JP4393335B2 - Manufacturing method of high cleanliness steel with excellent fatigue strength or cold workability - Google Patents

Description

  The present invention relates to a method for producing a high cleanliness steel excellent in cold workability and fatigue characteristics, and preferably when used as a high strength steel wire, an ultra fine steel wire, a high strength spring (particularly a valve spring), etc. The present invention relates to a method for producing a high cleanliness steel that is extremely useful.

  For ultra-fine steel wires drawn to 0.1 to 0.5 mm by cold working and spring steel materials that require high fatigue strength, it is necessary to reduce hard non-metallic inclusions present in the steel as much as possible. It is. This is because these non-metallic inclusions cause disconnection during wire drawing and cause a decrease in fatigue strength. From this point of view, high cleanliness steel in which non-metallic inclusions are reduced as much as possible is used as the steel material used for the above-described applications.

  In recent years, there has been an increasing demand for lighter and higher-powered automobiles for the purpose of reducing exhaust gas and improving fuel efficiency, and valve springs and suspension springs used for engines, suspensions, and the like are designed for high stress. For this reason, spring steel tends to be increased in strength and diameter, and the load stress increases more and more. Accordingly, there is a demand for a high performance spring steel that is further excellent in fatigue resistance and sag resistance, and in particular, valve springs are required to have the highest fatigue strength.

  On the other hand, ultra-thin steel wires represented by tire cords are also being increased in strength for the purpose of reducing the weight of the tire, and recently steel cords having a strength of 4000 MPa class have been used. However, the higher the strength of the ultrafine steel wire, the easier it is to break during cold working (at the time of wire drawing), so further cold workability is required.

  As described above, these spring steels and extra fine steel wires are subject to fatigue breakage and disconnection due to non-metallic inclusions as the strength of the material increases, and the main cause of this is the reduction and miniaturization of non-metallic inclusions. The demand for is getting stricter.

For reducing or reducing the size of hard non-metallic inclusions, many techniques have been proposed so far. For example, Non-Patent Document 1 discloses that CaO- has a melting point of 1400 to 1500 ° C. or less in spring steel. 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) 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 (in the case of Patent Document 2), cold It is described that inclusions can be made harmless because the inclusions are crushed and dispersed during processing or wire drawing. However, in view of the recent improvement in required characteristics, higher performance is required.

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 Al ₂ O ₃ and SiO ₂ hard non-metallic inclusions. As a result, the non-metallic inclusions are elongated like a thread during hot rolling. It has a form that 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%.
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 Japanese Examined Patent Publication No. 6-74484 Patent Claim etc. Japanese Patent Publication No. 6-74485 Patent Claim etc. JP-A-1-319623 Patent Claims JP, 2-15111, A Claims etc. JP, 2002-167647, A Claims etc. JP, 2002-194497, A Claims etc.

  The present invention has been made paying attention to the above-described circumstances, and an object thereof is to provide a method for producing a high cleanliness steel in which cold workability and fatigue characteristics are further improved.

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). Etc.) can be remarkably altered, and the effect specific to Li becomes prominent by devising the method of adding Li to the steel, and the cold workability and fatigue strength are remarkably improved. I found.

  More specifically, when adding Li to the molten steel, Li is efficiently contained in the molten steel by using a Li-containing material different from the conventional one, and the alteration of the complex oxide inclusions can be effectively performed. It has been found that it can proceed.

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 techniques aim to directly control the inclusion composition. In order to directly control the inclusion composition, harmless top slag is involved during slag refining, and the slag and harmful deoxidation products (especially SiO ₂ and Al ₂ O ₃ ) in 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. As a result, it becomes difficult to generate harmful SiO ₂ -based deoxidation products 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 early stage 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 cold workability and fatigue characteristics are not sufficiently improved.

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. For example, in Example 2, 700 kg of Li-Si alloy having a Li concentration of 2% (equivalent to 14 kg of Li content) is added to 240 ton of molten steel during refining (in slag). In Example 3, the Li concentration is 5%. Li-Si alloy (equivalent to 10 kg of pure Li) is added during refining (in slag). 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 high. As a result of actual confirmation by the inventors, the Li ₂ O concentration in the slag was about 1% (Example 2) to 1.5% (Example 3). Even if the Li concentration in the slag is about 1%, as in Patent Document 3, the melting point and viscosity of the slag are lowered, the refractory melt resistance is increased, and the external inclusions start to increase. And since Li is insufficient, the refinement | miniaturization effect of an inclusion becomes inadequate. As a result, the cold workability and fatigue characteristics are not sufficiently improved.

  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 meltability 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 ensured, the cold workability and fatigue characteristics are significantly reduced.

Unlike the above-described techniques, in the method of the present invention, the Li-containing material added to the molten steel is a Li-Si alloy and / or LiCO having a Li content of 20 to 40% (meaning mass%, the same applies hereinafter). Since it is set to ₃ , the Li concentration in the molten steel can be effectively increased. And furthermore, the addition method of the said Li containing material,
(A) It is added to the molten steel at a later stage during the addition treatment at the stage where a series of molten steel treatment of component adjustment, temperature adjustment and slag refining is completed.
Or (b) Li concentration in steel while controlling oxide inclusions derived from refractory etc. by adding to molten steel in the latter stage of a series of molten steel treatment of component adjustment, temperature adjustment, slag refining Is increased to a predetermined amount or more, and the effect of Li as described above is effectively exhibited. In addition to the Li-containing material, adding a substance containing one or more elements selected from the group consisting of Ca, Mg, Na and K to the molten steel is also effective for alteration of oxide inclusions. It is.

That is, the manufacturing method of high cleanliness steel excellent in fatigue strength or cold workability according to the present invention,
(1) As a Li-containing material, a Li-Si alloy and / or Li ₂ CO ₃ having a Li content of 20 to 40% (meaning mass%, hereinafter the same) is added to molten steel.
(2) Further, in addition to the Li-containing material, a substance containing one or more elements selected from the group consisting of Ca, Mg, Na and K is added to the molten steel.
(3) By adding the Li-containing material to the molten steel at the stage where a series of molten steel treatment of component adjustment, temperature adjustment, and slag refining is completed, the total-Li amount in the steel is 0.020 to 20 ppm (mass) Control) so that the number of oxide inclusions having a major axis of 20 μm or more is 1.0 or less per 50 g of steel, or (4) component adjustment, temperature adjustment, slag refining of the Li-containing material the addition of late in a series of molten steel processing, the oxide-based inclusions present in the steel _{CaO: 15~55%, SiO 2:} 20~70%, Al 2 O 3: 35% or less, MgO : Control to contain 20% or less, Li ₂ O: 0.5 to 20%,
Is a more effective method.

  On the other hand, examples of the addition location of the Li-containing material include one or more of a ladle, a continuous casting tundish (TD), and a continuous casting mold (MD), and the addition means includes (i) the Li-containing material. Is added to the molten steel while stirring the molten steel. (Ii) The Li-containing material is blown into the molten steel using an inert gas as a carrier gas.

  According to the method of the present invention, since the total-Li amount in the steel can be appropriately controlled by making the kind of Li-containing material and the addition means appropriate, the high cleanliness steel excellent in cold workability or fatigue characteristics. Can be efficiently manufactured.

In the method for producing 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 in this state, when the partial rolling or hot rolling is performed, the vitreous portion has a low melting point and a low viscosity and is highly stretchable. Since stress at the time of rolling concentrates on the interface of the glass phase and breaks easily in a breakthrough, 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 effectively exhibit the function of Li, Li must be efficiently added into the molten steel. For this purpose, it is necessary to use a Li—Si alloy or Li ₂ CO ₃ having a Li content of 20 to 40% as a Li-containing material, not a conventional method.

The Li content of the Li—Si alloy is set to 20 to 40% because the liquidus temperature can be lowered during the production of the Li—Si alloy, so that the evaporation of Li during the production of the Li—Si alloy can be prevented, and the yield. In addition, since the Li—Si-based intermetallic compound is present in the above composition, the yield of Li in the molten steel can be increased. The reason why Li carbonate (Li ₂ CO ₃ ) is used is that the yield of Li can be increased. In addition, it is preferable that Li content in a Li-Si alloy is 25 to 35%.

  Furthermore, in addition to the Li-containing material, it is also preferable to add a substance containing one or more elements selected from the group consisting of Ca, Mg, Na and K to the molten steel, and by adding such a substance And, the effect that Li is easily contained in the inclusion is exhibited. However, if these elements are excessive, the inclusion composition will not be the target complex oxide system, so it should be up to 50 ppm with respect to the molten steel. The addition time of these elements may be before or after the Li-containing material, but when adding the Li-containing material later in the molten steel treatment, simultaneously, when adding the Li-containing material after the molten steel treatment is completed. It is preferable to add to that before.

  The Li—Si alloy can be manufactured by premelt. If necessary, the Li—Si alloy may be pre-melted or mixed with Ca, Mg, and other alkali metals (Na, K, etc.), or may be pre-melted with a diluted metal (Fe, etc.). Also when Li carbonate is used, Ca, Mg, and other alkali metals (Na, K, etc.) may be appropriately mixed. However, since the function of Li is remarkably superior to other alkali metals, it is possible to sufficiently control inclusions without using other alkali metals in combination (premelt, mixing, etc.), and to improve cold workability and fatigue strength. It can be improved sufficiently.

In the high cleanliness steel obtained by the method of the present invention, in order to exert desired properties,
(1) The total-Li amount in the steel is 0.020 to 20 ppm (mass basis), and the oxide inclusions having a major axis of 20 μm or more are controlled to be 1.0 or less per 50 g of steel,
Or (2) an oxide-based inclusions present in the steel _{CaO: 15~55%, SiO 2:} 20~70%, Al 2 O 3: 35% or less, MgO: 20% or less, Li ₂ O: 0 Control to contain 5-20%,
Etc. are preferable. At this time, in order to control as in the above (1), it is necessary to add the Li-containing material into the molten steel at the stage where a series of molten steel treatment of component adjustment, temperature adjustment, and slag refining is completed. Since the yield of Li is increased, the amount of Li in the steel can be made a predetermined amount or more even after the molten steel treatment is completed, and addition during the molten steel treatment (in the slag) is avoided. This is because the inclusion of refractory-derived inclusions can be prevented from increasing.

In the molten steel controlled as in (1) above, the ratio of total-Li amount to Si amount [Li / Si ratio (mass ratio)] is 1 × 10 ⁻⁶ or more, preferably 10 × 10 ⁻⁶ or more. More preferably, it is recommended to be 50 × 10 ⁻⁶ or more, and for example, it may be about 100 × 10 ⁻⁶ or more (for example, 200 × 10 ⁻⁶ or more).

  On the other hand, in order to control as in the above (2), it is necessary to add the Li-containing material at a later stage during a series of molten steel processes of component adjustment, temperature adjustment, and slag refining. Here, the “late stage during molten steel treatment” refers to the latter half of the entire time required for a series of molten steel treatment of component adjustment, temperature adjustment, and slag refining. For example, when 90 minutes are required for a series of molten steel processes of component adjustment, temperature adjustment, and slag refining, this means the latter 45 minutes of the 90 minutes. In particular, it is recommended that it be performed within the last third of the total time (in the last 90 minutes, the last 30 minutes). In addition, the time until a later stage in a series of molten steel treatments is referred to as “the first half of the molten steel treatment”.

  In a series of molten steel treatments, when Li-containing materials are added from the previous period, Li is contained in the steel, but the inclusion of refractory is easy to enter by stirring of the molten steel, and partly altered by Li This is because the hard inclusions remain without receiving the effect.

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 the molten steel treatment stage, Ca and Mg are often taken into steel by the inclusion of top slag. If necessary, Ca or Mg may be added. There are cases where secondary deoxidation products generated during solidification become SiO ₂ -rich and Al ₂ O ₃ -rich, which may be problematic, and the addition of Ca, Mg, Li or the like may be effective for these. . 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 inclusions of high SiO ₂ or high Al ₂ O ₃ can be suppressed.

  The total-Ca in steel (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)].

  According to the production method of the present invention, a high cleanliness steel excellent in cold workability and fatigue characteristics can be obtained, which is advantageous for high-strength steel wires, ultrafine steel wires, high-strength springs (particularly valve springs) and the like. Available. When applying the high cleanliness steel obtained by the method of the present invention to these uses, C: 1.2% or less (preferably 0.1 to 1.0%, more preferably 0.3 to 0.9% ), Si: 0.1 to 4% (preferably 0.1 to 3%, more preferably 0.2 to 2.5%), Mn: 0.1 to 2% (preferably 0.2 to 1.%). 5%, more preferably 0.3-1.2%), 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) and O: 0.005% or less (preferably 0.004% or less, more preferably 0.003% or less) can be used. The preferable C content is 1.2% or less for applications from high-strength steel wire (C content: about 1.1% level) to ultra-fine mild steel wire (C content: about 0.01% level). This is because, when the carbon content is higher than 1.2%, the steel becomes excessively hardened and the workability is lowered, which makes it impractical.

  Further, if necessary, it may further contain Cr, Ni, V, Nb, Mo, W, Cu, Ti, etc. as physical property improving elements, and these elements are contained alone or in appropriate combination of two or more. Also good. Preferred contents of these elements are Cr: 3% or less (preferably 0.01 to 1%), Ni: 1% or less (preferably 0.05 to 0.5%), V: 0.5% or less (Preferably 0.005 to 0.2%), Nb: 0.1% or less (preferably 0.005 to 0.05%), Mo: 1% or less (preferably 0.01 to 0.5%) , W: 1% or less (preferably 0.01 to 0.5%), Cu: 2% or less (preferably 0.05 to 1%), Ti: 0.06% or less (preferably 0.005 to 0) 0.03%). The balance may be Fe and inevitable impurities.

  The high cleanliness steel most suitable as a high strength extra fine steel wire or a high strength valve spring has C, Si and Mn in the following ranges among the elements described above. For example, the optimum high cleanliness steel for high-strength ultrafine steel wire steel is C: 0.5 to 1.2% (preferably 0.7 to 1.1%), Si: 0.1 to 0.5% (Preferably 0.15 to 0.4%), Mn: 0.2 to 1% (preferably 0.3 to 0.8%). The optimum high cleanliness steel for high strength valve spring steel is C: 0.3 to 1.0% (preferably 0.4 to 0.8%), Si: 1 to 4% (preferably 1.2). -2.5%), Mn: 0.3-1.5% (preferably 0.4-1.0%).

  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.

Experimental example 1
The experiment was performed on a real machine (or laboratory level). That is, in the actual machine, the molten steel melted in the converter is put into a ladle (in the laboratory, 500 kg of molten steel simulating the molten steel discharged from the converter is melted), and various fluxes are added to the components. Adjustment, electrode heating, and argon bubbling were performed, and molten steel treatment (slag refining) was performed. Moreover, while adding Ca, Mg, etc. during the molten steel treatment as necessary, Li is in the state of Li ₂ O, lithium carbonate, Li—Si alloy, or LiF, before the molten steel treatment, during the molten steel treatment (however, It was added after the molten steel treatment). At this time, the addition place [ladder, continuous casting tundish (TD) or continuous casting mold (MD)] and addition form [wire, injection, charging] were variously changed. Next, the molten steel was cast (in the laboratory, cast into a mold capable of obtaining a cooling rate equivalent to that of an actual machine). The obtained steel ingot was forged and hot-rolled to obtain a wire having a diameter of 5.5 mm. Moreover, as a steel component, it implemented about the spring steel component and the steel cord component.

  While evaluating the Li content in each wire, and the microscopic observation and composition investigation of inclusions in the L cross section in each wire, each wire is acid-dissolved to investigate the number and size of hard inclusions, The spring steel was subjected to a rotating bending fatigue test, and the steel cord was subjected to an evaluation test by a wire drawing test.

[Li content in steel]
A sample of 0.5 g was taken from the target wire, taken into 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

[Number of oxide inclusions having a major axis of 20 μm or more]
The target wire 1500 g was cut every about 100 g, the scale was removed, and the steel was dissolved in acid by placing it in a warm nitric acid solution at about 90 ° C. This solution is filtered through a filter having a mesh size of 10 μm, and the inclusions remaining on the filter are analyzed for the composition by EPMA and the major axis is measured, whereby the oxide inclusions having a maximum major axis of 20 μm or more ( The number of hard inclusions) was measured, and the number per 50 g of steel was calculated.

[Wire drawing test (number of breaks)]
The hot-rolled wire (diameter 5.5 mm) was first drawn to a diameter of 2.5 mm, heat-treated (air patenting), and then secondarily drawn to a diameter of 0.8 mm. Subsequently, heat treatment (lead patenting) and brass plating were performed, followed by wet drawing to a diameter of 0.15 mm, and evaluation was performed in terms of the number of breaks per 10 tons of steel wire.

[Fatigue strength test (breaking rate)]
For each hot-rolled steel wire (diameter 5.5 mm), skin cutting (SV) → low temperature annealing (LA) → cold drawing (diameter 4.0 mm) → oil temper [oil quenching and lead bath (about 450 ° C.) tempering Continuous process] → Simple strain relief annealing (Bluing: approx. 400 ° C.) → Shot peening → Strain relief annealing, then take a 4.0mm x 650mm diameter wire as test material, and use Nakamura rotary bending tester The test was conducted at a nominal stress of 940 MPa, a rotational speed of 4000 to 5000 rpm, and a number of cancellations of 2 × 10 ⁷ times. And the fracture | rupture rate was calculated | required by the following formula about what the inclusion broke among the fracture | ruptured things.
Breaking rate (%) = [number of inclusions broken / (number of inclusions broken + number of broken inclusions reached a predetermined number of times)] × 100

[Maximum inclusion size]
In the wire drawing test and fatigue strength test, the cross section of the fracture due to inclusions was observed with an SEM, and the width of the largest inclusion (maximum inclusion) among the inclusions that appeared in the cross section was measured. The composition of maximum inclusions was examined by EPMA.

  The results are shown in Tables 1 and 2. Table 1 shows the results when the wire obtained in Experimental Example 1 was subjected to the wire drawing test by simulating a steel cord, and Table 2 shows the wire obtained in Experimental Example 1 with a valve spring. This is a result of simulating and subjected to the fatigue strength test.

  As is clear from these results, in the method of adding Li before the molten steel treatment or in the first half of the molten steel treatment (in the table, “first half of treatment”), when the amount of Li in the steel can be secured, the inclusion of refractories Increasing the number of substances (A12-14, A16, B20, B22, B24, B26), conversely, when trying to reduce inclusions derived from refractories, the amount of Li in the steel is insufficient and the inclusions become coarse ( A15, A17-23, B19, B21, B23, B25). On the other hand, 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 derived from refractories, and the Li / Si ratio can be made appropriate (A1 to 11). , B1-18). As a result, inclusions can be miniaturized, the number of oxide inclusions having a major axis of 20 μm or more and the maximum inclusion size can be reduced, and wire drawing workability (number of wire breaks) and fatigue strength (breaking rate) are improved.

Experimental example 2
Experiments were conducted on 90 ton and 250 ton real machines (or laboratory level). That is, in the actual machine, the molten steel melted in the converter is put into a ladle (in the laboratory, 500 kg of molten steel simulating the molten steel discharged from the converter is melted), and various fluxes are added to the components. Adjustment, electrode heating, and argon bubbling were performed, and molten steel treatment was performed. In slag refining, Li-70% Si (meaning 30% Li-70% Si), Ca-Si wire, and Li ₂ CO ₃ , Na ₂ CO ₃ , during the molten steel treatment (pre-treatment or late treatment) Addition of a mixture of K ₂ CO ₃ and Ca wire, Mg wire or the like was also performed. After the refining, the molten steel was cast (in the laboratory, it was cast into a mold capable of obtaining a cooling rate equivalent to that of the actual machine). The obtained steel ingot was forged and hot-rolled to obtain a wire having a diameter of 5.5 mm. At this time, as steel components, similar to Experimental Example 1, spring steel components and steel cord components were carried out.

  In the evaluation, while performing microscopic observation and composition investigation of inclusions in the L cross section in each wire, each wire is acid-dissolved and the composition, number and size of hard inclusions are examined. The spring steel was subjected to a rotating bending fatigue test, and the steel cord was subjected to an evaluation test by a wire drawing test.

[Inclusions in steel wire]
The L cross section of a 80 mm long wire (diameter 5.5 mm) was polished, and the thickness, length, number and inclusion composition of inclusions were determined.

[Inclusion composition analysis]
Since the Li ₂ O concentration of inclusions cannot be measured by conventional EPMA, it was measured by SIMS (secondary ion mass spectrometry) according to the following procedure.
(1) Primary standard sample
1) a synthetic oxides range covering the inclusions composition excluding Li ₂ O, this Li ₂ O to a synthetic oxide was many work plus, these Li ₂ O concentration was quantitatively analyzed by chemical analysis, Prepare a standard sample.
2) Measure the relative secondary ion intensity of Li with respect to Si of each prepared synthetic oxide.
3) Draw a calibration curve of the relative secondary ion intensity of Li with respect to Si and the Li ₂ O concentration chemically analyzed in 1) above.
(2) Secondary standard sample (for measurement environment correction)
1) When correcting the environment at the time of measurement, a standard sample in which Li is ion-implanted on a Si wafer is separately prepared, the relative secondary ion intensity of Li with respect to Si is measured, and the above (1) -2) is performed. To correct.
(3) Actual measurement
1) First, each concentration of inclusions in the steel, such as CaO, MgO, Al ₂ O ₃ , MnO, SiO ₂ , NaO, and K ₂ O, is analyzed by DEX, EPMA, or the like.
2) Measure the relative secondary ionic strength of Li with respect to Si in the steel inclusions. Of the calibration curves obtained in (1) -3) above, the calibration curve closest to the analysis results in (3) -1) above. Is selected, thereby obtaining the Li ₂ O concentration.

  The results are shown in Tables 3 and 4. Table 3 shows the results obtained when the wire obtained in Experimental Example 2 was subjected to the wire drawing test by simulating a steel cord, and Table 4 shows the wire obtained in Experimental Example 2 with a valve spring. This is a result of simulating and subjected to the fatigue strength test.

  As is clear from the results in Table 3 (steel wire for steel), the number of coarse hard inclusions of 20 μm or more is reduced by adding Li in the later stage of the molten steel treatment (“late stage in the table”). Moreover, it can be seen that the maximum inclusion size is also relatively small, and the number of wire breaks during wire drawing is reduced (A24-30). Judging from this table, it can be seen that the number of broken hard inclusions of 20 μm or more, especially 0.3 or less per 50 g of steel material, clearly has a reduced number of disconnections.

  On the other hand, in the thing of A31-39 which added Li in the first half of the molten steel process, the inclusion composition is out of the range defined in the present invention, and the number of breaks is 20 times in terms of 10 tons of steel wire. The number of coarse hard inclusions of 20 μm or more is decreasing.

  On the other hand, Table 4 is an example for steel wires for springs. Among them, B27 to 38 satisfy the requirements defined in the present invention, so the fracture rate is relatively small and the maximum fracture inclusion size is also small. On the other hand, B39 to 48 are comparative examples that deviate from the requirements defined in the present invention, have a relatively high fracture rate and a large maximum fracture inclusion size.

Claims (7)

When producing high cleanliness steel, adding Li-Si alloy and / or Li ₂ CO ₃ having a Li content of 20 to 40% (meaning mass%, the same shall apply hereinafter) to the molten steel as a Li content. A method for producing high cleanliness steel with excellent fatigue strength or cold workability.   The method for producing a high cleanliness steel according to claim 1, wherein a substance containing one or more elements selected from the group consisting of Ca, Mg, Na and K is further added to the molten steel.   By adding the Li-containing material to the molten steel at the stage where a series of molten steel treatment of component adjustment, temperature adjustment, and slag refining is completed, the total-Li amount in the steel is 0.020 to 20 ppm (mass basis). The method for producing a high cleanliness steel according to claim 1 or 2, wherein the oxide inclusions having a major axis of 20 µm or more are controlled to be 1.0 or less per 50 g of steel. By adding the Li-containing material at a later stage in the series of molten steel treatment of component adjustment, temperature adjustment, and slag refining, oxide inclusions present in the steel are CaO: 15 to 55%, SiO ₂ : 20 _{~70%, Al 2 O 3:} 35% or less, MgO: 20% or less, Li ₂ O: high cleanliness according to claim 1 or 2 and controls to contain 0.5 to 20% Steel manufacturing method.   The method for producing high cleanliness steel according to any one of claims 1 to 4, wherein the Li-containing material is added at one or more locations of a ladle, a continuous casting tundish, and a continuous casting mold.   The manufacturing method of the high cleanliness steel in any one of Claims 1-5 which fills the said Li content in an iron wire, and adds in molten steel, stirring molten steel.   The method for producing high cleanliness steel according to any one of claims 1 to 5, wherein the Li-containing material is blown into molten steel using an inert gas as a carrier gas.