EP0014086A1

EP0014086A1 - A method for the production of alloyed steel products and such products thereby obtained

Info

Publication number: EP0014086A1
Application number: EP80300187A
Authority: EP
Inventors: Ignace Lefever; Martin Bombeke
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 1979-01-22
Filing date: 1980-01-21
Publication date: 1980-08-06
Also published as: JPS55131127A; BE873620A; AU5482580A

Abstract

A method for the production of alloyed steel products having substantially no ferrite matrix phase and having Md and Ms conversion temperatures (as herein defined) such that plastic deformation may cause phase transformation, comprises effecting plastic deformation partly at a cryogenic temperature of from 0°C to the Ms conversion temperature (as herein defined) and partly at a low temperature of from 0°C to the recrystallisation temperature, the plastic deformation at said cryogenic temperature being effected prior to plastic deformation at said low temperature.

The method enables heat transfer and cooling problems inherent in known methods to be overcome and may also be employed to produce alloyed steel products having improved properties e.g. tenacity, creep resistance and/or fatigue strength over similar products produced according to known methods.

Description

The present invention relates to a method for the production of alloyed steel products and such products thereby obtained. In particular the present invention relates to the production of, for example, elongate e.g. wire-shaped and flat sheet products and the like from austenitic steels, especially stainless steel.
It is known that certain mechanical properties of alloyed steel products may be improved by a deformation treatment at a cryogenic temperature, optionally followed by an ageing treatment. Such a method results in heat transfer and cooling problems so that inter alia the speed of a continuous operation is seriously restricted.
The present invention is based on the discovery that the above-mentioned difficulties may, at least in part, be overcome by effecting plastic deformation partly at a cryogenic temperature of from 0°C to the Ms conversion temperature (as hereinafter defined) and partly at a low temperature of from 0°C to the recrystallisation temperature, the plastic deformation at said cryogenic temperature being effected prior to plastic deformation at said low temperature.
In particular, we have found that problems of lubrication may be alleviated by effecting the plastic deformation at cryogenic temperature before plastic deformation at said low temperature. Thus where plastic deformation is effected first at a low temperature (as herein defined) and then at a cryogenic temperature it is conventional to apply a lubricating coating before deformation at the low temperature, this coating subsequently being removed by an expensive degreasing operation in order to avoid discolouration during the ageing treatment, and then to reapply the coating. The coating is especially necessary where for example it is desired to make springs. The lubricating coating may for example be a thin layer of a lubricant carrier and/or appropriate soap. According to the method of the present invention a lubricating coating may if desired be applied after deformation at the cryogenic temperature especially where the metal is soft following annealing, with the advantage that the need to apply a second coating may be avoided.
The present invention may also be employed to produce elongate alloyed steel products having improved properties. e.g..tenacity, creep resistance and/or fatigue strength over similar products produced according to known methods.
Thus according to one feature of the present invention there is provided a method for the production of alloyed steel products having substantially no ferrite matrix phase and having Md and Ms conversion temperatures (as herein defined) such that plastic deformation may cause phase transformation, which method comprises effecting plastic deformation partly at a cryogenic temperature of from 0 C to the Ms conversion temperature (as hereinafter defined) and partly at a low temperature of from 0°C to the recrystallisation temperature, the plastic deformation at said cryogenic, temperature being effected prior to plastic deformation at said low temperature.
The products produced according to the method of the present invention may, for example, be in the form of elongate products e.g. flat sheet or round wire, the wire having a substantially round cross-section. Thus, for example, the elongate products may take the form that would be obtained by flat rolling into strip or drawing through a die into shaped wire or bundles of wires which can be jointly treated. Further examples include stranded wire cord and rope, and even narrow tubes, called hollow wires, which may for example be used as spirally wound springs and the like. Continuous treatments may, for example, be used in the basic manufacturing processes of such products. Such products may then, for example, be suitable for obtaining such useful objects as profiles, springs, nails, rivets, needles, etc.
The alloys which may for example be used in the method according to the invention include steel alloys of the AISI 200 and 300 series and other high-alloy steels, which after being annealed at an appropriate temperature, for example, between 820 C and 1200°C, and after beeing cooled to ambient temperature can adopt a predominantly austenitic structure. Thus alloys for use in the method of the present invention contain no more than 20% ferrite, and preferably no more than 5% ferrite, when the alloy is cooled in a regular manner as currently employed in relation to Cr-Ni alloyed steels in order to avoid carbide precipitation. The austenite content amounts to more than 80% by volume and preferably to more than 95%. The said alloys must also show an Ms temperature lower than minus 50°C and an Md temperature lower than 200°C and preferably lower than 100°C.
The term "Msconversion temperature"as used herein means the temperature at which the conversion of austenite into martensite can start spontaneously without the need for external mechanical deformation. Below the Ms conversion temperature thermal martensite is formed in substantial quantities.
The term "Md conversion temperature" as used herein means the temperature above which no conversion of austenite: into martensite can take place irrespective of the amount of mechanical deformation applied. Hence, below this temperature plastic deformation may cause phase transformation. For practical purposes this deformation is preferably at least 5% (as hereinafter defined).
The term "martensite" as used herein means mechanical martensite (i.e. martensite which may be formed by mechanical means as opposed to thermal martensite) unless specifically stated otherwise. Thus whilst the production of mechanical martensite in-the appropriate quantity is advantageous the production of thermal martensite should be avoided.
As stated the plastic deformation according to the invention is partly effected at a temperature in the temperature range of from 0°C to Ms, which range is termed the cryogenic temperature and partly in a subsequent plastic deformation step at a low temperature defined as being a temperature in the range from 0°C to the recrystallization temperature, preferably at about ambient temperature i.e. without external heating.
It will be appreciated that plastic deformation will dissipate an amount of energy in the form of heat and thus the temperature of the alloy may increase considerably. Recrystallization should be avoided in any case even if extensive cooling is necessary. Moderate external cooling is considered standard practice.
The method according to the invention is advantageously effected such that the plastic deformation at the cryogenic temperature is small, for example from 5% to 60%, preferably no more than 20% (as hereinafter defined). The precise preferred percentage deformation in practice depends on the alloy composition, the deformation temperature and the embrittlement as a result of the formation of martensite. Althouah the formation of martensite during the method of the present invention is not essential, it is preferred and the volumetric martensite content following plastic deformation at a cryogenic temperature will, in general, be from 10 to 15%, but may exceptionally be as high as 80% in the case of a very ductile martensite phase.
The indicated percentage plastic deformation as referred to herein is expressed in terms of the true elongation as measured in the uniaxial tensile test. Where more complex deformations are employed the associated total deformation is expressed as "equivalent uniaxial" strain or "effective" strain as described in "Mechanical Metallurgy" by G.E. Dieter, Jr., McGraw Hill Book Company Ed., 1961, p. 66 and p. 279 et seq.
In the majority of cases the plastic deformation at cryogenic temperature is, in general, effected so that under uniaxial or multiaxial stresses, the austenite transforms into martensite above the Ms temperature. Preferably however the cryogenic temperature will be very low, such as -65 to 200°C e.g. -70°C or -190^oC, if the alloy contains substantially stabilized austenite. The occurrence of thermal martensite should be avoided. In order to obtain improved mechanical characteristics, it is possible to deform "stabilized" alloys at cryogenic temperatures without obtaining substantial amounts of martensite because the Md temperature is too low. This is the case with regard to ferro-alloys resistant to a severe corrosive environment, e.g. 20% Cr - 30% Ni ferrous alloys and alloys of the AISI 304N, 305, 316, 316N, 317 Series. Thus, in this way the tensile strength of highly corrosion resistant alloys can be improved by the use of the method of the present invention.
It will be appreciated that where deformation is effected by torsioning or profile change, the maximum deformation of the outer fibres may be much greater than the deformation of the inner fibres depending on the shape of the object.
Plastic deformation may be effected by any convenient method and, for example, complex plastic deformation at croyogenic temperature may comprise wire drawing, torsioning, rolling, hammering, flattening or other methods if desired in combination with axial elongation. More particularly, the distortional deformation offers the further advantage of preferentially reinforcing the surface area of a wire, which, for example in springs is the area which is subjected to the greatest loads; the distortion of the alloy in such cases is conveniently along its longitudinal axis.
A torsioning treatment may, for example, also be employed in addition to axial elongation in order to reinforce the core area to a sufficient extent. A torsioning treatment may even lead to an increase in martensite content by the consecutive application of a deformation in both directions without effecting a net shape modification.
It will be appreciated by skilled persons which method of plastic deformation is appropriate to the manufacture of a particular product such as a flat sheet, stranded wire cable, nails, and the like.
Reduction of the plastic deformation effected at cryogenic temperature offers the advantage that the amount of deformation heat evolved is small, and thus the required cooling capacity of the cooling medium need not be so great, or alternatively, the energy to be exchanged at cryogenic temperature can be substantially reduced. This reduction of plastic deformation may enable a greater working speed to be employed.
The amount of deformation at low temperature is defined in the same way as the deformation at cryogenic temperature.
The plastic deformation effected at low temperature may, for example, be the same as that which is applied at cryogenic temperature. However, the deformation method preferably comprises a wire drawing operation which involves a deformation under the influence of triaxial stresses.
The plastic deformation effected at cryogenic temperature is preferably such that a martensite phase is formed in a volume fraction of from 20 to 80% of the metallic matrix. This deformation at cryogenic temperature is preferably followed by a multiaxial plastic deformation at low temperature such as a wire drawing operation which, for example, results in a reduction in cross-sectional area of the wire of more than 30%. This corresponds to a deformation in which the uniaxial elongation is 45%. The martensite volume fraction considered is the content as recorded by means of a magnetometer and is based on the magnetic properties. The magnetometer is calibrated on the basis of X-ray diffraction measurements on uniform samples.
The method according to the present invention may if desired be followed by an ageing treatment. This may be effected after the above-mentioned treatment or may be conducted on finished ready-for-use products such as for example spiral springs in a tensioned condition. Ageing is a thermal treatment between for example 200 and 6060C depending on the steel type and the treatment time.
The mechanical properties determined from stress-strain curves or from torsion curves, such as tenacity, creep resistance and fatigue strength of the products of alloyed steel made according to the method of the invention may be improved over the properties of analogous objects made of the same steel by known methods.
The present invention also relates to alloyed steel products as described above when made by the process of the present invention.
The following non-limiting Examples illustrate the present invention:-

Example 1

A continuous length of wire having a diameter of 4 mm is made of AISI 302 steel, (which is cooled sufficiently rapidly after annealing to contain an appropriate austenitic phase) and cooled by means of liquid nitrogen to a temperature of about -190°C. In order to improve the economy of the cooling process, it is preferred to effect a preliminary cooling with a mixture of foamite and alcohol to, for example, -80°C.
At the temperature of about -190°C the wire is uniaxially stretched to an elongation of 6 to 8% (as hereinbefore defined) and this results in a diameter reduction to 3.85 mm. This relatively small deformation causes a transformation to the martensite phase of approximately 45% by volume. The correct volume fraction also depends, of course, on the alloy composition.
A further reduction in the diameter of the wire is effected at room temperature. The diameter is reduced to 2.72 mm in three steps, which corresponds to a reduction in cross-sectional area of 50%, each step resulting in approximately the same reduction in cross-section. After this treatment the martensite content rises to an average value of 78%, but may vary between 70% and 90%. The mechanical properties can be summarized as follows:
τw* indicates the shear stress at the respective torsion deformation which after correction with the Nadai formula approximates to the real stress. The angular deformation δ is considered equivalent to 1.8 ε (elongation).

Example 2

A continuous length of wire having a diameter of 4 mm is made of AISI 302^-steel, (which is cooled sufficiently rapidly after annealing to contain an appropriate austenitic phase) is cooled by means of liquid nitrogen to a temperature of about -190°C.
At this temperature the wire is subjected to a torsioning treatment with a torsional deflection in the S direction so that the deformation X amounts to 15%. The torsional direction is then reversed and the torsioning continued until a resulting torsional deflection δ equal to 0 is reached. The wire is returned to room temperature and further drawn in four equal steps to a diameter of 2.43 mm, which corresponds to an overall reduction in cross-sectional area of 63%. The following mechanical properties were obtained:
After the formation of cylindrical spiral springs made of wire in one of the above ways, it is possible to age these springs for half an hour at 430°C according to known techniques so as to improve the stability of the spring.

Claims

1. A method for the production of alloyed steel products having substantially no ferrite matrix phase and having Md and Ms conversion temperatures (as herein defined) such that plastic deformation may cause phase transformation, which method comprises effecting plastic deformation partly at a cryogenic temperature of from 0°C to the Ms conversion temperature (as herein defined) and partly at a low temperature of from 0°C to the recrystallisation temperature, the plastic deformation at said cryogenic temperature being effected prior to plastic deformation at said low temperature.

2. A method as claimed in claim 1 wherein the plastic deformation at said cryogenic temperature is from 5% to 60% (as herein defined).

3. A method as claimed in claim 2 wherein the plastic deformation at said cryogenic temperature is no greater than 20% (as herein defined).

4. A method as claimed in any one of claims 1-3 wherein the plastic deformation at said cryogenic temperature comprises a distortion of the alloyed steel along its longitudinal axis.

5. A method as claimed in claim 4 wherein the distortion is applied consecutively in opposite directions.

6. A method as claimed in any one of the preceding claims wherein the deformation at cryogenic temperature generates a substantial amount of martensite in the metallic matrix of the alloyed steel.

7. A method as claimed in claim 1 wherein the plastic deformation at said cryogenic temperature is effected to obtain from 20 to 80% martensite by volume of the metallic matrix.

8. A method as claimed in any one of the preceding claims wherein the plastic deformation at cryogenic temperature is effected at a temperature of from -65 to -200°C.

9. A method as claimed in any one of the preceding claims wherein a plastic deformation of at least 45% (as herein defined) is effected at said low temperature.

10. A method as claimed in any one of the preceding claims wherein the said low temperature is ambient temperature.

11. A method as claimed in any one of the preceding claims wherein the plastic deformation at said .low temperature comprises wire drawing.

12. A method as claimed in any one of the preceding claims for the production of continuous lengths of wire of alloyed steel wherein a plastic deformation of no greater than 20% at the surface of said wire is applied to alloyed steel wire having a substantially austenitic metal matrix, said wire subsequently being subjected to a wire drawing operation to effect the equivalent of an uniaxial elongation of. at least 45%.

₁3. A method as claimed in any one of the preceding claims wherein the alloyed steel product
obtained following plastic deformation is subjected to an ageing treatment at a temperature below the recrystallization temperature of the said product.

14. An alloyed steel product having
substanially no ferrite matrix phase and having Md and Ms conversion temperatures (as herein defined) such that plastic deformation may cause phase transformation, when produced by a process which includes a method as claimed in any one of the preceding claims.