EP1565587A1

EP1565587A1 - Ready-use low-carbon steel mechanical component for plastic deformation and method for making same

Info

Publication number: EP1565587A1
Application number: EP03796115A
Authority: EP
Inventors: Bernard Resiak; Mario Confente
Original assignee: Mittal Steel Gandrange SA
Current assignee: ArcelorMittal Gandrange SA
Priority date: 2002-11-27
Filing date: 2003-11-27
Publication date: 2005-08-24
Anticipated expiration: 2023-11-27
Also published as: ES2338227T3; US20070051434A1; FR2847592B1; EP1565587B1; WO2004050935A1; JP5036967B2; AU2003298375A1; DE60331163D1; FR2847592A1; ATE456685T1; JP2006508248A

Abstract

A steel for the fabrication of a mechanical component by hot or cold plastic deformation has the following composition, in addition to iron and production impurities, (by wt %): (a) 0.02 at most C at most 0.15; (b) 1.3 at most Mn at most 2.0; (c) 0.04 at most Nb at most 0.10; (d) 0.10 at most Mo at most 0.35; (e) 0.001 at most B at most 0.005; (f) 0.15 at most Si at most 1.30; (g) 0.01 at most Al 0.08; (h) N at most 0.015 with Ti at least 3.5xN; (i) possibly up to 0.80 of chromium and/or up to 0.1 of sulfur. Independent claims are also included for: (a) the fabrication of a mechanical component by cold plastic deformation; (b) the fabrication of a mechanical component by hot plastic deformation; (c) a ready for use steel mechanical component.

Description

Ready-to-use mechanical part in low carbon steel for plastic deformation and its manufacturing process.

The invention relates to mechanical parts made of low carbon steel with high characteristics, such as ball joints of land vehicle wheels, pivots, axes, suspension triangles, rods, or other similar ready-to-use mechanical parts obtained by plastic deformation of '' a long steel product (wire, bar ...)

It is known that steels for plastic deformation must have properties of both deformability and resistance. Thus, during the manufacture of the mechanical parts for which some of them are intended, they must be able to withstand without breaking significant changes in shape while sometimes having in the end high mechanical characteristics. In fact, in certain cases, the required characteristics of parts obtained from these steels are close to those of class 10.9 according to ISO 898, namely a minimum breaking limit of 1000 MPa and a minimum elastic limit of 900 MPa. In addition, these steels must have good machinability characteristics, since a majority of applications require ultimate machining for final ribs.

It will be recalled that the plastic deformation operations are carried out on pieces of steel resulting from the cutting of wires or bars conventionally obtained by hot rolling of semi-continuous casting products (billets or blooms). In cold plastic deformation (striking, forging, etc.), the pieces are cold formed using a press, if necessary after a globulation annealing, and the parts obtained are then heat treated by quenching and tempering. For hot forging, the pieces are first heated to a temperature of around 1000-1200 ° C, hot formed and cooled. The parts thus obtained are then heat treated by quenching and tempering, the quenching can be done directly during cooling after forging.

Carrying out these different heat treatments presupposes operations, admittedly controlled, but nevertheless costly, the intended results of which are not always achieved and which, in any case, increase the time and cost of production. Also, in recent years, we have been looking for steel grades which allow us to overcome them and obtain parts with high "ready to use" characteristics, which can be used for the intended application without having to undergo a heat treatment to modify their metallurgical structure after the plastic deformation operation.

Regarding cold stamping for example, it is already known for example to use steel grades of essentially bainitic structure (ie containing more than 50% of bainite), having a good compromise between deformability and final mechanical characteristics. However, taking into account the capacities of the cooling means which are generally available on a hot rolling line, these grades make it possible to obtain an essentially bainitic structure only on rolled wires or bars of relatively small diameter, rarely exceeding 8 mm. in fact. Beyond this, a degenerate bainite or associated with ferrite is obtained, which leads to a marked deterioration in the mechanical properties of the rolled products. In addition, the structure not being well controlled, there is a risk of great dispersion of the mechanical characteristics within the same ring or between several rings of wound wires or between several bars or within the same bar to the outcome of hot rolling.

Similar problems are encountered with steel grades for hot forging for which the thickness of the forged piece often imposes severe cooling constraints in order to achieve the core cooling speed necessary for obtaining the bainitic structure referred to in the mass. In addition, the periphery of the part being inevitably cooled much more energetically than the heart, this results in internal tensions which can lead to unacceptable permanent deformations.

It can therefore be seen that conventionally, in the applications of grades for plastic deformation, a bainitic structure is sought which offers a good compromise between deformability and mechanical characteristics, as well as good machinability. In all cases, the success of obtaining this bainitic structure is subject to the cooling constraints of the steel at heart, whether this cooling occurs before the plastic deformation or after. These constraints imposed on cooling are so severe on the steel grades currently known and used that this bainitic structure may not be obtained directly in the hot rolling, or even after the forging operation, so that many mechanical parts must undergo heat treatment after they are shaped. The objective of the invention is to provide transformers with a low carbon steel grade capable of developing a bainitic, or essentially bainitic, structure with low cooling stresses, for the manufacture of parts ready for use both by cold press and hot forging.

More specifically, the object of the invention is the development of a low carbon steel grade specific to the manufacture of mechanical parts provided with a bainitic or essentially bainitic structure which can already be obtained with a low cooling rate at the core, which can go down to 1 ° C / s, and offering both good deformability and good machinability for the production of these parts by cold or hot deformation, without post-heat treatment, said grade having high mechanical characteristics allowing said parts to be in quality classes 8.8 to 12.9 according to ISO standard 898.

The subject of the invention is therefore a mechanical part with high characteristics in low carbon steel ready for use coming from the plastic transformation of a long rolled steel product, characterized in that: - the composition of said steel, in addition to iron and the inevitable residual impurities resulting from the production of steel, meets at least the following analysis, given in weight percentages relative to iron:

C <0.15% 0.04% <Nb <0.10%

0.001% <B <0.005% 0.10% <Mo <0.35% l, 3% ≤ Mn <2.0% 0, 15% ≤ If <1.30% 0.01% <A1 <0.08%

N <0.015% with Ti> 3.5 x% N;

- Said long product is obtained from a semi-product from continuous casting and hot rolled in the austenitic field, then heat treated to give it a bainitic, or essentially bainitic, structure, as well as shaped by plastic transformation to cold or hot to give it its final shape with a breaking strength greater than 800 MPa.

In a first preferred embodiment, the mechanical cold-deformed steel part defined above is characterized in that the long product from which it is obtained by plastic transformation is a rolled wire or bar heat treated by cooling in the hot rolling at a sufficient cooling rate to give it a bainitic or essentially bainitic structure.

In a second preferred embodiment of the invention, the mechanical hot-forged steel part defined above is characterized in that the long product from which it is obtained by plastic transformation is a bar or a rolled wire whose piece of forge which has been extracted therefrom has been heat treated by quenching under a cooling rate sufficient to give it a bainitic structure to the core, from a quenching temperature of the order of 1200 ° C. and above at which the slug has underwent a plastic transformation by forging bringing it to its desired final shape.

Preferably, in the two embodiments mentioned above, the heat treatment involved in the development of the mechanical part comprises a final phase of cooling at low speed, which can drop to around 1 ° C / s, to the core.

It will be noted that this cooling of the part is a gentle cooling, different in any case from a cooling operation which would quench the steel, which moreover would, in normal practice, be followed by tempering.

In a variant, the mechanical part is produced with a steel whose carbon content is between 0.06% and 0.10%.

In another variant, the mechanical part is made of steel, the molybdenum content of which does not exceed 0.30%, and that of manganese is less than 1.80%. The invention also relates to a method of manufacturing a mechanical part with high characteristics ready for use in low carbon steel having a breaking strength of more than 800 MPa, characterized in that it comprises the following steps : - from a long semi-finished product in low carbon steel, the composition of which, in addition to iron and the inevitable residual impurities resulting from the production of steel, meets at least the following analysis, given in weight percentages relative to iron: C <0.15%

0.04% <Nb <0.10% 0.001% <B <0.005%

0.10% <Mo <0.35% 1.3% <Mn <2.0%

0.15% <If <1.30%

0.01% <A1 <0.08% N <0.015% with Ti ≥ 3.5 x% N, a long product is hot rolled in the austenitic field according to the usual practice of hot rolling;

the long rolled product obtained is then heat treated, this heat treatment comprising a final low-speed cooling phase, which can drop to around 1 ° C / s, - to the core to obtain a bainitic, or essentially bainitic structure, and said long product is plastically deformed to bring it to the desired final shape, the plastic deformation operation being able to be performed after or during said heat treatment.

The invention also relates to a long steel product intended to obtain a mechanical steel part as defined above, characterized in that it is in the form of wire or hot-rolled bar and in that the The steel of which it is composed corresponds at least to the following analysis, given in weight percentages relative to iron:

C <0.15% 0.04% <Nb <0.10%

0.001% <B <0.005%

0.10% <Mo <0.35% 1.3% ≤Mn <2.0%

0.15% <If <1.30% 0.01% <A1 <0.08%

N <0.015% with Ti> 3.5 x% N.

As will be understood, the invention, in its essential characteristics, consists in the definition of an analysis of low carbon steel based on niobium, boron and molybdenum, which is specific to mechanical parts with high characteristics and able to develop a homogeneous bainitic (or essentially bainitic) structure in the mass of the part with few cooling requirements. This structure can in fact already be obtained from a low core cooling rate which can drop to around 1 ° C / s, a speed which can be reached, as we know, directly in the hot rolling mill - even for wires and bars with a diameter of around 20 mm and more depending on the installation.

Consequently, the invention opens up to large diameters the production range of long hot-rolled products intended for cold forging or forging workshops, and, for those reserved for hot forging, it provides the economy of a additional final heat-quenching heat treatment. To better fix the ideas, it will be noted that with the usual hot rolling, the limit diameters are around 20 to 25 mm for the grades according to the invention.

The vocabulary habits in the steel profession make what is called

- "wires or small bars" rolled products with diameters of up to around 30 mm (which are often packaged in the form of rings for delivery to processors);

- and "bars" those laminated from 18 mm in diameter and which are delivered straight after cutting to length at the exit of the train.

Furthermore, for the sake of clarity of the description, the expression "bainitic structure" will denote a "bainitic or essentially bainitic structure".

The invention will be well understood and other aspects and advantages will appear more clearly in the light of the detailed description which follows, given by way of exemplary embodiment.

Long semi-finished products (billets or blooms) produced from steel having, in addition to iron, the following composition, in weight content relative to iron:

From 0.02 to 0.15%, and preferably 0.08%, of carbon. The carbon at these contents is used to obtain a bainitic structure having the required mechanical properties. It provides good aptitude for work hardening during cold plastic deformation. Its low content also makes it possible to avoid the formation of large carbides unfavorable to ductility without it being necessary to carry out a globulization treatment.

From 0.04 to 0.10%, and preferably 0.06 to 0.08%, of niobium. Niobium acts in synergy with molybdenum and boron to widen the area of bainitic transformation. It increases the quenching effect of boron by increasing the effective boron content contained in the steel. Indeed, the formation of the carbides Fe ₂₃ (CB ₆ ) (trapping the boron and passive as for the hardenability of the steel) is made more difficult under the action of niobium which stabilizes the austenite and delays the diffusion of carbon. Furthermore, it makes it possible to increase the recrystallization temperature of the austenite which makes it possible to obtain a finer bainitic structure during controlled rolling, and thus to increase the resilience of the parts.

From 0.001 to 0.005% boron. Boron inhibits the germination of ferrite thus promoting the formation of a bainitic structure. It acts in synergy with niobium and molybdenum to widen the bainitic domain.

From 0.10 to 0.35%, and preferably less than 0.30% of molybdenum. Molybdenum is a carburizing element allowing to widen the bainitic domain by delaying the germination of ferrite. In addition, at these contents, its action on the hardenability of the steel makes it possible to obtain a steel of higher mechanical strength by lowering the temperature at the start of bainitic transformation. It thus tends to compensate for the low carbon content necessary for obtaining good ductility. Furthermore, it acts in synergy with boron and with niobium, which it strengthens its role. In addition, at these levels, it acts in synergy with niobium to increase the recrystallization temperature of austenite. From 1.30 to 2.00%, and preferably between 1.60 and 1.80%, of manganese. This manganese then makes it possible to obtain sufficient quenchability, helps in the formation of bainite and makes it possible to obtain the desired mechanical characteristics.

From 0.10 to 1.30%, and preferably from 0.20 to 0.35%, of silicon. At these contents, it makes it possible to obtain a moderate hardening of the steel. We can go up to a content of 1.30% if necessary, in particular to increase the mechanical strength of the steel. The silicon also makes it possible to deoxidize the steel during casting.

From 0.007 to 0.010% nitrogen, associated with a titanium content of the order of 3.5 times this nitrogen content to form a sacrificial screen for the benefit of boron. Titanium is used to fix nitrogen and thus protect the boron. Without titanium, boron would lose its quenching power by reacting with nitrogen. Titanium also makes it possible to obtain a fine austenitic grain which improves the ability to cold forming and to ductility.

Less than 0.08% aluminum. This residual dissolved aluminum, coming from the calming of the steel before casting, is a good deoxidizer for protecting titanium against oxidation by dissolved oxygen inevitably present, so that this titanium remains available to protect boron against nitrogen. This aluminum also serves to control the magnification of the austenitic grain during the hot rolling of the starting semi-finished product, and thus to give the steel good impact properties.

Optionally from 0.001 to 0.1% sulfur. This sulfur combines with manganese to form plastic and ductile manganese sulfides. It provides good machinability. It is possible, if one wishes to improve machinability further, to increase its content up to a maximum value of 0.1% but not beyond it if one wishes to guarantee good deformability. Cold.

This steel also has the inevitable impurities and residual elements resulting from its production, in particular phosphorus, the content of which must preferably remain below 0.02% to guarantee good ductility during and after setting. in cold form, as well as copper and nickel, the content of which should preferably be less than 0.30%.

This optimized composition allows the steel to have a very good capacity for plastic deformation at the same time as good machinability. Indeed, this shade not only promotes the production of bainite, but also reduces the risk of obtaining martensite, the presence of which can constitute a serious obstacle to a good machining operation.

Most of the time, we can also limit the molybdenum content to 0.30% and the manganese content to 1.80% in order to avoid the risk of the appearance of a martensitic-type quenching structure in certain cases. given local conditions.

An essential aspect of the invention is that the mechanical parts have a homogeneous bainitic structure in the mass at low cooling speed at the heart of the hot forged parts, or of the wires or bars from which they are obtained by cold striking, which can descend up to approximately 1 ° C / s.

When, in accordance with an implementation of the invention, the mechanical part is cold struck (or cold forged), the bainitic structure is obtained before shaping. The steel, after deformation, then has good ductility, measured by a necking much greater than 50%, a tensile strength greater than 650 MPa, and a mechanical resistance greater than 800 Mpa.

In this first embodiment, the part is in fact obtained by cold plastic deformation of the steel already having a bainitic structure. A long semi-finished product consisting of an analysis steel in accordance with the invention is supplied which is hot-rolled, if necessary after reheating above 1100 ° C., according to the usual practice of hot rolling up to obtaining a laminated wire 10 mm in diameter for example. The wire removal temperature is less than 1000 ° C. The laminated wire obtained is then air-cooled in the rolling hot water itself in the usual way ("stelmor" process for example), at a low core speed which can drop to around 1 ° C / s to obtain a homogeneous bainitic structure. The rolled wire is then delivered (or deliverable) to the transformer in the form of a crown. The transformer who receives the crown unwinds the wire, erects it if necessary, before cutting it into pieces of the desired length. Each piece is then subjected to a usual cold plastic deformation operation to obtain the final ready-to-use part (ball joints, pins, rods, screws ...), after a nominal rib machining if necessary. . The final mechanical characteristics will naturally be obtained by the work hardening resulting from the shaping.

In a second embodiment, the parts are hot deformed and the bainitic structure is obtained after this plastic deformation operation: a long semi-product consisting of an analysis steel in accordance with the invention is supplied which is rolled hot until a 30 mm laminated bar is obtained diameter for example. After possible cooling, the bar cut to length by cutting can be delivered rectilinear to the blacksmith with its ordinary metallographic structure acquired naturally during hot rolling.

The blacksmith who receives it cuts it into pieces and each piece is then brought to a temperature of around 1200 ° C. before being subjected to a hot plastic deformation operation at the forge. The parts are then cooled in the usual way, in two stages, with a first controlled cooling down to a temperature below 1000 ° C and a second cooling at low core cooling rate which can go down to 1 ° C / s approx. In this embodiment, the end of rolling conditions have no particular importance on obtaining the metallurgical structure, since the bainite, which gives the part most of its working properties, is reached at the very end, after hot forming and controlled cooling.

It will be recalled that the mechanical parts according to the invention are obtained by plastic deformation of laminated products without additional heat treatment of quenching and tempering.

Laboratory tests were carried out on a flow of the following composition:

The billets from the casting were hot rolled after reheating above 1100 ° C to form a wire 12 mm in diameter. The wire deposition temperature after rolling was 820 ° C. The rate of cooling of the wire in the hot end of rolling (air cooling type "stelmor") was of the order of 5 ° C / s. A homogeneous bainitic structure is obtained over the whole of the wire, at the periphery as well as at the core.

The mechanical characteristics of the wire are as follows:

We recall that:

- Rm represents the breaking strength corresponding to the maximum force before breaking reported in the initial section of the wire.

- E _O I represents the conventional elastic limit corresponding to the force related to the initial section of the wire causing a plastic elongation of 0.2%.

- A represents the elongation at break.

- Z represents the necking corresponding to the reduction in section of the wire after breaking.

The evolution of the mechanical characteristics according to the rate of deformation undergone by the wire is as follows:

The mechanical parts with high characteristics according to the invention are remarkable in that they make it possible in particular to save the quenching and tempering treatments currently used during striking or cold forging or hot forging operations.

On the other hand, by imposing less drastic cooling conditions, they are less likely to deform during the cooling operation, or else with equivalent cooling fluid they can have larger diameters or thicknesses. They are also remarkable for the very good machinability characteristics which they exhibit, which allows in cold applications to reduce the sulfur contents and therefore to limit the harmful influence of this element in the ability to deformability.

It goes without saying that the invention cannot be limited to the examples which have just been described, but that it extends to multiple variants and equivalents insofar as its definition given by the appended claims is respected.

Thus, for example, in hot forging applications, a person skilled in the art may choose to improve the machinability by varying the sulfur content or by adding other agents favoring machining such as tellurium, lead or selenium. Likewise, although being intended more particularly for stamping applications or cold forging or hot forging, the invention also applies to other plastic deformation applications such as wire drawing, drawing, stamping, etc. ..

Claims

1) Mechanical part with high characteristics in low carbon steel ready for use coming from the plastic transformation of a long rolled steel product characterized in that:

the composition of said steel, in addition to iron and the inevitable residual impurities resulting from the production of steel, meets at least the following analysis, given in weight percentages relative to iron: C <0.15% 0, 04% <Nb <0.10% 0.001% <B <0.005% 0, 10% <Mo <0.35% l, 3% ≤Mn <2.0%

0.15% <If <1.30% 0.01% <A1 <0.08% N <0.015% with Ti> 3.5 x% N;

- Said long product is obtained from a semi-product from continuous casting and hot rolled in the austenitic field then heat treated to obtain a bainitic, or essentially bainitic, structure and shaped by cold or cold plastic transformation hot to give it its final shape with a breaking strength greater than 800 MPa.

2) Mechanical part in low carbon steel cold deformed according to claim 1 characterized in that the long product from which it comes from plastic transformation is a rolled wire or bar heat treated by cooling in the hot rolling at a sufficient cooling speed to give it a bainitic or essentially bainitic structure.

3) Forged steel mechanical part according to claim 1 characterized in that the long product from which it is obtained by hot plastic transformation is a bar or a rolled wire from which the piece of forge which has been extracted from it has been heat treated by quenching under a sufficient cooling rate to give it a bainitic structure to the core, this from a quenching temperature of the order of 1200 ° C. and above at which the piece has undergone a plastic transformation by forging bringing it to its shape desired finish. 4) Mechanical steel part according to claim 2 or 3 characterized in that the heat treatment involved in its preparation comprises a final phase of low speed cooling, which can drop to 1 ° C / s, to the core.

5) Mechanical steel part according to one of claims 1 to 4 characterized in that the carbon content of the steel is between 0.06 and 0.10%.

6) Mechanical steel part according to one of claims 1 to 5 characterized in that the steel of which it is composed has a molybdenum content which does not exceed 0.30%, and a manganese content of less than 1, 80%

7) Process for manufacturing a ready-to-use mechanical part with high characteristics in low carbon steel having a breaking strength of more than 800 MPa, characterized in that it comprises the following stages:

- from a long semi-finished product, the composition of which, in addition to iron and the inevitable residual impurities resulting from the production of steel, corresponds at least to the following analysis, given in weight percentages relative to iron:

C <0.15% 0.04% <Nb <0.10%

0.001% <B <0.005%

0.10% <Mo <0.35% 1.3% ≤Mn <2.0%

0.15% <If <1.30% 0.01% <A1 <0.08%

N <0.015% with Ti> 3.5 x% N, a long product (wire or bar) is hot rolled, the temperature for depositing the wire after rolling being less than 1000 ° C .;

said long laminated product obtained is then heat treated, said heat treatment comprising a final low-speed cooling phase, which can drop to around 1 ° C./s, at the core to obtain a bainitic, or essentially bainitic, structure, and said long product is plastically deformed to bring it to its desired final shape, the plastic deformation operation being able to be performed after or during said heat treatment. 8) Long low carbon steel product intended to be transformed into a mechanical part with high characteristics ready for use according to claim 1, characterized in that it is in the form of wire or hot rolled bar and in that the steel which composes it answers at least the following analysis, given in percentages by weight compared to iron:

C <0.15% 1.3% ≤Mn <2.0%

0.04% <Nb <0.10%

0.10% ≤Mo <0.35% 0.001% <B <0.005%

0.15% <If <1.30%

0.01% <A1 <0.08%

N <0.015% with Ti> 3.5 x% N.