JP2006097039A - Free-cutting stainless steel with excellent corrosion resistance, cold forgeability and hot workability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide free-cutting stainless steel having excellent corrosion resistance, cold forgeability and hot workability. <P>SOLUTION: The free-cutting stainless steel with excellent corrosion resistance, cold forgeability and hot workability has a composition consisting of, by mass, ≤0.40% C, ≤2.0% Si, 0.20 to 0.90% Mn, 0.05 to 0.40% S, 10 to 30% Cr and the balance Fe with inevitable impurities and satisfying inequality expä(12+0.18×[%Cr]-36×[%S])/22}×[%S]≤[%Mn]≤expä(32+0.18×[%Cr]-36×[%S])/22}×[%S], where [%Cr], [%Mn] and [%S] represent the contents of Cr, Mn and S, respectively. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a free-cutting stainless steel excellent in corrosion resistance, cold forgeability and hot workability.

  In general, not only the amount of sulfide, which is a free-cutting material produced in steel, but also the sulfide composition has an important influence on the corrosion resistance and other properties of S-added free-cutting stainless steels such as SUS416, SUS430F, and SUS303. It is known. The sulfide composition greatly depends on the components of the steel itself, and in the case of stainless steel, Cr is naturally contained in the sulfide because of the Cr content of 12% or more. Therefore, conventionally, when emphasizing corrosion resistance, a method of limiting the Mn addition amount to reduce the sulfide composition to a low Mn concentration-high Cr concentration has been taken.

  Since sulfide containing Cr is excellent in corrosion resistance, it is suitable as a free-cutting material for stainless steel that is expected to have original corrosion resistance. However, when the Cr concentration in the sulfide fluctuates, there is a problem that the machinability changes remarkably due to a sudden change in the hardness of the sulfide. The sulfide composition largely depends on the components of the steel itself, but this also changes properties such as cold forgeability and hot workability. Conventionally, as a method for controlling the sulfide composition, for example, as disclosed in Japanese Patent Application Laid-Open No. 10-46292 (Patent Document 1), the factors represented by the Mn / S ratio and the Cr amount are controlled to control the sulfide. Substituting Mn in Cr to improve corrosion resistance, and the machinability is slightly reduced by changes in the sulfide structure, but it is compensated by the addition of Pb and Se, and the structure at the time of hot working is made a ferrite phase. Ferritic free-cutting stainless steel has been proposed that ensures sufficient hot workability.

  Further, as disclosed in Japanese Patent Application Laid-Open No. 2000-17401 (Patent Document 2), martensitic free-cutting stainless steel for controlling sulfide to Cr-rich as Cr-21Mn / S ≧ -15 has been proposed. ing. Further, JP-A-5-339680 (Patent Document 3) proposes a free-cutting austenitic stainless steel that has Mn / S ≦ 2 and suppresses the generation of MnS to improve the corrosion resistance.

Japanese Patent Laid-Open No. 10-46292 JP 2000-17401 A JP-A-5-339680

  However, as described above, there is only a method of adjusting the Mn / S ratio added to the steel, such as a method of controlling the sulfide composition, or a control formula that slightly expands it, and it has excellent quality stability, corrosion resistance, cold forging. Free-cutting stainless steel excellent in workability and hot workability has not been known, and the above method has a problem that it is not sufficient for quality stability that achieves both corrosion resistance and machinability.

  In order to solve the problems as described above, the inventors have conducted intensive research. As a result, in the S-added free-cutting stainless steel, which emphasizes corrosion resistance, the amount of Mn added is determined by a formula. Provided is a free-cutting stainless steel having excellent corrosion resistance, cold forgeability and hot workability, in which the Cr content in the sulfide is 15 to 35% and corrosion resistance can be improved without extreme sacrifice of machinability. Is.

The gist of the invention is that
(1) By mass%, C: 0.40% or less, Si: 2.0% or less, Mn: 0.20-0.90%, S: 0.05-0.40%, Cr: 10-30 %, And the balance Fe and inevitable impurities, and the content of Cr, Mn, and S is [% Cr], [% Mn], and [% S], respectively. A free-cutting stainless steel excellent in corrosion resistance, cold forgeability and hot workability characterized by satisfying 1).
exp {(12 + 0.18 × [% Cr] −36 × [% S]) / 22} × [% S] ≦ [% Mn] ≦ exp {(32 + 0.18 × [% Cr] −36 × [% S) ]) / 22} × [% S] (1)

(2) Ni: 25% or less, Mo: 3.0% or less, Cu: 3.0% or less, Co: 3.0% or less, containing 1 type or 2 types or more (1 ) Free-cutting stainless steel with excellent corrosion resistance, cold forgeability and hot workability.
(3) Ti: 0.50% or less, V: 0.50% or less, Nb: 0.50% or less, W: 0.50% or less, Ta: 0.50% or less, Hf: 0.50% or less The free-cutting stainless steel excellent in corrosion resistance, cold forgeability and hot workability as described in (1) or (2) above, comprising one or more of the above.

(4) Se: 0.20% or less, Te: 0.10% or less, Sn: 0.30% or less, Pb: 0.30% or less, Bi: 0.30% or less, Ca: 0.020% or less The free-cutting stainless steel excellent in corrosion resistance, cold forgeability and hot workability as described in (1) to (3) above.
(5) One or more of B: 0.020% or less, Al: 0.20% or less, Mg: 0.020% or less, Zr: 0.20% or less, REM: 0.020% or less A free-cutting stainless steel excellent in corrosion resistance, cold forgeability and hot workability as described in (1) to (4) above.

(6) In the above (1) to (5), containing one or more of P: 0.20% or less, N: 0.20% or less, O: 0.030% or less Free-cutting stainless steel with excellent corrosion resistance, cold forgeability and hot workability.
(7) The average value of the amount of Cr contained in the sulfide inclusions is 15 to 35% by mass%, and the corrosion resistance, cold forgeability and heat described in (1) to (6) above It is a free-cutting stainless steel with excellent hot workability.

  As described above, the amount of Mn added according to the present invention is determined by a formula, and the Cr content in the sulfide is controlled to 15 to 35% with high accuracy, so that it is suitable for applications in which corrosion resistance is important and cold forging. S-added free-cutting stainless steel excellent in workability and hot workability can be provided.

Hereinafter, the reasons for limiting the component composition according to the present invention will be described.
C: 0.40% or less C is an element that increases the strength with the addition amount thereof, and is an element effective for adjusting a desired strength. However, if it increases excessively, corrosion resistance and machinability decrease due to precipitation of carbides, so the upper limit was made 0.40%.
Si: 2.0% or less Si is effective as a deoxidizing element. However, if the amount is excessively increased, the hardness is increased and the toughness is deteriorated, so the upper limit was made 2.0%.

Mn: 0.20 to 0.90%
Mn is an element that not only is effective as a deoxidizing element but also generates sulfides and has an important influence on the composition thereof. As shown in Formula 1, although depending on the balance of other components, if it is less than 0.20%, the Mn concentration in the sulfide becomes too low and the machinability deteriorates. Further, since the Mn concentration in the sulfide becomes too high and the corrosion resistance is lowered, the range was made 0.20 to 0.90%.

S: 0.05-0.40%
S is an extremely effective element as a machinability improving element, but if the content is less than 0.05%, this effect is not sufficiently exhibited. Moreover, when adding over 0.40%, the machinability improvement effect corresponding to the addition amount cannot be obtained, and the hot workability is deteriorated, so the range is 0.05 to 0.40%. did.

Cr: 10-30%
Cr is an element that improves the corrosion resistance by forming a solid oxide film on the surface of the steel by dissolving in the matrix, and significantly improves the corrosion resistance when contained in sulfides. However, if it is less than 10%, a sufficient film cannot be formed and the corrosion resistance is insufficient, and if it exceeds 30%, the toughness and machinability deteriorate, so the range was made 10 to 30%.

Ni: 25% or less Ni is an austenite-forming element and is a basic element in austenitic stainless steel. It has the effect of improving ductility and improving corrosion resistance against non-oxidizing acids. However, if it exceeds 25%, the hot workability of the S-added free cutting steel of the present invention deteriorates, so the upper limit was made 25%.

Mo: 3.0% or less Mo has an effect of strengthening the oxidation protective coating of Cr and improving the corrosion resistance. However, if it exceeds 3.0%, the σ phase tends to precipitate and the mechanical properties are lowered, so the upper limit was made 3.0%.
Cu: 3.0% or less Cu, when added to austenitic stainless steel, functions to improve cold workability. However, if it exceeds 3.0%, the hot workability deteriorates, so the upper limit was made 3.0%.

Co: 3.0% or less Co is an element that increases strength and improves heat resistance and wear resistance. However, if it exceeds 3.0%, the hot workability deteriorates, so the upper limit was made 3.0%.
Ti: 0.50% or less, V: 0.50% or less, Nb: 0.50% or less, W: 0.50% or less, Ta: 0.50% or less, Hf: 0.50% or less These elements Has the effect of producing carbonitrides and improving corrosion resistance. However, if it exceeds 0.50%, the machinability deteriorates, so each upper limit was made 0.50%.

Se: 0.20% or less, Te: 0.10% or less, Sn: 0.30% or less, Pb: 0.50% or less, Bi: 0.30% or less, Ca: 0.020% or less These elements Has the effect of improving machinability. However, if the respective upper limit is exceeded, the effect is saturated or the hot workability is lowered, so each upper limit was made 0.020%.

B: 0.020% or less, Al: 0.20% or less, Mg: 0.020% or less, Zr: 0.20% or less, REM: 0.020% or less These elements improve hot workability. effective. However, if the respective upper limit is exceeded, the effect is saturated, or conversely the hot workability is lowered, so each upper limit is set.
P: 0.20% or less, N: 0.20% or less, O: 0.030% or less These elements are impurities, and their content is as low as possible in order to reduce hot workability and mechanical properties. However, it was determined as an acceptable range that does not significantly impair the effects provided by the present invention.

  exp {(12 + 0.18 × [% Cr] −36 × [% S]) / 22} × [% S] ≦ [% Mn] ≦ exp {(32 + 0.18 × [% Cr] −36 × [% S) ]) / 22} × [% S] This formula is an alloy element for controlling sulfide to a composition suitable for improving corrosion resistance and machinability, and having good cold forgeability and hot workability. The component adjustment method is shown and is the most important in the present invention. When the amount of Mn is less than the value calculated on the leftmost side, the amount of Cr in the sulfide increases and the machinability and cold forgeability decrease, and when the amount of Mn is larger than the rightmost side, As the amount of Mn increases, the corrosion resistance decreases and the cold forgeability and hot workability decrease due to solid solution strengthening in the matrix. Therefore, it was set as the said range.

  FIG. 1 shows 12% Cr-0.17% S steel, 12% Cr-0.32% S steel, 19% Cr-0.32% S steel, 18% Cr-8% Ni-0.32% S. It is a figure which shows the influence of Mn / S ratio, Cr amount, and S amount which has on the Cr amount in the sulfide type inclusion about each steel type system of steel. As can be seen from this figure, the Cr content in the sulfide increases with a decrease in the Mn / S ratio, an increase in the Cr content as the alloy element, and a decrease in the S content. Moreover, FIGS. 2-6 is a figure which shows the influence of the amount of Mn which exerts on machinability, corrosion resistance, cold forgeability, and hot workability about 12% Cr-0.17% S steel. When the Mn content is controlled and added according to the present invention, free-cutting stainless steel having the above characteristics can be obtained.

Hereinafter, the present invention will be specifically described with reference to examples.
Steels having the composition shown in Tables 1 and 2 were melted in a vacuum induction furnace to form a 100 kg steel ingot, subjected to heat treatment after forging, and subjected to various tests. The tests included sulfide inclusion investigation, salt spray test, cycle wetness test, pitting potential test, drill life test, turning tool wear test, cold forgeability test and hot workability test. The results are shown in Tables 3 and 4.

(1) Investigation of sulfide inclusions After φ20 mm forging and heat treatment, cut specimens parallel to the longitudinal direction for structure observation, polish to mirror surface using emery paper and buff, and then 30 per specimen The structure of the sulfide inclusions was quantitatively analyzed with an energy dispersive X-ray analyzer.

(2) Salt spray test After φ20 mm forging and heat treatment, φ12 mm × L21 mm corrosion resistance test pieces were prepared and used for the test. The test conditions were 5% NaCl aqueous solution sprayed at 35 ° C. for 24 hours, and the degree of galling after the test was evaluated by rating. Ratings were evaluated as A: no sprout, B: sprout area rate of less than 5%, C: sprout area rate of 5% to less than 20%, and D: sprout area rate of 20% or more.

(3) Cyclic wetting test After φ20 mm forging and heat treatment, φ12 mm × L21 mm corrosion resistance test pieces were prepared and used for the test. The test conditions were 90% relative humidity, [(20 ° C.-1.5 h) ← → (50 ° C.-4.5 h)] × 20 cycles, and the degree of wrinkling after the test was rated. Ratings were evaluated as A: no sprout, B: sprout area rate of less than 5%, C: sprout area rate of 5% to less than 20%, and D: sprout area rate of 20% or more.
(4) Pitting potential test After φ20 mm forging and heat treatment, a test piece was cut out parallel to the longitudinal direction, and pitting potential measurement was performed using a 3.5% NaCl aqueous solution at 30 ° C. Test conditions were based on JIS G0577. The pitting potential rating is A: 0.4 mV or more, B: 0.3 to less than 0.4 mV, C: less than 0.2 to 0.3 mV, D: less than 0.08 to 0.2 mV, E: 0 Evaluation was made at less than 0.08 mV.

(5) Drill life test For the φ60 mm forging-heat treated material, drilling was performed in parallel with the forging direction under certain conditions until the drill was broken or melted and could not be drilled. The test was repeated three times under the same conditions, and the average value was defined as the drill life. Drill used: SKH51, φ5 mm straight drill, drilling depth: 15 mm, cutting speed: 30 m / min, feed rate: 0.1 mm / rev, coolant: none.

(6) Turning tool wear test The φ60 mm forge-heat treated material was turned in the circumferential direction under certain conditions. Tool flank wear was measured when the cutting distance was 2000 m. Tool used: Carbide P20, square negative tip, cutting edge R 0.4 mm, cutting speed: 200 m / min, cutting amount: 1.0 mm, feeding amount: 0.2 mm / rev, coolant: none.
Drill life and turning wear are no. 1-19 are No.1. It was expressed as a ratio of the drill life or turning tool wear amount of each test material to 10 drill life (number of holes) or turning tool wear amount (mm). Similarly, no. For Nos. 20 to 36, no. 28, and No. 28. For Nos. 37-48, no. 46 was used as a reference.

(7) Cold forgeability test After φ20 mm forging-heat treatment, φ14 mm x L21 mm upsetting test piece is produced, and the compression rate until the crack is generated by compressing in the longitudinal direction in the cold (limit upsetting rate) Was measured.
About the limit upsetting rate, A: 70% or more, B: 65 to less than 70%, C: less than 60 to 65%, D: less than 55 to 60%, and E: less than 55%.

(8) Hot workability A test piece of φ8 mm × L100 mm was produced from the slab, heated to 1200 ° C., pulled and broken at 50 mm / s, and the drawing value was determined from the cross-sectional shrinkage ratio of the fractured part.
About hot workability, A: Drawing value in 1200 degreeC was 80% or more, B: 70-80 less, C: 60-70%, D: less than 60% evaluated.

  As shown in Table 3 and Table 4, 1-9, no. 20-27, no. Nos. 37 to 45 are steels of the present invention. 10-19, no. 28-36, no. 46-49 are comparative steels. Comparative steel No. Since No. 10 has a low S content, the machinability is inferior. Comparative steel No. Nos. 11 to 12 have a low Mn content. Similar to 10, the machinability is inferior. Comparative steel No. Since 13 does not satisfy the formula (1), the amount of Cr in the sulfide is insufficient and the corrosion resistance is inferior. No. Since all 14-15 have high Mn content, they are inferior in corrosion resistance and hot workability or limit upsetting rate.

  Comparative steel No. Since No. 16 does not satisfy the formula (1), the amount of Cr in the sulfide becomes excessive, and the machinability and the limit upsetting rate are poor. Comparative steel No. Since No. 17 has a high Mn content, the corrosion resistance is poor, and the limit upsetting rate and hot workability are poor. Comparative steel No. Since No. 18 has a high C content, it has poor corrosion resistance, machinability, limit upsetting rate and hot workability. Comparative steel No. Since No. 19 has a high S content, it has poor corrosion resistance, limit upsetting rate and hot workability.

  Comparative steel No. No. 28 has poor machinability due to its low S content. Comparative steel No. No. 29 has a low Mn content, so the limit upsetting rate is poor. On the other hand, 30 has a high Mn content, so that the corrosion resistance is inferior, and the limit upsetting rate and hot workability are inferior. Comparative steel No. Since No. 31 does not satisfy the formula (1), the limit upsetting rate is bad. Comparative steel No. Since 32 and 33 have a high Mn content, the corrosion resistance and the limit upsetting rate or hot workability are inferior. Comparative steel No. Since 34 has a high Si content, machinability and hot workability are inferior.

Comparative steel No. No. 35 has a low Cr content. On the contrary, since 35 is high, corrosion resistance and hot workability, or machinability are poor. Comparative steel No. No. 46 has poor machinability due to its low S content. Comparative steel No. 47 does not satisfy the formula (1), so the amount of Cr in the sulfide is excessive and the machinability or the limit upsetting rate is inferior. Comparative steel No. Since 48 has a high Mn content, the corrosion resistance is inferior, and the limit upsetting rate and hot workability are poor.
As described above, as can be seen from Tables 3 and 4, the corrosion resistance, machinability, and the amount of Mn added are regulated by the formula and the amount of Cr in the sulfide is controlled to be constant. A free-cutting stainless steel excellent in the limit upsetting rate and hot workability could be obtained.

12% Cr-0.17% S steel, 12% Cr-0.32% S steel, 19% Cr-0.32% S steel, 18% Cr-8% Ni-0.32% S steel It is a figure which shows the influence of Mn / S ratio, Cr amount, and S amount which has on the Cr amount in the sulfide type inclusion about a steel type system. It is a figure which shows the influence of the amount of Mn which gives to the drill life about 12% Cr-0.17% S steel. It is a figure which shows the influence of the amount of Mn which gives to the cutting tool wear about 12% Cr-0.17% S steel. It is a figure which shows the influence of the amount of Mn which gives to the pitting corrosion potential about 12% Cr-0.17% S steel. It is a figure which shows the influence of the amount of Mn which has on the cold forgeability about 12% Cr-0.17% S steel. It is a figure which shows the influence of the amount of Mn which exerts on the hot workability about 12% Cr-0.17% S steel.

Claims (7)

% By mass
C: 0.40% or less,
Si: 2.0% or less,
Mn: 0.20 to 0.90%,
S: 0.05-0.40%,
Cr: 10-30%
And the balance Fe and unavoidable impurities, and the content of Cr, Mn, and S is [% Cr], [% Mn], and [% S], respectively. ) Free-cutting stainless steel with excellent corrosion resistance, cold forgeability and hot workability.
exp {(12 + 0.18 × [% Cr] −36 × [% S]) / 22} × [% S] ≦ [% Mn] ≦ exp {(32 + 0.18 × [% Cr] −36 × [% S) ]) / 22} × [% S] (1) It contains one or more of Ni: 25% or less, Mo: 3.0% or less, Cu: 3.0% or less, and Co: 3.0% or less. Free-cutting stainless steel with excellent corrosion resistance, cold forgeability and hot workability. Ti: 0.50% or less, V: 0.50% or less, Nb: 0.50% or less, W: 0.50% or less, Ta: 0.50% or less, Hf: 0.50% or less The free-cutting stainless steel having excellent corrosion resistance, cold forgeability, and hot workability according to claim 1 or 2, further comprising two or more kinds. Se: 0.20% or less, Te: 0.10% or less, Sn: 0.30% or less, Pb: 0.30% or less, Bi: 0.30% or less, Ca: 0.020% or less Or the free-cutting stainless steel excellent in corrosion resistance, cold forgeability, and hot workability of Claims 1-3 containing 2 or more types. B: 0.020% or less, Al: 0.20% or less, Mg: 0.020% or less, Zr: 0.20% or less, REM: 0.020% or less The free-cutting stainless steel excellent in corrosion resistance, cold forgeability and hot workability according to claim 1. It contains one or more of P: 0.20% or less, N: 0.20% or less, O: 0.030% or less, corrosion resistance, cold forging according to claim 1-5 Free-cutting stainless steel with excellent workability and hot workability. The average amount of Cr contained in the sulfide-based inclusion is 15 to 35% by mass%, and excellent in corrosion resistance, cold forgeability and hot workability according to claim 1 Free-cutting stainless steel.

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