EP0187935A1

EP0187935A1 - Unstable steels

Info

Publication number: EP0187935A1
Application number: EP85115253A
Authority: EP
Inventors: Ursula Ruth Lenel
Original assignee: Chamber of Mines Services Pty Ltd
Current assignee: Chamber of Mines Services Pty Ltd
Priority date: 1984-12-07
Filing date: 1985-12-02
Publication date: 1986-07-23
Also published as: AU5057385A; ZA859030B; GB2168077A; JPS61179854A

Abstract

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.

Description

BACKGROUND TO THE INVENTION

THIS invention relates 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 centered cubic structure or a body centered tetragonal structure (both known as martensite), and ferritic, 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 martensitic steels. It is well known 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 M_sand the lower temperature of which is referred to as the M_f .Under certain circumstances the martensite transformation may be induced by deformation of the material at temperatures above the M. The upper s temperature at which transformation to martensite may be induced by deformation is known as the M_d. The M_d temperature is also dependent upon the amount of deformation applied, the M_d temperature being higher for heavier deformations and lower for lesser deformations. All of these temperatures also depend upon the composition of the steel.
For instance, an austentic steel can be considered to be a steel in - which the temperature at which the transformation from austenite to _I martensite starts (the M temperature) is below room temperature. Where the M 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 do, however, have the advantage of being non-magnetic. The properties of martensitic steels contrast 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 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.
It has recently been found that the addition of nitrogen to austenitic steel acts markedly to increase the strength of the austenitic steel. The addition of nitrogen to austenitic steels is known to decrease the M_s. In austenitic steels proposed to date which include added nitrogen the M has been such that the steel is stable at room temperature (20°C), i.e. the M_d is below room temperature.
ticket is generally added to stainless steels to provide an austenitic nicrostructure and imparts an improved 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.
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.

SUMMARY OF THE INVENTION

According to the invention a steel comprises by mass 9 to 15% chromium, 7 to 13% manganese and 0,05 to 0,35% nitrogen, while the sum of nitrogen and carbon does not exceed 0,35%, the balance being iron and unavoidable and incidental impurities, and the proportions of chromium, manganese, nitrogen and carbon being so chosen that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon deformation.
The amount of chromium is preferably selected to be in the range from 9 to 13%, and more preferably in the range from 11 to 13%. The amount of manganese is preferably selected to be in the range from 8 to 11% and more preferably in the range from 9 to 11%. The amount of nitrogen is selected to be in the range from 0,15 to 0,35% and more preferably in the range from 0,15 to 0,25% and in the latter case the sum of nitrogen and carbon should not exceed 0,25%.
The use of manganese and nitrogen 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 nitrogen which could provide substantial benefits during manufacture of the steel.
A typical 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 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 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.

DESCRIPTION OF THE DRAWING

It is a comparative stress/strain graph.

DESCRIPTION OF EMBODIMENTS

All percentages of compositions given are by mass.

Example 1

A 10kg steel ingot was made having the composition 12.8% Cr, 9.1% Mn i and 0.215% N. Levels of other elements were: 0.049%C; 0.01%S; 0.018%P; 0.05% Mo; 0.052%V; 0.58Ni; 0.67%Si;0.08% Cu; 0.06%Co.
The ingot was hot hammered and hot rolled to 12 mm thick plate and samples 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, abrading, impact or tensile testing) at room temperature, indicating that Md is above room temperature.
The following properties were measured:-
The mechanical tests were carried out in the standard manner. The wear tests were carried out on a pin-on-disc abrasion testing machine on dry abrasive papers at a load of lMNm and a velocity of 0.2 ms 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.
Corrosion tests were also carried out, comprising potentiodynamic polarisation tests in a typical mine 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

A steel was made having the composition 10.6%Cr, 9.5%Mn and 0.192%N.
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:-
The steel was fully austensitic at room temperature with an Ms temperature below -196°C. The steel transformed to martensite on deformation.

Example 3

A steel was made having the composition 11.8%Cr, 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:
The M_s 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.

Discussion:

The drawing is a schematic stress/strain graph having curves marked 1 to 5. The curve marked 5 represents a steel of example 2.
Considering firstly, in general terms, the transformation of austenitic steels, it is well known that appropriate selection of the composition of a steeel can provide marked changes in the M temperature. In the present invention, the composition of the steel is so chosen that the M temperaure 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 ustenite to a martensite can also be achieved by deformation of the steel. Again, the amount of deformation required to bring about the onset of transforation to martensite depends upon the precise composition-of the particular steel, the composition affecting the M_d. 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 containing austenitic steels which, even upon deformation, remain austenitic and may therefore_'.be termed "stable". As used herein, the term "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 study of the properties of such steels.
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 are generally hard but are generally not ductile and have only a limited capacity for work hardening. In 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.
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 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 Example 2 above.
As will be appreciated from Figure 1, a typical martensitic steel (lightly tempered) has little ductility or work hardening but is very hard and strong, the hardness and strength largely being due to the presence 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 carbon or nitrogen.
The illustrated nitrogen enhanced austenitic steel (304N) presents very similar properties to the unenhanced 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 effect 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 capacity for work hardening and has somewhat higher strength than the others, the increased strength 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 α martensite.
In contrast to all of the above steels, a steel of the invention provides very high work hardening, due to a combination of strengthening by transformation to martensite and strengthening by nitrogen, the nitrogen 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 1 on page 14, which shows the properties of a steel of example 1. The properties of a steel of example 2 were found to be closely similar.
Considering briefly some of the properties illustrated by the table:

Strength:

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.

Ductility:

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

Toughness:

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

Hardness:

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

Work hardening: -

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:

Steels of the invention provide a considerably superior wear resistance to the other stainless steels 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:

Steels of the invention may have slightly inferior corrosion resistance to presently available austenitic 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 resistance and being relatively cheap, by virtue of the use of manganese rather than nickel. Additionally a 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 resistant martensitic steels cannot easily be welded.

Claims

1. A steel comprising by mass 9 to 15% chromium, 7 to 13% manganese and 0,05 to 0,35% nitrogen, while the sum of nitrogen and carbon does not exceed 0,35%, the balance being iron and unavoidable and incidental impurities, and the proportions of chromium, manganese, nitrogen and carbon being so chosen that the resultant steel is austenitic at room temperature but transforms significantly to martensite upon deformation.

2. The steel according to claim 1 in which the amount of chromium is selected to be in the range from 9 to 13%.

3. The steel according to claim 2 in which the amount of chromium is selected to be in the range from 11 to 13%.

4. The steel according to any one of the above claims in which the amount of manganese is selected to be in the range from 8 to 11%.

5. The steel according to claim 4 in which the amount of manganese is selected to be in the range from 9 to 11%.

6. The steel according to any one of the above claims in which the amount of nitrogen is selected to be in the range from 0,15 to 0,35%.

7. The steel according to claim 6 in which the amount of nitrogen is selected to be in the range from 0,15 to 0,25%.

The steel according to claim 7 in which the sum of nitrogen and carbon does not exceed 0,25%.