JP2007247004A - Low yield ratio high tensile steel having excellent toughness of weld heat-affected zone and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low yield ratio high tensile steel having excellent toughness of a weld heat-affected zone. <P>SOLUTION: The low yield ratio high tensile steel having the excellent toughness of the weld heat-affected zone contains 0.03 to 0.2% C, ≤0.5% (not inclusive of 0%) Si, 1.0 to 2.0% Mn, and ≤0.01% (not inclusive of 0%) N, satisfies ≤0.02% (not inclusive of 0%) P, ≤0.015% (not inclusive of 0%) S, and ≤0.01% (not inclusive of 0%) Al, and contains 0.001 to 0.1% REM and/or 0.0003 to 0.005% Ca, and 0.001 to 0.05% Zr, and the balance iron and inevitable impurities, wherein when the composition of the entire oxide included in the steel is measured, the steel contains the oxide of the REM and/or CaO and ZrO<SB>2</SB>and the ferrite fraction occupying in the entire structure is 4 to 24% and the balance bainite structure and/or martensite structure. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a high-tensile steel material having a low yield ratio of 590 MPa or higher, which is mainly used for building structures and the like, and is particularly a part that is affected by heat (hereinafter referred to as “the heat input amount of 30 kJ / mm or more”). The present invention relates to a steel material with improved toughness of a “welding heat-affected zone” or “HAZ”) and a method for producing the same.

  In recent years, the characteristics required for steel materials mainly used for building structures have become increasingly severe, and particularly good toughness is required. In general, these steel materials are often joined by welding. In particular, HAZ has a problem that the toughness is easily deteriorated due to thermal influence during welding. This deterioration in toughness becomes more noticeable as the heat input during welding increases, and the cause is that the larger the heat input during welding, the slower the cooling rate of the HAZ, the lower the hardenability and the formation of coarse island martensite. It is thought that there is to do. Therefore, in order to improve the toughness of the HAZ, it is considered that the heat input during welding should be suppressed as much as possible. However, in order to increase the welding work efficiency, for example, the electrogas arc welding method, the electroslag welding method, the submerged arc welding method, etc. Such a high heat input welding method with a heat input of 30 kJ / mm or more is desired.

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

  Patent Document 2 discloses controlling the form of oxides and nitrides contained in steel as a technique for improving the toughness of the base material and the HAZ. In this document, Ti and Zr are used in combination to produce fine oxides and nitrides to improve the toughness of the matrix and HAZ, and to produce such fine oxides and nitrides. It is disclosed that Ti and Zr may be added in this order in the manufacturing process. However, as a result of investigations by the present inventors, it is only necessary to increase the amount of oxide in order to further increase the toughness of HAZ. However, in the technique of Patent Document 2 described above, if a large amount of Ti or Zr is added to increase the amount of oxide, It turned out that carbide | carbonized_materials, such as Ti and Zr, are formed and the toughness of steel materials (base material) falls on the contrary.

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

  By the way, in recent years, with the increase in the number of buildings and the increase in span, there has been an increase in the use of 590 MPa class high strength steel materials having higher strength than conventional 490 MPa class steel materials. In the technique of Patent Document 3 described above, improvement of HAZ toughness has been addressed, but steel materials having a low yield ratio (YR ≦ 80%) required for high-strength steel materials for construction have not been studied.

On the other hand, Patent Document 4 realizes a low yield ratio with a steel sheet having a tensile strength of 590 N / mm ² or more by dispersing fine carbonitrides and securing a certain amount or more of ferrite. However, it is difficult to say that the steel sheet is excellent in HAZ toughness when welding with a heat input of 30 kJ / mm or more, and the realization of a steel material excellent in both characteristics of low yield ratio and HAZ toughness is eagerly desired. Yes.
Japanese Patent Publication No. 55-26164 (refer to claims, page 3, etc.) JP 2003-213366 A (refer to claims, paragraph 0007, paragraph 0008, paragraph 0018, paragraph 0023, etc.) JP 2005-48265 A (see claims, paragraph 0013, paragraph 0052, paragraph 0056, etc.) Japanese Patent No. 2901890

  The present invention has been made in view of the above circumstances, and the object thereof is excellent in HAZ toughness when welding with a heat input of 30 kJ / mm or more and 80% or less in a high strength region of 590 MPa or more. An object of the present invention is to provide a steel material having a low yield ratio and a manufacturing method thereof.

That is, the steel material according to the present invention that has solved the above problems is
C: 0.03 to 0.2% (meaning “mass%”; the same shall apply hereinafter)
Si: 0.5% or less (excluding 0%),
Mn: 1.0-2.0%, and N: 0.01% or less (not including 0%),
P: 0.02% or less (excluding 0%),
S: 0.015% or less (not including 0%) and Al: 0.01% or less (not including 0%),
REM: 0.001-0.1% and / or Ca: 0.0003-0.005%,
Zr: 0.001 to 0.05% each contained,
The balance is steel consisting of iron and inevitable impurities,
When the composition of all oxides contained in the steel material is measured, it contains REM oxide and / or CaO and ZrO ₂ , and
The gist is that the ferrite fraction in the entire structure is 4 to 24% and the balance is a bainite structure and / or a martensite structure.

The steel material preferably satisfies a total of REM oxide and / or CaO of 5% or more and ZrO ₂ of 5% or more when the composition of all oxides contained in the steel material is measured.

The steel material further contains Ti: 0.08% or less (not including 0%) as another element, and contains Ti ₂ O ₃ when the composition of all oxides contained in the steel material is measured. It is preferable to do. It is because the toughness of the weld heat affected zone can be further improved by including Ti. As described above, when the steel material contains Ti, it is preferable that Ti ₂ O ₃ is 0.3% or more when the composition of all oxides is measured.

The steel material, as another element,
Cu: 2% or less (excluding 0%),
Ni: 2% or less (excluding 0%),
Cr: 1.5% or less (excluding 0%),
Mo: 1% or less (excluding 0%),
Nb: 0.05% or less (excluding 0%),
V: 0.1% or less (not including 0%), and B: 0.005% or less (not including 0%)
Those containing one or more elements selected from the group consisting of these elements are preferred, and the strength of the base material can be increased by containing these elements.

  The steel material according to the present invention can be manufactured, for example, by adding at least one element selected from the group consisting of REM and Ca and Zr to molten steel whose dissolved oxygen content is adjusted to a range of 0.0020 to 0.010%. . When the steel material contains Ti in particular, at least one element selected from the group consisting of REM and Ca, Ti, and Zr to a molten steel whose dissolved oxygen content is adjusted to a range of 0.0020 to 0.010%. Is preferably added. In this case, it is preferable to add Ti prior to adding at least one element selected from the group consisting of REM and Ca and Zr to the molten steel in which the amount of dissolved oxygen is adjusted.

  According to the present invention, in order to disperse, in the steel material, an oxide having a composition that does not dissolve in the steel material even when reaching a high temperature of 1400 ° C. in the high heat input welding, it is not limited to small to medium heat input welding. Even in heat input welding, it is possible to prevent toughness deterioration of the weld heat affected zone (HAZ). Further, by making the hard bainite structure and / or martensite structure a structure in which an appropriate amount of ferrite phase is mixed, the HAZ toughness is not impaired, and a low strength of 80% or less in a high strength region of 590 MPa or more. A steel material with a yield ratio is obtained.

The present inventors first studied whether or not improvement in HAZ toughness could be achieved by dispersing an oxide having a composition different from that of Patent Document 3 in the steel material in order to increase the toughness of HAZ. As a result, when REM and / or Ca and Zr are combined and added to the steel material and the composition of all oxides contained in the steel material is measured, the REM oxide and / or CaO and ZrO ₂ may be contained. If adjusted to, the toughness of the weld heat affected zone can be increased, and when the composition of all oxides contained in the steel material is measured by further adding Ti to such a component system, Ti ₂ O It has been found that the toughness of the weld heat-affected zone can be further improved by adjusting to contain ₃ . Moreover, in order to achieve a low yield ratio of 80% or less in a high-strength steel plate of 590 MPa or more without inhibiting the improvement of HAZ toughness by the above oxide, a soft bainite structure and / or martensite structure is used. It was found that an appropriate amount of the ferrite phase should be present, and the present invention was completed. Hereinafter, the present invention will be described in detail.

First, the steel material of the present invention contains REM oxide and / or CaO and ZrO ₂ when the composition of all oxides contained in the steel material is measured. As described above, if the oxide of REM and / or CaO and ZrO ₂ are included, the oxide does not disappear in a solid solution even at a high temperature of 1400 ° C. due to the thermal influence during welding. It is possible to prevent coarsening of austenite grains in the HAZ at the time of welding, and as a result, it is possible to further improve the toughness of the HAZ as compared with the case where an oxide is formed by adding REM, Ca, or Zr individually. .

  Moreover, if the oxides or composite oxides are combined and contained in the steel material, the absolute amount of all oxides contained in the steel material can be increased, and REM that causes toughness deterioration of the steel material (base material). As a result, the HAZ toughness can be improved while suppressing the deterioration of the toughness of the base metal.

The steel material of the present invention contains (a) an oxide of REM and / or CaO and ZrO ₂ , or (b) a composite oxide containing REM and / or Ca and Zr, or (c ) Any oxide containing REM and / or CaO and ZrO ₂ and any composite oxide containing REM and / or Ca and Zr may be used. Examples of the composite oxide containing REM and / or Ca and Zr include a composite oxide containing REM and Zr, a composite oxide containing Ca and Zr, and a composite oxide containing REM, Ca, and Zr.

The steel material of the present invention preferably further contains a Ti oxide in addition to the oxides described above. That is, when measuring the composition of the total oxides contained in the steel, as long as it contains the Ti ₂ O _3. By containing the Ti oxide, the amount of oxide dispersed in the steel material can be further increased, so that the toughness of the HAZ can be further improved.

The Ti oxide may be contained in the steel material as a single oxide (Ti ₂ O ₃ ), for example, the composite oxide (that is, a composite oxide containing REM and Zr, a composite containing Ca and Zr). Oxides, composite oxides containing REM, Ca, and Zr) may be included as composite oxides.

When the composition of the total oxide contained in the steel material is measured, the total amount of REM oxide and / or CaO in the total oxide is 5% or more, and ZrO _{2 in the} total oxide is ZrO _{2 in the} total oxide. It is preferable to satisfy 5% or more. The reason is to ensure the amount of oxide that contributes to the improvement of HAZ toughness. The total of REM oxides and / or CaO is preferably 10% or more, more preferably 15% or more, and still more preferably 20% or more. On the other hand, ZrO ₂ is preferably 10% or more, more preferably 15% or more, and further preferably 20% or more.

When the steel material contains Ti oxide, it is preferable that Ti ₂ O ₃ satisfies 0.3% or more when the composition of all oxides contained in the steel material is measured. More preferably, it is 1% or more, more preferably 3% or more, particularly preferably 5% or more, and most preferably 10% or more.

The steel material of the present invention preferably has a total of 55% or more of the oxide of REM and / or CaO, and ZrO ₂ and Ti ₂ O ₃ when the composition of all oxides contained in the steel material is measured. . This is because if the total of these oxides is less than 55%, the amount of oxide that contributes to improving the toughness of the HAZ is insufficient, and the toughness of the HAZ cannot be sufficiently improved. More preferably, it is 60% or more, More preferably, it is 65% or more.

The remaining components of the total oxide composition are not particularly limited, but may be, for example, SiO ₂ , Al ₂ O ₃ , or MnO. It is preferable to keep the “other” components other than SiO ₂ , Al ₂ O ₃ and MnO to less than 5%.

The composition of the oxide contained in the steel material is determined by observing the cross section of the steel material with, for example, EPMA (Electron Probe X-ray Micro Analyzer) and quantitatively analyzing the inclusions observed in the observation field. Can be measured. In the observation of EPMA, for example, the acceleration voltage is 20 kV, the sample current is 0.01 μA, the observation visual field area is 1 to 5 cm ^2, and the composition at the center of the inclusion is quantitatively analyzed by wavelength dispersion spectroscopy of characteristic X-rays.

  The size of inclusions to be analyzed is a maximum diameter of 0.2 μm or more, and the number of analyzes is at least 100.

  The analysis target elements are Al, Mn, Si, Ti, Zr, Ca, La, Ce, and O. Using a known substance, the relationship between the X-ray intensity and the element concentration of each element is obtained in advance as a calibration curve. From the X-ray intensity obtained from the inclusions to be measured and the calibration curve, the element concentration contained in the inclusions to be analyzed is quantified, and inclusions having an oxygen content of 5% or more are defined as oxides. However, when a plurality of elements are observed from one inclusion, the composition of the oxide is calculated in terms of a single oxide of each element from the ratio of X-ray intensity indicating the presence of these elements. In the steel material of the present invention, the average of the quantitative results thus obtained for the individual oxides is taken as the average composition of the oxides.

  Next, the component composition in the steel material (base material) of the present invention will be described. The steel material of the present invention is characterized in that it contains REM: 0.001 to 0.1% and / or Ca: 0.0003 to 0.005% and Zr: 0.001 to 0.05%. The reasons for setting these ranges are as follows.

REM, Ca, and Zr are elements that contribute to improving the toughness of HAZ by forming REM oxide, CaO, ZrO ₂ , or a composite oxide in the steel material. In the steel material of the present invention, REM and Ca may be used alone or in combination.

  When REM is contained, it should be 0.001% or more, preferably 0.006% or more, more preferably 0.010% or more. However, if added excessively, sulfide of REM is generated and the toughness of the base material deteriorates, so it should be suppressed to 0.1% or less. Preferably it is 0.09% or less, More preferably, it is 0.08% or less. In the present invention, REM means a lanthanoid element (15 elements from La to Ln) and Sc (scandium) and Y (yttrium). Among these elements, La, Ce and Y It is preferable to contain at least one element selected from the group consisting of, and more preferably La and / or Ce.

  When Ca is contained, it should be 0.0003% or more, preferably 0.0005% or more, more preferably 0.0008% or more. However, if excessively added, coarse Ca sulfide is generated and the toughness of the base material deteriorates, so it should be suppressed to 0.005% or less. Preferably it is 0.004% or less, More preferably, it is 0.003% or less.

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

  In addition to containing REM and / or Ca and Zr, the steel material of the present invention includes C: 0.03 to 0.2%, Si: 0.5% or less (not including 0%), Mn: 1.0 to 2.0%, and N: 0.01% or less (not including 0%). The reason for setting such a range is as follows.

  C is an element indispensable for securing the strength of a steel material (base material), and in order to exert such effects, it is necessary to contain 0.03% or more. Preferably it is 0.04% or more, More preferably, it is 0.05% or more. However, if it exceeds 0.2%, a lot of island martensite is generated in the HAZ at the time of welding and not only causes deterioration of the toughness of the HAZ, but also adversely affects the weldability. Therefore, C must be suppressed to 0.2% or less, preferably 0.18% or less, more preferably 0.15% or less.

  Si is an element that has a deoxidizing action and contributes to improving the strength of the steel (base material). In order to exhibit such an effect effectively, it is preferable to contain 0.02% or more, more preferably 0.05% or more, and still more preferably 0.1% or more. However, if it exceeds 0.5%, the weldability and base material toughness of the steel (base material) deteriorate, so it is necessary to keep it to 0.5% or less. Preferably it is 0.45% or less, More preferably, it restrains to 0.4% or less. In addition, when the further high toughness of HAZ is calculated | required, it is good to suppress Si to 0.3% or less. More preferably, it is 0.05% or less, More preferably, it is 0.01% or less. However, when the Si content is suppressed in this way, the toughness of the HAZ is improved, but the strength tends to decrease, so the addition of another strength increasing element is necessary.

  Mn is also necessary for deoxidation and securing strength in the same manner as Si, and is 1.0% or more to secure the minimum strength as a structural member. Preferably it is 1.2% or more, More preferably, it is 1.3% or more. However, since the HAZ toughness deteriorates when the content exceeds 2.0%, the Mn content is set to 2.0% or less. Preferably it is 1.8% or less, More preferably, it is 1.6% or less.

  N is an element that precipitates nitrides (for example, ZrN and TiN), and the nitrides prevent austenite grains formed in the HAZ during welding and promote ferrite transformation, so that the toughness of the HAZ is increased. Contributes to improvement. In order to exhibit such an effect effectively, it is preferable to make it contain 0.002% or more, More preferably, it is 0.003% or more. The more N, the more refined austenite grains are promoted, which effectively works to improve the toughness of HAZ. However, if it exceeds 0.01%, the amount of solute N increases and the toughness of the base material deteriorates. Therefore, N must be suppressed to 0.01% or less, preferably 0.009% or less, more preferably 0.008% or less.

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

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

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

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

  The contained elements defined in the present invention are as described above, and the balance is iron and unavoidable impurities, and as the unavoidable impurities, elements brought in depending on the situation of raw materials, materials, production facilities, etc. (for example, Mg and As , Se, etc.) can be permitted. Further, it is possible to further contain the following elements.

<Ti: 0.08% or less (excluding 0%)>
Ti is an element that contributes to improving the toughness of HAZ by generating Ti oxide in the steel material. In order to exert such an effect effectively, Ti is preferably contained in an amount of 0.005% or more, more preferably 0.007% or more, and further preferably 0.01% or more. However, if added excessively, a large amount of oxide is generated and the toughness of the steel (base material) is deteriorated, so it should be suppressed to 0.08% or less. Preferably it is 0.07% or less, More preferably, it is 0.06% or less.

  In order to increase the strength of the steel material of the present invention, Cu: 2% or less (excluding 0%), Ni: 2% or less (not including 0%), Cr: 1.5% or less (0% Mo: 1% or less (not including 0%), Nb: 0.05% or less (not including 0%), V: 0.1% or less (not including 0%), and B: 0 It is also effective to contain one or more elements selected from the group consisting of 0.005% or less (not including 0%). The reasons for setting these ranges are as follows.

<Cu: 2% or less (excluding 0%)>
Cu is an element for solid solution strengthening of the steel material, and in order to exhibit such an effect effectively, it is preferable to contain 0.05% or more. More preferably, it is 0.1% or more, More preferably, it is 0.2% or more. In particular, when 0.6% or more is contained, in addition to solid solution strengthening, aging precipitation strengthening is also exhibited, and a significant improvement in strength becomes possible. However, if the content exceeds 2%, the toughness of the steel material (base material) decreases, so Cu should be suppressed to 2% or less. Preferably it is 1.8% or less, More preferably, you may be 1.6% or less.

<Ni: 2% or less (excluding 0%)>
Ni is an element that effectively acts to increase the strength of the steel material and improve the toughness of the steel material. In order to exert such an effect, Ni is preferably contained in an amount of 0.05% or more. More preferably, it is 0.1% or more, More preferably, it is 0.2% or more. The more Ni, the better. However, since it is an expensive element, it is preferable to suppress it to 2% or less from an economical viewpoint. More preferably, it is 1.8% or less, More preferably, it is 1.6% or less.

<Cr: 1.5% or less (excluding 0%)>
In order to increase the strength by adding Cr, the content is preferably 0.01% or more. More preferably it is 0.02% or more, and still more preferably 0.03% or more. However, if it exceeds 1.5%, weldability deteriorates, so Cr is preferably suppressed to 1.5% or less. More preferably, it is 1.3% or less, More preferably, it is 1.1% or less.

<Mo: 1% or less (excluding 0%)>
In order to increase the strength by adding Mo, it is desirable to contain 0.01% or more. More preferably it is 0.02% or more, and still more preferably 0.03% or more. However, if it exceeds 1%, the weldability deteriorates, so Mo is preferably 1% or less. More preferably, it is 0.9% or less, and more preferably 0.8% or less.

<Nb: 0.05% or less (excluding 0%)>
In order to increase the strength by adding Nb, it is preferable to contain 0.005% or more. More preferably, it is 0.007% or more, More preferably, it is 0.01% or more. However, if it exceeds 0.05%, carbide (NbC) precipitates and the base material toughness deteriorates, so Nb is preferably suppressed to 0.05% or less. More preferably, it is 0.04% or less, More preferably, it is 0.03% or less.

<V: 0.1% or less (excluding 0%)>
In order to increase the strength by adding V, it is desirable to contain 0.005% or more. More preferably 0.01% or more, still more preferably 0.03% or more. However, if it exceeds 0.1%, the weldability deteriorates and the toughness of the base material deteriorates. Therefore, V is preferably 0.1% or less. More preferably, it is 0.08% or less, and more preferably 0.06% or less.

<B: 0.005% or less (excluding 0%)>
B increases the strength of the steel material and, in the process of cooling the HAZ heated during welding, combines with N in the steel to precipitate BN and promote ferrite transformation from within the austenite grains. In order to exhibit such an effect effectively, it is preferable to contain 0.0003% or more. More preferably, it is 0.0005% or more, More preferably, it is 0.0008% or more. However, if it exceeds 0.005%, the toughness of the steel (base material) deteriorates, so B is preferably 0.005% or less. More preferably, it is 0.004% or less, and further preferably 0.003% or less.

  In the present invention, in order to achieve both high strength and a low yield ratio, the metal structure needs to have a ferrite fraction of 4 to 24% in the entire structure and the balance being a bainite structure and / or a martensite structure. There is.

  FIG. 1 is a graph showing the relationship between the ferrite fraction and the yield ratio, and is a summary of the results of Examples described later. From FIG. 1, in order to achieve a yield ratio of 80% or less, ferrite is used. It can be seen that the fraction needs to be 4% or more. In order to further reduce the yield ratio, the ferrite fraction is preferably 7% or more, more preferably 10% or more.

  On the other hand, FIG. 2 is a graph showing the relationship between the ferrite fraction and the tensile strength (TS), which is a summary of the results of Examples to be described later. From FIG. 2, the tensile strength is reliably set to 590 MPa or more. It can be seen that the ferrite fraction must be 24% or less in order to increase it. In order to further increase the tensile strength, the ferrite fraction is preferably 22% or less, more preferably 20% or less.

  The “remaining bainite structure and / or martensite structure” as used in the present invention includes 76 to 96% of bainite structure and / or martensite structure, in addition to the bainite structure and / or martensite structure and the ferrite. It is intended to include other structures (cementite, MA) that may be inevitably formed in the manufacturing process.

Next, a production method that can be suitably employed in producing the steel material of the present invention will be described. As described above, in order to contain an appropriate amount of REM oxide and / or CaO and ZrO ₂ in the steel material, as is apparent from the examples described later, at least one selected from the group consisting of REM and Ca is used. The element and the amount of dissolved oxygen immediately before adding Zr are appropriately controlled, that is, the molten steel in which the amount of dissolved oxygen is appropriately controlled is combined with at least one element selected from the group consisting of REM and Ca, and Zr. It is very effective to add them. If manufactured by this method, the oxide can be formed reliably even if the amount of REM, Ca, or Zr added is increased to some extent. As a result, REM sulfide, Ca sulfide, or Zr carbide can be formed. This is because generation can be prevented.

At this time, if the amount of dissolved oxygen is less than 0.0020%, even if Zr is added in combination with at least one element selected from the group consisting of REM and Ca, the amount of oxygen becomes insufficient, so the HAZ toughness is improved. REM or Ca, which could not secure the amount of contributing oxides and could not form oxides, formed sulfides, or Zr formed carbides, deteriorating the toughness of the base material. The amount of dissolved oxygen before adding the above elements is preferably adjusted to 0.0025% or more, and more preferably 0.0030% or more. However, if the amount of dissolved oxygen exceeds 0.010%, the amount of oxygen in the molten steel is too large, and the reaction between the oxygen in the molten steel and the above elements becomes violent, which is not preferable for melting work. Oxides, Ca oxides, and ZrO ₂ are formed. Therefore, the amount of dissolved oxygen should be suppressed to 0.010% or less, preferably 0.008% or less, more preferably 0.007% or less.

  After at least one element selected from the group consisting of REM and Ca and Zr are added in combination, an alloy element may be added to adjust the components of the steel material.

  In addition, when adding the said element to the molten steel which adjusted the amount of dissolved oxygen, what is necessary is just to compound-add at least 1 sort (s) of elements selected from the group which consists of REM and Ca, for example, REM and Ca are compounded. In order to do this, (a) after adding REM, Ca, and Zr to the molten steel in which the amount of dissolved oxygen is adjusted, the alloy elements may be added to adjust the components of the steel material, and (b) the amount of dissolved oxygen is adjusted. After adding REM (or Ca) and Zr to the adjusted molten steel, an alloy element other than Ca (or REM) may be added to adjust the components of the steel material, and then Ca (or REM) may be added.

  The procedure for adding at least one element selected from the group consisting of REM and Ca and Zr to the molten steel with the dissolved oxygen content adjusted is not particularly limited. For example, (a) selected from the group consisting of REM and Ca Zr may be added after at least one element is added, or (b) at least one element selected from the group consisting of REM and Ca may be added after adding Zr. (C) At least one element selected from the group consisting of REM and Ca and Zr may be simultaneously added in combination. When REM and Ca are added in combination, (d) REM (or Ca) may be added, then Zr may be added, and then Ca (or REM) may be added. (E) REM and Ca Zr may be combined and added simultaneously.

  When the steel material of the present invention contains Ti, after adding Zr in combination with at least one element selected from the group consisting of REM and Ca to the molten steel in which the amount of dissolved oxygen is adjusted, (a) adjusting the components of the steel material At the same time, Ti may be added, or (b) Ti may be added after adjusting the components of the steel material. Preferably, at least one element selected from the group consisting of REM and Ca, and Ti and Zr are added to the molten steel in which the amount of dissolved oxygen is adjusted.

In this case, it is recommended to add Ti prior to adding at least one element selected from the group consisting of REM and Ca and Zr to the molten steel in which the amount of dissolved oxygen is adjusted. If Ti is added to the molten steel whose amount of dissolved oxygen is adjusted, Ti ₂ O ₃ is first formed, but since Ti ₂ O ₃ has a small interfacial energy with the molten steel, the size of the formed Ti ₂ O ₃ is It becomes fine. Then, by adding Zr in combination with at least one element selected from the group consisting of REM and Ca, the REM oxide, CaO, and ZrO ₂ grow using the Ti ₂ O ₃ as a production nucleus. In addition, the number of particles increases, and the effect of suppressing austenite grain coarsening increases.

  By the way, the amount of dissolved oxygen in molten steel primarily refined in a converter or electric furnace usually exceeds 0.010%. Therefore, in the manufacturing method of the present invention, the amount of dissolved oxygen in the molten steel is adjusted to the above range before adding Zr in combination with at least one element selected from the group consisting of REM and Ca, or before adding Ti. There is a need. Examples of the method for adjusting the amount of dissolved oxygen include a method of vacuum C deoxidation using an RH type degassing refining device, a method of adding a deacidifying element such as Si, Mn, Ti, Al, etc. The amount of dissolved oxygen may be adjusted by appropriately combining these methods. Moreover, you may adjust the amount of dissolved oxygen using a ladle heating type refining apparatus, a simple molten steel processing facility, etc. instead of the RH type degassing refining apparatus. In this case, since the amount of dissolved oxygen cannot be adjusted by vacuum C deoxidation, a method of adding a deacidifying element such as Si may be employed to adjust the amount of dissolved oxygen. When employing a method of adding a deoxidizing element such as Si, the deoxidizing element may be added when steel is removed from the converter to the ladle.

  The form of REM, Ca, Zr, Ti added to the molten steel is not particularly limited. For example, as REM, pure La, pure Ce, pure Y, or pure Ca, pure Zr, pure Ti, and further Fe-Si- A La alloy, Fe—Si—Ce alloy, Fe—Si—Ca alloy, Fe—Si—La—Ce alloy, Fe—Ca alloy, Ni—Ca alloy, or the like may be added. Moreover, you may add misch metal to molten steel. Misch metal is a mixture of cerium group rare earth elements, and specifically contains about 40 to 50% of Ce and about 20 to 40% of La. However, since misch metal often contains Ca as an impurity, when the misch metal contains Ca, the range specified in the present invention must be satisfied.

  In addition, in order to obtain the metal structure, a steel material satisfying the above component composition is used, and in the manufacturing process, after heating / hot rolling, quenching is performed in an austenite-ferrite two-phase region (hereinafter simply referred to as “two-phase region”). It is recommended to perform heat treatment (quenching) and tempering.

  FIG. 3 is a graph showing the relationship between the quenching start temperature (quenching start temperature in the case of direct quenching in FIG. 3) and the ferrite fraction, and is a summary of the experimental results of examples described later. From FIG. 3, it can be seen that the quenching start temperature is preferably set to be equal to or higher than the ferrite transformation start temperature (Ar3) in order to suppress the ferrite fraction to 24% or less in order to reliably achieve the tensile strength of 590 MPa or more.

  In addition to direct quenching (DQ) in which quenching is performed immediately after hot rolling, the quenching may be performed offline using a hot rolled material (RQ). In the case of the DQ process, since the process cannot be performed again, stricter temperature control of the quenching start temperature is required than in the case of the RQ process.

  In order to mix a prescribed amount of ferrite phase in the hard bainite structure and / or martensite structure, it is effective to perform heat treatment in a two-phase region. FIG. 4 is a graph showing the relationship between the heat treatment temperature and the ferrite fraction in the two-phase region (nearby), and is a summary of the experimental results of Examples described later. From this FIG. 4, the yield ratio: 80 It can be seen that heat treatment at Ac1 or more and Ac3 or less is necessary to secure 4% or more of ferrite in order to achieve% or less. In addition, it is desirable to hold at Ac1 or more and Ac3 or less (two-phase region temperature) for 5 minutes or more.

  After heating to the two-phase region, quenching (for example, RQ) is performed, and then the strength of the steel material is adjusted by tempering at a temperature equal to or lower than the ferrite transformation start temperature (Ac1).

  The steel material of the present invention thus obtained can be used as a material for structures (particularly high-rise buildings) such as bridges, high-rise buildings, and ships, and can be used not only in small to medium heat input welding but also in high heat input welding. While preventing toughness deterioration of the heat affected zone, it is possible to achieve both high strength and low yield ratio.

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

  After the hot metal was first refined in a 240-ton converter, the steel was removed from the converter into a ladle and subjected to secondary refining while adjusting the components and adjusting the temperature. Here, in the ladle, the amount of dissolved oxygen shown in Table 1 below was adjusted by the deoxidation method shown in Table 1 below. Thereafter, the elements were added in the order shown in Table 1 below. Subsequently, the remaining alloying elements were added as necessary to finally adjust the compositions shown in Table 2 below. In the secondary refining, dehydrogenation and desulfurization were performed using an RH type degassing refining apparatus. In Table 1, the steel type No. A dissolved oxygen amount “−” of 16 indicates that it is less than the limit of quantification.

  In Table 1, La is in the form of an Fe-La alloy, Ce is in the form of an Fe-Ce alloy, REM is in the form of a misch metal containing about 50% La and about 25% Ce, and Ca is Ni. Zr was added in the form of Zr alone, and Ti was added in the form of an Fe-Ti alloy in the form of -Ca alloy, Ca-Si alloy, or Fe-Ca compact. In Table 2, "-" indicates that no element was added, and "<" (less than) did not add an element but was inevitably contained, so detection was in the range below the limit of quantification. It means that it was done.

  Table 2 also shows Ac1, Ac3 and Ar3. The Ac1, Ac3 and Ar3 are measured by the following method.

<Method for measuring ferrite transformation start temperature (Ar3) during cooling>
The processed formaster test piece is heated to 1100 ° C. and held for 10 seconds, then processed at 1000 ° C. with a cumulative reduction of 25%, further processed at 900 ° C. with a cumulative reduction of 25%, and then at a cooling rate of 1 from 800 ° C. The temperature at which the volume began to expand during cooling was determined as the Ar3 transformation temperature.

<Measurement Method of Ferrite Transformation Start Temperature (Ac1) During Heating and Ferrite Transformation End Temperature (Ac3) During Heating>
In the process of heating the processed formaster specimen from room temperature to 1000 ° C. at a heating rate of 10 ° C./s, the temperature at which the volume begins to shrink is Ac1 transformation temperature, and the temperature at which the volume begins to expand further after heating is the Ac3 transformation temperature. did.

  After the above melting, the slab obtained by continuous casting was hot-rolled and then directly quenched (DQ) or offline quenched (RQ). Further, the steel was heated to the austenite-ferrite two-phase region or the vicinity of the two-phase region and then quenched, and then tempered to obtain steel plates having the thicknesses shown in Table 3. (Finish) rolling end temperature in the above hot rolling [temperature of t (sheet thickness) / 4 part at the end of rolling], quenching method and quenching start temperature (temperature of t / 4 part at the start of quenching), two-phase region Table 3 shows the heat treatment temperature (temperature at the t / 4 site) and the tempering temperature (temperature at the t / 4 site) in (near).

In addition, the temperature of the t / 4 site | part at the time of completion | finish of rolling of Table 3 is calculated | required in the way of the following (1)-(6).
(1) In the process computer, based on the atmospheric temperature from the start of heating to the end of heating and the in-furnace time, the heating temperature at an arbitrary position in the thickness direction from the front surface to the back surface of the steel slab is calculated.
(2) Using the calculated heating temperature, calculate the rolling temperature at any position in the plate thickness direction based on the rolling pass schedule during rolling and the cooling method between the passes (water cooling or air cooling). Rolling while calculating using a suitable method.
(3) The steel sheet surface temperature is measured using a radiation type thermometer installed on the rolling line (however, calculation is also performed on a process computer).
(4) The steel plate surface temperature measured at the start of rough rolling, at the end of rough rolling and at the start of finish rolling is collated with the calculated temperature on the process computer.
(5) When the difference between the calculated temperature at the start of rough rolling, at the end of rough rolling and at the start of finish rolling and the above measured temperature is ± 30 ° C or more, recalculate so that the measured surface temperature matches the calculated surface temperature. The calculated temperature on the process computer.
(6) The above calculated temperature is corrected to determine the finish rolling finish temperature at the t / 4 part.

In addition, the heat treatment temperature (temperature at the t / 4 site) in the two-phase region (near) is obtained by the following procedures (1) and (2). Further, the quenching temperature (temperature at the t / 4 site at the start of quenching) and the tempering temperature (temperature at the t / 4 site) were obtained in the same manner.
(1) In the process computer, based on the atmospheric temperature from the start of heating to the end of heating and the in-furnace time, the heating temperature at an arbitrary position in the thickness direction from the front surface to the back surface of the steel slab is calculated.
(2) The temperature of the t / 4 part is obtained from the calculated temperature.

  Using the steel sheet obtained as described above, a tensile test, structure observation, investigation of inclusion composition by EPMA, and evaluation of HAZ toughness were carried out in the following manner.

<Tensile test>
Sample No. 4 of JISZ 2201 was sampled in the direction perpendicular to the rolling direction from the t (plate thickness) / 4 part of each steel plate, and a tensile test was conducted in accordance with the procedure of JISZ 2241 to determine the tensile strength (TS). It was measured. And what TS was 590 Mpa or more and YR was 80% or less evaluated that it was excellent in the tensile characteristic.

<Observation of metal structure>
The ferrite fraction was measured as follows.
(I) A sample is taken from the steel plate so that a plate thickness cross section including the steel plate front and back surfaces parallel to the rolling direction and perpendicular to the steel plate surface can be observed.
(Ii) The observation surface is mirror-finished by polishing with wet emery polishing paper (# 150 to # 1000) or a polishing method having the same function (polishing using an abrasive such as diamond slurry).
(Iii) The polished sample is corroded using a 3% nital solution to reveal the grain boundaries of the ferrite structure.
(Iv) At t (plate thickness) / 4 portion, the exposed structure was photographed at a magnification of 100 times or 400 times (taken as a photograph of 6 cm × 8 cm in this example), and the ferrite structure was colored black To do.

Next, the photograph is taken into an image analysis apparatus (the area of the photograph corresponds to 600 μm × 800 μm when the magnification is 100 times, and 150 μm × 200 μm when the magnification is 400 times). Capture to the image analyzer at any magnification so that the total area is 1 mm x 1 mm or more (ie, at least 6 photos for 100x and above for 400x). Capture at least 35).
(V) In the image analysis apparatus, the black area ratio is calculated for each photograph, and the average value of all photographs is defined as the ferrite fraction.

  In addition, in the said microscope observation, in any Example, it was confirmed that the remainder is a bainite structure and / or a martensite structure.

<Investigation of inclusion composition>
A sample was cut out from the cross section at the t (plate thickness) / 4 position of each steel plate. The cut sample surface was observed 600 times using “EPMA-8705 (device name)” manufactured by Shimadzu Corporation, and the component composition of the precipitate having a maximum diameter of 0.2 μm or more was quantitatively analyzed. The observation conditions are an acceleration voltage of 20 kV, a sample current of 0.01 μA, an observation visual field area of 1 to 5 cm ² , and an analysis number of 100, and the component composition at the center of the precipitate is determined by wavelength dispersion spectroscopy of characteristic X-rays. analyzed. The analysis target elements are Al, Mn, Si, Ti, Zr, Ca, La, Ce, and O, and a relationship between the electron beam intensity and the element concentration of each element is obtained in advance using a known substance as a calibration curve. The element concentration of the precipitate was determined from the electron beam intensity obtained from the precipitate and the calibration curve.

  Of the obtained quantitative results, a precipitate having an oxygen content of 5% or more was defined as an oxide, and the average was defined as the average composition of the oxide. The average composition of all oxides is shown in Table 4 below. When a plurality of elements were observed from one inclusion, the composition of the oxide was calculated in terms of the X-ray intensity ratio indicating the presence of these elements, converted to a single oxide of each element.

As a result of observing the sample surface with EPMA, most of the observed oxides were REM and / or a composite oxide containing Ca and Zr, or a composite oxide containing Ti. The oxides of CaO, ZrO ₂ and Ti ₂ O ₃ were also produced.

<Evaluation of HAZ toughness>
Next, in order to evaluate the toughness of the HAZ, electroslag welding was performed at a heat input of 30 to 170 kJ / mm to produce a welded joint. And using the test piece prescribed | regulated by JISZ2242 (2006) which made the V notch in the bond part of this welded joint, the Charpy impact test was done at ₀ degreeC, and the absorbed energy (vE0) was measured. Example No. An average value of three test pieces collected every time was obtained, and those having vE ₀ of 150 J or more were evaluated as having excellent HAZ toughness.

  The measurement results are also shown in Table 3.

Tables 1 to 4 can be considered as follows (in addition, the following No. indicates Example No. in Table 3). No. 1 to 3, 6 to 17, and 20 to 22 are examples that satisfy the requirements defined in the present invention. Since the steel material contains REM oxide and / or CaO and ZrO ₂ , the influence of welding heat Steel material with good toughness of the part is obtained. Moreover, the ferrite fraction also satisfies the requirements defined in the present invention, and both a strength of 590 MPa or more and a yield ratio of 80% or less can be achieved.

On the other hand, no. 4, 5, 18, 19, 23 to 31 are examples that do not meet any of the requirements defined in the present invention. In particular, no. Since Nos. 23 to 30 do not contain any one of REM oxide and / or CaO and ZrO _{2 in} the steel material, the toughness of the weld heat affected zone is inferior.

  No. 4, 5, 28 and 29 have a high yield ratio because the ferrite fraction is below the specified range.

  No. Nos. 18, 19 and 27 have high strength because the ferrite fraction exceeds the specified range.

  No. No. 23 cannot achieve high strength because Mn is insufficient.

  No. No. 31 has an excess of Mn and Al and a small amount of dissolved oxygen, so that a prescribed oxide cannot be secured sufficiently and is inferior in HAZ toughness.

It is a graph which shows the relationship between a ferrite fraction and a yield ratio. It is a graph which shows the relationship between a ferrite fraction and tensile strength (TS). It is a graph which shows the relationship between the quenching start temperature after completion | finish of hot rolling, and a ferrite fraction. It is a graph which shows the relationship between the heat processing temperature and ferrite fraction in a two-phase area | region (near vicinity).

Claims (8)

C: 0.03 to 0.2% (meaning “mass%”; the same shall apply hereinafter)
Si: 0.5% or less (excluding 0%),
Mn: 1.0-2.0%, and N: 0.01% or less (not including 0%),
P: 0.02% or less (excluding 0%),
S: 0.015% or less (not including 0%) and Al: 0.01% or less (not including 0%),
REM: 0.001-0.1% and / or Ca: 0.0003-0.005%,
Zr: 0.001 to 0.05% each contained,
The balance is steel consisting of iron and inevitable impurities,
When the composition of all oxides contained in the steel material is measured, it contains REM oxide and / or CaO and ZrO ₂ , and
A low-yield ratio high-tensile steel material excellent in toughness of a weld heat-affected zone, characterized in that the ferrite fraction in the entire structure is 4 to 24% and the balance is a bainite structure and / or a martensite structure. The steel material according to claim 1, wherein the total of the REM oxide and / or CaO satisfies 5% or more and the ZrO ₂ satisfies 5% or more. The steel material further contains Ti: 0.08% or less (not including 0%) as another element, and also contains Ti ₂ O ₃ when the composition of all oxides contained in the steel material is measured. The steel material according to claim 1 or 2. The steel material according to claim 3, wherein the Ti ₂ O ₃ satisfies 0.3% or more. The steel material is still another element,
Cu: 2% or less (excluding 0%),
Ni: 2% or less (excluding 0%),
Cr: 1.5% or less (excluding 0%),
Mo: 1% or less (excluding 0%),
Nb: 0.05% or less (excluding 0%),
V: 0.1% or less (not including 0%), and B: 0.005% or less (not including 0%)
The steel material according to any one of claims 1 to 4, comprising at least one element selected from the group consisting of: A method for producing the steel material according to any one of claims 1 to 5,
To the molten steel with the dissolved oxygen content adjusted to the range of 0.0020-0.010%,
A method for producing a low-yield-ratio high-tensile steel material excellent in toughness of a weld heat-affected zone, characterized by adding at least one element selected from the group consisting of REM and Ca and Zr. A method for producing the steel material according to any one of claims 3 to 5,
To the molten steel with the dissolved oxygen content adjusted to the range of 0.0020-0.010%,
A method for producing a low-yield ratio high-tensile steel material excellent in toughness of a weld heat-affected zone, characterized by adding at least one element selected from the group consisting of REM and Ca, and Ti and Zr.   The process according to claim 7, wherein Ti is added to the molten steel with the dissolved oxygen content adjusted before adding at least one element selected from the group consisting of REM and Ca and Zr.

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