EP0474530B1 - Process for manufacturing products with very high tensile strength from unstable austenitic steel, and products obtained - Google Patents

Abstract

According to this process the steel is subjected to a first plastic deformation at a temperature above the limiting temperature (Md) of the formation of martensite by deformation and lower than the recrystallisation temperature, and then to a second deformation at a temperature lower than the said temperature (Md). During the second deformation the steel is deformed in a determined range of temperature of the formation of martensite so that, in the case of an additional rational deformation of 0.1, the increase in the content of martensite formed does not at any time exceed 20%.

Description

The present invention relates to a process for producing products with a very high breaking load from an unstable austenitic steel, and the product obtained by this process, in particular in the form of a wire or strip. This process is of the type in which the steel is subjected to a first plastic deformation at a temperature higher than the limit temperature (Md) of formation of martensite by deformation and lower than the recrystallization temperature and then to a second deformation of said steel to a temperature lower than said temperature (Md).

Generally, hot-rolled steel, prior to the first deformation, is subjected to a standard annealing heat treatment known as hyper-hardening consisting in placing said steel at a temperature between 1000 and 1100 ° C for about 30 minutes.

Such a method is known. A large number of studies have been made on the tensile behavior, below the limit temperature (Md) of formation of martensite by deformation, both at room temperature and below room temperature, of materials to unstable austenite, previously hardened above said limit temperature (Md) which can vary depending on the composition of the austenitic steel between -150 ° C and + 250 ° C.

• à un premier laminage au-dessus de la température limite (Md) puis,
• à un second laminage au-dessous de la température limite (Md).

Mention may be made of US Pat. No. 4,415,377 describing a process and a device for rolling unstable austenite steels and particularly stainless steel, process according to which the steel is subjected:

• to a first rolling above the limit temperature (Md) then,
• to a second rolling below the limit temperature (Md).

These two laminations make it possible to obtain products determined in size more easily, that is to say to reduce the number of rolling passes compared to obtaining said products by cold rolling, while keeping the products similar mechanical characteristics.

The process described in the cited patent does not make it possible to obtain particular mechanical characteristics in comparison with cold rolling.

The object of the invention is to control the formation of martensite so as to obtain products with particularly high tensile characteristics.

To this end, in a first step, the steel is subjected to plastic deformation at a temperature above the limit temperature (Md) for the formation of martensite by deformation and below the recrystallization temperature. In order to obtain very high tensile mechanical characteristics, it is ensured that after plastic deformation, the breaking load of the steel is greater than 1000 MPa, preferably greater than 1300 MPa.

In a second step, the steel is deformed at a temperature below (Md), in a determined range of martensite formation temperature so that for an additional rational deformation of 0.1, the increase in the rate of martensite formed does not exceed 20% at any time.

Preferably, the steel is deformed at a temperature below (Md) with a cumulative rational deformation of between 0.7 and 3, which corresponds to a reduction rate of section included between 50 and 95%.

A minimum cumulative rational deformation of 0.7 is necessary to reach or slightly exceed the breaking loads envisaged in the drawing processes of the prior art.

On the other hand, a cumulative rational deformation cannot exceed 3, a value corresponding to a maximum admissible threshold of brittleness of the deformed steel.

Preferably after the second deformation, the steel is subjected to an aging treatment.

A rational deformation greater than 1.65 corresponding to a reduction rate of section greater than 80% introduced into the steel, a quantity of crystallographic defects sufficient in the form of dislocations to promote accommodation, this is to say the development of martensite platelets under the effect of cold deformation. The reduction rate can be much higher than this minimum rate.

• l'acier austénitique est un acier comprenant dans sa composition :
     de 0,01 à 0,15 % de carbone
     de 13 à 22 % de chrome
     de 5 à 13 % de nickel
     de 0,2 à 2,5 % de manganèse
     de 0,2 à 3 % de silicium
     de 0,01 à 0,15 % d'azote,
     le cas échéant de 0,5 à 2 % d'aluminium et moins de 2 % de molybdène,
     le reste étant du fer, et en ce que sa charge à la rupture, avant vieillissement, est supérieure à 2300 MPa.
• l'acier austénitique est un acier comprenant dans sa composition :
     de 0,2 à 1 % de carbone
     de 15 à 30 % de manganèse
     de 0,01 à 0,7 % d'azote,
     le cas échéant moins de 5 % de chrome et moins de 7 % d'aluminium,
     le reste étant du fer, et en ce que sa charge à la rupture, avant vieillissement, est supérieure à 2300 MPa.
• l'acier austénitique est un acier comprenant dans sa composition :
     de 0,1 à 0,6 % de carbone
     de 6 à 25 % de nickel
     de 0 à 13 % de chrome
     de 0 à 4 % de molybdène
     de 0 à 3 % de silicium ;
     le reste étant du fer, et en ce que sa charge à la rupture, avant vieillissement, est supérieure à 2300 MPa.

The invention also relates to a product with very high breaking load obtained by deformation, from an unstable austenitic steel, in the form in particular of wire or strip. According to three variants, this product produced according to the process of the invention, is characterized in that:

• Austenitic steel is a steel comprising in its composition:
  from 0.01 to 0.15% carbon
  13 to 22% chromium
  5 to 13% nickel
  0.2 to 2.5% manganese
  0.2 to 3% silicon
  from 0.01 to 0.15% nitrogen,
  where appropriate from 0.5 to 2% of aluminum and less than 2% of molybdenum,
  the rest being iron, and in that its breaking load, before aging, is greater than 2300 MPa.
• Austenitic steel is a steel comprising in its composition:
  0.2 to 1% carbon
  15 to 30% manganese
  from 0.01 to 0.7% nitrogen,
  where appropriate, less than 5% of chromium and less than 7% of aluminum,
  the rest being iron, and in that its breaking load, before aging, is greater than 2300 MPa.
• Austenitic steel is a steel comprising in its composition:
  0.1 to 0.6% carbon
  6 to 25% nickel
  0 to 13% chromium
  0 to 4% molybdenum
  0 to 3% silicon;
  the rest being iron, and in that its breaking load, before aging, is greater than 2300 MPa.

The following description, as well as the accompanying drawings, all given by way of non-limiting example, will make the invention easier to understand.

• les courbes 1A à 3A représentent dans deux opérations de laminage à tiède puis à froid pour trois aciers pris en exemple, le taux de martensite obtenu en fonction de différentes déformations rationnelles cumulées;
• les courbes 1B à 3B représentent les caractéristiques en traction pour les trois aciers pris en exemple, en fonction de la déformation rationnelle cumulée, dans différentes conditions de traitement.

In these drawings:

• curves 1A to 3A represent in two lamination operations, lukewarm and then cold for three steels taken as an example, the rate of martensite obtained as a function of different cumulative rational strains;
• curves 1B to 3B represent the tensile characteristics for the three steels taken as an example, depending on the cumulated rational deformation, under different processing conditions.

The products with very high breaking load according to the invention, in particular in the form of wire or strip, are obtained by a plasticity effect induced by deformation.

Steels hardened at a temperature above the limit temperature (Md) for forming martensite by deformation, also called "warm" hardened, have no particular properties of use.

When the steels are subjected to lukewarm hardening and then to cold hardening, we observe, under certain conditions, a particularly high plasticity, which allows a cold mechanical transformation by successive passes, for example with reduction rates of 25% and obtaining load levels at break of more than 2300 Mpa, the product retaining an acceptable ductility from one pass to the next.

The effect of cold plasticity after warm hardening is greater the higher the breaking load of the cold worked steel.

Aging of steels subjected to warm work hardening, followed by cold work hardening according to the process can increase the breaking load by around 200 MPa and sometimes more depending on the grade of steel used.

The conditions for obtaining high tensile strengths are attached on the one hand to the mechanical characteristics obtained during lukewarm hardening, the steel having to have at least one tensile strength greater than 1000 MPa and on the other hand to the conditions of cold work hardening, according to which the work hardening of the steel is carried out in a determined temperature range, called critical field of temperature of deformation of martensite, so that for an additional rational formation of 0.1, the increase in the rate of martensite formed does not exceed, at any time, 20%.

By rational deformation is meant the logarithm of the ratio of the surface S of the section after deformation on the surface So of the initial section (ln S / So).

It should be noted that depending on the grade of steel or unstable austenitic alloy considered, the speed of martensite formation, linked to the critical range of martensite formation temperature and to the cumulative rational deformation of between 0 , 7 and 3, must be checked to obtain high mechanical characteristics, in other words it is necessary not to form the martensite too quickly during cold work hardening.

The determined range of martensite formation temperature is, for the steels taken as an example, included in the range -20 ° C. + 180 ° C. Indeed, beyond + 180 ° C there is no longer an appreciable amount of martensite, while below -20 ° C the formation of martensite is excessive.

Wire drawing tests were carried out and made it possible to highlight the surprising effect of the plasticity induced in unstable austenitic steels, treated according to the method of the invention.

Three embodiments demonstrating the invention will be described below:

The basic thread used in these three examples is a wire rod about 5.6mm in diameter; the wire is cold-worked at 250 ° C in several passes up to a diameter of approximately 2mm, then this wire is cold drawn at room temperature of 20 ° C in order to obtain a wire of approximately 0.5mm in diameter, the section reduction rates per drawing pass being approximately 25%. After each drawing pass the wire returns to room temperature before another pass. The number of lukewarm passes is approximately 20, the number of cold passes being approximately 10.

Example 1:

The steel used for the implementation of the process is a steel referenced 1, having the following composition by weight: 0.08% of carbon, 18.6% of chromium, 8.5% of nickel, 0.3% of manganese , 0.5% silicon, 0.04% nitrogen.

During lukewarm hardening, there is no formation of martensite and the breaking load reached is approximately 1500 Mpa; after cold drawing the breaking load of the steel wire is around 2500 MPa.

An aging treatment at 400 ° C. for one hour then gives it a breaking load of approximately 2700 MPa.

Curve 1A represents, for the steel referenced 1, the evolution of the rate of martensite as a function of the rational deformation, this rate of martensite reaching around 60% after cold drawing.

Curve 1B represents the evolution of the load at break during lukewarm hardening up to 1400 MPa then the evolution of the load at break during cold drawing thus reaching 2500 MPa; aging provides a breaking load of approximately 2700 Mpa.

The increase in the breaking load is obtained by controlling the increase in the formation of martensite from one wire drawing pass to another.

Example 2:

The steel used is a steel referenced 2, having the following weight composition: 0.09% carbon, 17.3% chromium, 8.3% nickel, 0.9% manganese, 0.8% silicon , 0.06% nitrogen.

Curve 2A illustrates for the steel referenced 2 the evolution of the rate of martensite as a function of the cumulative rational deformation, the rate of martensite reached being around 45%.

Curve 2B represents the evolution of the breaking load during lukewarm hardening, up to a breaking load of 1600 MPa, then the evolution of said breaking load during cold drawing, reaching about 2500 MPa.

Aging at 400 ° C for 1 hour improves the value of the breaking load by around 200 MPa.

Example 3:

The steel used is a steel referenced 3 and having the following composition: 0.09% carbon, 17.3% chromium, 7.7% nickel, 0.5% manganese, 0.8% silicon, 0.15% nitrogen, 1% aluminum.

The curve 3A represents for the steel referenced 3 the evolution of the rate of martensite as a function of the rational deformation, the rate of martensite being able to reach 95%.

Curve 3B represents the evolution of the breaking load during lukewarm hardening, up to the breaking load of approximately 1600 MPa, then the evolution of said breaking load during cold drawing, reaching around 2300 Mpa.

The gain after heat treatment at 480 ° C for 45 minutes is in this example about 300 MPa.

Such characteristics are obtained by the formation of martensite during cold drawing, slower formation on a product previously cold-worked at lukewarm than by direct cold drawing of the wire rod.

• en optimisant l'écrouissage à tiède;
• par addition dans les compositions d'éléments tels que molybdène, silicium, ... pour durcir la martensite formée;
• par addition d'éléments tels que cuivre, aluminium, ... favorisant des précipitations durcissantes lors du vieillissement final.

Even higher characteristics can be obtained:

• optimizing the warm-up;
• by adding elements such as molybdenum, silicon, etc. to the compositions to harden the martensite formed;
• by adding elements such as copper, aluminum, ... favoring hardening precipitation during final aging.

The first “lukewarm” deformation or work hardening operation may be a wire drawing, as described in the examples, but also a rolling, hammering or forging, twisting, alternating bending, or the like.

The second cold deformation operation can also take various aspects: drawing, rolling or other.

Claims (9)

1. Process for manufacturing products of very high tensile strength from unstable austenitic steel, according to which the steel is subjected to a first plastic deformation at a temperature higher than the limit temperature (Md) for the formation of martensite by deformation and lower than the recrystallisation temperature, and the steel is then subjected to a second deformation at a temperature lower than the said limit temperature (Md), characterised in that, during the second deformation, the steel is deformed within a determined temperature range for the formation of martensite in such a manner that, for an additional ratio of deformation of 0.1, the increase in the amount of martensite formed does not at any time exceed 20%.
2. Process according to Claim 1, characterised in that the steel is deformed at a temperature lower than (Md) with a cumulative ratio of deformation of between 0.7 and 3.
3. Process according to Claim 1, characterised in that, after the second deformation, the steel is subjected to an ageing treatment.
4. Process according to Claim 1, characterised in that the first plastic deformation is effected with a ratio of deformation greater than 1.65.
5. Process according to Claims 1 and 4, characterised in that, after the first plastic deformation, the tensile strength of the steel is greater than 1300 MPa.
6. Product of very high tensile strength obtained from unstable austenitic steel, especially in the form of wire or ribbon, characterised in that it is manufactured by the process according to any one of Claims 1 to 5, in that the austenitic steel is a steel comprising in its composition:
      from 0.01 to 0.15 % of carbon
      from 13 to 23 % of chromium
      from 5 to 13 % of nickel
      from 0.2 to 2.5 % of manganese
      from 0.2 to 3 % of silicon
      from 0.01 to 0.15 % of nitrogen
      where appropriate from 0.5 to 2 % of aluminium and less than 2 % of molybdenum,
      the remainder being iron, and in that its tensile strength, before ageing, is greater than 2300 MPa.
7. Product of very high tensile strength obtained from unstable austenitic steel, especially in the form of wire or ribbon, characterised in that it is manufactured by the process according to any one of Claims 1 to 5, in that the austenitic steel is a steel comprising in its composition:
      from 0.2 to 1 % of carbon
      from 15 to 30 % of manganese
      from 0.01 to 0.7 % of nitrogen
      where appropriate less than 5 % of chromium and less than 7 % of aluminium, and in that its tensile strength, before ageing, is greater than 2300 MPa.
8. Product of very high tensile strength obtained from unstable austenitic steel, especially in the form of wire or ribbon, characterized in that it is manufactured by the process according to any one of Claims 1 to 5, in that the austenitic steel is a steel comprising in its composition:
      from 0.1 to 0.6 % of carbon
      from 6 to 25 % of nickel
      from 0 to 13 % of chromium
      from 0 to 4 % of molybdenum
      from 0 to 3 % of silicon,
      the remainder being iron, and in that its tensile strength, before ageing, is greater than 2300 MPa.
9. Product according to any one of Claims 6 to 8, characterised in that its tensile strength after ageing is greater than 2500 MPa.

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