EP0446188A1

EP0446188A1 - Stainless steel

Info

Publication number: EP0446188A1
Application number: EP91850036A
Authority: EP
Inventors: Hakan Holmberg
Original assignee: Sandvik AB
Current assignee: Sandvik AB
Priority date: 1990-02-26
Filing date: 1991-02-13
Publication date: 1991-09-11
Anticipated expiration: 2011-02-13
Also published as: EP0446188B1; SE506886C2; SE9000673L; SE9000673D0; DE69128293T2; KR910021491A; ATE160827T1; DE69128293D1; JP3169978B2; KR100190442B1; JPH0598391A

Abstract

The invention relates to a high strength vanadium containing stainless steel alloy in which the amounts of the alloy elements have been balanced such that the austenite phase remains stable without being deformed into martensite even at largely extended reductions. The steel alloy should essentially consist of 0,04-0,25 % C, 0,1-2 % Si, 2-15 % Mn, 16-23 % Cr, 8-14 % Ni, 0,10-1,5 % N, 0,1-2,5 % V, and the remainder being iron and normal impurities.

Description

The invention relates to a non-magnetic high strength stainless steel in which the austenite phase is sufficiently stable so as to resist transformation into the ferromagnetic phase martensite even at extended reduction for instance by cold rolling of strips or drawing of wire.
The rapid development within computer and electronic industries have created increased demand of material with combination of properties not considered earlier or easily achievable. For instance, this refers to the combination of high mechanical strength and non-magnetic structure for materials to be used in for instance spring applications where the material must be magnetically inert. For many such products the manufacturing process includes various formation steps. Since it is common knowledge that increased strength leads to impaired ductility it is of substantial advantage if the formation can be carried out in as soft condition as possible and requisite strength can be achieved by a simple heat treatment.
Among stainless high strength steeel the so called non-stable austenitic spring steels SS 2331 with normal analysis 17 Cr, 7 Ni, 0,8 Si, 1,2 Mn, 0,1 C and 0,03 N are in a special position by their combination of high strength and good corrosion properties.
The very high strength achievable with this type of steel depends from the (para-magnetic) austenitic structure which during deformation transforms into the (ferromagnetic) martensite, a phase of exceptional hardness. When the amount of alloy elements is increased primarily of Ni and Mo such as in type SS 2343/2353 the tendency for the formation of deformation martensite is reduced but thereby has also the possibility of achieving high strength become limited.
Due to a systematic research it has been found possible, by carefully balancing the alloy elements and by cold working to achieve a remarkable work hardening whilst preserving a non-magnetic structure. In addition thereto it is found possible, without affecting the magnetic properties, to reach a precipitation hardening of the alloy to high strength by a simple heat treatment.
The strictly controlled optimized composition (in weight-%) of the alloy of this invention comprises following analysis:

C: 0,04-0,25
Si: 0,1-2
Mn: 2-15
Cr: 16-23
Ni: 8-14
N: 0,10-1,5
V: 0,1-2,5

The amounts of alloy elements, which are very critical, are governed by structural requirements which structure shall consist of an austenitic matrix with inclusions of vanadium nitrides. The structure should not comprise any amounts of ferrite. The austenite phase shall be sufficiently stable such that no significant portion thereof is transformed into ferromagnetic martensite at cooling from high temperature annealing or at substantial cold working, typically > 70 % thickness/reduction at cold rolling or corresponding degree of reduction at wire drawing. At the same time the austenite phase shall during deformation exhibit a substantial cold hardening which results in that high mechanical strength is achieved without presence of ferromagnetic phase. Of importance is also the possibility of additionally increasing the strength in the cold rolled condition by a simple heat treatment. In order to achieve these objectives at the same time the effects of the various alloy elements upon the material properties must be known. Certain of these alloy elements are ferrite formers and others are austenite formers at those temperatures that are relevant for hot working and annealing. Further, certain of these elements contributes positively to deformation hardening during cold working whereas others contributes negatively thereto.
The effects of the various alloy elements and an explanation of the limitations thereof is described below where the amounts are given in weight-%.
Carbon is an element which strongly contributes to austenite formation. Carbon also contributes to a stabilization of austenite against martensite transformation and it has consequently a double positive effect in this alloy. Carbon also positively contributes to the work hardenability at cold working. The carbon content should therefore exceed 0,04 %. High carbon amounts however leads to negative effects. The high chromium affinity results in an increased tendency for carbide precipitation with increased carbon content. This also leads to impaired corrosion properties embrittlement problems and a destabilization of the matrix which might lead to local martensite transformation which renders the material being partially ferromagnetic. The maximum content of C is therefore limited to 0,25 %, preferably below 0,20 %.
Si is an important element for the purpose of facilitating the manufacturing process. The amount of Si should therefore be at least 0,1 %. Si is however a ferrite stabilizer which rather drastically tends to increase the tendency for the formation of the ferromagnetic phase of ferrite. High Si amounts additionally promote the tendency of precipitating easily melting intermetallic phases and thereby impairs the hot working. The Si-content should therefore be limited to max 2 % preferably max 1,0 %.
Manganese has been found to contribute positively to several properties of the alloy of this invention. Mn stabilizes austenite without simultaneously negatively affecting the work hardening. Mn has the additional important ability of providing increased solubility of nitrogen, properties described more specifically hereunder, in melted and solid phase. The Mn content should therefore exceed 2 % and preferably exceed 4 %. Mn increases the coefficient of linear expansion and reduces electrical conductivity which could be of disadvantage for applications within electronics and computer areas. High amounts of Mn also reduce corrosion resistance in chloride containing environments. Mn is also much less efficient than nickel as a corrosion reducing element under oxidizing corrosion conditions. The Mn content should therefore not exceed 15 % and should preferably amount to 4-10 %, and more preferably 4,0-7,5 %.
Cr is an important alloy element from several aspect. Cr content should be high in order to achieve good corrosion resistance. Cr also increases nitrogen solubility in the melt and in the solid phase and thereby enables increased alloyed presence of nitrogen. Increased Cr content also contributes to stabilized austenite phase towards martensite transformation. The alloy of the present invention can, to advantage, as described below be subject of precipitation hardening and precipitate high chromium containing nitrides. In order to reduce the tendency for too strong local reduction of Cr-content with non-stabilization and reduction in corrosion resistance the Cr content should exceed 16 %.
Since Cr is a ferrite stabilizing element presence of very high Cr contents will lead to the presence of ferromagnetic ferrite. The Cr content should therefore be equal to or less than 21 %.
Ni is, next after carbon and nitrogen, the most efficient austenite stabilizing element Ni also increases austenite stability towards deformation into martensite. Ni is also, in contrast of Mn, known for efficiently contributing to corrosion resistance under oxidizing conditions. Ni is, however, an expensive alloy element at the same time as it has a negative impact on work hardening during cold working. In order to achieve a sufficiently stable non-magnetic structure the Ni-content should exceed 8 %. In order to achieve high strength after cold working the Ni-content should not exceed 14 %, preferably not exceed 12 % but preferably exceed 9 %.
N is a central alloy element in the present alloy. N is a strong austenite former, it promotes solution hardening and stabilizes the austenite phase strongly towards deformation into martensite. N is also of advantage for the purpose of achieving increased work hardening at cold working and it acts as a precipitation hardening element at heat treatment. Nitrogen can therefore contribute to a further increase of the cold rolled strength.
Nitrogen also increases resistance towards nodular corrosion. Chromium nitrides precipitated during heat treatment also appear to be less sensibilizing than corresponding chromium carbides.
In order to completely take advantage of its many good properties the N content should not be less than 0,10 %, preferably not less than 0,15 %.
When using very high nitrogen contents the solubility of N is exceeded in the melt. The N content should therefore be equal to or less than 1,5 %, and preferably amount to max 0,6 %, more preferably 0,2-0,5 %.
Vanadium is an element having several positive effects. Vanadium increases the solubility of nitrogen and contributes to the formation of vanadium nitrides which promotes fine grain formation during heat treatment. By optimizing the heat treatment the mechanical properties can also be improved by precipitation hardening. The content of V should be at least 0,1 %, preferably higher than 0,25 %. is also a ferrite stabilizing element and its content should therefore not exceed 2,5 %, preferably max 2,0 %.
The invention will in the following be disclosed by way of results from research carried out whereby further details about structure, work hardening, mechanical properties and magnetic properties will be disclosed.
Production of the testing materials included melting in a high-frequency induction furnace and casting to ingots at about 1600°C. These ingots were heated to about 1200°C and hot worked by forging the material into bars. The materials were then subject of hot rolling into strips which hereafter were quench annealed and clean pickled. The quench annual was carried out at 1080°-1120°C and quenching occurred in water.
The strips obtained after quench annealing were then cold rolled to various reduction degrees after which test samples were taken out for various tests. In order to avoid variations in temperature and its possible impact on magnetic properties the samles were cooled to room temperature after each cold rolling step.
The chemical analysis of the testing materials in weight-% appears from Table 1 below:

P,S < 0.030 weight-% is valid for all alloys above.
Samples were taken in quench annealed condition for control of ferrite and martensite content and for hardness measurement. The results are disclosed in Table 2.
All these test alloys fulfill the requirement of being free from ferrite and martensite in quench annealed condition. The annealed hardness is somewhat higher than that of the reference materials AISI 304/305.
As described above it is very essential that materials of this invention exhibit a substantial work hardening at cold working operation. After cold rolling to 75 % thickness reduction samples were taken for hardness measurement. Table 3 shows increase in hardness as a function of cold working.
All these testing alloys appear to have a substantial deformation hardening compared with reference materials AISI 304/305.
The strength of the alloys at uniaxial tensile testing as a function of cold working degree is disclosed in Table 4 wherein R_p 0,05 and R_p 0,2 correspond to the load which gives 0,05 % and 0,2 % residual elongation, and Rm corresponds to the maximum value of applied load in the load-elongation diagram and A10 corresponds with the ultimate elongation of the testing bar.
Table 4 shows that by using alloys of this invention very high strength levels can be achieved during cold working. Alloy AISI 305 appears to have a substantially slower work hardening due to its low amounts of interstitially dissolved alloy elements, i.e. nitrogen and carbon, combined with rather high nickel content.
Spring steel of the type SS 2331 are often annealed for the purpose of achieving an additional increase of the mechanical properties. This contributes favorably to several important spring properties such as fatigue strength and relaxation resistance and the ability of forming this material in a rather soft condition. The higher ductility at lower strength can hereby be used favorably to a more specific formation of the material.
Table 5 shows the effects of such annealing upon the mechanical properties after 75 % cold reduction. The annealing tests gave as result an optimal effect at a temperature of 450/500°C and 2 hours maintenance.
The alloys of this invention appear to have obtained a very good effect as a result of the anneal. It is of specific importance to notice the extremely high increase in R_p 0,05 value of 45-55 %. This is the value that is best correlated with the elastic limit which is an indication of how much a spring can be loaded without being subject of plastification. By having reached such an increase in the R_p 0,05 value a larger work area can be used for a spring made of such material. It is of specific interest to notice the rather minor increase in ultimate strength in AISI 304 and AISI 305. This is an essential disadvantage since the ultimate strength by experience is the value that is best correlated with the fatigue strength.
For a material according to this invention it is the objective to achieve the objective of a high strength material at the same time as the material exhibits paramagnetic behaviour, i.e. a magnetic permeability very close to 1. Table 6 discloses the magnetic permeability depending upon field strength the various alloys after 75 % cold reduction and annealing at 450/500 % / 2 hours.
Table 6 discloses that by cold working and precipitation hardening of an alloy of the invention it is possible, by strictly controlling the composition in cold rolled and precipitation hardened condition, to obtain a strength exceeding 1800 or even 1900 MPa combined with a very low value of the magnetic permeability 1,002-1,025. The inventive alloy thus enables using the property advantages given by a high strength for spring applications at the same time as the material is able to preserve its para-magnetic structure and thereby be useful in applications where a magnetic inert material is desired. The reference materials outside the composition ranges of this invention have lower values for both its mechanical properties and the effect of precipitation treatment while the magnetic permeability is higher. This is relevant for commercial alloys AISI 304/305.

Claims

Precipitation-hardenable non-magnetic steel alloy with high strength, characterized in that it consists essentially of the following elements by weight:
C 0,04-0,25 %

Si 0,1-2 %

Mn 2-15 %

Cr 16-23 %

Ni 8-14 %

N 0,10-1,5 %

V 0,1-2,5 %
the remainder being iron and normal impurities, the contents of said elements being balanced so that the austenite phase remains sufficiently stable so as to resist any transformation into martensite even during extended reduction.
The steel of claim 1, characterized in that the elements are so mutually balanced that the austenite phase remains sufficiently stable so as to resist any transformation into martensite at cold working > 70 % thickness reduction.
The steel of claim 1, characterized in that the amount of nitrogen is 0,15-0,6 %.
The steel of claim 1, characterized in that the amount of carbon is 0,04-0,20 %.
The steel of claim 1, characterized in that the amount of silicon is 0,1-1%.
The steel of claim 1, characterized in that the amount of manganese is 4-10 %, preferably 4-7,5 %.
The steel of claim 1, characterized in that the amount of chromium is 16-21 %.
The steel of claim 1, characterized in that the amount of nickel is 9-12 %.
The steel of claim 1, characterized in that the amount of vanadium is 0,25-2 %.
The steel of claim 1, characterized in that the amount of nitrogen is 0,2-0,5 %.