CN101184859A - Low carbon sulfur free cutting steel - Google Patents

Abstract

A low-carbon sulfur free-cutting steel, containing C: 0.05% to 0.20%, Si: less than 0.02%, Mn: 0.7-2.2%, P: 0.005-0.25%, S: 0.40%- 0.60% but not including 0.40%, Al: less than 0.003%, O: 0.0090-0.0280%, N: 0.0030-0.0250%, the balance is composed of Fe and impurities, and Ca, Mg, Ti, Zr and REM in impurities are , Ca: less than 0.001%, Mg: less than 0.001%, Ti: less than 0.002%, Zr: less than 0.002%, and REM: less than 0.001%, and satisfy Mn×O>0.018 and 2.5<Mn/( S+O) < 3.5, the low-carbon sulfur free-cutting steel has machinability equal to or higher than that of Pb free-cutting steel and Pb-added composite free-cutting steel in relatively low-speed cutting using HSS tools, and is also excellent in carburization. Since such steel has excellent continuous castability, it can be mass-produced at low cost, and can be used as a raw material for soft small parts such as brake parts for automobiles, personal computer peripheral parts, and electrical equipment parts.

Description

Low carbon sulfur free cutting steel

technical field

The present invention relates to a low-carbon sulfur free-cutting steel, and in detail relates to an existing lead free-cutting steel (hereinafter referred to as "Pb free-cutting steel") and a complex addition of Pb even without the addition of Pb. Composite free-cutting steel with other free-cutting elements such as , S and P (hereinafter referred to as "Pb-added composite free-cutting steel") is equivalent to or above low-carbon sulfur free-cutting steel with good machinability. More specifically, it relates to a Pb-free low-carbon sulfur free-cutting steel that can be mass-produced cheaply and mass-produced because it has good machinability when cutting with a high-speed steel tool, is excellent in carburization, and is also excellent in continuous castability.

Background technique

In the past, the raw material of soft small parts, such as brake parts for automobiles, personal computer peripheral equipment parts and electrical equipment parts, etc., used the so-called "free cutting machine" with excellent machinability in order to improve productivity. steel".

The cutting of this kind of soft small parts is mainly carried out in the relatively low speed area below 100m/min in industry. In addition, high-speed steel tools (hereinafter referred to as "HSS tools") that have not been coated are often used as tools for cutting. In addition, under such cutting conditions, as the "machinability" of the raw material steel, in addition to ensuring a long tool life, it is also required that the surface roughness of the steel material surface after cutting is small from the viewpoint of machining accuracy. , In addition, the property of finely cutting chips (hereinafter referred to as "chip disposability") is also valued. In particular, good chip disposability is indispensable for the automation of processing lines, and is an essential characteristic for improving productivity.

Free-cutting steels include sulfur free-cutting steels (hereinafter referred to as "S free-cutting steels") in which a large amount of S is added to improve machinability, Pb-added free-cutting steels, and Pb-added composite free-cutting steels. wait.

Among the above-mentioned free-cutting steels, Pb free-cutting steels and Pb-added composite free-cutting steels also have the characteristics of excellent chip disposability, long tool life, and excellent machined surface roughness of the processed steel surface.

Therefore, these Pb-added free-cutting steels are machined into the shapes of various soft small parts such as brake parts for automobiles, personal computer peripheral parts, and electrical equipment parts by cutting, and are used as the final production. In addition, for the purpose of ensuring strength, various small parts after cutting are sometimes subjected to carburizing treatment for surface hardening to increase surface hardness before being used as final products.

However, due to the increasing emphasis on global environmental issues in recent years, there is an extremely strong desire for free-cutting steel with reduced Pb content and free-cutting steel that does not contain Pb at all. For example, in Europe, according to RoHS (On the restriction of the use of certain hazardous Substances electrical and electronic equipment) directive and ELV (End of Life Vehicle) directive, the Pb content contained in steel is limited to 0.35% or less by mass %, etc., in order to reduce the Pb content as much as possible.

Also, since Pb has a low melting point and hardly dissolves in steel, steel containing a large amount of Pb is prone to cracks during rolling. Therefore, from the standpoint of stable production of steel, free-cutting steels with reduced Pb content and free-cutting steels that do not contain Pb at all are highly desired.

In order to meet such desires, in Patent Documents 1 to 10, various free-cutting steels that replace Pb free-cutting steels and Pb-added composite free-cutting steels are proposed.

The most famous ones, such as Patent Documents 1 to 4, are low-carbon sulfur free-cutting steels in which machinability is improved by increasing the amount of S instead of adding Pb.

In addition, as in Patent Documents 5 to 10, a large number of free-cutting steels in which the form of inclusions in the steel are controlled have been proposed in large quantities. For the purpose of improving machinability, the addition of B and Ti to the S free-cutting steel controls the amount of inclusions in the steel. form of inclusions.

Specifically, Patent Document 1 proposes a "low-carbon sulfur-based free-cutting steel" that contains more than 0.4% of S to increase MnS and does not add Pb.

In Patent Document 2, a "free cutting steel" is proposed which contains more than 0.50% of S and increases MnS to improve machinability.

In Patent Document 3, a "low-carbon sulfur free-cutting steel" is proposed, which contains 0.4% or more of S and further adds Sn to improve machinability.

Patent Document 4 discloses "low-carbon sulfur-based free-cutting steel and its manufacturing method" in which machinability is improved by adjusting the average width of sulfide and the yield ratio of the wire rod together.

Patent Document 5 discloses a "high-sulfur free-cutting steel" that improves machinability by refining sulfide-based inclusions by adding appropriate amounts of Ti, Al, and Zr.

In Patent Document 6, there is disclosed a "sulfur-containing free-cutting steel, a method for manufacturing the free-cutting steel, and a machining method for the free-cutting steel", which does not substantially add Al as a deoxidizer, but uses a sulfide The inclusions are oxysulfides to improve machinability.

Patent Documents 7 to 9 disclose a "steel excellent in machinability and its manufacturing method" or a "steel excellent in machinability" in which the microstructure is adjusted by adjusting the composition of the steel, or by Improve machinability by dispersing fine MnS.

In Patent Document 10, there is disclosed a "low-carbon free-cutting steel" proposed by the present inventors, which contains specific amounts of C, Mn, S, Ti, Si, P, Al, O, and N, The content of Ti and S satisfies the following formula (i), while the atomic ratio of Mn to S satisfies the following formula (ii), and Ti sulfide or/and Ti carbide contains inner MnS.

Ti(mass%)/S(mass%)＜1... (i)

Mn/S≥1%...(ii)

[Patent Document 1] JP-A-2000-319753

[Patent Document 2] JP-A-2000-160284

[Patent Document 3] JP-A-2002-249848

[Patent Document 4] JP-A-2003-253390

[Patent Document 5] JP-A-2004-269912

[Patent Document 6] JP-A-2002-363691

[Patent Document 7] Japanese Patent Laid-Open No. 2004-169051

[Patent Document 8] JP-A-2004-169052

[Patent Document 9] JP-A-2004-169054

[Patent Document 10] JP-A-2003-226933

In the "low-carbon sulfur-based free-cutting steel" disclosed in the aforementioned Patent Document 1, the composition of Mn, S, O, and N, etc. is not fully considered, and only the amount of S is added. Therefore, when cutting with an HSS tool at a relatively low speed of 100 m/min or less, the desired inclusions that can simultaneously improve the roughness of the machined surface and chip handling properties cannot be obtained, and therefore the desired good machinability cannot be ensured. .

In the case of the "free cutting steel" disclosed in Patent Document 2, the improvement of the machinability by the HSS tool was surely confirmed. However, since the amount of S is only increased without taking into account the form of sulfide, the roughness of the processed surface is large, and the desired low roughness of the processed surface may not be obtained.

"Low-carbon sulfur free-cutting steel" disclosed in Patent Document 3 discloses that increasing the amount of O affects the morphology of MnS to improve machinability. Indeed, even in steel containing a high content of S, it is important to increase the amount of O in order to optimize the morphology of MnS. However, if the amount of O is only increased, a large amount of coarse sulfides will be generated, so the chip treatability will deteriorate. Also, in the technology disclosed in Patent Document 3, the oxide composition is not optimized at the same time. Therefore, in the cutting using the HSS tool at a relatively low speed of 100 m/min or less, the desired good machinability cannot be ensured.

In the "low-carbon sulfur-based free-cutting steel" disclosed in Patent Document 4, the diameter of the steel wire rod is d, and only the form of sulfide in the area from 0.1mm below the outer peripheral surface to d/8 is controlled, while the sulfide form in the deeper area is controlled. Forms such as sulfide forms in the center of steel wire rods are not considered. Therefore, when other than the surface portion is cut by machining using an HSS drill, etc., it is not possible to achieve both excellent chip control and long tool life.

In the "high-sulfur free-cutting steel" disclosed in Patent Document 5, the forms of generated sulfides and oxides are not preferable for cutting using HSS tools. Therefore, in the cutting using the HSS tool at a relatively low speed of 100 m/min or less, the desired small roughness of the machined surface cannot be obtained.

In the "sulfur-containing free-cutting steel" disclosed in Patent Document 6, machinability can be improved by the presence of oxysulfides, but the chemical composition of the steel is not carefully considered. Therefore, for cutting using HSS tools in a relatively low-speed range of 100 m/min or less, sulfides and oxides with suitable morphology cannot be obtained, and the desired good machinability cannot be ensured.

In the "steel excellent in machinability" disclosed in Patent Documents 7 to 9, component elements such as Al, Ti, and Zr, which greatly affect the morphology of MnS, are added without sufficiently considering the component composition. In this case, for cutting using the HSS tool, sulfides and oxides having appropriate morphologies cannot be obtained, chip disposability and machined surface roughness deteriorate, and desired good machinability cannot be ensured.

The "low-carbon free-cutting steel" disclosed in Patent Document 10 is definitely superior in tool life r in high-speed cutting using a cemented carbide tool compared with Pb free-cutting steel, and can obtain excellent chip controllability. However, when cutting with an HSS tool at a relatively low speed of 100 m/min or less, the sulfide form is not suitable for improving the machinability, so the roughness of the machined surface is large, and it is found that the desired small roughness of the machined surface cannot be obtained. .

As mentioned above, the conventionally proposed free-cutting steels have various characteristics of machinability required as raw materials for soft small parts such as brake parts for automobiles, personal computer peripheral equipment parts, and electrical equipment parts, that is, At least any one of tool life, chip disposability, and machined surface roughness in cutting using HSS tools in a relatively low speed range of 100 m/min or less is inferior to Pb free-cutting steel and Pb-added composite free-cutting steel. That is, none of the previously proposed free-cutting steels can be combined with Pb free-cutting steels containing a large amount of Pb and Pb-added composites in terms of machinability in cutting using HSS tools in a relatively low-speed region of 100 m/min or less. Free cutting steel is completely equivalent.

Furthermore, the above-mentioned conventionally proposed free-cutting steels do not have good continuous castability required at the manufacturing stage for inexpensive mass production and heat treatment characteristics required as products. That is, for various small parts after cutting, carburizing treatment is performed for the purpose of ensuring strength, and when the surface hardness is increased before being used as a product, good "carburization properties" are required. However, the above-mentioned conventionally proposed free-cutting steels are not necessarily excellent in "carburization properties". In addition, the "continuous castability" required in the production stage for inexpensive mass production is not necessarily excellent.

Contents of the invention

Therefore, the object of the present invention is to provide a low-carbon sulfide free-cutting steel whose machinability when cutting using an HSS tool at a relatively low speed range of 100 m/min or less even without adding Pb is comparable to that of the existing steel. Some Pb free-cutting steels and Pb-added composite free-cutting steels are at the same level or higher, have excellent carburization properties, and are also suitable for mass production by continuous casting.

The present inventors first investigated the machinability in cutting with an HSS tool in a relatively low speed range of 100 m/min or less using Pb-free S free-cutting steel.

As a result, the following conclusion (a) was obtained. Also, in the "Mn-based sulfides" described below, unless otherwise specified, a composite compound containing MnS and Mn expressed by the chemical formula of Mn(S, X), such as Mn(S, Te), Mn(S , Se), Mn (S, O) and Mn (S, Se, O), etc. X is Te, Se and O, and is an element other than S that combines with Mn.

(a) In the case of S free-cutting steel, by increasing the amount of O (oxygen) in the steel, it is considered that coarse Mn-based sulfides are generated and the machinability is improved. However, in cutting using HSS tools in the aforementioned cutting speed range, simply increasing the amount of O will only coarsen the Mn-based sulfides, making it difficult to improve machinability, especially chip disposability.

Therefore, next, the relationship between the morphology of the Mn-based sulfide and the machinability in cutting using the HSS tool in the aforementioned cutting speed range was studied in detail. The results show that not only the size of the Mn-based sulfide greatly affects the roughness of the machined surface and the chip handling properties, but also its dispersion form greatly affects it, and the following (b) to (d) are obtained. ) conclusion.

(b) When the amount of O in the steel is increased to coarsely crystallize the Mn-based sulfide, the roughness of the machined surface is reduced and improved, but the chip treatability deteriorates. That is, when coarse Mn-based sulfides are plastically deformed as chips during cutting, they function as stress concentration points, and cracks originating from the Mn-based sulfides occur, thereby suppressing the growth of built-up edge and improving the appearance of the machined surface. The roughness is reduced and improved, the resistance force in the chip shearing area is weakened, the cutting resistance is reduced, and the tool life is prolonged. On the other hand, when the Mn-based sulfides are coarsened, cracks propagate inefficiently inside the chips, and the chips are broken, thereby degrading the cutting performance.

(c) When the Mn-based sulfide is crystallized in a fine form, the chip treatability is improved, but the roughness of the machined surface is increased and deteriorated. That is, a large amount of fine Mn-based sulfides crystallized by the eutectic reaction at the time of solidification have high deformability, so they are cut in the state of being elongated by casting and rolling, or by further rolling after elongation. And become a subtle state to be observed. These finely crystalline Mn-based sulfides are easily deformed when they are plastically deformed as chips during cutting, and therefore deform when shear stress is applied to the chips during deformation. Furthermore, starting from the deformed Mn-based sulfide, the chip handleability is improved until the chips are brittle and fractured. On the other hand, since the secondary shear domain around the built-up edge is further subjected to strong processing, the above-mentioned Mn-based sulfide is further cut and miniaturized, and cracks effective for cutting built-up edge and chips do not occur, but into the built-up edge. As a result, the growth of built-up edge cannot be suppressed, so the roughness of the machined surface becomes large and deteriorates.

(d) From the conclusions of (b) and (c) above, it can be known that in order to reduce and improve the roughness of the machined surface, it is necessary to disperse Mn-based sulfides. The role of Mn-based sulfides is to cut off built-up edge and chips. Sufficiently large cracks are generated, that is, Mn-based sulfides having a large width are dispersed in the state before cutting, which will not be cut even if subjected to strong processing in the secondary shear region. In addition to improving the roughness of the machined surface, in order to improve the chip disposability and efficiently propagate cracks originating from Mn-based sulfides, it is necessary to increase the width of the coarse cracks generated by Mn-based sulfides. The distribution density increases.

Therefore, the conditions for increasing the distribution density of the wide Mn-based sulfides to improve the two characteristics of the machined surface roughness and chip disposability were further studied in detail. As a result, the conclusions (e) to (i) were obtained, and the following conclusion (j) was obtained regarding the tool life.

(e) In order to increase the distribution density of wide Mn-based sulfides, it is necessary to increase the absolute amount of Mn-based sulfides. For this reason, S must be contained in a range exceeding 0.4%.

(f) For Mn-based sulfides with a large width, when the "shortest average inter-particle distance" which is the average distance to the nearest Mn-based sulfide is small, cracks propagate efficiently and the cutting of chips is promoted, even when Pb is not contained , It is also possible to obtain the same chip disposability as Pb free-cutting steel and Pb-containing composite free-cutting steel.

(g) In order to increase the distribution density of Mn-based sulfides with a large width, it is necessary to disperse a large amount of Mn-based sulfides that are the nuclei of Mn-based sulfides in the solidification stage according to the amount of S. The amount of steel and the amount of O increase, and it is necessary to balance the composition of the steel, especially the content of Mn, S, and O, as well as the content of Al, Si, and the content of Ca, Mg, Ti, Zr, and REM in impurities. In addition, the generation frequency of Mn-based oxides is related to the concentration product of Mn and O, that is, "Mn×O".

(h) Distributing the Mn-based oxides formed in the early stage of solidification with a sufficient density, and using them as nuclei to form Mn-based sulfides through a monotectic reaction, thereby enabling a large distribution density, a large width, Mn-based sulfides with a small shortest average interparticle distance exist.

(i) N does not affect the appropriate form of Mn-based sulfides and oxide composition in improving machinability, but is solid-soluted in ferrite to improve chip disposability, so it can be contained in a sufficient amount.

(j) If a large amount of fine Mn-based oxides are dispersed in ferrite grains, the tool life is improved and prolonged.

The present invention is complete based on the above-mentioned conclusions, and its gist lies in the low-carbon sulfur free-cutting steel shown in the following (1) to (4).

(1) A low-carbon sulfur free-cutting steel, wherein, by mass%, C: 0.05% to less than 0.20%; Si: less than 0.02%; Mn: 0.7-2.2%; P: 0.005-0.25% %; S: 0.40% to 0.60% but not including 0.40%; Al: less than 0.003%; O: 0.0090 to 0.0280%; , Ti, Zr and REM are, Ca: less than 0.001%; Mg: less than 0.001%; Ti: less than 0.002%; Zr: less than 0.002% and REM: less than 0.001%, and satisfy the following formula (1) and (2).

Mn×O>0.018...(1)

2.5＜Mn/(S+O)＜3.5...(2)

However, the symbols of the elements in the formulas (1) and (2) represent the content of the element in the steel in mass %.

(2) The low-carbon sulfur free-cutting steel according to the above (1), wherein the content of N is N: 0.0060% to 0.0250% in mass %.

(3) The low-carbon sulfur free-cutting steel according to the above (1) or (2), wherein Te: 0.0005 to 0.03%; Sn: 0.001% or more and less than 0.50%; and Se : One or more of 0.0005% or more and less than 0.30%.

(4) The low-carbon sulfur free-cutting steel according to any one of the above (1) to (3), wherein Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cr : 0.01 to 1.0% and Mo: 0.01 to 0.5% or more.

Hereinafter, the inventions of the low-carbon sulfur free-cutting steels of the above (1) to (4) are referred to as "the present invention (1)" to "the present invention (4)", respectively. In addition, it is collectively referred to as "the present invention".

In the present invention, "REM" is a general term for a total of 17 elements including Sc, Y, and lanthanide elements, and the content of REM refers to the total content of the above elements.

Although the steel of the present invention is a "free-cutting steel that is beneficial to the global environment" without adding Pb, when it is cut with an HSS tool at a relatively low speed of 100m/min, it has the same properties as the existing free-cutting steel with Pb and the composite addition of Pb. Pb added composite free-cutting steel with other free-cutting elements such as Pb, S, and P has good machinability equal to or better than that of long tool life, good chip handling and small machined surface roughness, and infiltration It has excellent carbon properties, and because it is excellent in continuous castability, it can be mass-produced at low cost. Therefore, it can be used as a raw material of soft small parts such as brake parts for automobiles, personal computer peripheral parts, and electrical equipment parts.

Detailed ways

First, the chemical composition of the low-carbon sulfur free cutting steel of the present invention and the reason for its limitation will be described. In addition, in the following description, "%" of content of each element shows "mass %".

C: More than 0.05% and less than 0.20%

C is an important element that greatly affects the pluckability. When machinability is emphasized in the application of steel, if C is contained at 0.20% or more, the strength of the steel will increase and the machinability will deteriorate. Cracking will accelerate tool wear, and the roughness of the processed surface will also increase and deteriorate. Therefore, the content of C is not less than 0.05% and less than 0.20%. In addition, in order to obtain better machinability, the content of C is preferably 0.06 to 0.18%.

Si: less than 0.02%

Si is a powerful deoxidizing element with a strong affinity with O (oxygen). If it is contained at 0.02% or more, the form and oxide composition of Mn-based sulfides suitable for improving machinability cannot be obtained. Therefore, at 100m/min The machinability of HSS tools deteriorates in the following relatively low-speed ranges. Therefore, the content of Si is less than 0.02%. In addition, since Si greatly affects the form and oxide composition of Mn-based sulfides, it should not be added but must be removed as much as possible during refining. In order to obtain more excellent machinability, the content of Si is preferably less than 0.01%.

Mn: 0.7-2.2%

Mn forms Mn-based sulfides together with S, and is an important element that greatly affects machinability. When the content is less than 0.7%, the absolute amount of Mn-based sulfides is insufficient, and desired good machinability cannot be obtained, and hot workability also deteriorates. Mn also has the effect of improving carburization. Therefore, to obtain good carburization, the content of Mn can be increased. However, Mn is a Mn-based sulfide-forming element, and it also contributes to deoxidation. Therefore, in order to improve carburization Simply increasing the content of Mn for the purpose of safety still cannot obtain the desired morphology of inclusions. In order to obtain the desired shape of the inclusions, it is necessary to add Mn on the basis of taking sufficient care of the mass balance of S and O (oxygen). However, even in this case, if the Mn content exceeds 2.2%, the desired shape of inclusions cannot be obtained, and the machinability will deteriorate. Therefore, the content of Mn is 0.7 to 2.2%. In addition, in order to achieve both desired good machinability and good carburization properties, the content of Mn is preferably 1.2 to 1.8%.

Also, the above-mentioned so-called "Mn-based sulfide" refers to a composite compound of MnS and Mn expressed by the chemical formula of Mn(S, X), such as Mn(S, Te), Mn(S, Se), Mn( S, O) and Mn (S, Se, O), etc., X is Te, Se, and O, and is an element other than S that binds to Mn.

P: 0.005～0.25%

P has the effect of weakening the strength of grain boundaries and improving machinability. In order to obtain the above effects, the content of P needs to be 0.005% or more. On the other hand, if the P content is too high, the strength of the steel will increase, which will cause a decrease in machinability. In particular, if the P content exceeds 0.25%, the strength will become too high and the machinability will decrease significantly. In addition, when the content of P exceeds 0.25%, the segregation of the steel ingot is promoted, so that the hot workability also decreases. Therefore, the content of P is 0.005 to 0.25%. In order to stably obtain more excellent machinability, the content of P is preferably 0.03 to 0.15%.

S: 0.40%～0.60% but not including 0.40%

S forms a Mn-based sulfide together with Mn, and is an essential element for improving machinability. The effect of improving the machinability by the Mn-based sulfide is not only determined by the amount of its formation, but also changes according to its form and dispersion state. For this reason, the balance between the content of S and the content of Mn and O (oxygen) becomes important. However, when the content of S is 0.40% or less, even if the balance with the content of Mn and O (oxygen) is properly adjusted, the If the amount of the Mn-based sulfide is less than a sufficient amount, the dispersion form of the Mn-based sulfide for obtaining the desired good machinability cannot be obtained. In addition, in general, if the content of S exceeds 0.35%, the hot workability is reduced, so it becomes the cause of the so-called "internal cracks" inside the slab, but by adjusting the content of Mn and O (oxygen) If the balance is appropriate, even if the S content exceeds 0.35%, internal cracks will not be caused, and high machinability can be achieved. However, when the content of S exceeds 0.60%, it is necessary to contain a large amount of Mn so that the deterioration of hot ductility does not occur. However, since Mn functions as a deoxidizing element, sufficient oxygen cannot be secured, which impairs Mn-based vulcanization. In essence, it is difficult to obtain the desired form and dispersion state of the Mn-based sulfide. In addition, the addition of excess S exceeding 0.60% in terms of content causes deterioration of yield and leads to increase in yield. Therefore, the S content is made to be more than 0.40% and not more than 0.60%. In addition, in order to ensure more stable and excellent machinability and to obtain a desired form of Mn-based sulfide without deteriorating manufacturability, the S content is preferably 0.45 to 0.55%.

Al: less than 0.003%

Al is a powerful deoxidizing element with a strong affinity for O (oxygen). If it is contained above 0.003%, the form and oxide composition of Mn-based sulfides suitable for improving machinability cannot be obtained. Therefore, it should be below 100 m/min The machinability deterioration under the use of HSS tools in the relatively low-speed area. Therefore, the content of Al is less than 0.003%. In addition, Al has a great influence on the form and oxide composition of Mn-based sulfides, so not only is it not added, but it needs to be removed as much as possible during refining. In order to obtain more excellent machinability, the content of Al is preferably less than 0.002%.

O: 0.0090～0.0280%

After optimizing the balance of the contents of Mn and S, by increasing the O (oxygen) content, the morphology of the Mn-based sulfide can be changed to improve the machinability. However, when the O content is less than 0.0090%, the shape of inclusions for obtaining desired good machinability cannot be obtained, and sufficient machinability cannot be ensured. On the other hand, if the content of O exceeds 0.0280%, not only the desired shape of inclusions cannot be obtained, but also coarse oxides are formed to induce cracks during rolling. Therefore, the content of O is 0.0090 to 0.0280%. In addition, in order to stably secure the desired shape and dispersion state of inclusions, the content of O is preferably 0.0100 to 0.0200%.

N: 0.0030～0.0250%

Even if the content of N is increased, it will not affect the form and oxide composition of Mn-based sulfides suitable for improving machinability. Moreover, in the present invention that does not contain Al and Ti substantially, since hard Al and Ti Since nitrides of Ti are hardly formed, N exists in a solid solution state in ferrite. The above-mentioned N dissolved in ferrite has the effect of improving chip treatability. However, when the content of N is less than 0.0030%, the effect of sufficiently improving the chip treatability cannot be obtained. On the other hand, if the N content exceeds 0.0250%, not only the above-mentioned effects are saturated, but also the production cost increases. Therefore, the content of N is 0.0030 to 0.0250%. In addition, in order to obtain good machinability, N is 0.0060% or more, and in order to obtain good machinability more effectively, N is preferably contained in an amount of 0.0080% or more.

In the low-carbon sulfur free-cutting steel of the present invention, the contents of Ca, Mg, Ti, Zr, and REM among impurities are limited as follows.

Ca: less than 0.001%, Mg: less than 0.001%, Ti: less than 0.002%, Zr: less than 0.002%, and REM: less than 0.001%

In general free cutting steels, Ca, Mg, Ti, Zr and REM are all elements added to improve machinability. However, the above-mentioned elements from Ca to REM all have adverse effects on the morphology and oxide composition of Mn-based sulfides and the dispersion state of these inclusions. Machinability in cutting is reduced. In particular, among the impurities, with respect to the above-mentioned Ca, Mg, Ti, Zr, and REM, when any one of Ca, Mg, and REM is contained at 0.001% or more, and any one of Ti and Zr is contained at 0.002% or more, Then, the machinability during cutting using the HSS tool in the above-mentioned cutting speed range is significantly reduced. Therefore, the content of Ca, Mg, Ti, Zr and REM in impurities needs to be, Ca: less than 0.001%, Mg: less than 0.001%, Ti: less than 0.002%, Zr: less than 0.002% and REM: low at 0.001%. All of the above-mentioned Ca, Mg, Ti, Zr, and REM among impurities are preferably 0.0005% or less.

In addition, as already mentioned, "REM" is a generic term for a total of 17 elements including Sc, Y, and lanthanide elements, and the content of REM refers to the total content of these elements.

Concentration product of Mn and O (Mn×O): more than 0.018

Contains elements from C to N in the above range, and the balance is composed of Fe and impurities. Among the impurities, Ca, Mg, Ti, Zr, and REM are Ca: less than 0.001%, Mg: less than 0.001%, and Ti: less than 0.002%, Zr: less than 0.002% and REM: less than 0.001%, when the concentration product of Mn and O, that is, the value of "Mn×O" exceeds 0.018, use it in a relatively low speed area below 100m/min. In the cutting of HSS tools, the desired excellent machinability can be ensured.

Therefore, the value of Mn×O, which is the concentration product of Mn and O, needs to exceed 0.018, that is, the above formula (1) needs to be satisfied. In addition, the element symbols in the above formula "Mn×O" indicate the content of the element in steel in mass %, and the upper limit of the value of Mn×O is preferably 0.30. When the value of Mn×O exceeds 0.30, the distribution density of wide Mn-based sulfides is not so high, and it may be difficult to obtain good machined surface roughness and chip handling properties.

Mn/(S+O): more than 2.5, less than 3.5

Contains elements from C to N in the above range, and the balance is composed of Fe and impurities. Among the impurities, Ca, Mg, Ti, Zr, and REM are Ca: less than 0.001%, Mg: less than 0.001%, and Ti: less than 0.002%, Zr: less than 0.002%, and REM: less than 0.001% of steel, when the value of Mn/(S+O) exceeds 2.5, the Mn-based oxides that are the nuclei of Mn-based sulfides can be formed during the solidification stage. A large amount of dispersion can increase the distribution density of the wide-width Mn-based sulfides, so desired good machinability can be obtained. In addition, when the value of Mn/(S+O) is 2.5 or less, cracks or the like will occur inside the slab when it is manufactured by continuous casting, thereby reducing the hot workability. However, Mn/(S+O) When the value of exceeds 2.5, sufficient hot workability suitable for mass production on an industrial scale can be ensured.

On the other hand, when the value of Mn/(S+O) is 3.5 or more, excess Mn is contained relative to the contained S and O, and the amount of solid-dissolved Mn in the structure is excessive, resulting in poor machinability and tool life. deteriorating. In addition, in the present invention that does not substantially contain Ca, Mg, Ti, Zr, and REM, since Mn also functions as a deoxidizing element, if Mn is contained excessively, it is not possible to obtain a suitable material for improving machinability. Due to the sufficient amount of O in the form of the Mn-based sulfide, the chip disposability decreases and the roughness of the machined surface also increases.

Therefore, the value of Mn/(S+O) exceeds 2.5 and is less than 3.5, that is, the above formula (2) needs to be satisfied. In addition, the symbol of an element in the above formula "Mn/(S+O)" indicates the content of the element in steel in mass %.

From the above-mentioned reasons, the chemical composition of the low-carbon sulfur free-cutting steel of the present invention is specified to contain elements from C to N in the above-mentioned range, and the balance is composed of Fe and impurities. Among the impurities, Ca, Mg, Ti, Zr and REM is Ca: less than 0.001%, Mg: less than 0.001%, Ti: less than 0.002%, Zr: less than 0.002%, and REM: less than 0.001%, and satisfies the above formulas (1) and (2) .

In addition, the chemical composition of the low-carbon sulfur free-cutting steel of the present invention (2) is defined such that the content of N in the low-carbon sulfur free-cutting steel of the present invention (1) is N: 0.0060 to 0.0250%.

In the low-carbon sulfur free-cutting steel of the present invention, if necessary, instead of a part of Fe, one or more elements selected from the first group described later and one or more elements selected from the second group may be added and contained. One or both of the elements can be added as arbitrary elements.

Hereinafter, arbitrary additive elements of the above-mentioned first group and second group will be described.

Group 1: Te: 0.0005 to 0.03%; Sn: 0.001% or more and less than 0.50%; Se: 0.0005% or more and less than 0.30%

None of Te, Sn, and Se will damage the shape of inclusions suitable for improving machinability, and have the effect of improving machinability. Therefore, when an HSS tool is used in a relatively low-speed range of 100 m/min or less, it may be contained in the following range when more excellent machinability is desired.

Te: 0.0005～0.03%

Te forms Mn(S, Te) together with Mn, and this Mn(S, Te) functions similarly to a lubricating effect in cutting. Moreover, even adding Te only increases the ratio of the wide Mn-based sulfides without affecting the oxide morphology, so the machinability in cutting using HSS tools in the above-mentioned cutting speed range is improved. . However, when its content is less than 0.0005%, the effect of addition is lacking. On the other hand, even if Te is contained in excess of 0.03%, the effect is saturated, the cost increases, and the hot workability also deteriorates. Therefore, the Te content at the time of addition is 0.0005 to 0.03%. In addition, in order to achieve both good hot workability and good machinability more stably, the Te content is preferably 0.003 to 0.02%, more preferably 0.003 to 0.01%.

Sn: more than 0.001% and less than 0.50%

Sn has the effect of improving the machinability of steel. This is believed to be due to its embrittlement effect on the matrix. However, when its content is less than 0.001%, the effect of addition is lacking. On the other hand, even if Sn is contained at 0.50% or more, the effect is saturated, and the hot workability also deteriorates. Therefore, the content of Sn at the time of addition is 0.001% or more and less than 0.50%. In addition, in order to achieve both good hot workability and good machinability, the content of Sn is preferably 0.03% or more and 0.30% or less.

Se: more than 0.0005% and less than 0.30%

Se forms Mn(S, Se) together with Mn, and this Mn(S, Se) acts like a lubricating effect in cutting. Moreover, even if Se is added, the ratio of the wide Mn-based sulfides will only be increased without affecting the oxide morphology. Therefore, the machinability of HSS tools in cutting in the above-mentioned cutting speed range is improved. . However, when its content is less than 0.0005%, the effect of addition is lacking. On the other hand, even if Se is contained at 0.30% or more, the effect is saturated, the cost increases, and the hot workability also deteriorates. Therefore, the content of Se at the time of addition is 0.0005% or more and less than 0.30%. In addition, in order to achieve both good hot workability and good machinability more stably, the Se content is preferably 0.005% or more and 0.15% or less.

Any one of the aforementioned Te, Sn, and Se may be added alone, or two or more of them may be added in combination.

Group 2: Cu: 0.01-1.0%, Ni: 0.01-1.0%, Cr: 0.01-1.0%, Mo: 0.01-0.5%

Cu, Ni, Cr, and Mo all have the effect of increasing the strength of steel. Therefore, when it is desired to improve the product strength, it may be contained within the following ranges.

Cu: 0.01 to 1.0%

Cu has the effect of increasing the strength of steel by precipitation strengthening. However, when its content is less than 0.01%, the effect of addition is lacking. On the other hand, if the content of Cu exceeds 1.0%, the hot workability will be deteriorated, and Cu precipitates will be coarsened. Therefore, not only the above-mentioned effect is saturated, but also the machinability will be reduced. Therefore, the content of Cu at the time of addition is 0.01 to 1.0%. In addition, in order to stably combine good strength and good hot workability, the content of Cu is preferably 0.03 to 0.50%, and in order to stably combine better strength and good hot workability, it is more preferable that the content of Cu is 0.05-0.50%.

Ni: 0.01 to 1.0%

Ni has a function of increasing the strength of steel by precipitation strengthening. However, when its content is less than 0.01%, the effect of addition is lacking. On the other hand, if the Ni content exceeds 1.0%, the machinability will be deteriorated, and the hot workability will also be deteriorated. Therefore, the content of Ni at the time of addition is 0.01 to 1.0%. In addition, in order to stably have good strength, machinability, and hot workability, the Ni content is preferably 0.03 to 0.50%.

Cr: 0.01 to 1.0%

Cr has the effect of increasing the strength of steel. Cr also has the effect of improving the hardenability of steel, thereby improving carburization. However, when its content is less than 0.01%, the effect of addition is lacking. On the other hand, even if Cr is contained in excess of 1.0%, the above effects are saturated, and the machinability is lowered in addition to cost increase. Therefore, the content of Cr at the time of addition is 0.01 to 1.0%. In addition, in order to stably have good strength, hardenability and machinability, the content of Cr is preferably 0.02 to 0.5%, and in order to stably have better strength, hardenability and machinability, it is more preferable that the content of Cr is 0.03%. ~0.5%.

Mo: 0.01 to 0.5%

Mo has the effect of increasing the strength of steel. Mo also has the effect of improving the hardenability of steel to improve carburization, and the effect of refining the structure to improve toughness. However, when its content is less than 0.01%, the effect of addition is lacking. On the other hand, even if Mo is contained in excess of 0.5%, the above-mentioned effect is saturated, and the machinability is lowered in addition to cost increase. Therefore, the content of Mo at the time of addition is 0.01 to 0.5%. In addition, in order to stably have good strength, hardenability, toughness and machinability, the content of Mo is preferably 0.05 to 0.5%. In addition, in order to have good strength, hardenability, toughness, and machinability while suppressing the production cost, the content of Mo is preferably 0.02 to 0.3%.

Any one of the above-mentioned Cu, Ni, Cr, and Mo may be added alone, or two or more of them may be added in combination.

Based on the above reasons, the chemical composition of the low-carbon sulfur free-cutting steel of the present invention (3) is defined as containing Te: 0.0005% to 0.03%, Sn: 0.001% or more and less than 0.50% and Se: 0.0005% or more and less than One or more of 0.30% is substituted for a part of Fe in the low-carbon sulfur free-cutting steel of the present invention (2).

In addition, the chemical composition of the low-carbon sulfur free-cutting steel of the present invention (4) is specified to contain Cu: 0.01-1.0%, Ni: 0.01-1.0%, Cr: 0.01-1.0%, and Mo: 0.01-0.5%. One or more kinds of Fe in any one of the low-carbon sulfur free-cutting steels of the present invention (1) to the present invention (3) is substituted for a part of Fe.

In addition, the dispersion form and oxide composition of Mn-based sulfides depend on the solidification rate and production conditions. Therefore, the low-carbon sulfur free cutting steel of the present invention can be industrially mass-produced as follows, for example.

First, when the low-carbon sulfur free-cutting steel of the present invention is produced by the continuous casting method, the state of the tapping stage from the steelmaking furnace such as a converter to the ladle and the slag refining stage in the ladle are adjusted.

Specifically, at the start of ladle refining, the amount of Mn contained in molten steel is adjusted to less than 1.5%, preferably less than 1.2%. At this stage, even if the molten steel contains 1.5% or more Mn, it can be adjusted to the above-mentioned range in the end, but in order to obtain the appropriate form of oxides and Mn-based sulfides, it is preferable to adjust the Mn at the beginning of refining in the above-mentioned manner. Mn content. While adjusting the Mn content, it is more preferable to adjust the MnO content in the slag at the start of refining to an appropriate range, specifically, to a range of 25 to 40%. Then, from the second half to the end of refining, it is sufficient to make Mn a predetermined content by adding alloy iron.

Next, in order to obtain an appropriate form of Mn-based sulfide, the cooling rate during casting is adjusted.

That is, since the cooling rate of the slab differs greatly between the skin and the center, in order to stably make Mn-based sulfides with a large width and a small shortest average interparticle distance exist at a high distribution density, the cooling rate at the center must be at least Cooling is performed at 1° C./minute or more, more preferably at 2° C./minute or more.

Also, when producing a steel ingot by the ingot casting method, as in the case of casting a small steel ingot, when the cooling rate is fast, the cooling rate at the center of the steel ingot may be 20° C./min or less. On the contrary, in the case of casting a huge steel ingot, when the cooling rate is slow, the casting mold may be arranged so that the cooling rate of the central part is 1° C./min or more.

Hereinafter, the present invention will be described in more detail by way of examples.

【Example】

Using a high-frequency induction furnace, steels 1-57 having the chemical compositions shown in Tables 1-3 were smelted to produce 150-180 kg steel ingots with a diameter of about 220 mm.

Steels 1 to 23 in Table 1 are steels having a chemical composition within the range specified by the present invention (hereinafter referred to as "examples of the present invention"). On the other hand, steels 24 to 41 in Table 2 and steels 42 to 57 in Table 3 are steels of comparative examples whose chemical compositions deviate from the conditions specified in the present invention. In addition, among the steels of the comparative examples, steels 55 to 57 are steels corresponding to conventional Pb free-cutting steels.

Among the above-mentioned steels, steels 29, 30, 34, 36, and 50 to 57, which are examples of the present invention 1 to 23 and comparative examples, control the amount of dissolved oxygen and solidification in the smelting stage. Ingots crafted for speed. That is, after the raw material iron is burned through, MnO powder sold in the state of being coated in iron foil is added to the molten steel at the stage of adding auxiliary raw materials, and then the composition is adjusted, and the steel is tapped at a temperature around 1600°C to In the mold. In addition, in order to adjust the solidification rate of steel, the method of adjustment is to tap steel into a ceramic crucible surrounded by sand so that the solidification rate becomes an appropriate rate.

On the other hand, steels 24 to 28, steels 31 to 33, and 35, and steels 37 to 49 were not melted by the above-mentioned special method among the steels of the comparative examples. That is, after adding the auxiliary raw material, casting is performed in a normal mold without putting MnO powder or a ceramic crucible surrounded by sand.

【Table 1】

steel Chemical composition (mass%) balance: Fe and impurities The value of Mn×O The value of Mn/(S+O) C Si Mn P S Al N o Ca Mg Ti Zr REM other 1234567891011121314151617181920212223 0.080.100 090.120.180.060.070.080.070.100.090.070.090.100.080.100.120.090.080.080.080.090.08 0.0050.0040.0070.0060.0090.0120.0100.0040.0100.0090.0070.0050.0060.0110.0050.0070.0030.0010.0090.0070.0040.0060.008 1.451.381.661.891.491.521.151.351.491.441.491.641.701.491.541.421.511.711.511.361.511.441.41 0.0670.0950.0870.0920.0650.0720.1200.0720.0810.0750.0930.0740.0780.0850.0770.0540.1390.0830.0340.0530.0700.0620.079 0.500.480.550.580.500.460.420.460.410.560.410.460.510.430.450.460.460.540.540.470.540.510.49 0.0010.0010.0020.0010.0020.0020.0010.0010.0020.0010.0020.0020.0010.0020.0020.0010.0010.0010.0010.0010.0010.001 0.00900.01050.00850.00950.01300.00820.00700.01020.01200.01020.01110.00980.00940.00840.00950.01390.01970.01150.05760.012000.01 0.01880.01680.01520.01090.01250.01350.02000.01720.01370.01450.01900.01600.01680.01340.01950.01420.01390.01310.01470.016470.01 0.00040.00020.00030.00030.00010.00030.00030.00020.00040.00020.00020.00020.00020.00060.00020.00050.00040.00020.00020.00020.0020.00 0.00020 00020.00010.00030.00030.00030.00020.00020.00010.00030.00020.00040.00020.00030.00030.00040.00020.00060.00020.00010020.0000 0.00020.00020.00030.00120.00110.00030.00020 00040.00040.00010.00030.00050.00030.00040.00070.00030.00010.00060.00060.00020.0020.00 0 00020 00010.00010 00030.00040.00120 00030.00030.00030.00040.00020.00040.00010.00020.00030.00030.00070.00060.00020.000300.000 0.00010.00050.00040.00030.00030.00040.00040.00040.00010.00010.00030.00030.00040.00010.00040.00020.00040.00020.050010.000040.0000 -------Cr: 0.30Mo: 0.15Cu: 0.26Ni: 0.20Te: 0.008Te: 0.015Sn: 0.25Se: 0.042Te: 0.005, Cr: 0.09Te: 0.010, Mo: 0.05Te: 0.015, Cu : 0.29Se: 0.055, Ni: 0.15Te: 0.007, Sn: 0.25Cu: 0.20, Ni: 0.15Cr: 0.25, Cu: 0.20, Te: 0.0008- 0.0270.0230.0250.0210.0190 0210.0230.0230.0200.0210.0290.0290.0290.0200.0300.0200.0210.0220.0220.0200.0250.0220.020 2.792.782.943.202.913.212.612.833.362.513.473.453.233.363.282.993.183.082.702.802.712.742.80

【Table 2】

steel Chemical composition (mass%) balance: Fe and impurities The value of Mn×O The value of Mn/(S+O) C Si Mn P S Al N o Ca Mg Ti Zr REM other 242526272829303132333435363738394041 0.090.070.080.090.09*0.04*0.230.100.090.060.080.070.090.080.120.080.100.09 0.0040.0150.0080.0070.0080.0040.004*0.2500.0090.0040.0040.0070.0030.0060.0040.0050.0090.009 1.030.991.231.271.901.581.681.35*2.55*0.551.531.801.231.341.071.510.951.17 0.0320.0260.0610.0580.0490.0720.0750.0960.0870.041*0.3200.1290.0640.0270.0320.0700.0680.095 *0.29*0.24*0.34*0.35*0.720.510.500.520.520.510.490.520.460.420.410.500.410.53 0.0010.0010.0020.0010.0020.0020.0020.0020.0020.0020.002*0.0150.0020.0010.0010.0010.0020.001 0.00700.00610.00650.00860.01100.01100.01270.00850.00790.00850.01330.0098*0.00200.00610.00640.00700.01150.0104 *0.0080*0.00480.01680.01880.01480.01340.0120*0.00420.01010.01820.0160*0.00340.01870.00990.01340.0108*0.02950.0162 0.00020.00010.00030.00030.00040.00030.00010.00030.00030.00020.00050.00050.00020.00020.00030.00030.00020.0005 0.00020.00040.00020.00010.00010.00050.00050.00010.00040.00020.00020.00030.00050.00030.00030.00040.00040.0002 0.00020.00020.00010.00070.00070.00020.00020.00010.00030.00020.00040.00040.00040.00050.00030.00040.00050.0002 0.00020.00030.00040.00050.00020.00020.00030.00050.00010.00020.00020.00040.00040.00040.00020.00030.00020.0005 0.00020.00040.00030.00040.00040.00030.00030.00040.00040.00020.00050.00020.00030.00050.00050.00050.00010.0003 ------------------ *0.008*0.0050.0210.0240.0280.0210.020*0.0060.026*0.0100.024*0.0060.023*0.013*0.014*0.0160.0280.019 3.46*4.043.453.442.593.023.302.56*4.81*1.043.023.442.573.122.532.96*2.16*2.14 *Indicates that it is outside the specified conditions of the present invention.

【table 3】

steel Chemical composition (mass%) balance: Fe and impurities The value of Mn×O The value of Mn/(S+O) C Si Mn P S Al N o Ca Mg Ti Zr REM other 42434445464748495051525354555657 0.080.090.090.070.100.090.070.080.090.070.070.080.090.100.100.07 0.0110.0080.0040.0050.0010.0180.0140.0010.0050.0060.0030.0070.0020.0060.0070.007 2.040.981.381.651.731.471.351.561.681.511.491.471.561.301.141.02 0.0780.0770.0330.0750.0580.0340.0450.0380.0600.0490.0550.0610.0780.0700.0720.070 0.410.520.450.470.510.470.420.530.470.490.420.500.48*0.31*0.32*0.32 0.0020.0020.0010.0020.0010.0010.0010.0010.0020.0010.0020.0020.0010.0010.0010.002 0.00860.01250.00650.00870.00990.00650.00850.00830.00900.01020.01040.01270.0084*0.0057*0.0056*0.0052 0.00980.01270.0122*0.0065*0.0081*0.0019*0.00240.01070.01130.01630.01360.01540.01330.0112*0.00850.0160 0.00020.00010.0003*0.00200.00040.00030.00030.00010.00040.00020.00020.00020.00040.00030.00030.0002 0.00030.00040.00020.0001*0.00150.00040.00020.00030.00030.00040.00020.00020.00050.00010.00040.0004 0.00040.00020.00020.00040.0002*0.0540.00030.00010.00020.00050.00030.00010.00040.00030.00040.0003 0.00020.00020.00030.00030.00030.0004*0.0460.00020.00060.00040.00040.00030.00020.00050.00020.0003 0.00020.00010.00020.00010.00030.00040.0006*0.00130.00050.00040.00010.00030.00020.00030.00040.0005 --------*Cr: 1.30*Mo: 0.70*Te: 0.058*Sn: 0.65*Se: 0.35*Pb: 0.25*Pb: 0.20*Pb: 0.31 0.020*0.012*0.017*0.011*0.014*0.003*0.003*0.0170.0200.0250.0200.0230.021*0.015*0.011*0.016 *4.86*1.842.993.433.333.123.202.913.492.983.442.853.16*4.053.473.04 *Indicates that it is outside the specified conditions of the present invention.

A high temperature tensile test piece with a diameter of 10 mm and a length of 130 mm was extracted from the direction of the steel ingot around the position of the Di/8 portion (where "Di" is the diameter of the steel ingot) close to the surface of the steel ingot of each of the above-mentioned steels, and the thermal conductivity was investigated. Processability. That is, using a thermal processing reproduction test device, after high-frequency heating to 1250°C in the atmosphere and keeping it for 5 minutes, cooling to 900°C at a rate of 10°C/min, and keeping it for 10 seconds, the strain rate is 10 seconds ^-1 at 900 ℃ high temperature tensile test to adjust the hot workability. In addition, the heating area of the above-mentioned rod-shaped test piece was about 20 mm in the central part in the longitudinal direction, and it was rapidly cooled immediately after the high-temperature tensile test. In the above, the reason why 900°C is selected as the temperature for the high-temperature tensile test is because generally in the case of low-carbon free-cutting steel, the necking value of high-temperature stretching at 900°C is at the minimum point.

The hot workability was evaluated by the neck shrinkage ratio (%) in the above-mentioned high-temperature tensile test. In addition, the target of hot workability is to have a neck-in value of 40% or more in the above-mentioned high-temperature tensile test. Also, in this case, even if the steel contains a high amount of S exceeding 0.4%, it is possible to stably produce cast slabs without generating internal cracks during continuous casting.

In addition, the machinability and carburization properties of each steel were investigated by the following method.

That is, the remainder of the steel ingot with a diameter of about 220 mm of each steel was heated to 1200° C. and kept for 2 hours or more, then hot forged at a final temperature of 1000° C. or higher, and air-cooled after forging to produce a round bar with a diameter of 40 mm. Next, each of the above-mentioned round rods was heated to 950° C. and held for 1 hour, then air-cooled and normalized. In addition, Steel 33 was not subjected to the following investigations because cracks occurred during hot forging.

Next, a part of the above-mentioned round bar with a diameter of 40 mm was peeled to form a round bar with a diameter of 31 mm, which was subjected to cold drawing processing to finally become a round bar with a diameter of 28 mm.

The thus obtained round bar with a diameter of 28 mm was used as a test material, and was turned with an uncoated HSS tool, specifically, an SKH4 (JIS G 4403 (2000)) turning insert under the following conditions, and the investigation was carried out. Being cut.

Cutting speed: 100m/min,

Feed rate: 0.05mm/rev,

Cutting depth: 0.5mm,

Lubrication: Wet lubrication using water-soluble lubricating oil.

That is, after turning continuously for 1 minute under the above conditions, the maximum height roughness Rz was measured using a stylus type roughness meter, and the processed surface roughness was evaluated.

In addition, the chip disposability was evaluated by measuring the mass of 20 chips discharged in the above-mentioned 1 minute from the long chip in order. The smaller the value of this mass, the better the chip disposability can be judged. In addition, regarding the poor chip handling property, long chips were discharged, and as a result, 20 chips could not be obtained, and the mass when converted from the number and quality of the chips to 20 chips.

In addition, tool wear was evaluated by measuring the amount of tip wear after cutting for 30 minutes under the same conditions as above. The above-mentioned roughness of the processed surface, chip disposability, and amount of tool wear were evaluated based on the worst property among the properties of steels 55 to 57, which correspond to conventional Pb free-cutting steels. That is, the roughness of the machined surface was evaluated based on the Rz value of 7.8 μm of steel 56, the amount of tool wear was evaluated based on the value of 175 μm of steel 57, and the chip disposability was evaluated based on the chip weight of 2.7 g of steel 55. base value. Then, when the roughness of the machined surface is 7.8 μm or less in Rz, the tool wear amount is 175 μm or less, and the chip mass is 2.7 g or less, it has machinability equal to or higher than that of Pb free-cutting steel.

In addition, a columnar test piece having a diameter of 24 mm and a length of 50 mm was extracted from the remainder of each normalized round bar having a diameter of 40 mm, and carburization properties were investigated.

That is, the above-mentioned cylindrical test piece with a diameter of 24 mm and a length of 50 mm was heated to 900°C and carburized, then diffused at 850°C, and then quenched by cooling in oil at 80°C. . Next, the above-mentioned test piece was heated to 190° C. and kept for 60 minutes, then air-cooled and tempered. In addition, the carbon potential value during the above-mentioned carburization was 0.8%, and the treatment time was 75 minutes. In addition, the carbon potential at the time of diffusion was 0.7%, and the treatment time was 20 minutes.

From the position of 25 mm from the end of the above-mentioned carburized quenching-tempering test piece, that is, the cross-section at the central position in the longitudinal direction of the test piece, from the surface to the inside, the test force is 2.94N, and measured For the Vickers hardness distribution, the carburizing property was evaluated from the position of the surface with a Vickers hardness of 550 as the "effective hardened layer depth".

In addition, regarding the carburizing property, the worst carburizing property among steels 55 to 57 corresponding to conventional Pb free-cutting steels was used as an evaluation criterion. That is, 0.15 mm of the effective hardened layer depth of steel 57 was used as a reference value for evaluation. Then, when the effective hardened layer depth is 0.15±0.05 mm, that is, 0.10 to 0.20 mm, it has the same carburizing properties as Pb free-cutting steel.

Table 4 and Table 5 collectively show the above test results. "◎" in the column of carburization in Table 4 and Table 5 indicates that the depth of the effective hardened layer exceeds 0.20mm, and has better carburization than Pb free-cutting steel, and "○" indicates that the concentration of the effective hardened layer is 0.10-0.20mm , has the same carburizing property as Pb free-cutting steel, then, "×" indicates that the effective hardened layer depth is lower than 0.10mm, and has poorer carburizing property than Pb free-cutting steel. In addition, "-" of steel 33 in Table 5 indicates that hot forging cannot be performed, so investigation was not carried out.

【Table 4】

steel machinability Carburizing  Hot workability [neck shrinkage rate] (%) Processing surface roughness [Rz] (μm)   Chip mass (g) Built-up edge wear (μm) 1234567891011121314151617181920212223*24*25*26*27*28*29*30 5.85.55.96.17.36.85.85.25.86.46.15.55.15.65.86.15.95.55.45.66.25.76.215.016.57.37.46.89.28.0 1.51.61.51.10.71.61.81.40.80.91.82.12.42.32.52.62.12.01.22.61.21.82.42.82.04.85.00.62.31.0 1121048374100881131051151101201181051169911311110310910511211310518417911912678193222 ○○○○○○○◎◎○○○○○○◎◎○○○○◎○○◎○○○○○ 525559696670576172536950465645484644424361505959726568196967 *Indicates that it is outside the specified conditions of the present invention. "◎", "○", and "×" in the column of carburization indicate that the effective hardened layer is "more than 0.20mm", "0.10～0.20mm", and "less than 0.10mm".

【table 5】

steel machinability Carburizing  Hot workability [neck shrinkage rate] (%) Processing surface roughness [Rz] (μm)   Chip mass (g) Built-up edge wear (μm) *31*32*33*34*35*36*37*38*39*40*41*42*43*44*45*46*47*48*49*50*51*52*53*54*55 *56*57 15.88.4-8.915.97.713.312.412.77.57.69.59.311.313.012.916.715.411.98.56.94.85.46.75.8#7.84.1 0.90.8-1.50.93.21.11.60.75.23.02.53.42.11.21.80.70.71.91.71.62.92.52.3#2.71.11.8 116152-1992371231188668121111157128108172195179206168275299107137118117165#175 ○◎-○○○○○○××◎×○○○○○○◎◎○○○○○○ 61725.2425948615357373575305859645858586864111512383935 *Indicates that it is outside the scope of the present invention. # in the machinability column indicates a standard value. "◎", "○", and "×" in the column of carburization indicate that the effective hardened layer is "more than 0.20mm", "0.10～0.20mm", and "less than 0.10mm". "-" in Steel 33 indicates that investigation was not carried out because hot forging was not possible.

It can be seen from Table 4 and Table 5 that although the low-carbon sulfur free-cutting steels of the present invention of steels 1 to 23 do not contain Pb, they have a long tool life, good chip disposability, and small roughness of the machined surface. Carbon properties are also excellent. In addition, it can be seen that its hot workability is superior to conventional Pb free-cutting steels, and there is no problem in industrial mass production.

On the other hand, the steels of the comparative examples deviated from the specified conditions of the present invention were inferior in at least one of tool life, chip handling properties, machined surface roughness, carburization properties, and hot workability.

Also, for each round bar having a final diameter of 28 mm by the aforementioned cold drawing process, a test piece for microscopic observation was cut from the longitudinal cross-sectional direction of the portion of Df/4 (where "Df" is the diameter of the round bar), Investigation of Mn-based sulfides was carried out. That is, the above-mentioned test piece was embedded in resin and mirror-polished, and an automatic image analysis device incorporating image analysis software was used to measure the width and the shortest average particle size of Mn-based sulfides present in a field of view with a detection area of 5.2 mm ² . distance and distribution density. In addition, the "shortest average inter-particle distance" refers to the distance between the central coordinates of the observed Mn-based sulfides and the closest Mn-based sulfides for each Mn-based sulfide. , and averaged the value of it.

As a result, in the case of the low-carbon sulfur free-cutting steels of the present invention of steels 1 to 23, it was found that Mn-based sulfides with a large width and a small shortest average interparticle distance existed in a large distribution density. Specifically, the width Mn-based sulfides having a diameter of 4 μm or more and a shortest average interparticle distance of 50 μm or less exist at a distribution density as high as 80 particles/mm ^{2 or} more.

As mentioned above, the present invention has been concretely explained by examples, but the present invention is not limited by these examples, and those not disclosed as examples are naturally included in the present invention as long as they satisfy the requirements of the present invention.

【Industrial Utilization Possibility】

Although the steel of the present invention is a "free-cutting steel that is beneficial to the global environment" without adding Pb, when cutting with an HSS tool at a relatively low speed of 100 m/min, it has the same fast-cutting properties as the existing Pb-added free-cutting steel and the composite free-cutting steel with Pb. Good machinability equal to or better than that of steel, that is, long tool life, good chip handling, and small machined surface roughness, and excellent carburization properties, and because of excellent continuous casting properties, it can be produced in large quantities at low cost Production. Therefore, it can be used as a raw material of soft small parts such as brake parts for automobiles, personal computer peripheral parts, and electrical equipment parts.

Claims (4)

1. low carbon sulfur free-machining steel, it is characterized in that, contain in quality % and to be lower than 0.20% more than the C:0.05%, Si: be lower than 0.02%, Mn:0.7～2.2%, P:0.005～0.25%, S:0.40%～0.60% but do not comprise 0.40%, Al: be lower than 0.003%, O:0.0090～0.0280%, N:0.0030～0.0250%, surplus is made of Fe and impurity, Ca in the impurity, Mg, Ti, Zr and REM are, Ca: be lower than 0.001%, Mg: be lower than 0.001%, Ti: be lower than 0.002%, Zr: be lower than 0.002%, and REM: be lower than 0.001%, and satisfy following formula (1) and (2)

Mn×O＞0.018…(1)

2.5＜Mn/(S+O)＜3.5…(2)

Wherein, the symbol of element in (1) formula and (2) formula represents that this element is in content in the steel of quality %.

2. low carbon sulfur free-machining steel according to claim 1, wherein, the content of N is counted N:0.0060～0.0250% with quality %.

3. low carbon sulfur free-machining steel according to claim 1 and 2 wherein, substitutes the part of Fe, contain Te:0.0005～0.03%, be lower than 0.50% more than the Sn:0.001%, and be lower than more than the Se:0.0005% among 0.30% more than a kind.

4. want each described low carbon sulfur free-machining steel in 1～3 according to right, wherein, substitute the part of Fe, contain Cu:0.01～1.0%, Ni:0.01～1.0%, Cr:0.01～1.0%, and Mo:0.01～0.5% among more than a kind.