EP0663018A1

EP0663018A1 - Tool steel compositions.

Info

Publication number: EP0663018A1
Application number: EP93921994A
Authority: EP
Inventors: Jean Bourrat
Original assignee: Aubert and Duval SA
Current assignee: Industrielle De Metallurgie Avancee Sim Ste
Priority date: 1992-10-09
Filing date: 1993-10-05
Publication date: 1995-07-19
Anticipated expiration: 2013-10-05
Also published as: CA2144654A1; WO1994009170A1; ATE153709T1; FR2696757B1; DE69311125D1; ES2104178T3; EP0663018B1; US5622674A; FR2696757A1; DE69311125T2

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

TOOL STEEL COMPOSITIONS

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.

Such steels generally contain from 3 to 5% by weight of chromium, although contents of 2 to 6% can be observed. More precisely, 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 casings of gearboxes, clutch or engine. Some of these steels are designated in the United States of America AISI nomenclature by the names H 11, H 12, H 13, in the DIN nomenclature by the names 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

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 now 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 hydrocarbons M ₂₃ C ₆ (M = Cr, Fe, Mo, ...) during subsequent heat treatments. The Applicant has, on the other hand, been able to show that certain areas of coarser appearance, sometimes needle-like with a bainitic appearance, especially on products of large cross 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.

The first type of tool steel composition according to the invention 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 applicable,

at most 0.8 of Mn, and / or at most 1.5% of W,

the balance consisting of Fe, usual additives and impurities,

Hi possibly constituting an impurity, at a rate of 0.5% at most.

The second type of tool steel composition according to the invention 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 lerritic zones. rich in silicon, as well as primary carbons, the solution of which 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. -to say in the field of forging (600-650º0. On the other hand, the homogeneity of macrostructure (less marked band structure) and microstructure are guarantors of good behavior in service, ie good characteristics with regard to toughness, mechanical fatigue, thermal fatigue.

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.

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

B = (10 P + 5 Sb + 4 Sn + As) × 10- ²

being at most equal to 0.10%.

A second main object of the present invention 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 fαrgeage 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 1100ºC, preferably between 980 and 1010ºC, then

- air quenching in a fluid up to room temperature, or stepped quenching between 250 and 450ºC, preferably between 250 and 280ºC, then

- a series of at least two incomes to adjust the target hardness.

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

At least two incomes are recommended, the first at the peak of secondary hardening (550 / 560º0, the second in the area of aging or "overaging", that is to say at a temperature greater than or equal to 570ºC. the adjustment of the temperature of the second tempering which gives hardness to the treated product.

A third object of the invention is a piece of steel of composition previously described, preferably manufactured according to a method also described above.

Such parts are especially intended, suitably, 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 and 4 according to the invention, whose compositions in% by weight are recorded in table 1 below, were compared with those of steels 1 and 2 representative of the prior art, the steel 1 being steel DIN W-1.2343, steel 2 being steel 1 having undergone reflow

Example 1

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 necking

Z (%), after double income at 550-560ºC,

- 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 / 550ºC, 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 breaking energy (in joules) on uncracked test pieces - ENC - of the Charpy V type, that is to say on V-notch 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

The fracture energy (in joules) obtained by extrapolation for a crack depth tending towards zero, or zero crack energy - ECO -, is measured on the same steels as in the previous examples, using 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 piece is pre-cracked at the bottom of the notch and broken after pre-cracking on a Charpy V sheep of 30 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 less than the breaking energy on a conventional uncracked test tube. 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 given in table 5 below.

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 550º, the ECO / ENC ratio is less favorable but the values of ECO, although reduced, are still very high.

Claims

1.- Composition of tool steel comprising, expressed by weight,

4.5 to 5.8% Cr,

0.75 to 1.75% of Mo,

at most 1.3% of V,

at most 0.8% of Si,

0.3 to 0.4% of C,

the balance consisting of Fe, usual additives and impurities.

2.- Composition according to claim 1, characterized in that it comprises at most 0.35% by weight of Si.

3.- Composition according to claim 1 or 2, characterized in that it further comprises, expressed by weight,

- at most 0.8% of Mn, and / or

- at most 1.5% of W.

4.- Composition according to any one of claims 1 to 3, characterized in that it comprises

0.25 to 50% by weight of V.

5.- Composition according to any one of claims 1 to 4, characterized in that Ni, constituting an impurity, is present in an amount of 0.5% by weight at most.

6.- Composition of tool steel including, expressed by weight - 2.5 to 5.5% of 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,

- the remainder consisting of Fe, usual additives and impurities,

the contents of this composition in P, Sb, Sn and As, expressed in% by weight, satisfying the relationships

- P ≤ 0.008%,

- Sb ≤ 0.002%,

- Sn ≤ 0.003%,

- As ≤ 0.005%,

- the value expressed by the Bruscato relation

B = (10 P + 5 Sb + 4 Sn + As) × 10 ^-2

being at most equal to 0.10%.

7.- Composition according to claim 6, characterized in that it further comprises at most 5% by weight of Co.

8.- Composition according to any one of claims 1 to 7, characterized in that it comprises 0.32 to 0.38% by weight of C.

9.- Composition according to claim 8, characterized in that it comprises 0.34 to 0.36% by weight of C.

10.- Composition according to any one of claims 1 to 5, characterized in that its contents of P, Sb, Sn and As, expressed in% by weight, satisfy the relationships

- P ≤ 0.008%,

- Sb ≤ 0.002%,

- Sn ≤ 0.003%,

- As ≤ 0.005%,

- the value expressed by the Bruscato relation

B = (10 P + 5 Sb + 4 Sn + As) × 11 ^-2

being at most equal to 0.10%.

11.- A method of preparing and shaping steel composition according to any one of claims 1 to 10, characterized in that it comprises a remelting by electrode consumable under vacuum or by electrode consumable under slag or by these two means combined.

12.- Method according to claim 11, characterized in that the shaping of the steel is carried out by thermomechanical transformation such as forging or rolling, or by molding.

13.- Method according to claim 11 or 12, characterized in that it comprises

a complete heat treatment by dissolving at temperatures between 950 and 1100ºC, then

- quenching in air or in a fluid up to room temperature, or stepped quenching between 250 and 450ºC, then

- a series of at least two incomes to adjust the target hardness.

14.- Method according to claim 13, characterized in that the dissolution is carried out at temperatures between 980 and 1010ºC.

15.- Method according to claim 13 or 14, characterized in that a stepped quenching is carried out between 250 and 280ºC.

16. A piece of steel of composition according to any one of claims 1 to 10.

17.- Steel part according to claim 16, characterized in that it is manufactured by a process according to any one of claims 11 to 15.