FR2479854A1 - STAINLESS STEEL, FREE OF FERRITE, FOR STRUCTURAL CURING - Google Patents

Abstract

STAINLESS STEEL, FERRITE FREE, SUITABLE FOR STRUCTURAL HARDENING. A TYPICAL COMPOSITION CONSISTS OF BY WEIGHT, 0.07 TO 0.09 CARBON, 1.0 MAXIMUM MANGANESE, 0.04 MAXIMUM PHOSPHORUS, 0.15 MAXIMUM SULFUR, 1.0 MAXIMUM SILICON, 16 , 0 TO 17.0 CHROME, 6.5 TO 8.0 NICKEL, 1.05 TO 1.75 ALUMINUM, 0.02 TO 0.08 NITROGEN, 0.50 MAXIMUM MOLYBDENE, 0 , 50 AT THE MAXIMUM OF COPPER, 0.12 AT THE MAXIMUM OF TITANIUM, THE TITANIUM REPRESENTING ABOUT 3.5 TIMES THE NITROGEN CONTENT WHEN THE NITROGEN DOES NOT EXCEED 0.035, 0.001 TO 0.05 OF BORON, THE REMAINING ESSENTIAL FORM OF IRON.

Description

The invention relates to a stainless steel capable of

  structural crisis which is in a single phase at its various

  states and which has resistance, hardness and

  superior to those of 17-7 PH nominal stainless steels which usually contain up to about 15%

  ferrite delta to all states. Although this is not

  As a result, the steel of the invention is particularly useful as a material capable of yielding with an elastic characteristic. In the

  description, the abbreviation PH stands for "hardenable

structurally ".

  The steel of the invention has a good heat-formability, it can be cold-reduced by up to 90% without intermediate annealing, it has a

  high proportional or elastic resistance and a temperature

  uniformity of martensitic transformation, lower than

ambient temperatures.

  In a generally known manner, the heat treatment of steels 17-7 PH requires the following steps:

  Solution annealing at 1038 + 14 ° C (for barbs)

  or wires) or at 1066 + 14'C (for sheets or foils)

  lard) for a minimum of 30 minutes to achieve state A. Conditioning the austenite at 760 + 14 ° C for 90 minutes.

  Transformation of austenite to martensite by cooling

  at a temperature of 10 to 60C maintained for a minimum of 30 minutes to obtain the state T. Structural hardening by heating at 565.6 ± 5.60C for 90 minutes, followed by air cooling up

  the ambient temperature, to obtain the TH 1050 state.

  Better mechanical properties are obtained by a treatment variant applied to a material of state A and which is as follows: Conditioning of the austenite at 954 + 90C for 10

  minutes and air-cooling to obtain state A 1750.

  Transformation of austenite to martensite by cooling

  in one hour at a temperature of -73.3 + 5.5 C and hold for 8 hours to obtain the

R 100.

  Structural curing by heating at 510 + 5.50C for 60 minutes, followed by cooling in air at room temperature, to obtain the RH 950 state. The highest mechanical properties are obtained by a treatment variant, applied to a matter of state A and which is the following:

  Cold reduction of at least 50% (at the rolling mill),

  austenite to martensite to obtain state C. Structural curing by heating at 482.2 + 5.50C for 30 minutes (bars and wires) at 60 minutes (sheets and strips), followed by cooling at air at temperature

  ambient to obtain the CH 900 state.

  Typically, the steel manufacturer supplies 17-7 PH stainless steel in the A or C state as sheet, strip, plate, bar or wire. The buyer then usually performs the desired transformation of the material in state A or state C and conducts the treatments described above on the products manufactured for

  reach TH 1050, RH 950 or CH 900.

  The 17-7 PH steel has the following composition, by weight, as determined by reference to AMS 5528D (revised 15.1.78): Carbon 0.09% maximum, manganese 1.00% maximum, silicon 1.00% maximum, phosphorus up to 0.040%, sulfur up to 0.030%, chromium 16.00 to 18.00%, nickel 6.50 to 7.75%,

  aluminum 0.75 to 1.50%, the balance being iron.

  The standard 17-7 PH stainless steel manufactured according to the above standards has several drawbacks including stress cracking during cold drawing, transformation of a state material A to about -180C, embrittlement by the hydrogen component of the ferrite delta component during copper plating (applied as a lubricant for

  wire drawing), non-uniform properties in state C and in

  CH 900 and the need for dual conversion

  or triple for the ingot transformation of 33 cm

square billets of 10 cm.

  The above drawbacks are mainly due to the presence of about 10 to 15% of delta ferrite, which remains

  unchanged in all the various states of steel processing.

  Delta ferrite is the fracture site during mechanical deformation. In the cold drawing of bar and wire sections, a longitudinal crack appears which propagates through the delta ferrite regions. In addition, since delta ferrite is not reinforced by cold work

  heat treatment to the same extent as the

  austenite, the delta ferrite content

  dye the maximum possible mechanical properties of 17-7 PH.

  The presence of about 10 to 15% of ferrite in steel

  17-7 PH classic, in the state resulting from the annealing of setting

  lution, is a direct result of the lack of metal stability

  lurgique. The aluminum content of the 17-7 PH steel lowers the minimum temperature at which the delta ferrite is a stable phase. Thus, heating and hot working do not cause sufficient chemical homogenization to give a fully austenitic structure such as

  that we obtain in the 301 AISI type (containing very few

luminium).

  Despite the above disadvantages, the pre-

  delta ferrite in amounts up to about 15% in conventional 17-7 PH steels because it is considered

  It is necessary to specify fairly wide commercial ranges

  to enable the production of steel between

  ticables. Prior patents describing steels 17-7 PH are in particular US Pat. Nos. 2,505,763, 2,505,764 and 2,506,558.

and 2,553,707.

  As far as the Applicant is aware, it appears that previous attempts to modify the conventional commercial ranges of 17-7 PH steels have failed to eliminate delta ferrite or have resulted in

  detrimental effects on other desired properties of the steel.

  U.S. Patent 3,253,908 discloses a hardenable steel.

  structural composition whose broad composition range comprises mainly 9.00 to 20.00% chromium, 2.50 to 8.00% nickel, 0.70 to 2.50% aluminum, 1 to 5% molybdenum , the total contents of chromium and molybdenum being 14 to 21%, 0,10 to 0,40% of nitrogen, 0,12% maximum of carbon, 8,00% maximum of manganese, the content of manganese being inversely proportional to the nickel content, 2.00%

  of silicon, maximum 0,050% of phosphorus, 0,050% maximum

  mum of sulfur, the rest being essentially formed of iron.

  Nitrogen and molybdenum are essential elements of

  according to the cited patent, but despite the great austenite formation power of nitrogen, it has been indicated

  that this steel contains delta ferrite in quantities not

  not exceeding 10% in volume. The high nitrogen content is a disadvantage because it reacts with the added aluminum for the structural hardening and the formed aluminum nitrides are occluded in the solidified metal which has an effect

  harmful to ductility and cold forming ability.

  The essential presence of molybdenum is another disadvantage because it is a ferrite trainer still

more powerful than chrome.

  US Pat. No. 3,071,460 also discloses a stainless steel

  chromium-nickel-aluminum fiber suitable for structural hardening,

  voluntarily added 0.10 to 0.4% of nitrogen and in the

  which carbon is restricted to a critically low level of

  0,03% maximum, the steel of this patent not containing any

  lybdène. U.S. Patent No. 3,117,861 discloses a stainless steel

  chromium-nickel-aluminum, suitable for structural hardening,

  additionally of at least one of the titanium, zircon

  niu # and uranium to react on the nitrogen dissolved in the

  to prevent harmful occlusions of nitrides

  aluminum. The main purpose of this amendment was to

  avoid porosity, during welding, along the joint

  between the caterpillar and the base metal.

  An article by W.C. Clarke Jr. and H. W. Garvin, inti

  "ASTM Special Technical Publication No. 369 (1965)," pages 151 to 151, entitled "Effect of Composition and Section Size on Mechanical Properties of Some Precipitation Hardening Stainless Steels".

  158, discusses the disadvantage of poor trans-ductility

  in large sections and in the trans-

  short versal thin sections of alloys such as 17-7 PH and PH 15-7 MB. It is said that this disadvantage is due to

  the presence of significant amounts of delta ferrite and

  at the edges of the ferrite delta masses. Studies have

  determined that thermal treatments of all kinds do not

  were not effective in improving transverse ductility

  and that likewise a vacuum reflow was ineffective.

  A modified composition has been described in which the

  luminium is still used as a curing agent, which is called PH 13-8 MB and which is free of delta ferrite and

  therefore has good transverse ductility, especially

  especially when melted twice under vacuum by induction and remelting techniques. The variant called PH 13-8 Mo has a notable reduction in the carbon content, to a level of about 0.025%, as well as a decrease in the chromium content of up to 13%, an increase of the nickel content up to 8% and the addition of about 2.25% molybdenum,

  compared to 17-7 PH steel. The modified alloy is martensi-

  after the solution treatment and structural hardening can be achieved by simple

low temperature.

  Speaking of 17-7 PH and PH 15-7 MB, the authors indicate: "At the chromium levels in question, the necessary composition equilibrium leads to a structural phase

  secondary formed of fairly significant amounts of delta ferrite.

  This phase can constitute up to 12% to 30% of the structure of these two alloys. It has been observed that delta ferrite

  the transverse ductility of other martensi- stainless steels

  ticks and this is the case in these alloys.

  The article cited, although published in 1965,

  a current description of the state of the art in

  which concerns alloys of this type.

  It appears that the solution to which the previous researchers arrived mainly consisted of reducing the carbon and chromium contents in order to eliminate the existence of delta ferrite. These modifications result in the loss of a stable austenitic structure and a sharp increase in the achievable minimum hardness in the alloy under various processing conditions. The hot forming ability of the low carbon and low chromium variant is further limited. It is therefore clear that there is still a need for a structural hardenable stainless steel that avoids the disadvantages of prior art alloys of this type.

  aluminum as a curing agent.

  The subject of the invention is essentially a stainless steel

  chromium-nickel-aluminum-iron dable suitable for structural hardening

  containing less than 5% ferrite delta at all

  treatment, presenting as a result of the treatment of

  Increased tensile ductility and lower tensile strength than conventional stainless steel, with no stress cracking due to ferrite

  delta in cold work operations and endowed with stabi-

  metallurgical unit at temperatures down to about

-30 C.

  The invention also relates to a steel of this kind which has an improved ability to hot work ingots and billets and lends itself to higher degrees of work

  cold than a 17-7 PH, without mechanical breakage, and, more

  that can be stretched cold with a reduction in

  Viron 90% of the section without intermediate annealing.

  The above objects are achieved in the steel according to the invention by critical balancing of carbon, manganese, silicon, chromium, nickel, aluminum and nitrogen contents. In addition, the molybdenum must be limited to residual levels not exceeding 0.5%. Boron may optionally be added as a carbide precipitation aid in the heat treatment of austenite conditioning. Titanium and / or cerium can be added to

  limit the formation of aluminum nitride.

  The invention relates to a stainless steel capable of

  substantially at a single phase at all treatment states, essentially comprising, by weight, 0.07 to 0.12% carbon, 0.20 to 3.0% manganese, 0.07% maximum phosphorus, not more than 0.15% sulfur, not more than 2.0% silicon, 15.5 to 17.5% chromium, 6.0 to 9.0% nickel,

  0.95 to 2.50% aluminum, 0.005 to 0.20% nitrogen, 0.50% maximum

  molybdenum, up to 0,75% copper, up to 0,07% boron, up to 0,12% titanium when the nitrogen does not exceed 0,035%, up to 0,05% of cerium when the nitrogen exceeds 0.035%,

  the rest being essentially iron.

  Carbon is essential in the range indicated below.

  above for its great austenitic and stabilizing power

austenite.

  Manganese is necessary for microbial stabilization

  pre-established austenitic structures.

  Silicon is limited to a maximum of 2.0% and pre-

  at a maximum of 1.0% because it is a ferrite trainer.

  Chromium is of course essential for resistance to

  corrosion. It is limited between 15.5 and 17.5% and preferably

  between 16.0 and 17.0% because it is a ferrite trainer.

  Nickel is essential for its austenite forming power and as a structural curing agent. It is balanced with chromium in the range indicated above, from

preferably from 6.5 to 8.0%.

  Aluminum is essential as a structural curing agent. With less than 0.95% aluminum, the proportion

  volume of nickel-aluminum compounds available for dura-

  Interference creep is not sufficient to give a minimum hardness of Rockwell C38 at TH1050 or Rockwell C42 at RH 950. Since aluminum is

  a ferrite trainer, it is limited to a maximum of 2.50%.

  Nitrogen is necessary for its great power of

  of austenite and is preferably from 0.02 to

0.08%.

  Aluminum is more effective than copper as a precipitation hardener because copper has a

- 2479854

8-

  density 3.3 times higher than that of aluminum.

  Thus, 1.0% by weight of aluminum represents as many atoms as 3.3% by weight of copper. Since the solubility limit of copper in iron-chromium-nickel alloys is about 4% and "free" copper has a very detrimental influence on hot malleability, the volume proportion of nickel-copper compounds available for hardening by

  interference is very limited in comparison to aluminum.

  Copper is also much more expensive than aluminum.

  A preferred composition comprises essentially by weight, 0.07 to 0.09% carbon, 1.0% maximum manganese, 0.04% maximum phosphorus, 0.025% maximum sulfur, 1.0% maximum of silicon, 16.0 to 17.0% of chromium, 6.5 to

  8.00% nickel, 1.05 to 1.75% aluminum, 0.02 to 0.08% of

  zote, 0.50% maximum of molybdenum, 0.50% maximum of copper, 0.12% maximum of titanium, titanium representing approximately 3.5 times the nitrogen content when nitrogen does not exceed 0.035% , 0.001 to 0.05% boron, the balance being essentially iron. A preferred composition in which the content

  carbon is slightly increased, nitrogen is added vol-

  cerium replaces titanium so as to prevent the formation of aluminum nitrides, essentially comprises

  by weight, 0,07 to 0,12% of carbon, 1,0% at most of manganese

  0.04% maximum phosphorus, 0.03% maximum sulfur, 1.0% maximum silicon, 16.5% to 17.5% chromium, 6.75% to 7.75% nickel , 1.05 to 1.75% aluminum, more than 0.035 to 0.20% nitrogen, 0.50% maximum molybdenum, 0.50% maximum copper, 0.004 to 0.02% cerium, 0.001 to 0.05% boron, the

  the remainder being essentially formed of iron.

  In the first preferential embodiment above, optimum results are obtained by maintaining the carbon at a maximum value of 0.09% or in the vicinity thereof, in

  limiting manganese and silicon to the lowest levels

  ticable, keeping nickel at about 7.25% minimum, restricting aluminum to 1.4% or near, limiting molybdenum to the lowest practicable level, maintaining nitrogen at or near 0.035% and by keeping the titanium to

  about 3.5 times the nitrogen content.

  In conventional 17-7 PH steel, a maximum of 0.030%

  sulfur content is specified in order to minimize the influence of

  Sulfur particles on the trend of longitudinal crack during cold work operations. Since the steel of the invention is substantially free of ferrite, a feature of the invention is that it is possible to increase

  sulfur up to about 0.15%, both in the composi-

  than in the preferred composition, to obtain

good machinability.

  In the preferential composition o car rates

  Bone and nitrogen are increased above the maximum permitted under the current conventional specifications, titanium is omitted and replaced by cerium to prevent the formation of aluminum nitrides. Virtually all aluminum is so

  available to act as a structural curing agent

  ral. We have found that it is not necessary to add cerium

  in a stoichiometric proportion relative to nitrogen as

  cerium forms with nitrogen a complex of one or

the other kind.

  Thanks to its properties, the steel of the invention is suitable

  ideally for processing into products such as

  cold rolled and / or machined, elastically yieldable products, including coil springs, antenna rods, fishing rods, leaf springs, bellows, grinder balls, bearings

  roller and ball bearings.

  The steel of the invention can be processed according to the

  conventional elements to obtain the various states described above. More specifically, a state A steel can be subjected to dissolution annealing (to obtain the A 1750 state) in

  heating it between 955 and 1120 ° C for a sufficient time

  to dissolve the carbon and nitrogen compounds, then cool to room temperature,

  medium of air, oil or water. As mentioned above, the addi-

  boron to the steel according to the invention promotes the precipitation of

  carbides in the conditioning of austenite. The transformation of the material resulting from solution annealing to alpha martensity can then be carried out, or by heating between 650 and 9250C for about 3

  maximum hours and cooling to between 10 and 15

  ron (T state), ie by cooling to about -730C and

  for periods of up to 8 hours (to obtain

  state R 100), either by cold working the material re-

  sultant solution annealing with reduction of 50 to

  % of the thickness or area of section, to obtain

  In addition, structural hardening is obtained by heating at approximately 425-525 ° C for periods of 8 minutes.

  maximum hours and then cool to room temperature

  ature. The improved ductility of the steel of the invention offers a clear advantage in that the steel manufacturer can supply it to transformers in state C or in the state CH and that they can transform it into final products which are subject to structural hardening. As mentioned above, another advantage is that the steel of the invention can be cold reduced to a degree of up to 90% and brought to the C state by

  a single stage of cold reduction, without intermedia-

  res, a process that can not be followed with conventional 17-7 PH steel.

  In known manner, improvements in

  strength, resistance to intergranular corrosion and

  resistance to fatigue by repeating the treatment of trans-

  formation of the T state or R100 state material prior to processing

structural hardening.

  Experimental fillers falling within the preferred ranges of compositions were made and these compositions are indicated in Table I. The mechanical properties at various states were determined and are indicated in Table II for sample 3 (preferential embodiment containing a addition of nitrogen as well as cerium) and for a

17-7 PH classic steel.

  Table 3 shows the tensile strength and yield strength of samples 3, 4 and 5, which are voluntarily added nitrogen and cerium, for cold drawing reductions ranging from 89% to 89%. in state A after solution treatment, at

  the transformed state and after structural hardening.

  The data in Tables II and III show that

  the steel of the invention has a tensile breaking load

  and elastic limit comparable to conventional 17-7 PH steel at the same time as higher levels of elongation and surface reduction. The ductility improvement of the steel of the invention is important in the manufacture of cold-drawn or cold-rolled materials.

  in order to minimize the mechanical breakages of those

  this. This makes it possible to obtain elastic characteristics of

  income in a wide range of wire and bar sizes.

  Proportional limit or elastic resistance levels are the basis of the load capacities of yield products with elastic characteristics. As a result, the levels

  from about 1170 to more than 1310 MPa of the steel of the invention.

  provide a significant improvement over the 17-7 PH

  which has an elastic limit of about 724 to 758 MPa.

  As indicated above, the relatively high ductility of the steel according to the invention allows the transformation into end products in the C state or even in the CH state. The rate of

  soluble carbon of the steel of the invention is such that the temperature

  Martensitic transformation temperature is about -340C, which prevents processing during cold weather transport but does not constitute an excessively expensive processing treatment for a transformer if a state A material is supplied and if the products manufactured are subject to

  treatments leading to state A1750 and state R100.

  The increase in tensile strength and yield strength at various treatment states is accompanied by

  corresponding resistance increases. As an example

  a material in state A has a Brinnel hardness of

  180, which is increased to about 260 for a subject in the field of

  T tat and at about 430 for the TH state and the C state. To illustrate the critical balancing of elements that virtually eliminate delta ferrite in the steel of the invention, Table IV gives the comparison.

  between the conventional 17-7 PH and steels of the invention.

  Other steel charges have been prepared according to the invention.

  and their mechanical properties were determined at various processing conditions. The compositions of these charges are

  shown in Table V and their mechanical properties are

  capitulated in Table VI, the samples having been converted

  billets 5.1 cm thick in 2.54 mm thick strip.

TABLE I

  Composition,% by weight Sample C Mn P Si Si Cr Ni Al N Ti Ce

  1 0.095 0.54 0.020 0.014 0.34 16.89 7.28 1.24 0.022 0.060 -

  2 0.10 0.50 0.022 0.023 0.34 16.82 7.31 1.21 0.031 0.067 -

  3 0.076 0.87 0.012 0.014 0.60 17.08 7.34 1.07 0.10 - 0.005

  4 0.081 0.85 0.012 0.019 0.50 16.72 7.26 1.02 0.11 -

  0.083 1.07 0.012 0.013 0.81 17.07 7.20 1.19 0.095 -

  Mo <0.5%, Cu <0.5% W% ferrite delta: samples 1 and 2 <0.5%, samples 3 to 5 = 0% Co Ln I0

TABLE II

  Sample State Load of. Tensile strength, MPa Mechanical properties Limit proportional elastic resistance Elongation% to 0.2%, MPa or elastic, MPa in 5.1 cm 3 C (6.35 mm diameter hot-rolled bar reel; dissolved at 1038 C, cold drawn into a 4.45 mm diameter rod coil, erected and cut lengthwise

1430 1374

  average of 2 average of 2.0 average of 42.0 2 average of 2 3 CH (state C + 454 C - 1 hour, air cooling) 3 CH (state C + 482 C - 1 hour, cooling at room temperature air) 3 CH (state C + 510 C - 1 hour, air-cooling) average of 2 2 i average of 2 mean of 2 average of 2, 0 mean of 2 12,2 average of 2 41 1 average of 2 to 39.4 mean of 2 17-7 PH C (treatment as above conventional in example 3)

* 17-7 PH

  classical CH (state C + 482 C - 1 hour, air-cooled) Reduction of area,%, 5 38.9 4.0 4.0, 0, 0 -4% 0 co in State A (bar 6 , 35 mm diameter, pickled) C (cold drawn, 50% reduction) CH (C + 482 C - 1 h, air-cooled) C '(cold drawn, 60% reduction) C'H (C '+ 482 C - 1 h, air-cooled) C "(cold drawn, 89% reduction) C" H (C "+ 482 C 1 h, air-cooled)

TABLE III

  Mechanical Properties, MPa Sample 3 Sample 4 Sample 5 Limit Load Limit Load Breakdown Limit Load in Elastic Breakdown Elastic Breakage Tensile Strength 0.2% Tensile 0.2% Tensile 0_2%, A D N% 0 Co L4 n

TABLE IV

Sample

17-7 PH

  6 * 7 * C 0.064 0.087 0.083 Mn 0.54 0.92 0.84 Si 0.25 0.39 0.57 * Steels of the invention

TABLE V

  Composition,% by weight Sample C 0.091 0.082 0.087 Mn 0.91 0.89 0.80 Si 0.38 0.33 0.32 Cr 16.87 16.90 16.28 Ni 7.41 7.60 7.50 Al 1.09 1.26 1, 12 Ferrite delta,% (billet, 2 cm) o0 N 0.030 0.029 0.032 Ferrite delta,% (bar 0.64 cm), 0 1.7 Cr 16.92 17.10 17, 01 Ni 7.51 7.36 7.57 Al 1.121.19 1.17 Mo 0.21 0.14 0.15 Cu 0.32 0.11 0.12 N 0.031 0.032 0.024 Ti 0.046 0.064 0.062% ri% O CO Ln

TABLE VI

  Mechanical Properties Sample 8- Condition

TH 1050

RH 950

  I. State C * Tensile tensile strength, MPa Elastic limit Elongation 0.2%, MiPa% over 5.1 cm

503 17

862 13

883 12

  State CH-900 l * State C = cold reduction 60% 3.0 4.0 2.0

Claims (7)

R E V E N D I C A T I ONS   1. Structurally hardenable stainless steel, containing mehs of 5% by volume of ferrite at all states of   treatment and characterized by the fact that it essentially comprises   by weight, 0.07 to 0.12% carbon, 0.20 to 3.0% manganese, 0.07% maximum phosphorus, 0.15% maximum sulfur, 2.0 % silicon, 15.5 to 17.5% chromium, 6.0 to 9.0% nickel, 0.95 to 2.50% aluminum, 0.005 to 0.20% nitrogen, 0.50% maximum molybdenum, maximum 0.75% copper, up to 0.07% boron, up to 0.12% titanium when nitrogen does not exceed 0.035%, up to 0.05% cerium when the nitrogen exceeds 0.035%, the remainder being essentially formed of iron. 2. Stainless steel capable of structural hardening, practically a single phase at all the treatment states and characterized in that it comprises essentially, by weight, 0.07 to 0.09% of carbon, 1.0% at manganese, maximum 0.04% phosphorus, maximum 0.025% sulfur, maximum 1.0% silicon, 16.0 to 17.0% chromium, 6.5 to   8.00% nickel, 1.05 to 1.75% aluminum, 0.02 to 0.08% of   zote, 0.50% maximum molybdenum, 0.50% maximum copper, 0.12% maximum titanium, titanium being approximately 3.5 times the nitrogen content when the nitrogen does not exceed 0.035%, 0.001 to 0.05% boron, the balance being essentially iron, 3. Stainless steel capable of structural hardening, practically a single phase at all treatment states and characterized in that it comprises essentially, by weight, 0.07 to 0.12% of carbon, 1.0% at maximum manganese, maximum 0.04% phosphorus, maximum 0.03% sulfur, maximum 1.0% silicon, 16.5 to 17.5% chromium, 6.75 to 7.75% nickel, 1.05 to 1.75% aluminum, more than 0.035 to 0.20% nitrogen, 0.50% maximum molybdenum, 0.50% maximum copper, 0.004 to 0.02 % of cerium, 0.001 to 0.05% boron, the   the remainder being essentially formed of iron.   4. Steel according to claim 2, characterized in that it contains about 0.09% carbon, at least about 7.25% nickel, about 1.4% aluminum and about 0.035% nitrogen. 5. Steel according to claim 1, characterized in that it comprises essentially, by weight, 0.07 to 0.09% carbon, 1.0% maximum manganese, 0.04% maximum of   phosphorus, maximum sulfur content not exceeding 0.15%, maximum   licium, 16.0 to 17.0% chromium, 6.5 to 8.0% nickel, 1.05 to 1.75% aluminum, 0.02 to 0.08% nitrogen, 0, 50% maximum of molybdenum, 0.50% maximum of copper, 0.12% maximum of titanium, titanium representing approximately 3.5 times the nitrogen content when the nitrogen does not exceed 0.035%, 0.001 to 0 , 05% of   boron, the remainder being essentially iron.   6. Steel according to claim 1, characterized in that it comprises essentially, by weight, 0.07 to 0.12% of carbon, maximum 1.0% of manganese, 0.04% maximum of   phosphorus, maximum sulfur content not exceeding 0.15%, maximum   licium, 16.5 to 17.5% chromium, 6.75 to 7.75% nickel, 1.05 to   1.75% aluminum, more than 0.035 to 0.20% nitrogen, 0.50% maximum   mum of molybdenum, 0.50% maximum of copper, 0.004 to 0.02% of cerium, 0.001 to 0.05% of boron, the remainder being essentially formed of iron.   7. Steel according to claim 3, having an allon-   5.1 cm of at least 10% after transformation to a substantially martensitic state in total by section reduction at least 50% cold.