EP1918408B1

EP1918408B1 - Martensitic free cutting stainless steel

Info

Publication number: EP1918408B1
Application number: EP07019360.2A
Authority: EP
Inventors: Koichi Ishikawa; Tetsuya Shimizu
Original assignee: Daido Steel Co Ltd
Current assignee: Daido Steel Co Ltd
Priority date: 2006-10-03
Filing date: 2007-10-02
Publication date: 2014-11-26
Anticipated expiration: 2027-10-02
Also published as: EP1918408A3; CN101158011A; US20080089804A1; JP5135918B2; CN101158011B; JP2008111186A; EP1918408A2

Description

FIELD OF THE INVENTION

The present invention relates to a martensitic free cutting stainless steel. More particularly, it relates to a martensitic free cutting stainless steel which does not contain Se which is one of free cutting elements.

BACKGROUND OF THE INVENTION

Conventionally, as the elements for improving the machinability of a stainless steel, for example, S, Pb, Se, and Te have been known.
For example, S generally forms a sulfide type inclusion such as MnS or MnSe. Accordingly, stress concentrates to the inclusion upon forming chips, thereby to improve the machinability. Further, for example, Pb is present in the form of a simple substance in the steel, and serves as a lubricant between a tool and chips. As a result, the machinability is improved.
In recent years, there has been an increase in components requiring accurate finishing for ensuring the dimensional accuracy, components in complicated shapes with a large machining allowance, or the like. For the production of these components, the machinability is demanded to be improved as much as possible. For this reason, the content of the elements for improving the machinability tends to be increased. Further, these elements may be added not alone, but in the combination thereof.
For example, JP-A-2002-38241 discloses a free cutting stainless steel which contains, by mass percent, C: 0.50% or less, Si: 0.05 to 2.00%, Mn: 0.05 to 1.00%, S: 0.05 to 0.50%, Se: 0.02 to 0.20%, Te: 0.01 to 0.10%, and Cr: 10.00 to 30.00%, in which component ratios of a Mn/S ratio: 2 or less, a Se/S ratio: 0.2 or more, and a Te/S ratio: 0.04 or more are satisfied, with the balance including Fe and inevitable impurities.
Further, for example, JP-A-8-134602 discloses a martensitic stainless steel which contains, by weight percent, C: 0.5% or less, Si: 0.05 to 2.00%, Mn: 0.10 to 3.00%, P: 0.20% or less, Ni: 2.00% or less, Cr: 12.0 to 25.0%, Mo: 0.10 to 3.00%, S: 0.40 to 0.50%, Al: 0.10% or less, N: 0.10% or less, 0.60 to 200ppm, Pb: 0.03 to 0.30%, and Te: 0.02 to 0.15%, with the balance including Fe and inevitable impurities, and has a Mn/S ratio of 4.5 to 6.5, and a Te/S ratio of less than 0.07.
Herein, when the content of S which is the main element for improving the machinability is excessive, the hot workability or the cold workability of the stainless steel may be deteriorated.
Whereas, the machinability is improved by the stress concentration to the sulfide. For this reason, the size or form of the formed sulfide directly affects the machinability. Further, when the formed sulfide is too large, it operates as the breakage starting point, resulting in the deterioration of the strength of the stainless steel. Especially, when the formed sulfide extends extremely in one direction, the anisotropy occurs in the stainless steel, resulting in the deterioration of the toughness. Also from these viewpoints, it is important to control the size and the form of the sulfide.
However, for conventional free cutting stainless steels, the balance therebetween has not been sufficiently achieved. Actually, there have not been obtained the free cutting stainless steels excellent in all of the machinability, the hot workability, the cold workability, and the toughness.
Incidentally, the free cutting stainless steel of JP-A-2002-38241 has a Mn/S ratio of as small as 2.0 or less, and hence it is inferior in hot workability. This tends to cause an increase in manufacturing cost as well. Whereas, when Se is essentially added for the purpose of improving the machinability, the inclusions increase in size. Therefore, the sulfides extend long in the longitudinal direction of the steel material. Accordingly, anisotropy occurs in the toughness, the fatigue strength, or the like, and thus the characteristics are largely degraded. In addition, unfavorably, selenium ions detrimental to a human body are generated due to the corrosion of selenides.
On the other hand, the free cutting stainless steel of JP-A-8-134602 has a largely increased S content, and whereby it has been improved in machinability due to the increase in size of sulfides. However, sulfides are excessively formed, and in addition, the inclusions increase in size. Therefore, the sulfides extend long in the longitudinal direction of the steel material. Accordingly, anisotropy occurs in the toughness, the fatigue strength, or the like, and thus the characteristics are largely degraded. Further, it also causes the deterioration of the corrosion resistance, the hot workability, or the cold workability.
The patent application US 2002/0170638 A1 discloses martensitic stainless steel parts having high hardness and corrosion resistance, which can be produced with high finishing accuracy, and from which release of sulfide gas is effectively suppressed. The parts are produced by forging and/or machining a steel of a specific alloy composition to the shape of the part followed by quenching and tempering, or vice versa, and applying a solution of oxidative acid to the surface of the part so as to dissolve and remove sulfides existing on the surface of the part.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a martensitic free cutting stainless steel excellent in the machinability, the hot workability, the cold workability, and the toughness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing the test piece collection direction in the Charpy impact test.

DETAILED DESCRIPTION OF THE INVENTION

Namely, the present invention relates to a martensitic free cutting stainless steel according to claim 1.
The martensitic free cutting stainless steel may further comprise:

by weight percent,
at least one element selected from the group consisting of
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%, and
Mo: 0.01 to 1.0%.

The martensitic free cutting stainless steel may further comprise:

by weight percent,
Bi: 0.01 to 0.30%.

The martensitic free cutting stainless steel may further comprise:

by weight percent,
at least one element selected from the group consisting of
Ca: 0.0001 to 0.05%,
Mg: 0.0001 to 0.02%, and
REM: 0.0001 to 0.02%.

The martensitic free cutting stainless steel may further comprise:

by weight percent,
W: 0.01 to 2.0%,

The martensitic free cutting stainless steel may further comprise:

by weight percent,
at least one element selected from the group consisting of
Nb: 0.01 to 0.50%,
Ta: 0.01 to 0.50%, and
V: 0.01 to 0.50%.

A martensitic free cutting stainless steel in accordance with the present invention satisfies the foregoing component composition, and S, Mn, Te, and O satisfy the foregoing respective formulae, whereby the good balance is achieved. Accordingly, in the steel, a sulfide having a specific size and form is present in a specific area ratio. Therefore, the martensitic free cutting stainless steel in accordance with the invention is excellent in machinability, hot workability, cold workability, and toughness in comparison with the conventional martensitic free cutting stainless steels.
In this regard, when at least one element selected from the group consisting of Cu, Ni, and Mo is/are contained in a specific ratio, it becomes more likely to improve the corrosion resistance.
Further, when Bi is contained in a specific ratio, it becomes more likely to improve the machinability.
Still further, when at least one element selected from the group consisting of Ca, Mg, and REM are contained in a specific ratio, it becomes more likely to improve the hot workability.
Moreover, when W is contained in a specific ratio, it becomes more likely to improve the strength and the corrosion resistance.
Additionally, when at least one element selected from the group consisting ofNb, Ta, and V are contained in a specific ratio, it becomes more likely to improve the toughness.
Hereinafter, a martensitic free cutting stainless steel in accordance with one embodiment of the present invention (which may be referred to as "this stainless steel") will be described in details. The stainless steel of the invention contains the following elements, with the remainder being Fe and inevitable impurities. The types of the addition elements, the component ratio, the reason for limitation, and the like are as follows. Incidentally, the unit of the component ratio is weight percent. Herein, in the present specification, all the percentages defined by weight are the same as those defined by mass, respectively.

(1) C: 0.10 to 1.20%

C is an element which is solid-solved in the base metal, and enhances the hardness. Accordingly, in order to obtain a sufficient hardness, the lower limit of the content of C is set at 0.10% or more.
However, when the content of C exceeds 1.20%, a large quantity of carbides in a simple substance form which are harmful for the improvement of the machinability are generated, or the deterioration of the corrosion resistance is caused. The upper limit of the content of C is preferably 1.00% or less, and more preferably 0.50% or less.

(2) Si: 0.10 to 2.00%

Si operates as a deoxidizer of the steel. In order to obtain the effect, the lower limit of the content of Si is set at 0.10% or more. The lower limit of the content of Si is preferably 0.2% or more.
However, when the content of Si is excessive, the amount of δ-ferrite to be formed increases. Thus, it is observed that the hot workability tends to be deteriorated. Accordingly, the upper limit of the content of Si is set at 2.00% or less. When the hot workability is important, the upper limit of the content of Si is preferably 1.20% or less, and more preferably 0.50% or less.

(3) Mn: 0.80 to 1.50%

Mn operates as a deoxidizer of the steel. In addition, Mn forms a compound effective for the improvement of the machinability by the coexistence with S. However, when the content of Mn is small, S is in excess amounts. Accordingly, the hot workability becomes deteriorated. For this reason, the lower limit of the content of Mn is set at 0.80% or more.
On the other hand, among the compounds for improving the machinability, especially, MnS largely reduces the corrosion resistance, and inhibits the cold workability, and in addition, excessively reduces the Ms point. Accordingly, the upper limit of the content ofMn is set at 1.50% or less. Particularly, when the machinability and the corrosion resistance are important, the upper limit of the content of Mn is preferably 1.20% or less.

(4) S:0.15 to 0.30%

S binds to Mn, Cr, or the like to form a compound effective for the improvement of the machinability. In order to obtain the effect, the lower limit of the content of S is set at 0.15% or more.
On the other hand, when the content of S is excessive, it is observed that the hot workability tends to be deteriorated. Accordingly, the upper limit of the content of S is set at 0.30% or less. The upper limit of the S content is preferably 0.25% or less, and more preferably 0.20% or less from the viewpoints of achieving the excellent balance with the hot workability.

(5) Cr: 10.5 to 15.0%

Cr is an element for improving the corrosion resistance. In order to obtain the effect, the lower limit of the content of Cr is set at 10.5% or more. The lower limit of the content of Cr is preferably 11.0% or more, and more preferably 12.0% or more.
On the other hand, when the content of Cr is excessive, the manufacturing cost increases, and in addition, the hot workability is also deteriorated. Accordingly, the upper limit of the content of Cr is set at 15.0% or less, and preferably 14.0% or less.

(6) Pb: 0.03 to 0.23%

Pb is an element effective for the improvement of the machinability. In order to obtain the effect, the lower limit of the content of Pb is set at 0.03% or more. The lower limit of the content of Pb is preferably 0.10% or more, and more preferably 0.13% or more from the viewpoints of ensuring a sufficient amount for the improvement of the machinability, and the like.
On the other hand, when the content of Pb is excessive, it is observed that the hot workability tends to be deteriorated. Accordingly, the upper limit of the content of Pb is set 0.23% or less from the viewpoints of facilitating the improvement of the hot workability, and the like.

(7) Te: 0.01 to 0.10%

Te is an element effective for improving the machinability, and it also suppresses the extension of sulfide due to rolling. In order to obtain the effect, the lower limit of the content of Te is set at 0.01% or more.
On the other hand, when the content of Te is excessive, it is observed that the hot workability tends to be deteriorated. Accordingly, the upper limit of the content of Te is preferably 0.10% or less. The upper limit of the content of Te is preferably 0.08% or less, and more preferably 0.05% or less from the viewpoints of facilitating the improvement of the hot workability, and the like.

(8) B: 0.0005 to 0.010%

B is an element effective for improving the hot workability. Accordingly, it may be optionally added in order to obtain the effect, the lower limit of the content ofB is set at 0.0005% or more. The lower limit of the content of B is preferably 0.0010% or more.
On the other hand, when the content of B is excessive, the manufacturing cost increases. Accordingly, the upper limit of the content of B is set at 0.010% or less. The upper limit of the content ofB is preferably 0.008% or less.

(9) O: 0.005 to 0.030%

O is an element which is involved in the formation of a sulfide necessary for the improvement of the machinability. Accordingly, in order to obtain the effect, the lower limit of the content of O is set at 0.005% or more. The lower limit of the content of O is preferably 0.007% or more.
On the other hand, when the content of O is excessive, an oxide which is not effective for the improvement of the machinability becomes more likely to be formed. Accordingly, the upper limit of the content of O is set at 0.030% or less. The upper limit of the content of O is preferably 0.020% or less.

(10) P: 0.005 to 0.10%

P is segregated in the grain boundary, and enhances the sensitivity for the grain boundary corrosion. In addition, it incurs the deterioration of the toughness. Therefore, the content thereof is preferably low. Accordingly, the upper limit of the content ofP is set at 0.10% or less. The upper limit of the content ofP is preferably 0.050% or less.
On the other hand, a larger reduction of the content of P than necessary incurs an increase in manufacturing cost. Accordingly, in view of the refining technology or the like, the lower limit of the content of P is set at 0.005% or more.

(11) N: present up to 0.050%

N forms a nitride which is harmful for the improvement of the machinability. Therefore, the content thereof is desirably controlled to the minimum. Accordingly, the upper limit of the content ofN is set at 0.050% or less. The upper limit of the content ofN is preferably 0.030% or less, although it depends on the even balance with the manufacturing cost.
The stainless steel of the present invention may further arbitrarily contain one or two or more elements selected from the following elements in addition to the foregoing essential elements. The component ratio, the reason for limitation of each element, and the like are as follows.

<1> At least one element selected from Cu: 0.01 to 2.0%, Ni: 0.01 to 2.0%, and Mo: 0.01 to 1.0%

Cu is an element which is effective for the improvement of the corrosion resistance, especially the corrosion resistance in a reducing acid environment. In order to obtain the effect, the lower limit of the content of Cu is set at 0.01% or more. The lower limit of the content of Cu is preferably 0.05% or more, and more preferably 0.10% or more.
On the other hand, when the content of Cu is excessive, it is observed that the hot workability tends to be deteriorated. Accordingly, the upper limit of the content of Cu is set at 2.0% or less. The upper limit of the content of Cu is preferably 1.0% or less, and more preferably 0.8% or less.
Ni is an element which is effective for enhancing the corrosion resistance imparted by Cr. In order to obtain the effect, the lower limit of the content ofNi is set at 0.01% or more. The lower limit of the content of Ni is preferably 0.05% or more, and more preferably 0.10% or more.
On the other hand, when the content ofNi is excessive, the manufacturing cost increases. Accordingly, the upper limit of the content ofNi is set at 2.0% or less. The upper limit of the content ofNi is preferably 1.0% or less, and more preferably 0.5% or less.
Mo is an element which is capable of improving the corrosion resistance and the strength. In order to obtain the effect, the lower limit of the content of Mo is set at 0.01% or more. The lower limit of the content of Mo is preferably 0.05% or more, and more preferably 0.10% or more.
On the other hand, when the content of Mo is excessive, the hot workability is deteriorated, and in addition, the manufacturing cost increases. Accordingly, the upper limit of the content of Mo is set at 1.0% or less. The upper limit of the content of Mo is preferably 0.60% or less, and more preferably 0.50% or less.

<2> Bi: 0.01 to 0.30%

Bi is an element which is capable of further improving the machinability. Accordingly, it may be optionally added. In order to obtain the effect, the lower limit of the content of Bi is set at 0.01% or more. The lower limit of the content of Bi is preferably 0.05% or more.
On the other hand, when the Bi content is excessive, the hot workability becomes more likely to be deteriorated. Accordingly, the upper limit of the content ofBi is set at 0.30% or less. The upper limit of the content ofBi is preferably 0.20% or less.

<3> At least one element selected from Ca: 0.0001 to 0.05%, Mg: 0.0001 to 0.02%, and REM: 0.0001 to 0.02%

Ca, Mg, and REM are elements which are effective for the improvement of the hot workability. Accordingly, they may be optionally added. In order to obtain the effect, all of the content of Ca, the content of Mg, and the content of REM are set at 0.0001% or more, respectively.
On the other hand, when the content of Ca, the content of Mg, and the content of REM are excessive, the effect is saturated. In addition, it is observed that the hot workability adversely tends to be deteriorated. Accordingly, the content of Ca is set at 0.05% or less, and preferably 0.01% or less. The content ofMg is set at 0.02% or less, and preferably 0.01% or less. The content of REM is set at 0.02% or less.

<4> W: 0.01 to 2.0%

W has an effect of further improving the corrosion resistance and the strength. Accordingly, it may be optionally added. In order to obtain the effect, the content of W is set at 0.01% or more. The lower limit of the content ofW is preferably 0.05% or more, and more preferably 0.10% or more.
On the other hand, when the content of W is excessive, not only the hot workability is deteriorated, but also an increase in cost is incurred. Accordingly, the content of W is set at 2.0% or less. The upper limit of the content of W is preferably set at 1.0% or less.

<5> At least one element selected from Nb: 0.01 to 0.50%, Ta: 0.01 to 0.50%, and V: 0.01 to 0.50%

Nb, Ta, and V each form a carbonitride to thereby refine the grain size, and thus have an effect of enhancing the toughness. In order to obtain the effect, all of the content of Nb, the content of Ta, and the content of V are set at 0.01% or more, respectively. All of the content ofNb, the content of Ta, and the content of V are preferably 0.05% or more, and more preferably 0.10% or more, respectively.
On the other hand, when the content ofNb, the content of Ta, and the content of V are excessive, an increase in cost is incurred. Accordingly, all of the content ofNb, the content of Ta, and the content of V are set at 0.50% or less, respectively. All of the content ofNb, the content of Ta, and the content of V are preferably 0.40% or less, respectively.
Herein, the stainless steel of the invention satisfies the respective formulae of 3.0≤ [Mn]/[S] ≤ 10.0, 0.10 ≤ [Te]/[S] ≤0.50 and 15 ≤ [S]/[O] ≤ 30. Incidentally, each parenthesis [ ] in the formulae indicates the weight percent of each element. Hereinafter, the technical meanings of respective formulae will be described.

3.0 ≤ [Mn]/[S] ≤ 10.0

When the value of [Mn]/[S] is less than 3.0, the content of Mn is extremely low, or the content of S is extremely high. Accordingly, production of the stainless steel of the invention tends to become difficult. Therefore, the lower limit of the value of [Mn]/[S] is set at 3.0 or more. The lower limit of the value of [Mn]/[S] is preferably 4.0 or more.
On the other hand, when the value of [Mn]/[S] exceeds 10.0, the content of Mn in the sulfide is so high that it becomes difficult to ensure the content of Cr in the sulfide. Thus, it is observed that the corrosion resistance tends to be remarkably deteriorated. Therefore, the upper limit of the value of [Mn]/[S] is set at 10.0 or less.

0.10 ≤ [Te]/[S]≤0.50

When the value of [Te]/[S] is less than 0.10, the content of Te is too low, or the content of S is too high. Accordingly, the sulfide extends, and the spindle shape which is an effective shape for the machinability is difficult to be formed, thereby resulting in the deterioration of the machinability. Additionally, anisotropy occurs in the toughness, the fatigue strength, or the like. Like this, it is observed that the characteristics tend to be largely degraded. Therefore, the lower limit of the value of [Te]/[S] is set at 0.10 or more.
The upper limit of the value of [Te]/[S] is 0.50 or less from the viewpoint of the hot workability.

15 ≤ [S]/[O] ≤ 30

When the value of [S]/[O] is less than 15, a low proportion of [S] makes it difficult to obtain sufficient machinability. Whereas, a high proportion of [O] results in an increase in amount of hard oxides. As a result, it is observed that the machinability tends to be deteriorated. Therefore, the lower limit of the value of [S]/[O] is set at 15 or more.
On the other hand, in the case where the value of [S]/[O] exceeds 30, when the proportion of [S] is high, it is observed that the manufacturability of this stainless steel tends to be deteriorated. When the proportion of [O] is low, it is observed that a sulfide having a size effective for the machinability tends to be difficult to obtain. From these viewpoints, the upper limit of the value of [S]/[O] is 30 or less.
Further, in the stainless steel of the invention, sulfides having a circle equivalent diameter of 2.0 µm or more and an aspect ratio of 10 or less are present in a total amount of 0.50% or more by area ratio. As a result, an excellent machinability can be exerted.
The upper limit of the above-mentioned area ratio is 10.0% or less. When it exceeds the value, anisotropy occurs in the toughness, the fatigue strength, or the like. Like this, it is observed that the characteristics tend to be largely degraded.
Indeed, when the above-mentioned specific sulfides are present in an amount equal to or more than the above-mentioned area ratio in the stainless steel of the invention, it does not matter if there are sulfides which are not the above-mentioned specific sulfides such as sulfides having a circle equivalent diameter of less than 2.0 µm, and sulfides having an aspect ratio of more than 10.
The above-mentioned area ratio can be determined in the following manner. Namely, for the mirror-polished surface of the stainless steel of the invention, typical microphotographs are taken at a 200-fold magnification in 50 visual fields. Then, color extraction of the sulfide (inclusion) is carried out. Thus, the circle equivalent diameter and the aspect ratio of each sulfide are measured by image processing. Out of these, the total area ratio of the sulfides having a circle equivalent diameter of 2 µm or more and an aspect ratio of 10 or less can be determined.
Further, the aspect ratio indicates the value of the longer diameter of the sulfide/shorter diameter of the sulfide.
Then, a typical method for manufacturing the stainless steel of the invention will be described.
By the use of, for example, a high frequency induction furnace, respective raw materials are dissolved and molten into a steel ingot, and then, cooled to manufacture an ingot so as to achieve a stainless steel satisfying the above-mentioned component composition and respective formulae. Subsequently, the resulting ingot is subjected to hot processing, hot forging or hot rolling, followed by an annealing heat treatment. Consequently, the stainless steel of the invention can be manufactured.
In the foregoing manufacturing process, the heating temperature during the hot processing, hot forging or hot rolling, is in a temperature range of 950 to 1250°C.
Annealing, is carried out at 750 to 900°C for 3 to 5 hours. Then, furnace cooling is carried out to around 600°C at a rate of 10 to 20°C/hour. Thereafter, air cooling is carried out.
Moreover, in the manufacturing process, pickling or polishing for removal of the surface oxide layer may be optionally carried out, and cold rolling may be optionally carried out.
The application of the stainless steel of the invention as described above is not particularly limited. For the applications of the stainless steel of the invention, specifically, for example, the stainless steel of the invention can be preferably used for members required to be subjected to cold cutting processing (such as finishing processing), and to have a corrosion resistance, a high strength, and the like, such as a motor shaft, a pump shaft, a valve component, a screw, a bolt, and a nut.

EXAMPLES

Next, the invention will be specifically described by way of Examples.

1. Manufacturing of stainless steels in accordance with Examples and Comparative Examples

First, respective stainless steels of component compositions shown in Tables 1 to 3 were molten into their respective 50-kg steel ingots by the use of a high frequency induction furnace. Then, the steel ingots were cooled to manufacture respective ingots.
Then, respective ingots were heated to 1000 to 1200°C, and processed into round bar steels with a diameter of 60 mm and with a diameter of 20 mm, and square bar steels with a width of 60 mm and a height of 30 mm by hot forging.
Then, the bar steels were further heated at 860°C for another one hour, followed by slow cooling (an annealing treatment). As a result, stainless steels in accordance with Examples and Comparative Examples to be subjected to each test were obtained.
Incidentally, the stainless steels in accordance with Comparative Example 1 and Comparative Example 2 are SUS410 and SUS416 specified according to JIS, respectively. The stainless steels in accordance with Comparative Example 9 and Comparative Example 10 are SUS420J2 and SUS420F specified according to JIS, respectively.
Further, the stainless steels in accordance Comparative Examples 1 to 8 are for the comparison with the stainless steels in accordance with Examples 1 to 15, and 31 to 35. The stainless steels in accordance Comparative Examples 9 to 16 are for the comparison with the stainless steels in accordance with Examples 16 to 30, and 36 to 40.

2. Sulfide characteristics

Then, for the sulfides present in each stainless steel in accordance with Examples and Comparative Examples, the circle equivalent diameter, the aspect ratio, and the area ratio were measured.
The measurement method was as follows. Samples with a length of 10 mm per side were collected from the 20-mm round bar steels, and each sample was embedded in a resin so that the longitudinal direction is the measurement surface. Then, for the mirror-polished surface of each stainless steel, typical microphotographs were taken at a 200-fold magnification in 50 visual fields by means of an optical microscope.

Thereafter, color extraction of each sulfide (inclusion) was carried out. Thus, the circle equivalent diameter and the aspect ratio of each sulfide were measured by image processing. Out of these, the total area ratio of the sulfides having a circle equivalent diameter of 2 µm or more and an aspect ration of 10 or less was determined. The results are shown in Tables 4 to 6. The circle equivalent diameter indicates the diameter of a circle having the same area as the sulphide inclusions cross section in the measurement surface.

Table 4

		[Mn]/[S]	[Te]/[S]	[S]/[O]	Area ratio of sulfide (%)
Ex.	1	5.44	0.17	22.5	1.77
	2	4.48	0.17	19.2	1.33
	3	6.94	0.19	17.8	1.03
	4	7.11	0.26	12.7	1.44
	5*	4.05	0.14	31.4	1.56
	6*	5.81	0.30	33.8	1.90
	7*	9.46	0.15	11.8	0.98
	8	5.39	0.17	20.0	1.32
	9*	8.30	0.10	16.7	0.64
	10*	4.44	0.12	10.4	1.57
	11*	12.45	0.45	22.0	0.89
	12*	3.54	0.14	31.1	2.31
	13	3.27	0.12	26.0	1.63
	14*	6.70	0.22	20.8	1.43
	15	4.07	0.14	26.4	1.59
Comp. Ex.	1	-	-	0.3	-
	2	-	-	25.6	1.68
	3	6.52	0.07	7.9	1.57
	4	11.57	1.00	6.4	0.41
	5	15.60	0.03	11.1	0.67
	6	6.85	0.15	43.3	1.02
	7	4.96	0.09	56.3	2.03
	8	1.71	0.45	20.7	1.89

(*1) [X] denotes the content (wt%) of element X.
[X]/[Y] denotes the ratio (wt%) of elements X to Y
* Illustrative Example

Table 5

		[Mn]/[S]	[Te]/[S]	[S]/[O]	Area ratio of sulfide (%)
Ex.	16	6.38	0.25	22.9	2.31
	17	5.84	0.26	21.1	1.38
	18	6.29	0.14	21.0	1.78
	19	5.44	0.22	22.5	1.47
	20*	4.92	0.13	34.3	1.55
	21*	6.75	0.17	12.0	1.04
	22*	5.90	0.24	13.8	1.83
	23	6.47	0.33	25.0	1.22
	24	5.19	0.31	20.0	1.09
	25*	10.80	0.10	16.7	0.67
	26	5.64	0.12	25.0	1.65
	27	5.00	0.21	21.1	1.32
	28*	6.23	0.38	11.8	1.02
	29*	6.59	0.14	10.0	1.25
	30	4.11	0.30	16.9	1.83
Comp. Ex.	9	-	-	0.3	-
	10	-	-	17.2	1.31
	11	6.42	0.13	5.9	1.88
	12	25.00	1.60	5.0	0.34
	13	3.15	0.02	32.5	1.74
	14	12.82	0.18	55.0	0.62
	15	5.34	0.07	58.6	2.44
	16	1.40	0.29	31.8	2.12

Table 6

		[Mn]/[S]	[Te]/[S]	[S]/[O]	Area ratio of sulfide (%)
Ex.	31	7.00	0.18	18.9	1.21
	32	5.42	0.11	17.3	1.33
	33	6.05	0.18	27.5	1.53
	34*	5.75	0.19	12.3	1.28
	35*	7.75	0.20	28.6	1.44
	36	7.00	0.24	21.3	1.23
	37	4.65	0.13	16.4	1.08
	38	6.40	0.30	22.2	1.12
	39	7.19	0.19	16.0	1.33
	40*	8.06	0.22	13.8	1.43

3. Evaluation test

Then, by using the resulting respective round bar steels and square bar steels, relative evaluations were carried out for the machinability (machinability in turning, machinability in drilling), the hot workability, the cold workability, and the toughness (anisotropy). At this step, as the reference data, the corrosion resistance and the quenching/tempering hardness, which are the basic characteristics of the stainless steel, were also measured, respectively.

1) Machinability

The machinability evaluations were carried out for two of the machinability in turning and the machinability in drilling.
The machinability in turning was evaluated in the following condition. By the use of a 60-mm round bar steel, the tool wear amount and the chip shape were relatively evaluated.
Namely, by the use of a carbide tool (UTi20T)as a turning machining tool, turning was carried out without a lubricant under the conditions of a cutting speed of 150 mm/min, a depth of cut per revolution of 1.0 mm, and a feed per revolution of 0.2 mm/rev.
Herein, the tool wear amount was evaluated by measuring the wear amount of the average tool flank wear (see, JIS B170 section "(5) Tool damage No. 5005"). The case where the tool wear amount was 50 µm or less was rated as "A"; the case of 51 to 100 µm was rated as "B"; and the case of 101 µm or more was rated as "C".
Further, the chip shape was confirmed by visual observation. The one which was good in chip breakability was rated as "A"; the one which was broken into coils of approximately several turns was rated as "B"; and the one which was inferior in breakability and provided continuous chips was rated as "C". Incidentally, continuous chips unfavorably make automation difficult.
On the other hand, the machinability in drilling was evaluated in the following condition. By the use of a test piece with a width of 60 mm, a height of 30 mm, and a length of 200 mm, such a cutting speed as to result in a tool life (undrillable) of 5000 mm and the chip shape were relatively evaluated.
Namely, by the use of a high speed steel drill SKH51 (diameter of 5 mm), drilling was carried out without a lubricant under the conditions of a hole depth of 15 mm, and a feed per revolution of 0.07 mm/rev. Thus, the tool life distance was measured by changing the cutting speed.
Herein, the case of a cutting speed of more than 50 m/min, a high speed, was rated as "A"; the case of a cutting speed of 20 to 50 m/min, an intermediate speed, was rated as "B"; and the case of a cutting speed of less than 20 m/min, a low speed, was rated as "C".
Further, the chip shape was confirmed by visual observation of the initial chips when drill machining was carried out at a cutting speed of 20 m/min. The one which was good in chip breakability was rated as "A"; the one which was broken into coils of approximately several turns was rated as "B"; and the one which was inferior in breakability and provided continuous chips was rated as "C". Incidentally, continuous chips unfavorably make automation difficult.

2) Hot workability

The hot workability was evaluated by the appearance after hot forging of each ingot into a 20-mm round bar steel and the high-temperature high-speed tensile test (Gleeble test).
The appearance after forging was evaluated based on the degree of occurrence of crack. The one with no crack was rated as "A"; the one with slight crack of such a degree as to be able to be cut by a grinder, occurred therein, was rated as "B"; and the one with large crack occurred therein was rated as "C".
The high-temperature high-speed tensile test was carried out in the following condition. Each high-temperature high-speed tensile test piece with a diameter of 6 mm and a length of 110 mm was cut out from the steel in the as-casted state, and relatively evaluated based on the value of reduction of area at 1000°C.

3) Cold workability

The cold workability evaluation was carried out in the following condition. Namely, for each stainless steel, a cylindrical test piece with a diameter of 12 mm and a height of 18 mm was prepared from a 20-mm round bar steel was prepared. Then, for each test piece, a single-step compression test was carried out by a 600-t hydraulic press to measure the limit compressibility. Incidentally, a higher limit compressibility indicates excellent cold workability.

4) Toughness (anisotropy)

The toughness evaluation was carried out at room temperature (24°C) according to JIS Z2202. A material was collected as shown in FIG. 1 from a square bar steel with a width of 60 mm and a height of 30 mm, and roughly machined. Then, the material was heated to 950°C, and kept for 30 minutes. Then, the material was oil cooled, and thus subjected to a quenching treatment. Then, the material was heated to 180°C, and kept for 1 hour. Then, the air-cooled and tempered test piece was machined into a JIS No. 4 test piece, so that a Charpy impact test in the L and T directions was carried out.

5) Corrosion resistance

The corrosion resistance test was carried out in accordance with a high-temperature high-humidity test. Namely, for each stainless steel, a cylindrical test piece with a diameter of 10 mm and a height of 50 mm was prepared from a 20-mm round bar steel. Then, the surface of the test piece was polished with emery papers of up to No. 400 count, and degreased and washed. Then, these respective test pieces were held in a constant-temperature constant-humidity bath at a temperature of 50°C, and a relative humidity of 98% RH for 96 hours. Then, for each test piece after holding, whether rust occurred or not was visually observed. The stainless steels in accordance with Examples 1 to 15, and 31 to 35 were compared with the stainless steel in accordance with Comparative Example 2. On the other hand, the stainless steels in accordance with Examples 16 to 30, and 36 to 40 were compared with the stainless steel in accordance with Comparative Example 10. The case where rust occurred in a very small amount was judged as "A⁺"; the case where rust occurred in a comparable amount was judged as "A"; and the case where rust occurred in a large amount was judged as "B".

6) Quenching/tempering hardness

For each stainless steel, a cylindrical test piece with a diameter of 20 mm and a height of 10 mm was prepared from a 20-mm round bar steel. Then, each sample was heated to 950°C, and kept for 30 minutes. Then, it was oil cooled, and thus subjected to a quenching treatment. Then, each test piece was heated to 180°C, and kept for 1 hour. Then, it was air cooled and subjected to a tempering treatment. Thereafter, the hardness was measured in terms of Rockwell hardness (C scale).
The summary of the results are shown in Tables 7 to 9.
The tables indicate the followings. Namely, the stainless steel in accordance with Comparative Example 1 is SUS410 specified according to JIS, and contains a very small amount of S which is one of free cutting elements, and is not a so-called free cutting stainless steel. Accordingly, it contains almost no sulfides, and is very excellent in corrosion resistance, and has almost no anisotropy, and is also excellent in toughness. Further, it is also excellent in hot workability and cold workability. However, it is very inferior in machinability.
On the other hand, the stainless steel in accordance with Comparative Example 2 is SUS416 specified according to JIS. As compared with SUS410, it contains S which is one of free cutting elements, added therein (it also contains no Pb nor Te which is another free cutting element). Accordingly, the machinability is improved by the presence of sulfides which are inclusions. However, this is due to a mere increase in amount of S to be added to SUS410. Therefore, anisotropy occurs, so that the toughnesses in the L direction and in the T direction are badly balanced. This is presumably due to the fact that there occurred a sulfide which extended in the form of a string in the longitudinal direction. Further, the cold workability is also deteriorated as compared with SUS410.
In addition, the stainless steel in accordance with Comparative Example 9 is SUS420J2 specified according to JIS. This is also not a free cutting stainless steel as with the SUS410. Accordingly, it contains almost no sulfides, and is very excellent in corrosion resistance, and has almost no anisotropy, and is also excellent in toughness. Further, it is also excellent in hot workability and cold workability. However, it is very inferior in machinability.
Furthermore, the stainless steel in accordance with Comparative Example 10 is SUS420F specified according to JIS. As compared with SUS420J2, it contains S which is one of free cutting elements, added therein (it contains no Pb nor Te which is another free cutting element). Accordingly, the machinability is improved by the presence of sulfides which are inclusions. However, this is due to a mere increase in amount of S to be added to SUS420J2. Therefore, anisotropy occurs, so that the toughnesses in the L direction and in the T direction are badly balanced. This is presumably due to the fact that there occurred a sulfide which extended in the form of a string in the longitudinal direction. Further, the cold workability is also deteriorated as compared with SUS420J2.
The stainless steels in accordance with Comparative examples 3 and 11 each contain O in a larger proportion than the range specified in this application. Accordingly, as compared with the stainless steels in accordance with Examples 1 to 15, and 31 to 35, and the stainless steels in accordance with Examples 16 to 30, and 36 to 40, the machinability is lower. This is presumably due to the fact that excessive addition of O resulted in the formation of oxides which are not effective for the improvement of the machinability.
As indicated, the stainless steels in accordance with Comparative Examples 4 and 12 each contain S which is one of free cutting elements in a smaller proportion than the range specified in this application. Accordingly, the total area ratio of a specific sulfide is also lower than the range specified in this application. As compared with the stainless steels in accordance with Examples 1 to 15, and 31 to 35, and the stainless steels in accordance with Examples 16 to 30, and 36 to 40, the effect of improving the machinability cannot be sufficiently obtained.
As indicated, the stainless steels in accordance with Comparative Examples 5 and 13 each contain Te which is one of free cutting elements in a smaller proportion than the range specified in this application. Accordingly, as compared with the stainless steels in accordance with Examples 1 to 15, and 31 to 35, and the stainless steels in accordance with Examples 16 to 30, and 36 to 40, the effect of improving the machinability cannot be sufficiently obtained. Further, it is indicated that the cold workability is deteriorated, and that the toughness is also deteriorated due to the anisotropy.
The stainless steels in accordance with Comparative Examples 6 and 14 each contain O in a smaller proportion than the range specified in this application. Accordingly, as compared with the stainless steels in accordance with Example 1 to 15, and 31 to 35, and the stainless steels in accordance with Examples 16 to 30, and 36 to 40, the machinability is deteriorated. This is presumably due to the fact that too small amount of O inhibited the sufficient formation of sulfides with such a size as to improve the machinability.
As indicated, the stainless steels in accordance with Comparative Examples 7 and 15 each contain S in an extremely larger proportion than the range specified in this application. Accordingly, the machinability is comparable to that of each stainless steel in accordance with Examples 1 to 15, and 31 to 35, and that of each stainless steel in accordance with Examples 16 to 30, and 36 to 40. However, the anisotropy has occurred, resulting in a deterioration of the toughness. Further, the cold workability is also extremely deteriorated.
The stainless steels in accordance with Comparative Examples 8 and 16 each has, especially, an extremely smaller value of [Mn]/[S] than the range specified in this application. Accordingly, the machinability is deteriorated, and in addition, the hot workability is also inferior as compared with each stainless steel in accordance with Examples 1 to 15, and 31 to 35, and each stainless steel in accordance with Examples 16 to 30, and 36 to 40.
In contrast to these, the inventive stainless steels in accordance with Examples 1-4, 8, 13, 15-19, 23, 24, 26, 27, 30-33, 36-39 satisfy the component composition specified in this invention. In addition, S, Mn, Te, and O each satisfy the above-mentioned respective formulae. Furthermore, a specific sulfide is present in a specific area ratio. Accordingly, the machinability, the hot workability, the cold workability, and the toughness were excellent.

Claims

A martensitic free cutting stainless steel, consisting of, by weight percent:
C: 0.10 to 1.20 %,

Si: 0.10 to 2.00 %,

Mn: 0.80 to 1.50 %,

S: 0.15 to 0.30 %,

Cr: 10.5 to 15.0 %,

Pb: 0.03 to 0.23 %,

Te: 0.01 to 0.10 %,

P: 0.005 to 0.10 %,

N: present up to 0.050 %,and

O: 0.005 to 0.030 %,
and optionally
B: 0.0005 to 0.010 %,

Cu: 0.01 to 2.0 %,

Ni: 0.01 to 2.0 %,

Mo: 0.01 to 1.0 %,

Bi: 0.01 to 0.30 %,

Ca: 0.0001 to 0.05 %,

Mg: 0.0001 to 0.02 %,

REM:0.0001 to 0.02%,

W: 0.01 to 2.0 %,

Nb: 0.01 to 0.50 %,

Ta: 0.01 to 0.50 %, and

V: 0.01 to 0.50 %
with the remainder being Fe and inevitable impurities,
wherein respective formulae of $3.0 \leq [Mn] / [S] \leq 10.0,$
$0.10 \leq [Te] / [S] \leq 0.50,$

and $15 \leq [S] / [O] \leq 30$

are satisfied; and
wherein a sulfide having a circle equivalent diameter of 2.0 µm or more and aspect ratio of its longer diameter/its shorter diameter of 10 or less is present in a total amount of 0.50 to 10 % by area ratio.
The martensitic free cutting stainless steel according to claim 1, which comprises
at least one element selected from the group consisting of

Cu: 0.01 to 1.0 %,

Ni: 0.01 to 1.0 %, and

Mo: 0.01 to 0.60%.
The martensitic free cutting stainless steel according to claim 1 or 2, which comprises: Bi: 0.01 to 0.20 %.
The martensitic free cutting stainless steel according to any one of claims 1 to 3, which comprises at least one element selected from the group consisting of
Ca: 0.0001 to 0.01 %,

Mg: 0.0001 to 0.01 %, and

REM: 0.0001 to 0.02%.
The martensitic free cutting stainless steel according to any one of claims 1 to 4, which comprises
W: 0.01 to 1.0 %.
The martensitic free cutting stainless steel according to any one of claims 1 to 5, which comprises at least one element selected from the group consisting of
Nb: 0.01 to 0.40 %,

Ta: 0.01 to 0.40 %, and

V: 0.01 to 0.40 %
The martensitic free cutting stainless steel according to one of claims 1 to 6, in which 4.0 ≤ [Mn]/[S].
Use of the martensitic stainless steel of one of the preceding claims, for a motor shaft, a pump shaft, a valve component, a screw, a bolt, or a nut.
A method of manufacturing martensitic stainless steel, comprising the use of a composition as set out in one of claims 1 to 7, and further comprising at least a hot processing in a temperature range of 950-1250°C, an annealing treatment performed at 750-900°C.
The method according to claim 9, wherein the annealing treatment is performed for 3-5 hours.
The method according to claim 10, wherein the annealed steel is furnace cooled to around 600°C at a rate of 10-20°C/hour.
The method according to claim 11, wherein the furnace cooling at the prescribed rate is followed by air cooling.
The method according to one of claims 9 to 12, wherein the hot processing is hot forging or hot rolling.