EP0550294A1 - 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

Abstract

Elongate product such as a wire or a rod for the manufacture by cold forming, especially by cold coining, of wrought products which have a tensile strength higher than 1000 MPa and lower than 1170 MPa and an elasticity limit higher than 900 MPa. The elongate product is made of a steel in the quenched and annealed state, containing 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 % the remainder being iron and impurities resulting from the melting of the raw materials.

Description

The invention relates to a long product such as a wire or a bar for the manufacture by cold forming, in particular by cold striking, of processed products such as bolts, said processed products having a tensile strength greater than 1000 MPa and less than 1170 MPa and a yield strength R _e0.002 , greater than 900 MPa.

To manufacture bolts, cold deformation by cold striking strips of steel wire which one chooses 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 cut sections that 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.

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.

le reste étant du fer et des impuretés résultant de la fusion des matières premières.To this end, the subject of the invention is a long product such as a wire or a bar for the manufacture by cold deformation, in particular by cold stamping of produced products such as bolts, said produced produced having resistance to tensile strength greater than 1000 MPa and less than 1170 MPa and an elastic limit R _e0.002 , greater than 900 MPa, made of steel in the quenched and tempered state containing by weight:

the rest being iron and impurities resulting from the fusion of raw materials.

The steel may also contain 0.12% to 0.25% nickel and 0.04% to 0.15% molybdenum.

le reste étant du fer et des impuretés résultant de la fusion des matières premières.Preferably, the chemical composition of the steel comprises:

the rest being iron and impurities resulting from the fusion of raw materials.

The wire or bar has a martensitic micrographic structure returned with possibly islands of bainite returned.

The quenching and tempering are carried out continuously.

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 required that their contents are between 0.12% and 0.25% for nickel and between 0.04% and 0.15% (preferably 0.08%) for the 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, temperature at which it remains less than 5 min. This very short time has the advantage of not enlarging the austenitic grain, which in the end makes it possible to obtain a very fine structure favorable to good resistance to fatigue, good resilience and an aptitude for shaping. cold. The controlled atmosphere makes it possible 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. The chromium content being low, it is inexpensive. Thanks to its low carbon content, it has good resistance to hydrogen cracking. 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 limit crushing rates of between 70% and 76% while the globulized 38C4 wire had 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 part produced cold. 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.

Similarly, 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 (6)

1.- Long product for the manufacture of elaborate products such as bolts, by cold forming in particular by cold stamping, said elaborate products having a tensile strength greater than 1000 MPa and less 1170 MPa and an elastic limit R _e0.002 greater than 900 MPa characterized in that it consists of a steel in the quenched and tempered state containing by weight:

the rest being iron and impurities resulting from the fusion of raw materials. 2.- long product according to claim 1, characterized in that it consists of a steel further containing 0.12% to 0.25% nickel and 0.04% to 0.15% molybdenum . 3. Long product according to any one of claims 1 or 2, characterized in that its chemical composition preferably comprises:

the rest being iron and impurities resulting from the fusion of raw materials. 4. A long product according to any one of the preceding claims, characterized in that its micrographic structure is martensitic returned with possibly islands of bainite returned. 5. Long product according to any one of the preceding claims, characterized in that the quenching and tempering are carried out continuously. 6.- Method of manufacturing a product 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 cold forming of the long steel product is carried out which contains:

after continuous quenching and long product tempering, without subsequent heat treatment of the product produced by cold forming.