CN1711367A - Steel excellent in machinability and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a steel with excellent cuttability, and a method for producing the same, where the steel comprises, in weight%, C: 0.005-0.2%, Si: 0.001-0.5%, Mn: 0.2-3.0%, P: 0.001-0.2%, S: 0.03-1.0%, B: 0.0005-0.05%, Total-N: 0.002-0.02%, Total-O: 0.0005-0.035% and the balance being Fe and unavoidable impurities, and satisfying one or both of the conditions of (1) Mn/S: 1.2-2.8 in the steel, (2) an area ratio of pearlite having a diameter more than 1 mum of less than 5%.

Description

Steel excellent in machinability and manufacturing method thereof

technical field

The present invention relates to a steel used in automobiles and general machinery, and a method for producing the steel, and in particular, to a steel having excellent machinability with good tool life during cutting, smoothness of the cut surface, and chip disposability, and a method for producing the same.

Background technique

General machines and automobiles are manufactured by combining various parts, but in many cases, the parts are manufactured through a chipping process from the viewpoint of required precision and manufacturing efficiency. At this time, cost reduction and productivity improvement are required, and for steel, improvement of machinability is also required. Especially in the past, SUM23 and SUM24L were developed with emphasis on machinability. It has been known so far that in order to improve machinability, it is effective to add machinability-improving elements such as S and Pb. However, depending on the needs, Pb is often avoided as an environmental burden, and there is a tendency to reduce its usage.

Even when Pb is not added until now, a technique of improving machinability by forming MnS soft inclusions like S in a cutting environment has been used. However, in the so-called low-carbon lead-containing free-cutting steel SUM24L, the same amount of S as that of the low-carbon sulfur-containing free-cutting steel SUM23 is added. Therefore, it is necessary to add a larger amount of S than before. However, addition of a large amount of S not only makes MnS coarser, but not only does not form an effective MnS distribution for improving machinability, but also becomes a starting point of failure in rolling, casting, etc., causing many manufacturing problems such as rolling defects. The problem. In addition, the sulfur-containing free-cutting steel based on SUM23 is prone to build-up edge adhesion, and with the built-up edge falling off and chip separation, unevenness occurs on the cutting surface and the surface finish deteriorates. Therefore, from the standpoint of machinability, there is also a problem of a decrease in precision due to deterioration of surface roughness. In terms of chip disposability, chips that are short and easy to cut are also considered to be good, but only adding S increases the ductility of the matrix, so they cannot be cut sufficiently, and cannot be greatly improved.

In addition, elements other than S, such as Te, Bi, P, etc., are also known as machinability-improving elements. Although they can improve machinability to a certain extent, they are also prone to cracks during rolling or hot forging, so it is desirable The content should be as small as possible, which has been disclosed in JP-A-9-71840, JP-A-2000-160284, JP-2000-219936, and JP-2001-329335.

Furthermore, in Japanese Patent Application Laid-Open No. 11-222646, it is proposed that by connecting sulfides greater than or equal to 20 μm alone or a plurality of sulfides connected in series to form a group of sulfides with a length greater than or equal to 20 μm in 1 mm ² There are 30 or more pieces within the field of view of the cross-section in the rolling direction to improve chip handling. However, neither the dispersion nor the production method of the ultramicron-order sulfide that is actually most effective for machinability is mentioned, and it cannot be expected from its component system.

In addition, Japanese Patent Application Laid-Open No. 11-293391 proposes that the average size of the sulfide-based inclusions be 50 μm or less, and there are 750 or more of the sulfide-based inclusions per 1 mm ² , so as to improve chip removal. processing method. However, Japanese Unexamined Patent Publication No. 11-222646 also does not mention anything about the dispersion of ultramicron-order sulfides that are actually the most effective for machinability, and there is no description about the production technology and investigation method that realized this.

On the other hand, cutting tool life is often concerned because it directly affects manufacturing efficiency, etc. However, in machinability, the technical difficulty is also the surface finish, and the surface finish is affected by the nature of the cutting material. , so it is difficult to achieve a surface finish that is higher than that of conventional steel. The surface roughness is directly related to the performance of parts, so deterioration of the surface roughness is often a cause of lowering of parts performance and an increase in defective rate during product manufacturing, and is often more important than tool life. In this sense, conventional lead-containing free-cutting steels are excellent, and have better tool life and surface finish than free-cutting steels containing only sulfur, so they are often used to prevent deterioration in component performance.

In the technology of steel for improving the surface roughness, generally, free-cutting elements such as Pb and Bi are often added, but in addition, as seen in JP-A No. The average size of MnS inclusions is refined to less than or equal to 50 μm to ensure surface roughness, tool life and machined surface characterized by graphite with an average cross-sectional area of 0.20 to 1.0% in the ferrite matrix: 5 to 30 μm ² Graphite free-cutting steel with excellent smoothness, etc.

However, even with these methods, it is difficult to obtain a surface finish greater than or equal to that of conventional leaded free-cutting steels. So far, the so-called low-carbon leaded free-cutting steel SUM24L has excellent surface finish. The reason is presumed that the fine dispersion level of inclusions under these regulations is only processed for particles with an average diameter of about 3 μm, and the uniform dispersion is not sufficient, so built-up edge is prone to occur, which cannot be achieved as in the past. Improves surface finish like leaded free-cutting steels, etc.

Contents of the invention

The present invention provides a steel having a machinability equal to or higher than conventional low-carbon lead-containing free-cutting steels and a good surface finish while avoiding defects in rolling or hot forging while improving both tool life and surface roughness, and its Manufacturing method.

Chipping is a destructive phenomenon that separates chips, and it is important to promote chipping. In particular, in order to obtain a good surface finish, by embrittlement of the matrix, it is easy to break, and while prolonging the tool life, by suppressing unevenness in the steel as much as possible, stable breakage occurs microscopically. Suppresses unevenness on the cut surface. Specifically, it provides a method that focuses on the distribution of pearlite in steel, and forms stable fracture by uniformly dispersing C in steel as fine pearlite (cementite, strictly speaking), thereby creating a cutting surface with less unevenness. , and a feasible manufacturing method. The gist of the present invention is as follows.

(1) A steel excellent in machinability, characterized in that it is composed of

C: 0.005～0.2%,

Si: 0.001 to 0.5%,

Mn: 0.2～3.0%,

P: 0.001～0.2%,

S: 0.03～1.0%,

Total N: 0.002～0.02%,

Total O: 0.0005～0.035%,

The balance is steel composed of Fe and unavoidable impurities, satisfying that Mn/S in the steel is 1.2 to 2.8, or the area ratio of pearlite with a particle size exceeding 1 μm in the steel microstructure is 5% or less Either or both, and the steel surface roughness Rz is less than or equal to 11 μm.

(2) A steel excellent in machinability, characterized in that it contains C: 0.005 to 0.2%, Mn: 0.3 to 3.0%, and S: 0.1 to 1.0% by mass %. For the MnS adopted by the type method, in the section parallel to the rolling direction of the steel, the MnS density with an equivalent circle diameter of 0.1-0.5 μm is greater than or equal to 10,000 pieces/ ^mm2 , and the steel’s cutting surface roughness Rz becomes less than Or equal to 11 μm.

(3) The steel described in (1) or (2), which further contains B: 0.0005 to 0.05% by mass.

(4) The steel as described in (1), wherein the MnS collected by the extraction replica method observed with a transmission electron microscope has an equivalent circle diameter of 0.1 in a section parallel to the rolling direction of the steel material. The MnS density of ~0.5 μm is greater than or equal to 10,000/mm ² .

(5) The steel described in (1), characterized in that it further contains the amount of S limited to 0.25 to 0.75% by mass, the amount of B to be limited to 0.002 to 0.014% by mass, and the contents of S and B satisfy the following formula 1 The amount of S and B in the area surrounded by A, B, C, and D shown in Figure 4, and the sulfide that precipitates BN is included in MnS.

(B-0.008) ² /0.006 ² +(S-0.5) ² /0.25 ² ≤1 …Formula (1)

(6) In the steel described in (1) or (2), it is characterized in that it further contains

V: 0.05～1.0%,

Nb: 0.005 to 0.2%,

Cr: 0.01 to 2.0%,

Mo: 0.05 to 1.0%,

W: 0.05～1.0%,

Ni: 0.05 to 2.0%,

Cu: 0.01 to 2.0%,

Sn: 0.005～2.0%,

Zn: 0.0005～0.5%,

Ti: 0.0005～0.1%,

Ca: 0.0002 to 0.005%,

Zr: 0.0005～0.1%,

Mg: 0.0003～0.005%,

Te: 0.0003～0.05%,

Bi: 0.005 to 0.5%,

Pb: 0.01 to 0.5%,

Al: 1 type of ≤0.015% or 2 or more types.

(7) A method for producing steel excellent in machinability described in any one of (1) to (3), characterized in that, after casting molten steel having the composition described in (1), the Cooling at a cooling rate of 0.5 °C/s or more in the range from A ₃ point to 550 °C after hot rolling.

(8) A method for producing a steel excellent in machinability according to (4) or (5), characterized in that, after casting molten steel having the composition described in (2), cooling at a cooling rate of 10 to 100°C/min After cooling, the final temperature of hot rolling is limited to 1000°C or higher, and cooling after hot rolling is performed at a cooling rate of 0.5°C/sec or higher in the range from _A3 point to 550°C.

(9) A method for producing steel excellent in machinability according to any one of (1) to (6), wherein the heating temperature for adjusting the hardness is limited to 750°C or below.

(10) A method for producing a steel excellent in machinability as described in any one of (7) to (9), wherein the steel further contains

V: 0.05～1.0%,

Nb: 0.005 to 0.2%,

Cr: 0.01 to 2.0%,

Mo: 0.05 to 1.0%,

W: 0.05～1.0%,

Ni: 0.05 to 2.0%,

Cu: 0.01 to 2.0%,

Sn: 0.005～2.0%,

Zn: 0.0005～0.5%,

Ti: 0.0005～0.1%,

Ca: 0.0002 to 0.005%,

Zr: 0.0005～0.1%,

Mg: 0.0003～0.005%,

Te: 0.0003～0.05%,

Bi: 0.005 to 0.5%,

Pb: 0.01 to 0.5%,

Al: 1 or more of ≤0.015%.

Description of drawings

Fig. 1 is a micrograph showing the ferrite-pearlite structure of the steel of the present invention.

Fig. 2 (a) is a micrograph showing the finely dispersed state of MnS in the steel of the present invention,

Fig. 2(b) is a micrograph showing the state of existence of coarse MnS in conventional steel.

Fig. 3 is a graph showing the relationship between the pearlite area ratio and the surface roughness.

Fig. 4 is a graph showing optimum ranges of the S content and the B content of the steel of the present invention.

Fig. 5 is a transmission electron microscope replica photograph showing the form of sulfide compounded and precipitated BN with MnS of the present invention as a main component.

Fig. 6 is a graph showing the results of EDX analysis of BN.

Fig. 7 is a diagram showing a plunge cutting method.

Detailed ways

In the present invention, lead is not added, and a large amount of B is added in order to obtain sufficient machinability, especially good surface roughness, while embrittlement of the substrate, and in order to improve the lubrication of the contact surface of the tool/cutting material. In addition, S is also added in a relatively large amount, and the ratio of the addition amounts of Mn and S is precisely controlled in order to finely disperse it. In addition, regarding the microstructure of steel, pearlite seen in conventional carbon steel is also controlled. That is, the amount of C added in the chemical composition is controlled to suppress the precipitation of coarse pearlite, or when there is a lot of C, the formation of coarse pearlite particles is suppressed by heat treatment, that is, the formation of pearlite bands often seen in natural cooling is suppressed Steel with excellent machinability.

Next, reasons for limiting the steel components specified in the present invention will be described.

C is related to the basic strength of the steel and the amount of oxygen in the steel, so it has a great influence on the machinability. Adding a large amount of C improves the strength but lowers the machinability, so the upper limit is made 0.2%. On the other hand, in order to prevent the formation of hard oxides that reduce the machinability and to suppress the disadvantages of solid dissolved oxygen at high temperatures such as fine pores during solidification, it is necessary to control the amount of oxygen appropriately. If the amount of C is reduced too much simply by blowing, not only will the cost increase, but also a large amount of oxygen will remain in the steel, causing defects such as fine pores. Therefore, the lower limit is made 0.005% of the amount of C that can easily prevent troubles such as fine pores. The optimum lower limit of the amount of C is 0.05%.

Excessive addition of Si will generate hard oxides, which will reduce the machinability, but moderate addition will soften the oxides without reducing machinability. The upper limit is 0.5%, and above that, hard oxides are formed. When it is 0.001% or less, softening of oxides becomes difficult, and industrial cost increases.

Mn is necessary for fixing and dispersing sulfur in steel as MnS. In addition, it is necessary to soften oxides in steel and make oxides harmless. The effect also depends on the amount of added S, but if it is less than 0.2%, the added S cannot be sufficiently fixed as MnS, and S becomes FeS to become brittle. If the amount of Mn increases, the hardness of the matrix increases and the machinability or cold workability decreases, so the upper limit is made 3.0%.

In steel, P increases the hardness of the matrix and degrades not only cold workability but also hot workability and casting properties, so the upper limit must be made 0.2%. On the other hand, it is an element effective in improving machinability, so the lower limit value is made 0.001%.

S combines with Mn and exists as MnS inclusions. MnS can improve machinability, but stretched MnS is one of the causes of anisotropy during forging. Large MnS must be avoided, but it is preferable to add a large amount from the viewpoint of improving machinability. Therefore, it is preferable to finely disperse MnS. Addition of 0.03% or more is necessary to improve the machinability above that of conventional sulfur-containing free-cutting steels in which no Pb is added. On the other hand, if it exceeds 1%, not only the formation of coarse MnS cannot be avoided, but also cracks will occur during production due to the deterioration of casting characteristics and thermal deformation characteristics caused by FeS and the like, so 1% is made the upper limit.

When B is precipitated as BN, it is effective in improving machinability. These effects are insignificant at less than 0.0005%, and even if added at more than 0.05%, the effect is saturated. If excessive BN is precipitated, the casting properties and thermal deformation properties will deteriorate instead, and cracks will occur during production. Therefore, 0.0005 to 0.05% is made into the range.

In the present invention, in particular, the area surrounded by A, B, C, and D in the ellipse shown in FIG. 4 that extremely limits the above-mentioned S amount and B amount, that is, by the following formula (1)

(B-0.008) ² /0.006 ² + (S-0.5) ² /0.25 ² ≤ 1 ... the region of formula (1), and the best characteristics are obtained.

N (total N) hardens the steel in the presence of solid solution N. Especially in cutting, due to dynamic strain aging, hardening occurs near the cutting edge, which reduces the life of the tool, but it also has the effect of improving the surface finish of the cutting. In addition, it combines with B to form BN, which improves machinability. If it is less than 0.002%, the effect of improving the surface roughness by solid solution nitrogen or the effect of improving machinability by BN is not seen, so 0.002% is made the lower limit. On the other hand, if it exceeds 0.02%, a large amount of solid-solution nitrogen will exist, so the life of the tool will be reduced on the contrary. In addition, air bubbles are generated during casting, causing defects and the like. Therefore, in the present invention, 0.02% is made the upper limit.

When O (total O) exists in a free state, it becomes air bubbles during cooling and causes fine pores. In addition, in order to soften oxides and suppress hard oxides that are harmful to machinability, it is also necessary to suppress them. Furthermore, when MnS is finely dispersed, oxides are used as precipitation nuclei. If it is less than 0.0005%, MnS cannot be sufficiently finely dispersed, and coarse MnS is generated, which adversely affects mechanical properties, so 0.0005% is made the lower limit. Furthermore, if the oxygen content exceeds 0.035%, air bubbles will be formed during casting to become fine pores, so the upper limit is made 0.035%.

Next, the reason for setting the pearlite area ratio to 5% or less will be described. In general, when carbon-containing steel is cooled from a temperature above the transformation point, it forms a ferrite-pearlite structure. Steel with a small amount of C, which is the object of the present invention, is cut from a temperature above the transformation point (A ₃ point) after air cooling, and then polished to a mirror surface. If etched with nital, the inside can be observed. Microstructure as shown in Figure 1. The black grains are a composite structure of ferrite and cementite called pearlite, but generally, the black grains seen through nital etching are harder than the white ferrite grains seen. Qualitatively, in the deformation/fracture behavior of steel, it locally shows behavior different from that of ferrite grains. This hinders uniform deformation/fracture in the chip breaking behavior during cutting, and thus is largely related to the generation of built-up edge, which in turn degrades the surface finish of the cutting face. Therefore, it is important to rule out tissue heterogeneity due to C as much as possible. Therefore, the black particles etched with nital are regarded as pearlite particles. If the pearlite particles are too large, the structure will be uneven and cause the deterioration of the surface finish. Therefore, the area ratio is limited to 5% or less. , the surface finish Rz is limited to less than or equal to 11 μm. The relationship between the pearlite area ratio and the surface roughness is shown in FIG. 3 .

Here, details of the measurement method are described. A sample cut along the longitudinal section (L section) of rolled or forged steel and embedded in resin was mirror-polished and etched with nital. Using an image processing device, analyze particles with a particle size (circle-equivalent diameter) greater than or equal to 1 μm after removal of gray MnS in the black matter etched with nital, and obtain the area ratio. In the image processing of the area ratio measurement, by setting the "critical value" corresponding to the visible black pearlite, the image intensity is adjusted, and by eliminating the visible gray inclusions (MnS, etc.) from the image, only the pearlite body as the object of measurement. At this time, the smallest pearlite to be recognized is about 1 μm, but pearlite of less than 1 μm does not affect the machinability, so even if it cannot be recognized, it has no effect.

For the measurement field of view in the present invention, with a magnification greater than or equal to 400 times, 20 field of view measurements are carried out for a field of view of 0.2 ^mm (0.4 mm × 0.5 mm), which is a total area of ⁴ mm, and the pearlite area is calculated. Rate.

As for Mn/S, it is known that it has a great influence on hot rolling properties. Generally, if Mn/S>3 is not achieved, the manufacturability of steel will be greatly reduced. The reason for this is the formation of FeS, but it was found in the present invention that the ratio can be further reduced to Mn/S: 1.2 to 2.8 in a low C and high S region. When Mn/S is less than 1.2, a large amount of FeS is formed, which reduces the hot rolling property extremely and greatly reduces the manufacturability.

FIG. 2 shows examples of fine MnS observed with a transmission electron microscope when Mn/S≦2.8 and Mn/S>2.8 using the replica method. When Mn/S>2.8, only coarse MnS as shown in FIG. 2(b) is formed, and the surface roughness cannot be reduced. On the other hand, when Mn/S is limited to 1.2 to 2.8, fine MnS is formed as shown in FIG. 2( a ).

The number of fine MnS objects can be increased by continuous casting or by repeated heating at 900° C. or higher after casting of a steel ingot.

Next, the reason why the existence density of MnS is defined as 10,000/mm ² or more at a circle-equivalent diameter of 0.1 to 0.5 μm in the form, size, and distribution of MnS will be described.

MnS is an inclusion that improves machinability, and is dispersed finely and at a high density to remarkably improve machinability. In order to exert its effect, the density of MnS having a circle-equivalent diameter of 0.1 to 0.5 μm must be greater than or equal to 10,000/mm ² . The distribution of MnS sulfides is usually observed with an optical microscope to measure their size and density. For MnS sulfides of this size, observation under an optical microscope is impossible to confirm, and it can only be observed with a transmission electron microscope (TEM). Even if there is no difference in size and density under an optical microscope, it is a sulfide whose main component is MnS of a size that is clearly different in TEM observation. In the present invention, by controlling MnS, the existing form is quantified, To seek differentiation from existing technologies.

In order to make MnS exceeding the above-mentioned size exist at a density of 10,000 pieces/ ^mm2 or more, it is necessary to add a large amount of S exceeding the range of the present invention, but if a large amount is added, the probability that a large amount of coarse MnS exists also becomes high, and It becomes the cause of anisotropy at the time of forging. With the addition amount of S within the range specified in the present invention, if MnS exceeds this size, the amount of MnS will be insufficient, and the density necessary for improving machinability cannot be maintained. In addition, MnS having a minimum diameter of 0.1 μm or less does not substantially affect machinability. Therefore, it is necessary to exist at a density of 10,000 pieces/mm ² of MnS having a circle-equivalent diameter of 0.1 to 0.5 μm. In order to obtain the size and density of such MnS, in addition to controlling the cooling rate, it is more effective if the ratio of Mn to S contained is 1.5 to 2.5.

Furthermore, in the present invention, it is important that 10% by mass or more of boron nitride (BN) in the above-mentioned MnS has the form of complex precipitated sulfides, as shown in FIG. 5 .

BN is usually easy to precipitate at the grain boundary, and it is difficult to disperse uniformly in the matrix. For this reason, the uniform embrittlement of the matrix necessary for improving machinability cannot be produced, that is, the effect of BN cannot be fully exhibited. In order to uniformly disperse BN in the matrix, it is necessary to uniformly disperse MnS, which serves as a precipitation site of BN and is also effective in improving machinability, in the matrix. By precipitating BN and MnS compositely, uniform dispersion of BN is achieved, and the machinability is greatly improved. For this reason, less than 10% or more of BN and MnS complex precipitation is required.

The BN mentioned here refers to the compounds of B and N, which are shown in the TEM replica photograph in FIG. 5 and have clearly confirmed the peaks of B and N in the EDX analysis of FIG. 6 .

Furthermore, the so-called MnS includes not only pure MnS, but also inclusions in which sulfides such as Fe, Ca, Ti, Zr, Mg, REM (rare earth elements) and MnS are solid-solubilized and co-exist with MnS in the main body, Or, like MnTe, elements other than S form compounds with Mn, and inclusions coexist in solid solution and combination with MnS, or the above-mentioned inclusions precipitate with oxides as nuclei, which means that the chemical formula can be (Mn, X) (S, Y) (where X is a sulfide-forming element other than Mn, and Y is an element other than S that binds to Mn) is a general term for Mn sulfide-based inclusions.

Next, in the present invention, in addition to the above-mentioned components, V, Nb, Cr, Mo, W, Ni, Sn, Zn, Ti, Ca, Zr, Mg, Te, Bi, and Pb may be added as necessary. species or more than one species.

V forms carbonitrides and can strengthen steel by secondary precipitation hardening. If it is less than 0.05%, there is no effect on strengthening, and if it is added in excess of 1.0%, a large amount of carbonitrides will be precipitated, conversely impairing the mechanical properties, so 1.0% is made the upper limit.

Nb also precipitates carbonitrides, and steel can be strengthened by secondary precipitation hardening. If it is less than 0.005%, there is no effect on strengthening, and if it is added in excess of 0.2%, a large amount of carbonitrides will be precipitated, conversely impairing the mechanical properties, so 0.2% is made the upper limit.

Cr is an element that improves hardenability and imparts temper softening resistance. Therefore, it is added to steels that require high strength. In this case, addition of 0.01% or more is necessary. However, if added in large amounts, Cr carbides are formed to cause embrittlement, so 2.0% is made the upper limit.

Mo is an element that improves hardenability while imparting temper softening resistance. When it is less than 0.05%, the effect cannot be obtained, and even if it is added in excess of 1.0%, the effect is saturated, so the addition range is 0.05% to 1.0%.

W forms carbides and can strengthen steel by secondary precipitation. If it is less than 0.05%, it has no effect on strengthening, and if it is added in excess of 1.0%, a large amount of carbides will be precipitated, conversely impairing the mechanical properties, so 1.0% is made the upper limit.

Ni-strengthening ferrite is effective in improving ductility, hardenability, and corrosion resistance. If it is less than 0.05%, the effect cannot be obtained, and even if it is added in excess of 2.0%, the effect on mechanical properties will be saturated, so 2.0% is made the upper limit.

Cu strengthens ferrite and is also effective in improving hardenability and improving corrosion resistance. If it is less than 0.01%, the effect cannot be obtained, and even if it is added in excess of 2.0%, the effect on mechanical properties will be saturated, so 2.0% is made the upper limit. In particular, hot rolling ductility is lowered, and it is likely to cause defects during rolling, so it is preferably added simultaneously with Ni.

Sn embrittles ferrite and prolongs tool life, and is also effective in improving surface roughness. If it is less than 0.005%, the effect cannot be obtained, and even if it is added in excess of 2.0%, the effect on the mechanical properties will be saturated, so 2.0% is made the upper limit.

Zn embrittles ferrite and prolongs tool life, and is also effective in improving surface roughness. If it is less than 0.0005%, the effect cannot be obtained, and even if it is added in excess of 0.5%, the effect on mechanical properties will be saturated, so 0.5% is made the upper limit.

Ti also forms carbonitrides to strengthen steel. It is also a deoxidizing element and improves machinability by forming soft oxides. When it is less than 0.0005%, the effect cannot be obtained, and even if it is added in excess of 0.1%, the effect is saturated, and Ti also forms nitrides at high temperatures to suppress the growth of austenite grains. Therefore, 0.1% is made the upper limit. In addition, Ti and N are combined to form TiN, but TiN is a hard substance and reduces machinability. In addition, the amount of N necessary to produce BN effective for improving machinability is reduced. Therefore, the amount of Ti added is preferably 0.010% or less.

Ca is a deoxidizing element and forms soft oxides, which not only improve the machinability, but also dissolve in MnS to reduce the deformation ability, and even if it is rolled or hot forged, it also has the effect of inhibiting the elongation of the MnS shape. Therefore, it is an element effective in reducing anisotropy. When it is less than 0.0002%, the effect is insignificant, and even if it is added more than 0.005%, not only the yield is extremely deteriorated, but also a large amount of hard CaO is formed, which reduces the machinability on the contrary. Therefore, the addition range is specified as 0.0002 to 0.005%.

Zr is a deoxidizing element and forms oxides. Oxides serve as precipitation nuclei of MnS, and are effective in finely and uniformly dispersing MnS. Solid solution in MnS reduces its deformability, even if it is rolled or hot forged, it also has the effect of inhibiting the extension of MnS shape. Therefore, it is an element effective in reducing anisotropy. When it is less than 0.0005%, the effect is insignificant, and even if it is added more than 0.1%, not only the yield is extremely poor, but also a large amount of hard ZrO ₂ , ZrS, etc. are formed, which reduces the machinability on the contrary. Therefore, the addition range is specified as 0.0005 to 0.1%. In addition, in order to achieve fine dispersion of MnS, composite addition of Zr and Ca is preferable.

Mg is a deoxidizing element and forms oxides. Oxides serve as precipitation nuclei of MnS, are effective in finely and uniformly dispersing MnS, and are elements effective in reducing anisotropy. When it is less than 0.0003%, the effect is insignificant, and even if it is added more than 0.005%, not only the yield is extremely deteriorated, but also the effect is saturated. Therefore, the addition range is specified as 0.0003 to 0.005%.

Te is an element improving machinability. And MnTe is generated, or the deformation energy of MnS is reduced due to the coexistence with MnS, and it has the effect of suppressing the extension of the shape of MnS. Therefore, it is an element effective for anisotropy reduction. When the content is less than 0.0003, this effect cannot be obtained, and if it exceeds 0.05%, the effect becomes saturated.

Bi and Pb are elements effective in improving machinability. If it is less than 0.005%, this effect cannot be obtained, and even if it is added in excess of 0.5%, not only the effect of improving machinability is saturated, but also the hot forging properties are lowered, which tends to cause defects.

Al is a deoxidizing element that forms Al ₂ O ₃ and Al in steel. However, Al ₂ O ₃ is hard, causes tool damage during cutting, and promotes wear. Therefore, it is limited to less than or equal to 0.015% of Al ₂ O ₃ generated in large quantities. Especially when giving priority to tool life, it is preferably 0.005% or less.

In addition, in the present invention, when the avoidance of machinability failures is prioritized rather than the avoidance of failures during quenching, the amount of B is reduced within the allowable range of machinability. For example, in the composition specified in the present invention, it is specified that B: 0.0005% to 0.005%, and the amount of S is also specified to be 0.5% to 1.0%, and it can also be used as a steel excellent in machinability. In the case of a large amount of B, B remains in solid solution, and the hardenability increases. After heat treatment such as carburizing and quenching, the hardened layer becomes too deep, or the strain increases in the performance of the part, or the hardened part becomes brittle. Therefore, the above-mentioned provisions of the present invention can prevent various failures such as quench cracking. Furthermore, in the present invention, in cold forging or wire drawing and other processing methods for free-cutting steel other than cutting, MnS is likely to become the starting point of destruction and cause cracks, so the mechanical properties tend to decrease. The minimum machinability S content of steel is controlled to 0.03 to 0.5% by mass, and surface cracks generated by cold forging or high frequency can also be suppressed.

Next, a method for producing steel in which the above-mentioned MnS and BN are finely dispersed will be described.

Fine dispersion of sulfides containing MnS as the main component and complex-precipitating BN is effective in improving machinability. In order to finely disperse the sulfide, it is necessary to control the crystallization of the sulfide containing MnS as the main component and complex-precipitating BN. In this control, it is necessary to specify the cooling rate range during casting. When the cooling rate is below 10°C/min, the solidification is too slow, and the composite precipitated BN sulfides mainly composed of crystallized MnS are coarsened and cannot be finely dispersed. When the cooling rate exceeds 100°C/min, the density of the generated fine sulfide reaches saturation, the hardness of the steel ingot increases, and the risk of cracking increases. In order to obtain this cooling rate, it can be easily obtained by controlling the size of the cross-section of the mold, the casting rate, etc. to appropriate values. This can be applied to both the continuous casting method and the ingot casting method.

The cooling rate mentioned here means the rate at the time of cooling from the liquidus temperature in the thickness direction Q part of an ingot to a solidus temperature. The cooling rate is calculated from the interval of the secondary dendrite arms of the solidified structure in the thickness direction of the ingot after solidification according to the following formula:

$\mathrm{Rc}={\left(\frac{\mathrm{\λ}2}{770}\right)}^{\frac{1}{0.41}}$ Wherein, Rc: cooling rate (° C./min), λ2: distance between arms of secondary dendrites (μm).

That is, the distance between the arms of the secondary dendrite varies depending on the cooling conditions. Therefore, by measuring this change, the controlled cooling rate can be confirmed.

BN dissolves in austenite at temperatures higher than or equal to 1000°C. At a temperature lower than 1000°C, the BN precipitated from casting to rough rolling remains in the grain boundaries, and sulfides containing MnS as the main component and complex-precipitating BN cannot be precipitated. In the finish rolling process of hot rolling, by rolling at a temperature higher than or equal to 1000° C., the re-solid-solution BN becomes easy to compound precipitate with MnS sulfide as a precipitation nucleus. If finish rolling is performed at a temperature lower than 1000° C., complex precipitation of sulfides mainly composed of BN and MnS becomes less likely to occur.

Next, a method for producing a microstructure in order to obtain a pearlite area ratio of 5% or less in the present invention will be described.

The build-up behavior of built-up edge onto the tool has a large impact on the cut surface finish. Originally, mechanically, directly above the cutting tool is the harshest environment for the material, and it is thought that damage/separation of the material is likely to occur, so there should be no built-up edge adhesion, but in fact, because of the strong adhesion between the tool and the cutting material With the uneven organization of the cutting material, built-up edge occurs. It is therefore considered important to maximize the homogeneity of the microstructure of the material. As a result, the present inventors found that the distribution of pearlite, which was thought to have little relationship until now, is largely related to the uniformity of the microstructure.

Here, the term "pearlite" refers to a black structure seen after nital etching is carried out on the mirror-polished surface. Strictly speaking, the so-called pearlite refers to a group composed of ferrite and plate cementite arranged alternately, but it looks like a single particle under an optical microscope. Furthermore, as shown in FIG. 1 , when the usual manufacturing process of rolling and cooling is used, such pearlite grains are arranged and precipitated in bands (hereinafter referred to as pearlite bands). The mechanical properties of this pearlite are different from those of the single-phase ferrite in the matrix, so the deformation and failure near the tip of the tool are made uneven, which in turn promotes the growth of built-up edge.

Therefore, it became clear when investigating the critical region for obtaining a good surface finish by controlling the pearlite area ratio in the observation field with a measurement field of view of 4 ^mm2 in relation to pearlite particles larger than or equal to 1 μm by adjusting the steel composition or thermal history. , in order to suppress deterioration of the surface finish, the area ratio occupied by pearlite particles of 1 μm or more is 5% or less. The relationship between the pearlite area ratio and the surface roughness is shown in FIG. 2 .

As shown in Fig. 1, it can be seen that the free-cutting steel according to the present invention has very few black structures. Strictly speaking, in the present invention, it becomes tempered martensite or tempered bainite structure, and the carbide is not pearlite (in other words, a striped structure produced by platy cementite and ferrite), but it cannot The possibility of existence in the form of cementite particles is denied. However, such iron-based carbides are collectively referred to as pearlite here.

Next, the manufacturing method of the free cutting steel of this invention is demonstrated.

[Quenching heat history: 0.5°C/sec from the temperature at or above point _A3 to 550°C or below]

In the present invention, as the heat history after hot rolling, it is important to cool at a cooling rate of 0.5°C/s or more from the temperature at or above _A3 point to 550°C or below after hot rolling.

Conventionally, so-called low-carbon free-cutting steels have not been quenched. Low-carbon free-cutting steel has a small amount of C, so there is little change in hardness even after quenching. Therefore, the use of conventional "quenching and tempering" has no effect on strength and toughness, so free-cutting steels are bound by so-called unnecessary fixed concepts. However, considering the essence of cutting and pursuing the uniformity of the material, by quenching from A ₃ point, the movement of C in the steel can be frozen, and the formation of coarse cementite caused by the phase transformation during air cooling can be suppressed , and then suppress the generation of pearlite. In this case, hardening by quenching is not the purpose, so even if a quenched structure having a martensitic structure is not formed, it is sufficient to freeze the movement of C in the steel and prevent the formation of coarse cementite or pearlite. For this reason, as shown in FIG. 3 , it is necessary to cool at a rate of 0.5° C./second or more from point A ₃ to 550° C. or lower. When there are few hardenability-enhancing elements, etc., a cooling rate of 1° C./second or more is preferable. When the temperature after cooling exceeds 550°C or the cooling rate is slower than 0.5°C/sec, coarse pearlite is formed. Generally, most of the precipitation bands are called pearlite bands. Of course, if a large amount of alloying elements are added like stainless steel, pearlite bands will not be formed even if the cooling rate is slower than 0.5°C/sec.

Next, in the present invention, the structure of the free-cutting steel can be further homogenized by performing heat treatment at a temperature lower than or equal to 750° C. following the above-mentioned rapid cooling treatment.

In the actual manufacturing process, in order to further increase the stability of the product, although the amount of C is small, it is preferable to reduce the hardness deviation in the steel. For this reason, material unevenness can be reduced by maintaining high temperature again. First, in order to suppress coarse pearlite, it is important to rapidly cool from a temperature higher than or equal to _A3 point to lower than or equal to 550° C. at which coarse pearlite does not form. Then, as shown in Fig. 4, by maintaining the predetermined temperature T ₂ °C again and adjusting the hardness to meet the requirements of the customer, the variation in hardness can be reduced. By heating and maintaining at a temperature lower than or equal to 750°C, the hardness is adjusted to meet the requirements of those who need it.

As for the holding temperature T ₂ ℃, the holding temperature and holding time should be determined to meet the hardness requirements of those who need it. However, if the holding temperature T ₂ °C exceeds 750 °C, transformation to austenite begins, so if the cooling rate during recooling is slow, pearlite bands are formed. Therefore, the holding temperature T ₂ °C is specified to be lower than or equal to 750°C. Secondary processing such as wire drawing is often applied in subsequent processes, so it is preferable to adjust the holding temperature T ₂ °C of hardness suitable for the operation of these subsequent processes. The holding time is preferably 3 minutes or more since there is no change in hardness or the like compared to the case where the holding time is 3 minutes or less in industrial production.

Furthermore, in industrial production, due to the different dimensions of rolling or forging, temperature unevenness occurs even inside the steel. Therefore, it is necessary to consider the temperature T lower than or equal to 550°C after rapid cooling of coarse pearlite. Hold time at ₁ °C. After rapid cooling, the temperature T ₁ °C lower than or equal to 550°C is preferably maintained for 5 minutes or more, so that uniform ferrite transformation can be promoted regardless of material size or segregation bands. In this way, no coarse pearlite or pearlite bands will be formed even if the temperature is raised to the holding temperature T ₂ °C (≤750 °C) later. Conversely, in the case of a large size after rolling or forging, if the holding time at 550°C or less is shorter than 1 minute, the internal phase transformation will not be completed, so hold at a temperature higher than 550°C thereafter. When , coarse pearlite or coarse pearlite bands are formed.

Example

Example 1

The effects of the present invention will be described using examples. Table 1, Table 2 (continued from 1 of Table 1), Table 3 (continued from 2 of Table 1), Table 4 (continued from 3 of Table 1), Table 5 (continued from 4 of Table 1), Table 6 (continued from 5) Among the test materials shown, No.13 was smelted in a 270-ton converter, and the others were smelted in a 2-ton vacuum melting furnace, rolled into steel plates in pieces, and then rolled into 60 mm.

In the item of heat treatment in the table, the examples described as normalizing were kept at 920° C. for 10 minutes or more, and then air-cooled. In the example of the present invention, which is referred to as QT (Quenching and Tempering), it is quenched from 920° C. in a water tank at the rear end of the rolling line, and then kept at 710° C. for 1 hour or more during annealing. This adjusts the pearlite area ratio. In the example of the present invention, although the amount of C is low, the pearlite area ratio can also be reduced during normalizing.

The machinability evaluation of the materials shown in Examples 1 to 81 in Tables 1 to 6 was performed using a drill piercing test, and Table 7 shows the cutting conditions. Machinability was evaluated at the highest cutting speed (so-called VL1000, unit: m/min) capable of cutting to a cumulative hole depth of 1000 mm.

Further, the cut surface roughness, which indicates the surface quality during cutting, was evaluated. The cutting conditions are shown in Table 8, and the outline of the evaluation method (hereinafter referred to as plunge cutting test) is shown in Fig. 7(a) and Fig. 7(b). In the plunge cutting test, the tool cuts repeatedly for short periods of time. In one cutting, the tool does not move longitudinally along the cutting material, but moves toward the center of the rotating cutting material. Therefore, after a short period of cutting, the tool is pulled out, but its shape, basically the shape of the tip of the tool, is transferred to the surface of the cutting material. The surface finish of the transferred cutting face is affected due to built-up edge buildup or tool wear. The surface finish was measured with a surface finish meter. The 10-point surface roughness Rz (μm) is used as an index representing the surface roughness.

Inventive Examples 1 to 75 were all good in the life of the drilling tool compared to Comparative Examples 76 to 81, and at the same time, the surface roughness in plunge cutting was good. This is considered to be due to local embrittlement of the B ferrite and smooth surface creation, resulting in good surface roughness.

These surface roughness improvement effects are remarkable when S exceeds 0.5%, and even when the S content is less than 0.5%, the effect is seen in chip disposability.

In addition, the ratio of Mn and S is about 3, which is often seen in conventional steels, and the effect can be seen. However, if Mn/S is reduced, the tool life is further improved, and the surface finish is also improved. The reason for this is considered to be that fine MnS is also finely dispersed in ferrite in an environment where a large amount of B is added, and it effectively exerts both the lubricating effect and the embrittlement effect. However, as in Example 80, if the Mn/S ratio is too small, FeS is formed, thereby causing rolling cracks. Regarding the evaluation of the present invention, in Example 70, the machinability and the like were not evaluated at all due to rolling cracks, and the evaluation results are not shown in the table.

Even when the amount of C is changed (Tables 1 to 6, Examples 37 to 75), by adding a large amount of B and further controlling the pearlite area ratio, good tool life and smoothness of the cutting surface can be obtained.

Furthermore, in terms of chip handling properties, chips with a small curvature when the chips are curled, or chips that are divided are preferable. Therefore, chips whose curvature radius exceeds 20 mm and which are continuously curled for 3 or more rolls and are extended are regarded as defective. Chips with a large number of turns but a small radius of curvature, or chips with a large radius of curvature but a chip length of less than 100 mm are good.

Table 1

Table 2 (Table 1 continued 1) Chemical composition (mass%) heat treatment Pearlite area ratio (%) VL1000m/min Surface roughness Rz(μm) Chip disposal Example distinguish Ti Ca Zr Mg Te Bi Pb al Mn/S 1 Invention example 0.0011 3.26 Normalizing 1.5 147 10.5 ○ 2 Invention example 0.0013 2.84 Normalizing 0.6 155 10.4 ○ 3 Invention example 0.0023 2.98 QT 1.9 144 7.3 ○ 4 Invention example 0.0018 3.13 QT 0.7 157 6.6 ○ 5 Invention example 0.0013 3.13 QT 0.7 142 7.8 ○ 6 Invention example 0.0021 2.83 QT 2.0 152 6.2 ○ 7 Invention example 0.0019 3.24 QT 2.0 147 6.6 ○ 8 Invention example 0.0020 2.88 QT 1.4 157 7.4 ○ 9 Invention example 0.0017 3.11 QT 2.6 141 6.8 ○ 10 Invention example 0.0013 2.91 QT 0.6 145 6.5 ○ 11 Invention example 0.0020 3.19 Normalizing 5.5 130 10.8 ○ 12 Invention example 0.0017 2.92 QT 2.3 131 6.4 ○ 13 Invention example 0.0026 3.08 QT 2.7 126 6.3 ○ 14 Invention example 0.0024 3.14 QT 0.8 145 7.5 ○ 15 Invention example 0.0025 3.13 QT 2.6 146 7.7 ○ 16 Invention example 0.0023 2.84 QT 0.7 144 6.6 ○ 17 Invention example 0.0012 3.14 QT 2.8 147 6.8 ○ 18 Invention example 0.0025 3.29 QT 0.5 145 7.5 ○ 19 Invention example 0.0025 3.01 QT 1.5 147 7.0 ○ 20 Invention example 0.0023 2.89 QT 2.5 145 7.0 ○ twenty one Invention example 0.0016 3.03 QT 3.0 146 6.9 ○ twenty two Invention example 0.0011 3.24 QT 0.8 143 7.2 ○ twenty three Invention example 0.026 0.0030 2.96 QT 1.0 143 8.0 ○ twenty four Invention example 0.0037 0.0028 3.26 QT 1.3 145 7.2 ○ 25 Invention example 0.0037 0.0021 3.09 QT 3.0 144 6.9 ○ 26 Invention example 0.0025 0.0027 3.25 QT 2.9 146 7.7 ○ 27 Invention example 0.0030 0.0022 2.94 QT 1.0 144 7.9 ○ 28 Invention example 0.16 0.0012 3.02 QT 1.2 170 7.3 ○ 29 Invention example 0.283 0.0018 3.29 QT 1.3 170 6.4 ○ 30 Invention example 0.0153 2.88 QT 0.8 128 7.1 ○ 31 Invention example 0.0019 1.82 Normalizing 1.4 154 10.2 ○ 32 Invention example 0.0030 2.16 Normalizing 1.4 165 11.7 ○ 33 Invention example 0.0013 2.42 QT 2.0 156 3.9 ○ 34 Invention example 0.0020 2.25 QT 1.4 167 4.5 ○ 35 Invention example 0.0027 2.39 QT 0.7 153 4.1 ○

Table 3 (Table 1 continued 2)

Table 4 (Table 1 continued 3) Chemical composition (mass%) heat treatment Pearlite area ratio (%) VL1000m/min Surface roughness Rz(μm) Chip disposal Example distinguish Ti Ca Zr Mg Te Bi Pb Al Mn/S 36 Invention example 0.0028 2.01 QT 3.0 168 3.5 ○ 37 Invention example 0.0018 2.39 QT 2.2 154 3.4 ○ 38 Invention example 0.0014 2.11 QT 2.1 170 3.7 ○ 39 Invention example 0.0024 2.39 QT 0.5 156 3.5 ○ 40 Invention example 0.0027 2.00 QT 0.7 168 3.9 ○ 41 Invention example 0.0014 1.95 Normalizing 5.2 135 3.9 ○ 42 Invention example 0.0023 1.90 QT 2.5 131 3.6 ○ 43 Invention example 0.0029 1.95 QT 2.0 133 3.1 ○ 44 Invention example 0.0016 1.92 QT 1.0 155 3.4 ○ 45 Invention example 0.0015 1.82 QT 2.8 156 3.7 ○ 46 Invention example 0.0026 2.00 QT 1.9 155 3.3 ○ 47 Invention example 0.0012 2.39 QT 1.4 156 3.7 ○ 48 Invention example 0.0026 2.09 QT 0.6 155 3.6 ○ 49 Invention example 0.0012 2.00 QT 2.8 154 4.1 ○ 50 Invention example 0.0030 2.31 QT 1.4 156 4.2 ○ 51 Invention example 0.0019 2.02 QT 2.6 155 3.3 ○ 52 Invention example 0.0029 2.27 QT 0.8 153 4.8 ○ 53 Invention example 0.036 0.0016 2.12 QT 1.3 156 4.7 ○ 54 Invention example 0.0033 0.0017 1.89 QT 2.5 156 4.5 ○ 55 Invention example 0.0035 0.0024 2.14 QT 2.1 154 3.0 ○ 56 Invention example 0.0020 0.0013 1.82 QT 2.6 154 4.3 ○ 57 Invention example 0.0061 0.0022 2.21 QT 2.4 154 3.6 ○ 58 Invention example 0.16 0.0017 2.37 QT 2.8 182 2.6 ○ 59 Invention example 0.266 0.0031 2.02 QT 2.5 189 2.2 ○ 60 Invention example 0.0208 1.96 QT 1.9 136 3.5 ○ 61 Invention example 0.005 0.0010 2.70 QT 2.3 146 6.5 ○ 62 Invention example 0.0009 0.0021 2.95 QT 3.4 145 6.4 ○ 63 Invention example 0.0022 0.0025 0.0010 2.68 QT 2.9 145 6.6 ○ 64 Invention example 0.0018 0.0012 0.0011 2.45 QT 3.0 139 6.5 ○ 65 Invention example 0.0016 2.00 QT 2.5 172 7.3 ○ 66 Invention example 0.0030 0.0015 2.00 QT 2.8 134 6.5 ○ 67 Invention example 0.0012 2.00 QT 3.6 131 8.9 ○ 68 Invention example 0.0025 0.0015 0.0019 2.00 QT 2.1 130 6.1 ○ 69 Invention example 0.0016 2.50 QT 3.9 135 9.9 ○ 70 Invention example 0.0017 2.51 QT 2.3 133 7.2 ○ 71 Invention example 0.0025 0.0010 2.50 QT 3.9 132 6.5 ○

Table 5 (Table 1 continued 4) Chemical composition (mass%) Example distinguish C Si mn P S B Total N Total O V Nb Cr Mo W Ni Cu sn Zn 72 Invention example 0.059 0.009 1.38 0.075 0.55 0.0092 0.0132 0.0173 73 Invention example 0.069 0.009 1.62 0.076 0.54 0.0089 0.0095 0.0160 74 Invention example 0.062 0.006 1.80 0.090 0.60 0.0100 0.0106 0.0181 75 Invention example 0.058 0.002 1.65 0.079 0.55 0.0110 0.0122 0.0173 76 comparative example 0.045 0.007 1.00 0.084 0.35 0.0076 0.0074 0.0183 77 comparative example 0.050 0.005 1.79 0.074 0.59 0.0067 0.0062 0.0180 78 comparative example 0.049 0.008 0.96 0.077 0.34 0.0129 0.0141 0.0205 79 comparative example 0.055 0.009 1.78 0.080 0.59 - 0.0123 0.0151 80 comparative example 0.047 0.011 0.48 0.085 0.53 0.0089 0.0090 0.0167 81 comparative example 0.048 0.008 0.93 0.089 0.53 - 0.0139 0.0151

Table 6 (Table 1 continued 5) Chemical composition (mass%) heat treatment Pearlite area ratio (%) VL1000m/min Surface roughness Rz(μm) Chip disposal Example distinguish Ti Ca Zr Mg Te Bi Pb al Mn/S 72 Invention example 0.0016 2.51 QT 2.2 132 7.2 ○ 73 Invention example 0.0016 0.0010 0.0006 3.00 QT 2.6 134 9.1 ○ 74 Invention example 0.0010 3.00 QT 1.9 130 8.2 ○ 75 Invention example 0.0022 0.0017 0.0009 2.00 QT 2.9 130 6.4 ○ 76 comparative example 0.0012 2.90 Normalizing 5.8 97 17.0 x 77 comparative example 0.0013 3.05 Normalizing 5.8 119 21.1 ○ 78 comparative example 0.0017 2.83 Normalizing 5.8 100 24.4 ○ 79 comparative example 0.0011 3.03 Normalizing 5.3 119 24.2 ○ 80 comparative example 0.0013 0.90 - - - - - 81 comparative example 0.0027 2.81 Normalizing 5.9 117 24.6 x

Table 7 Cutting conditions drilling other Cutting speed 10-200 m/min Feed rate 0.33 mm/rev Water-soluble cutting oil 5mm NACHI ordinary drill overtravel 60mm Hole depth 15mm Tool life to breakage

Table 8 Plunge Cutting Conditions Cutting conditions tool other Cutting speed 80 m/min Feed rate 0.05 mm/rev Water-soluble cutting oil Equivalent to SKH57 Front Angle 20° Rear Angle 6° Overrun evaluation timing 200 cycles

Example 2

Table 9, Table 10 (Continued from 1 of Table 9), Table 11 (Continued from 2 of Table 9), Table 12 (Continued from 3 of Table 9), Table 13 (Continued from 4 of Table 9), Table 14 (Continued from Table 9 5) A part of the test material shown in 5) was smelted in a 270-ton converter, and cast at a cooling rate of 10 to 100° C./min. Rolled into steel plates in blocks, and then rolled into 50 mm. Others are smelted in a 2-ton vacuum melting furnace and rolled into 50 mm. At this time, the cooling rate of the ingot is adjusted by changing the cross-sectional size of the mold. The machinability of the material was evaluated by the drill piercing test under the conditions shown in Table 7 and the plunge cutting under the conditions shown in Table 8. The drilling and piercing test is a method of evaluating machinability at the highest cutting speed (so-called VL1000, unit: m/min) capable of cutting to a cumulative hole depth of 1000 mm. Plunge cutting is a method of evaluating the surface finish by transferring the shape of the cutting tool to the cutting tool. The outline of this experimental method is shown in FIG. 7( a ) and FIG. 7( b ). In this experiment, the surface roughness of 200 grooves was measured with a surface roughness meter. The 10-point surface roughness Rz (unit: μm) is used as an index representing the surface roughness.

The determination of the sulfide density with MnS having an equivalent circle diameter of 0.1 to 0.5 μm as the main component is taken from the Q part of the cross-section parallel to the rolling direction after 50 mm rolling by using the extraction replica method. Carried on. The measurement was performed at 10,000 times, 1 field of view 80 μm ² , and 40 fields or more, and this was converted into the number of sulfides having MnS as a main component per 1 square centimeter. Among the calculated values of formula (1) in Table 10, Table 12 and Table 14, those less than or equal to 1 are development steels satisfying the present invention.

As shown in Fig. 2(a) and Fig. 2(b), the size of MnS cannot be confirmed at the optical microscope level, but observed by transmission electron microscope, the inventive example and the comparative example have obvious differences in size and density.

In addition, the cutting resistance and cutting layer treatability in Table 10, Table 12, and Table 14 are as follows. A piezoelectric element type tool dynamometer (manufactured by Kisura Co., Ltd.) was attached to the turret of the lathe, and the tool was fixed thereon at the same position as in normal cutting, and plunge cutting was performed to measure cutting resistance. Thus, the main component force and the back component force of the load applied to the tool can be measured as voltage signals respectively. Cutting conditions such as cutting speed and feed rate are the same as evaluating the cutting surface finish.

With regard to chip disposability, it is preferable to select chips that have a small curvature or are broken when cutting curl. Therefore, chips whose curvature radius exceeds 20 mm and which are continuously curled for 3 or more rolls and are extended are regarded as defective. Chips with a large number of turns but a small radius of curvature, or chips with a large radius of curvature but a chip length of less than 100 mm are good.

With regard to machinability, both the drilling tools of the inventive examples and the comparative examples were excellent in life, and at the same time, the surface roughness in plunge cutting was also good. Especially for the surface roughness, a very good value can be obtained due to the complex precipitation effect of fine MnS and BN.

Table 9 distinguish steel Chemical composition (mass%) C Si mn P S Total N Total O B V Nb Cr Mo W Ni Cu Su Zn Ti Ca Invention example 1 0.051 0.012 0.83 0.076 0.56 0.0140 0.0202 0.0070 2 0.031 0.003 0.76 0.084 0.52 0.0124 0.0153 0.0066 3 0.021 0.005 1.05 0.079 0.54 0.0044 0.0177 0.0061 4 0.052 0.010 0.91 0.075 0.47 0.0148 0.0157 0.0059 5 0.053 0.009 1.45 0.071 0.61 0.0125 0.0184 0.0079 6 0.021 0.012 1.41 0.077 0.62 0.0051 0.0207 0.0079 7 0.053 0.005 1.72 0.077 0.60 0.0044 0.0202 0.0077 8 0.021 0.014 1.31 0.081 0.46 0.0113 0.0187 0.0068 9 0.057 0.013 1.07 0.080 0.54 0.0126 0.0181 0.0070 0.10 10 0.055 0.008 1.10 0.078 0.56 0.0051 0.0175 0.0079 0.005 11 0.052 0.011 1.17 0.079 0.59 0.0082 0.0202 0.0056 0.41 12 0.051 0.006 1.15 0.080 0.58 0.0121 0.0209 0.0066 0.36 13 0.029 0.010 0.93 0.089 0.48 0.0118 0.0194 0.0053 0.10 0.23 14 0.059 0.012 0.90 0.077 0.46 0.0110 0.0190 0.0057 0.11 0.28 15 0.055 0.005 0.98 0.076 0.50 0.0069 0.0208 0.0066 0.28 16 0.021 0.008 1.03 0.087 0.52 0.0078 0.0200 0.0078 0.23 17 0.031 0.010 0.90 0.088 0.48 0.0067 0.0158 0.0054 0.03 0.0065 18 0.052 0.004 0.89 0.078 0.45 0.0071 0.0181 0.0073 0.0100 19 0.053 0.011 0.95 0.086 0.49 0.0120 0.0190 0.0073 0.038 20 0.023 0.008 1.04 0.077 0.53 0.0135 0.0205 0.0079 0.0018 twenty one 0.039 0.002 1.09 0.061 0.55 0.0128 0.0151 0.0062 twenty two 0.051 0.008 1.05 0.076 0.54 0.0102 0.0208 0.0051 twenty three 0.053 0.008 1.11 0.083 0.57 0.0077 0.0162 0.0078 twenty four 0.029 0.010 0.98 0.088 0.50 0.0065 0.0184 0.0057 25 0.053 0.004 1.13 0.080 0.57 0.0169 0.0109 0.0066 26 0.051 0.011 1.04 0.077 0.53 0.0092 0.0160 0.0076 27 0.065 0.005 0.67 0.087 0.46 0.0152 0.0165 0.0050 28 0.064 0.010 0.75 0.082 0.52 0.0048 0.0161 0.0075 29 0.111 0.010 1.03 0.071 0.53 0.0053 0.0200 0.0056 30 0.055 0.014 1.12 0.080 0.57 0.0064 0.0162 0.0075

Table 10 (Table 9 continued 1) distinguish steel Chemical composition (mass%) Cooling rate during casting (°C/min) Rolling final temperature (°C) TEM replica MnS density (pieces/mm2) BN composite precipitation rate (%) VL1000(m/min) Surface finish (μmRz) Cutting resistance (N) Chip disposal Calculated value of formula (1) Zr Mg Te Bi Pb Al back force main component Invention example 1 0.002 100 1097 353565 20 145 6.7 65 390 ○ 0.09 2 0.004 72 1073 249998 15 149 5.4 73 342 ○ 0.06 3 0.004 64 1020 328542 29 142 7.0 86 358 ○ 0.13 4 0.003 55 1035 262595 25 148 4.1 64 383 ○ 0.14 5 0.003 47 1029 166778 16 149 8.9 87 385 ○ 0.19 6 0.002 34 1055 178854 29 133 8.4 72 352 ○ 0.16 7 0.002 37 1079 148887 12 142 7.4 71 332 ○ 0.16 8 0.001 92 1031 305248 28 140 7.9 67 339 ○ 0.07 9 0.004 66 1176 299171 18 131 5.2 84 331 ○ 0.05 10 0.004 14 1104 82353 twenty two 136 5.9 90 350 ○ 0.06 11 0.005 37 1098 186895 16 141 8.8 80 368 ○ 0.29 12 0.002 28 1181 142954 28 140 4.6 83 342 ○ 0.16 13 0.002 82 1173 384851 27 144 4.5 72 381 ○ 0.21 14 0.005 88 1096 394447 20 132 4.4 62 336 ○ 0.17 15 0.003 97 1145 432218 18 141 5.0 67 367 ○ 0.05 16 0.003 67 1101 260532 26 139 4.4 72 380 ○ 0.01 17 0.001 39 1165 120677 twenty two 143 6.7 62 342 ○ 0.19 18 0.003 77 1116 266822 12 137 4.2 78 355 ○ 0.05 19 0.002 87 1012 407007 twenty one 135 5.8 69 377 ○ 0.02 20 0.002 86 1001 333280 11 148 6.1 73 346 ○ 0.01 twenty one 0.0020 0.003 92 1153 366185 12 147 4.5 69 380 ○ 0.13 twenty two 0.0038 0.002 54 1103 303000 twenty three 138 5.3 69 367 ○ 0.26 twenty three 0.0029 0.0026 0.006 82 1124 285444 twenty four 147 4.3 62 379 ○ 0.08 twenty four 0.0020 0.005 38 1129 243854 10 134 6.1 74 360 ○ 0.15 25 0.256 0.002 80 1018 365823 twenty two 145 5.6 66 332 ○ 0.13 26 0.16 0.001 95 1199 309532 10 139 4.7 75 387 ○ 0.02 27 0.002 77 1131 255448 13 134 6.7 83 363 ○ 0.28 28 0.003 20 1173 146979 20 145 4.3 84 366 ○ 0.01 29 0.002 47 1089 260872 18 145 8.9 66 332 ○ 0.17 30 0.004 91 1133 281096 twenty two 145 6.9 65 369 ○ 0.09

Table 11 (Table 9 continued 2) distinguish steel Chemical composition (mass%) C Si mn P S Total N Total O B V Nb Cr Mo W Ni Cu Su Zn Ti Ca Invention example 31 O.116 O.003 1.37 0.073 0.55 0.0119 0.0208 0.0078 32 0.077 0.004 1.39 0.070 0.56 0.0089 0.0168 0.0060 33 0.071 0.007 1.32 0.084 0.46 0.0135 0.0154 0.0063 34 0.102 0.013 1.36 0.088 0.48 0.0140 0.0177 0.0077 35 0.054 0.003 1.59 0.073 0.56 0.0133 0.0163 0.0067 36 0.056 0.007 1.57 0.075 0.55 0.0139 0.0183 0.0060 37 0.159 0.011 0.74 0.084 0.51 0.0115 0.0194 0.0054 38 0.176 0.004 0.73 0.072 0.50 0.0147 0.0167 0.0059 39 0.177 0.014 0.97 0.071 0.49 0.0053 0.0177 0.0075 40 0.182 0.004 1.04 0.080 0.53 0.0105 0.0166 0.0053 41 0.150 0.004 1.29 0.073 0.49 0.0124 0.0189 0.0056 42 0.199 0.012 1.42 0.087 0.57 0.0120 0.0174 0.0075 43 0.189 0.015 1.30 0.073 0.45 0.0104 0.0160 0.0076 44 0.165 0.010 1.33 0.080 0.46 0.0148 0.0209 0.0067 45 0.171 0.007 1.34 0.077 0.47 0.0177 0.0156 0.0078 46 0.19l 0.009 1.56 0.089 0.55 0.0112 0.0153 0.0065 47 0.051 0.008 1.03 0.086 0.51 0.0110 0.0050 0.0072 O.005 48 0.031 0.003 1.03 0.078 0.52 0.0100 0.0185 0.0115 0.0020 49 0.053 0.004 1.02 0.080 0.53 0.0103 0.0159 0.0078 0.0019 50 0.084 0.008 1.01 0.082 0.52 0.0084 0.0040 0.0112 51 0.065 0.006 1.01 0.08l 0.46 0.0110 0.0152 0.0100 52 0.057 0.008 1.03 0.080 0.53 0.0109 0.0156 0.0132 53 0.049 0.008 1.05 0.082 0.50 0.0112 0.0125 0.0112 54 0.079 0.010 0.99 0.072 0.47 0.0113 0.0145 0.0108 55 0.082 0.008 1.34 0.080 0.67 0.0106 0.012l 0.0035 56 0.064 0.010 1.12 0.079 0.50 0.0112 0.0134 0.0105 0.006 57 0.055 0.010 1.15 0.074 0.49 0.0108 0.0127 0.0114 O.0015 58 0.070 0.010 1.20 0.071 0.51 0.0112 0.0184 0.0112 0.0018 59 0.076 0.009 0.81 0.077 0.30 0.0111 0.0147 0.0121 60 0.081 0.008 1.34 0.079 0.64 0.0109 0.0156 0.0121

Table 12 (Table 9 continued 3) distinguish steel Chemical composition (mass%) Cooling rate during casting (°C/min) Rolling final temperature (°C) TEM replica MnS density (pieces/mm2) BN composite precipitation rate (%) VL1000(m/min) Surface finish (μmRz) Cutting resistance (N) Chip disposal Calculated value of formula (1) Zr Mg Te Bi Pb al back force main component Invention example 31 0.003 16 1057 86221 14 132 7.6 82 386 ○ 0.04 32 0.002 45 1120 142738 15 147 7.9 79 338 ○ 0.18 33 0.002 16 1017 61245 10 149 7.0 65 371 ○ 0.11 34 0.004 78 1110 272514 28 133 7.8 70 349 ○ 0.01 35 0.17 0.002 77 1168 262609 15 135 4.9 63 344 ○ 0.10 36 0.298 0.002 twenty one 1106 81541 18 146 5.0 61 335 ○ 0.15 37 0.003 52 1100 194907 16 145 5.5 73 351 ○ 0.19 38 0.002 59 1085 301851 15 132 6.9 80 378 ○ 0.13 39 0.001 twenty two 1191 125206 30 145 6.7 74 382 ○ 0.01 40 0.003 74 1125 262061 11 135 5.0 75 358 ○ 0.21 41 0.003 twenty three 1036 108319 19 144 7.6 67 331 ○ 0.16 42 0.002 50 1163 170214 17 133 8.7 87 379 ○ 0.09 43 0.003 11 1171 50750 25 137 6.7 67 366 ○ 0.04 44 0.004 69 1098 234200 10 138 7.0 83 388 ○ 0.07 45 0.286 0.004 53 1095 289829 14 148 6.8 89 332 ○ 0.02 46 0.20 0.003 53 1089 186791 twenty two 147 6.0 80 333 ○ 0.10 47 0.002 89 1011 416010 26 140 5.5 66 354 ○ 0.02 48 0.0018 0.001 85 1000 333350 13 144 6.2 72 344 ○ 0.35 49 0.0021 0.001 86 1003 353921 12 139 6.1 70 352 ○ 0.02 50 0.0010 0.003 20 1173 146542 twenty two 145 4.5 84 366 ○ 0.29 51 0.002 78 1130 253458 twenty one 145 4.0 81 352 ○ 0.13 52 0.001 79 1126 262337 20 140 4.1 82 362 ○ 0.77 53 0.001 65 1002 189562 20 140 4.1 82 345 ○ 0.28 54 0.001 82 1121 252563 twenty one 135 4.4 84 361 ○ 0.23 55 0.001 54 1056 164512 20 140 4.1 81 361 ○ 1.02 56 0.001 77 1096 132654 17 135 5.1 82 375 ○ 0.17 57 0.0012 0.001 78 1059 192563 14 135 5.6 84 375 ○ 0.32 58 0.0014 0.001 62 1100 189562 15 135 5.7 81 352 ○ 0.29 59 0.0011 0.001 50 1058 123654 16 140 4.9 86 362 ○ 1.11 60 0.001 51 1123 165842 14 135 5.2 83 374 ○ 0.78

Table 13 (Table 9 continued 4) distinguish steel Chemical composition (mass%) C Si mn P S Total N Total O B V Nb Cr Mo W Ni Cu Su Zn Ti Ca Invention example 61 0.060 0.008 1.45 0.080 0.65 0.0112 0.0132 0.0050 62 0.061 0.011 0.75 0.076 0.33 0.0104 0.0112 0.0110 63 0.068 0.008 1.51 0.081 0.58 0.0132 0.0156 0.0110 64 0.072 0.009 0.71 0.072 0.30 0.0122 0.0125 0.0110 65 0.082 0.008 0.88 0.077 0.34 0.0118 0.0135 0.0043 comparative example 66 0.081 0.003 0.93 0.077 0.31 0.0099 0.0170 67 0.072 0.010 0.75 0.076 0.24 0.0069 0.0184 68 0.097 0.017 0.90 0.072 0.30 0.0095 0.0175 69 0.067 0.006 0.92 0.077 0.30 0.0142 0.0168 70 0.069 0.011 0.84 0.088 0.28 0.0130 0.0177 71 0.089 0.012 0.37 0.070 0.12 0.0103 0.0191 72 0.092 0.019 0.31 0.079 0.11 0.0166 0.0174 73 0.096 0.014 0.40 0.089 0.13 0.0173 0.0177 74 0.064 0.035 0.94 0.070 0.01 0.0133 0.0158 0.0035 75 0.079 0.036 0.50 0.071 0.17 0.0126 0.0178 0.0013 76 0.090 0.012 0.34 0.081 0.12 0.0167 0.0183 0.0030 77 0.089 0.015 0.98 0.073 0.32 0.0134 0.0205 0.0038

Table 14 (Table 9 continued 5) distinguish steel Chemical composition (mass%) Cooling rate during casting (°C/min) Rolling final temperature (°C) Density of TEM replica MnS (pieces/mm2) BN composite precipitation rate (%) VL1000(m/min) Surface finish (μmRz) Cutting resistance (N) Chip disposal Calculated value of formula (1) Zr Mg Te Bi Pb Al back force main component Invention example 61 0.002 71 1005 212365 16 140 5.0 81 366 ○ 0.61 62 0.001 70 1022 196354 14 140 6.2 86 379 ○ 0.71 63 0.002 56 1006 156235 20 145 5.1 82 354 ○ 0.35 64 0.001 69 1215 142562 19 140 4.9 83 362 ○ 0.89 65 0.001 72 1231 212365 17 135 5.1 85 374 ○ 0.79 comparative example 66 0.004 6 865 232 0 92 17.7 173 451 x 2.36 67 0.004 7 820 194 0 95 19.4 169 512 x 2.82 68 0.002 5 784 214 0 66 18.2 188 452 ○ 2.45 69 0.001 2 831 53 0 83 15.5 201 466 ○ 2.41 70 0.002 5 814 192 0 99 15.4 217 497 x 2.54 71 0.001 8 763 227 0 73 18.7 210 454 x 4.03 72 0.003 4 799 161 0 79 18.5 155 524 ○ 4.24 73 0.004 3 821 141 0 66 19.9 189 464 ○ 3.95 74 0.002 8 844 207 0 75 17.8 152 500 x 4.39 75 0.001 2 774 57 0 93 16.9 209 481 x 3.02 76 0.003 6 891 180 1 93 17.9 217 486 ○ 3.07 77 0.004 6 827 154 1 83 15.3 199 523 ○ 1.10

Industrial Utilization Possibility

As described above, the present invention can be applied to automotive component materials and general machine component materials because of the characteristics of excellent tool life during cutting, smoothness of the cut surface, and chip disposability.

Claims (10)

1. the steel of an excellent in machinability is characterized in that, it be by quality % by

C：0.005～0.2％、

Si：0.001～0.5％、

Mn：0.2～3.0％、

P：0.001～0.2％、

S：0.03～1.0％、

Total N:0.002～0.02%,

Total O:0.0005～0.035%,

Surplus is the steel that Fe and unavoidable impurities constitute, satisfy Mn/S in the steel and be 1.2～2.8 or particle diameter surpasses 1 μ m in the microstructure of steel pearlitic area occupation ratio be either party or two sides that are less than or equal in 5%, and the surface smoothness Rz of steel is less than or equal to 11 μ m.

2. the steel of an excellent in machinability, it is characterized in that, contain C:0.005～0.2%, Mn:0.3～3.0%, S:0.1～1.0% by quality %, about using the MnS that takes with the extraction replica method of transmission electron microscope observation, in the section parallel, be that to have density be more than or equal to 10000/millimeter for the MnS of 0.1～0.5 μ m with diameter of equivalent circle with the rolling direction of steel ², and the cutting surface smooth finish Rz of steel becomes and is less than or equal to 11 μ m.

3. the steel of excellent in machinability as claimed in claim 1 or 2 is characterized in that, also contains B:0.0005～0.05 quality %.

4. the steel of excellent in machinability as claimed in claim 1, it is characterized in that, about using the MnS that takes with the extraction replica method of transmission electron microscope observation, in the section parallel, be that to have density be more than or equal to 10000/millimeter for the MnS of 0.1～0.5 μ m according to diameter of equivalent circle with the rolling direction of steel ²

5. the steel of excellent in machinability as claimed in claim 1, it is characterized in that, also contain the S amount is limited in 0.25～0.75 quality %, the B amount is limited in 0.002～0.014 quality %, and S and B content satisfy the interior S and the B amount of A shown in Figure 4, B, C, D area surrounded of following formula 1, and in MnS, comprise the sulfide of separating out BN

(B-0.008) ²/ 0.006 ²+ (S-0.5) ²/ 0.25 ²≤ 1 ... formula (1).

6. the steel of the excellent in machinability described in claim 1 or 2 is characterized in that, also contains by quality %

V：0.05～1.0％、

Nb：0.005～0.2％、

Cr：0.01～2.0％、

Mo：0.05～1.0％、

W：0.05～1.0％、

Ni：0.05～2.0％、

Cu：0.01～2.0％、

Sn：0.005～2.0％、

Zn：0.0005～0.5％、

Ti：0.0005～0.1％、

Ca：0.0002～0.005％、

Zr：0.0005～0.1％、

Mg：0.0003～0.005％、

Te：0.0003～0.05％、

Bi：0.005～0.5％、

Pb：0.01～0.5％、

Al：≤0.015％

More than a kind or a kind.

7. as the manufacture method of the steel of each described excellent in machinability in the claim 1～3, it is characterized in that the molten steel with the described composition of steel of claim 1 is after casting, with 10～100 ℃/minute speed of cooling cooling, from A ₃O'clock carry out cooling after the hot rolling with 0.5 ℃/s or the speed of cooling more than it to 550 ℃ scope.

8. as the manufacture method of the steel of claim 4 or 5 described excellent in machinability, it is characterized in that, will have the molten steel casting of the composition that claim 2 puts down in writing after, with 10～100 ℃/minute speed of cooling cooling, then the hot rolled outlet temperature is limited in 1000 ℃ or more than it, from A ₃O'clock carry out cooling after the hot rolling with 0.5 ℃/second or speed of cooling more than it to 550 ℃ scope.

9. as the manufacture method of the steel of each described excellent in machinability in the claim 1～6, it is characterized in that, after the cooling after hot rolling, will be limited in 750 ℃ or below it again for the Heating temperature of adjusting hardness.

10. as the manufacture method of the steel of each described excellent in machinability in the claim 7～9, it is characterized in that above-mentioned steel also contains by quality %

V：0.05～1.0％、

Nb：0.005～0.2％、

Cr：0.01～2.0％、

Mo：0.05～1.0％、

W：0.05～1.0％、

Ni：0.05～2.0％、

Cu：0.01～2.0％、

Sn：0.005～2.0％、

Zn：0.0005～0.5％、

Ti：0.0005～0.1％、

Ca：0.0002～0.005％、

Zr：0.0005～0.1％、

Mg：0.0003～0.005％、

Te：0.0003～0.05％、

Bi：0.005～0.5％、

Pb：0.01～0.5％、

Al：≤0.015％

In more than a kind or a kind.

Images