EP0663018B1 - Tool steel compositions - Google Patents

Abstract

PCT No. PCT/FR93/00979 Sec. 371 Date Apr. 3, 1995 Sec. 102(e) Date Apr. 3, 1995 PCT Filed Oct. 5, 1993 PCT Pub. No. WO94/09170 PCT Pub. Date Apr. 28, 1994A tool steel composition including, expressed by weight: 2.5% to 5.8% Cr, not more than 1.3% V, not more than 0.8% Si, with an Mo content lying in the range 0.75% to 1.75%, and not more than 0.35% Si, when the Mo content is 2.5% to 3.5%, with 0.3% to 0.4% by weight of C. Also disclosed is a method of preparing and of shaping such steels, as well as parts obtained thereby.

Description

The present invention relates to a family of steels known as 3-5% by weight of chromium which are used for the manufacture of tools resistant to heat and under high stresses, such as stamping and forging dies and dies. pressure casting or static casting of various alloys such as aluminum or titanium alloys.

• 5% de Cr, 1,3% de Mo, 0,5 à 1,3% de V, ou d'autre part,
• 3% de Cr, 3% de Mo, 0,5% de V, ou enfin,
• 5% de Cr, 3% de Mo, 0,8% de V.

Such steels generally contain from 3 to 5% by weight of chromium, although contents of 2 to 6% can be observed. More specifically, they essentially comprise three families of compositions which, although slightly different from each other, confer similar physical properties, so that these steel compositions are used for the same applications. These are compositions comprising, expressed by weight, of the order of

• 5% Cr, 1.3% Mo, 0.5-1.3% V, or on the other hand,
• 3% Cr, 3% Mo, 0.5% V, or finally,
• 5% Cr, 3% Mo, 0.8% V.

For several decades, the use of these steels has been generalized in forges, for the production of forged or stamped parts on presses and pestles, and in light alloy foundries, for example for the production of dies for molded parts in steels or light alloys intended for the automobile, such as gearbox, clutch or engine casings.

Some of these steels are designated in the United States of America AISI nomenclature by the denominations H 11, H 12, H 13, in the DIN nomenclature by the denominations W-1.2343, W-1.2606 and W-1.2344. The French standard NFA 35590 also defines similar compositions.

A silicon content, hardening element, close to 1% by weight gives mechanical parts high strengths, of the order of 1800 MPa, or more. This resistance is not sought in the envisaged jobs of forging, except for very flat parts, and never in dies under aluminum pressure, for which a Rockwell C hardness (HRC) less than or equal to 48 is sufficient.

It is known, in particular for so-called steels with 3-5% chromium, that a successful annealing heat treatment inevitably implies a successful quality heat treatment. Thus, the fineness and the homogeneity of the microstructure of the finished product treated for use derive from those observed after annealing. This is why professionals commonly use a charter of microphotographs of structures in the annealed state, defining conforming microstructures and non-conforming microstructures.

This use, which is currently widespread, has gradually frozen the conditions for production, thermomechanical transformation and annealing. In addition, it has been observed that the refining of the annealing structure is conditioned by the homogenization of the structure in the austenite domain, which makes it necessary to avoid the presence of primary carbides, and by the dispersion coherent precipitates of secondary carbides M ₂₃ C ₆ (M = Cr, Fe, Mo, ...) during subsequent heat treatments.

The Applicant has been able, on the other hand, to show that certain areas of coarser appearance, sometimes needle-like with a bainitic appearance, especially on products of large section, have higher silicon contents.

On the basis of these fundamental considerations, steels with suitable homogeneous annealing structures have been developed.

To this end, the invention relates to two types of tool steel compositions.

• 4,5 à 5,8% de Cr,
• 0,75 à 1,75% de Mo,
• au plus 1,3%, et de préférence 0,25 à 0,50% de V,
• au plus 0,8%, et de préférence au plus 0,35% de Si,
• 0,3 à 0,4% de C, et en outre, le cas échéant,
• au plus 0,8 de Mn, et/ou au plus 1,5% de W,
• le complément étant constitué de Fe, d'additifs et impuretés habituels,
• Ni constituant, éventuellement, une impureté, à raison de 0,5% au plus.

The first type of tool steel composition according to claim 1 comprises, expressed by weight

• 4.5 to 5.8% Cr,
• 0.75 to 1.75% of Mo,
• at most 1.3%, and preferably 0.25 to 0.50% of V,
• at most 0.8%, and preferably at most 0.35% of Si,
• 0.3 to 0.4% of C, and further, if necessary,
• at most 0.8 of Mn, and / or at most 1.5% of W,
• the balance consisting of Fe, usual additives and impurities,
• Nor constituting, possibly, an impurity, at a rate of 0.5% at most.

• 2,5 à 5,5% de Cr,
• 2,5 à 3,5% de Mo,
• au plus 1,3% de V,
• au plus 0,35% de Si,
• 0,3 à 0,4% de C, et en outre, le cas échéant,
• au plus 5% de Co,
• le complément étant constitué de Fe, d'additifs et d'impuretés habituels.

The second type of tool steel composition according to claim 4 comprises, expressed by weight

• 2.5 to 5.5% Cr,
• 2.5 to 3.5% of Mo,
• at most 1.3% of V,
• at most 0.35% of Si,
• 0.3 to 0.4% of C, and further, if necessary,
• at most 5% of Co,
• the balance consisting of Fe, usual additives and impurities.

Such compositions provide homogenization of the annealing structure, which is all the more difficult to obtain when the section of the parts is larger, by suppressing the formation of more ferritic zones. rich in silicon, as well as primary carbides whose dissolution is always difficult.

In addition, these two modifications do not cause a significant revision of the range of heat treatment in the fields of use: the difference which one could note, on the values of resistance and elastic limit can easily be compensated by a regulation of the temperature of the second income, which is within the reach of any specialist.

On the other hand, the lowering of the silicon content has little or no influence on the oxidation resistance of the steel, up to its maximum working temperatures, i.e. - say in the field of forging (600-650 ° C). On the other hand, the homogeneity of macrostructure (less marked band structure) and of microstructure are guarantees of good behavior in service, that is to say of good characteristics with regard to toughness, mechanical fatigue, fatigue. thermal.

According to a preferred embodiment, the compositions according to the invention comprise 0.32 to 0.38, and in a particularly preferred manner, 0.34 to 0.36% by weight of C.

• P ≤ 0,008 %,
• Sb ≤ 0,002 %,
• Sn ≤ 0,003 %,
• As ≤ 0,005 %,
• la valeur exprimée par la relation de Bruscato $${\text{B = (10 P + 5 Sb + 4 Sn + As) x 10}}^{\text{-2}}$$
  
  étant au plus égale à 0,10%.

In addition, the proportions of phosphorus, antimony, tin and arsenic, expressed in% by weight, advantageously satisfy the relationships

• P ≤ 0.008%,
• Sb ≤ 0.002%,
• Sn ≤ 0.003%,
• As ≤ 0.005%,
• the value expressed by the Bruscato relation $${\text{B = (10 P + 5 Sb + 4 Sn + As) x 10}}^{\text{-2}}$$
  
  being at most equal to 0.10%.

• un traitement thermique complet par mise en solution à des températures comprises entre 950 et 1100°C, de préférence entre 980 et 1010°C, puis
• une trempe à l'air dans un fluide jusqu'à la température ambiante, ou une trempe étagée entre 250 et 450°C, de préférence entre 250 et 280°C, puis
• une série d'au moins deux revenus pour régler la dureté visée.

A second main object of the present invention according to claim 7 consists in a process for the preparation and shaping of steel having a composition in accordance with the above description, a process which comprises a reflow by consumable electrode under vacuum or by consumable electrode under slag or by these two combined means, the shaping being preferably carried out by thermomechanical transformation such as forging or rolling, or by molding. The method according to the invention also advantageously comprises

• a complete heat treatment by dissolving at temperatures between 950 and 1100 ° C, preferably between 980 and 1010 ° C, then
• air quenching in a fluid up to room temperature, or stepped quenching between 250 and 450 ° C, preferably between 250 and 280 ° C, then
• a series of at least two incomes to adjust the target hardness.

The quenching is advantageously carried out between 250 and 280 ° C., that is to say below the point M _s (Martensite start) in a fluid, for example a nitrate bath.

At least two incomes are recommended, the first at the peak of secondary hardening (510/560 ° C), the second in the area of over-aging or "overaging", that is to say at a temperature greater than or equal to 570 ° vs. It is the adjustment of the temperature of the second tempering which gives the hardness to the treated product.

A third object of the invention, according to claims 12 and 13, is a forging and stamping die or a die casting under pressure or by gravity, made of a steel of composition previously described, preferably manufactured in accordance with a method also described previously.

Such dies are in particular intended, in an appropriate manner, for the manufacture of tools or mechanical parts working at high temperatures and under high stresses, and, in particular, forging, stamping, die casting or gravity dies various light steels and alloys such as aluminum or zamak alloys or titanium alloys.

The following examples illustrate the present invention.

The mechanical properties of two steels 3 not in accordance with the invention and 4 according to the invention, the compositions of which by weight are given in table 1 below, were compared with those of steels 1 and 2 representative of the prior art, steel 1 being steel DIN W-1.2343, steel 2 being steel 1 having undergone remelting

Example 1

• pour un niveau de résistance totale à la traction de 1700-1800 MPa (tableau 2), et
• pour un niveau de résistance totale à la traction de 1300-1400 MPa (tableau 3).
  

For steels 1, 2, 3 and 4, the total tensile strength R (MPa), the elastic limit E (MPa) at 0.2% elongation, the elongation A (%) were measured at different temperatures. ) and the necking Z (%), after double tempering at 550-550 ° C,

• for a level of total tensile strength of 1700-1800 MPa (table 2), and
• for a level of total tensile strength of 1300-1400 MPa (table 3).
  

For the level of total tensile strength of 1700-1800 MPa, the characteristics of the steels according to the invention are slightly deficient. For the level of 1300-1400 MPa, the differences are blurred.

For the two levels, the values of the characteristics (rapid traction in temperature) are identical from 500/550 ° C, that is to say in the field of industrial exploitation.

It should be noted, moreover, that the mechanical characteristics which are deficient in certain cases, of the steels according to the invention, are however sufficient in terms of tools, where superior characteristics are only rarely required.

Example 2

Bending tests are carried out on steels 1, 2, 3 and 4 by measuring the fracture energy (in joules) on uncracked test pieces - ENC - of the Charpy V type, that is to say on V-notched test pieces, for a level of total tensile strength R = 1300-1400 MPa (42 ± 1 HRC). The results are reported in Table 4 below.

Example 3

We measure, on the same steels as in the previous examples, the fracture energy (in joules) obtained by extrapolation for a crack depth tending towards zero, or zero crack energy - ECO -, by means of test pieces treated with level of 42 ± 1 HRC.

This test is used as a criterion to measure the sensitivity to cracking in the presence of a crack. It can be summarized as follows.

The Charpy V test tube is prefissured at the bottom of the notch and broken after prefissuring on a Charpy V sheep of 300 Joules.

After rupture, one can characterize on the fracture, the initial depth of the fatigue crack and that of the brutal rupture. We also demonstrate that there is proportionality between failure energies and failure surfaces.

The determination of the zero crack energy is done by extrapolating the line measuring the total rupture energy as a function of the depth of pre-cracking from the point (energy 0 / crack depth = 8mm), up to crack depth zero, that is, up to the y axis.

The zero crack energy represents the tear energy; it is always lower than the energy of rupture on conventional uncracked specimen. The difference measures the plastic deformation energy located at the bottom of the notch.

Some test pieces, after treatment for a hardness of 42 HRC, have not undergone aging; some others have been aged for 50 hours at 550 ° C. These tests made it possible to assess the susceptibility to cracking which decreases when one progresses from shade 1 and 2 to shade 3 and finally to shade 4. Their results are recorded in table 5 below. TABLE 5 Meaning LONG CROSS Steel Not aged Aged 550 ° / 50h Not aged Aged 550 ° / 50h 1 17.5 18 15.5 17.5 2 29.5 20 19.5 16 3 69 35 23 25 4 90 60 61 36

In particular, we note that for the test pieces in grades 3 and 4, the values of ENC and ECO are very close (and the ratio is close to 1 for steel 4, which implies that the plastic strain energy localized at notch bottom is weak.

After prolonged maintenance for 50 hours at 550 °, the ECO / ENC ratio is less favorable but the values of ECO, although reduced, are still very high.

Claims (14)

1. A tool steel composition comprising, expressed by weight:

   . 4.5% to 5.8% Cr;

   . 0.75% to 1.75% Mo;

   . not more than 1.3% V

   . not more than 0.8% Si;

   . 0.3% to 0.4% C;

   and where applicable

   . not more than 0.8% Mn; and/or

   . not more than 1.5% W; and/or

   . not more than 0.5% Ni; and

   . the balance being constituted by Fe, and inevitable impurities;

   the concentrations in said composition of P, Sb, Sn, and As, expressed in percentages by weight, satisfying the following relationships:

   . P ≤ 0.008%;

   . Sb ≤ 0.002%;

   . Sn ≤ 0.003%;

   . As ≤ 0.005%;

   . with the value expressed by the Bruscato relationship $${\text{B = (10 P + 5 Sb + 4 Sn + As) x 10}}^{\text{-2}}$$

   

   being no greater than 0.10%.
2. A composition according to claim 1, characterized in that it comprises not more than 0.35% by weight of Si.
3. A composition according to claim 1 or 2, characterized in that it comprises 0.25% to 0.50% by weight of V.
4. A tool steel composition comprising, expressed by weight:

   . 2.5% to 5.5% Cr;

   . 2.5% to 3.5% Mo;

   . not more than 1.3% V

   . not more than 0.35% Si;

   . 0.3% to 0.4% C;

   and where applicable

   . not more than 5% Co;

   . the balance being constituted by Fe, and inevitable impurities;

   the concentrations in said composition of P, Sb, Sn, and As, expressed in percentages by weight, satisfying the following relationships:

   . P ≤ 0.008%;

   . Sb ≤ 0.002%;

   . Sn ≤ 0.003%;

   . As ≤ 0.005%;

   . with the value expressed by the Bruscato relationship $${\text{B = (10 P + 5 Sb + 4 Sn + As) x 10}}^{\text{-2}}$$

   

   being no greater than 0.10%.
5. A composition according to any one of claims 1 to 4, characterized in that it includes 0.32% to 0.38% by weight of C.
6. A composition according to claim 5, characterized in that it includes 0.34% to 0.36% by weight of C.
7. A method of preparing and shaping steel of a composition according to any one of claims 1 to 6, characterized in that it includes remelting by means of a consumable electrode under a vacuum or by means of a consumable electrode under slag, or by both means in combination.
8. A method according to claim 7, characterized in that the steel is shaped by thermomechanical transformation such as forging or rolling, or by molding.
9. A method according to claim 7 or 8, characterized in that it comprises:

   . complete solution heat treatment at temperatures lying in the range 950°C to 1100°C, followed by

   . quenching in air or in a fluid down to ambient temperature, or staged quenching in the range 250°C to 450°C; and then

   . a series of at least two annealings to adjust the intended hardness.
10. A method according to claim 9, characterized in that solution treatment is performed at temperatures lying in the range 980°C to 1010°C.
11. A method according to claim 9 or 10, characterized in that a staged quenching operation is performed in the range 250°C to 280°C.
12. A die for stamping and forging steels and light alloys, the die being constituted by a steel having the composition according to any one of claims 1 to 6.
13. A die for casting under pressure or by gravity steels and light alloys, the die being constituted by a steel having the composition according to any one of claims 1 to 6.
14. A die according to claim 12 or 13, characterized in that it is manufactured by a method according to any one of claims 7 to 11.