WO2009046484A1

WO2009046484A1 - Austenitic manganese steel alloy and method for making same

Info

Publication number: WO2009046484A1
Application number: PCT/AU2008/001483
Authority: WO
Inventors: Peter Duecker; Glenn Dickson
Original assignee: Steelfinne Fabrications Pty Ltd; Bohler Uddeholm (Australia) Pty Ltd
Priority date: 2007-10-08
Filing date: 2008-10-07
Publication date: 2009-04-16

Abstract

An improved Austenitic Manganese Steel Alloy having a composition substantially as follows (all percentages by weight of the following compositions): carbon 1.30%; silicon 0.50%; manganese 13.00%; phosphorous 0.035%; sulphur 0.004%; chromium 0.60%; molybdenum 0.15%; nickel 0.4%; vanadium 0.15%; tungsten 0.15%; with the balance being iron and other impurities, and being formed from ingots cast at a temperature of about 1460°C, the ingots then being formed into billets and then bars and, the bars being air quenched immediately to about 20°C over a period of at least one hour, and subsequently subjected to straightening, peeling and roll polishing at a pressure of at least 50 Bar or 5N/mm2.

Description

AUSTENITIC MANGANESE STEEL ALLOY AND METHOD FOR

MAKING SAME

FIELD OF THE INVENTION

The present invention relates, in general terms, to products formed from manganese-containing alloy of steel. More particularly, but not exclusively, the invention relates to austenitic manganese steel alloy products demonstrating improved surface hardness and penetration resistance. The invention further relates to a method for the manufacture of such alloy products with improved surface hardness and penetration resistance.

BACKGROUND OF THE INVENTION

The improved alloy in accordance with the present invention is particularly, but not exclusively, intended for use in establishments such as prisons, correctional facilities, remand centres, detention centres, asylums, etc., all being establishments wherein a high degree of security is required. In such establishments past experiences have shown that bars, wires, indeed all structures used to prevent or inhibit unwanted/unauthorised ingress or egress, have to be capable of withstanding attack and/or damage, as by cutting, bending, impact or the like. For that reason there have been ongoing efforts made to develop improved materials for use in the construction of such establishments, and elements for use therein such as doors, walls, barriers, cell bars, fences, wires, etc. More specifically, alloys have been developed which exhibit improved resistance to cutting, bending and/or impact. It has been found that continual improvements in materials to be employed in such contexts have been needed, in order to meet new challenges posed by those seeking to make unauthorised ingress or egress into establishments of this general type.

With materials, more specifically steel alloys, intended for usage in establishments of the aforementioned type, it should be realised that the end product - as formed using such an alloy - has to provide security against attack as by cutting, bending and/or impact in order to allow for the maintenance of "a secure envelope" for the appropriate correctional authority, police or other end user. The preferred balance of mechanical, chemical and environmental properties has been achieved, in accordance with the present invention, by a careful development of the chemical analysis/composition, utilising or realising the importance of temperature transfers in the creation of a material/alloy exhibiting a micro-structure suited to deliver an improved product. In that regard such an improved product should include an outer sheath layer having a typical hardness of 500 Vickers [HV 300], whilst maintaining ductility of the material inner core of a hardness of the order of 260 Vickers [HV 300], to allow the product to be subjected to hydraulic bending loads up to at least 30 KN and yet return to its original shape, and to resist dynamic cumulative impact with a strike energy of the order of at least 200 J, with minimal deflection and with no susceptibility to brittleness.

To date no steel alloy, in particular as austenitic manganese steel alloy, has been produced which exhibits these improved characteristics.

The present invention relates to an improvement over existing materials, whereby to provide a steel alloy exhibiting enhanced strength capabilities, in terms of resistance to bending and/or impact, as well as resistance to damage as by cutting. It should be understood, however, that the improved alloy in accordance with the present invention is also suitable for use in contexts other than security establishments of the aforementioned general type.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided an improved austenitic manganese steel alloy products having a composition as follows (all percentages are by weight of the final composition): carbon from 1.1 to 1.3%; silicon from 0.25% to 0.50%; manganese from 12.0% to 13.0%; phosphorous from 0.00% to 0.035%; sulphur from 0.00% to 0.004%; chromium from 0.00% to 0.60%; molybdenum from 0.00% to 0.15%; nickel from 0.00% to 0.040%; vanadium from 0.00% to 0.15%; tungsten from 0.00% to 0.15%; with the balance being iron (and other impurities) said alloy being cast into ingots at a temperature of about 1460°C, then formed into billets, the billets being then formed into bars, said bars being air quenched immediately thereafter to about 20⁰C over a period of at least one hour, and subsequently subjected to straightening, peeling and roll polishing at a pressure of at least 50 Bar or 5N/mm².

Throughout the description including the claims the value "0.00%" defines a value from actual zero to values undetectable by conventional analytic means.

According to an especially preferred embodiment of the invention there is provided an improved austenitic manganese steel alloy having a composition substantially as follows (all percentages are by weight of the final composition): carbon 1.30%; silicon 0.50%; manganese 13.00%; phosphorous 0.035%; sulphur 0.004%; chromium 0.60%; molybdenum 0.15%; nickel 0.40%; vanadium 0.15%; and tungsten 0.15%, with the balance being iron (and other impurities) said alloy being cast into ingots at a temperature of about 1460⁰C, then formed into billets, the billets being then formed into bars, said bars being air quenched immediately thereafter to about 20°C over a period of at least one hour, and subsequently subjected to straightening, peeling and roll polishing at a pressure of at least 50 Bar or 5N/mm².

According to another aspect of the invention there are provided products formed from anisotropic austenitic manganese steel alloys having compositions substantially as disclosed above, and being treated by rolling, air quenching, compression rolling, peeling and roll polishing to increase the hardness of the outer sheath of the products.

According to another aspect of the invention there is provided a process for forming anisotropic austenitic manganese steel products, said process including the steps of forming a liquid phase of an alloy having a composition as follows (all percentages are by weight of the final composition): carbon from 1.1 to 1.3%; silicon from 0.25% to 0.50%; manganese from 12.0% to 13.0%; phosphorous from 0.00% to 0.035%; sulphur from 0.00% to 0.004%; chromium from 0.00% to 0.60%; molybdenum from 0.00% to 0.15%; nickel from 0.00% to 0.040%; vanadium from 0.00% to 0.15%; tungsten from 0.00% to 0.15%; with the balance being iron (and other impurities) in an electric furnace at about 1750°C, transferring the liquid to a ladle furnace and adjusting the temperature to about 1460°C, casting the liquid alloy into ingots, forming billets from the ingots, subsequently forming bars from the billets by reduction rolling, air quenching the bars to about 20°C on a cooling bed immediately after rolling over a period of at least one hour, followed by straightening, peeling to initiate work hardening and subsequently pressure rolling the bars at a pressure at least 50 Bar or 5N/mm² (roll polishing).

This further cold working mechanically removes machine marks resulting from the peeling process and creates a highly intensified hardness on the outer sheath of the product, thereby giving rise to the final product's unique properties. The macro-deformation and consolidation of the surface structure adds to the resistance to penetration.

The atypically low concentration of sulphur decreases the machinability of the material thus resulting in the product being harder to cut or penetrate.

It has been found, with austenitic manganese steels, that heat transfer changes during the formation thereof can have a significant impact on the final micro-structure of the material, and hence on the properties thereof in terms of resistance to impact, bending and/or cutting.

In terms of mechanical properties, testing has revealed that products made from an alloy treated in accordance with the present invention, typically exhibit improvements in terms of:

(i) resistance to cutting or penetration; (ii) resistance to deflection under load; and (iii) resistance, as by reduced deformation, under impact.

DESCRIPTION OF PREFERRED EMBODIMENT

Hereinafter reference will be made in more detail to a preferred embodiment of the alloy in accordance with the present invention. It should be understood, however, that the ensuing description is given by way of non-limitative example only and is with reference to the attached representations, wherein:

Figure 1 depicts hardness test results at the surface of two samples of austenitic manganese alloy steel bars according to the invention;

Figure 2 depicts transverse hardness test results in HV 300 in cross-sectional depth readings for one sample of austenitic manganese steel alloy bar according to the invention;

Figure 3 depicts transverse hardness test results for another sample of austenitic manganese steel alloy bar according to the invention;

Figure 4 depicts a pictorial micro-analysis of the crystal grain structure of a sample of austenitic manganese steel alloy bar according to the invention;

Figure 5 contains a pictorial micro-analysis of the crystal grain structure of a preferred sample of austenitic manganese steel alloy bar according to the invention, at various depths from the surface;

Figure 6 contains a pictorial micro-analysis of the crystal grain structure of a second sample of austenitic manganese steel alloy 0.5 mm from the surface; and

Figure 7 contains a pictorial micro-analysis of the crystal grain structure of a second sample of austenitic manganese steel alloy at various distances from the surface. Since any alloys of metal, with the exception of 99.99% pure metals, comprise mixtures of host ions (eg iron) and guest ions (eg carbon), convenient and reproducible means of accommodating the guest ions within the host ions are required. In the present instance this is done in a liquid state, wherein the components of the alloy are added to a converter through an electric arc furnace at a temperature of preferably 1750°C. Subsequently the liquid alloy is transferred to a ladle furnace. Samples are taken and checked with a spectrograph to ensure the correct composition has been achieved. Any shortfall of any one of the components required for the alloy can be corrected by the introduction of that component to meet the specification requirements.

The temperature in the ladle furnace is brought to preferably 1460°C to finalise the homogeneous structure of the alloy for subsequent casting into ingots.

In arriving at the unique properties of this improved austenitic manganese steel alloy it is intended that the balance of the host ions in the matrix (as a resulting substrate) form in conversion to an ingot the resulting substrate with properties in which random crystal formation is tightly intertwined. This makes the substrate anisotropic. A result is that the balance of mechanical properties an equal in all directions of load application, with one exception, that being that in the reheated hot formed work pieces they are slightly stronger in the rolling or forging direction.

In one preferred embodiment in accordance with the present invention the alloy may have the following composition (weight percent): Carbon 1.3

Tungsten 0.15

Silicon 0.5

Manganese 13.0

Phosphorus 0.035 Sulphur 0.004

Chromium 0.6

Molybdenum 0.15 Nickel 0.4

Vanadium 0.15, with the balance being iron (and other impurities).

Ingots of the alloy are cast at the temperature of about 1460°C in nominal 3 tonne weights, with a minimal square cross-section of about 465 mm. Each ingot comprises a multitude of individual crystals, or grains, each having individual crystal planes. The growth planes differ from the failure, twinning, and resistance to tension (compression or shear) planes.

By convention the planes are placed within a three-dimensional coordinate system. The unit distances along three axes, counting from the origin, locate the origin and planes inside the crystal structure. It is to be understood that all crystals (or grains) are in a completely random arrangement with respect to each other.

Following casting, the ingots are rolled in a cogging mill to billet size, with the billets being substantially square in cross-section with a nominal side size of 130 mm. Subsequently the billets can be rolled in a multi-line rolling mill to black bar size, the black bars being substantially round with a nominal diameter of the order of 21.5 mm. Immediately after the black bars are formed by reduction rolling, a controlled air quench from 850-900°C to 20°C is carried out on a cooling bed. This method of precipitation by air cooling provides maximum yield of mechanical properties which afford the ultimate tensile strength for the final product. The controlled cooling can preferably be carried out on a rake-type bed with at least 50 mm spacing between the black bars and with a minimum cooling time of about 60 minutes. As a result of this process a microstructure is created in the black bars that allows further manipulation by subsequent processes.

The black bars can then be peeled to produce substantially round bars of about 20 mm diameter. The black bars may be peeled from the roll size of about 21.5 mm down to about 20 mm in one pass. The peeling can be effected by a lathe. The bars are fed through a fixed head where the hot rolled surface from hot rolling is removed. This operation initiates work hardening of the austenitic manganese steel. The work hardening surface of the bars continues to renew itself by further mechanical or impact working. After peeling, the surface of the bars may be straightened and smoothed with pressure rolls of at least 50 Bar or 5 N/mm². This further cold working mechanically removes machine marks resulting from the peeling process and creates a highly intensified hardness on the outer sheath of the product, thereby giving rise to the final product's unique properties. The macro- deformation and consolidation of the surface structure adds to the resistance to penetration.

The heat transfer changes during the formation of the bars have a significant impact on the final micro-structure of the material, and hence on the properties thereof in terms of resistance to impact, bending and/or cutting.

Products made from an alloy according to the present invention have an outer sheath layer of a typical hardness of at least [HV 300] 500 Vickers, yet maintain ductility in the material inner core for the hardness [HV 300] 260 Vickers. This allows any such product to perform under hydraulic bending loads of the order of up to and greater than 30KN and to return to its original straightness and shape and to withstand dynamic cumulative impact with a strike energy of up to at least 200J with minimal limited deflection and with no susceptibility to brittleness. Such products containing the alloy according to the invention have been found to be extremely robust and are well suited for use in high-security environments.

In terms of mechanical properties, testing has revealed that an alloy in accordance with the present invention, and products made therefrom, typically exhibit improvements in terms of:

(iv) resistance to cutting or penetration; (v) resistance to deflection under load; and (vi) resistance, as by reduced deformation, under impact. As to (i), a saw penetration test was carried out on a 20 mm bar fabricated from an alloy in accordance with the invention, being a bar of the type which would be used, for example, in a cell window. Such a bar, when clamped and subjected to attack by a power hacksaw with a force of from 50 and 60 KN for a period of at least 10 minutes, revealed a depth of penetration of only 1.4 mm.

In terms of impact load the testing regime, again on a bar of 20 mm diameter, involved subjecting such a bar to the impact from a dead weight of 20.05 kg. (150 mm diameter) released from a height of at least one metre, such involving a strike energy of at least 200 Joule. The bar was subjected to repeated such impacts, yet exhibited minimal damage and/or deflection.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form or suggestion that the prior art forms part of the common general knowledge in Australia.

Finally, it is to be understood that the foregoing description refers merely to preferred embodiments of the invention, and that variations and modifications will be possible thereto without departing from the spirit and scope of the invention, the ambit of which is to be determined from the following claims.

Claims

1. An austenitic manganese steel alloy product having a composition as follows (all percentages being by weight of the final composition): carbon from 1.1 to 1.3%; silicon from 0.25% to 0.50%; manganese from 12.0% to 13.0%; phosphorous from 0.00% to 0.035%; sulphur from 0.00% to 0.004%; chromium from 0.00% to 0.60%; molybdenum from 0.00% to 0.15%; nickel from 0.00% to 0.040%; vanadium from 0.00% to 0.15%; tungsten from 0.00% to 0.15%; the balance being iron (and other impurities); being formed in the following manner the components are heated to approximately 1750° C to form a liquid phase, and the alloy is then cast into ingots with the temperature of approximately 146O⁰C, then formed into billets, the said billets are then formed into bars, said bars are then air quenched immediately thereafter to a temperature of approximately 2O⁰C over a period of at least an hour, and are subsequently subjected to straightening, peeling and roll polishing at a pressure of at least 50 BAR or 5 N/mm².

2. An improved austenitic manganese steel alloy product as claimed in Claim 1, wherein the composition is substantially as follows: carbon 1.3%; silicon 0.50%; manganesel3.0%; phosphorous 0.035%; silver 0.004%; molybdenum 0.15%; nickel 0.40%; vanadium 0.15%; tungsten 0.15%; the balance being iron (and other impurities).

3. A process for forming an anisotropic austenitic manganese steel alloy product, said process including the steps of forming a liquid phase of an alloy having a composition as follows (all percentages by weight of the final composition): carbon from 1.1 to 1.3%; silicon from 0.25% to 0.50%; manganese from 12.00% to 13.00%; phosphorous from 0.00% to 0.035%; sulphur from 0.0% to 0.004%; chromium from 0.0% to 0.60%; molybdenum from 0.0% to 0.15%; nickel from 0.0% to 0.40%; vanadium from 0.0% to 0.15%; and tungsten from 0.0% to 0.15%, the balance being iron (and other impurities) by heating in an electric arc furnace to a temperature of approximately

175O⁰C, transferring said liquid phase to a ladle furnace and adjusting the temperature of said liquid phase to about 1460⁰C, casting the liquid phase alloy into ingots; forming billets from said ingots; subsequently forming bars from said billets by reduction rolling; and air quenching the resulting bars to approximately 20⁰C on a cooling bed immediately after rolling, said cooling taking place over a period of at least one hour; followed by straightening and peeling to initiate work hardening and pressure rolling at a pressure of at least 50 Bar or 5 N/ram²

4. A process for forming an anisotropic austenitic manganese steel alloy product, said process including the steps of forming a liquid phase of an alloy having a composition as follows (all percentages being by weight of the final composition): carbon 1.3%; silicon 0.50%; manganese 13.00%; phosphorous 0.035%; sulphur 0.004%; chromium 0.60%; molybdenum 0.15%; nickel 0.40%; vanadium 0.15%; tungsten 0.15%; the balance being iron (and other impurities) in an electric furnace at a temperature of about 1750⁰C, transferring said liquid phase to a ladle furnace and adjusting the temperature at said liquid phase to approximately 1460⁰C casting the liquid phase alloy into ingots, forming billets from said ingots, subsequently forming bars from said billets by reduction rolling, and air quenching the bars to approximately 2O⁰C on a cooling bed immediately after rolling over a period of at least one hour, followed by straightening and peeling to initiate work hardening and pressure rolling at a pressure of at least 50 BAR or 5 N/mm²