GB1564737A - Composition for low alloy steel powder and method of producing same - Google Patents

Description

(54) IMPROVED COMPOSITION FOR LOW ALLOY STEEL POWDER AND METHOD OF PRODUCING SAME (71) We, HOGANAS AB, a Company organised under the laws of Sweden, of Fack, 263 01 Hogan s, Sweden do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to compositions for use in powder metallurgy processes and concerns annealed steel powders containing relatively high percentages of elements which are more oxidisable than iron.

Alloy steel powders for use in powder metallurgy processes are well known in the art. Further, the making of steel powders through the consecutive steps of melting, atomising and annealing are also known in the art. In prior annealed alloy steel powder compositions, the main alloying elements have almost exclusively been elements less oxidisable than iron such as nickel and molybdenum. Manganese and chromium, which are common alloying elements in wrought steels due to the fact that they are of low cost and substantially raise the hardenability of the steel, have been used to very little extent as alloying elements in steel powder. The low extent of use of such elements is due to the fact that they have a high sensitivity to oxygen which is higher than that of iron. Thus, to avoid substantial amounts of oxide inclusions in the sintered or powdered forged parts, prior art compositions have generally contained less than 0.3% by weight of each of these elements and preferably less than 0.1% by weight chromium and less than 0.3% by weight manganese.

In some prior art compositions, it has been stated that manganese weight percentages as high as 0.50% may be used when the alloy contains substantial amounts of chromium, nickel and/or molybdenum.

However, it is to be noted that nickel and molybdenum are extremely expensive and therefore economically restrict the applications of the alloy steel powders so produced.

In other prior art, the manganese and chromium levels are maintained in a low percentage, but nickel and molybdenum are used to give acceptable hardenability characteristics. As has hereinbefore been described, this provides for an expensive powder which may restrict the ultimate number of uses.

When iron or iron alloy powder is used in powder forging to form high strength finished parts, it is important that substantially all of the porosity is effectively removed in the part during the manufacturing process. The removal of porosity is critical in obtaining a high mechanical strength from the material.

Impact strength vs. the percent voids in powder parts is a substantially exponential function. Thus, in one type of powder part where 10.0% voids are found, the impact strength would approximate to 10.0 ft.pounds. If the voids were reduced to 5.0% the impact strength would go up to approximately 20.0 ft-lbs. Further, if the porosity were reduced to substantially 0.0%, the impact strength would rise to approximately 160.0 ft.-lbs. The sensitivity that the impact strengths of powder forgoings show to porosity is analogous to that which they show to foreign inclusions in the material. These inclusions are typically oxides formed during the entire manufacturing process.

In prior art powder compositions for use in conventional powdered metals for the production of sintered parts, presence of oxide inclusions was not of such a critical concern owing to the fact that the presence of porosity precluded the obtaining of high impact strength. Thus, the presence of such oxide inclusions were overwhelmed by the effect of porosity. However, the oxide inclusions do have an important influence when the porosity of the material is reduced to a low level between 0.0 ' and 5.0 ".

Oxide inclusions in the finally formed mechanical part have also been found to affect the hardenability. The degree of hardenability is controlled by the alloy content of the steel. If any of the alloy content is oxidised, the hardenability of the manufactured part will be found to be reduced and the strength of the part will be decreased. Thus, a low oxide inclusion percentage increases the impact strength and hardenability of parts manufactured from alloy powders. However, as has been hereinbefore described, a number of useful alloying elements in steel are more oxidisable than iron and thus in prior cases have not been able to be used in substantial amounts to provide the increased mechanical properties necessary in forged parts.

According to one aspect of the invention there is provided an annealed alloy steel powder consisting of particles which are agglomerated and/or separate, the particles comprising the following composition, expressed as a percentage of the weight of the power:-- (a) 0.10 to 0.7n e carbon; (b) less than 2.00 " nickel: (c) less than 1.0 ó molybdenum; (d) 0.04 to 1.82 " in total of an element or elements which is or are more oxidisable than iron; (e) less than 0.10% silicon; (f) less than 5000 ppm by weight of oxygen: and (g) the balance being iron.

According to a second aspect of the invention there is provided an annealed alloy steel powder consisting of atomised alloy steel particles which particles subsequent to annealing consist of by weight from: (a) 0.l00,/o to 0.70% carbon; (b) less than 2.0 " nickel; (c) less than 1.0 X" molybdenum; (d) 0.20% to 1.75 ," manganese; (e) 0.04 Ó to 1.51 fn chromium; (f) less than 0.10 " silicon; (g) less than 5000 ppm by weight of oxygen; and (h) the balance being iron.

According to a third aspect of the invention there is provided a method of producing an alloy steel powder comprising the steps of: (a) providing an alloy steel composition comprising by weight from: ( I ) greater than 0.10 " carbon; (2) 0.04 ,/, to 1.8 ," of at least one element which is more oxidizable than iron; (3) 0.01? to 0.10 silicon; and, (4) the balance being iron: (b) melting the steel alloy composition to form a steel alloy melt; (c) atomizing the steel alloy melt to form an atomized alloy steel powder; and, (d) annealing the atomized steel powder to a carbon content within the range of 0.100 to 0.70 ", carbon and less than 5000 ppm oxygen by weight of the annealed powder.

The invention makes it possible to provide an annealed alloy steel powder for use in powder metallurgy processes requiring increased hardenability, consisting of atomized alloy steel particles.

The particles subsequent to annealing consist of, by weight: (a) 0.10 " to 0.700;, carbon; (b) less than 2.0An nickel: (c) less than 1.0 O molybdenum: (d) 0.20? to 1.75"" manganese; (e) 0.049, to l.5l?o chromium; (f) less than 0.10 Ó silicon; (g) less than 5000 ppm by weight of oxygen; and, (h) the balance being iron.

One particular preferred embodiment of the invention will now be described.

As hereinafter described, the description is directed to an improved annealed alloy steel powder and to a method producing it for use in a multiplicity of power metallurgy processes. A particular area of use to which the improved composition of the subject alloy steel powders may be applied advantageously is in the area of the manufacturing of sintered and/or powder forged components. In the formation of such components in both kinds of manufacture, the steps of cold compaction of the powder and sintering of the compacted powder at an elevated temperature are performed.

In the case of sintering, the powder is cold compacted into a predetermined shape or contour which is generally similar in contour dimensions to the desired contour of the end product of the sintering step which follows. Generally, the sintering step is followed by the component being hot compacted. In general, a sintered part or component contains a large amount of voids, in contrast to a powder forged component which generally contains few if any voids. As an example, in the case of powder forged parts, the voids may typically be less than 0.5% by volume.

Thus, the invention is directed to an improved annealed alloy steel powder for use in powder metallurgy and preferably in powder forging processes. As will be described in the following, an annealed alloy steel powder embodying the invention is able to include a higher content of manganese and chromium, as well as other elements which are more oxidisable than iron, when compared with other alloy steel powders which have been used in the prior art.

Basically, the method of making such an alloy steel powder is to provide an annealed alloy powder having an increased carbon content sufficient to allow the use of elements more oxidisable than iron. For use in powder forging processes, the carbon content in the annealed alloy powder should be at or above 0.lQ by weight of the annealed alloy steel powder. In order to provide an improved powder forged component the carbon content is preferably in the range of 0.21 ó to 0.60% although a range of anywhere between greater than 0.10 ' and less than 0.70% has been successfully used in production of acceptable powder forged components.

The annealed alloy steel powder is derived from an alloy steel which may be produced through standard processes such as basic oxygen, electric furnace, open hearth, and the like standard methods. The initial steel alloy contains approximately the same percent by weight of carbon as is found in the unannealed alloy steel powder, however, such weight percentage of carbon found in the steel powder is only important in that the annealed alloy steel powder has a carbon content between 0.10% and 0.70% of the weight of the annealed alloy steel powder with a preferred range of between 0.21 to 0.60% of the weight of the annealed steel powder.

As will be hereinafter explained, the extended and unexpectedly high percentage of carbon within the alloy powder correspondingly permits the annealed steel powder to contain a higher percentage of manganese, chromium and other elements which oxidise more readily than iron. In addition to the extremely high content of carbon used in such steel powders, the annealed alloy steel powder may further contain one or more of the following elements, the percentages being by weight of the annealed alloy steel powder: (a) less than 2.00/, nickel; (b) less than 1.0% molybdenum; (c) 0.20% to 1.73% manganese with a preferred range between 0.55% to 1 75 /ó; (d) 0.04% to 1.51% chromium with a preferred range between 0.04% to 0.8 /n; and (e) less than 0.10% silicon.

Still further, traces of phosphorus may be found in such powders up to 0.005% by weight of the annealed alloy steel powder.

Additionally, traces of sulphur up to 0.020% by weight as well as copper having a trace amount up to 0.05% as well as titanium and aluminium each having a percentage weight of 0.01% may be incorporated in the alloy steel powder. Although not critical the sulphur content will generally be less than 0.04% by weight of the annealed steel powder and the phosphorus content is usually maintained less than 0.035 2n by weight of the annealed alloy steel powder.

Initially, the alloy steel is brought to a molten state in a tundish at a temperature approximating 17000C. (31000 F). The molten steel is then allowed to flow from the tundish under gravity through slots or other openings formed therein. Such apparatus are well known in the art. The molten steel stream is atomised by impinging jets of water on it. Through this atomisation step which is well known in the art, the steel is formed into irregularly shaped particles.

Further, although this too is not critical, atomisation in general is controlled in a manner so as to allow a majority of the atomised particles to pass through an 80 mesh sieve.

Subsequent to the atomisation of the alloy steel, the resulting alloy steel powder is subjected to the critical step of annealing. In this step the carbon content is decreased, but most importantly maintained at the conclusion of the annealing step in excess of 0.10 ,/, by weight of the annealed alloy steel powder. The final carbon content is within the range between 0.10% and 0.70% with a preferred range within the limits 0.21 to 0.60 /n of the annealed alloy steel powder.

This important maintenance of the carbon content will be further described in the following.

Further, the annealing step serves to soften the particles and reduce the oxide film formed on the particles during the atomisation step. In general, the oxygen content of the annealed alloy steel powder is found to be below 5000 parts per million (ppm). During the annealing step the powder is heated to a temperature in the range of 750"C (1382"F) to 1200"C (2192"F). The annealing atmosphere is a reducing atmosphere such as hydrogen, dissociated ammonia, or some other well known reducing gas or gases.

The annealing step as herein described is critical in the production of atomised alloy steel powders containing relatively high percentages of manganese and/or chromium.

During the annealing step, the carbon content of the steel particles is maintained in excess of 0.10%. In doing so the oxygen potential of the atmosphere is kept low, which leads to a lesser degree of oxidation of the sensitive alloying elements, e.g.

manganese and chromium. Moreover, the presence of more than 0.10% carbon in alloyed form in. the steel particles ensures a more efficient reduction of harmful oxides during the sintering treatment subsequent to powder compaction as will be shown in the examples. In consequence, it is possible to produce alloy steel powder with higher effective manganese and/or chromium contents than heretofore and use them in the manufacture of sintered or forged components with desired high hardenability and high mechanical properties.

The particles are generally found to be caked together after annealing. They may be broken apart by hammer milling or some like technique well known in the art. Once this is done the powder is then ready for use in the production of sintered or powder forged parts.

In prior art alloy steel powders made in the conventional way a very low carbon content was desired and provided. Such prior annealing steps in the powder production processes has reduced the carbon content of the powder as much as is reasonably possible. In most of these prior cases, the carbon contents have been brought to below 0.02% by weight of the powder and generally into a preferred range below 0.01% by weight of the annealed alloy steel powder.

In order to provide a desired carbon content of a finished part, graphite has generally been mixed with the powder and is alloyed with the compacted powder during a sintering step. The temperature of the sintering step is in an approximate range of between 1090 C and 1315"C with the sintering temperature most common being approximately 1120"C. A preform is usually sintered for a time between 5.0 and 90.0 minutes. When powder forging parts are being made, the sintered member may be cooled down to room temperature and then heated to the forging temperature. The forging temperatures are generally in an approximate range of 900"C to 11000C.

However, in some cases the sintered part is taken directly from the sintering furnace to the forging tool where it is forged to its final shape. As has hereinbefore been described, the amount of oxide inclusions in the forged product is of critical importance for the mechanical properties, such as impact strength and for the hardenability properties of the final component. -The oxygen content of the powder forged component is thus considered as a sensitive indicator of the relative quality of the forged component.

An improved annealed alloy steel powder embodying the invention and to be used in powder metallurgy processes requiring increased hardenability consists of atomised alloy steel particles. The composition of these particles subsequent to annealing consist of by weight from: (a) 0.10 ,; to 0.70% carbon; (b) less than 2.0% nickel; (c) less than 1.0% molybdenum; (d) 0.20% to 1.75% manganese; (e) 0.04% to 1.51% chromium; (f) less than 0.10% silicon; (g) less than 5000 ppm by weight of oxygen; and, (h) the balance being iron.

As has been found when using the alloy steel powder in accordance with the invention as to composition and the process steps, as opposed to the proposed low carbon powder of the prior art, an unexpectedly large reduction of the oxygen content of the powder forged parts has been found to be achieved. This has been achieved by maintaining carbon content of the annealed alloy steel powder in an amount greater than 0.1% by weight of the annealed powder. It will be understood that when the combined carbon content of the forged part should necessarily exceed the carbon content of the powder, graphite may be admixed in the conventional manner as has been described.

Forgings of alloy steel powder with differing chromium and manganese contents within the ranges previously described have been analysed for oxygen.

The oxygen content of the forgings was empirically found to be lower than the oxygen content given by the equation: Contgent (ppm)=l 11 00(WMn+WCr)+200 Where: WMn=manganese weight percentage in the annealed steel alloy powder Wcrchromium weight percentage in the annealed alloy steel powder.

From experimental results, it has been found that oxygen contents of forged parts should not generally exceed 2200 ppm.

Where the oxygen content has been found to be greater than 2200 ppm, the mechanical properties of the parts have been found to decrease to -a great extent. Thus, a maximum of the sum of chromium and manganese contents can be calculated from the equation with a combined summed value of 1.82% by weight of the annealed steel powder. Further, it has been found that carbon contents of the powder higher than 0.6% by weight do not apparently decrease the oxygen content of the powder forged parts further. Thus, the preferred carbon content of the annealed steel powder according to the empirically derived equations and experimental results have been found to lie between 0.1% and 0.6% by weight.

In the following examples, a steel alloy composition was used as the molten steel alloy supplied to the tundish. The steel alloy composition differs from standard steel alloy compositions in that for such powder metallurgy powders, a high content of carbon is utilised as well as a high content of elements which oxidise more readily than iron are provided. The percentage refers to weight percent. The steel alloy composition is as follows: (a) carbon: a sufficiently high content to yield a final carbon content after annealing of greater than 0. 0.10 ', and usually between 0.21 to 0.60%; (b) phosphorus 0.005 /"; (c) sulphur: 0.020%; (d) silicon, 0.01%; (e) copper; 005 /n; (f) vanadium, less than 001 /n; (g) titanium, less than 0.01An: (h) aluminium, less than 0.01An: (i) molybdenum 0.4% unless changed in a particular example; (j) nickel, 0.3% unless changed in a particular example; (k) manganese, as stated in each example; (1) chromium, as stated in each example; (m) iron, the balance.

It is to be noted that the examples as shown and described in the following paragraphs are for the purpose of illustrating the effect of the high carbon content on the oxygen content of the final powder forged parts. Thus, for illustration purposes, manganese and chromium are shown in amounts far in excess of what has been proposed for such annealed steel powders in prior art. Certain other elements more oxidisable than iron, in other words those which have a greater tendency to form oxides than does iron, may give analogous results. Moreover it is not critical that the powder forgings are used for the oxygen analysis. The same effect would equally have been seen if the analysis had been performed on conventionally sintered parts.

EXAMPLE I Molten steel was supplied to a tundish and included the following alloy element weight percentages: Manganese 1.64% Chromium 0.04% The temperature of the molten alloy steel was maintained at approximately 16500C (30000 F). The molten steel was passed through openings in a tundish. Atomisation was effected by jets of water impinging on the molten steel stream passing downwards under gravity, which resulted in the formation of an alloy steel powder which was then annealed in a dissociated ammonia atmosphere at a temperature of approximately 9000C (1650 F) for 120 minutes (2 hours).

The powder was then annealed to a carbon content of 0.59% by weight of the powder. The oxygen level of the annealed powder was found to be 4000 ppm. In the same example, the alloy steel powder was annealed to 0.02% carbon and the powder was found to have an oxygen content of 5000 ppm.

The low carbon powder (0.02% carbon) was mixed with 0.6% graphite so that forged parts made from the two powders would provide the same carbon content. The powders were then compacted to preforms.

The preforms were sintered at 1100"C (20100 F) for 15 minutes in a dry atmosphere of dissociated ammonia and then hot forged to full density. The forged parts were analysed for oxygen and the content was found to be 1900 ppm for the part made from the high carbon powder (0.59 " carbon) and 2500 ppm for the part made from the low carbon powder (.02or carbon).

EXAMPLE II Molten steel alloy supplied to the tundish included the following alloy elements weight percentages: Manganese 0.02 ," Chromium 0.42 ,/n The temperature of the molten steel was approximately 17000C (31000F). The molten steel was then passed through passages in the tundish. Atomisation was affected by jets of water impinging on the falling molten steel. Although not important to the invention per se the initial water temperature was found to be approximately 21"C (700 F) and the final water recovered after striking the molten steel was found to be approximately 630C (145 F).

The resulting alloy steel powder was then annealed in a dissociated ammonia atmosphere at a temperature of approximately 1000"C (1832"F) for approximately 105 minutes. The powder was annealed to a carbon content of 0.14% by weight of the annealed alloy steel powder. The dew point was approximately 15"C. The oxygen level of the annealed powder was found to be 1400 ppm.

When an identical powder composition was annealed to a carbon content of 0.0159; the powder was found to have an oxygen level of 2400 ppm.

The powders were forged in a manner similar to the forging process of Example I and the oxygen content was analysed to be 650 ppm for the member made from the high carbon powder (0.14%). The oxygen content was analysed to be 1100 ppm for the part made from the low carbon powder (0.015% carbon).

In order to provide the same combined carbon content of the forgings, graphite was admixed to the annealed powder. The addition of the graphite was 0:46% graphite for the high carbon powder and 0.59"i, graphite for the low carbon powder.

EXAMPLE III Molten steel alloy as previously described as supplied to the tundish and included the following alloy element weight percentages: Manganese 0.75% Molybdenum 0.25% Chromium 0.05% The temperature of the molten alloy steel was found to be approximately 16500C (30000 F). After atomisation, as has been previously described, the powder was annealed to a 0.1 " carbon content by weight of the powder. The annealing atmosphere was once again dissociated ammonia and the annealing temperature was approximately 9000 C. The powder was found to contain 2500 ppm of oxygen and the forgings made from the powder contained 1000 ppm of oxygen. It will be noted that graphite was added to the powder in an amount sufficient to render the forged parts a combined carbon content of 0.21F6.

EXAMPLE IV Molten steel alloy supplied to the tundish included the following alloy element weight percentages: Manganese 1.75% Chromium 0.05 /" The temperature of the molten alloy steel was approximately 17000C (3100 F). After atomisation, as previously described, the powder was annealed to 0.1% carbon content. The annealing atmosphere was hydrogen and the annealing temperature was approximately 9250C (1697 F). The dewpoint temperature was measured at 10 C. The powder was found to have an acceptable oxygen level of 4000 parts per million. Graphite was added as in Example Ill and the forgings fabricated out of this powder had an oxygen content of 2000 ppm.

EXAMPLE V Molten steel alloy supplied to the tundish included the following alloy element weight percentages: Manganese 0.75% Chromium 0.8% Molybdenum 0.25% Nickel 1.8% The temperature of the molten steel was approximately 16800C (3050 F). After atomisation, the powder was annealed to 0.1% carbon content. The annealing atmosphere was hydrogen and the annealing temperature was approximately 9000 C (1652"F) with a dewpoint of 12"C. The powder was found to have an acceptable oxygen level of 3500 parts per million. The oxygen content of the powder forgings made of this powder in the way described in the first Example was found to be 2000 ppm.

The addition of 0.50% graphite to the annealed powder gave the final combined carbon content of 0.49% in the forged part.

The previous Examples as herein described are illustrative in showing that by annealing an alloy steel powder to a carbon content of 0.109/, or in excess thereof, higher quantities of elements more oxidisable than iron can be used for the composition of an annealed alloy steel powder. It will be understood in particular, that the invention provides a method where the alloy steel powder containing a high percentage by weight of manganese and chromium also has a carbon content in excess of 010An by weight of the annealed steel powder. Still further, the invention provides a method and composition of an improved alloy steel powder where the annealed steel powder has a carbon content at a predetermined level within the approximating range of 0.10% to 0.70% and preferably between 0.21 , to 0.60 ,' and where the carbon is homogeneously distributed throughout the steel powder particles.

WHAT WE CLAIM IS: 1. An annealed alloy steel powder consisting of particles which are agglomerated and/or separate, the particles comprising the following composition, expressed as a percentage of the weight of the powder: (a) 0.10 to 0.7 /O carbon; (b) less than 2.00% nickel; (c) less than 1.0% molybdenum; (d) 0.04 to 1.82% in total of an element or elements which is or are more oxidisable than iron; (e) less than 0.10% silicon; (f) less than 5000 ppm by weight of oxygen; and (g) the balance being iron.

2. An annealed alloy steel powder consisting of atomised alloy steel particles which particles subsequent to annealing consist of by weight from: (a) 0.10% to 0.70 carbon; (b) less than 2.0% nickel; (c) le

Claims (26)

**WARNING** start of CLMS field may overlap end of DESC **. The temperature of the molten alloy steel was found to be approximately 16500C (30000 F). After atomisation, as has been previously described, the powder was annealed to a 0.1 " carbon content by weight of the powder. The annealing atmosphere was once again dissociated ammonia and the annealing temperature was approximately 9000 C. The powder was found to contain 2500 ppm of oxygen and the forgings made from the powder contained 1000 ppm of oxygen. It will be noted that graphite was added to the powder in an amount sufficient to render the forged parts a combined carbon content of 0.21F6. EXAMPLE IV Molten steel alloy supplied to the tundish included the following alloy element weight percentages: Manganese 1.75% Chromium 0.05 /" The temperature of the molten alloy steel was approximately 17000C (3100 F). After atomisation, as previously described, the powder was annealed to 0.1% carbon content. The annealing atmosphere was hydrogen and the annealing temperature was approximately 9250C (1697 F). The dewpoint temperature was measured at 10 C. The powder was found to have an acceptable oxygen level of 4000 parts per million. Graphite was added as in Example Ill and the forgings fabricated out of this powder had an oxygen content of 2000 ppm. EXAMPLE V Molten steel alloy supplied to the tundish included the following alloy element weight percentages: Manganese 0.75% Chromium 0.8% Molybdenum 0.25% Nickel 1.8% The temperature of the molten steel was approximately 16800C (3050 F). After atomisation, the powder was annealed to 0.1% carbon content. The annealing atmosphere was hydrogen and the annealing temperature was approximately 9000 C (1652"F) with a dewpoint of 12"C. The powder was found to have an acceptable oxygen level of 3500 parts per million. The oxygen content of the powder forgings made of this powder in the way described in the first Example was found to be 2000 ppm. The addition of 0.50% graphite to the annealed powder gave the final combined carbon content of 0.49% in the forged part. The previous Examples as herein described are illustrative in showing that by annealing an alloy steel powder to a carbon content of 0.109/, or in excess thereof, higher quantities of elements more oxidisable than iron can be used for the composition of an annealed alloy steel powder. It will be understood in particular, that the invention provides a method where the alloy steel powder containing a high percentage by weight of manganese and chromium also has a carbon content in excess of 010An by weight of the annealed steel powder. Still further, the invention provides a method and composition of an improved alloy steel powder where the annealed steel powder has a carbon content at a predetermined level within the approximating range of 0.10% to 0.70% and preferably between 0.21 ,ó to 0.60 ,' and where the carbon is homogeneously distributed throughout the steel powder particles. WHAT WE CLAIM IS:

1. An annealed alloy steel powder consisting of particles which are agglomerated and/or separate, the particles comprising the following composition, expressed as a percentage of the weight of the powder: (a) 0.10 to 0.7 /O carbon; (b) less than 2.00% nickel; (c) less than 1.0% molybdenum; (d) 0.04 to 1.82% in total of an element or elements which is or are more oxidisable than iron; (e) less than 0.10% silicon; (f) less than 5000 ppm by weight of oxygen; and (g) the balance being iron.

2. An annealed alloy steel powder consisting of atomised alloy steel particles which particles subsequent to annealing consist of by weight from: (a) 0.10% to 0.70 carbon; (b) less than 2.0% nickel; (c) less than 1.0% molybdenum; (d) 0.20% to 1.75% magnanese; (e) 0.04% to 1.51% chromium; (f) less than 0.10 Y, silicon; (g) less than 5000 ppntby weight of oxygen; and (h) the balance being iron.

3. An annealed alloy steel powder according to Claim 2, in which the carbon content is within the range of 0.21% to 0.60% by weight of the steel powder.

4. An annealed alloy steel powder according to Claim 2 or Claim 3, in which the chromium content is within the range of 0.04 ,' to 0.8% by weight of the steel powder.

5. An annealed alloy steel powder according to any one of Claims 2 to 4, in which the manganese content is within the range of 0.55 /O to 1.75% by weight of the steel powder.

6. An annealed alloy steel powder according to any of Claims 2 to 5, in which the nickel content is 1.8% by weight of the steel powder.

7. An annealed alloy steel powder according to any one of Claims 2 to 6, in which the molybdenum content is 0.25% by weight of the steel powder.

8. An annealed alloy steel powder

according to any one of the preceding Claims, in which the maximum oxygen content is 4000 ppm of the steel powder.

9. An annealed alloy steel powder according to any one of the preceding Claims, which includes up to 0.005% of phosphorus by weight of the steel powder.

10. An annealed alloy steel powder according to any one of the preceding Claims, including up to 0.0200/, of sulphur by weight of the steel powder.

I I. An annealed alloy steel powder according to any one of the preceding Claims, including up to 0.05% of copper by weight of the steel powder.

12. An annealed alloy steel powder according to any one of the preceding Claims, including vanadium in an amount which is less than 0.01% by weight of the steel powder.

13. An annealed alloy steel powder according to any one of the preceding Claims, including titanium in an amount which is less than 0.01% by weight of the steel powder.

14. An annealed alloy steel powder according to any one of the preceding Claims, including aluminium in an amount which is less than 0.01% by weight of the steel powder.

15. An annealed alloy steel powder according to Claims 1 or 2, substantially as ereinbefore described.

16. An annealed alloy steel powder according to Claims I or 2, substantially as hereinbefore described with reference to any one of the Examples.

17. An article made in a powder metallurgy process using an annealed alloy steel powder according to any one of the preceding Claims.

18. A method of producing an alloy steel powder comprising the steps of: (a) providing an alloy steel composition comprising by weight from: (1) greater than 0.10 /0 carbon; (2) 0.04 /O to 1.8% of at least one element which is more oxidisable than iron; (3) 0.01% to 0.10% silicon; and, (4) the balance being iron; (b) melting the steel alloy composition to form a steel alloy melt: (c) atomising the steel alloy melt to form an atomised alloy steel powder; and (d) annealing the atomised steel powder to a carbon content within the range of 0.10 to 0.70% carbon and less than 5000 ppm oxygen by weight of the annealed powder.

19. A method according to Claim 18, in which the at least one element which is more oxidisable than iron is manganese or chromium.

20. A method of producing alloy steel powder according to Claim 19, in which annealing takes place in an annealing furnace and includes the step of maintaining the powder at a predetermined temperature within the range of 750"C to 1200"C within the furnace.

21. A method of producing alloy steel powder according to Claim 20, in which the annealing step includes the establishment of a reducing atmosphere within the furnace.

22. A method of producing alloy steel powder according to Claim 21, in which the reducing atmosphere is hydrogen.

23. A method of producing alloy steel powder according to Claim 21, in which the reducing atmosphere is dissociated ammonia.

24. A method of producing alloy steel powder according to Claim 18, substantially as hereinbefore described.

25. A method of producing alloy steel powder according to claim 18, substantially as hereinbefore described with reference to any one of the Examples.

26. An alloy steel powder whenever produced by a method according to any one of Claims 18 to 25.