Spring steel

Abstract

There is provided a spring steel including predetermined chemical composition, in which ([Ti mass %]−3.43×[N mass %])/[S mass %]>4.0, and [Ni mass %]+[Cu mass %]<0.75 are satisfied, and an appearance frequency of MnS is less than 20% among inclusions having an equivalent circle diameter of 1 μm or more which are observed at a ¼ position of a diameter from a surface.

Claims

1. A spring steel comprising, as a chemical composition, by mass %: C: 0.40% to 0.60%; Si: 0.90% to 3.00%; Mn: 0.10% to 0.60%; Cr: 0.10% to 1.00%; Al: 0.010% or more and less than 0.050%; Ti: 0.040% to 0.100%; B: 0.0010% to 0.0060%; N: 0.0010% to 0.0070%; V: 0% to 1.00%; Mo: 0% to 1.00%; Ni: 0% or more and less than 0.45%; Cu: 0% to 0.50%; Nb: 0% to 0.10%; P: limited to less than 0.020%; S: limited to less than 0.020%; and a remainder including Fe and impurities, wherein Expressions 1 and 2 are satisfied, and an appearance frequency of MnS is less than 20%, wherein said appearance frequency is defined as a number of MnS inclusions observed at a ¼ position of a diameter from a surface divided by a number of total inclusions having an equivalent circle diameter of 1 μm or more observed at said ¼ position of a diameter from a surface, expressed as a percentage,
([Ti mass %]−3.43×[N mass %])/[S mass %]>4.0  Expression 1
[Ni mass %]+[Cu mass %]<0.75  Expression 2 here, [Ni mass %], [Cu mass %], [Ti mass %], [N mass %], and [S mass %] in Expressions 1 and 2 respectively represent a Ni content, a Cu content, a Ti content, a N content, and a S content in unit mass%, and when the spring steel is heated at temperature of 900° C. to 1,050° C. and quenched, and then is tempered such that the tensile strength is 1900 MPa to 2000 MPa, the impact value is 70.0 J/cm.sup.2 or more.

2. The spring steel according to claim 1 comprising, as the chemical composition, by mass%, one or more selected from the group consisting of: V: 0.05% to 1.00%; Mo: 0.10% to 1.00%; Ni: 0.05% or more and less than 0.45%; Cu: 0.05% to 0.50%; and Nb: 0.01% to 0.10%.

3. The spring steel according to claim 1, comprising, as the chemical composition, by mass %, Si: 0.9 to 2.5%.

Description

EXAMPLES

(1) Subsequently, examples of the present invention are described. The conditions in the examples are one condition example employed in order to confirm the applicability and the effect of the present invention, and the present invention is not limited to this one condition example. The present invention may employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(2) The chemical compositions of each example and comparative example and ([Ti mass %]−3.43×[N mass %])/[S mass %] (in tables (Ti−3.43×N)/S), [Cu mass %]+[Ni mass %] (in tables, Cu+Ni) are provided in Tables 1 and 2. In Tables 1 and 2, the reference symbol “-” represents that the corresponding element is not added. The remainders in Tables 1 and 2 are Fe and impurities.

(3) The steel ingot having the chemical composition shown in Tables 1 and 2 were heated at the temperature of 950° C. to 1200° C. for the time not more than 120 min, so as to perform hot rolling, such that a steel (spring steel) having φ (diameter) of 12 mm to 18 mm.

(4) TABLE-US-00001 TABLE 1 Chemical Composition (mass %) Cu + C Si Mn P S Cr Al Ti N B Mo V Cu Ni Nb (Ti—3.43N)/S Ni Exam- 1 0.50 1.99 0.52 0.007 0.008 0.85 0.020 0.070 0.0032 0.0025 0.20 — — — — 7.4 — ple 2 0.56 1.50 0.31 0.005 0.009 0.30 0.025 0.095 0.0040 0.0030 — — 0.20 0.25 — 9.0 — 3 0.59 1.80 0.25 0.006 0.005 0.70 0.022 0.056 0.0030 0.0035 — — — — — 9.2 — 4 0.43 2.19 0.50 0.006 0.008 0.69 0.023 0.059 0.0034 0.0027 — 0.19 0.10 0.15 — 9.3 0.25 5 0.49 2.40 0.30 0.012 0.010 0.18 0.030 0.061 0.0040 0.0023 0.20 0.16 — — — 4.7 — 6 0.55 1.05 0.48 0.011 0.011 0.75 0.031 0.068 0.0032 0.0025 — — — — — 5.2 — 7 0.50 1.40 0.55 0.010 0.012 0.60 0.025 0.070 0.0035 0.0031 — 0.22 — — — 4.8 — 8 0.50 2.00 0.19 0.011 0.010 0.75 0.021 0.070 0.0028 0.0030 0.20 0.15 — — — 6.0 — 9 0.48 1.50 0.30 0.012 0.008 0.95 0.035 0.071 0.0027 0.0026 — — — — — 7.7 — 10 0.50 2.00 0.50 0.009 0.010 0.15 0.025 0.069 0.0035 0.0025 0.20 0.31 — — — 5.7 — 11 0.49 1.79 0.50 0.010 0.011 0.50 0.025 0.080 0.0042 0.0024 0.75 — — — 6.0 — 12 0.52 1.80 0.49 0.008 0.010 0.50 0.025 0.080 0.0045 0.0030 — 0.78 — — — 6.5 — 13 0.49 1.79 0.50 0.010 0.011 0.50 0.025 0.080 0.0063 0.0032 — 0.22 — — — 5.3 — 14 0.49 1.79 0.50 0.010 0.011 0.40 0.025 0.080 0.0063 0.0051 — — — — — 5.3 — 15 0.50 1.80 0.49 0.008 0.006 0.69 0.029 0.071 0.0034 0.0026 — — — 0.41 — 9.9 0.41 16 0.51 1.79 0.49 0.008 0.005 0.71 0.022 0.049 0.0035 0.0024 — — 0.40 0.19 — 7.4 0.59 17 0.50 2.00 0.30 0.006 0.007 0.85 0.028 0.069 0.0041 0.0030 0.20 0.18 — — 0.07 7.9 — 18 0.51 2.21 0.34 0.009 0.008 0.75 0.023 0.055 0.0038 0.0027 — — — — 0.03 5.3 —

(5) TABLE-US-00002 TABLE 2 Chemical Composition (mass %) Cu + C Si Mn P S Cr Al Ti N B Mo V Cu Ni Nb (Ti—3.43N)/S Ni Compar- 21 0.50 1.99 0.52 0.007 0.008 0.85 0.020 0.001 0.0032 0.0025 0.20 — — — — −1.2 — ative 22 0.52 1.50 0.31 0.005 0.009 0.30 0.025 0.020 0.0040 0.0030 — 0.15 — — — 0.7 — Exam- 23 0.51 1.90 0.32 0.012 0.012 0.71 0.018 0.042 0.0052 0.0025 — 0.15 0.25 0.50 — 2.0 0.75 ple 24 0.62 1.49 0.48 0.015 0.010 0.95 0.020 0.070 0.0045 0.0022 — — — — — 5.5 — 25 0.38 2.48 0.40 0.008 0.007 0.57 0.025 0.055 0.0025 0.0018 0.25 — — — — 6.6 — 26 0.50 3.15 0.31 0.005 0.012 0.70 0.024 0.070 0.0028 0.0023 — — — — — 5.0 — 27 0.55 0.50 0.52 0.008 0.015 0.70 0.001 0.021 0.0035 0.0024 — — — — — 0.6 — 28 0.52 1.60 0.85 0.012 0.009 0.75 0.001 0.065 0.0028 0.0025 — — — — — 6.2 — 29 0.52 1.50 0.51 0.025 0.015 0.50 0.021 0.070 0.0035 0.0024 — 0.12 — — — 3.9 — 30 0.51 1.50 0.50 0.012 0.030 0.52 0.020 0.070 0.0028 0.0025 — 0.15 — — — 2.0 — 31 0.54 1.80 0.30 0.009 0.010 1.21 0.020 0.062 0.0042 0.0019 — 0.10 — — — 4.8 — 32 0.54 1.75 0.41 0.008 0.010 0.81 0.035 0.062 0.0042 0.0029 1.21 — — — — 4.8 — 33 0.50 1.51 0.30 0.005 0.005 0.50 0.025 0.042 0.0029 0.0020 — 1.50 — — — 6.4 — 34 0.50 1.78 0.29 0.007 0.012 0.32 0.085 0.072 0.0042 0.0030 — — — — — 4.8 — 35 0.51 1.80 0.49 0.009 0.007 0.50 0.025 0.151 0.0051 0.0025 0.15 0.20 — — — 19.1 — 36 0.49 2.15 0.51 0.006 0.008 0.29 0.023 0.042 0.0092 0.0033 — — — — — 1.3 — 37 0.50 2.13 0.49 0.005 0.005 0.33 0.022 0.063 0.0042 — — — — — — 9.7 — 38 0.52 2.20 0.51 0.008 0.006 0.70 0.001 0.045 0.0052 0.0025 — — — — — 4.6 — 39 0.51 1.80 0.50 0.007 0.012 0.71 0.024 0.042 0.0068 0.0028 — — — — — 1.6 —

(6) With respect to the obtained spring steel, properties after quenching and tempering were evaluated, and thus a step of heating at the temperature of 900° C. to 1,050° C. and quenching, and a step of tempering such that the tensile strength was 1,900 MPa to 2,000 MPa were performed. The tempering condition was determined by tempering at 300° C., 400° C., and 500° C. as a preliminary test, measuring the strength, and estimating the tempering temperature at which the predetermined strength was obtained.

(7) A test piece was obtained from the obtained steel after quenching and tempering, and a tensile test, a charpy impact test, observation of inclusions, and the constant temperature and constant humidity test were performed.

(8) <Tensile test>

(9) A tensile test was performed by manufacturing a test piece having a parallel portion diameter of 8 mm in conformity with No.14 test piece of “JIS Z 2241”. In a case where the tensile strength was 1,800 MPa or more, it was determined that sufficient strength was able to be obtained.

(10) <Charpy Impact Test>

(11) A charpy impact test is performed at room temperature (23° C.) by manufacturing a U-notched test piece (notch lower height of 8 mm and a width of 5 mm sub size) in conformity with “JIS Z 2242”. In a case where the impact value (absorbed energy) was 70.0 J/cm.sup.2 or more, it was determined that sufficient toughness was able to be obtained.

(12) <Observation of Inclusion>

(13) The steel was cut in parallel to the rolling direction, the cut surface was mirror-polished, 20 or more inclusions having an equivalent circle diameter of 1 μm or more were observed with a metallographic microscope, and the appearance frequency of MnS in the inclusions having an equivalent circle diameter of 1 μm or more was calculated from the number of MnS with respect to the number of observed inclusions. At this point, the observation visual field was set to the ¼ position of the diameter, and 10 or more visual fields were observed at observation magnification of 1,000 times while moving in the rolling direction. The determination of MnS was estimated from the color (MnS is gray and Ti-based sulfide is a color from white to yellow through pink) during observation with the metallographic microscope and was confirmed by EPMA or SEM-EDS. An appearance frequency of MnS of less than 20% was accepted.

(14) <Constant Temperature and Constant Humidity Test>

(15) The test piece was exposed to the constant temperature and constant humidity (temperature of 35° C. and humidity of 95%) for one week, and whether rusting was generated was visually observed. In a case where rusting was not generated, it was determined that corrosion resistance was excellent.

(16) In the examples and the comparative examples, the mechanical properties (tensile strength and impact value), MnS appearance frequency in the inclusion, and whether the rusting was generated or not generated after the constant temperature and constant humidity test (temperature of 35° C. and humidity of 95%) for one week are shown in Tables 3 and 4.

(17) TABLE-US-00003 TABLE 3 Rusting generated or not Tensile Impact MnS generated after constant strength value appearance temperature and MPa J/cm.sup.2 frequency constant humidity test Exam- 1 1933 81.1 5% Not generated ple 2 1928 70.3 5% Not generated 3 1942 82.5 5% Not generated 4 1935 79.3 5% Not generated 5 1946 82.5 10%  Not generated 6 1937 82.4 5% Not generated 7 1927 70.3 10%  Not generated 8 1940 87.5 0% Not generated 9 1942 92.1 5% Not generated 10 1972 77.7 5% Not generated 11 1964 82.7 5% Not generated 12 1902 72.3 5% Not generated 13 1915 70.3 10%  Not generated 14 1931 87.5 5% Not generated 15 1920 81.4 5% Not generated 16 1932 83.2 10%  Not generated 17 1965 75.9 5% Not generated 18 1948 72.4 15%  Not generated

(18) TABLE-US-00004 TABLE 4 Rusting generated or not generated after Tensile Impact MnS constant temperature strength value appearance and constant humidity MPa J/cm.sup.2 frequency test Compar- 21 1992 81.1 95% Generated ative 22 1925 70.3 95% Generated Example 23 1952 75.2 65% Generated 24 1944 48.7  5% Not generated 25 1932 37.9 10% Not generated 26 1944 62.3  5% Not generated 27 1936 42.1 90% Generated 28 1966 55.5 30% Not generated 29 1932 27.1 10% Not generated 30 1924 16.3 35% Generated 31 1954 62.1 10% Not generated 32 1945 52.4 10% Not generated 33 1954 40.3  5% Not generated 34 1912 28.3  5% Not generated 35 1925 48.7  0% Not generated 36 1934 72.3 90% Generated 37 1965 52.8  5% Not generated 38 1927 72.4 30% Generated 39 1950 64.2 55% Generated

(19) All examples had the tensile strength of 1,900 MPa to 2,000 MPa and an impact value of 70.0 J/cm.sup.2 or more and thus exhibited the compatibility between the strength and the toughness at a high level. In all examples, the appearance frequency of MnS was less than 20% and rusting in the constant temperature and constant humidity test was not recognized.

(20) On the other hand, in Comparative Examples 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 37, and 39, the C content, the Si content, the Mn content, the P content, the S content, the Cr content, the Mo content, the V content, the Al content, the Ti content, the B content, and ([Ti mass %]−3.43×[N mass %])/[S mass %] were excessive or deficient, and as a result, the steel was embrittled or the structure became coarse, such that the impact value was decreased.

(21) Since, in Comparative Examples 21, 22, and 27, Ti was deficient, in Comparative Examples 23 and 39, ([Ti mass %]−3.43×[N mass %])/[S mass %] was deficient, in Comparative Example 30, S was excessive, in Comparative Example 36, N was excessive, and in Comparative Example 38, Al was deficient, corrosion resistance was decreased, and thus the rusting was recognized.

Industrial Applicability

(22) The prior austenite grain after quenching and tempering is refined, and thus the spring steel according to the present invention has excellent mechanical properties after quenching and tempering. Therefore, according to the present invention, it is possible to obtain the spring steel in which the impact value having the high strength of 1,800 MPa or more is secured, and further in which corrosion resistance is also high.