EP1085105B1

EP1085105B1 - Free cutting alloy

Info

Publication number: EP1085105B1
Application number: EP20000118990
Authority: EP
Inventors: Kiyohito Ishida; Katsunari Oikawa; Takashi c/o Tohoku Tokushuko K.K. Ebata; Takayuki Inoguchi; Tetsuya Shimizu; Michio Okabe
Original assignee: Daido Steel Co Ltd; Tohoku Tokushuko KK; Tohoku Steel Co Ltd; Tohoku Techno Arch Co Ltd; Japan Research Industries and Industrial Technology Association (JRIA)
Current assignee: ISHIDA, KIYOHITO; OIKAWA, KATSUNARI; Daido Steel Co Ltd; Tohoku Tokushuko KK; Tohoku Techno Arch Co Ltd; Japan Research Industries and Industrial Technology Association (JRIA)
Priority date: 1999-09-03
Filing date: 2000-09-01
Publication date: 2006-06-28
Anticipated expiration: 2020-09-01
Also published as: EP1431412B1; DE60029261T2; EP1431410B1; DE60029063T2; EP1431411B1; DE60029364T2; DE60030175D1; EP1431412A1; EP1085105A3; DE60029260T2; EP1431409B1; EP1431409A1; EP1085105A2; EP1431410A1; EP1431411A1; DE60029261D1; DE60029063D1; DE60030175T2; DE60029260D1; DE60029364D1

Description

The present invention relates to free cutting alloy excellent in machinability.
Alloy has widespread applications because of a variety of characteristics thereof. A free cutting alloy excellent in machinability is, in a case, selected for improvement of productivity. In order to improve machinability, for example, free cutting alloy containing an element improving machinability such as S, Pb, Se or Bi (hereinafter referred to as machinability-improving element) is widely used. Especially in a case where machinability is particularly required because of precise finishing in machining or for other reasons, not only is a content of such a machinability-improving element increased in an alloy, but the elements are also added to an alloy in combination.
While S, which has widely been used for improvement of machinability, is in many cases added in the form of MnS, addition thereof in an alloy in a large content causes for degrading corrosion resistivity, hot workability and cold workability of the alloy. Moreover, when the alloy is exposed to the air, a sulfur component included in the alloy is released into the air in the form of a sulfur containing gas, which causes sulfur contamination in peripheral areas of parts with ease. Therefore, there arises a necessity of suppressing release of sulfur containing gas (hereinafter referred to as improvement on out-gas resistivity). Elements such as S, Se and Te, however, deteriorate magnetic properties to a great extent in an electromagnetic stainless steel and the like.
Therefore, various proposals have been made: a Mn content is limited, a Cr content in sulfide is increased or in a case where S is contained, Ti is added in combination with S in order to disperse sulfide in the shape of a sphere (for example, see JP-A-98-46292 or JP-A-81-16653). To increase a Cr content in sulfide, however, tends to greatly decrease in machinability and hot workability and therefore, such a alloy has been restricted on its application in many cases.
Specific steels having improved machinability and/or fatigue resistance and wear resistance and/or workability in general have been disclosed in JP 11229032, EP 0 903 418, JP 11229082, and EP 0 767 247.
It is accordingly an object of the present invention is to provide free cutting alloy excellent in machinability, showing outstanding characteristics as an alloy such as corrosion resistivity, hot workability and cold workability or specific magnetic characteristics, which are comparable to those of conventional alloys.

Summary of the Invention

In order to achieve the above described object, a free cutting alloy of the present invention is characterized by the free cutting alloy of claim 1. "(Ti,Zr)" means one or two of Ti and Zr.
Machinability of an alloy can be improved by forming the above described (Ti, Zr) based compound in a matrix metal phase of the alloy. Furthermore, by forming this compound in the alloy, formation of compounds such as MnS and (Mn,Cr)S, easy to reduce corrosion resistivity and hot workability of the alloy, can be prevented or suppressed, thereby enabling corrosion resistivity, hot workability and cold workability to be retained at good levels. That is, according to the present invention, a free cutting alloy excellent in machinability can be realized without any degradation in useful characteristics as an alloy such as hardness, corrosion resistivity, hot workability, cold workability and specific magnetic characteristics.
Further, a (Ti,Zr) based compound formed in a free cutting alloy of the present invention is dispersed in the alloy structure. Machinability of an alloy can be further increased especially by dispersing the compound in an alloy structure. In order to increase the effect, a particle size of the (Ti,Zr) based compound as observed in the structure of a polished section of the alloy is preferably, for example, approximately in the range of 0.1 to 30 µm on the average and further, an area ratio of the compound in the structure is preferably in the range of 1 to 20 %, wherein the particle size is defined by the maximum distance between two parallel lines circumscribing a particle in observation when parallel lines are drawn intersecting on a region including the particle in observation while changing a direction of the parallel lines.
The above described (Ti,Zr) based alloy can include at least a compound expressed in a composition formula (Ti,Zr)₄(S,Se,Te)₂C₂ (hereinafter also referred to as carbo-sulfide/selenide), wherein one or more of Ti and Zr may be included in the compound and one or more of S, Se and Te may be included in the compound. By forming a compound in the form of the above described composition formula, not only can machinability of an alloy be improved, but corrosion resistivity is also improved.
It should be appreciated that identification of a (Ti,Zr) based compound in an alloy can be performed by X-ray diffraction (for example, a diffractometer method), an electron probe microanalysis method (EPMA) and the like technique. For example, the presence or absence of the compound of (Ti,Zr)₄(S,Se,Te)₂C₂ can be confirmed according to whether or not a peak corresponding to the compound appear in a diffraction chart measured by an X-ray diffractometer. Further, a region in the alloy structure in which the compound is formed can also be specified by comparison between two-dimensional mapping results on characteristic X-ray intensities of Ti, Zr, S, Se or C obtained from a surface analysis by EPMA conducted on a section structure of the alloy.

Brief Description of the Drawings

Fig. 1 is a graph showing an X-ray diffraction chart of an inventive steel specimen No. 5 in experiment of Example 1;
Fig. 2 is a graph showing an example of Schaeffler diagram;
Fig. 3 is an optical microphotograph of the first selection inventive steel specimen No.5 in Example 1;
Fig. 4 is a graph showing dependencies of solubility products on temperature of components of TiO, TiN, Ti₄C₂S₂, TiC, TiS and CrS in γ-Fe;

Preferred Embodiments of the Invention

The present invention, to be concrete, is applied on an alloy constituted as stainless steel. In this case, in order to form a (Ti,Zr based compound without any degradation in characteristics as stainless steel, such an alloy contains one or more of Ti and Zr such that W_Ti + 0.52 W_Zr = 0.03 to 3.5 mass %, wherein W_Ti and W_Zr denote respective contents in mass % of Ti and Zr; and one or more of S and Se in the respective ranges of 0.01 to 1.0 mass % for S and 0.01 to 0.8 mass % for Se.
The reason why the elements and contents thereof are selected as follows:

(1) The Ti and Zr content being defined such that W_Ti + 0.52 W_Zr = 0.03 to 3.5 mass %, wherein W_Ti and W_Zr denote respective contents in mass % of Ti and Zr

Ti and Zr are indispensable elements for forming a (Ti,Zr) based compound playing a central role in exerting the effect of improving machinability of a free cutting alloy of the present invention. When a value of W_Ti + 0.52 W_Zr is lower than 0.03 mass %, the (Ti,Zr) based compound is insufficiently formed in amount, thereby disabling the effect of improving machinability to be satisfactorily exerted. On the other hand, when in excess of the value, machinability is reduced on the contrary. For this reason, the value of W_Ti + 0.52 W_Zr is required to be suppressed to 3.5 mass % or lower. The above effect exerted when Ti and Zr are added into an alloy is determined by the sum of the numbers of atoms (or the sum of the numbers of values in mol), regardless of kinds of metals, Ti or Zr. Since a ratio between atomic weights is almost 1 : 0.52, Ti of a smaller atomic weight exerts a larger effect with a smaller mass. Thus, a value of W_Ti + 0.52 W_Zr is said to be compositional parameter reflects the sum of the numbers of atoms of Zr and Ti included in an alloy.

(2) One or more of S and Se in the respective ranges of 0.01 to 1 mass % for S and 0.01 to 0.8 mass % for Se

S and Se are elements for useful in improving machinability. By adding S and Se into an alloy, in an alloy structure, formed is a compound useful for improving machinability (for example, a (Ti,Zr) based compound expressed in the form of a composition formula (Ti,Zr)₄(S,Se)₂C₂). Therefore, contents of S and Se are specified 0.01 mass % as the lower limit. When the contents are excessively large, there arises a chance to cause a problem of deteriorating hot workability and therefore, there have to be the upper limits: The S content is set to 1 mass % and the Se content is set to 0.8 mass % as the respective upper limits. Further, S and Se are both desirably added into an alloy in a necessary and sufficient amount in order to form a compound improving machinability of the alloy, such as the above described (Ti,Zr) based compound. An excessive addition of S results in deterioration of the out-gas resistivity.
A free cutting alloy constituted as stainless steel of the present invention is to be more detailed, ferrite containing stainless steel (hereinafter referred to as a first selection invention), wherein a composition of the free cutting alloy of the present invention is as follows:
The free cutting alloy contains: 2 mass % or lower, including zero, Ni; 12 to 35 mass % Cr; and 0.005 to 0.4 mass % C.
The reason why the constituting elements and contents thereof in the first selection invention constituted as ferrite containing stainless steel are determined is as follows:

(3) 0.005 to 0.4 mass % C

C is an important element forming a compound improving machinability. When a content thereof is lower than 0.005 mass %, however, an effect exerting sufficient machinability can not be imparted to the alloy, while when in excess of 0.4 mass %, much of a single carbide not effective for improving machinability is formed. Addition of C is preferably set in the range of 0.01 to 0.1 mass %, wherein it is preferable that addition of C is adjusted so properly that the effect of imparting machinability on the alloy is optimized depending on an amount of a constituting element of a compound improving machinability such as a (Ti,Zr) based compound.

(4) 2 mass % or lower, including zero Ni

Ni can be added according to a necessity since the element is effective for improving corrosion resistivity, particularly in an environment of a reducing acid. Excessive addition, however, not only reduce stability of a ferrite phase, but also causes cost-up and therefore, a content thereof has the upper limit of 2 mass %, wherein a case of no addition of Ni may be included.

(5) 12 to 35 mass % Cr

Cr is an indispensable element for ensure corrosion resistivity and is added in the range of 12 mass % or higher. On the other hand, excessive addition is not only harmful to hot workability but also causes reduction in toughness and therefore the upper limit is set to 35 mass %.
Further, free cutting alloys of the first selection invention of the present invention constituted as ferrite containing stainless steel can contain: 2 mass % or lower, including zero Si; 2 mass % or lower, including zero Mn; 2 mass % or lower, including zero Cu; and 2 mass % or lower, including zero Co. In addition, the free cutting alloys can further contain one or more of Mo and W in the respective ranges of 0.1 to 4 mass % for Mo and 0.1 to 3 mass % for W.
Description will be given of the reason why the elements and contents thereof are defined as follows:

(6) 2 mass % or lower, including zero Si

Si is added as a deoxidizing agent for steel. That Si is added in excess, however, is unfavorable because not only cold workability is deteriorated, but formation of δ ferrite increases in amount, thereby degrading hot workability of steel. Consequently, a Si content has the upper limit of 2 mass %. In a case where cold workability is particularly regarded as important, the Si content is preferably set 0.5 mass % or lower, including zero.

(7) 2 mass % or lower, including zero Mn

Mn acts an deoxidizing agent for steel. In addition, since a compound useful for increase in machinability in co-existence with S or Se, there arises a necessity of addition when machinability is highly thought of. On the other hand, since MnS especially deteriorates corrosion resistivity, affects cold workability adversely. Therefore a Mn content has the upper limit of 2 mass %. Especially when corrosion resistivity and cold workability are regarded as important, a Mn content is desirably limited to 0.4 mass % or lower, including zero.

(8) 2 mass % or lower, including zero Cu

Cu can be added according to a necessity since the element is effective for improving corrosion resistivity, particularly in an environment of a reducing acid. It is preferable to contain 0.3 mass % or higher in order to obtain a more conspicuous effect of the kind. When in excess, hot workability decreases whereby it is preferable for a Cu content to be set 2 mass % or lower, including zero. Especially when hot workability is regarded as important, it is more desirably to suppress the Cu content to 0.5 mass % or lower, including zero.

(9) 2 mass % or lower, including zero Co

Co is an element effective for improving corrosion resistivity, particularly in an environment of a reducing acid. To contain Co in content equal to 0.3 mass % or higher is preferable in order to obtain more of conspicuousness in the effects. When added in excess, however, not only does hot workability decrease, but a raw material cost increases, and therefore, it is preferable to set a content of Co in the range of 2 mass % or lower, including zero. Especially when hot workability and decrease in raw material cost are regarded as important, a content of Co is more desirably suppressed to 0.5 mass % or lower, including zero.

(10) One or more of Mo and W in the respective ranges of 0.1 to 4 mass % for Mo and 0.1 to 3 mass % for W

Since Mo and W can further increase corrosion resistivity and a strength, the elements may be added according to a necessity. The lower limits are both 0.1 %, where the effects thereof become clearly recognized. On the other hand, when added in excess, hot workability is deteriorated. Therefore, the upper limits of Mo and W are set 4 mass % and 3 mass %, respectively.
In the ferrite containing stainless steel, described above, contents of other elements are as follows: the stainless steels can contain: 0.05 mass % or lower P; and 0.03 mass % O; and 0.05 mass % or lower N. Moreover, the stainless steels can further contain one or more of Te, Bi and Pb in the respective ranges of 0.005 to 0.1 mass % for Te; 0.01 to 0.2 mass % for Bi; and 0.01 to 0.3 mass % for Pb. Description will be given of the reason why the elements and contents thereof are defined as follows:

(11) 0.05 mass % or lower, including zero P

P is segregated at grain boundaries and not only increases intergranular corrosion sensibility but also sometimes reduces toughness. Therefore, a P content is preferably set as low as possible and to 0.05 mass % or lower, including zero. Although the P content is more desirably set to 0.03 mass % or lower, including zero, reduction in content more than necessary has a chance to be reflected on increased production cost.

(12) 0.03 mass % or lower, including zero O

O combines with Ti or Zr both of which are constituting elements of a compound useful for improving machinability and forms oxides not useful for improving machinability. Therefore, an O content should be suppressed as low as possible and is set to 0.03 mass % as the upper limit. The O content is desirably set to 0.01 mass % or lower if allowable in consideration of increase in production cost.

(13) 0.05 mass % or lower, including zero N

N combines with Ti or Zr both of which are constituting elements of a compound useful for improving machinability and forms nitrides not useful for improving machinability. Therefore, a N content should be suppressed as low as possible and is set to 0.05 mass % as the upper limit. The N content is desirably set to 0.03 mass % or lower, including zero and more desirably to 0.01 mass %, if allowable in consideration of increase in production cost.

(14) One or more of Te, Bi and Pb in the respective ranges of 0.005 to 0.1 mass % for Te; 0.01 to 0.2 mass % for Bi; and 0.01 to 0.3 mass % for Pb

Since Te, Bi and Pb can further improve machinability, the elements may add if necessary. The lower limits thereof at which the respective effects are exerted to clearness are as follows: 0.005 mass % Te; 0.01 mass % Bi and 0.01 mass % Pb, respectively. On the other hand, since excessive addition reduces hot workability, the upper limits are set as follows: 0.1 mass % Te; 0.2 mass % Bi; and 0.3 mass % Pb.
Furthermore, the free cutting alloy of the present invention constituted as stainless steel can contain one or more selected from the group consisting of Ca, Mg, B and REM (one or more of metal elements classified as Group 3A in the periodic table of elements) in the range of 0.0005 to 0.01 mass % for one element or as a total content in a case of two or more elements. The elements are useful for improving hot workability of steel. The effect of improving hot workability obtainable by addition of the elements is more conspicuously exerted in the range of 0.0005 mass % or higher for one element or as a total content of more than one elements combined. On the other hand, when the elements are added in excess, the effect is saturated and hot workability is then reduced on the contrary. Therefore, the content of a single element or total content of the elements combined is set to 0.01 mass % as the upper limit. As for REM, since low radioactivity elements are easy to be handled when being mainly used, from this viewpoint, it is useful to use one or more selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. It is desirable to use light rare earth elements, especially La or Ce from the viewpoint of conspicuous exertion of the effect and price. However, there arises no trouble with mixing in of a trace of radioactive rare earth elements such as Th and U inevitably remaining, without being excluded, in a process to separate rare earth elements. Further, from the viewpoint of reduction in raw material cost, there can be used not-separated rare earth elements such as mish metal and didymium.
A free cutting alloy of the present invention constituted as stainless steel can contain one or more selected from the group consisting of Nb, V, Ta and Hf in each range of 0.01 to 0.5 mass %. Since Nb, V, Ta and Hf has an effect of forming carbo-nitrides to miniaturize crystalline particles of steel and increase toughness. Hence, the elements can add in each content up to 0.5 mass % and desirably contain 0.01 mass % or higher in the range.
A free cutting alloy of the present invention constituted as the above described stainless steel can contain the Wso value of which is less than 0.035 mass % when the following test is performed: an alloy test piece of said free cutting alloy is prepared so as to have the shape of rectangular prism in size of 15 mm in length, 25 mm in width and 3 mm in thickness with the entire surface being polished with No. 400 emery paper; a silver foil in size of 10 mm in length, 5 mm in width and 0.1 mm in thickness with a purity of 99.9 % or higher as a S getter; 0.5 cm³ of pure water are sealed in a vessel of an inner volume of 250 cm³ together with said test piece; the temperature in said vessel is raised to 85°C and said temperature is then kept there for 20 hr; and thereafter, the S content in mass % in said silver foil piece is analyzed, then S content obtained is defined as said Wso.
A (Ti, Zr) based compound being a feature of the present invention is formed and in the course of the formation, added S is included in the stainless steel as a constituting element of the (Ti, Zr) based compound which is more stable chemically than MnS or the like. And therefore, a S amount released into the air from the stainless steel decreases. Consequently, an out-gas resistivity of the stainless steel can also be improved by formation of the(Ti, Zr) based compound.
In this case, when the out-gas resistivity test is performed, a S component released from the test piece as a sulfur containing gas is forced to be absorbed in the silver foil as a getter and a sulfur content Wso in the silver foil is measured to quantitatively determine the out-gas resistivity of a material. A S content absorbed in the silver foil is defined using the Wso value and set to 0.035 mass % or lower in Wso. Stainless steel of the present invention controlled so as to be 0.035 mass % or lower in Wso is hard to cause sulfur contamination in the peripheral parts when exposed to the air since a S component released from the stainless steel into the air is very small and thereby the stainless steel can be preferably used as parts of industrial equipment requiring the out-gas resistivity.
While a factor determining out-gas resistivity of a material mainly is a composition of the material, it is desirable to fix S as carbo-sulfides of Ti and Zr for improvement on out-gas resistivity of the material. For the purpose, a S content is desirably determined such that a value of W_S/(W_Ti + 0.52W_Zr) is 0.45 or less, or alternatively a value of W_S/W_C is 0.4 or less and W_S/(W_Ti+ 0.52W_Zr) is 0.45 or less, wherein W_S and W_C denote a S content and a C content, respectively. With such a range of components adopted, a S content which is chemically in unstable condition can be limited and thereby, the out-gas resistivity of the matrix metal phase of stainless steel can be improved.

Examples

The following experiments were performed in order to confirm the effects of the present invention. It should be appreciated that in the following description, test alloy relating to the present invention is referred to as inventive steel or inventive alloy, or as a selection inventive steel or a selection inventive alloy.

Example 1 Ferrite containing stainless steel

The effects of a free cutting alloy constituted as ferrite containing stainless steel (a first selection inventive steel) were confirmed by the following experiment. First, 50 kg steel blocks with respective compositions in mass % shown in Table 1 were molten in a high frequency induction furnace and ingots prepared from the molten blocks were heated at a temperature in the range of from 1050 to 1100°C and the ingots were forged in a hot state into rods with a circular section of 20 mm diameter and the rods were further heated at 800°C for 1 hr, followed by air cooling (annealing) as a source for test pieces.

Table 1

While main inclusions of an inventive steel of the present invention was (Ti,Zr)₄(S,Se)₂C₂, other inclusions such as (Ti,Zr)S and (Ti,Zr)S₃ are locally recognized in the matrix. Further, in a specimen No. 7 high in Mn content, (Mn, Cr)S is recognized, though in a trace amount. An identification method for inclusions was performed in the following way: A test piece in a proper amount was sampled from each of the rods. A metal matrix portion of the test piece was dissolved by electrolysis using a methanol solution including tetramethylammonium chloride and acetylaceton at 10 % as a electrolytic solution. The electrolytic solution after the electrolysis was subjected to filtration and compounds not dissolved in steel were extracted from the filtrate. The extract was dried and subjected to chemical analysis by an X-ray diffraction method with a diffractometer. A compound was identified based on peaks of a diffraction chart. A composition of a compound particle in the steel structure was separately analyzed by EPMA and a compound with a composition corresponding to a compound observed by X-ray diffraction was confirmed based on formation from two dimensional mapping results. Fig. 1 shows an X-ray diffraction chart of an inventive steel No. 5 by a diffractometer and Fig. 3 is an optical microphotograph of an inventive steel specimen No. 5. Further, specimens Nos. 1 to 14 in Table 1 are kinds of steel corresponding to the first selection inventive steel and specimens Nos. 15 to 28 are kinds of steel as comparative examples.
The following experiments were performed on the above described test pieces:

1) Hot workability test

Evaluation of hot workability was effected based on visual observation of whether or not defects such as cracks occur in hot forging. [○] indicates that substantially no defect occurred in hot forging, [×] indicates that large scale cracks were recognized in hot forging and △ indicates that small cracks occurred in hot forging.

2) Evaluation of machinability

Evaluation of machinability was collectively effected based on cutting resistance in machining, finished surface roughness and chip shapes. A cutting tool made of cermet was used to perform machining under a dry condition at a circumferential speed of 150 m/min, a depth of cutting per revolution of 0.1 mm and a feed rate per revolution of 0.05 mm. A cutting resistance in N as a unit was determined by measuring a cutting force generating in the machining. The finished surface roughness was measured by a method stipulated in JIS B 0601 and a value thereof was an arithmetic average roughness (in µm Ra) on a test piece surface after the machining. Moreover, chip shapes were visually observed and when friability was good, the result is indicated by [G] and when friability is bad and all chips are not separated but partly connected, the result is indicated by [B] and when evaluation of chip shapes is intermidiate of [G] and [B], the result is indicated by [I].

3)

4)Evaluation of out-gas resistivity

Evaluation of out-gas resistivity was performed by determining an amount of released S. To be concrete, test pieces in use each had the shape of a rectangular prism of 15 mm in length, 25 mm in width and 3 mm in thickness and the entire surface of each were polished with No. 400 emery paper. A test piece was placed in a sealed vessel having an inner volume of 250 cm³ together with a silver foil having a size of 10 mm in length, 5 mm in width and 0.1 mm in thickness and 0.5cm³ of pure water, and a temperature in the vessel was maintained at 85°C for 20 hr. A S content Wso in the silver foil after the process for the test was measured by a combustion type infrared absorbing analysis method.

4) Cold workability test

Evaluation of cold workability was performed by measuring a threshold compressive stain in a compression test on specimens Nos. 1 to 5 and 13. Test pieces for compression each had the shape of a cylinder of 15 mm in diameter and 22.5 mm in height and each piece was compressed by a 600 t oil hydraulic press to obtain a threshold compressive strain, wherein the threshold compressive strain is defined as In (H0/H) or a natural logarithm of H0/H, H0 being an initial height of the test piece and H being a threshold height which is a maximum height at which no cracking has occurred. First selection inventive alloys of the specimens Nos. 1 to 5 were confirmed to have high threshold compressive ratios almost equal to comparative steel specimen No. 15 and higher than comparative steel specimen No. 16 by about 20 %, and have a good cold workability as well.

5) Evaluation of corrosion resistivity

Evaluation of corrosion resistivity was performed by a salt spray test. Test pieces each were prepared so to have the shape of a cylinder of 10 mm in diameter and 50 mm in height. The entire surface of each test piece was polished with No. 400 emery paper and cleaned. A test piece was exposed to a fog atmosphere of 5 mass % NaCl aqueous solution at 35°C for 96 hr. Final evaluation was visually performed with the naked eye. As a result, the inventive steel of the present invention was confirmed to maintain good corrosion resistivity. The results are shown in Table 2.

Table 2

It is found from Table 2 that first selection inventive steel of the present invention is comparable with conventional ferrite containing stainless steel in hot workability, cold workability and corrosion resistivity and moreover, is better in machinability than the conventional ferrite containing stainless steel. Further, it is found from Table 2 when comparing with comparative steel specimens Nos. 16 and 18 that the first selection inventive steel of the present invention is smaller in Wso and better in out-gas resistivity. The reason why kinds of steel of comparative alloy specimens Nos. 16 and 18 each have a high Wso is considered that since the steel of the kinds has neither Ti nor Zr, carbo-sulfide is hard to be formed, whereby a S amount in the matrix is excessively high. In comparative alloy specimen No. 18, hot workability is poor and therefore, evaluation of machinability was not performed.

Claims

Free cutting alloy constituted as ferrite containing stainless steel containing:
2 mass % or lower, including zero, Ni; 12 to 35 mass % Cr; and 0.005 to 0.4 mass % C;

one or more of Ti and Zr such that W_Ti + 0.52 W_Zr = 0.03 to 3.5 mass %, wherein W_Ti and W_Zr denote respective contents in mass % of Ti and Zr;

and one or more of S and Se in the respective ranges of 0.01 to 1 mass % for S and 0.01 to 0.8 mass % for Se and

a (Ti,Zr) based compound containing one or more of Ti and Zr as a metal element component,

and wherein C is an indispensable element as a bonding component with the metal element component, and one or more of S, Se and Te is dispersed in a matrix metal phase; optionally further containing:
2 mass % or lower, including zero Si; 2 mass % or lower, including zero Mn; 2 mass % or lower, including zero Cu; 2 mass % or lower, including zero Co;

one or more of Mo and W in the respective ranges of 0.1 to 4 mass % for Mo and 0.1 to 3 mass % for W;

0.05 mass % or lower, including zero P; 0.03 mass % or lower, including zero O; 0.05 mass % or lower, including zero N;

one or more of Te, Bi and Pb in the respective ranges of 0.005 to 0.1 mass % for Te; 0.01 to 0.2 mass % for Bi; and 0.01 to 0.3 mass % for Pb;

one or more selected from the group consisting of Ca, Mg, B and REM (one or more of metal elements classified as Group 3A in the periodic table of elements) in the range of 0.0005 to 0.01 mass % for one element or as a total content of more than one elements combined;

one or more selected from the group consisting of Nb, V, Ta and Hf in each range of 0.01 to 0.5 mass %, the balance being Fe and inevitable impurities.
Free cutting alloy according to claim 1, wherein W_S/(W_Ti+0.52 W_Zr) is 0.45 or less, wherein W_S, W_Ti and W_Zr denote a S content, a Ti content and a Zr content, respectively.
Free cutting alloy according to any of claims 1 to 2, the Wso value representing the S amount released is less than 0.035 mass % when the following test is performed:
an alloy test piece of said free cutting alloy is prepared so as to have the shape of rectangular prism in size of 15 mm in length, 25 mm in width and 3 mm in thickness with the entire surface being polished with No. 400 emery paper;

a silver foil in size of 10 mm in length, 5 mm in width and 0.1 mm in thickness with a purity of 99.9 % or higher as a S getter;

0.5 cm³ of pure water are sealed in a vessel of an inner volume of 250 cm³ together with said test piece;

the temperature in said vessel is raised to 85°C and said temperature is then kept there for 20 hr;

and thereafter, the S content in mass % in said silver foil piece is analyzed, then S content obtained is defined as said Wso.
Free cutting alloy according to any one of claims 1 to 3, wherein a particle size of the (Ti, Zr) based compound as observed in the structure of a polished section of the alloy is in the range of 0.1 to 30 µm on the average, and further an area ratio of the compound in the structure is in the range of 1 to 20%.