HYPOEUTECTOID BEARING STEEL

Abstract

A steel alloy for bearings contains: 0.6 to 0.9 wt. % carbon, 0.1 to 0.5 wt. % silicon, 0.1 to 1.5 wt. % manganese, 1.5 to 2.0 wt. % chromium, 0.2 to 0.6 wt. % molybdenum, 0 to 0.25 wt. % nickel, 0 to 0.3 wt. % copper, 0 to 0.2 wt. % vanadium, 0 to 0.2 wt. % cobalt, 0 to 0.2 wt. % aluminium, 0 to 0.1 wt. % niobium, 0 to 0.2 wt. % tantalum, 0 to 0.05 wt. % phosphorous, 0 to 0.03 wt. % sulphur, 0 to 0.075 wt. % tin, 0 to 0.075 wt. % antimony, 0 to 0.075 wt. % arsenic, 0 to 0.01 wt. % lead, up to 350 ppm nitrogen, up to 100 ppm oxygen, up to 50 ppm calcium, up to 50 ppm boron, up to 50 ppm titanium, the balance being iron, together with any other unavoidable impurities. Furthermore, the steel alloy contains (i) molybdenum and silicon in a weight ratio of 0.4<Mo/Si<6.0 and (ii) molybdenum and chromium in a weight ratio of 0.1<Mo/Cr<0.4.

Claims

1. A steel alloy comprising: 0.6 to 0.9 wt. % carbon, 0.1 to 0.5 wt. % silicon, 0.1 to 1.5 wt. % manganese, 1.5 to 2.0 wt. % chromium, 0.2 to 0.6 wt. % molybdenum, 0 to 0.25 wt. % nickel, 0 to 0.3 wt. % copper, 0 to 0.2 wt. % vanadium, 0 to 0.2 wt. % cobalt, 0 to 0.2 wt. % aluminium, 0 to 0.1 wt. % niobium, 0 to 0.2 wt. % tantalum, 0 to 0.05 wt. % phosphorous, 0 to 0.03 wt. % sulphur, 0 to 0.075 wt. % tin, 0 to 0.075 wt. % antimony, 0 to 0.075 wt. % arsenic, 0 to 0.01 wt. % lead, up to 350 ppm nitrogen, up to 100 ppm oxygen, up to 50 ppm calcium, up to 50 ppm boron, up to 50 ppm titanium, the balance iron, together with any other unavoidable impurities, wherein the alloy comprises molybdenum and silicon in a weight ratio of 0.4<Mo/Si<6.0 and molybdenum and chromium in a weight ratio of 0.1<Mo/Cr<0.4.

2. The steel alloy of claim 1, comprising from 0.7 to 0.8 wt. % carbon, more preferably from 0.72 to 0.78 wt. % carbon.

3. The steel alloy of claim 1, comprising from 50 to 350 ppm nitrogen.

4. The steel alloy of claim 1, comprising molybdenum and silicon in a weight ratio of 0.5<Mo/Si<3.0.

5. The steel alloy of claim 1, comprising molybdenum and chromium in a weight ratio of 0.15<Mo/Cr<0.3.

6. The steel alloy of claim 1, wherein the alloy comprises at least 100 ppm nitrogen.

7. The steel alloy of claim 1, wherein the alloy comprises vanadium and niobium in a weight ratio of 2.0<V/Nb<6.0.

8. The steel alloy of claim 7, wherein the alloy comprises vanadium and niobium in a weight ratio of 3.0<V/Nb<5.0.

9. The steel alloy of claim 1, wherein the alloy comprises vanadium and tantalum in a weight ratio of 1.0<V/Ta<12.0.

10. The steel alloy of claim 9, wherein the alloy comprises vanadium and tantalum in a weight ratio of 1.5<V/Ta<4.0.

11. The steel alloy of claim 1, comprising from 50 to 350 ppm nitrogen and up to 0.1 wt. % aluminium, wherein the weight ratio of aluminium to nitrogen in the alloy is 0.014<Al/N<0.6.

12. A bearing component made from the steel alloy of claim 1.

13. An engine component, an armour component, a gear component or a rail track component made from the steel alloy of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0077] The invention will now be described in relation to the figures annexed hereto by way of non-limiting examples.

[0078] FIG. 1: Mass fraction of M7C3 with temperature for Steel 1;

[0079] FIG. 2: Mass fraction of M7C3 and vanadium-nitrogen-rich precipitates with temperature for Steel 3;

[0080] FIG. 3: Plot showing an example of the adopted spheroidise-annealing schedule;

[0081] FIG. 4: SEM micrographs showing Steel A before (left) and after (right) spheroidise-annealing;

[0082] FIG. 5: Comparison of hardenability data for Steel A and Steel B and conventional 100CrMo7-3;

[0083] FIG. 6: Plot showing an example of how the steel alloy can be transformed into bainite; and

[0084] FIG. 7: SEM micrographs showing the bainitic microstructure obtained from Steel A austenitised at 885 C./120 min.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Examples

[0085] The invention will now be explained with reference to the following non-limiting examples.

Steel 1, Comprising in Wt. %

[0086] C: 0.75

[0087] Si: 0.2

[0088] Mn: 0.8

[0089] Mo: 0.35

[0090] Cr: 1.65

[0091] Ni: max 0.25

[0092] Cu: max 0.30

[0093] P: max 0.01

[0094] S: max 0.015

[0095] As+Sn+Sb: max 0.075

[0096] Pb: max 0.002

[0097] Al: max 0.050

[0098] Fe: Balance

[0099] Oxygen level should be less than 10 ppm, Ti level less than 30 ppm and Ca level less than 10 ppm. Nitrogen is present as a trace element (at least 50 ppm). The maximum limit for As is 0.04 wt. %. The Mo/Si ratio is 1.75. The Mo/Cr ratio is 0.21.

[0100] The thermodynamic calculations with Thermo-Calc (TCFE6) are shown in FIG. 1, which plots the mass fraction of M7C3 with temperature.

Steel 2, comprising in wt. %

[0101] C: 0.75

[0102] Si: 0.2

[0103] Mn: 0.8

[0104] Mo: 0.35

[0105] Cr: 1.65

[0106] V: 0.1

[0107] N: 0.02

[0108] Ni: max 0.25

[0109] Cu: max 0.30

[0110] P: max 0.01

[0111] S: max 0.015

[0112] As+Sn+Sb: max 0.075

[0113] Pb: max 0.002

[0114] Al: max 0.001

[0115] Fe: Balance

[0116] Oxygen level should be less than 10 ppm, Ti level less than 30 ppm and Ca level less than 10 ppm. The maximum limit for As is 0.04 wt. %. The Mo/Si ratio is 1.75. The Mo/Cr ratio is 0.21. The Al/N ratio is 0.05.

Steel 3, Comprising in Wt. %

[0117] C: 0.75

[0118] Si: 0.2

[0119] Mn: 0.8

[0120] Mo: 0.35

[0121] Cr: 1.65

[0122] V: 0.08

[0123] Nb: 0.02

[0124] N: 0.02

[0125] Ni: max 0.25

[0126] Cu: max 0.30

[0127] P: max 0.01

[0128] S: max 0.015

[0129] As+Sn+Sb: max 0.075

[0130] Pb: max 0.002

[0131] Al: max 0.001

[0132] Fe: Balance

[0133] Oxygen level should be less than 10 ppm, Ti level less than 30 ppm and Ca level less than 10 ppm. The maximum limit for As is 0.04 wt. %. The Mo/Si ratio is 1.75. The Mo/Cr ratio is 0.21. The Al/N ratio is 0.05. The V/Nb ratio is 4.

[0134] The thermodynamic calculations with Thermo-Calc (TCFE6) are shown in FIG. 2, which plots the mass fraction of M7C3 and vanadium-nitrogen-rich precipitates with temperature.

Steel 4, Comprising in Wt. %

[0135] C: 0.75

[0136] Si: 0.2

[0137] Mn: 0.8

[0138] Mo: 0.35

[0139] Cr: 1.65

[0140] V: 0.08

[0141] Ta: 0.02

[0142] N: 0.02

[0143] Ni: max 0.25

[0144] Cu: max 0.30

[0145] P: max 0.01

[0146] S: max 0.015

[0147] As+Sn+Sb: max 0.075

[0148] Pb: max 0.002

[0149] Al: max 0.001

[0150] Fe: Balance

[0151] Oxygen level should be less than 10 ppm, Ti level less than 30 ppm and Ca level less than 10 ppm. The maximum limit for As is 0.04 wt. %. The Mo/Si ratio is 1.75. The Mo/Cr ratio is 0.21. The Al/N ratio is 0.05. The V/Ta ratio is 4.

Steel 5, Comprising in Wt. %

[0152] C: 0.75

[0153] Si: 0.4

[0154] Mn: 0.8

[0155] Mo: 0.35

[0156] Cr: 1.65

[0157] Ni: max 0.25

[0158] Cu: max 0.30

[0159] P: max 0.01

[0160] S: max 0.015

[0161] As+Sn+Sb: max 0.075

[0162] Pb: max 0.002

[0163] Al: max 0.050

[0164] Fe: Balance

[0165] Oxygen level should be less than 10 ppm, Ti level less than 30 ppm and Ca level less than 10 ppm. The maximum limit for As is 0.04 wt. %. The Mo/Si ratio is 0.875. The Mo/Cr ratio is 0.21.

[0166] The following further experimental results further describe the present invention by way of example. Examples of steels (with and without vanadium) were melted. The chemical compositions are presented in Table 1 below (all in wt. %, except * in ppm). The chemical composition of a reference steel, 100CrMo7-3, is also given. Each steel type was supplied in the as hot-forged condition in the form of 1 m long, 30 mm bars. The total number of steel bars was 7 each. The steels in the as hot-forged condition exhibited fully pearlitic structures, as expected.

TABLE-US-00001 TABLE 1 Element, wt % Steel A Steel B 100CrMo7-3 C 0.73 0.73 0.97 Si 0.20 0.21 0.26 Mn 0.81 0.82 0.66 Cr 1.63 1.68 1.79 Ni <0.001 <0.001 0.11 Mo 0.36 0.36 0.26 Cu 0.006 0.005 0.206 V 0.106 0.003 0.009 P 0.007 0.006 0.006 S 0.010 0.010 0.004 Al 0.006 0.033 0.028 As 0.001 0.001 0.015 Sn 0.001 0.001 0.011 Sb <0.0015 <0.0015 0.0025 Ti* 15 15 16 B* 4 4 2 Pb* <5 <5 <5 Ca* 1 1 2 N* 118 79 50 O* 8.3 6.0 3.7

[0167] The steels were then spheroidise-annealed to facilitate easier machining and for better response to subsequent heat treatment (hardening) steps. The plot in FIG. 3 shows an example of the adopted spheroidise-annealing schedule.

[0168] The hardenability of the steels according to the present invention is an important aspect and was assessed according to the ASTM standard test specification A255-10. The assessed structures were untempered martensite.

[0169] The plots in FIG. 5 clearly demonstrate the superior hardenability of the Steel A and Steel B with ca. 0.75 wt. % carbon, compared with the reference existing bearing steel with ca. 1 wt % carbon.

[0170] The introduction of microalloying additions such as, for example, vanadium and nitrogen means that it is possible to raise the austenitisation temperature compared with the reference steel, without the risk of excessive austenite grain growth. This can result in superior hardenability at greater depths, or, for thicker bearing components.

[0171] Additionally, the more gradual decrease in hardenability demonstrated by the steel alloy compositions according to the present invention allows for more hardness uniformity across bearing component sections. This enables better predictability regarding component growth during heat treatments, which results in easier to set grinding allowances for the subsequent stages.

[0172] As well as transformation into martensite, the steels were also transformed into bainite following the schematic schedule presented in FIG. 6 and Table 2 below.

TABLE-US-00002 TABLE 2 Steel A Steel B 100CrMo7-3 T.sub.1/t.sub.1 885 C./ 885 C./ 865 C./ 50 min 885 C./120 min 50 min 50 min Bainite Identical transformation stage variables HV10 685 690 681 707

[0173] As can be seen in Table 2, the hardness of the bainitic structures obtained from the steels according to the present invention was not significantly different from that measured on the reference bearing steel. However, for improved hardness, it was found that slightly longer transformation time into bainite was necessary.

[0174] FIG. 7 presents SEM micrographs showing the bainitic microstructure obtained from Steel A, austenitised at 885 C./120 min.

[0175] The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.