KR20090128559A - Low-carbon sulphur free-cutting steel - Google Patents

Abstract

A low-carbon sulphur free-cutting steel which contains C: 0.02% to less than 0.20%, Si: more than 0.10% to 1.5%, Mn: 0.8 to 2.2%, P: 0.005 to 0.25%, S: more than 0.40% to 0.8%, O: <0.010%, N: <= 0.025%, and Ca: 0.0003 to 0.005% with the balance consisting of Fe and impurities and in which the impurities contain Al < 0.005%, Mg < 0.0003%, Ti <= 0.002%, Zr <= 0.002%, and REM < 0.0003% and the relationships: 2.0 < Mn/S < 4.0, and 0.0005 <= 10Ca x Si <= 0.050 are satisfied attains a smaller finished-surface roughness than that of conventional non-Pb low-carbon free-cutting steel and excellent chip disposability in the finish cutting process by turning with a carbide tool. The low-carbon sulphur free-cutting steel may contain one or more of Te <= 0.05%, Bi <= 0.15%, and Sn <= 0.5% and/or one or more of Cr <= 2.0%, Mo <= 0.5%, V <= 0.3%, and Nb <= 0.3%.

Description

LOW-CARBON SULPHUR FREE-CUTTING STEEL}

TECHNICAL FIELD This invention relates to low carbon sulfur free-cutting steel, and has low Pb non-additive low carbon suitable for the use of OA parts, such as a printer shaft of a narrow diameter, which has outstanding machinability, especially when finishing turning using a carbide tool. It is about sulfur free cutting steel.

Background Art Conventionally, so-called "free-cutting steels", which are steel materials excellent in machinability, have been used for materials such as soft small parts, for example, brake parts of automobile parts, printer shafts of OA parts, and electrical equipment parts.

As the free-cutting steel, the steel specified in JIS G 4804 (1999), that is, "sulfur free-cutting steel" which improved the machinability by MnS by adding a large amount of S (hereinafter, referred to as "S free-cutting steel"), S and "Sulfur composite free-cutting steel" which added both of Pb (henceforth "S composite free-cutting steel"), or "lead free cutting steel" which added Pb (henceforth "Pb free-cutting steel") is well. Known.

Among the above free cutting steels, Pb containing, that is, Pb free cutting steels and S composite free cutting steels are easily broken into chips, so that they are excellent in the so-called "crude treatment property", have a long tool life, and have a long tool life. It has the characteristic that the finish surface is excellent in roughness.

However, in recent years, due to the increase in global environmental problems, the movement to remove Pb from the product is becoming stronger. For example, in Europe, the content of Pb contained in steel is limited to 0.35% or less by mass%. It is desired to reduce the content of Pb as much as possible.

In addition, since Pb has a low melting point and a low solubility in steel, steel containing Pb poses a problem in terms of production, such as being prone to breakage during rolling.

Therefore, in order to solve the problem mentioned above, for example, Patent Documents 1 to 5 describe a low carbon free cutting steel containing no Pb, while increasing the amount of S and sulfide-based inclusions such as MnS (hereinafter, Techniques for controlling the shape of the pattern (hereinafter referred to as "MnS") to improve the machinability, or techniques for improving the machinability by controlling the structure have been proposed.

Specifically, in Patent Document 1, in the manufacturing method of high S free cutting steel having a Mn / S ratio (mass ratio) of 3.5 or more, the free oxygen concentration of molten steel immediately before casting is set to 0.004% or more by mass%. The technique regarding the "high S free cutting steel" of low carbon is disclosed.

"Low-carbon sulfur-based free-cutting steel wire" which defines the width of the sulfide-based inclusions contained in steel while containing high amount of S up to 0.50% and high amount of O of 0.01 to 0.03% in Patent Document 2. A technique is disclosed.

Patent Literature 3 discloses a technique relating to “steel having excellent machinability”, which contains 0.1% to 0.5% of S in mass% and has a pearlite area ratio of 5% or less.

Patent Literature 4 discloses a technique related to “steel having excellent machinability”, wherein the pearlite area ratio is 5% or less while containing 0.5% to 1.0% of S in mass%.

Patent Document 5 contains S in the range of 0.1% to 1.0% by mass, and the presence density of MnS of 0.1 to 0.5 µm in a circular equivalent diameter is 10000 pieces / mm ² or more. Steel "is disclosed.

[Patent Document 1: Japanese Patent Laid-Open No. 2005-23342]

[Patent Document 2: Japanese Patent Laid-Open No. 2003-253390]

[Patent Document 3: Japanese Patent Application Laid-Open No. 2004-176176]

[Patent Document 4: Japanese Patent Application Laid-Open No. 2004-169054]

[Patent Document 5: Japanese Patent Application Laid-Open No. 2004-169052]

However, the above technique relates to improving the machinability at the time of forming by a high-speed steel tool (hereinafter, referred to as an "HSS tool") mainly intended for machining automotive parts such as brake parts.

In the case of machining on automobile parts such as brake parts, an automatic lathe is used as a processing machine, and mainly a HSS tool is used to press the tool in a direction perpendicular to the rotation axis of the workpiece to send the tool, thereby transferring the tool shape to the workpiece, thereby forming the part shape. There is a lot of finish processing by processing method such as processing by so-called forming (cutting) processing.

In the case of forming, the surface of the part is finished by the entire cutting edge of the tool, so that the "constituent cutting edge" formed on the cutting edge of the tool greatly affects the finish surface roughness. And in order to obtain the component of a small finish surface roughness, the constituent cutting edge formed in the cutting edge of a tool is small, it does not fall out during cutting, and the size must be stabilized.

And all of the free cutting steel materials proposed by the above-mentioned patent documents 1-5 form a small, stable component knife point as mentioned above in the tool tip by controlling a chemical component or a manufacturing method, and finish surface roughness in forming process It was intended to improve.

On the other hand, since so-called "OA parts" such as a printer shaft are precision parts, a smaller finish surface roughness is required than automobile parts such as brake parts.

In the case of such an OA component, in order to obtain a smaller finish surface roughness, turning is performed using a carbide tool or a carbide tool coated on the surface at a small feed amount of 0.07 mm / rev or less under wet conditions using a lubricant. It is often finished by. And for steel materials used for such a use, it is required to maintain small finish surface roughness for a long time under the same tool in an automated processing line.

Although the techniques proposed in the above-mentioned patent documents 1 to 5 can all improve the machinability at the time of forming using an HSS tool, in the case of finishing turning using such a cemented carbide tool, the finish surface roughness is always sufficient. I could not say that.

An object of the present invention is to provide a superior machinability compared to conventional Pb non-added low carbon free-cutting steel, in particular, when finishing turning using a cemented carbide tool. It is to provide a low carbon sulfur free cutting steel.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly examined for the said objective.

As a result, first, knowledge of the following <1> to <4> was obtained.

In the case of turning using a <1> cemented carbide tool, since the workpiece is hardly adhered to the tool, a constituent knife tip is hardly formed on the tool tip. Therefore, in the case of finishing turning by a cemented carbide tool, determining the finishing surface roughness of the workpiece is not the same as a constituent knife tip as in the case of forming by the HSS tool.

As a turning condition for finishing OA parts requiring a smaller finish surface roughness, such as <2> printer shafts, a small feed amount of 0.07 mm / rev or less is often set under wet conditions. The surface of the workpiece is finished by the nose R portion of the tool.

<3> When it processes for a long time in such a state, as shown typically in FIG. 1, several groove-like abrasions generate | occur | produce in the nose R part of the tip of the cemented carbide tool which contact | abuts the surface of a workpiece, and the feed amount.

In the case of finishing turning with a small amount of feed using a <4> carbide tool, the groove-shaped abrasion generated on the tool side is transferred to the surface on the workpiece side, whereby the roughness of the finished surface of the workpiece is determined, and the groove-side on the tool side is determined. The greater the wear, the greater the finish surface roughness of the workpiece. Immediately after tool change, i.e., at the initial stage of the start of the use of a new tool, groove-shaped wear hardly occurs at the tip of the tool, so the difference due to the material of the work material is not so recognized in the finished surface roughness obtained. However, as the machining time becomes longer, the groove-shaped wear of the tool tip develops, and the finish surface roughness of the workpiece gradually increases. Therefore, in order to maintain a smaller finish surface roughness for a long time under the same tool, it is important to make the groove-shaped abrasion occurring at the tip of the tool smaller and less prone to progress, and the same is true for the free cutting steel used in such a machining method. Properties are required for the material design.

Therefore, the present inventors examined the influence of the inclusion shape and composition of free cutting steel on the groove-shaped wear of the tip of the tool in finishing turning with a small amount of feed by the cemented carbide tool. Specifically, first, the form of MnS of the workpiece was changed in various ways, and a detailed study was conducted on how the distribution form of MnS affects the wear of the cemented carbide tool.

As a result, new knowledge of the following <5> to <7> was obtained.

The groove-shaped wear of the tool tip is hardly affected by the relatively small MnS, but is affected by the coarse MnS. When there is almost no MnS exceeding 10 micrometers in terms of a circular equivalent diameter, specifically, when the total area of MnS exceeding 10 micrometers in terms of a circular equivalent diameter is 10% or less among MnS observed in 1 mm <^2> of steel longitudinal cross sections. The groove shape at the tip of the tool can be suppressed.

In order to suppress generation of coarse MnS as much as possible, it is effective to reduce the amount of 0 contained in the steel as much as possible. By reducing the content of 0, the amount of 0 dissolved in MnS, that is, the amount of solid solution of 0 in MnS decreases, and the deformation resistance of MnS can be reduced.

<7> Even in the casting piece after solidification, even if coarse MnS exceeding 10 μm exists, MnS having a small deformation resistance with a small amount of O is stretched and finely divided when hot worked, so that fine MnS can be obtained. .

Therefore, the present inventors further examined oxides by changing various deoxidation elements in order to reduce the amount of 0 contained in steel.

As a result, knowledge of the following <8>-<11> was obtained.

<8> Elements having a high affinity to O, such as Al, Mg, Ti, Zr, and REM (rare earth elements), can reduce the content of O and reduce the coarse MnS, but all of these elements contain hard oxides. Since it is easy to form, it is not possible to suppress the groove-shaped wear which arises in a tool tip.

<9> Although it is an element effective for reducing Si and O content, when used alone as a deoxidation element, hard SiO ₂ is formed. For this reason, it is not effective for suppressing the groove shape wear of a tool tip.

<10> However, by using Si and Ca together to adjust the mass balance and limiting the content of elements having a high affinity with 0, such as Al, Mg, Ti, Zr, and REM contained in the impurities, It is possible to have a soft composite oxide composition of Al ₂ O ₃ -MnO-SiO ₂ -CaO-based containing at least 5% by mass of CaO and SiO ₂ as an average composition, and greatly suppress the groove shape wear at the tip of the tool. Can be.

<11> As described above, when the finish turning process using a cemented carbide tool is performed by reducing the coarse MnS and using a soft oxide composition, the machinability is superior to that of a low carbon free cutting steel without conventional Pb. Especially, the low carbon sulfur free cutting steel of the non-Pb addition which can obtain the outstanding surface property with a small finish surface roughness can be provided.

This invention is completed based on said knowledge, The summary is in the low carbon sulfur free cutting steel shown to following (1)-(3).

(1) In mass%, C: 0.02% or more but less than 0.20%, Si: 0.10% or more and 1.5% or less, Mn: 0.8 to 2.2%, P: 0.005 to 0.25%, S: 0.40% or more and 0.8% or less, O: Less than 0.010%, N: 0.025% or less, Ca: 0.0003 to 0.005%, the balance is made of Fe and impurities, and Al, Mg, Ti, Zr, and REM contained in the impurities are respectively less than 0.005%. , Mg: less than 0.0003%, Ti: 0.002% or less, Zr: 0.002% or less, and REM: less than 0.0003% and satisfy the following formulas (1) and (2).

2.0 <Mn / S <4.0... (One),

0.0005 ≦ 10 Ca × Si ≦ 0.050... (2).

However, the element symbol in Formula (1) and Formula (2) shows the content in steel in the mass% of the element.

(2) The low-carbon sulfur free-cutting steel according to the above (1), which contains one or more of Te: 0.05% or less, Bi: 0.15% or less, and Sn: 0.5% or less by mass%.

(3) Said (1) or (2) containing 1 or more of Cr: 2.0% or less, Mo: 0.5% or less, V: 0.3% or less, and Nb: 0.3% or less by mass% instead of a part of Fe. Low carbon sulfur free cutting steel described in

In addition, "REM" said by this invention is a general term of 17 elements of Sc, Y, and a lanthanoid in total, and content of REM points out the total content of the said element.

Hereinafter, the low carbon sulfur free cutting steel of said (1)-(3) is called "this invention (1)"-"this invention (3)", respectively. Moreover, generically, it may be called "this invention."

The steel of the present invention has a smaller finish surface roughness than the conventional low carbon free cutting steel of Pb non-addition, when subjected to finish turning using a cemented carbide tool, regardless of the Pb non-adding "free cutting steel excellent in the global environment". , Good surface properties are obtained. Moreover, since it is excellent also in hot workability, it can manufacture at low industrial scale. Therefore, it can use as a raw material of OA parts, such as a narrow-diameter printer shaft, which requires the finish surface roughness smaller smaller than automobile parts, such as a brake part.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which demonstrates typically groove-shaped wear of the space | interval equal to the feed amount formed in the nose R part of the tip of the cemented carbide tool which contacts the surface side of a workpiece.

First, the chemical composition and the reason for limitation in the low carbon sulfur free cutting steel of this invention are described. In addition, in the following description, "%" display of content of each element means the "mass%."

In the low carbon sulfur free-cutting steel of the present invention, in finish turning, it is most required to maintain a small finish surface roughness for a long time under the same tool, and in order to do so, it is necessary to suppress the groove shape wear of the tool tip. .

C: More than 0.02% Less than 0.20%

C is an element having a great influence on machinability and strength. In finish turning using a cemented carbide tool, in order to reduce the finish surface roughness, the C content must be less than 0.20%. This is because, when the content of C is 0.20% or more, the hardness of the steel is increased, so that the groove-shaped wear of the tool tip easily proceeds. On the other hand, when the C content is less than 0.02%, in addition to the increase in manufacturing cost, the hardness becomes too weak, so that good debris treatment property cannot be obtained. Therefore, in order to ensure the outstanding debris processing property, it is unpreferable since the hardness must be repeated several times in cold processing like extension wire processing. Therefore, content of C was made into 0.02% or more and less than 0.20%. Moreover, the range of preferable C content is 0.03 to 0.18%. It is more preferable if it is 0.05 to 0.12%.

Si: 0.10% or more and 1.5% or less

The role of Si in this invention is important, and it must be contained after fully considering mass balance with Ca mentioned later. However, when the content of Si is 0.10% or less, since the content of O cannot be lowered to a sufficient amount, a large number of MnS exceeding 10 μm exists, so that the groove-shaped wear of the tip of the tool tends to proceed, and the good finish surface roughness Not obtained. On the other hand, when Si is contained in an amount exceeding 1.5%, since Si is solid-solution in ferrite to increase the strength of the steel, groove-like wear tends to proceed on the contrary, and a good finish surface roughness is not obtained. Therefore, the range of content of Si was made into more than 0.10% and 1.5% or less. In addition, when the mass balance with Ca mentioned later is considered, since Ca yields a bad yield, it is preferable to obtain a deoxidation effect by Si rather than Ca, and in that case, it is preferable to contain Si exceeding 0.15%, and it is 0.20. It is more preferable to contain more than%. On the other hand, when the hardness rises, the groove-shaped wear of the tool tip is easily promoted, and a good finish surface roughness cannot be obtained. Therefore, the upper limit of the Si content is preferably 1.0%, more preferably 0.5% or less. .

In addition, content of Si in the said range needs to satisfy 0.0005-0.050 also as a value of 10Ca (%) x Si (%), as mentioned later.

Mn ： 0.8-2.2%

Mn is an important element which forms MnS together with S and greatly influences overall machinability, that is, finish surface roughness, debris treatability, and cutting resistance. If the content is less than 0.8%, the absolute amount as MnS will be insufficient, and desired desired machinability cannot be obtained. In addition, splitting occurs in the casting piece during continuous casting, or deteriorates hot workability. Moreover, since Mn also has the effect | action which raises a carburization characteristic, what is necessary is just to increase content of Mn in order to acquire favorable carburization characteristic. However, in the case of containing a large amount of Mn of more than 2.2%, Mn is solid-solution and the hardness is high, so that the groove shape wear is likely to proceed, a good finish surface roughness is not obtained, and the cold workability is lowered. Also causes. Therefore, content of Mn was made into 0.8 to 2.2%. It is preferable that it is 1.0 to 1.8%, and, as for content of Mn, it is more preferable if it is 1.2 to 1.7%.

In addition, content of Mn in the said range needs to satisfy more than 2.0 and less than 4.0 as a value of Mn / S as mentioned later.

P ： 0.005 ~ 0.25%

Since P has the effect of increasing the strength, in the present invention having a low content of C, P is an element effective in adjusting the hardness to ensure the strength as a component and to obtain good finish surface roughness and debris treatment. For that purpose, P may be contained 0.005% or more. However, when P content becomes excess, hardness will become high too much and groove-shaped abrasion will advance easily, and, as a result, a favorable finish surface roughness will not be obtained. In particular, when it exceeds 0.25%, not only groove-shaped wear becomes remarkable but also cold workability and hot workability also fall. Therefore, the range of content of P was made into 0.005 to 0.25%. In addition, it is preferable to make content of P into 0.03 to 0.15%.

S: 0.40% or more and 0.8% or less

S is an essential element for forming MnS together with Mn to secure good overall machinability, that is, finish surface roughness, crumbness and cutting resistance. If the content of S is 0.40% or less, a sufficient amount of MnS cannot be generated, and desired finish surface roughness and debris treatment cannot be obtained. In addition, if the content of S is high, it may cause breakage inside the casting piece during continuous casting or deteriorate hot workability. However, by optimizing the balance with the content of Mn, internal breakdown and hot workability are not degraded. The desired finish surface roughness and cracking treatment can be secured. However, in the case where S is contained in excess of 0.8%, it is necessary to contain a large amount of Mn suitable for the S content. In this case, coarse MnS is formed in a large amount, so that groove-shaped abrasion tends to proceed easily and is satisfactory. Finish surface roughness cannot be obtained. Moreover, addition of excess S exceeding 0.8% by content leads to cost increase by deterioration of a yield. Therefore, content of S was made into more than 0.40% and 0.8% or less.

In order to acquire more machinability more stably, it is preferable to contain S exceeding 0.50%, and the upper limit is preferable to be less than 0.70%, and when it is 0.6% or less, it is still more preferable.

In addition, content of S in the said range needs to satisfy more than 2.0 and less than 4.0 as a value of Mn (%) / S (%), as mentioned later.

O: less than 0.010%

In the present invention, O (oxygen) is a very important element. If the content of O is high, coarse MnS is formed in a large amount. If a large amount of coarse MnS is formed, it advances the progress of the groove-shaped wear of the tool tip, and as a result, the finish surface roughness becomes large. Therefore, in finishing turning, in order to maintain a small finish surface roughness for a long time under the same tool, to reduce the coarse MnS as much as possible, specifically, steel longitudinal section 1 mm ² It is important to reduce the total area of MnS exceeding 10 micrometers in conversion of a circular equivalent diameter among MnS observed in the inside, and to do this, it is necessary to reduce content of O as much as possible. If the content of O is less than 0.010%, even in the casting piece after solidification, even though coarse MnS is present in the steel, it is drawn in the subsequent hot working and finely divided, so that MnS observed in steel longitudinal section 1 mm ² is equivalent. In terms of diameter, the total area of MnS exceeding 10 µm is 10% or less, and coarse MnS is hardly observed in steel. Therefore, small finish surface roughness can be ensured. However, when the content of O increases to 0.010% or more, the amount of O dissolved in MnS also increases, so that the deformation resistance of MnS increases, the MnS is difficult to divide and remains in a coarse state, and thus the finish surface roughness increases. Therefore, content of O was made into less than 0.010%.

In addition, in order to reduce coarse MnS and obtain a small finish surface roughness, the lower the content of O is, the better. The lower the content of O, the less the content of coarse MnS can be reduced stably. If the content of O is less than 0.005%, better machinability can be obtained.

N: 0.025% or less

N is an element inevitably contained as an impurity. In addition, in the present invention, since Al and Ti are substantially not contained, N hardly forms nitrides of Al or Ti and exists in a solid solution in ferrite. When N is positively contained, N dissolved in ferrite has the effect of increasing the strength. In addition, N also has an effect of reducing the finish surface roughness. However, containing N in excess of 0.025% not only saturates the above effects, but also leads to a decrease in cold workability and increases production costs. Therefore, content of N was made into 0.025% or less. In order to improve intensity | strength more effectively, and to make finish surface roughness small, and to provide favorable cold workability, it is preferable to make content of N into 0.005-0.015%.

Ca ： 0.0003 ~ 0.005%

Ca is an important element in the present invention, and if it is contained 0.0003% or more after sufficiently considering the mass balance with Si, the content of O can be lowered to a sufficient amount and the formation of a hard oxide can be suppressed. In addition, the finish surface roughness can be remarkably reduced with the debris treatment. However, since the addition yield is very low, Ca is not preferable because the production cost is too high to contain more than 0.005%. Therefore, content of Ca was made into 0.0003 to 0.005%. Moreover, it is preferable to make content of Ca into 0.0005% or more and less than 0.0045%, and it is more preferable to set it as 0.001% or more and less than 0.0040%.

In addition, content of Ca in the said range needs to satisfy 0.0005-0.050 also as a value of 10Ca (%) x Si (%), as mentioned later.

In the low carbon sulfur free cutting steel according to the present invention, the Al, Mg, Ti, Zr and REM contents in the impurities are respectively less than Al: 0.005%, less than Mg: 0.0003%, less than Ti: 0.002%, and less than Zr: 0.002. It is limited to less than% and REM: less than 0.0003%.

This will be described below.

Al: less than 0.005%

Al is a deoxidation element having a strong affinity with O (oxygen), and when it is contained 0.005% or more as an impurity, it is 0.0005 at a value of Si content, Ca content, and 10 Ca (%) x Si (%) described later, Even if it satisfies -0.050, hard oxide is formed. Therefore, since the groove shape wear of the tool tip cannot be suppressed, in finish turning, a small finish surface roughness cannot be maintained for a long time under the same tool. For this reason, content of Al contained in an impurity needs to be less than 0.005%.

Moreover, it is preferable that content of Al contained in an impurity is less than 0.003%, and it is still more preferable if it is less than 0.002%.

Mg: Less than 0.0003%

Mg is also a deoxidation element with strong affinity with O (oxygen), and when it is contained 0.0003% or more as an impurity, it is 0.0005 at a value of Si content, Ca content, and 10 Ca (%) x Si (%) described later. Even if it satisfies -0.050, hard Mg oxide is formed. Therefore, since the groove shape wear of the tool tip cannot be suppressed, in finish turning, a small finish surface roughness cannot be maintained for a long time under the same tool. For this reason, content of Mg contained in an impurity needs to be less than 0.0003%.

Ti: 0.002% or less

Ti is also a deoxidation element having a strong affinity with O (oxygen), and when it exceeds 0.002% as an impurity, the Si content, Ca content described above, and the value of 10Ca (%) × Si (%) described later are 0.0005. Even if it satisfies -0.050, hard oxide of Ti is formed. Therefore, since the groove shape wear of the tool tip cannot be suppressed, in finish turning, a small finish surface roughness cannot be maintained for a long time under the same tool. For this reason, content of Ti contained in an impurity needs to be 0.002% or less. Preferable content of Ti contained in an impurity is 0.001% or less, and it is more preferable in it being 0.0005% or less.

Zr ： 0.002% or less

Zr is also a deoxidation element having a strong affinity with O (oxygen), and when it exceeds 0.002% as an impurity, it is 0.0005 at a value of Si content, Ca content, and 10 Ca (%) x Si (%) described later. Even if it satisfies -0.050, hard Zr oxide is formed. Therefore, since the groove shape wear of the tool tip cannot be suppressed, in finish turning, a small finish surface roughness cannot be maintained for a long time under the same tool. For this reason, content of Zr contained in an impurity needs to be 0.002% or less. Preferable content of Zr contained in an impurity is 0.001% or less, and it is more preferable if it is 0.0005% or less.

REM: Less than 0.0003%

REM is also a deoxidation element having a strong affinity with O (oxygen), and when it is contained 0.0003% or more as an impurity, it is 0.0005 at a value of Si content, Ca content and 10 Ca (%) x Si (%) described later. Even if it satisfies -0.050, hard REM oxide is formed. Therefore, since the groove shape wear of the tool tip cannot be suppressed, in finish turning, a small finish surface roughness cannot be maintained for a long time under the same tool. For this reason, content of REM contained in an impurity needs to be less than 0.0003%.

In addition, as already described, "REM" is a general term of 17 elements of Sc, Y, and lanthanoid, and content of REM points out the total content of the said element.

Value of "Mn / S": More than 2.0 Less than 4.0

C, Si, Mn, P, S, 0, N, Ca in the above-mentioned range, the balance is made of Fe and impurities, and Al, Mg, Ti, Zr, and REM contained in the impurities are Al: In steels having less than 0.005%, less than Mg: less than 0.0003%, less than Ti: 0.002%, less than Zr: less than 0.002%, and less than REM: 0.0003%, the value of the "Mn / S" is greater than 2.0 and less than 4.0, that is, the above (1 It is necessary to satisfy the expression.

That is, the present invention contains a high amount of S exceeding 0.40%. When the value of "Mn / S" is less than 2.0, since hot ductility is inadequate, breakage tends to generate | occur | produce inside the casting piece at the time of continuous casting. Moreover, even if splitting does not occur inside the casting piece, hot workability is insufficient. On the other hand, when the value of "Mn / S" is 4.0 or more, excessive Mn is contained, so that the amount of Mn dissolved in the matrix becomes excessive and the groove-shaped wear of the tool tip cannot be suppressed. Thus, small finish surface roughness cannot be maintained during prolonged processing under the same tool. Therefore, the value of "Mn / S" is more than 2.0 and less than 4.0, that is, it is necessary to satisfy the above formula (1). In addition, the element symbol in "Mn / S" in Formula (1) shows the content in steel in the mass% of the element.

The value of "Mn / S" is preferably 2.5 or more and less than 3.5, and more preferably 2.8 or more and less than 3.5.

Value of "10Ca x Si": 0.0005 or more and 0.050 or less

C, Si, Mn, P, S, 0, N, Ca in the above range, the balance is made of Fe and impurities, Al, Mg, Ti, Zr and REM contained in the impurities, respectively: Al: 0.005 For steels with less than%, less than Mg: less than 0.0003%, less than Ti: 0.002%, less than Zr: less than 0.002%, and less than REM: 0.0003%, the value of the "10Ca x Si" is 0.0005 or more and 0.050 or less, that is, the above (2) It is also necessary to satisfy the equation.

As already described, the coarse MnS employs a large amount of O, and during the final turning operation, the groove shape wear of the tool tip is likely to proceed. For this reason, a MnS reduces employment O amount to reduce the deformation resistance, it is necessary to finely by hot working, be appropriate in addition to this, the oxide composition, specifically, the sum of CaO and SiO ₂ as an average composition of the oxide In the case of a soft oxide containing at least 5% by mass, it is possible to maintain a small finish surface roughness for a long time under the same tool in finish turning. To this end, as described above, Al, Mg, Ti, Zr and REM contained in impurities, which are elements having high affinity with oxygen, are respectively less than Al: 0.005%, less than Mg: 0.0003% and Ti: 0.002%. Hereinafter, in addition to restricting to Zr: 0.002% or less and REM: less than 0.0003%, the value of "10Ca x Si" needs to be 0.0005 or more and 0.050 or less.

When the value of "10Ca x Si" is less than 0.0005, it becomes difficult to lower the content of O. In addition, even if the content of O can be lowered, the average composition of the oxide is not a soft oxide containing at least 5% by mass of CaO and SiO ₂ in total, and the groove-like wear of the tool tip is likely to proceed. On the other hand, when the value of &quot; 10Ca x Si &quot; exceeds 0.050, a soft oxide composition is hardly obtained, and the excessive Si content causes the hardness of the ferrite to be too high, which leads to the progression of the groove shape wear of the tool tip. Easier In addition, considering the addition yield of Ca, the manufacturing cost increases. Therefore, the value of "10Ca x Si" needs to satisfy 0.0005 or more and 0.050 or less, that is, Formula (2). In addition, the element symbol in "10Ca * Si" in Formula (2) shows the content in steel in the mass% of the element.

In addition, in order to obtain a stable small finish surface roughness, the value of "10Ca x Si" is preferably 0.001 or more and 0.030 or less, and more preferably 0.002 or more and 0.010 or less.

From the above reasons, the low carbon sulfur free cutting steel according to the present invention (1) contains C, Si, Mn, P, S, 0, N, and Ca in the above-described range, and the balance is made of Fe and impurities, and impurities Al, Mg, Ti, Zr and REM contained in the inside of the above-mentioned range were prescribed | regulated as the thing which satisfy | fills said Formula (1) and (2), respectively.

In the low carbon sulfur free cutting steel which concerns on this invention, instead of a part of Fe in the said this invention (1) as needed,

1st group: Te: 0.05% or less, Bi: 0.15% or less, Sn: 0.5% or less 1 or more types,

The second group: Cr: 2.0% or less, Mo: 0.5% or less, V: 0.3% or less, and Nb: 0.3% or less.

That is, in order to obtain more outstanding characteristic, even if it contains at least 1 type of elements of the at least 1 group of the said 1st group and 2nd group instead of a part of Fe in the low carbon sulfur free cutting steel of this invention (1), do.

Hereinafter, the above elements will be described.

1st group: Te: 0.05% or less, Bi: 0.15% or less, Sn: 0.5% or less 1 or more types

Te, Bi, and Sn are all elements which have the effect | action which improves machinability, and you may contain in the following ranges, in order to acquire more machinability.

Te: 0.05% or less

Te produces Mn (S, Te) together with Mn and S, making it difficult to advance the groove shape wear of the tool tip and improving the finish surface roughness. You may contain in the said range. However, even if it contains Te exceeding 0.05%, since the effect is saturated, it becomes inexpensive and hot workability also deteriorates. Therefore, content of Te in the case of adding was made into 0.05% or less.

In order to secure the above-mentioned effects of Te, it is preferable to make content of Te into 0.002% or more. For this reason, more preferable Te content in the case of adding is 0.002-0.05%. In addition, it is preferable that the upper limit of Te content shall be 0.03%.

Bi: 0.15% or less

Bi has the embrittlement effect as the low melting point metal inclusion similar to Pb, and has the effect of improving all the machinability of the steel, namely, finish surface roughness, debris treatment and cutting resistance. However, even if Bi is contained in excess of 0.15%, the effect is saturated, the cost increases, and the hot workability also deteriorates. Therefore, content of Bi when adding is made into 0.15% or less.

In order to ensure the effect of Bi mentioned above, it is preferable to make Bi content into 0.01% or more. For this reason, more preferable Bi content in the case of adding is 0.01 to 0.15%. In addition, it is preferable that the upper limit of Bi content shall be 0.10%.

Sn: 0.5% or less

Sn has an effect of improving the finish surface roughness and the debris treatment. It is considered that this is because Sn embrittles the matrix. However, even if it contains Sn exceeding 0.5%, the effect will be saturated, cost will increase, and hot workability will also deteriorate. Therefore, content of Sn in the case of adding was made into 0.5% or less.

In order to ensure the effect of Sn mentioned above, it is preferable to make content of Sn into 0.05% or more. For this reason, more preferable Sn content in the case of adding is 0.05 to 0.5%. In addition, it is preferable that the upper limit of Sn content shall be 0.3%.

Said Te, Bi, and Sn can contain any 1 type or in combination of 2 or more types.

Group 2: Cr: 2.0% or less, Mo: 0.5% or less, V: 0.3% or less, and Nb: 0.3% or less

Cr, Mo, V, and Nb all have the effect | action which raises intensity | strength. For this reason, in the part obtained by the finishing turning process using a cemented carbide tool, when especially wanting to raise strength, you may contain in the following ranges.

Cr: 2.0% or less

Cr has the effect | action which raises intensity | strength. Cr also has an effect of improving hardenability and improving carburizing characteristics. However, when Cr content increases, especially when it exceeds 2.0%, the groove-shaped wear of a tool tip will become easy to advance, and in finish turning, a small finish surface roughness cannot be maintained for a long time under the same tool. . In addition, since the above effects are saturated, the cost also increases. Therefore, content of Cr in the case of adding was made into 2.0% or less.

In order to ensure the effect of Cr mentioned above, it is preferable to make content of Cr into 0.02% or more, and it is still more preferable if it is 0.05% or more. For this reason, preferable Cr content is 0.02 to 2.0%. Moreover, as for Cr content, it is more preferable to set it as 0.05 to 1.5%.

Mo: 0.5% or less

Mo has the effect | action which raises intensity | strength. Mo also has the effect | action which raises hardenability. However, when the content of Mo increases, in particular, when it exceeds 0.5%, the groove shape wear of the tool tip tends to proceed, and in finish turning, the small finish surface roughness cannot be maintained during the prolonged processing under the same tool. . In addition, since the above effects are saturated, the cost also increases. Therefore, content of Mo at the time of adding was made into 0.5% or less.

In order to secure the above-mentioned effect of Mo, it is preferable to make Mo content into 0.02% or more. For this reason, more preferable Mo content in the case of adding is 0.02 to 0.5%. In addition, it is preferable to make Mo content into 0.05 to 0.3%.

V: 0.3% or less

V has the effect of increasing strength by strengthening precipitation, and furthermore, V does not significantly affect the form of MnS. For this reason, it is a very effective element for securing machinability and then improving strength. However, when the content of V increases, in particular, exceeding 0.3%, the groove-shaped wear of the tool tip tends to proceed, and in finish turning, a small finish surface roughness cannot be maintained during the prolonged processing under the same tool. . Therefore, content of V in the case of adding was made into 0.3% or less.

In order to ensure the effect of V mentioned above, it is preferable to make content of V into 0.02% or more. For this reason, preferable V content at the time of adding is 0.02 to 0.3%. Moreover, as for V content, it is more preferable to set it as 0.05 to 0.2%.

Nb: 0.3% or less

Nb has the effect of increasing strength by precipitation strengthening in the same manner as V, and furthermore, Nb does not significantly affect the form of MnS. For this reason, it is a very effective element for securing machinability and then improving strength. However, when Nb content increases, especially when it exceeds 0.3%, the groove shape wear of a tool tip will become easy to advance, and in finish turning, a small finish surface roughness cannot be maintained for a long time under the same tool. . Therefore, content of Nb at the time of adding was made into 0.3% or less.

In order to secure the above-mentioned effect of Nb, it is preferable to make content of Nb into 0.02% or more. For this reason, more preferable Nb content at the time of adding is 0.02 to 0.3%. In addition, it is preferable to make Nb content into 0.05 to 0.2%.

Said Cr, Mo, V, and Nb can contain any 1 type or in combination of 2 or more types.

From the above-mentioned reason, the chemical composition of the low carbon sulfur free cutting steel which concerns on this invention (2) is replaced with a part of Fe of the low carbon sulfur free cutting steel which concerns on this invention (1), and Te: 0.05% or less, Bi: 0.15% or less, and Sn: It was prescribed | regulated to contain 1 or more types of 0.5% or less.

Moreover, the chemical composition of the low carbon sulfur free cutting steel which concerns on this invention (3) is replaced with a part of Fe in the low carbon sulfur free cutting steel of this invention (1) or this invention (2), and is Cr: 2.0% or less, Mo: 0.5%. Hereinafter, it defined that it contains 1 or more types of V: 0.3% or less and Nb: 0.3% or less.

In addition, Cu and Ni are impurity elements which may be mixed from raw material scraps, but are each 0.3% or less from the viewpoint of avoiding unnecessary cost increase in the steelmaking process and preventing degradation of machinability due to excessive content. It is preferable.

Hereinafter, the present invention will be described in more detail with reference to Examples.

<Example>

Using a high frequency induction furnace, steel (1 to 25) having various chemical compositions shown in Tables 1 and 2 was dissolved to prepare a steel ingot of 180 kg. The steel ingot has a cylindrical shape with a thin taper of 250 mm in diameter on the upper side and 210 mm in diameter on the lower surface.

Steels 1 to 14 in Table 1 are steels (hereinafter referred to as "steel of the present invention example") in which the chemical composition is within the range defined by the present invention. It is steel of the comparative example which shifted under the conditions prescribed | regulated by this invention. The steels 15 and 16 of the comparative example are conventional low carbon free cutting steel without Pb.

Subsequently, these ingots were heated to high temperature of 1200 degreeC, hold | maintained for 2 hours or more, and hot forging which finish temperature becomes 1000 degreeC or more was performed, and after forging, air cooling was performed and the round bar of diameter 40mm was obtained.

The chemical component extract | required the test piece for analysis from D / 4 part of said round bar of diameter 40mm ("D" shows the diameter of round bar.), And calculated | required by chemical analysis. The component of each steel shown in the said Table 1 and Table 2 is based on this chemical analysis result.

Moreover, since the hot workability was bad with respect to the steel 25, and splitting generate | occur | produced at the time of forging, only the said chemical analysis was investigated and subsequent investigation was not performed.

Moreover, the test piece for micro observation was cut out from the D / 4 part of the said round bar of diameter 40mm, the longitudinal cross section was buried in resin, and mirror-polishing was performed and the distribution form and oxide composition of MnS were investigated.

That is, 64 parts | regions of about 1.4 mm <^2> were image | photographed by the optical microscope which can be patchwork, and the image analysis was performed from the received photographic image, and the distribution form of MnS was measured. When the area ratio of MnS was calculated | required, the above-mentioned operation | work was performed at least twice about the other area | region in a micro observation sample, and it converted into the value per 1 mm <^2> . In addition, MnS made into the measurement object exceeded 1 micrometer in conversion of the original equivalent diameter calculated | required from the area.

Of the MnS observed in the steel longitudinal section 1 mm ² , the total area of MnS exceeding 10 μm in terms of equivalent equivalent diameter is set to [A], and the total area of all MnS observed in the steel longitudinal section 1 mm ² is defined as [B], [ The amount of coarse MnS was evaluated by A] / [B].

Moreover, the composition of the oxide was investigated by performing quantitative analysis using the EPMA (electron probe micro analyzer) and EDS (energy dispersive X-ray spectroscopy apparatus) using the test piece for micro observation prepared above. Moreover, the composition was checked about 10 or more oxides observed randomly, and the arithmetic mean was made into the average composition of oxides.

Moreover, each round bar of diameter 40mm obtained by said hot forging was peeled and it turned into a round bar of diameter 31mm, cold extension line process was carried out to this round bar of diameter 28mm, and the machinability test was done.

The machinability test was carried out using the throwaway type carbide tool (material: JIS K class equivalent, nose R: 0.2 mmR) subjected to PVD coating under the following conditions, and examined the finish surface roughness and debris treatment property. .

Cutting speed: 100 m / min

Feed rate: 0.03 mm / rev,

Cutting depth: 1.0 mm

Lubrication: Wet lubrication using water-insoluble coolant.

The finish surface roughness measured each three points of the surface after cutting 6000m by cutting distance on the said conditions using the stylus roughness system, and evaluated by the arithmetic mean value of average finish surface roughness Ra.

Moreover, the waste processing property collected the waste discharged | emitted while cutting 200m by the cutting distance on the said conditions, measured 20 masses sequentially from the long waste, respectively, and evaluated with the mass of 20 sum total. Since the debris treatment property can be judged to be good as the mass is a small value, the debris treatability when the mass is 5.0 g or less equivalent to that of the steel (15) and the steel (16), which is a low carbon free cutting steel without conventional Pb. It judged that this was favorable. Moreover, when waste processing property was bad and long waste was discharged, what was not able to collect 20 waste was converted into the mass per 20 from the number and mass.

In Table 3, each said test result is put together and shown.

Further, in the "average composition of oxide" means in the Table 3, "○" is CaO and SiO ₂ is to be not less than 5% by mass, "×" is a total content of CaO and SiO ₂ 5 parts by mass in total It shows less than%. In either case, SiO ₂ and CaO alone did not become 90% or more in the average composition.

In addition, "○" in the "crude treatment property" column of Table 3 has the debris-processability equivalent to the steel 15 and steel 16 which are the low carbon free cutting steel of the conventional non-Pb addition, since the mass of debris is 5.0g or less. Moreover, "x" shows that the mass of debris exceeds 5.0 g, and waste processing property is inferior to the said steel 15 and steel 16 which are the low carbon free cutting steel of the said conventional Pb non-addition.

"-" In the test number 25 using the steel 25 of Table 3 shows that hot workability was bad, and since division generate | occur | produced at the time of forging, it did not irradiate.

From Table 3, the low-carbon sulfur free cutting steel according to the present invention of Test Nos. 1 to 14 has a smaller finish when compared to a conventional low carbon free cutting steel without Pb when a finish turning is performed under a condition where the feed amount is small using a carbide tool. It is clear that the surface roughness can be maintained for a long time under the same tool, and also has good debris treatment.

On the other hand, the steel of the comparative example which shifted under the conditions prescribed | regulated by this invention of the test numbers 15-24 has a large finish surface roughness, and is inferior to surface property in all compared with the low carbon sulfur free cutting steel which concerns on this invention of the test numbers 1-14. In the above, the steel of the test number 17 and the test number 18 is inferior to waste processing property.

The steel of this invention is compared with the conventional low carbon free cutting steel of Pb non-addition when the finish turning process using a cemented carbide tool is carried out on condition that feed amount is small, regardless of the Pb non-additive "free cutting steel excellent in the earth environment", It is possible to maintain a small finish surface roughness for a long time under the same tool, and also have good debris treatment. Moreover, since it is excellent also in hot workability, it can manufacture at low industrial scale. Therefore, it can use as a raw material of "OA parts", such as a printer shaft with a narrow diameter which requires much smaller finish surface roughness compared with automobile parts, such as a brake part.

Claims (3)

In mass%, C: 0.02% or more, less than 0.20%, Si: more than 0.10% and 1.5% or less, Mn: 0.8 to 2.2%, P: 0.005 to 0.25%, S: more than 0.40% and 0.8% or less, O: less than 0.010% , N: 0.025% or less, Ca: 0.0003% to 0.005%, the balance consists of Fe and impurities, and Al, Mg, Ti, Zr, and REM contained in the impurities are Al: less than 0.005%, and Mg: A low-carbon sulfur free-cutting steel characterized by satisfying the following formulas (1) and (2) below 0.0003%, Ti: 0.002% or less, Zr: 0.002% or less, and REM: less than 0.0003%. 2.0 <Mn / S <4.0... (One), 0.0005 ≦ 10 Ca × Si ≦ 0.050... (2). However, the element symbol in Formula (1) and Formula (2) shows the content in steel in the mass% of the element. The method according to claim 1, A low-carbon sulfur free-cutting steel containing at least one of Te: 0.05% or less, Bi: 0.15% or less, and Sn: 0.5% or less in place of a part of Fe. The method according to claim 1 or 2, A low carbon sulfur free-cutting steel containing at least one of Cr: 2.0% or less, Mo: 0.5% or less, V: 0.3% or less, and Nb: 0.3% or less in place of a part of Fe.

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