KR101197872B1 - Steel materials excellent in toughness in weldheat-affected zone and fatigue characteristics of base material, and manufacturing method of the same - Google Patents

Abstract

The steel material of the present invention comprises (a) an oxide containing Zr, REM and Ca, and (b) an oxide of ZrO ₂ : 5 to 50% and REM when the composition of all the oxides is measured and converted into a single oxide: 10 to 50%, and CaO: 5.0 to 50%, (c) 120 or more oxides per 1 mm ² with 0.1-2.0 μm in equivalent circle diameter, and 1 mm ² per oxide with more than 5.0 μm in circle equivalent diameter (D) When the metallographic structure was observed by the EBSP method, the average circle equivalent diameter D of the crystal grains enclosed by the diagonal grain boundary whose crystal orientation difference was 15 degrees or more was 25 µm or less, and the random grain boundary occupied at the diagonal grain boundary. The ratio R is 70 area% or less, and (e) average hardness is 170 Hv or more. The steel material of the present invention is excellent in HAZ toughness even when high heat input welding with a heat input amount of 50 kJ / mm or more is performed, and furthermore, fatigue characteristics of the base material itself are improved.

Description

STEEL MATERIALS EXCELLENT IN TOUGHNESS IN WELDHEAT-AFFECTED ZONE AND FATIGUE CHARACTERISTICS OF BASE MATERIAL, AND MANUFACTURING METHOD OF THE SAME}

The present invention relates to steel materials used in bridges, high-rise buildings, ships, and the like, particularly the toughness of the site affected by heat when welded (hereinafter sometimes referred to as "welding heat affected zone" or "HAZ"), The present invention relates to a steel having excellent fatigue properties of the steel itself and a manufacturing method thereof.

The characteristics required for steel materials used in bridges, high-rise buildings, ships, and the like have become increasingly strict in recent years, and particularly good toughness is required. Although such steel materials are often welded and joined, there is a problem that the weld joints, in particular HAZ, are easily affected by toughness due to heat effects during welding. This toughness deterioration is more pronounced as the amount of heat input during welding increases. The reason for this is that as the amount of heat input during welding increases, the cooling rate of HAZ decreases, the hardenability decreases, and the coarse island martensite is reduced. It is thought to be in generating. Therefore, in order to improve the toughness of HAZ, it is considered that heat input during welding may be suppressed as much as possible. On the other hand, however, in order to improve the welding work efficiency, it is desired to employ a high heat input welding method having a welding heat input amount of 50 kJ / mm or more, for example, electro gas welding, electro slag welding, and submerged welding.

Therefore, the present applicant has disclosed a steel material which suppresses the deterioration of the HAZ toughness when the high heat input welding method is adopted, Japanese Patent Laid-Open No. 2007-100213, Japanese Patent Laid-Open No. 2007-247004, and Japanese Patent Laid-Open No. 2007-247005 Proposed in the issue. Such steels are characterized by containing oxides and / or CaO of REM and ZrO ₂ as oxides. Since the said oxide exists in a liquid phase in molten steel, it is disperse | distributed finely in steel. Moreover, since the oxide is thermally stable and does not dissolve even after being exposed to high temperature of 1400 ° C. for a long time, it contributes greatly to the improvement of HAZ toughness.

By the way, structures such as bridges, high-rise buildings, ships, and the like are subject to cyclic stress and are destroyed by fatigue. Therefore, it is desired that steel materials used as raw materials have good fatigue characteristics from the viewpoint of ensuring the safety of the structure. The fatigue process of steel is considered to be roughly divided into two processes, the generation of a crack in a stress concentration part and the generated crack advancing in the steel material.

Regarding the occurrence of cracks, fatigue cracks in structures are often generated from the start end of the weld joint, and therefore, in order to suppress the occurrence of the fatigue crack, improving the start shape of the weld joint has been studied. .

On the other hand, the fatigue crack generated in the steel advances in the steel, but when the crack develops, the steel itself is finally broken, so that the generated crack stops rapidly, or the crack progresses. It is desired to make it as small as possible to make the growth of the crack difficult. Techniques for reducing the growth rate of fatigue cracks have been proposed in the following three documents. Nakaji Maki-Yodaka et al., `` Influence of Hard Second Phase on Fatigue Crack Propagation Characteristics and Weld Joint Fatigue Characteristics of Thick Steel Sheets '', Welded Structure Symposium 2004 Proceedings, 2004, p. The technique which makes a mixed structure of a layer (ferrite) and a hard layer (martensite), flattens a hard layer (martensite), bypasses a fatigue crack, and reduces the growth rate of a fatigue crack. Honda-no-boru et al., "A New Functional Thick Steel Plate for Self-Inhibiting Fatigue Crack Growth and Its Seam Characteristics," Journal of Welding Vol. 74, No. 4, 2005, p. 25, Nikisato-shi et al., JFE Gazette No. 5, JFE Steel, August 2004, p. 13, "High Performance Steels for Shipbuilding-JFE Steel", Bypassing Fatigue Cracks by Dispersing Hard Layers in Soft Layers (Ferrite) A technique for reducing the growth rate of fatigue cracks is disclosed. Here, in the former, bainite is produced as a hard layer, and in the latter, pearlite is produced as a hard layer.

This invention is made | formed in view of such a situation, The objective is to provide the steel material which was excellent in HAZ toughness and improved the fatigue characteristic of the base material itself even when the high heat input welding of a heat input amount of 50 kJ / mm or more was performed. Another object of the present invention is to provide a method for producing the steel.

The steel material which concerns on this invention which could solve the said subject is C: 0.02-0.12% (The meaning of "mass%. Hereinafter, the same with respect to a chemical component and an oxide.), Si: 0.02-0.5%, Mn: 1-2 %, Zr: 0.0002 to 0.050%, REM: 0.0002 to 0.050%, Ca: 0.0005 to 0.010%, Ti: 0.005 to 0.02%, N: 0.0040 to 0.01%, O: 0.0005 to 0.010%, P: 0.02% or less, S: 0.015% or less, Al: 0.05% or less, the remainder being iron and inevitable impurities,

(a) the steel material comprises an oxide containing Zr, REM and Ca,

(b) the mass ratio of ZrO ₂ in the average composition of the oxide contained in the steel is 5-50%, the mass ratio of the oxide of REM (M ₂ O _{3 when} REM is represented by the symbol M) is 10-50%, And the mass ratio of CaO is 5.0 to 50%,

(c) to the total of oxides, to the circle-equivalent diameter of 0.1 to 2.0㎛ of this oxide exceeds the 5.0㎛ oxide to 1mm more than 120 per ^second, the equivalent circle diameter of not more than 5 per 1mm ² contained in the steel material,

(d) When the metal structure of the steel is observed by the backscattered electron diffraction method (EBSP method), the following equations (1) and (2) are satisfied,

(e) The average hardness of the steel is 170 Hv or more.

[Equation 1]

D ≤ 25 μm

&Quot; (2) "

R ≤ 70 area%

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

In addition, in Equation 2, R denotes the ratio (area%) of the random grain boundary to the diagonal grain boundary.]

The steel is further, as other elements,

(1) at least one element selected from the group consisting of Ni: 0.4% or less, Cu: 0.3% or less, Cr: 1.5% or less, and Mo: 1% or less,

(2) It may contain one or more elements selected from the group consisting of Nb: 0.1% or less, V: 0.1% or less, and B: 0.005% or less.

In the steel material of the present invention, after adjusting the dissolved oxygen amount of molten steel in the range of 0.0010 to 0.0060%, and then adjusting the total oxygen amount in the range of 0.0010 to 0.0070% by stirring the molten steel to separate the oxides in the molten steel, Ti, Subsequently, Zr, REM, and Ca were added to adjust the composition of the composition, followed by casting. When hot rolling, the steel pieces were heated to 1100 to 1250 ° C, and then 1050 ° C or less and Ar ₃ point + 100 ° C. The first rolling is performed so that the cumulative reduction ratio is 40% or more while cooling so that the average cooling rate per one pass is 1.5 ° C / sec or more in the temperature range, and then the Ar ₃ point + 100 ° C. or less and the Ar ₃ point are exceeded. After performing the second rolling so that the maximum reduction rate per pass in the temperature range is 12% or less and the cumulative reduction ratio is 50% or more, the average cooling rate is 5 ° C / sec or more to the temperature range where the surface temperature becomes 500 ° C or less. Manufactured by cooling Can.

After adding Zr, REM and Ca, it is preferable to perform casting after stirring molten steel in the range which does not exceed 40 minutes. It is preferable to perform the said 1st rolling in the austenite recrystallization temperature range. After cooling at an average cooling rate of 5 ° C / sec or more to a temperature range where the surface temperature of the steel material after the second rolling is 500 ° C or less, tempering treatment may be performed at a temperature range of 500 ° C or more and less than Ac ₁ point. It may be.

According to the present invention, while a predetermined amount of oxides (oxides containing Zr, REM and Ca), which are nuclei of ferrite transformation in the particles, is produced, the size and number (particle size distribution) of oxides present in the steel are also properly controlled. Therefore, the steel material excellent in HAZ toughness at the time of high heat input welding can be provided. In particular, the steel material of the present invention has a coarse oxide having a circle equivalent diameter of more than 5.0 μm, which has been found to not only have a predetermined amount of fine oxide having a circle equivalent diameter of 0.1 to 2 μm useful for improving HAZ toughness, but also adversely affect HAZ toughness improvement. Since the number of oxides is considerably suppressed, the HAZ toughness can be improved even if welding is performed at a heat input larger than the HAZ toughness evaluation method disclosed in the examples of Japanese Patent Laid-Open No. 2007-100213.

According to the present invention, when the metal structure is observed by the EBSP method, the average circle equivalent diameter D of crystal grains surrounded by diagonal grain boundaries (hereinafter sometimes referred to simply as "diagonal grain boundaries") whose crystal orientation difference is 15 ° or more is determined. Since the ratio R of the random grain boundary occupies 25 micrometers or less and diagonal grain boundary is controlled to 70% or less, and the average hardness of a base material is controlled to 170 Hv or more, the fatigue characteristic of a base material itself can also be improved.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic explanatory drawing which shows the shape of the compact test piece (CT test piece) used in the Example.
2 is a graph showing the relationship between the average circle equivalent diameter D and the impact characteristic (vE- ₆₀ ) of crystal grains surrounded by diagonal grain boundaries.
3 is a graph showing the relationship between the ratio R of the random grain boundaries at the diagonal grain boundaries and the fatigue crack growth rate (da / dN).
4 is a graph showing the relationship between the average hardness of the rolled material and the fatigue crack growth rate (da / dN).
5 is a graph showing the relationship between the cumulative reduction ratio at the Ar ₃ point + 100 ° C. or lower and the average circle equivalent diameter D of crystal grains surrounded by diagonal grain boundaries.
Fig. 6 is a graph showing the relationship between the average circle equivalent diameter D of crystal grains surrounded by a? Particle size and a diagonal grain boundary.
7 is a graph showing the relationship between the cumulative reduction ratio at 1050 ° C. or lower and the γ particle size.
8 is a graph showing the relationship between the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundaries and the ratio R of the random grain boundaries in the diagonal grain boundaries.

The present invention improves the intra-particle ferrite transformation technique disclosed in Japanese Patent Application Laid-Open No. 2007-100213, and obtains a steel having good HAZ toughness even when welding with a larger heat input and excellent fatigue properties of the base material itself. It relates to technology for.

That is, the present invention has been studied based on the technique disclosed in Japanese Patent Application Laid-Open No. 2007-100213 to further improve HAZ toughness. As a result, all oxides in steel materials (the oxides that become nuclei of ferrite transformation in particles) The size and number of all the oxides) are deeply involved in the improvement of the HAZ toughness, and in particular, when the coarse oxides having a diameter equivalent to more than 5.0 µm are suppressed to 5 or less, the amount of heat input It was found that steel materials having excellent HAZ toughness can be obtained even by conducting high heat input welding of approximately 50 kJ / mm. In addition, the present inventors repeated examination for improving the fatigue characteristic of the base material itself in the state which improved HAZ toughness as mentioned above. As a result, it was discovered that fatigue property can be improved by controlling the metal structure and hardness of steel materials in a balanced manner, and completed the present invention. Specifically, when the orientation difference between two crystals adjacent to the metal structure of the steel material was measured by the EBSP method, the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundary with the crystal orientation difference of 15 ° or more was 25 μm or less, and the diagonal It was found that the fatigue property of the base material itself can be improved by setting the ratio R of the random grain boundary at the grain boundary to 70 area% or less and the hardness of the steel to 170 Hv or more.

In other words, in order to improve the toughness of the base material itself and to improve the fatigue properties, it is effective to refine the metal structure, and in particular, when the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundaries having a crystal orientation difference of 15 ° or more is set to 25 μm or less. good. However, when the structure is refined, the grain boundary increases, so that the growth rate of cracks generated in the steel increases. This is because cracks tend to advance grain boundaries, and as the grain boundaries increase, the growth path of the cracks increases. Therefore, as a result of the study aiming at improving the fatigue characteristics of the base material itself while miniaturizing the structure, it has been found that the random grain boundary, which has high grain boundary energy and tends to develop fatigue cracking, may be reduced. That is, even at diagonal grain boundaries where the crystal orientation difference is 15 ° or more, the energy is not high at all the difference angles, and at some specific difference angles, there exist grain boundaries called "corresponding grain boundaries" with extremely low grain boundary energy (for example, "materials" Histology: Takashi Setsuo, Tsuzaki Kaneaki Asakura Bookstore, page 45). That is, diagonal grain boundaries are roughly classified into "corresponding grain boundaries" having low grain boundary energy and "random grain boundaries" having high grain boundary energy, and fatigue cracks are known to be easy to progress in random grain boundaries, but they are difficult to progress. Accordingly, the present invention also presupposes that the ratio R is reduced to 70 area% or less on the basis of reducing the ratio R of the random grain boundary occupying the diagonal grain boundary having a crystal orientation difference of 15 ° or more, thereby improving the fatigue properties of the base metal. have.

However, as a result of further investigation by the present inventors, as mentioned above, the ratio of the random grain boundary to the average circle equivalent diameter D of the crystal grain enclosed by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more is 25 micrometers or less, and occupies at the said diagonal grain boundary. It was proved to be very important to make the hardness of the steel material 170 Hv or more because it is insufficient to improve the fatigue properties of the base material only by setting R to 70 area% or less.

In other words, if the ratio R of the random grain boundary occupies the diagonal grain boundary decreases, the path which tends to become a fatigue crack decreases, and thus the fatigue crack progresses in the steel while continuously bypassing the grain boundary to select the random grain boundary. However, the fatigue crack which became unable to advance a grain boundary may advance in a crystal grain rather than a grain boundary. Therefore, as described above, even if the ratio R of the random grain boundary occupying the diagonal grain boundary is reduced to reduce the fatigue crack propagation rate, the fatigue characteristics of the base material itself are eventually reduced unless the rate of fatigue crack propagation in the crystal grains is reduced together. It cannot be improved. Therefore, in the present invention, the hardness of fatigue cracks advancing in the crystal grains is based on the viewpoint that the softer the grains become, the steel hardness useful for preventing the growth of fatigue cracks in the crystal grains is set at 170 Hv or more. .

In the present specification, in order to distinguish between oxides which become nuclei of ferrite transformation in particles, that is, oxides containing Zr, REM and Ca, and all oxides contained in the steel, the former is particularly referred to as “Zr? REM? Ca”. System oxide ”, and the latter may be referred to as“ all oxides ”in particular.

In addition, the essential components (Zr, REM and Ca) constituting the above-mentioned "Zr-REM-Ca-based oxide" may be called especially the ferrite transformation nucleation element in a particle | grain.

In addition, in this specification, when all the oxide contained in steel materials is an oxide with a round equivalent diameter of 0.1-2.0 micrometers, it is called "fine oxide", and the oxide with a round equivalent diameter of more than 5.0 micrometers is called "coarse oxide". There is.

According to the present invention, since the number of coarse oxides is remarkably suppressed, the HAZ toughness evaluation method disclosed in the examples of Japanese Patent Laid-Open No. 2007-100213 [maintained at a heating temperature of 1400 ° C. for 5 seconds and then at 500 ° C. at 500 ° C. Heat cycle (cooling condition: 1400 ° C. × 5 seconds, cooling time Tc = 300 seconds) to cool the temperature up to 300 ° C., and the absorption energy at −40 ° C. is greater than the heat input amount. Even if welding is performed, the HAZ toughness can be improved. Specifically, the steel materials of the present invention provide absorbed energy when the heat cycle (heating condition: 1400 ° C. × 30 seconds, cooling time Tc = 300 seconds) in which the holding time of 1400 ° C. is extended for 30 seconds is described above. Even if it evaluates by the method of measuring similarly, it shows favorable HAZ toughness (refer the Example mentioned later).

EMBODIMENT OF THE INVENTION Hereinafter, the requirements of said (a)-(e) which comprise this invention are demonstrated in detail.

[(a) About Zr-REM-Ca-based Oxides]

First, the Zr-REM-Ca type oxide which is a starting point of ferrite transformation in a particle | grain is demonstrated. The Zr-REM-Ca oxide described above includes an oxide of Zr, an oxide of REM, and an oxide of Ca. The elements constituting the Zr-REM-Ca-based oxides (ferrite-transformed nucleation elements in the particles) are Zr, REM, and Ca. In addition to the above, oxide-forming elements such as Ti, Mn, Si, Al, and others It may contain components in the steel.

The presence form of the Zr-REM-Ca-based oxide is not particularly limited, and may be present as a single oxide containing ferrite-transformed nucleation elements in the particles alone, or two or more kinds of ferrite-transformed nucleation elements in the particles. It may exist as a complex oxide to contain. Examples of the single oxides include ZrO _{2 in} Zr; Ca is CaO; The REM, when the REM shown by the symbol of "M", and the like include _{M 2 O 3, M 3 O} 5, MO 2. In addition, such oxides may be present in aggregate with each other, or may be present in the form of complex precipitates of other compounds such as sulfides and nitrides in the oxides.

It is preferable that said Zr-REM-Ca type | system | group oxide further contains the oxide of Ti. The presence of further Ti oxide promotes ferrite transformation in the particles, further improving the HAZ toughness. The oxide of Ti may exist as a single oxide (for example, Ti ₂ O ₃ , Ti ₃ O ₅ , TiO ₂ ), or may exist in the form of a complex oxide with the Zr-REM-Ca oxide. .

[(b) About Average Composition of Oxide]

In the steel of the present invention, the mass ratio of ZrO ₂ is 5 to 50% in the average composition of the oxide contained in the steel, and the mass ratio of the oxide of REM (M ₂ O ₃ when REM is represented by the symbol M) is 10. It is 50-50%, and the mass ratio of CaO is 5.0-50%, and this becomes effective as a nucleus of ferrite transformation in a particle | grain. If it is less than the lower limit of each oxide, the amount of oxide which becomes a nucleus of ferrite in particle at the time of welding is insufficient, and the improvement effect of HAZ toughness is not exhibited. On the other hand, when the upper limit of each oxide is exceeded, the oxide is coarsened, and the number of fine oxides that effectively act as a production nucleus of ferrite in the particles is reduced, so that the HAZ toughness improving effect is not effectively exhibited.

The ZrO ₂ is at least 5%, preferably at least 10%, more preferably at least 13%, even more preferably at least 15%. On the other hand, the upper limit is 50%, preferably 45%, more preferably 40%.

The oxide of REM is 10% or more, preferably 15% or more, more preferably 20% or more, even more preferably 30% or more. On the other hand, the upper limit is 50%, preferably 45%, more preferably 40%. On the other hand, when the REM is represented by the symbol M, the oxide of REM exists in the form of M ₂ O ₃ , M ₃ O ₅ , MO _2, etc. in the steel, but when all the oxides of the REM are converted to M ₂ O ₃ . Means quantity.

The CaO is at least 5.0%, preferably at least 10%, more preferably at least 15%, even more preferably at least 18%. On the other hand, the upper limit is 50%, preferably 45%, more preferably 40%, particularly preferably 30%.

On the other hand, the remaining components of the total oxide composition are not particularly limited, and examples thereof include oxides of oxide-forming elements (for example, SiO ₂ , Al ₂ O ₃ , MnO, and the like) included in the steel of the present invention.

The composition of all the oxides contained in the steel is measured by observing the surface of the steel with, for example, an Electron Probe X-ray Micro Analyzer (EPMA) and quantitatively analyzing the oxides visible in the observation field. The detail of a measurement condition is demonstrated in the Example column mentioned later.

[(c) About Particle Size Distribution of All Oxides]

Next, the number and size of all the oxides (all oxides present in the steel, not limited to the Zr-REM-Ca-based oxides described above), which characterize the present invention, will be described.

As for all the oxides contained in the steel of this invention, 120 or more fine oxides of 0.1-2.0 micrometers in circular equivalent diameter are 120 or more per 1 mm <^2> observation field area, and coarse oxides larger than 5.0 micrometers in circular equivalent diameter observe field It satisfies five or less per 1mm <^2> area.

The inventors have investigated in detail the relationship between the particle size distribution of the entire oxide and the HAZ toughness, in particular, that the fine oxide having a circle equivalent diameter of 0.1 to 2.0 µm and the coarse oxide larger than 5.0 µm are deep in the HAZ toughness of the high heat input welding. It has been found that the number of fine oxides is largely contributing to the improvement of the HAZ toughness, and that the coarse oxide is a starting point of brittle fracture and causes a decrease in the HAZ toughness. It has also been found that fine oxides having a circle equivalent diameter of less than 0.1 μm contribute little to the HAZ toughness enhancing action by oxide dispersion. Therefore, in order to increase the HAZ toughness, the number of fine oxides is preferably as much as possible. However, when the number of fine oxides increases, the number of coarse oxides also tends to increase. It is necessary to control the number of.

The preferable number of fine oxides is 200 or more per 1mm <^2> observation field area, More preferably, it is 500 or more, More preferably, it is 1000 or more.

The smaller the coarse oxide is, the better. Preferably it is 3 or less, more preferably 1 or less, and most preferably 0 per 1 mm ² observation field area. With respect to the number of oxides other than the above, the present invention is not limited at all, and it has been confirmed by experiment that the desired HAZ toughness can be obtained if only the oxides of the above sizes are controlled.

Said "circle equivalent diameter" is a diameter of the circle assumed so that the area of an oxide may become the same, and is seen on a transmission electron microscope (TEM) observation surface.

[(d) About the average circle equivalent diameter D of the crystal grains enclosed by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more, and the ratio R of the random grain boundary occupied by the diagonal grain boundary]

The steel material of this invention needs to satisfy following formula (1) and formula (2) when a metal structure is observed by a backscattering electron diffraction image method (EBSP method). By satisfying the two equations, the fatigue property of the base material itself is improved.

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

In this invention, D value shall be 25 micrometers or less. By setting D value to 25 micrometers or less, the toughness of a base material itself can be ensured. In addition, it is generally known that the fatigue crack bends, bypasses, or stops at diagonal grain boundaries having a crystal orientation difference of 15 ° or more. Therefore, by miniaturizing the crystal grains surrounded by the diagonal grain boundary having a crystal orientation difference of 15 ° or more, the position at which the crack cracks, bypasses, and rectifies increases, and as a result, the impact property of the base material rises, and the fatigue property of the base material itself is increased. It becomes favorable (refer to a later figure 2). In addition, as described later, in order to reduce the ratio R of the random grain boundary to a predetermined value or less, it is effective to refine the crystal grains surrounded by the diagonal grain boundary (see later FIG. 8). This is because the ratio R of the random grain boundary is more easily reduced as the crystal grains surrounded by the diagonal grain boundary become smaller. The smaller the D value, the better. Preferably it is 23 micrometers or less, More preferably, it is 20 micrometers or less. In addition, the minimum of D value is about 13 micrometers.

D value measures and measures the metal structure in the t / 2 position of a plate | board thickness direction, when the plate | board thickness of steel materials is t (mm). The D value at the t / 2 position is based on the fact that the deformation becomes more difficult as the plate thickness increases, which is why the control of the metal structure is the most difficult. The specific measurement procedure is demonstrated in the Example column of the latter.

In the above formula (2), R means the ratio (area%) of the random grain boundary occupied at the diagonal grain boundary where the crystal orientation difference is 15 ° or more when the azimuth difference between two adjacent crystals is measured by the EBSP method.

In this invention, R value is made into 70 area% or less. As described above, the diagonal grain boundary is divided into the corresponding grain boundary and the random grain boundary, but by reducing the random grain boundary where fatigue cracks tend to grow, the path through which the crack is bypassed becomes long, so that the crack growth rate can be reduced, so that Properties can be improved. In addition, since the energy of the cracks is attenuated in the process of propagating, the cracks are stopped by decreasing the energy by lengthening the propagation path of the cracks, so that the fatigue characteristics of the base material itself can be improved (late FIG. 3). Reference).

The smaller the R value is, the better it is, preferably 65 area% or less, and more preferably 60 area% or less. In addition, the minimum of R value is about 40 area%.

When the plate | board thickness of steel materials is t (mm), R value observes and measures the metal structure in the surface of the plate | board thickness direction, t / 4 position, and t / 2 position, respectively. Observation of the surface of the steel is because fatigue cracking is likely to occur on the surface of the steel, and observation of the t / 4 position is because it is a general reference point for evaluating the characteristics of the entire steel. The reason for this is that the larger the plate thickness, the harder the deformation is, and therefore, the position where control of the metal structure is most difficult. The specific measurement procedure is demonstrated in the Example column of the latter.

[(e) about average hardness]

In this invention, the average hardness of steel materials shall be 170 Hv or more. By hardening a steel material to some extent, even when the crack which became difficult to advance a grain boundary advances in an inside of a particle | grain, the speed which advances in an inside of a particle can be made small. As a result, the growth rate of the crack which advances in steel materials can be made small (refer FIG. 4 later).

The larger the average hardness is, the better, preferably at least 180 Hv, more preferably at least 190 Hv, even more preferably at least 200 Hv.

In order to make the average hardness of steel materials 170 or more, it is recommended that the metal structure be the bainite main body. This is because the hardness of the steel tends to decrease as the ferrite fraction occupies in the metal structure increases. The bainite principal means that the bainite fraction is about 60 area% or more when the metal structure is observed under an electron microscope. On the other hand, the upper limit of average hardness is about 260 Hv. It is because when it becomes hard too much, a crack will not grow in a particle | grain, and it will become difficult to make a crack growth speed small. The upper limit of this average hardness is a value substantially equal to the average hardness of bainite structure.

The hardness of a steel material is measured at the t / 2 position of a plate thickness direction, when making the plate | board thickness of steel materials t (mm). The hardness at the t / 2 position is based on the fact that as the plate thickness increases, cooling to the center of the plate thickness becomes difficult, and it is difficult to properly control the metal structure. In general, the hardness of the steel depends on the form of the metal structure, and the form of the metal structure depends on the cooling rate. The specific measurement procedure is demonstrated in the Example column of the latter.

Next, the component composition in the steel material (base material) of this invention is demonstrated. Steel materials of the present invention, as a basic component, C: 0.02 to 0.12%, Si: 0.02 to 0.5%, Mn: 1 to 2%, Zr: 0.0002 to 0.050%, REM: 0.0002 to 0.050%, Ca: 0.0005 to 0.010 %, Ti: 0.005 to 0.02%, N: 0.0040 to 0.01%, O: 0.0005 to 0.010%, P: 0.02% or less, S: 0.015% or less, Al: 0.05% or less (including 0%) . The reason for determining this range is as follows.

C is an element which cannot be removed in order to secure the strength of the steel material (base material). In order to exhibit such an effect, it is necessary to contain 0.02% or more. C is preferably 0.04% or more, more preferably 0.05% or more. However, when C exceeds 0.12%, not only does the island-like martensite (MA) generate | occur | produce in HAZ at the time of welding, but also leads to toughness deterioration of HAZ, and also adversely affects weldability. Therefore, C is at most 0.12%, preferably at most 0.1%, more preferably at most 0.08%.

Si is an element which contributes to securing the strength of steel by solid solution strengthening. However, when Si exceeds 0.5%, not only the island-like martensite (MA) is generated in the HAZ during welding, but also deteriorates the toughness of the HAZ, and also adversely affects the weldability. Therefore, Si is made into 0.5% or less. Preferably it is 0.3% or less, More preferably, it is 0.2% or less, More preferably, it is 0.18% or less. On the other hand, in order to add Si to secure the strength of the steel, it is preferable to contain 0.02% or more, more preferably 0.05% or more, even more preferably 0.1% or more.

Mn is an element which contributes to the strength improvement of steel materials (base materials). In order to exhibit these effects effectively, it is necessary to contain 1% or more. Mn is preferably 1.2% or more, more preferably 1.4% or more. However, if it exceeds 2%, the weldability of the steel material (base material) is deteriorated. Therefore, Mn needs to be suppressed to 2% or less. Preferably it is 1.8% or less, More preferably, you may be 1.6% or less.

Zr is an element necessary to produce ZrO ₂ . By containing ZrO ₂ , oxides tend to be finely dispersed, and the finely dispersed oxides become nuclei for ferrite in the particles, thereby contributing to the improvement of HAZ toughness. However, when Zr is less than 0.0002%, HAZ toughness cannot be improved because the amount of fine oxides that form nuclei of ferrite in the particles decreases. Therefore, it is necessary to contain Zr 0.0002% or more. Preferably it is 0.0005% or more, More preferably, you may be 0.0010% or more. However, excessive addition of Zr produces many coarse oxides and deteriorates HAZ toughness. In addition, it forms a fine Zr carbide which brings precipitation strengthening, leading to a decrease in toughness of the base material itself. Therefore, Zr is suppressed to 0.050% or less. Preferably it is 0.04% or less, More preferably, you may be 0.01% or less, More preferably, you may be 0.008% or less.

REM (rare earth element) and Ca are elements necessary for producing each oxide. By containing these oxides, the oxides tend to be finely dispersed, and the finely dispersed oxides become the nuclei of ferrite in the particles, thereby contributing to the improvement of the HAZ toughness.

REM should be contained at least 0.0002%, preferably at least 0.0005%, more preferably at least 0.0010%, even more preferably at least 0.0015%. However, when REM is excessively added, coarse oxides are formed and HAZ toughness deteriorates on the contrary. Therefore, REM should be suppressed to 0.050% or less. Preferably it is 0.04% or less, More preferably, it is 0.01% 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, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, and more preferably La and / or Ce.

Ca should be contained 0.0005% or more, preferably 0.001% or more, and more preferably 0.0015% or more. However, when Ca is excessively added, coarse oxides are formed and HAZ toughness is deteriorated. Therefore, Ca needs to be suppressed to 0.010% or less. Preferably it is 0.008% or less, More preferably, it is 0.005% or less.

Ti is an element that forms nitrides such as TiN and Ti oxides in steel and contributes to the improvement of HAZ toughness. In order to exhibit such an effect, Ti needs to be contained 0.005% or more. Preferably it is 0.007% or more, More preferably, you may be 0.010% or more. However, excessive addition of Ti deteriorates the toughness of the steel (base metal), and therefore Ti should be suppressed to 0.02% or less. Preferably it is 0.018% or less, More preferably, it is 0.017% or less.

N is an element that precipitates nitride (eg, ZrN, TiN, etc.), and the nitride is ferritic transformation by preventing flux coarsening of austenite particles generated in HAZ during welding by flux pinning. By promoting it, it contributes to the improvement of HAZ toughness. In order to exhibit these effects effectively, it is necessary to contain 0.0040% or more. Preferably it is 0.005% or more, More preferably, it is 0.006% or more. As the N content increases, the Ti-containing nitride is formed to promote the miniaturization of the austenite particles. Therefore, N effectively acts to improve the toughness of the HAZ. However, when N exceeds 0.01%, the amount of solid solution N increases, the toughness of the base material itself deteriorates, and HAZ toughness also falls. Therefore, N needs to be suppressed to 0.01% or less. Preferably it is 0.0095% or less, More preferably, you may be 0.009% or less.

O (oxygen) is an element necessary for producing a fine oxide that becomes a nucleus of ferrite in particles that contributes to the improvement of HAZ toughness. However, if O is less than 0.0005%, the fine oxide amount is insufficient, and HAZ toughness cannot be improved. Therefore, O needs to be contained 0.0005% or more. Preferably it is 0.0010% or more, More preferably, it is 0.0015% or more. However, excessive addition leads to coarsening of the oxides, which deteriorates HAZ toughness. Therefore, O should be suppressed to 0.010% or less. Preferably it is 0.008% or less, More preferably, it is 0.005% or less.

P is an element which is easy to segregate, and in particular, it segregates at grain boundaries in steel materials and deteriorates the toughness of the base material. Therefore, P should be suppressed to 0.02% or less. Preferably it is 0.018% or less, More preferably, you may be 0.015% or less.

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

Al is an element having a strong deoxidizing power and acting as a deoxidizing agent. However, when excessively added, the oxide is reduced, making it difficult to produce the Zr-REM-Ca oxide which contributes to the improvement of HAZ toughness. Therefore, Al needs to be suppressed to 0.05% or less. Al becomes like this. Preferably it is 0.04% or less, More preferably, it is 0.035% or less. In addition, Al may be 0%.

The steel material of this invention contains the said element as an essential component, and remainder may be iron and an unavoidable impurity (for example, Mg, As, Se, etc.).

The steel of the present invention is also effective to contain, as other elements, an element (Ni, Cu, Cr, Mo) for improving the strength of the steel, or an element (Nb, V, B) for further improving the HAZ toughness. Specifically,

(1) at least one element selected from the group consisting of Ni: 0.4% or less, Cu: 0.3% or less, Cr: 1.5% or less, and Mo: 1% or less,

(2) It is preferable to contain at least one element selected from the group consisting of Nb: 0.1% or less, V: 0.1% or less, and B: 0.005% or less. The reason for determining this range is as follows.

[(1) Ni, Cu, Cr, Mo]

Ni, Cu, Cr, and Mo are all elements which contribute to raising the intensity | strength of steel materials, and can be added individually or in combination, respectively.

In particular, Ni is an element that contributes to increasing the strength of the steel and improving the toughness of the steel itself. 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. It is preferable to contain Ni as much as possible, but since it is an expensive element, when it contains excessively, cost will become high. Therefore, for economic reasons, the upper limit is preferably 0.4%. More preferably, it is 0.38% or less, More preferably, it is 0.35% or less.

Cu is an element which solid-solution strengthens and raises the strength of steel materials. 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. However, since the toughness of steel materials deteriorates when it contains exceeding 0.3%, Cu is preferable to be 0.3% or less. More preferably, it is 0.28% or less, More preferably, it is 0.25% or less.

It is preferable to contain Cr 0.1% or more. More preferably, it is 0.2% or more, More preferably, it is 0.5% or more. However, when Cr exceeds 1.5%, HAZ toughness is lowered because the strength of the steel (base material) is significantly increased so that the toughness of the base material is degraded. Therefore, Cr is preferably at most 1.5%. More preferably, it is 1.2% or less, More preferably, it is 1% or less.

It is preferable to contain Mo 0.1% or more. More preferably, it is 0.2% or more, More preferably, it is 0.3% or more. However, if it exceeds 1%, the strength of the steel (base material) becomes significantly too high and the base material toughness deteriorates rather than the HAZ toughness. Therefore, it is preferable to make Mo 1% or less. More preferably, it is 0.7% or less, More preferably, it is 0.5% or less.

[(2) Nb, V, B]

Nb, V, and B are all elements which improve HAZ toughness, and can be added alone or in combination.

In particular, Nb and V suppress the coarsening of austenite particles during slab heating before rolling due to the solute drag effect due to solid solution and the flux fixation effect due to precipitation of carbonitride, thereby miniaturizing the structure. An element that acts to improve HAZ toughness.

In order to effectively exhibit such an effect by Nb addition, it is preferable to contain 0.002% or more. More preferably, it is 0.005% or more, More preferably, it is 0.008% or more. However, when Nb exceeds 0.1%, precipitated carbonitrides coarsen and deteriorate the toughness of the base metal. Therefore, Nb is preferably made 0.1% or less. More preferably, it is 0.08% or less, More preferably, it is 0.05% or less.

If V is added in excess of 0.1% as in Nb, the deposited carbonitride is coarsened and the toughness of the base material is deteriorated. Therefore, V is preferably made 0.1% or less. More preferably, it is 0.08% or less, More preferably, it is 0.05% or less. On the other hand, in order to exhibit the effect | action by V addition effectively, it is preferable to contain 0.002% or more. More preferably, it is 0.005% or more, More preferably, it is 0.01% or more.

On the other hand, B is an element which suppresses generation of grain boundary ferrite and improves HAZ toughness. However, when B exceeds 0.005%, it precipitates as BN in an austenite grain boundary and causes the fall of HAZ toughness. Therefore, it is preferable to make B into 0.005% or less. More preferably, it is 0.004% or less. On the other hand, in order to exhibit the effect by B addition effectively, it is preferable to contain 0.001% or more. More preferably, it is 0.0015% or more.

Next, the manufacturing method which can be suitably employ | adopted in manufacturing the steel material of this invention is demonstrated.

First, in casting, after adjusting the dissolved oxygen amount of molten steel to the range of 0.0010 to 0.0060%, after stirring molten steel and floating-separating oxide in molten steel, the total oxygen amount is adjusted to 0.0010 to 0.0070%, and then Ti, Zr, REM and Ca are added to adjust to the component composition. Thereby, the requirements of (a) to (c) can be satisfied.

Then, when the hot rolling, the slabs (鋼片) After heating to the 1100 to 1250 ℃, 1050 ℃ or less, Ar ₃ point + in the temperature range exceeding 100 ℃ 1 pass 1.5 ℃ / average cooling rate per second First rolling (corresponding to rough rolling) is carried out so that the cumulative reduction ratio is 40% or more while cooling to the above-mentioned, and then the maximum reduction ratio per pass in the temperature range exceeding Ar ₃ point + 100 ° C or less and Ar ₃ point. After performing 2nd rolling (equivalent to finishing rolling) so that 12% or less of this cumulative reduction may be 50% or more, it cools to an average cooling rate of 5 degrees C / sec or more to the temperature range which surface temperature becomes 500 degrees C or less. Thereby, the requirements of (d) and (e) can be satisfied.

The outline of the production method of the present invention is as follows.

First, by adding a predetermined alloy element to a molten steel in which the dissolved oxygen amount and the total oxygen amount are adjusted in a predetermined order, it is possible to produce a desired oxide serving as a nucleus of ferrite in the particles. In particular, in the present invention, it is very important to adjust the total amount of oxygen after adjusting the amount of dissolved oxygen so that coarse oxide is not produced.

Dissolved oxygen means oxygen in the free state which does not form oxide and exists in molten steel. Total oxygen means the sum total of all the oxygen contained in molten steel, ie, the oxygen which forms free oxygen and oxide.

Subsequently, although the steel piece obtained by casting is hot-rolled, in order to make the average circle equivalent diameter D of the crystal grain enclosed by diagonal grain boundary to 25 micrometers or less, and to make ratio R of the random grain boundary occupied by diagonal grain boundary to 70 area% or less, the controls, first the maximum reduction ratio per pass to 12%, the cumulative rolling reduction in the temperature range of from Ar ₃ point in the Ar ₃ point + 100 ℃, as defined as a second rolling with 50% or more to the hot rolling It is important. And, in order while still achieving the R≤70 area% in the hot rolling in the temperature range to satisfy D≤25㎛, as to define a first rolled, in a temperature range of from 1050 ℃ to Ar ₃ point + 100 ℃ Rolling is performed such that the cumulative reduction ratio is 40% or more while cooling so that the average cooling rate per one pass is 1.5 ° C / sec or more. Thus, by performing 1st rolling and making austenite (Y) particle diameter 50 micrometers or less, metal structure is controlled suitably in the 2nd rolling following 1st rolling, and it is adjusted so that it may satisfy the requirements prescribed | regulated by this invention. . Hereinafter, the manufacturing method of this invention is explained in full detail in order.

First, the dissolved oxygen amount of molten steel is adjusted to 0.0010 to 0.0060% of range. If the amount of dissolved oxygen in molten steel is less than 0.0010%, the amount of dissolved oxygen in molten steel is insufficient, so that a predetermined amount of Zr-REM-Ca oxide, which is a nucleus of ferrite transformation in particles, cannot be secured, and excellent HAZ toughness cannot be obtained. If the amount of dissolved oxygen is insufficient, Zr, which cannot form oxides, forms carbides, or REM or Ca forms sulfides, which causes deterioration of the toughness of the base material itself. Therefore, the said dissolved oxygen amount shall be 0.0010% or more. The dissolved oxygen is preferably 0.0013% or more, and more preferably 0.0020% or more.

On the other hand, if the amount of dissolved oxygen exceeds 0.0060%, the amount of oxygen in molten steel is too high, so that the reaction of oxygen in the molten steel with the element becomes severe, which is undesirable in solvent operation, and coarse oxides are formed, and thus HAZ toughness is produced. Deteriorate Therefore, the dissolved oxygen amount should be suppressed to 0.0060% or less. The dissolved oxygen amount is preferably 0.0055% or less, and more preferably 0.0053% or less.

By the way, the amount of dissolved oxygen in the molten steel refine | purified primarily by the converter and the electric furnace usually exceeds 0.010%. Therefore, in the manufacturing method of this invention, it is necessary to adjust the dissolved oxygen amount in molten steel to the said range by some method.

As a method of adjusting the amount of dissolved oxygen in molten steel, the method of vacuum C deoxidation using a RH type degassing refiner, the method of adding deoxidative elements, such as Si, Mn, Ti, Al, etc. are mentioned, It is also possible to adjust the dissolved oxygen amount by appropriately combining these methods. In addition, instead of the RH degassing refinery, the dissolved oxygen amount may be adjusted using a blow-up heating refining apparatus, a simple molten steel treatment plant, or the like. In this case, since the dissolved oxygen amount cannot be adjusted by vacuum C deoxidation, a method of adding a deoxidizing element such as Si may be employed to adjust the dissolved oxygen amount. When employ | adopting the method of adding deoxidizing element, such as Si, you may add a deoxidizing element at the time of tapping out from a converter to the brisk.

After adjusting the dissolved oxygen amount of molten steel to the range of 0.0010 to 0.0060%, the molten steel is stirred and the oxide in molten steel is floated and separated, and the total amount of oxygen in molten steel is adjusted to 0.0010 to 0.0070%. As described above, in the present invention, since the molten steel in which the amount of dissolved oxygen is properly controlled is stirred to remove unnecessary oxide, ferrite transformation nucleation elements in particles such as Zr are added, and thus, formation of coarse oxide can be prevented. . In the above-mentioned Japanese Patent Laid-Open Publication No. 2007-100213, since this flotation separation process is not performed, coarse oxides are generated, and good HAZ toughness cannot be secured (see later examples).

If the total amount of oxygen is less than 0.0010%, the desired amount of oxide becomes insufficient, and therefore, the amount of oxide serving as a nucleus for the formation of ferrite in the particles that contributes to the improvement of HAZ toughness cannot be ensured. Therefore, the said total amount of oxygen is made into 0.0010% or more. The total amount of oxygen is preferably 0.0015% or more, and more preferably 0.0018% or more.

On the other hand, when the total amount of oxygen exceeds 0.0070%, the amount of oxide in the molten steel becomes excessive, coarse oxide is formed, and HAZ toughness is deteriorated. Therefore, the total amount of oxygen should be suppressed to 0.0070% or less. The total amount of oxygen is preferably 0.0060% or less, and more preferably 0.0050% or less.

Since the total amount of oxygen in molten steel changes generally with respect to the stirring time of molten steel, it can control by adjusting stirring time. Specifically, the molten steel is stirred to appropriately control the total amount of oxygen in the molten steel while appropriately measuring the total amount of oxygen in the molten steel after removing the floating oxide.

After adjusting the total amount of oxygen in molten steel to the said range, Ti is added, and then Zr, REM, and Ca are added, and it casts. By adding said element to molten steel which adjusted the total amount of oxygen, the Zr-Ca-REM type | system | group oxide used as the nucleus of ferrite transformation in a desired particle | grain is obtained. The Ti oxide has a smaller interfacial energy with molten steel than the Zr-REM-Ca oxide, and when the alloying elements are added in this order, the Ti oxide becomes fine, and as a result, fine oxides that contribute to HAZ toughness can be produced. have.

The forms of REM, Ca, Zr, and Ti added to molten steel are not particularly limited. For example, as REM, pure La, pure Ce, pure Y, or the like, or pure Ca, pure Zr, pure Ti, and Fe-Si-La It is preferable to add an alloy, a Fe-Si-Ce alloy, a Fe-Si-Ca alloy, a Fe-Si-La-Ce alloy, a Fe-Ca alloy, a Ni-Ca alloy, or the like. It is also possible to add misch metal to molten steel. A misch metal is a mixture of cerium group rare earth elements, specifically, it 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 misch metal contains Ca, it is necessary to satisfy the range prescribed | regulated by this invention.

In this invention, after adding Zr, REM, and Ca for the purpose of promoting removal of coarse oxide, it is preferable to stir molten steel in the range which does not exceed 40 minutes. If the stirring time exceeds 40 minutes, fine oxides aggregate and coalesce in molten steel, so that the oxides coarsen and HAZ toughness deteriorates. Therefore, it is preferable to make stirring time into 40 minutes or less. Stirring time becomes like this. More preferably, it is within 35 minutes, More preferably, it is within 30 minutes. Although the lower limit of the stirring time of molten steel is not specifically limited, When stirring time is too short, the density | concentration of an additional element will become nonuniform and a desired effect as a whole steel material cannot be acquired. Therefore, the desired stirring time depends on the vessel size.

As mentioned above, molten steel with a component composition adjusted can be obtained by adding Zr, REM, and Ca. It casts using the obtained molten steel, and obtains a steel piece.

Although the obtained slab is hot rolled, the heating temperature at the time of hot rolling shall be 1100-1250 degreeC. The lower limit of the heating temperature is set at 1100 ° C in order to make the metal structure of the steel piece austenite. However, if the heating temperature is too high, the initial austenite structure becomes too coarse, making it difficult to sufficiently refine the tissue after transformation. Therefore, the upper limit of heating temperature shall be 1250 degreeC.

After heating, the average cooling rate per one pass is 1.5 ° C / sec or more in the temperature range where the t / 2 position of the steel slab (t is the thickness of the steel slab) is 1050 ° C or less and exceeds Ar ₃ points + 100 ° C. 1st rolling is performed so that a cumulative reduction may be 40% or more while cooling.

While by 1050 ℃ or less, cooling in the temperature range exceeding the Ar ₃ point + 100 ℃ the rolling line, it is possible to prevent the coarsening of austenite may be the austenite grain size to less than 50㎛, the second in an appropriate condition to be described later By rolling, the average circle equivalent diameter D of the crystal grains enclosed by the diagonal grain boundary can be controlled to 25 micrometers or less (refer FIG. 6 later).

However, if the average cooling rate at the time of rolling in the said temperature range is less than 1.5 degree-C / sec, or if the cumulative reduction rate is less than 40%, austenite will coarsen and an austenite particle size will exceed 50 micrometers, and will be described later. As described above, even if the second rolling is performed under appropriate conditions, the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundary cannot be controlled to 25 µm or less (see later FIG. 7). Therefore, the fatigue property of a base material cannot be improved. Therefore, the average cooling rate at the time of rolling in the said temperature range shall be 1.5 degree-C / sec or more. Preferably it is 2 degree-C / sec or more, More preferably, it is 2.5 degreeC or more.

In addition, a cumulative reduction ratio is 40% or more. Preferably it is 50% or more, More preferably, it is 60% or more. The upper limit of the cumulative reduction ratio is not particularly limited, but is generally about 65%.

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

The cumulative reduction ratio is a value obtained by the following equation (4). t ₀ is the rolling start thickness (mm), t ₁ at which the average temperature of the billet is in the temperature region of the object means the rolling end thickness (mm) at which the average temperature of the billet is in the temperature region of the object.

1050 ℃ below, subsequent to the first rolling in a temperature region exceeding the Ar ₃ point + 100 ℃ is also performed a second rolling are continuously described below. However, the first rolling, 1050 ℃ or less, Ar ₃ point + 100 It is preferable to carry out in the temperature range which austenite recrystallizes among the temperature range exceeding ° C. In order to roll from the austenite recrystallization temperature to more than Ar ₃ point + 100 degreeC (this temperature range may be called the austenite unrecrystallization temperature range below), it is necessary to raise a reduction ratio and an installation load is applied. .

Although the upper limit of the austenite recrystallization temperature is somewhat changed depending on the chemical composition of the steel piece, the steel grade targeted by the present invention is generally 1060 ° C. By finishing the first rolling in the austenite recrystallization temperature range, the austenite grain size can be reduced.

It is preferable to make the temperature which complete | finish 1st rolling into the temperature of a substantially lower limit (temperature just before reaching an austenite uncrystallized region) in an austenite recrystallization temperature region. This is because after a long rolling in the austenite recrystallization temperature range, the austenite is recrystallized and coarsened.

After finishing the first rolling, the maximum rolling reduction per pass in the temperature range where the temperature at the t / 2 position (t is the thickness of the steel sheet) of the steel piece exceeds Ar ₃ points + 100 ° C. or less and Ar ₃ points. 2nd rolling is performed so that 12% or less and cumulative reduction may be 50% or more.

When 2nd rolling is performed at the temperature exceeding Ar <_3> +100 degreeC, the crystal grain enclosed by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more cannot refine | miniaturize. On the other hand, even if it carries out 2nd rolling at the temperature below Ar <_3> , the crystal grain enclosed by a diagonal grain boundary cannot be refined | miniaturized and rolling temperature is too low and installation load becomes large.

The maximum rolling reduction per pass when rolling in the temperature range is made 12% or less. When the maximum reduction ratio exceeds 12%, excessive strain accumulates and causes recrystallization, so that the ratio R of the random grain boundaries occupying the diagonal grain boundaries becomes large. Therefore, the maximum reduction ratio is 12% or less. Preferably it is 11.5% or less, More preferably, it is 11% or less.

The cumulative reduction ratio at the time of rolling in the said temperature range shall be 50% or more. If the cumulative reduction ratio is less than 50%, the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundaries having a crystal orientation difference of 15 ° or more becomes large (see later FIG. 5), and the ratio R of the random grain boundaries occupying the diagonal grain boundaries increases. The fatigue crack growth rate is increased, and the fatigue properties of the base metal cannot be improved. Therefore, the cumulative reduction ratio is 50% or more. Preferably it is 55% or more, More preferably, it is 60% or more.

On the other hand, when 1st rolling is complete | finished in an austenite recrystallization temperature range, 2nd rolling is started after cooling to the temperature range where the temperature of the t / 2 position of a steel piece becomes Ar <_3> +100 degreeC or less.

In addition, for the convenience of production or the convenience of equipment, when it takes time until the start of the second rolling after the end of the first rolling, after the end of the first rolling, it is rapidly cooled to the austenite unrecrystallized temperature region, and this temperature range After waiting, the second rolling may be started when ready. This is to prevent the micronized austenite from coarsening again by setting the atmospheric temperature of the steel piece to the austenite microcrystalline temperature range.

After completion of the second rolling, at the surface temperature of the steel piece, the average cooling rate is 5 ° C./sec or more from the temperature exceeding the Ar ₃ point to 5 ° C./sec or more, thereby actively cooling (accelerated cooling). If the cooling start temperature below the Ar ₃ point, ferrite is generated a lot can not be allocated in the hardness. Therefore, cooling start temperature shall be temperature exceeding Ar <_3> point. In addition, cooling stop temperature shall be 500 degrees C or less in order to complete transformation.

The average cooling rate from the temperature exceeding Ar ₃ point to 500 degrees C or less shall be 5 degrees C / sec or more. If the average cooling rate is less than 5 DEG C / sec, a large amount of ferrite is produced and hardness cannot be secured. Therefore, average cooling rate shall be 5 degrees-C / sec or more. Preferably it is 7 degrees C / sec or more, More preferably, it is 9 degrees C / sec or more.

After cooling to the temperature range which the surface temperature of a steel piece becomes 500 degrees C or less, tempering treatment can also be performed in the temperature range of 500 degreeC or more and less than Ac ₁ point. By tempering, since the strain introduced by rolling or transformation is lost, the base metal toughness can be improved.

The temperature of said Ac ₁ point can be computed from following formula (5). In formula, [] has shown content (mass%) of each element.

The steel according to the present invention can be used, for example, as a material for structures such as bridges, high-rise buildings, ships, and so on. You can stop it.

The steel material of this invention targets steel materials, such as a thick steel plate whose plate | board thickness is about 3.0 mm or more. The excellent HAZ toughness improvement effect which concerns on this invention is exhibited effectively when it is set as the thick steel plate of 20 mm or more (especially 40 mm or more), and the large heat input welding of 50 kJ / mm or more of heat input is performed.

[Example]

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

By using a vacuum melting furnace (capacity 150 kg), the steel (a steel grade a to w. Remainder is iron and unavoidable impurities) containing the chemical steel (mass%) shown in Table 2 under the conditions shown in Table 1 below, Cooled by casting into 150 kg of ingots. After heating the obtained ingot to the heating temperature shown in following Table 3 or 4, it cools until the average temperature in t / 2 position of a slab (t is the thickness of slab. Or less the same) will be 1050 degrees C or less, and then Hot rolling produced a rolled material having a product thickness of 10 to 80 mm. Hereinafter, manufacturing conditions are demonstrated concretely.

In dissolving the test steel in a vacuum furnace, the amount of dissolved oxygen in the molten steel is deoxidized using at least one element selected from C, Si, and Mn while simultaneously adjusting the components for elements other than Ti, Zr, Al, REM, and Ca. Adjusted. The dissolved oxygen amount (ppm) after adjustment is shown in Table 1 below.

The total amount of oxygen in molten steel was adjusted by stirring molten steel which adjusted the amount of dissolved oxygen for about 1 to 10 minutes, and floating the oxide in molten steel. The total amount of oxygen (ppm) after the adjustment is shown in Table 1 below.

After adding Ti to the molten steel which adjusted the total amount of oxygen, molten steel component-adjusted was obtained by adding Zr, REM, and Ca. On the other hand, Ti is in the form of Fe-Ti alloy, Zr is in the form of Fe-Zr alloy, REM is in the form of misch metal containing about 25% La and about 50% Ce, Ca is Ni-Ca alloy, or In the form of Ca-Si alloy or Fe-Ca green compact, it added, respectively.

However, No. 22 of Table 1 added FeO in order to adjust the total amount of oxygen. In addition, No. 7 of Table 1 added Ti immediately, without stirring the molten steel which adjusted the amount of dissolved oxygen.

The molten steel which added the said element (Zr, REM, and Ca) and adjusted the component was cast into an ingot and cooled after stirring time shown in Table 1 below.

After heating the obtained ingot to the heating temperature shown in following Table 3 or Table 4, it hot-rolled. In the present Example, 1st rolling was performed using the roughing mill, and 2nd rolling was performed using the finishing mill.

The rolling start temperature of rough rolling is shown in following Table 3 or Table 4. In addition, rolling start temperature was managed by the average temperature in the t / 2 position of a steel piece.

The rough rolling was performed while cooling so that the average cooling rate per one pass may be the speed shown in Table 3 or Table 4 below. End temperature of the rough rolling was set to more than Ar ₃ + 120 ℃. The cumulative reduction ratio in the rough rolling is shown in Table 3 or Table 4 below.

After completion of the rough rolling, the steel sheet was cooled until the average temperature at the t / 2 position of the steel piece became Ar ₃ points + 100 ° C. or lower, and finish rolling was performed. The finish rolling start temperature was Ar ₃ points + 100 ° C. or less, and the finish rolling finish temperature was Ar ₃ + 40 ° C. or more. The maximum rolling reduction and cumulative rolling reduction per pass in finish rolling are shown in Table 3 or Table 4 below.

After finishing rolling, it cooled until the surface temperature of a hot rolling material became 500 degrees C or less. The cooling start temperature after finishing rolling is shown in following Table 3 or Table 4. In addition, the average cooling rate from cooling start temperature to 500 degreeC is shown in following Table 3 or Table 4.

On the other hand, No.1-2 to 1-8 of Table 3 are steel grades a shown in Table 2, No.2-2 of Table 3 is steel grade b shown in Table 2, and No.3-2 of Table 3 are shown in Table 2 It is an example which changed hot rolling conditions etc. using the steel grade c, respectively.

In addition, No.1-2 and No.1-3 of Table 3 temper temperature shown in following Table 3 after cooling to the temperature range which the surface temperature of a hot rolling material becomes 500 degrees C or less, and it manages by surface temperature. This is an example of tempering treatment.

The temperature at the t / 2 position of the steel piece was calculated in the following procedure.

? Calculation method of average temperature

(1) Using a process computer, the heating temperature of arbitrary positions in the plate thickness direction from the surface of a steel piece to the back surface according to the atmospheric temperature and ashing time from a heating start to extraction is computed.

(2) Using the calculated heating temperature, the rolling temperature at any position in the sheet thickness direction is suitable for calculation, such as the differential method, according to the rolling pass schedule being rolled and the data of the cooling method (water cooling or air cooling) between the passes. It rolls, calculating using a method.

(3) The surface temperature of a steel piece is measured using the radial thermometer provided on the rolling line (however, it computes also on a process computer).

(4) The surface temperature of the steel slab measured at the start of rough rolling, at the end of rough rolling, and at the start of finish rolling is compared with the calculated surface temperature on the process computer.

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

(6) Using the corrected calculated surface temperature, find the temperature at the t / 2 position.

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

In following Table 3 or 4, the product thickness (mm) of the rolling material obtained by cooling was also shown. In addition, Table 3 or Table 4 also show the values of Ar ₃ point and Ac ₁ point calculated using the equations (3) and (5) according to the chemical component compositions shown in Table 2 above.

The sample was cut out from the cross section in the t / 4 position (t is the thickness of a rolled material) of the obtained rolling material, the composition of all the oxides contained in the said sample was measured, mass conversion into the single oxide was carried out, and the average composition of the oxide was computed.

The sample surface cut out was observed at 600 times using "EPMA-8705 (device name) by Shimadzu Corporation, and the component composition was quantitatively analyzed about the particle | grains whose maximum diameter is 0.2 micrometer or more. Observation conditions made 20 kV of acceleration voltages, 0.01 microamperes of sample currents, 1-5 cm <^2> of observation field areas, and 100 analysis numbers, and quantitatively analyzed the component composition in particle center part by wavelength dispersion spectroscopy of characteristic X-rays. As the element to be analyzed, Mn, Ti, Zr, La, Ce, Ca, Si, Al, and O (oxygen), the relationship between the electron beam intensity of each element and the element concentration is determined in advance as a calibration curve using a known substance. Subsequently, the element concentration of the particle was quantified from the electron beam intensity obtained from the particle and the calibration curve in advance.

In the obtained quantitative results, particles having an oxygen content of 5% or more were recognized as oxides. About each particle recognized as an oxide, it further recognized that it was an oxide of the element detected by the said method. However, when a plurality of elements were detected from one particle, it was calculated to divide "one" according to the ratio of X-ray intensities indicating the presence of those elements (for example, 0.6 elements A and 0.4 elements B). Like a dog). Thus, the number of particles was obtained for each element. For an oxide of a certain element, the ratio of the sum of all the detection elements of (molecular weight of the oxide) x (number of particles) of (molecular weight of the element oxide) x (number of particles of the element oxide) to the average composition Therefore, it was set as the mass ratio of the oxide of the element. The average composition of all the oxides is shown in Table 3 or Table 4 below. On the other hand, an oxide of REM are expressed as the metal element M, the steel material in the form of M ₂ O ₃ and M ₃ O _5, MO _2, but with all of the oxide M ₂ O ₃ was subjected to the calculation.

As a result of observing the sample surface with EPMA, most of the observed oxides were composite oxides containing Ti, Zr, REM and Ca, but Ti ₂ O ₃ , ZrO ₂ , oxides of REM and CaO were produced as single oxides. there was.

In addition, in the obtained quantitative results, the circle equivalent diameter of the oxide having an oxygen content of 5% or more was measured, and the number of oxides having a circle equivalent diameter (particle diameter) of 0.1 to 2.0 μm and the oxide having a circle equivalent diameter (particle diameter) of more than 5.0 μm. The number of was calculated. Table 5 shows the number of oxides converted to 1 mm ² per field of view.

Next, in order to evaluate the toughness of HAZ affected by the heat at the time of welding, the welding heat test was simulated and the welding reproduction test shown below was performed. The welding reproduction test was heated so that the sample cut out from the rolled material might be 1400 ° C, held at this temperature for 30 seconds, and then given a heat cycle to cool. The cooling rate was adjusted so that cooling time from 800 degreeC to 500 degreeC might be 300 second.

The impact characteristic of the sample after cooling was evaluated by measuring the absorption energy (vE- ₄₀ ) in -40 degreeC by the V notch Charpy test. Let vE- ₄₀ be 100 J or more to pass (good HAZ toughness). The measurement results are shown in Table 5 below.

Next, the metal structure of the obtained rolling material was observed in the following order, and the average circle equivalent diameter D of the crystal grain enclosed by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more, and the ratio R of the random grain boundary occupied by diagonal grain boundary were calculated | required. The values of D (μm) and R (area%) are shown in Table 6 below.

? Calculation method of D

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

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

(3) Two mirror polished surfaces were determined by an EBSP (Electron Back Scattering Pattern) device manufactured by TexSEM Laboratories, with a measurement range of 200 μm × 200 μm and a pitch of 0.5 μm at the t / 2 position in the sheet thickness direction. The azimuth difference is measured, and the boundary where the crystal orientation difference is 15 degrees or more is assumed to be a diagonal boundary. On the other hand, the measuring point whose confidence index indicating the reliability of the measuring direction is smaller than 0.1 is excluded from the analysis target.

(4) In the grain distribution map, the maximum width (usually length along the plate thickness direction) and the maximum length (usually along the rolling direction) of crystal grains surrounded by diagonal grain boundaries with a crystal orientation difference of 15 ° or more are determined. By measuring, the area of crystal grains is calculated, the circular equivalent diameter of crystal grains is computed, and an average value is calculated | required.

? R calculation method

(1) The ratio R of the random grain boundary in the diagonal grain boundary is an EBSP device manufactured by TexSEM Laboratories, using a sample subjected to specular finishing under the same conditions as when the above D is calculated. The azimuth difference between the two crystals is measured by setting the measurement range at the t / 2 position and the t / 4 position to 200 µm x 200 µm and the pitch to 0.5 µm. On the other hand, the measuring point whose confidence index indicating the reliability of the measuring direction is smaller than 0.1 is excluded from the analysis target.

(2) In the measurement result, the thing with a crystal orientation difference of less than 5.5 degrees is regarded as noise, and it removes, and the distribution in each orientation difference up to 62.5 degrees is calculated | required.

(3) The random grain boundary fraction R was computed by making the crystal orientation difference of said (2) correspond with a corresponding map. The correspondence map is a table in which the number (fraction) of each corresponding grain boundary is described. Corresponding grain boundaries are grain boundaries in a particular direction so that a certain proportion of the lattice points among the crystal lattice points of the two crystal grains sandwiching the grain boundaries is common to both crystal grains. Specifically, by dividing each corresponding grain boundary (Σ1 to 49) by the number of diagonal grain boundaries of 15 degrees or more obtained by the crystal direction distribution, the distribution (fraction) of each corresponding grain boundary is obtained, and each corresponding grain boundary is 100%. By subtracting the distribution (fraction) of [meaning other than the corresponding grain boundary to be a random grain boundary (> Σ49)], the random grain fraction R (average value) is measured. That is, the diagonal grain boundary consists of a corresponding grain boundary and a random grain boundary, and in order to calculate the random grain boundary fraction which is this time, the fraction (distribution) of a corresponding grain boundary is needed. Therefore, it is necessary to obtain each fraction (distribution) of Σ1 to 49 that are considered to be corresponding grain boundaries. Here, Σ is the inverse of the ratio of the number of A to the total lattice points in which A and B are combined, when a lattice point occupying a common position in both crystals having grain boundaries is set to A, and other lattice points are B. .

On the other hand, "TSL OIM Data Collection ver 5.2" of TSL Corporation was used for the measurement of a corresponding grain boundary, and "TSL OIM Analysis ver 5.0" of TSL Corporation was used for analysis.

Moreover, the metal structure of the obtained rolling material was observed in the following procedure, and the austenite particle diameter ((gamma) particle diameter) and hardness were calculated | required. The value of γ particle diameter is shown in Table 6 below.

? Measuring method of γ particle size

(1) The γ particle size of the rolled material is corroded using an ultra-low carbon corrosion solution using a sample subjected to mirror finish under the same conditions as when the above D is calculated, and after exposing a sphere γ grain boundary, the plate thickness direction The area | region at the t / 2 position was image | photographed with the optical magnification 100 times or 400 times, and was made into the photograph of 6 cm x 8 cm. That is, it corresponds to 600 micrometers x 800 micrometers at 100 times the observation magnification, and 150 micrometers x 200 micrometers at 400 times the observation magnification.

(2) The 6 cm side of the photograph corresponds to the plate thickness direction, and the 8 cm side corresponds to the rolling direction. The photographed picture was analyzed for images, and the average particle diameter of the γ particles seen in the observation field was obtained.

? Measuring Hardness

The hardness of the rolled material was measured with a Vickers hardness tester using a sample subjected to mirror finish under the same conditions as when the above D was calculated. The measurement position was made into t / 2 position of the plate | board thickness direction of a rolling material, the load was 10 kgf, the measurement frequency was made into 20 points, and the average value was made into the hardness of the rolling material.

Next, the fatigue characteristic of the obtained rolling material was evaluated in the following procedure.

? Evaluation of Shock Characteristics

The impact characteristic of the rolled material was evaluated by performing the V-Nupey Charpy test, and measuring the impact characteristic of the rolled material by absorbed energy (vE- ₆₀ ) at -60 ° C. The measurement of vE- ₆₀ collected the U4 test piece which the NK (Japan Maritime Association) classification Society determines from t / 4 position, and performed it according to JISZ2242. The measurement results are shown in Table 6 below. On the other hand, in shipbuilding E grade in NK classification, since the impact temperature of a base material evaluates a test temperature at -40 degreeC, in this experiment example, conditions were made more strictly as the test temperature as -60 degreeC, and absorbed energy ( vE- ₆₀ ) was measured and this average value made 100J or more pass (good low-temperature toughness of a base material).

? Measurement of Fatigue Crack Growth Rate

From the t / 4 position of the rolled material, a compact test piece (CT test piece) having a thickness of 12.5 mm was taken. The shape of the collected CT test piece is shown in FIG. The fatigue crack growth rate was calculated | required by performing a fatigue crack growth test based on ASTME647 using this CT test piece. The test conditions are as follows.

Test Method: Using an electro-hydraulic servo type ± 10 ton fatigue tester, the crack length was measured by a computer controlled compliance method. Compliance means the ratio (delta / P) of the crack opening displacement (delta) and the load P, and a crack length is automatically measured from this compliance.

Test environment: room temperature, air

Control method: load control

Control waveform: sine wave

Stress ratio: R = 0.1

Test speed: 600 ~ 1200cpm

The crack was generated from the weld end portion, and was assumed to be developed with the HAZ and the base metal, and evaluated at a value of ΔK = 20 (MPa? √m), which is the middle ΔK region. On the other hand, in the ΔK region of this test, it was found that the Paris rule defined by the following equation (6) is a stable stable growth region.

However, a: crack length, n: number of repetitions, C, m: constants determined in the case of material, load, and the like, ΔK: stress intensity factor ranges, respectively.

On the other hand, since it is known that the value of the fatigue crack growth rate by CT test is uneven, it evaluated based on the average value of the data of the conventional steel plate. Since it is the average value of the prior data of the steel sheet 5.4 × 10 ^-5 mm / cycle or so, in the present invention, was less than 5.4 × 10 ^-5 mm / cycle to pass. On the other hand, the average value of the data of the conventional steel sheet was determined according to "metal material fatigue crack growth resistance data" published by the Japanese Society of Materials, Inc.

Based on these results, the relationship between the average circle equivalent diameter D and the impact characteristic (vE- ₆₀ ) of the crystal grain enclosed by the diagonal grain boundary is shown in FIG. 2 shows that it is useful to make the said average circle equivalent diameter D into 25 micrometers or less in order to improve the impact characteristic at -60 degreeC.

3 shows the relationship between the ratio R of the random grain boundaries at the diagonal grain boundaries and the fatigue crack growth rate (da / dN). 3 shows that in order to make fatigue crack growth rate less than 5.4x10 <^-5> mm / cycle, it is useful to make ratio R of the said random grain boundary below 70 area%.

The relationship between the average hardness of a rolled material and the fatigue crack growth rate (da / dN) is shown in FIG. 4 shows that in order to make fatigue crack growth rate less than 5.4x10 <^-5> mm / cycle, it is useful to make the average hardness of a rolling material into 170 Hv or more.

The relationship between the cumulative reduction ratio at Ar ₃ points + 100 ° C. or lower and the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundaries is shown in FIG. 5. From Figure 5, to the average circle-equivalent diameter D below the 25㎛, it may be useful to know that the cumulative rolling reduction at Ar ₃ point + 100 ℃ or less to over 50%.

The relationship between the average circle equivalent diameter D of the crystal grain enclosed by (gamma) particle diameter and a diagonal grain boundary is shown in FIG. 6 shows that it is useful to make (gamma) particle diameter 50 micrometers or less in order to make the said average circle equivalent diameter D into 25 micrometers or less.

The relationship between the cumulative reduction ratio at 1050 ° C. or lower and the γ particle size is shown in FIG. 7. 7 shows that in order to make a (gamma) particle diameter into 50 micrometers or less, it is useful to make the cumulative reduction ratio in 1050 degreeC or less 40% or more.

8 shows the relationship between the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundaries and the ratio R of the random grain boundaries in the diagonal grain boundaries. From Fig. 8, the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundary and the ratio R of the random grain boundary occupying the diagonal grain boundary are correlated, and as the crystal grains surrounded by the diagonal grain boundary become smaller, the random grain boundary can be reduced. Able to know.

Next, according to the result shown in Table 5, the HAZ toughness at the time of welding is considered.

No.1-1, No.2-1, No.3-1, and No.23 are examples of satisfying the requirements defined in the present invention. When the composition of all oxides is measured and mass converted as a single oxide, ZrO ₂ , after adjusting to contain a predetermined amount of the oxide and CaO of REM, many fine oxides having a circle equivalent diameter of 0.1 to 2.0 µm are produced so that coarse oxides having a circle equivalent diameter of more than 5.0 µm are not produced. Steel having good HAZ toughness is obtained.

On the other hand, Nos. 4 to 22 are examples that deviate from the requirements of any of the conditions specified in the present invention.

No. 4 is an example in which the amount of dissolved oxygen in molten steel is insufficient. As a result, the amount of fine oxide serving as a nucleus for the formation of ferrite in the particles, which contributes to the improvement of HAZ toughness, cannot be ensured, and the HAZ toughness is not improved. No. 5 is an example in which the dissolved oxygen amount of molten steel is excessive, and coarse oxide is produced and HAZ toughness deteriorates. No. 6 is an example in which the amount of dissolved oxygen in molten steel is insufficient, and in which the total amount of oxygen in molten steel before addition of Ti or the like is also insufficient. Therefore, the amount of the fine oxide which becomes the nucleus of the ferrite in the particle which contributes to the improvement of the HAZ toughness cannot be secured, and the HAZ toughness is not improved.

No. 7 is an example which simulated the composition of the steel material disclosed by the said Unexamined-Japanese-Patent No. 2007-100213. Since after adjusting the dissolved oxygen amount of molten steel, the total amount of oxygen is not adjusted by stirring molten steel, the total amount of oxygen before adding Ti exceeds the range prescribed | regulated by this invention. Thereby, coarse oxide increases and HAZ toughness deteriorates.

In Nos. 8 and 9, since molten steel is stirred for a long time after Zr, REM, and Ca are added, oxides in molten steel aggregate to each other to produce a large amount of coarse oxides. Therefore, HAZ toughness deteriorates.

As an example of No. 10, too much Al produces coarse oxide, and HAZ toughness deteriorates. No. 11 is an example in which Ti is too small. HAZ toughness is not improved because fine nitrides that form nuclei of ferrite in particles that contribute to the improvement of HAZ toughness are reduced. No. 12 is an example in which there is too much Ti, and the coarsening of oxides deteriorates the HAZ toughness. No. 13 is an example in which the amount of REM is too small, and the amount of oxides of REM when converted into a single oxide is small, and the amount of fine oxides that form nuclei of ferrite in particles, which contributes to the improvement of HAZ toughness, is small. Not improved No. 14 is an example in which there are too many REMs, and the amount of oxides of REM when converted into a single oxide is large, and coarse oxides are produced to deteriorate HAZ toughness. No. 15 is an example in which Zr is too small, and the amount of ZrO ₂ when converted into a single oxide is small, and the amount of fine oxides that form nuclei of ferrite in particles, which contributes to the improvement of HAZ toughness, decreases, thereby improving HAZ toughness. It is not. In No. 16, too many examples of Zr have a large amount of ZrO ₂ when converted into a single oxide, and coarse oxides are produced to deteriorate HAZ toughness. No. 17 is an example in which Ca is too small, the amount of Ca 0 when converted into a single oxide is small, and the amount of fine oxides that form nuclei of ferrite in the particles, which contributes to the improvement of HAZ toughness, decreases, thereby improving HAZ toughness. Not. No. 18 is an example in which there are too many Ca, and the amount of CaO when converted into a single oxide is large, and coarse oxides are generated to deteriorate HAZ toughness. No. 19 is an example where there is too much N, and the amount of solid solution N increases, the toughness of the base material itself deteriorates, and HAZ toughness also falls. As No. 20 is an example in which N is too small, since the production of Ti-containing nitride is suppressed, coarsening of austenite particles due to the magnetic flux fixing effect cannot be prevented, and HAZ toughness is deteriorated. No. 21 is an example in which O is too small, and the amount of fine oxides that form nuclei for ferrite in particles is insufficient, and the HAZ toughness is not improved. No. 22 is an example in which there is too much O, and an oxide coarsens and HAZ toughness deteriorates.

Next, according to the result shown in Table 6, the fatigue characteristic of a base material itself is considered.

No.1-1 to 1-6, No.2-1, No.2-2, No.3-1, No.3-2, and No.23 are all steel grades satisfying the requirements specified in the present invention. As an example using a to c, since the metal structure of the steel is properly controlled, the fatigue characteristics of the base metal are excellent.

Although No. 1-7 and No. 1-8 are examples using the steel grade a which satisfies the requirements specified in the present invention, the steel materials are so hard that the fatigue properties of the base material are inferior.

Nos. 15 to 17 are inferior in fatigue characteristics because the metal structure of the steel is not properly controlled. In particular, since the average circle equivalent diameter D of the crystal grains enclosed by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more exceeds 25 micrometers, the impact characteristic is also inferior.

Nos. 10, 12 to 14, 18, 21, and 22 have good impact characteristics among the fatigue properties because the metal structure of the steel is not properly controlled, but the fatigue crack growth rate is not small.

On the other hand, Nos. 4 to 9, 11, 19, and 20 have good fatigue properties of the base material because the metal structure of the steel is properly controlled, but the component composition of the steel does not satisfy the requirements specified in the present invention. Therefore, as mentioned above, the HAZ toughness at the time of welding is not improved.

Next, the result shown in Tables 5-6 is put together and considered.

No.1-1 to 1-6, No.2-1, No.2-2, No.3-1, No.3-2 and No.23 in Table 6 satisfy the requirements specified in the present invention. As an example of using steel grades a to c and steel grade w, steel materials obtained by using such steel grades have good HAZ toughness as evident from Table 5 and excellent fatigue properties of the base metal as evident from Table 6.

On the other hand, Nos. 1-7, Nos. 1-8, and Nos. 4 to 22 in Table 6 are examples that do not satisfy any of the requirements defined in the present invention. It is a backward example.

Claims (7)

C: 0.02 to 0.12% (the meaning of "mass%". Hereinafter, the same as for chemical components and oxides),
Si: 0.02 to 0.5%,
Mn: 1-2%,
Zr: 0.0002 to 0.050%,
REM: 0.0002 to 0.050%,
Ca: 0.0005 to 0.010%,
Ti: 0.005 to 0.02%,
N: 0.0040 to 0.01%,
0: 0.0005 to 0.010%,
P: 0.02% or less,
S: 0.015% or less,
Al: 0.05% or less is satisfied,
The balance is made of iron and inevitable impurities,
(a) the steel material comprises an oxide containing Zr, REM and Ca,
(b) the mass ratio of ZrO ₂ in the average composition of the oxide contained in the steel is 5-50%, the mass ratio of the oxide of REM (M ₂ O _{3 when} REM is represented by the symbol M) is 10-50%, And the mass ratio of CaO is 5.0 to 50%,
(c) of all the oxides contained in the steel,
120 or more oxides of 0.1 to 2.0 탆 in 1 mm ² of equivalent circle diameter,
The 5.0㎛ than an oxide with a circle equivalent diameter is not more than 5 per 1mm ^2,
(d) When the metal structure of the steel is observed by the backscattered electron diffraction method (EBSP method), the following equations (1) and (2) are satisfied,
(e) Steel having an average hardness of 170 Hv or more.
[Equation 1]
D ≤ 25 μm
&Quot; (2) &quot;
R ≤ 70 area%
[Equation 1] In Equation (1), D denotes an average circle equivalent diameter (mu m) of crystal grains surrounded by diagonal grain boundaries having a crystal orientation difference of 15 ° or more by measuring the orientation difference between two adjacent crystals by the EBSP method.
In addition, in Equation 2, R denotes the ratio (area%) of the random grain boundary to the diagonal grain boundary.] The method of claim 1,
In addition, as other elements,
Ni: 0.4% or less,
Cu: 0.3% or less,
Cr: 1.5% or less, and
Mo: Steel containing 1 or more types chosen from the group which consists of 1% or less. The method of claim 1,
In addition, as other elements,
Nb: 0.1% or less,
V: 0.1% or less, and
B: Steel materials containing at least one member selected from the group consisting of 0.005% or less. As a manufacturing method of the steel materials in any one of Claims 1-3,
After adjusting the dissolved oxygen amount of molten steel in the range of 0.0010 to 0.0060%,
After stirring the molten steel to separate the oxides in the molten steel to adjust the total amount of oxygen in the range of 0.0010 to 0.0070%,
After adding Ti, then Zr, REM and Ca, and adjusting to the component composition as described in any one of Claims 1-3,
With casting,
When hot rolling,
After heating the slabs to 1100 to 1250 ℃,
While cooling the 1050 ℃ or less, Ar ₃ point + in the temperature range exceeding 100 ℃ 1 pass per average cooling rate is at least 1.5 ℃ / second cumulative rolling reduction is subjected to a first rolling is at least 40%,
Then after the Ar ₃ point + 100 ℃ or less, per one-pass reduction rate is up to 12% or less, the cumulative rolling reduction in the temperature range exceeding the Ar ₃ point is subjected to a second rolling is at least 50%,
The manufacturing method of steel materials cooled by the average cooling rate of 5 degrees C / sec or more to the temperature range which surface temperature becomes 500 degrees C or less. The method of claim 4, wherein
After adding Zr, REM, and Ca, casting is performed after stirring molten steel in the range which does not exceed 40 minutes. The method of claim 4, wherein
A method for producing steel, wherein the first rolling is performed in an austenite recrystallization temperature range. The method of claim 4, wherein
Steel material subjected to tempering treatment at a temperature range of 500 ° C or higher and less than Ac ₁ point after cooling at an average cooling rate of 5 ° C / sec or more to a temperature range where the surface temperature of the steel material after the second rolling becomes 500 ° C or lower. Manufacturing method.

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