JP4515428B2 - Steel material excellent in toughness and brittle fracture occurrence characteristics of weld heat affected zone and its manufacturing method - Google Patents

Description

  The present invention relates to a steel material used for structures such as bridges, high-rise buildings, and ships, and more specifically, a part that is affected by heat in welding (hereinafter referred to as a welding heat-affected zone or HAZ). The steel has improved toughness and the brittle fracture occurrence characteristics of the base metal, and its manufacturing method.

  The properties required for steel materials used in bridges, high-rise buildings, ships and the like have become increasingly severe in recent years, and particularly good toughness is required. In general, these steel materials are often joined by welding. 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 HAZ toughness, it is considered that the heat input during welding should be suppressed as much as possible. However, on the other hand, in order to increase the welding work efficiency, it is desired to employ a high heat input welding method in which the heat input of welding is 40 kJ / mm or more, for example, electrogas welding, electroslag welding, submerged welding and the like.

  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 becomes a high temperature of 1400 ° C. or higher, the TiN is dissolved in the HAZ by the heat received during welding, particularly in a portion (bond portion) close to the weld metal. Therefore, it has been found that the deterioration of the HAZ toughness cannot be sufficiently 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. This document describes that Ti and Zr are used in combination to produce fine oxides and nitrides, thereby improving the toughness of the base material and the HAZ. Further, it is disclosed that such fine oxides and nitrides may be added in the order of Ti and Zr in the manufacturing process. However, as a result of studies by the present inventors, it is sufficient to increase the amount of oxide in order to further increase the toughness of HAZ. However, if a large amount of Ti or Zr is added to increase the amount of oxide in the technique of Patent Document 2, Ti It turned out that carbides, such as Zr and Zr, are formed, and the toughness of the steel (base metal) is decreased instead.

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. This composite oxide is in a liquid state in molten steel, so it is finely dispersed in the steel, and it does not disappear in solid solution even if it is affected by heat at the time of welding. This contributes to improving the toughness of HAZ. . 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 is also disclosed that Ti may be added to the molten steel.

By the way, in order to ensure the safety as a structure, it is necessary to suppress the occurrence of brittle fracture cracks in the steel [hereinafter referred to as brittle fracture generation characteristics or CTOD (Crac-Tip Opening Displacement) characteristics]. desired. This is because if a brittle fracture crack occurs, the structure itself is destroyed. However, a steel material that has improved the HAZ toughness during high heat input welding while suppressing the occurrence of brittle fracture cracks has not been known so far.
Japanese Patent Publication No.55-26164 JP 2003-213366 A JP 2005-48265 A

  The present invention has been made in view of the above circumstances, and the purpose thereof is excellent in HAZ toughness particularly when welding with a heat input of 40 kJ / mm or more, and brittle fracture cracks are generated in the steel material. An object of the present invention is to provide a steel material that suppresses the above and a method for producing the same.

The steel material excellent in the toughness and brittle fracture occurrence characteristics of the weld heat-affected zone according to the present invention that has solved the above-mentioned problems is C: 0.03 to 0.12% (meaning “mass%”. The same applies hereinafter. ), Si: 0.5% or less (excluding 0%), Mn: 1.4-1.8%, and N: 0.003-0.01%, P: 0.02% or less ( 0% is not included), S: 0.015% or less (not including 0%), and Al: 0.01% or less (not including 0%) are satisfied, and REM: 0.001 to 0.001 0.1% and / or Ca: 0.0003 to 0.02% and Zr: 0.001 to 0.05%, respectively, and the balance is iron and inevitable impurities, REM and / Or contains Ca and Zr as a single oxide or a composite oxide, and has a thickness t (mm Of when observed with metal structure of a plane perpendicular to and steel surface parallel to the rolling direction of the steel, with the gist in that satisfies the following (1) to (3).
(1) The ferrite area ratio is 75% or more.
(2) The average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less.
(3) The average aspect ratio of the ferrite grains at the t / 2 position is 1.6 or less.

The steel material has a total REM oxide and / or CaO content of 5% or more and ZrO ₂ of 5% or more when the composition of all oxides contained in the steel material is measured and converted into mass as a single oxide. It is preferable that

  In order to further improve the HAZ toughness, the steel material further contains Ti: 0.08% or less (not including 0%) as another element, and contains the Ti as a single oxide or a composite oxide. Preferably it is. The Ti oxide is desirably 0.3% or more when the composition of all oxides contained in the steel material is measured and converted to mass as a single oxide.

  In order to further increase the strength of the steel material, as other elements, Cu: 2% or less (not including 0%), Ni: 3.5% or less (not including 0%), Cr: 3% or less ( 0% not included), Mo: 1% or less (not including 0%), Nb: 0.25% or less (not including 0%), V: 0.1% or less (not including 0%), And B: One or more elements selected from the group consisting of 0.005% or less (not including 0%) may be included.

  When manufacturing the steel material, to the molten steel whose dissolved oxygen amount is adjusted to a range of 0.0020 to 0.010%, (a) at least one element selected from the group consisting of REM and Ca, and Zr By adding at least one element selected from the group consisting of (b) REM and Ca, and Ti and Zr, the steel material can contain a predetermined oxide. In the procedure (b), Ti may be added prior to 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.

  According to the present invention, an oxide having a composition that does not dissolve and disappear in the steel material even when reaching a high temperature of 1400 ° C. when high heat input welding is performed is dispersed in the steel material. Even in high heat input welding, it is possible to prevent toughness deterioration of the weld heat affected zone (HAZ). Moreover, since the metal structure of the steel material is appropriately controlled, it is possible to prevent the occurrence of brittle fracture cracks in the steel material. Therefore, according to the present invention, a steel material having both the toughness of the weld heat affected zone and the brittle fracture occurrence characteristics can be provided.

  The present inventors first studied whether or not improvement of HAZ toughness could be achieved by dispersing an oxide having a composition different from that of Patent Document 3 in a steel material. As a result, if REM and / or Ca and Zr are combined and added to the steel, and the REM and / or Ca and Zr are adjusted to be contained in the steel as a single oxide or a composite oxide, the HAZ toughness is improved. It has been found that the HAZ toughness can be further improved by adjusting Ti so as to contain Ti as a single oxide or composite oxide by further adding Ti to such a component system. Furthermore, in order to prevent the occurrence of brittle fracture cracks without hindering the improvement of HAZ toughness due to the above oxide, the metal structure of the steel material is mainly composed of ferrite, and the form of ferrite grains is appropriately controlled. As a result, the present invention was completed. Hereinafter, the present invention will be described in detail.

  First, the steel material of the present invention contains REM and / or Ca and Zr as a single oxide or a composite oxide. If such an oxide is included, the oxide does not disappear in a solid solution even at a high temperature of 1400 ° C. due to the heat effect during welding, so the austenite grains are coarsened in the HAZ during welding. As a result, the toughness of the HAZ can be further improved compared to the case where REM, Ca, and Zr are added individually to form an oxide.

  Moreover, if the above single oxide or composite oxide is combined and contained in the steel material, the absolute amount of the oxide contained in the steel material can be increased, and the REM causes the deterioration of the toughness 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 an oxide of Ti in addition to the oxides described above. By containing the oxide of Ti, the amount of oxide dispersed in the steel material can be further increased, so that the HAZ toughness can be further improved.

The Ti oxide may be contained in the steel material as a single oxide (Ti ₂ O ₃ , Ti ₃ O ₅ , TiO ₂ ), for example, the composite oxide (that is, a composite containing REM and Zr). Oxide, composite oxide containing Ca and Zr, composite oxide containing REM, Ca and Zr), and may be contained as a composite oxide.

When the composition of all oxides contained in the steel material is measured and converted into mass as a single oxide, the total amount of oxides of REM and / or CaO is 5% or more, and ZrO ₂ is 5%. It is preferable to satisfy the above. The reason is to ensure the amount of oxide that contributes to the improvement of HAZ toughness. The total amount of REM oxides and / or CaO is preferably 10% by mass or more, more preferably 15% by mass or more, and still more preferably 20% by mass or more. On the other hand, it is preferred that ZrO ₂ is not less than 10 wt%, more preferably 15 mass% or more, more preferably 20 mass% or more.

When the steel material contains an oxide of Ti, the composition of all oxides contained in the steel material is measured, and when converted to mass as a single oxide, the oxide of Ti satisfies 0.3% or more. It is preferable. More preferably, it is 1 mass% or more, More preferably, it is 3 mass% or more, Most preferably, it is 5 mass% or more, Most preferably, it is 10 mass% or more. Incidentally, alone oxides of Ti is present as a Ti ₂ O ₃ and Ti ₃ O _5, TiO ₂ in the steel, the value of the oxide was calculated as Ti ₂ O ₃ for all of Ti contained in the steel material, What is necessary is just to satisfy the said range.

The steel material of the present invention is a REM oxide and / or CaO and a ZrO ₂ and Ti oxide (Ti ₂ ) when the composition of all oxides contained in the steel material is measured and converted into mass as a single oxide. The total (O ₃ conversion) is preferably 55% or more. If the total is less than 55% by mass, the amount of oxide contributing to the improvement of HAZ toughness is insufficient, and the HAZ toughness cannot be sufficiently improved. More preferably, it is 60 mass% or more, More preferably, it is 65 mass% or more.

The remaining component composition of the oxide is not particularly limited, but may be, for example, SiO ₂ , Al ₂ O ₃ , or MnO when converted as a single oxide. The “other” component other than SiO ₂ , Al ₂ O ₃ and MnO is preferably less than 5% by mass.

The composition of the oxide contained in the steel is determined by observing the cross section of the steel 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, REM (for example, La and Ce) and O, and using a known substance, the relationship between the X-ray intensity and the element concentration of each element is used as a calibration curve in advance. The X-ray intensity obtained from the inclusions to be analyzed and the concentration of elements contained in the inclusions to be analyzed are determined from the calibration curve, and the inclusions having an oxygen content of 5% or more are determined as oxides. To do. 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.

  As described above, in the steel material of the present invention, in order to improve the HAZ toughness, REM and / or Ca and Zr are combined and added to the steel material, and REM and / or Ca and Zr are added to the steel material as a single oxide or Although it adjusts so that it may contain as complex oxide, even if it adjusts the composition of the oxide contained in steel materials, a brittle fracture generating characteristic cannot be improved.

  Therefore, the present inventors have studied in order to improve the brittle fracture occurrence characteristics without deteriorating the HAZ toughness improved by containing a predetermined oxide. When the metal structure of the plane parallel to the surface of the steel material and perpendicular to the steel surface is observed, (1) the ferrite area ratio is 75% or more, and (2) the average equivalent circle diameter of the ferrite grains at the t / 2 position is 20 If the average aspect ratio of ferrite grains at 0.03 μm or less and (3) t / 2 position is 1.6 or less, it is clear that the brittle fracture occurrence characteristics of the steel can be improved and the HAZ toughness is not deteriorated. Became. Hereinafter, the reason for this definition will be described in detail.

  The metal structure of the steel material according to the present invention is mainly composed of ferrite in order to ensure the strength of the steel material. The ferrite main body means that the ferrite fraction in the steel material is 75% by volume or more, and the ferrite area ratio may be 75% or more when the metal structure of the steel material cross section is observed. The area ratio of ferrite is preferably 80% or more, and more preferably 85% or more.

  The remainder of the metal structure is not particularly limited as long as pearlite, bainite, martensite, or the like is generated as the second phase. The area ratio of the second phase may be less than 25%, preferably less than 20%, more preferably less than 15%.

  The metal structure of the steel material is mainly composed of ferrite, and in order to improve CTOD characteristics, it is important to appropriately adjust both the equivalent circle diameter and the aspect ratio of the ferrite grains. That is, as a result of repeating various experiments by the present inventors, the average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less, and the average aspect ratio of the ferrite grains at the t / 2 position is 1.6 or less. Need to be.

This is clear from the examples described later, and FIG. 1 shows the relationship between the average equivalent circle diameter, average aspect ratio, and CTOD characteristics of the ferrite grains at the t / 2 position of the steel material. In FIG. 1, the X-axis shows the average equivalent circle diameter of the ferrite grains at the t / 2 position, the Y-axis shows the CTOD characteristics (δc _{-40 ° C.} ), and ◯ shows the average aspect ratio of the ferrite grains at the t / 2 position. 1.6 or less, and ● indicate the results when the average aspect ratio of the ferrite grains at the t / 2 position exceeds 1.6, respectively.

As can be seen from FIG. 1, the smaller the average equivalent circle diameter of the ferrite grains at the t / 2 position, the more the CTOD characteristics tend to be improved (the value of δc _{−40 ° C.} tends to increase). At this time, if the average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less and the average aspect ratio is 1.6 or less, δc _{−40 ° C.} becomes 0.20 mm or more, and the CTOD characteristic is ensured. Can improve.

  The reason is considered as follows. That is, in brittle fracture, the boundary between crystal grains (crystal grain boundary) serves as resistance to crack propagation. Therefore, if the grain boundaries exist densely, brittle fracture itself is less likely to occur, Even if a brittle fracture occurs, the propagation of cracks can be prevented if the grain boundaries are densely present in the direction in which the cracks propagate. However, since the ferrite grains extend in the rolling direction in the rolling process, the aspect ratio of the ferrite grains increases. Therefore, the major axis of the ferrite grains is aligned in the rolling direction, and the minor axis is easily aligned in the plate thickness direction. Therefore, although the crystal grain boundaries are densely present in the plate thickness direction, the crystal grain boundaries in the rolling direction are sparse, so that the density of the crystal grain boundaries is likely to vary, and brittle fracture is likely to occur. Further, once brittle fracture occurs, cracks easily propagate along the grain boundary in the rolling direction. On the other hand, if the average equivalent circle diameter of the ferrite grains is reduced and the average aspect ratio is reduced, there is almost no variation in the density of the crystal grain boundaries, so that brittle fracture hardly occurs. The boundary acts as a resistance and can prevent crack propagation.

  In the present invention, the average equivalent circle diameter of the ferrite grains is 20.0 μm or less and the average aspect ratio of the ferrite grains is 1.6 or less. The position where the average equivalent circle diameter and the average aspect ratio are controlled is the thickness of the steel material. Where t is the t / 2 position. Since it is generally known that brittle fracture occurs near the center of the plate thickness, the occurrence of brittle fracture can be suppressed by appropriately controlling the structure at the t / 2 position.

  As the plate thickness increases, the difference between the temperature at the t / 2 position of the steel material and the strain introduced at the t / 2 position and the temperature near the surface of the steel material (for example, the t / 4 position) and the introduced strain increase. By controlling the temperature at the t / 2 position and appropriately controlling the structure at the t / 2 position, the brittle fracture crack characteristics can be improved.

  The average equivalent circle diameter of the ferrite grains at the t / 2 position is preferably 17.5 μm or less, and more preferably 16 μm or less. The lower limit of the average equivalent circle diameter of the ferrite grains is not particularly specified and is preferably as small as possible, but is usually about 7 μm or more (particularly 10 μm or more) because there is a limit to the reduction. The equivalent circle diameter means the diameter of a circle when ferrite grains are converted into a circle having the same area.

  On the other hand, the average aspect ratio of the ferrite grains at the t / 2 position is preferably 1.5 or less, and more preferably 1.4 or less. The aspect ratio of the ferrite grains means the ratio (Dl / Dt) of the grain diameter (Dl) in the rolling direction of the ferrite grains and the grain diameter (Dt) in the plate thickness direction.

  The average equivalent circle diameter and average aspect ratio of the ferrite grains can be calculated, for example, by the following procedure. First, a sample is cut out so that a surface that includes the front and back surfaces of the steel material is parallel to the rolling direction and is perpendicular to the steel surface (the front surface of the steel material), and this exposed surface is polished. And mirror finish.

  The method for polishing the exposed surface is not particularly limited. For example, polishing may be performed using a wet emery polishing paper of # 150 to # 1000 or a polishing method having an equivalent function. In addition, when performing mirror finish, an abrasive such as diamond slurry may be used.

  The mirror-finished sample is corroded using a 3% nital solution to reveal the grain boundary of the ferrite structure, and then photographed at a magnification of 100 or 400 and taken into an image analyzer. The image is captured so that the area corresponds to 1 mm × 1 mm or more at any magnification.

  Next, in the image analysis device, the ferrite grain region (area) surrounded by the grain boundary is converted into a circle having an equivalent area, and the diameter of the converted circle is defined as the equivalent circle diameter of the ferrite grain. Measure the equivalent diameter. This is measured for all observation visual fields, and the average is calculated by averaging the results.

  On the other hand, as for the aspect ratio of the ferrite grains, for the ferrite grains surrounded by the grain boundaries, the grain size Dl in the rolling direction and the grain size Dt in the plate thickness direction are measured, and the ratio of Dl to Dt (Dl / Dt) is determined. Calculate as aspect ratio. This is performed for all observation fields, and the average aspect ratio is calculated by averaging the results.

  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 0.02% and Zr: 0.001 to 0.05%. The reasons for setting these ranges are as follows.

REM, Ca, and Zr are REM single oxide, Ca single oxide (CaO), Zr single oxide (ZrO ₂ ), or a composite oxide of REM and / or Ca and Zr. It is an element that forms and contributes to the improvement of HAZ toughness. 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 it is added excessively, sulfide of REM is generated and the base material toughness 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 base metal toughness deteriorates, so it should be suppressed to 0.02% or less. Preferably it is 0.015% or less, More preferably, you may be 0.01% 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 0.12%, Si: 0.5% or less (not including 0%), Mn: It includes 1.4 to 1.8% and N: 0.003 to 0.01%. 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, leading to deterioration of the HAZ toughness, and also adversely affecting the weldability. Therefore, C must be suppressed to 0.12% or less, preferably 0.11% or less, more preferably 0.10% or less.

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

  Mn is an element that contributes to improving the strength of the steel material (base material), and in order to exert such effects effectively, it is necessary to contain 1.4% or more. Preferably it is 1.45% or more, more preferably 1.50% or more. However, if the content exceeds 1.8%, the HAZ toughness deteriorates, so the Mn content is 1.8% or less. Preferably it is 1.75% or less, More preferably, it is 1.70% or less.

  N is an element that precipitates nitrides (for example, ZrN and TiN). The nitrides prevent austenite grains formed in the HAZ during welding and promote ferrite transformation, thereby improving HAZ toughness. Contributes to In order to exhibit such an effect effectively, it is contained 0.003% or more. Preferably it is 0.004% 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 base material toughness 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 generate sulfide (MnS) and degrades the base material toughness and ductility in the thickness direction. S combines with La and Ce to form LaS and CeS, and inhibits the formation of REM 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, Ti can be further actively contained.

Ti: 0.08% or less (excluding 0%)
Ti is an element that contributes to the improvement of HAZ toughness by generating an oxide of Ti 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: 3.5% or less (not including 0%), Cr: 3% or less (0% Mo: 1% or less (not including 0%), Nb: 0.25% 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 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 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 3.5% or less from an economical viewpoint. More preferably, it is 3.3% or less, More preferably, it is 3% or less.

  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 3%, weldability deteriorates, so Cr is preferably suppressed to 3% or less. More preferably, it is 1.5% or less, More preferably, it is 1% or less.

  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%, weldability is deteriorated, so Mo is preferably 1% or less. More preferably, it is 0.9% or less, and more preferably 0.8% or less.

  In order to increase the strength by adding Nb, it is preferable to contain 0.005% or more. More preferably, it is 0.01% or more, More preferably, it is 0.03% or more. However, if it exceeds 0.25%, carbide (NbC) precipitates and the base material toughness deteriorates, so Nb is preferably suppressed to 0.25% or less. More preferably, it is 0.23% or less, and still more preferably 0.20% or less.

  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 base material toughness 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 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.

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

  At this time, the amount of dissolved oxygen before the combined addition of the above elements is preferably adjusted to 0.0020% or more. 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 that the oxidation contributes to the improvement of HAZ toughness. This is because REM or Ca, which cannot ensure the amount of material and cannot form an oxide, forms a sulfide, or Zr forms a carbide to deteriorate the base material toughness. The dissolved oxygen content is preferably adjusted to 0.0025% or more, 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. 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) component adjustment of the steel material In addition, Ti may be added together, or (b) after adjusting the components of the steel material, Ti may be added. 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.

When Ti is contained, 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 in which the amount of dissolved oxygen is adjusted, Ti ₂ O ₃ is first formed, but since Ti ₂ O ₃ has a small interface 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 the coarsening of austenite grains in the HAZ during welding 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 deoxidation 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 deoxidizing 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.

  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- REM alloys such as La alloy, Fe-Si-Ce alloy, Ca alloys such as Fe-Si-Ca alloy, Fe-Ca alloy, Ni-Ca alloy, REM-Ca alloys such as Fe-Si-La-Ce alloy, etc. 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.

  On the other hand, the metal structure of the steel material is mainly ferrite, the average equivalent circle diameter of ferrite grains at the t / 2 position is 20.0 μm or less, and the average aspect ratio of ferrite grains at the t / 2 position is 1.6 or less. The slab obtained by casting may be heated to 1000 to 1200 ° C., then roughly rolled, and then finish rolled in the austenite non-recrystallization temperature range. In the following, description will be given in order.

  It is preferable that the temperature which heats a slab shall be 1000-1200 degreeC. This is to reversely transform austenite in order to refine the ferrite structure obtained after rough rolling and subsequent cooling (natural cooling or forced water cooling). That is, normally, the ferrite is transformed back to austenite by heating to about 900 ° C., but it is effective to roll and recrystallize the austenite structure in order to refine the ferrite structure after rolling. Therefore, although the lower limit of the recrystallization temperature of austenite depends on the chemical composition of the steel material, it is usually 850 to 900 ° C. Therefore, in order to roll and recrystallize the austenite structure above this lower temperature, the heating temperature is 1000 ° C. It is good to be the above. Preferably it shall be 1050 degreeC or more. In addition, the said heating temperature is good to manage the average temperature (calculated value) in the plate | board thickness direction of steel materials using a process computer, and to manage by this average temperature.

  However, if the temperature exceeds 1200 ° C., the initial austenite structure becomes too coarse, and it is difficult to sufficiently refine the austenite structure even if the austenite structure is rolled and recrystallized. Also, heating at a high temperature is uneconomical in terms of energy. Therefore, the heating temperature is preferably 1200 ° C. or lower. More preferably, it shall be 1100 degrees C or less.

  The heated slab may be roughly rolled at a cumulative reduction ratio of 40% or more in the austenite recrystallization temperature range. This is because the austenite structure can be recrystallized by rough rolling at a cumulative reduction ratio of 40% or more in the austenite recrystallization temperature region, and the ferrite structure after rolling can be refined. That is, even if the cumulative reduction ratio in the recrystallization temperature range of austenite is less than 40%, the ferrite grains can be refined by increasing the cumulative reduction ratio in the austenite non-recrystallization temperature range described later. However, if the ferrite grains are already coarsened at the time of starting rolling in the austenite non-recrystallization temperature range, the metal structure finally obtained is coarse even if appropriately rolled in the austenite non-recrystallization temperature range. This is because it tends to be in a mixed state in which ferrite grains and fine ferrite grains are mixed. Thus, when the metal structure is in a mixed grain state, the CTOD characteristics tend to be difficult to stabilize. Therefore, in order to sufficiently refine the austenite structure in the austenite recrystallization temperature range, it is recommended that the cumulative rolling reduction in the austenite recrystallization temperature range be 40% or more. Preferably it is 45% or more, more preferably 50% or more.

  The cumulative rolling reduction is preferably as large as possible, and as the cumulative rolling reduction increases, the equivalent circle diameter of the ferrite grains can be reduced to about 25 to 30 μm. However, even if the cumulative reduction rate in the recrystallization temperature range of austenite exceeds 70%, the effect is almost saturated, so the cumulative reduction rate may be about 70% or less.

  The temperature range at which the rough rolling is performed is the austenite recrystallization temperature range, but this temperature range varies somewhat depending on the chemical composition of the steel material. However, since the lower limit of the recrystallization temperature of austenite is usually about 850 to 900 ° C., the cumulative rolling reduction in the temperature range of 900 ° C. or higher may be adjusted to the above range. However, if the rolling temperature range is too high, the austenite grains grow faster following the recrystallization after rolling, so that it may not be possible to effectively reduce the size. Therefore, the rolling start temperature is preferably 1000 ° C. or less.

The cumulative reduction rate, t ₀ the thickness at the temperature (calculated value) is 1000 ° C. at t / 2 position of the steel material, the thickness at a temperature at t / 2 position of the steel material (calculated value) of 900 ° C. t _{When set to 1} , it can be calculated by the following equation (a).
Cumulative rolling reduction (%) = [(t ₀ −t ₁ ) / t ₀ ] × 100 (a)

However, when the rough rolling start temperature is lower than 1000 ° C., the steel thickness at the start of the rough rolling is t _0, and when the rough rolling start temperature exceeds 1000 ° C., the temperature at the t / 2 position of the steel is 1000. The cumulative rolling reduction is calculated with the steel material thickness at ₀ ° C. as t ₀ . On the other hand, when the rough rolling end temperature does not reach 900 ° C. (when it exceeds 900 ° C.), the steel material thickness at the end of rough rolling is t _1, and when the rough rolling end temperature is lower than 900 ° C., 900 ° C. The cumulative rolling reduction is calculated with the steel material thickness at ₁ as t ₁ .

  The temperature at the time of rough rolling is preferably based on the temperature calculated by calculating the temperature at the t / 2 position using a process computer. This is for appropriately controlling the metal structure at the t / 2 position. In addition, compared with the temperature of t / 2 position (calculated value), the temperature of the steel sheet surface (actually measured value) is lower by about 50 to 70 ° C. when the steel material thickness is 150 mm, and when the steel material thickness is 100 mm. Is about 40-50 ° C. lower. Therefore, the temperature at which the rough rolling is performed may be controlled using the temperature of the steel sheet surface (actually measured value) as a reference in consideration of such a temperature difference.

  After rough rolling with a cumulative reduction ratio of 40% or more in the recrystallization temperature range of austenite, cooling to the austenite non-recrystallization temperature range, and finish rolling to a true strain of 0.5 or more in the austenite non-recrystallization temperature range. It is recommended. This is because the ferrite grains can be further refined by finish rolling in the austenite non-recrystallization temperature range. That is, since the metal structure obtained by rolling in the austenite recrystallization temperature range is an austenite structure having an average particle size of about 25 to 30 μm, ferrite grains obtained by air cooling the steel material as it is or by forced cooling. The average equivalent-circle particle diameter of is no more than about 25 μm. Therefore, CTOD characteristics cannot be improved sufficiently. On the other hand, if the finish rolling is performed in the austenite non-recrystallization temperature range, strain is introduced into the ferrite grains, so that the ferrite grains can be further refined.

  In this finish rolling, the true strain is preferably 0.5 or more. If the true strain amount is less than 0.5, the ferrite grains may be insufficiently refined, and the CTOD characteristics may not be sufficiently improved. The larger the true strain amount, the better. The larger the amount, the smaller the ferrite grains.

The austenite non-recrystallization temperature range is a temperature range where the austenite structure is not recrystallized even when the steel material is rolled. Although this temperature range varies somewhat depending on the chemical composition of the steel material, in the present invention, finish rolling is performed by setting the true strain amount to be 0.5 or more in the region where the temperature at the t / 2 position of the steel material is 850 ° C. or less. However, if the temperature range of finish rolling becomes too low, the flatness (ie, aspect ratio) of the ferrite grains tends to be extremely large, and the CTOD characteristics tend to deteriorate. Therefore, the finish rolling end temperature is preferably “Ar ₃ transformation point + 10 ° C.” or higher. The temperature of the Ar ₃ transformation point can be calculated by the following equation (b) based on the content of chemical components contained in the steel material. However, [] has shown content (mass%) of each element.
Ar ₃ transformation point (° C.) = 868−369 × [C] + 24.6 × [Si] −68.1 × [Mn] −36.1 × [Ni] −20.7 × [Cu] −24.8 × [Cr] + 190 × [V] (b)

The true strain amount is, t ₂ thickness when the temperature (calculated) of 850 ° C. at t / 2 position of the steel material, the thickness of the temperature (calculated value) of the finish rolling end temperature of t / 2 position of the steel material when the t _3, can be calculated by the following equation (c).
True strain = ln (t ₂ / t ₃ ) (c)

However, when the finish rolling start temperature is less than 850 ° C., the thickness of the steel material at the start of finish rolling is t _2, and when the finish rolling start temperature exceeds 850 ° C., the temperature at the t / 2 position of the steel material is 850. The true strain is calculated with the steel material thickness at ° C. being t ₂ . On the other hand, when the finish rolling end temperature is not reached "Ar ₃ transformation point + 10 ° C." (if more than "Ar ₃ transformation point + 10 ° C."), the steel thickness during the finish rolling end and t _3, the finish rolling end If the temperature falls below "Ar ₃ transformation point + 10 ° C." calculates the true strain steel thickness in the "Ar ₃ transformation point + 10 ° C." as t _3.

  The temperature at which the finish rolling is performed is based on the temperature calculated by calculating the temperature at the t / 2 position using a process computer.

The temperature at the time of finish rolling is preferably based on the temperature calculated by calculating the temperature at the t / 2 position using a process computer when the thickness of the steel material is t (mm). This is for appropriately controlling the metal structure at the t / 2 position. When the thickness of the steel material is about 40 to 80 mm, the temperature difference between the temperature inside the steel plate (temperature at the t / 2 position) and the surface temperature of the steel plate is at most about 10 to 40 ° C. In view of the above, the surface temperature (actually measured value) of the steel sheet may be used as a reference (for example, “850 ° C.−temperature difference”, “Ar ₃ transformation point + 10 ° C.−temperature difference”).

  What is necessary is just to cool in accordance with a conventional method after completion | finish of finish rolling. The cooling method is not particularly limited, and air cooling or forced cooling may be used. The cooling rate at this time is not particularly limited, but the present inventors have confirmed that the size of the ferrite grains is not affected as long as it is about 4 ° C./second or less.

  The steel material of the present invention thus obtained can be used, for example, as a material for structures such as bridges, high-rise buildings, ships, and the like in small to medium heat input welding as well as large heat input welding (for example, 40 kJ / mm or more), It is possible to prevent toughness deterioration of the weld heat affected zone and to have excellent brittle fracture crack characteristics.

  The plate thickness of the steel material of the present invention is not particularly limited, but even if the final product thickness is 40 mm or more (especially 50 mm or more), the steel material of the present invention can prevent toughness deterioration of the weld heat affected zone and has brittle fracture crack characteristics. Will also be excellent. The upper limit of the plate thickness is about 80 mm, for example.

  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.

Example 1
After the hot metal was first refined in a 240-ton converter, the steel was removed from the converter into a ladle and subjected to secondary refining while adjusting the components and adjusting the temperature.

  In the ladle, the amount of dissolved oxygen shown in Table 1 was adjusted by the deoxidation method shown in Table 1 below. Thereafter, the elements were added in the order shown in Table 1 below. 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-Ca. Zr was added in the form of an alloy, Ca—Si alloy, or Fe—Ca green compact, Zr alone, and Ti was added in the form of an Fe—Ti alloy. In Table 1, No. A dissolved oxygen amount “−” of 16 indicates that it is less than the limit of quantification.

  Subsequently, the remaining alloying elements were added as necessary to finally adjust to the composition shown in Table 2 below (the balance being iron and inevitable impurities). In the secondary refining, dehydrogenation and desulfurization were performed using an RH type degassing refining apparatus. In Table 2, “-” indicates that no element was added, and “less than” did not add the element but was inevitably contained, so it was detected but was in the range below the limit of quantification. It means that.

  The component-adjusted molten steel was cast into a slab with a continuous casting machine, and the resulting slab was heated to the temperature shown in Table 3 below and then roughly rolled. The heating temperature shown in Table 3 is an actual measurement value (surface temperature) measured using a radiation thermometer installed on the rolling line. Table 3 shows the steel thickness (mm) when the temperature (calculated value) at the t / 2 position of the steel is 1000 ° C. and the steel thickness (mm) when the temperature is 900 ° C., respectively, and 1000 to 900 ° C. The cumulative rolling reduction in the range is calculated using the above equation (a), and the results are shown in Table 3.

After rough rolling, finish rolling was performed to produce a hot-rolled steel sheet having a thickness (product thickness) shown in Table 3. Table 3 shows the finish rolling end temperature (° C.). In addition, the Ar ₃ transformation point temperature calculated from Table 2 and the above equation (b) and the value of “Ar ₃ transformation point temperature + 10 ° C.” are also shown.

The temperature (calculated value) at the t / 2 position is obtained in the following manners (1) to (6).
(1) Using a process computer, the heating temperature at an arbitrary position in the thickness direction from the front surface to the back surface of the steel material is calculated based on the atmospheric temperature from the start of heating to the end of heating and the in-furnace time.
(2) Using the calculated heating temperature, calculate the rolling temperature at any position in the plate thickness direction based on the rolling pass schedule during rolling and the cooling method between the passes (water cooling or air cooling). Roll while calculating using a suitable method.
(3) The steel sheet surface temperature is measured using a radiation thermometer installed on the rolling line (however, it is also calculated on the 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 obtain the finish rolling end temperature at the t / 2 position.

  Using the obtained steel sheet, the investigation of the inclusion composition by EPMA, the evaluation of HAZ toughness, the observation of the metal structure, and the evaluation of the brittle fracture crack generation characteristics were carried out as follows.

<Investigation of inclusion composition>
A sample was cut out from the cross section at the t / 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. Note that the oxide of the oxide and REM of Ti, expressed a metal element in M, is present in the form of M ₂ O ₃ and M ₃ O _5, MO ₂ in the steel material, these oxides M ₂ The oxide composition was calculated in terms of O ₃ . When a plurality of elements were observed from one inclusion, the composition of the oxide was calculated in terms of a single oxide of each element from the ratio of X-ray intensity indicating the presence of these elements. Note that “others” in Table 4 is the total amount of oxides of elements not analyzed (for example, MgO).

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

<Evaluation of HAZ toughness>
Next, in order to evaluate the toughness of the HAZ that is affected by heat during welding, a welding reproduction test was performed by simulating large heat input welding (corresponding to a heat input of 40 to 60 kJ / mm). The welding reproduction test was performed by heating so that the entire steel material was 1400 ° C., holding at this temperature for 40 to 60 seconds, and then cooling. The cooling rate was adjusted so that the cooling time (Tc) from 800 ° C. to 500 ° C. was 400 seconds. Table 5 below shows thermal cycle conditions in the welding reproduction test.

About the steel material after cooling, three V notch Charpy test pieces prescribed | regulated by JISZ2242 (2006) were extract | collected from the site | part centering on the 7-mm position from the back surface of a plate | board thickness direction. Next, a Charpy impact test was performed at −40 ° C. using the test piece, the absorbed energy (vE ₋₄₀ ) was measured, and the average value of the three test pieces was obtained. Those having a vE- ₄₀ of 100 J or more were evaluated as having excellent HAZ toughness. The measurement results are shown in Table 5 below.

<Observation of metal structure (measurement procedure of equivalent circle diameter and aspect ratio)>
A sample is cut out so that a surface including the front surface and the back surface of the steel plate is parallel to the rolling direction and perpendicular to the steel surface (the front surface of the steel material), and the exposed surface is polished. Mirror finish. The exposed surface was polished using # 150 to # 1000 wet emery polishing paper and then mirror-finished using diamond slurry as an abrasive.

  The mirror-finished sample was corroded using a 3% nital solution to reveal the grain boundary of the ferrite structure, and then photographed at a magnification of 400 × to obtain a photograph of 6 cm × 8 cm (that is, 150 μm × at 400 ×) Equivalent to 200 μm). The 6 cm side of the photo corresponds to the plate thickness direction, and the 8 cm side corresponds to the rolling direction. This was taken into the image analysis apparatus so that the area corresponds to 1 mm × 1 mm or more at any magnification.

  Next, in the image analysis device, the ferrite grain region (area) surrounded by the grain boundary is converted into a circle having an equivalent area, and the diameter of the converted circle is defined as the equivalent circle diameter of the ferrite grain. The equivalent diameter was measured. This was measured for all observation visual fields, and the average equivalent circle diameter was calculated by averaging the results.

  On the other hand, as for the aspect ratio of the ferrite grains, for the ferrite grains surrounded by the grain boundaries, the grain size Dl in the rolling direction and the grain size Dt in the plate thickness direction are measured, and the ratio of Dl to Dt (Dl / Dt) is determined. Calculated as aspect ratio. This was performed for all observation visual fields, and the average aspect ratio was calculated by averaging the results.

  In addition, the measurement position of the circle equivalent diameter and aspect ratio of the ferrite grain was set to the t / 2 position when the thickness of the steel material was t (mm). The number of observation fields was 35.

  When calculating the average equivalent circle diameter and aspect ratio of the ferrite grains, the ferrite area ratio in the metal structure was also measured. The results are also shown in Table 5 below.

<Evaluation of brittle fracture crack initiation characteristics>
The brittle fracture initiation characteristics are determined based on the crack tip opening displacement test based on the “crack tip opening displacement test (CTOD test)” defined by WES1108 (established on February 1, 1995) issued by the Japan Welding Association (WES). The opening displacement (δc) at the start of unstable fracture was measured and evaluated based on this result. In conducting the crack tip opening displacement test, the “Guidelines for the Weld Heat Affected Zone CTOD Test Method” defined by WES1109 (established on April 1, 1995) was also taken into consideration.

The test piece is a P.S. of WES1108 (established on February 1, 1995). 6 “standard three-point bending test piece” shown in FIG. 6 was used. The test temperature was −40 ° C., and δc _{−40 ° C.} (mm) was measured. In this invention, the case where (delta) c- ₄₀ degreeC is 0.20 mm or more is set as a pass. The results of the CTOD test are shown in Table 5 below.

It can consider as follows from Tables 1-5. No. Since Nos. 1 to 14 contain REM oxide and / or CaO and ZrO ₂ as oxides in the steel material, steel materials having good toughness in the weld heat affected zone are obtained. Moreover, since the metal structure is also appropriately controlled, it is possible to prevent the occurrence of brittle fracture.

On the other hand, no. Since Nos. 15 to 22 do not contain any one of REM oxide and / or CaO and ZrO ₂ as oxides in the steel material, the toughness of the weld heat affected zone is inferior. No. In No. 23, Mn, P and Al are excessive and the amount of dissolved oxygen is small, so that the amount of oxide produced is small and the HAZ toughness is poor.

Example 2
No. in Example 1 above. In Example 8, a steel plate having a plate thickness (product thickness) of 65 mm was manufactured by appropriately changing the conditions for hot rolling the slab obtained by the continuous casting machine to the conditions shown in Table 6 below.

  Using the obtained steel plate, the metal structure was observed and the brittle fracture crack generation characteristics were evaluated in the same manner as in Example 1. The results are shown in Table 6 below. FIG. 1 shows the relationship between the average equivalent-circle diameter, average aspect ratio, and CTOD characteristics of ferrite grains at the t / 2 position of the steel material.

  It can be considered as follows from Table 6 and FIG. No. 31, 34, 35, 37, 39, 40 and 42 to 45 can prevent brittle fracture from occurring because the metal structure is appropriately controlled. On the other hand, no. Nos. 32, 33, 36, 38, and 41 can prevent brittle fracture from occurring because the average equivalent circle diameter or average aspect ratio of the ferrite grains at the t / 2 position does not satisfy the requirements defined in the present invention. Absent.

FIG. 1 is a graph showing the relationship between the average equivalent circle diameter, average aspect ratio, and CTOD characteristics of ferrite grains at the t / 2 position of a steel material.

Claims (8)

C: 0.03 to 0.12% (meaning “mass%”; the same shall apply hereinafter)
Si: 0.5% or less (excluding 0%),
Mn: 1.4-1.8%, and N: 0.003-0.01%,
P: 0.02% or less (excluding 0%),
S: satisfying 0.015% or less (excluding 0%) and Al: 0.01% or less (not including 0%),
Furthermore, REM: 0.001-0.1% and / or Ca: 0.0003-0.02%,
Zr: 0.001 to 0.05% each contained,
The balance is steel consisting of iron and inevitable impurities,
REM and / or Ca and Zr were contained as a single oxide or a composite oxide, and the metal structure of a plane parallel to the rolling direction of the steel material having a thickness t (mm) and perpendicular to the steel material surface was observed. A steel material excellent in toughness and brittle fracture occurrence characteristics of a weld heat-affected zone characterized by satisfying the following (1) to (3).
(1) The ferrite area ratio is 75% or more.
(2) The average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less.
(3) The average aspect ratio of the ferrite grains at the t / 2 position is 1.6 or less. When the composition of all oxides contained in the steel material is measured and converted to mass as a single oxide, the total of REM oxides and / or CaO is 5% or more and ZrO ₂ satisfies 5% or more The steel material according to claim 1.   The steel material further contains Ti: 0.08% or less (not including 0%) as another element and contains the Ti as a single oxide or a composite oxide. Steel materials described.   The steel material according to claim 3, wherein when the composition of all oxides contained in the steel material is measured and converted to mass as a single oxide, the oxide of Ti satisfies 0.3% or more. The steel material is still another element,
Cu: 2% or less (excluding 0%),
Ni: 3.5% or less (excluding 0%),
Cr: 3% or less (excluding 0%),
Mo: 1% or less (excluding 0%),
Nb: 0.25% 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:   It is a method of manufacturing the steel material according to any one of claims 1 to 5, wherein at least selected from the group consisting of REM and Ca to a molten steel whose dissolved oxygen amount is adjusted to a range of 0.0020 to 0.010%. A method for producing a steel material excellent in toughness and brittle fracture occurrence characteristics of a weld heat-affected zone, characterized by adding one element and Zr.   It is a method of manufacturing the steel material according to any one of claims 3 to 5, wherein at least selected from the group consisting of REM and Ca to a molten steel whose dissolved oxygen content is adjusted to a range of 0.0020 to 0.010%. A method for producing a steel material excellent in toughness and brittle fracture occurrence characteristics of a weld heat-affected zone, characterized by adding one element 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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