EP1431412B1

EP1431412B1 - Free cutting alloy

Info

Publication number: EP1431412B1
Application number: EP04004046A
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: Daido Steel Co Ltd; Tohoku Tokushuko KK; Tohoku Steel Co Ltd; 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: 2006-08-16
Anticipated expiration: 2020-09-01
Also published as: EP1431409B1; DE60029063D1; DE60029261D1; DE60029364T2; DE60029260T2; EP1431411A1; EP1431410A1; DE60030175D1; EP1085105A3; EP1431411B1; EP1085105A2; DE60030175T2; DE60029261T2; EP1431409A1; EP1431412A1; DE60029260D1; DE60029364D1; EP1085105B1; EP1431410B1; DE60029063T2

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.
R. Kiessling et al.,"Non metallic inclusions in steel", 1978 relates to sulphide inclusions in steel.
GB 1 519 313 relates to a stainless steel alloy and to a ferritic free-machining steel having an excellent machinability and a high corrosion resistance.
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 (Reference);
Fig. 2 is a graph showing EDX analytical results of a second selection inventive specimen No.2 in Example 2;
Fig. 3 is optical microphotograph of second selection inventive steels specimen Nos. 2 and 13 in Example 2;
Fig. 4 is a representation describing measuring points for a hardness test in Example 2;
Fig. 5 is a graph showing an example of Schaeffler diagram;
Fig. 6 is an optical microphotograph of the reference steel specimen No.5 in Example 1 (Reference)
Fig. 7 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 preferably 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 of the present invention, constituted as stainless steel is martensite containing stainless steel (hereinafter referred to a second selection invention), wherein a composition of the free cutting alloy of the present invention is as follows:
The free cutting alloy can contain: 2 mass % or lower, including zero, Ni; 9 to 17 mass % Cr; and C satisfying the following formulae: $0.375 (W_{S} + 0.4 W_{S e}) < W_{C} \leq 1.5$
and $0.125 (W_{T i} + 0.52 W_{Z r}) < W_{C} \leq 1.5$

, wherein W_C, W_S and W_Se denote respective contents of C, S and Se, all in mass %, and W_Ti and W_Zr are as defined as described above.
Martensitic stainless steel is in more of cases used in equipment and parts requiring hardness and corrosion resistivity as performances. Since martensitic stainless steel increases hardness thereof by a quenching heat treatment, there was a case where machining was performed in an annealed state and thereafter, quenching and tempering were performed, such that workability was improved. However, in the case, strain was produced in stainless steel by a quenching heat treatment and thereby, machining had to be, in a case, performed after a quenching heat treatment when precision processing was intended. Furthermore, when in order to increase machinability, machinability improving elements such as S, Se, Pb and Bi were added into a stainless steel, there arose a problem specific to martensite containing stainless steel since not only corrosion resistivity, hot workability and the like but quenchability were also deteriorated, thereby disabling sufficient hardness to be acquired. It should be appreciated that martensite containing stainless steel is a generic name for stainless steel forming a martensitic phase in the matrix by a quenching heat treatment.
As examples of compositions of the martensite containing stainless steel: there can be named: corresponding kinds of stainless steel, such as SUS 403, SUS 410, SUS 410S, SUS 420J1, SUS 420J2, SUS 429J1, SUS 440C and the like, all shown within JIS G 4304. Moreover, it should be appreciated that in the present invention, martensitic heat resisting steel is handled as conceptually included in martensite containing stainless steel. As examples of composition of martensitic heat resisting steel, there can be named corresponding kinds of steel whose compositions are defined in JIS G 4311 and G 4312, such as SUH 1, SUH 3, SUH 4, SUE 11, SUH 600 and SUH 616. However, neither of elements Ti, Zr, S and Se as essential features of the present invention is naturally expressed in compositions described in the standard. In this case, it should be understood that part of Fe content of each of the above described kinds of stainless steel is replaced with the above described elements in the respective above described compositions and thereby martensite containing stainless steel of the present invention is obtained. Therefore, while in description of the present specification, the same JIS Nos. are used, those actually means alloys specific to the present invention, which alloys have compositions defined in JIS standards as a base only.
Since martensite containing stainless steel changes a martensitic transformation temperature (Ms point) and quenchability depending on components included therein, attention has to be paid to ranges of the components in content. For this reason, the ranges of components in content of a (Ti,Zr) based compound described above are required to be set considering the following conditions: First, contents of components are desirably determined such that a (Ti,Zr) based compound is not formed so excessively that a martensitic formation temperature (Ms point) and quenchability are affected. Since atoms included in the (Ti,Zr) based compound exerts almost no influence on characteristics of the stainless steel, such as a hardness of the martensitic phase and quenchability thereof, it is considered that the elements left behind by excluding the amount of the elements included in the (Ti,Zr) based compound from the elements added originally in the martensite containing stainless steel (hereinafter referred to as residual elements) are dissolved as solid solution in the matrix phase and the residual elements exert an influence on martensitic transformation. Accordingly, ranges in content of the respective elements are preferably set, considering an influence on martensitic transformation using a continuous cooling transformation diagram of stainless steel having a composition analogous to a composition of the residual elements. Especially, since C has a great influence on martensitic transformation, a C content is adjusted such that the above described formulae A and B are satisfied. As a result, not only is machinability is improved, but hardness after quenching, quenchability and the like become compatible with conventional martensite containing stainless steel.
Below, description will be given of the reason why the components and contents thereof in the second selection invention of the present invention constituted as martensite containing stainless steel are selected and limited as follows:

(3) 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 reduces a martensitic transformation temperature (Ms point), but also increases stability of an austenitic phase of the matrix phase excessively, whereby a case arises in which an amount of martensite necessary to ensure hardness is hard to be obtained. Moreover, hardness after annealing becomes high producing a solid solution hardening effect caused by Ni in excess, which sometimes makes performances such as machinability decrease. For the above described reason, a Ni content has the upper limit of 2 mass %.

(4) 9 to 17 mass % Cr

Cr is an indispensable element for ensuring corrosion resistivity and added 9 mass % or higher in content. However, when a content is in excess of 17 mass %, phase stability is deteriorated and thereby high temperature brittleness occurs with ease, leading to poor hot workability. Moreover, it is considered that as the content increases, toughness decreases- Especially, when a stainless steel including Cr in excess receives a long heat treatment at a temperature in the intermediate range of 400 to 540°C, toughness at room temperature is lost with ease. A Cr content is desirably set in the range of 11 to 15 mass % and more desirably in the range of 12 to 14 mass %.
Further, free cutting alloys of the second selection invention of the present invention constituted as martensite 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:

(5) 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. Moreover, a Ms point decreases in excess in a case of martensite containing stainless 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.

(6) 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 and moreover, reduces a Ms point excessively in martensite containing stainless steel, 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.

(7) 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, however, not only does hot workability decrease, but in martensite containing stainless steel, a Ms point decreases and quenchability is also deteriorated, 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.

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

Co is an element effective for improving corrosion resistivity, particularly in an environment of a reducing acid and in addition, can also be added to martensite containing stainless steel depending on a necessity since Co increases a Ms point and improves quenchability. 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.

(9) 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, not only is hot workability deteriorated, but in martensite containing stainless steel, a Ms point decrease excessively and further cost increases and therefore, the upper limits of Mo and W are set 4 mass % and 3 mass %, respectively.
In the martensite 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:

(10) 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.

(11) 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.

(12) 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.

(13) 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 W_SO.
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 W_SO 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 W_SO value and set to 0.035 mass % or lower in W_SO. 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 + 0.52 W_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 Ws 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 (Reference only)

The effects of a free cutting alloy constituted as ferrite containing stainless steel (a reference 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 a reference steel 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 a steel No. 5 by a diffractometer and Fig. 6 is an optical microphotograph of a steel specimen No. 5. Further, specimens Nos. 1 to 14 in Table 1 are kinds of steel corresponding to the reference 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 0.5 cm³ 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. Reference 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 a reference steel 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 reference steel is smaller in W_SO and better in out-gas resistivity. The reason why kinds of steel of comparative alloy specimens Nos. 16 and 18 each have a high W_SO 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.

Example 2 Martensite containing stainless steel

The following experiment was performed on martensite containing stainless steel and second selection inventive steel of the present invention. First, 50 kg steel blocks of compositions in mass % shown in Table 3 were molten in a high frequency induction furnace to form respective ingots. The ingots were heated at temperature in the range of from 1050 to 1100°C to be forged in a hot state and be formed into rods each with a circular section, of a diameter of 20 mm. The rods were further heated at 750°C for 1 hr, followed by air cooling to be applied to the test.

Table 3

In Table 3, specimens Nos. 1 to 19 are second selection inventive steels of the present invention constituted as martensite containing stainless steel. Further, in comparative examples, specimens correspond to stainless steel: a specimen No. 20 corresponds to SUS 410, a specimen No. 21 to SUS 416, a specimen No. 22 to SUS 420F and a specimen No. 23 to SUS 440F. Further, specimens Nos. 24 to 26 are of stainless steel, i.e. reference specimens.
While main inclusions of the inventive steel of the present invention was of (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. 9 high in a Mn content and the like, (Mn,Cr)S was recognized, though in a small amount. An identification of 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 EDX (Energy Dispersive X-ray spectrometer). 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 EDX and a compound with a composition corresponding to a compound observed by EDX was confirmed based on formation from two dimensional mapping results. Fig. 2 shows EDX analytical results of arbitrary inclusions in a second selection inventive steel specimen No.2 and from the results, formation of (Ti,Zr) based compound can be recognized. Further, Fig. 3 shows optical microphotograph of second selection inventive steel specimens Nos. 2 and 13.
The following experiment was 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. While workability in hot forging was at levels at which processing can be performed with no problem, as not only inclusions but an amount of alloy elements increase, deterioration in the workability was a tendency observed in the test. It was found that kinds of steel of the present invention in which one or more of Ca, B, Mg and REM was included had good hot workability when comparing with a kind of steel in which none of the elements was included.

2) Evaluation of machinability

Evaluation of machinability was collectively effected based on tool ware loss in machining, finished surface roughness and ship shapes. A cutting tool made of cermet was used to perform machining under a wet condition by water-soluble cutting oil at a circumferential speed of 120 m/min, a depth of cutting per revolution of 0.1 mm and a feed rate per revolution of 0.05 mm. The tool ware loss was measured at a flank of the cutting tool after 60 min machining with µm as a unit of the tool wear loss. The finished surface roughness and chip shapes were evaluated by a method similar to that in Example 1 (Reference).
The following evaluations were performed using material subjected to treatments in which the material is kept at 980 to 1050°C for 30 min, thereafter subjected to a quenching heat treatment and still further subjected to a tempering treatment of holding at 180°C for 1 hr, followed by air cooling.

3) Hardness test

Measurement of hardness on a test piece was performed on a C scale Rockwell hardness by the Rockwell hardness test stipulated in JIS Z 2245. The Rockwell hardness was obtained as the average of measurements at arbitrary 5 measuring points S on a circle drawn on a cross section of a rod test piece having a circular section, the circle drawn on the cross section being a circle satisfying a relation of PS = 0.25 PG, wherein G denotes a point almost coinciding with a center of the circular section, P denotes an arbitrary point on the outer periphery of the test piece and a point S is on a line segment PG

4) Evaluation of out-gas resistivity

Evaluation of out-gas resistivity was performed similar to in Example 1 (Reference).

5) Evaluation of corrosion resistivity

Evaluation of corrosion resistivity was performed by a method similar to in Example 1 (Reference). Test pieces each were prepared so to have the shape of a cylinder of 15 mm in diameter and 50 mm in height. The entire surface of each test piece was polished. Each test piece was polished and thereafter, a test piece was held in a thermohygrostat at a temperature of 60°C and a relative humidity of 90 % RH for 168 hr. An evaluation method was such that when no rust was confirmed, the test piece was evaluated [A], when dot-like stains were recognized at several points on a test piece, the test piece was evaluated [B], when red rust was recognized in an area of an area ratio of 5 % or less, the test piece was evaluated [C] and when red rust was recognized in an area wider than an area ratio of 5 %, the test piece was evaluated [D]. The results are shown in Table 4.

Table 4

It is found from Table 4 that while in stainless steel of comparative specimens Nos. 20 to 23, hardness is sufficiently ensured, machinability is poor. It is further found that specimens Nos. 21 to 23 are inferior in corrosion resistivity and out-gas resistivity. When a reference steel is compared with a second selection inventive steel, it is found that the reference steel has improved machinability, while the second selection inventive steel has improved hardness, improved corrosion resistivity and improved out-gas resistivity. The reason why the second selection inventive steel was improved in hardness as compared with the reference steel is considered that a C content satisfies the formulae A and B and thereby, a C content constituting a (Ti,Zr) based compound and a C content as additive establishes an adjusted balance and thereby, a C component is sufficiently dispersed in a Fe based matrix phase. Further, the reason why out-gas resistivity was improved is considered that S is added excessively relative to an amount of a (Ti,Zr) based compound that can be formed.

Claims

Free cutting alloy constituted as martensite containing stainless steel containing:
2 mass % or lower, including zero, Ni; 9 to 17 mass % Cr;

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 C satisfying the following formulae: $0.375 (W_{S} + 0.4 W_{S e}) < W_{C} \leq 1.5$
and $0.125 (W_{T i} + 0.52 W_{Z r}) < W_{C} \leq 1.5$

wherein W_Ti, W_Zr, W_C, W_S and W_Se denote respective contents of Ti, Zr, C, S and Se, all in mass %;

and wherein a (Ti,Zr) based compound containing one or more of Ti and Zr as a metal element component, C being 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 W_SO 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 W_SO.
Free cutting alloy according to any 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 structures is in the range of 1 to 20%.