JP2007327122A - TREATMENT METHOD FOR MOLTEN IRON BY Nd AND Ca ADDITION - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a treatment method for molten iron by which soluble P concentration in the molten iron can be reduced by producing P compound by the addition of a little quantity of Nb and the molten iron excellent in continuous castability and high in cleanliness can be produced. <P>SOLUTION: In this treatment method, the Nd is added into the molten iron containing ≥0.0001% to ≤0.5% P, ≤0.005% S and ≤0.005% O(oxygen), then the Nd concentration in the molten iron is controlled so as to satisfy inequalities (1) and (2) according to P, S and O concentrations in the molten iron, and then Ca is added into the molten metal and the Ca concentration is controlled so as to satisfy ineqality (3) according to the Nd concentration in the molten iron. A=0.24[P]+0.82[S]+0.85[O]...(1). A+0.005≤[Nd]≤A+0.03...[2]. 1.2×10<SP>-2</SP>×[Nd]<SP>2/3</SP>≤[Ca]≤1.6×10<SP>-2</SP>×[Nd]<SP>2/3</SP>+0.15...(3). Wherein, [P], [S], [O], [Nd] and [Ca] denote concentrations (mass%) of respective elements. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for refining molten iron that reduces the P concentration dissolved in steel or the P concentration dissolved in steel, and more specifically, Nd and then Ca are added to the molten iron to add P to the Nd-P system. The present invention relates to a refining method for reducing the concentration of dissolved or dissolved P by using inclusions and Ca-P inclusions.

  Phosphorus (P) dissolved in the steel (hereinafter also referred to as “solid solution P”) is concentrated at the grain boundaries or at the center of the slab, and the properties of the steel such as high temperature ductility, corrosion resistance, and weldability are remarkably increased. In order to make it worse, it is important to reduce the concentration of solute P in steel. Solid solution P exists as a single element in steel, but this solid solution P exists as dissolved P (hereinafter also referred to as “dissolved P”) in molten iron.

  In general, as a method for reducing the concentration of solute P in steel, a dephosphorization process for removing dissolved P from molten iron in a steelmaking stage is used. The dephosphorization process is a method in which slag or flux suitable for dephosphorization is added to molten iron, and P in the molten iron is transferred as oxide to slag or flux in an oxidizing atmosphere.

  Conventionally, a large number of hot metal dephosphorization techniques and molten steel dephosphorization techniques have been developed for the purpose of increasing the dephosphorization efficiency and dephosphorizing to a lower dissolved P concentration. On the other hand, in recent years, the demand for high-grade steel has increased at the same time as the required performance for steel materials has increased. In order to meet this required performance and demand, it has become necessary to reduce the concentration of solute P in steel more than before by a simpler method.

  However, conventional dephosphorization treatment has (a) an economical and thermodynamic limit at the lower limit of the P concentration that can be reduced, and (b) exhaust slag increases when thorough dephosphorization treatment is performed. c) There was a problem that dephosphorization was not possible under reducing refining conditions, and it was difficult to meet the above requirements.

  As described above, in order to cope with the recent low phosphatization by the dephosphorization treatment that reduces the dissolved P concentration by using flux or slag based on the conventional concept, (1) a decrease in productivity due to the limit of dephosphorization ability Further, there have been problems such as an increase in refining costs, (2) excessive dephosphorization treatment due to the fact that dephosphorization treatment cannot be performed at the end of refining, and (3) an increase in waste amount due to an increase in the amount of discharged slag.

  On the other hand, as a new approach to dephosphorization, a technology has been developed to reduce the concentration of dissolved P by generating a compound (inclusion) consisting of P and REM in molten iron and fixing a part of the dissolved P as a compound. It was done. For example, Patent Document 1 discloses that slag produced by adding 0.1% by mass or more of rare earth metal (hereinafter also referred to as “REM”) to molten metal maintained in a non-oxidizing atmosphere and stirring the molten metal. A dephosphorization method is disclosed in which oxidative refining is carried out after removing slag. In this method, a compound of REM and P is generated in the molten metal, stirred and floated, and then the slag containing the P compound is removed, and then the REM in the molten metal is removed by oxidation refining. It is a method to do.

  However, in the conventional method of reducing dissolved P concentration by using dissolved REM as a compound with REM using REM, a plurality of treatments such as P compound generation and levitation treatment, slag removal, and subsequent oxidation refining are performed. Therefore, there is room for improvement in terms of processing costs.

In order to solve the above problems, the present inventors have investigated in detail the formation reaction of NdP inclusions in molten iron when Nd is added to molten iron, and appropriately controlling the P concentration and the Nd concentration. , New melted P that does not require flotation of inclusions, slag removal and oxidative refining
A method for reducing the concentration was previously proposed as Patent Document 2. Furthermore, the inhibitory action of oxygen in molten iron (hereinafter also referred to as “O”) and sulfur (hereinafter also referred to as “S”) on the reduction of dissolved P concentration can be eliminated, and an excellent effect of reducing dissolved P concentration can be ensured. The processing method of the molten iron was proposed as Patent Document 3.

  These methods are high in productivity, can reduce processing costs, and are excellent processing methods that can secure the cleanliness of steel by eliminating the inhibitory action of O and S on the reduction of P concentration. A method for reducing the dissolved P concentration with excellent continuous castability is desired.

Japanese Patent Publication No. 6-21288 (Claims and page 3, left column, lines 8 to 13) Japanese Patent Application No. 2006-003257 (claims, etc.) Japanese Patent Application No. 2006-003489 (claims, etc.) JP-A-5-43929 (Claims and paragraphs [0010] to [0012])

  According to the treatment method by adding Nd to molten iron proposed by the inventors in Patent Document 2 and Patent Document 3, NdP inclusions are efficiently generated without deteriorating the cleanliness of the molten iron, and the dissolved P concentration Can be reduced. Moreover, since it avoids that the Nd density | concentration in molten iron becomes high too much and the removal process of the inclusion and slag is made unnecessary, it is excellent at the point that productivity is high and processing cost can be reduced. In addition, it is possible to avoid the inhibitory effect due to O and S in the molten iron on the Nd addition effect, and achieve a reduction in dissolved P concentration with higher efficiency.

  On the other hand, since the method presented so far was a technique for reducing the dissolved P concentration only by Nd, it was necessary to make the Nd concentration in the molten iron relatively high. The method of treating molten iron by adding Nd can reduce the refining cost compared to thorough dephosphorization of molten iron, but if the amount of Nd used can be reduced in addition to this, the treatment cost of molten iron can be further reduced. it can.

  In addition, when the Nd concentration in the molten iron is increased, oxide-based Nd inclusions are generated together with P-based Nd inclusions, and these inclusions tend to cause nozzle clogging during continuous casting. there were. That is, although a casting operation by a continuous casting machine is possible, the nozzle may tend to be blocked, which causes a problem in that the number of continuous castings is limited, and the productivity is limited. For this reason, if it is possible to develop a processing method that does not restrict the number of continuous castings even if Nd addition processing is performed, that is, a molten iron processing method that is excellent in castability, it is possible to further reduce costs and improve productivity. it can.

  The present invention has been made in view of the above problems, and the problem is to achieve a high cleanliness of molten iron that can achieve a reduction in the dissolved P concentration under a small Nd addition amount and is excellent in continuous castability. It is providing the processing method of the molten iron which can be manufactured.

  The present invention does not belong to a general dephosphorization treatment method in which dissolved P is transferred into slag and the like to reduce the concentration of dissolved P. Nd is added to molten iron, and then Ca is added to produce a P compound. This is a refining method in which the dissolved P concentration is reduced, and as a result, the solid solution P concentration is reduced. That is, the gist of the present invention resides in a molten iron treatment method shown below.

  “After adding Nd to molten iron containing, by mass%, P: 0.0001% or more and 0.5% or less, S: 0.005% or less, and O (oxygen): 0.005% or less, Ca is added. A method for treating the molten iron to be added, wherein the Nd concentration in the molten iron is set so as to satisfy the relationship expressed by the following formulas (1) and (2) according to the P concentration, S concentration and O concentration in the molten iron. After the control, Ca is added to the molten iron, and the Ca concentration in the molten iron is controlled so as to satisfy the relationship represented by the following formula (3) according to the Nd concentration in the molten iron. Processing method.

A = 0.24 [P] +0.82 [S] +0.85 [O] (1)
A + 0.005 ≦ [Nd] ≦ A + 0.03 (2)
1.2 × 10 ⁻² × [Nd] ^2/3 ≦ [Ca] ≦ 1.6 × 10 ⁻² × [Nd] ^2/3 +0.0015 (3)
Here, [P], [S], [O], [Nd] and [Ca] represent the concentration (mass%) of each element in the molten iron. "
In the following description, the description of “%” in the component composition display of steel means “mass%”.

  In order to solve the above-mentioned problems, the present inventors have investigated and analyzed efficient production conditions and inclusion control effects of Nd-P compounds and Ca-P compounds, and have the following (a) to ( The above-mentioned present invention was completed by obtaining the knowledge of c).

(A) Appropriate range of P, S and O concentration of the target molten iron P: 0.0001% or more and 0.5% or less P is an impurity element that deteriorates properties such as high temperature ductility, corrosion resistance and weldability of the steel material, The lower the concentration, the better. However, in reality, it is rarely necessary to further reduce the dissolved P concentration when the solid solution P concentration is less than 0.0001% in terms of material characteristics and the like. That is all. On the other hand, when the P concentration exceeds 0.5%, it can be easily reduced to 0.5% by ordinary dephosphorization treatment without using the method of the present invention. Therefore, an appropriate range of P concentration is set to 0.0001% to 0.5%.

S: 0.005% or less Sulfur (hereinafter, also simply referred to as “S”) is an element that is considered to have a stronger affinity for Nd than for P and Nd in molten iron. If the S concentration exceeds 0.005%, the amount of Nd sulfide produced in preference to the production of NdP increases, and the production of NdP is suppressed. Similarly, since S has a strong affinity for Ca, the amount of CaS produced increases, preventing an increase in Ca concentration, which will be described later. Therefore, the appropriate range of S concentration is set to 0.005% or less.

O (oxygen): 0.005% or less Oxygen (hereinafter also simply referred to as “O”) is an element considered to have a stronger affinity for Nd than for P and Nd in molten iron. When the O concentration exceeds 0.005%, the amount of Nd oxide produced in preference to the production of NdP increases, and the production of NdP is suppressed. Similarly, since O has a strong affinity for Ca, the amount of CaO produced increases, preventing an increase in Ca concentration described later. Therefore, the appropriate range of O concentration is set to 0.005% or less.

(B) Conditions for generating Ca-P compound 1) Increase in Ca concentration in molten iron by reducing oxygen activity As a method for improving castability, for example, as disclosed in Patent Document 4, Ca is converted into molten iron. A method of adding and modifying inclusions in molten iron to a low melting point composition is known. However, even if the Nd-based inclusion is modified to a Ca—Nd-based inclusion, the melting point is higher than the molten steel temperature, and it is difficult to improve the castability. That is, simply adding Ca to molten iron containing Nd cannot improve castability as in conventional steel.

  Furthermore, in the molten iron containing Nd, it is considered that the oxygen activity is reduced to a very low value due to Nd-O deoxidation equilibrium. When the oxygen activity due to the Nd-O deoxidation equilibrium is lower than the oxygen activity due to the Ca-O deoxidation equilibrium, the Nd inclusions are modified with Ca even if Ca is added to the Nd-containing steel. I can't even do that. Therefore, for these reasons described above, it is difficult to improve castability by simply adding Ca.

On the other hand, it is known that Ca and P form a compound called Ca ₃ P ₂ , which means that Ca, like Nd, forms a phosphorus compound in molten iron and can reduce the dissolved P concentration. It has shown that it has. However, there has been no report on the formation of Ca-P inclusions in molten iron or the confirmation thereof.

  The following is a summary of conventional knowledge. That is, (1) improvement in castability cannot be expected even if Ca addition treatment is simply performed on Nd-containing steel, and (2) Ca—P inclusions are generated by addition of Ca as in Nd. Although there is a possibility that the dissolved P concentration can be reduced, there is no report that the formation of Ca-P inclusions in the molten iron has been confirmed. According to the above knowledge, even if Nd and Ca are used in combination, almost no improvement effect due to improvement in castability or reduction in dissolved P concentration can be expected.

  On the other hand, as a result of repeated examinations by the present inventors without being bound by the above-described conventional knowledge, the following idea has been reached.

  That is, the oxygen activity determined by the Ca—O deoxidation equilibrium (hereinafter referred to as “Ca—O deoxidation equilibrium oxygen activity”, and the same notation is used for the oxygen activity determined by other equilibrium) is Nd. Although higher than the -O deoxidation equilibrium oxygen activity, this varies with the balance between dissolved Nd concentration and dissolved Ca concentration. For example, when the dissolved Ca concentration is very high and the dissolved Nd concentration is very low, the Ca—O deoxidation equilibrium oxygen activity is lower. However, it is difficult to increase the dissolved Ca concentration having high evaporability to such an extent that the above conditions are satisfied by a normal method. Therefore, if the dissolved Ca concentration can be sufficiently increased, Ca-P inclusions can be generated by adding Ca, and as a result, the dissolved P concentration can be reduced.

  Here, a problem is a method of ensuring a high dissolved Ca concentration that cannot be obtained by a normal method. The Ca concentration in the molten iron is governed by an equilibrium reaction represented by CaO = Ca + O. Usually, the oxygen activity is determined by adding Ca. Therefore, if the oxygen activity in molten iron can be made lower than the Ca—O deoxidation equilibrium oxygen activity by another means before the addition of Ca, the equilibrium Ca concentration can be increased, As a result, the Ca concentration in molten iron can be increased to a very high concentration. Therefore, if this can be realized, improvement of castability and reduction of the dissolved P concentration in the molten iron can be achieved at the same time by adding Ca to the molten iron.

2) Increase in dissolved Ca concentration due to addition of Nd Prior to the addition of Ca, a method for lowering the oxygen activity in molten iron over the Ca—O deoxidation equilibrium oxygen activity was examined. In order to make the oxygen activity in molten iron lower than the Ca—O deoxidation equilibrium oxygen activity before the addition of Ca, an element having a stronger deoxidizing power than Ca may be added. Examples of such elements include Nd and Mg. Since Nd produces a P compound, it is preferably used to reduce the dissolved P concentration. However, when Nd is not used in an appropriate manner, the above-described problems occur. That is, when Nd is used, there is an appropriate Nd concentration range.

  Obtaining this condition by thermodynamic calculation is difficult for the following reasons. That is, Nd and Ca both react with S in addition to O. Moreover, Ca is an element that is very easily evaporated. Furthermore, in the method of the present invention, since it is necessary to consider the reaction with P in molten iron, the reaction system becomes extremely complicated. However, the thermodynamics for calculating such a complex multi-component system is necessary. This is because there is a lack of target quantities.

  Therefore, these conditions were determined by experiments. The purpose of the experiment is to determine conditions for generating Ca-P inclusions in the molten iron.

  The experiment included C: 0.0015 to 0.5%, Si: 0.008 to 0.8%, Mn: 0.001 to 1.5%, and P in the range of 0.0001 to 0.5%. Then, 15 kg of molten iron having Nd in the range of 0.007 to 0.1%, S in the range of 0.005% or less, and O in the range of 0.005% or less was used.

  The molten iron temperature was set to 1560 to 1630 ° C., the Nd concentration was adjusted to a predetermined concentration, and then Ca was added. After holding for a certain time, a sample was taken from the molten iron and rapidly solidified. Moreover, after completion | finish of sample collection, the molten iron was cooled at the speed | rate of 20 degree-C / min, and the sample was also cut out from the obtained steel ingot. Nonmetallic inclusions (hereinafter also referred to as “inclusions”) in the samples collected by the above two methods were observed by SEM and the composition of inclusions was quantified using EPMA.

  First, an increase in the Ca concentration will be described. FIG. 1 is a diagram showing the relationship between the Ca concentration in molten iron and the CaO concentration in inclusions present in a quenched sample taken from molten iron. In all the experiments shown in the figure, the Al concentration in the molten iron was 0.04%, the oxygen concentration in the molten iron was adjusted to 0.0011 to 0.0045%, and the P concentration was 0.001%, and The S concentration was 0.0004%, and the Nd concentration was 0.025% as indicated by ▲ and no additive as indicated by ◯.

  As shown in the results of the figure, when Nd is not added, the CaO concentration in the inclusions increases as the Ca concentration in the molten iron increases. This shows that the oxygen activity in molten iron was higher than the Ca-O deoxidation equilibrium oxygen activity, so that Ca reacted with oxygen, and as a result of Ca reacting with oxygen, the dissolved Ca concentration decreased. ing.

  On the other hand, when the Nd concentration is 0.025%, the Ca concentration in the inclusion does not increase even if the Ca concentration is increased in the range where the Ca concentration is less than 0.002%, and the Ca concentration is 0.0035%. However, the CaO concentration in inclusions is only about 6%. This indicates that the oxygen activity in molten iron was lower than the Ca—O deoxidation equilibrium oxygen activity, and that Ca was present as dissolved Ca without reacting with inclusions.

  The dissolved Ca concentration in molten iron is obtained by subtracting the Ca concentration in inclusions from the Ca concentration in molten iron. Therefore, from the above experimental results, it was confirmed that the dissolved Ca concentration can be increased by adding Nd to the molten iron.

3) Nd concentration range for generating P-based inclusions Conditions for generating Ca-P-based inclusions will be described. As described above, since the target reaction system is complicated, it is not possible to grasp appropriate conditions simply by increasing the number of experiments. Therefore, it was decided to conduct experiments by the following method and organize the results.

  In order to increase the Ca concentration, the adjustment of the dissolved Nd concentration is important in order to lower the oxygen activity before adding Ca. However, it is considered that Nd added to the molten iron before Ca addition reacts with S, O, and P in the molten iron before Ca addition. Therefore, in order to obtain the dissolved Nd concentration, it is necessary to reduce the amount of Nd consumed by reacting with these elements.

  When the decrease in Nd consumed by the above reaction is A, A is represented by the following formula (1).

A = 0.24 [P] +0.82 [S] +0.85 [O] (1)
Here, [P], [S] and [O] represent the concentration (%) of each element in the molten iron. Each coefficient of [S] and [O] is a value obtained by dividing the weight of Nd atoms in sulfide and oxide by the molecular weight of sulfide and oxide, respectively. However, the coefficient of [P] is not a value obtained by dividing the Nd atomic weight in the phosphorus compound by the molecular weight of the phosphorus compound. This is because, as already shown by the present inventors, there is a certain limit in the reaction between Nd and P, and the added Nd does not react with the total amount of P. Therefore, the value obtained by dividing the atomic weight of P by the atomic weight of Nd (0.21), and further considering the reaction limit of Nd and P, the reaction ratio of Nd and P is 0.24, and [P] The coefficient was determined.

  Next, the dissolved Nd concentration for increasing the dissolved Ca concentration was determined. Since the dissolved Nd concentration is a value obtained by subtracting the value of A from the Nd concentration in the molten iron, the relationship between the value of ([Nd] -A) and the dissolved Ca concentration was determined. Here, since it is difficult to directly analyze the dissolved Ca concentration, a value obtained by dividing the difference between the Ca concentration in molten iron and the Ca concentration contained in inclusions by the Ca concentration in molten iron, that is, B = (in molten iron Evaluation was made based on the value of Ca concentration-Ca concentration contained in inclusions / Ca concentration in molten iron. When the value of B is 1, the Ca concentration in the molten iron is equal to the dissolved Ca concentration and the dissolved Ca concentration is high. Conversely, when the B value is small, the dissolved Ca concentration is low. Indicates.

  FIG. 2 is a graph showing the relationship between dissolved Nd concentration ([Nd] -A) and dissolved Ca concentration (B). In the figure, B means the value of (Ca concentration in molten iron−Ca concentration contained in inclusions) / (Ca concentration in molten iron) as described above.

  From the results shown in FIG. 9, when the value of ([Nd] -A) is 0.005 or more, the value of B increases to the vicinity of 1, so the value of ([Nd] -A) needs to be 0.005 or more. I understand that there is. On the other hand, if the value of ([Nd] -A) increases beyond 0.03, the value of B slightly decreases, so the value of ([Nd] -A) needs to be 0.03 or less. I understand that. The reason why the value of B slightly decreases in the range where ([Nd] -A) exceeds 0.03 is considered to be that the Ca activity is changed by increasing the dissolved Nd concentration. I'm not sure.

  From the above results, the appropriate range of the Nd concentration in the molten iron is expressed by the following equation (2).

A + 0.005 ≦ [Nd] ≦ A + 0.03 (2)
4) Ca concentration range for generating Ca-P inclusions Conditions for generating Ca-P inclusions will be described. As described above, the effects of the concentrations of P, S and O in the molten iron and the dissolved Nd concentration on the dissolved Ca concentration are taken into account by the equations (1) and (2). What is necessary is just to obtain | require the relationship between Ca density | concentration and Nd density | concentration on the density | concentration conditions which satisfy | fill a formula (1) and (2) type | formula.

  FIG. 3 is a diagram showing the results of experimentally determining the influence of the Ca concentration in molten iron and the Nd concentration satisfying the relationship of the expressions (1) and (2) on the presence or absence of Ca—P inclusions. . In the figure, the circles indicate Ca-P inclusions, that is, CaP, Ca-OP, Ca-OP-S, Ca-P-Nd-O, Ca-P-Nd-O-X. (-S) (wherein X represents Si or Al) was generated, and the X mark indicates that no Ca-P inclusion was generated.

  From the results shown in the figure, it was found that the boundary line between the region where the Ca—P inclusions are generated and the region where the Ca—P inclusions are not generated is expressed by the following equation (4A). Moreover, in order to produce | generate Ca-P type inclusions, the relationship in which the Nd concentration in molten iron that satisfies the relationship of the equations (1) and (2) and the Ca concentration in the molten iron is represented by the following equation (4): It was confirmed that it was necessary to satisfy.

1.2 × 10 ⁻² × [Nd] ^2/3 = [Ca] (4A)
1.2 × 10 ⁻² × [Nd] ^2/3 ≦ [Ca] (4)
Therefore, by controlling each component of P, S, O, Nd and Ca in the molten iron so as to satisfy the relationship represented by the above formulas (1), (2) and (4) simultaneously, The effects (1) to (3) are obtained.

  (1) The dissolved P concentration can be reduced by generating Ca-P inclusions in molten iron.

  (2) Since the dissolved P concentration can be reduced by Ca, the Nd concentration in the molten iron can be kept low as compared with the conventional molten iron processing method.

  (3) Since inclusions generated in the molten iron are Ca-based inclusions, there is no possibility of closing the casting nozzle.

  As described above, by adding and using Nd under an appropriate condition as an auxiliary role of added Ca, it is possible to reduce the dissolved P concentration by adding Ca that has not been confirmed so far.

  Furthermore, the inventors of the present invention focused on the fact that the appropriate range of Ca concentration is an order of magnitude smaller than the appropriate range of Nd concentration in FIG. After the Ca—P inclusions are formed in the molten iron, the molten iron is cast and solidified, so that the reaction between Ca and P in the molten iron further proceeds with the temperature drop at this time. At this time, since the concentration of Ca is low, when Ca is lowered to a certain temperature, Ca is consumed by the reaction and disappears. Thereafter, in order to continue to reduce the P concentration, Nd may react with P instead of Ca. That is, in order for Nd to start reacting with P instead of Ca, it is advantageous that the Ca concentration is not too high.

  Therefore, in order to investigate this condition, the inclusions in the steel ingot obtained by the experiment were investigated. As a result, when the Ca concentration is equal to or lower than the predetermined concentration, in addition to the Ca-P inclusions generated in the molten iron, Nd-P inclusions generated at the time of temperature drop due to casting solidification coexist. I found it. This phenomenon is caused by reducing the dissolved P concentration by generating Ca-P inclusions with Ca in the molten iron stage, and further reducing the dissolved P concentration by generating Nd-P inclusions with Nd in the solidification process. This indicates that “two-stage P concentration reduction” can be achieved.

  FIG. 4 is a diagram showing the influence of the Ca concentration in molten iron and the Nd concentration satisfying the relationship of the equations (1) and (2) on the presence or absence of the formation of Ca—P inclusions and Nd—P inclusions. is there. Since the results in the figure are based on data collected from the steel ingot after solidification in the same experiment as the experiment shown in FIG. 3, the Ca concentration and the Ca concentration are the same as those in FIG. In the figure, a circle indicates a Ca-P inclusion (that is, CaP, Ca-OP, Ca-OP-S, Ca-P-Nd-O, Ca-P-Nd-O-X). (-S) (wherein X represents Si or Al)), Nd-P inclusions were generated, and Δ marks that only Ca-P inclusions were generated, And x mark shows that the Ca-P inclusion was not generated, respectively.

  From the results of FIG. 4, only Ca-P inclusions were generated in the region where the Ca concentration was high because the temperature at which Ca disappears by reaction with P becomes too low, and Nd and P react instead. It is estimated that the sufficient reaction rate was not obtained. Further, in order to reduce the P concentration in the steel in two steps with Ca and Nd, the following equation (5A) is used with respect to the Nd concentration that satisfies the relationship of the equations (1) and (2). It has been found that it needs to be present in a region where the Ca concentration is lower than the curve represented. That is, it was confirmed that the Nd concentration in molten iron that satisfies the relationship of the equations (1) and (2) and the Ca concentration in the molten iron must satisfy the relationship represented by the following equation (5).

[Ca] = 1.6 × 10 ⁻² × [Nd] ^2/3 +0.0015 (5A)
[Ca] ≦ 1.6 × 10 ⁻² × [Nd] ^2/3 +0.0015 (5)
By summarizing the relationship between the equation (4) and the equation (5), the relationship of the equation (3) can be obtained. Therefore, after controlling the Nd concentration in the molten iron so as to satisfy the relationship of the equations (1) and (2), Ca is added, and the Ca concentration is satisfied so as to satisfy the relationship represented by the equation (3) By controlling the Nd, a good castability is ensured by a low Nd concentration, and at the molten iron stage, Ca-P inclusions are generated by Ca to reduce the dissolved P concentration. Further, in the solidification process, Nd- It is possible to achieve “two-stage P concentration reduction” in which P-based inclusions are generated to reduce the solid solution P concentration. Thereby, compared with the case where Nd is added and used alone, it is possible to manufacture a steel material in which the solid solution P concentration is further reduced with a higher productivity.

(C) Confirmation of Inclusion Control Effect The following test was conducted on structural high strength CrMo steel SCM430 equivalent steel to confirm the inclusion control effect. Five types of steel having chemical composition shown in Table 1 and Table 2 were melted in an amount of 20 kg using a vacuum high frequency induction melting furnace to produce an ingot.

  In the same table, Expressions (6) and (7) indicate the relational expressions disclosed by the inventors in Patent Document 2. That is, the equation (6) shows the relationship between Nd concentration in molten iron: [Nd] and P concentration: [P] necessary for the formation of NdP inclusions by the reaction of P and Nd in molten iron. In addition, the formula (7) further shows the relationship between both concentrations necessary to suppress the coarsening of inclusions.

  In the same table, the test steel used for the test number A1 is an extremely low phosphorus steel thoroughly dephosphorized in advance and added with Ca, but the formula (2) and ( It is a test steel that does not satisfy the relationship represented by the equation (3), and does not satisfy the relationship between the equations (6) and (7). The test steel used for test number A2 is a test steel to which Nd and Ca are added, and although the relationship between the formulas (6) and (7) is satisfied, the formula (2) and ( 3) It is a test steel that does not satisfy the relationship of the formula. The test steel used for test number A3 is a test steel to which Nd and Ca are added, and the relationship between the formulas (6) and (7) is satisfied, but the formula (3) defined in the present invention is satisfied. It is a test steel that does not satisfy the relationship. The test steel used for test number A4 satisfies the relationship of formula (3) after controlling the Nd concentration so as to satisfy the relationship of formula (1) and formula (2) defined in the present invention. Thus, it is a test steel in which the Ca concentration is controlled and, of course, satisfies the relationship of the formulas (6) and (7). And the test steel used for test number A5 is a steel with a high P concentration that is generally used, and the relationship between formulas (6) and (7) and formulas (2) and (3) This is a test steel that does not satisfy.

  The obtained ingot was subjected to soaking treatment at 1000 ° C. for 2 hours, and then forged into a plate material having a thickness of 25 mm and a width of 60 mm by hot pressing to obtain a test material. The material was soaked at 820 ° C. for 1 hour and then quenched with oil, followed by tempering that was held at 550 ° C. for 1 hour and air-cooled to obtain a test material. A Charpy impact test piece (10 mm × 10 mm × 55 mm, with a V notch of 2 mm depth) defined by JIS Z 2202 is taken from the specimen in a direction perpendicular to the forging direction, and the test temperature is 0 ° C. to 100 ° C. The impact characteristics (impact absorption energy) were evaluated by performing an impact test while changing between.

  Table 3 shows the impact absorption energy at each impact test temperature.

  FIG. 5 shows the relationship between the Charpy impact test temperature and impact absorption energy for each test steel.

  From the results shown in Table 3 and FIG. 5, the following became clear. The impact properties of the material after tempering are inferior impact properties with the lowest impact absorption energy in the case of test number A5 using ordinary high P concentration steel. On the other hand, a test number using a test steel that was an ultra-low phosphorus steel dephosphorized in advance and added Ca, but did not satisfy the relationship of the formulas (2) and (3) defined in the present invention. In A1, the impact absorption energy increases and the impact characteristics are remarkably improved. Furthermore, Nd and Ca were added, and a test steel that satisfies the relationship of the formulas (6) and (7) but does not satisfy the relationship of the formulas (2) and (3) defined in the present invention was used. In test number A2, the impact absorption energy is further increased and the impact characteristics are improved.

  Moreover, it was a test steel to which Nd and Ca were added, and the test steel that satisfied the relationship of the formulas (6) and (7) but did not satisfy the relationship of the formula (3) defined in the present invention was used. In the test number A3, impact characteristics almost the same as the test number A2 were obtained. In addition to the relationship between the equations (6) and (7), the Ca concentration control was performed after the Nd concentration control so as to satisfy the relationships (1) to (3) defined in the present invention. In the test number A4 using the test steel, the impact absorption energy is further increased and becomes the highest, and the value of the upper shelf energy is increased in addition to the increase of the transition temperature, which shows extremely good impact characteristics. .

  From the above results, by performing the Nd addition treatment or the Nd and Ca addition treatment to generate inclusions containing the P compound, it is possible to obtain a good effect of reducing the dissolved P concentration and the solid solution P concentration, and In addition, by processing molten iron by the method defined in the present invention to efficiently produce Nd-P compounds and Ca-P compounds, it is possible to achieve a better inclusion control effect that can achieve even higher impact properties of steel materials. It was confirmed that it was obtained.

  According to the molten iron treatment method of the present invention, the Nd concentration in the molten iron is controlled according to the P, S, and O concentrations in the molten iron, and then the Ca concentration in the molten iron is controlled according to the Nd concentration. Since Ca-P inclusions and Nd-P inclusions can be efficiently generated in the molten iron under the Nd concentration, the dissolved P concentration and thus the solid solution P concentration in the product can be effectively reduced. The steel quality hindering action by P can be reduced to an extremely low level unprecedented. Moreover, since the method of this invention produces | generates Ca inclusions in molten iron, it can manufacture the molten iron excellent in the continuous castability which has the obstruction | occlusion prevention effect of the nozzle for casting.

  As described above, the present invention, after adding Nd to molten iron containing P: 0.0001% or more and 0.5% or less, S: 0.005% or less, and O (oxygen): 0.005% or less , A treatment method of molten iron to which Ca is added, and in the molten iron so as to satisfy the relationship represented by the equations (1) and (2) according to the P concentration, S concentration and O concentration in the molten iron After controlling Nd density | concentration, the process method of the molten iron which controls Ca density | concentration in molten iron so that Ca may be added to molten iron and the relationship represented by said Formula (3) may be satisfied according to Nd density | concentration in molten iron It is. Hereinafter, the method of the present invention will be described in more detail.

(A) Method of adding Nd and Ca In the present invention, after controlling the Nd concentration in the molten iron so as to satisfy the relationship of the formulas (1) and (2) according to the P, S, and O concentrations in the molten iron A specific method of adding Ca to the molten iron and controlling the Ca concentration in the molten iron so as to satisfy the relationship of the expression (3) according to the Nd concentration in the molten iron will be described below.

  Generally, the highest cost is required to control the P concentration in the molten iron, and then the high cost is required to control the S concentration for performing the desulfurization treatment. The deoxidation treatment in which the deoxidizer is added is relatively easy, and the Nd addition treatment that can be performed only by adding Nd is the easiest. Therefore, before the addition of Nd to the molten iron or after the hot metal pretreatment where the P concentration changes, or after the converter, AOD or VOD treatment, the P concentration in the molten iron is quickly analyzed by emission spectroscopic analysis and the value is grasped. . Similarly, the S concentration is also measured. If there is no restriction on the oxygen concentration in the molten iron, a deoxidizer such as Al or Si is introduced to adjust the oxygen concentration in the molten iron.

  In this way, prior to the addition of Nd, the P concentration and the S concentration are measured during the treatment of the molten iron, and after adjusting the oxygen concentration by adding a deoxidizing agent, the above-mentioned formulas (1) and (2) What is necessary is just to add Nd in molten iron so that it may become the Nd density | concentration range calculated | required by a type | formula. The deoxidation element concentration and oxygen concentration in the molten iron, or the yield of added Nd can be obtained from past operation results for each processing apparatus. In the case of a steel type having a standard for oxygen concentration in molten iron, after adjusting the oxygen concentration so as to satisfy the oxygen concentration standard by the conventional method, the oxygen concentration after the adjustment is used (1) What is necessary is just to obtain | require the addition amount of Nd by Formula and Formula (2).

  For example, in the process using the converter, the molten iron is processed in the order of hot metal pretreatment, converter blowing, vacuum degassing treatment such as RH, and continuous casting, so that the P concentration and S in the molten iron are increased during the RH treatment. Measure the concentration. Further, the oxygen concentration is obtained from the deoxidation conditions, the amount of Nd added is obtained from the equations (1) and (2) based on these concentrations, and Nd is added at the end of the RH treatment. Further, since the P concentration and the S concentration hardly change after the RH treatment, Nd may be added during the treatment using the ladle refining device after the RH treatment, and further, the molten steel in the tundish of the continuous casting machine is added. Nd addition may be performed.

  The same applies when an electric furnace, AOD or VOD is used. That is, the concentration of P in molten iron is measured at the end of the treatment step before casting to determine the amount of Nd added, and then Nd is added at the end of the treatment or immediately before casting. However, it is necessary to add Nd prior to the addition of Ca, as is apparent from the Nd addition action described above.

  As a form of Nd to be added, it is preferable to use metal Nd for the purpose of reducing the total addition amount, but an alloy or a mixture of Al, Si, Fe, or the like and Nd may be used. As a method for adding Nd, commonly used methods such as batch addition using a charging device such as a hopper, addition by injection, and addition by a wire supply method can be used.

  On the other hand, since Ca is a highly evaporable metal, its addition amount may be determined in consideration of the addition yield for each facility. Ca may be added at any time before casting as long as Nd is added, or may be added to the molten iron in the refining apparatus or to the molten iron in the tundish.

  As a form of Ca to be added, in addition to metal Ca, an alloy such as a general Ca-Si alloy, or a mixture of metal Ca or Ca alloy and a flux may be used. In addition, as in the case of Nd, commonly used methods such as batch addition using a charging device such as a hopper, addition by injection, and addition by a wire supply method can be used as the Ca addition method.

  When carrying out the method of the present invention, it is not necessary to carry out a dephosphorization treatment so as to greatly reduce the P concentration, but in order to reduce the amount of Ca and Nd added, a dephosphorization treatment is carried out in a hot metal pretreatment or the like. May be. In order to improve the yield of addition of Ca and Nd, it is preferable to modify the slag before adding Nd.

(B) Preferred Component Composition Range of Molten Iron and Slag 1) Preferred Component Composition Range of Molten Iron As described above, the method of the present invention uses molten iron having an S concentration of 0.005% or less and an O concentration of 0.005% or less. Although it is the object, preferably, the Sd concentration before addition of Nd is 0.0025% or less and the O concentration is 0.0030% or less, whereby the Nd addition amount and the Ca addition amount can be further reduced.

The S concentration is 0.0003% or more in order to further disperse the Nd-P inclusions and Ca-P inclusions, and the O concentration is the same reason. Is preferably 0.0005% or more.
For alloy components other than Nd, P, S, and O that do not affect the composition of these components, the components may be adjusted after Nd is added.

  Further, Al, Si, Mg, etc. that can effectively reduce the oxygen concentration are preferably added before the addition of Nd. By adding these elements before the addition of Nd, the oxygen concentration in the molten iron can be reduced, so that fluctuations in the oxygen concentration can be reduced and the addition amounts of Nd and Ca can be reduced. Furthermore, the generation of Nd-P inclusions and Ca-P inclusions can be promoted as long as the oxygen concentration is within a range that easily satisfies the relationship of the expressions (2) and (3).

  Next, preferred ranges of C, Si, Mn, Si, Al and other component compositions will be described.

C: 3.5% or less and Si: 2.5% or less C and Si are elements having an action of increasing the activity of P in steel when the concentration is high. If the C concentration is higher than 3.5%, the effect on the activity of P becomes significant, and the production conditions of the P compound may change. Therefore, the C concentration may be 3.5% or less. preferable. For the same reason, the Si concentration is preferably 2.5% or less.

  The C concentration is more preferably in the range of 0.0015% or more from the viewpoint of securing the steel material characteristics and the stable deoxidation action, and the Si concentration is 0 from the viewpoint of performing preliminary deoxidation. More preferably, it is in the range of 0.01% or more.

Mn: 3% or less Mn is an element having an action of reducing the activity of P in steel when its concentration is high. If the Mn concentration is higher than 3%, the activity of P is remarkably reduced, and thus it may be difficult to produce a P compound. Therefore, the Mn concentration is preferably 3% or less. The Mn concentration is more preferably in the range of 0.2% or more from the viewpoint of securing the steel material strength.

Al: 3% or less Al has an extremely large influence on the dissolved oxygen concentration from the equilibrium relationship with dissolved oxygen in the steel. If the Al concentration exceeds 3%, the equilibrium dissolved oxygen concentration increases rapidly, and the alumina oxide inclusions increase, which may deteriorate the cleanliness of the steel. Therefore, the Al concentration is 3% or less. It is preferable that The Al concentration is more preferably in the range of 0.0035% or more from the viewpoint of improving the yield of Nd and ensuring its stability.

  In the present invention, the Al concentration means the concentration of acid-soluble Al (sol. Al).

  In said molten iron, it replaces with a part of iron and elements, such as following Ni, Mo, V, Ti, Cr, may contain. This is because these elements hardly affect the reaction between Nd and P in molten iron. That is, Ni in a concentration range of 0.01 to 30%, Mo in a concentration range of 0.01 to 1%, V in a concentration range of 0.001 to 0.1%, and a concentration of 0.005 to 0.3% Ti in the range, Cr in the concentration range of 0.001 to 35%, and the like.

2) Preferred component composition range of slag Since the present invention uses a highly reactive Ca together with Nd, it is not necessary to control the component composition of slag, unlike the case of adding Nd alone, but it is more reproducible. From the viewpoint of generating Nd—P inclusions well, the value of (CaO / Al ₂ O ₃ ), which is the mass concentration ratio of CaO and Al ₂ O _{3 in} the slag, is preferably 0.7 or more and 9 or less. The above value is more preferably 1 or more and 2 or less. The value of (CaO / SiO ₂ ), which is the mass concentration ratio of CaO and SiO ₂ , is preferably 0.65 or more. T. in slag The concentration of the lower oxide in the slag, which is the sum of the Fe concentration and the MnO concentration, is preferably 10% or less, and more preferably 5% or less. This is because the amount of oxidation loss of Nd and Ca is reduced and the yield of the added metal is improved.

  In the present invention, the influence of the slag component composition on the treatment effect of the molten iron is small, but a more stable treatment effect can be obtained by adjusting to the above slag component composition range.

(Test method)
The C, Si, Mn, Cr, Mo, and V concentrations in the steel components were set to the same ranges as the component concentration ranges of the test steel described in Table 1, and the P, S, O, Nd, and Ca concentrations were varied. 180 kg of steel was melted in a vacuum high frequency induction melting furnace. At the time of melting, the melting temperature was 1600 ° C., and after adjusting the P, S, and O concentrations in the steel, the metal Nd was added, and then the metal Ca was added to adjust the component concentration. The O concentration was controlled by increasing the O concentration by blowing up oxygen gas and then reducing the O concentration by performing deoxidation treatment with C or deoxidation treatment with Al to obtain a target value. After adjusting the component composition of the molten steel, it was cast into a mold through a refractory casting nozzle having an inner diameter of 16 mm and a length of 200 mm to produce an ingot.

  The obtained ingot was subjected to a soaking treatment at 1000 ° C. for 2 hours, and then forged into a plate material having a thickness of 25 mm and a width of 60 mm by a hot press to obtain a test material. Thereafter, the test material was soaked at 820 ° C. for 1 hour, then oil-quenched, and then subjected to a tempering process that was held at 550 ° C. for 1 hour and air-cooled to obtain a test material. A Charpy impact test piece (10 mm × 10 mm × 55 mm, with a V notch of 2 mm depth) specified in JIS Z 2202 is taken from the specimen in a direction perpendicular to the forging direction, and Charpy is tested at a test temperature of 20 ° C. An impact test was performed to evaluate the impact absorption energy. Further, the inclusions in the sample cut out separately were observed by SEM and the component composition was quantified by EPMA, and the inner surface of the casting nozzle was observed to check for the presence or absence of deposits.

(Test results)
In Table 4, as test conditions, P, S, O, Nd and Ca concentrations in steel, A value calculated by equation (1), lower limit value and upper limit value of Nd concentration in equation (2), (2) Satisfaction with the formula, lower and upper limits of the Ca concentration in the formula (3), and satisfaction with the formula (3) were shown.

  Table 5 shows the presence or absence of Ca-P inclusions and Nd-P inclusions, impact absorption energy, and the presence or absence of deposits in the casting nozzle as test results.

  Test numbers 1 to 8 are tests for examples of the present invention that satisfy all of the relationships represented by the expressions (1) to (3). On the other hand, Test Nos. 9 to 15 of Comparative Example 1 defined the appropriate range of the Ca concentration (3) while satisfying the relationship between the expressions (1) and (2) that defined the appropriate range of the Nd concentration (3 ) Is a test that does not satisfy the relationship of the formula, and the test numbers 16 to 23 of Comparative Example 2 are the proper ranges of the Ca concentration in the relationships of the formulas (1) and (2) that define the appropriate range of the Nd concentration. This is a test that does not satisfy the relationship of the expression (3) that defines Test number 24 of Comparative Example 3 is a test in which the P concentration was reduced to an extremely low phosphorus level in advance but Nd was not added.

  The present invention in which the Nd concentration in the steel is adjusted so as to satisfy the relationship of the formula (3) after adding and adjusting so as to satisfy the relationship of the formulas (1) and (2) defined in the present invention. In Test Nos. 1 to 8, which are tests for the examples, both Ca—P inclusions and Nd—P inclusions were produced, and steel materials with high impact absorption energy and extremely good impact characteristics were obtained. Furthermore, according to the result of the investigation of the inner surface of the nozzle after casting, it was confirmed that no deposit was formed and the method was excellent in castability. This is because, in addition to the effect of efficiently reducing the dissolved P concentration, the effect of preventing nozzle clogging due to the formation of Ca-P inclusions in the molten steel was exhibited.

  On the other hand, the relationship between the expressions (1) and (2) that define the appropriate range of the Nd concentration is satisfied, but the relationship of the expression (3) that defines the appropriate range of the Ca concentration is not satisfied. In test numbers 9 to 15 which are tests, only one of Ca-P inclusions and Nd-P inclusions is produced, and the impact absorption energy of the obtained steel is an example of the present invention. It is lower than test numbers 1-8. In Test Nos. 9, 11 and 14 in which only Nd-P inclusions were generated, deposits were observed in the nozzle, resulting in poor castability.

  Among the tests of Comparative Example 2, Nd concentration is lower than the concentration range defined by Equations (1) and (2), and Ca concentration is outside the concentration range defined by Equation (3). Then, neither Nd-P inclusion nor Ca-P inclusion is generated, and the impact absorption energy of the obtained steel material is extremely low.

  Furthermore, among the tests of Comparative Example 2, in test numbers 20 to 23, Nd-P inclusions were generated, but the Nd concentration exceeded the upper limit of the concentration range defined by the formula (2). The cleanliness of steel deteriorates, and as a result, the impact absorption energy is lower than that of Comparative Example 1. Moreover, in test numbers 20-23, the deposit | attachment produced | generated in the nozzle.

  When comparing the present invention example with test number 24 of comparative example 3, in the present invention example, a higher shock absorption energy of the steel material than test number 24 in which the P concentration was reduced to an extremely low level in advance was obtained. It can be seen that it is possible to supply a high-performance steel material at a lower cost by treating the molten iron by using the method of the present invention than by carrying out thorough dephosphorization treatment.

  From the above test results, it was confirmed that by using the molten iron treatment method of the present invention, it is possible to manufacture a steel material having excellent continuous castability and having a greatly reduced solid solution P concentration with high productivity. .

  According to the molten iron treatment method of the present invention, the Nd concentration in the molten iron is controlled according to the P, S, and O concentrations in the molten iron, and then the Ca concentration in the molten iron is controlled according to the Nd concentration. Since Ca-P inclusions and Nd-P inclusions can be efficiently generated in the molten iron under the Nd concentration, the dissolved P concentration and thus the solid solution P concentration in the product can be effectively reduced. The steel quality hindering action by P can be reduced to an extremely low level unprecedented. Moreover, since the method of this invention produces | generates Ca inclusions in molten iron, it can manufacture the molten iron excellent in the continuous castability which has the obstruction | occlusion prevention effect of the nozzle for casting. Therefore, the method of the present invention can be widely applied in the steelmaking technical field as a molten iron treatment method capable of reducing the dissolved P concentration with high productivity and refining and supplying molten iron with good continuous castability.

It is a figure which shows the relationship between Ca density | concentration in molten iron, and the CaO density | concentration in the inclusion which existed in the rapidly cooled sample extract | collected from molten iron. It is a figure which shows the relationship between melt | dissolution Nd density | concentration ([Nd] -A) and melt | dissolution Ca density | concentration (B). It is a figure which shows the influence of Nd density | concentration which satisfies the relationship of Ca density | concentration in molten iron and the presence or absence of the production | generation of Ca-P type inclusion, and (1) Formula and (2) Formula. It is a figure which shows the influence of the Nd density | concentration which satisfies the relationship of Ca density | concentration in molten iron and the relationship of (1) type | formula and (2) type | formula to the presence or absence of the production | generation of Ca-P type inclusion and Nd-P type inclusion. It is a figure which shows the relationship between Charpy impact test temperature and impact absorption energy.

Claims (1)

After adding Nd to molten iron containing, by mass%, P: 0.0001% to 0.5%, S: 0.005% and O (oxygen): 0.005% or less, Ca is added A method of treating molten iron, wherein the Nd concentration in molten iron is controlled so as to satisfy the relationship expressed by the following formulas (1) and (2) according to the P concentration, S concentration and O concentration in the molten iron Then, Ca is added to the molten iron, and the concentration of Ca in the molten iron is controlled so as to satisfy the relationship represented by the following formula (3) according to the Nd concentration in the molten iron. Method.
A = 0.24 [P] +0.82 [S] +0.85 [O] (1)
A + 0.005 ≦ [Nd] ≦ A + 0.03 (2)
1.2 × 10 ⁻² × [Nd] ^2/3 ≦ [Ca] ≦ 1.6 × 10 ⁻² × [Nd] ^2/3 +0.0015 (3)
Here, [P], [S], [O], [Nd], and [Ca] represent the concentration (mass%) of each element in the molten iron.

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