JP2006028575A - Method for producing high sulfur-containing steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high sulfur-containing steel containing no lead at the high stable sulfur content and at low cost, and a method for producing the high sulfur-containing steel containing Te at high yield. <P>SOLUTION: (1) The producing method for high sulfur-containing steel is performed, in which this steel contains 0.01-0.15% C, ≤0.03% Si, 0.9-2.0% Mn, 0.40-0.70% S, 0.01-0.2% P, 0.003-0.03% N and 0.008-0.020% O, and Mn content in the molten steel at the slag refining in a ladle is controlled to ≤1.5% and MnO content in the slag is controlled to 25-40%, and in the above refining, a stirring index of the molten steel is adjusted to 10-100 (J/kg of molten steel). (2) In the above steel, further, Te is contained at 0.001-0.07% and after adding Te, the stirring index of the molten steel is adjusted to ≥10 (J/kg of molten steel). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention has a high sulfur and Mn content and an oxygen content of 0.008 to 0.020 in the production of a high-sulfur-containing free-cutting steel that does not substantially contain lead, which is an element with a large environmental load. The present invention relates to a method for producing low-cost and reliable high-sulfur-containing steel by reliably and stably controlling the mass% range and further controlling the sulfur yield at the steelmaking stage at a high level and stably.

  Furthermore, the present invention relates to a melting method in which expensive Te can be effectively added in as small a quantity as possible in the production of high sulfur content free-cutting steel containing Te that further improves machinability.

  In conventional low-carbon sulfur-based free-cutting steel, the alloy component composition ranges are generally C: 0.01 to 0.15%, Si: ≦ 0.03%, Mn: 0.6 to 1.% by mass. 2%, S: 0.15 to 0.40%, P: 0.01 to 0.2%, N: 0.003 to 0.03%, and in order to further improve the machinability, Pb: 0.05 to 0.4% was also used, and the balance was made of iron. Further, based on such a steel component composition, it is effective to coarsen MnS inclusions in order to stabilize the machinability. For that purpose, O (oxygen) is set to 0.008 to 0. It has been considered to be contained in the range of 0.020%.

  However, in recent years, low carbon sulfur-based free-cutting steel containing no lead has been demanded from the viewpoint of reducing the burden on the environment. Such low-carbon sulfur-based free-cutting steel improves machinability by increasing the content of sulfur (hereinafter also referred to as “S”) instead of containing lead substantially. Development of high S steel exceeding the standard and its processing method has been advanced.

  In addition, in order to stably and reliably manufacture low-carbon S-based free-cutting steel that exceeds the conventional S content, it has been necessary to review and optimize the conventional steelmaking conditions for the steelmaking method in the molten steel stage. It was.

  As a conventional method for producing steel, for example, Patent Document 1 discloses a method for producing low-carbon sulfur-based free-cutting steel that adjusts the amount of oxygen in steel by controlling the amount of MnO in slag and the amount of Mn in steel. It is disclosed. However, according to the relational expression of the oxygen content in steel and the MnO concentration in slag (molar fraction of MnO) and the Mn content in steel shown in Patent Document 1, the Mn content is 1.5. %, If the MnO molar fraction does not exceed 0.3, the oxygen concentration in the steel cannot be ensured to be 80 ppm or more, and a large amount of MnO needs to coexist in the slag. Furthermore, under the conventional steelmaking conditions, it is considered that the S yield deteriorates with the increase of S and the S content becomes unstable, but this document does not consider such points.

  Moreover, since the manufacturing method of steel in the case of adding Te based on such high S steel is not known, steel that contains Te, which is an expensive additive element, under effective and high controllability. The melting method was completely unknown.

JP-A-9-31522 (Claims and paragraph [0006])

  The present invention has been made in order to solve the above-mentioned problems, and the problem is that the steel is substantially free of lead and has a high S content exceeding conventional low-carbon sulfur free-cutting steel. To provide an inexpensive and reliable method for melting high-sulfur steel that can obtain the target oxygen content and obtain the desired S content in a stable and reliable manner during the melting stage. It is in.

  Furthermore, another object of the present invention is to provide a melting method capable of effectively adding expensive Te with a high yield in the production of high sulfur-containing free-cutting steel containing Te that further improves machinability. is there.

  In order to solve the above-mentioned problems, the present inventors have reexamined the melting process in the actual production process of conventional low-carbon sulfur-based free-cutting steel, and obtained the following knowledge (a) to (f). The present invention was completed.

  (A) The oxygen content in low-carbon S-based free-cutting steel is determined by the refining performed in contact with the slag in the ladle stage from the steelmaking furnace such as a converter to the ladle (hereinafter referred to as “ladder slag refining”). Are also governed by the conditions at the stage. In the steelmaking stage, oxygen exceeding the thermodynamic equilibrium value is supersaturated or suspended as an oxide in the steel, and in the slag refining stage, the excess oxygen gradually decreases. It is in.

  (B) The same applies to the sulfur content. In the process in which the excess S gradually decreases in the slag refining stage after an excessive sulfur source such as pyrite is added in the steelmaking stage, a predetermined S content is obtained. An operation to reach is performed.

  (C) The target steel of the present invention does not substantially contain lead, and the S content is 0.40 to 0.70 mass%, which is higher than that of conventional steel, so the slag after the sulfur source is added The rate of decrease in the S content during refining is further increased. In order to effectively form MnS inclusions for improving machinability, it is necessary to increase the Mn content in proportion to the increase in the S content.

  (E) Since the deoxidation reaction and the desulfurization reaction both proceed easily under a high Mn content as in (c) above, the rate of decrease in oxygen content and S content in the molten steel at the slag refining stage Must be controlled appropriately and reliably. These controls are possible by optimizing the main component composition in the steelmaking stage and the stirring strength in the slag refining stage.

  (F) Furthermore, when adding Te as a machinability improving element, it is necessary to reliably dissolve Te in steel. By adding Te at the latter stage of the slag refining stage and optimizing the stirring strength, loss due to evaporation of Te is prevented, and loss due to transfer into slag is prevented, so that Te is surely contained in the molten steel. Can be dissolved.

  The present invention has been completed on the basis of the above findings, and the gist of the present invention resides in a method for producing high sulfur steels shown in the following (1) and (2).

  (1) By mass%, C: 0.01 to 0.15%, Si ≦ 0.03%, Mn: 0.9 to 2.0%, S: 0.40 to 0.70%, P: 0 0.01 to 0.20%, N: 0.003 to 0.030%, and O (oxygen): 0.008 to 0.020%, and the method for producing a high-sulfur steel containing the balance of Fe and impurities In the refining, the Mn content in the molten steel when refining in contact with the slag in the ladle is 1.5% by mass or less and the MnO content in the slag is 25 to 40% by mass, and in the refining A method for producing a high sulfur content steel (hereinafter also referred to as “first invention”) in which the value of the stirring index of molten steel, which is the stirring energy per unit mass of molten steel, is adjusted to a range of 10 to 100 (J / kg-molten steel). ).

  (2) By mass%, C: 0.01 to 0.15%, Si ≦ 0.03%, Mn: 0.9 to 2.0%, S: 0.40 to 0.70%, P: 0 0.01 to 0.2%, N: 0.003 to 0.03%, O (oxygen): 0.008 to 0.020% and Te: 0.001 to 0.07%, with the balance being Fe And a method for producing high-sulfur steel comprising impurities, wherein the Mn content in the molten steel when the molten steel not containing Te is brought into contact with the slag in the ladle and refined is 1.5% by mass or less and in the slag In the refining, the stirring index value of the molten steel, which is the stirring energy per unit mass of the molten steel, is adjusted to a range of 10 to 100 (J / kg-molten steel). In addition, after the addition of Te in the latter stage of refining, the value of the stirring index of the molten steel is 10 (J / The method of manufacturing high sulfur-containing steel is adjusted to a range of g- molten steel) or more (hereinafter, also referred to as "second invention").

  In the present invention, the “ladder” means a refining vessel that can be used for the purpose of refining ladle slag in the present invention, such as a refining vessel of a converter type as well as a general ladle.

  The “stirring index” means stirring energy per unit mass of molten steel, for example, an amount obtained by the following equation (1).

Z = 10 ⁻³ × ∫εdθ (1)
Here, Z represents the stirring index (J / kg-molten steel), ε represents the stirring power density (W / t-molten steel), and θ represents the ladle refining time (s).

  The stirring power density in the above formula (1) means stirring energy per unit time (1 s) per unit mass (1 t) of molten steel. For example, stirring is performed by blowing an inert gas from the bottom of the ladle. Means an amount calculated by the following equation (2).

  In addition, the “second stage of refining” is a stage in which the refining operation is almost completed so that the main component has the target content rate, and the content of trace elements including Te is adjusted and the temperature is adjusted to perform casting. means.

  In the following description of the specification, “%” of the content ratio representing the component composition means “mass%”.

  According to the method of the present invention, in the production of high S content steel, it has been difficult to control conventionally by adjusting the Mn content in molten steel, the MnO content in slag and the stirring index of molten steel in ladle slag refining. In addition, the controllability of the oxygen content and S content in molten steel can be improved, and high quality S steel with high S yield and stable quality can be produced at low cost. Therefore, high-S content free-cutting steel that replaces Pb free-cutting steel can be supplied at low cost, and stable Te-containing high-S free-cutting steel can be supplied with high Te yield and controllability.

  In the present invention, by mass%, C: 0.01 to 0.15%, Si ≦ 0.03%, Mn: 0.9 to 2.0%, S: 0.40 to 0.70%, P: Production of steel containing high sulfur containing 0.01 to 0.2%, N: 0.003 to 0.03% and O (oxygen): 0.008 to 0.020%, with the balance being Fe and impurities In the ladle, the Mn content in the molten steel when refining is brought into contact with the slag in the ladle is 1.5% by mass or less, and the MnO content in the slag is 25 to 40% by mass. Is a method for producing high-sulfur steel, wherein the value of the stirring index of the molten steel, which is the stirring energy per unit mass of the molten steel, is adjusted to a range of 10 to 100 (J / kg-molten steel). The reason why the scope of the present invention is limited as described above and the preferred range will be described below.

(A) Component composition of high sulfur steel C: 0.01 to 0.15%
C is an element necessary for obtaining the strength and toughness of steel. In free-cutting steel having free-cutting properties as the most important characteristic, in order to obtain necessary tensile strength and fatigue strength, the C content needs to be 0.01% or more. However, if the content exceeds 0.15%, the workability of the base material, which is a basic characteristic of free-cutting steel, deteriorates. Therefore, the appropriate range of the C content is set to 0.01 to 0.15%.

Si: ≦ 0.03%
Si is an element having a deoxidizing action and a solid solution strengthening action of steel. In order to secure the oxygen content in the steel in the range described later, the Si content needs to be 0.03% or less.

Mn: 0.9 to 2.0%
Mn is an element that combines with S in steel to form MnS to enhance machinability and also functions as a deoxidizing element. In order to suppress the formation of FeS that causes brittleness during rolling by forming MnS, it is necessary to contain 0.9% or more of Mn. On the other hand, when the Mn content is higher than 2.0%, the deoxidizing action becomes strong, and it becomes difficult to stably secure an oxygen content of 0.008% or more, which is a requirement of the present invention. Therefore, the appropriate range of the Mn content is set to 0.9 to 2.0%.

S: 0.40 to 0.70%
S in steel is an essential element for forming MnS and improving machinability. In order to obtain an effect of improving machinability comparable to that of a conventional free-cutting steel containing Pb in a state where Pb is not substantially contained, the S content needs to be 0.40% or more. However, if the content exceeds 0.70%, the occurrence of cracks during rolling becomes significant, and the mechanical properties of the steel deteriorate significantly. Therefore, the appropriate range of the S content is set to 0.40 to 0.70%. In addition,
From the viewpoint of balancing the improvement in machinability and the anisotropy in the rolling direction of the mechanical properties as a steel material, the preferred content range is 0.50 to 0.60%.

P: 0.01-0.20%
P is an element that segregates at the grain boundaries and embrittles the steel, and has an effect of improving machinability. In order to obtain the effect, it is necessary to contain 0.01% or more. On the other hand, when the P content is increased, the toughness of the steel is deteriorated and the ductility is lowered. Therefore, the content is set to 0.20% or less. P is an element having a solid solution strengthening action if its content is in the range of 0.20% or less, and its content can be determined in consideration of the necessary strength as a material. In addition, P is often brought along with iron ore and scrap, and when dephosphorization or phosphorus addition, these increase production costs. It is necessary to decide. For the above reason, the appropriate range of the P content is set to 0.01 to 0.20%.

N: 0.003-0.030%
N is an element that forms nitrides in steel, is unevenly distributed at grain boundaries, and contributes to improvement of machinability together with MnS. In order to ensure the effect, the N content needs to be 0.003% or more. On the other hand, when the N content exceeds 0.030%, the nitride becomes coarse, and the wear of the tool becomes apparent. Therefore, the appropriate range of N content is set to 0.003 to 0.030%.

O (oxygen): 0.008 to 0.020%
Oxygen is an element that exists together with MnS as an oxide inclusion in steel and has a useful effect on the improvement of machinability. That is, when oxygen coexists with MnS as oxide inclusions, MnS exhibits a relatively coarse spherical shape, and MnS does not extend much even during rolling of a steel material, and maintains an isolated and thick shape. Inclusions having such a shape become a starting point of embrittlement during cutting, resulting in a significant improvement in machinability. In addition, MnS moderately suppresses the formation of the constituent cutting edge at the tip of the cutting tool, thereby improving the finished surface roughness. The above effect can be enjoyed when the oxygen content is 0.008% or more. However, when the oxygen content is higher than 0.020%, the presence of oxide inclusions adversely affects the quality of the steel surface. Therefore, the appropriate range of the oxygen content is set to 0.008 to 0.020%. In addition, the range of a preferable content rate is 0.010 to 0.018% from a viewpoint which balances the improvement effect of a machinability, and suppression of the surface flaw generation resulting from a large sized oxide type inclusion.

  In the present invention, the oxygen content means the total amount of dissolved oxygen in steel and the amount of oxygen contained in inclusions, that is, the total oxygen content.

  By the way, in consideration of the above-described range of the high Mn content, it is difficult to control oxygen, which is a useful element, by a conventional component adjustment method. One of the effects that can be enjoyed by the present invention is that this useful oxygen can be stably and reliably controlled in the steelmaking stage.

Te: 0.001 to 0.070%
Te in steel is an element that has the effect of improving machinability by forming MnTe around and inside the MnS inclusions. Such a machinability improving effect is obtained when the Te content is 0.001% or more. On the other hand, when Te is contained in a large amount exceeding 0.070%, embrittlement occurs starting from the MnTe phase around the MnS inclusions in the hot state. Therefore, the appropriate range of Te content is set to 0.001 to 0.070%. A suitable Te content range that can achieve both machinability improvement and hot embrittlement suppression is 0.004 to 0.050%.

(B) Mn in molten steel and MnO composition in slag in ladle slag refining In the method for producing high sulfur-containing steel of the present invention, the stage of steel removal from a steelmaking furnace such as a converter to a ladle and slag refining in a ladle The definition of the state at the stage is important. In the steelmaking stage, there is generally an operation of adding raw materials such as new alloy iron and ironmaking agent and auxiliary materials after oxidative refining, and the steeling time and stirring state differ depending on the form of the furnace. Therefore, the Mn content in molten steel and the MnO content in slag, which are strong variables at the time of refining ladle slag, were defined as the state quantities that define the state in this steelmaking stage. The ladle slag refining in this case means a period in which molten steel is accommodated in the ladle and slag refining is performed. The component composition of molten steel and the component composition of slag at the time of slag refining are determined mainly by addition of alloy iron and a faux-forming agent at the time of steelmaking. Therefore, the above rules are effective.

  In the present invention, the Mn content in the molten steel at the start of the above ladle refining needs to be 1.5% or less. The reason for this is that if the Mn content is higher than 1.5% at an early stage of ladle slag refining, even if the Mn content is decreased during the subsequent slag refining, the initial oxygen content is decreased. This is because it is difficult to stably obtain an oxygen content of 0.008% or more in the ladle refining stage. Moreover, if the Mn content exceeds 1.5%, desulfurization with slag also proceeds, so that it becomes necessary to adjust the S content each time. Preferably, the Mn content is maintained at 1.2% or less, and by adding iron alloy or the like, the Mn content necessary for the slag refining is adjusted from the latter half to the final stage. The Mn content is preferably 0.5% or more.

  The MnO content in the slag at the start of the above refining needs to be in the range of 25-40%. The reason is that when the MnO content is less than 25%, it becomes difficult to stably obtain an oxygen content in the molten steel of 0.008% or more, while when the MnO content is higher than 40%, it is stable. This is because the Mn content in the steel cannot be maintained.

(C) Stirring conditions during ladle slag refining The amount of stirring per unit mass of molten steel during ladle slag refining will be described. During ladle refining, the molten steel is agitated by gas agitation in order to make the composition and temperature of the molten steel uniform. Generally, it is often expressed by stirring energy per unit time (also referred to as “stirring power density”) (ε) per unit mass of molten steel called stirring power, and the calculation method is not particularly limited. When gas stirring is performed by blowing an inert gas from the bottom of the ladle, the following equation (2) can be used.

ε = (371 × Q × T / m) × [ln {1+ (9.8 × 7 × 10 ³ × h) / p} + (1−298 / T)] (2)
Where ε is the stirring power density (W / t-molten steel), Q is the blown gas flow rate (Nm ³ / s), T is the molten steel temperature (K), m is the molten steel mass (t), and h is the bath depth of the molten steel. (M) and p represent the atmospheric pressure (Pa), respectively.

  Since the gas stirring is performed during the ladle slag refining period, by integrating the above stirring power density with respect to the slag refining time (θ), the unit mass of molten steel ( Stir energy (Z) per kg) is obtained.

Z = 10 ⁻³ × ∫εdθ (1)
In this invention, Z calculated | required by the said (1) Formula can be used as a stirring index. When the flow rate of the blown gas is constant with respect to the refining time, the stirring index is expressed as the following equation (3).
Z = 10 ⁻³ × ε × θ (3)
That is, Z in the above formula (1) or (3) is an index having a dimension of (J / kg-molten steel) and representing the stirring energy per unit mass of the molten steel. Therefore, this index can be an index for controlling the decrease rate of the oxygen content rate and the decrease rate of the S content rate.

  Next, the reason for limiting the stirring index will be described. In controlling the oxygen content in steel, it was confirmed by the following test that the agitation index had a critical value.

Using a graphite heating element resistance heating furnace capable of melting steel in a MgO refractory container, C: 0.06-0.10%, Si ≦ 0.03%, Mn: 1.2-1.5%, 10 kg of molten steel containing S: 0.40 to 0.55% and P: 0.05 to 0.07% was held in an Ar gas atmosphere of 1.013 × 10 ⁵ Pa at a temperature of 1873K. After preliminarily adjusting the oxygen content in the steel to a range of 0.015 to 0.025%, 300 g of slag composed of CaO, SiO ₂ , MnO, MgO, Al ₂ O ₃ is added, and Mn in the steel is added. Metal Mn was added so that the content would be 1.5-1.6%. The chemical composition of the slag, MnO: 30~35%, MgO: 10%, Al 2 O 3: Adjust the mixing ratio so that about 3%, also for residue, CaO and SiO ₂ are the same It adjusted so that it might become a content rate.

  At the bottom of the MgO refractory holding container, a porous brick for gas stirring is installed, and Ar gas is supplied at a flow rate of 0.24 to 0.4 NL / min to stir the molten steel. The transition of oxygen content was investigated.

  FIG. 1 is a graph showing the relationship between the stirring index of molten steel and the total oxygen content in the molten steel. In the figure, the total oxygen content on the vertical axis means the total amount of dissolved oxygen in steel and oxygen contained in inclusions.

According to the results in the figure, the total oxygen content in the molten steel decreases as the stirring index increases. Based on this relationship, it can be seen that the oxygen content in the molten steel can be controlled using the stirring index as an index. Furthermore, it has been found that in order to ensure that the oxygen content in the steel of the present invention is 0.008% or more, the stirring index value must be 100 or less. On the other hand, in order to make the component composition and temperature of the molten steel uniform, a minimum amount of stirring is required, and a stirring index value of 10 or more is required. Therefore, the appropriate range of the stirring index (Z) was set to 10-100. In addition, since the stirring for a long time or intensity causes an increase in steelmaking cost, the preferable range of the stirring index is 10 to 80.

(D) Stirring conditions when adding Te The stirring conditions when adding Te will be described. Te is extremely expensive as an alloy element for steel, and pure metal is often used for adjusting steel components. Te has a melting point of 449.5 [deg.] C. and a boiling point of 989.5 [deg.] C., both of which are lower than the steelmaking temperature, and therefore, there is a large evaporation loss when added to steel. Furthermore, its chemical behavior is similar to the homologous element S, and is believed to have absorption and distribution into the slag. Therefore, when adding Te to the high-sulfur steel that is the subject of the present invention, appropriate stirring conditions for suppressing evaporation loss to a minimum amount and suppressing absorption into the slag, that is, a stirring index I guessed there existed.

Therefore, a predetermined amount of Te is added using molten steel and slag having the same composition as in the oxygen content control test using 10 kg of molten steel described above, and the relationship between the subsequent decrease in Te content and the stirring conditions investigated.
FIG. 2 is a diagram showing the influence of the stirring index of molten steel on the Te residual ratio in molten steel and the distribution ratio of Te between slag and molten steel. In the figure, the Te residual rate is a value expressed by percentage (%) by dividing the Te residual amount in the molten steel calculated from the Te content rate (%) in the molten steel by the Te addition amount. The value of (Te) / [Te] is also expressed as Te content (%) in slag (hereinafter also referred to as (Te)) and Te content (%) in molten steel (hereinafter referred to as [Te]. The ratio of Te between the slag and the molten steel (−).

  From the results shown in FIG. 9, the Te residual rate increases with the stirring time, and when the stirring index is 10 or more, the Te residual rate is secured at 30% or more. On the other hand, the Te content in the slag and the Te content in the molten steel decrease with the elapse of the stirring time, that is, with an increase in the stirring index, but the distribution ratio of Te between the slag and the molten steel, (Te) / [Te] The value decreases to 70 (−) or less when the stirring index is 10 or more, and it becomes easy to secure the Te content in the molten steel. Therefore, the appropriate range of the stirring index after Te addition is set to 10 or more.

  However, when the stirring index is higher than 100, the residual ratio of Te in the molten steel is saturated at 50 to 60%, and the distribution ratio of Te between the slag and the molten steel, the value of (Te) / [Te] is also 20 Since it becomes a constant value before and after (−), the increase in the refining effect by increasing the stirring time becomes small, which is not preferable because it increases the refining cost. Therefore, the stirring index is preferably 100 or less.

(E) Embodiments and effects of the present invention in actual production scale For molten steel decarburized by top blowing oxygen in a steelmaking furnace such as a converter and having a C content of about 0.03 to 0.13% At the time of steel production, Mn alloy iron, pyrite ore as a sulfur source, P alloy iron, and the like are added. Here, the Mn alloy iron is added so that the Mn content in the molten steel at the stage of being accommodated in the ladle is 1.5% or less, preferably 1.2% or less, more preferably 1.0% or less. Need to be done. The reason for this is to maintain a high oxygen content in the molten steel at the time of steel output as described above. Next, after removing the converter slag as necessary, a fossilizing agent such as quick lime as a CaO source and silica sand as a SiO ₂ source is added again. At this time, the slagging agent needs to be added so that the MnO content in the slag at the stage of being accommodated in the ladle is in the range of 25 to 40%. In addition, since MnO is formed by the reaction between oxygen in the molten steel and the Mn alloy iron at the time of converter steel, after considering that MnO and CaO and SiO ₂ are diluted by the converter slag, In order to achieve the target MnO content, the addition amounts of Mn alloy iron and each of the faux agents must be controlled. Moreover, Mn ore etc. can also be added as a MnO source as needed.

  Next, the molten steel accommodated in the ladle is subjected to a stirring operation such as gas stirring at a position where the ladle refining is performed. The power of stirring may be obtained by, for example, a formula for calculating the stirring power density as shown by the above formula (2). In addition, ladle slag refining time means the time that steel is extracted from a refining furnace such as a converter to a ladle and subjected to processing such as stirring at the ladle refining position. Time such as addition is also included. On the other hand, the waiting time until casting which does not receive operation, such as stirring, or the time conveyed by a crane etc. is not included.

  About the ladle slag refining of this invention, the case where gas stirring is performed by supplying gas from the gas blowing tuyere provided in the ladle bottom part is demonstrated as an example. It is necessary to make the Mn content in the molten steel 1.5% or less at the stage of being accommodated in the ladle. Preferably, the Mn content in the molten steel is 1.2% or less, more preferably 1.0% or less, and the ladle is stored in a ladle and the stirring index value is maintained at 6 or more. Therefore, it is preferable to adjust to 0.9 to 2.0% which is the desired Mn content.

As a slag component composition at this time, MnO content rate is adjusted so that it may become the range of 25-40%. Other major components of the _{slag, CaO, SiO 2, MgO,} Al 2 O 3, a FeO and S. The amount of slag and the composition of the slag component are determined in consideration of the interruption of the atmosphere with the outside air during refining, the wear of the ladle refractory, the ease of adjusting the S content in the molten steel, and the like. To exemplify the preferred conditions, the amount of slag is 2kg~20kg per molten steel 1t, CaO: 15~35%, SiO 2: 15~35%, MgO: 5~20%, Al 2 O 3: 3~15% and FeO: It is 3 to 8% of range.

  Gas blow stirring is usually performed when a ladle is installed at the slag refining processing position, and then gas blowing is started, followed by addition of alloyed iron and heat refining, and further addition of a slagging agent as necessary. The operation is continued while. Therefore, the stirring index can be obtained by integrating the stirring power density with respect to the ladle refining time. In particular, when the amount of stirring gas is constant, the stirring power density and the stirring time (that is, ladle slag refining). Time).

  By adjusting the value of the stirring index to 10 to 100, the oxygen content in the molten steel at a high concentration in a non-equilibrium state is appropriately reduced, and the final oxygen content is 0.008 to 0.020%. High S content steel in the range of can be obtained. Furthermore, by performing such control, it becomes possible to suppress an excessive decrease in the S content in the molten steel and fluctuations in the Mn content, and a low-cost and stable quality high-S content steel by ladle slag refining. Can be manufactured.

  In addition, when Te is added in an amount of 0.001 to 0.070% instead of a part of Fe in steel, it is added after the latter half of ladle slag refining in view of the tendency of Te evaporation loss to occur. The addition form of Te in this case is not particularly limited, but it is preferable to use an alloy form of Te because it is easy to prevent evaporation loss in the initial stage of addition. Here, when Te is added, Te remains in the molten steel as much as possible, and the stirring condition of the molten steel capable of stable component composition control is 10 or more in terms of the stirring index. On the other hand, the upper limit is not particularly limited from the viewpoint of improving the Te yield, but an increase in the stirring time causes an increase in the refining cost, so the stirring index is preferably 100 or less.

  As described above, an inexpensive and stable high S content steel melted through ladle slag refining is commercialized through ordinary continuous casting and rolling processes.

  In order to confirm the effect of the manufacturing method of the high sulfur content steel of the present invention, the melting test described below was performed and the result was evaluated.

Example 1
1. Test method Ladle slag was refined using molten steel obtained by decarburization blown by a converter having a capacity of molten steel of 78 t, and further continuous casting was performed. The target steel composition is C: 0.01 to 0.15%, Si ≦ 0.03%, Mn: 0.9 to 2.0%, S: 0.40 to 0.70%, P: The range is 0.01 to 0.20%, N: 0.003 to 0.030%, and O: 0.008 to 0.020%. Moreover, about the steel grade which adds Te, it is the range of Te: 0.001-0.070%.

  Molten steel decarburized to about C: 0.01-0.15% by converter blowing was steeled in a ladle having a gas blown tuyere at the bottom of the ladle. The components were adjusted by adding alloy iron such as Mn alloy iron and P-alloy iron as necessary at the time of steel production. Further, in order to adjust the slag amount and the component composition, after removing from the converter after steel removal, predetermined amounts of quick lime and quartz sand were added as a slagging agent. Thereafter, the ladle containing the molten steel was transported to a ladle refining position where electric heating was possible, Ar gas piping was connected, and ladle slag refining was carried out.

  At the time of refining, in order to confirm the composition of the molten steel in the ladle after ladle and the composition of the ladle slag, samples of the molten steel and slag are collected and analyzed before the time when the stirring index becomes 5 The composition was investigated. Furthermore, after adjusting the amount of Ar gas for stirring and the ladle slag refining time so that the stirring index is in the range of 10 to 100, samples for analysis of molten steel and slag were appropriately collected. In the middle, the component composition was adjusted as necessary to achieve the desired composition, and the temperature was adjusted by conducting heating to achieve the desired molten steel temperature. And after this, the ladle which accommodated the molten steel was conveyed to the continuous casting machine, and the molten steel was continuously cast.

2. Test results
The test conditions and test results in the above test are shown in Table 1. As test conditions, Ar gas flow rate for stirring in ladle slag refining, stirring power density, slag refining time, stirring index, Mn content in molten steel: [Mn], and MnO content in slag: (MnO) As test results, the total oxygen content in steel after refining: [O] and the sulfur content: [S] were shown.

  In the table, the Mn content in the molten steel before refining and the MnO content in the slag are both analytical results of samples taken before the stirring index value of 5 during the ladle slag refining Based on. The stirring power density (ε) was a value obtained by the above equation (2), and the stirring index (Z) was a value obtained by the above equation (1) or (3).

  Test Nos. 1 to 10 are tests for examples of the present invention that satisfy all of the conditions defined in the first invention, and Test Nos. 11 to 16 satisfy at least one of the conditions defined in the first invention. This is a test for a comparative example which is not satisfactory.

  In test numbers 1 to 10 in which the Mn content in the molten steel before refining, the MnO content in the slag, and the stirring index in the ladle slag refining are all within the ranges defined in the first invention, the oxygen content in the steel after refining All the rates are in the range of 0.008 to 0.020%, and the target component composition of the high sulfur content steel is achieved.

  On the other hand, the test number 11 in which the Mn content in the steel exceeds 1.5% is high, the test number 12 in which the stirring index exceeds 100 and the Mn content after refining exceeds 2.0% and the high test No. 14 is the oxygen content after refining is too low, and Test No. 13 whose stirring index is less than 10 is too high after refining. The result was out of the range. Furthermore, the test number 15 in which the MnO content in the slag is less than 25% is too low after the refining, and the test number 16 in which the MnO content in the slag is higher than 40% is the oxygen after the refining. The content rate was too high, and both results were out of the component composition of the high sulfur steel.

(Example 2)
1. Test method As in the case of Example 1, ladle slag refining with stirring is performed using molten steel obtained by decarburization blowing using a converter having a capacity of 78 t of molten steel, and Te is added in the latter half of the refining. Then, Ar gas was blown in at a predetermined flow rate for a predetermined time, and stirring was further continued. In order to investigate the Te content in the molten steel and slag after the addition of Te, samples for component analysis were taken over time as appropriate. The steel containing high S after refining was subjected to a continuous casting process.

  Te is added in an amount of nominally 0.030 to 0.085% to 78t of molten steel, and the actual value of components in the molten steel is in the range of 0.011 to 0.056%. It was. Thus, by analyzing the content of Te in the steel and slag, the Te residual ratio and the Te distribution ratio ((Te) / [Te]) between the slag and the molten steel were obtained. Here, the Te residual ratio is expressed as a percentage (%) by dividing the Te residual amount in the molten steel calculated from the Te content (%) in the molten steel by the Te addition amount, as described in the description of FIG. did.

2. Test Results FIG. 3 is a diagram illustrating the influence of the stirring index of molten steel on the Te residual ratio in molten steel and the distribution ratio of Te between slag and molten steel.

  According to the result of FIG. 6, when the value of the stirring index (Z) is less than 10 which is the lower limit value of the stirring index defined in the first invention, the Te residual ratio is low, and the Te distribution ratio Since the value of ((Te) / [Te]) is also high, the yield of Te in the molten steel is low, and the Te content in the molten steel is not stable.

  On the other hand, when the value of the stirring index (Z) exceeds 10 and the Te residual ratio increases, the value of the Te distribution ratio ((Te) / [Te]) is also around 20 to 40 (-). It became clear that the controllability of the component composition was improved.

  According to the method of the present invention, in the production of high S content steel, by adjusting the Mn content in the molten steel and the MnO content in the slag and the stirring index of the molten steel in the ladle slag refining, it has been difficult to control conventionally. The controllability of the medium oxygen content and the S content can be improved, and a high quality S steel with a high S yield and a stable quality can be produced at low cost. Therefore, high-S content free-cutting steel that replaces Pb free-cutting steel can be provided at a low cost, and stable Te-containing high-S free-cutting steel with high Te yield and controllability can also be provided. Therefore, the manufacturing method of the high S content steel of the present invention can be widely applied as a melting method of molten steel in the manufacturing field of the high S content free cutting steel not containing Pb.

It is a figure which shows the relationship between the stirring index of molten steel, and the total oxygen content rate in molten steel. It is a figure which shows the influence of the stirring index of molten steel on the Te residual rate in molten steel, and the distribution ratio of Te between slag and molten steel. It is a figure which shows the influence of the stirring index of molten steel on the Te residual rate in molten steel, and the distribution ratio of Te between slag and molten steel.

Claims (2)

  In mass%, C: 0.01 to 0.15%, Si ≦ 0.03%, Mn: 0.9 to 2.0%, S: 0.40 to 0.70%, P: 0.01 to 0.2%, N: 0.003-0.03% and O (oxygen): 0.008-0.020%, a method for producing a high-sulfur steel containing the balance of Fe and impurities In the ladle, the Mn content in the molten steel when refining in contact with the slag is 1.5% by mass or less, and the MnO content in the slag is 25 to 40% by mass. A method for producing high-sulfur steel, characterized by adjusting the value of the stirring index of molten steel, which is stirring energy per unit mass, to a range of 10 to 100 (J / kg-molten steel). In mass%, C: 0.01 to 0.15%, Si ≦ 0.03%, Mn: 0.9 to 2.0%, S: 0.40 to 0.70%, P: 0.01 to 0.2%, N: 0.003 to 0.03%, O (oxygen): 0.008 to 0.020% and Te: 0.001 to 0.07%, with the balance being Fe and impurities A method for producing a high sulfur content steel, comprising a molten steel not containing Te in a ladle in contact with slag and refining Mn content in the molten steel of 1.5% by mass or less and containing MnO in the slag In the refining, the stirring index value of the molten steel, which is the stirring energy per unit mass of the molten steel, is adjusted to a range of 10 to 100 (J / kg-molten steel) and refined. After Te addition in the latter period, the stirring index value of the molten steel was 10 (J / kg- Method for producing a high sulfur content steel and adjusting to a range of steel) or more.

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