furnace or an electric furnace exceeds 0.015%. When REM or Zr is added to the molten steel, the amount of oxygen in the molten steel is excessively large, so that the reaction between REM and Zr and oxygen becomes severe, which is not preferable for the working of the solvent. In addition, oxides of coarse REM and coarse ZrO ₂ are produced, and the toughness of the base material itself deteriorates.

Therefore, in the present invention, REM and Zr are added to molten steel in which the total amount of oxygen [O] ₁ is adjusted to be slightly smaller than that of the prior art, so that REM oxide as an inclusion of REM, Zr oxide as inclusion of Zr, or REM And Zr. &Lt; / RTI &gt;

On the other hand, from the viewpoint of increasing the amount of oxide, especially REM and Zr, it is preferable to add a large amount of REM and Zr to the molten steel in which the total amount of oxygen [O] ₁ is adjusted. However, excess REM and Zr It is dissolved in steel. However, when the amount of the employed REM or the employed Zr increases, the HAZ toughness is varied as described above.

Therefore, in the present invention, by adjusting the amount of REM and the amount of Zr added to molten steel, the amount of dissolved oxygen [O] ₂ after the addition of REM and Zr is adjusted to be slightly larger than that of the prior art to prevent the REM and Zr from being dissolved during casting .

The total amount of oxygen [O] ₁ before the addition of REM and Zr is less than the total amount of oxygen contained in the molten steel after the first smelting and should be suppressed to 0.015% or less, preferably 0.01% or less, more preferably 0.008 %. However, if the total amount of oxygen [O] ₁ is excessively reduced to less than 0.0020%, the amount of oxygen becomes insufficient. Therefore, even if REM and Zr are added in combination, an amount of oxide contributing to toughness improvement of the HAZ can not be secured, REM or Zr which could not be solved is dissolved in the steel material, or Zr forms carbide or the like to deteriorate toughness of the base material. Therefore, the total amount of oxygen [O] ₁ before the combined addition of REM and Zr is preferably adjusted to 0.0020% or more, and more preferably 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 (so-called free oxygen) contained as dissolved atoms in molten steel and the amount of oxygen existing as oxide- Means the total amount of oxygen added. The amount of oxygen contained as dissolved atoms in molten steel can be measured by using an oxygen sensor using a solid electrolyte. The total oxygen amount can be measured by a general inert gas fusion-infrared absorption method or the like.

In order to adjust the total oxygen amount [O] ₁ in the molten steel to the above-described range, for example, a method of deoxidizing by using an RH type degassing refining apparatus, a method of heating by using a ladle heating type (ladle heating type) A method of deoxidation, and a method of adding deoxidation elements such as Si, Mn, Ti, and Al to molten steel to perform deoxidation. Of course, the total oxygen amount [O] ₁ may be adjusted by appropriately combining these methods. When a method of adding a deoxidizing element is employed, a deoxidizing element may be added when the molten metal is introduced from the converter.

The order of adding REM and Zr to the molten steel in which the total amount of oxygen [O] ₁ is adjusted is not particularly limited. For example, (a) Zr may be added after REM is added, (b) (C) REM and Zr may be added in combination at the same time. When a plurality of REMs are added, they may be added simultaneously or separately. For example, Ce and La may be used as REM, and Ce and Zr + La may be added in this order.

The form of REM or Zr to be added to molten steel is not particularly limited. For example, REM may be pure La, pure Ce, pure Y, or pure Zr, further Fe-Si-La alloy, Fe- -Si-La-Ce alloy or the like may be added. In addition, misal metal may be added to molten steel. Mismetal is a mixture of rare earth elements, specifically containing about 40 to 50% of Ce and about 20 to 40% of La.

After the REM and Zr are mixedly added, if the amount of dissolved oxygen [O] ₂ immediately before casting is not affected, an alloy element may be added to adjust the composition of the steel.

The dissolved oxygen amount [O] ₂ immediately before casting is 0.0010% or more. If it is less than 0.0010%, the oxygen amount becomes insufficient, so that REM or Zr is solidified in the steel during casting, which causes deviation of 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 is excessive, a large amount of coarse oxide is produced 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, 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 is determined so as to satisfy the formula 4, and the element may be added in the range of the added amount of REM and Zr determined. In the formula (4), [REM] and [Zr] are amounts of addition of REM or Zr respectively (mass%) and [O] ₁ is total oxygen content (mass%) of molten steel before addition of REM and Zr. The coefficient 15 on the right side is the value determined by repeating the experiment.

Equation 4

[REM] + [Zr]? 15 x [O] ₁

However, the amount of REM (total REM) and the amount of Zr (total Zr) contained in the steel need to satisfy the range specified by the above composition of the components.

On the other hand, when REM or Zr is slightly added to the total oxygen amount [O] ₁ and the dissolved oxygen amount [O] ₂ is less than 0.0010%, an oxide (e.g., MnO or iron oxide )] May be added.

Next, a method for producing the first rolled steel sheet of the present invention obtained from the steel material (piece) of the present invention will be described. The steel strip obtained by casting as described above was hot rolled at a rolling finish temperature of 870 DEG C or higher and then quenched from a temperature range of Ar ₃ point or higher, and was then quenched in a temperature range from Ac ₁ point to Ac ₃ point (austenite- hereinafter, only the quenching from that there is a case called "sangyeok 2"), and then tempering is carried out at a temperature range of less than Ac ₁ point.

As described above, the steel material of the present invention is characterized in that inclusions containing REM and Zr are dispersed in the steel material in order to improve the HAZ toughness after welding. Since such inclusions are dispersed in the steel material, It is recognized that in the quenching process after the quenching, the transformation in the grain is promoted and the quenched structure tends to become finer. The finer structure effectively works to improve the toughness of the base material itself. However, if the structure becomes finer, the yield point rises from the rule of the hole patch, and the yield ratio increases. Therefore, in order to realize the yield ratio (resistance ratio) of 80% or less, the structure after the quenching is excessively fine more than necessary and the crystal grains surrounded by the diagonal grain boundary having the crystal orientation difference of 15 degrees or more are not reduced more than necessary. It is necessary to perform rolling at a relatively high temperature.

Concretely, in the present invention, it is important to perform hot rolling so that the rolling finish temperature is not lower than 870 캜. 4 is a graph showing the relationship between the rolling finish temperature and the average circle-equivalent diameter D of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 15 degrees or more, and summarizes the experimental results of examples described later. In order to achieve an average circle equivalent diameter D of crystal grains surrounded by a diagonal grain boundary having a crystal orientation difference of 15 degrees or more in order to achieve a yield ratio of 80% or less, the rolling end temperature is preferably 870 DEG C or higher .

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

As the quenching method, in addition to direct quenching (DQ) for quenching the hot rolled material immediately after hot rolling, the hot rolled material may be quenched (RQ) offline. On the other hand, strict temperature control of the quenching start temperature is required in the DQ process as compared with the case of the RQ process because the process can not be retried.

Further, in order to mix a specified amount of ferrite phase in the hard bainite structure and / or the martensite structure, it is necessary to carry out the second quenching from the two-phase station. FIG. 6 is a graph showing the relationship between the ferrite content and the temperature (hereinafter, sometimes referred to as heating temperature) at the time of quenching from the temperature region in the vicinity of the two-phase region, 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, it is necessary to maintain the temperature at Ac ₁ point or more and Ac ₃ point or less (2-phase temperature), as is apparent from Fig. The holding time at the two-phase temperature may be, for example, 5 minutes or more.

After heating to the bimetallic zone, quenching (for example, RQ) is performed, and then tempering is performed at a temperature lower than the ferrite transformation starting temperature (temperature lower than Ac ₁ point). Thus, the strength of the steel material can be adjusted to about 590 MPa or more.

The temperature of the steel strip is controlled by the temperature at the t / 4 position calculated in the order described in the later embodiment. t is the thickness (mm) of the slab. The temperatures of the Ar ₃ point, the Ac ₃ point and the Ac ₁ point can be measured in the order shown in the later examples.

The first rolled steel sheet of the present invention can be used as a material for structures such as bridges, high-rise buildings, ships, and the like, and can prevent deterioration of the toughness of the weld heat affected zone as well as small-to-middle heat-welding.

Next, a method for manufacturing the second rolled steel strip of the present invention obtained from the steel material (steel strip) of the present invention will be described. (For example, a slab) obtained by casting as described above is formed so that the steel strip obtained is in a temperature range of Ac ₃ point or more and 1200 ° C or less (hereinafter, referred to as &quot; Is sometimes referred to as &quot; heating temperature &quot; or &quot; T1 &quot;), followed by hot rolling. In the hot rolling, in the temperature range where the average temperature of the steel strip is Ar ₃ point + 10 ° C or more and 900 ° C or less (hereinafter sometimes referred to as "T2" in this temperature range), the maximum reduction rate per pass is 12 % Or less, and the cumulative reduction ratio is 40% or more. (Hereinafter, the temperature in this temperature range may be referred to as &quot; T3 &quot; hereinafter) at which the average temperature of the obtained hot rolled material is at least _three Ar points, And the temperature in this temperature range is sometimes referred to as &quot; T4 &quot;). Hereinafter, reasons for defining such a range will be described.

The steel piece obtained by casting is heated at a heating temperature (T1) of not less than Ac ₃ point and not more than 1200 ° C. The heating temperature (T1) needs to be heated to Ac ₃ or higher in order to make the metal structure of the billet austenite. However, if the heating temperature exceeds 1200 ° C, the initial austenitic grains are coarsened, so that the transformed structure can not be sufficiently refined. Therefore, the heating temperature (T1) should be 1200 ° C or less.

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

&Quot; (5) &quot;

_{Ac 3 (℃) = 908-223.7 ×} [C] + 438.5 × [P] + 30.49 × [Si] -34.43 × [Mn] -23 × [Ni]

A billet heated to the heating temperature (T1) is there is hot rolled, hot rolled in, the first pass per maximum reduction ratio In the average temperature of the billet to the Ar ₃ point + more than 10 ℃, not more than 900 ℃ temperature range more than 12%, the cumulative It is necessary to set the reduction rate to 40% or more. The growth of the austenite grains can be suppressed and the strain can be efficiently introduced into the austenite grains before the transformation by controlling the rolling conditions at the Ar ₃ point + 10 ° C or higher and 900 ° C or lower. Therefore, The crystal grains surrounded by the diagonal grain boundary having the azimuth difference of 15 degrees or more can be made finer and the low temperature toughness of the base material itself can be enhanced.

If the maximum reduction rate per pass at a temperature of Ar ₃ point + 10 ° C or higher and 900 ° C or lower is more than 12%, the deformation is excessively accumulated in the austenite lips and the deformation recovery phenomenon occurs, Since the structure (crystal grains surrounded by the diagonal grain boundary) is coarsened, the low-temperature toughness of the base material itself deteriorates. Therefore, in order to suppress the recovery of deformation, the maximum reduction ratio per pass at a temperature range of Ar ₃ point + 10 ° C or higher and 900 ° C or lower is 12% or less. , Preferably not more than 11%, and more preferably not more than 10%. The effect of suppressing the coarsening of crystal grains surrounded by the diagonal grain boundary is increased by decreasing the maximum reduction rate per pass. However, if the maximum reduction ratio is made too small, the production time becomes longer and the productivity becomes worse. Therefore, the lower limit of the maximum reduction rate per pass is preferably 6%.

The cumulative reduction ratio at the Ar ₃ point + 10 ° C or higher and 900 ° C or lower is 40% or more. If the cumulative rolling reduction is less than 40%, the amount of deformation introduced into the austenite lips is reduced and the nucleation sites after the transformation become small, so that the crystal grains surrounded by the diagonal grain boundary are coarsened and the low temperature toughness of the base material itself It gets worse. Therefore, the cumulative reduction ratio at Ar ₃ point + 10 ° C or higher and 900 ° C or lower is 40% or more. , Preferably at least 45%, and more preferably at least 50%. The upper limit of the cumulative reduction ratio at the Ar ₃ point + 10 ° C or higher and 900 ° C or lower is not particularly limited, but is usually about 60%.

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

&Quot; (6) &quot;

_{Ar 3 (℃) = 910-310 ×} [C] -80 × [Mn] -20 × [Cu] -55 × [Ni] + 0.35 × (t-8)

The cumulative reduction factor can be calculated by the following equation (7). t ₀ is the rolling start thickness (mm) of a billet at the average temperature of not more than 900 ℃ temperature region of the slabs, t ₁ is rolling end thickness of the slabs in the average temperature of the billet temperature region not less than Ar ₃ point + 10 ℃ (mm).

&Quot; (7) &quot;

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

The average temperature of the steel strip is controlled by the temperature at the t / 4 position calculated in the order described in the later section of the embodiment. t is the thickness of the slab (mm).

On the other hand, the maximum reduction ratio and cumulative reduction ratio per pass in the temperature range (austenite recrystallization region) where the average temperature of the billet exceeds 900 deg. C is not particularly 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 crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 55 degrees or more can be increased, and toughness of the base material itself can be improved.

In other words, although the second rolled steel sheet of the present invention is mainly composed of bainite structure, in general, bainite has a KS (KS) method in which the closest surface of the crystal lattice of austenite and bainite is substantially parallel to the most closely- (Kurdjumov-Scahs) relationship. In this relation, bainite is produced by selecting any one of a maximum of 24 orientations with respect to austenite, but it is said that the tendency to be selected is changed by changing the temperature of the bainite transformation, and the bainite form is changed (Kawada et al .: CAMP-ISJ vol 16, No. 3 (2003), PS30). This is because, as the transformation temperature decreases, the bainite changes from the diffusion transformation represented by ferrite transformation to the shear transformation represented by martensite, or the transformation performance decreases due to the nucleation ability of the transformation, And the structure after the transformation is largely changed. In view of the above, by increasing the average cooling rate from the temperature range T ₃ above the Ar ₃ point to the temperature range T 4 of 500 ° C or less, it is possible to increase the fraction of crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 55 ° or more .

After cooling to an average cooling rate of 5 deg. C / sec or more up to a temperature range T4 of 500 DEG C or less, tempering may be performed if necessary. Since the deformation introduced by hot rolling or transformation is lost by tempering, the low temperature toughness of the base material can be further increased. The tempering may be performed, for example, by heating at a temperature of 500 ° C or higher and lower than Ac ₁ point.

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

&Quot; (8) &quot;

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

The second rolled steel strip of the present invention can be used as a material for a structure such as a bridge, a high-rise building, a ship, and the like, and can prevent the deterioration of the toughness of the weld heat affected zone as well as small- and medium-

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the scope of the present invention, but may be appropriately modified within the scope of the present invention, All of which 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 the deviation (Experimental Example 1-1) of the steel material and the strength and yield ratio (Experimental Example 1-2) of the rolled steel sheet were examined using the same steel grade , And the characteristics of the steel material (rolled steel sheet) were evaluated by combining Experimental Example 1-1 and Experimental Example 1-2.

[Experimental Example 1-1 (Evaluation of HAZ toughness and its deviation)

The molten iron was firstly refined in a 240-ton electric furnace, followed by pouring from the electric furnace, and secondary refining was carried out while adjusting the components and adjusting the temperature.

In the course, deoxidation was performed using Si and Mn, and the chemical composition was adjusted while adjusting the total oxygen amount [O] ₁ shown in Table 1 below. The total oxygen amount [O] ₁ means the total amount of oxygen combined with the amount of oxygen contained as dissolved atoms in molten steel and the amount of oxygen present as oxide inclusions, and the amount of oxygen contained as molten atoms in molten steel is measured using an oxygen sensor using a solid electrolyte And the total oxygen amount was measured by a general inert gas fusion-infrared absorption method. On the other hand, in Table 1 below, in addition to the total oxygen amount [O] ₁ , the dissolved oxygen amount of molten steel before addition of REM and Zr was also shown.

The addition amount of REM and Zr was calculated so as to satisfy the formula (2) according to the total oxygen amount [O] ₁ , and REM and Zr were added to adjust the dissolved oxygen amount [O] ₂ as shown in Table 1 below. Table 1 shows the sum ([REM] + [Zr]) of the amount of addition of REM, the amount of addition of Zr, and the amount of addition of REM and Zr. The ratio ([REM] + [Zr]) / [O] ₁ of the total amount of REM and Zr addition to the total amount of oxygen [O] ₁ is also shown.

After adjusting the dissolved oxygen amount to [O] ₂ , the chemical components were adjusted so as not to affect the [O] ₂ amount and then cast.

On the other hand, for the secondary refining, dehielding, de-sintering and the like were performed using a RH type degassing refining apparatus or the like.

In Table 1, REM is added in the form of micro metal containing about 50% of La and about 25% of Ce, and Zr is added as Zr alone.

FIG. 1 is a graph showing the relationship between the total amount of oxygen [O] ₁ before addition of REM and Zr and the sum of the amounts of REM and Zr added ([REM] + [Zr]). In Fig. 1, &amp; cir &amp; 1 &amp; tilde &amp; 5, &amp; cir &amp; 11 to 15, respectively. On the other hand, in FIG. 1, the unit of total oxygen amount [O] ₁ is expressed in ppm.

In Table 2, the composition of the steel after the composition adjustment (the balance is iron and unavoidable impurities) is shown in Table 2 below.

The molten steel after the component adjustment was cast into a slab in a continuous casting machine, and a sample was cut from a cross section at a t / 4 (t is the thickness of the slab) of the slab. The surface of the cut sample was observed at 10,000 times using EPMA "JXA-8500F (apparatus name)" manufactured by Japan Electronics Corporation, and the composition of the inclusions having a maximum diameter of 0.2 μm or more was quantitatively analyzed. Observation conditions were quantitative analysis of the composition of the inclusions at the central portion of the inclusion by means of wavelength dispersive spectroscopy of characteristic X-rays, with an acceleration voltage of 7 kV, a sample current of 0.003 μA, an observation field area of 1 cm ² and a number of analysis randomly selected 100. When the elements to be analyzed are Al, Mn, Si, Ti, Zr, Ca, La, and Ce, the molar ratio of the element to be analyzed is 1 mol and the total amount of the elements after conversion is 1 mol, The mole fraction of each element contained in the sample was calculated. The calculation results of the mole fractions are shown in Table 3 below.

The surface of the sample was observed with EPMA. As a result, the observed inclusions were mostly complex inclusions including REM and Zr, but inclusions of REM and inclusions of Zr were also generated as single inclusions.

In addition, the amount of solidified REM and solid Zr contained in the steel were calculated in the following order. First, the amount of REM and the amount of Zr contained as inclusions in the steel were measured by an electrolytic extraction method. Electrolytic extraction was carried out by using a solution containing 2 cc of triethanolamine and 1 g of tetramethylammonium chloride in 100 cc of methanol as an electrolytic solution under the current of 500 A / m ² or less. As a result, the matrix was dissolved, and solid solution REM and solid solution Zr were also extracted into the electrolyte solution. The size of the sample was 15 mm × 15 mm × 5 mm in length.

Subsequently, the electrolytic solution after the extraction was filtered using a membrane filter (filter diameter: 47 mm, hole size: 0.1 탆), and the residue was transferred to a platinum crucible for each filter and heated by a gas burner to separate. Subsequently, an alkali flux (mixture of sodium carbonate and sodium tetraborate) was added, and the mixture was heated with a gas burner to melt the residue. Next, 18% by volume hydrochloric acid was added to dissolve the fused product, and the fused product was transferred to a measuring flask. Further, pure water was added to increase the scale to obtain an analysis solution. The REM and Zr concentrations in the analytical solution were measured by the ICP-MS method.

The amount of REM and the amount of Zr contained in the inclusions thus obtained were separately calculated from the amount of REM (total REM amount) or the amount of Zr (total amount of Zr) analyzed by ordinary ICP-MS to obtain the amount of employed REM and the amount of employed Zr . The calculated results are shown together in Table 3 below. In Table 3, &quot; 0.0001 &quot; means that no element was detected.

FIG. 2 is a graph showing the relationship between the amount of dissolved oxygen [O] ₂ contained in molten steel before casting and the amount of solidified REM or solid Zr contained in the steel. On the other hand, in FIG. 2, the unit of dissolved oxygen amount [O] ₂ is expressed in ppm. Further, in Fig. 2, only the data in which the employment REM or employment Zr is detected was plotted.

Next, in order to evaluate the toughness of the HAZ subjected to the heat at the time of welding, the weld recreation test shown below was carried out by simulating large heat welding. In the welding reproduction test, the sample cut from the slab was heated so as to be 1400 占 폚, maintained at this temperature for 5 seconds, and cooled. The cooling was adjusted so that the cooling time from 800 ° C to 500 ° C was 300 seconds.

The impact characteristics of the sample after cooling were evaluated by measuring the absorbed energy (vE- ₄₀ ) at -40 DEG C by performing the V-notch Charpy test.

Three samples were sampled from the same steel grade in accordance with JIS Z2242 &quot; Charpy impact test method for metallic materials &quot;, and the results of measurement of vE- ₄₀ for each sample and their average values are shown in Table 4 below. and the average value of vE- ₄₀ is 150 J or more (HAZ toughness is good).

Further, for each sample, the deviation of toughness was evaluated on the basis of the maximum value and the minimum value of vE- ₄₀ value by the following criteria. The evaluation results are shown in Table 4 below.

[Evaluation Criteria of Maximum and Minimum Values]

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

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

[Evaluation Criteria]

?: The minimum value of the three measurements was 150 J or more, and high HAZ toughness was stably secured.

?: At least one of the three measured results is 150 J or more, but the deviation of HAZ toughness is large and the minimum value is less than 150 J.

X: All of the three measurements were less than 150J.

3 shows the average values of the HAZ toughness (marks in the drawing) and the widths of the maximum value and the minimum value of the HAZ toughness for each sample shown in Table 4 below.

From the above results, it can be considered as follows. As is apparent from FIGS. 1 and 3, REM and Zr (molar ratio) were added to molten steel in which total oxygen amount [O] ₁ before addition of REM and Zr was adjusted to 0.0020 to 0.015% (20 to 150 ppm) It is understood that the HAZ toughness is improved and the deviation of the HAZ toughness is also reduced. On the other hand, also the expression ([REM] + [Zr] ) of the straight line shown in Fig. 1 = a ^{15 × 10 -4 × [O]} 1.

As apparent from Tables 1 and 3 and FIG. 2, when the amount of dissolved oxygen before casting [O] ₂ is adjusted within the range of 0.0010 to 0.0035% (10 to 35 ppm) and then cast, It can be understood that the amount of the employed Zr can be reduced to a predetermined value or less.

As is clear from Tables 2 to 4 and Fig. 1 to 5 are examples satisfying the requirements defined in the present invention. In particular, since the REM amount and the Zr amount are appropriately controlled among the chemical components of the steel material, and the amount of solid solution REM and the solid Zr amount are appropriately controlled, An average value of 150 J or more is excellent in HAZ toughness. In addition, the deviation of the HAZ toughness is also reduced.

On the other hand, 6 to 15 are examples deviating from the requirements specified in the present invention. In particular, it is preferable that the amount of REM or Zr in the chemical composition of the steel material deviates from the range specified in the present invention (No. 6 to 10, 15) Since the amount of solid solution Zr deviates from the range specified by the present invention (Nos. 11 to 14), the average value of HAZ toughness is less than 150 J, and the HAZ toughness is falling. In addition, a large variation in HAZ toughness is also increasing.

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

The Experimental Example Slabs (steel grade a1~o1) obtained by casting under the conditions described in 1-1, the finish rolling end temperature is subjected to hot rolling such that the temperature shown in Table 5, the hot-rolled plate Ar ₃ point or more of the obtained And quenched from the temperature range. After quenching, the hot rolled material obtained by hot rolling was quenched directly from the quenching start temperature shown in Table 5 below (denoted as DQ in Table 5 below), or hot rolled by hot rolling to the quenching start temperature shown in Table 5 below Followed by quenching (denoted RQ in Table 5 below).

After the quenching, heating was maintained at a temperature range from Ac ₁ point to Ac ₃ point, and quenching was performed from this temperature range. The maintenance time was 5 minutes. Table 5 shows heating temperatures.

Then it was subjected to tempering in a temperature range lower than the Ac ₁ point. Table 5 shows the tempering temperature.

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

&Quot; Measurement method of Ar ₃ point (ferrite transformation start temperature at cooling) &quot;

A machined forma master test piece having a diameter of 8 mm and a length of 12 mm taken from the slab was heated to 1100 占 폚 and held for 10 seconds in a processed foam master tester and then processed at 1000 占 폚 at a cumulative reduction of 25% ° C to 25%, and thereafter 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 the time of heating) and Ac ₃ point (ferrite transformation end temperature at the time of heating)

The temperature at which the volume starts to decrease at the time of heating from room temperature to 1000 占 폚 at an average heating rate of 10 占 폚 / sec is defined as Ac ₁ point temperature, and the temperature at which the volume starts to expand Was measured as the Ac _three- point temperature.

The rolling finish temperature, the quenching start temperature, the heating temperature, and the tempering temperature were controlled by the average temperature at the t / 4 position when the thickness of the hot rolled material was t. The temperature at the t / 4 position was calculated in the following order.

&Quot; Method of calculating rolling finish temperature &quot;

(1) Using a process computer, the heating temperature at an arbitrary position in the thickness direction from the surface to the back surface of the billet is calculated on the basis of the atmospheric temperature from the start of heating to extraction and the furnace stay time.

(2) Using the calculated heating temperature, the rolling temperature at an arbitrary position in the plate thickness direction based on the rolling pass schedule during rolling and the cooling method between the passes (water-cooling or air-cooling) And rolled while calculating.

(3) The surface temperature of the steel sheet is measured using a radial thermometer installed on the rolling line (but calculated on the process computer).

(4) The surface temperature of the steel sheet 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) If the difference between the calculated surface temperature and the measured steel surface temperature is more than ± 30 ℃, substitute the measured steel surface temperature with the calculated surface temperature to obtain the calculated surface temperature on the process computer.

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

&Quot; Calculation method of quenching start temperature, heating temperature, tempering temperature &quot;

(1) Using a process computer, the heating temperature at an arbitrary position in the thickness direction from the surface to the back surface of the billet is calculated on the basis of the atmospheric temperature from the start of heating to the end of heating and the furnace stay time.

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

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

Next, the metal structure of the obtained rolled material was observed in the following order to measure the ferrite fraction.

&Quot; Observation of metal structure &quot;

(1) A sample cut parallel to the rolling direction (long direction) is prepared so as to include both the front and back surfaces of the rolled material.

(2) Polishing is carried out using a wet emery abrasive of # 150 to # 1000 or a polishing method having a function equivalent to that, and mirror finishing is performed using an abrasive such as a diamond slurry.

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

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

(5) Next, the photographed photograph is input to the image analyzing apparatus (the area of the photograph corresponds to 600 占 퐉 800 占 퐉 when the magnification is 100 times, and 150 占 占 200 占 when the magnification is 400x). Inputs to the image analysis apparatus are input so that the sum of the areas is equal to or larger than 1 mm x 1 mm (in other words, when the number of the images is 100, the number of the photographs is at least 6, ).

(6) In the image analyzer, the black area ratio is calculated for each photograph, and the average value of all the photographs is taken as the ferrite fraction.

On the other hand, in the above-mentioned microscopic observation, it was confirmed that the remainder in any of the examples was a bainite structure and / or a martensitic structure.

Next, 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 boundary having the crystal orientation difference of 15 degrees or more was obtained. The values of D (占 퐉) are shown in Table 6 below.

"Method of calculating D"

(1) A sample cut in a direction parallel to the rolling direction (long direction) is prepared so as to include both the front surface and the back surface of the rolled material.

(2) Polishing is carried out using a wet emery abrasive of # 150 to # 1000 or a polishing method having a function equivalent to that, and mirror finishing is performed using an abrasive such as a diamond slurry.

(3) The mirror polished surface was measured with an EBSP (Electron Back Scattering Pattern) apparatus manufactured by TexSEM Laboratories with a measuring range of 200 탆 x 200 탆 and a pitch of 0.5 탆 at t / 4 in the plate thickness direction The orientation difference of the crystal was measured, and the boundary having the crystal orientation difference of 15 degrees or more was set as a diagonal boundary system. On the other hand, a measurement point with a confidence index of less than 0.1, which indicates the reliability of the measurement direction, is excluded from the analysis target.

(4) The maximum width (length along the thickness direction) and the maximum length (length along the normal rolling direction) of crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 15 degrees or more in the grain distribution map were measured , The area of the crystal grains was calculated, and the circle equivalent diameter of the crystal grains was calculated to obtain an average value.

Next, the yield strength and the tensile strength of the obtained rolled material were measured in the following order to calculate the yield ratio.

"Measurement of yield strength and tensile strength"

No. 4 specimen of JIS Z2201 was taken from the t / 4 position of the rolled material (t is the thickness of the rolled material) so as to be perpendicular to the rolling direction (longitudinal direction) and subjected to tensile test 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, it is evaluated that the tensile properties are excellent (acceptable).

Based on the above results, FIG. 4 to FIG. 9 were prepared.

4 is a graph showing the relationship between the rolling finish temperature and the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 15 degrees or more. On the other hand, in Fig. Only results of 33 to 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, among the examples in which direct quenching (DQ) is performed after hot rolling, Only results of 28-32 were shown.

Fig. 6 is a graph showing the relationship between the heating temperature and the ferrite fraction when heated and maintained at a temperature near the bimodal. Fig. On the other hand, in Fig. Only results of 21 to 25 were shown.

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

8 is a graph showing the relationship between the ferrite fraction and the yield ratio. On the other hand, in Fig. 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 crystal grains surrounded by the diagonal grain boundary. On the other hand, Fig. Only results of 33 to 38 were shown.

The results of Experimental Example 1-1 and Experimental Example 1-2 are summarized as follows from Table 4 and Table 6.

No. 21 to 23, 26 to 30, and 35 to 38 are examples using the steel types a1 to e1 satisfying the requirements specified in the present invention. As can be seen from Table 4, the HAZ toughness is good, the deviation of the HAZ toughness is small, 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. 24, and 25 use the steel grade a1 that satisfies the requirements specified in the present invention. Therefore, as is evident from Table 4, since the HAZ toughness is good and the deviation of the HAZ toughness is small but ferrite is not produced, The ratio exceeds 80%.

No. 31 and 32 use a steel grade d1 which satisfies the requirements specified in the present invention. Therefore, as is evident from Table 4, although the HAZ toughness is good and the deviation of the HAZ toughness is small, The tensile strength is less than 590 MPa.

No. 33 and 34 use a steel grade e1 that satisfies the requirements specified in the present invention. Therefore, as is evident from Table 4, although the HAZ toughness is good and the deviation of the HAZ toughness is small, Since the mean circle equivalent diameter D of the crystal grains surrounded by the system is less than 35 mu m, the yield ratio of the base material is higher than 80%.

No. 39 to 48 use the steel types f1 to o1 which do not satisfy the requirements specified in the present invention. Therefore, as apparent from Table 4, the HAZ toughness is poor and the deviation of the HAZ toughness is also large. In particular, 43, 45, and 46, since the ferrite fraction is not properly controlled, the tensile strength is low and the yield ratio is increasing.

[Example 2]

In the following Experimental Examples 2-1 and 2-2, HAZ toughness and deviation (Experimental Example 2-1) and low temperature toughness of the steel itself (Experimental Example 2-2) were examined using the same steel grade, The characteristics of the steel material (rolled steel sheet) were evaluated by combining Experimental Example 2-1 and Experimental Example 2-2.

[Experimental Example 2-1 (Evaluation of HAZ toughness and its deviation)

A steel material was obtained in the same manner as in Experimental Example 1-1, and the steel material was evaluated by the same measurement and test method as in Experimental Example 1-1. The steel materials having the composition shown in Table 8 were evaluated for the steel material under the conditions shown in Table 7. [ The measurement and evaluation results are shown in Tables 9 and 10.

10 shows the relationship between the total oxygen amount [O] ₁ before addition of REM and Zr and the sum of the amounts of REM and Zr added ([REM] + [Zr]). In Fig. 10, &amp; cir &amp; The results of 101 to 104 are shown in Table 7 below. 109 to 112, respectively. On the other hand, in FIG. 7, the unit of the total oxygen amount [O] ₁ is expressed in ppm.

Fig. 11 is a graph showing the relationship between the amount of dissolved oxygen [O] ₂ contained in molten steel before casting and the amount of solidified REM or solid Zr contained in the steel. On the other hand, in Fig. 11, the unit of dissolved oxygen amount [O] ₂ is expressed in ppm. In Fig. 2, only the data in which the employment REM or employment Zr is detected is plotted.

From the results shown in Tables 7 and 9 and Figs. 10 to 12, it can be said that the same thing as described in the discussion of Example 1-1.

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, since the REM amount and the Zr amount are appropriately controlled among the chemical components of the steel material and the amount of solid solution REM and the solid Zr amount are appropriately controlled, An average value of 150 J or more is excellent in HAZ toughness. In addition, the deviation of the HAZ toughness is also reduced.

On the other hand, 105 to 113 are examples deviating from the requirements specified in the present invention. In particular, when the REM amount or the Zr amount out of the chemical composition of the steel material deviates from the range specified in the present invention (No. 105 to 108, 113) Since the amount of solid solution Zr deviates from the range specified by the present invention (No. 109 to 112), the average value of HAZ toughness is less than 150 J, and HAZ toughness is poor. In addition, a large variation in HAZ toughness is also increasing.

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

The slabs (steel types a2 to m2) obtained by casting under the conditions described in Experimental Example 2-1 were heated to a heating temperature (T1) shown in Table 5 below, and hot rolled to obtain a hot rolled steel sheet. In the hot rolling, the maximum rolling reduction and the cumulative rolling reduction per pass in a temperature range (T2) where the average temperature of the slab was Ar ₃ point + 10 ° C or higher and 900 ° C or lower were measured under the conditions shown in Table 5 below. The cumulative reduction factor was calculated using the above-mentioned 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 during cooling are shown in Table 5 below.

No. 11 of Table 11 below. 123 was cooled to the temperature range T4 where the surface temperature of the hot rolled material was 500 DEG C or lower, and then heated at 580 DEG C for tempering.

On the other hand, the average temperature of the slab or the hot rolled sheet was controlled to the temperature at the t / 4 position when the thickness of the slab or the hot rolled sheet 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 at an arbitrary position in the thickness direction from the surface to the back surface of the billet is calculated on the basis of the atmospheric temperature from the start of heating to extraction and the furnace stay time.

(2) Using the calculated heating temperature, the rolling temperature at an arbitrary position in the plate thickness direction based on the rolling pass schedule during rolling and the cooling method between the passes (ice-cooling or air-cooling) And rolled while calculating.

(3) The surface temperature of the steel sheet is measured using a radial thermometer installed on the rolling line (but calculated on the process computer).

(4) The surface temperature of the steel sheet 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) If the difference between the calculated surface temperature and the measured steel surface temperature is more than ± 30 ℃, substitute the measured steel surface temperature with the calculated surface temperature to obtain the calculated surface temperature on the process computer.

(6) Calculate the temperature at the t / 4 position using the corrected calculated surface temperature.

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

In Table 11, the product thickness (mm) of the rolled material obtained by cooling is also shown. In Table 11, the values of Ac ₃ point, Ar ₃ point and Ac ₁ point calculated using the above-mentioned Equations 5, 6, and 8 based on the chemical composition shown in Table 8 .

Next, the specimen was taken out from the t / 4 position (t is the plate thickness) of the obtained rolled material, and the specimen was then polished with a 2% nitric acid-ethanol solution (Na dissolution solution) , And the bainite fraction (area%) in the steel structure was measured by image analysis. At this time, all the structures other than ferrite and martensite were regarded as bainite. The bainite fraction (area%) is shown in Table 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 boundary having the crystal orientation difference of 15 degrees or more and the crystal grains surrounded by the diagonal grain boundary having the crystal orientation difference of 55 degrees or more occupied the entire steel material The ratio M was obtained. Values of D (占 퐉) and M (area%) are shown in Table 12 below.

"Method of calculating D"

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

"Method of calculating M"

The ratio M in which the crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 55 degrees or more occupies the entire steel material was calculated by analyzing the text data of the crystal orientation difference in the step (3) in the calculation method of D above. Analysis of the text data was performed by removing noise having a crystal orientation difference of 5 degrees or less as noise and calculating the area fraction of the crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 55 degrees or more in the entire metal structure.

Fig. 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 rolling reduction ratio in the non-recrystallized region is 40% or more, the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundary having the crystal orientation difference of 15 or more can be made 30 占 퐉 or less.

A decision between the average cooling rate in the temperature region (T4) of less than 500 ℃ from rate M and, Ar ₃ point or more temperature range (T3) the crystal grains surrounded to step diagonal mouth or more bearing car 55 ° in the entire steel material in Fig. 5 . As it is apparent from Figure 5, by controlling the Ar ₃ point or more temperature range (T3) average cooling rate of 5 ℃ / sec or more to a temperature region (T4) of less than 500 ℃ from, determines the diagonal grain boundaries than azimuth difference 55 ° The ratio M of the surrounding crystal grains in the entire steel material can be set to 50% or more by area.

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

&Quot; Evaluation method of low temperature toughness &

The low-temperature toughness of the rolled material was evaluated by carrying out the V-notch Charpy test and measuring the impact characteristics of the rolled material by measuring the absorbed energy (vE- ₆₀ ) at -60 캜. The measurement of vE- ₆₀ was carried out in accordance with JIS Z2242 by taking U4 test specimen specified by NK (Japan Maritime Safety Agency) from the t / 4 position. The measurement results are shown in Table 6 below.

On the other hand, since the impact characteristic of the base material is evaluated at the test temperature of -40 ° C in the shipboard E class in the NK class, the absorbed energy (vE _-60 ) is set to -60 ° C, And the average value of 100 J or more was determined as acceptable (low temperature toughness of the base material was good).

Fig. 15 shows the relationship between the average circle-equivalent diameter D and the rolled material vE- ₆₀ . 15, when the average circle-equivalent diameter D of crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 15 degrees or more is 30 占 퐉 or less, vE- ₆₀ can be made 100 J or more and the low temperature toughness of the base material itself can be improved Can be seen.

FIG. 16 shows the relationship between the ratio M of the crystal grains surrounded by the diagonal grain boundaries with the crystal orientation difference of 55 degrees or more and the vE- ₆₀ of the rolled material. 16, when the ratio M of the crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 55 degrees or more is occupied by the entire steel material at 50 area% or more, vE- ₆₀ can be made to be 100 J or more and the low temperature toughness of the base material itself Can be improved.

The results of Experimental Example 2-1 and Experimental Example 2-2 are summarized as follows from Table 10 and Table 12.

No. 121 to 125 and 127 to 133 are examples using the steel types a2 to d2 satisfying the requirements specified in the present invention. As is clear from Table 4, the HAZ toughness is good, the deviation of the HAZ toughness is small, As described above, the low temperature toughness of the base material itself is also good.

No. 126, since the steel type b2 satisfying the requirements specified in the present invention is used, as is evident from Table 10, although the HAZ toughness is good and the deviation of the HAZ toughness is small, the maximum value per pass Since the reduction rate exceeds 12%, the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 15 degrees or more exceeds 30 占 퐉 and the low temperature toughness of the base material deteriorates.

No. 134 and No. 135, as is apparent from Table 12, the low temperature toughness of the base material is good, but since the steel type e2 and the steel type f2 which do not satisfy the requirements specified in the present invention are used, as is clear from Table 10, the HAZ toughness , And the deviation of the HAZ toughness is also increasing.

No. 136 to 142 use a steel grade g2 to steel grade m2 that do not satisfy the requirements specified in the present invention. Therefore, as is evident from Table 10, the HAZ toughness is poor and the deviation of the HAZ toughness is large. Further, as is apparent from Table 12, since the metal structure is not properly controlled, the low temperature toughness of the parent material itself also deteriorates.

Claims (6)

C: 0.03 to 0.2% (meaning "mass%", the same applies hereinafter)
Si: not more than 0.5% (not including 0%),
Mn: 1.0 to 2% or less,
Ti: not more than 0.03% (not including 0%), and
N: not more than 0.01% (not including 0%),
P: not more than 0.02% (not including 0%),
S: 0.015% or less (not including 0%), and
Al: not more than 0.01% (including 0%),
Add to
0.0010 to 0.1% of REM and 0.0010 to 0.05% of Zr,
And the remainder is iron and unavoidable impurities,
(A) The steel material includes an inclusion containing REM and Zr,
(B) Employment of REM and Zr in steel
Employment REM: 0.0010% or less (including 0%),
And Zr: 0.0010% or less (including 0%),
(C) the structure comprises bainite or martensite or both of them and ferrite, the balance being inevitable, the ferrite fraction of the whole structure being 4 to 24% by area, the total fraction of bainite and martensite Is 74% by area or more and less than 96% by area,
(D) When a metal structure of a steel material is observed by a back scattering electron diffraction method (EBSP method), the following formula (1) is satisfied.
Equation 1
35 D
(In the formula (1), D means the average circle equivalent diameter (占 퐉) of the crystal grains surrounded by the diagonal grain boundary having the crystal orientation difference of 15 占 or more by measuring the azimuth difference between two adjacent crystals by the EBSP method. The method according to claim 1,
The composition of the inclusions contained in the steel material is measured and the molar ratio of the element other than O, C, N and S among the elements contained in the inclusion is converted into molar amount, 0.05 or more and the molar fraction of Zr is 0.04 or more. The method according to claim 1,
The steel further comprises, as another element,
Cu: 2% or less (not including 0%),
Ni: 2% or less (not including 0%),
Cr: 3% or less (not including 0%),
Mo: 1% or less (not including 0%),
Nb: not more than 0.05% (not including 0%),
V: not more than 0.1% (not including 0%), and
B: not more than 0.005% (not including 0%)
And at least one element selected from the group consisting of: The method according to claim 1,
Wherein said steel further comprises, as another element, Ca: not more than 0.01% (not including 0%). A method of manufacturing a rolled steel strip as set forth in claim 1,
REM and Zr are added to molten steel whose total oxygen amount [O] ₁ is adjusted in the range of 0.0020 to 0.015% to adjust the dissolved oxygen amount [O] ₂ in the range of 0.0010 to 0.0035%, followed by casting to obtain a steel material ,
After the steel material is hot-rolled to a rolling finish temperature of 870 DEG C or higher,
Ar annealing from a temperature range of Ar ₃ or higher,
From the temperature range of Ac ₁ point to Ac ₃ point,
Method of producing a step-down series performed at a temperature range lower than the Ac ₁ point for each of the tempering step in order. 6. The method of claim 5,
Method of producing a step-down series of the measured total oxygen content [O] _1, and the total oxygen content [O] according to the _first to adjust the REM and Zr was added to the dissolved oxygen content [O] ₂ so as to satisfy the equation (4).
Equation 4
[REM] + [Zr]? 15 x [O] ₁
In the formula (4), [REM] and [Zr] are the addition amounts (mass%) of REM or Zr, respectively, and [O] ₁ is the total oxygen amount (mass%) of the molten steel before adding REM and Zr. ]

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