EP1426452B1 - Manufacturing process of a bainitic steel article - 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 specifically the field of steels for the manufacture of parts to withstand significant stresses.

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

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

It has already been proposed to make such bainitic steel pieces from a grade of 25MnSiCrVBS, the cooling after forging or rolling taking place in the air. The withstand performance is significantly improved over the previous examples, but remain relatively limited compared to what can be achieved on a tempered and tempered steel.

• coulée d'un acier de composition, en pourcentage de poids: C 0,04-0,08%; Mn 1,20-1,70%; Si 0,10-0,30%; Mo 0,100-0,500%; Ti 0,020-0,070; Nb 0.030-0,080%; balance Fe et impuretés;
• chauffage entre 1200 et 1300°C;
• laminage à chaud; et
• bobinage entre 450 et 560°C.

The document JP-A-10-102184 discloses a method of manufacturing a steel part comprising the following steps:

• casting of a composition steel, in percent by weight: C 0.04-0.08%; Mn 1.20-1.70%; If 0.10-0.30%; Mo 0.100-0.500%; Ti 0.020-0.070; Nb 0.030-0.080%; Fe balance and impurities;
• heating between 1200 and 1300 ° C;
• hot rolling; and
• winding between 450 and 560 ° C.

The object of the invention is to propose an association between a steel grade and a part manufacturing process, having economic advantages over existing associations without the metallurgical performance being altered, or even improving these performances. The part thus manufactured will have to withstand heavy fatigue stresses. In the case of 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 à une vitesse inférieure à 3°C/s entre 600 at 300° C, de manière à lui conférer une microstructure bainitique;
• et on réchauffe l'acier pour effectuer un revenu de précipitation, précédant ou suivant l'usinage de la pièce à partir de ladite ébauche.

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

• a composition steel is produced and cast in percentages by weight, 0.06% ≤ C ≤ 0.25%; 0.5% ≤ Mn ≤ 2%; traces ≤ If ≤ 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 rest being iron and impurities resulting from the elaboration;
• at least one hot deformation of the cast steel is performed to obtain a blank of the workpiece at a temperature of 1100 to 1300 ° C;
• a controlled cooling of the blank of the workpiece is carried out in still air or in pulsed air at a speed of less than 3 ° C./s between 600 and 300 ° C., so as to give it a bainitic microstructure;
• and heating the steel to effect a precipitation of precipitation, preceding or following machining of the workpiece from said blank.

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

Preferably the steel contains 0.005 to 0.04% Ti.

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

Preferably the steel contains from 0.005 to 0.06% 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 be preferably 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 yield is in the general case preferably between 425 and 600 ° C.

When the steel contains 0.5 to 3.5% Cu, the precipitation yield is preferably between 425 and 500 ° C for 1 to 10h.

When the steel contains 0.5 to 2% of V, the precipitation is preferably 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 precipitation is preferably between 500 and 550 ° C for more than 1 hour.

Said hot deformation may be a rolling.

Said hot deformation may be forging.

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

A steel part obtained by the process according to the invention has a bainitic microstructure, and typically a tensile strength Rm of 750 to 1300 MPa and a yield strength Re greater than or equal to 500 MPa.

As will have been understood, the invention consists of the combination of a steel grade and a post-casting treatment process comprising a hot forming step of the part, a controlled cooling being carried out in calm air or in pulsed air and a precipitation income preceding or following the machining of the room. The composition of the selected steel ensures that, regardless of the cooling mode, the fatigue strength results of the parts manufactured from this steel will be sufficient to meet the requirements of the users.

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

The chemical characteristics of the steel and its heat treatments after casting aim at obtaining a bainitic microstructure, and also at obtaining optimized mechanical characteristics. This bainitic microstructure must be able to be obtained after cooling in still air, but must also be compatible with forced air cooling. In this way, the parts of the process of the invention can be produced on any existing installation, that it allows after forging or rolling a forced air cooling, or that it allows only a cooling in the calm air . Thus, a forging installation initially designed to treat ferritic-pearlitic microstructure steel parts can without difficulty, and without special adaptations, treat bainitic microstructure parts of the process of the invention. The bainitic microstructure steels previously used for these purposes required pulsed air cooling, and therefore could not always be processed on standard design facilities.

According to the invention, we therefore start by developing a steel whose composition will be detailed and justified further, then it is cast, in ingots or continuously according to the format of the final part, and more generally it is rolled so as to get a half-product.

It is then possible to perform a forging operation of the semi-finished product.

The last hot deformation is carried out at 1100-1300 ° C and is followed by air-controlled cooling in hot rolling or forging, in still air or forced air. This gives a rough sketch of the piece.

By the term "blank", it should be understood that here is meant a bar, or a half-product in another form, from which the final piece will be obtained by machining, and this regardless of the mode of hot deformation practiced: rolling, forging or their combination.

A precipitation income is then made. This is either before or after machining the workpiece from said blank.

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

The carbon content is between 0.06 and 0.25%. This content makes it possible to govern the type of microstructure obtained. Less than 0.06%, the microstructure obtained would not be interesting for the objectives. Above 0.25%, in combination with the other elements, a sufficiently bainitic microstructure would not be obtained after cooling with still air.

The manganese content is between 0.5 and 2%. This element added to more than 0.5% gives its quenchability to the material, and allows to obtain a broad bainitic range regardless of the cooling mode. A content greater than 2% would, however, be likely to cause excessive segregation.

The silicon content is between traces and 3%. This element, which is not obligatory, is advantageous in that it hardens the bainite by passing it in solid solution. In addition, in the case where copper is present in a relatively large amount, silicon avoids the problems associated with the presence of copper during hot forming. A content greater than 3% can however pose problems of machinability of the material.

The nickel content is between traces and 4.5%. This non-obligatory element promotes quenchability and stabilization of austenite. If the aluminum content allows it, it can form very hardening NiAl precipitates, giving the metal high mechanical characteristics. In the case where copper is present in a relatively large quantity, nickel can play the same role as silicon. Above 4.5%, the addition of nickel is unnecessarily expensive in view of metallurgical objectives.

The aluminum content is between traces and 3%. This non-obligatory element is a strong deoxidizer, and even added at a low level, it makes it possible to limit the amount of dissolved oxygen in the liquid steel, thus improving the inclusiveness of the room if it has been possible to avoid reoxidation too much. important during casting. At high levels, as has been said, it is likely to form NiAl precipitates if nickel is present in large quantities. It is not useful for the aluminum content to exceed 3%.

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

The molybdenum content is between traces and 0.30%. This element, which is not mandatory, prevents the formation of coarse-grained ferrite and makes it possible to obtain the bainitic structure more definitely. Its addition is unnecessarily expensive beyond 0.30%.

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

The copper content is between traces and 3.5%. This element, which is not mandatory, can improve machinability and, by precipitating, cause secondary hardening of the material. But beyond 3.5% it makes hot formatting of the problematic part. As has been said, it is advisable to associate a significant nickel or silicon content to minimize hot forming problems. Beyond 3.5% its addition is in any case 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 respected:

• a copper content between 0.5 and 3.5%
• a vanadium content 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 have just been mentioned are those whose metallurgical role is or may be the most important for the invention, but other elements that will be mentioned may also be optionally present to improve certain properties of the steel.

The boron content can be between 5 and 50 ppm. It can improve the hardenability, but must be in solid solution to be effective. In other words, it must be avoided that almost all the boron is found in the form of nitrides or carbonitrides of boron. For this purpose, it is advisable to associate the addition of boron with an addition of titanium, preferably in a proportion such that 3.5 × N% ≤ Ti%. In this latter condition, all the dissolved nitrogen can be captured and the formation of nitrides or carbonitrides of boron can be avoided. The minimum titanium content, for this purpose, is 0.005%, for the lowest nitrogen contents usually encountered. However, it is advisable not to exceed a titanium content of 0.04%, otherwise we obtain titanium nitrides too large.

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

Niobium may also be added at levels of between 0.005 and 0.06%. He too can precipitate in the form of carbonitrides in the austenite, and can thus bring about a hardening of the material.

Finally, in a conventional way, the machinability of the material can be improved by adding sulfur (from 0.005% to 0.2%), to which a calcium addition can also be added (up to 0.007%), and / or 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 above-mentioned composition, the part blank is forged or not according to the usual methods. It is heated to 1100-1300 ° C, then the deformations giving rise to the piece blank are carried out.

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

Then immediately after rolling, or after forging if this operation was carried out, controlled cooling of the room is carried out, either in still air or in forced air. In general, the part is forced to cool 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 before or after the machining of the part which gives it its final dimensions, the steel is hardened by precipitation by means of an income, that is to say from a heat treatment following heating from a temperature equal to or slightly greater than the ambient; for that three options are possible, and can be combined:

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

In general, the precipitation yield is preferably between 425 and 600 ° C. But the temperature of the income and its duration are optimally adapted to the targeted characteristics. By way of example, the precipitation of copper is preferably obtained by treatment at 425-500 ° C for 1 to 10 hours. Precipitation of vanadium is preferably obtained by treatment at 500-600 ° C for more than 1 hour. Precipitation of NiAl is preferably obtained by treatment at 500-550 ° C for more than 1h.

• 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:

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

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

Optimally, the carbon content is limited to 0.06-0.2%, so as to obtain a bainite of hardness limited to 300-330 Hv30. Optimally, the manganese content should be between 0.5 and 1.5%, the chromium content between 0.3 and 1.2%, and the nickel content can be up to 1% if it is not that a good quenchability, or go from 2 to 4% if one seeks a NiAl precipitation as we have 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 with controlled air are not particularly high: typically the tensile strength Rm is of the order of 750 -1050 MPa and the elasticity limit Re of the order of 500 to 750 MPa. But these steels have good machinability.

By way of examples of implementation of the invention and comparative example, mention may be made of the following tests,

Example 1 (invention)

This example is representative of the variant of the invention for which a relatively low carbon content can be used, and precipitation hardening is achieved by 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 with still air (average cooling rate of 1 ° C / s between 700 and 300 ° C) a bainitic microstructure is obtained with a moderate hardness of 265Hv30, providing a resistance of less than 900 MPa. With this level of mechanical characteristics, machinability is not a problem. Then, an income at 450 ° C, with a hold time of one hour, allows to 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 a relatively low carbon content can be used, and precipitation hardening is carried out by 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 forging piece of diameter equivalent to 15 mm, a microstructure mainly bainitique is obtained with already a significant hardness of 300-320Hv30, providing a resistance of about 1000MPa, which is currently the limit high still allowing good machinability on conventional machining means. After a period of 2 hours at 580 ° C., the hardening with vanadium makes it possible to reach a hardness of the order of 400Hv30, corresponding to a resistance greater than 1200 MPa.

Example 3 (Invention)

This example is representative of the variant of the invention for which a relatively low carbon content can be used, and precipitation hardening is carried out by means of conjugate 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 with still air (average cooling rate of 1 ° C / s between 700 and 300 ° C) a bainitic microstructure is obtained with a moderate hardness of 240Hv30, providing a resistance of less than 800 MPa. With this level of mechanical characteristics, machinability is not a problem. Then, an income at 520 ° C, with a holding time of 10 hours, increases the strength 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 cooling in a still air of a piece of diameter equivalent to 25 mm, a predominantly bainitic microstructure is obtained with a hardness close to 320 Hv30, providing a resistance of about 1050Mpa. An income of one hour between 300 and 450 ° C does not significantly increase the resistance.

Claims (12)

1. Method for producing a steel component, characterised in that:

   - a steel is produced and cast having a composition, in percentages by weight, 0.06% s C ≤ 0.25%; 0.5% ≤ Mn ≤ 2%; trace levels ≤ Si ≤ 3%; trace levels ≤ Ni ≤ 4.5%; trace levels ≤ Al ≤ 3%; trace levels ≤ Cr ≤ 1.2%; trace levels ≤ Mo ≤ 0.30%; trace levels ≤ V ≤ 2%; trace levels ≤ Cu ≤ 3.5%; and complying with at least one of the conditions:

   * 0.5% ≤ Cu ≤ 3.5%

   * 0.5% ≤ V ≤ 2%

   * 2% ≤ Ni ≤ 4.5% and 1% ≤ Al ≤ 2%

   optionally from 5 to 50 ppm of B;
   optionally from 0.005 to 0.04% of Ti;
   optionally from 0.005 to 0.06% of Nb;
   optionally from 0.005 to 0.2% of S;
   optionally 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%; the remainder being iron and impurities resulting from the production operation;

   - at least one hot-deformation of the cast steel is carried out in order to obtain a blank of the component at a temperature of from 1100 to 1300°C;

   - a controlled cooling of the blank of the component is carried out at a rate less than or equal to 3°C/s between 600 and 300°C in still air or pulsed air in order to confer a bainitic microstructure thereon;

   - and the steel is reheated in order to carry out a precipitation tempering operation prior to or following the machining of the component from the blank.
2. Method according to claim 1, characterised in that the steel contains B and Ti and that the content of Ti is equal to at least 3.5 times the N content of the steel.
3. Method according to either claim 1 or claim 2, characterised in that the C content of the steel is between 0.06 and 0.20%.
4. Method according to claim 3, characterised 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%.
5. Method according to claim 3 or claim 4, characterised in that the Ni content of the steel is between trace levels and 1%.
6. Method according to claim 3 or claim 4, characterised 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%.
7. Method according to any one of claims 1 to 6, characterised in that the precipitation tempering operation is carried out between 425 and 600°C.
8. Method according to claim 7, characterised in that the steel contains from 0.5 to 3.5% of Cu, and in that the precipitation tempering operation is carried out between 425 and 500°C for from 1 to 10h.
9. Method according to claim 7, characterised in that the steel contains from 0.5 to 2% of V, and in that the precipitation tempering operation is carried out between 500 and 600°C for more than 1 hour.
10. Method according to claim 7, characterised in that the steel contains from 2 to 4.5% of Ni and from 1 to 2% of Al, and in that the precipitation tempering operation is carried out between 500 and 550°C for more than 1 hour.
11. Method according to any one of claims 1 to 10, characterised in that the hot-deformation is a rolling operation.
12. Method according to any one of claims 1 to 10, characterised in that the hot-deformation is a forging operation.