CN101400814A - Low-carbon resulfurized free-cutting steel material - Google Patents

Abstract

A low-carbon resulfurized free-cutting steel material which contains less than 0.05% C, less than 0.05% Si, 0.7-2.2% Mn, 0.03-0.20% P, 0.40-0.70%, excluding 0.40% and 0.70%, S, less than 0.005% Al, 0.0050-0.0380% O, and 0.0020-0.0250% N, with the remainder being iron and impurities, wherein Ca, Mg, Ti, Zr, and REM among the impurities satisfy the following relationships: Ca<0.001%, Mg<0.001%, Ti<0.002%, Zr<0.002%, and REM<0.001%, provided that 0.010<O/S<0.080 and 2.5<Mn/(S+O)<4.0. When cut with a high-speed steel tool, the steel material has the same chip disposability as conventional Pb-free free-cutting steel materials and has better finished-surface properties than those. It is excellent also in continuous castability and hence suitable for mass production. This material may contain at least one of Te=0.05%, Bi=0.15%, and Se<0.30%.

Description

Low carbon sulfur free cutting steel

technical field

The present invention relates to a low-carbon sulfur free-cutting steel, and more specifically, to a lead free-cutting steel (hereinafter referred to as "Pb free-cutting steel") and a composite free-cutting steel added with S and Pb, namely "sulfur composite free-cutting steel". Low-carbon sulfur free-cutting steel that has better machinability than existing free-cutting steel without adding Pb, which is used as an alternative material to "Steel" (hereinafter referred to as "S composite free-cutting steel"). More specifically, it relates to a low-carbon sulfur free-cutting steel without adding Pb, which uses a high-speed steel tool (hereinafter referred to as "HSS" tool.) in a relatively low speed range of 100 m/min or less, And when cutting under wet conditions with the supply of cutting oil, it has the property that the chips can be divided into finer pieces (hereinafter referred to as "chip disposability"), and has the property that compared with the above-mentioned conventional free-cutting steel without adding Pb , good surface texture with small final processing surface roughness, and because of its excellent continuous castability, it can be mass-produced at low cost.

Background technique

At present, steel materials with excellent machinability can be used for the raw materials of soft small parts such as brake parts for automobiles, personal computer peripheral equipment parts, and electrical equipment parts in order to improve productivity. The so-called "free cutting steel".

The above-mentioned cutting of small soft parts on an industrial scale is mainly performed under wet conditions in a relatively low speed range of 100 m/min or less using HSS tools. Moreover, on the basis of such cutting processing conditions, the "machinability" of the steel as the raw material of the part, in particular, requires that the final processed surface roughness of the processed steel be small (that is, the surface of the steel is smooth, the surface shape Excellent), in addition, it is considered that excellent chip control is also important.

In addition, among the above-mentioned characteristics, good chip disposability is indispensable for the automation of processing lines, and is a characteristic required for improving productivity.

As the free cutting steel, the steel specified in JIS G 4804 (1999), that is, the "sulfur free cutting steel" that contains a large amount of S and improves the machinability by using MnS (hereinafter sometimes referred to as "S free cutting steel") is well known. That is, "S-combined free cutting steel" containing both S and Pb, and "Pb free cutting steel" which can improve the machinability by Pb is also known as a general free cutting steel.

Among the above-mentioned free-cutting steels, free-cutting steel containing Pb, that is, Pb free-cutting steel or S-composite free-cutting steel, is used for cutting with HSS tools under wet conditions in a relatively low speed range below 100m/min. , has the characteristics of excellent chip disposability, and can obtain a smooth final machined surface with small surface roughness.

Therefore, these Pb-containing free-cutting steels are mostly processed into soft various small parts such as the aforementioned brake parts for automobiles, personal computer peripheral parts, ie, electrical equipment parts, etc. by cutting under the aforementioned conditions. , used as the final product.

In addition, high dimensional accuracy after cutting is required for such small parts. Therefore, it is believed that it is important that the steel material used as a raw material has less warpage in the stage before cutting, that is, good "straightness". Therefore, when the free-cutting steel material containing Pb is used as the raw material of the above-mentioned small parts, for example, cold working such as wire drawing is performed to ensure high straightness before cutting.

However, in recent years, the wave of protection of the global environment has been increasing, so activities to exclude Pb from products are being strengthened. For example, in Europe, it is desired that the content of Pb contained in steel materials be limited to 0.35% by mass. Below, etc., the content of Pb is reduced as much as possible.

In addition, Pb has a low melting point and hardly dissolves in steel, so steel containing a large amount of Pb tends to generate cracks during rolling. Therefore, from the viewpoint of stable production of so-called steel materials, there is an urgent need for free-cutting steel without adding Pb.

In response to such urgent needs, Patent Documents 1 to 9 have proposed various steel materials that increase the amount of S while controlling the form of MnS to improve machinability, or control the structure to improve machinability, as an alternative to Pb. Pb-free free-cutting steel and S-composite free-cutting steel free-cutting steel.

Specifically, Patent Document 1 discloses "low-carbon sulfur-based free-cutting steel and its manufacturing method", which improves the final surface roughness and final dimensional accuracy by adjusting the average width of sulfide and the yield ratio of the wire rod.

Patent Document 2 discloses "a high-S free-cutting steel that is a method for producing a high-S free-cutting steel excellent in machinability", and by adjusting the average area of MnS in the steel on the basis of containing 0.38% or more of S, the The prevention of surface flaws coexists with the improvement of the final machined surface roughness.

Patent Document 3 discloses "Low-carbon composite free-cutting steel with excellent final surface roughness and its manufacturing method", which has a specific chemical composition and metal structure, and the average width and proeutectoid The hardness of ferrite is adjusted to 133 to 150 in terms of Vickers hardness (hereinafter referred to as "Hv hardness"), thereby improving the final surface roughness.

Patent Documents 4 to 7 disclose "steel excellent in machinability and its manufacturing method" or "steel excellent in machinability", in which the area ratio of pearlite is adjusted after a specific amount of S is contained in the steel , or disperse fine MnS to improve machinability.

Patent Document 8 discloses a "low-carbon free-cutting steel" characterized by containing specific amounts of C, Mn, S, Ti, Si, P, Al, O, and N, Ti, and S proposed by the present inventors. content satisfies the following (I) formula, and at the same time, the atomic ratio of Mn and S satisfies the following (ii) formula, and contains Ti sulfide or/and MnS inherent in Ti carbosulfide.

Ti (mass %)/S (mass %)<1...(i),

Mn/S≥1...(ii).

Patent Document 9 discloses "low-carbon free-cutting steel", which is characterized by containing specific amounts of C, Mn, S, Ti, Si, P, Al, O, and N proposed by the present inventors. Contains impurities that satisfy two specific formulas.

[Patent Document 1] Japanese Patent Laid-Open No. 2003-253390

[Patent Document 2] Japanese Unexamined Patent Publication No. 2005-23342

[Patent Document 3] Japanese Unexamined Patent Publication No. 2005-187935

[Patent Document 4] Japanese Unexamined Patent Publication No. 2004-169052

[Patent Document 5] Japanese Unexamined Patent Publication No. 2004-169054

[Patent Document 6] Japanese Unexamined Patent Publication No. 2004-176176

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

[Patent Document 8] Japanese Patent Laid-Open No. 2004-226933

[Patent Document 9] Japanese Unexamined Patent Publication No. 2004-54227

The "low-carbon sulfur-based free-cutting steel" disclosed in Patent Document 1 does not consider reducing the content of elements having a high affinity for oxygen, such as Ca, Mn, Ti, Zr, and REM. Therefore, when HSS tools are used for cutting under wet conditions in a relatively low speed range of 100 m/min or less, as shown in Table 3 of the examples, the final machined surface roughness is only about 35 μm. In order to obtain the surface smoothness required for the above-mentioned small parts, it cannot be said that the surface roughness of the final processing of such steel materials can be sufficiently satisfied.

In the case of the "high-S free-cutting steel" disclosed in Patent Document 2, the content reduction of elements having a high affinity for oxygen, such as Ca, Mn, Ti, Zr, and REM, is not considered. Therefore, under the wet conditions of the relatively low speed region below 100m/min, the final machined surface roughness when using the HSS tool for cutting is as shown in Table 1 of its embodiments, in terms of ten-point average roughness, It also reaches 10.5-15 μm. In order to obtain the surface smoothness required for the above-mentioned small parts, it cannot be said that the surface roughness of the final processing of such steel materials can be sufficiently satisfied.

The "low-carbon composite free-cutting steel" disclosed in Patent Document 3 does not consider reducing the contents of Ca and Mg, which are elements with a high affinity for O. In addition, the hardness of the steel material specified here is the Hv hardness of the proeutectoid ferrite in the ferrite-pearlite structure intact after hot working, and the Hv hardness of the steel material itself is not considered. Therefore, when the HSS tool is used for cutting under the wet conditions of the relatively low speed range below 100m/min, as shown in Table 3, Table 6 and Table 10 of the examples, the final machined surface roughness Ra is relatively low. Large, 27.6-37.6 μm in terms of centerline average roughness. In order to obtain the surface smoothness required for the above-mentioned small parts, it cannot be said that the surface roughness of the final processing of such steel materials can be sufficiently satisfied.

The "steel excellent in machinability" disclosed in Patent Document 4 not only does not take into account the content balance of Mn, S, and O that affect the form of sulfides and oxides, but also allows the addition of Al, Ti , Zr, Ca, and Mg are component elements that have a large influence on the morphology of sulfides and oxides. Therefore, when HSS tools are used for cutting under wet conditions in the relatively low speed range below 100m/min, as shown in Table 2 and Table 4 of the examples, the final surface roughness is higher than that of the plunge-cut method. When the cutting distance is short such as the 200 slot machining, the ten-point average roughness reaches 4.1-11.2μm. In this way, the above-mentioned "steel with excellent machinability" cannot clearly obtain a small final surface roughness after cutting for a long distance. In addition, in order to obtain the surface smoothness required for the aforementioned small parts, It cannot be said that the ten-point average roughness of this level can be sufficiently satisfied.

The "steel excellent in machinability" disclosed in Patent Document 5 and Patent Document 6 not only does not consider the content balance of Mn, S, and O, which affect the form of sulfides and oxides, but also can be set Component elements such as Ti, Zr, Ca, and Mg that greatly affect the morphology of sulfides and oxides are added. Therefore, under the above-mentioned wet conditions in the relatively low speed range of 100m/min or less, the final machined surface roughness when cutting with HSS tools is shorter than the cutting distance such as 200 groove machining in plunge cutting. In this case, the ten-point average roughness is 4.1 to 8.8 μm (Patent Document 5) or 4.3 to 12.1 μm (Patent Document 6). In this way, it is not clear that the above-mentioned "steel with excellent machinability" can obtain a small final surface roughness after long-distance cutting. In addition, in order to obtain the smooth surface required for the aforementioned small parts degree, it cannot be said that the ten-point average roughness of this degree can be sufficiently satisfied.

The "steel with excellent machinability" invented by the same inventor at the same time as Patent Documents 4 to 6 and disclosed in Patent Document 7 is also similar to Patent Documents 4 to 6. Even after long-distance cutting, it cannot It is clear that a small final surface roughness can be obtained. In addition, in order to obtain the surface smoothness required for the aforementioned small parts, it cannot be said that this level of ten-point average roughness is sufficient.

In addition, the "low-carbon free-cutting steel" disclosed in Patent Document 8 and Patent Document 9 proposed by the present inventors can reliably obtain excellent chip disposability and final machined surface roughness when cutting with a cemented carbide tool. Therefore, under wet conditions in a relatively low speed range of 100 m/min or less, when cutting with an HSS tool, the final machined surface roughness increases, and it can be seen that the desired good surface texture may not be obtained.

That is, the "low-carbon free-cutting steel" disclosed in Patent Document 8 can reliably obtain chip handling properties superior to those of Pb free-cutting steel when high-speed cutting is performed under non-lubricated, ie, dry conditions using cemented carbide tools. Therefore, it can be seen that under wet conditions in the relatively low speed range of 100 m/min or less, when HSS tools are used for cutting, the final machined surface roughness increases, and desired good surface properties may not be obtained.

In addition, the "low-carbon free-cutting steel" disclosed in Patent Document 9 can obtain a good surface texture with a small final surface roughness and excellent chip controllability even when it is reliably cut with a cemented carbide tool. Therefore, it can be seen that in the relatively low speed range of 100 m/min or less, when HSS tools are used for cutting, the final machined surface roughness increases, and the desired good surface texture may not be obtained.

As mentioned above, the free-machining steel without adding Pb that has been proposed so far has machinability necessary as a raw material for soft small parts such as brake parts for automobiles, personal computer peripheral parts, and electrical equipment parts. , In the relatively low speed area below 100m/min, the final machined surface properties after using HSS tools to cut a long distance, that is, the final machined surface roughness, cannot be said to be able to meet the requirements of the industry.

In addition, it cannot be said that the above-mentioned free-cutting steel without adding Pb that has been proposed so far is necessarily excellent in "continuous castability" required in the manufacturing stage for inexpensive mass production.

Contents of the invention

Therefore, the object of the present invention is to provide a low-carbon free-cutting steel without adding Pb, which still has the same properties as the above-mentioned steel after cutting a long distance with HSS tools under wet conditions in a relatively low speed range below 100m/min. Compared with the conventional free-cutting steels without adding Pb, which have been proposed in Patent Documents 1 to 9, etc., the conventional free-cutting steels without adding Pb can ensure the roughness of the final machined surface. Good surface texture with small degree of hardness, and suitable for mass production in continuous casting.

The inventors of the present invention used Pb free-cutting steel and existing free-cutting steel without adding Pb as the workpiece, and performed cutting using HSS tools under wet conditions in a relatively low speed range of 100 m/min or less. In addition, the fine structure of built-up edge formed on the edge of the tool was observed using a transmission electron microscope (TEM), and the built-up edge was investigated using a scanning electron microscope (SEM) or an electron probe. The tissue observation and composition analysis of the built-up edge itself have carried out active research on the relationship between the structure and composition of the built-up edge itself and the roughness of the final processed surface.

As a result, first, the following findings of <1> to <3> were obtained. In the following description, "MnS" which is not particularly limited in advance includes, in addition to pure MnS, complex compounds of Mn, such as Mn(S, Te), Mn(S, Se), Mn(S , O) or Mn(S, Se, O) and other elements in X that combine with Mn in addition to S, namely Te, Se and O, are described by the chemical formula of Mn(S, X).

<1> When cutting Pb free-cutting steel, the dissolution embrittlement effect based on the low melting point of Pb is obtained, so that it is easy to break even under a small shear stress, and HSS tools are used for cutting under the above conditions , can also obtain excellent chip disposal. In addition, due to the above-mentioned effect of Pb, the friction force between the tool and the material to be cut, that is, the interface with Pb free-cutting steel is reduced, and the material to be cut is difficult to adhere to the tool, so the built-up edge is difficult to grow, and only small Therefore, it is possible to obtain a good surface texture with a small final surface roughness.

<2>According to the detailed observation by TEM, in the small built-up edge formed after cutting Pb free-cutting steel, there are more particles with a particle size of several numbers than in the Pb free-cutting steel as the workpiece As a result of compositional analysis of fine cementite grains of about 100 nm, C in the built-up edge is concentrated 5 to 6 times the amount of C contained in the material to be cut.

<3> On the other hand, after cutting the existing Pb free-cutting steel, the final machined surface roughness is relatively large, and according to the observation with SEM, it can be seen after cutting the above-mentioned Pb free-cutting steel On the small built-up edge where C is concentrated, the material to be cut (existing free-cutting steel without adding Pb) is still glued and accumulated, thus forming a coarse built-up edge.

Furthermore, the inventors of the present invention estimated the following <4> to <8> based on the above findings.

<4> After cutting the existing free-cutting steel without adding Pb, the coarse built-up edge grows from the small built-up edge with concentrated C as the nucleus. From the results, the roughness of the final machined surface increases. Large, making the surface properties worse.

<5> On the other hand, after cutting Pb free-cutting steel, in the stage where the built-up edge further grows with the small built-up edge formed in the cutting process as the nucleus, the growth is suppressed by the action of Pb and MnS. Inhibition, so only small built-up edge was observed.

<6>Therefore, if Pb and MnS play a role in inhibiting the growth of built-up edge in the growth stage of built-up edge so that built-up edge does not grow, it can be obtained as consistent experience with free-cutting steel containing Pb or increasing O(oxygen ) content to form coarse MnS to improve the final machined surface properties (that is, to reduce the effect of the final machined surface roughness).

<7> In the case where Pb is not added, by reducing the built-up edge which is the initial stage of the nucleus for the growth of the built-up edge to a coarse tip, in other words, by reducing the built-up edge with C concentrated, it is possible to prevent The built-up edge grows larger, ensuring a good final machined surface texture, that is, a small final machined surface roughness can be ensured.

<8> The reduction of C-enriched built-up edge, which is the nucleus for the built-up edge to grow into a coarse tool tip, can be achieved by reducing the C content of the steel material to be cut.

Then, the inventors of the present invention conducted various studies on the influence of the amount of C on the machinability based on the above-mentioned estimation.

As a result, the following insights <9> and <10> were obtained.

<9> By reducing the content of C, the final surface roughness can be reduced without adding Pb.

<10> On the other hand, when the C content is lowered and compared at the same S content, the chip disposal property tends to deteriorate.

Therefore, the inventors of the present invention conducted further studies in order to obtain both excellent surface properties with a small final surface roughness and good chip disposability. As a result, the following <11> and <12> were obtained. opinion.

<11> As mentioned in <1>, the fact that Pb free-cutting steel can ensure both good chip handling and excellent surface properties with small surface roughness in final processing is based on the melting embrittlement caused by the low melting point of Pb. chemical effect. Therefore, when Pb is not added, it is preferable to embrittle the steel material itself in order to obtain both good chip disposability and excellent surface texture, that is, small final surface roughness.

<12> In order to embrittle the steel itself, it is enough to use cold working starting from wire drawing. Pb free-cutting steel does not cause significant embrittlement because the hardness of the workpiece does not increase when it is cold-worked, and therefore sufficient chip disposability cannot be obtained.

Then, wire drawing was adopted as an example of cold working of low C steel without adding Pb, and the machinability when the reduction of area at that time was changed in various ways was also investigated.

As a result, the following <13> insight was obtained.

<13> Even for low-C steel without Pb added, by increasing the degree of processing at low temperatures, as long as the hardness after processing reaches 180 or more in terms of Hv hardness, excellent chip disposability can be ensured. However, if the Hv hardness after cold working is too high, especially if it exceeds 230, the roughness of the final machined surface will increase when a constant distance is cut.

In addition, since the improvement of chip disposability by increasing the processing degree at low temperature as described above decreases the ductility due to processing variation, it is presumed that the chips are easily broken due to the concentration of shear stress.

<14> Next, the inventors of the present invention improved the processing degree at low temperature and adjusted the hardness after processing to 180-230 in terms of Hv hardness for the final processing of low-C free-cutting steel without adding Pb and Pb free-cutting steel. The surface roughness and chip disposability were comparatively studied. As a result, it was found that in order to simultaneously improve the final machined surface properties and chip disposability, it is necessary not only to reduce the amount of C in the free-cutting steel without adding Pb, but also to adjust the degree of processing at low temperatures to a high level, and to adjust the Hv hardness after cold working to 180 to 230, and, as shown in the following <15>, the morphology and dispersion state of MnS are also important.

<15> The final machined surface roughness and chip treatability of low-C free-cutting steel without adding Pb vary with the morphology and dispersion state of MnS. That is, when MnS is fine, the chip disposability is improved, but the surface roughness of the final machined surface is increased, and the surface quality is deteriorated. On the other hand, when the O content in the steel is increased to coarsely crystallize MnS, the final surface roughness is reduced and the surface texture is improved, but the chip treatability is deteriorated.

Then, in order to optimize the form and dispersion state of MnS, detailed studies were also carried out.

As a result, the following important insights <16> to <20> were obtained. Combining the above insights, compared with conventional steel materials without adding Pb, it is possible to obtain excellent final surface roughness and chip handling properties. Low carbon sulfur free cutting steel.

<16> When cutting steel containing coarse MnS, microcracks are generated around the built-up edge of the tool tip from the coarse MnS. Since the microcracks function to divide the chip and the built-up edge, Thus, the growth of built-up edge is inhibited. Therefore, to reduce the surface roughness of the final processing and improve the surface properties, as long as the MnS is coarse. On the other hand, in order to improve the chip disposability, it is necessary to embrittle and split the generated chips, and to embrittle the chips, it is necessary for the microcracks generated from MnS to spread over a wide area. Therefore, in order to spread the microcracks generated from each MnS to a wide area in the chip shear range, it is necessary to reduce the ductility of the steel itself and make it brittle, and at the same time, the distance between the MnSs must be small, in other words, the number of MnSs must be small. high density. According to the above findings, increasing the number density of MnS capable of suppressing the growth of built-up edge is related to both the reduction of the final surface roughness, the improvement of the surface texture, and the improvement of chip disposability.

<17> By adjusting the composition of low-C free-cutting steel without adding Pb, on the basis of increasing the content of S, adjusting the content of Mn, S and O to a specific range, to increase the ability to inhibit the growth of built-up edge The number density of MnS, thus, can further improve the final machined surface properties and improve chip handling.

<18> In order to obtain the above-mentioned preferred form of MnS, it is necessary to crystallize a large amount of MnS at the stage of rapid solidification, and for this reason, the control of the oxide as the crystallization nucleus of MnS becomes an important point. That is, the crystal nuclei for controlling the form of MnS, which is largely crystallized in low-carbon sulfur free cutting steel, must form Mn-based oxides such as MnO, Mn ₃ O ₄ , or Mn ₂ O. This is because, in the case of other than Mn-based oxides, it is impossible to exist in molten steel in an amount sufficient to control the form of MnS that is largely crystallized in low-carbon sulfur-based free-cutting steel.

<19> In order to use Mn-based oxides such as MnO, Mn ₃ O ₄ or Mn ₂ O as crystallization nuclei for controlling the morphology of MnS, Ca, Mg, zr, and Ti that have a high affinity for O are not added to molten steel and REM, in addition, the content of these elements in impurities must be controlled.

<20> As long as the content balance of Mn, S and O is properly balanced, defects caused by low hot ductility such as internal cracks will not occur in the case of mass production in continuous casting equipment.

The present invention was completed based on the above-mentioned knowledge, and its gist is a low-carbon sulfur free-cutting steel shown in the following (1) and (2).

(1) A low-carbon sulfur free-cutting steel excellent in machinability, characterized by containing, in mass %, C: less than 0.05%, Si: less than 0.05%, Mn: 0.7 to 2.2%, and P: 0.03 ~0.20%, 0.40%<S<0.70%, Al: less than 0.005%, O: more than 0.0050 but less than 0.0380%, N: 0.0020~0.0250%, the balance is composed of Fe and impurities, Ca, Mg in impurities , 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%, REM: less than 0.001%, and satisfy the following formula (1 ) and (2), in addition, the Vickers hardness after cold working is 180-230.

0.010<O/S<0.080...(1)

2.5<Mn/(S+O)<4.0...(2)

Wherein, the symbols of the elements in the formulas (1) and (2) represent the contents of the elements in the steel in mass %.

(2) The low-carbon sulfur free-cutting steel as described in the above (1), wherein instead of a part of Fe, Te: 0.05% or less, Bi: 0.15% or less, and Se: less than 0.30% are contained in mass % more than one.

Hereinafter, the inventions of the low-carbon sulfur free-cutting steel of the above (1) and (2) are respectively referred to as "the present invention (1) and the present invention (2). In addition, they are collectively referred to as "the present invention".

Regardless of whether the steel of the present invention is free-cutting steel that is superior to the earth's environment without adding Pb or not, after using HSS tools to cut a long distance in a relatively low speed area below 100m/min, it still has the same performance as the existing Pb-free steel. Compared with the above-mentioned conventional free-cutting steel without adding Pb, it can ensure a good surface texture with a small surface roughness in the final machining, and it is excellent in continuous casting , can be mass-produced at low cost. Therefore, it 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 of drawings

1 is a graph showing the relationship between the Hv hardness after drawing and the average roughness Rz of the final machined surface in Example 2;

FIG. 2 is a graph showing the relationship between the Hv hardness after drawing and the average roughness Ra of the finished surface in Example 2. FIG.

Detailed ways

First, the chemical composition in 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 means "mass %".

C: less than 0.05%

C is an element that has a great influence on the strength and machinability of steel, especially on the final surface roughness. When the content is more than 0.05%, the growth nucleus of built-up edge increases, and the built-up edge is easy. growth, resulting in an increase in the final machined surface roughness. Therefore, in order to obtain a good finished surface texture, that is, to obtain a small finished surface roughness, the C content is set below 0.05%. From the viewpoint of obtaining good final processed surface properties, the lower the C content, the better. Therefore, it is preferably less than 0.04%. In addition, it is more preferable if the content of C is 0.03% or less. However, when the C content is too low, not only does the production cost increase, but also in cold working such as wire drawing, it is necessary to greatly increase the degree of working to increase the hardness in order to ensure excellent chip disposability, so it is not preferable. The lower limit of the C content is preferably 0.005% from the viewpoint of obtaining a good final machined surface texture and excellent chip disposability.

Si: less than 0.05%

Although Si has the effect of solid-solution in ferrite to increase the strength of steel, it has a strong deoxidation effect, so when it is contained above 0.05%, the O content is reduced. Therefore, it cannot be obtained to improve the machinability. It is the form and dispersion state of the above-mentioned MnS required for the final machined surface properties and chip disposability. Therefore, the content of Si is set to be less than 0.05%. In addition, from the viewpoint of further improving the machinability, the lower the Si content, the better, so it is preferably less than 0.02%. In addition, the content of Si is more preferably less than 0.01%.

Mn: 0.7-2.2%

Mn is an element that forms MnS together with S and has a large influence on the final surface roughness. When the content thereof is less than 0.7%, hot workability deteriorates. In addition, Mn causes deoxidation at the same time as forming MnS. Therefore, simply increasing its content for the purpose of improving hot workability cannot obtain the aforementioned MnS form and dispersed state. Therefore, Mn must be contained in consideration of the mass balance of S and O (oxygen). However, even in this way, when the Mn content exceeds 2.2%, the desired MnS morphology and dispersion state cannot be obtained. Therefore, when the chip length is long, the final machined surface roughness increases and the surface texture decreases. Therefore, the content of Mn is set to 0.7 to 2.2%. In addition, in order to more stably and reliably obtain a good surface texture with a small final surface roughness, the content of Mn is preferably 1.2 to 1.8%.

As mentioned above, in addition to pure MnS, the above-mentioned "MnS" also contains complex compounds of Mn, such as Mn(S, Te), Mn(S, Se), Mn(S, O) or Mn(S, Se, O) and other elements that combine with Mn in addition to S in X, that is, Te, Se, and O, are described by the chemical formula of Mn(S, X).

P: 0.03～0.20%

P has the effect of increasing the strength of steel. Therefore, in order to ensure good final machined surface properties, that is, a small final machined surface roughness and the occasion of the present invention where the content of C is controlled to be low, in order to maintain the strength of the small parts as the final product, it is necessary to increase the content of P The content is set to 0.03% or more. However, if the P content is too high, the strength of the steel increases and the machinability decreases, and especially if it exceeds 0.20%, the strength of the steel becomes too high and the machinability, especially the final machined surface properties, remarkably decreases. In addition, when the content of P exceeds 0.20%, hot workability also deteriorates. Therefore, the content of P is set to 0.03 to 0.20%. In addition, in order to ensure better machinability, the content of P is preferably 0.05 to 0.15%.

S: 0.40%<S<0.70%

S is an element necessary to form the above-mentioned MnS together with Mn, and to obtain good machinability, especially good surface texture with a small final machined surface roughness, and excellent chip disposability. The machinability-improving effect of MnS varies not only with the generated amount but also with the form, that is, the dispersed state. Therefore, the balance between the content of S and the content of Mn and O (oxygen) is very important. However, if the content of S is 0.40% or less, for example, even if the balance between the content of Mn and O (oxygen) is appropriate, it will not be obtained. A sufficient amount of MnS, so that the morphology and dispersion state of MnS for obtaining a small final machined surface roughness and good chip disposability cannot be obtained. In addition, in general, when the content of S exceeds 0.35%, the hot workability is reduced, so it becomes the main cause of the so-called "cracking" inside the slab, and by adjusting the content of Mn and O (oxygen) When the balance is appropriate, even when the S content exceeds 0.35%, internal cracking will not occur, so that a small final machined surface roughness and good chip disposability can be ensured. However, when the S content exceeds 0.70%, a large amount of Mn needs to be contained so that the hot workability does not decrease, but since Mn acts as a deoxidizing element, sufficient oxygen cannot be secured, and the form of MnS is damaged. , it is substantially difficult to obtain the aforementioned desired form and dispersion state of MnS. In addition, the addition of excess S exceeding 0.70% of the content leads to a cost increase due to a decrease in yield. Therefore, the content of S is set to be 0.40%<S<0.70%.

In order to more stably ensure excellent machinability, that is, good surface texture with small final machined surface roughness and excellent chip controllability, S is preferably more than 0.50%. In addition, in order to suppress the manufacturing cost without lowering the manufacturability and to obtain the desired form and dispersed state of MnS, S is preferably less than 0.60%, more preferably S is 0.55% or less.

Al: less than 0.005%

Al is a powerful deoxidizing element with a high affinity to O (oxygen). When it is contained in an amount of 0.005% or more, the aforementioned form, dispersion state, and oxide composition of MnS suitable for improving machinability cannot be obtained, and therefore, the desired Good machinability, that is, small surface roughness for manufacturing. Therefore, the content of Al is set to be less than 0.005%. In addition, since Al has a great influence on the form, dispersion state, and oxide composition of MnS, it must be added, and needs to be removed when it should be refined. In order to obtain more excellent final processed surface properties, the content of Al is preferably lower than 0.003%, more preferably lower than 0.002%.

O: More than 0.0050 but less than 0.0380%

O (oxygen): By increasing its content, the morphology, machinability, and especially the final surface roughness of MnS can be changed. However, simply increasing the O content without adding a deoxidizing element cannot obtain the desired good machinability, that is, the above-mentioned MnS required for a good surface texture with a small final machined surface roughness and excellent chip handling properties. form and dispersion. That is, increasing the O content while optimizing the balance of Mn and S can change the morphology and dispersion state of MnS, thereby improving the machinability. However, when the content of O is 0.0380% or more, for example, even if the balance of the content of Mn and S is appropriate, not only the form and dispersion state of MnS cannot be obtained, but also coarse oxides are formed, which induces the problem of cold working starting from wire drawing. crack. On the other hand, when the content of O is less than 0.005%, the above-mentioned morphology and dispersion state of MnS required to obtain the desired good final machined surface properties and excellent chip disposability cannot be obtained. Therefore, the content of O is set to be 0.0050% or more but less than 0.0380%. In addition, in order to stably secure the desired form and dispersed state of MnS, the content of O is preferably 0.0080% to 0.0280%.

N: 0.0020～0.0250%

In the present invention, since Al and Ti are not substantially contained, hard nitrides of Al and Ti are hardly formed, and N exists as a solid solution in ferrite. The N dissolved in the ferrite has no influence on the form of MnS and can increase the strength of the steel, thereby improving chip disposability. In addition, N also has the effect of reducing the roughness of the final processed surface. However, when the N content is less than 0.020%, sufficient chip treatability and final machined surface properties cannot be obtained. On the other hand, even if the content of N exceeds 0.0250%, not only the above-mentioned effect is saturated, but also an increase in production cost is incurred. Therefore, the content of N is set to 0.0020 to 0.0250%. In order to more effectively improve the machinability, especially chip controllability, and final machined surface properties, it is preferable to contain 0.0050% or more of N, and it is more preferable to contain 0.0095% or more of N.

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%, REM less than 0.001%

Ca, Mg, Ti, Zr, and REM are all elements that are often added to improve the machinability of free-cutting steels. However, the above-mentioned Ca-REM elements all have a high affinity to O, so they all affect the form of Mn or the composition of oxides and the dispersion state of their inclusions, making the machinability especially at 100m/min In the following relatively low speed range, the final machined surface properties are reduced when cutting with HSS tools under wet conditions. Especially for the above-mentioned Ca, Mg, Ti, Zr and REM among the impurities, when containing any one of Ca, Mg and REM at 0.001% or more, and any one of Ti and Zr at 0.002% or more, the above-mentioned The reduction of the final machined surface properties in the cutting area using HSS tools is more significant. Therefore, the contents of Ca, Mg, Ti, Zr and REM in impurities must be less than 0.001% for Ca, less than 0.001% for Mg, less than 0.002% for Ti, less than 0.002% for Zr, and less than 0.001% for REM. The above-mentioned contents of Ca, Mg, Ti, Zr and REM among impurities are preferably: Ca less than 0.0005%, Mg less than 0.0005%, Ti less than 0.0010%, Zr less than 0.0010%, and REM less than 0.0005%.

In addition, as described above, "REM" is a generic term for a total of 17 elements including Sc, Y, and rare earth elements, and the content of REM refers to the total content of these elements.

Value of "O/S": more than 0.010 but less than 0.080

Contains elements 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 less than 0.001% of Ca, less than 0.001% of Mg, less than 0.002% of Ti, and Zr Steel materials with less than 0.002% and REM less than 0.001%, when the "O/S" value is greater than 0.10 and less than 0.080, the workability in cold working starting from wire drawing is good, and no cracks will occur, and, In the cutting using the HSS tool in the relatively low speed range of 100 m/min or less, the desired final surface roughness can be ensured with excellent surface texture. Next, the mechanism thereof will be described.

During the solidification process of MnS, the Mn-based oxides are used as growth nuclei for crystallization, and finally become a form with O in solid solution. Therefore, in order to form the aforementioned preferred MnS form and dispersed state, it is necessary to form Mn-based oxides as growth nuclei at the stage of fast solidification, so the O content must be increased according to the S content.

On the other hand, the low content specified in the present invention can hardly expect the deoxidation effect brought by C, so the content of O tends to increase. When the content of O is too high, pores will be generated in the slab, resulting in the following Wire drawing is the initial cold working to induce defects such as cracks.

Therefore, the range of the O content that should eventually remain in the steel material is limited based on the S content. Furthermore, when the value of "O/S" is 0.010 or less, the above-mentioned preferable form and dispersion state of MnS cannot be obtained, and a good surface roughness after finishing cannot be obtained.

On the other hand, when the value of "O/S" is more than 0.080, the amount of O exceeds the amount of generated MnS to form coarse oxides, so the reduction of cracks during cold working starting from wire drawing is induced. Cold workability, and the roughness of the final machined surface after cutting for a long distance is deteriorated.

Therefore, the value of "O/S" must satisfy more than 0.010 but less than 0.080, that is, satisfy the above formula (1). In addition, the symbol of the O element in the above-mentioned formula "O/S" represents the content of the element in the steel by mass %, and the value of "O/S" is preferably 0.020 to 0.060.

"Mn/(S+O)" value: more than 2.5 and less than 4.0

Contains elements 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 less than 0.001% of Ca, less than 0.001% of Mg, less than 0.002% of Ti, and Zr The steel with less than 0.002% and REM less than 0.001% has good hot workability when its "Mn/(S+O)" value exceeds 2.5 and is less than 4.0, so during continuous casting, the interior of the cast sheet is not Cracks will occur, and in the cutting with HSS tools in the relatively low speed range of 100m/min or less, the desired final machined surface roughness is small, excellent surface texture and good chip controllability can be ensured. Next, the mechanism thereof will be described.

The effect of Mn in the present invention is extremely important. For low-carbon free-cutting steel that does not add deoxidizing elements during melting, the deoxidizing treatment is mainly performed by C and Mn, but as mentioned above, the aforementioned low The deoxidation effect of C can hardly be expected in the C content, so the deoxidation effect of Mn becomes important. In addition, in high S-content steel materials containing more than 0.40% specified in the present invention, in order to suppress the formation of FeS and suppress the reduction of hot workability, the content of Mn must be fully considered.

That is, Mn reacts with O during deoxidation, and then combines with S to suppress the formation of FeS and improve hot workability. Also, when the value of "Mn/(S+O)" is greater than 2.5, solidification ensures sufficient hot workability suitable for industrial large-scale mass production. However, when the value of "Mn/(S+O)" is 2.5 or less, sufficient hot workability cannot be obtained, and defects such as internal cracks may occur during mass production with continuous casting equipment.

On the other hand, when the value of "Mn/(S+O)" is 4.0 or more, Mn is contained in excess of S or O, and the amount of solid-dissolved Mn in the billet becomes excessive, resulting in a decrease in machinability, especially Decrease in surface texture due to reduction in chip controllability and increase in final machined surface roughness. In addition, in the low-carbon sulfur free-cutting steel of the present invention that does not substantially contain Al, Si, Ca, Mg, Ti, or REM, Mn functions as a deoxidizing element. O, and the excess Mn increases the strength of the steel, and the hardness rises sharply when cold working is performed at a high degree of processing, so after cutting a long distance, a good final surface roughness cannot be obtained.

Therefore, the value of "Mn/(S+O)" must satisfy more than 2.5 and less than 4.0, that is, satisfy the above formula (2). In addition, the symbol of the element in the above-mentioned formula "Mn/(S+O)" indicates the content of the element in the steel in mass %, and the value of "Mn/(S+O)" is preferably 2.7 or more and less than 3.5.

Based on the above-mentioned reasons, the chemical composition of the low-carbon sulfur free-cutting steel of the present invention (1) is defined as: containing the elements 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 are: Ca less than 0.001%, Mg less than 0.001%, Ti less than 0.002%, Zr less than 0.002%, REM less than 0.001%, and satisfy the formulas (1) and (2).

In the low-carbon sulfur free-cutting steel of the present invention, one or more elements selected from Te, Bi, and Se described later may be optionally added as necessary to replace a part of Fe.

Next, the optional additional elements mentioned above will be described.

Te: 0.05% or less, Bi: 0.15% or less, Se: less than 0.30%

Te, Sn, and Se all have the effect of improving machinability. Therefore, in order to further improve the machinability, especially in the wet condition of the relatively low speed range of 100m/min or less, the final machined surface properties and chip handling properties when using HSS tools for cutting can also be contained in the following ranges: .

Te: 0.05% or less

Te and Mn jointly produce Mn(S, Te), which has the effect of improving the machinability in cutting with HSS tools, especially the final surface roughness. That is, even if Te is added, the proportion of Mn (S, Te) with a large width is only increased without affecting the oxide form, so the machinability in cutting using HSS tools in the low cutting speed region can be improved. , especially the final processed surface properties. In order to obtain this effect, its content is preferably 0.0005% or more. On the other hand, if more than 0.05% of Te is contained, the effect is saturated, the cost increases, and the hot workability deteriorates. Therefore, when Te is contained, the Te content is 0.05% or less. In addition, in order to achieve both good hot workability and good workability more stably, the Te content is preferably 0.0005 to 0.03%, more preferably 0.003 to 0.03%.

Bi: 0.15% or less

Bi has an embrittlement effect as a low-melting-point metal inclusion like Pb, and has an effect of improving the machinability of steel. In order to obtain this effect, its content is preferably 0.01% or more. On the other hand, if more than 0.15% of Bi is contained, the effect is saturated, the cost increases, and the hot workability deteriorates. Therefore, when Bi is contained, the Bi content is 0.15% or less. In addition, in order to achieve both good hot workability and good workability more stably, the Bi content is preferably 0.01 to 0.10%, more preferably 0.02 to 0.10%.

Se: less than 0.30%

Se and Mn together form Mn(S, Se), which has the effect of improving the machinability in cutting with HSS tools, especially the final surface roughness. That is, even if Se is added, the proportion of Mn (S, Se) with a width of 4 μm or more is only increased without affecting the oxide morphology, so the machinability in cutting using HSS tools in the low cutting speed region can be improved , especially the final processed surface properties. In order to obtain this effect, its content is preferably 0.0005% or more. On the other hand, when Se is contained in an amount of 0.30% or more, the effect is saturated, the cost increases, and the hot workability deteriorates. Therefore, when Se is contained, the Se content is less than 0.30%. In addition, in order to achieve both good hot workability and good workability more stably, the Se content is preferably 0.0005 to 0.15%, more preferably 0.005 to 0.15%.

The aforementioned Te, Bi, and Se may be added alone or in combination of two or more.

Based on the above reasons, the chemical composition of the low-carbon sulfur free-cutting steel of the present invention (2) is defined as containing any one of Te: 0.05% or less, Bi: 0.15% or less, and Se: less than 0.30%, instead of Part of Fe in the low-carbon sulfur free-cutting steel of the present invention (1).

In addition, Cr, Mo, Cu, and Ni hardly affect the machinability as long as the content is within the range of Cr: 0.25% or less, Mo: 0.10% or less, Cu: 0.20% or less, and Ni: 0.20% or less. Allowed to exist as an impurity.

Next, the Hv hardness after cold working of the low carbon sulfur free cutting steel of the present invention and the reasons for its limitation will be described.

Hv hardness after cold working: 180～230

In order to have the desired machinability of the low-carbon sulfur free-cutting steel of the present invention without adding Pb after cutting a long distance with an HSS tool under wet conditions in a relatively low speed range of 100 m/min or less, that is, with Compared with the existing free-cutting steel without Pb, which has the same chip disposability as the existing free-cutting steel without Pb, and has a good surface texture with less surface roughness after final processing, it is necessary to embrittle the steel itself by cold working.

That is, the steel of the present invention aims to further reduce the nuclei for the growth of built-up edge by reducing the content of C, and as a result, suppress the size of built-up edge and obtain a good surface texture with a small final surface roughness. Therefore, , in the state without cold working, the ductility is high, and the chip handling property tends to be deteriorated compared with the conventional free-cutting steel without adding Pb.

Therefore, in order to embrittle the steel itself and improve the chip handling properties, real-time cold working is necessary. When the Hv hardness after cold working is 180 or more, the desired chip handling properties as described above can be obtained, that is, it can be obtained. Chip control performance equivalent to that of free-cutting steel. However, due to the large degree of processing at low temperatures, the Hv hardness after cold working is too large, especially when it is greater than 230, the final machined surface roughness increases, and the desired final machined surface properties as described above cannot be obtained, that is, it is different from the existing ones. Compared with the free-cutting steel to which Pb is added, a favorable surface texture with a small final surface roughness cannot be obtained.

For the reasons described above, in the low-carbon sulfur free-cutting steel of the present invention, the Hv hardness after cold working is set to 180-230.

In addition, as long as the Hv hardness after cold working is 180-230, the method of cold working is not particularly specified. For example, ordinary cold working such as real-time wire drawing to ensure high straightness may be used.

The low-carbon sulfur free-cutting steel of the present invention is preferably industrially mass-produced as follows, for example.

First, in the case of producing the low-carbon sulfur free-cutting steel of the present invention 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 must be adjusted .

Specifically, the amount of Mn contained in molten steel at the start of ladle refining is adjusted to be less than 1.5%, preferably less than 1.2%. At this stage, even if the molten steel contains 1.5% or more of Mn, it can be adjusted to the above-mentioned range in the end. However, in order to obtain an appropriate form of oxides and MnS, it is preferable to adjust the Mn content at the beginning of refining as described above. Make adjustments. It is preferable to adjust the Mn content in the slag at the beginning of refining to an appropriate range, specifically, to a range of 25 to 40%, while adjusting the Mn content. In addition, it is only necessary to adjust to a predetermined Mn content by adding alloy iron from the second half stage to the final stage of refining.

Next, adjust the cooling rate during casting to obtain an appropriate form of MnS.

That is, the cooling rate of the slab differs greatly between the skin and the center. Therefore, in order to obtain the preferred form of MnS, the cooling rate of the center is set to be at least 1°C/min or more, more preferably 2°C/min or more. Allow to cool.

In addition, in the case of producing a steel ingot by the agglomeration method, as in the case of casting in a small mold, if the cooling rate is fast, as long as the cooling rate at the center of the steel ingot is set to 20°C/min or less That's it. On the contrary, when the cooling rate is slow as in the case of casting in a huge mold, it is only necessary to design the mold so that the cooling rate at the central part is 1° C./min or more.

Hereinafter, the present invention will be described in more detail through examples.

[Example]

(Example 1)

Steels 1 to 16 having the chemical compositions shown in Table 1 were melted using a high-frequency induction furnace to produce steel ingots with a diameter of about 220 mm.

Steels 1 to 8 in Table 1 are steels whose chemical compositions are within the range specified in the present invention (hereinafter referred to as "steels of the present invention examples"), and steels 9 to 16 are steels whose chemical compositions deviate from the conditions specified in the present invention. The steel of the comparative example. In addition, steel 9 among the steels of the comparative example is a steel corresponding to a conventional free-cutting steel to which Pb is not added.

Taking the position of Di/8 part (where "Di" is the diameter of the steel block) close to the surface of the steel block of each of the above steels as the center, a high-temperature tensile test with a diameter of 10 mm and a length of 130 mm is taken from the height direction of the steel block. A test piece was used to investigate its hot workability. That is, using a thermal processing reproduction test device, after high-frequency heating to 1250°C in vacuum and holding for 5 minutes, cooling to 900°C at a rate of 10°C/min, holding ^for 10 seconds, and setting the deformation speed to 10 seconds- ¹ Conduct a high temperature tensile test at 900°C to investigate hot workability. In addition, the heating area of the above-mentioned bar-shaped test piece was about 20 mm in the central part in the longitudinal direction, and the high-temperature tensile test was directly quenched. Among the above, 900°C was selected as the temperature for the high-temperature tensile test because the tensile value of the high-temperature tensile, which is an index of hot ductility, is extremely small at 900°C in the case of low-carbon sulfur free-cutting steel.

The hot workability was evaluated by the elongation (%) in the above-mentioned high temperature tensile test. In addition, the target of hot workability is to have a tensile value of 30% or more in the above-mentioned high temperature tensile test. This is because steel containing more than 0.4% of S does not generate internal cracks during continuous casting even when the above-mentioned stress value is present, and cast slabs can be stably produced.

For cracks, the Hv hardness of each steel after cold working by the following method, that is, the machinability was investigated.

That is, after heating the remaining amount of the above-mentioned steel ingot with a diameter of about 220mm of each steel to 1200°C and holding it for more than 2 hours, hot forging is carried out so that the final temperature reaches 1000°C or higher, and after forging, it is air-cooled to produce a steel block with a diameter of 40mm. round stick. In addition, Steel 13 had a low tensile value and was judged to be poor in productivity, so the following investigation was not carried out.

Next, peeling was performed on the above-mentioned round bar with a diameter of 40 mm, and it was processed into a round bar with a diameter of 31 mm, which was subjected to cold drawing. In addition, based on the results of the preliminary investigation, the reduction of area was adjusted and the drawing process was performed so that the Hv hardness after processing satisfied the range of 180 to 230 specified in the present invention, and the Hv hardness measurement and analysis were performed using the drawn round bar. Machinability survey.

The measurement of the Hv hardness is to cut out the test piece from the longitudinal section direction of the drawn Df/4 (here, "Df/4" represents the diameter of each round bar.) part, embed it in the resin, and carry out the mirror surface After grinding, measure its Vickers hardness with a test force of 9.807N. In addition, five points were measured for each steel, and the average value thereof was defined as the Hv hardness.

In terms of machinability, the round bars obtained by the above-mentioned cold drawing process were used as test materials, and HSS tools without coating treatment were used, specifically, SKH4 (JIS G 4403 (2000) turning tools were used. The inserts were turned under the following conditions, and the final surface roughness and chip controllability were investigated.

Cutting speed: 100m/min,

Feed rate: 0.05mm/rev,

Cutting depth: 1.0mm,

Lubrication: Wet lubrication with water-soluble lubricating oil.

Use a stylus roughness meter to measure 3 points on the surface after cutting according to the cutting distance of 100m, 700m, 1500m and 2000m under the above conditions, and calculate the maximum roughness Rz and average roughness of the final machined surface at each cutting distance above Ra, and then average these roughnesses, and use the averaged value as the maximum roughness Rz and average roughness Ra of the final machined surface of each test material after long-distance cutting, and evaluate the final machined surface roughness .

In addition, the chips discharged during the cutting distance of 100 m under the above conditions were collected, and the mass of 20 chips was measured sequentially from the long chips, and the chip handling properties were evaluated based on the masses. That is, the smaller the value of the mass, the better the chip disposability. Therefore, when the mass is 5.0 g or less, which is equivalent to Steel 9, which is a conventional free-cutting steel without adding Pb, it is judged that the chip disposability is good. . In addition, as a result of discharging long chips due to poor chip disposability, for 20 pieces of steel from which no chips were obtained, the number and mass of the steels were converted into mass per 20 pieces.

Table 2 summarizes the above test results.

"○" in the "hot workability" column in Table 2 indicates that the steel has a tensile value of 30% or more in the high temperature tensile test and has good hot workability, and "×" indicates that the tensile value in the high temperature test is lower than 30%, steel with low hot workability.

In addition, "○" in the column of "chip disposability" in Table 2 indicates that the mass of the chips is 5.0 g or less, and the steel has the same chip disposability as Steel 9, which is equivalent to the conventional free-cutting steel without adding Pb, In addition, "x" indicates a steel whose chip quality is higher than 5.0 g, and whose chip treatability is inferior to the above-mentioned steel 9 corresponding to the conventional free-cutting steel without adding Pb. "-" of steel 13 in Table 2 indicates a steel that was judged to be poor in productivity due to low hot ductility and was not investigated.

Table 2

It can be seen from Table 2 that although the low-carbon sulfur free-cutting steel of the present invention does not contain Pb, it still has the same performance as the current one after cutting a long distance with HSS tools under wet conditions in a relatively low speed range below 100 m/min. It has the same chip disposability as free cutting steel without adding Pb, and has a good surface texture with less surface roughness after finishing than the above-mentioned conventional free cutting steel without adding Pb. In addition, it was also found that the low-carbon sulfur free-cutting steel of the present invention has good hot workability, and there is no problem in performing industrial mass production required for continuous casting.

On the other hand, the steel of the comparative example deviated from the conditions specified in the present invention was inferior to the low-carbon sulfur free-cutting steel of the present invention in at least one of chip disposability, final machined surface texture, and hot workability.

(Example 2)

Steel 17 and Steel 18 having the chemical compositions shown in Table 3 were melted using a high-frequency induction furnace to produce steel ingots with a diameter of about 220 mm.

In addition, steel 17 is steel whose chemical composition falls within the range prescribed|regulated by this invention. On the other hand, Steel 18 is a steel in which the C content in the chemical composition deviates from the conditions specified in the present invention, and corresponds to a conventional free-cutting steel to which Pb is not added. In addition, the S contents of the above two steels were adjusted to be approximately the same level.

table 3,

The above-mentioned steel block with a diameter of about 220mm was heated to 1200°C and held for more than 2 hours, then hot forged so that the final temperature reached 1000°C or higher, and air-cooled after forging to produce a round bar with a diameter of 40mm.

Next, peeling was performed on the above-mentioned round bar with a diameter of 40 mm, processed into a round bar with a diameter of 31 mm, cold-drawn at each reduction of area shown in Table 4, and Hv was performed using each round bar after drawing. Measurement of hardness and investigation of machinability. In addition, in the case where the reduction of area is greater than 40%, a round bar is produced by two-stage drawing (two passes).

The measurement of the Hv hardness is to cut out the test piece from the longitudinal section direction of the drawn Df/4 (here, "Df/4" represents the diameter of each round bar.) part, embed it in the resin, and carry out the mirror surface After grinding, measure its Vickers hardness with a test force of 9.807N. In addition, 5 points were measured for each steel after each drawing process, and the average value was made into Hv hardness.

In terms of machinability, the round bars obtained by the above-mentioned cold drawing process were used as test materials, and HSS tools without coating treatment were used, specifically, SKH4 (JIS G 4403 (2000) turning tools were used. The inserts were turned under the following conditions, and the final surface roughness and chip controllability were investigated.

Cutting speed: 100m/min,

Feed rate: 0.05mm/rev,

Cutting depth: 1.0mm,

Lubrication: Wet lubrication with water-soluble lubricating oil.

Use a stylus roughness meter to measure 3 points on the surface after cutting according to the cutting distance of 100m, 700m, 1500m and 2000m under the above conditions, and calculate the maximum roughness Rz and average roughness of the final machined surface at each cutting distance above Ra, and then average these roughnesses, and use the averaged value as the maximum roughness Rz and average roughness Ra of the final machined surface of each test material after long-distance cutting, and evaluate the final machined surface roughness .

In addition, the chips discharged during the cutting distance of 100 m under the above conditions were collected, and the mass of 20 chips was measured sequentially from the long chips, and the chip handling properties were evaluated based on the masses. That is, the smaller the value of the mass, the better the chip disposability. Therefore, when the mass is 5.0 g or less which is equivalent to Steel 18, which is a conventional free-cutting steel without adding Pb, it is judged that the chip disposability is good. . In addition, as a result of discharging long chips due to poor chip disposability, for 20 pieces of steel from which no chips were obtained, the number and mass of the steels were converted into mass per 20 pieces.

Table 4 summarizes the above test results. 1 and 2 respectively show the relationship between the Hv hardness after drawing and the maximum roughness Rz and Ra of the final machined surface. In addition, in each figure of the coral tree, the result of steel 17 is described with a symbol as "invention example", and the result of steel 18 is described with a symbol of "comparative example".

Table 4

From Table 4, Fig. 1 and Fig. 2, it can be seen that the steel whose chemical composition is within the range specified in the present invention can be cut with HSS tools for a long distance under wet conditions in a relatively low speed range below 100m/min. Finally, good chip disposability and good surface texture with small final machined surface roughness can also be obtained. It is necessary to adjust the Hv hardness after cold working to be within the range specified in the present invention.

As mentioned above, although an Example demonstrated this invention concretely, this invention is not limited to these Examples. What is not disclosed as an embodiment is of course included in the present invention as long as it satisfies the main requirements of the present invention.

Industrial availability

Although the steel of the present invention is a "free-cutting steel that is beneficial to the global environment" without adding Pb, it still has the same performance as the existing steel without adding Pb after cutting for a long distance in a relatively low speed range below 100m/min using HSS tools. Pb-free cutting steels have the same chip handling properties as compared with conventional free-cutting steels without adding Pb. At the same time, it can ensure a good surface texture with a small surface roughness in the final machining, and because of its excellent continuous casting, it can Produced in large quantities cheaply. 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 (2)

1. A low-carbon sulfur free-cutting steel with excellent machinability, characterized in that it contains C: less than 0.05%, Si: less than 0.05%, Mn: 0.7-2.2%, and P: 0.03- 0.20%, S: more than 0.40% and less than 0.70%, Al: less than 0.005%, O: more than 0.0050% and less than 0.0380%, N: 0.0020 to 0.0250%, and the balance is composed of Fe and impurities. Ca, Mg, 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), in addition, the Vickers hardness after cold working is 180～230, 010<O/S<0.080...(1) 2.5<Mn/(S+O)<4.0...(2) Wherein, the symbols of the elements in the formulas (1) and (2) represent the mass percentage content of the element in the steel. 2. The low-carbon sulfur free-cutting steel according to claim 1, wherein instead of a part of Fe, one of Te: 0.05% or less, Bi: 0.15% or less, and Se: less than 0.30% is contained in mass % above.

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