EP0787812B1 - Process for manufacturing steel forging - Google Patents

Description

The present invention relates to the manufacture of forged parts at high characteristics, in steel.

High-characteristic forgings, made of steel, and in particular the forgings with high characteristics for the automobile, are manufactured according to different techniques which each have drawbacks.

According to a first technique, the parts are made of steel of the chromium-molybdenum type, the chemical composition of which comprises, by weight, 0.25% to 0.45% carbon, approximately 1% chromium and approximately 0.25% carbon molybdenum. The parts are forged and then subjected to a heat treatment of quenching and tempering intended to give them a tempered martensitic structure to obtain in particular a tensile strength Rm of the order of 1000 MPa. This technique has the disadvantage of being costly and sometimes of generating deformations of the geometry of the parts.

• il n'est pas possible d'obtenir une résistance à la traction Rm supérieure à 1000 MPa,
• le rapport limite d'élasticité sur résistance à la traction Rp_0,2/Rm est inférieur à 0,75 ce qui limite les possibilités d'allégement des pièces lorsque celles-ci sont dimensionnées en particulier par référence à la limite d'élasticité,
• la température de transition de la résilience est supérieure à 50°C ce qui conduit à une résistance aux chocs faible,
• il est parfois nécessaire d'adapter les installations de fabrication en ajoutant des tunnels de refroidissement pour obtenir un refroidissement adapté après forgeage.

According to another technique, the parts are made of steel containing from 0.3% to 0.4% of carbon, from 1% to 1.7% of manganese, from 0.25% to 1% of silicon and up to '' 0.1% vanadium. After forging, the pieces are cooled slowly to give them a ferrito-pearlitic structure. This less expensive technique than the previous one however has several drawbacks:

• it is not possible to obtain a tensile strength Rm greater than 1000 MPa,
• the yield strength to tensile strength ratio Rp _0.2 / Rm is less than 0.75, which limits the possibilities of lightening the parts when they are dimensioned in particular by reference to the elasticity limit,
• the resilience transition temperature is greater than 50 ° C. which leads to low impact resistance,
• it is sometimes necessary to adapt the manufacturing facilities by adding cooling tunnels to obtain suitable cooling after forging.

The forged parts can also be made of a steel containing less carbon than in the previous case, and be soaked with water in the hot forging to give them a bainitic or bainito-martensitic structure. This technique makes it possible to obtain a tensile strength Rm greater than 1000 MPa and an elastic limit Rp _0.2 greater than 800 MPa, but it has the drawback of requiring water quenching which, sometimes, generates geometric deformations imposing the need for a straightening operation or which may even be unacceptable.

Some parts, finally, are made of steel containing between 0.3% and 0.4% carbon and between 1.9% and 2.5% manganese. They are cooled to the air after forging so as to present a bainitic structure to high mechanical characteristics. But, these pieces often have segregated bands with martensitic structure making machining difficult.

The aim of the present invention is to propose a method for the manufacture of forgings with high characteristics which remedies these disadvantages.

• éventuellement de 0,005% à 0,06% d'aluminium,
• éventuellement du bore en des teneurs comprises entre 0,0005% et 0,01 %,
• éventuellement entre 0,005% et 0,03% de titane,
• éventuellement entre 0,005% et 0,06% de niobium,
• éventuellement de 0,005% à 0,1% de soufre, éventuellement jusqu'à 0,006% de calcium, éventuellement jusqu'à 0,03% de tellure, éventuellement jusqu'à 0,05% de sélénium, éventuellement jusqu'à 0,05% de bismuth, éventuellement jusqu'à 0, 1 % de plomb,

le reste étant du fer et des impuretés résultant de l'élaboration. Nuances similaires sont déjà divoulguées dans EP-A-717116, JP-A-6017188 et JP-A-60096718.To this end, the invention applies to a steel for the manufacture of forgings whose chemical composition comprises, by weight: 0.1% ≤ C ≤ 0.4% 1% ≤ Mn ≤ 1.8% 0.15% ≤ If ≤ 1.7% 0% ≤ Ni ≤ 1% 0% ≤ Cr ≤ 1.2% 0% ≤ Mo ≤ 0.3% 0% ≤ V ≤ 0.3% Cu ≤ 0.35%

• optionally from 0.005% to 0.06% aluminum,
• optionally boron in contents of between 0.0005% and 0.01%,
• optionally between 0.005% and 0.03% of titanium,
• optionally between 0.005% and 0.06% niobium,
• optionally from 0.005% to 0.1% sulfur, optionally up to 0.006% calcium, optionally up to 0.03% tellurium, optionally up to 0.05% selenium, optionally up to 0.05 % bismuth, possibly up to 0.1% lead,

the remainder being iron and impurities resulting from processing. Similar nuances are already disclosed in EP-A-717116, JP-A-6017188 and JP-A-60096718.

Preferably, the carbon content is less than or equal to 0.3%, of preferably also, the manganese content is less than 1.6%. According to nature of the envisaged applications, the silicon content can be, preferably, either greater than 1.2% or less than 0.8%.

• on approvisionne un lopin en un acier ayant la composition citée et on le forge à chaud pour obtenir une pièce,
• on soumet la pièce à un traitement thermique comportant un refroidissement depuis une température à la quelle l'acier est entièrement austénitique jusqu'à une température Tm comprise entre Ms+100°C et Ms-20°C à une vitesse de refroidissement Vr supérieure à 0,5°C/s, suivi d'un maintien de la pièce entre Tm et Tf, avec Tf ≥ Tm-100°C, et de préférence Tf > Tm-60°C, pendant au moins 2 minutes de façon à obtenir une structure essentiellement bainitique comportant au moins 15% de bainite inférieure, et de préférence, au moins 30% de bainite formée entre Tm et Tf.

The invention relates to a method for manufacturing a forged part according to which:

• a piece of steel is supplied with the composition mentioned and it is hot forged to obtain a part,
• the part is subjected to a heat treatment comprising cooling from a temperature at which the steel is entirely austenitic up to a temperature Tm of between Ms + 100 ° C and Ms-20 ° C at a cooling rate Vr greater than 0.5 ° C / s, followed by maintaining the part between Tm and Tf, with Tf ≥ Tm-100 ° C, and preferably Tf> Tm-60 ° C, for at least 2 minutes so as to obtain an essentially bainitic structure comprising at least 15% of lower bainite, and preferably, at least 30% of bainite formed between Tm and Tf.

Preferably, the cooling rate Vr is greater than 2 ° C / s.

After maintaining between Tm and Tf, the part can be cooled to the ambient temperature and possibly subject to tempering between 150 ° C and 650 ° C.

After the hold between Tm and Tf, the part can also be reheated to a temperature below 650 ° C, then cooled to the ambient temperature.

The heat treatment can be carried out either after heating the forged part to a temperature higher than AC ₃ , or directly after forging.

The invention will now be described in more detail, but not limiting and illustrated by the examples which follow.

• plus de 0,1% de carbone, et de préférence plus de 0,15%, pour obtenir une dureté suffisante, mais, moins de 0,4%, et de préférence moins de 0,3 %, afin de limiter la résistance à la traction Rm à 1200 MPa;
• plus de 1 % de manganèse pour obtenir une trempabilité suffisante, mais moins de 1,8%, et de préférence moins de 1,6 % pour éviter la formation de bandes ségrégées;
• plus de 0,15% de silicium pour durcir la ferrite et, éventuellement, pour favoriser la formation d'austénite résiduelle ce qui améliore la limite d'endurance en fatigue, mais moins de 1,7%, car, au delà, le silicium fragilise l'acier; entre 0,15 % et 0,8% le silicium durcit la ferrite sans favoriser la formation d'austénite résiduelle; entre 1,2 % et 1,7 % le silicium favorise suffisamment la formation d'austénite résiduelle pour améliorer la limite d'endurance en fatigue; selon les applications, la teneur en silicium peut être choisie dans l'une ou l'autre de ces plages;
• de 0% à 1% de nickel, de 0% à 1,2% de chrome et de 0% à 0,3% de molybdène pour ajuster la trempabilité;
• éventuellement du titane en des teneurs comprises entre 0,005% et 0,03%;
• éventuellement du niobium en des teneurs comprises entre 0,005% et 0,06%;
• éventuellement du bore en des teneurs comprises entre 0,0005 % et 0,01% pour compléter l'effet des élément précédents sur la trempabilité; dans ce cas, il est préférable que l'acier contienne du titane pour renforcer l'effet du bore;
• de 0% à 0,3% de vanadium pour obtenir un durcissement complémentaire et améliorer la trempabilité;
• moins de 0,35% de cuivre, élément résiduel présent fréquemment dans l'acier élaboré à partir de ferrailles, mais qui, en trop grande quantité, a l'inconvénient de détériorer la forgeabilité;
• éventuellement de 0,005% à 0,06% d'aluminium pour assurer la désoxydation de l'acier et pour contrôler le grossissement du grain austénitique, notamment lorsque la teneur en silicium est inférieure à 0,5 %;
• éventuellement de 0,005% à 0,1% de soufre, éventuellement jusqu'à 0,006% de calcium, éventuellement jusqu'à 0,03% de tellure, éventuellement jusqu'à 0,05% de sélénium, éventuellement jusqu'à 0,05% de bismuth, éventuellement jusqu'à 0,1 % de plomb, pour améliorer l'usinabilité;

le reste étant du fer et des impuretés résultant de l'élaboration.The chemical composition of the steel used for the process of the invention comprises, by weight:

• more than 0.1% carbon, and preferably more than 0.15%, to obtain sufficient hardness, but less than 0.4%, and preferably less than 0.3%, in order to limit the resistance to the traction Rm at 1200 MPa;
• more than 1% of manganese to obtain sufficient quenchability, but less than 1.8%, and preferably less than 1.6% to avoid the formation of segregated bands;
• more than 0.15% of silicon to harden the ferrite and, possibly, to favor the formation of residual austenite which improves the limit of endurance in fatigue, but less than 1.7%, because, beyond, silicon weakens steel; between 0.15% and 0.8% the silicon hardens the ferrite without promoting the formation of residual austenite; between 1.2% and 1.7% the silicon sufficiently promotes the formation of residual austenite to improve the limit of fatigue endurance; depending on the applications, the silicon content can be chosen from one or other of these ranges;
• from 0% to 1% nickel, from 0% to 1.2% chromium and from 0% to 0.3% molybdenum to adjust the hardenability;
• optionally titanium in contents of between 0.005% and 0.03%;
• optionally niobium in contents of between 0.005% and 0.06%;
• optionally boron in contents of between 0.0005% and 0.01% to complete the effect of the preceding elements on the quenchability; in this case, it is preferable that the steel contains titanium to enhance the effect of boron;
• from 0% to 0.3% vanadium to obtain additional hardening and improve hardenability;
• less than 0.35% copper, a residual element frequently present in steel produced from scrap, but which, in too large a quantity, has the disadvantage of deteriorating the forgeability;
• optionally from 0.005% to 0.06% of aluminum to ensure the deoxidation of the steel and to control the magnification of the austenitic grain, in particular when the silicon content is less than 0.5%;
• optionally from 0.005% to 0.1% sulfur, optionally up to 0.006% calcium, optionally up to 0.03% tellurium, optionally up to 0.05% selenium, optionally up to 0.05 % bismuth, possibly up to 0.1% lead, to improve machinability;

the remainder being iron and impurities resulting from processing.

To manufacture a forged piece, a piece of steel is supplied with this composition and it is hot forged after having heated it to a temperature above AC _3, preferably above 1150 ° C., and better still, between 1200 ° C and 1280 ° C, so as to give it an entirely austenitic structure and a sufficiently low flow stress. After forging, the part is subjected to a heat treatment which can be carried out either directly in the hot forging, or after cooling of the part and reheating above the temperature AC ₃ of the steel.

The heat treatment involves cooling at a speed of cooling Vr measured at the passage at 700 ° C greater than 0.5 ° C / s, and preferably greater than 2 ° C / s, up to a temperature Tm between Ms + 100 ° C and Ms-20 ° C, Ms being the temperature at the start of transformation martensitic of steel. This cooling is followed by holding for a time greater than 2 min between the temperature Tm and a temperature Tf ≥ Tm-100 ° C, and preferably Tf ≥ Tm-60 ° C. Maintenance is followed by either cooling to room temperature possibly supplemented by a heat between 150 ° C and 650 ° C, i.e. reheating to a temperature less than or equal to 650 ° C before cooling to temperature room.

The purpose of this heat treatment is to give the room a structure essentially bainitic with less than 20% ferrite and at least 15%, and preferably at least 30%, of lower bainite formed between Tm and Tf. he can be carried out on the whole part or simply on a part having a special functionality.

The conditions of maintenance (Tm, Tf, duration), as well as the proportions of each of the structures, and in particular the proportion of lower bainite, can be determined, in a manner known to those skilled in the art, using dilatometric measurements on test bars.

The parts thus obtained have the advantage of having a tensile strength Rm between 950 MPa and 1150 MPa, an elastic limit Rp _0.2 greater than 750 MPa, a Mesnager K resilience greater than 25 Joules / Cm ² at 20 ° C, machinability at least equal to that of parts with a ferritic-pearlitic structure and good fatigue strength: σ _D / Rm> 0.5 in rotary bending at 2x10 ⁶ cycles.

As a first example, an axle was manufactured with a steel whose chemical composition included in% by weight: VS Yes mn Or Cr MB Cu V al B Ti Nb 0.25 0.5 1.67 0.09 0.52 - 0.199 0.2 0.03 - 0.02 - this steel also contained 0.065% S to improve machinability. Its Ms temperature was 380 ° C.

The part was hot forged between 1280 ° C and 1050 ° C. Directly after forging, the part was cooled with blown air at a speed of 2.6 ° C / s until the temperature of 425 ° C, then maintained between 425 ° C and 400 ° C for 10 min; finally, the part was cooled to room temperature by natural air cooling.

The part thus obtained had a structure comprising at least 80% of bainite. Its characteristics were: Rm = 1100 MPa Rp _0.2 = 870 MPa A% = 10% Z = 60%

As a second example, a rocket was made from steel, the chemical composition of which, in% by weight: VS Yes mn Or Cr MB Cu V al B Ti Nb 0.25 0.5 1.63 0.006 0.51 0.09 0.196 0,107 0,038 0,003 0,023 - this steel also contained 0.05% S to improve machinability. Its Ms temperature was 385 ° C.

The part was hot forged between 1270 ° C and 1040 ° C. Directly after forging, the part was cooled with blown air at a speed of 2.6 ° C / s until the temperature of 400 ° C, then maintained between 400 ° C and 380 ° C for 10 min; the room was then brought to the temperature of 550 ° C for 1 hour, then it has been cooled to room temperature by natural air cooling.

The part thus obtained had a structure comprising at least 80% of bainite. Its characteristics were: Rm = 967 MPa Rp _0.2 = 822 MPa A% = 12% Z = 60%

As a third example, a ball joint was made with a steel whose chemical composition included, in% by weight: VS Yes mn Or Cr MB Cu V al B Ti Nb 0.28 0.79 1.63 0.05 0.5 0.09 0.19 - 0.04 0.0033 0,023 - this steel also contained 0.06% S to improve machinability. Its Ms temperature was 350 ° C.

The part was hot forged between 1270 ° C and 1060 ° C. Directly after forging, the workpiece was cooled in still air at a speed of 1.19 ° C / s until the temperature of 380 ° C, then maintained between 380 ° C and 360 ° C for 10 min; finally, the part was cooled to room temperature by natural air cooling.

The part thus obtained had a structure comprising at least 80% of bainite. Its characteristics were: Rm = 1170 MPa Rp _0.2 = 947 MPa A% = 8% Z = 50%

The parts thus obtained can in particular be parts for automotive such as wishbones, drive shafts, connecting rods, but they can also be shafts, cams or any another forged part for various machines.

Claims (13)

1. Process for producing a forged part characterised in that

   a steel bar is supplied, the chemical composition of which comprises by weight: 0.1% ≤ C ≤ 0.4% 1% ≤ Mn ≤ 1.8% 0.15% ≤ Si ≤ 1.7% 0% ≤ Ni ≤ 1% 0% ≤ Cr ≤ 1.2% 0% ≤ Mo ≤ 0.3% 0% ≤ V ≤ 0.3% Cu ≤ 0.35%

   optionally 0.005% to 0.06% of aluminium,

   optionally boron in amounts between 0.0005% and 0.01%,

   optionally between 0.005% and 0.03% of titanium,

   optionally between 0.005% and 0.06% of niobium,

   optionally from 0.005% to 0.1% of sulphur, optionally up to 0.006% of calcium, optionally up to 0.03% of tellurium, optionally up to 0.05% of selenium, optionally up to 0.05% of bismuth, optionally up to 0.1% of lead,
   the remainder being iron and impurities resulting from the production process,

   the bar is hot-forged to obtain a part,

   the part is subjected to heat treatment comprising cooling from a temperature at which the steel is entirely austenitic to a temperature Tm of between Ms+100°C and Ms-20°C at a cooling rate Vr greater than 0.5°C/s, followed by maintaining the part between the temperature Tm and a temperature Tf which is higher than or equal to Tm-100°C for at least 2 minutes in order to obtain an essentially bainitic structure comprising at least 15% of lower bainite formed between Tm and Tf and less than 20% of perlite ferrite, Ms being the temperature of the start of martensitic transformation of the steel.
2. Process according to claim 1, characterised in that the steel contains less than 0.3% of carbon.
3. Process according to claim 1 or 2, characterised in that the steel contains less than 1.6% of manganese.
4. Process according to claim 1, 2 or 3, characterised in that the steel contains less than 0.8% of silicon.
5. Process according to claim 1, 2 or 3, characterised in that the steel contains more than 1.2% of silicon.
6. Process according to any one of claims 1 to 5, characterised in that the maintenance is such that the structure comprises at least 30% of lower bainite formed between Tm and Tf.
7. Process according to any one of claims 1 to 6, characterised in that Tf is greater than or equal to Tm-60°C.
8. Process according to any one of claims 1 to 7, characterised in that the cooling rate Vr is greater than 2°C/s.
9. Process according to any one of claims 1 to 8, characterised in that after being maintained between Tm and Tf the part is cooled to ambient temperature.
10. Process according to claim 9, characterised in that the heat treatment further comprises quenching between 150°C and 650°C.
11. Process according to any one of claims 1 to 8, characterised in that after being maintained between Tm and Tf the part is heated to a temperature below 650°C and then cooled to ambient temperature.
12. Process according to any one of claims 1 to 11, characterised in that the heat treatment is carried out after heating the part to a temperature higher than AC₃.
13. Process according to any one of claims 1 to 11, characterised in that the heat treatment is carried out directly after forging.