EP0851038A1 - Steel and process for forming a steel article by cold plastic working - Google Patents

Abstract

Steel contains by weight 0.03-0.16% carbon, 0.5-2% manganese, 0.05-0.5% silicon, 0-1.8% chromium, 0-0.25% molybdenum, 0.001-0.05% aluminium, 0.001-0.05% titanium, 0-0.15% vanadium, 0.0005-0.005% boron, 0.004-0.012% nitrogen and 0.001-0.09% sulphur, optionally also up to 0.005% calcium, 0.01% tellurium, 0.04% selenium and 0.3% lead and the rest iron and impurities. Proportions are governed by the following: Mn+0.9 Cr+1.3 Mo+1.6 V=at least 2.2%, and Al+Ti=at least 3.5 N. Also claimed are a hot-rolled semi-product of the steel with a bainitic structure and processes wherein pieces of the steel are hot-rolled, cut to size and then cold-formed, a bainitic structure being conferred by heating above 940 deg. C and quenching, either during and after hot-rolling or after cold-forming, also pieces of steel so formed.

Description

The invention relates to a steel and a method for manufacturing a steel part shaped by cold plastic deformation.

Many steel parts, in particular mechanical parts with high characteristics, are produced by cold forging or cold forging, and more generally by cold plastic deformation of pieces of rolled steel hot. The steel used has a carbon content of between 0.2% and 0.42% (in weight). It is alloyed either with chromium, or with chromium - molybdenum, or nickel - chromium, either with nickel - chromium - molybdenum, or, finally, with manganese - chromium, so as to be sufficiently soaking to allow after quenching a martensitic structure, structure necessary to obtain after income the desired mechanical characteristics which are on the one hand resistance to high tensile, on the other hand good ductility. To be able to be shaped at cold, the steel must first be subjected to a heat treatment of globulation or "maximum softening" consisting in maintaining temperature above 650 ° C for a long time up to several tens of hours. This treatment gives the steel a structure globularized pearlitic, easy to deform when cold. This technique presents the disadvantage, in particular, of requiring three heat treatments, which complicates manufacturing and increases costs.

The object of the present invention is to remedy this drawback by proposing a means for manufacturing, by shaping by plastic deformation cold, a mechanical part made of high characteristics steel, without being necessary to do neither a globulization heat treatment or maximum softening or heat treatment.

• éventuellement jusqu'à 0,005 % de calcium, jusqu'à 0,01 % de tellure, jusqu'à 0,04 % de sélénium, jusqu'à 0,3 % de plomb,
  le reste étant du fer et des impuretés résultant de l'élaboration. La composition chimique de l'acier satisfaisant, en outre les relations : Mn + 0,9 x Cr + 1,3 x Mo + 1,6 x V ≥ 2,2 % et Al + Ti ≥ 3,5 x N

To this end, the subject of the invention is a steel for the manufacture of a steel part shaped by cold plastic deformation, the chemical composition of which comprises, by weight: 0.03% ≤ C ≤ 0.16% 0.5% ≤ Mn ≤ 2% 0.05% ≤ If ≤ 0.5% 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% calcium, up to 0.01% tellurium, up to 0.04% selenium, up to 0.3% lead,
  the remainder being iron and impurities resulting from processing. The satisfactory chemical composition of the steel, furthermore the relationships: Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V ≥ 2.2% and Al + Ti ≥ 3.5 x N

• éventuellement jusqu'à 0,005 % de calcium, jusqu'à 0,01 % de tellure, jusqu'à 0,04 % de sélénium, jusqu'à 0,3 % de plomb,
  le reste étant du fer et des impuretés résultant de l'élaboration.

Preferably, the chemical composition of the steel is such that: 0.06% ≤ C ≤ 0.12% 0.8% ≤ Mn ≤ 1.7% 0.1% ≤ If ≤ 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% calcium, up to 0.01% tellurium, up to 0.04% selenium, up to 0.3% lead,
  the remainder being iron and impurities resulting from processing.

Ni ≤ 0,25 %

Cu ≤ 0,25%

P ≤ 0,02 %

It is preferable that the contents of impurities or residual elements are, simultaneously or separately, such that:

Ni ≤ 0.25%

Cu ≤ 0.25%

P ≤ 0.02%

The invention also relates to a method for manufacturing a part. in steel formed by cold plastic deformation which has for only heat treatment, quenching. The term "quenching" is used, here and throughout the rest, in the broad sense, that is to say that it is cooling sufficiently fast to obtain a structure which is practically not ferrito-pearlitic and which is also not essentially martensitic.

In addition to quenching, the process consists of hot rolling a semi-finished product into steel to obtain a hot rolled product, possibly to cut a piece in the hot rolled product, and to be shaped by plastic deformation to cold the piece or the rolled product.

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

Finally, the invention relates to a steel part obtained by shaping cold formed of a steel according to the invention, such that the Z necking of the steel is greater than 45%, preferably greater than 50%, and the tensile strength Rm greater than 650 MPa, and even, for certain applications, greater than 1200 MPa. In general, and it is desirable, the room has a structure essentially bainite, that is to say, made up of more than 50% bainite.

The invention will now be described in more detail and illustrated by 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 0,05 % à 0,5 %, et, de préférence, 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 according to the invention comprises, in% by weight:

• from 0.03% to 0.16%, and preferably from 0.06% to 0.12% of carbon to obtain a high workability when cold forming, avoid the formation of large carbides unfavorable to ductility, and allow cold forming to be carried out without it being necessary to carry out a globalization or maximum softening annealing;
• from 0.5% to 2%, and preferably from 0.8% to 1.7% of manganese, to ensure good flowability, obtain sufficient quenchability and the desired mechanical characteristics;
• from 0.05% to 0.5%, and preferably from 0.1% to 0.35% of silicon, an element necessary for ensuring the deoxidation of the steel, in particular when the aluminum content is low, but which, in too large a quantity, favors a hardening detrimental to the ability to cold forming and to ductility;
• from 0% to 1.8%, and preferably from 0.1% to 1.5% chromium to adjust the hardenability and the mechanical properties to the level desired for the parts, without exceeding a value which would harden it too much steel in the rough state of rolling or would lead to the formation of martensite detrimental to the ability to cold forming and to ductility;
• from 0% to 0.25%, and preferably from 0.07% to 0.15% of molybdenum to, in synergy with the boron, ensure homogeneous quenchability in the various sections of the part;
• optionally, from 0% to 0.15%, and preferably, less than 0.1% of vanadium to obtain high mechanical characteristics (tensile strength) when these are desired;
• from 0.0005% to 0.005%, and preferably from 0.001% to 0.004% boron to increase the quenchability required;
• from 0% to 0.05%, and preferably from 0.001% to 0.035% of aluminum, and from 0% to 0.05%, and preferably from 0.001% to 0.03% of titanium, the sum of the aluminum and titanium contents having to be greater than or equal to 3.5 times the nitrogen content, in order to obtain a fine grain necessary for good aptitude for cold forming and good ductility;
• from 0.004% to 0.012%, and preferably from 0.006% to 0.01% nitrogen, to control the grain size by the formation of aluminum, titanium or vanadium nitrides, without forming boron nitrides ;
• more than 0.001% of sulfur in order to ensure a minimum of machinability to allow final touch-ups on the parts, but less than 0.09% to guarantee a good aptitude for cold forming ; the workability combined with good workability 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 for the Te / S ratio to remain close to 0.1, or by adding selenium up to 0.05%, it is then preferable for the selenium content to remain close to the sulfur content, ie , finally, by adding lead up to 0.3%, in this case, the sulfur content must be reduced;

the rest is iron and impurities resulting from processing.

• 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 should preferably remain less than or equal to 0.02% to guarantee 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 manganese, chromium, molybdenum and vanadium contents makes it possible to obtain the desired resistance characteristics and an essentially bainitic structure.

This steel has the advantage of having a very good aptitude for plastic deformation when cold and of making it possible to obtain, without the need to make an income, a structure of the bainitic type 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, as when it is carried out after austenitization by heating above AC ₃ , before or after cold forming.

To manufacture a cold-formed part, we supply a semi-finished product steel according to the invention and hot rolled after reheating above 940 ° C in order to obtain a hot rolled product such as a bar, a billet, or wire rod.

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

In a second embodiment, after hot rolling, either the rolled product is quenched after austenitization and then cut into pieces which are shaped by cold plastic deformation, or the pieces are cut before quenching and then setting in cold form. In both cases, the austenitization consists of heating between AC ₃ and 970 ° C., and the quenching is carried out by cooling with blown air, oil, mist, water or water. added with polymers, depending on the product section. The final mechanical characteristics, obtained directly after cold forming, result, in particular, from 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 carried out on a piece cut from the hot rolled product and the quenching is carried out after cold forming. As in the previous case, 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 of no particular importance.

The invention intended more particularly for the manufacture of parts for mechanical, also applies to the manufacture of cold drawn bars, wire drawn and unwound rods, cold drawn, drawn and unwound being particular modes of shaping by cold plastic deformation. Drawn bars and machine or drawn wire can be peeled, shaved or rectified so as to present a surface condition free from defects. The term "cold formed steel part" covers all these products, and the term "piece" covers, in particular, any section of bar or wire; in some cases, bars or wires are not cut into pieces before being cold formed.

The invention can, finally, be used to manufacture pre-treated bars or pretreated wires, or more generally pretreated steel products, intended to be used as is for the manufacture of parts by shaping cold without additional heat treatment. These steel products are soaked after hot rolling either directly in the hot rolling, either after austenitization, in order to present a structure essentially bainitic (bainite ≥ 50%). They can be peeled or shaved to present a surface condition free of defects.

The invention will now be illustrated by examples.

1st example :

C = 0,065 %

Mn = 1,33 %

Si = 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 %

remplissant donc les conditions : Mn + 0,9 x Cr + 1,3 x Mo + 1,6 x V = 2,27 % ≥ 2,2 % et Al + Ti = 0,040% ≥ 3,5 x N = 0,028% A steel was produced in accordance with the invention, the chemical composition of which 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%

thus fulfilling the conditions: Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V = 2.27% ≥ 2.2% and Al + Ti = 0.040% ≥ 3.5 x N = 0.028%

With this steel we produced billets 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) 16 mm diameter rounds:

A : refroidissement à la vitesse de 5,3°C/s équivalent à une trempe à l'air soufflé,

B : refroidissement à la vitesse de 26°C/s équivalent à une trempe à l'huile,

C : refroidissement à la vitesse de 140°C/s équivalent à une trempe à l'eau.

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

A: cooling at the speed of 5.3 ° C / s equivalent to quenching with blown air,

B: cooling at a speed of 26 ° C / s equivalent to oil quenching,

C: cooling at the speed of 140 ° C / s equivalent to water quenching.

The mechanical characteristics before cold forming of the hardened rounds and their fitness for forming by cold plastic deformation were evaluated by tensile tests and by cold breaking torsion tests (the results of the torsion tests are expressed in "number of turns before rupture of the test piece"). The results were as follows: Tempering conditions Round hardness before torsion (HV) Resistance before torsion (MPa) Striction Z before twist (%) Number of rounds at break AT 234 734 69 4.7 B 318 1001 73 5.2 VS 350 1103 69

Hardness and tensile strength, which vary considerably with the quenching conditions, are all the greater as the speed of cooling is high. However, in all cases, the ductility and cold deformation capacity are excellent since Z necking is always significantly greater than 50%, and the number of ruptured turns is always significantly higher than 3.

In order to determine the mechanical characteristics which it is possible to obtain on parts manufactured by shaping by cold plastic deformation from these same rounds, cold torsion-tensile tests were carried out, the results of which were The following : Tempering conditions Resistance after 3 turns of rotation (MPa) Striction after 3 turns Z (%) Strength increase after 3 twists (%) AT 919 66 25% B 1189 67 19% VS 1245 68 13%

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

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

2) Rounds of 25.5 mm in diameter:

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

D: supply air cooling (average cooling rate of 3.3 ° C / s between 950 ° C and the 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 shaping tests by forging at cold consisting in the measurement of the Limit Crushing Rate (T.E.L.) by crushing of notched cylinders according to a generator. The Crushing Rate Limit, expressed in%, is the overwrite rate above which the first crack by cold forging with the press in the notch practiced following the generator of the cylinder.

C = 0,37 %

Mn = 0,75 %

Si = 0,25 %

S = 0,005 %

Cr = 1 %

Mo = 0,02%

Al = 0,02 %

By way of comparison, the TEL was also measured on a steel for cold forging according to the prior art, the composition of which 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 been previously annealed globulation of 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 Limit Crushing Rates it appears that the following steel the invention has an aptitude for shaping by cold forging significantly greater than steel according to the prior art, despite a hardness higher, regardless of the resistance level, even if it is high (treatment F).

3) Rounds of 24.8 mm in diameter :

G : trempe à l'air soufflé

H : : trempe à l'huile

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

G: blown air quenching

H:: oil quenching

The rounds thus treated were cold forged to manufacture automobile wheel rockets whose mechanical characteristics were as follows: treatment Resistance (MPa) Striction Z (%) G 741 71 H 984 74

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

In addition, in both cases the rounds were entirely suitable for placing in shape by cold forging since the parts were found to be free from any defect, both internal and external.

With other rounds of 24.8 mm in diameter (identical to the previous ones), the same rockets were manufactured by cold forging of the rough rounds of rolling by quenching after the cold forming operation. The 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 according to the invention by performing a quenching after cold forging of a rough round of hot rolling, we can obtain very good ductility (Z ≥ 50%) despite a high level of resistance. By elsewhere the steel according to the invention has proved to be perfectly suitable for shaping by cold forging in the rolling state without requiring treatment globulization prerequisite as is practiced with steels according to art rockets have, in fact, been found to be free from all defects as long as internal than external.

C = 0,195 %

Mn = 1,25%

Si = 0,25 %

S = 0,005 %

Ni = 0,25 %

Cr = 1,15 %

Mo = 0,02 %

Cu = 0,2 %

Al = 0,02 %

By way of comparison, according to the prior art, a steel of composition 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 production range:

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

2nd example :

Mechanical parts were also made by cold stamping using steels 1 and 2 in accordance with the invention, 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% thus 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%

In accordance with the invention, these steels were hot rolled in the form of bars with a diameter of 28 mm. After rolling and before cold forming, the bars were subjected to a quenching treatment with lukewarm oil at 50 ° C. after austenitization at 950 ° C. The bars were cut to obtain pieces from which the parts were formed by cold striking at a rate of deformation of 60%. The mechanical characteristics obtained on the plots before cold stamping and on the parts after cold stamping were as follows: Steel HV hardness before cold stamping Rm of steel before cold stamping (MPa) Rm on part after cold stamping (MPa) Z on part after cold stamping (%) Rm increase on strike (%) 1 323 1019 1380 61 35 2 331 1038 1430 59 38

These results show that the ductility is high (Z ≥ 50%) despite a rate very high cold deformation, regardless of the level of initial (before cold stamping) and final (after cold stamping) resistance 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 impact resistance at cold, is important.

Furthermore, the aptitude for shaping by cold striking is excellent. since, despite high initial resistance levels and strong deformation (60%) when cold, the cold-pressed parts were found to be free from defects both internal and external.

These examples show that the steel and the processes according to the invention allow very good ductility (Z ≥ 50%) to be obtained by manufacturing a part shaped by cold plastic deformation, without the need for it to perform an expensive globalization treatment or an income treatment. This high ductility (Z ≥ 50%) combined with mechanical characteristics on very high parts (Rm ≥ 1200 MPa) can be obtained in particular thanks to the significant steel hardening capacity. Finally, the very good aptitude for shaping by forging or cold striking can be observed even if the level of initial strength (or hardness) of steel and the rate of cold deformation are high.

Claims (11)

Steel for the manufacture of a steel part formed by cold plastic deformation, characterized in that its chemical composition comprises, by weight: 0.03% ≤ C ≤ 0.16% 0.5% ≤ Mn ≤ 2% 0.05% ≤ If ≤ 0.5% 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% calcium, up to 0.01% tellurium, up to 0.04% selenium, up to 0.3% lead,
the rest being iron and impurities resulting from the production, the chemical composition of the steel satisfying, in addition the relationships: Mn + 0.9 x Cr + 1.3 x Mo + 1.6 x V ≥ 2.2% and Al + Ti ≥ 3.5 x N Steel according to claim 1 characterized in that its chemical composition is such that: 0.06% ≤ C ≤ 0.12% 0.8% ≤ Mn ≤ 1.7% 0.1% ≤ If ≤ 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% calcium, up to 0.01% tellurium, up to 0.04% selenium, up to 0.3% lead,
the remainder being iron and impurities resulting from processing. Steel according to claim 2 characterized in that its chemical composition is such that: Ni ≤ 0.25% Cu ≤ 0.25% Steel according to claim 2 or claim 3 characterized in that its chemical composition is such that: P ≤ 0.02% Process for the manufacture of a steel part formed by cold plastic deformation, characterized in that: a semi-finished steel product according to any one of claims 1 to 4 is supplied, the semi-finished product is hot rolled after having heated it to a temperature above 940 ° C. and the rolling is terminated at a temperature between 900 ° C. and 1050 ° C. in order to obtain a laminated product, the laminated product is dipped directly into the hot rolling, so as to give it an essentially bainitic structure, optionally, a piece is cut from the rolled product, and the plastic or the rolled product is shaped by cold plastic deformation to obtain the part having its final mechanical characteristics. Process for the manufacture of a steel part formed by cold plastic deformation, characterized in that: a semi-finished steel product according to any one of claims 1 to 4 is supplied, the semi-finished product is hot rolled in order to obtain a laminated product, the laminated product is soaked after having reheated it above the point AC ₃ , so as to give it an essentially bainitic structure. optionally, a piece is cut from the rolled product, and the plastic or the rolled product is shaped by cold plastic deformation to obtain the part having its final mechanical characteristics Process for the manufacture of a steel part formed by cold plastic deformation, characterized in that: a semi-finished steel product according to any one of claims 1 to 4 is supplied, the semi-finished product is hot rolled in order to obtain a laminated product, optionally, a piece is cut from the rolled product, the piece or the rolled product is formed by cold plastic deformation to obtain the part,. and the part is quenched after having heated it above the point AC ₃ , so as to give it an essentially bainitic structure and its final mechanical characteristics. Cold-formed steel part characterized in that it is made of steel according to any one of claims 1 to 4, in that the Z necking of the steel is greater than 45% and in that the resistance to Rm tensile strength of the steel is greater than 650 MPa. Piece according to claim 8 characterized in that the resistance to Rm tensile strength of the steel is greater than 1200 MPa. Piece according to claim 8 or claim 9 characterized in that it has an essentially bainitic structure. Hot rolled steel product characterized in that it is consisting of a steel according to any one of claims 1 to 4 and in that it has an essentially bainitic structure.