EP3392355A1

EP3392355A1 - Ferrite-based free-machining stainless steel and method for producing same

Info

Publication number: EP3392355A1
Application number: EP16875188.1A
Authority: EP
Inventors: Chihiro Furusho; Hiroyuki Takabayashi
Original assignee: Daido Steel Co Ltd
Current assignee: Daido Steel Co Ltd
Priority date: 2015-12-18
Filing date: 2016-09-20
Publication date: 2018-10-24
Also published as: EP3392355A4; WO2017104202A1; JP6631234B2; JP2017110285A

Abstract

The present invention is characterized by obtaining a steel having a ferrite cross-sectional area ratio of 95% or more through hot-forging, in a ferrite single-phase region, a steel having a composition makeup containing, in mass%, 0.015% or less of C, 0.02-0.60% of Si, 0.2-2.0% of Mn, 0.050% or less of P, 1.5% or less of Cu, 1.5% or less of Ni, 10.0-25.0% of Cr, 2.0% or less of Mo, 0.30-2.50% of Al, 0.0030-0.0400% of O, 0.035% or less of N, and 0.10-0.45% of S, and in addition, two or more selected from 0.03-0.40% of Pb, 0.03-0.40% of Bi, and 0.01-0.10% of Te, the balance being Fe and incidental impurities, where the masses by percent of the respective elements of C, N, Si, Cr, Mo, and Al, satisfy a predetermined relationship.

Description

TECHNICAL FIELD

The present invention relates to a free-cutting ferritic stainless steel and a method for producing the same, and particularly to a free-cutting ferritic stainless steel excellent in machinability to a small-diameter drill and in hot workability, and a method for producing the same.

BACKGROUND ART

A free-cutting ferritic stainless steel contains a so-called "free-cutting element" such as S added for the purpose of improving machinability, and on the other hand, the balance of component design is made in order to maintain or improve other properties.
For example, Patent Document 1 discloses a steel containing, in mass %, C: 0.1% or less, Si: 2.0% or less, Mn: 2.0% or less, Cr: 19 to 25%, and S: 0.20 to 0.35%, as a free-cutting ferritic stainless steel having excellent corrosion resistance. It is disclosed that the corrosion resistance is improved by setting the lower limit of the content of Cr to higher value. On the other hand, excessive addition of Cr deteriorates hot workability, and for this reason, the upper limit thereof is specified.
Cr can increase corrosion resistance as a free-cutting ferritic stainless steel, but on the other hand, sometimes deteriorates machinability.
Patent Document 2 describes that corrosion resistance as a free-cutting stainless steel can be improved by adjusting Cr amount in a sulfide without excessively deteriorating machinability. It is disclosed that such a steel contains, in mass %, C: 0.40% or less, Si: 2.0% or less, Mn: 0.20 to 0.90%, S: 0.05 to 0.40%, and Cr: 10 to 30%, and satisfies exp{(12+0.18[Cr]-36[S])/22}[S]≤[Mn]≤exp{32+0.18[Cr]-36[S])/22}[S] when mass % of an element M is denoted by [M]. Specifically, Mn gives important effect to the composition of a sulfide, but in the case where the content of Mn is smaller than of the most left side in the above formula, Cr concentration in a sulfide increases, thereby deteriorating machinability. On the other hand, in the case where the content of Mn is larger than the most right side, Mn concentration in the sulfide increases, thereby deteriorating corrosion resistance, and additionally a matrix is solid-solution hardened, thereby deteriorating hot workability.
Patent Document 3 discloses a steel containing, in vol%, C: 0.200% or less, Si: 0.01 to 5.00%, Mn: 0.01 to 5.00%, Ni: 5.00% or less, Cr: 7.50 to 30.00%, N: 0.027% or less, Al: 0.300% or less, O: 0.0050 to 0.1000%, and B: 0.0020 to 0.1000, in which a ratio of the O concentration to the B concentration is set to 0.60 to 2.50, as a free-cutting ferritic stainless steel having excellent corrosion resistance while having machinability equal to that of a general free-cutting steel without containing Pb. Generally, in a free-cutting ferritic stainless steel, the content of S is increased from the standpoint of maintaining machinability, instead of adding Pb, but on the other hand, S deteriorates corrosion resistance. If the content of other elements improving machinability, such as Se, is increased, hot workability is deteriorated. In view of the above, this Patent Document proposes to adjust the amounts of B and oxygen and disperse an oxide of B in a steel. Specifically, it is disclosed that the oxide of B has a low melting point similar to Pb, and as a result, it is melted by the heat during cutting whereby melting embrittlement effect can be obtained, and machinability equal to that of a Pb-containing free-cutting steel is achieved.

PRIOR ART DOCUMENTS

PATENT DOCUMENT

Patent Document 1: JP-A H11-140597
Patent Document 2: JP-A 2006-097039
Patent Document 3: JP-A 2008-274361

SUMMARY OF THE INVENTION

PROBLEMS THAT THE INVENTION IS TO SOLVE

A ferritic stainless steel containing a large amount of Cr generally has excellent corrosion resistance and is therefore widely used in precision parts. In small diameter drilling such as a diameter of 2 mm or less for producing precision parts, when a hole having a depth of 2 times or more of a drill diameter is tried to obtain, the life of a tool is remarkably deteriorated as compared with a large diameter drilling, thereby deteriorating roughness of the machined surface. For this reason, in the machining of precision parts requiring high level of surface roughness, particularly the machining of precision parts such as a pen point of a ballpoint pen, small diameter drilling workability should be improved by using a free-cutting stainless steel having higher machinability, and additionally the life of a tool should not be decreased. However, when elements such as S, Se, Pb, Bi, and Te improving the small diameter drilling workability are tried to be contained more, the problem on production occurs such that hot forging becomes difficult due to the deterioration of properties other than machinability, particularly hot workability.
The present invention has been made in view of the above circumstances, and the object thereof is to provide a free-cutting ferritic stainless steel excellent in machinability to a small diameter drill and in hot workability, and a method for producing the same.

MEANS FOR SOLVING THE PROBLEMS

A method for producing a free-cutting ferritic stainless steel according to the present invention containing hot forging a steel having a component composition containing, in mass %, C: 0.015% or less, Si: 0.02 to 0.60%, Mn: 0.2 to 2.0%, P: 0.050% or less, Cu: 1.5% or less, Ni: 1.5% or less, Cr: 10.0 to 25.0%, Mo: 2.0% or less, Al: 0.30 to 2.50%, O: 0.0030 to 0.0400%, N: 0.035% or less, and S: 0.10 to 0.45%, and further containing at least two selected from Pb: 0.03 to 0.40%, Bi: 0.03 to 0.40% and Te: 0.01 to 0.10%, with a balance being Fe and unavoidable impurities, in which when mass % of an element M is denoted by [M], 900([C]+[N])+170[Si]+12[Cr]+30[Mo]+10[Al]≤300 is satisfied, in a ferrite single phase region, thereby obtaining a steel having a ferrite cross-sectional area percentage of 95% or more.
According to the present invention, a free-cutting ferritic stainless steel having excellent machinability to small diameter drill can be provided with high productivity.
The above-described invention may further satisfy ([Cr]+[Mo]+1.5[Si]+4[Al])/([Ni]+0.5[Mn]+30[C]+30[N])≥7. This invention can enhance ferrite stability and expand a ferrite single phase temperature region, and as a result, higher productivity can be achieved.
In the above-described invention, the steel may further contain one kind or two or more kinds selected from B: 0.0001 to 0.0080%, Mg: 0.0005 to 0.0100% and Ca: 0.0005 to 0.0100%. According to this invention, hot workability is improved and higher productivity can be achieved.
A free-cutting ferritic stainless steel according to the present invention is a free-cutting ferritic stainless steel having a component composition containing, in mass %, C: 0.015% or less, Si: 0.02 to 0.60%, Mn: 0.2 to 2.0%, P: 0.050% or less, Cu: 1.5% or less, Ni: 1.5% or less, Cr: 10.0 to 25.0%, Mo: 2.0% or less, Al: 0.30 to 2.50%, O: 0.0030 to 0.0400%, N: 0.035% or less, and S: 0.10 to 0.45%, and further containing at least two selected from Pb: 0.03 to 0.40%, Bi: 0.03 to 0.40% and Te: 0.01 to 0.10%, with a balance being Fe and unavoidable impurities, in which when mass % of an element M is denoted by [M], 900([C]+[N])+170[Si]+12[Cr]+30[Mo]+10[Al]≤300 is satisfied, and has a ferrite cross-sectional area percentage being 95% or more.
According to the present invention, machinability to a small-diameter drill can be enhanced as a free-cutting ferritic stainless steel.
The above-described invention may further satisfy ([Cr]+[Mo]+1.5[Si]+4[Al])/([Ni]+0.5[Mn]+30[C]+30[N])≥7. This invention can enhance ferrite stability and can produce the steel by expanding a ferrite single phase temperature region, and as a result, a free-cutting ferritic stainless steel with higher productivity can be provided.
In the above-described invention, the steel may further contain one kind or two or more kinds selected from B: 0.0001 to 0.0080%, Mg: 0.0005 to 0.0100% and Ca: 0.0005 to 0.0100%. This invention can enhance hot workability and can give a free-cutting ferritic stainless steel having higher productivity.

ADVANTAGEOUS EFFECTS OF THE INVENTION

According to the present invention, a free-cutting ferritic stainless steel excellent in machinability to a small diameter drill and in hot workability, and a method for producing the same can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] It is a view of component compositions of Examples and Comparative Examples.
[FIG. 2] It is a view of component compositions of Examples and Comparative Examples.
[FIG. 3] It is a view showing the results of evaluation tests of Examples and Comparative Examples.
[FIG. 4] It is a view showing the relationship between the value of Formula 1 and Vickers hardness.
[FIG. 5(a)] It is a microstructure photograph of Example 13.
[FIG. 5(b)] It is a microstructure photograph of Comparative Example 16.

MODE FOR CARRYING OUT THE INVENTION

The present inventors first made investigations on a component composition of a free-cutting stainless steel having higher machinability that improves small diameter drilling workability and additionally does not decrease the life of a drill tool.
Regarding the life of a drill tool, the consideration can be given to decrease matrix strength of a steel to reduce thrust resistance of a drill and additionally stabilize this. This can be achieved by decreasing the addition amounts of solid-solution hardening elements such as Si, Cr and Mo. On the other hand, those elements are also elements stabilizing a ferrite phase, and thus, the temperature at which a ferrite single phase can be maintained is decreased. That is, a two-phase state of ferrite-austenite is easy to be formed in a hot forging temperature region, and hot workability is deteriorated. When the hot forging temperature region is tried to be decreased, deformation resistance is increased and hot workability is deteriorated.
Regarding the small diameter drilling workability, the consideration can be given to make a matrix of a steel brittle and improve chip breakability. This can be achieved by adding elements such as Si, P and V, but on the other hand, those elements decrease the life of a drill tool by solid-solution hardening.
In view of the above, the present inventors conceived of an idea of enhancing phase stability of ferrite by increasing the content of Al as a strong ferrite stabilizing element while decreasing matrix strength by decreasing the addition amount of solid-solution hardening element so as not to decrease the life of a drill tool. In addition, brittleness-ductility transition temperature can be shifted to a high temperature side by containing Al, thereby effectively making a matrix brittle and improving chip breakability, and small diameter drilling workability can be improved. Furthermore, Al has a small solid-solution hardening amount as compared with Si, V and the like and thus, can suppress the increase in strength of matrix, and does not decrease the life of a drill tool.
In free-cutting ferritic stainless steels of plural component compositions in which the addition amounts of solid-solution hardening elements such as Si, Cr and Mo were decreased and the content of Al was increase, they calculated the values of the following formula land formula 2 predicting matrix strength and phase stability of ferrite phase, and evaluated hot workability and machinability. Based on the results, they have found ranges of the contents of Al capable of improving machinability while maintaining hot workability and other elements, and ranges of the value (MS value) of Formula 1 and the value (FS value) of Formula 2. The results of the evaluation tests and the like are described below.
When mass % of an element M is denoted by [M], Formula 1 and Formula 2 are as follows. $MS = 900 ([C] + [N]) + 170 [Si] + 12 [Cr] + 30 [Mo] + 10 [Al]$
$FS = ([Cr] + [Mo] + 1.5 [Si] + 4 [Al]) / ([Ni] + 0.5 [Mn] + 30 [C] + 30 [N])$
Formula 1 is a formula predicting matrix strength and is constituted of solid-solution hardening elements. Formula 2 is a formula predicting phase stability of ferrite phase in a hot forging temperature region, in which the numerator is constituted of elements stabilizing ferrite and the denominator is constituted of elements stabilizing austenite.
In the evaluation tests, each of 150 kg of steel ingots having respective component compositions shown in Examples 1 to 25 and Comparative Examples 1 to 16 of FIG. 1 and FIG. 2 was melted and hot forged. A part of the as-hot forged material was directly subjected to the tests described hereinafter, and the remainder was hot rolled to form a round bar having a diameter of 20 mm and a timber of 60 mm square. As an annealing treatment, they were maintained at a temperature of 740 to 800°C for 4 hours and then air cooled. The following test pieces were appropriately cut out of the annealed materials of the round bar and timber obtained, and subjected to the tests. The results obtained were evaluated. The unit of the component compositions in FIG. 1 and FIG. 2 is mass %.

[Vickers hardness]

Vickers hardness of each annealed material was measured at a portion corresponding to the "middle part" when ingoting after melting. The measurement was conducted at 5 points, and its average value is shown in FIG. 3.

[Hot workability]

Greeble test piece was collected from the as-hot forged material, and was subjected to a high speed tensile test at high temperature. A parallel part of the test piece had a size of 4.5 mm diameter × 20 mm L, and a grip part thereof had a size of M6 × 10 mm L (6 mm diameter × 10 mm length). The test piece was heated up to 1100°C in 100 seconds and maintained at the temperature for 60 minutes. The temperature of the test piece was changed to each test temperature in a rate of 10°C/sec, and the test piece was maintained at the temperature for 60 seconds, pulled in a rate of 50.8 mm/sec and forced to break. The test temperatures are set 7 points of from 900°C to 1200°C by 50°C. After breaking, the reduction rate in area at the break position was measured. As hot workability at a temperature of 900 to 1200°C, the case where the reduction rate in area was 50% or more in all of the 7 test temperatures was evaluated as Good (A), and the case where the reduction rate in area was less than 50% in any of the 7 test temperatures was evaluated as Poor (C). Those evaluation results are shown in FIG. 3.

[Ferrite amount]

A plate-shaped sample having a size of 15mm square × 2 mm T was collected from the as-hot forged material, the surface thereof was mirror polished and etched, and the microstructure of the surface was observed at 25°C. In the microstructure observation, the case where martensite structure was 5% or less in terms of the cross-sectional area percentage in the ferrite structure was evaluated as Good (A), and the case where the martensite structure exceeded 5% was evaluated as Poor (C). Those evaluation results are shown in FIG. 3. Specifically, it is evaluated such that the formation of martensite structure is due to a two-phase state of ferrite-austenite during hot forging at which a temperature is the highest temperature in the production process after melting steel ingot and the two-phase state adversely affects hot workability.

[Machinability to small-diameter drill]

To evaluate machinability to a small-diameter drill, the life of a drill tool and chip breakability were evaluated. In detail, the annealed material was perforated in a feed rate of 0.03 mm/rev and a cutting rate of 70 m/min by using a high-speed drill having a diameter of 1 mm without using a lubricant, and the life of the drill tool was evaluated. The case where the perforation exceeding 4000 mm was possible without breakage of a drill was evaluated as Good (A), the case where the perforation of 2000 to 4000 mm was possible was evaluated as Acceptable (B), and the case where the perforation of less than 2000 mm was possible was evaluated as Poor (C). Those evaluation results are shown in FIG. 3. The chip breakability was evaluated as follows. When the chips were observed, the case where 80% or more of the chips was cut within 1 or 2 curls was evaluated as Good (A), the case where chips were cut with 3 to 5 curls was evaluated as Acceptable (B), and the case where chips continued with 6 or more curls was evaluated as Poor (C). Those evaluation results are shown in FIG. 3.
The respective results of Vickers hardness, hot workability, ferrite amount, life of drill toll, and chip breakability of Examples 1 to 25 and Comparative Examples 1 to 16 are described below based on FIG. 3.
In Examples 1 to 25, Vickers hardness was within a range of 131 to 169 HV, and matrix strength was decreased in each Example. It is therefore considered to contribute to the improvement of machinability. The evaluation of Good (A) or Acceptable (B) was obtained in the life of a drill tool and the chip breakability. The evaluation of the ferrite amount was all Good (A), and it is considered that ferrite single phase could be maintained in hot forging. The evaluation of the hot workability was all Good (A), and therefore high hot workability can be maintained in the test temperature. In other words, according to Examples 1 to 25, machinability could be improved while maintaining hot workability. The value (MS value) of Formula 1 predicting matrix strength was 188 to 287. The value (FS value) of Formula 2 predicting phase stability of the ferrite phase in the hot forging temperature region was 8.2 to 27.5.
In the life of a drill tool, Examples 3, 4, 6, 11 to 16, 20 to 22, and 25 having the evaluation of Acceptable (B) were that the MS value was large as 235 or more, as compared with Examples having the evaluation of Good (A). In the chip breakability, Examples 2 and 12 having the evaluation of Acceptable (B) were that the content of Al was small as 0.31 mass %, as compared with Examples having the evaluation of Good (A).
On the other hand, Comparative Example 1 has the component composition corresponding to SUS 430F as the representative free-cutting ferritic stainless steel, but the content of C was large as 0.044 mass % as compared with Examples, and the MS value was large as 316 as compared with Examples. In other words, it was predicted that matrix strength was increased. As is understood from that none of Pb, Bi and Te were added and both the life of a drill tool and chip breakability were evaluated as Poor C, machinability is poor. The FS value was 5.4 and was smaller than the above-described Examples, and it was predicted that phase stability of the ferrite phase in the hot forging temperature region was poor. Although the hot workability was evaluated as Good (A), the ferrite amount was evaluated as Poor (C).
In Comparative Example 2, the content of C was large as 0.039 mass % as compared with Examples, and the FS value was small as 5.9 as compared with Examples. Therefore, it was predicted that phase stability of the ferrite phase in the hot forging temperature region was poor, and the ferrite amount was evaluated as Poor (C). The hot workability was also evaluated as Poor (C), and in fact, the working was impossible. For this reason, the machinability with a small-diameter drill was not evaluated.
In Comparative Example 3, the content of Si was large as 1.22 mass % as compared with Examples, and the MS value was large as 404 as compared with Examples. Therefore, it was predicted that matrix strength was increased. It is deemed that thrust resistance during cutting with a small-diameter drill was increased, and the life of a drill tool was evaluated as Poor (C).
In Comparative Example 4, the content of S was small as 0.02 mass % as compared with Examples, and the life of a drill tool and chip machinability were evaluated as Poor (C) so that machinability was poor.
In Comparative Example 5, the content of S was large as 0.58 mass % as compared with Examples, and the hot workability was evaluated as Poor (C). In fact, the working was impossible. For this reason, the machinability with a small-diameter drill was not evaluated.
In Comparative Example 6, the content of Ni was large as 2.2 mass % as compared with Examples, and the FS value was small as 4.6 as compared with Examples. It was predicted that phase stability of the ferrite phase in the hot forging temperature region was poor, and the ferrite amount was evaluated as Poor (C). The hot workability was also evaluated as Poor (C), and in fact, the working was impossible. For this reason, the machinability with a small-diameter drill was not evaluated.
In Comparative Example 7, the content of Mo was large as 2.2 mass % as compared with Examples, the MS value was large as 313 as compared with Examples, and it was predicted that matrix strength was increased. The chip breakability was evaluated as Acceptable (B), but the life of a drill tool was evaluated as Poor (C), and it is considered that thrust resistance was increased.
In Comparative Example 8, the content of Al was small as 0.03 mass % as compared with Examples, the FS value was small as 6.2 as compared with Examples, and it was predicted that phase stability of the ferrite phase in the hot forging temperature region was poor. The ferrite amount was evaluated as Poor (C). The hot workability was also evaluated as Poor (C), and in fact, the working was impossible. For this reason, the machinability with a small-diameter drill was not evaluated.
In Comparative Example 9, the content of O was small as 0.0025 mass % as compared with Examples, and it is considered that S-based inclusion was converted into a needle shape. The life of a drill tool was evaluated as Poor (C).
In Comparative Example 10, the content of Pb was large as 0.45 mass % as compared with Examples, and the hot workability was evaluated as Poor (C). In fact, the working was impossible. For this reason, the machinability with a small-diameter drill was not evaluated.
In Comparative Example 11, the content of Bi was large as 0.41 mass % as compared with Examples, and the hot workability was evaluated as Poor (C). In fact, the working was impossible. For this reason, the machinability with a small-diameter drill was not evaluated.
In Comparative Example 12, the content of Al was small as 0.03 mass % as compared with Examples and instead thereof, Nb was contained in an amount of 0.35 mass %. It is considered that phase stability of the ferrite phase in the hot forging temperature region was secured by containing Nb but matrix strength was increased by refinement of grains due to fine dispersion of carbonitrides and by solid-solution hardening. The life of a drill tool was evaluated as Poor (C), and it is considered that thrust resistance was increased. The chip breakability was also evaluated as Poor (C), and it is considered that Nb is poorer than Al in the effect of embrittlement of a matrix.
In Comparative Example 13, the content of Al was small as 0.01 mass % as compared with Examples and instead thereof, Ti was contained in an amount of 0.31 mass %. It is considered that phase stability of the ferrite phase in the hot forging temperature region was secured by containing Ti but matrix strength was increased by refinement of grains due to fine dispersion of carbonitrides and by solid-solution hardening. The life of a drill tool was evaluated as Poor (C), and it is considered that thrust resistance was increased. The chip breakability was also evaluated as Poor (C), and it is considered that Ti is poorer than Al in the effect of embrittlement of a matrix.
In Comparative Example 14, the content of Al was small as 0.02 mass % as compared with Examples and instead thereof, V is contained in an amount of 0.32 mass %. It is considered that phase stability of the ferrite phase in the hot forging temperature region was secured by containing V but matrix strength was increased by refinement of grains due to fine dispersion of carbonitrides and by solid-solution hardening. The life of a drill tool was evaluated as Poor (C), and it is considered that thrust resistance was increased. The chip breakability was also evaluated as Poor (C), and it is considered that V is poorer than Al in the effect of embrittlement of a matrix.
Comparative Example 15 has the component composition nearly equal to that of Examples. However, the MS value was large as 329 as compared with Examples, and it was predicted that matrix strength was increased. The chip breakability was evaluated as Acceptable (B) but the life of a drill tool was evaluated as Poor (C), and it is considered that thrust resistance was increased.
Comparative Example 16 has the component composition nearly equal to that of Examples. However, the FS value was small as 6.1 as compared with Examples, and it was predicted that phase stability of the ferrite phase in the hot forging temperature region was poor. The ferrite amount was evaluated as Poor (C). The hot workability was also evaluated as Poor (C), and in fact, the working was impossible. For this reason, machinability with a small-diameter drill was not evaluated.
Based on the above-described results and some other similar test results, the value (MS value) of Formula 1 predicting matrix strength for achieving machinability with a small-diameter drill, required in steels having the same component compositions as Examples was defined as 300 or less. The MS value is preferably 230 or less from the evaluation of the life of a drill toll of Examples. The MS value is preferably 180 or more.
The relationship between the MS value (the value of Formula 1) and Vickers hardness is shown in FIG. 4. The Vickers hardness is a main factor affecting matrix strength, and therefore had certain correlation with the MS value as in Examples 1 to 25 and Comparative Examples 1 to 11, 15, and 16. Thus, matrix strength can be predicted by the MS value in the alloy system of the present invention. Comparative Examples 12, 13 and 14 contain Nb, Ti and V, respectively, and are alloy systems different from the present invention. For this reason, matrix strength cannot be predicted by Formula 1, and the correlation differs from Examples and other Comparative Examples. In other words, the free-cutting ferritic stainless steel according to the present invention does not contain Nb, Ti and V, excluding the case of being contained in the level of unavoidable impurities. The "level of unavoidable impurities" of those elements as used herein means Nb≤005%, Ti≤0.05% and V≤0.05%, in mass %. Even in the component composition nearly equal to Examples, as in Comparative Example 15, in the case where the MS value is larger than 300, it is predicted that matrix strength is increased. As a result, in Comparative Example 15, Vickers hardness was high and the required machinability with a small-diameter drill was not obtained.
Hot forging must be conducted in a ferrite single phase region in order to maintain the hot workability, as explained above. Therefore, the value (FS value) of Formula 2 for achieving preferable phase stability of the ferrite phase was defined as 7 or more. In other words, in the case where the FS value is 7 or more, phase stability of the ferrite phase is increased to increase the upper limit temperature of the ferrite single phase temperature region, thereby facilitating the forging in the ferrite single phase temperature region.
Microstructures at 25°C of Examples 13 and Comparative Example 16 are shown in FIG. 5(a) and FIG. 5(b). Example 13 shows the representative structure of Examples, and has the structure having a ferrite cross-sectional area percentage at 25°C of 95% or more. In other words, it is assumed that the hot forging could be performed in a ferrite single phase region. The free-cutting ferritic stainless steel according to the present invention only has to have the structure having a ferrite cross-sectional area percentage at 25°C of 95% or more, and may contain another phase within the cross-sectional area percentage being less than 5%. On the other hand, in Comparative Example 16, even though the component composition nearly equals to that of Examples, the FS value (the value of Formula 2) was less than 7, and it was predicted that phase stability of the ferrite phase in the hot forging temperature region was poor. In the microstructure, many martensite was observed, and it is apparent that it does not satisfy the requirement that the ferrite cross-sectional area percentage at 25°C is 95% or more. It is considered in Comparative Example 16 that even though the hot forging was performed under the same conditions as in Examples, two phase state of ferrite-austenite was formed when hot forged.
The composition range of the alloy capable of giving hot workability and machinability nearly equal to those in the above evaluation tests is defined as follows.
C is a representative solid-solution hardening element and has the possibility of increasing matrix strength and decreasing machinability. For this reason, C is, in mass %, 0.015% or less, and preferably 0.012% or less.
Si is an element necessary as a deoxidizing agent. On the other hand, Si is a representative solid-solution hardening element, and has the possibility of increasing matrix strength and decreasing machinability when excessively added. For this reason, Si is, in mass %, in a range of 0.02 to 0.60%, and preferably in a range of 0.02 to 0.40%.
Mn forms a compound together with S, and is an element necessary to improve machinability. Furthermore, it suppresses grain boundary segregation of S and improves hot workability. On the other hand, Mn is an element stabilizing austenite, and destabilizes the ferrite phase in the hot forging temperature region when excessively added. For this reason, Mn is, in mass %, in a range of 0.2 to 2.0%.
P is a solid-solution hardening element and has the possibility of increasing matrix strength and decreasing machinability. For this reason, P is, in mass %, 0.050% or less, and preferably 0.040% or less.
Cu is an element stabilizing austenite, and destabilizes the ferrite phase in the hot forging temperature region. For this reason, Cu is, in mass %, 1.5% or less.
Ni is an element stabilizing austenite, and destabilizes the ferrite phase in the hot forging temperature region. For this reason, Ni is, in mass %, 1.5% or less.
Cr is an element necessary for improving corrosion resistance. On the other hand, excessive addition of Cr has the possibility of increasing matrix strength and decreasing machinability. For this reason, Cr is, in mass %, in a range of 10.0 to 25.0%, and preferably in a range of 10.0 to 17.0%.
Mo is an element contributing to the improvement of corrosion resistance, and can be added as necessary. On the other hand, it is a representative solid-solution hardening element and has the possibility of increasing matrix strength and decreasing machinability. For this reason, Mo is, in mass %, 2.0% or less.
Al is the most important element in the present invention. Al is an element necessary to shift a brittleness-ductility transition temperature to a high temperature side, accelerate embrittlement of a matrix and improve chip breakability. Furthermore, it is an element strongly stabilizing the ferrite phase in the forging temperature region, and is necessary to maintain hot workability. On the other hand, excessive addition of Al causes cooling cracks of a steel ingot and has the possibility of adversely affecting productivity. For this reason, Al is, in mass %, in a range of 0.30 to 2.50%, and preferably in a range of 0.35 to 2.50%.
O is an element necessary to decrease an acicular ratio of S-based inclusion. On the other hand, excessive addition of O accelerates the formation of an oxide and deteriorates machinability. For this reason, O is, in mass %, in a range of 0.0030 to 0.0400%.
N is a representative solid-solution hardening element and increases matrix strength. It further forms a hard nitride, thereby decreasing machinability. For this reason, N is, in mass %, 0.035% or less, and preferably 0.025% or less.
S is an element necessary to form a sulfide and improve machinability. On the other hand, excessive addition of S remarkably deteriorates hot workability. For this reason, S is, in mass %, in a range of 0.10 to 0.45%, and preferably in a range of 0.10 to 0.40%.
Pb is an element contributing to the improvement of machinability by a melting embrittlement action due to the heat during cutting. On the other hand, excessive addition of Pb remarkably deteriorates hot workability. For this reason, Pb is, in mass %, in a range of 0.03 to 0.40%, and preferably in a range of 0.03 to 0.30%.
Bi is an element contributing to the improvement of machinability by a melting embrittlement action due to the heat during cutting. On the other hand, excessive addition of Bi remarkably deteriorates hot workability. For this reason, Bi is, in mass %, in a range of 0.03 to 0.40%, and preferably in a range of 0.03 to 0.30%.
Te is an element contributing to the improvement of machinability by a melting embrittlement action due to the heat during cutting and by an action of decreasing the acicular ratio of a sulfide. On the other hand, excessive addition of Te remarkably deteriorates hot workability. For this reason, Te is, in mass %, in a range of 0.01 to 0.10%, and preferably in a range of 0.01 to 0.08%.
It only has to contain at least two elements of the above-described three elements of Pb, Bi and Te. Elements that may be selectively added are described below.
B is an element effective to secure hot workability. On the other hand, excessive addition of B rather deteriorates hot workability. For this reason, B can be contained, in mass %, in a range of 0.0001 to 0.080%, and preferably in a range of 0.0003 to 0.0060%.
Mg is an element effective to secure hot workability. On the other hand, excessive addition of Mg saturates the effect of improving hot workability. For this reason, Mg can be contained, in mass %, in a range of 0.0005 to 0.0100%, and preferably in a range of 0.0010 to 0.0100%.
Ca is an element effective to secure hot workability. On the other hand, excessive addition of Ca saturates the effect of improving hot workability. For this reason, Ca can be contained, in mass %, in a range of 0.0005 to 0.0100%, and preferably in a range of 0.0010 to 0.0100%.
The present invention is described above based on the representative examples, but the present invention is not necessarily limited to those. One skilled in the art might be able to find out various substitute examples and modification examples without departing from the scope of the claims attached.

INDUSTRIAL APPLICABILITY

According to the present invention, a free-cutting ferritic stainless steel excellent in machinability to a small-diameter drill and in hot workability, and a method for producing the same can be provided.
Although the present invention has been described in detail and by reference to the specific embodiments, it is apparent to one skilled in the art that various modifications or changes can be made without departing the spirit and scope of the present invention.
This application is based on Japanese Patent Application (No. 2015-247973) filed on December 18, 2015 , the disclosure of which is incorporated herein by reference.

Claims

A method for producing a free-cutting ferritic stainless steel, comprising hot forging a steel having a component composition comprising, in mass %:
C: 0.015% or less,

Si: 0.02 to 0.60%,

Mn: 0.2 to 2.0%,

P: 0.050% or less,

Cu: 1.5% or less,

Ni: 1.5% or less,

Cr: 10.0 to 25.0%,

Mo: 2.0% or less,

Al: 0.30 to 2.50%,

O: 0.0030 to 0.0400%,

N: 0.035% or less, and

S: 0.10 to 0.45%, and
further containing at least two selected from:
Pb: 0.03 to 0.40%,

Bi: 0.03 to 0.40% and

Te: 0.01 to 0.10%,
with a balance being Fe and unavoidable impurities,
wherein when mass % of an element M is denoted by [M],
900([C]+[N])+170[Si]+12[Cr]+30[Mo]+10[Al]≤300 is satisfied,
in a ferrite single phase region, thereby obtaining a steel having a ferrite cross-sectional area percentage of 95% or more.
The method for producing a free-cutting ferritic stainless steel according to Claim 1, further satisfying ([Cr]+[Mo]+1.5[Si]+4[Al])/([Ni]+0.5[Mn]+30[C]+30[N])≥7.
The method for producing a free-cutting ferritic stainless steel according to Claim 2, wherein the steel further comprises, in mass %, one kind or two or more kinds selected from:
B: 0.0001 to 0.0080%,

Mg: 0.0005 to 0.0100% and

Ca: 0.0005 to 0.0100%.
A free-cutting ferritic stainless steel,
having a component composition comprising, in mass %:
C: 0.015% or less,

Si: 0.02 to 0.60%,

Mn: 0.2 to 2.0%,

P: 0.050% or less,

Cu: 1.5% or less,

Ni: 1.5% or less,

Cr: 10.0 to 25.0%,

Mo: 2.0% or less,

Al: 0.30 to 2.50%,

O: 0.0030 to 0.0400%,

N: 0.035% or less, and

S: 0.10 to 0.45%, and
further containing at least two selected from:
Pb: 0.03 to 0.40%,

Bi: 0.03 to 0.40% and

Te: 0.01 to 0.10%,
with a balance being Fe and unavoidable impurities,
wherein when mass % of an element M is denoted by [M], 900([C]+[N])+170[Si]+12[Cr]+30[Mo]+10[Al]≤300 is satisfied, and having a ferrite cross-sectional area percentage being 95% or more.
The free-cutting ferritic stainless steel according to Claim 4, further satisfying ([Cr]+[Mo]+1.5[Si]+4[Al])/([Ni]+0.5[Mn]+30[C]+30[N])≥7.
The free-cutting ferritic stainless steel according to Claim 5, wherein the steel further comprises, in mass %, one kind or two or more kinds selected from:
B: 0.0001 to 0.0080%,

Mg: 0.0005 to 0.0100% and

Ca: 0.0005 to 0.0100%.