EP0550294B1 - Elongated product for cold forming manufacturing process, more particularly for cold coining of shaped products such as bolts, and process for manufacturing this cold formed product - Google Patents

Description

The invention relates to a method for manufacturing a steel wire and an elaborate product, such as a bolt, from steel wire, and to the steel used, said elaborate product having a tensile strength greater than 1000 MPa and less than 1170 MPa and an elastic limit R _e0.002 greater than 900 MPa.

To manufacture bolts, cold deformation by cold striking strips of steel wire which is chosen according to the mechanical characteristics sought for the bolts. There are at least three classes of steel for cold stamping, class 8.8, class 10.9, class 12.9; the first number represents the minimum tensile strength divided by 100, the second number represents the ratio of the yield strength to the strength multiplied by ten; these quantities being measured on the products obtained by cold stamping.

To make bolts corresponding to class 10.9, steel 38C4 is generally used (according to AFNOR standard) containing approximately 0.38% carbon and approximately 1% chromium.

According to a first technique, the steel is hot rolled to make a wire which is subjected to a globulation treatment to make it suitable for cold striking, it is drawn to give it the desired final diameter, then cutting of sections which are shaped by cold striking, the parts obtained are then hardened and returned and as the hardening treatment deforms the parts, the bolts are straightened. This technique has the disadvantage of comprising numerous operations, in particular two heat treatments, and of being expensive. In addition, 38C4 steel containing 1% chromium is expensive; in addition, it contains a high carbon content, which makes it relatively sensitive to the delayed rupture caused by hydrogen.

According to another technique, the steel is hot rolled, it is optionally drawn, it is put into a crown and the crowns are subjected to a quenching and tempering treatment. Plots are sheared and shaped by cold striking; there is no need to heat treat or straighten the parts. This technique has the advantage of reducing the manufacturing operations, but on the one hand the steel used which is of the 38C4 type has the same drawbacks as in the previous technique and on the other hand the crown heat treatment leads to heterogeneities features along the wire; this results in dispersions of mechanical characteristics on the bolts.

GB-A-1,193,995 discloses a high-strength steel containing, by weight, 0.003 to 0.18% C, 0.1 to 1.0% Si, 0.5 to 1.5% Mn, 0.05 to 5% Cr, 0.03 to 0.5% Mo, 0.0003 to 0.003% B, 0.04 to 0.25% Al and at least one element of the group comprising Nb (0.005 to 0 , 05%), Ti (0.005 to 0.10%) and Zr (0.005 to 0.10%), the total of Nb + Ti + Zr being less than 0.1% and the balance consisting of iron. The composition of the steel is adjusted to obtain good weldability and in no case in consideration of forming characteristics which may make it possible to envisage the manufacture of wires or products produced by cold stamping.

US-A-4,537,644 also discloses a steel having a tensile strength greater than 1000 MPa, containing, by weight, 0.15 to 0.5% C, up to 1.5% Si, 0.20 to 1.5% Mn, up to 2% Cr, 0.0005% to 0.0030% B, 0.005% to 0.10% Al, up to 0.010% P, up to 0.0090 % N, 0.010% to 0.050% Ti, the balance being made up of iron. Steel can also contain up to 0.50% Mo.

Although this steel is intended inter alia for the manufacture of high-strength bolts, its composition and its treatment are not optimized optically the manufacture of a wire by rolling and drawing and a bolt by cold striking.

The object of the invention is to propose a long product such as a wire or a bar for cold forming, in particular by cold striking, of class 10.9, which makes it possible to eliminate the heat treatment on the parts, at the developed state, after cold forming, which has homogeneous characteristics over its entire length, which is economical and which has improved resistance to cracking by hydrogen.

• on approvisionne un demi-produit en un acier dont la composition chimique comprend en poids :
  • 0,15 % ≤ C ≤ 0,3 %
  • 1 % ≤ Mn ≤ 1,8 %
  • Si ≤ 0,2 %
  • S ≤ 0,03 %
  • 0,15 % ≤ Cr ≤ 0,5 %
  • 0,0005 % ≤ B ≤ 0,005 %
  • 0,01 % ≤ Ti ≤ 0,03 %,
  • éventuellement : 0,12 % ≤ Ni ≤ 0,25 %
  • éventuellement : 0,04 % ≤ Mo ≤ 0,15 %
  le reste étant du fer et des impuretés résultant de l'élaboration,
• on lamine le demi-produit sur un train à fil pour obtenir un fil laminé,
• on tréfile le fil laminé pour obtenir un fil tréfilé,
• on fait passer en continu le fil tréfilé dans un four d'austénitisation, puis dans un bac de trempe, puis dans un équipement de revenu, de façon à lui faire subir un traitement de trempe et revenu.

To this end, the subject of the invention is a method of manufacturing a steel wire having a tensile strength of between 800 MPa and 1000 MPa and an elastic limit R _e0.002 greater than 850 MPa and a necking greater than 65%, characterized in that:

• a semi-finished product is supplied with a steel whose chemical composition comprises by weight:
  • 0.15% ≤ C ≤ 0.3%
  • 1% ≤ Mn ≤ 1.8%
  • If ≤ 0.2%
  • S ≤ 0.03%
  • 0.15% ≤ Cr ≤ 0.5%
  • 0.0005% ≤ B ≤ 0.005%
  • 0.01% ≤ Ti ≤ 0.03%,
  • optionally: 0.12% ≤ Ni ≤ 0.25%
  • optionally: 0.04% ≤ Mo ≤ 0.15%
  the rest being iron and impurities resulting from the production,
• the semi-finished product is laminated on a wire train to obtain a laminated wire,
• the rolled wire is drawn to obtain a drawn wire,
• the drawn wire is passed continuously through an austenitization oven, then through a quenching tank, then in tempering equipment, so as to subject it to quenching and tempering treatment.

• dans le four d'austénitisation, le fil tréfilé est porté à une température supérieure à 900°C pendant un temps supérieur à 5 mn.
• la trempe se fait à l'eau additionnée de polymères du type PAG ou PVP ou à l'huile de trempe accélérée.
• le revenu se fait à une température comprise entre 450°C et 600°C par passage dans un bac contenant un métal fondu tel qu'un alliage de plomb.
• l'acier contient entre 0,12 % et 0,25 % de Nickel.

Preferably:

• in the austenitization oven, the drawn wire is brought to a temperature above 900 ° C. for a time greater than 5 min.
• quenching is carried out with water containing polymers of the PAG or PVP type or with accelerated quenching oil.
• tempering takes place at a temperature between 450 ° C and 600 ° C by passing through a tank containing a molten metal such as a lead alloy.
• the steel contains between 0.12% and 0.25% of Nickel.

• on approvisionne un fil tréfilé obtenu par le procédé selon l'invention ;
• on effectue une opération de recouvrement du fil tréfilé ;
• on forme le produit élaboré par frappe à froid, sans traitement thermique ultérieur.

The invention also relates to a process for the manufacture of an elaborated product such as a bolt, produced by cold forming a long steel product, having a tensile strength greater than 1000 MPa and less than 1170 MPa and an elastic limit R _e0.002 greater than 900 MPa, characterized in that:

• a drawn wire obtained by the process according to the invention is supplied;
• an operation of covering the drawn wire is carried out;
• the product produced is formed by cold pressing, without subsequent heat treatment.

• 0,15 % ≤ C ≤ 0,3 %
• 1 % ≤ Mn ≤ 1,8 %
• Si ≤ 0,2 %
• S ≤ 0,03 %
• 0,15 % ≤ Cr ≤ 0,5 %
• 0,12 % ≤ Ni ≤ 0,25 %
• éventuellement : 0,04 % ≤ Mo ≤ 0,15 %
• 0,0005 % ≤ B ≤ 0,005 %
• 0,01 % ≤ Ti ≤ 0,03 %,

le reste étant du fer et des impuretés résultant de l'élaboration.Finally, the invention relates to a steel containing, by weight

• 0.15% ≤ C ≤ 0.3%
• 1% ≤ Mn ≤ 1.8%
• If ≤ 0.2%
• S ≤ 0.03%
• 0.15% ≤ Cr ≤ 0.5%
• 0.12% ≤ Ni ≤ 0.25%
• optionally: 0.04% ≤ Mo ≤ 0.15%
• 0.0005% ≤ B ≤ 0.005%
• 0.01% ≤ Ti ≤ 0.03%,

the remainder being iron and impurities resulting from processing.

The invention will now be described in more detail, but without limitation.

• plus de 0,15 % de carbone pour avoir suffisamment de dureté et de trempabilité et moins de 0,3 % pour assurer une bonne aptitude à la déformation à froid et pour réduire la sensibilité à l'hydrogène ; de préférence on choisit la teneur en carbone comprise entre 0,18 % et 0,28 %,
• plus de 1 % de Mn pour obtenir une certaine trempabilité mais moins de 1,8 % et préférentiellement moins de 1,5 % car de trop fortes teneurs en manganèse fragilisent l'acier,
• plus de 0,15 % et de préférence plus de 0,2 % de chrome pour augmenter la trempabilité mais moins de 0,5 % car cet élément est cher,
• de 0,0005 % à 0,005 % de bore qui augmente la trempabilité sans modifier les autres propriétés de l'acier. La fourchette de composition correspond au domaine dans lequel le bore est efficace,
• de 0, 01 % à 0, 03 % de titane pour piéger l'azote et éviter qu'il ne combine au bore de façon à conserver l'ef ficacité de cet élément,
• moins de 0,2 % de silicium et préférentiellement moins de 0,15 % car de trop fortes teneurs diminuent la ductilité de l'acier.

To make a wire according to the invention, a steel containing:

• more than 0.15% of carbon to have sufficient hardness and hardenability and less than 0.3% to ensure good suitability for cold deformation and to reduce the sensitivity to hydrogen; preferably the carbon content of between 0.18% and 0.28% is chosen,
• more than 1% of Mn to obtain a certain quenchability but less than 1.8% and preferably less than 1.5% because too high manganese contents weaken the steel,
• more than 0.15% and preferably more than 0.2% of chromium to increase the hardenability but less than 0.5% because this element is expensive,
• from 0.0005% to 0.005% boron which increases the hardenability without modifying the other properties of the steel. The composition range corresponds to the field in which boron is effective,
• from 0.01% to 0.03% of titanium to trap nitrogen and prevent it from combining with boron so as to maintain the effectiveness of this element,
• less than 0.2% of silicon and preferably less than 0.15% because too high contents reduce the ductility of the steel.

Steel also contains sulfur and phosphorus which are impurities.

The sulfur content is limited to 0.015% at most but in the steels intended to produce parts which will be machined, the sulfur content can go up to 0.025%, because this element improves the machinability of the steels.

Phosphorus is an embrittling element and its content must remain below 0.015%.

Finally, especially when produced from scrap, the steel may contain nickel and molybdenum. These elements have a favorable effect on the hardenability and it is imposed that their contents are between 0.12% and 0.25% for nickel and between 0.04% and 0.15% (preferably 0.08%) for molybdenum.

The steel is quenched with aluminum and contains low levels of this element. It also contains nitrogen introduced by the process.

The steel which has just been defined is used in the form of a semi-finished product such as a billet which is rolled on a wire train to produce a wire with a diameter of for example between 5.5 mm and 18 mm .

The rolled wire is then drawn to obtain the final diameter and a good surface condition; the diameter can be for example between 4.5 mm and 17.5 mm.

• un four d'austénitisation à atmosphère contrôlée,
• un bac de trempe,
• un équipement de revenu constitué d'un bac contenant un alliage de plomb fondu.

The wire then undergoes a continuous quenching and tempering treatment carried out by running the wire through an installation comprising:

• a controlled atmosphere austenitization oven,
• a quench tank,
• tempering equipment consisting of a tank containing a molten lead alloy.

In the austenitization oven the wire is brought to a temperature above 900 ° C., a temperature at which it remains less than 5 min. This very short time has the advantage of not enlarging the austenitic grain, which makes it possible to obtain in the end a very fine structure favorable to good resistance to fatigue, good resilience and an aptitude for shaping. cold. The controlled atmosphere allows not to decarburize on the surface.

The quenching is carried out with water containing polymers of the PAG or PVP type or with accelerated quenching oil.

Water is used with additives or oil to avoid causing streaks on the wires.

Tempering is carried out at a temperature between 450 ° C and 600 ° C.

A wire is thus obtained, the structure of which is returned martensite possibly containing islands of returned bainite.

This wire has a tensile strength Rm between 900 MPa and 1000 MPa, an elastic limit R _e0.002 , greater than 850 MPa and a necking greater than 65%.

With this wire, after having made a covering operation which consists of pickling followed by phosphating and soaping, it is possible to produce, by cold striking and without heat treatment, parts such as bolts which have a resistance of between 1000 MPa and 1170 MPa and a yield strength greater than 900 MPa.

The yarn thus obtained is of class 10.9; it has the advantage of not requiring heat treatment after cold stamping. As it was produced by a continuous process, it has very homogeneous characteristics over its entire length. As the chromium content is low, it is inexpensive. Thanks to its low carbon content, it has good resistance to cracking by hydrogen. Finally, this wire has a ductility at least as good as that of 38C4 globulated steel wires.

By way of example, during crushing tests known to a person skilled in the art, the wire according to the invention exhibited limiting crushing rates of between 70% and 76% while the globulated 38C4 wire was limit crushing rates between 60% and 80% although its resistance is between 530 and 540 MPa only.

This wire can be used for any type of shaping by cold deformation and to make any type of cold-worked part. It can for example be used in cold bending or in cold forging or in cold drawing.

The heat treatment must be carried out continuously, but the austenitization oven can be replaced by an induction heating device.

Likewise, any type of continuous device can be used for quenching and for tempering.

Finally, the steel grade claimed can also be used to manufacture quenched quenched bars intended for cold deformation.

Claims (7)

1. Process for the production of a steel wire having a tensile strength between 800 and 1000 MPa and a yield strength R_e0.002 above 850 MPa and a necking above 65%, characterized in that:

   - a semifinished product is supplied with a steel having the following chemical composition by weight:

   0,15% ≤ C ≤ 0.3%

   1% ≤ Mn ≤ 1.8%

   Si ≤ 0.2%

   S ≤ 0.03%

   0.15% ≤ Cr ≤ 0.5%

   0.005% ≤ B ≤ 0.005%

   0.01% ≤ Ti ≤ 0.03%

   optionally : 0,12% ≤ Hi ≤ 0.25%

   optionally : 0.04% ≤ Mo ≤ 0.15%,

   the remainder being iron and impurities resulting from the preparation,

   - the semifinished product is rolled on a wire train to obtain a rolled wire,

   - the rolled wire is drawn to obtain a drawn wire,

   - the drawn wire is continuously passed into an austenitization furnace and then into a hardening tank, followed by passage into a tempering equipment, so that it undergoes a hardening and tempering treatment.
2. Process according to claim 1, characterized in that, in the austenitization furnace, the drawn wire is heated to a temperature above 900°C for a time exceeding 5 minutes.
3. Process according to claims 1 or 2, characterized in that hardening takes place with water to which have been added polymers of the type PAG or PVP or with accelerated hardening oil.
4. Process according to any one of the claims 1 to 3, characterized in that tempering takes place at a temperature between 450 and 600°C by passing into a tank containing a molten metal such as a lead alloy.
5. Process according to any one of the claims 1 to 4, characterized in that the steel contains between 0.12 and 0.25% nickel.
6. Process for the production of a prepared product such as a bolt, prepared by the cold shaping of a long steel product, having a tensile strength above 1000 MPa and below 1170 MPa and a yield strength R_e0.0.₀₂ above 900 MPa, characterized in that a drawn wire according to any one of the claims 1 to 6 is supplied, a drawn wire covering operation is performed and the prepared product is shaped by cold stamping without any subsequent heat treatment.
7. Long steel product, characterized in that its chemical composition comprises by weight:

   0.15% ≤ C ≤ 0.3%

   1% ≤ Mn ≤ 1.8%

   Si ≤ 0.2%

   S ≤ 0.03%

   0.15% ≤ Cr ≤ 0.5%

   0.12% ≤ Ni ≤ 0.25% optionally : 0.04% ≤ Mo ≤ 0.15%

   0.005% ≤ B ≤ 0.005%

   0.01% ≤ Ti ≤ 0.03%,

   the remainder being iron and impurities resulting from the preparation.