FERRITIC FREE-CUTTING STAINLESS STEEL

Abstract

 The present invention relates to a ferritic free-cutting stainless steel consisting of: C ≤ 0.02 mass%, Si ≤ 0.50 mass%, 0.20 ≤ Mn ≤ 1.00 mass%, P ≤ 0.05 mass%, 0.20 ≤ S ≤ 0.70 mass%, Cu ≤ 1.5 mass%, Ni ≤ 1.5 mass%, 10.0 ≤ Cr ≤ 20.0 mass%, Mo ≤ 2.0 mass%, 0.30 ≤ Al ≤ 1.00 mass% O ≤ 0.010 mass%, N ≤ 0.030 mass%, and optionally at least one selected from the group consisting of: B ≤ 0.0100 mass%, Mg ≤ 0.0100 mass%, and Ca ≤ 0.0100 mass%, with a balance being Fe and unavoidable impurities, in which the following three expressions are satisfied: ([Cr]+[Mo]+1.5[Si]+4[Al])/([Ni]+0.5[Mn]+30[C]+30[N]) > 7, 900([C]+[N])+170[Si]+450[P]+12[Cr]+30[Mo]+10[Al] <_ 300, and 0.285 <_ [Mn]/[S] <_ 1.5, where [X] represents a content (mass%) of an element X.

Description

TECHNICAL FIELD

[0001] The present invention relates to a ferritic free-cutting stainless steel, and more particularly to a ferritic free-cutting stainless steel that has excellent machinability while reducing environmental load.

BACKGROUND ART

[0002] The ferritic stainless steel refers to a stainless steel having a ferrite structure at room temperature.

[0003] The ferritic stainless steel has excellent corrosion resistance, oxidation resistance, heat resistance, or the like, and is therefore used in various parts to be used in precision instruments, automobiles, water-heating equipment, chemical plants, or the like.

[0004] In the case of producing various parts using the ferritic stainless steel, cutting is usually performed. Therefore, the ferritic stainless steel is required to have high machinability.

[0005] In order to solve this problem, various proposals have been made in the related art. For example, Patent Literature 1 discloses

a ferritic free-cutting stainless steel containing predetermined amounts of C, Si, Mn, P, Cu, Ni, Cr, Mo, Al, O, N, S, Pb, Bi, and Te, with the balance being Fe and unavoidable impurities, in which

900([C]+[N])+170[Si]+12[Cr]+30[Mo]+10[Al] < 300 is satisfied, and a ferrite cross-sectional area percentage is 95% or more.

[0006] The same literature discloses that

(A) when the amount of solid solution-strengthening elements added decreases, the matrix strength decreases; and when the content of Al increases, the brittleness-ductility transition temperature shifts to a high temperature side and the chip crushability is improved, which improves the machinability of the ferritic stainless steel with respect to a small diameter drill, and

(B) when the stability of a ferrite phase in a hot-forging temperature range is improved, the hot-workability of the ferritic stainless steel is improved.

[0007] Patent Literature 2 discloses

a highly corrosion-resistant free-cutting stainless steel containing predetermined amounts of C, Si, Mn, S, Cr, Al, and O, with the balance being Fe and unavoidable impurities, in which

a Cr/Mn ratio in a sulfide is 1 or more.

[0008] The same literature discloses that

(A) when the oxygen concentration is increased in a S free-cutting stainless steel, large granular sulfides that are highly effective in improving the machinability are likely to be generated, and at the same time, coarse oxides are also likely to be generated, which may cause a decrease in surface quality after surface finishing, and

(B) when the Cr/Mn ratio is 1 or more, sulfides are not finely dispersed even in the case where the oxygen concentration is low, so that a decrease in machinability is prevented.

[0009] Patent Literature 3 discloses a corrosion-resistant and weather-resistant steel having excellent machinability, which contains predetermined amounts of C, Si, Mn, P, Cr, Ni, Cu, Mo, S, Pb, Se, Te, and Bi, with the balance substantially being Fe.

[0010] The same literature discloses that

(A) when 4.0 mass% to 10.0 mass% of Cr is contained in the steel, it exhibits corrosion resistance and weather resistance comparable to a Ni-plated steel, and

(B) when one or two or more of S, Pb, Se, Te, and Bi are added to the steel, the machinability of the steel is improved.

[0011] S, Pb, Bi, and the like are known as elements that improve machinability of the stainless steel. However, S can improve the machinability, but it may decrease mechanical properties and corrosion resistance. Therefore, in the ferritic free-cutting stainless steel, Pb and Bi, which have the effect of improving lubricity with respect to a cutting tool, may be added as free-cutting elements.

[0012] However, due to the tightening of environmental regulations such as the Restriction of Hazardous Substances (RoHS) in European Union and the Consumer Product Safety Improvement Act (CPSIA) in the United States, requests for Pb-free products are becoming more apparent from user companies. Although Bi, which exhibits properties similar to Pb, is not currently listed as a regulated substance, it is a heavy metal like Pb, so that there is concern that Bi may become a regulated substance in the future.

[0013] Further, in the ferritic stainless steel, in the case where the stability of the ferrite phase decreases, the hot-workability may decrease.

[0014] However, there has never been proposed an example of a Pb- and Bi-free ferritic free-cutting stainless steel that has excellent corrosion resistance and high hot-workability, and has machinability equivalent to a Pb-based free-cutting stainless steel in the related art.

CITATION LIST

PATENT LITERATURE

[0015] 

Patent Literature 1: JP2017-110285A

Patent Literature 2: JP2000-144339A

Patent Literature 3: JPH06-033186A

SUMMARY OF INVENTION

[0016] The problem to be solved by the present invention is to provide a ferritic free-cutting stainless steel that has excellent machinability, corrosion resistance, and hot-workability while reducing environmental load.

[0017] In order to solve the above-mentioned problem, a ferritic free-cutting stainless steel of the present invention consists of:























and optionally at least one selected from the group consisting of:



and

with a balance being Fe and unavoidable impurities, in which the following expression (1) to expression (3) are satisfied:





where [X] represents a content (mass%) of an element X.

[0018] The present invention also provides a method for producing the above-described ferritic free-cutting stainless steel, the method including:

a first step of producing a steel ingot or a billet having a predetermined component composition;

a second step of subjecting the steel ingot or the billet to a homogenization heat treatment at 1250°C to 1300°C for 1 hour or longer; and

a third step of subjecting the steel ingot or the billet after the homogenization heat treatment to hot-working at 700°C to 1000°C.

[0019] The ferritic free-cutting stainless steel according to the present invention contains S as a free-cutting element and does not substantially contain Pb or Bi. Therefore, it exhibits excellent machinability while reducing environmental load.

[0020] Since in the ferritic free-cutting stainless steel according to the present invention, the contents of the respective components are optimized to satisfy the expression (1) (ferrite stabilization index), the phase stability of a ferrite phase in a hot-working temperature range is enhanced. Therefore, it exhibits excellent hot-workability.

[0021] Since in the ferritic free-cutting stainless steel according to the present invention, the contents of the respective components are optimized to satisfy the expression (2) (matrix strength index), the matrix strength is moderately decreased. Therefore, it exhibits excellent machinability.

[0022] Further, since in the ferritic free-cutting stainless steel according to the present invention, the contents of Mn and S are optimized to satisfy the expression (3) (sulfide form index), a Cr-rich MnS-based sulfide is likely to be formed. Therefore, it exhibits excellent corrosion resistance.

DESCRIPTION OF EMBODIMENTS

[0023] Hereinafter, an embodiment of the present invention will be described in detail.

[1. Ferritic Free-cutting Stainless Steel]

[1.1. Main Constituent Elements]

[0024] A ferritic free-cutting stainless steel according to the present invention contains the following elements, with the balance being Fe and unavoidable impurities. Types of added elements, component ranges thereof, and reasons for limitation thereof are as follows.

(1) C ≤ 0.02 mass%:

[0025] C is a typical solid solution-strengthening element. Therefore, in the case where the amount of C is excessive, there is a possibility that the matrix strength increases and the machinability decreases. In addition, in the case where the amount of C is excessive, hard carbides to be abrasive particles are likely to be formed. When hard carbide particles fall off from a workpiece during cutting and the hard carbide particles enter a friction surface between a cutting tool and the workpiece, abrasive wear of the cutting tool is accelerated. Therefore, excessive hard carbides cause a decrease in machinability of the ferritic stainless steel. Therefore, the amount of C needs to be 0.02 mass% or less. The amount of C is preferably 0.015 mass% or less.

[0026] Note that, reducing the amount of C more than necessary may lead to an increase in cost. In consideration of these points, it is preferable to select an optimum amount of C.

(2) Si ≤ 0.50 mass%:

[0027] Si is an element effective as a deoxidizer. On the other hand, Si is also a typical solid solution-strengthening element.

[0028] Therefore, in the case where the amount of Si is excessive, there is a possibility that the matrix strength increases and the machinability decreases. Therefore, the amount of Si needs to be 0.50 mass% or less. The amount of Si is preferably 0.4 mass% or less.

[0029] Note that, reducing the amount of Si more than necessary may lead to an increase in the amount of O due to insufficient deoxidizer. In consideration of these points, it is preferable to select an optimum amount of Si.

(3) 0.20 mass% ≤ Mn ≤ 1.00 mass%:

[0030] Mn is an element that forms a compound with S and contributes to improving the machinability. Mn is also an element that prevents grain boundary segregation of S and contributes to improving the hot-workability. In order to obtain such an effect, the amount of Mn needs to be 0.20 mass% or more. The amount of Mn is preferably 0.30 mass% or more.

[0031] On the other hand, Mn is an austenite stabilization element. Therefore, in the case where the amount of Mn is excessive, there is a possibility that the ferrite phase becomes unstable in a hot-working temperature range and the hot-workability decreases. Therefore, the amount of Mn needs to be 1.00 mass% or less. The amount of Mn is preferably 0.80 mass% or less.

(4) P ≤ 0.05 mass%:

[0032] P is a solid solution-strengthening element. Therefore, in the case where the amount of P is excessive, there is a possibility that the matrix strength increases and the machinability decreases. Therefore, the amount of P needs to be 0.05 mass% or less. The amount of P is preferably 0.04 mass% or less.

[0033] Note that, reducing the amount of P more than necessary may lead to an increase in cost. In consideration of these points, it is preferable to select an optimum amount of P.

(5) 0.20 mass% ≤ S ≤ 0.70 mass%:

[0034] S is an element that forms a sulfide and is effective in improving the machinability. In order to obtain such an effect, the amount of S needs to be 0.20 mass% or more.

[0035] On the other hand, in the case where the amount of S is excessive, the hot-workability may be remarkably decreased. Therefore, the amount of S needs to be 0.70 mass% or less. The amount of S is preferably 0.60 mass% or less.

(6) Cu ≤ 1.5 mass%:

[0036] Cu is an austenite stabilization element. In the case where the amount of Cu is excessive, there is a possibility that the ferrite phase becomes unstable in a hot-working temperature range and the hot-workability decreases. Therefore, the amount of Cu needs to be 1.5 mass% or less. The amount of Cu is preferably 1.0 mass% or less.

(7) Ni ≤ 1.5 mass%:

[0037] Ni is an austenite stabilization element. In the case where the amount of Ni is excessive, there is a possibility that the ferrite phase becomes unstable in a hot-working temperature range and the hot-workability decreases. Therefore, the amount of Ni needs to be 1.5 mass% or less. The amount of Ni is preferably 1.0 mass% or less.

(8) 10.0 mass% ≤ Cr ≤ 20.0 mass%:

[0038] Cr is an element that contributes to improving the corrosion resistance. In addition, Cr can replace a part of Mn in a compound containing Mn and S (hereinafter, also referred to as a "MnS-based sulfide"), and can form a Cr-rich MnS-based sulfide. Since the Cr-rich MnS-based sulfide is less likely to corrode than MnS that does not contain Cr, a stainless steel containing the Cr-rich MnS-based sulfide exhibits high corrosion resistance. In order to obtain such an effect, the amount of Cr needs to be 10.0 mass% or more. The amount of Cr is preferably 12.0 mass% or more.

[0039] On the other hand, in the case where the amount of Cr is excessive, there is a possibility that the matrix strength increases due to solid solution-strengthening and the machinability decreases. Therefore, the amount of Cr needs to be 20.0 mass% or less. The amount of Cr is preferably 18.0 mass% or less.

(9) Mo ≤ 2.0 mass%:

[0040] Mo is an element that contributes to improving the corrosion resistance. On the other hand, in the case where the amount of Mo is excessive, there is a possibility that the matrix strength increases due to solid solution-strengthening and the machinability decreases. Therefore, the amount of Mo needs to be 2.0 mass% or less. The amount of Mo is preferably 1.5 mass% or less.

(10) 0.30 mass% ≤ Al ≤ 1.00 mass%:

[0041] Al is an element that shifts the brittleness-ductility transition temperature to a high temperature side, promotes embrittlement of the matrix, and contributes to improving the chip crushability. In addition, Al is a strong ferrite phase stabilization element in a hot-working temperature range, and is an element necessary to ensure the hot-workability. In order to obtain such an effect, the amount of Al needs to be 0.30 mass% or more.

[0042] On the other hand, in the case where the amount of Al is excessive, there is a possibility that it causes cooling cracks of a steel ingot and the manufacturability is adversely influenced. The reason is considered to be that AlN is generated during the cooling process after casting, and AlN becomes a starting point of cracks. Further, in the case where the amount of Al is excessive, there is a possibility that the matrix strength becomes excessively high, and the machinability decreases. Therefore, the amount of Al needs to be 1.00 mass% or less. The amount of Al is preferably 0.8 mass% or less.

(11) O ≤ 0.010 mass%:

[0043] O promotes the generation of hard oxides to be abrasive particles. In the case where the amount of O is excessive, there is a possibility that abrasive wear of a tool due to oxide particles progresses and the machinability of the stainless steel decreases. Therefore, the amount of O needs to be 0.010 mass% or less. The amount of O is preferably 0.008 mass% or less.

[0044] Note that, reducing the amount of O more than necessary may lead to an increase in cost. In consideration of these points, it is preferable to select an optimum amount of O.

(12) N ≤ 0.030 mass%:

[0045] N is a typical solid solution-strengthening element. In the case where the amount of N is excessive, there is a possibility that the matrix strength increases and the machinability decreases. In addition, N promotes the formation of hard nitrides to be abrasive particles. In the case where the amount of N is excessive, there is a possibility that abrasive wear of a tool due to nitride particles progresses and the machinability of the stainless steel decreases. Therefore, the amount of N needs to be 0.030 mass% or less. The amount of N is preferably 0.020 mass% or less.

[0046] Note that, reducing the amount of N more than necessary may lead to an increase in cost. In consideration of these points, it is preferable to select an optimum amount of N.

[1.2. Sub Constituent Element]

[0047] The ferritic free-cutting stainless steel according to the present invention may further contain one or two or more of the following elements in addition to the above-described main constituent elements. Types of added elements, component ranges thereof, and reasons for limitation thereof are as follows.

(13) 0.0001 mass% ≤ B ≤ 0.0100 mass%:

[0048] B is an element effective in ensuring the hot-workability. The reason why the hot-workability is improved by the addition of B is considered to be that B increases grain boundary strength. In order to obtain such an effect, the amount of B is preferably 0.0001 mass% or more. The amount of B is more preferably 0.002 mass% or more.

[0049] On the other hand, in the case where the amount of B is excessive, the hot-workability may be rather deteriorated. Therefore, the amount of B is preferably 0.0100 mass% or less. The amount of B is more preferably 0.0080 mass% or less.

(14) 0.0005 mass% ≤ Mg < 0.0100 mass%:

[0050] Mg is an element effective in ensuring the hot-workability. The reason why the hot-workability is improved by the addition of Mg is considered to be that Mg traps excessive S and prevents the formation of sulfides at grain boundaries. In order to obtain such an effect, the amount of Mg is preferably 0.0005 mass% or more. The amount of Mg is more preferably 0.0010 mass% or more.

[0051] On the other hand, in the case where the amount of Mg is excessive, the hot-workability may be rather deteriorated. Therefore, the amount of Mg is preferably 0.0100 mass% or less. The amount of Mg is more preferably 0.0080 mass% or less.

(15) 0.0005 mass% ≤ Ca ≤ 0.0100 mass%:

[0052] Ca is an element effective in ensuring the hot-workability. The reason why the hot-workability is improved by the addition of Ca is considered to be that Ca traps excessive S and prevents the formation of sulfides at grain boundaries. In order to obtain such an effect, the amount of Ca is preferably 0.0005 mass% or more. The amount of Ca is more preferably 0.0010 mass% or more.

[0053] On the other hand, in the case where the amount of Ca is excessive, the hot-workability may be rather deteriorated. Therefore, the amount of Ca is preferably 0.0100 mass% or less. The amount of Ca is more preferably 0.0080 mass% or less.

[1.3. Unavoidable Impurities]

[0054] The unavoidable impurities mean elements mixed in from ores or scraps used as steel raw materials, or from the environment of a production process or the like. Examples of the unavoidable impurities include the following elements in addition to the above-described C, Si, P, O, and N. In the present invention, the following elements may be contained as unavoidable impurities in the contents shown below:

[1.4. Component Balance]

[0055] The ferritic free-cutting stainless steel according to the present invention not only has the components within the above-described ranges, but also satisfies the following expression (1) to expression (3).





[X] represents a content (mass%) of an element X.

[1.4.1. Expression (1): Ferrite Stabilization Index]

[0056] In the present invention, the "ferrite stabilization index" is an empirical formula for predicting the phase stability of the ferrite phase in a hot-working temperature range, and is expressed by the left side of the expression (1). Here, the numerator on the left side of the expression (1) represents the sum of the products of the contents of the ferrite stabilization elements and the weighting coefficients. In addition, the denominator on the left side of the expression (1) represents the sum of the products of the contents of the austenite stabilization elements and the weighting coefficients. A large ferrite stabilization index indicates that the ferrite phase has high stability in a hot-working temperature range.

[0057] When the ferrite phase becomes unstable in the hot-working temperature range and becomes a mixed phase of the ferrite phase and an austenite phase, the stress applied to the steel material during hot-working may be non-uniform. As a result, there is a possibility that plastic deformation becomes non-uniform and the steel material cracks. In order to prevent cracks of the steel material during hot-working, the higher the stability of the ferrite phase, the better. For this purpose, the ferrite stabilization index needs to be 7 or more. The ferrite stabilization index is preferably 10 or more, more preferably 12 or more, and further preferably 15 or more.

[0058] On the other hand, in order to increase the ferrite stabilization index, it is necessary to reduce the contents of the austenite stabilization elements (Ni, Mn, C, N). However, excessive reduction in the contents of the austenite stabilization elements may lead to an increase in cost. Therefore, the ferrite stabilization index is preferably 267.5 or less. The ferrite stabilization index is more preferably 50.0 or less, and further preferably 30.0 or less.

[1.4.2. Expression (2): Matrix Strength Index]

[0059] In the present invention, the "matrix strength index" is an empirical formula for predicting the matrix strength of the stainless steel, and is expressed by the left side of the expression (2). Here, the left side of the expression (2) represents the sum of the products of the contents of the solid solution-strengthening elements and the weighting coefficients. A large matrix strength index indicates high matrix strength.

[0060] Generally, the higher the matrix strength, the lower the machinability. In order to obtain practically sufficient machinability, the matrix strength index needs to be 300 or less. The matrix strength index is preferably 290 or less, and more preferably 280 or less.

[0061] On the other hand, in the case where the matrix strength index is too small, the content of elements (for example, Cr) that have an effect of improving corrosion resistance may be excessively low, and the corrosion resistance may decrease. Therefore, the matrix strength index is preferably 123 or more.

[1.4.3. Expression (3): Sulfide Form Index]

[0062] In the present invention, the "sulfide form index" is an empirical formula for predicting the form of the sulfide (that is, whether the Cr-rich MnS-based sulfide is formed), and is expressed by the middle of the expression (3). Here, the middle of the expression (3) represents a ratio of the concentration of Mn to the concentration of S contained in the stainless steel. A small sulfide form index indicates that the Cr-rich MnS-based sulfide, (Cr, Mn)S, is likely to be formed.

[0063] In the case where the sulfide form index is too large, a generation rate of the Cr-rich MnS-based sulfide decreases. Since a Cr-poor MnS-based sulfide has poor corrosion resistance, the smaller the generation rate of the Cr-rich MnS-based sulfide, the lower the corrosion resistance of the stainless steel. Therefore, the sulfide form index needs to be 1.5 or less. The sulfide form index is preferably 1.4 or less.

[0064] On the other hand, in the case where the sulfide form index is too small, the total amount of the MnS-based sulfides that have an effect of improving machinability may be excessively small, and the machinability may decrease. Therefore, the sulfide form index needs to be 0.285 or more. The sulfide form index is preferably 0.3 or more, and more preferably 0.4 or more.

[1.5. Metallographic Structure]

[1.5.1. Number Proportion of MnS-based Sulfide]

[0065] The "MnS-based sulfide" refers to a compound containing Mn and S. In other words, the "MnS-based sulfide" refers to

(a) MnS, and

(b) a sulfide in which a part of Mn in MnS is replaced by another metal element M. Examples of the other metal element M include Cr.

[0066] The "number proportion of the MnS-based sulfide" refers to the number of the MnS-based sulfide contained within an observation view field of 10,000 µm² (= 100 µm × 100 µm).

[0067] Generally, the larger the number proportion of the MnS-based sulfide, the higher the machinability. In order to obtain such an effect, the number proportion of the MnS-based sulfide is preferably 100 or more. The number proportion of the MnS-based sulfide is more preferably 200 or more, and further preferably 300 or more.

[0068] On the other hand, in the case where the number proportion of the MnS-based sulfide is too large, the sulfide may segregate at grain boundaries and the hot-workability may decrease. Therefore, the number proportion of the MnS-based sulfide is preferably 10,000 or less. The number proportion of the MnS-based sulfide is more preferably 9,000 or less, and further preferably 8,000 or less.

[1.5.2. Number Proportion of Inclusion]

[0069] The "inclusion" refers to a compound other than the MnS-based sulfide. Examples of the inclusion include a carbide, an oxide, a nitride, a carbonitride, and a sulfide other than the MnS-based sulfide.

[0070] The "number proportion of the inclusion" refers to the number of the inclusion contained within an observation view field of 10,000 µm² (= 100 µm × 100 µm).

[0071] The inclusion may have an adverse influence on the machinability, the mechanical properties, or the like of the ferritic stainless steel. Therefore, the smaller the number proportion of the inclusion, the better. In order to obtain a ferritic stainless steel having excellent machinability or the like, the number proportion of the inclusion is preferably 20 or less. The number proportion of the inclusion is more preferably 15 or less, and further preferably 10 or less.

[2. Method for Producing Ferritic Free-cutting Stainless Steel]

[0072] A method for producing a ferritic free-cutting stainless steel according to the present invention includes:

a first step of producing a steel ingot or a billet having a predetermined component composition;

a second step of subjecting the steel ingot or the billet to a homogenization heat treatment;

a third step of subjecting the steel ingot or the billet after the homogenization heat treatment to hot-working; and

if necessary, a fourth step of annealing the hot-worked body.

[2.1. First Step]

[0073] First, a steel ingot or a billet having a predetermined component composition is produced. The method for producing the steel ingot or the billet is not particularly limited.

[0074] Examples of the method for producing the steel ingot or the billet include

(a) a method of obtaining a steel ingot by melting and casting raw materials that have been blended to have a predetermined component composition,

(b) a method of obtaining a billet by performing blooming-forging or blooming-rolling on the steel ingot, and

(c) a method of obtaining a billet by continuously casting a molten metal having a predetermined component composition.

[2.2. Second Step]

[0075] Next, the steel ingot or the billet is subjected to a homogenization heat treatment. The conditions for the homogenization heat treatment are not particularly limited as long as component segregation can be eliminated. In the case of the ferritic free-cutting stainless steel according to the present invention, the temperature in the homogenization heat treatment is preferably 1250°C to 1300°C. In addition, the time for the homogenization heat treatment is preferably 1 hour to 24 hours.

[2.3. Third Step]

[0076] Next, the steel ingot or the billet after the homogenization heat treatment is subjected to hot-working. The hot-working is performed to make the steel ingot or the billet into a shaped material having a desired shape. The conditions for the hot-working are not particularly limited as long as a shaped material having a predetermined shape can be efficiently produced. In the case of the ferritic free-cutting stainless steel according to the present invention, the temperature in the hot-working is preferably 700°C to 1000°C. A cooling method after the hot-working is not particularly limited, and air cooling is preferred.

[2.4. Fourth Step]

[0077] Next, if necessary, the hot-worked body is annealed. The annealing is performed in the case where it is necessary to soften the hot-worked body. The annealing conditions are not particularly limited as long as the hot-worked body can be softened. In the case of the ferritic free-cutting stainless steel according to the present invention, the annealing temperature is preferably 730°C to 870°C. In addition, the time for the annealing is preferably 0.5 hours to 10 hours.

[3. Effects]

[0078] The ferritic free-cutting stainless steel according to the present invention contains S as a free-cutting element and does not substantially contain Pb or Bi. Therefore, it exhibits excellent machinability while reducing environmental load.

[0079] Since in the ferritic free-cutting stainless steel according to the present invention, the contents of the respective components are optimized to satisfy the expression (1) (ferrite stabilization index), the phase stability of the ferrite phase in a hot-working temperature range is enhanced. Therefore, it exhibits excellent hot-workability.

[0080] Since in the ferritic free-cutting stainless steel according to the present invention, the contents of the respective components are optimized to satisfy the expression (2) (matrix strength index), the matrix strength is moderately decreased. Therefore, it exhibits excellent machinability.

[0081] Further, since in the ferritic free-cutting stainless steel according to the present invention, the contents of Mn and S are optimized to satisfy the expression (3) (sulfide form index), the Cr-rich MnS-based sulfide is likely to be formed. Therefore, it exhibits excellent corrosion resistance.

[Examples]

(Examples 1 to 11 and Comparative Examples 1 to 14)

[1. Preparation of Sample]

[0082] Steel ingots (150 kg) respectively having the component compositions shown in Table 1 were melted and subjected to hot-forging. Next, each of the hot-forged products was subjected to hot-rolling to obtain a round bar having a diameter of 20 mm and a square bar having a cross-section of 60 mm square. Further, the obtained round bar and square bar were annealed. The annealing temperature was 750°C, the annealing time was 4 hours, and the cooling method after annealing was air cooling.

Table 1

	Composition (mass%)
	Fe	C	Si	Mn	P	S	Cu	Ni	Cr	Mo	Al
Ex. 1	bal.	0.018	0.28	0.46	0.04	0.33	0.1	0.1	14.5	0.2	0.61
Ex. 2	bal.	0.012	0.45	0.57	0.03	0.41	0.2	0.1	13.7	0.0	0.51
Ex. 3	bal.	0.019	0.30	0.88	0.02	0.61	0.1	0.1	14.5	0.4	0.80
Ex. 4	bal.	0.008	0.26	0.25	0.03	0.22	0.1	0.2	13.4	0.2	0.44
Ex. 5	bal.	0.005	0.31	0.38	0.03	0.68	0.1	0.1	15.2	0.2	0.31
Ex. 6	bal.	0.010	0.40	0.49	0.04	0.66	0.1	0.2	11.0	0.4	0.47
Ex. 7	bal.	0.008	0.13	0.52	0.02	0.49	0.0	0.2	19.2	0.2	0.40
Ex. 8	bal.	0.007	0.27	0.30	0.04	0.38	0.1	0.1	12.0	0.3	0.35
Ex. 9	bal.	0.004	0.47	0.32	0.02	0.41	0.2	0.1	13.3	0.2	0.90
Ex. 10	bal.	0.009	0.35	0.33	0.04	0.35	0.2	0.2	11.3	0.5	0.67
Ex. 11	bal.	0.011	0.32	0.48	0.03	0.45	0.1	0.0	14.2	0.2	0.35
Comp. Ex. 1	bal.	0.042	0.22	0.43	0.03	0.35	0.1	0.1	14.3	0.3	0.62
Comp. Ex. 2	bal.	0.007	0.92	0.39	0.03	0.50	0.2	0.0	13.5	0.1	0.79
Comp. Ex. 3	bal.	0.013	0.43	1.22	0.03	0.54	0.2	0.1	10.7	0.4	0.47
Comp. Ex. 4	bal.	0.014	0.24	0.57	0.02	1.15	0.1	0.3	12.8	0.4	0.52
Comp. Ex. 5	bal.	0.009	0.34	0.21	0.04	0.16	0.2	0.2	13.7	0.2	0.57
Comp. Ex. 6	bal.	0.012	0.42	0.28	0.03	0.48	0.0	0.1	9.6	0.5	0.66
Comp. Ex. 7	bal.	0.015	0.29	0.33	0.04	0.35	0.1	0.1	21.3	0.1	0.86
Comp. Ex. 8	bal.	0.006	0.31	0.25	0.05	0.34	0.1	0.2	14.1	0.0	0.12
Comp. Ex. 9	bal.	0.013	0.38	0.28	0.02	0.61	0.1	0.1	14.9	0.3	1.42
Comp. Ex. 10	bal.	0.017	0.32	0.51	0.04	0.42	0.2	0.1	12.9	0.3	0.45
Comp. Ex. 11	bal.	0.008	0.29	0.31	0.03	0.62	0.2	0.1	14.6	0.0	0.95
Comp. Ex. 12	bal.	0.008	0.37	0.48	0.03	0.45	1.7	0.3	14.4	0.4	0.57
Comp. Ex. 13	bal.	0.012	0.28	0.39	0.04	0.52	0.2	1.8	11.9	0.3	0.48
Comp. Ex. 14	bal.	0.009	0.41	0.33	0.02	0.39	0.2	0.2	12.5	2.2	0.34

Table 1 (Continued)

	Composition (mass%)					Expression 1	Expression 2	Mn/S
	O	N	B	Mg	Ca
Ex. 1	0.006	0.020	0.004			12.14	283.57	1.39
Ex. 2	0.005	0.008		0.007		16.35	277.33	1.40
Ex. 3	0.007	0.008			0.008	13.48	278.21	1.44
Ex. 4	0.006	0.023	0.008	0.003		12.76	255.57	1.14
Ex. 5	0.008	0.014		0.004	0.006	19.38	275.70	0.56
Ex. 6	0.006	0.005	0.005		0.003	15.28	248.52	0.74
Ex. 7	0.006	0.011				21.11	286.77	1.06
Ex. 8	0.008	0.010	0.002			17.51	234.61	0.79
Ex. 9	0.005	0.019		0.007		18.85	286.31	0.77
Ex. 10	0.009	0.007	0.004	0.005	0.002	18.03	247.48	0.96
Ex. 11	0.006	0.026		0.002	0.007	11.97	279.88	1.05
Comp. Ex. 1	0.006	0.007	0.002			9.65	281.15	1.24
Comp. Ex. 2	0.005	0.018	0.006		0.004	19.12	367.83	0.79
Comp. Ex. 3	0.006	0.007			0.007	10.26	250.88	2.26
Comp. Ex. 4	0.009	0.016	0.005	0.005		10.74	246.83	0.50
Comp. Ex. 5	0.007	0.012			0.009	17.13	271.37	1.31
Comp. Ex. 6	0.005	0.018		0.007	0.005	11.57	251.45	0.59
Comp. Ex. 7	0.009	0.029	0.004	0.005	0.003	15.56	369.86	0.94
Comp. Ex. 8	0.007	0.025	0.002			12.41	272.48	0.73
Comp. Ex. 9	0.008	0.022		0.003		16.16	306.64	0.46
Comp. Ex. 10	0.035	0.020		0.006	0.005	10.81	276.01	1.20
Comp. Ex. 11	0.006	0.044			0.008	10.35	295.58	0.50
Comp. Ex. 12	0.009	0.017				13.84	287.96	1.07
Comp. Ex. 13	0.007	0.021			0.006	4.87	251.62	0.75
Comp. Ex. 14	0.008	0.013	0.005		0.007	16.91	319.08	0.85

[2. Test Method]

[2.1. Vickers Hardness]

[0083] The round bar after annealing was cut in the longitudinal direction so as to pass through a central axis of the round bar. The Vickers hardness was measured in a middle region between the surface and the central axis in the longitudinal cross-section of the round bar. The "middle region" refers to a region located near a position corresponding to 1/4 of the diameter D of the round bar from the surface of the round bar. The middle region is a region corresponding to the "middle part" of the cylindrical steel ingot before hot-forging. The measurement was performed at 5 points, and the average value thereof was calculated. Samples having an average value of Vickers hardness of 170 HV or less were evaluated as passed.

[2.2. Corrosion Resistance]

[0084] A cylindrical sample having a diameter of 10 mm and a height of 50 mm was taken from the round bar after annealing. The surface of the sample (side surface of the cylinder) was dry polished by using #400 paper. The obtained sample was kept in a constant temperature and humidity chamber under a humid environment of a temperature of 50°C and a relative humidity of 98% for 96 hours. After 96 hours, the presence or absence of rust on the surface of the sample was visually observed.

[0085] The corrosion resistance was evaluated based on the following criteria, and samples having an evaluation of "A" or "B" were evaluated as passed.

A: no rust is observed on the surface of the sample.

B: one to three spots of rust are observed on the surface of the sample.

C: four or more spots frust are observed on the surface of the sample.

[2.3. Hot-Workability]

[0086] A Greeble test piece was taken from the round bar after annealing and was subjected to a high-speed tensile test at high temperature. In the test piece, a parallel portion has a diameter of 4.5 mm and a length of 20 mm, and a grip portion was a shape of M6 (outer diameter: 6 mm, inner diameter: 4.9 mm, pitch: 1 mm) and a length of 10 mm.

[0087] The temperature of the test piece was raised to 1100°C in 100 seconds and held at 1100°C for 60 seconds. Next, the temperature was raised or lowered from 1100°C to each test temperature at a rate of 10°C/s, and held at the test temperature for 60 seconds. Further, at the test temperature, the test piece was pulled at a speed of 50.8 mm/s to be broken. The test temperature was set at 7 points from 900°C to 1200°C in an increment of 50°C. After breaking, the amount of contraction at the breaking position was measured.

[0088] The hot-workability at 900°C to 1200°C was evaluated based on the following criteria, and samples having an evaluation of "A" were evaluated as passed.

A: the amount of contraction is 50% or more at all 7 test temperatures.

C: the amount of contraction is less than 50% at any one or more of the 7 test temperatures.

[2.4. Machinability]

[0089] Regarding the machinability, the drill tool life and the chip crushability were evaluated.

[0090] A cutting test was performed on the square bar after annealing by repeatedly drilling holes in the longitudinal direction by using a high-speed drill having a diameter of 1 mm. The point at which the drill broke was defined as the "drill tool life" and the drilling distance until the drill reached the end of the drill tool life was measured. The drill-cutting conditions were a feed of 0.05 mm/rev, a cutting speed of 100 mm/min, and no lubrication.

[0091] The drill tool life was evaluated based on the following criteria, and samples having an evaluation of "A" or "B" were evaluated as passed.

A: the drilling distance at the end of the tool life is more than 1000 mm.

B: the drilling distance at the end of the tool life is 500 mm or more and 1000 mm or less.

C: the drilling distance at the end of the tool life is less than 500 mm.

[0092] In addition, the chip crushability was evaluated based on the following criteria, and samples having an evaluation of "A" or "B" were evaluated as passed.

A: 80% or more of chips during drilling are separated within 1 or 2 curls.

B: 80% or more of chips during drilling are separated within 3 to 5 curls.

C: 80% or more of chips during drilling are separated within 6 or more curls.

[2.5. Number Proportions of Compounds (MnS sulfides and inclusions)]

[0093] The metallographic structure of the annealed round bars was observed by using an automated particle analysis function of scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX). In the automated particle analysis, a photograph was first taken at any magnification, and compounds were identified from the photograph by image analysis. Size measurement and EDX were performed to determine the size and composition of the compound. By repeating this process for multiple photographs, it is possible to quantitatively represent the number, size, and composition of compounds in a large area. In the present invention, the magnification was set to 10,000 times and the automated particle analysis was performed on 100 images.

[0094] In the results of the EDX analysis, compounds containing 5 at% or more of Mn and 5 at% or more of S as constituent elements were classified as "MnS-based sulfides", whereas the other compounds detected (such as carbides, oxides, nitrides, carbonitrides, and sulfides other than the MnS-based sulfides) were classified as "Inclusions".

[3. Results]

[0095] The results are shown in Table 2. In Table 2, "Vickers hardness" represents the average value of Vickers hardness. "MnS-based sulfides" and "Inclusions" represent the number proportion calculated from the number of MnS-based sulfides and inclusions detected in 100 images, respectively (the number of MnS-based sulfides and inclusions in the observation field of view of 10000 µm², respectively).

[0096] The followings can be understood from Table 2.

(1) In Comparative Example 1, the drill tool life decreased. The reason is considered to be that the amount of C was excessive and a large amount of hard carbide was generated.

(2) In Comparative Example 2, the drill tool life decreased and the chip crushability also decreased. The reason is considered to be that the amount of Si was excessive and the matrix strength was excessively high.

(3) In Comparative Example 3, the corrosion resistance decreased. The reason is considered to be that the amount of Mn was excessive and the sulfide form index was more than 1.5.

(4) In Comparative Example 4, the hot-workability decreased. The reason is considered to be that the amount of S was excessive and S segregated at grain boundaries. Note that, in Comparative Example 4, the machinability test was not conducted because the hot-workability decreased.

(5) In Comparative Example 5, the drill tool life decreased. The reason is considered to be that the amount of S was small and the amount of the MnS-based sulfide was insufficient.

(6) In Comparative Example 6, the corrosion resistance decreased. The reason is considered to be that the amount of Cr was small.

(7) In Comparative Example 7, the drill tool life decreased and the chip crushability also decreased. The reason is considered to be that the amount of Cr was excessive and the matrix strength was excessively high.

(8) In Comparative Example 8, the hot-workability decreased. The reason is considered to be that the amount of Al was small. Note that, in Comparative Example 8, the machinability test was not conducted because the hot-workability decreased.

(9) In Comparative Example 9, the drill tool life decreased. The reason is considered to be that the amount of Al was excessive and the matrix strength was excessively high.

(10) In Comparative Example 10, the drill tool life decreased. The reason is considered to be that the amount of O was excessive.

(11) In Comparative Example 11, the drill tool life decreased. The reason is considered to be that the amount of N was excessive.

(12) In Comparative Example 12, the hot-workability decreased. The reason is considered to be that the amount of Cu was excessive and the ferrite phase became unstable. Note that, in Comparative Example 12, the machinability test was not conducted because the hot-workability decreased.

(13) In Comparative Example 13, the hot-workability decreased. The reason is considered to be that the amount of Ni was excessive and the ferrite phase became unstable. Note that, in Comparative Example 13, the machinability test was not conducted because the hot-workability decreased.

(14) In Comparative Example 14, the drill tool life decreased. The reason is considered to be that the amount of Mo was excessive and the matrix strength was excessively high.

(15) Examples 1 to 11 were all excellent in corrosion resistance, hot-workability, drill tool life, and chip crushability.

(16) In Example 6, the corrosion resistance slightly decreased. The reason is considered to be that the amount of Cr was slightly small.

(17) In Examples 8 and 10, the corrosion resistance slightly decreased. The reason is considered to be that the amount of Cr was slightly small.

(18) In Examples 1, 7 and 9, the drill tool life slightly decreased. The reason is considered to be that the matrix strength was slightly high (the matrix strength index is more than 280).

(19) In Comparative Examples 10,11, the number proportion of inclusions exceeded 30. The reason is considered to be that the amount of C or N was excessive.

Table 2

	MnS-based sulfides	Inclusions	Vickers hardness	Corrosion resistance	Hot workability	Drill tool life	Chip crushability
Ex. 1	566	9	161	A	A	B	A
Ex. 2	704	4	158	A	A	A	A
Ex. 3	1083	5	158	A	A	A	A
Ex. 4	308	7	146	A	A	A	A
Ex. 5	468	6	157	A	A	A	A
Ex. 6	609	3	142	B	A	A	A
Ex. 7	640	4	163	A	A	B	A
Ex. 8	369	9	135	B	A	A	A
Ex. 9	388	3	162	A	A	B	A
Ex. 10	406	11	142	B	A	A	A
Ex. 11	588	16	159	A	A	A	A
Comp. Ex. 1	529	5	160	A	A	C	A
Comp. Ex. 2	480	12	206	A	A	C	C
Comp. Ex. 3	1501	3	144	C	A	A	A
Comp. Ex. 4	701	8	142	B	C	-	-
Comp. Ex. 5	258	6	155	A	A	C	A
Comp. Ex. 6	343	7	144	C	A	A	A
Comp. Ex. 7	406	16	207	A	A	C	C
Comp. Ex. 8	304	15	155	A	C	-	-
Comp. Ex. 9	345	14	173	A	A	C	B
Comp. Ex. 10	628	33	157	B	A	C	A
Comp. Ex. 11	382	41	167	A	A	C	A
Comp. Ex. 12	591	11	163	A	C	-	-
Comp. Ex. 13	480	13	144	B	C	-	-
Comp. Ex. 14	404	5	180	B	A	C	B

[0097] Although the embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the gist of the present invention.

[0098] The present application is based on the Japanese patent application No. 2023-128870 filed on August 7, 2023 and Japanese patent application No. 2024-106556 filed on July 2, 2024, which contents are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

[0099] The ferritic free-cutting stainless steel according to the present invention can be used in various parts to be used in precision instruments, automobiles, water-heating equipment, chemical plants, or the like.

Claims

1. A ferritic free-cutting stainless steel consisting of:























and

optionally at least one selected from the group consisting of:



and

with a balance being Fe and unavoidable impurities, wherein

the following expression (1) to expression (3) are satisfied:

where [X] represents a content (mass%) of an element X.

2. The ferritic free-cutting stainless steel according to Claim 1, further satisfying at least one selected from the group consisting of:



and

3. The ferritic free-cutting stainless steel according to Claim 1 or 2, having:

a number proportion of a MnS-based sulfide being 100 or more and 10,000 or less, and

a number proportion of an inclusion being 20 or less,
where

the "MnS-based sulfide" refers to a compound containing Mn and S,

the "inclusion" refers to a compound other than the MnS-based sulfide,

the "number proportion" of the MnS-based sulfide refer to the number of the MnS-based sulfides contained within an observation view field of 10,000 µm² (= 100 µm × 100 µm),
and

the "number proportion" of the inclusion refer to the number of the inclusions contained within an observation view field of 10,000 µm² (= 100 µm × 100 µm).

4. A method for producing the ferritic free-cutting stainless steel described in any one of Claims 1 to 3, the method comprising:

a first step of producing a steel ingot or a billet having a predetermined component composition;

a second step of subjecting the steel ingot or the billet to a homogenization heat treatment at 1250°C to 1300°C for 1 hour or longer; and

a third step of subjecting the steel ingot or the billet after the homogenization heat treatment to hot-working at 700°C to 1000°C.

5. The method for producing the ferritic free-cutting stainless steel according to Claim 4, further comprising a fourth step of annealing the hot-worked body at 730°C to 870°C.

6. Use of the ferritic free-cutting stainless steel according to one of claim 1 to 3, or as manufactured according to claim 4 or 5, for making precision instrument parts.

7. Use of the ferritic free-cutting stainless steel according to one of claim 1 to 3, or as manufactured according to claim 4 or 5, for making automobile parts.

8. Use of the ferritic free-cutting stainless steel according to one of claim 1 to 3, or as manufactured according to claim 4 or 5, for making parts of water heating equipment.

9. Use of the ferritic free-cutting stainless steel according to one of claim 1 to 3, or as manufactured according to claim 4 or 5, for making chemical plant parts.