EP1426452A1 - Cooled and annealed article in bainitic steel, and production process thereof - Google Patents

Abstract

Fabrication of a steel component consists of: (a) production and casting of a steel with a given composition; (b) effecting at least one hot deformation of the cast steel to produce a component blank at a temperature of 1100 to 1300 degrees C; (c) effecting a controlled cooling of the blank in still or pulsed air; (d) effecting a precipitation tempering, preceding or following the machining of the component from the blank. An Independent claim is also included for a steel component obtained by this method.

Description

The invention relates to metallurgy, and more precisely the field steels intended for the manufacture of parts having to withstand significant solicitations.

Often such parts are made of hardened and tempered steel or, as far as possible, forged steel with ferrito-pearlitic structure which is supposed to offer a better technical and economic compromise, but whose mechanical performance is still limited.

Ferrito-pearlitic steels often used for this purpose are types XC70, 45Mn5, 30MnSiV6 and 38MnSiV5, and undergo after rolling or forging a simple in-line cooling with calm air. Their method of implementation is therefore relatively economical, but their duration life in the presence of heavy loads is limited.

It has already been proposed to produce such bainitic steel parts at from a grade of 25MnSiCrVBS type, cooling after forging or air rolling. The holding performance is significantly improved compared to previous examples, but remain relatively limited compared to what can be achieved on hardened steel and returned.

The object of the invention is to propose an association between a nuance of steel and a method of manufacturing a part, having advantages economical compared to existing associations without performance metallurgical are altered, or even improving these performances. The room thus manufactured must withstand significant fatigue stresses. In the case forgings, this manufacturing process should, in particular, be adaptable to any forging line.

• on élabore et on coule un acier de composition, en pourcentages pondéraux, 0,06% ≤ C ≤ 0,25% ; 0,5% ≤ Mn ≤ 2% ; traces ≤ Si ≤ 3% ; traces ≤ Ni ≤ 4,5% ; traces ≤ Al ≤ 3% ; traces ≤ Cr ≤ 1,2% ; traces ≤ Mo ≤ 0,30% ; traces ≤ V ≤ 2% ; traces ≤ Cu ≤ 3,5% ; et respectant l'une au moins des conditions :
  • 0,5% ≤ Cu ≤ 3,5%
  • 0,5% ≤ V ≤ 2%
  • 2% ≤ Ni ≤ 4,5% et 1% ≤ Al ≤ 2%
  le reste étant du fer et des impuretés résultant de l'élaboration ;
• on effectue au moins une déformation à chaud de l'acier coulé pour obtenir une ébauche de la pièce à une température de 1100 à 1300°C ;
• on effectue un refroidissement contrôlé de l'ébauche de la pièce à l'air calme ou à l'air pulsé ;
• et on réchauffe l'acier pour effectuer un revenu de précipitation, précédant ou suivant l'usinage de la pièce à partir de ladite ébauche.

To this end, the subject of the invention is a method for manufacturing a steel part, characterized in that:

• a composition steel is produced and poured, in weight percentages, 0.06% ≤ C ≤ 0.25%; 0.5% ≤ Mn ≤ 2%; traces ≤ Si ≤ 3%; traces ≤ Ni ≤ 4.5%; traces ≤ Al ≤ 3%; traces ≤ Cr ≤ 1.2%; traces ≤ Mo ≤ 0.30%; traces ≤ V ≤ 2%; traces ≤ Cu ≤ 3.5%; and respecting at least one of the conditions:
  • 0.5% ≤ Cu ≤ 3.5%
  • 0.5% ≤ V ≤ 2%
  • 2% ≤ Ni ≤ 4.5% and 1% ≤ Al ≤ 2%
  the remainder being iron and impurities resulting from processing;
• at least one hot deformation of the cast steel is carried out in order to obtain a blank for the part at a temperature of 1100 to 1300 ° C;
• a controlled cooling of the workpiece blank is carried out with calm air or forced air;
• and the steel is heated to effect precipitation precipitation, preceding or following the machining of the part from said blank.

Preferably, the steel contains from 5 to 50 ppm of B.

Preferably, the steel contains from 0.005 to 0.04% of Ti.

If B is present, the Ti content is preferably at least equal to 3.5 times the N content of the steel.

Preferably, the steel contains from 0.005 to 0.06% of Nb.

Preferably, the steel contains from 0.005 to 0.2% of S.

In this case, preferably, the steel contains at least one of the elements Ca up to 0.007%, Te up to 0.03%, Se up to 0.05%, Bi up to 0.05% and Pb up to 0.1%.

According to a variant of the invention, the C content of the steel is between 0.06 and 0.20%.

The Mn content of the steel is then preferably between 0.5 and 1.5%, and the Cr content is preferably between 0.3 and 1.2%.

The Ni content of the steel can then preferably be between traces and 1%.

The Ni content of the steel can then also be between 2 and 4.5%, and the Al content is then between 1 and 2%.

The precipitation income is in the general case carried out from preferably between 425 and 600 ° C.

When the steel contains 0.5 to 3.5% Cu, the precipitation income is preferably carried out between 425 and 500 ° C for 1 to 10 hours.

When the steel contains 0.5 to 2% V, the precipitation income is preferably performed between 500 and 600 ° C for more than 1 hour.

When the steel contains 2 to 4.5% Ni and 1 to 2% Al, the income of precipitation is preferably carried out between 500 and 550 ° C for more than 1 hour.

Said hot deformation can be a rolling.

Said hot deformation can be forging.

Preferably, the controlled cooling of the blank is carried out at a speed below 3 ° C / s between 600 and 300 ° C.

The invention also relates to a steel part obtained by the previous process which typically has a bainitic microstructure, a tensile strength Rm from 750 to 1300MPa and a yield strength Re greater than or equal to 500MPa.

As will be understood, the invention consists of the combination of a steel grade and a post-casting treatment process comprising a stage of hot shaping of the part, controlled cooling which can be performed in calm air or forced air and a precipitation income preceding or according to the machining of the part. The composition of the steel chosen guarantees that, whatever the cooling mode, the fatigue resistance results of parts made from this steel will be sufficient to meet the user requirements.

The hot forming operation may consist of one or more rolling, or in a rolling followed by a forging, or in a forging alone. The main thing is that the last hot deformation brings the steel between 1100 and 1300 ° C, and that the controlled cooling takes place from this temperature.

The chemical characteristics of steel and its heat treatments after casting aim at obtaining a bainitic microstructure, and also to obtain optimized mechanical characteristics. This bainitic microstructure must be obtainable after cooling in calm air, but must also be compatible with forced air cooling. In this way, the parts concerned by the invention can be produced on any existing installation, whether this allows after forging or rolling a forced air cooling, or that it only allows air cooling calm. Thus, a forging installation originally designed to process steel parts with ferrito-pearlitic microstructure can easily and without special adaptations, treat parts with bainitic microstructure according to the invention. The bainitic microstructure steels previously used for these uses required forced air cooling, and therefore could not always be treated on commonly designed installations.

According to the invention, we therefore begin by developing a steel whose composition will be detailed and justified later, then poured, in ingots or in continuous according to the format of the final piece, and more generally it is laminated so as to obtain a semi-finished product.

We can then perform a forging operation of the semi-finished product.

The last hot deformation is carried out at 1100-1300 ° C and is followed by controlled air cooling in the hot rolling or forge, with calm air or forced air. A blank of the part is thus obtained.

By the term "draft", it should be understood that one designates here a bar, or a semi-finished product in another form, from which the final part will be obtained by machining, regardless of the deformation mode at hot practiced: rolling, forging or a combination thereof.

A precipitation income is then carried out. This is located either before or after machining the part from said blank.

The analytical ranges required are as follows for different chemical elements in front of or which may be present (all percentages are by weight).

The carbon content is between 0.06 and 0.25%. This content governs the type of microstructure obtained. At less than 0.06%, the microstructure obtained would not be interesting for the objectives sought. Beyond 0.25%, in combination with the other elements, we would not get a sufficiently bainitic microstructure after cooling in still air.

The manganese content is between 0.5 and 2%. This element added to more than 0.5% provides its hardenability to the material, and allows to obtain a wide bainitic range whatever the cooling mode. A content greater than 2% would however be likely to cause segregation too important.

The silicon content is between traces and 3%. This element, not strictly speaking, is advantageous in that it hardens the bainite by its passage into solid solution. In addition, in case copper is present in relatively large quantities, silicon avoids problems associated with this presence of copper during hot forming. A content higher than 3% can however pose machinability problems of the material.

The nickel content is between traces and 4.5%. This non-compulsory element promotes hardenability and stabilization of austenite. Yes aluminum content allows, it can form very precipitated NiAl hardening, providing the metal with high mechanical properties. In case where copper is present in relatively large quantities, nickel can play the same role as silicon. Above 4.5%, the addition of nickel is unnecessarily expensive in view of the metallurgical objectives targeted.

The aluminum content is between traces and 3%. This non-compulsory element is a strong deoxidizer, and even added at low content, it limits the amount of oxygen dissolved in the liquid steel, so to improve the inclusiveness of the room if we were able to avoid excessive reoxidation during casting. With a high content, as we said, it is likely to form NiAl precipitates if nickel is present in large quantity. It is not useful that the aluminum content exceeds 3%.

The content of chromium, a non-compulsory element, is between traces and 1.2%. Like manganese, chromium contributes to the improvement of hardenability. Its addition becomes unnecessarily expensive beyond 1.2%.

The molybdenum content is between traces and 0.30%. This element, not compulsory, prevents the formation of coarse-grained ferrite and allows to obtain more assuredly the bainitic structure. Its addition is unnecessarily costly above 0.30%.

The vanadium content is between traces and 2%. This element, not obligatory, serves to harden the bainite by its passage in solution solid. With a high content, it also makes it possible to obtain a hardening by precipitation of carbides and / or carbonitrides. Its addition is unnecessarily expensive beyond 2%.

The copper content is between traces and 3.5%. This element, not compulsory, can improve the machinability and, by precipitating, cause secondary hardening of the material. But above 3.5% it makes the implementation hot form of the problematic part. As we said, it is advisable to him combine a significant nickel or silicon content to minimize hot formatting problems. Beyond 3.5% its addition is of all unnecessarily expensive.

• une teneur en cuivre comprise entre 0,5 et 3,5%
• une teneur en vanadium comprise entre 0,5 et 2%
• une teneur en nickel comprise entre 2 et 4,5% et une teneur en aluminium comprise entre 1 et 2%.

In addition, at least one of the following three conditions must be met:

• a copper content of between 0.5 and 3.5%
• a vanadium content of between 0.5 and 2%
• a nickel content of between 2 and 4.5% and an aluminum content of between 1 and 2%.

The elements that we have just mentioned are those whose role metallurgical is or may be most important to the invention, but others elements which we will cite may also be optionally present for improve certain properties of steel.

The boron content can be between 5 and 50 ppm. he can improve hardenability, but must be in solid solution to be effective. In other words, we must avoid that almost all of the boron ends up under the form of boron nitrides or carbonitrides. To this end, it is advisable to associate to the addition of boron an addition of titanium, preferably in a proportion such that 3.5 x N% ≤ Ti%. With this last condition, we can capture all the nitrogen dissolved and avoid the formation of boron nitrides or carbonitrides. Content minimum titanium, for this purpose, is 0.005%, for the most nitrogen contents bass usually encountered. It is however advisable not to exceed a titanium content of 0.04%, otherwise we obtain titanium nitrides of size too high.

Titanium also has the function of limiting the magnification of the austenitic grain at high temperature, and can be added for this independently of boron, at a content of between 0.005 and 0.04%.

Niobium can also be added, at levels between 0.005 and 0.06%. It too can precipitate in the form of carbonitrides in austenite, and can thus harden the material.

Finally, conventionally, the machinability of the material can be improved by adding sulfur (from 0.005% to 0.2%), which can also be combined addition of calcium (up to 0.007%), and / or tellurium (up to 0.03%) and / or selenium (up to 0.05%), and / or bismuth (up to 0.05%) and / or lead (up to 0.1%).

Once obtained after rolling the semi-finished product having the composition previously mentioned, forging or not proceeding to forging the blank of the part according to the usual procedures. It is heated to 1100-1300 ° C, then performs the deformations giving rise to the part blank.

In the absence of forging, rolling must end at a temperature of 1100-1300 ° C.

Then immediately after rolling, or after forging if this operation has been carried out, a controlled cooling of the part is carried out, in calm air, or in forced air. In general, we impose on the piece a cooling at a speed less than or equal to 3 ° C / s between 600 and 300 ° C.

• la précipitation de cuivre, si la teneur en cuivre est comprise entre 0,5 et 3,5% ;
• la précipitation de vanadium si sa teneur est comprise entre 0,5 et 2% ;
• la précipitation de NiAl si la teneur en nickel est comprise entre 2 et 4,5% et la teneur en aluminium comprise entre 1 et 2%.

According to the invention, and this before or after machining the part which gives it its final dimensions, the steel is hardened by precipitation by means of tempering, that is to say of a heat treatment following reheating from a temperature equal to or slightly above ambient; for this three options are possible, and can also be combined:

• copper precipitation, if the copper content is between 0.5 and 3.5%;
• the precipitation of vanadium if its content is between 0.5 and 2%;
• NiAl precipitation if the nickel content is between 2 and 4.5% and the aluminum content between 1 and 2%.

Generally, precipitation income is made from preferably between 425 and 600 ° C. But the temperature of the income and its duration are optimally to adapt to the targeted characteristics. For example, the copper precipitation is preferably obtained by treatment at 425-500 ° C for 1 to 10 hours. The vanadium precipitation is preferably obtained by a treatment at 500-600 ° C for more than 1 hour. NiAl's precipitation is preferably obtained by treatment at 500-550 ° C for more than 1 hour.

• soit après l'usinage de façon à avoir un métal pas trop dur pendant l'usinage ;
• soit après le refroidissement contrôlé à l'air et avant l'usinage ; on réalise alors l'usinage sur une pièce à hautes caractéristiques mécaniques, ce qui le rend particulièrement précis.

This income can be made:

• either after machining so as to have a metal which is not too hard during machining;
• either after the controlled air cooling and before machining; machining is then carried out on a part with high mechanical characteristics, which makes it particularly precise.

Thanks to this income, mechanical characteristics can be obtained high for the product obtained. Typically, the tensile strength Rm ranges from 1000 to 1300 MPa and the elastic limit Re is of the order of 900 MPa or more.

Optimally, the carbon content is limited to 0.06-0.2%, so to obtain a bainite of hardness limited to 300-330 Hv30. Optimally, the content in manganese must be between 0.5 and 1.5%, the chromium content between 0.3 and 1.2%, and the nickel content can either be up to 1% if only one is targeted good hardenability, ie go from 2 to 4% if we are looking for a precipitation of NiAl as we've seen. In the latter case, the aluminum content is between 1 and 2%.

For these steels, the tensile characteristics (yield strength, strength) of the product obtained after rolling or forging and cooling to the controlled air is not particularly high: typically the resistance to tensile Rm is around 750-1050 MPa and the yield strength Re is around from 500 to 750MPa. However, these steels have good machinability.

As examples of implementation of the invention and as an example comparative, we can cite the following tests,

Example 1 (invention)

This example is representative of the variant of the invention for which one can use a relatively low carbon content, and where one hardens by precipitation with the addition of copper.

The composition of the steel is as follows, expressed in 10 ^-3 % by weight: VS mn Yes S P Or Cu Cr MB al Ti B NOT 80 1500 300 85 10 1500 2500 280 50 25 - - 6

After hot forging at a temperature of 1250-1200 ° C and cooling in still air (average cooling rate of 1 ° C / s between 700 and 300 ° C) a bainitic microstructure is obtained with a hardness moderate of 265Hv30, providing resistance less than 900 MPa. With this level of mechanical characteristics, machinability poses no problems. Then, an income at 450 ° C., with a hold time of one hour, allows increase the resistance characteristics to reach more than 340Hv30 hardness, providing a resistance of 1100MPa.

Example 2 (invention)

This example is representative of the variant of the invention for which one can use a relatively low carbon content, and where one hardens by precipitation with the addition of vanadium.

The composition of the steel is as follows, expressed in 10 ^-3 % by weight: VS mn Yes S P Or Cu Cr MB al Ti V 150 1230 250 80 20 150 200 205 50 30 - 820

After hot forging at a temperature of 1250-1200 ° C and cooling in still air (on average 1 ° C / s between 700 and 300 ° C) of a room of forge with a diameter equivalent to 15mm, a microstructure mainly bainitic is obtained with already a significant hardness of 300-320Hv30, providing a resistance of around 1000 MPa, which is currently the limit high still allowing correct machinability on machining means classics. After tempering for 2 hours at 580 ° C, hardening with vanadium achieves a hardness of around 400Hv30, corresponding to a resistance greater than 1200MPa.

Example 3 (invention)

This example is representative of the variant of the invention for which one can use a relatively low carbon content, and where one achieves precipitation hardening by means of combined additions of nickel and aluminum.

The composition of the steel is as follows, given in 10 ^-3 % by weight: VS mn Yes S P Or Cu Cr MB al Ti B NOT 95 1150 200 80 10 3000 206 220 60 1500 - 3 3

After hot forging at a temperature of 1250-1200 ° C and cooling in still air (average cooling rate of 1 ° C / s between 700 and 300 ° C) a bainitic microstructure is obtained with a hardness moderate 240Hv30, providing resistance less than 800 MPa. With this level of mechanical characteristics, machinability poses no problems. Then, an income at 520 ° C, with a holding time of 10 hours, allows to increase the resistance characteristics to reach more than 370Hv30 hardness, providing a resistance of the order of 1200MPa.

Example 4 (reference)

The composition of the steel is as follows, given in 10 ^-3 % by weight: VS mn Yes S P Or Cu Cr MB al Ti V B 230 1500 700 80 11 150 150 800 70 20 25 190 3

After hot forging at 1250 - 1200 ° C and air cooling calm of a part with a diameter equivalent to 25 mm, a microstructure mainly bainitic is obtained with a hardness close to 320 Hv30, providing a resistance of around 1050Mpa. One hour income between 300 and 450 ° C does not significantly increase the resistance.

Claims (20)

Method for manufacturing a steel part, characterized in that : a steel of composition is worked out and poured, in percentages. by weight, 0.06% ≤ C ≤ 0.25%; 0.5% ≤ Mn ≤ 2%; traces ≤ Si ≤ 3%; traces ≤ Ni ≤ 4.5%; traces ≤ Al ≤ 3%; traces ≤ Cr ≤ 1.2%; traces ≤ Mo ≤ 0.30%; traces ≤ V ≤ 2%; traces ≤ Cu ≤ 3.5%; and respecting at least one of the conditions: 0.5% ≤ Cu ≤ 3.5% 0.5% ≤ V ≤ 2% 2% ≤ Ni ≤ 4.5% and 1% ≤ Al ≤ 2% the remainder being iron and impurities resulting from processing; at least one hot deformation of the cast steel is carried out in order to obtain a blank for the part at a temperature of 1100 to 1300 ° C; a controlled cooling of the workpiece blank is carried out with calm air or forced air; and the steel is heated to effect precipitation precipitation, preceding or following the machining of the part from said blank. Process according to claim 1, characterized in that the steel contains from 5 to 50 ppm of B. Process according to claim 1 or 2, characterized in that the steel contains from 0.005 to 0.04% of Ti. Process according to claims 2 and 3 taken together, characterized in that the Ti content is at least 3.5 times the N content of the steel. Method according to one of claims 1 to 4, characterized in that the steel contains from 0.005 to 0.06% of Nb. Method according to one of claims 1 to 5, characterized in that the steel contains from 0.005 to 0.2% of S. Process according to Claim 6, characterized in that the steel contains at least one of the elements Ca up to 0.007%, Te up to 0.03%, Se up to 0.05%, Bi up to 0, 05% and Pb up to 0.1%. Method according to one of claims 1 to 7, characterized in that the C content of the steel is between 0.06 and 0.20%. Process according to claim 8, characterized in that the Mn content of the steel is between 0.5 and 1.5%, and in that the Cr content is between 0.3 and 1.2%. Process according to claim 8 or 9, characterized in that the Ni content of the steel is between traces and 1%. Process according to claim 8 or 9, characterized in that the Ni content of the steel is between 2 and 4.5%, and in that the Al content is between 1 and 2%. Method according to one of claims 1 to 11, characterized in that the precipitation income is carried out between 425 and 600 ° C. Process according to Claim 12, characterized in that the steel contains 0.5 to 3.5% of Cu and in that the precipitation income is carried out between 425 and 500 ° C for 1 to 10 hours. Process according to claim 12, characterized in that the steel contains 0.5 to 2% of V and in that the precipitation income is carried out between 500 and 600 ° C for more than 1 h. Process according to Claim 12, characterized in that the steel contains from 2 to 4.5% of Ni and 1 to 2% of Al and in that the precipitation income is carried out between 500 and 550 ° C for more than '1 hr. Method according to one of claims 1 to 15, characterized in that said hot deformation is a rolling. Method according to one of claims 1 to 15, characterized in that said hot deformation is a forging. Method according to one of claims 1 to 17, characterized in that the controlled cooling of the blank is carried out with a speed less than or equal to 3 ° C / s between 600 and 300 ° C. Steel part, characterized in that it was obtained by the process according to one of claims 1 to 17. Steel part according to claim 18, characterized in that it has a bainitic microstructure, a tensile strength Rm of 750 to 1300MPa and an elastic limit Re greater than or equal to 500MPa.