GB2168077A

GB2168077A - Improvements in or relating to stainless steels

Info

Publication number: GB2168077A
Application number: GB08430962A
Authority: GB
Inventors: Ursula Ruth Lenel
Original assignee: Fulmer Research Institute Ltd
Current assignee: Fulmer Research Institute Ltd
Priority date: 1984-12-07
Filing date: 1984-12-07
Publication date: 1986-06-11
Also published as: AU5057385A; ZA859030B; EP0187935A1; JPS61179854A

Description

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GB 2 168 077 A

1

SPECIFICATION

Improvements in or relating to steels

5 This invention relates to steels, and in particular, to corrosion resistant steels, known generally as "stainless" steels.

Corrosion resistance in steels is achieved by providing chromium as a major constituent of the steel. Whilst many corrosion resistant steels are available, the commonly used steels are classified by their crystal structure as austenitic, which has a face centered cubic structure, martensitic which has a body 10 centered cubic structure or a body centered tetragonal structure (both known as a martensite), and fern-tic, which again has a body centered cubic structure.

Another common constituent of austenitic stainless steels is nickel. However, nickel is a relatively expensive material and, thus, the use of nickel inevitably increases the cost of the resultant steel.

The present invention is concerned particularly with austenitic and a martensitic steels. It is well known 15 that the same steel can present either an austenitic or a martensitic microstructure, depending upon the ambient temperature. For example, a typical austenitic steel can be transformed to a martensitic steel by decreasing the ambient temperature by an appropriate amount.

The transformation from austenite to martensite occurs over a temperature range the upper temperature of which is referred to as the Ms and the lower temperature of which is referred to as the Mf. Under 20 certain circumstances the martensite transformation may be induced by deformation of the material at temperatures above the Ms. The upper temperature at which transformation to martensite may be induced by deformation is known as the Md. The Md temperature is also dependent upon the amount of deformation applied, the Md temperature being higher for heavier deformations and lower for lesser deformations. All of these temperatures also depend upon the composition of the steel.

25 For instance, an austentic steel can be considered to be a steel in which the temperature at which the transformation from austenite to martensite starts (the Ms temperature) is below room temperature. Where the Ms temperature is above room temperature the steel will be martensitic, at least in part.

Austenitic steels (such as AISI 304 and 316) have the advantage of high ductility and toughness but are not generally very hard and not very wear-resistant whilst being relatively expensive. Austenitic steels 30 do, however, have the advantage of being non-magnetic. The properties of martensitic steels constrast markedly to those of austenitic steels. In the quenched state, martensitic steels are strong and hard, the strength and hardness increasing with increasing content of the interstitial alloying elements, such as carbon or nitrogen. In this state, martensitic steels lack ductility and toughness. Whilst ductility and toughness can be improved by tempering this leads to a reduction of the strength and hardness of the 35 steel. A significant advantage of martensitic steels is that the high hardness generally results in a high wear resistance. Martensitic steels are magnetic.

In the past, it has usually been desired practice to select the composition of a steel to ensure that the steel, whether austenitic or martensitic, is very stable, i.e. is unlikely to transform, particularly where the magnetic properties of the steel are important such as in the electronics industry.

40 It has recently been found that the addition of nitrogen to austenitic steels acts markedly to increase the strength of the austenitic steel. The addition of nitrogen to austenitic steels is known to decrease the Ms. In austenitic steels proposed to date which include added nitrogen the Ms has been such that the steel is stable at room temperature (20°C), i.e. the Md is below room temperature.

Nickel is generally added to stainless steels to provide an austenitic microstructure and imports an im-45 proved corrosion resistance. It is also known to use manganese to render a stainless steel austenitic but manganese does not have the same beneficial effects on corrosion resistance as previously achieved using nickel. Thus, whilst specialised manganese steels are known, manganese has not generally been considered to be a suitable element to replace the more costly nickel in a corrosion resistant steel for use in aggressive environments.

50 It is an object of the present invention to provide a steel that has significant wear resistance but is ductile and tough and which has corrosion resistant properties.

Accordingly, the present invention provides a steel comprising selected amounts of chromium, manganese and nitrogen and/or carbon, the amounts of chromium, manganese, and nitrogen and/or carbon being selected such that the resultant steel is austenitic at room tempeature but transforms significantly 55 to martensite upon deformation.

In a second aspect, the invention provides a steel comprising selected amounts of chromium, manganese and nitrogen and/or carbon, the amounts of chromium, manganese and nitrogen and/or carbon being, elected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon deformation, the amount of nitrogen and/or carbon being selected to provide the re-60 sultant steel with significant strength.

In a further aspect, the invention provides a steel comprising selected amounts of chromium, manganese and at least 0.05wt.% nitrogen and/or carbon, the amounts of chromium, manganese and nitrogen and/or carbon being selected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite steel upon deformation.

65 According to a fourth aspect, the present invention provides a steel comprising selected amounts of

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chromium, manganese and nitrogen, the amounts of chromium, manganese and nitrogen being selected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon deformation.

According to a fifth aspect, the present invention provides a steel comprising selected amounts of 5 chromium, manganese and nitrogen, the amounts of chromium, manganese and nitrogen being selected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon deformation, the amount of nitrogen being selected to provide the resultant steel with significant strength.

According to a sixth aspect, the invention provides a steel comprising selected amounts of chromium 10 and manganese and at least 0.05wt.% nitrogen, the amounts chromium, manganese and nitrogen being selected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon deformation.

According to a still further aspect, the invention provides a steel comprising selected amounts of chromium, manganese and carbon, the amounts of chromium, manganese and carbon being selected such 15 that the resultant steel is austentic at room temperature but transforms significantly to martensite upon deformation.

According to a still further aspect, the invention provides a steel comprising selected amounts of chromium, manganese and carbon, the amounts of chromium, manganese and carbon being selected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon 20 deformation and wherein the amount of carbon is selected to provide the resultant steel with significant strength.

According to yet another aspect, the invention provides a steel comprising selected amounts of chromium, manganese and at least 0.05wt.% carbon, the amounts of chromium, manganese and carbon being selected such that the resultant steel is austenitic at room temperature but transforms significantly 25 to martensite upon deformation.

Whilst it is desirable to use manganese in place of the more typical, but expensive, nickel, steels of the invention may also contain nickel. It will also be appreciated that as steels of the invention will generally be made in standard steel works, they will include typical steel working impurities.

Desirably, the amount of chromium present is selected to be in the range of from 9 to 15 wt.%. 30 More desirably, the amount of chromium present is selected to be in the range of from 9 to 13 wt.%.

Preferably the amounts of chromium present is selected to be in the range of from 11 to 13 wt.%.

Desirably the amount of manganese present is in the range of from 7 to 13 wt.%.

More desirably the amount of manganese present is in the range of from 8 to 11 wt.%.

Preferably the amount of manganese present is in the range of from 9 to 11 wt.%.

35 Desirably the amount of nitrogen and/or carbon present is in the range of from 0.05 to 0.35 wt.%.

More desirably the amount of nitrogen and/or carbon present is in the range of from 0.15 to 0.35 wt.%.

Preferably the amount of nitrogen and/or carbon present is in the range of from 0.15 to 0.25 wt.%.

Considering the properties of steels of the invention, the nitrogen (and/or carbon) present provides the steel with significant strength, chromium provides the desired corrosion resistance properties and the 40 combined effect of manganese and nitrogen (and/or carbon) promotes the formation of austenite and reduces the Ms to a temperature well below 20°C.

Preferably the Ms is less than -50°C. More preferably the Ms is below -100°C. Desirably the Ms is below -190°C. In any case, the Md of the steel is desirably controlled to be close to but above 20°C.

The use of manganese and nitrogen (or manganese and carbon or manganese and carbon plus nitro-45 gen) to promote the formation of austenite provides the advantage of being much less costly than using nickel. An additional advantage of the use of manganese over the use of nickel is that manganese has the effect of reducing the stacking fault energy of the austenite which acts to increase the work hardening rate of the steel.

A still further advantage of the use of manganese is that it increases the solubility of the steel for nitro-50 gen which could provide substantial benefits during manufacture of the steel.

Atypical steel of the invention has been found to provide very good wear resistance.

It is known in a traditional austenitic wear resistant steel to use manganese to confer a low stacking fault energy, for example Hadfield's 14% manganese steel. The presence of manganese results in a lowering of the stacking fault energy which leads to heavy faulting of the austenite under deformation and 55 thus provides a high capacity for work hardening with corresponding high wear resistance. A steel of the invention provides these advantageous features but, in addition, provides further work hardening due to the deformation induced transformation to a martensite thus leading to even higher wear resistance.

Another significant advantage is that a typical nitrogen containing steel of the invention is readily weldable, which contrasts markedly with traditional wear resistant steels which are not readily weldable. 60 in order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described, by way of example, with reference to the accompanying figure, which is a schematic stress/strain graph, and by reference to the accompanying examples.

Considering firstly, in general terms, the transformation of austenitic steels to martensitic steels, it is well known that appropriate selection of the composition of a steel can provide marked changes in the 65 Ms temperature. In the present invention, the composition of the steel is so chosen that the Ms tempera5

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ture lies below room temperature and thus, in normal (unstressed) circumstances the steel is austenitic and provides many of the advantages of known nitrogen enhanced austentic steels. However, in addition to transformation by changes in temperature, transformation from an austenite to a martensite can also be achieved by deformation of the steel. Again, the amount of deformation required to bring about the 5 onset of transformation to martensite depends upon the precise composition of the particular steel, the 5 composition affecting the Md. Thus, in steels of the present invention, the composition is selected such that, at room temperature, the steel is austenitic but, again at room temperature, the application of deformation causes the onset of transformation. Such steels are termed "unstable" steels. This selection of composition to provide an unstable steel is in marked distinction to presently available nitrogen contain-10 ing austenitic steels which, even upon deformation, remain austenitic and may therefore be termed "sta- 10 ble". As used herein, the term "an unstable austenitic steel" is intended to refer to an austenitic steel which can be transformed from an austenitic microstructure to a martensitic microstructure upon deformation at room temperature.

The benefits of providing a steel composition of the present invention can readily be appreciated by a 15 study of the properties of such steels. 15

One of the most significant advantages of the present invention is the provision of a steel with a very high capacity for work hardening. This can be best understood by referring to the schematic stress/strain graph of Figure 1. Work hardening can be considered as the hardening of a steel that is achieved during working thereof. As is readily apparent from Figure 1, and as will be explained further, martensitic steels 20 are generally hard but are generally not ductile and have only a limited capacity for work hardening. In 20 contrast, austenitic steels have a very reasonable capacity for work hardening but cannot achieve a degree of hardness approaching that of a martensitic steel. It has been found that the wear resistance of a steel is greatly enhanced if that steel has a large capacity for work hardening, and achieves a high surface hardness during wear.

25 Referring again to Figure 1, line 1 is a plot for a typical austenitic steel (A.I.S.I.304), line 2 is a plot for a 25 typical nitrogen enhanced austenitic steel, (304N), line 3 is a plot for Hadfield's manganese steel, line 4 is a plot for a typical martensitic steel (A.I.S.I. 410) and line 5 is a plot for a chromium-manganese-nitrogen steel of the present invention.

As will be appreciated from Figure 1, a typical martensitic steel (lightly tempered) has little ductility or 30 work hardening but is very hard and strong, the hardness and strength largely being due to the presence 30 of carbon. In contrast the typical austenitic steel (A.I.S.I.304) has considerable ductility and work hardening but the hardness achieved, even after full work hardening, does not approach that of a martensitic steel. The work hardening of the illustrated austenitic steel may be partly due to transformation to martensite (as A.I.S.I. 304 is unstable) but the martensite produced is not very hard as it contains little or no 35 carbon or nitrogen. 35

The illustrated nitrogen enhanced austenitic steel (304N) presents very similar properties to the unenh-anced A.I.S.I. 304 but is somewhat stronger. The strengthening is due to the presence of nitrogen but, in this steel the nitrogen has the affect of making the alloy stable and thus there is no additional strengthening due to transformation. As seen from line 3, Hadfield's manganese steel does provide a fairly high 40 capacity for work hardening and has somewhat higher strength than the others, the increased strength 40 being due to a high level of carbon (usually greater than 1%). Again the Hadfield's manganese steel is stable and thus no strengthening or increase in work hardening occurs due to transformation to a martensite.

In contrast to all of the above steels, a steel of the invention provides very high work hardening, due to 45 a combination of strengthening by transformation to martensite and strengthening by nitrogen, the nitro- ,45 gen acting to strengthen both the austenitic and martensitic forms of the steel.

The substantial benefits of steels of the invention, in comparison with known steels, will be better appreciated by referring to some typical properties of the steels and these are shown in Table I.

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TABLE I Typical Properties

5 0.2% PS UTS Elongation Charpy V Hardness 5

(MPa) (MPa) % Impact (HVj

(J)

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Austenitic stainless 250 600

Steel (304)

Nitrogen strength- 350 700

ened austenitic stainless steel (304N)

Martensitic stain- 950 1250

less steel (410)(3)

Hadfield's mangan- 370 810

ese steel

New steel 350 1200

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Hardness after working (HV)(1)

370

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440 40 600

Work hardening

Moderate

Moderate Low High V.High

Relative Wear

Resistance(2) 1.6*

Approx 1.5 Approx 2 2.3*

(1) Cold rolled to 50% reduction

(2) Wear resistance relative to mild steel *Abraded on Ottawa sand, others estimated

45 (3) Tempered at 300°C.

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Corrosion resistance very good very good Good Poor Good

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Considering briefly some of the properties illustrated by the table:

50 Strength: 50

A) The proof stress (PS) of steels of the invention is similar or superior to other austenitic steels but is lower than martensitic steels.

B) The ultimate tensile strength (UTS) of steels in the invention is similar to martensitic steels but is much higher than for austenitic steels.

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Ductility:

Steels of the invention have similar ductility to other austenitic steels but are much more ductile than available martensitic steels.

60 Toughness: 60

Steels of the invention have a toughness similar or superior to other austenitic steels but are very much tougher than available martensitic steels.

Hardness:

65 A) Before working - steels of the invention have a similar or superior hardness to other austenitic steels 65

GB 2 168 077 A

but are less hard than available martensitic steels.

B) After working - steels of the invention have a hardness similar to available martensitic steels and higher than other austensitic steels.

5 Work hardening: 5

Steels of the invention provide considerably superior work hardening to all of the other steels shown in the Table and, in fact, to all known austenitic and martensitic steels.

Wear Resistance:

10 Steels of the invention provide a considerably superior wear resistance to the other stainless steels 10 shown in the Table and a wear resistance that is similar to Hadfield's manganese steel (a proprietary wear resistant steel that contains no chromium and is thus not corrosion resistant).

Corrosion Resistance:

15 Steels of the invention may have slightly inferior corrosion resistance to presently available austenitic 15 stainless steels but have corrosion resistance that is similar to available martensitic stainless steels and very much superior to Hadfield's manganese steel.

Thus, a preferred steel of the invention has been found to have a very high resistance to wear, much higher than previously available in useful corrosion resistant alloys, whilst retaining the desired corrosion 20 resistance and being relatively cheap, by virtue of the use of manganese rather than nickel. Additionally a 20 steel of the invention has been found to be tough and weldable. This provides another significant contrast to known martensitic steels which may be expected to have a wear resistance similar to steels of the invention. Such known martensitic steels contain appreciable amounts of carbon and thus have a major drawback in that they are not ductile or tough. Moreover, wear resistance martensitic steels cannot 25 easily be welded. 25

The invention will be better understood by reference to the following Examples.

The following steels can be prepared using standard steel manufacturing techniques, it should be noted that nitrogen can be introduced in any convenient way, for example by flushing the steel melt with a nitrogen gas or a nitrogen/inert gas mixture or by adding nitrided ferro-alloys (e.g. nitrided ferro-30 chrome) to the steel melt. Solution annealing at temperatures normal for conventional austensitic stain- 30 less steels, e.g. 1050°C, serves to ensure that all nitrogen present in the steel is taken into solution.

Example 7

A 10kg steel ingot was made having the composition 12.8% Cr, 9.1% Mn and 0.215% N. Levels of other 35 elements were: 0.049%C; 0.01%S; 0.018%P; 0.05% Mo; 0.052%V; 0.58% Ni; 0.67% Si; 0.08%Cu; 0.06%Co. 35 The ingot was hot hammered and hot rolled to 12 mm thick plate and sample were heat treated at 1050°C for one hour and air cooled. The steel was austensitic at room temperature, and cooling to liquid nitrogen temperatures revealed that the Ms temperature in this steel is below -196°C. Nevertheless, the austensitic steel is unstable and transforms to a martensitic steel under deformation (e.g. by cold rolling, 40 abrading, impact or tensile testing) at room temperature, indicating that Md is above room temperature. 40 The following properties were measured:-

0.2% Proof Stress 361 MPa

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Ultimate Tensile Strength 1178 MPa

Elongation 44%

50 Reduction in Area 34% 50

Charpy V Impact Energy:

Longitudinal 246J

Transverse 195J

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Hardness 229HV

Hardness after 45% reduction by cold rolling 564 HV

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Relative wear resistance:

on 50/70 Ottawa sand 2.41

on 50/70 Crushed Quartz 1.65

gij The mechanical tests were carried out in the standard manner. The wear tests were carried out on a 65

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pin-on-disc abrasion testing machine on dry abrasive papers at a load of IMNrrr2 and a velocity of 0.2 ms1. The above results are quoted relative to mild steel (i.e. mild steel = 1). A standard AISI 304 austensitic stainless steel yielded RWR (relative wear resistance) values of 1.55 on sand and 1.48 on quartz in the same conditions.

5 Corrosion tests were also carried out, comprising potentiodynamic polarisation tests in a typical mine 5 water at pH 5.9 and in 10 wt.% sulphuric acid. There was no significant difference between the corrosion resistance of an AISI 410 martensitic stainless steel and the exemplary steel of the invention.

Example 2

10 A steel was made having the composition 10.6%Cr, 9.5%Mn and 0.192%N. 10

The steel contained 0.046%C and levels of other elements were similar to those given in Example 1.

The steel was treated and tested as in Example 1, giving the following properties:-

0.2% Proof Stress 320 MPa

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Ultimate Tensile Strength 1225MPa

Elongation 47%

20 Reduction in area 40% 20

Charpy V Impact Energy:

Longitudinal 235J

Transverse 259J or not broken (>298J)

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Hardness 223HV

Hardness after 50% reduction by cold rolling 620HV

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Relative Wear Resistance:

on 50/70 Ottawa sand 2.28

on 50/70 Crushed quartz 1.62

35 Corrosion resistance Similar to AISI 410. 35

The steel was full austensitic at room temperature with an Ms temperature below —196°C. The steel transformed to martensite on deformation.

40 Example 3 40

A steel was made having the composition 11.8%Ch, 8.1%Mn and 0.203%N. Levels of other elements were similar to those given in Example 1. The steel was treated and tested as in Example 1, giving the following properties:

45 0.2% Proof Stress 297 MPa 45

Ultimate Tensile Strength 1024 MPa

Elongation 9%

50 50

Reduction in area 8%

Charpy V impact energy 123J (Longitudinal)

55 Hardness 287 HV 55

Hardness after 49% reduction by cold rolling 564 HV

60 Relative Wear Resistance: 60

on 50/70 Ottawa sand 2.15

on 50/70 Crushed quartz 1.48

Corrosion resistance

Similar to AISI 410.

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The Ms temperature of this steel was measured by a resistivity technique to be 30°C. At room temperature therefore the steel comprises austenite and some martensite, and further martensite formed readily on deformation.

Claims

5 CLAIMS

1. A steel comprising selected amounts of chromium, manganese and nitrogen and/or carbon, the amounts of chromium, manganese, and nitrogen and/or carbon being selected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite steel upon deformation.

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2. A steel comprising selected amounts of chromium, manganese and nitrogen and/or carbon, the amounts of chromium, manganese and nitrogen and/or carbon being, elected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon deformation, the amount of nitrogen and/or carbon being selected to provide the resultant steel with significant strength.

3. A steel comprising selected amounts of chromium, manganese and at least 0.05wt.% nitrogen and/

1j5 or carbon, the amounts of chromium, manganese and nitrogen and/or carbon being selected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon deformation.

4. A steel comprising selected amounts of chromium, manganese and nitrogen, the amounts of chromium, manganese and nitrogen being selected such that the resultant steel is austenitic at room temper-

20 ature but transforms significantly to martensite upon deformation.

5. A steel comprising selected amounts of chromium, manganese and nitrogen, the amounts of chromium, manganese and nitrogen being selected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon deformation, the amount of nitrogen being selected to provide the resultant steel with significant strength.

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6. A steel comprising selected amounts of chromium and manganese and at least 0.05wt.% nitrogen, the amounts of chromium, manganese and nitrogen being selected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon deformation.

7. A steel comprising selected amounts of chromium, manganese and carbon, the amounts of chromium, manganese and carbon being selected such that the resultant steel is austentic at room tempera-

30 ture but transforms significantly to martensite upon deformation.

8. A steel comprising selected amounts of chromium, manganese and carbon, the amounts of chromium, manganese and the carbon being selected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon deformation and wherein the amount of carbon is selected to provide the steel with significant strength.

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9. A steel comprising selected amounts of chromium, manganese and at least 0.05wt.% carbon, the amounts of chromium, manganese and carbon being selected such that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon deformation.

10, A steel according to any one of claims 1 to 9, wherein the amount of chromium present is selected to be in the range of from 9 to 15 wt.%.

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11. A steel according to claim 10, wherein the amount of chromium present is selected to be in the range of from 9 to 13 wt.%.

12. A steel according to claim 11, wherein the amount of chromium present is selected to be in the range of from 11 to 13 wt.%.

13. A steel according to any one of claims 1 to 12, wherein the amount of manganese present is in

45 the range of from 7 to 13 wt.%.

14. A steel according to claim 13, wherein the amount of manganese present is in the range of from 8 to 11 wt.%.

15. A steel according to claim 14, wherein the amount of manganese present is in the range of from 9 to 11 wt.%.

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16. A steel according to any one of claims 1 to 15, wherein the amount of nitrogen and/or carbon present is in the range of from 0.05 to 0.35 wt.%.

17. A steel according to claim 16, wherein the amount of nitrogen and/or carbon present is in the range of from 0.15 to 0.35 wt.%.

18. A steel according to claim 17, wherein the amount of nitrogen and/or carbon present is in the

55 range of from 0.15 to 0.25 wt.%.

19. A steel according to any one of claims 1 to 18, wherein the Ms is less than -50°C.

20. A steel according to claim 19, wherein the Ms is below -100°C.

21. A steel according to claim 20, wherein Ms is below -190°C.

22. A steel according to any one of claims 1 to 21, wherein the Md of the steel is controlled to be

60 close to but above 20°C.

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23. A steel according to any one of claims 1 to 22, further comprising a selected amount of nickel.

24. A steel substantially as herein described with reference to Example 1.

25. A steel substantially as herein described with reference to Example 2.

26. A steel substantially as herein described with reference to Example 3.

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27. Any novel feature or combination of features disclosed herein. 5

Printed in the UK for HMSO, D8818935, 4/86,7102.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.