JP6631234B2 - Ferritic free-cutting stainless steel and its manufacturing method. - Google Patents

Description

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

  Ferritic free-cutting stainless steels contain so-called “free-cutting elements” such as S to improve machinability, but on the other hand, balance the component design to maintain or improve other properties. Is made.

  For example, in Patent Document 1, as a ferritic free-cutting stainless steel excellent in corrosion resistance, C: 0.1% or less, Si: 2.0% or less, Mn: 2.0% or less, Cr: 19 to A steel containing 25% and S: 0.20-0.35% is disclosed. By setting the lower limit of the Cr content higher, the corrosion resistance is improved. On the other hand, excessive addition of Cr degrades hot workability, so the upper limit is defined.

  While Cr can improve the corrosion resistance as a ferritic free-cutting stainless steel, the machinability may be reduced.

  Patent Literature 2 describes that by adjusting the amount of Cr in a sulfide, the corrosion resistance as a free-cutting stainless steel can be improved without significantly reducing the machinability. As such steel, in mass%, C: 0.40% or less, Si: 2.0% or less, Mn: 0.20 to. 090%, S: 0.05 to 0.40%, Cr: 10 to 30%, and assuming that the mass% of the element M is [M], exp ｛(12 + 0.18 [Cr] -36 [S] ) / 22 {[S] ≦ [Mn] ≦ exp} (32 + 0.18 [Cr] −36 [S]) / 22} [S]. That is, Mn has an important influence on the sulfide composition, but if the content of Mn in the above formula is smaller than the leftmost side, the Cr concentration in the sulfide increases and the machinability decreases. On the other hand, when the content of Mn is larger than the rightmost side, the Mn concentration in the sulfide becomes high and the corrosion resistance is lowered, and the matrix is solid-solution strengthened and the hot workability is lowered. .

  Further, in Patent Document 3, as a ferritic free-cutting stainless steel having the same machinability as a general free-cutting steel without containing Pb and having excellent corrosion resistance, C: 0.200 by volume% %, 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% In the following, Al: 0.300% or less, O: 0.0050 to 0.1000%, and B: 0.0020 to 0.1000%, and the ratio of the O concentration to the B concentration is 0.60 to 2.0. No. 50 is disclosed. In general, ferritic free-cutting stainless steel increases the content of S instead of not containing Pb from the viewpoint of maintaining machinability, but on the other hand, reduces the corrosion resistance. In addition, when the content of other elements that improve machinability such as Se is increased, hot workability is reduced. Therefore, this document proposes adjusting the amounts of B and oxygen to disperse the oxides of B in the steel. That is, since the oxide of B has a low melting point like Pb, it can be melted by heat at the time of cutting to obtain a melt embrittlement effect, and the machinability equivalent to that of the Pb-containing free-cutting steel can be obtained. It is said that it can be obtained.

JP-A-11-140597 JP 2006-097039 A JP 2008-274361 A

  Generally, ferritic stainless steel containing a large amount of Cr is often used for precision parts because of its excellent corrosion resistance. Here, in the drilling of a small diameter, for example, 2 mm or less in diameter for manufacturing precision parts, the tool life is remarkably reduced as compared with the drilling of a large diameter when trying to obtain a hole depth more than twice the drill diameter. And deteriorates the roughness of the machined surface. Therefore, in the processing of precision parts that require high surface roughness, especially in the processing of precision parts such as the tip of a ballpoint pen, small diameter drilling is performed using free-cutting stainless steel with higher machinability. It is required that the properties are improved and the tool life is not reduced. However, when an attempt is made to include more elements such as S, Se, Pb, Bi, and Te that improve the workability of such small-diameter drills, properties other than machinability, in particular, a decrease in hot workability cause a decrease in heat. This causes a manufacturing problem that makes forging difficult.

  The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a ferritic free-cutting stainless steel excellent in machinability and hot workability for a small diameter drill and a method for producing the same. It is in.

  The method for producing a ferritic free-cutting stainless steel according to the present invention is as follows: 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, S: 0.10 to 0.45%, and further, Pb: 0.03 to 0.40%, Bi: When at least two elements selected from 0.03 to 0.40% and Te: 0.01 to 0.10% are included, and the mass% of the element M is [M], 900 ([C] + [N]) + 170 [Si] +12 [Cr] +30 [Mo] +10 [Al] ≦ 300, the remainder being Fe and unavoidable impurities The minute composition the steel was hot forged ferrite single phase region, and wherein the obtaining a ferrite sectional area of 95% or more of the steel.

  According to this invention, a ferritic free-cutting stainless steel excellent in machinability for a small diameter drill is provided with high productivity.

  In the above invention, ([Cr] + [Mo] +1.5 [Si] +4 [Al]) / ([Ni] +0.5 [Mn] +30 [C] +30 [N]) ≧ 7 is further satisfied. May be characterized. According to the invention, since the ferrite stability can be enhanced and the ferrite single-phase temperature range can be expanded, higher manufacturability can be obtained.

  In the above invention, the steel is one or more selected from B: 0.0001 to 0.0080%, Mg: 0.0005 to 0.0100%, and Ca: 0.0005 to 0.0100%. It may be characterized by further containing two or more kinds. According to the invention, hot workability is improved, and higher manufacturability can be obtained.

  The ferritic free-cutting stainless steel according to the present invention is a ferritic free-cutting stainless steel, which is expressed by mass%: C: 0.015% or less, Si: 0.02 to 0.60%, Mn: 0.2 to 0.2%. 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, S: 0.10 to 0.45%, and further, Pb: 0.03 to 0.40%, Bi: 0.03 to 0.40%, and Te: Two or more selected from 0.01 to 0.10%, and the mass% of the element M is [M]. Then, 900 ([C] + [N]) + 170 [Si] +12 [Cr] +30 [Mo] +10 [Al] ≦ 300 is satisfied, and the remainder is F And together consist component composition and inevitable impurities, characterized in that the ferrite sectional area ratio was 95% or more.

  According to the invention, as a ferritic free-cutting stainless steel, the machinability of a small-diameter drill can be improved.

  In the above invention, ([Cr] + [Mo] +1.5 [Si] +4 [Al]) / ([Ni] +0.5 [Mn] +30 [C] +30 [N]) ≧ 7 is further satisfied. May be characterized. According to this invention, since the ferrite stability can be enhanced and the ferrite single-phase temperature region can be expanded to produce a ferritic free-cutting stainless steel with higher productivity.

  In the above invention, the steel is one or more selected from B: 0.0001 to 0.0080%, Mg: 0.0005 to 0.0100%, and Ca: 0.0005 to 0.0100%. It may be characterized by further containing two or more kinds. According to this invention, it is possible to improve the hot workability and provide a ferritic free-cutting stainless steel with higher productivity.

It is a figure of the component composition of an example and a comparative example. It is a figure of the component composition of an example and a comparative example. It is a figure which shows the result of the evaluation test of an Example and a comparative example. FIG. 3 is a diagram illustrating a relationship between a value of Expression 1 and Vickers hardness. It is a microstructure photograph of an example and a comparative example.

  The present inventors first studied the component composition of free-cutting stainless steel having higher machinability so as to improve the small-diameter drill workability and not to shorten the life of the drill tool.

  Here, regarding the tool life of the drill, it is possible to consider reducing the matrix strength of the steel, reducing the thrust resistance of the drill, and stabilizing the same. This can be achieved by reducing the amount of solid solution strengthening elements such as Si, Cr, and Mo. On the other hand, since these are also elements for stabilizing the ferrite phase, the temperature at which the ferrite single phase can be maintained is lowered. Would. That is, in the hot forging temperature range, a two-phase state of ferrite-austenite is easily formed, and the hot workability is reduced. When the hot forging temperature range is reduced, the deformation resistance increases, and the hot workability also decreases. .

  Next, regarding the small-diameter drill workability, it can be considered that the steel matrix is embrittled to improve chip crushability. For this purpose, elements such as Si, P and V may be added, but on the other hand, the life of the drill tool is shortened by solid solution strengthening.

  Therefore, the present inventor reduced the amount of the solid solution strengthening element so as not to reduce the life of the drill tool, and lowered the matrix strength, while increasing the content of Al, which is a strong ferrite stabilizing element, to increase the ferrite content. I came up with the idea of increasing the phase stability. In addition, the inclusion of Al can shift the brittle-ductile transition temperature to a higher temperature side, thereby effectively embrittle the matrix, improve chip crushability, and improve small-diameter drill workability. . Further, Al has a small amount of solid solution strengthening as compared with Si, V, etc., and can suppress an increase in the strength of the matrix and does not shorten the life of the drill tool.

  The matrix strength and the phase stability of the ferrite phase in a ferritic free-cutting stainless steel having a plurality of component compositions in which the content of Al is increased by decreasing the amount of solid solution strengthening elements such as Si, Cr, and Mo are reduced. The values of Equations 1 and 2 to be predicted were calculated, and hot workability and machinability were evaluated. Further, based on the results, the ranges of the contents of Al and other elements capable of improving the machinability while maintaining the hot workability, and the value of Formula 1 (MS value) and the value of Formula 2 (FS value) Value). The results of this evaluation test are shown below.

In addition, about Formula 1 and Formula 2, let the mass% of the element M be [M], and it is as follows.

Formula 1: MS = 900 ([C] + [N]) + 170 [Si] +12 [Cr] +30 [Mo] +10 [Al]

Formula 2: FS = ([Cr] + [Mo] +1.5 [Si] +4 [Al]) / ([Ni] +0.5 [Mn] +30 [C] +30 [N])

Here, Equation 1 is an equation for estimating the matrix strength and is based on the solid solution strengthening element. Equation 2 predicts the phase stability of the ferrite phase in the hot forging temperature range. The numerator is based on the ferrite stabilizing element, and the denominator is based on the austenite stabilizing element.

  In the evaluation test, 150 kg of a steel ingot having the component composition shown in Examples 1 to 25 and Comparative Examples 1 to 16 of FIGS. 1 and 2 was respectively melted and hot forged, and a part was used as a hot forged material. It was subjected to the test described below, and the remainder was hot-rolled to obtain a round bar having a diameter of 20 mm and a square bar having a square of 60 mm. Further, as an annealing treatment, the sample was kept at a temperature of 740 to 800 ° C. for 4 hours and air-cooled. Using the obtained round bar and annealed material consisting of square bars, the following test pieces were appropriately cut out and tested, and the results were evaluated.

[Vickers hardness]
The Vickers hardness was measured from the annealed material at a position corresponding to the “middle portion” at the time of the ingot after smelting. The measurement was performed at five points, and the average value is shown in FIG.

[Hot workability]
A grease test specimen was collected from the hot forged material and subjected to a high temperature high speed tensile test. The parallel part of the test piece was φ4.5 mm × 20 mmL, and the grip part was M6 × 10 mmL. The test piece was heated to 1100 ° C. in 100 seconds, held for 60 seconds, then changed to the test temperature at 10 ° C./s, held for 60 seconds, pulled at a speed of 50.8 mm / s, and broken. The test temperature is 7 points from 900 ° C to 1200 ° C in 50 ° C increments. After breaking, the amount of drawing at the breaking position was measured, and the hot workability at 900 to 1200 ° C was evaluated as good when the amount of drawing was 50% or more at all seven test temperatures. Was evaluated as defective when any of the seven test temperatures resulted in a reduction of less than 50%, and "C" is shown in FIG.

[Amount of ferrite]
From the as-forged hot forged material, a plate-like sample of 15 mm square × 2 mmT was sampled, the surface was mirror-polished and etched, and the microstructure was observed at 25 ° C. In such microstructure observation, when the cross-sectional area ratio of the martensite structure in the ferrite structure is 5% or less, it is evaluated as good, and “A” is evaluated as poor when it exceeds 5%, and “C” is evaluated as poor. Each is shown in FIG. That is, the martensitic structure is formed due to the two-phase state of ferrite-austenite during hot forging, which is the highest temperature in the manufacturing process after the ingot smelting. They evaluate it as having an adverse effect on sex.

[Small diameter drill machinability]
In order to evaluate the machinability of a small diameter drill, the life of a drill tool and chip crushability were evaluated. Specifically, a drilling was performed on the annealed material using a φ1 mm high speed steel drill at a feed rate of 0.03 mm / rev and a cutting speed of 70 m / min without using a lubricant to evaluate the drill tool life. A case where drilling is possible beyond 4000 mm without breakage of the drill is evaluated as good and “A” is evaluated. A case where drilling of 2000 to 4000 mm is possible is evaluated as “B” and a case where less than 2000 mm is poor. And "C" is shown in FIG. In addition, the chip crushability is evaluated as good if 80% or more of the chips are separated within 1 or 2 curls by observing the chips, and "A" is divided by 3 to 5 curls. FIG. 3 shows "B", which was evaluated as acceptable, and "C", which was evaluated as defective if continuous for 6 or more curls.

  Next, the results of Vickers hardness, hot workability, ferrite amount, drill tool life, and chip crushability of Examples 1 to 25 and Comparative Examples 1 to 16 will be described with reference to FIG. I do.

  In Examples 1 to 25, the Vickers hardness was in the range of 131 to 169 HV, and it is considered that the matrix strength was reduced in each of the examples, which contributed to the improvement of machinability. In addition, it was evaluated as good (A) or acceptable (B) in both the drill tool life and the chip crushability. The evaluation of the amount of ferrite was all good (A), and it is considered that the ferrite single phase could be maintained in hot forging. In addition, as shown in the evaluation of hot workability, which was good (A), high hot workability was maintained at this test temperature. That is, according to Examples 1 to 25, the machinability was able to be improved while maintaining the hot workability. The value (MS value) of Equation 1 for estimating the matrix intensity was 188 to 287. Furthermore, the value of Formula 2 (FS value) for predicting the phase stability of the ferrite phase in the hot forging temperature range was 8.2 to 27.5.

  In addition, in Examples 3, 4, 6, 11, 16, 20, 22, and 25, which were evaluated as acceptable (B), compared to the examples as evaluated as good (A) in terms of drill tool life, The value was large and was 235 or more. In addition, in Examples 2 and 12, which were evaluated as good (B), the content of Al was as low as 0.31% by mass as compared with the examples obtained as good (A) in chip crushing property. .

  On the other hand, Comparative Example 1 has a component composition corresponding to SUS430F, which is a typical ferritic free-cutting stainless steel, but has a C content of 0.044% by mass as compared with the example. The MS value is as large as 316 as compared with the example. That is, it was predicted that the matrix intensity was high. Furthermore, none of Pb, Bi and Te was added, and the machinability was poor as can be seen from the fact that all of the evaluations of the drill tool life and chip crushing property were poor (C). Further, the FS value was 5.4, which was smaller than that of the above example, and it was predicted that the phase stability of the ferrite phase in the hot forging temperature range was inferior, and the evaluation of hot workability was good (A). However, the evaluation of the amount of ferrite was poor (C).

  In Comparative Example 2, the content of C was as large as 0.039% by mass as compared with the Example, and the FS value was as small as 5.9 as compared with the Example. It was predicted that the phase stability of the ferrite phase was inferior, and the evaluation of the amount of ferrite was poor (C). In addition, the hot workability was evaluated as poor (C), and the work was not actually possible. Therefore, evaluation of the machinability of the small diameter drill is not performed.

  In Comparative Example 3, the Si content was increased to 1.22% by mass as compared with the Example, and the MS value was 404, which was larger than that of the Example, and it was predicted that the matrix strength was increased. Was. It is considered that the thrust resistance at the time of cutting with a small diameter drill was increased, and the drill tool life was poor (C).

  In Comparative Example 4, the S content was as small as 0.02% by mass as compared with the Example, and the machinability was inferior as in the evaluation of poor (C) in drill tool life and chip crushability. .

  In Comparative Example 5, the content of S was increased to 0.58% by mass as compared with the Example, and the hot workability was evaluated as poor (C), and the work was not actually possible. Therefore, evaluation of the machinability of the small diameter drill is not performed.

  Comparative Example 6 had a higher Ni content of 2.2% by mass than the Example, a smaller FS value of 4.6 than the Example, and a ferrite phase in the hot forging temperature range. Was predicted to be inferior in phase stability, and the evaluation of the amount of ferrite was poor (C). In addition, the hot workability was evaluated as poor (C), and the work was not actually possible. Therefore, evaluation of the machinability of the small diameter drill is not performed.

  Comparative Example 7 is predicted to have a higher Mo content of 2.2% by mass than the example, an MS value of 313 as compared to the example, and an increased matrix strength. Was. Although the chip crushing property was evaluated as good (B), the evaluation of the drill tool life was poor (C), and it is considered that the thrust resistance was increased.

  In Comparative Example 8, the Al content was as low as 0.03% by mass as compared with the Example, and the FS value was 6.2, which was smaller than that of the Example. The phase stability was predicted to be inferior, and the evaluation of the amount of ferrite was poor (C). In addition, the hot workability was evaluated as poor (C), and the work was not actually possible. Therefore, evaluation of the machinability of the small diameter drill is not performed.

  Comparative Example 9 had a lower O content of 0.0025% by mass than the example, and it is considered that the S-based inclusions had become acicular. )Met.

  In Comparative Example 10, the content of Pb was as high as 0.45% by mass as compared with the Example, and as a result, the hot workability was poor (C) as in the evaluation of hot workability. Was. Therefore, evaluation of the machinability of the small diameter drill is not performed.

  In Comparative Example 11, the Bi content was as high as 0.41% by mass as compared with the Example, and as a result, the hot workability was poor (C) as in the evaluation of hot workability. Was. Therefore, evaluation of the machinability of the small diameter drill is not performed.

  Comparative Example 12 has a smaller Al content of 0.03% by mass than that of the Example, and contains 0.35% by mass of Nb instead. Although the Nb content ensures the phase stability of the ferrite phase in the hot forging temperature range, the matrix strength is increased by the refinement of crystal grains and the solid solution strengthening by fine dispersion of carbonitride. Conceivable. The evaluation of the drill tool life was poor (C), and it is considered that the thrust resistance was increased. In addition, the evaluation of the chip crushing property is also poor (C), and it is considered that Nb is inferior to Al with respect to the effect of matrix embrittlement.

  In Comparative Example 13, the content of Al was as small as 0.01% by mass as compared with the example, and 0.31% by mass of Ti was contained instead. Although the phase stability of the ferrite phase in the hot forging temperature range is ensured by the inclusion of Ti, the matrix strength has increased due to the refinement of crystal grains due to the fine dispersion of carbonitride and the solid solution strengthening. Conceivable. The evaluation of the drill tool life was poor (C), and it is considered that the thrust resistance was increased. In addition, the evaluation of chip crushing property is also poor (C), and it is considered that Ti is inferior to Al with respect to the effect of matrix embrittlement.

  In Comparative Example 14, the content of Al was reduced to 0.02% by mass as compared with the Example, and instead, V was contained at 0.32% by mass. Although the phase stability of the ferrite phase in the hot forging temperature range is ensured by the inclusion of V, the matrix strength is increased by the refinement of crystal grains due to the fine dispersion of carbonitrides and the solid solution strengthening. Conceivable. The evaluation of the drill tool life was poor (C), and it is considered that the thrust resistance was increased. In addition, the evaluation of chip friability was also poor (C), and it is considered that V is inferior to Al with respect to the effect of matrix embrittlement.

  In Comparative Example 15, although the component composition was almost the same as that of the above-described example, the MS value was 329, which was larger than that of the example, and it was predicted that the matrix strength was high. Although the chip crushing property was evaluated as good (B), the evaluation of the drill tool life was poor (C), and it is considered that the thrust resistance was increased.

  In Comparative Example 16, although the component composition was almost equal to that of the above-described example, the FS value was 6.1, which was smaller than that of the example, and it was predicted that the phase stability of the ferrite phase was poor in the hot forging temperature range. The ferrite content was poor (C). In addition, the hot workability was evaluated as poor (C), and the work was not actually possible. Therefore, evaluation of the machinability of the small diameter drill is not performed.

  Based on the above results and some other similar test results, Equation 1 for predicting the matrix strength for obtaining the required small diameter drill machinability in steel having the same composition as in the above-described example. (MS value) was determined to be 300 or less. The MS value is preferably 230 or less from the evaluation of the drill tool life of the above-described embodiment.

  Here, FIG. 4 shows the relationship between the MS value (the value of Equation 1) and Vickers hardness. Since Vickers hardness is a main factor affecting the matrix strength, it has a certain correlation with the MS value as in Examples 1 to 25 and Comparative Examples 1 to 11, 15, and 16. Thus, in the alloy system of the present invention, the matrix value can be predicted from the MS value. Comparative Examples 12, 13, and 14 contain Nb, Ti, and V, respectively, and are alloy systems different from the present invention. For this reason, the matrix intensity cannot be predicted in the above equation 1, and the correlation is different from the correlation between the embodiment and other comparative examples. That is, the free-cutting ferritic stainless steel according to the present invention does not contain Nb, Ti and V, except when it is contained at an unavoidable impurity level. Here, the unavoidable impurity level of each element is Nb ≦ 0.05%, Ti ≦ 0.05%, and V ≦ 0.05% in mass%. Also, as in Comparative Example 15, even if the component composition is almost the same as that of the Example, it is predicted that if the MS value is larger than 300, the matrix strength will be increased. As a result, Comparative Example 15 However, the Vickers hardness was high and the required small-diameter drill machinability could not be obtained.

  In addition, in order to maintain hot workability, it is necessary to perform hot forging in a ferrite single phase region as described above. Therefore, the value (FS value) of Equation 2 for obtaining a preferable phase stability of the ferrite phase is determined to be 7 or more. That is, by setting the FS value to 7 or more, the phase stability of the ferrite phase is increased, the upper limit temperature of the ferrite single phase temperature region is increased, and forging in the ferrite single phase temperature region is facilitated.

  FIG. 5 shows the microstructures of Example 13 and Comparative Example 16 at 25 ° C. Example 13 shows a structure of a typical example, and shows a structure in which the ferrite cross-sectional area ratio at 25 ° C. is 95% or more. That is, it is estimated that hot forging was performed in the ferrite single phase region. The ferritic free-cutting stainless steel according to the present invention may have a structure in which the ferrite cross-sectional area ratio at 25 ° C. is 95% or more, and may include other phases as long as the cross-sectional area ratio is less than 5%. Good. On the other hand, in Comparative Example 16, the FS value (the value of Equation 2) was less than 7 even if the component composition was almost the same as that of the Example, indicating that the phase stability of the ferrite phase in the hot forging temperature range was low. It was predicted to be inferior. Then, a large amount of martensite was observed in the microstructure, and it is considered that the two-phase state of ferrite-austenite was obtained at the time of hot forging although hot forging was performed under the same conditions as in the example.

  By the way, the composition range of the alloy which can provide the hot workability and the machinability almost equivalent to the above evaluation test is determined as follows.

  C is a typical solid solution strengthening element, and can increase the matrix strength and reduce the machinability. Therefore, C is 0.015% or less, preferably 0.012% or less by mass%.

  Si is an element necessary as a deoxidizing agent. On the other hand, Si is also a typical solid solution strengthening element, and if added in excess, may increase the matrix strength and reduce the machinability. Therefore, Si is in a range of 0.02 to 0.60% by mass, preferably in a range of 0.02 to 0.40%.

  Mn forms a compound with S and is an element necessary for improving machinability. Further, it suppresses grain boundary segregation of S and improves hot workability. On the other hand, Mn is an austenite stabilizing element, and when added in excess, makes the ferrite phase unstable in the hot forging temperature range. Thus, Mn is in the range of 0.2 to 2.0% by mass.

  P is a solid solution strengthening element and may increase the matrix strength and reduce the machinability. Therefore, P is 0.050% or less, preferably 0.040% or less by mass%.

  Cu is an austenite stabilizing element and makes the ferrite phase unstable in the hot forging temperature range. Therefore, Cu is 1.5% or less by mass%.

  Ni is an austenite stabilizing element and makes the ferrite phase unstable in the hot forging temperature range. Therefore, Ni is not more than 1.5% by mass%.

  Cr is an element necessary for improving corrosion resistance. On the other hand, if Cr is added excessively, the matrix strength may be increased and the machinability may be reduced. Therefore, Cr is in a range of 10.0 to 25.0% by mass, preferably in a range of 10.0 to 17.0%.

  Mo is an element that contributes to the improvement of corrosion resistance and can be added as needed. However, Mo is also a typical solid solution strengthening element, and may increase the matrix strength and decrease the machinability. Therefore, Mo is 2.0% or less by mass%.

  Al is the most important element in the present invention. Al is an element that shifts the brittle-ductile transition temperature to a higher temperature side, promotes the embrittlement of the matrix, and is necessary for improving the chip crushability. Further, it is a strong ferrite phase stabilizing element in the forging temperature range, and is necessary for maintaining hot workability. On the other hand, if Al is added in excess, it may cause cooling cracks of the steel ingot, which may adversely affect the productivity. Therefore, Al is in the range of 0.30 to 2.50% by mass%, preferably in the range of 0.35 to 2.50%.

  O is an element necessary for reducing the acicular ratio of S-based inclusions. On the other hand, if O is added excessively, it promotes the formation of oxides and lowers machinability. Therefore, O is in the range of 0.0030 to 0.0400% by mass%.

  N is a typical solid-solution strengthening element, which increases the matrix strength, and also generates hard nitride to lower the machinability. Therefore, N is not more than 0.035% by mass%, and preferably not more than 0.025%.

  S is an element that generates sulfide and is necessary for improving machinability. On the other hand, if S is added excessively, the hot workability will be significantly deteriorated. Therefore, S is in the range of 0.10 to 0.45% by mass, preferably in the range of 0.10 to 0.40%.

  Pb is an element that contributes to improvement in machinability due to a melt embrittlement effect due to heat during cutting. On the other hand, if Pb is added excessively, the hot workability will be significantly deteriorated. Therefore, Pb is in the range of 0.03 to 0.40% by mass%, and preferably in the range of 0.03 to 0.30%.

  Bi is an element that contributes to the improvement of machinability due to the melt embrittlement due to heat during cutting. On the other hand, when Bi is excessively added, the hot workability is remarkably deteriorated. Therefore, Bi is in the range of 0.03 to 0.40% by mass%, and preferably in the range of 0.03 to 0.30%.

  Te is an element that contributes to the improvement of machinability by the action of melting embrittlement due to heat during cutting and the action of lowering the needle ratio of sulfide. On the other hand, if Te is added excessively, the hot workability will be significantly deteriorated. Therefore, Te is in the range of 0.01 to 0.10% by mass, preferably in the range of 0.01 to 0.08%.

  The above-mentioned Pb, Bi, and Te may be added in two or more of the three types. Hereinafter, elements which may be selectively added will be described.

  B is an element effective for ensuring hot workability. On the other hand, if B is excessively added, the hot workability is rather deteriorated. Therefore, B may be contained in a range of 0.0001 to 0.0080% by mass, preferably in a range of 0.0003 to 0.0060%.

  Mg is an element effective for ensuring hot workability. On the other hand, if Mg is added excessively, the effect of improving hot workability is saturated. Therefore, Mg can be contained in a range of 0.0005 to 0.0100% by mass, preferably in a range of 0.0010 to 0.0100%.

  Ca is an element effective for ensuring hot workability. On the other hand, when Ca is excessively added, the effect of improving hot workability is saturated. Therefore, Ca can be contained in a range of 0.0005 to 0.0100% by mass, preferably in a range of 0.0010 to 0.0100%.

  The representative embodiments according to the present invention have been described so far, but the present invention is not necessarily limited to these. Those skilled in the art will be able to find various alternative embodiments and modifications without departing from the scope of the appended claims.

Claims (6)

A method for producing a ferritic free-cutting stainless steel,
In mass%,
C: 0.015% or less,
Si: 0.02 to 0.60%,
Mn: 0.2-2.0%,
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,
S: 0.10 to 0.45%
Furthermore,
Pb: 0.03 to 0.40%,
Bi: 0.03 to 0.40%, and
Te: contains two or more kinds selected from 0.01 to 0.10%, and
When the mass% of the element M is [M],
900 ([C] + [N]) + 170 [Si] + 12 [Cr] + 30 [Mo] + 10 [Al] ≤ 300, and the balance is Fe and unavoidable impurities. A method for producing a free-cutting ferritic stainless steel, comprising hot forging to obtain a steel having a ferrite cross-sectional area ratio of 95% or more.   ([Cr] + [Mo] +1.5 [Si] +4 [Al]) / ([Ni] +0.5 [Mn] +30 [C] +30 [N]) ≧ 7. Item 4. The method for producing a ferritic free-cutting stainless steel according to Item 1. The steel is
B: 0.0001 to 0.0080%,
Mg: 0.0005 to 0.0100%, and
Ca: 0.0005 to 0.0100%
The method for producing a ferritic free-cutting stainless steel according to claim 2, further comprising one or more selected from the group consisting of: Ferritic free-cutting stainless steel,
In mass%,
C: 0.015% or less,
Si: 0.02 to 0.60%,
Mn: 0.2-2.0%,
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,
S: 0.10 to 0.45%
Furthermore,
Pb: 0.03 to 0.40%,
Bi: 0.03 to 0.40%, and
Te: contains two or more kinds selected from 0.01 to 0.10%, and
When the mass% of the element M is [M],
900 ([C] + [N]) + 170 [Si] +12 [Cr] +30 [Mo] +10 [Al] ≦ 300, and the balance is composed of Fe and unavoidable impurities.
A ferritic free-cutting stainless steel having a ferrite cross-sectional area ratio of 95% or more.   ([Cr] + [Mo] +1.5 [Si] +4 [Al]) / ([Ni] +0.5 [Mn] +30 [C] +30 [N]) ≧ 7. Item 4. A ferritic free-cutting stainless steel according to item 4. The steel is
B: 0.0001 to 0.0080%,
Mg: 0.0005 to 0.0100%, and
Ca: 0.0005 to 0.0100%
The ferritic free-cutting stainless steel according to claim 5, further comprising one or more selected from the group consisting of: