Material, Method & Component

Abstract

Austempered steel for components requiring high strength and high ductility and/or fracture toughness, which has a silicon content of 3.1 weight-% to 4.4 weight-% and a carbon content of 0.4 weight-% to 0.6 weight-%. The microstructure of the austempered steel is ausferritic or superbainitic.

Claims

1. An austempered steel composition for components requiring high strength and high ductility and/or fracture toughness comprising a silicon content of 3.1 weight-% to 4.4 weight-% and a carbon content of 0.4 weight-% to 0.6 weight-%, and a microstructure that is ausferritic or superbainitic.

2. An austempered steel composition according to claim 1, obtained by a means of an austempering heat treatment including complete austenitization, whereby the higher the silicon content of the steel, the higher an austenitization temperature of the austempering heat treatment.

3. An austempered steel composition according to claim 1, comprising: TABLE-US-00005 C 0.4-0.6 weight-% Si 3.1-4.4 weight-% Mn max 4.0 weight-% Cr max 25.0 weight-% Cu max 2.0 weight-% Ni max 20.0 weight-% Al max 2.0 weight-% Mo max 6.0 weight-% V max 0.5 weight-% Nb max 0.2 weight-% wherein the remaining weight-% of the austempered steel steel composition comprises Fe and normally occurring impurities.

4. An austempered steel composition according to claim 1 comprising a microstructure that is substantially carbide-free.

5. An austempered steel composition according to claim 1 comprising a microstructure that contains less than 5 vol-% carbides.

6. Method for producing an austempered steel composition according to claim 1 comprising austempering heat treatment including complete austenitization, wherein the higher the silicon content of the steel, the higher an austenitization temperature of the austempering heat treatment, and wherein the resulting microstructure of the austempered steel composition is ausferritic or superbainitic.

7. Method for producing an austempered steel composition for components requiring high strength and high ductility and/or fracture toughness, comprising the producing the austempered steel composition from an alloy having a silicon content of 3.1 to 4.4 weight-% and a carbon content of 0.4 to 0.6 weight-%, wherein the austempered composition steel is obtained by austempering heat treatment including complete austenitization, wherein the higher the silicon content of the steel, the higher an austenitization temperature of the austempering heat treatment, and wherein that the resulting microstructure of the austempered steel composition is ausferritic or superbainitic.

8. Method according to claim 7, wherein the austempered steel composition comprises: TABLE-US-00006 C 0.4-0.6 weight-% Si 3.1-4.4 weight-% Mn max 4.0 weight-% Cr max 25.0 weight-% Cu max 2.0 weight-% Ni max 20.0 weight-% Al max 2.0 weight-% Mo max 6.0 weight-% V max 0.5 weight-% Nb max 0.2 weight-% wherein the remaining weight-% of the austempered steel steel composition comprises Fe and normally occurring impurities.

9. A method according to claim 7 comprising the steps of: a) forming a melt comprising steel with a silicon content of 3.1 to 4.4 weight-% and a carbon content of 0.4 to 0.6 weight-%; b) casting from said melt a component or a semi-finished bar; c) allowing said component or semi-finished bar to be forged before cooling or to cool directly, optionally followed by forging and subsequent cooling; d) heat treating said cooled component, semi-finished bar or forging at a first temperature and holding said component, semi-finished bar or forging at said temperature for a predetermined time to completely austenitize said component, semi-finished bar or forging, wherein the higher the silicon content of the steel, the higher the austenitization temperature; e) quenching said heat treated component, semi-finished bar or forging at a quenching rate sufficient to prevent the formation of pearlite during quenching down to an intermediate temperature below the pearlite region in the continuous cooling transformation (CCT) diagram but above the M.sub.s temperature, such as a quenching rate of at least 150° C./min; f) heat treating the component, semi-finished bar or forging at one or several temperatures above the M.sub.s temperature for a predetermined time to austemper said component, semi-finished bar or forging, resulting in an ausferritic or superbainitic steel.

10. Method according to claim 9, wherein machining is performed before, after or both before and after the heat treatment performed in steps d-f.

11. An ausferritic steel component according to the austempered steel composition of claim 1.

12. A semi-finished bar, comprising ausferritic steel according to the austempered steel composition of claim 1.

13. A forging comprising ausferritic steel according to the austempered steel composition of claim 1.

14. An ausferritic steel component manufactured according to the method of claim 7.

15. A semi-finished bar manufactured according to the method of claim 7.

16. A forging manufactured according to the method of claim 7.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended figure where;

[0041] FIG. 1 schematically shows the austempering heat treatment cycle according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0042] FIG. 1 shows an austempering heat treatment cycle according to an embodiment of the invention. A steel component, semi-finished bar or forging having a silicon content of 3.1 weight-% to 4.4 weight-% and a carbon content of 0.4 weight-% to 0.6 weight-% is heated [step (a)] and held at a sufficiently high austenitizing temperature (depending on silicon content) for a time [step (b)] until the component, semi-finished bar or forging becomes fully austenitic and saturated with carbon. The component, semi-finished bar or forging may for example be used in a suspension or powertrain-related component for use in a heavy goods vehicle, such as a spring hanger, bracket, wheel hub, brake calliper, timing gear, cam, camshaft, annular gear, clutch collar, bearing, or pulley.

[0043] According to an embodiment of the invention the method comprises the step of maintaining said austenitization temperature for a period of at least 30 minutes. According to another embodiment of the invention the austenitizing step is carried out in a nitrogen atmosphere, an argon atmosphere or any reducing atmosphere, such as a dissociated ammonia atmosphere to prevent the oxidation of carbon. The austenitizing may be accomplished using a high temperature salt bath, a furnace or a localized method such as flame or induction heating.

[0044] After the component, semi-finished bar or forging is austenitized, preferably after the component is fully austenitized, it is quenched at a high quenching rate [step (c)], such as 150° C./min or higher in a quenching medium and held at an austempering temperature above the M.sub.s temperature of the alloy [step (d)] for a predetermined time, such as 30 minutes to two hours depending on section size. The expression “a predetermined time” in this step is intended to mean a time sufficient to produce a matrix of ausferrite/superbainite in the component or at least one part thereof. The austempering step may be accomplished using a salt bath, hot oil or molten lead or tin. The complete heat treatment may be performed under Hot Isostatic Pressing (HIP) conditions in equipment capable of quenching under very high gas pressure.

[0045] The austempering treatment is preferably, but not necessarily, isothermal. A multi-step transformation temperature schedule may namely be adopted to tailor the phase fractions and their resulting carbon contents in a component's microstructure and to reduce the processing time by increasing nucleation rate of acicular ferrite at lower temperature and growth at higher temperature.

[0046] After austempering, the component, semi-finished bar or forging is cooled to room temperature [step (e)]. The steel component, semi-finished bar or forging may then be used in any application in which it is likely to be subjected to stress, strain, impact and/or wear under a normal operational cycle.

[0047] According to an embodiment of the invention the method comprises the step of machining the component, semi-finished bar or forging after it has been cast but before the austenitizing step until the desired tolerances are met. It is namely favourable to carry out as much of the necessary machining of the component, semi-finished bar or forging as possible before the austenitization and austempering steps. Alternatively or additionally, the component, semi-finished bar or forging may be machined after the austempering step, for example, if some particular surface treatment is required. A component, semi-finished bar or forging may for example be finished by machining and/or grinding to the required final dimensions and, optionally, honing, lapping or polishing can then be performed.

[0048] According to an embodiment of the invention austempered steel having the following composition in weight-% may be manufactured using a method according to an embodiment of the present invention:

TABLE-US-00003 C 0.4-0.6 Si 3.1-4.4 Mn max 4.0 Cr max 25.0 Cu max 2.0 Ni max 20.0 Al max 2.0 Mo max 6.0 V max 0.5 Nb max 0.2
balance Fe and normally occurring impurities. Phosphorous and sulphur are preferably kept to a minimum.

[0049] It will be appreciated that the austempered steel according to the present invention may contain unavoidable impurities, although, in total, these are unlikely to exceed 0.5 weight-% of the composition, preferably not more than 0.3 weight-% of the composition, and more preferably not more than 0.1 weight-% of the composition. The austempered steel alloy may consist essentially of the recited elements. It will therefore be appreciated that in addition to those elements that are mandatory, other non-specified elements may be present in the composition provided that the essential characteristics of the composition are not substantially affected by their presence.

EXAMPLE

[0050] Austempered steel having the following composition in weight-% may be manufactured using a method according to an embodiment of the present invention:

TABLE-US-00004 C 0.5 Si 3.5 Mn 0.1 Cr 1.0 Ni 2.0 Mo 0.2
balance Fe and normally occurring impurities. Phosphorous and sulphur are preferably kept to a minimum.

[0051] Such a steel may be austenitized at an austenitizing temperature of 920° C. for half an hour until the steel is fully austenitized. After quenching in a quenching medium, the steel may be austempered at 320° C. for two hours. After isothermal austempering, the component, semi-finished bar or forging may be cooled to room temperature.

[0052] An austenitizing temperature greater than 920° C. would have to be used to completely austenitize austempered steel having a silicon content greater than 3.5 weight-% silicon.

[0053] Further modifications of the invention within the scope of the claims would be apparent to a skilled person. For example, it should be noted that any feature or method step, or combination of features or method steps, described with reference to a particular embodiment of the present invention may be incorporated into any other embodiment of the present invention.