TWI717990B - Free-cutting steel and its manufacturing method - Google Patents

Abstract

The present invention provides a free-cutting steel with Pb added in a substantially reduced amount from the previous addition and having machinability equal to or higher than that of low-carbon, sulfur-lead composite free-cutting steel. It contains C: 0.15% or less, Mn: 0.5% or more and 2.0% or less, S: 0.200% or more and 0.650% or less, O: more than 0.01% and less than 0.05%, Cr: 0.05% or more and 2.0% or less, Pb : 0.02% or more and less than 0.10% and N: 0.005% or more and 0.015% or less, and the A value defined by the following formula (1) satisfies 4.0 or more and 20.0 or less, and the remainder is composed of Fe and unavoidable impurities, Pb with an equivalent circle diameter of less than 1 μm: 1000 pieces/mm ² or more, with a circle equivalent diameter of 1 μm or more and 5 μm or less: 500 pieces/mm ² or more; and Pb with an equivalent circle diameter of 1 μm or less It is a tissue of 1000 pieces/mm ² or more. A value=(Mn+5Cr)/S···(1) Here, the element symbol in the formula represents the content (mass%) of this element.

Description

Free-cutting steel and its manufacturing method

The present invention relates to a free-cutting steel, in particular to a free-cutting steel containing sulfur and a trace amount of lead as machinability-enhancing elements instead of the previous free-cutting steel and a method for manufacturing the same.

Sulfur-lead composite free-cutting steel containing low-carbon sulfur and lead represented by Japanese Industrial Standards (JIS) 4804 SUM24L by adding large amounts of lead (Pb) and sulfur (S) as machinability improvement elements , Which ensures excellent machinability. Among them, lead is an extremely important and useful element for industrial products. That is, lead is reused as an element that greatly improves the machinability of materials, such as reduction of tool wear and improvement of chip handling properties in the cutting of steel materials.

However, with the increasing awareness of environmental protection in recent years, the trend of abolishing or restricting the use of environmentally hazardous substances is expanding worldwide. Lead (Pb) is also listed as one of them and is required to restrict its use.

For example, Patent Document 1 discloses a Pb non-additive type fast-cutting non-quenched and tempered steel. Similarly, Patent Document 2 also discloses a Pb non-additive free-cutting steel. Furthermore, Patent Document 3 discloses that by adding Cr, which is easier to form a compound with S than Mn, it changes into Mn-Cr-S-based inclusions, thereby improving machinability Free-cutting steel.

[Prior Art Literature]

[Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. 9-25539

[Patent Document 2] Japanese Patent Laid-Open No. 2000-160284

[Patent Document 3] Japanese Patent Publication No. 2-6824

In the technique described in Patent Document 1, since the target steel type is non-quenched and tempered steel containing C: 0.2% or more, it is hard, and furthermore, since the special element Nd is used, there is a problem of high manufacturing cost. In addition, the technique described in Patent Document 2 has low hot ductility due to the addition of a large amount of S, and cracks are likely to occur during continuous casting or hot rolling, which poses a problem in terms of surface properties. On the other hand, in the technique described in Patent Document 3, the addition of Mn is reduced and Cr and S are added. However, the addition of Cr is as high as 3.5% or more, which makes it difficult to reduce costs and produces a large amount of CrS. Therefore, there is a manufacturing problem that the material melting process of the steelmaking step is difficult. Moreover, it is clear that the steel materials of any document do not describe the sulfur-lead composite free-cutting steel for heavy cutting represented by JIS SUM24L (requiring cutting processing) in the comparative examples in the examples, but only the machinability For steel materials with relatively little necessity, the machinability of any of the invention steels is insufficient.

The present invention was completed to solve the above-mentioned problems, and its purpose is to provide A free-cutting steel with Pb added in a drastically reduced amount from the previous addition and having machinability equal to or higher than that of low-carbon, sulfur-lead composite free-cutting steel.

The inventors have conducted intensive research to solve the above-mentioned problems, and as a result, obtained the following findings.

(1) It was found that by adding appropriate amounts of Cr, Mn, and S and rationalizing the ratio of (Mn+5Cr)/S, the composition of an appropriate amount of sulfide becomes a composite system of Mn-Cr-S, and the composite system is composed of The sulfide is refined during hot working. With this fine composite sulfide, it is possible to produce free-cutting steel with excellent lubricity during cutting.

(2) The finer the sulfide, the greater the lubrication effect, and therefore the tool life and surface properties after cutting are significantly improved.

(3) On the other hand, with regard to chips during cutting, if only fine sulfides are used, the chips will continue and the handling properties will deteriorate. However, by dispersing fine sulfides while allowing a certain range of relatively large sulfides to exist, chip handling during cutting can also be significantly improved.

(4) If Pb is added in a certain range at the same time, it can be finely dispersed at the same time as the sulfide, so that the amount of Pb can be smaller than before and the machinability can be improved compared to the previous material.

The present invention is based on the above knowledge, and its gist is as follows.

1. A free-cutting steel containing C: 0.15% or less, Mn: 0.5% or more and 2.0% or less, S: 0.200% or more and 0.650% or less, O: more than 0.01% and less than 0.05% by mass , Cr: 0.05% or more and 2.00% or less, Pb: 0.02% or more and less than 0.10%, and N: 0.005% or more and 0.015% or less, and the A value defined by the following formula (1) satisfies 4.0 or more and 20.0 or less, the remainder Part is composed of Fe and unavoidable impurities, and has a sulfide with an equivalent circle diameter of less than 1μm at 1000 pieces/mm ² or more, and a sulfide with a circle equivalent diameter of 1μm or more and 5μm or less at 500 pieces/mm ² or more and Pb with a circle-equivalent diameter of 1μm or less is 1000 pcs/mm ² or more; A value=(Mn+5Cr)/S. . . (1)

Here, the element symbol in the formula represents the content (mass %) of this element.

2. The free-cutting steel according to the above 1, wherein the component composition is further in terms of mass% and contains any of Si: 0.10% or less, P: 0.01% or more and 0.15% or less, and Al: 0.010% or less the above.

3. The free-cutting steel according to 1 or 2, wherein the component composition is further calculated by mass% and contains Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.

4. A method for manufacturing free-cutting steel, which has C: 0.15% or less, Mn: 0.5% or more and 2.0% or less, S: 0.200% or more and 0.650% or less, O: more than 0.0100% and 0.0500% or less, Cr: 0.05% or more and 2.00% or less, Pb: 0.02% or more and less than 0.10%, and N: 0.005% or more and 0.015% or less, and the A value defined by the following formula (1) satisfies 4.0 or more and 20.0 or less, the remainder is composed of Fe and unavoidable impurities, and the cross section perpendicular to the length direction A rectangular cast slab with a length of 200mm or more on one side, this cast slab is hot-rolled with a reduction of area of 60% or more to make a billet, and the billet is heated at a temperature of 1050°C or more and a reduction of area of 65% or more It is made into steel bars by hot working.

5. The method for producing free-cutting steel according to the above 4, wherein the component composition further by mass% contains Si: 0.10% or less, P: 0.01% or more and 0.15% or less, and Al: 0.010% or less Any one of more than one.

6. The method for producing free-cutting steel according to 4 or 5, wherein the component composition further contains Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, and Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.

According to the present invention, a low-carbon free-cutting steel having excellent machinability even if the lead content is low can be obtained.

Next, the free-cutting steel of the present invention will be described in detail. First, the reason for limiting the content of each component in the component composition of the free-cutting steel will be explained in order. In addition, as long as there is no special description, the% representation of the components means mass %.

C: 0.15% or less

C is an important element that greatly affects the strength and machinability of steel. However, if its content exceeds 0.15%, it will harden, the strength will become too high, and the machinability will deteriorate. Therefore, the C content is set to 0.15% or less, preferably 0.10% or less. In addition, from the viewpoint of ensuring strength, it is preferable to set the C content to 0.02% or more, and further Above 0.04%.

Mn: 0.5% or more and 2.0% or less

Mn is a sulfide-forming element important for machinability. However, if the content is less than 0.5%, the amount of sulfide decreases and sufficient machinability cannot be obtained, so the lower limit is made 0.5%. On the other hand, if the content exceeds 2.0%, in addition to coarsening, the sulfide will also elongate and reduce the machinability. Furthermore, since the mechanical properties will decrease, the upper limit of the Mn content is set to 2.0%. More preferably, it is set to 0.6% or more and less than 1.8%.

S: 0.200% or more and 0.650% or less

S is an element that contributes to the formation of sulfides effective for machinability. If the S content is less than 0.200%, the amount of sulfide is small, so the effect of improving machinability is small. On the other hand, if the S content exceeds 0.650%, the sulfide becomes too coarse and the amount decreases, so the machinability decreases. In addition, hot workability and ductility, which are important mechanical properties, decrease. Therefore, the S content is set to the range of 0.200% or more and 0.650% or less. The S content is preferably 0.250% or more. Moreover, the S content is preferably 0.500% or less.

O: more than 0.01% and below 0.05%

In addition to forming oxides and becoming the precipitation nuclei of sulfides, O is also an effective element for suppressing the elongation of sulfides during hot working such as rolling, and this effect can improve machinability. However, if the content is 0.01% or less, the effect of suppressing the elongation of the sulfide is insufficient, and the sulfide after the elongation remains, and the original effect cannot be expected. On the other hand, even if it is added in excess of 0.05%, the elongation suppression effect of the sulfide is saturated, the amount of hard oxide-based inclusions increases, and addition of the excess amount becomes economically disadvantageous. Therefore, O is set to exceed 0.01% and 0.05% or less. Better O content It is above 0.012%. Moreover, the O content is preferably 0.030% or more.

Cr: 0.05% or more and 2.00% or less

Cr has the function of forming sulfides and improving the machinability by the lubricating effect during cutting. Moreover, since the elongation of sulfides during hot working such as rolling is suppressed, the machinability can be improved. If the Cr content is less than 0.05%, the formation of sulfide is insufficient, and the sulfide after elongation is likely to remain, so the original sufficient effect cannot be expected. On the other hand, if the addition exceeds 2.00%, in addition to hardening, the sulfide becomes coarser, the effect of suppressing the elongation of the sulfide is saturated, and the machinability is conversely reduced. Moreover, the addition of an excessive amount of alloy components is economically disadvantageous. Therefore, the Cr content is set to 0.05% or more and 2.00% or less. The Cr content is preferably set to 0.06% or more. Furthermore, the Cr content is preferably set to 1.80% or less.

Pb: 0.02% or more and less than 0.10%

When Pb is finely dispersed, it promotes the lubrication effect during cutting, and the machinability improvement effect is great. However, when 0.10% or more is added, Pb aggregates and coarsens, and the effect disappears. Furthermore, if it is less than 0.02%, even if it is finely dispersed, the amount of dispersion is too small, and the effect cannot be obtained.

N: 0.005% or more and 0.015% or less

N forms nitrides with Cr, etc., and decomposes the nitrides by temperature rise during cutting, thereby forming an oxide film called belag on the surface of the tool. Since the thin layer protects the surface of the tool, it contains 0.005% or more in terms of improving the life of the tool. On the other hand, if more than 0.015% is added, the effect of the thin layer is saturated and the material becomes hardened, so the tool life is shortened. Therefore, N The content of is 0.005% or more and 0.015% or less. The N content is preferably 0.006% or more. Furthermore, the N content is preferably 0.012% or less.

Contains the above components, and contains the remainder of Fe and unavoidable impurities. Or it further contains the optional containing components mentioned later. Here, it is preferable to contain the above-mentioned components, or any containing components mentioned later, the remainder of Fe and unavoidable impurities.

Here, it is important that in the above component composition, the A value defined by the following formula (1) satisfies 4.0 or more and 20.0 or less.

A value = (Mn+5Cr)/S. . . (1)

Here, the element symbol in the formula represents the content (mass %) of this element.

That is, the A value is an important index that influences the refinement of sulfides and the refinement of sulfides and Pb during hot working such as rolling, and by limiting this ratio, the machinability can be improved. If the A value is less than 4.0, sulfides of Mn-S alone will be generated, coarse sulfides will increase, and the machinability will deteriorate. Furthermore, the sulfide is also a precipitation nucleus of Pb, so if the sulfide becomes coarse, it is difficult for Pb to be finely dispersed. On the other hand, if the A value exceeds 20.0, in addition to the saturation of the effect of making the sulfide and Pb finer, the sulfide-forming element becomes too large for the sulfur, and the sulfide becomes coarse. Therefore, the A value is set to the range of 4.0 or more and 20.0 or less. In addition, the A value is preferably 4.5 or more. Furthermore, the A value is preferably 18.0 or less.

Next, the optional contained components will be described. In the present invention, in addition to the above In addition to the basic ingredients of, the following ingredients may also be included as needed.

Si: 0.10% or less, P: 0.01% or more and 0.15% or less, and Al: 0.010% or less.

Si: 0.10% or less

Si is an element used in deoxidation before refining. However, if too much is added, a large amount of hard oxides after deoxidation will be present, and the tool life will be deteriorated due to abrasive wear. Therefore, the content of Si is set to 0.10% or less. It is preferably set to 0.03% or less.

P: 0.01% or more and 0.15% or less

P is an effective element for reducing the roughness of the machined surface by suppressing the generation of the constituent tool nose during cutting. Therefore, it is preferable to contain 0.01% or more. On the other hand, if the content exceeds 0.10%, the hot workability and ductility decrease significantly while hardening. Therefore, the P content is set to 0.15% or less, preferably 0.10% or less.

Al: less than 0.010%

Al is a deoxidizing element like Si, and Al ₂ O _{3 is} produced. Since the oxide is hard, the life of the cutting tool is deteriorated due to so-called abrasive wear. Therefore, it is preferable to reduce the addition amount to 0.010% or less, preferably 0.005% or less.

Furthermore, the following components can be contained as needed.

Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: Any one or more of 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.

That is, Ca, Se, Te, Bi, Sn, Sb, B, Cu, Ni, Ti, V, Zr, and Mg are all preferably contained in the case where machinability is important. In the case of containing elements selected from these, from the viewpoint of exhibiting the effect of improving machinability, the respective contents are preferably Ca: 0.0001% or more, Se: 0.02% or more, and Te: 0.10% Above, Bi: 0.02% or more, Sn: 0.003% or more, Sb: 0.003% or more, B: 0.004% or more, Cu: 0.05% or more, Ni: 0.05% or more, Ti: 0.003% or more, V: 0.005% or more, Zr: 0.005% or more and Mg: 0.0005% or more.

On the other hand, if it is Ca: more than 0.0010%, Se: more than 0.30%, Te: more than 0.15%, Bi: more than 0.20%, Sn: more than 0.020%, Sb: more than 0.025%, B: more than 0.010%, Cu: More than 0.50%, Ni: more than 0.50%, Ti: more than 0.100%, V: more than 0.20%, Zr: more than 0.050%, and Mg: more than 0.0050%, the effect is saturated and it is economically disadvantageous.

Therefore, the content range of each element is set to Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less , Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.200% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.

(organization)

Sulfides with a circle equivalent diameter of less than 1μm are 1,000 pieces/mm ² or more, sulfides with a circle equivalent diameter of 1μm or more and 5μm or less are 500 pieces/mm ² or more, and Pb with a circle equivalent diameter of 1μm or less is 1,000 pieces/mm ² The above organization

Regarding the structure of free-cutting steel, the fine dispersion of sulfide or Pb is beneficial to promote the lubrication between the tool and the material to be cut during cutting. The more dispersed fine sulfides, the greater the lubricating effect, and the better the tool life and surface properties after cutting. Furthermore, in the steel containing Pb in the above-mentioned range, when a large amount of fine sulfides are dispersed, Pb and sulfides are finely dispersed at the same time. If Pb is finely dispersed, the steel to be cut per Pb content in the steel increases and hardens. For this reason, it is necessary to disperse a sulfide having an equivalent circle diameter of less than 1 μm and a Pb having an equivalent circle diameter of 1 μm or more by a fixed amount or more. Specifically, it is necessary to make the steel equivalent circular diameter of less than 1μm exist sulfide 1000 / mm ² or more and a circle equivalent diameter of 1μm or less of Pb 1000 / mm ² or more. Regarding chips during cutting, if only sulfides with a circle-equivalent diameter of less than 1 μm are used, the chips will continue and the handling properties will deteriorate. However, by dispersing fine sulfides with an equivalent circle diameter of less than 1 μm, a certain range of relatively large sulfides are present. Specifically, 500 sulfides with an equivalent circle diameter of 1 μm or more and 5 μm or less are present. mm ² or more, chip handling during cutting can also be significantly improved.

Hereinafter, the conditions for manufacturing the free-cutting steel of the present invention will be described.

That is, a rectangular cast slab having the above-mentioned composition and having a side of a cross section perpendicular to the longitudinal direction of 200 mm or more is used, and the cast slab is hot-rolled with a reduction of area of 60% or more to produce a billet. The billet is hot processed at a heating temperature of 1050°C or more and a reduction in area of 65% or more to produce steel bars.

First, molten steel (molten steel) adjusted to the above-mentioned component composition is cast to produce a cast slab. As the cast slab, it is preferable to use a rectangular cast with a side of a cross section perpendicular to the longitudinal direction of 200 mm or more. Billet.

The cast slab is a cast slab with a rectangular section manufactured by a continuous casting method or an ingot method. At this time, if the length of one side of the rectangular cross section is less than 200 mm, the size of sulfide or Pb when the cast slab is solidified becomes larger. Therefore, coarse sulfide or Pb will remain after the billet is continuously rolled into the billet, which is not conducive to the miniaturization of the wire rod after rolling. Therefore, the length of one side in the cross section of the cast slab is set to 200 mm or more. More preferably, it is 250 mm or more.

Reduction of area of hot rolling from cast slab to billet: 60% or more

The size of the sulfide or Pb crystallized during casting and solidification is relatively large, so hot rolling is required to reduce the size to a certain extent. If the reduction of area in the hot rolling to the billet (hereinafter also referred to as slab rolling) is small, the sulfide and Pb directly become the billet in a large state. Therefore, it is difficult to continue heating when the billet is made into steel bar by hot working And make it finer during processing such as rolling. Therefore, the reduction of area in slab rolling is set to 60% or more. Preferably it is 70% or more. In addition, the upper limit does not need to be particularly limited, but from the viewpoint of the surface properties of the final product, it is preferably set to 90% or less.

The billet is rolled into a billet. Regarding the size of the blank, as long as the reduction in area of the final product can be ensured, there is no need to limit it, but it is more preferably formed into a blank having a cross section perpendicular to the longitudinal direction of 120 mm×120 mm or more.

That is, if the cross-sectional area of the blank is less than 120 mm×120 mm, when the steel bar is produced by the subsequent hot working, the cross-sectional reduction rate cannot be obtained, which is disadvantageous to the miniaturization of Pb. Therefore, the cross-sectional area of the blank is preferably 120 mm×120 mm or more. More preferably, it is 150 mm × 150 mm or more.

The heating temperature of the blank: above 1050℃

The heating temperature when the billet is made into steel bars is an important factor. If the heating temperature is less than 1050°C, the sulfide and Pb will not be finely dispersed, so the lubricating effect during cutting is reduced. As a result, tool wear becomes larger, so tool life becomes shorter. Therefore, the heating temperature of the billet is set to 1050°C or higher. More preferably, it is 1080°C or higher. In addition, the upper limit does not need to be particularly limited, but from the viewpoint of suppressing the decrease in yield due to scale loss, it is preferably set to 1250°C or lower.

The reduction of area during the hot working of the billet into steel bars: 65% or more

The reduction of area during hot working of the billet into steel bars is also an important factor for the miniaturization of sulfides and Pb. If the reduction of area is less than 65%, the miniaturization of sulfide and Pb is insufficient, so the lower limit of the reduction of area is set to 65%. more Preferably, it is set to 70% or more.

Example

Next, the present invention will be described in detail according to embodiments.

The steel of the chemical composition shown in Table 1 was produced by a continuous casting machine into a rectangular cast slab whose cross section perpendicular to the longitudinal direction is the size shown in Table 2. The obtained cast slab was rolled under the manufacturing conditions shown in Table 2 into steel bars. That is, the cast slab was hot-rolled at the heating temperature and the reduction of area shown in Table 2 to produce a billet whose long side dimensions and short side dimensions are shown in Table 2. The obtained billet was heated at the heating temperature shown in Table 2 and hot rolled to produce steel bars with diameters shown in Table 2. The obtained steel bars (invention steel and comparative steel) were used in the tests shown below.

Collecting test from a section parallel to the rolling direction of the obtained steel bar The sheet was observed with a scanning electron microscope SEM (Scanning Electron Microscope, SEM) at the position on the 1/4 axis side of the radial diameter from the periphery of the section to investigate the sulfide in the steel and the circle equivalent of Pb Diameter and number density. Furthermore, the composition analysis of sulfide and Pb is performed by energy dispersive X-ray spectrometry (EDX). After confirming that it is sulfide or Pb by EDX, the obtained SEM image is binarized by image analysis, and the circle equivalent diameter and number density are calculated.

The machinability is evaluated by the peripheral turning test. That is, BNC-34C5 manufactured by Citizen Machinery was used as the cutting machine, the superhard EX35 machine tool (bite) TNGG160404R-N manufactured by Hitachitool was used for the turning insert, and the holder was KYOCERA. ) DTGNR2020 manufactured. In addition, the lubricant used a 15-fold diluted emulsion of Yushiroken FGE283PR manufactured by Yushiro Chemical. The cutting conditions were performed with a cutting speed of 100m/min, a feed rate of 0.05mm/rev, a cutting amount of 2.0mm, and a machining length of 10m.

The evaluation of the machinability was performed by the tool belly wear Vb after the cutting test with a length of 10 m was completed. The case where the blade wear Vb after the end of the cutting test was 200 μm or less was regarded as good "○", and the case where the blade wear exceeded 200 μm was regarded as poor "×", as shown in Table 2.

Table 2 shows the test results of the invention steel and the comparative steel. It can be seen from Table 2 that the steel of the present invention has good machinability relative to the comparative steel.

Claims (6)

A kind of free-cutting steel, including C: 0.15% or less, Mn: 0.5% or more and 2.0% or less, S: 0.200% or more and 0.650% or less, O: more than 0.01% and less than 0.05%, Cr : 0.05% or more and 2.00% or less, Pb: 0.02% or more and less than 0.10%, and N: 0.005% or more and 0.015% or less, and the A value defined by the following formula (1) satisfies 4.0 or more and 20.0 or less, and the remainder is The composition of Fe and unavoidable impurities, and the sulfide with a circle equivalent diameter of less than 1μm is 1000 pieces/mm ² or more, and the sulfide with a circle equivalent diameter of 1μm or more and 5μm or less is 500 pieces/mm ² or more and the circle equivalent Pb with a diameter of 1μm or less is 1000 pcs/mm ² or more; A value=(Mn+5Cr)/S. . . (1) Here, the element symbol in the formula represents the content (mass %) of this element. The free-cutting steel according to claim 1, wherein the component composition is further calculated by mass%, and contains Si: 0.10% or less, P: 0.01% or more and 0.15% or less, and Al: 0.010% or less. The free-cutting steel according to claim 1 or claim 2, wherein the component composition is further calculated by mass%, and contains Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less , Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less and Mg : Any one or more of 0.0050% or less. A method for manufacturing free-cutting steel, which has C: 0.15% or less in mass% Mn: 0.5% or more and 2.0% or less, S: 0.200% or more and 0.650% or less, O: more than 0.0100% and less than 0.0500%, Cr: 0.05% or more and 2.00% or less, Pb: 0.02% or more and less than 0.10% And N: 0.005% or more and 0.015% or less, and the A value defined by the following formula (1) satisfies 4.0 or more and 20.0 or less, the remainder is composed of Fe and unavoidable impurities, and is a section perpendicular to the length direction A rectangular cast slab with a length of 200mm or more on one side, the cast slab is rolled with a reduction of area of 60% or more to make a billet, and the billet is heated at a temperature of 1050°C or more and the section is hot processed The shrinkage rate is above 65% to make bar steel; A value = (Mn+5Cr)/S. . . (1) Here, the element symbol in the formula represents the content (mass %) of this element. The method of manufacturing a free-cutting steel according to claim 4, wherein the component composition further comprises any of Si: 0.10% or less, P: 0.01% or more and 0.15% or less, and Al: 0.010% or less in mass% More than one kind. The manufacturing method of free-cutting steel as described in claim 4 or claim 5, Wherein the composition of the component is further calculated by mass%, containing Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.