EP1339880B1 - Method for making a strip or a workpiece cut out from a cold rolled maraging steel strip - Google Patents

Description

The present invention relates to a maraging steel particularly suitable for the manufacture of parts requiring a very good resistance to fatigue.

Many parts are made from maraging steel strips containing, in% by weight, about 18% nickel, 9% cobalt, 5% molybdenum, 0.5% titanium and 0.1% aluminum, treated to have a yield strength greater than 1800 MPa. These strips are obtained by hot rolling and cold rolling. The strips or cut pieces in the strips are then cured by a curing heat treatment at about 500 ° C. The pieces are optionally nitrided on the surface to improve their fatigue resistance. However the fatigue resistance of these parts is insufficient.

In order to improve the fatigue strength of the parts, it has been envisaged to use maraging steels having different chemical compositions and mechanical characteristics, such as maraging steels containing 18% of nickel, 12% of cobalt, 4% of molybdenum, 1.6% titanium and 0.2% aluminum, or maraging steels containing 18% nickel, 3% molybdenum, 1.4% titanium and 0.1% aluminum, or maraging steels containing 13% chromium, 8% nickel, 2% molybdenum and 1% aluminum. But none of these steels gave satisfactory results. Fatigue suits are always lower than those made with the usual steel.

The object of the present invention is to overcome this disadvantage and to propose a band or piece of maraging steel having improved fatigue strength.

$$2,5\%\le \mathrm{Mo}\le 12\%$$

$$4,17\%\le \mathrm{Co}\le 20\%$$

$$\mathrm{Al}\%\le 0,15\%$$

$$\mathrm{Ti}\le 0,1\%$$

$$\mathrm{N}\le 0,003\%$$

$$\mathrm{Si}\le 0,1\%$$

$$\mathrm{Mn}\le 0,1\%$$

$$\mathrm{C}\le 0,005\%$$

$$\mathrm{S}\le 0,001\%$$

$$\mathrm{P}\le 0,005\%$$

$$\mathrm{H}\le 0,0003\%$$

$$\mathrm{O}\le 0,001\%$$

le reste étant du fer et des impuretés résultant de l'élaboration, la composition chimique satisfaisant en outre les relations : $$20\%\le \mathrm{Ni}+\mathrm{Mo}\le 27\%$$

$$50\le \mathrm{Co}\times \mathrm{Mo}\le 200$$

$$\mathrm{Ti}\mathrm{\times }\mathrm{N}\mathrm{\le }2\times {\mathrm{10}}^{\mathrm{-}\mathrm{4}}$$

For this purpose, the subject of the invention is a method for manufacturing a strip or a piece cut from a cold-rolled maraging steel strip. According to this process, before carrying out the curing heat treatment, the strip or the part is subjected to a cold plastic deformation with a hardening rate greater than 30% and the strip or part is annealed. of recrystallization so as to obtain a fine grain of ASTM index greater than 8. The chemical composition of the steel comprises, by weight: $$12\%\le \mathrm{Or}\le 24,5\%$$

$$2,5\%\le \mathrm{MB}\le 12\%$$

$$4,17\%\le \mathrm{Co}\le 20\%$$

$$\mathrm{al}\%\le 0,15\%$$

$$\mathrm{Ti}\le 0,1\%$$

$$\mathrm{NOT}\le 0,003\%$$

$$\mathrm{Yes}\le 0,1\%$$

$$\mathrm{mn}\le 0,1\%$$

$$\mathrm{VS}\le 0,005\%$$

$$\mathrm{S}\le 0,001\%$$

$$\mathrm{P}\le 0,005\%$$

$$\mathrm{H}\le 0,0003\%$$

$$\mathrm{O}\le 0,001\%$$

the rest being iron and impurities resulting from the preparation, the chemical composition further satisfying the relations: $$20\%\le \mathrm{Or}+\mathrm{MB}\le 27\%$$

$$50\le \mathrm{Co}\times \mathrm{MB}\le 200$$

$$\mathrm{Ti}\mathrm{\times }\mathrm{NOT}\mathrm{\le }2\times {\mathrm{10}}^{\mathrm{-}\mathrm{4}}$$

Optionally, after the recrystallization annealing, the strip or the part is subjected to cold rolling with a reduction rate of between 1% and 10%.

Preferably, the maraging steel is remelted under vacuum by the VAR method or remelted a first time under vacuum by the VAR method or electroconductive slag by the ESR process and remelted a second time under vacuum by the VAR method.

The invention also relates to a strip or piece, less than 1 mm thick, maraging steel having a fine grain ASTM index greater than 8 and a yield strength after curing greater than 1850 MPa.

The strip or the piece thus obtained can be used for the manufacture of parts such as belts. These pieces are cured by a curing treatment at 450-550 ° C for 1-10 hours which may be followed by surface nitriding.

The invention will now be described in more detail but in a nonlimiting manner.

To manufacture a cold rolled strip of maraging steel according to the invention, the steel is made by targeting a carbon less than 0.005% and then deoxidizing aluminum.

The steel thus produced is cast in the form of reflow electrodes. These electrodes are either remelted under vacuum (VAR method, "Vacuum Arc Remelting" known in itself) to form ingots or slabs, or remelted under vacuum (VAR) or electrically conductive slag (ESR process, "Electro Slag Remelting ", known in itself) to form second electrodes which are themselves remelted under vacuum (VAR) to form ingots or slabs. This is done either a simple VAR reflow or a double reflow VAR + VAR or ESR + VAR. These refusals make it possible to purify the metal and to improve the quality of the solidification by reducing the segregations. In particular, ESR remelting makes it possible to lower the sulfur content, and VAR remelting makes it possible to lower the nitrogen and hydrogen content.

The ingots or slabs are then hot-rolled after reheating at about 1200 ° C., and for example between 1150 ° C. and 1250 ° C. to obtain hot-rolled strips a few millimeters thick, and for example about 4 , 5 mm thick.

The hot rolled strips are pickled then cold rolled with one or more recrystallization anneals to obtain cold-rolled strips less than 1 mm thick, for example 0.4 mm or 0.2 mm thick. .

The last intermediate recrystallization annealing treatment is carried out at such a thickness that the cold rolled strip has a work hardening rate of greater than 30% and more preferably greater than 40%.

The thus-hardened strip is annealed, for example in the passage oven, to obtain a fine grain of ASTM index greater than 8 (corresponding to an average grain diameter of less than 20 microns), and better than 10 (corresponding to a diameter average grain less than 10 microns); the grain size being determined according to ASTM E112.

The annealing treatment intended to obtain a fine grain is carried out under a protective atmosphere by suitably adjusting the temperature and duration parameters. These parameters depend on the particular conditions of realization of the heat treatment and those skilled in the art know how to determine these parameters in each particular case. In the case of a treatment carried out in a continuous passage furnace, the duration (ie the residence time of any point of the strip in the oven) is between 10s and 1 minute, and the set temperature of the oven is between 900 ° C and 1100 ° C; the furnace atmosphere may be argon with a lower dew temperature preferably at -50 ° C.

In order to improve the flatness of the band and, if necessary, to perfect the martensitic transformation, the band can, in addition, be subjected to a light cold rolling with a reduction ratio of between 1% and 10%, which leads to a work hardening rate of the same value.

It is then possible to cut a piece into the strip and to shape this piece, for example by folding, then to carry out thereon a hardening treatment consisting of a holding between 450 and 550 ° C. for 1 to 10 hours. Note that when the processing temperature is in the upper part of the temperature range (500 to 550 ° C), the ductility is improved and the yield strength is lowered slightly.

The hardening treatment may also be carried out in the oven at a temperature of between 600 ° C. and 700 ° C. for a time of between 30 seconds and 3 minutes.

This gives a piece made of a metal having a high yield strength and excellent fatigue resistance.

During the curing treatment or after this, the part can be cured on the surface by a nitriding treatment carried out by maintaining a few hours at 500 ° C in a nitrogen-rich reactive gas mixture.

Alternatively, blanks of parts may be cut into cold-rolled strips of greater thickness than the desired final thickness for the parts. These blanks are shaped, possibly welded, then cold rolled to the final thickness so as to have a work hardening rate greater than 30% or better than 40%. The parts are then annealed under the same conditions as described above, so as to obtain a fine grain of ASTM index greater than 8, or better than 10, and then subjected to a hardening treatment as indicated below. above. The yield strength obtained is high and the resistance to fatigue is excellent.

Pieces can also be made by cutting, for example by chemical cutting, into hardened strips. The entire process, including the curing heat treatment, is then applied to the web. These parts are, for example, integrated circuit support grids.

• de 12% à 24,5% de nickel,
• de 2,5% à 12% de molybdène,
• de 4,17% à 20% de cobalt,

le reste étant du fer et des impuretés ou des éléments résiduels en faible quantité résultant de l'élaboration.Maraging steel, which is best used to obtain very good fatigue properties and a yield strength greater than 1850 MPa, mainly contains, in% by weight:

• from 12% to 24.5% nickel,
• from 2.5% to 12% molybdenum,
• from 4.17% to 20% cobalt,

the remainder being iron and impurities or residual elements in small quantities resulting from the elaboration.

In order to obtain a point Ms (martensitic transformation start temperature) close to 200 ° C., the contents of nickel and molybdenum must be such that 20% ≤ Ni + Mo ≤ 27%, and preferably such that 22% ≤ Ni + Mo ≤ 25%.

In order to obtain a yield strength, after curing heat treatment, greater than 1850 MPa, the cobalt and molybdenum contents must be such that Co x Mo ≥ 50, and preferably such that Co x Mo ≥ 70. Indeed the higher the product, the higher the yield strength. But, in order to obtain sufficient ductility, the cobalt and molybdenum contents must be such that Co x Mo ≤ 200, and preferably such that Co x Mo ≤ 120. These values respectively correspond to yield strengths of less than approximately 3000 MPa. and 2500 MPa.

• 4,17 à 6 % de Mo qui correspond à des produits possédant une très bonne aptitude à la transformation à chaud et à froid ainsi qu un très bon compromis entre limite élastique élevée et bonne ductilité et ténacité.
• 6 à 8 % de Mo qui correspond à des aciers à très haute limite élastique ou plus économiques car de teneur réduite en cobalt.

Molybdenum has a favorable effect on surface nitriding hardening. To achieve good cure, the molybdenum content should preferably be greater than 4%, and more preferably greater than 6%. However, it is preferable that it remains below 8% in order to limit the segregation problems and to facilitate the hot processing operations as well as to improve the ductility of the final product. Two preferred ranges of molybdenum contents can be defined:

• 4.17 to 6% Mo which corresponds to products having a very good ability to heat and cold transformation and a very good compromise between high yield strength and good ductility and toughness.
• 6 to 8% of Mo which corresponds to steels with very high elastic limit or more economical because of reduced cobalt content.

1. 1) afin d'obtenir une limite d'élasticité supérieure à 1850 MPa et une aptitude moyenne au durcissement par nitruration: $$17\%\le \mathrm{Ni}\le 20\%$$
   
   $$4,17\%\le \mathrm{Mo}\le 6\%$$
   
   $$13\%\le \mathrm{Co}\le 17\%$$
   
   $$20\%\le \mathrm{Ni}+\mathrm{Mo}\le 27\%$$
   
   $$\mathrm{Co}\times \mathrm{Mo}\ge 50$$
   
2. 2) afin d'obtenir une limite d'élasticité supérieure à 1850 MPa et une aptitude forte au durcissement par nitruration: $$15\%\le \mathrm{Ni}\le 17\%$$
   
   $$6\%\le \mathrm{Mo}\le 8\%$$
   
   $$8,75\%\le \mathrm{Co}\le 13\%$$
   
   $$20\%\le \mathrm{Ni}+\mathrm{Mo}\le 27\%$$
   
   $$\mathrm{Co}\times \mathrm{Mo}\ge 50$$
   
3. 3) afin d'obtenir une limite d'élasticité supérieure à 2000 MPa et un point Ms plus favorable: $$15\%\le \mathrm{Ni}\le 21\%$$
   
   $$4,17\%\le \mathrm{Mo}\le 8\%$$
   
   $$8,75\%\le \mathrm{Co}\le 17,5\%$$
   
   $$22\%\le \mathrm{Ni}+\mathrm{Mo}\le 25\%$$
   
   $$\mathrm{Co}\times \mathrm{Mo}\ge 70$$
   
4. 4) afin d'obtenir une limite d'élasticité supérieure à 2000 MPa et un point Ms plus favorable et une aptitude moyenne au durcissement par nitruration: $$17\%\le \mathrm{Ni}\le 20\%$$
   
   $$4\%\le \mathrm{Mo}\le 6\%$$
   
   $$13\%\le \mathrm{Co}\le 17,5\%$$
   
   $$22\%\le \mathrm{Ni}+\mathrm{Mo}\le 25\%$$
   
   $$\mathrm{Co}\times \mathrm{Mo}\ge 70$$
   
5. 5) afin d'obtenir une limite d'élasticité supérieure à 2000 MPa et un point Ms plus favorable et une aptitude forte au durcissement par nitruration: $$15\%\le \mathrm{Ni}\le 17\%$$
   
   $$6\%\le \mathrm{Mo}\le 8\%$$
   
   $$8,75\%\le \mathrm{Co}\le 13\%$$
   
   $$22\%\le \mathrm{Ni}+\mathrm{Mo}\le 25\%$$
   
   $$\mathrm{Co}\times \mathrm{Mo}\ge 70$$
   

By combining all these conditions, the following preferred areas of composition can be defined for the main elements:

1. 1) in order to obtain a yield strength greater than 1850 MPa and an average ability to cure by nitriding: $$17\%\le \mathrm{Or}\le 20\%$$
   
   $$4,17\%\le \mathrm{MB}\le 6\%$$
   
   $$13\%\le \mathrm{Co}\le 17\%$$
   
   $$20\%\le \mathrm{Or}+\mathrm{MB}\le 27\%$$
   
   $$\mathrm{Co}\times \mathrm{MB}\ge 50$$
   
2. 2) in order to obtain a yield strength greater than 1850 MPa and a strong ability to cure by nitriding: $$15\%\le \mathrm{Or}\le 17\%$$
   
   $$6\%\le \mathrm{MB}\le 8\%$$
   
   $$8,75\%\le \mathrm{Co}\le 13\%$$
   
   $$20\%\le \mathrm{Or}+\mathrm{MB}\le 27\%$$
   
   $$\mathrm{Co}\times \mathrm{MB}\ge 50$$
   
3. 3) in order to obtain a yield strength greater than 2000 MPa and a more favorable Ms point: $$15\%\le \mathrm{Or}\le 21\%$$
   
   $$4,17\%\le \mathrm{MB}\le 8\%$$
   
   $$8,75\%\le \mathrm{Co}\le 17,5\%$$
   
   $$22\%\le \mathrm{Or}+\mathrm{MB}\le 25\%$$
   
   $$\mathrm{Co}\times \mathrm{MB}\ge 70$$
   
4. 4) in order to obtain a yield strength greater than 2000 MPa and a more favorable Ms point and an average ability to cure by nitriding: $$17\%\le \mathrm{Or}\le 20\%$$
   
   $$4\%\le \mathrm{MB}\le 6\%$$
   
   $$13\%\le \mathrm{Co}\le 17,5\%$$
   
   $$22\%\le \mathrm{Or}+\mathrm{MB}\le 25\%$$
   
   $$\mathrm{Co}\times \mathrm{MB}\ge 70$$
   
5. 5) in order to obtain a yield strength greater than 2000 MPa and a more favorable Ms point and a strong aptitude for hardening by nitriding: $$15\%\le \mathrm{Or}\le 17\%$$
   
   $$6\%\le \mathrm{MB}\le 8\%$$
   
   $$8,75\%\le \mathrm{Co}\le 13\%$$
   
   $$22\%\le \mathrm{Or}+\mathrm{MB}\le 25\%$$
   
   $$\mathrm{Co}\times \mathrm{MB}\ge 70$$
   

$$\mathrm{Ti}\le 0,1\%$$

$$\mathrm{N}\le 0,003\%$$

$$\mathrm{Si}\le 0,1\%$$

$$\mathrm{Mn}\le 0,1\%$$

$$\mathrm{C}\le 0,005\%$$

$$\mathrm{S}\le 0,001\%$$

$$\mathrm{P}\le 0,005\%$$

$$\mathrm{H}\le 0,0003\%$$

$$\mathrm{O}\le 0,001\%$$

Pour chacun de ces éléments la teneur minimale peut être 0% ou des traces. De plus, et pour obtenir une tenue à la fatigue améliorée des courroies, les teneurs en azote et titane doivent être telles que : Ti x N ≤ 2 x 10^-4, ou mieux, ≤ 1 x 10^-4.In addition to the main elements whose composition areas have just been described, the residual elements must be rigorously controlled to obtain good ductility and fatigue resistance properties. These limitations include: $$\mathrm{al}\%\le 0,15\%$$

$$\mathrm{Ti}\le 0,1\%$$

$$\mathrm{NOT}\le 0,003\%$$

$$\mathrm{Yes}\le 0,1\%$$

$$\mathrm{mn}\le 0,1\%$$

$$\mathrm{VS}\le 0,005\%$$

$$\mathrm{S}\le 0,001\%$$

$$\mathrm{P}\le 0,005\%$$

$$\mathrm{H}\le 0,0003\%$$

$$\mathrm{O}\le 0,001\%$$

For each of these elements the minimum content may be 0% or traces. In addition, and to obtain improved fatigue life of the belts, the nitrogen and titanium contents must be such that: Ti x N ≤ 2 x 10 ^-4 , or better, ≤ 1 x 10 ^-4 .

By way of example and comparison, we made maraging steel strips of composition:
Ni = 18.1% Co = 16.2% Mo = 5.3% Al = 0.020% Ti = 0.013% Si = 0.03% Mn = 0.03% C = 0.003% Ca <0.0005% S = 0.0007% P = 0.002 N = 0.0023% O <0.001% H <0.0001%, the remainder being iron and impurities. These impurities are in particular copper and chromium whose contents are: Cu = 0.07% and Cr = 0.06%.
The martensitic transformation point Ms of this casting is equal to + 195 ° C.

These strips were cold-rolled to a thickness of 0.4 mm, with a final hardening rate of 70%.

A first band A, given by way of example, was annealed with hydrogen passage at 1020 ° C for 1 minute to obtain a fine grain of ASTM 11 index and then cured by holding at 490 ° C for 3 hours.

A second strip B, given for comparison, was pass-annealed at 1150 ° C for 1 minute to obtain a coarse grain of ASTM 7 index and then cured by holding at 490 ° C for 3 hours.

Comparative tests of fatigue strength were carried out with the bands A and B by undulating traction, at 25 hertz, with a maximum stress of 750 MPa and a minimum stress of 75 MPa.

For the band A according to the invention, the fatigue limit was greater than 8 × 10 ⁸ cycles, whereas for the band B the fatigue limit was 5 × 10 ⁸ cycles. These results show the interest of a fine grain to improve the fatigue resistance of these bands.

Both bands A and B had a yield strength greater than 1850 MPa.

In order to demonstrate the particular interest of the preferred chemical composition of maraging steel in accordance with the invention, a maraging steel strip containing 18% nickel, 9% cobalt and 5% molybdenum was also manufactured. 0.5% titanium and 0.1% aluminum. This tape was made by the process according to the invention, the grain had an ASTM index of 10 and the yield strength was 1910 MPa. The fatigue limit measured under the same test conditions as in the previous case was 2 x 10 ⁸ cycles.

These strips can advantageously be used to manufacture belts or any other product, such as integrated circuit support grids.

By way of example, with belts in accordance with the invention, transmission belts for an internal combustion engine consisting of jumpers held by rings consisting of narrow strips in accordance with the invention and whose two ends are welded have been manufactured. These belts have a lifespan more than ten times longer than the life of identical belts but manufactured with maraging steel strips according to the prior art.

Claims (10)

1. Process for the manufacture of a strip or of a part cut from a strip made of cold-rolled maraging steel hardened by a hardening heat treatment, characterized in that, before the hardening heat treatment is carried out, the strip or the part undergoes cold plastic deformation with a reduction ratio of greater than 30% and the strip or the part undergoes recrystallization annealing so as to obtain a fine grain corresponding to an ASTM index of greater than 8, the chemical composition of the steel comprising, in % by weight: $$12\%\le \mathrm{Ni}\le 24.5\%$$

   

   $$2.5\%\le \mathrm{Mo}\le 12\%$$

   

   $$4.17\%\le \mathrm{Co}\le 20\%$$

   

   $$\mathrm{Al}\%\le 0.15\%$$

   

   $$\mathrm{Ti}\le 0.1\%$$

   

   $$\mathrm{N}\le 0.003\%$$

   

   $$\mathrm{Si}\le 0.1\%$$

   

   $$\mathrm{Mn}\le 0.1\%$$

   

   $$\mathrm{C}\le 0.005\%$$

   

   $$\mathrm{S}\le 0.001\%$$

   

   $$\mathrm{P}\le 0.005\%$$

   

   $$\mathrm{H}\le 0.0003\%$$

   

   $$\mathrm{O}\le 0.001\%$$

   

   the balance being iron and impurities resulting from the smelting, the chemical composition furthermore satisfying the relationships: $$20\%\le \mathrm{Ni}+\mathrm{Mo}\le 27\%$$

   

   $$50\le \mathrm{Co}\times \mathrm{Mo}\le 200$$

   

   $$\mathrm{Ti}\mathrm{\times }\mathrm{N}\mathrm{\le }\mathrm{2}\mathrm{\times }{\mathrm{10}}^{\mathrm{-}\mathrm{4}}\mathrm{.}$$

   
2. Process according to Claim 1, characterized in that, after the recrystallization annealing, the strip or the part undergoes cold rolling with a reduction ratio of between 1% and 10%.
3. Process according to Claim 1 or Claim 2, characterized in that the maraging steel is vacuum-remelted by the VAR process or is remelted a first time in a vacuum by the VAR process or electroslag-remelted by the ESR process and remelted a second time in a vacuum by the VAR process.
4. Process according to any one of Claims 1 to 3, characterized in that the hardening heat treatment consists of a soak between 450°C and 550°C for 1 to 10 hours.
5. Process according to Claim 4, characterized in that, during the hardening heat treatment or after it, the surface of the part is hardened by nitriding.
6. Process according to any one of Claims 1 to 3, characterized in that the hardening heat treatment is carried out in a continuous furnace at a temperature of between 600°C and 700°C for a time of between 30 seconds and 3 minutes.
7. Process according to Claim 6, characterized in that after the hardening heat treatment the surface of the part is hardened by nitriding.
8. Strip or part, having a thickness of less than 1 mm, made of maraging steel, characterized in that the steel of which the strip or part is made has a fine grain corresponding to an ASTM index of greater than 8 and in that the composition of the steel comprises, in % by weight: $$12\%\le \mathrm{Ni}\le 24.5\%$$

   

   $$2.5\%\le \mathrm{Mo}\le 12\%$$

   

   $$4.17\%\le \mathrm{Co}\le 20\%$$

   

   $$\mathrm{Al}\%\le 0.15\%$$

   

   $$\mathrm{Ti}\le 0.1\%$$

   

   $$\mathrm{N}\le 0.003\%$$

   

   $$\mathrm{Si}\le 0.1\%$$

   

   $$\mathrm{Mn}\le 0.1\%$$

   

   $$\mathrm{C}\le 0.005\%$$

   

   $$\mathrm{S}\le 0.001\%$$

   

   $$\mathrm{P}\le 0.005\%$$

   

   $$\mathrm{H}\le 0.0003\%$$

   

   $$\mathrm{O}\le 0.001\%$$

   

   the balance being iron and impurities resulting from the smelting, the chemical composition furthermore satisfying the relationships: $$20\%\le \mathrm{Ni}+\mathrm{Mo}\le 27\%$$

   

   $$50\le \mathrm{Co}\times \mathrm{Mo}\le 200$$

   

   $$\mathrm{Ti}\mathrm{\times }\mathrm{N}\mathrm{\le }\mathrm{2}\mathrm{\times }{\mathrm{10}}^{\mathrm{-}\mathrm{4}}$$

   

   the steel having a yield strength after hardening of greater than 1850 MPa.
9. Transmission belt comprising at least one strip or part according to Claim 8.
10. Leadframe for integrated circuits, consisting of a part according to Claim 1.