WO2000023632A1

WO2000023632A1 - Case hardening structural steel, method for obtaining same and parts formed with same

Info

Publication number: WO2000023632A1
Application number: PCT/FR1999/002368
Authority: WO
Inventors: André GRELLIER
Original assignee: Aubert & Duval
Priority date: 1998-10-20
Filing date: 1999-10-05
Publication date: 2000-04-27
Also published as: EP1042524A1; FR2784692A1; FR2784692B1; BR9916944A; AR020905A1

Abstract

The invention concerns a case hardening structural steel comprising, expressed in wt. %: 0.16 to 0.25 % of C; 0.90 to 1.40 % of Cr; 0.90 to 1.50 % of Mn; 0.30 to 1.10 % of Ni; 0.010 to 0.050 % of Al; and, if required, not more than 0.20 % of Mo; not more than 0.40 % of Si; not more than 0.40 % of Cu; not more than 0.050 % of S; the rest consisting of iron and residual impurities, the value expressed by the following relationship: KC = 8.34 x %C + 0.60 x %Si + %Mn- 1.69 x %S + 0.30 x %Ni + 0.64 x %Cr + 2.55 x %Mo + 0.37 x %Cu ranging between 4.00 and 4.65. The invention also concerns a method for making case hardening and treated parts, in particular toothed meshing gears, produced with said compositions.

Description

Cementable structural steel, process for obtaining it and parts formed therefrom

The present invention relates to a low alloyed and economical cementable structural steel, usable in particular for the manufacture of pinion parts for automobile gearboxes, as well as for other applications such as the manufacture of mechanical transmissions or reducers, for example.

Indeed, these mechanical energy transmission assemblies consist of toothed pinions arranged on shafts, the movement being transmitted from one shaft to another by association of pairs of pinions chosen respectively from one and the other. tree.

The pinions are made of steel with a suitable compromise in properties between toughness and mechanical resistance. The steel must be tough enough not to break during accidental events, or transient regimes leading to shocks or to large instantaneous variations in the force supported.

However, these basic properties are not sufficient for the teeth, the surface of which withstands severe stresses. This is why these teeth undergo a surface treatment by case hardening, followed by a heat treatment of quenching, with or without relaxation income. This gives a resistant and tenacious base material and a toothing surface resistant to contact fatigue.

The steels traditionally used for these applications are distributed between the families chromium-molybdenum (Cr Mo 4, that is to say 1% by weight of chromium), nickel-chromium (Ni Cr 6, that is to say 1.5% by weight of nickel) or manganese- chromium (Mn Cr 5, ie 1.25% by weight of manganese), families which are partially described in standard NFA 03.151 and the draft standard EN 10084. In each family, the steels are staggered according to varying carbon contents from 0.10% to 0.35% by weight, the choice of carbon governing the undercoat resistance of the part for the application targeted. Consequently, the steel user must, in order to manufacture a part according to the desired properties, choose the steel and its mode of implementation in a coherent manner as a function of the quenchability of this material. The hardenability of a given grade of steel, i.e. the hardness that can be expected after quenching as a function of quenching rates, can be determined by means of a test called the Jominy test. This test consists of austenitizing a standard cylindrical test tube, then cooling it with a jet of water which strikes its lower end, from bottom to top. After cooling, hardness measurements are carried out at points located at increasing distances from the cooled end and which are identified by the terms J1, J3, J5, etc.

The Jominy curve obtained using these measurements is characteristic of the steel grade. Other general documents, such as annex A of standard EN 10083-1, indicate the point of the curve

Jominy representative of the application that we consider.

All the steels of the abovementioned families have a Jominy curve with a zone with strong decay, and this all the more since their carbon content is low, which means that, when the representative point of the application crosses this zone, the properties of hardness of the parts undergo a strong variation.

Jominy curves are highly dependent on the composition and even variations of compositions in the normalized bands. The data available in the normative documents are, for critical cases, supplemented by specific specifications agreed between the steelmaker and the user.

In particular, steels of the 25 Cr Mo 4 or 25 Mn Cr 5 family, commonly used for applications with an aiming of an underlying hardness of 450 to 500 HV (Vickers hardness), exhibit a strongly decreasing Jominy curve between the points J5 and J15, which has drawbacks. The first drawback is the strong dependence between the hardness obtained in fine underlayment and the size of the part, which can oblige the user to use different steels for parts of different sizes. The second drawback is the strong dependence between the hardness obtained in fine in the undercoat and the quenching medium used, itself resulting from the industrial means available: quenching in liquid medium (oil), gas quenching under different pressure conditions.

The third drawback is the generation of stresses and deformations on the parts because the metallurgical transformations are not simultaneous between different points of the part, due to the temperature gradients during cooling.

In addition, in the case hardened state, all of these low-alloy steels have poor toughness, which means that in the presence of impacts or violent instantaneous forces, a sudden rupture can occur.

Both tough and tough steels have been developed for more critical applications, for example on aeronautical parts. The 16 Ni Cr Mo 13 grade defined by AIR 9160 C standard, or by standard

NF A 35-565, is representative of this class of more alloyed steels, in particular nickel, and of an inevitably higher price.

The objectives of the present invention are therefore to provide a steel:

- remaining little alloyed in order to maintain a low cost price,

- offering improved toughness compared to reference steels,

- can be manufactured by the industrial means usually available, and

- presenting a more progressive hardenability than the reference steels, hardenability illustrated by a slightly decreasing Jominy curve according to the following table:

To this end, the first object of the invention is a cementable structural steel composition comprising, expressed in% by weight:

0.16 to 0.25% of C,

0.90 to 1.40% Cr,

0.90 to 1.50% Mn,

0.30 to 1.10% Ni,

0.010 to 0.050% of Al, and, where appropriate, at most 0.20% of Mo, at most 0.40% of Si, at most 0.40% of Cu, at most 0.050% of S, the remainder being consisting of iron and impurities due to processing, the quantities of alloying elements being balanced so that the value expressed by the following relationship:

KC = 8.34 x% C + 0.60 x% Si +% Mn - 1.69 x% S + 0.30 x% Ni + 0.64 x% Cr + 2.55 x% Mo + 0.37 x% Cu

is between 4.00 and 4.65, preferably between 4.20 and 4.45, which improves the appearance of the Jominy curve.

The various elements of alloys each fulfill various roles. The carbon fixes the hardness under full quenching and, consequently, the hardness at point J3. Its concentration preferably respects the range 0.20% - 0.24% by weight.

Manganese, silicon, nickel, chromium, carbon contribute positively to the increase in hardenability.

The chromium is preferably adjusted between 1.00% and 1.30% by weight. Molybdenum, added at a concentration of less than 0.20% by weight and preferably between 0.04% and 0.12% by weight, completes the above elements to adjust the quenchability.

The nickel is intentionally added preferably between 0.50% and 0.90% by weight for its favorable effect on the toughness.

Manganese is maintained above 0.90% by weight for its favorable action against contact fatigue, but an excess beyond 1.50% by weight would be detrimental to the toughness. Its concentration is preferably set between 1.10% and 1.40% by weight. Aluminum is added at a level greater than 0.010% by weight to prevent the grain from growing during cementation and quenching. An excess of more than 0.050% by weight would lead to embrittlement and to difficulties in preparing the steel. It is preferably aimed at a concentration of 0.015% to 0.030% by weight. The sulfur can be either kept at a low level, which gives the best mechanical properties, or fixed in the range 0.020% to 0.050% by weight to improve the machinability without unduly handicapping the properties of use. It is preferably aimed at a concentration of 0.02% to 0.040% by weight. Copper is a residual element which contributes to hardenability: it is limited to 0.40% by weight and preferably to 0.25% by weight maximum.

A second object of the invention is a method of manufacturing cemented and treated parts comprising the following operations: a - constitution of a filler intended to obtain a composition in accordance with the present invention, as described above, b - fusion of said charge in an arc furnace, c - heating and hot processing of the ingot, d - heat treatment for homogenization of the structure and refinement of the grain, e - case hardening, f- machining of the parts, and g - treatment thermal use. The case hardening, step e of the process according to the invention, can be carried out using conventional means, the case hardening cycle being to be defined by a person skilled in the art as a function of the desired depth of hardening, in a completely classic. In a preferred embodiment, this step e of carburizing is carried out under reduced pressure between 850 and 900 ° C., in two successive steps followed by a return to ambient temperature. The first step is intended to enrich the surface layer of carbon with carbon, using hydrocarbons, and the second is a step of diffusion under vacuum. In a preferred embodiment, step g of heat treatment of use comprises an austenitization between 800 and 900 ° C, then a quenching between 800 and 900 ° C, followed by tempering between 150 and 200 ° C.

A third object of the invention consists of the case-hardened and treated parts produced with the case-hardening steel according to the invention, and more particularly toothed pinions as described above. These parts can advantageously be manufactured by means of the manufacturing method according to the invention, but also by any other method chosen according to the final application.

The exemplary embodiments of the invention which follow show that the combination of the various elements of the composition according to the invention, in the proportions by weight indicated above, results in a steel meeting the various aims sought.

Examples The symbols used in the following have the following meanings:

R _m = maximum resistance

R _p o, ₂ = conventional elastic limit at 0.2% deformation

A = elongation

Z = necking H V = Vickers hardness

HRC = Rockwell C hardness

KV = breaking energy in impact bending on V-notch test piece. The examples presented below were carried out, with regard to the grade according to the invention, with an industrial casting the composition of which, expressed in% by weight, is as follows:

0.209% C,

1.08% Cr,

1.21% Mn,

0.72% of Ni,

0.029% Al,

0.065% of Mo,

0.359% of Si,

0.105% Cu,

0.028% S, the balance consisting of iron and residual impurities.

Example 1 - Jominy curves

The industrial casting produced according to the above indications made it possible to obtain the following results, on Jominy test specimens soaked at 850 ° C., compared with reference compositions:

Example 2 - Heat treatment

To evaluate the response to the heat treatment in the mass of the parts, bars of respective diameters 20 and 35 mm, having undergone a heat treatment comprising a quenching at 850 ° C in the oil followed by tempering at 180/200 ° C , provided the results below:

These results show that:

the mechanical characteristics obtained are very little dependent on the dimension of the hardened part, this in comparison with the nuances currently known and intended for this application,

the steel which is the subject of the invention exhibits ductility values (necking during the tensile test, fracture energy and toughness) greater than those of reference steels with comparable strength level, this in unexpected proportions given the current state of the art.

Example 3 - Tenacity in the case hardened state

The toughness in the case-hardened state can be measured by rupture in impact bending, using an instrumented pendulum sheep, an appendage in the form of a pinion tooth, machined at the end of a bar with a diameter of 25 mm. .

The maximum force recorded at the level of the pendulum knife then constitutes a significant toughness criterion for the classification of materials.

Three series of test pieces were tested, the first being produced in the shade according to the invention and the other two in reference shades, namely the shades 25 Mn Cr 5 and 16 Ni Cr Mo 13.

The three series of test pieces tested were previously completely cemented under reduced pressure at 880 ° C, for a total effective time of 85 minutes, divided into a first sequence of 55 minutes of carbon enrichment by a hydrocarbon, followed by a second sequence of 30 minutes of diffusion under vacuum, then return to ambient temperature. The test pieces produced in the 25 Mn Cr 5 and

16 Ni Cr Mo 13 were then subjected to a conventional heat treatment, while the test pieces produced in the grade according to the invention were austenitized at 850 ° C, quenched in oil and then returned to 180 ° C for 2 hours.

The results obtained are as follows, for an average of six measurements:

The toughness of the steel according to the invention is consequently higher than that of the reference steel 25 Mn Cr 5, and is situated at a level very close to that of the steel 16 Ni Cr Mo 13, which is much richer in elements of alloys. The steel compositions according to the invention are more particularly intended for the manufacture of toothed pinions, but can naturally be used for all the parts subjected to similar stresses.

It also goes without saying that the embodiments of the various objects of the invention which have been described above have been given for purely indicative and in no way limitative, and that numerous modifications can be easily made by those skilled in the art. art without departing from the scope of the invention.

Claims

1. Composition of case-hardened structural steel comprising, expressed in% by weight,

0.16 to 0.25% of C,

0.90 to 1.40% Cr, 0.90 to 1.50% Mn,

0.30 to 1.10% Ni,

0.010 to 0.050% Al, and, where applicable, at most 0.20% Mo, at most 0.40% Si, at most 0.40% Cu, at most 0.050% S, the balance being consisting of iron and residual impurities, the value expressed by the following relationship:

being between 4.00 and 4.65.

2. Composition of tool steels according to claim 1, characterized in that the value of KC is between 4.20 and 4.45.

3. A cemented structural steel composition according to claim 1, characterized in that it comprises from 0.20 to 0.24% by weight of carbon.

4. A cemented structural steel composition according to claim 1, characterized in that it comprises from 1.00 to 1.30% by weight of chromium.

5. A cementable structural steel composition according to claim 1, characterized in that it comprises from 0.04 to 0.12% by weight of molybdenum.

6. Composition of cementable structural steel according to claim 1, characterized in that it comprises from 0.50 to 0.90% by weight of nickel.

7. A cemented structural steel composition according to claim 1, characterized in that it comprises from 1.10 to 1.40% by weight of manganese.

8. Composition of cementable structural steel according to claim 1, characterized in that it comprises from 0.015 to 0.030% by weight of aluminum.

9. A cemented structural steel composition according to claim 1, characterized in that it comprises at most 0.25% by weight of copper.

10. A cemented structural steel composition according to claim 1, characterized in that it comprises from 0.02 to 0.04% by weight of sulfur.

11. Method for manufacturing cemented and treated parts, comprising the following operations: a - constitution of a charge intended to obtain a chemical composition according to any one of claims 1 to 10, b - preparation of said charge in a arc, c - reheating and hot transformation of the ingot, d - heat treatment for homogenization of the structure and refinement of the grain, e - case hardening, f - machining of parts and g - heat treatment for use.

12. A method of manufacturing case-hardened and treated parts according to claim 11, characterized in that the case-hardening (step e) is carried out under reduced pressure between 850 and 900 ° C in two successive steps followed by a return to ambient temperature and in that the heat treatment for use (step g) comprises an austenitization between 800 and 900 ° C, then a quenching between 800 and 900 ° C, followed by tempering between 150 and 200 ° C.

13. Steel parts, in particular toothed pinions, having a composition according to any one of claims 1 to 10.

14. Steel parts, in particular toothed pinions, obtained by the process according to any one of claims 11 or 12.