JP4606321B2 - High cleanliness steel with excellent fatigue strength - Google Patents

Description

  The present invention relates to a high cleanliness steel excellent in fatigue characteristics, particularly a forging steel, and more particularly to a forging steel in which inclusions present in a steel material are refined. The forged product manufactured from the steel of the present invention is useful in a wide range of machines, ships, electric machines, automobiles, etc., but in the following, as a typical application example, a crankshaft used as a transmission member for a ship drive source The explanation will be focused on the case of applying to the above.

  What is manufactured using forging steel, for example, a large crankshaft which is a transmission member of a marine drive source, is required to have excellent fatigue characteristics that do not cause fatigue failure even under severe use environments. Furthermore, in recent years, generators and marine diesel engines have been aimed at higher output and lighter weight, and the integral crankshaft used therefor is increasingly required to have high fatigue strength. As the demand for higher fatigue strength increases, fatigue breakage due to non-metallic inclusions becomes a problem, and the demand for reduction and miniaturization of non-metallic inclusions becomes more severe.

For reducing and reducing the size of hard non-metallic inclusions, many techniques have been proposed so far. For example, in Non-Patent Document 1, inclusions in spring steel have a melting point of 1400-1500 ° C. It has been disclosed that if controlled to Al ₂ O ₃ —SiO ₂ system, it does not become a starting point of fatigue failure, and that non-ductile inclusions such as Al ₂ O ₃ may be reduced in the tire cord. In Patent Documents 1 and 2, if the average composition of inclusions is SiO ₂ : 20 to 60%, MnO: 10 to 80%, CaO: 50% or less, MgO: 15% or less (in the case of Patent Document 1) ), Or if the average composition of inclusions is SiO ₂ : 35 to 75%, Al ₂ O ₃ : 30% or less, CaO: 50% or less, MgO: 25% or less, intervening during cold working or wire drawing It describes that it can be rendered harmless because it breaks up and disperses.

In Patent Document 3, a clean steel is manufactured while adding a mixture of a Si-based deoxidizer and an alkali metal compound to molten steel and controlling the deoxidation product to a composition containing an alkali metal. These alkali metals are used to lower the melting point of hard non-metallic inclusions such as alumina and SiO _2. As a result, the non-metallic inclusions can be elongated like a thread during hot rolling. The form is harmless to the drawability and fatigue resistance. For example, Na or Li is used as the alkali metal, and Na and Li are considered to be synergistic elements. Alkaline metals are recommended to be added together with a deoxidizer because the yield is poor even if it is added to molten steel as it is. For example, at the beginning of the molten steel treatment (LF) process after steel is removed from the converter to the ladle. Li is added in the form of LiF together with sodium silicate at the position where the stirring Ar bubbles rise.

  Also in Patent Document 4, an alkali metal is added to molten steel for the purpose of lowering the melting point of inclusions and deforming the inclusions during hot rolling. As the alkali metal, Li, Na, K and the like are used, and these are considered to be synergistic elements. Further, since alkali metals do not dissolve in molten steel, it is recommended to dilute them with Si. Specifically, a Si alloy containing Li in a range of 12% or less is added as a deoxidizer.

In order to make ductile inclusions in Patent Document 5, alkali metal oxides are included in inclusions mainly composed of SiO ₂ . In this document, it is explained that the improvement in ductility of inclusions is not due to the melting point decrease as described in the above documents 3 to 4, but due to the decrease in interfacial energy between inclusions and molten iron due to alkali metal. However, in any case, the alkali metals Na, K, and Li are described as being equivalent. Moreover, the alkali metal is added as much as about 10% (concentration in slag) by adding slag. Actually, only Na is used.

Patent Document 6 proposes to use an alkali metal oxide when deoxidizing Si. The reason for using alkali metal oxide in this document is that the SiO ₂ activity in the ladle slag can be made sufficiently low, and as a result, the total oxygen concentration in the molten steel can be reduced. There is. In this document, Na ₂ O, K ₂ O, Li ₂ O, and the like are listed as alkali metal oxides, but these are described as effective elements. This document is different from the above-mentioned patent document 5 in that Li is actually added. Specifically, Li ₂ O is blended in the slag in a carbonate state, and the concentration in the case of Li (in the slag) Has reached up to 8%.

However, these inclusion control techniques are not intended for forgings in which large components are used in harsh environments, such as a transmission member of a ship drive source. Therefore, in order to enhance the fatigue characteristics of large forgings, it is required to study and establish a unique inclusion control method for forging steel.
Edited by the Japan Iron and Steel Institute “126th and 127th Nishiyama Memorial Technology Course”, published by the Japan Iron and Steel Institute, November 14, 1988, pages 145 to 165 JP-A-62-99436 JP-A-62-99437 JP-A-1-319623 Japanese Patent Laid-Open No. 2-15111 JP 2002-167647 A JP 2002-194497 A

  The present invention has been made paying attention to the above circumstances, and an object thereof is to provide a high cleanliness steel having further improved fatigue characteristics.

As a result of intensive studies to solve the above problems, the present inventors have found that Li has a unique action and effect not found in other alkali metals (Na, K, etc.). That is, Li is the same as Na and K in that the melting point of inclusions is lowered, but only Li is a complex oxide-based inclusion (for example, CaO—Al ₂ O ₃ —SiO ₂ —MnO—MgO-based complex oxide). And the like, and the effect peculiar to the above-mentioned Li becomes remarkable when the Li in the steel is set to a predetermined amount or more, and it has been found that the fatigue strength is remarkably improved. More specifically, when total-Li (meaning dissolved Li and Li in inclusions, hereinafter the same) is set to a predetermined concentration or more, the composite oxide is a single-phase liquid composite oxidation containing Li in the molten steel. objects (e.g., such as _{CaO-Al 2 O 3 -SiO 2} -MnO-MgO-Li 2 O -based composite oxide), and although even during slab after casting is present as a complex oxide of a single phase, When the steel having the composite oxide is heated for rolling, the inclusions shift to an equilibrium state at a rolling temperature or a forging temperature. As a result, in the single-phase composite oxide, phase separation proceeds into a glassy phase and a crystalline phase, and the crystalline phase as an equilibrium phase is finely precipitated in the glassy single-phase inclusions. In this state, when rolling, hot rolling, or hot forging is performed, the vitreous part is highly stretchable due to its low melting point and low viscosity, while it stretches well, while at the interface between the crystal phase and the glass phase Stress at the time of rolling or forging concentrates, and breaks down easily. In other words, not only the diameter is reduced, but also the dividing effect works, so that the inclusions become extremely fine. Such a phenomenon occurs because Li ₂ O has an effect of promoting fine crystallization. Na ₂ O, K ₂ O, and the like have the effect of lowering the melting point and viscosity even with the same alkali metal. However, the effect of promoting crystallization is weak, so even if these Na ₂ O and K ₂ O are used, the effect of refining inclusions is insufficient.

  Therefore, in the present invention, the total-Li concentration in the steel is set to a predetermined amount or more in order to exert the refinement effect using the crystallization promotion as described above. And in order to fully exhibit the improvement effect of the fatigue characteristic by this Li, it was decided to suppress the oxide type inclusion derived from a refractory etc.

In contrast to the present invention, the invention disclosed in Non-Patent Document 1 and Patent Documents 1 and 2 (CaO—Al ₂ O ₃ —SiO ₂ inclusions have a melting point of 1400 to 1500 ° C. or lower. In the invention that controls the composition of inclusions in the region, etc., the inclusions are reduced in size to some extent, but since the effect of promoting crystallization by Li is not used, the reduction in size is insufficient. Moreover, these documents aim to directly control the inclusion composition. In order to directly control the composition of inclusions, harmless top slag is involved during slag refining, and harmful oxidization products (especially SiO ₂ and Al ₂ O ₃ ) in the molten steel are combined. It is important to react and detoxify. By this operation, total-O does not decrease so much, but dissolved oxygen decreases thermodynamically, and as a result, it becomes difficult to generate a harmful deoxidation product of SiO ₂ system generated during solidification. However, when the inclusion composition is directly controlled (that is, using the slag reaction) in this way, it is necessary to vigorously stir the molten steel or slag, so that inclusions derived from refractories are easily mixed.

  Although Patent Documents 3 to 6 refer to Li, these Patent Documents 3 to 6 are insufficient. For example, in Patent Document 3, Li is added together with sodium silicate in the form of LiF, but LiF has a melting point of 842 ° C. and a boiling point of 1676 ° C., which are close to the steelmaking temperature, and the yield is insufficient. Therefore, like this patent document 3, it is necessary to add LiF to the position where Ar bubbles for stirring rise in the early stage of the molten steel processing (LF) process after steel is discharged from the converter to the ladle.

  However, even in this case, it is still difficult to secure a sufficient amount of Li in the steel, and the Li concentration in the slag becomes too high. As a result of confirmation by the present inventors, the LiF concentration in the slag is as high as 4%. When strong stirring is performed using slag having a high Li concentration from the beginning of the molten steel treatment (LF), the refractory melts violently, and foreign inclusions originating from the refractory begin to increase. Furthermore, Li is insufficient and the inclusion refinement effect becomes insufficient. As a result, the fatigue characteristics are not improved sufficiently.

  Also in Patent Document 4, the Li concentration in the slag is high. That is, since the Si-Li alloy used in Patent Document 4 has a Li concentration of 12% or less, the yield of Li is low, and in order to perform inclusion control with such a Si-Li alloy, Li in the slag It is necessary to increase the concentration.

  In Patent Documents 5 to 6, the Li concentration in the slag is as extremely high as 8 to 10% at the maximum. When the Li concentration in the slag is so high, the amount of Li in the steel can finally be ensured, but conversely, the melting point and viscosity of the slag are remarkably lowered, and the refractory melt resistance is remarkably increased. If such a slag is made from the initial stage of the molten steel treatment and is vigorously stirred, the refractory is severely damaged, and when the amount of Li is secured, the fatigue characteristics are significantly lowered.

  Unlike the above-mentioned documents, the steel of the present invention succeeded in increasing the Li concentration in the steel to a predetermined amount or more while suppressing oxide inclusions derived from refractories and the like. And when total-Li density | concentration in steel was made into predetermined amount or more, it discovered that the crystallization promotion effect of Li was exhibited as mentioned above, and the inclusion could be refined | miniaturized very much, and completed this invention. Such steel can be produced, for example, by devising the form of addition of Li to increase the yield and adding Li when the molten steel treatment (LF) process is completed.

That is, the high cleanliness steel excellent in fatigue strength according to the present invention includes (1) total-Li in the range of 0.020 ppm to 20 ppm (mass basis) and / or (2) total-Li. Si is contained in a range of total-Li / Si (mass ratio) = 5 × 10 ^{−6 to} 9000 × 10 ⁻⁶ , and the cross-sectional inclusion size (√inclusion cross-sectional area) becomes 160 μm or less. There is a feature in that. The cross-sectional inclusion size (√inclusion cross-sectional area) is more preferably 100 μm or less, and still more preferably 60 μm or less.

  In the high cleanliness steel of the present invention, the chemical components are usually C: 1.0% (meaning mass%, hereinafter the same) or less (not including 0%), Si: 0.6% or less (0%) Mn: 1.5% or less (not including 0%), Ni: 4% or less (not including 0%), Cr: 12% or less (not including 0%), Mo: 1. 0% or less (not including 0%), total-Al: 0.01% or less (not including 0%), O: 0.005% or less (not including 0%), total-Mg: 0.1 -15 ppm (mass basis) and total-Ca: 0.1-40 ppm (mass basis), and V, Nb, W, Cu, Ti, etc. may be added as appropriate, and the balance is Fe And unavoidable impurities.

  According to the present invention, while suppressing the increase of oxide inclusions such as those derived from refractories, the amount of total-Li in the steel is appropriately controlled, so that the fatigue properties of high cleanliness steel can be further improved. .

In the high cleanliness steel of the present invention, Li is effectively used. Unlike other alkali metals (Na, K, etc.), Li significantly alters complex oxide inclusions (for example, CaO—Al ₂ O ₃ —SiO ₂ —MnO—MgO based oxide). Is possible. That is, during steelmaking, Li is taken into the complex oxide to form a single-phase complex oxide (for example, a CaO—Al ₂ O ₃ —SiO ₂ —MnO—MgO—Li ₂ O-based complex oxide). When this steel is heated to a hot temperature, the Li-containing composite oxide inclusions undergo phase separation into a vitreous phase and a crystalline phase, and in an equilibrium phase in a vitreous single-phase inclusion. When a certain crystal phase is finely precipitated, and when rolling, hot rolling or hot forging is performed in this state, the vitreous part is highly extensible due to its low melting point and low viscosity. The stress at the time of rolling or forging is concentrated at the interface between the crystal phase and the glass phase, and the breakage is easily broken, so that the inclusions become extremely fine.

In addition, since Li is a strong deoxidizing element, it also has the effect of reducing dissolved oxygen in the steel, and the amount of oxide itself can be reduced. In addition, when Li is present in the molten steel, it also has an effect of suppressing the generation of high SiO ₂ -based harmful oxides generated during solidification.

  In order to appropriately control Li, it is necessary to control Li by an index having excellent correlation with the fineness of inclusions. As such an index, (1) the total-Li amount in the steel, and (2) the ratio between the total-Li amount and the Si amount in the steel [Li / Si ratio (mass ratio)] can be used. May be used as an index, or a combination of both. The latter (total-Li / Si ratio) refers to Si as an oxide-forming element modified by Li, and defines the total-Li amount relative to Si, and is particularly effective for Si-deoxidized steel. It is. The former (total-Li amount in steel) can be widely applied besides Si deoxidized steel.

  In order to effectively exhibit the function of Li, the total-Li amount in the steel is 0.020 ppm (mass basis) or more, preferably 0.03 ppm (mass basis) or more, more preferably 0.1 ppm (mass). (Standard) or more, for example, 0.5 ppm (mass standard) or more [for example, 1 ppm (mass standard) or more] may be used.

The mass ratio between total-Li and Si in steel (total-Li / Si) is 5 × 10 ⁻⁶ or more, preferably 10 × 10 ⁻⁶ or more, more preferably 50 × 10 ⁻⁶ or more. For example, it may be about 100 × 10 ⁻⁶ or more (for example, 200 × 10 ⁻⁶ or more).

Even when total-Li becomes excessive, the number of oxide inclusions (hard inclusions) increases, coarse inclusions also increase, and fatigue strength decreases. Also, if the total-Li exceeds 20 ppm (mass basis), the Li addition means such as adding a Li-Si alloy to the molten steel after the molten steel treatment is completed, for example, the slag Li moves and slag contains a lot of Li ₂ O, and its melting point and viscosity are remarkably lowered. As a result, the refractory melts severely, and the hard coarse oxide derived from the refractory increases. Accordingly, the total-Li amount in the steel is 20 ppm (mass basis) or less, preferably 9 ppm (mass basis) or less, and more preferably 6 ppm (mass basis) or less. The mass ratio (total-Li / Si) between total-Li and Si in the steel is 9000 × 10 ⁻⁶ or less, preferably 8000 × 10 ⁻⁶ or less, and more preferably 6000 × 10 ⁻⁶ or less.

  Furthermore, in the high cleanliness steel of the present invention, an increase in inclusions derived from refractories is also suppressed. That is, the cross-sectional inclusion size (√inclusion cross-sectional area) is, for example, 160 μm or less, preferably 100 μm or less, and more preferably 60 μm or less.

  Here, the value of “cross-sectional inclusion size (√inclusion cross-sectional area)” in the present invention is the fatigue test described below. The observed location is observed with an SEM (observation magnification of about 200 to 2000 times), the length and width of the maximum inclusion region (breaking origin inclusion region) in the observation field are measured, and the product (inclusion cross-sectional area) To the power of 1/2. The location of the fracture starting point of steel can be identified relatively easily from the pattern of the fracture surface. The number of observations by SEM may be one. This is because the fractured part is observed starting from the inclusion, so that the size of the cross-sectional inclusion that affects the fatigue strength can be measured more accurately without underestimating.

In the fatigue test (difference method) in the above method for measuring the inclusion size (√inclusion cross-sectional area), a smooth test piece (diameter 10 mm x gauge length 30 mm) is cut out from steel, and the test piece is turned ono-type by rotating bending. While maintaining the number of revolutions at 3600 rpm with a tester and increasing the stress by 20 MPa every 3 × 10 ⁶ cycles, the test is performed until the specimen breaks.

  While suppressing inclusions derived from refractory as described above, fatigue characteristics can be improved by controlling the total-Li amount to refine the inclusions.

  The high cleanliness steel of the present invention is a steel that employs Si deoxidation, such as stainless steel, and can be widely used for applications to make inclusions finer, and has particularly excellent fatigue characteristics. It can be advantageously used for generators and integrated crankshafts used in marine diesel engines. When applying the high cleanliness steel of the present invention to these uses, C: 1.0% or less (0% not included) (preferably 0.2 to 0.6%), Si: 0.6% or less (0% not included) (preferably 0.1 to 0.4%), Mn: 1.5% or less (0% not included) (preferably 0.5 to 1.5%), Ni: 4 % Or less (not including 0%) (preferably 0.1 to 2.5%), Cr: 12% or less (not including 0%) (preferably 1 to 3.5%), Mo: 1.0 % Or less (excluding 0%) (preferably 0.1 to 0.7%), total-Al (the sum of dissolved Al and Al in inclusions, the same shall apply hereinafter): 0.01% or less ( Preferably 0.008% or less, more preferably 0.005% or less), O: 0.005% or less (preferably 0.004% or less, more preferably 0.003%) Steel is below) can be used.

The present invention modifies complex oxide inclusions (such as CaO—Al ₂ O ₃ —SiO ₂ —MnO—MgO complex oxide) with Li, and Ca and Mg constituting the inclusion are In many cases, it is taken into steel by the top slag entrainment at the stage of the molten steel treatment. If necessary, Ca or Mg may be added. When the secondary deoxidation product generated during solidification becomes hard SiO ₂ -rich, Al ₂ O ₃ -rich, which may be a problem, and the addition of Ca, Mg, Li, etc. is effective for these. There is. The secondary deoxidation product is generated by using the primary product inclusions as a core or alone, and compared with the inclusion composition in molten steel such as tundish (TD), SiO ₂ -rich or Al ₂ O. There are cases where _3- rich is likely to occur, but if Ca, Mg, Li or the like is added, the secondary deoxidation product also contains SiO ₂ , Al ₂ O ₃ , CaO, MgO, Li ₂ O or the like. It becomes a complex oxide (inclusion), and generation of hard, high SiO ₂ or high Al ₂ O ₃ inclusions can be suppressed.

  The total-Ca in steel (meaning the total of dissolved Ca and Ca in inclusions, hereinafter the same) is about 0.1 to 40 ppm (mass basis) [preferably 0.2 to 25 ppm (mass basis)], total-Mg (meaning the total of dissolved Mg and Mg in inclusions, hereinafter the same): 0.1 to 15 ppm (mass basis) [preferably 0.2 to 10 ppm (mass basis)].

  If necessary, V, Nb, W, Cu, Ti and the like may be further contained as physical property improving elements, and these elements may be contained alone or in combination of two or more. Preferred contents of these elements are V: 0.5% or less (preferably 0.005 to 0.35%), Nb: 0.1% or less (preferably 0.005 to 0.05%), W : 1% or less (preferably 0.01 to 0.5%), Cu: 2% or less (preferably 0.005 to 0.1%), Ti: 0.15% or less (preferably 0.005 to 0) .08%).

  The balance may be Fe and inevitable impurities. P, S and N are components that are easily segregated, but in consideration of segregation, P is allowed to be 0.05% or less, S is 0.007% or less, and N is allowed to be 0.015% or less. The

  Optimum high cleanliness steel as an integral crankshaft is C: 0.2 to 0.6% (preferably 0.3 to 0.5%), Si: 0.1 to 0.5% (preferably 0) 0.1-0.4%), Mn: 0.3-1.5% (preferably 0.5-1.5%).

  High cleanliness steel like this invention can be manufactured by devising the addition means of Li.

  First, for example, a Li—Si alloy (Li content: 20 to 40% by mass) or lithium carbonate may be used. The reason why the Li content of the Li—Si alloy is 20 to 40% by mass (preferably 25 to 35% by mass) is that the liquidus temperature can be lowered during the production of the Li—Si alloy. This is because the evaporation of Li can be prevented and the yield can be increased. Further, when the composition is used, the Li-Si intermetallic compound is present, so that the yield of Li in the molten steel can be increased. This is because it can. The reason why lithium carbonate is used is that the yield of Li can be increased.

  Second, Li, which has a high yield, may be added not after the molten steel treatment (in the slag) but after the molten steel treatment. Since the yield of Li is increased, the amount of Li in the steel can be set to a predetermined amount or more even after completion of the molten steel treatment. And since the addition during a molten steel process (in slag) is avoided, it can prevent that the inclusion from a refractory increases.

  The Li—Si alloy can be manufactured by premelt. If necessary, the Li—Si alloy may be premelted or mixed with Ca, Mg, and other alkali metals (Na, K, etc.), or may be premelted with a diluted metal (Fe, etc.). Also when lithium carbonate is used, Ca, Mg, and other alkali metals (Na, K, etc.) may be mixed as appropriate. However, the high cleanliness steel of the present invention is significantly superior in function of Li compared to other alkali metals, so that inclusions can be sufficiently controlled without using other alkali metals together (premelt, mixed, etc.) The fatigue strength can be sufficiently improved.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention. In the following Examples, “%” means “mass%” and “ppm” means “ppm (mass basis)” unless otherwise specified.

Experimental example 1
In the experiment, a 20-ton real machine was experimentally used. That is, the molten steel melted in an electric furnace was taken out into a ladle, various fluxes were added, component adjustment, electrode heating, and argon bubbling were performed, and molten steel treatment was performed. Moreover, while adding Ca, Mg, etc. during a molten steel process as needed, Li was added during the molten steel process or after the molten steel process in the state of lithium carbonate or Li—Si alloy. The molten steel was then cast. The obtained steel ingot was forged into a round bar having a diameter of 600 mm. Table 1 shows the steel component composition of the obtained round bar, and Table 2 shows the alkali metal addition method, total-Li / Si (mass ratio), and Li ₂ O concentration in the slag.
The evaluation of the round bar was as follows.

[Li content in steel] (lower limit of determination 0.01 ppm)
A 0.5 g sample was collected from the target round bar, placed in a beaker, and mixed acid (H ₂ O + HCl + HNO ₃ ) was added for thermal decomposition. After allowing to cool, the decomposition solution was transferred to a separatory funnel and hydrochloric acid was added to make it 9N-hydrochloric acid. Methyl isobutyl ketone (MIBK) was added and shaken to extract the iron content into the MIBK phase. After standing, the MIBK phase was discarded, MIBK was added again, and the same extraction / separation operation was repeated three times in total to completely remove iron. The acidic phase of 9N-hydrochloric acid was diluted to a volume of 100 mL to obtain an alkali measurement solution.

Using an ICP mass spectrometer (model SPQ8000) manufactured by Seiko Instruments Inc., the concentration of Li (mass number 7) in the alkali measurement solution was measured, and the Li content in the steel was calculated. The IPC mass spectrometry conditions are as follows.
High frequency output: 1.2kW
Carrier gas flow rate: 0.4 L / min

[Na content in steel] (lower limit of determination 0.01 ppm)
A sample was collected from the target round bar and weighed in a beaker, and then subjected to thermal decomposition by adding pure water, hydrochloric acid and nitric acid. After decomposition, pure water and hydrochloric acid were added to make a constant volume, MIBK was added and the mixture was shaken well. After standing, the MIBK layer was extracted, MIBK was added again, and the same extraction / separation operation was repeated twice to completely extract and remove iron. The aqueous layer was diluted with pure water and quantitatively analyzed using a graphite furnace atomic absorption method.

[Fatigue test (difference method)]
A smooth specimen (diameter 10 mm × gauge length 30 mm) was cut out in the radial direction from the obtained round bar (diameter 600 mm). A fatigue test (difference method) was performed using an Ono-type rotary bending tester while maintaining the rotational speed at 3600 rpm and increasing the stress by 20 MPa every 3 × 10 ⁶ cycles.
Fatigue properties were evaluated by the value (durability limit ratio) obtained by dividing the stress at break by the tensile strength of the test piece. In addition, the tensile strength was calculated | required by performing a tensile test at normal temperature according to JISZ2241, using the test piece (diameter 6mm x gauge length 30mm) cut out from the same location as the said test piece for rotation bending tests.

[Cross-sectional inclusion size (√inclusion cross-sectional area)]
In the fatigue test, in the cross section of the test piece fractured due to the inclusion, the fractured portion starting from the inclusion was observed with an SEM, and the maximum inclusion area in the observation field (breaking origin inclusion area) The cross-sectional inclusion size (√inclusion cross-sectional area) was determined by measuring the length and width. The composition of the maximum inclusion was examined by EPMA.

  Results when the obtained round bar is regarded as an integrated crankshaft and subjected to the fatigue test (step difference method) (endurance limit ratio, maximum inclusion, cross-sectional inclusion size (√inclusion cross-sectional area)) ) Is shown in Table 2.

  As is apparent from Tables 1 and 2, when Li is added after the molten steel treatment in a form with a good yield, the amount of Li in the steel can be secured while suppressing inclusions from the refractory, and the Li / Si ratio is appropriately set. it can. As a result, as in Invention Examples A1 to A9, inclusions can be miniaturized, and the cross-sectional inclusion size (√inclusion cross-sectional area) can be reduced, thereby improving fatigue characteristics (endurance limit ratio) (Table 2). And FIG. 1).

It is a graph which shows the relationship between the cross-sectional inclusion size ((root) inclusion cross-sectional area) and durability ratio in an Example.

Claims (2)

Ingredients
C: 1.0% (meaning mass%, hereinafter the same) or less (excluding 0%),
Si: 0.6% or less (excluding 0%),
Mn: 1.5% or less (excluding 0%),
Ni: 4% or less (excluding 0%),
Cr: 12% or less (excluding 0%),
Mo: 1.0% or less (excluding 0%),
total-Al: 0.01% or less (excluding 0%),
O: 0.005% or less (excluding 0%),
total-Mg: 0.1 to 15 ppm (mass basis),
total-Ca: 0.1~40ppm (by weight),
total-Li: 0.020-20 ppm (mass basis), and
total-Li and Si: total-Li / Si (mass ratio) = 5 × 10 ^{−6 to} 9000 × 10 ^−6, and the balance is Fe and inevitable impurities,
A high cleanliness steel with excellent fatigue strength, characterized in that the cross-sectional inclusion size (√inclusion cross-sectional area) is 160 μm or less. further,
V : 0.5% or less ,
Nb : 0.1% or less ,
W : 1% or less ,
The high cleanliness steel according to claim 1, comprising at least one selected from the group consisting of Cu : 2% or less and Ti : 0.15% or less .

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