EP0851038B2 - Steel and process for forming a steel article by cold plastic working - Google Patents

Description

The invention relates to a use of a steel and a method for the manufacture of a steel part shaped by cold plastic deformation.

Many steel parts, and in particular mechanical parts with high characteristics, are manufactured by cold forging or cold forging, and more generally by cold plastic deformation of hot rolled steel slabs. The steel used has a carbon content of between 0.2% and 0.42% (by weight). It is combined with either chromium, chromium - molybdenum, nickel - chromium or nickel - chromium - molybdenum or, finally, with manganese - chromium, so as to be sufficiently soaking to obtain after quenching a structure. martensitique, structure necessary to obtain after income the desired mechanical characteristics which are on the one hand a high tensile strength, on the other hand a good ductility. In order to be able to be cold-formed, the steel must first be subjected to a heat treatment of globulization or "maximum softening" consisting of holding at a temperature above 650 ° C. for a long time that can reach several dozens of hours. This treatment gives the steel a pearlitic structure globulized, easy to deform cold. This technique has the disadvantage, in particular, of requiring three heat treatments, which complicates the manufacture and increases costs.

The document EP-A-0 775 756 , cited under Art.54 (3) EPC, describes a steel for the manufacture of forgings of composition by weight: C = 0.08%; Si = 0.40%; Mn = 1.30%; Cr = 0.092%; Mo = 0.1%; Cu = 0.22%; Ni = 0.095%; Ti = 0.023%; Al = 0.032%; N = 0.0075%; B = 0.0028%; P = 0.015%; S = 0.072%.

The document JP-A-62 483 41 discloses a high strength steel and tenacity of composition by weight C = 0.05-0.25%; Si = 0.10-1.00%; Mn = 0.50-3.00%; P ≤ 0.025%; Cr = 0.30-3.00%; Mo ≤ 1.00%; B = 0.0005-0.0050%; Ti = 0.01-0.1%; Al = 0.01-0.10%; N ≤ 0.02%, the remainder being iron and impurities, hot deformed, and to which a bainitic structure is imparted. It is then cold deformed to less than 10% and then returned to 50-300 ° C for more than 10 minutes.

The document JP-A-50 655 40 discloses a method of manufacturing a high strength bolt of composition by weight:

C = 0.04-0.15%; Si = 0.05-1.5%; Mn = 0.5-1.5%; Cr = 0.1-1.5%; B = 0.0003-0.005%; Ti = 0.01-0.04%; Al = 0.01 - 0.06% the remainder being iron and impurities. After being cold-formed, the bolt is heated above Ac ₃ , then cooled to less than 400 ° C to above 1 ° C / sec and cured to a martensite-based structure and / or bainite.

The document EP-A-0 747 496 discloses hot rolled sheets with high strength and high drawability of composition by weight C ≤ 0.12%; Mn = 0.5-1.5%; If ≤ 0.3%; P ≤ 0.1%; S ≤ 0.05%; Al = 0.01-0.1%; Cr ≤ 1%; Ti _eff = 0.03-0.15%; Nb ≤ 0.05%, the structure of which comprises at least 75% of ferrite hardened by precipitation of carbides or carbonitrides of Ti or Ti and Nb, the remainder of the structure comprising at least 10% of martensite and optionally bainite and residual austenite.

The object of the present invention is to overcome this disadvantage by providing a means for manufacturing, by forming by cold plastic deformation, a high-grade steel mechanical part, without the need to do or treatment. maximum heat transfer or softening temperature or heat treatment of income.

For this purpose, the subject of the invention is a use of a steel according to claim 1.

The invention also relates to a method for the manufacture of a steel part shaped by cold plastic deformation which comprises for heat treatment only, quenching. The term "quenching" is used, here and throughout, in the broad sense, that is to say that it is a fast enough cooling to obtain a structure which is practically not ferrito-pearlite and which is not essentially martensitic either.

In addition to quenching, the process consists in hot rolling a half-product made of steel to obtain a hot-rolled product, possibly cutting a slug in the hot-rolled product, and shaping the slug or slab by cold plastic deformation. rolled product.

Quenching, which is intended to give the part an essentially bainitic structure, can be carried out both before and after the cold forming. When carried out before cold forming, quenching can be carried out both directly in the hot rolling mill and after reheating austenitization over AC ₃ . When performed after cold forming, quenching is performed after austenitization by reheating above AC ₃ .

Finally, the invention relates to a cold-formed steel part according to claim 8.

The invention will now be described in more detail and illustrated by the following examples.

• de 0,03 % à 0,16 %, et, de préférence, de 0,06 % à 0,12 % de carbone pour obtenir une importante aptitude à l'écrouissage lors de la mise en forme à froid, éviter la formation de gros carbures défavorables à la ductilité, et permettre d'effectuer une mise en forme à froid sans qu'il soit nécessaire de réaliser un recuit de globulisation ou d'adoucissement maximal ;
• de 0,5 % à 2 %, et, de préférence, de 0,8 % à 1,7 % de manganèse, pour assurer une bonne coulabilité, obtenir une trempabilité suffisante et les caractéristiques mécaniques souhaitées ;
• de 0,1 % à 0,35 % de silicium, élément nécessaire pour assurer la désoxydation de l'acier, en particulier lorsque la teneur en aluminium est faible, mais, qui, en trop forte quantité, favorise un durcissement préjudiciable à l'aptitude à la mise en forme à froid et à la ductilité ;
• de 0 % à 1,8 %, et, de préférence, de 0,1 % à 1,5 % de chrome pour ajuster la trempabilité et les caractéristiques mécaniques au niveau souhaité pour les pièces, sans dépasser une valeur qui durcirait trop l'acier à l'état brut de laminage ou conduirait à la formation de martensite préjudiciable à l'aptitude à la mise en forme à froid et à la ductilité;
• de 0 % à 0,25 %, et, de préférence, de 0,07 % à 0,15 % de molybdène pour, en synergie avec le bore, assurer une trempabilité homogène dans les diverses sections de la pièce ;
• éventuellement, de 0 % à 0,15 %, et de préférence, moins de 0,1 % de vanadium pour obtenir de hautes caractéristiques mécaniques (résistance à la traction) lorsque celles-ci sont recherchées ;
• de 0,0005 % à 0,005 %, et, de préférence, de 0,001 % à 0,004 % de bore pour augmenter la trempabilité nécessaire ;
• de 0 % à 0,05 %, et, de préférence, de 0,001 % à 0,035 % d'aluminium, et de 0 % à 0,05 %, et, de préférence, de 0,001 % à 0,03 % de titane, la somme des teneurs en aluminium et en titane devant être supérieure ou égale à 3,5 fois la teneur en azote, afin d'obtenir un grain fin nécessaire pour une bonne aptitude à la mise en forme à froid et une bonne ductilité;
• de 0,004 % à 0,012 %, et, de préférence, de 0,006 % à 0,01 % d'azote, pour contrôler la taille du grain par formation de nitrures d'aluminium, de titane ou de vanadium, sans former de nitrures de bore ;
• plus de 0,001 % de soufre afin d'assurer un minimum d'aptitude à l'usinage pour permettre d'effectuer des retouches finales sur les pièces, mais moins de 0,09 % pour garantir une bonne aptitude à la mise en forme à froid ; l'aptitude à l'usinage combinée avec une bonne aptitude à la mise en forme par déformation plastique à froid peut être améliorée soit par une addition de calcium jusqu'à 0,005 %, soit par une addition de tellure jusqu'à 0,01 %, il est alors préférable que le rapport Te/S reste voisin de 0,1, soit par une addition de sélénium jusqu'à 0,05 %, il est alors préférable que la teneur en sélénium reste voisine de la teneur en soufre, soit, enfin, par une addition de plomb jusqu'à 0,3 %, dans ce cas, la teneur en soufre doit être réduite ;

le reste est du fer et des impuretés résultant de l'élaboration.The chemical composition of the steel used in the context of the invention comprises, in% by weight:

• from 0.03% to 0.16%, and preferably from 0.06% to 0.12% carbon to obtain a high work hardening ability during cold forming, to avoid the formation of large carbides disadvantageous to ductility, and allow cold forming without the need to perform a globulization annealing or maximum softening;
• from 0.5% to 2%, and preferably from 0.8% to 1.7% of manganese, to ensure good flowability, to obtain sufficient quenchability and the desired mechanical characteristics;
• from 0.1% to 0.35% of silicon, which element is necessary to ensure the deoxidation of the steel, in particular when the aluminum content is low, but which, in excessive amounts, favors a hardening which is detrimental to ability to cold form and ductility;
• from 0% to 1.8%, and preferably from 0.1% to 1.5% of chromium to adjust the quenchability and mechanical characteristics to the desired level for the pieces, without exceeding a value which would harden too much steel in the raw state of rolling or would lead to the formation of martensite detrimental to the ability to cold forming and ductility;
• from 0% to 0.25%, and preferably from 0.07% to 0.15% molybdenum for, in synergy with boron, to ensure homogeneous quenchability in the various sections of the room;
• optionally from 0% to 0.15%, and preferably less than 0.1% vanadium to obtain high mechanical characteristics (tensile strength) when these are sought;
• from 0.0005% to 0.005%, and preferably from 0.001% to 0.004% boron to increase the necessary quenchability;
• from 0% to 0.05%, and preferably from 0.001% to 0.035% aluminum, and from 0% to 0.05%, and preferably from 0.001% to 0.03% titanium, the sum of aluminum and titanium contents to be greater than or equal to 3.5 times the nitrogen content, in order to obtain a fine grain necessary for good cold forming ability and good ductility;
• from 0.004% to 0.012%, and preferably from 0.006% to 0.01% nitrogen, to control grain size by forming aluminum, titanium or vanadium nitrides, without forming boron nitrides ;
• more than 0.001% sulfur to ensure a minimum of machinability to allow final touch-ups on parts, but less than 0.09% to ensure good cold forming ability ; Combined machinability with good formability by cold plastic deformation can be improved either by adding calcium up to 0.005% or by adding tellurium up to 0.01%. , it is then preferable that the Te / S ratio remains close to 0.1, or by adding selenium up to 0.05%, it is preferable for the selenium content to remain close to the sulfur content, ie finally, by adding lead up to 0.3%, in which case the sulfur content must be reduced;

the rest is iron and impurities resulting from the elaboration.

• le phosphore, dont la teneur doit rester, de préférence, inférieure ou égale à 0,02 % pour garantir une bonne ductilité pendant et après mise en forme à froid ;
• le cuivre et le nickel, considérés comme des résiduels, dont la teneur doit, de préférence, rester inférieure à 0,25 % pour chacun.

The impurities are, in particular:

• phosphorus, the content of which must remain preferably less than or equal to 0.02% to ensure good ductility during and after cold forming;
• copper and nickel, considered as residuals, the content of which should preferably remain below 0.25% for each.

Finally, the chemical composition of steel must satisfy the relationship:

Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V ≥ 2.2%

which ensures that the combination of the manganese, chromium, molybdenum and vanadium contents makes it possible to obtain the desired strength characteristics and an essentially bainitic structure.

This steel has the advantage of having a very good ability to cold plastic deformation and to obtain, without the need for income, a bainitic type structure having excellent ductility and high characteristics mechanical. In particular, the ductility can be measured by the necking Z which is greater than 45%, and even greater than 50%. The tensile strength Rm is greater than 650 MPa and can exceed 1200 MPa. These characteristics can be obtained both when the quenching is carried out in the hot rolling before cold forming, and when it is carried out after austenitization by heating above AC ₃ , before or after cold forming.

To manufacture a cold-formed part, a semi-finished steel product having the above-mentioned composition is supplied and hot-rolled after reheating above 940 ° C to obtain a hot-rolled product such as a bar , a billet, or a wire rod.

In a first embodiment, the hot rolling is finished at a temperature of between 900 ° C. and 1050 ° C. and the hot-rolled product is immersed directly in the hot rolling mill by cooling with the blown air, at oil, mist, water or water with polymers, according to its section. The product thus obtained is then cut into pieces and then shaped in the cold form, for example by cold forging or cold forging. The final mechanical characteristics, obtained directly after cold forming, result, in particular, the work hardening generated by the cold forming operation.

In a second embodiment, after hot rolling, the rolled product is quenched after austenitization and then cut into pieces which are shaped by cold plastic deformation, or the slugs are cut before the quenching and then the setting in cold form. In both cases, the austenitization consists of a heating between AC ₃ and 970 ° C, and the quenching is carried out by cooling with the blown air, the oil, the fog, the water or the water added with polymers, depending on the product section. The final mechanical characteristics, obtained directly after cold forming, result, in particular, the work hardening generated by the forming operation. In this embodiment, the end-of-rolling conditions are not of particular importance.

In a third embodiment, the cold forming operation is performed on a cut slug in the hot rolled product and quenching is carried out after cold forming. As in the preceding case, the quenching is carried out after heating between AC ₃ and 970 ° C and by cooling with blown air, oil, mist, water or water containing polymers. The end-of-rolling conditions are also not of particular importance.

The invention intended more particularly for the manufacture of mechanical parts, is also applicable to the manufacture of cold drawn bars, wire drawn and machine son unwound, cold drawing, drawing and unwinding being modes. particular shaping by cold plastic deformation. Drawn bars and machine or drawn wire may be peeled, shaved or ground to a flawless surface condition. The term "cold-formed steel part" covers all these products, and the term "slug" covers, in particular, any section of bar or wire; in some cases, the bars or wires are not cut into pieces before being cold-formed.

The invention may, finally, be used to manufacture pretreated bars or pretreated yarns, or more generally pretreated steel products, intended to be used in the state for the manufacture of parts by cold forming without additional heat treatment . These steel products are quenched after hot rolling either directly in the hot rolling or after austenitization, in order to have a substantially bainitic structure (bainite ≥ 50%). They can be peeled or shaved to have a flawless surface condition.

The invention will now be illustrated by examples.

1st example :

A steel was developed whose chemical composition included by weight:

C = 0.065%

Mn = 1.33%

If = 0.34%

S = 0.003%

P = 0.014%

Ni = 0.24%

Cr = 0.92%

Mo = 0.081%

Cu = 0.23%

V = 0.003%

Al = 0.02%

Ti = 0.02%

N = 0.008,%

B = 0.0035%

fulfilling the conditions

Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V = 0.27% ≥ 2.2%

and

Al + Ti = 0.040% ≥ 3.5 x N = 0.028%

With this steel billets were manufactured which were hot rolled after reheating above 940 ° C to form rounds (or bars) of diameter 16 mm, 25.5 mm and 24.8 mm.

1) rounds of 16 mm in diameter:

1. A : refroidissement à la vitesse de 5,3°C/s équivalent à une trempe à l'air soufflé,
2. B : refroidissement à la vitesse de 26°C/s équivalent à une trempe à l'huile,
3. C : refroidissement à la vitesse de 140°C/s équivalent à une trempe à l'eau.

The rolling of rounds with a diameter of 16 mm was completed at 990 ° C. and the rounds were soaked in hot rolling under the following three conditions (in accordance with the invention):

1. A: cooling at a speed of 5.3 ° C / s equivalent to blown air quenching,
2. B: cooling at 26 ° C / s equivalent to oil quenching,
3. C: Cooling at a rate of 140 ° C / s equivalent to quenching with water.

Mechanical characteristics prior to cold forming of hardened rounds and their ability to form by cold plastic deformation were evaluated by tensile tests and cold-break torsion tests (torsion test results). are expressed in "number of revolutions before failure of the specimen"). The results were as follows: Quenching conditions Hardness of the round front torsion (HV) Resistance before torsion (MPa) Striction Z before torsion (%) Number of laps to break AT 234 734 69 4.7 B 318 1001 73 5.2 VS 350 1103 69

The hardness and the tensile strength, which vary considerably with the quenching conditions, are all the greater as the cooling rate is high. Nevertheless, in all cases, the ductility and cold deformation capacity are excellent since the necking Z is always substantially greater than 50%, and the number of turns to failure is still significantly greater than 3.

In order to determine the mechanical characteristics that can be obtained on parts made by forming by cold plastic deformation from these same rounds, cold tensile torsion tests were carried out, the results of which were The following : Quenching conditions Resistance after 3 turns of torsion (MPa) Striction after 3 turns of twist Z (%) Increase in resistance after 3 turns of torsion (%) AT 919 66 25% B 1189 67 19% VS 1245 68 13%

The cold tensile-torsion test consists of subjecting a specimen to 3 turns of cold twisting to simulate the shaping by plastic deformation, before performing a tensile test at ambient temperature. The increase in resistance corresponds to the relative increase in resistance between the hardened state (after 3 turns of torsion) and the normal state (before the 3 turns of torsion).

The results obtained show that, even after a large cold deformation (three turns of torsion), the necking remains greater than 50%, and that the tensile strength can exceed 1200 MPa. The work hardening capacity, measured by the increase in resistance after deformation by cold twisting, is high in all cases.

2) 25.5 mm diameter rounds :

• D : refroidissement à l'air soufflé (vitesse de refroidissement moyenne de 3,3°C/s entre 950 °C et l'ambiante)
• E : refroidissement à l'huile (vitesse de refroidissement moyenne de 22°C/s entre 950 °C et l'ambiante)
• F : refroidissement à l'eau (vitesse de refroidissement moyenne de 86°C/s entre 950 °C et l'ambiante)

The rounds 25.5 mm in diameter were soaked before cold forming, after austenitization at 950 ° C., under the following conditions (in accordance with the invention):

• D: blast chilling (average cooling rate of 3.3 ° C / s between 950 ° C and ambient)
• E: oil cooling (average cooling rate of 22 ° C / s between 950 ° C and ambient)
• F: water cooling (average cooling rate of 86 ° C / s between 950 ° C and ambient)

The rounds were subjected to cold forging shaping tests consisting of the measurement of the Limited Crush Rate (T.E.L.) by crushing rolled cylinders along a generator. The Crush Rate Limit, expressed in%, is the crush rate above which appears the first crack by cold press forging in the notch made following the generatrix of the cylinder.

By way of comparison, the TEL was also measured on a steel for cold forging according to the prior art whose composition was:

C = 0.37%

Mn = 0.75%

If = 0.25%

S = 0.005%

Cr = 1%

Mo = 0.02%

Al = 0.02%

This steel according to the prior art had previously been subjected to a globularization annealing of the perlite to make it suitable for cold deformation.

The results obtained were as follows: Steel Heat treatment HV hardness Resistance (MPa) Crush Rate Limit% Steel according to the invention D 249 793 52 E 303 954 52 F 355 1115 52 Steel according to the prior art Globulation annealing 174 547 44

In view of the Crush Rate Limit it appears that the steel used according to the invention has a cold forging shaping capacity substantially greater than the steel according to the prior art, despite a higher hardness, and this whatever the level of resistance, even if it is high (treatment F).

3) Rounds 24.8 mm in diameter :

• G : trempe à l'air soufflé
• H: trempe à l'huile

After rolling and before cold forming, rounds 24.8 mm in diameter were quenched after austenitization at 930 ° C. under the following conditions according to the invention:

• G: air-blast quenching
• H: oil quenching

The rounds thus treated were cold-forged to make automobile wheel-flares whose mechanical characteristics were as follows: treatment Resistance (MPa) Striction Z (%) BOY WUT 741 71 H 984 74

These results show that, regardless of the initial treatment, the ductility obtained on the cold forged part is very high (Z ≥ 50%), and this, independently of the level of resistance.

On the other hand, in both cases the rounds were entirely suitable for cold forging shaping since the parts proved to be free of any internal and external defect.

With other rounds 24.8 mm in diameter (identical to the previous ones), the same rockets were made by cold forging of the rough rolling rounds by quenching after the cold forming operation. Quenching was carried out with water after austenitization at 940 ° C.

• Rm = 1077 MPa
• Z = 73%

Under these conditions, the characteristics obtained on rockets were as follows:

• Rm = 1077 MPa
• Z = 73%

These results show that with the steel used according to the invention by performing a quenching after cold forging of a hot rolling round, one can obtain a very good ductility (Z ≥ 50%) despite a level of resistance. Student. Furthermore, the steel according to the invention has proved perfectly suitable for shaping by cold forging in the raw state of rolling without the need for prior treatment of globulization as is practiced with steels according to the prior art. the rockets have, in fact, been shown to be free from any defect, both internal and external.

By way of comparison, according to the prior art, a composition steel is used to manufacture the same rockets.

C = 0.195%

Mn = 1.25%

If = 0.25%

S = 0.005%

Ni = 0.25%

Cr = 1.15%

Mo = 0.02%

Cu = 0.2%

Al = 0.02%

• Recuit globulaire de l'acier pour le rendre apte à la mise en forme à froid.
• Forgeage à froid des fusées.
• Trempe à l'huile de l'acier suivant l'art antérieur.
• Revenu de l'acier suivant l'art antérieur.

To obtain mechanical characteristics similar to those obtained with the invention, it is necessary to use the following manufacturing range:

• Globular annealing of steel to make it suitable for cold forming.
• Cold forging of rockets.
• Oil quenching of the steel according to the prior art.
• Steel income according to the prior art.

2nd example :

Mechanical parts were also cold-worked using steel 1 and 2, the chemical compositions of which were in% by weight: Steel 1 Steel 2 C = 0.061% 0.062% Mn = 1.6% 1.57% If = 0.28% 0.29% S = 0.021% 0.021% P = 0.004% 0.004% Ni = 0.11% 0.11% Cr = 0.81% 0.8% Mo = 0.081% 0.128% Cu = 0.2% 0.2% Al = 0.028% 0.025% Ti = 0.017% 0.016% V = 0.002% 0.084% B = 0.0039% 0.0038% N = 0.007% 0.008% fulfilling the conditions:

For steel 1:

Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V = 2.43 ≥ 2.2%

Al + Ti = 0.045% ≥ 3.5 x N = 0.024%

For steel 2:

Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V = 2.59 ≥ 2.2%

Al + Ti = 0.041% ≥ 3.5 x N = 0.028%

According to the invention, these steels were hot-rolled in the form of bars of diameter 28 mm. After rolling and before cold forming, the bars were subjected to a warm oil quenching treatment at 50 ° C after austenitization at 950 ° C. The bars were cut to obtain pieces from which the pieces were shaped by cold stamping at the deformation rate of 60%. The mechanical characteristics obtained on the plots before cold stamping and on the pieces after cold stamping were as follows: Steel HV hardness before cold stamping Rm of steel before cold knocking (MPa) Rm on piece after cold stamping (MPa) Z on piece after cold stamp (%) Rm increase on strike (%) (*) 1 323 1019 1380 61 35 2 331 1038 1430 59 38 (*) = work hardening ability by cold forming.

These results show that the ductility is high (Z ≥ 50%) despite a very high rate of cold deformation, this independently of the level of initial strength (before cold stamping) and final (after cold stamping) of the steel, even if the final resistance level is very high. They also show that the work hardening capacity, measured by the increase in resistance to cold striking, is important.

In addition, cold forming ability is excellent since, despite high initial strength levels and high cold deformation (60%), cold-worked parts have been found to be free of internal defects. external.

These examples show that the steel and the processes according to the invention make it possible to obtain a very good ductility (Z ≥ 50%) by the manufacture of a workpiece shaped by cold plastic deformation, without it being necessary. to perform expensive treatment of globulization or treatment of income. This high ductility (Z ≥ 50%) combined with very high mechanical characteristics on parts (Rm ≥ 1200 MPa) can be obtained thanks to the high work hardening capacity of the steel. Finally, the very good fitness for shaping by cold forging or striking is observed even if the initial resistance level (or hardness) of the steel and the rate of cold deformation are high.

Claims (10)

1. Use, for the manufacture of a steel component formed by cold plastic deformation, of a steel the chemical composition of which comprises, by weight:
   
           0.03% ≤ C ≤ 0.16%
   
           0.5% ≤ Mn ≤ 2%
   
           0.1% ≤ Si ≤ 0.35%
   
           0% ≤ Cr ≤ 1.8%
   
           0% ≤ Mo ≤ 0.25%
   
           0.001% ≤ Al ≤ 0.05%
   
           0.001% ≤ Ti ≤ 0.05%
   
           0% ≤ V ≤ 0.15%
   
           0.0005% ≤ B ≤ 0.005%
   
           0.004% ≤ N ≤ 0.012%
   
           0.001% ≤ S ≤ 0.09%
   
   

   - optionally up to 0.005% of calcium, up to 0.01% of tellurium, up to 0.04% of selenium and up to 0.3% of lead,
   the balance being iron and impurities resulting from the smelting, the chemical composition of the steel furthermore satisfying the relationships:
   
           Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V ≥ 2.2%
   
   and
   
           Al + Ti ≥ 3.5 x N,
   
   said cold plastic deformation consisting of cold forging, or cold striking or cold drawing, wire drawing or peeling.
2. Use of a steel according to Claim 1, characterized in that the chemical composition of the steel is such that:
   
           0.06% ≤ C ≤ 0.12%
   
           0.8% ≤ Mn ≤ 1.7%
   
           0.1% ≤ Si ≤ 0 .35%
   
           0.1% ≤ Cr ≤ 1.5%
   
           0.07% ≤ Mo ≤ 0.15%
   
           0.001% ≤ Al ≤ 0.035%
   
           0.001% ≤ Ti ≤ 0.03%
   
           0% ≤ V ≤ 0.1%
   
           0.001% ≤ B ≤ 0.004%
   
           0.004% ≤ N ≤ 0.01%
   
           0.001% ≤ S ≤ 0.09%
   
   

   - optionally up to 0.005% of calcium, up to 0.01% of tellurium, up to 0.04% of selenium and up to 0.3% of lead,
   the balance being iron and impurities resulting from the smelting.
3. Use of a steel according to Claim 2,
   characterized in that the impurity contents of the steel are such that:
   
           Ni ≤ 0.25%
   
           Cu ≤ 0.25%.
   
   
4. Use of a steel according to Claim 2 or Claim 3, characterized in that the impurity content of the steel is such that:
   
           P ≤ 0.02%.
   
   
5. Process for the manufacture of a steel component formed by cold plastic deformation,
   characterized in that:

   - a semi-finished product made of a steel having the composition cited in any one of Claims 1 to 4 is provided;

   - the semi-finished product is hot rolled after it has been reheated to a temperature above 940°C and the rolling is stopped at a temperature of between 900°C and 1050°C in order to obtain a rolled product;

   - the rolled product is quenched immediately, whilst still hot from rolling, so as to give it an essentially bainitic structure;

   - optionally, a blank is cut from the rolled product; and

   - the blank or the rolled product is formed by cold plastic deformation in order to obtain the component having its final mechanical properties, said cold plastic deformation consisting of cold forging or cold striking or cold drawing, wire drawing or peeling.
6. Process for the manufacture of a steel component formed by cold plastic deformation, characterized in that: the

   - a semi-finished product made of a steel the composition cited in any one of Claims 1 to 4 is provided;

   - the semi-finished product is hot rolled so as to obtain a rolled product;

   - the rolled product is quenched after it has been reheated to above the AC₃ point, so as to give it an essentially bainitic structure;

   - optionally, a blank is cut from the rolled product; and

   - the blank or the rolled product is formed by cold plastic deformation in order to obtain the component having its final mechanical properties, said cold plastic deformation consisting of cold forging or cold striking or cold drawing, wire drawing or peeling.
7. Process for the manufacture of a steel component formed by cold plastic deformation, characterized in that:

   - a semi-finished product made of a steel having the composition cited in any one of Claims 1 to 4 is provided,

   - the semi-finished product is hot rolled so as to obtain a rolled product;

   - optionally, a blank is cut from the rolled product;

   - the blank or the rolled product is formed by cold plastic deformation in order to obtain the component; said cold plastic deformation consisting of cold forging or cold striking or cold drawing, wire drawing or peeling and

   - the component, after it has been reheated to above the AC₃ point, is quenched so as to give it an essentially bainitic structure and its final mechanical properties.
8. Cold-formed steel component, characterized in that it is made of a steel having the composition cited in any one of Claims 1 to 4, in that the reduction in section Z of the steel is greater than 45%, in that the tensile strength R_m of the steel is greater than 650 MPa and in that it is cold-formed by cold forging or cold striking or cold drawing or wire drawing or peeling.
9. Component according to Claim 8, characterized in that the tensile strength R_m of the steel is greater than 1200 MPa.
10. Component according to Claim 8 or Claim 9 , characterized in that it has an essentially bainitic structure.