JP2005059096A - Continuous casting method of low-carbon sulfur-based free cutting steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method which is capable of continuous casting of low-carbon sulfur-based free cutting steel without causing an internal crack or a surface crack. <P>SOLUTION: The continuous casting method is used for casting a molten low-carbon sulfur-based free cutting steel which contains 0.05-0.19% C , 1.0% or less Si, 0.4-2.0% Mn, 0.001-0.2% P, 0.2-0.69% S, less than 0.01% Pb, 0.2% or less Al, 0.001 to 0.02% O and 0.001 to 0.02% N, with the balance being Fe and impurities and in which the contents of Mn and S meet relations of [Mn%]×[S%]<0.9 and [S%]<0.32×[Mn%]<SP>5</SP>. Further the above low-carbon sulfur-based free cutting steel may contain one or more elements out of either or both of groups (a) and (b) mentioned below: group (a) consisting of Cu, Ni, Cr, Mo, V and Nb and group (b) consisting of Ti, Se, Te, Bi, Sn, Ca, Mg and rare earth elements. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention is a low-carbon sulfur-based material that does not contain lead and has superior machinability and hot workability compared to conventional lead free-cutting steel and composite free-cutting steel in which lead and other free-cutting elements are added in combination. More particularly, the present invention relates to a continuous casting method for obtaining a slab of low-carbon sulfur free-cutting steel having good quality by preventing or reducing internal cracking during continuous casting.

  Conventionally, in order to improve the machinability of steel, lead free-cutting steel in which lead (hereinafter also referred to as “Pb”) is added to the steel has been used. However, in recent years, the addition of Pb to free-cutting steel has become a problem from the viewpoint of environmental protection, and the development of free-cutting steel that does not contain Pb has been promoted. These Pb-free free-cutting steels employ a method of increasing the sulfur content, but increasing the sulfur content increases internal cracking susceptibility and causes internal cracking during continuous casting. This adversely affects the mechanical properties and internal quality of the steel, and is therefore difficult to manufacture by continuous casting.

  This will be described in more detail below. As mentioned above, lead free-cutting steel with Pb added to the steel is highly effective in extending the tool life by reducing chip cutting performance and tool wear, so it aims to improve productivity and reduce production costs. Has been used extensively. However, Pb adversely affects the human body and the natural world. Therefore, not only large-scale exhaust equipment is required for the manufacturing process of steel containing Pb, but also when re-melting lead-containing steel as scrap. There is a problem that the contained vapor is released. For this reason, in recent years, due to the growing interest in environmental problems, there has been a strong demand for free-cutting steel having machinability equivalent to or higher than that of lead-containing free-cutting steel without containing Pb.

  In response to such a request, for example, in Patent Documents 1 to 3, the machinability is improved by increasing the content of sulfur (S) and increasing the amount of MnS in the steel as an alternative to lead free cutting steel. Sulfur-based free-cutting steel is disclosed. Further, the present applicant clarified that machinability is improved when MnS containing Ti sulfide and / or Ti carbon sulfide is contained in steel. Proposed as 4. This document proposes a range of components such as C, Mn, S, Ti, Si, P, Al, O, and N in steel, and the mass concentration of Ti and S as conditions for obtaining good machinability. As a condition for obtaining a ratio of less than 1 and good hot workability, a range in which the atomic ratio of Mn and S exceeds 1 is proposed.

  However, these materials are all proposed purely in terms of machinability and mechanical properties of the product, and should be improved with regard to quality problems when actually produced by continuous casting. There is.

  Conventionally, steel has been cast by the ingot method, but continuous casting is being promoted from the viewpoint of productivity and yield improvement. However, during continuous casting of steel, internal cracking may occur in the slab because it is subjected to distortion caused by bending and straightening of the slab and bulging and reduction that do not occur in ingot casting. This crack occurs when the amount of strain experienced during the solidification process of the slab during continuous casting exceeds the limit strain inherent to the steel type.

  As a continuous casting method for preventing the above internal crack, for example, Patent Document 5 discloses that the total amount of strain received during the temperature range between the tensile strength appearance temperature and the ductility appearance temperature is the limit of the steel type to be cast. A continuous casting method of steel that is cast under conditions that do not exceed strain is disclosed. Furthermore, in Patent Document 6, the total strain acting on the solidification interface of the slab during casting is an estimation formula in the unsolidified reduction continuous casting method in which the reduction is performed in the thickness direction while leaving the unsolidified molten steel inside the slab. An unsolidified reduction continuous casting method is disclosed in which the slab reduction amount is controlled so as to be equal to or less than the internal crack limit strain estimated by

  The fact that the risk of internal cracking increases as the S concentration in the steel increases is also disclosed in Patent Document 5, but the S content disclosed in the Examples of Patent Documents 5 and 6 is 0. There is no disclosure about a method for preventing internal cracking of a steel type that is 020 mass% or less and contains 0.2 mass% or more of S.

JP 2000-319753 A (Claims and [0011])

JP-A-7-305110 (Claims etc.) JP-A-9-31522 (Claims etc.) Japanese Patent Application No. 2002-26368 (Claims and [0018] to [0023]) Japanese Patent Laid-Open No. 3-174962 (claims, etc.) JP-A-8-281400 (Claims and [0009])

  An object of the present invention is to provide a method capable of continuously casting a steel type having improved machinability by increasing the sulfur content as an alternative to lead free-cutting steel without causing internal cracks and surface cracks. .

  In order to solve the above-mentioned problems, the present inventors have taken into consideration the above-mentioned conventional problems, and a composition that can prevent MnS formation and internal cracking that cause deterioration of mechanical properties of sulfur-based free-cutting steel. The conditions were examined and the following findings (A) to (E) were obtained.

  (A) The formation form of MnS can be classified by the concentration product values of [Mn%] and [S%] in steel. (A) When the concentration product value is 1.5 or more, a large number of coarse products MnS is formed. (B) When the equivalence is 0.9 or more and less than 1.5, a small number of coarse MnS is produced. (C) When the equivalence is less than 0.9, almost coarse MnS is produced. do not do.

  (B) Therefore, if the Mn content and the S content satisfy the relationship of the following formula (1) that defines the condition of (C) in (A) above, the primary crystal becomes a metal phase, and the formation of coarse MnS Can be suppressed.

[Mn%] × [S%] <0.9 (1)
(C) The concentration ratio of the solute component in the solidification process of steel is more remarkable in S than in Mn, and the concentration ratio with respect to the initial concentration of S is approximately the fifth power of the concentration ratio with respect to the initial concentration of Mn. Proportional.

(D) In the process of concentrating Mn and S components in continuous casting solidification, when the value of the concentration product of [Mn%] and [S%] reaches 0.9 or more, Mn is stoichiometrically changed to S. On the other hand, if it is excessive, there is no risk of internal cracks occurring in the slab.
(E) From the above (A) to (D), if the content of Mn and S before solidification, that is, the initial content satisfies the relationship of the following equation (2) in addition to the relationship of the above equation (1), the internal Continuous casting is possible without generating cracks.

[S%] <0.32 × [Mn%] ⁵ (2)
The present invention has been completed based on the above findings, and the gist thereof is the continuous casting method for low-carbon sulfur-based free-cutting steel shown in the following (1) to (3).

  (1) By mass%, C: 0.05 to 0.19%, Si: 1.0% or less, Mn: 0.4 to 2.0%, P: 0.001 to 0.2%, S: 0.2 to 0.69%, Pb: less than 0.01%, Al: 0.2% or less, O (oxygen): 0.001 to 0.02% and N: 0.001 to 0.02% Low-carbon sulfur-based free-cutting, characterized by comprising continuously casting molten steel that contains Fe and impurities in the balance, and the content ratio of Mn and S satisfies the relationship of the following formulas (1) and (2) Steel continuous casting method.

[Mn%] × [S%] <0.9 (1)
[S%] <0.32 × [Mn%] ⁵ (2)
Here, [Mn%] represents the Mn content (mass%), and [S%] represents the S content (mass%).

(2) The low-carbon sulfur-based free-cutting steel in (1) may further contain one or more component elements selected from one or more of the following groups (a) and (b). .
(A) Cu: 0.01-1.0%, Ni: 0.01-1.0%, Cr: 0.01-2.0%, Mo: 0.01-1.0%, V: 0 0.005-0.5% and Nb: 0.005-0.1%,
(B) Ti: 0.005 to 0.30%, Se: 0.001 to 0.01%, Te: 0.001 to 0.07%, Bi: 0.005 to 0.3%, Sn: 0 0.005-0.3%, Ca: 0.0001-0.01%, Mg: 0.0001-0.01%, and rare earth elements: 0.0005-0.01%.

  (3) The continuous casting method for low-carbon sulfur-based free-cutting steel according to (1) or (2), wherein the C content is 0.05 to 0.10% by mass.

  In the present invention, “Al” means acid-soluble Al in steel (hereinafter also referred to as “sol.Al”).

  Further, “O (oxygen)” means total oxygen in the steel (hereinafter also referred to as “TO”).

  According to the continuous casting method of the present invention, it becomes possible to perform continuous casting of free-cutting steel containing low carbon and high sulfur without causing internal cracks.

  As described above, various free-cutting steels that can obtain good cutting performance without adding Pb to the steel have been proposed. However, the high S content steel often used in these steel types has a problem that internal cracks occur during continuous casting. On the other hand, the present inventors have found that if the component design is performed in consideration of the segregation behavior during solidification, the occurrence of internal cracks can be suppressed or prevented, and continuous casting can be performed. The details will be described below.

1. Relationship between Mn and S Content Considering Concentration of Solute Components Accompanying Solidification FIG. 1 is a diagram schematically showing the concentration distribution of the solute in the solid phase and the liquid phase in the solidification process. At the solid-liquid interface, an equilibrium is established locally between the solid phase and the liquid phase, and the solute element is distributed according to the equilibrium distribution coefficient unique to the solute element. In the liquid phase, the diffusion rate of the solute element is sufficiently fast, so that the solute concentration is almost evenly distributed. In the solid phase, the solid-liquid solution follows the diffusion coefficient of the solute element in the solid phase and the concentration gradient of the solute. It diffuses from the interface toward the center of the solidified phase.

  As the solidification progresses, the solid-liquid interface advances to the right in the figure while maintaining the distribution ratio according to the equilibrium distribution coefficient at the solid-liquid interface, that is, maintaining the relationship represented by Cs = k × Cl. . Here, Cs represents the solute concentration on the solid phase side at the solid-liquid interface, k represents the equilibrium partition coefficient of the solute element, and Cl represents the solute concentration on the liquid phase side at the solid-liquid interface. For this reason, a solute component that cannot be completely dissolved in the solid phase at the solid-liquid interface part is discharged to the liquid phase side, and the solute concentration in the residual molten steel increases. On the other hand, segregation components diffuse in the solid phase after solidification. In this way, microsegregation of solute components occurs. That is, the segregation of each solute component is determined by the equilibrium partition coefficient at the solid-liquid solidification interface and the diffusion rate in the solid phase.

  FIG. 2 is a diagram showing an example of analyzing the concentration change of Mn and S in the residual molten steel in the solidification process in consideration of the micro segregation mechanism as described above. As shown in the figure, the concentration of Mn and S in the liquid phase increases as the solid fraction increases, that is, the solidification progresses, but the concentration of S is more significant than the concentration of Mn. This is because the equilibrium distribution coefficient of Mn at the solid-liquid interface is about 0.76, whereas the equilibrium distribution coefficient of S is very low, about 0.05. This is because the amount of solute components discharged to the liquid phase side is large. This analysis result well reproduces the phenomenon in which the S component is significantly concentrated in the micro-segregation part.

  On the other hand, a steel type containing Mn and S forms a eutectic structure of Fe and MnS. When the initial composition of Mn and S is higher than the eutectic point composition, it becomes a hypereutectic region, and the primary crystal upon solidification is not a metal phase but MnS. When MnS is formed as a primary crystal, MnS grows freely and coarse MnS is generated, so that good mechanical properties cannot be obtained. In addition, when such a structure is generated by continuous casting, it becomes brittle and not only deteriorates mechanical properties such as fatigue properties and toughness, but also, in particular, MnS is stretched in the subsequent rolling process, and between the metal phase. Peeling occurs, causing internal cracks and surface cracks.

  Therefore, the present inventors conducted solidification tests on molten steels having various Mn and S contents, and investigated the production status of MnS. FIG. 3 is a view showing a result of cutting a slab obtained by solidifying molten steel produced by variously changing the Mn and S content ratios and examining the generation state of MnS in the steel with an optical microscope. .

  As shown in the result of the figure, the generation state of coarse MnS of 30 μm or more could be classified into the following three stages according to the concentration product value of [Mn%] and [S%]. That is, when [Mn%] × [S%] ≧ 1.5, a large number of coarse MnS is generated, and when 0.9 ≦ [Mn%] × [S%] <1.5, a small number of coarse MnS is formed. It was found that when [Mn%] × [S%] <0.9, almost no coarse MnS was generated.

  The above results correspond to the fact that the eutectic line of Fe and MnS can be approximated by [Mn%] × [S%] = 1.5, which is not limited to [Mn%] × [S %] ≧ 1.5 indicates that MnS was solidified as the primary crystal in the initial composition region. In addition, when 0.9 ≦ [Mn%] × [S%] <1.5, coarse MnS crystallizes when the solute component is concentrated by macrosegregation such as center segregation accompanying solidification, and [Mn %] × [S%] <0.9 indicates that the formation of coarse MnS can be stably prevented. Thus, if the relationship of the following formula (1) was satisfied, the primary crystal became a metal phase, and it was found that the formation of coarse MnS could be suppressed, and the relationship of the following formula (1) was defined.

[Mn%] × [S%] <0.9 (1)
When it is used for applications with even more severe conditions and the required level for MnS size and billet quality is high, the value of [Mn%] x [S%] in equation (1) should be less than 0.8. Is preferred.

  In an actual continuous cast slab, the concentrated microsegregated molten steel accumulates near the center of the slab thickness by bulging of the slab between guide rolls and solidifies as it is. Since macrosegregation may occur, coarse MnS may be generated even when the initial composition is smaller than the concentration product represented by the above formula (1). The present inventors investigated the concentration state of the solute in the macro segregation part in the actual continuous cast slab using the high S content steel, and Mn is about 1.1 times the average composition in the central segregation part, It was found that S was also concentrated to about 1.6 times.

  When the boundary line of the primary crystal in the macro segregation part is calculated based on the above knowledge, 1.5 / (1.1 × 1.6) = 0.9 is obtained. That is, when the product of the Mn concentration and the S concentration, that is, the value of [Mn%] × [S%] is 0.9 or more, coarse MnS may be generated as a primary crystal in the macrosegregation part. Therefore, also from this point, it is judged that the definition of the above formula (1) as a condition for preventing the formation of coarse MnS in the macro-segregation part is appropriate. If coarse MnS as described above is generated even in the center segregation part, it becomes a factor that significantly lowers the material performance. In addition, these coarse MnS may appear on the surface of the steel slab in rolling or forging processes after continuous casting, which may deteriorate the surface properties.

  Next, the initial Mn concentration and the S concentration were variously changed, and the analysis of the residual molten steel concentration in the solidification process was performed in the same manner as in FIG.

  FIG. 4 is a diagram showing an example of transition of Mn and S concentrations in the residual molten steel in the solidification process. As shown in the figure, as the solidification progresses, Mn and S are concentrated, and the coordinate points ([Mn%], [S%]) represented by both concentrations move to the upper right in the figure. . Here, since S has a smaller equilibrium partition coefficient than Mn and becomes more conspicuous, the solute concentration ratio in the residual network as the solidification progresses, the value of [Mn%] / [S%] Gradually gets smaller.

  As described above, the eutectic line can be approximated by [Mn%] × [S%] = 1.5, and the eutectic structure is generated when the value of the concentration product is 1.5 or more. Since macrosegregation or the like is formed, coarse MnS may begin to be generated when [Mn%] × [S%] ≧ 0.9. Therefore, crystallization of MnS starts when the concentration of Mn and S crosses the curve of [Mn%] × [S%] = 0.9. If MnS starts to crystallize during the solidification process, further increase in solute concentration can be suppressed.

  Further, a straight line a in the figure shows a relationship in which the atomic ratio of Mn to S is 1: 1, that is, a relationship in which the stoichiometric ratio of Mn to S for generating MnS is 1 (hereinafter referred to as “straight line”). a) ". In the molten steel having the initial composition of A in the figure, since it intersects the curve of [Mn%] × [S%] = 0.9 in the region above the straight line, at the start of crystallization of MnS, S is stoichiometrically excessive. For this reason, the concentration of S proceeds even after the start of crystallization of the eutectic structure, and the solidification temperature of the metal phase further decreases. When the solidification temperature is lowered, the ductile appearance temperature is lowered accordingly, so that the temperature range in which the internal crack occurs is expanded and the internal crack is deteriorated.

  On the other hand, in the molten steel having the initial composition of B in the same figure, the atomic ratio of Mn and S intersects with the curve of [Mn%] × [S%] = 0.9 in the region below the 1: 1 straight line. Therefore, at the start of crystallization of MnS, the value of the Mn / S ratio is 1 or more, that is, Mn is stoichiometrically excessive, S is fixed with MnS crystallization, and S no longer concentrates. Excess Mn is present in the residual molten steel, but the decrease in solidification temperature due to the concentration of Mn is small, and internal cracks are not deteriorated. That is, in order to prevent internal cracks during continuous casting of high S content steel, it is sufficient that Mn is stoichiometrically excessive with respect to S at the start of crystallization of MnS.

  Thus, when the solute concentration increases, the solidification temperature corresponding to the component decreases and the ductility appearance temperature decreases, so the temperature range in which internal cracks occur is expanded. Since S has a small equilibrium partition coefficient and greatly reduces the solidification temperature, it increases the internal cracking sensitivity. If Mn is stoichiometrically excessive with respect to S at the start of crystallization of MnS, the decrease in solidification temperature due to Mn concentration is small, and internal cracks are not further deteriorated.

Changes in the solute concentration in the solidification process of molten steel having various initial Mn concentrations and S concentrations were analyzed, and Mn and S concentrations at the start of crystallization of MnS were obtained. FIG. 5 is a diagram showing the relationship between the initial Mn and S concentrations in molten steel and the Mn and S concentrations at the start of MnS crystallization. In the figure, [Mn%] ₀ and [S%] ₀ represent the initial concentrations of Mn and S, respectively, and the ratio [Mn%] / [Mn%] ₀ and the ratio [S%] / [S%] _0. Represents the ratio of Mn and S to the initial concentration, that is, the concentration ratio. As shown in the figure, the relationship between the concentration ratio of Mn and the concentration ratio of S is roughly arranged by a single curve, and the ratio [S%] / [S%] ₀ is approximately the ratio [Mn %] / [Mn%] It was found to have a relationship proportional to the fifth power of ₀ .

  Therefore, from the above results, when the value of [Mn%] × [S%] is 0.9 or more, that is, at the start of MnS generation, Mn becomes excessive and the initial Mn and S concentrations that can prevent internal cracking Sought the area.

  FIG. 6 is a diagram illustrating an appropriate initial concentration range of Mn and S that can prevent generation of internal cracks and generation of coarse MnS. When the initial concentrations of Mn and S are in the region under the quintic curve PQR in the same figure and the hatched area in the lower part of the equation (1), at the start of crystallization of MnS, Mn is more than S Stoichiometric excess, that is, the atomic ratio of Mn and S becomes 1 or more, and continuous casting is possible without problems of internal cracks. On the other hand, when the initial concentrations of Mn and S are present in the region above the quintic curve PQR, S becomes stoichiometrically excessive with respect to Mn at the start of crystallization of MnS. As cracks worsen, surface defects during rolling also worsen.

As described above, the concentration of Mn and S components in the solidification process is approximately represented by [S%] / [S%] ₀ = C × {[Mn%] / [Mn%] ₀ } ^5. A relationship is established. Here, C is a constant. The boundary line of this curve, that is, Mn excess at the beginning of MnS crystallization, is a curve PQ ₁ R ₁ passing through the intersection R ₁ of the curve of [Mn%] × [S%] = 0.9 and the straight line a. Therefore, the value of the constant C is determined, and the range in which continuous casting can be performed without causing internal cracks and rolling without generating surface flaws even during rolling is expressed by the following equation (3). .

[S%] <0.25 × [Mn%] ⁵ (3)
As a result of investigating various slabs produced in the actual process, the allowable range for obtaining good slabs is slightly wider than the range defined by the above formula (3), and [Mn%] and [S %] Is adjusted to the region below the curve PQR as described above, that is, within the range defined by the following equation (2), the inner slab is virtually free of internal cracks and surface defects during rolling. It was found that can be manufactured.

[S%] <0.32 × [Mn%] ⁵ (2)
As described above, the reason why the range in which a slab that does not actually generate internal cracks or surface defects during rolling can be produced is wider than the theoretically derived range is presumed as follows. . That is, an error is included in the approximate expression in which the ratio [S%] / [S%] ₀ is approximately proportional to the fifth power of the ratio [Mn%] / [Mn%] ₀ , and Mn In a slab having a high initial and solidification composition close to the relationship represented by [Mn%] × [S%] = 0.9, the solidification until the start of crystallization of MnS causing internal cracks It is presumed that the concentration of Mn and S components at that time is small.

Furthermore, in order to more reliably prevent the occurrence of internal cracks and surface flaws, it was similarly determined under the condition of passing the intersection R ₂ between the curve of [Mn%] × [S%] = 1.5 and the straight line a. It is preferable that continuous casting is performed in a region below the curve PQ ₂ R ₂ , that is, in a range represented by the following expression (4).

[S%] <0.08 × [Mn%] ⁵ (4)
The knowledge that if Mn is excessive at the start of MnS generation, there is no decrease in the temperature of the solidus line, and the internal cracking of the slab at the time of continuous casting can be reduced even in a steel type having a lower S concentration, In steel types with low sulfur concentration, the amount of decrease in solidus temperature is small and the amount of expansion of internal cracks is also small, so the effect is not great. The effect of preventing internal cracks by defining the relationship between the formulas (1) and (2) becomes remarkable in the steel type having an S content of 0.05% by mass or more. The object of the present invention is to provide a continuous casting technique of free-cutting steel containing no Pb. Therefore, in order to ensure machinability as will be described later, the S content is further increased to 0.2 mass. % Or more is necessary.

2. Reasons for Limiting Chemical Component Composition and Preferred Ranges Reasons for limiting the component composition range of steel defined in the present invention and preferred ranges will be described below. In addition, all component composition is displayed by the mass%.

C: 0.05 to 0.19%:
Generally, C is known as an element having a great influence on the strength of steel. There is a strong correlation between strength and machinability, which also has a large effect on machinability. That is, if the C content exceeds 0.19%, the strength of the steel increases and the machinability decreases. For this reason, it cannot be used for applications in which machinability is important. On the other hand, when the C content is less than 0.05%, the strength is lowered and stickiness is generated during cutting, so that the machinability is lowered and the chip disposability is also deteriorated. Therefore, the appropriate range of the C content is set to 0.05 to 0.19%.

  Further, a steel type containing more than 0.10% and not more than 0.18% of C begins to solidify with a δ phase as an initial crystal, and causes a peritectic reaction in the solidification process, and a subperitectic steel in which the δ phase remains at the end of solidification. Become. Hypoperitectic steel exhibits non-uniform solidification due to the generation of stress associated with the peritectic reaction. For this reason, problems such as depletion and vertical cracking may occur in a continuous cast slab. In order to prevent such defects, it is necessary to lower the casting speed as compared with other steel types, and productivity is hindered. In addition, when a high S content steel containing 0.2% or more of S specified in the present invention is continuously cast, internal stress may be deteriorated by the stress. For this reason, the preferable range of C content was made 0.05 to 0.10%.

Mn: 0.4 to 2.0%:
Mn is an element that greatly affects the strength of the steel material. However, in free-cutting steel, sulfide is formed together with S to greatly affect the machinability. If the content is less than 0.4%, the absolute amount of sulfide is small and satisfactory machinability cannot be obtained. Further, when the content rate exceeds 2.0%, MnS is generated as the primary crystal as described above, so the S content rate must be relatively reduced, and the total amount of sulfides is reduced. For this reason, the machinability improving effect is saturated, or rather the machinability is deteriorated. On the other hand, if the content exceeds 2.0%, the strength increases, so that the machinability decreases. Therefore, the upper limit of the Mn content is set to 2.0%.

  Furthermore, from the viewpoint of material properties such as machinability and hot workability, the atomic ratio of Mn to S, Mn / S, needs to be 1 or more. This condition is a condition that is satisfied as a result if the Mn and S contents satisfy the relations of the expressions (1) and (2). In addition, the preferable range of Mn content for obtaining said effect more reliably is 0.6 to 1.8%.

S: 0.2 to 0.69%:
S is an element necessary for improving the machinability by forming a sulfide with an element such as Ti depending on Mn or steel type. If the content is less than 0.2%, a sufficient amount of sulfide is not generated, and the machinability improving effect cannot be obtained. Therefore, the content is set to 0.2% or more. Generally, when the S content exceeds 0.35%, hot workability deteriorates. Further, if the S content is increased, the internal cracking sensitivity is increased, and cracks are induced not only during continuous casting but also during rolling or forging, which are subsequent processes. However, if the content range defined in the present invention is maintained, free-cutting steel can be produced by a continuous casting process without causing such problems.

  In the present invention, the upper limit of the S content is set to 0.69% from the relationship defined by the formulas (1) and (2). In order to obtain a more reliable and stable crack prevention effect, the upper limit of the S content is preferably 0.55%.

Si: 1.0% or less:
Si is one of the effective elements used as a deoxidizing element in order to reduce the oxygen concentration in steel in the steel manufacturing process. Continuous casting in a state where the molten steel has not been sufficiently deoxidized not only generates bubbles in the steel, resulting in product defects, but also induces breakout in some cases, making operation impossible. There's a problem. However, when the content exceeds 1.0%, there is a problem that the hot workability of the steel is lowered and the cutting resistance is increased. Therefore, the upper limit of the content rate is defined as 1.0%. In order to further improve the machinability, the content is preferably less than 0.1%. In addition, if appropriate deoxidation is performed by deoxidation elements other than Si, such as Al and Ti, even if the Si content is a value of an impurity level of 0.01% or less, the performance of free-cutting steel and There is no problem in the manufacturing process.

P: 0.001 to 0.2%:
P is one of impurity elements in steel, but is also an element having an effect of improving machinability. Therefore, the lower limit of the content rate is set to 0.001%. Preferably it contains 0.01% or more. On the other hand, since P has a small distribution coefficient at the solidification interface, it promotes segregation, deteriorates internal cracks, and deteriorates hot workability. Therefore, the upper limit of the P content is set to 0.2%. In order to reliably prevent internal cracking due to P segregation, the upper limit of the content is preferably 0.1%.

Al: 0.2% or less:
Al is one of the effective elements used as a deoxidizing element in order to reduce the oxygen concentration in steel. In the present invention, Al is sol. It means Al, and its content necessary for deoxidation is 0.003% or more. However, when other deoxidizing agents such as Si described above are used, the content is less than 0.003%. There is no problem. On the other hand, since the alumina produced by deoxidation reduces machinability, the Al content is set to 0.2% or less. In order not to lower the machinability, the content is preferably 0.1% or less.

O (oxygen): 0.001 to 0.02%:
In the present invention, the O (oxygen) content means the total oxygen (TO) content. As described above, when the deoxidation in the steel is insufficient and the O content is high, Manufacturing trouble occurs. For this reason, the upper limit of the O content is set to 0.02%. However, the O content has a correlation with the surface roughness of the steel material, which is a factor of the cutting performance, and the surface roughness increases when the O content is low. Therefore, it is preferable that the O content is 0.01% or more for applications in which importance is attached to the small surface roughness of the steel material.

  On the other hand, when the O content exceeds 0.01%, it reacts with C in the steel to generate CO gas. For example, it reacts with other alloy components to lower the content, and it also enters the inside of the mold. Bubbles may be generated to deteriorate the quality of the slab, or the bubbles may cause breakout. Therefore, in order to continuously cast a steel type in which the O content exceeds 0.01%, other component adjustments such as C may be necessary. The oxygen content is usually determined by the selection of deoxidation element and deoxidation conditions, but oxygen present as a solid solution or oxide in MnS produced in steel is the extension of MnS during hot working of steel. In order to obtain this effect, O needs to be contained in an amount of 0.001% or more.

N: 0.001 to 0.02%:
N is an element that inevitably penetrates into steel when it is melted in an air atmosphere such as an electric furnace or converter, but in steel it forms a nitride by combining with Al, Ti, etc. Since nitride has an effect of refining crystal grains as pinned particles in the process of hot working, it has a favorable influence on the mechanical properties of the steel material. For this reason, N needs to contain 0.001% or more. On the other hand, if a large amount of these nitrides are produced in steel, not only the machinability is deteriorated but also the surface crack of the slab is caused, so the upper limit of the content is set to 0.02%. A preferable range of the content is 0.002 to 0.015%.

  The invention according to claims 2 to 4 of the present invention is a low carbon containing one or more elements selected from one or more of the following groups (a) and (b) in addition to the above components: This is a continuous casting method of sulfur-based free-cutting steel.

  That is, (a) group element consists of Cu, Ni, Cr, Mo, V, and Nb, and is an element that improves the mechanical properties of steel. The (b) group element is composed of Ti, Se, Te, Bi, Sn, Ca, Mg, and rare earth elements, and is an element that further improves the machinability of steel.

Cu: 0.01 to 1.0%:
Cu is an element that improves the hardenability of steel, and in order to obtain the effect, it is preferable to contain 0.01% or more. On the other hand, when the content exceeds 1.0%, the hot workability and machinability of the steel material are degraded. Therefore, when Cu is contained, the range of the content is set to 0.01 to 1.0%. In addition, since it is an element that induces surface cracking called “star cracking” during continuous casting, when Cu is contained in an amount of 0.03% or more, Ni with a content ratio of 1/3 or more is also contained. Is preferred.

Ni: 0.01 to 1.0%:
Ni is an element having an effect of improving the strength of steel by solid solution strengthening. It also has the effect of improving hardenability and toughness. In order to obtain these effects, the content is preferably 0.01% or more. On the other hand, if the content exceeds 1.0%, the effect is saturated and the machinability is lowered. Therefore, when Ni is contained, the content rate range is set to 0.01 to 1.0%.

Cr: 0.01 to 2.0%:
Cr is an element having an effect of improving the hardenability of steel. In order to acquire the effect, it is preferable to contain 0.01% or more. However, if the content exceeds 2.0%, the machinability is deteriorated. Therefore, when Cr is contained, the content range is set to 0.01 to 2.0%.

Mo: 0.01 to 1.0%:
Mo has the effect of refining the steel structure and improving toughness. In order to acquire the effect, it is preferable to contain 0.01% or more. However, even if the content exceeds 1.0%, the effect is saturated, and Mo is an expensive element, leading to an increase in cost. Therefore, when Mo is contained, the content rate range is set to 0.01 to 1.0%.

V: 0.005-0.5%, Nb: 0.005-0.1%:
V and Nb are elements having the effect of forming carbonitrides in steel and increasing the strength of the steel. In order to acquire the effect, it is preferable to contain 0.005% or more of each. However, if the V content exceeds 0.5% and the Nb content exceeds 0.1%, not only the above effects are saturated, but also carbides and nitrides are generated excessively, and machinability is reduced. Causes deterioration. Therefore, when these elements are contained, the range of the content is set to 0.005 to 0.5% for V and 0.005 to 0.1% for Nb.

Ti: 0.005 to 0.30%:
Ti forms Ti sulfide or Ti carbon sulfide together with C or S and is finely dispersed in the steel, thereby improving the machinability and hot workability of the steel. In order to acquire this effect, it is preferable to contain 0.005% or more of Ti. Furthermore, when the Ti content is increased, a eutectic structure of Ti sulfide or Ti carbon sulfide and a metal phase is formed in the steel, and the machinability of the steel is further improved. In order to acquire this effect, it is more preferable to make it contain 0.03% or more. However, if it is contained in a large amount exceeding 0.30%, carbides are formed and the machinability is deteriorated. For this reason, the range of the content rate in the case of containing Ti is set to 0.005 to 0.30%. A more preferable upper limit of the Ti content is 0.20%.

Se: 0.001 to 0.01%:
Se is an element that forms Mn (S, Se) together with Mn and is effective in improving machinability. Further, since the sulfide containing Se has an action of suppressing the elongation of sulfide during hot working, it has an action of reducing the anisotropy of mechanical properties of the steel material after hot working. In order to obtain these effects, it is preferable to contain 0.001% or more. However, if Se is contained in a large amount exceeding 0.01%, the effect is saturated, and the cost is high because it is an extremely expensive element. Then, when making it contain, the range of the content rate was made into 0.001-0.01%.

Te: 0.001 to 0.07%:
Te is an element that forms Mn (S, Te) together with Mn and is effective in improving machinability. Moreover, since the sulfide containing Te has the effect | action which suppresses the elongation of the sulfide at the time of hot working, it has the effect | action which reduces the anisotropy of the mechanical characteristic of the steel materials after hot working. Further, when Te is contained, an effect of remarkably improving the surface roughness which is one factor of the cutting performance can be obtained. In order to acquire these effects, it is preferable to contain 0.001% or more, and it is preferable to contain 0.03% or more especially in the case of the use which attaches importance to small surface roughness.

  However, since Te is an expensive element, if it is contained in a large amount, the cost becomes high, and if it is contained in a large amount exceeding 0.07%, the ductility at high temperature is lowered, and casting is performed during continuous casting. Cracks occur on one surface. When a crack occurs in the slab, not only the surface of the slab needs to be cleaned, but also in the case of a severe crack, the maintenance may be impossible or the casting may be impossible. Therefore, when Te is contained, the range of the content is set to 0.001 to 0.07%.

  Here, the effect of Te and oxygen content on the surface roughness of the steel material will be further described. FIG. 7 is a graph showing the effect of Te content on the relationship between the oxygen content and the surface roughness of the steel material. The Te content was changed in the range of 0.04 to 0.07%. In addition, the surface roughness is determined by manufacturing a round bar having a diameter of 60 mmφ by cutting and subjecting it to a cutting test, measuring the roughness of the finished surface with a contact-type roughness meter, and measuring the maximum roughness according to the method of JIS B 0601. (Rmax).

According to the result of FIG. 6, the maximum surface roughness (Rmax) decreases with an increase in the O content and an increase in the Te content, and the surface roughness is improved. In conventional Pb free cutting steel, it is possible to obtain a maximum roughness of about 4 to 6 μm. Therefore, in order to obtain a finished surface property equivalent to that, the O content is set to 0.01% or more. In addition, Te is 0.03-0.07%
It is preferable to contain in the range.

Bi: 0.005-0.3%, Sn: 0.005-0.3%
Bi and Sn are both low melting point metal inclusions, exhibit a lubricating effect when cutting steel, and improve machinability. The effect becomes remarkable when each content rate is 0.005% or more. On the other hand, during the continuous casting, these inclusions may become the starting point of the surface crack, which causes the surface quality to deteriorate. For this reason, the upper limit of the content rate of these elements was 0.3%, respectively.

Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%:
Ca and Mg are powerful deoxidizing elements, and generate a large number of fine oxides in molten steel, which becomes the core of MnS generation. MnS having these oxides as nuclei is restrained from being stretched during hot working. Moreover, Ca and Mg form sulfides and become nuclei for generating MnS. Thus, Ca and Mg have the effect of finely dispersing sulfides and controlling the form to improve machinability. To obtain this effect, 0.0001% or more of Ca and Mg is contained. More preferably, the content may be 0.0005% or more. On the other hand, even if these elements are contained in a large amount exceeding 0.01%, the effect is saturated, and at a high temperature where molten steel exists, the vapor pressure is high and the yield is also deteriorated. Therefore, when Ca and Mg are contained, the range of their content is set to 0.0001 to 0.01% for all.

Rare earth elements: 0.0005 to 0.01%:
Rare earth elements are a group of elements classified as lanthanoids. When these elements are contained, they are usually added by using an inexpensive misch metal containing these elements as main components. In the present invention, the rare earth element content is represented by the total content of one or more elements in the rare earth elements. Rare earth elements form sulfides or oxides with S and oxygen. Since the oxide is finely dispersed and becomes a nucleus of MnS formation, the stretching during hot working is suppressed, the sulfide is finely dispersed, and the form is controlled to improve the machinability. In order to acquire the effect, it is preferable to contain 0.0005% or more. However, if the content exceeds 0.01%, not only the above effects are saturated, but also a large amount of oxide is generated, which causes clogging of the immersion nozzle in the continuous casting process. Moreover, since the addition yield is low like Ca and Mg, it is disadvantageous in terms of cost to contain a large amount. Therefore, when the rare earth element is included, the range of the content is set to 0.0005 to 0.01%.

Pb: less than 0.01%:
In the present invention, the content of Pb is not particularly specified, but the object of the present invention is to provide a method for producing free-cutting steel that can obtain good cutting performance even if Pb is not contained. The method for producing free-cutting steel containing Pb is included in the method of the present invention. Considering that the Pb content contained in steel as an impurity due to contamination from scrap and the like is about 0.01% at most, the method of the present invention has a Pb content of less than 0.01%. The method for producing free-cutting steels is the target.

  When Ti and Cr are contained in the steel, Ti sulfide, Ti carbon sulfide, Cr sulfide and the like are generated in addition to MnS. However, in the component composition range defined by the method of the present invention, MnS begins to preferentially generate over those sulfides. Therefore, if Mn is excessive with respect to S at the start of MnS generation, internal cracks will occur. Can be prevented, and a good slab can be obtained under the conditions specified by the method of the present invention. Further, as described above, Ca and Mg are elements that control the form of sulfide, and form sulfide in preference to MnS. As described above, the content is at most 0.01%. Furthermore, since the oxide is also generated at the same time, the amount of sulfur fixed as CaS or MgS is a small part of the total amount of sulfur. Therefore, the method of the present invention can also be applied to steel types containing Ca and Mg.

  In order to confirm the effects of the present invention, test steels having the composition shown in Table 1 were melted and subjected to a continuous casting test. The resulting slab sample was used to investigate the occurrence of internal cracks, and the material performance such as machinability was also investigated, and the test results were evaluated.

  For the casting test, a 5-point bend with a slab cross section of width 400mm x thickness 300mm was used, and a 1-point straight vertical bending type bloom continuous casting machine was used. Casting speed was in the range of 0.55 to 0.75m / min. A test was conducted.

(Internal crack length)
Samples of the cross section and vertical section of the slab were collected, subjected to sulfur printing, and the maximum internal crack occurrence length was measured visually.

  If the length of the internal crack is less than 10 mm, the material properties and surface properties after rolling will not be adversely affected, and no echo that poses a problem in ultrasonic inspection of the product will be detected. However, if the crack length exceeds 10 mm, it may cause surface flaws of the product or adversely affect material performance such as fatigue strength and machinability depending on the application and processing process. When the length of the internal crack exceeds 20 mm, the yield extends significantly to the surface during rolling, and the possibility of causing surface defects of the product is remarkably increased.

(Survey of surface properties)
The slab obtained by continuous casting was subjected to block rolling to obtain a square billet having a cross-sectional shape of 160 mm × 160 mm. The surface of this square billet was visually inspected to evaluate the surface quality. If surface flaws exist, they must be removed, so they were maintained with a grinder. The surface quality was evaluated as follows. In other words, the case where no maintenance is required is “good”, the case where partial care is required is “light care”, the case where full care is required is “heavy care”, and the wrinkles are deep, A case where a billet that can be rolled in the next process could not be secured even though it was maintained was classified as “uncleanable” and divided into four stages.

(Ultrasonic inspection)
The billet that had been rolled in pieces was punch-rolled and processed into round bar steel pieces having a diameter of 65 mm. About this round bar steel piece, the ultrasonic inspection was performed on the following conditions, the presence or absence of an internal defect was determined, the defect rate was calculated | required, and the internal defect was evaluated. That is, the frequency was set to 5 MHz, and the flaw detection sensitivity was set to V5 = 100% + 6 dB using a G-type standard test piece defined in JIS. Measurement was performed under these conditions. A case where the echo height was 50% or higher was regarded as defective, a case where the defect rate with respect to the total number of inspections was less than 5% was determined as “good”, and a case where the echo height was 5% or higher was determined as “bad” .

(Investigation of machinability and measurement of surface roughness)
The round bar obtained as described above was externally cut to 60 mmφ and then subjected to a cutting test. The machinability test was performed using a JIS P type cemented carbide tool not subjected to TiN coating treatment. Cutting is dry (no lubricating oil), and the conditions are cutting speed: 150 m / min, feed: 0.10 mm / rev, cutting: 2.0 mm. After cutting for 30 minutes under these conditions, the cutting tool Average flank wear (VB) was measured. Furthermore, the maximum roughness of the finished surface was measured using a contact-type roughness meter.

(Evaluation of hot workability)
Evaluation of hot workability was performed as follows. That is, a high-temperature tensile test piece having a diameter of 10 mm and a length of 130 mm was taken from a position 120 mm thick from the skin of the slab obtained by the continuous casting test. This was fixed at a fixing interval of 110 mm, heated to 1100 ° C. by direct energization, held for 20 seconds, and then subjected to a tensile test at a strain rate of 10 ⁻³ / sec. The hot workability was evaluated by measuring the squeezing of the rupture portion of the test piece after rupture.

  Table 2 shows the test results for the items described above.

  Test numbers 1 to 4 using steel numbers A to D, test number 8 using steel numbers H and test numbers 10 to 17 using steel numbers J to Q are examples of the present invention, and steel numbers E to G Test Nos. 5-7 using No. and Test No. 9 using Steel No. I are comparative examples. Test numbers 1 to 7 using steel numbers A to G, test number 12 using steel number L, and test number 13 using steel number M are optional added elements such as Cr, Nb, Ti, Ca, or Te. The test numbers 8 to 11 using steel numbers H to K and the test numbers 14 to 17 using steel numbers N to Q are any of the above optional additive elements. It is a test in a component system containing one or more kinds.

  Furthermore, the test numbers 12 to 17 using the steel numbers L to Q increase the O (oxygen) content, or include Te, or increase the O content and include Te. In comparison with Test Nos. 1 to 11 that do not contain Te and do not increase the O content, the evaluation was performed with a particular focus on the roughness of the finished surface after the cutting test.

  Among the tests using component steels that do not contain the optional additive elements, the test numbers 1 to 4 that are examples of the present invention all have a maximum internal crack length of less than 10 mm. There was no problem in either quality, and the result was good. However, in test number 3 using steel number C with a C content of 0.12%, depletion and corner cracks associated therewith occurred on the slab surface. Further, normally, the largest internal crack occurs near the center of the slab due to bulging and bending stress of the slab, whereas in test number 3, the slab corresponding to the depletion is inside. An internal crack having a length of 5 mm occurred. This is presumed to be due to stress generated due to non-uniform solidification accompanying the peritectic reaction.

  In test number 5 using steel number E in which the concentration product of Mn content and S content exceeds 0.9 and does not satisfy the relationship of the formula (1), the internal crack remains within the allowable range of less than 10 mm. However, coarse MnS of 100 μm or more was generated. When such coarse MnS was generated, they became the starting point of cracks in the steel slab during rolling, and the defect rate of ultrasonic inspection deteriorated. Moreover, since the number of fine MnS produced decreases with the production of coarse MnS, this has an adverse effect on the machining performance, and the amount of tool wear increases. On the other hand, in test number 4 using steel number D in which the concentration product of Mn content and S content is close to 0.9, an internal crack of 6 mm occurs, and the amount of tool wear tends to increase. Good quality was obtained.

  Moreover, in the test numbers 6 and 7 using the steel numbers F and G, which have a high S content relative to the Mn content and do not satisfy the relationship of the formula (2), long internal cracks are generated, respectively. The drawing value at the fracture in the high-temperature tensile test was low, surface flaws occurred during rolling, and the defect rate of ultrasonic inspection was extremely high, resulting in inferior results.

  The amount of tool wear is in the range of 72 to 84 μm, except for test number 5 using steel number E, in the component system that does not contain the optional additive element. Is obtained. In test number 5 using steel number E in which coarse MnS was generated, machinability deteriorated and the amount of tool wear increased.

  Also in the test using the component steel containing the optional additive element, the test numbers 8, 10, and 11 using the steel numbers H, J, and K satisfying the relationship of the formulas (1) and (2). In either case, the maximum internal crack length was less than 10 mm, and there was no problem in the casting situation and the quality of the steel slab after rolling, which was a good result. Although it is a component system containing Ti as in Steel No. H, Test No. 9 using Steel No. I, which has a low Mn content and does not satisfy the relationship of the above formula (2), has a long internal crack inside the slab. As a result, surface defects occurred during rolling, and the defect rate of ultrasonic inspection also increased.

  In addition, when Ti is contained, the amount of tool wear is remarkably reduced as compared with the case where steel numbers A to G, which are component systems not containing the optional additive element, are used, and machinability is reduced. It is understood that it is effective to contain Ti when emphasizing the above. The amount of tool wear also decreases when other elements that improve machinability such as Ca are contained.

  Furthermore, in test numbers 12 and 13 using steel numbers L and M having an increased O content, and test numbers 14 to 17 using steel numbers N to Q containing Te, the Te content increased. At the same time, the surface wrinkles of the billet were slightly increased. However, the surface roughness of the finished surface was good as compared with the test numbers 1 to 11 using the steel numbers A to K. In particular, in the test numbers 16 and 17 using steel numbers P and Q in which the O content is 0.008% or more and the Te content is 0.03 to 0.07%, the conventional Pb free cutting steel is used. A very good surface texture was obtained. However, in the test number 16 using the steel number P having a high Te content, the fracture portion drawing value was lowered, the billet surface properties were slightly deteriorated, and the defect rate of the ultrasonic inspection was also slightly deteriorated.

  In addition, in test numbers 14 to 17 using steel numbers N to Q containing Te, steel numbers A to G are set in the same manner as test numbers 8 to 10 using steel numbers H to J containing Ti. Compared with the test numbers 1 to 7 used, the amount of wear of the tool is remarkably reduced, and it can be seen that inclusion of Te is also an effective means when emphasizing machinability.

  In test numbers 6, 7, and 9 using steel numbers F, G, and I that do not satisfy the relationship of the formula (2), the squeezing value for fracture at which the hot workability is evaluated is significantly reduced, and is less than 60%. It has become. In these tests, long internal cracks have occurred, and it is assumed that liquid film embrittlement has occurred along with the occurrence of internal cracks due to the microsegregation behavior during solidification.

  According to the continuous casting method of the present invention, by taking into account the concentration of segregation components such as Mn and S in the solidification process, the formation of coarse MnS is prevented by optimizing the composition of the steel components, and internal cracks are prevented. It is possible to perform continuous casting of a low-carbon, high-sulfur-containing free-cutting steel without generating. Therefore, the method of the present invention can be widely used as a casting method for improving the productivity and quality of sulfur-based free-cutting steel, which has been difficult to continuously cast, and greatly contributes to the development of this technical field.

It is a figure which shows typically the concentration distribution of the solute in the solid phase in a solidification process, and a liquid phase. It is a figure which shows the concentration of Mn and S in the residual molten steel in the solidification process. It is a figure which shows the production | generation condition of MnS in the slab obtained by the test which solidified the molten steel which has various Mn and S content rate. It is a figure which shows an example of the density | concentration transition of Mn and S in the residual molten steel in the solidification process. It is a figure which shows the relationship between the initial Mn and S density | concentration and the Mn and S density | concentration at the time of the MnS crystallization start. It is a figure which shows the appropriate initial concentration range of Mn and S which can prevent generation | occurrence | production of an internal crack and the production | generation of coarse MnS. It is a figure which shows the effect of Te containing on the relationship between oxygen content rate and surface roughness.

Claims (5)

In mass%, C: 0.05 to 0.19%, Si: 1.0% or less, Mn: 0.4 to 2.0%, P: 0.001 to 0.2%, S: 0.2 -0.69%, Pb: less than 0.01%, Al: 0.2% or less, O (oxygen): 0.001-0.02% and N: 0.001-0.02%, A continuous low-carbon sulfur free-cutting steel characterized by continuously casting a molten steel comprising the balance Fe and impurities and having a Mn and S content satisfying the relationship of the following formulas (1) and (2): Casting method.
[Mn%] × [S%] <0.9 (1)
[S%] <0.32 × [Mn%] ⁵ (2)
Here, [Mn%] represents the Mn content (mass%), and [S%] represents the S content (mass%). In mass%, C: 0.05 to 0.19%, Si: 1.0% or less, Mn: 0.4 to 2.0%, P: 0.001 to 0.2%, S: 0.2 ~ 0.69%, Pb: less than 0.01%, Al: 0.2% or less, O (oxygen): 0.001-0.02% and N: 0.001-0.02%, and Cu: 0.01-1.0%, Ni: 0.01-1.0%, Cr: 0.01-2.0%, Mo: 0.01-1.0%, V: 0.005-0. 5% and Nb: containing one or more selected from the group consisting of 0.005 to 0.1%, the balance is made of Fe and impurities, and the contents of Mn and S are expressed by the following formula (1) And a continuous casting method for low-carbon sulfur-based free-cutting steel, characterized by continuously casting molten steel that satisfies the relationship of formula (2).
[Mn%] × [S%] <0.9 (1)
[S%] <0.32 × [Mn%] ⁵ (2)
Here, [Mn%] represents the Mn content (mass%), and [S%] represents the S content (mass%). In mass%, C: 0.05 to 0.19%, Si: 1.0% or less, Mn: 0.4 to 2.0%, P: 0.001 to 0.2%, S: 0.2 -0.69%, Pb: less than 0.01%, Al: 0.2% or less, O (oxygen): 0.001-0.02% and N: 0.001-0.02%, and Ti: 0.005-0.30%, Se: 0.001-0.01%, Te: 0.001-0.07%, Bi: 0.005-0.3%, Sn: 0.005-0. Contains one or more selected from the group consisting of 3%, Ca: 0.0001-0.01%, Mg: 0.0001-0.01%, and rare earth elements: 0.0005-0.01% And continuously casting molten steel in which the balance is Fe and impurities, and the content ratio of Mn and S satisfies the relationship of the following formulas (1) and (2): Continuous casting method of low carbon sulfur-free-cutting steel according to symptoms.
[Mn%] × [S%] <0.9 (1)
[S%] <0.32 × [Mn%] ⁵ (2)
Here, [Mn%] represents the Mn content (mass%), and [S%] represents the S content (mass%). In mass%, C: 0.05 to 0.19%, Si: 1.0% or less, Mn: 0.4 to 2.0%, P: 0.001 to 0.2%, S: 0.2 ~ 0.69%, Pb: less than 0.01%, Al: 0.2% or less, O (oxygen): 0.001-0.02% and N: 0.001-0.02%, and Cu: 0.01-1.0%, Ni: 0.01-1.0%, Cr: 0.01-2.0%, Mo: 0.01-1.0%, V: 0.005-0. 1% or more selected from the group consisting of 5% and Nb: 0.005 to 0.1%, Ti: 0.005 to 0.30%, Se: 0.001 to 0.01%, Te : 0.001 to 0.07%, Bi: 0.005 to 0.3%, Sn: 0.005 to 0.3%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0 .01% and rare Analogous elements: containing one or more selected from the group consisting of 0.0005 to 0.01%, the balance consisting of Fe and impurities, and the contents of Mn and S having the following formula (1) and ( 2) A continuous casting method of low-carbon sulfur-based free-cutting steel, characterized by continuously casting molten steel that satisfies the relationship of formula (2).
[Mn%] × [S%] <0.9 (1)
[S%] <0.32 × [Mn%] ⁵ (2)
Here, [Mn%] represents the Mn content (mass%), and [S%] represents the S content (mass%). C content rate is 0.05-0.10 mass%, The continuous casting method of the low carbon sulfur type free cutting steel in any one of Claims 1-4 characterized by the above-mentioned.

Images