JPH11217650A - Free-drilling steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a free-drilling steel capable of facilitating the discharge of chips at the time of deep hole working of a steel material for use in frinctional parts, producing excellent fatigue strength, preventing the stoppage of a production line, and attaining improvement in productivity. SOLUTION: The steel has a composition consisting of, by weight ratio, 0.05-0.50% C, 0.03-1.00% Si, 0.50-2.00% Mn, 0.01-0.11% S, 0.005-0.040% Al, either or both of 0.05-0.30% Pb and 0.01-0.30% Bi, and the balance Fe with impurities. This steel has spherical or rod-like inclusions in its matrix. The spherical inclusions have oxides as nuclei and the rod-like inclusions have sulfides as nuclei, and at least a part of the outside peripheral part of each nucleus has a layer containing either or both of Pb and Bi. Further, the size of these inclusions are regulated to 9 to 50 μm in width and 25 to 50 μm in length, and the number of the inclusions is regulated to >=8 pieces per square millimeter at an arbitrary cross section parallel to rolling direction, in a position at a depth one-fourth the diameter of the steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to steel which facilitates the discharge of chips during deep hole drilling during machining in a steel material used as a functional component.
In addition, it provides steel material that facilitates chip discharge such as napping, reaming, boring, etc., and prevents the production line from stopping due to sudden chipping. This is a free-perforated steel that prevents winding around a tool or a work material, reduces wear of a tool to be used, and improves productivity by reducing tool cost.

[0002]

2. Description of the Related Art Steels to which low-melting elements such as Pb, Bi, Te, ln, and Se are added (for example, JP-A-56-38453, JP-A-60-152653,
Japanese Patent Application Laid-Open Nos. 1-165749, 1-168849, and 9-13119). The purpose of these series of steels is to prevent chips from being cut off during cutting and prevent the chips from wrapping around the tool or work material, thereby preventing a sudden stop of the production line. Further, since the deformation in the shearing region in the work material is facilitated, the load at the time of generation of the chips is reduced, and the life of the cutting tool is extended. On the other hand, there is a steel, such as Ca deoxidized steel, which generates a sulfide or a low melting point composite oxide on a tool rake face to extend tool life.

[0003] However, if the chips are divided in an unstable manner, for example, finely divided chips may accumulate in the grooves of the drill during deep hole drilling, causing breakage of the drill. Low melting point
When Pb, Bi, Te, etc. are added, the shear area in the work material near the cutting edge is severely deformed, so chips are generated on the ridge side of the cutting edge, and tool rake surface wear occurs in a narrow area near the cutting edge Therefore, there is a case where the cutting edge is lost and the tool life is shortened. In order to prevent this phenomenon, there are steels that simultaneously add S, Ca, etc., accumulate sulfides or low-melting-point composite oxides on the rake face to extend tool life (
Public notice 7-56061).

[0004] In this case, however, the cutting of the chips is not sufficiently stable, and the drill may be broken during deep hole drilling. As described above, conventional steels are proposed to be mixed with those intended to extend tool life and those intended to cut chips, and it seems that no steel achieves both of them sufficiently.

[0005] Further, there has been proposed a method in which sulfides having an average particle diameter of 10 μm to 30 μm are contained in a steel in an amount of 0.01 to 0.09% (weight ratio) to obtain cutting chips and extend tool life (Japanese Patent Laid-Open No. 5-163550). In this steel, sulfides having a particle size exceeding 50 μm are present, and when fatigue strength is required as a functional component, inclusions with a size of 50 μm or more draw out a notch phenomenon when stress is concentrated, and the strength decreases. The adoption of steel is difficult.

[0006]

Generally, hardness is increased to improve fatigue strength. However, on the other hand, the life of the cutting tool is shortened due to the high hardness. Also, to get free cutting
When elements such as S, Ca, Pb, and Bi are added, they cause notches and reduce fatigue strength. As described above, the fatigue strength and the machinability are contradictory characteristics. The present invention provides a high-fatigue-strength perforated steel that has excellent performance in both of these conflicting fatigue strengths and machinability in response to these problems.

In the present invention, the behavior of inclusions in the shear region in the work material generated in the vicinity of the cutting edge is observed in detail. An object of the present invention is to prevent the production line from being stopped from being wound around a work material or from a breakage of a drill due to a chip pinch during deep hole drilling. In particular, it is an object of the present invention to smoothly discharge chips when deep drilling, for example, when the drilling depth is 10 times or more the drill diameter, and to prevent sudden breakage of the drill.
Further, by adding an alloy element for extending the tool life and analyzing the interaction between the above-mentioned chip behavior and the tool contact portion, the extension of the tool life can be achieved at the same time.

[0008]

According to a first aspect of the present invention, C: 0.05-0.
50% (weight ratio, the same applies hereinafter), Si: 0.03-1.00%, Mn: 0.50
~ 2.00%, S: 0.01 ~ 0.11%, Al: 0.005 ~ 0.040%, Pb: 0.05 ~ 0.30%, Bi: 0.01 ~ 0.30%, at least one of which contains Fe and impurities, The matrix has spherical or rod-shaped inclusions, the spherical inclusions having oxides as nuclei and the rod-shaped inclusions having sulfides as nuclei, and at least a part of the outer periphery of each nucleus is composed of one kind of Pb or Bi. It has a layer containing the above, and the size of these inclusions is 9-50 μm in width.
m, length 25-50 μm, the number of which is 1 /
1 mm ² per in any cross section parallel to the rolling direction at 4
It is a free-drilling steel characterized by the presence of eight or more pieces.

The second invention of the present invention is characterized in that C: 0.05 to 0.50% (weight ratio, the same applies hereinafter), Si: 0.03 to 1.00%, Mn: 0.50 to 2.00
%, S: 0.01-0.11%, Al: 0.005-0.040%, plus Pb:
0.05 to 0.30%, Bi: 0.01 to 0.30%
The balance consists of Fe and impurities, and has spherical or rod-shaped inclusions in this matrix, and the spherical inclusions are (Al, Si) 0xide
And the rod-like inclusions have MnS as nuclei, and at least a part of the outer periphery of each nucleus has a layer containing at least one of Pb and Bi, and the size of these inclusions is 9 to 50 μm in width.
m, length 25-50 μm, the number of which is 1 /
1 mm ² per in any cross section parallel to the rolling direction at 4
It is a free-drilling steel characterized by the presence of eight or more pieces.

In the third invention, C: 0.05 to 0.50% (weight ratio, the same applies hereinafter), Si: 0.03 to 1.00%, Mn: 0.50 to 2.00%, S: 0.
01-0.11%, Al: 0.005-0.040%, Ca: 0.001-0.020%, Pb: 0.05-0.30%, Bi: 0.01-0.30%, with the balance being Fe and impurities. Has spherical or rod-shaped inclusions in this matrix, and the spherical inclusions are (Al
, Si, Ca) 0xide, (Al, Si) 0xide, (Al, Ca) 0xide,
One or more of (Si, Ca) Oxide, (Mn, Ca) S, and CaS are nuclei, and the rod-like inclusions are nuclei of MnS, and Pb or Bi is at least partially outside and outside of each nucleus. A layer containing more than one species, and the size of these inclusions is 9-50 μm in width.
m, length 25-50 μm, the number of which is 1 /
1 mm ² per in any cross section parallel to the rolling direction at 4
It is a free-drilling steel characterized by the presence of eight or more pieces.

In a fourth aspect of the present invention, in the first to third aspects, Cr: 2
% Or less, Mo: 1% or less, Ni: 3% or less, V: 0.5% or less 1
It is a free-drilling steel characterized by containing one or more types.

According to a fifth aspect of the present invention, the solidification rate at the center of the steel ingot is 0.1
By measuring the temperature of the ingot case so that it becomes 02 to 1.3 mm / sec, the size and amount of the inclusions are controlled by controlling the amount of thermionic emission from the strength of the electromagnetic field by the electromagnetic field generating means installed on the upper part of the ingot and the outer wall. The method for producing free-perforated steel according to any one of claims 1 to 4, wherein the number is controlled.

That is, (Al, Si, Ca) 0xide, (Al, Si) 0
xide, (Al, Ca) 0xide, (Si, Ca) 0xide, (Mn, Ca) S,
One or more inclusions of CaS and MnS extend the tool life during cutting and exhibit a lubricating function between the tool and steel.

If low-melting Pb or Bi or both are present in the outer shell of these composite inclusions, these elements are dissolved by an increase in the cutting temperature, and a gap is formed at the boundary between the inclusion and the base metal. Notch phenomena that promote deformation of inclusions in a manner similar to the state in which cracks occur. As a result, the inclusions serve as nuclei that promote stable deformation during chip generation, and promote the cutting of chips.

In order to exhibit such a phenomenon, a sufficiently large number of inclusions are required. The reasons for limiting the size and quantity are described below. First, regarding the width of inclusions, inclusions with a width of 9 μm or less have a large shear deformation resistance in the shear zone.
Alternatively, even if Bi is melted, the above-described deformation of the inclusion is unlikely to occur. However, when the thickness is 50 μm or more, the upper limit is set to 50 μm because it becomes the starting point of fatigue cracks or facilitates the propagation thereof. 25μ for inclusion length
In the case of m or less, the cracks generated by the notch effect of the inclusions are hard to grow into cracks crossing the chips by being connected with the cracks generated from other inclusions, and as a result, the chips are not divided. However, when the thickness is 50 μm or more, the fatigue strength decreases, so the upper limit is set to 50 μm.

With respect to the number of inclusions, chip breaking is caused by the growth of large cracks up to chip breaking by connection of cracks originating from a plurality of inclusions, but even if the inclusions are large enough. However, if the number is small, crack connection as described above is unlikely to occur. Therefore, the above-mentioned inclusions having a sufficient size for cutting chips are required to have at least 8 inclusions per mm ² in any cross section in the rolling direction.
You need more than one. Therefore, in order to effectively generate the above phenomenon in a stable state, the size and number of inclusions are defined as described for the limiting reason, and the width of the inclusions is defined as a research result that resulted in the present invention. It is necessary that at least 8 of the steels having a diameter of 9 μm to 50 μm and a length of 25 μm to 50 μm exist in 1 mm ² in an arbitrary cross section parallel to the rolling direction at the diameter / 4 position in the steel.

That is, in order to achieve the above-mentioned stable chip breaking phenomenon and tool wear reduction phenomenon, the cutting temperature is set so as to facilitate the generation and deformation of the chip in the shear region of the work material near the cutting edge. Add an element that easily dissolves and has a low melting point, and an element that has a lubricating function between the tool and the work material.

Further, in order to make the inclusions of an appropriate size as described above exist in an appropriate amount in the steel, the molten steel in the melting furnace is blown into the molten steel from the bottom of the ladle by a means such as blowing an inert gas. The above-mentioned elements are added while stirring to control the solidification rate in continuous casting or ingot casting, or control of the amount of oxide-based inclusions that become the core of sulfide-based inclusions by controlling the amount of oxygen in molten steel, etc. Can be controlled by various means. For example, by measuring the temperature of the ingot case, a manufacturing method of controlling the size, quantity and number of inclusions by controlling the amount of thermionic emission from the strength of the electromagnetic field by the electromagnetic field generating means installed on the upper part and the outer wall of the ingot case The solidification speed at the center of the steel ingot by 0.02-1.3 mm / sec
It is possible to control so that

Next, by analyzing the deformation behavior of inclusions made of the composite compound containing each of the above-mentioned elements and the generation phenomenon of chips in the shear region in the work material near the cutting edge, a stable cutting phenomenon of the chips can be obtained. A method for reducing tool wear is obtained. At that time, observe the composition and shape of the inclusions, and the deformation of the inclusions during chip generation differs depending on the difference in composition and shape, and the purpose of the present invention is to optimize the cutting of chips and reduce tool wear. Obtain the composition and shape of the resulting inclusions.

Next, the reasons for limiting each element will be described.
First, C increases the mechanical strength of steel and is at least 0.
However, if the content is 0.5% or more, the effect is remarkable and the workability deteriorates, so the upper limit is set to 0.5%. Si is added to the molten steel as a deoxidizing agent and must be at least 0.03%, but if added in a large amount, the toughness is lowered and the machinability is also deteriorated, so the upper limit was made 1.00%. Mn has a solid solution hardening effect and is required to be 0.5% or more in order to secure hardness. However, when added in a large amount, Mn is excessively hardened and the workability is reduced, so the upper limit is 2.
00%.

S must be at least 0.01% in order to form Mn and MnS and improve machinability. However, excessive addition of S lowers the hot workability, so the upper limit was made 0.12%. Al needs at least 0.005% as a deoxidizing agent, but Al ₂ O ₃
The upper limit is 0.04 because machinability is deteriorated by the formation of
0%.

Pb is required to be at least 0.05% in order to improve the chip controllability and the perforation property, but the upper limit is set to 0.30% because excessive addition deteriorates the mechanical properties. Bi has an effect similar to that of Pb in a small amount, and at least 0.01% is required to exert its effect. However, an excessive addition deteriorates the mechanical properties, so the upper limit was set to 0.30%.

Ca changes oxide-based inclusions into inclusions with a relatively low melting point to reduce the harmfulness in machinability, and also improves the mechanical properties of steel by spheroidizing sulfides. It has the effect of forming a protective film on the surface of the cutting tool to prevent wear of the tool, but at least 0.001% is required to achieve the effect, but if it exceeds 0.020%, the effect is saturated, so the upper limit is 0.020%. %.

The Cr according to the fourth aspect of the present invention has the function of improving the strength, but if added excessively, the machinability deteriorates, so the upper limit was made 2%. Mo has the effect of improving the hardenability, but if it exceeds 1%, the cost increases for the effect, so the upper limit was made 1%. Ni has the effect of improving toughness, but if it exceeds 3%, the cost increases for the effect, so the upper limit was set to 3%. V has the effect of improving the strength by precipitating in the ferrite structure during cooling after forging, but the effect is saturated even if added by 0.5%, so the upper limit was made 0.5%.

In a shear zone in the work material near the cutting edge, a cutting temperature is generated due to the deformation of the work material, and Pb covering the surfaces of the sulfide-based inclusions and the composite inclusions of sulfide and oxide. , B
Equivalent to a state in which an element with a low melting temperature such as i or its alloy melts first, and a gap is created in the gap between the composite of the element with a high melting temperature, which is the core of inclusions, and the base metal of the work material Becomes For this reason, these voids cause a stress concentration effect, and increase the deformation of sulfides, oxides, or a composite thereof serving as nuclei of inclusions, and promote the deformation of the shearing region for obtaining the optimal cutting chips. In addition, deformation or crushing of these inclusions becomes a composite of alloying elements that cause a lubricating function when the tool rubs against the chip or the finished surface of the work material, thereby extending the tool life.

The composition of the inclusions causing the above-mentioned phenomenon is a compound having a lubricating function in the nucleus and an alloy element having a low melting point in the outer shell. The shape of the inclusions is preferably large to facilitate the shearing phenomenon in the shearing region, that is, to facilitate the generation of chips. However, the size of the inclusions is 50 μm to prevent the inclusions from drawing out the stress concentration notch effect during use as a functional component and reducing the fatigue strength.
Is the upper limit.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The features of the present invention will be described below by comparison with comparative steel. Table 1 shows the chemical composition of these test steels.

[0028]

[Table 1]

In Table 1, A to M steels indicate the steels of the present invention, in which H steel is the first invention steel, A to D steels are the second invention steels, and E to G steels are the third invention steels. , And I to M steels are comparative steels. In the casting of the above component steel, the complete solidification time of the central part of the ingot is 5 to 16 by selecting several types of ingot cases and heat insulating methods to control the distribution of inclusion sizes.
Changed to 0 minutes.

The cast ingot is rolled or forged at a forging pressure ratio of 10, and A to D steel, and H steel, I steel, L steel, and M steel are 850 ° C.
Air cooling after holding for 60 minutes, and also E steel, and F steel, J steel,
K steel was kept at 1200 ° C for 30 minutes and air-cooled, and G steel was kept at 925 ° C for 60 minutes and air-cooled. After that, while turning to φ40 for drilling test, diameter / 4
In the section parallel to the rolling or forging direction at the position
The distribution of the size of the inclusions was examined over 50 fields of view of 0 μm × 100 μm, and the distribution was converted to 1 mm ² .

Table 2 shows the complete solidification time at the center of the ingot of A to M steel and the arbitrary cross section parallel to the rolling direction at the position of diameter / 4 in the steel.
9 μm to 50 μm width and 25 μm to length at 1 mm ²
The number of relatively large inclusions of 50 μm and the drilling depth are shown.

[0032]

[Table 2]

As for the perforation depth, a method shown in FIG. 1 is employed in order to focus on the evaluation of chips, and a stable perforation depth is obtained. That is, when the chips accumulate in the grooves of the drill and exhibit a phenomenon of clogging, the torque rises more than the thrusts associated with the drill, clearly indicating the degree of chip discharge. Under the adopted drill test conditions, the drilling depth is 12 times as large as the drill diameter, and the drilling property of each work material is evaluated by adopting the test method for non-through holes.

The drill life test was performed using a SKH9 φ6 straight drill at a cutting speed of 18.8 m / min and a feed speed of 0.1 mm /
rev Under no lubrication conditions, the wear of the cutting edge margin was measured after drilling a 50 mm deep hole 300 times. Table 3 shows the results.
Shown in

[0035]

[Table 3]

In addition, steel A, steel C, steel E, steel F, steel J, steel L
The fatigue limit was investigated in steel. That is, A steel, C
For steel and L steel, they were quenched at 850 ° C and then tempered at 550 ° C, and for E steel, F steel, and J steel, as in the drill test, they were kept at 1200 ° C for 30 minutes and then air-cooled. A fatigue test was performed on the test piece. For the fatigue test method,
With smooth specimen parallel section 8 mm diameter, a limit fatigue values of 10 ⁷ times in rotating bending fatigue test Ono equation to organize results with divided by endurance ratio tensile strength and fatigue limit. Table 4 shows the results and the number of inclusions having a width or length exceeding 50 μm in an arbitrary cross section of 1 mm ² in the rolling or forging direction at the position of diameter / 4.

[0037]

[Table 4]

As shown in FIG. 2, the composition of the inclusions is such that the nuclei of the spherical composite inclusions are (Al, Ca) 0xide, (C
a, Mn) S, the outer surface of which is coated with Bi or Pb. In the rod-shaped MnS system, the nucleus is MnS, and some (Al, Ca) 0
It may contain xide, and its outline is covered with Bi or Pb.

As shown by the above results, in the steel of the present invention, the stable piercing depth is all 60 mm or more, and stable and excellent piercing properties are obtained. This is because in the shear zone in the work material near the cutting edge, Bi or Pb present in the outer shell of the inclusion is melted by the cutting temperature, and a gap is generated between the core of the inclusion and the metal of the work material, Further, when an external force is applied by a cutting tool, this gap shows a notch effect of stress concentration, and (Al, Ca) 0xide, (Ca, Mn) S or MnS in the nucleus of the inclusion.
This facilitates the deformation and crushing of the material and facilitates the deformation of the shear zone in the work material.

Looking at the comparative steels, despite the fact that the steel I was coated with sulfides or composite inclusions of sulfides and oxides, the inclusions were relatively small due to the short solidification time. The depth did not reach the steel of the present invention. In the J steel, the inclusions are coated with Pb and the piercing property is good, but since the solidification time is excessively long, the inclusions exceeding 50 μm are present, which significantly lowers the durability ratio. In K steel, since the solidification time is very short, there are few moderately large inclusions and the piercing property is poor. Pb or Bi for L steel
Is not added, and since the solidification time is remarkably long, the piercing property and the durability ratio are deteriorated. In the M steel, the inclusions were sufficiently large, but Pb or Bi was not added, so that the stable drilling depth was lower than that of the steel of the present invention. In drill wear, the solidification time is long, the inclusions are moderately large, and
Steels A and C containing Pb and Bi showed good results. This is not only due to the lubricating action of Pb and Bi itself,
This is a result that the inclusion having the above-described components has a lubricating function in a process of rubbing against a tool surface from deformation and crushing in a shear region.

[0041]

According to the present invention, if inclusions show a notch effect of sufficient stress concentration in a shearing region in a work material near a cutting edge, it is easy to generate chips when drilling a deep hole of 10 times or more the tool diameter. Become.
In addition, the tool life is extended because the tool contact surface exhibits a wide lubrication function in the process of rubbing against the chip or the finished surface due to the deformation and crushing of the inclusions. And high fatigue strength was also obtained. That is, according to the present invention, a steel material used for a functional component can easily discharge chips during deep hole drilling, has excellent tool life, and has excellent fatigue strength. It is a free-drilling steel that can improve the properties.

[Brief description of the drawings]

FIG. 1 shows a method for determining a stable drilling depth.

FIG. 2 is a diagram showing the composition of a typical inclusion in the free-perforated steel of the present invention. Table 1: Chemical composition of the free-perforated steel of the present invention Table 2: Solidification time of the free-perforated steel of the present invention, the number of inclusions of a specific size, drilling depth Table 3: Drill life of the free-perforated steel of the present invention Table 4: Present invention Durability ratio of free-piercing steel

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shoji Miyamoto 1 Aomachi Wanowari, Tokai City, Aichi Prefecture Inside Aichi Steel Co., Ltd. (72) Inventor Masao Uchiyama 1 Wanowari Araocho, Tokai City, Aichi Prefecture Aichi Steel Inside (72) Inventor Naoki Iwama 1 Wanowari, Arao-cho, Tokai-shi, Aichi Prefecture Inside Aichi Steel Corporation (72) Inventor Kazuhiko Hiraoka 3007 Nakashima-shi, 1 character, Shikarima-ku, Himeji-shi, Hyogo Sanyo Special Steel Co., Ltd. (72) Inventor Norimasa Join 3007 one character, Nakajima-shi, Shima, Himeji City, Hyogo Prefecture Inside Sanyo Special Steel Co., Ltd. (72) Inventor Kazutaka Okochi 41-Cho, Yukumichi, Yoji, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Laboratory (72) Inventor Kunio Naito 41-41 Yokomichi, Nagakute-machi, Aichi-gun, Aichi Prefecture (72) Inventor Motohide Mori 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation

Claims (5)

[Claims] (1) C: 0.05 to 0.50% (weight ratio, the same applies hereinafter);
Si: 0.03-1.00%, Mn: 0.50-2.00%, S: 0.01-0.11%
, Al: 0.005 to 0.040%, Pb: 0.05 to 0.30%, Bi:
The matrix contains one or more of 0.01 to 0.30%, with the balance being Fe and impurities, with spherical or rod-like inclusions in the matrix.Spherical inclusions have oxides as nuclei and rod-like inclusions have sulfides. The core has a layer containing at least one type of Pb or Bi at least at a part of the outer periphery of each nucleus. The size of these inclusions is 9 to 50 μm in width and 25 to 50 μm in length. there free drilling steel, characterized in that there are eight or more per 1 mm ² at any cross-section parallel to the rolling direction at a quarter of the diameter of the steel. 2. C: 0.05 to 0.50% (weight ratio, hereinafter the same),
Si: 0.03-1.00%, Mn: 0.50-2.00%, S: 0.01-0.11%
, Al: 0.005 to 0.040%, Pb: 0.05 to 0.30%, Bi:
0.01 to 0.30% or more, the balance consisting of Fe and impurities, with spherical or rod-shaped inclusions in the matrix, the spherical inclusions having (Al, Si) 0xide as the core and rod-like inclusions Has a layer containing MnS as a nucleus and at least a part of the outer periphery of each nucleus containing at least one kind of Pb or Bi,
The size of these inclusions is 9-50 μm in width and 25-50 μm in length
Wherein the number is at least 8 per 1 mm ² in an arbitrary cross section parallel to the rolling direction at 1/4 of the diameter in the steel. 3. C: 0.05 to 0.50% (weight ratio, the same applies hereinafter), S
i: 0.03 to 1.00%, Mn: 0.50 to 2.00%, S: 0.01 to 0.11%
, Al: 0.005 to 0.040%, Ca: 0.001 to 0.020%, and Pb:
0.05 to 0.30%, Bi: 0.01 to 0.30%, at least one of which contains Fe and impurities, has a spherical or rod-shaped inclusion in the matrix, and the spherical inclusion is (Al, Si,
Ca) 0xide, (Al, Si) 0xide, (Al, Ca) 0xide, (Si, C
a) Oxide, (Mn, Ca) S, CaS as one or more nuclei and rod-shaped inclusions having MnS as a nucleus, and at least a part of the outer periphery of each nucleus contains at least one of Pb or Bi The size of these inclusions is 9-50 μm in width and 25 in length.
A free-drilling steel, characterized in that the number is at least 8 per 1 mm ² in an arbitrary cross section parallel to the rolling direction at 1/4 of the diameter in the steel. 4. The method according to claim 1, wherein Cr: 2% or less, M
o: 1% or less, Ni: 3% or less, V: 0.5% or less
Free piercing steel characterized by containing at least one kind. 5. The solidification rate at the center of the ingot is 0.02 to 1.3 mm /
By measuring the temperature of the ingot case to be sec
The size, quantity and number of inclusions are controlled by controlling the amount of thermionic emission from the strength of the electromagnetic field generated by the electromagnetic field generating means installed on the upper part of the ingot case and the outer wall. 4. The method for producing free-piercing steel according to any one of (1) to (4).