EP1379706B1 - Tool steel having high toughness, method for producing parts made of said steel, and parts thus obtained - Google Patents

Description

The present invention relates to a tool steel composition having a toughness reinforced compared to the shades of the prior art, a process of making this composition as well as the parts that can be thus obtained.

Tool steels are widely used in many applications involving in particular relative movements between parts in contact, where one of the parts must maintain its integrity geometric as long as possible. Examples of these are realization, machining and cutting tools and equipment metrological.

Preserving the geometric integrity of these parts requires good resistance to wear, good resistance to deformation and rupture under static or dynamic stresses, which implies that steel used has a high toughness and hardness.

In addition, the grade must have good hardenability, so that the structure as homogeneous as possible over large thicknesses after the tempering.

These different requirements are often contradictory. Thus, there is known a tool steel cold working steel grade called AISI D2 and widespread, containing 1.5% by weight of carbon and 12% of chromium with some additional additions of hardening carburizing elements such as Mo or V The high carbon and chromium contents lead to a high precipitation of M ₇ C ₃ type eutectic carbides which are formed at high temperature at the end of solidification and are therefore coarse and heterogeneously distributed in the metal matrix.

If the presence of a large volume fraction of hard carbides in steel is conducive to strengthening the wear resistance, their bad distribution, in turn, is a factor of tenacity.

To overcome this problem, it was then proposed to reduce the levels of carbon and chromium of this type of grades at respective levels of about 1 and 8% with compensation for a higher molybdenum content, of the order 2.5% (EP 0 930 374). The reduction of the carbon content allows reduce the volume fraction of eutectic carbides which is favorable for tenacity. The enrichment of these carbides in molybdenum which increases their hardness, allows in turn to maintain the hardness of the steel and its resistance to wear.

However, it would still be necessary to further refine the distribution of these carbides to increase toughness without reducing the hardness characteristics and Wear resistance of steel.

The inventors have found that a further improvement of the compromise toughness - mechanical and wear resistance results unexpectedly from a sufficient nitrogen content with a minimum content of titanium and / or zirconium, itself a function of the nitrogen content.

• d'une part N ≥ 0,004%, de préférence ≥ 0,006%,
• d'autre part (Ti + Zr/2) x N ≥ 2,5.10^-4 %²,

les teneurs en Ti, Zr et N étant exprimés en % pondéral.Specifically, refinement of chromium, molybdenum and tungsten carbides, and joint reinforcement of toughness have been observed when:

• N ≥ 0.004%, preferably ≥ 0.006%,
• on the other hand (Ti + Zr / 2) x N ≥ 2.5.10 ^-4 % ² ,

the contents of Ti, Zr and N being expressed in% by weight.

This joint requirement in nitrogen and titanium or zirconium suggests that the active factor is the presence of nitrides of titanium and or zirconium, supposed play the role of refiner of the size of chromium carbides, molybdenum and tungsten. The average size of the large carbides of chromium, molybdenum and tungsten and pass a typical value of about 10 microns according to the prior art, to a value of about 4 microns, according to the present invention.

0,8 ≤ C ≤ 1,5

5,0 ≤ Cr ≤ 14

0,2 ≤ Mn ≤ 3

Ni ≤ 5

V ≤ 1

Nb ≤ 0,1

Si+Al ≤ 2

Cu ≤ 1

S ≤ 0,3

Ca ≤ 0,1

Se ≤ 0,1

Te ≤ 0,1

1,0 ≤ Mo+W/2 ≤ 4

0,06 ≤ Ti+Zr/2 ≤ 0,15

0,004 ≤ N ≤ 0,02

   le reste de la composition étant constitué de fer et d'impuretés résultant de l'élaboration,
   étant en outre entendu que : 2,5.10^-4 %² ≤ (Ti + Zr/2) x N.A first object of the invention is thus constituted by a steel whose composition comprises, the percentages being expressed in% by weight:

0.8 ≤ C ≤ 1.5

5.0 ≤ Cr ≤ 14

0.2 ≤ Mn ≤ 3

Ni ≤ 5

V ≤ 1

Nb ≤ 0,1

If + Al ≤ 2

Cu ≤ 1

S ≤ 0.3

Ca ≤ 0.1

Se ≤ 0.1

Te ≤ 0.1

1.0 ≤ Mo + W / 2 ≤ 4

0.06 ≤ Ti + Zr / 2 ≤ 0.15

0.004 ≤ N ≤ 0.02

the remainder of the composition consisting of iron and impurities resulting from the preparation,
being further understood that: 2.5 × 10 ^-4 % ² ≤ (Ti + Zr / 2) x N.

0,8 ≤ C ≤ 1,2

7,0 ≤ Cr ≤ 9

0,2 ≤ Mn ≤ 1,5

Ni ≤ 1

0,1 ≤ V ≤ 0,6

Nb ≤ 0,1

Si+Al ≤ 1,2

Cu ≤ 1

S ≤ 0,3

Ca ≤ 0,1

Se ≤ 0,1

Te ≤ 0,1

2,4 ≤ Mo+W/2 ≤ 3

0,06 ≤ Ti+Zr/2 ≤ 0,15

0,004 ≤ N ≤ 0,02

   le reste de la composition étant constitué de fer et d'impuretés résultant
   de l'élaboration,
   étant en outre entendu que : 2,5.10^-4 %² ≤ (Ti + Zr/2) x N .In a preferred embodiment of the invention, the steel composition comprises, the percentages being expressed in% by weight:

0.8 ≤ C ≤ 1.2

7.0 ≤ Cr ≤ 9

0.2 ≤ Mn ≤ 1.5

Ni ≤ 1

0.1 ≤ V ≤ 0.6

Nb ≤ 0,1

If + Al ≤ 1.2

Cu ≤ 1

S ≤ 0.3

Ca ≤ 0.1

Se ≤ 0.1

Te ≤ 0.1

2.4 ≤ Mo + W / 2 ≤ 3

0.06 ≤ Ti + Zr / 2 ≤ 0.15

0.004 ≤ N ≤ 0.02

the remainder of the composition consisting of iron and resulting impurities
of the elaboration,
it being further understood that: 2.5 × 10 ^-4 % ² ≤ (Ti + Zr / 2) x N.

The titanium and / or zirconium content of the steel according to the invention must be between 0.06 and 0.15% by weight. Indeed, beyond 0.15% by weight, the Precipitation of titanium nitride and / or zirconium tends to coalesce and lose its effectiveness. On the other hand, if the content is less than 0.06% by weight, the amount of titanium and / or zirconium present is insufficient to form enough nitrides of titanium and / or zirconium to obtain the improvement sought after in toughness and resistance to wear. It will be noted that zirconium may be substituted in whole or in part for titanium in the proportion of two parts of zirconium for a part of titanium.

The nitrogen content of the steel according to the invention must be between 0.004 and 0.02% by weight, preferably between 0.006 and 0.02% by weight. We limits its content to 0.02% by weight because beyond, the tenacity tends to decrease.

The carbon content of the steel according to the invention must be between 0.8 and 1.5% by weight, preferably between 0.8 and 1.2% by weight. Carbon must be present in an amount sufficient to form carbides and reach the level of hardness that one wishes to obtain for the shade.

In another preferred embodiment, the carbon content of the steel according to the invention is between 0.9% and 1.5% by weight, in order to ensure improved hardness, unchanged heat treatment, and strength to wear by increasing the volume fraction of hard carbides.

The chromium content of the steel according to the invention must be between 5 and 14% by weight, preferably between 7 and 9% by weight. This element allows on the one hand to increase the quenchability of the grade and, on the other hand, to form hardening carbides.

The manganese content of the steel according to the invention must be understood between 0.2 and 3% by weight, preferably between 0.2 and 1.5% by weight. We add it in the shade according to the invention because it is a soaking element, but it is limited its content to limit the segregation which would lead to a bad forgeability and a tenacity too weak.

The steel can contain up to 5% by weight of nickel. Preferably, the The content of this element must remain below 1% by weight. We can add it in the shade according to the invention because it is a soaking element and which does not pose no problem of segregation. However, its content is limited because it is a gammagene element favorable to the formation of residual austenite.

To enhance resistance to softening in the frequent case where Steel is subject to income before use, it is helpful to add to the composition strong carburigenic elements forming at the income of fine carbides of types MC.

Among them vanadium is preferred, and it is then used in grades at least 0.1% but not exceeding 1%, preferentially less than 0.6%.

Niobium, which tends to precipitate at higher temperatures and which, from this fact, night strongly to the forgeability of steel is to be avoided and will not exceed in in any case 0.1%, and will preferably be less than 0.02% by weight.

The silicon and / or aluminum content of the steel according to the invention be less than 2% by weight. In addition to their role of deoxidizing the nuance, these elements allow to slow the coalescence of carbides in temperature and thus reduce the kinetics of softening to income. We limit their content because beyond 2% by weight, they weaken the shade.

The molybdenum and / or tungsten content of the steel according to the invention must be between 1 and 4% by weight, preferably between 2.4 and 3% by weight. weight. It should be noted that tungsten may be substituted in whole or in part for molybdenum in the proportion of two parts of tungsten for a part of molybdenum. These two elements make it possible to improve the quenchability of the shade and form hardening carbides. Their content is limited because they are at the origin of segregations.

Copper may be present in the steel in content nevertheless lower than 1% not to affect the forgeability of the shade.

Moreover, in order to improve the machinability of steel, sulfur, in one content not exceeding 0,3% may be added, possibly accompanied calcium, selenium, tellurium in contents each less than 0.1%.

The development of the steel grade according to the invention, including the mode addition of titanium and / or zirconium, can be done by any conventional method, but can advantageously be carried out by the process according to the invention which constitutes a second object of the invention.

This part manufacturing process includes a first step consisting in developing a liquid steel by melting all the elements of the grade according to the invention, with the exception of titanium and / or zirconium, and then add to the molten steel bath titanium and / or zirconium avoiding at any time local overconcentrations of titanium and / or zirconium in the steel bath molten.

Indeed, the present inventors have found that the methods of addition according to the prior art, titanium and zirconium in the form of massive ferro-alloy or metallic elements, generated nitrides titanium and / or zirconium coarse and consequently fewer, especially since some of them can even decant. This This situation seems to be related to the fact that these addition strong local overconcentrations of titanium and / or zirconium in the liquid at neighborhood of the added elements.

One of the embodiments of this first step of the method according to the invention consists in adding titanium and / or zirconium in the slag covering the bath of liquid steel continuously, titanium and / or zirconium then spreading gradually in the steel bath.

Another embodiment of this first step of the method according to the invention consists in adding titanium and / or zirconium by introducing continues a wire composed of this or these elements in the molten steel bath, while by stirring the bath by bubbling or by any other suitable method.

Another embodiment of this first step of the method according to the invention consists in adding titanium and / or zirconium by blowing a powder containing this or these elements in the molten steel bath, while stirring the bath by bubbling or by any other suitable method.

In the context of the present invention, it is preferred to use the different embodiments that have just been described, but it is understood that any process to avoid local overconcentration of titanium and / or zirconium may be used.

The elaboration is generally carried out in an arc furnace, or in a induction furnace.

At the end of this preparation, the liquid steel is poured into ingots or slabs. In order to refine its structure, we can perform a brewing in the ingot mold or even use the slag remelting process with consumable electrode.

These ingots or slabs are then processed by means of heat-forming plastic deformation treatments adapted such hot forging or rolling, for example.

The steel can then be subjected to heat treatment according to conventional channels for tool steels. Such a heat treatment can possibly include annealing to facilitate cutting and machining, then austenitization followed by cooling according to a mode adapted to thickness, such as air or oil cooling, possibly followed by income according to the level of hardness one wishes to achieve.

A third object of the invention is constituted by a steel piece of composition according to the invention or obtained by the implementation of the process according to the invention and whose average size of carbide precipitates of chromium, molybdenum or tungsten resulting from the solidification is between 2.5 and 6 μm, preferably between 3 and 4.5 μm.

The present invention is illustrated from the following observations and examples, Table 1 giving the chemical composition of the tested steels, among which the casting 1 is in accordance with the present invention, while the casting 2 is given for comparison. Composition
(% in weight) Casting 1 Casting 2 VS 0.98 0.96 Cr 8.40 8.20 mn 0.79 0.83 Or 0.35 0.31 Cu 0.26 0.22 V 0.37 0.40 Nb 0.01 0.09 Yes 0.97 0.94 al 0.03 0.03 MB 2.60 2.50 W - - Ti 0.11 0,004 Zr - - NOT 0,011 0,009

Abbreviations used

Pv:

volume loss, expressed in mm ³ ,

KV:

breaking energy, expressed in J / cm ² ,

T:

toughness, expressed in J / cm ² .

Example 1 - Tenacity

Two pieces are manufactured from casting 1 according to the invention and of the comparative casting 2, by hot rolling at 1150 ° C of the ingots made in these compositions. The samples are then austenitized at 1050 ° C. for one hour, soaked in oil and then subjected to a double income of 525 ° C for one hour to obtain a hardness of 60 Hrc.

• un essai de flexion par choc sur éprouvette Charpy prenant la forme d'un barreau entaillé en V selon la norme NF EN 10045-2, qui fournit l'énergie de rupture KV et
• un essai de flexion par choc sur barreau non entaillé (barreau de 10mm sur 10mm), qui fournit la ténacité T.

Les résultats obtenus sont rassemblés dans le tableau suivant :

Two series of tests are then performed using different methods to measure the tenacity:

• a Charpy impact test on the test piece taking the form of a V-notched bar in accordance with standard NF EN 10045-2, which provides the KV and
• a non-slotted bar bending test (10mm x 10mm bar), which provides T toughness.

The results obtained are summarized in the following table:

It can be seen that, whatever the method employed, the casting 1 according to the invention has improved toughness compared to casting 2 comparative.

Example 2 - Wear resistance

Two pieces are manufactured in a manner similar to that used in Example 1, and a measurement of the wear resistance is made by following the ASTM G52, which allows the determination of the volume loss experienced by samples tested. This test consists of measuring the weight loss of the sample subjected to abrasive wear of a quartz sand calibrated granulometry introduced between a rubber wheel and the sample fixed.

The results obtained are summarized in the following table: 1

It can be seen that the casting 1 according to the invention has a resistance to slightly improved wear compared to comparative casting 2.

Claims (10)

1. Tooling steel with a composition comprising the following, the percentages being expressed as % by weight:

   0.8 ≤ C ≤ 1.5

   5.0 ≤ Cr ≤ 14

   0.2 ≤ Mn ≤ 3

   Ni ≤ 5

   V ≤ 1

   Nb ≤ 0.1

   Si+Al ≤ 2

   Cu- ≤ 1

   S ≤ 0.3

   Ca ≤ 0.1

   Se ≤ 0.1

   Te ≤ 0.1

   1,0 ≤ Mo+W/2 ≤ 4

   0.06 ≤ Ti+Zr/2 ≤ 0.15

   0.004≤N ≤ 0.02

   the rest of the composition being made up of iron and foreign matter resulting from the processing, and further it is understood that: 2.5x10^-4 %² ≤ (Ti + Zr/2) x N.
2. Steel according to Claim 1, characterised in addition in that the composition comprises the following, the percentages being expressed as % by weight:

   0.8 ≤ C ≤ 1.2

   7.0 ≤ Cr ≤ 9

   0.2 ≤ Mn ≤ 1.5

   Ni ≤ 1

   0.1 ≤ V ≤ 0.6

   Nb ≤ 0.1

   Si+Al ≤ 1.2

   Cu ≤ 1

   S ≤ 0.3

   Ca ≤ 0.1

   Se ≤ 0.1

   Te ≤ 0.1

   2.4 ≤ Mo+W/2 ≤ 3

   0.06 ≤ Ti+Zr/2 ≤ 0.15

   0.004 ≤ N ≤ 0.02

   the rest of the composition being made up of iron and foreign matter resulting from the processing, and further it is understood that: 2.5 x 10^-4 %² ≤ (Ti + Zr/2) x N.
3. Steel according to Claim 1 or 2, further characterised in that the niobium content is below or equal to 0.02% by weight.
4. Steel according to any one of the Claims 1 to 3, characterised further in that the nitrogen content is between 0.006 and 0.02% by weight inclusive.
5. Process for manufacturing a steel component with a composition according to any one of the Claims 1 to 4, characterised in that

   a liquid steel is worked by melting all of the elements of the said composition, with the exception of the titanium and/or zirconium, then the titanium and/or zirconium is added to the melted steel bath while avoiding at any time local overconcentrations of titanium and/or zirconium in the melted steel bath;

   the said liquid steel is poured to make an ingot or a slab;

   the said ingot or slab is subjected to a forming treatment by hot plastic deformation, then possibly to heat treatment to make the said component.
6. Process according to Claim 5, characterised in that the titanium and/or zirconium are added continuously into a slag covering the bath of liquid steel, the titanium and/or zirconium then spreading out gradually in the said steel bath.
7. Process according to Claim 5, characterised in that the titanium and/or zirconium are added by continuous introduction of a titanium and/or zirconium wire into the steel bath, while the said bath is being stirred.
8. Process according to Claim 5, characterised in that the titanium and/or zirconium are added by blowing a powder containing the titanium and/or zirconium into the melted steel bath, while the bath is being stirred.
9. Steel component with a composition in accordance with any one of the Claims 1 to 4 or made by using the process according to any one of the Claims 5 to 8, characterised in that the average size of the precipitates of molybdenum or tungsten or chromium carbides from the solidification is between 2.5 and 6 µm.
10. Steel component according to Claim 9, characterised further in that the average size of the molybdenum or tungsten or chromium carbides from the solidification is between 3 and 4.5 µm.