KR100622887B1 - Metastable stainless steel strip having excellent fatigue property - Google Patents

Abstract

(Object) It is an object of the present invention to provide a metastable austenitic stainless steel having excellent fatigue characteristics by an index in consideration of the working history of fatigue characteristics.

(Measures) In Md given by the amount of martensite (= Ms amount), strength (= TS), and the following formula, the TS / Ms is 40 or less, and the Md / Ms is 0.16 or more. Metastable austenitic stainless steel strip with excellent fatigue characteristics,

Md = 551-462 (C + N)-9.2 Si-8.1 Mn-29 (Ni + Cu)-13.7 Cr-18.5 Mo.

Fatigue Properties, Metastable Austenitic Stainless Steels

Description

Metastable austenitic stainless steel strip with excellent fatigue properties {METASTABLE STAINLESS STEEL STRIP HAVING EXCELLENT FATIGUE PROPERTY}

The present invention relates to a quasi-stable austenitic stainless steel strip for switches requiring high strength and high fatigue properties.

Background Art In recent years, high-density mounting has been advanced in electronic devices such as mobile phones and personal computers. Therefore, miniaturization and thinning of electronic parts used in these electronic devices have been advanced. Among them, particularly in mobile phones, mobile communication, etc., there is a strong demand for miniaturization and thinning of the push spring. In addition, not only the stress repeatedly applied to the pressing spring member is increased, but also the number of times of the load tends to be increased. In order to meet such stringent demands, the metal material used for the press spring needs to raise the strength and improve the fatigue characteristics.

In such a use, the metastable austenitic stainless steel in which the austenite phase is transformed into a martensite phase by cold rolling and cured is used as a spring material having high strength and hardness (for example, Japanese Patent Laid-Open See Japanese Patent Application Laid-open No. Hei 10-140294, Japanese Unexamined Patent Publication No. 2002-275591 and Japanese Unexamined Patent Publication No. 2002-146483.)

In Japanese Unexamined Patent Publications No. Hei 10-140294, Japanese Unexamined Patent Publication No. 2002-275591, and Japanese Unexamined Patent Application Publication No. 2002-146483, "Md" is defined as a method for improving fatigue properties, and martensite is appropriately generated during processing. A method of leaving an austenite phase to some extent in a product is disclosed. Md is a value which shows the ease of production of processed organic martensite by the chemical composition of the material. Although Md differs according to the literature, For example, Unexamined-Japanese-Patent No. 2002-275591 has Md (N) = 580- (520C + 2Si + 16Mn + 16Cr + 23Ni + 300N + 10Mo), and Unexamined-Japanese-Patent No. 2002-146483 has Md = 551-462 ( C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo is used.

When Md is small, it means that processed organic martensite is hard to be formed at the time of deformation, and conversely, when the value is large, it is easy to form.

In Japanese Unexamined Patent Application Publication No. 10-140294, regarding the correlation between the Md and the fatigue life, in consideration of the fact that the transformation of austenite to martensite during deformation in the fatigue test tends to dominate the fatigue properties of the material. Therefore, it is described that when Md is less than 20, martensite transformation does not occur during deformation in the fatigue test, and on the contrary, when Md exceeds 100, martensite is formed quickly, so that sufficient fatigue characteristics are not obtained, respectively. This reason is considered to be because martensite is formed quickly and growth of martensite is completed at the beginning of the fatigue test.                         

However, the fatigue characteristics are not sufficient only by the provision of Md, and there is still room for improvement. Fatigue properties are inherently positively correlated with strength, and generally high strength materials have a long fatigue life. For example, even if they have the same Md value, the strength varies depending on the machining history, and the fatigue characteristics are greatly changed. In addition, the generation of martensite amount also varies depending on the processing history.

An object of the present invention is to provide a metastable austenitic stainless steel strip having excellent fatigue characteristics by an index in consideration of the processing history with respect to fatigue characteristics.

As a result of intensive studies, the present inventors have found that by specifying the values of Md / Ms and TS / Ms in place of Md, the good fatigue properties of stainless steel can be exhibited to the maximum.

In other words,

(1) In the amount of martensite (Ms (%)), tensile strength (TS (MPa)) and Md (° C.) given by the following equation, TS / Ms is 40 or less, and Md / Ms is 0.16 or more. Metastable austenitic stainless steel strip, excellent in fatigue characteristics, characterized in that

Md = 551-462 (C + N)-9.2 Si-8.1 Mn-29 (Ni + Cu)-13.7 Cr-18.5 Mo,

Unit of mass is% by mass.

(2) In mass%, C: 0.07 to 0.15%, Si: 0.45 to 1.00%, Mn: 0.60 to 1.20%, P: ≤0.040%, S: ≤0.015%, Ni: 6.00 to 7.00%, Cr: 16.00 It is a metastable austenitic stainless steel strip with excellent fatigue characteristics, consisting of -18.00%, residual Fe and unavoidable impurities, and having TS / Ms of 40 or less and Md / Ms of 0.16 or more.

Embodiment of the invention

The reason for limitation of this invention is demonstrated below.

(1) About Md / Ms

As described above, the magnitude of the Md value affects the fatigue characteristics, but the fatigue characteristics greatly change depending on the machining history. In quasi-austenitic stainless steels, the amount of martensite (Ms) varies depending on the working history, and there is a limit to the amount of S generated. When martensite grows in cold working in a manufacturing process, the amount of martensite newly generated in the fatigue process at the use stage is limited and good fatigue characteristics cannot be obtained. This is because, in the case of metastable austenitic stainless steel, even if a crack occurs in the stainless steel strip during the fatigue process at the use stage, a work-organic martensite transformation occurs at the crack tip, and the part hardens and the crack is difficult to develop. This is because the fatigue life is long.

Even if Md is small, if the growth of martensite phase is small by cold working in the manufacturing process, growth of martensite phase in the fatigue process in the use step can be obtained, and favorable fatigue characteristics can be obtained. On the other hand, even when Md is large, if martensite is excessively grown by cold working in the manufacturing process, the growth of new martensite cannot be afforded in the fatigue process in the use step, and good fatigue characteristics cannot be obtained.

Therefore, in order to improve a fatigue characteristic, since Md is large and it needs to have a margin in martensite amount Ms, the index of the following formula is proposed.

Md / Ms≥0.16

That is, fatigue characteristics can be improved by increasing Md for arbitrary Ms or making Ms small for arbitrary Md. In addition, although Md changes with literatures, it is assumed that the following formula used in Japanese Unexamined Patent Publication No. 2002-146483 is used in the present invention.

Md = 551-462 (C + N)-9.2 Si-8.1 Mn-29 (Ni + Cu)-13.7 Cr-18.5 Mo

(2) About TS / Ms

On the other hand, in quasi-austenitic stainless steel, the fatigue property is inherently correlated with the strength, and the higher the strength is to the extent of the strength, the better the fatigue property. However, since there is a strong positive correlation between the martensite ratio and the strength, it tends to be contrary to the knowledge that the above-mentioned occurrence of martensite phase is suppressed and fatigue characteristics are not improved when Ms cannot be afforded. In short, the good and bad of fatigue characteristics are also limited by the relationship (TS / Ms) of strength and martensite ratio, and this is the intention of this invention. At TS / Ms> 40, since the processing is small, the Ms is small, and the strength is not sufficient, the fatigue characteristics deteriorate.

As described above, the martensite amount is the minimum value of the martensite amount necessary to secure the strength and obtain good fatigue characteristics, and there is no growth of the new martensite phase in the fatigue process, as shown by 0.025TS≤Ms≤6.25Md. The range intermediate to the maximum value of the amount of martensite which cannot be improved is good.

Therefore, in the present invention, Md / Ms? 0.16 and TS / Ms? 40 are specified.

(3) About chemical composition

Metastable austenitic stainless steels are austenitic stainless steels in which the austenite phase of a structure is unstable at room temperature and a processed organic martensite is produced by cold rolling. Examples thereof include SUS301 and SUS304.

In the present invention, attention was paid to SUS301, and the content of C, Si, P, S, and Cr in the composition was defined in JIS as the claims.

On the other hand, with respect to Ni and Mn, which are substantially adjustable components in quasi-austenitic stainless steel, austenite becomes unstable by reducing the content thereof, and martensite transformation due to processing is likely to occur. Therefore, when the crack tip portion of the material surface layer occurring at the initial stage of fatigue is deformed, martensite phases are liable to occur, and new martensite phases are less likely to cause breakage due to fatigue, thereby improving fatigue characteristics.

However, when the Ni and Mn contents are lowered, the strength is also lowered. Therefore, it is necessary to increase the amount of martensite by processing to increase the strength. In short, a metastable austenitic stainless steel having high strength and excellent fatigue characteristics can be obtained by increasing the amount of martensite by machining so that TS / Md is 40 or less. However, when Mn and Ni content is made too low, austenite will become very unstable and workability will worsen, Therefore, about the minimum, the quantity prescribed | regulated to JIS was made into the range.

Therefore, the range of Mn and Ni when TS / Ms is 40 or less and Md / Ms is 0.16 or more is defined as 0.60 to 1.20% and 6.00 to 7.00%.

(Example)

The metastable austenitic stainless steel used in the examples is SUS301. The component composition is shown in Table 1. After forging the ingot of SUS301 obtained by melt casting in the range of the component based on JIS specification, it hot-rolls to 3 mm of plate | board thickness, and repeats cold rolling and recrystallization annealing twice, respectively, after solution treatment, and The final rolling was carried out to a plate thickness of 0.1 mm.

Chemical composition (mass%) C N Si Mn P S Ni Cu Cr Composition 1 0.11 0.05 0.6 One 0.03 0.002 7.2 0.16 17 Composition 2 0.11 0.06 0.6 1.5 0.03 0.002 6.7 0.16 16.9 Composition 3 0.11 0.06 0.6 0.8 0.03 0.002 6.3 0.14 16.6

The evaluation of the amount of martensite, tensile strength and fatigue characteristics was measured as follows.

(1) Martensite content

The amount of martensite is measured by using a ferrite content meter (ferrite scope) by magnetic induction by using the martensite phase paramagnetic while the austenite phase is nonmagnetic. Obtained by volume ratio.

(2) tensile strength

The plate-shaped test piece based on 13B prescribed | regulated to JIS Z 2201 was punched out, and the tension test based on JIS Z 2241 was performed.

(3) fatigue characteristics

Fatigue characteristics measured the repeated endurance count to break at the maximum load stress of 1150 MPa using the ultra-thin plate life tester which used the pulley based on JISZ2273.

No. Chemical composition Grain size ^{* 1} ㎛ Machinability ^{* 2} % TS MPa Ms% Md ℃ TS / Ms Md / Ms Repetition Count to Break   Inventive Example One  Composition 1 1.2 40 1595 80.5 15.27 20 0.19 〉 1.0 × 10 ⁸ 2 2.4 55 1697 84.1 15.27 20 0.18 9.2 × 10 ⁷ 3 11 55 1470 43.9 15.27 33 0.35 2.7 × 10 ⁷ 4  Composition 2 1.4 20 1344 44.3 22.47 30 0.507 〉 1.0 × 10 ⁸ 5 3.1 55 1569 78.8 22.47 20 0.285 7.6 × 10 ⁷ 6 16.8 55 1482 37.8 22.47 20 0.594 1.2 × 10 ⁷ 7  Composition 3 One 20 1457 52.3 44.43 28 0.85 〉 1.0 × 10 ⁸ 8 2.8 40 1721 96.5 44.43 18 0.46 〉 1.0 × 10 ⁸ 9 18.3 40 1588 59.5 44.43 27 0.75 1.9 × 10 ⁷   Comparative example 10  Composition 1 1.2 90 2142 100 15.27 21 0.15 6.1 × 10 ⁶ 11 2.4 5 1105 10.2 15.27 108 1.5 8.8 × 10 ⁵ 12 11 10 1068 7.7 15.27 139 1.98 4.1 × 10 ⁵ 13 Composition 2 3.1 5 1036 9.8 22.47 106 2.29 7.4 × 10 ⁵ 14 16.8 10 998 7.2 22.47 139 3.12 4.9 × 10 ⁵ 15  Composition 3 One 0 1115 0.2 44.43 5575 222 7.7 × 10 ⁶ 16 2.8 0 1052 0.1 44.43 10520 444 8.4 × 10 ⁶ 17 18.3 5 1105 4.1 44.43 270 11 1.1 × 10 ⁶

* 1 Crystal grain size in final annealing

* 2 Final rolling work drawing

Table 2 shows the results of the evaluation. Inventive Examples No. 1 to No. 9 satisfy TS / Ms 40 or less, Md / Ms 0.16 or more, and neither Comparative TS No. 40 or Md / Ms 0.16 or more. Compared with the material of the same composition in 10-17, the fatigue characteristic excellent in the invention example rather than the comparative example is obtained.

In addition, when the intensity | strength which concerns on a fatigue characteristic is paid attention, in Comparative Example No. 10, although the intensity | strength is high, Md / Ms is smaller than 0.16, ie, since the martensite amount is large, the fatigue characteristic deteriorates. In Comparative Examples Nos. 11 to 17, although the Md / Ms was 0.16 or more, the tensile strength was low, so the fatigue characteristics also deteriorated.

Comparing compositions 1 and 2 and composition 3, by comparison of invention examples Nos. 1 to 6 and Nos. 7 to 9, and Comparative Examples Nos. 10 to 14 and Nos. 15 to 17, composition 3 is more than composition 1 and 2 This fatigue property shows an excellent tendency. This is because Md was decreased by increasing the concentration of Ni in composition 1 and Mn in composition 2.

As described above, a metastable austenitic stainless steel for a switch having high strength and fatigue characteristics can be provided by an index for obtaining a metastable austenitic stainless steel strip having high strength and excellent fatigue characteristics.

Claims (2)

In the amount of martensite (Ms (%)), tensile strength (TS (MPa)), and Md (° C.) given by the following equation, TS / Ms is 40 or less and Md / Ms is 0.16 or more. Metastable austenitic stainless steel strip, excellent in fatigue characteristics, Md = 551-462 (C + N)-9.2 Si-8.1 Mn-29 (Ni + Cu)-13.7 Cr-18.5 Mo, Unit of mass is% by mass. In mass%, C: 0.07 to 0.15%, Si: 0.45 to 1.00%, Mn: 0.60 to 1.20%, P: ≤0.040%, S: ≤0.015%, Ni: 6.00 to 7.00%, Cr: 16.00 to 18.00% A metastable austenitic stainless steel strip having excellent fatigue characteristics, comprising a balance of Fe and unavoidable impurities, and having a TS / Ms of 40 or less and a Md / Ms of 0.16 or more.