JP2005048265A - Steel material superior in toughness of weld heat-affected zone, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material superior in the toughness of a weld heat-affected zone, and to provide a manufacturing method therefor. <P>SOLUTION: The steel material superior in the toughness of the weld heat-affected zone comprises 0.01-0.2% C (which means mass%, and hereafter the same), 0.05-0.5% Si, 0.5-2.5% Mn, 0.005-0.06% Ti and 0.002-0.01% N; 0.02% or less P (including 0%), 0.008% or less S (including 0%) and 0.01% or less Al (including 0%), which are inhibited elements; further 0.001-0.1% La and/or Ce in total; and the balance Fe with unavoidable impurities. Furthermore, in the steel material, the total mass of La<SB>2</SB>O<SB>3</SB>and Ce<SB>2</SB>O<SB>3</SB>occupies 55% or more in the average composition of oxide-based inclusions contained in the steel material, and a ratio of the total mass of La<SB>2</SB>O<SB>3</SB>and Ce<SB>2</SB>O<SB>3</SB>to the mass of SiO<SB>2</SB>in the above average composition, [(La<SB>2</SB>O<SB>3</SB>+Ce<SB>2</SB>O<SB>3</SB>)/SiO<SB>2</SB>], occupies 5.0 to 105. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention has improved toughness in a portion affected by heat (hereinafter referred to as “welding heat affected zone” or “HAZ”) when welding steel materials used in bridges, high-rise buildings, ships, and the like. The present invention relates to a steel material and a manufacturing method thereof.

  The properties required for steel materials used in bridges, high-rise buildings, ships and the like have become increasingly severe in recent years, and particularly good toughness is required. These steel materials are generally often joined by welding, but particularly HAZ has a problem that the toughness tends to deteriorate due to thermal influence during welding. This deterioration in toughness is considered to be caused by the fact that the larger the heat input during welding, the slower the cooling rate of the HAZ, the lowering the hardenability and generating coarse island martensite. Therefore, in order to improve the HAZ toughness, the heat input during welding may be suppressed. On the other hand, in order to increase the welding work efficiency, it is desired to employ a high heat input welding method such as an electrogas arc welding method or a flux copper backing welding method.

  Therefore, some steel materials that can suppress the deterioration of the HAZ toughness even 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 large 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, heat that is received during welding in a portion of the HAZ that is particularly close to the weld metal (hereinafter sometimes referred to as “bond portion”). As a result, TiN disappeared in solid solution, and it was found that the toughness of the base material could not be maintained.

  On the other hand, Patent Document 2 proposes a technique for achieving high toughness by optimizing steel components and making crystal grains fine. That is, by containing an appropriate amount of Cu in the steel and making use of precipitation hardening of Cu to compensate for the deterioration of hardenability, the formation of fine acicular ferrite with Ti oxide as the core is promoted, and as a result As described above, the HAZ structure is refined to improve toughness. However, since Ti oxide tends to agglomerate in molten steel when producing a steel material, it is difficult to finely disperse it in the steel material, and thus the HAZ toughness may not be sufficiently improved.

  In Patent Document 3, Al—Mn-based oxide particles are dispersed in a steel material, and the oxide particles are used as nucleation sites of intragranular acicular ferrite in HAZ, thereby reducing the HAZ structure. Techniques for increasing toughness have been proposed. However, since Al—Mn-based oxides are likely to agglomerate in molten steel, it is not a technique that can sufficiently cope with HAZ toughness during high heat input welding.

By the way, Patent Document 4 discloses a technique for dispersing a MnO.SiO ₂ -based composite oxide that hardly aggregates in molten steel as a measure for improving the quality of steel. In this document, MnS is precipitated using this complex oxide as a nucleus, and fine ferrite is generated in the austenite grains using this MnS as a ferrite transformation nucleus, thereby refining the structure and improving toughness. However, in this technique, since the deoxidizing power of Mn and Si is weak, it must be added in a large amount into the molten steel, and as a result, the MnO.SiO ₂ -based composite oxide becomes coarse and deteriorates the HAZ toughness.

Furthermore, in Patent Document 5, as a technique for improving HAZ toughness, rare earth oxides and sulfides are used as pinning particles for suppressing austenite grain coarsening, and the HAZ structure is refined to improve toughness. It is described. However, since rare earth oxides and sulfides are likely to agglomerate in the molten steel like the Ti oxide, the HAZ toughness may not be sufficiently improved. Further, in this technique, since Al and Si are added before the rare earth element is added, the oxide easily aggregates in the molten steel.
Japanese Patent Publication No. 55-26164 (see “Claims” on page 3) Japanese Patent Publication No. 7-57886 (see “Claims” on page 3) Japanese Patent Laid-Open No. 7-278736 (see [Claims], [0023] to [0029]) JP-A-3-287711 (see “Claims” on page 2) JP 2002-363687 A (see [Claims], [0009], [0019] to [0020])

  This invention is made | formed in view of such a condition, The objective is to provide the steel material excellent in the toughness of a welding heat affected zone, and its manufacturing method.

The steel material excellent in the toughness of the weld heat-affected zone that has solved the above problems is mass%, C: 0.01 to 0.2%, Si: 0.05 to 0.5%, Mn: 0 0.5 to 2.5%, Ti: 0.005 to 0.06%, N: 0.002 to 0.01%, and P: 0.02% or less (including 0%), In steel materials suppressed to S: 0.008% or less (including 0%), Al: 0.01% or less (including 0%), respectively, La and / or Ce: 0.001 to 0 in total 0.1% and the balance is Fe and inevitable impurities, and the total amount of La ₂ O ₃ and Ce ₂ O ₃ in the average composition of oxide inclusions contained in the steel is 55% or more. And the total amount of La ₂ O ₃ and Ce ₂ O _{3 in} the average composition and the mass ratio of SiO ₂ [(La ₂ O ₃ + Ce ₂ O ₃ ) / SiO ₂ ] is 5.0 to 105. In the steel material, as other elements, Cu: 0.05-2%, Ni: 0.05-3.5%, Cr: 0.01-1.5%, Mo: 0.01-1 %, Nb: 0.005 to 0.06%, V: 0.005 to 0.1%, and B: 0.0003 to 0.005%. Since the intensity | strength of can be raised further, it is preferable. Further, the steel material further preferably contains Ca: 0.0005 to 0.005% as another element. In this case, the amount of CaO in the average composition should be suppressed to 10% or less. is there.

  The method according to the present invention capable of reliably producing the steel material is the point of adding Si after adding La and / or Ce to the molten steel whose dissolved oxygen content is adjusted to a range of 0.0020 to 0.010%. Has a summary. However, when adding Ca, it is necessary to add Si, after adding La and / or Ce, and Ca to the molten steel which adjusted the amount of dissolved oxygen to the range of 0.0020-0.010%.

  ADVANTAGE OF THE INVENTION According to this invention, the steel materials excellent in the toughness of a welding heat affected zone and its manufacturing method can be provided. In particular, the steel material of the present invention can be suitably used not only for small to medium heat input welding but also for large heat input welding.

  As described above, Ti oxides and the like are likely to aggregate in the molten steel, and therefore HAZ toughness may not be improved. Moreover, in the steel material in which TiN is dispersed and reprecipitated, when the weld metal reaches a high temperature of 1400 ° C. or higher, the TiN dispersed in the bond portion disappears in a solid solution, and the HAZ toughness may not be improved.

Therefore, the present inventors, when producing the steel material, if it is difficult to agglomerate in the molten steel, and the inclusions that do not disappear in solid solution even if heated under the influence of heat during welding is dispersed in the steel material, I thought that the above problems could be solved, and I have studied from various angles. As a result, the total amount of La ₂ O ₃ and / or Ce ₂ O ₃ accounts for the average composition of the oxide inclusions contained in the steel material is not less than the predetermined amount, La ₂ O _3, and in said average composition / Alternatively, it has been found that a steel material in which the total amount of Ce ₂ O ₃ and the mass ratio of SiO ₂ [(La ₂ O ₃ + Ce ₂ O ₃ ) / SiO ₂ ] is within an appropriate range can satisfactorily solve the above problems. The present invention has been completed. Hereinafter, the function and effect of the present invention will be described.

In the steel material of the present invention, the total amount of La ₂ O ₃ and / or Ce ₂ O ₃ in the average composition of oxide inclusions contained in the steel material is 55% or more, and La accounts in the average composition. It is important that the total amount of ₂ O ₃ and / or Ce ₂ O ₃ and the mass ratio of SiO ₂ is 5.0 to 105.

The present inventors have found that a total of La ₂ O ₃ and Ce ₂ O ₃ accounts for the average composition of the oxide inclusions contained in the steel material, of La ₂ O ₃ and Ce ₂ O ₃ accounts for the average composition the total amount in the case where the mass ratio of SiO ₂ to satisfy the respective above range, La ₂ O ₃ and Ce ₂ O _3, and from the presence of SiO ₂ in the steel material of low melting point La ₂ O ₃ -SiO ₂ Oxides, Ce ₂ O ₃ —SiO ₂ oxides, La ₂ O ₃ —Ce ₂ O ₃ —SiO ₂ oxides, etc. are produced, and these composite oxides have a significant effect on improving HAZ toughness. I think it will affect.

  That is, when the average composition of oxide inclusions dispersed in the steel material and the composition ratio in the average composition satisfy the preferred range, the composition of the individual oxide inclusions dispersed in the steel material is also described above. It is considered that the average composition is approximated, and individual oxide inclusions having this composition are considered to exist in liquid form in the molten steel.

Here, La ₂ O ₃ —SiO ₂ oxide, Ce ₂ O ₃ —SiO ₂ oxide, La ₂ O ₃ —Ce ₂ O ₃ —SiO ₂ oxide are contained in molten steel (melting temperature: about The reason why it is considered to exist in a liquid state at 1500 to 1600 ° C. is that the molten steel to which La, Ce, Ca and Si are added is collected as is clear from the examples described later, and this is rapidly cooled to obtain a test piece. The shape of oxide inclusions containing La and Ce present in the test piece was observed with an electron microscope. As shown in FIG. This is because it has been found that the object is substantially spherical. That is, it is because the substantially spherical oxide inclusions have a typical shape suggesting that they exist in liquid form in the molten steel.

  And the present inventors think that the complex oxide which exists in a liquid state in molten steel hardly aggregates and is finely dispersed in the steel as oxide inclusions. This is because the liquid oxide present in the molten steel is less likely to agglomerate and coarsen in the molten steel because the interfacial energy between the molten steel and the oxide is generally smaller than that of the solid oxide.

  In addition, La and Ce have extremely strong affinity with oxygen, so that they can be immediately combined with oxygen in molten steel after addition, and can be quickly deoxidized with a small amount of addition. As a result, the oxide itself does not grow and become coarse.

However, since the melting points of La ₂ O ₃ itself and Ce ₂ O ₃ itself are much higher than the melting temperature and exist in a solid state in the molten steel, these oxides are the above-mentioned Al ₂ O ₃ , TiO _2, etc. Similarly, it tends to agglomerate in molten steel. Therefore, when manufacturing the steel material of the present invention, as will be described later, La ₂ O ₃ —SiO ₂ oxide that exhibits liquid in molten steel by adding La and Ce to the molten steel and then adding Si. , Ce ₂ O ₃ —SiO ₂ oxide, La ₂ O ₃ —Ce ₂ O ₃ —SiO ₂ oxide, etc. are produced in molten steel and cooled to disperse fine oxide inclusions. Obtained steel. And even if this oxide is heated at the time of welding, since the oxide does not disappear into the steel material, toughness deterioration at the bond portion can be prevented in addition to HAZ.

In the steel material of the present invention, it is important that the total amount of La ₂ O ₃ and / or Ce ₂ O ₃ in the average composition of oxide inclusions contained in the steel material is 55% or more. If the total amount is less than 55%, the oxide inclusions present in solid form in the molten steel increase, and these oxides aggregate and coarsen in the molten steel, so the HAZ toughness cannot be improved. Preferably, the total amount is 70% or more. As will be described later, when either La or Ce is added to the molten steel, the content of La ₂ O ₃ or Ce ₂ O _{3 in} the average composition of oxide inclusions contained in the steel material is 55%. % Or more.

In the steel material of the present invention, the total amount of La ₂ O ₃ and / or Ce ₂ O ₃ in the average composition of oxide inclusions and the mass ratio of SiO ₂ must be 5.0 to 105. If this mass ratio is less than 5.0, it is considered that a large amount of SiO ₂ that tends to agglomerate in the molten steel is generated. On the other hand, if this mass ratio exceeds 105, an oxide of La ₂ O ₃ having a high melting point or Ce _{2 is used.} This is because it is considered that many oxides of O ₃ alone are generated. A more preferred lower limit of the mass ratio is 10, and an upper limit is 85. In the case containing only La ₂ O ₃ free of Ce ₂ O _3, when the mass ratio of La ₂ O ₃ and SiO ₂ is, containing only Ce ₂ O ₃ containing no La ₂ O ₃ is, Ce ₂ The mass ratio between O ₃ and SiO ₂ only needs to satisfy the above range.

As a composition of the steel material to be used in the present invention, Mn, Ti, and Al as necessary are contained. Therefore, in the molten steel, Mn, Ti, Al, etc. are used in addition to La ₂ O ₃ , Ce ₂ O ₃ , and SiO ₂ . Although oxides are also produced, the present inventors have confirmed that the effects of the present invention are not impaired as long as the above requirements are satisfied even if some of these oxides are produced.

  Next, the component composition in the steel material of the present invention will be described.

  The steel material of the present invention contains, as essential components, C: 0.01 to 0.2%, Si: 0.05 to 0.5%, Mn: 0.5 to 2.5%, Ti: 0.005 to 0.06% and N: 0.002 to 0.01%, And La and / or Ce: 0.001 to 0.1% in total, P: 0.02% or less (including 0%), S: 0.008% or less (including 0%), and Al: 0.01% or less (0 % Are included), respectively. The reason why these ranges are determined will be described below.

C: 0.01 to 0.2%
C is an element indispensable for ensuring the strength of the steel material, and in order to exert this effect effectively, it is necessary to contain 0.01% or more, preferably 0.02% or more. However, if the C content exceeds 0.2%, the amount of island-like martensite generated in the HAZ increases, which not only degrades the HAZ toughness, but also adversely affects weldability, so it is necessary to keep it below 0.2%. . It is more recommended that the content be 0.15% or less.

Si: 0.05-0.5%
Si has a deoxidizing action and contributes to improving the strength of the steel material, and further, La ₂ O ₃ —SiO ₂ -based oxide, Ce ₂ O ₃ —SiO ₂ -based oxide, La ₂ which is essential for improving the HAZ toughness. It is an indispensable element for producing O ₃ —Ce ₂ O ₃ —SiO ₂ oxides and the like. In order to exert such an effect, the content should be 0.05% or more, preferably 0.07% or more. However, if the Si content exceeds 0.5%, the weldability and toughness of the steel material deteriorate, so it is necessary to keep it to 0.5% or less, preferably 0.4% or less.

Mn: 0.5-2.5%
Mn is an element that contributes to improving the strength of the steel material, but if the content is less than 0.5%, sufficient strength cannot be obtained, so it is necessary to contain 0.5% or more, preferably 0.7% or more. However, if the Mn content exceeds 2.5%, the weldability of the steel material is deteriorated, so it is necessary to keep it to 2.5% or less. Preferably it is 2.0% or less.

Ti: 0.005-0.06%
Ti is a useful element that precipitates TiN in the steel structure to prevent coarsening of austenite grains generated by heating during welding, promote ferrite transformation, and improve HAZ toughness. In order to exhibit such an effect effectively, it must be contained 0.005% or more. Preferably it is 0.007% or more. However, if the Ti content exceeds 0.06%, TiC precipitates and deteriorates the toughness of the steel material, so the Ti content should be suppressed to 0.06% or less. Preferably it is 0.05% or less.

N: 0.002 to 0.01%
N is an element that combines with Ti to precipitate TiN in the steel material, prevents coarsening of austenite grains generated in the HAZ during welding, promotes ferrite transformation, and improves HAZ toughness. In order to exhibit this effect effectively, it must be contained by 0.002% or more. As the N content increases, the austenite grain refinement is promoted and is effective in improving toughness. However, if the N content exceeds 0.01%, the toughness deteriorates due to an increase in the amount of solid solution N. Accordingly, the N content must be suppressed to 0.01% or less. A preferred lower limit is 0.003% and a preferred upper limit is 0.008%.

La and / or Ce: 0.001 to 0.1% in total
La and Ce are La ₂ O ₃ —SiO ₂ oxide, Ce ₂ O ₃ —SiO ₂ oxide, La ₂ O ₃ —Ce ₂ O ₃ —SiO ₂ oxide, which are essential for improving HAZ toughness. Are indispensable elements for generating the like and can be used alone or in combination. In order to exert such an effect effectively, La and / or Ce should be contained in a total of 0.001% or more, and preferably in a total of 0.002% or more. However, if the total amount of La and Ce exceeds 0.1%, oxides existing in a solid state in molten steel (that is, La ₂ O ₃ and Ce ₂ O ₃ single oxides) are likely to be formed. It is necessary to suppress it to 0.1% or less. It is recommended that the total content is 0.05% or less.

P: 0.02% or less (including 0%)
P is an element that easily segregates, and particularly segregates at a grain boundary in the steel material to deteriorate toughness. Therefore, the P content needs to be suppressed to 0.02% or less, and preferably 0.015% or less.

S: 0.008% or less (including 0%)
S is a harmful element that combines with Mn to generate MnS inclusions and degrades the toughness of the steel material and the ductility in the plate thickness direction. Further, S combines with La and Ce to produce LaS and CeS, and the desired La ₂ O ₃ —SiO ₂ oxide, Ce ₂ O ₃ —SiO ₂ oxide, La ₂ O ₃ —Ce ₂ O ₃ Inhibits the formation of SiO ₂ -based oxides. Therefore, the S content should be suppressed to 0.008% or less, preferably 0.006% or less.

Al: 0.01% or less (including 0%)
Al is an element having a stronger deoxidizing power than Si, and when added in excess, SiO ₂ is reduced and it becomes difficult to produce a desired oxide. Therefore, the Al content needs to be suppressed to 0.01% or less. Preferably it is 0.007% or less.

  The steel material of the present invention contains the above-described elements as essential components, and the balance is Fe and inevitable impurities (for example, Mg, Zr, As, Se, etc.), but further increases the strength of the steel material as other elements. Therefore, Cu: 0.05-2%, Ni: 0.05-3.5%, Cr: 0.01-1.5%, Mo: 0.01-1%, Nb: 0.005-0.06%, V: 0.005-0.1%, and B: 0.0003 It is also effective to include any one or more of -0.005%, and the reason for setting these ranges is as follows.

Cu: 0.05-2%
Cu can be solid solution strengthened by containing 0.05% or more. Moreover, when 0.6% or more is contained, the aging precipitation strengthening is exhibited and the strength can be greatly improved. However, if it exceeds 2%, it causes a reduction in toughness. Therefore, the Cu content is preferably suppressed to 2% or less. More preferably, it is recommended to be 1.5% or less.

Ni: 0.05-3.5%
Ni is an element effective for increasing the strength of the steel material and improving the toughness of the steel material, and in order to effectively exhibit this action, it is preferable to contain 0.05% or more. The higher the Ni content, the better. However, since it is an expensive element, it is recommended to keep it to 3.5% or less from the economical viewpoint. More preferably it is 3.0% or less.

Cr: 0.01-1.5%
In order to increase the strength, it is preferable to contain 0.01% or more of Cr. More preferably it is 0.02% or more. However, if the Cr content exceeds 1.5%, the weldability deteriorates, so the Cr content is preferably suppressed to 1.5% or less. More preferably, it should be suppressed to 1.2% or less.

Mo: 0.01 to 1%
In order to increase the strength, it is desirable to contain 0.01% or more of Mo. More preferably 0.02% or more is recommended. However, if the content exceeds 1%, weldability deteriorates, so the Mo content is preferably 1% or less. More preferably, it is recommended to keep it below 0.8%.

Nb: 0.005-0.06%
In order to increase the strength, Nb is preferably contained in an amount of 0.005% or more. More preferably, the content is 0.006% or more. However, if the Nb content exceeds 0.06%, NbC precipitates and deteriorates the toughness of the base metal. Therefore, the Nb content is preferably suppressed to 0.06% or less. More preferably, it is desirable to suppress it to 0.05% or less.

V: 0.005-0.1%
In order to increase the strength, it is desirable to contain V by 0.005% or more. More preferably, it is recommended to contain 0.006% or more. However, if the V content exceeds 0.1%, the weldability is deteriorated and the toughness of the steel material is deteriorated. Therefore, the V content is preferably 0.1% or less. More preferably, it is recommended to keep it to 0.08% or less.

B: 0.0003-0.005%
B increases the strength of the steel material and precipitates BN by combining with N in the process of cooling the HAZ heated during welding, thereby promoting ferrite transformation from within the austenite grains. In order to exhibit these effects effectively, it is preferable to contain 0.0003% or more. However, if the content exceeds 0.005%, the toughness deteriorates, so the B content is preferably 0.005% or less. More preferably, it is desirable to suppress it to 0.003% or less.

  Moreover, it is also effective that the steel material of the present invention contains Ca: 0.0005 to 0.005% as another element. In this case, the amount of CaO in the average composition should be suppressed to 10% or less. The reasons for setting these ranges are as follows.

Ca: 0.0005 to 0.005%
Ca _{is, La 2 O 3 -CaO-SiO} 2 based oxide and Ce ₂ O ₃ -CaO-SiO ₂ based _{oxide, La 2 O 3 -Ce 2 O} 3 -CaO-SiO 2 which is present in liquid form in the molten steel It is an element that generates a system oxide or the like and contributes to the improvement of HAZ toughness. In order to effectively exhibit this effect, the content is preferably 0.0005% or more, and more preferably 0.0008% or more. However, if the Ca content exceeds 0.005%, an oxide that exists in a solid state in the molten steel (that is, an oxide of CaO alone) is likely to be generated. Therefore, the Ca content is preferably suppressed to 0.005% or less. It is more preferable to suppress to% or less.

CaO amount: 10% or less When Ca is contained in a steel material, the CaO amount in the average composition of oxide inclusions contained in the steel material needs to be suppressed to 10% or less. If the amount of CaO is more than _{10%, La 2 O 3 -CaO} -SiO 2 based oxide and Ce ₂ O ₃ -CaO-SiO ₂ based _{oxide, La 2 O 3 -Ce 2 O} 3 -CaO-SiO 2 system This is because the amount of CaO contained in oxide inclusions such as oxides increases, and these oxide inclusions are present in a solid state in the molten steel and aggregate. Therefore, the improvement of HAZ toughness will be inhibited. Therefore, the CaO content in the average composition of oxide inclusions contained in the steel material needs to be 10% or less, more preferably 8% or less.

  Examples of means for controlling the amount of CaO produced include a method of adjusting the amount of Ca added to molten steel, and a method in which the amount of La and / or Ce added is at least twice the amount of Ca added.

The average composition of the oxide inclusions contained in the steel material may be quantitatively analyzed by observing the cross section of the steel material with EPMA. EPMA is an Electron Probe X-ray Micro Analysis. The observation conditions of EPMA are, for example, acceleration voltage: 20 kV, sample current: 0.01 μA, observation field area: 1 to 5 cm ^2, and quantitative analysis of the composition at the center of the inclusion by wavelength dispersion spectroscopy of characteristic X-rays. The size of inclusions to be analyzed is a maximum diameter of 5 μm or more, and the number of analyzes is at least 100. Inclusions with a maximum diameter of less than 5 μm were too small to be accurately quantified, and were excluded from the analysis.

  The analysis target elements are La, Ce, Al, Mn, Si, Mg, Ca, Ti, and O, and the relationship between the X-ray intensity and the element concentration of each element is obtained in advance as a calibration curve using a known substance. Next, the element concentration contained in the inclusion is quantified from the X-ray intensity obtained from the inclusion to be analyzed and the calibration curve.

Inclusions to be analyzed include single oxides such as La ₂ O ₃ , Ce ₂ O ₃ , SiO ₂ , and CaO. The amount of such single oxides generated is complex oxide (for example, La ₂ O ₃ -SiO ₂ -based oxides, Ce ₂ O ₃ -SiO ₂ -based oxides, La ₂ O ₃ -Ce ₂ O ₃ -SiO ₂ -based oxides, or those oxides containing CaO) It is considered that the effects of the present invention are not impaired even if these single oxides are present in some amount. In the steel material of the present invention, inclusions having an oxygen content of 15% or more are used as oxide inclusions. In addition, inclusions such as nitrides (eg, TiN) and sulfide inclusions (eg, MnS, LaS, CeS, etc.) contained in the steel are also analyzed, but inclusions that are not bonded to oxygen are Since it is considered that the effect of the present invention is not exhibited, it is excluded from the calculation of the average composition.

  The average of the quantitative results obtained for the individual oxide inclusions is calculated as the average composition of the oxide inclusions.

  Next, a method for reliably manufacturing the steel material will be described.

The total of La ₂ O ₃ and Ce ₂ O ₃ accounts for the average composition of the oxide inclusions contained in the steel material, the total amount and the SiO ₂ of La ₂ O ₃ and Ce ₂ O ₃ accounts for the average composition In order to control the mass ratio to satisfy the above range, La and / or Ce is added to the molten steel whose dissolved oxygen content is adjusted to a range of 0.0020 to 0.010%, and then Si is added. is important.

That is, when the amount of dissolved oxygen in the molten steel before addition of La and / or Ce is less than 0.0020%, a predetermined amount of oxides of La ₂ O ₃ and Ce ₂ O ₃ are not formed, and sulfides such as LaS and CeS are not formed. Since the production rate becomes high, La and / or Ce should be added to the molten steel in which the dissolved oxygen content is 0.0020% or more. Preferably, La and / or Ce is added to the molten steel whose dissolved oxygen amount is adjusted to 0.0025% or more. However, if the amount of dissolved oxygen before the addition of La and / or Ce exceeds 0.010%, the reaction between the molten steel and La and / or Ce becomes violent, which is not preferable for melting work and is not preferable for coarse La ₂ O ₃ or Ce _2. Since an oxide of O ₃ is formed, the amount of dissolved oxygen at the time of adding La and / or Ce should be suppressed to 0.010% or less. Preferably, it is recommended to add La and / or Ce to the molten steel whose dissolved oxygen content is adjusted to 0.008% or less.

The reason for adding La and / or Ce to the molten steel whose dissolved oxygen amount is adjusted to the above range is that when Al or Ti is added before La and / or Ce, the oxygen in the molten steel is combined with Al or Ti. Since Al ₂ O ₃ and TiO _x are produced, when La and / or Ce is added to the molten steel produced by these oxides, La ₂ O ₃ —Al ₂ O ₃ or Ce ₂ O ₃ —Al ₂ O is added. This is because the composite oxide intended in the present invention is not formed by generating ₃ , La ₂ O ₃ —TiO _x , Ce ₂ O ₃ —TiO _{x and the} like. Therefore, when producing the steel material of the present invention, it is important to first add La and / or Ce to the molten steel with the dissolved oxygen content adjusted to generate oxides of La ₂ O ₃ and Ce ₂ O _3. . However, the order of addition when La and Ce are used in combination is not particularly limited, and La and Ce may be added simultaneously, or La and Ce may be added in any order. When adding La and Ce in an arbitrary order, La and Ce may be added alternately several times. Of course, even when La and Ce are added simultaneously, they may be added in a plurality of times.

Next, by adding Si into molten steel in which oxides of La ₂ O ₃ and Ce ₂ O ₃ are formed, La ₂ O ₃ —SiO ₂ oxide, Ce ₂ O ₃ —SiO ₂ oxide, La ₂ O ₃ —Ce ₂ O ₃ —SiO ₂ oxide or the like is formed. This is because, after the addition of Si to the molten steel, the addition of La or Ce, the composite oxide having a SiO ₂ is previously generated and aggregation and coarsening, a predetermined component ratio be added La and Ce Thereafter This is because is not formed.

  Then, the components may be adjusted by adding Mn, Ti, Al or the like to the molten steel produced by these composite oxides.

On the other hand, when adding Ca to the steel material, it is important to add Si after adding La and / or Ce and Ca to the molten steel whose dissolved oxygen content is adjusted to a range of 0.0020 to 0.010%. . That is, the reason for adding Si after adding La or Ce to the molten steel is as described above. However, when Ca is added after adding Si as well as La and Ce, La ₂ O ₃ − having a predetermined component ratio is added. This is because composite oxides such as CaO—SiO ₂ oxide, Ce ₂ O ₃ —CaO—SiO ₂ oxide, La ₂ O ₃ —Ce ₂ O ₃ —CaO—SiO ₂ oxide and the like are not formed. However, the order of adding La and / or Ce, and Ca is not particularly limited, and these elements may be added simultaneously or in any order. La and Ce may be used alone or of course in combination.

  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 production method of the present invention, it is necessary to adjust the amount of dissolved oxygen in the molten steel within the above range by employing a known method before adding Ca. Examples of the method for adjusting the dissolved oxygen amount include a method of performing vacuum C deoxidation using an RH type degassing refining device, a method of adding a deacidifying element such as Si, Ti, and Al. Of course, the dissolved oxygen amount may be adjusted by combining these methods.

  Further, instead of the RH type degassing refining apparatus, secondary refining may be performed using a ladle heating type refining apparatus, a simple molten steel processing facility, or the like. 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 adopted 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.

  La, Ce, and Ca added to the molten steel are not particularly limited. For example, pure La, pure Ce, pure Ca, Fe-Si-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 it contains Ca, it is necessary to satisfy the range defined in the present invention. Si added to the molten steel is not particularly limited, and Si, Fe—Si alloy, Fe—Si—Mn alloy, or the like may be added.

  The steel material of the present invention can be used as a material for structures such as bridges, high-rise buildings and ships, and can be suitably used not only for small to medium heat input welding but also for large heat input welding.

  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 discharged from the converter to a ladle and secondary refined in the ladle using an RH degassing refiner. In the secondary refining, the components were adjusted by any one of the following procedures A to C.

[procedure]
A: After adding La and / or Ce and Ca to the molten steel adjusted to the dissolved oxygen amount shown in Table 1 by the deoxidation method shown in Table 1, Si is added, and then other elements are added. Component adjustment B: After adjusting the components of elements other than La and / or Ce and Ca to the deoxidized molten steel shown in Table 1, the amount of dissolved oxygen in the molten steel was measured, and then La and / or Alternatively, Ce and Ca are added. C: After secondary refining using a RH-type degassing refining apparatus in a ladle, first, Al is added, and then other elements are added to adjust the components. However, the amount of dissolved oxygen is not adjusted.

  Table 1 shows the component adjustment procedure, deoxidation method, La and / or Ce, and the amount of dissolved oxygen immediately before adding Ca. Table 2 shows the component composition of the steel material after component adjustment. In Table 2, "-" indicates that no element was added, and "less than 0.0003 mass%" indicates that the element was added but was below the limit of quantification.

  The molten steel after component adjustment was cast into a slab with a continuous casting machine, and then a sample was cut out from the slab. An example of inclusions taken by observing a section of the cut sample with a scanning electron microscope at a magnification of 10,000 is shown in FIG. FIG. 1 is a photograph of inclusions observed in the cross section of No. 3 steel shown in Table 2.

Moreover, the cross section of the cut sample was observed with EPMA-8705 manufactured by Shimadzu Corporation, and the component composition was quantitatively analyzed for inclusions having a maximum diameter of 5 μm or more. The observation conditions are: acceleration voltage: 20 kV, sample current: 0.01 μA, observation field area: 1 to 5 cm ² , analysis number: 100, and quantitative determination of component composition at the center of the inclusion by wavelength dispersion spectroscopy of characteristic X-rays analyzed. The analysis target elements are La, Ce, Al, Mn, Si, Mg, Ca, Ti, 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 inclusion was quantified from the electron beam intensity obtained from the inclusion and the calibration curve. Specifically, FIG. 2 (a) shows the result of electron beam intensity in the portion a shown in FIG. 1, and FIG. 2 (b) shows the result of electron beam intensity in the portion b.

Of the obtained quantitative results, inclusions having an oxygen content of 15% or more are defined as oxide inclusions, and the average of the quantitative results is defined as the average composition of oxide inclusions. The average composition is shown in Table 3 below. Further, the total amount of La ₂ O ₃ and Ce ₂ O ₃ in the average composition of oxide inclusions and the mass ratio of SiO ₂ [(La ₂ O ₃ + Ce ₂ O ₃ ) / SiO ₂ ] were calculated and This is also shown in Table 3.

The individual oxide inclusions whose average composition was calculated are La ₂ O ₃ —SiO ₂ oxide, Ce ₂ O ₃ —SiO ₂ oxide, La ₂ O ₃ —Ce ₂ O ₃ —SiO _2. Most of these oxides or those oxides containing CaO were included, but some oxides such as La ₂ O ₃ , Ce ₂ O ₃ , CaO, Al ₂ O ₃ and SiO ₂ were also included. . However, when the average composition is calculated, the quantitative results are averaged without distinguishing between the CaO—SiO ₂ oxide and the like and the single oxide.

In addition, the total amount of components having a detected concentration of less than 1% (excluding SiO ₂ ) is shown as “Others” in Table 3.

  Next, in order to evaluate the toughness of HAZ which is affected by heat during welding, a welding reproduction test shown below was performed by simulating high heat input welding.

  The entire sample cut out from the slab was heated to 1400 ° C., kept at this temperature for 5 seconds, and then cooled. The cooling rate 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 ₀ ) at 0 ° C. by the V-notch Charpy test. A sample having a vE ₀ of 150 J or more is considered acceptable (haz toughness is good). Table 3 shows the measurement results.

As is apparent from FIG. 1, substantially spherical inclusions are observed in the cross section of the No. 3 steel material. Further, as is apparent from FIG. 2, this inclusion includes a portion where a peak indicating the presence of La, Ce, Si, S, Fe and O is observed, and a peak indicating the presence of La, Ce, Fe and S. It can be seen that the part of b in which s is recognized is a mixed oxide inclusion, the part of a is La ₂ O ₃ —Ce ₂ O ₃ —SiO ₂ based oxide from the peak ratio, and the part of b is its peak From the ratio, it is considered to be a composite sulfide of LaS and CeS. And from the shape of this oxide inclusion, it is considered that it is liquid in molten steel.

  Moreover, it can consider as follows from Tables 2-3. Nos. 1 to 10 are examples that satisfy the requirements of the present invention, and steel materials having good toughness in the weld heat affected zone are obtained. On the other hand, Nos. 11 to 17 are examples that deviate from any of the requirements of the present invention, and the toughness of the weld heat affected zone is inferior.

It is the drawing substitute photograph which image | photographed steel material cross section with the scanning electron microscope at 10000 times. It is a chart figure showing a result of electron beam intensity.

Claims (5)

C: 0.01 to 0.2% (meaning “mass%”; the same shall apply hereinafter),
Si: 0.05 to 0.5%,
Mn: 0.5 to 2.5%,
Ti: 0.005 to 0.06%,
N: 0.002 to 0.01%, respectively,
P: 0.02% or less (including 0%),
S: 0.008% or less (including 0%),
In steel materials suppressed to Al: 0.01% or less (including 0%),
Further, La and / or Ce: 0.001 to 0.1% in total is included,
The balance is a steel material composed of Fe and inevitable impurities,
The total amount of La ₂ O ₃ and Ce ₂ O ₃ in the average composition of oxide inclusions contained in the steel material is 55% or more, and
The total amount of La ₂ O ₃ and Ce ₂ O _{3 in} the average composition and the mass ratio of SiO ₂ [(La ₂ O ₃ + Ce ₂ O ₃ ) / SiO ₂ ] is from 5.0 to 105. Steel material with excellent toughness of weld heat affected zone. The steel material is still another element,
Cu: 0.05-2%,
Ni: 0.05-3.5%,
Cr: 0.01 to 1.5%
Mo: 0.01 to 1%,
Nb: 0.005 to 0.06%,
V: 0.005 to 0.1%, and
B: 0.0003 to 0.005%,
The steel material according to claim 1, comprising at least one of the following. The steel material is still another element,
Ca: 0.0005 to 0.005% is included,
The steel material according to claim 1 or 2, wherein an amount of CaO in the average composition is 10% or less. A method for producing the steel material according to claim 1 or 2,
A steel material with excellent toughness of the heat affected zone of welding, characterized by adding Si after adding La and / or Ce to a molten steel whose dissolved oxygen content is adjusted to a range of 0.0020 to 0.010%. Manufacturing method. A method for producing the steel material according to claim 3,
To the molten steel with the dissolved oxygen content adjusted to the range of 0.002 to 0.010%,
A method for producing a steel material excellent in toughness of a weld heat affected zone, characterized by adding Si after adding La and / or Ce and Ca.

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