EP0638658A1 - Nitrogen-containing martensilic steel with low carbon content and process for its manufacture - Google Patents

Abstract

Martensitic stainless steel including: a - from 0.5 to 0.85 % by weight of carbon + nitrogen b - from 0.15 to 0.24 % by weight of nitrogen c - from 0 to 0.50 % by weight of silicon d - from 0 to 0.70 % by weight of manganese e - from 0 to 0.50 % by weight of nickel f - from 14 to 17 % by weight of chromium g - from 1 to 2.5 % by weight of molybdenum h - from 0.10 to 0.70 % by weight of vanadium, the remaining fraction consisting of iron and of residues and the respective contents of each of these elements being determined so as to restrict the residual austenite content to a value lower than 10 % by volume.

Description

The present invention relates to a low-carbon nitrogen nitrogen martensitic stainless steel and its manufacturing process.

There are already steels of the type (1% by weight of carbon - 17% by weight of chromium and 0.5% by weight of molybdenum) which are stainless and traditionally used for applications in tooling or for making mechanical parts. subject to corrosive conditions and / or relatively severe mechanical stresses.

However, such steels do not have sufficient resistance to corrosion, in particular with respect to pitting, to satisfy certain applications. In addition, their structure, which is too rich in coarse carbides, does not allow the required toughness to be obtained for certain parts subjected to impacts, or the obtaining of a good polishability.

On the other hand, this steel, after tempering at 500/525 ° C which are necessary to undergo several types of thermochemical and / or thermophysical deposits, no longer makes it possible to obtain sufficient hardness and sees its resistance to corrosion at temperature room further decreased.

• a - de 0,5 à 0,85% en poids de carbone + azote
• b - de 0,15 à 0,24% en poids d'azote
• c - de 0 à 0,50% en poids de silicium
• d - de 0 à 0,70% en poids de manganèse
• e - de 0 à 0,50% en poids de nickel
• f - de 14 à 17% en poids de chrome
• g - de 1 à 2,5% en poids de molybdène
• h - de 0,10 à 0,70% en poids de vanadium

la fraction restante étant constituée de fer et de résidus et les teneurs respectives de chacun de ces éléments étant déterminées de façon à limiter la teneur en austénite résiduelle à une valeur inférieure à 10% en volume.The invention aims to satisfactorily solve the technical problems of eliminating the above drawbacks. This object is achieved in accordance with the invention by a martensitic stainless steel comprising:

• a - from 0.5 to 0.85% by weight of carbon + nitrogen
• b - from 0.15 to 0.24% by weight of nitrogen
• c - from 0 to 0.50% by weight of silicon
• d - from 0 to 0.70% by weight of manganese
• e - from 0 to 0.50% by weight of nickel
• f - from 14 to 17% by weight of chromium
• g - from 1 to 2.5% by weight of molybdenum
• h - from 0.10 to 0.70% by weight of vanadium

the remaining fraction consisting of iron and residues and the respective contents of each of these elements being determined so as to limit the residual austenite content to a value of less than 10% by volume.

Thanks to the invention, a martensitic stainless steel of high purity nitrogen is obtained, having a high hardness (HRC ≧ 58 after heat treatment) and having a fine structure (grade ES 2299 W). This steel also guarantees good reproducibility of the results in response to heat treatments as well as great structural and, consequently, dimensional stability, which makes this steel more particularly suitable for the manufacture of bearings.

According to an advantageous characteristic, the nitrogen content is between 0.18 and 0.23% by weight.

According to another characteristic, the carbon content is between 0.30 and 0.60% by weight.

Preferably, the carbon + nitrogen content is between 0.55 and 0.80% by weight.

According to yet another characteristic, the Rockwell C hardness of the steel of the invention is greater than or equal to 58 HRC after heat treatment.

Another object of the invention is a process for manufacturing a martensitic stainless steel with nitrogen and low carbon content, comprising in particular steps of preparing a metal bath, refining this steel bath to the liquid state, of casting the steel into ingots and of thermomechanical transformation of the solidified ingots, characterized in that the carbon content in the liquid steel is lowered to values between 0.30 and 0, 60% of the total weight of the steel and an amount of nitrogen is added to it so that the solubility limit of the latter is not exceeded in liquid steel.

According to an advantageous characteristic, the total carbon and nitrogen content is adjusted to a value between 0.50 and 0.85% of the weight of the steel.

According to a particular embodiment, nitrogen is introduced in gaseous form into the liquid steel by bubbling.

According to another mode of implementation, nitrogen is introduced in solid form by nitrided charges.

According to yet another characteristic, the partial pressure of nitrogen exerted above the liquid steel is maintained at a value less than or equal to 1 bar.

According to a first variant, a heat treatment is carried out after thermomechanical transformation comprising a step of heating prior to an austenitization temperature of between 1020 ° C. and 1070 ° C. followed by quenching and then a tempering cycle between 130 ° C. and 180 ° C for 2 hours.

According to another variant, after thermomechanical transformation, a heat treatment of the ingots is carried out comprising a preliminary heating step at an austenitization temperature of between 1070 ° C. and 1120 ° C. followed by quenching and then a double tempering cycle. between 500 ° C and 525 ° C for 2 hours.

According to yet another variant, a cryogenic treatment is carried out at -80 ° C. after quenching and before tempering.

Yet another object of the invention lies in the use of the above steel to produce mechanical parts, ball bearings, surgical tools, or intended for the chemical, food industries for the transformation of plastics.

The steel of the invention has a low concentration of coarse carbides and also offers the possibility of being treated with tempering temperatures up to 500 ° C. in order to have a hardness greater than or equal to 58 HRC in order to allow the performing thermochemical and / or thermophysical surface treatments and use in mechanical elements working up to around 450 ° C.

In order to obtain a structure poor in coarse eutectic carbides, it is necessary to limit the carbon content. On the other hand, to reach the objective of hardness greater than or equal to 58 HRC, it is possible to harden the steel matrix by adding nitrogen. However, the importance of the latter is conditioned by the limit of solubility of this element in the metal bath resulting from an elaboration which does not involve partial nitrogen pressures greater than 1 bar (both at the level of the phase of fusion only from the refining phase). A carbon + nitrogen value of 0.50-0.85% by weight was chosen, favoring a high nitrogen content (0.15-0.24%).

To obtain a high potential for resistance to corrosion, particularly with regard to pitting, the chromium content was fixed between 14 and 17% by weight and the molybdenum at a value of 1-2.5% by weight. . Their action is reinforced by the presence of nitrogen.

The purpose of adding vanadium to less than 1% by weight is to limit softening during tempering and to promote the secondary hardening necessary for obtaining high hardness when this treatment is carried out at around 500 ° C.

Finally, the balance between all of the constituents has been determined so as to limit the residual austenite content, to a value of less than 10% by volume in order to respond to the concerns of manufacturers, in particular of bearings, to guarantee very good repeatability of response to heat treatments as well as great structural and therefore dimensional stability.

The composition of the steel of the invention is thus adjusted within the following limits (the% being by weight): Carbon 0.30-0.60% Silicon ≦ 0.50% Manganese ≦ 0.70% Nickel ≦ 0.50% Chromium 14-17% Molybdenum 1.00-2.50% Vandadium 0.10-0.70% or better 0.20-0.60% Nitrogen 0.15-0.24% or better 0.18-0.23% Carbon + nitrogen 0.50-0.85% or better 0.55-0.80%

The balance is made up of iron and conventional residues linked to the method of production.

The invention will be better understood on reading the description which follows, accompanied by the appended photographs representing the micrographic structure of the steel of the invention after treatment at magnification × 100 (FIG. 1) and at magnification × 500 (FIG. 2).

FIG. 3 symbolizes an ingot transformed into a billet with the location of the five sampling zones used for the chemical analyzes, the results of which appear in Tables 1 and 2.

Two 1 t castings were prepared, the nominal compositions of which are as follows:

Preparation in an electric oven and remelting under slag using a consumable electrode (ESR Electro Slag Remelting) took place under controlled conditions. The ingots obtained are healthy, without shrinkage or blowing.

• passage dans un four à arc puis dans un convertisseur A.O.D (Argon Oxygène Decarburizing).
• passage dans un four à arc puis dans un convertisseur A.O.D. et refusion sous laitier au four à électrode consommable
• passage dans un four à induction puis refusion sous laitier au four à électrode consommable.

The manufacturing methods include at least one of the following cycles:

• passage in an arc furnace then in an AOD (Argon Oxygen Decarburizing) converter.
• passage in an arc furnace then in an AOD converter and remelting under slag in the furnace with consumable electrode
• passage in an induction furnace then reflow under slag in the furnace with a consumable electrode.

The addition of nitrogen is carried out directly in the bath constituted by the steel in the liquid state, by conventional means of the bubbling type for the introduction in gaseous form or of the nitrided charges type for the solid form.

The hot transformation operations were carried out according to processes identical to those usually carried out with steels of the same family.

By forging and rolling operation, the ingots were transformed into 150 x 135 mm billets then into bars of diameters 90 mm, 24 mm and 14 mm.

Annealing treatments are then carried out on all diameters, according to the ranges provided.

The manufacturing controls are then carried out as described below.

a- Chemical composition

The chemical composition is checked on the billet from samples representative of various zones of the initial ingot (see Tables 1 and 2 and Figure 3).

Between the head area T and the foot area P are the intermediate areas MT, M and MP.

For the two flows, it can be seen that the values obtained for nitrogen and nitrogen + carbon are stable and in accordance with the targeted objectives, including in the case of flow B having a particularly high carbon + nitrogen value.

b- Internal health check by ultrasound

The ultrasonic checks on a 150 x 135 mm billet, 90 mm in diameter, did not detect indications equivalent to or greater than the 1.2 mm flat-bottom hole taken as a reference.

c- Macrography

All the macrographies carried out on 90 mm diameter billets and bars have revealed after attack (hot HCl) a homogeneous structure whatever the position of the sample relative to the ingot. There is no crack, no blowing, no segregation.

d- Hardnesses

In the annealed condition, the hardness noted on the diameters of 90 mm, 24 mm and 14 mm is uniform.

The values obtained are:
HV₅₀ 192 - 212 for casting A,
HV₅₀ 207 - 229 for casting B.

On each of the two flows A and B we tested the influence of the austenitization temperature and tempering on the hardness and the residual austenite content.

Austenitization has always been followed by oil quenching and cryogenic treatment at -75 / -80 ° C before tempering, with the aim of eliminating the residual austenite as much as possible. Voluntarily studying tempering temperatures has been limited to two reduced temperature zones corresponding to two areas of steel use:

Single cycle tempering 130-180 ° C / 2 h : this tempering meets the usual practice on steels of the same family and is suitable in the case of use at temperatures below 120 ° C.

Double cycle tempering of 500-525 ° C / 2 h : this tempering allows considering the use of steel up to service temperatures of 450 ° C and makes it possible to carry out thermochemical and / or thermophysical treatments requiring processing temperatures up to 500 ° C.

These tests made it possible to obtain simultaneously, for the two castings (A and B), a hardness at least equal to 58 HRC and a residual austenite content of less than 10%, provided that, for each type of income, a suitable austenitization temperature:
1020-1070 ° C in the case of the 130-180 ° C tempering cycle
1070-1120 ° C in the case of the tempering cycle at 500-525 ° C.

These values are explained by the fact that the final hardness as well as the residual austenite value are very strongly conditioned by the importance of the quantity of carbon dissolved in the austenitization, on the one hand, and by the possibility thermal decomposition of austenite during the income cycles on the other hand. These considerations have led to limiting the carbon content to 0.60% by weight.

Regarding the structure, the size of the carbides observed on the two ends of the 90 mm diameter bars reaches 35 µm for the coarsest, in the case of casting A and 40 µm in the case of casting B.

For this latter casting, a higher density of these carbides is observed, but their distribution is more homogeneous than in the case of casting A. In all cases, many other globular carbides of size ≦ 5 μm are uniformly distributed in the matrix . For the two castings tested, the structure is much finer than that observed on "1% carbon - 17% chromium - 0.5% molybdenum" steel, with a less "angular" appearance for larger carbides.

FIGS. 1 and 2 represent the micrographic structures of the steel of casting B, respectively at x100 and x500 magnification. This structure was photographed at mid-ray after heat treatment first comprising a step at 1050 ° C for 30 min in oil, then at -80 ° C for 2 h and finally at 180 ° C for 2 h at 1 'air.

The toughness was determined on the steel of casting B, having the least favorable structure, after treatment with double tempering at 500-525 ° C. The values of K1C (critical stress coefficient) obtained are as follows: 22 to 25 MPa.m ^1/2 . For a hardness level of 58/60 HRC this result is equivalent to that of non-stainless steel 0.8% carbon - 4% molybdenum - 4% chromium - 1% vanadium, widely used in aeronautics and higher than that of l '' reference stainless steel 1% carbon - 17% chromium - 0.5% molybdenum, for which the literature gives an average value of K1C of 15 MPa.m ^1/2 .

le plus faible, donc le moins favorable.Corrosion tests were carried out on casting B, the composition of which presents the report $$\text{R =}\frac{\text{\% chrome}}{\text{\% carbon +\% nitrogen}}$$

the weakest, therefore the least favorable.

The hardness HRC ≧ 58 was obtained by heat treatment with tempering at 130-180 ° C. The tests were carried out in parallel under the same conditions with the reference steel 1% carbon - 17% chromium - 0.5% molybdenum treated for HRC ≧ 58 by quenching and tempering at 150 ° C.

The results obtained are as follows:

Salt spray corrosion tests according to the conditions described in standard NFX 41 002 (which provides for 11 levels of corrosion identified from 0 to 10):

After 95 h of exposure.

Reference steel a few scattered pits (without rings of corrosion products - level 9).

Casting B : no trace of corrosion (level 10)

After 145 h of exposure

Reference steel : pitting with some "spots" of corrosion products around the pitting (level 8).

Casting B : no trace of corrosion (level 10).

Potentiokinetic electrochemical test in a non-chlorinated medium. This test takes place in an aqueous solution of sulfuric acid at 1% by weight, deaerated by bubbling argon. The test piece of the steel to be tested is polarized for 15 min at 550 mV relative to the potential of the saturated calomel electrode (E.C.S.). Then there is a back and forth potentiokinetic scan between -550 and +500 mV / DHW with a scanning speed of 60 mV / min.

The criterion used to characterize the corrosion resistance is the value of the current density at the reactivation peak of the return curve. The results obtained are as follows:
Reference steel : I = 520 µA / cm²
Casting B : I ≦ 20 µA / cm² (i.e. at the measurement threshold).

These two tests clearly show the better corrosion resistance of casting B compared to the reference stainless steel 1% carbon - 17% chromium - 0.5% molybdenum, treated for an equivalent hardness level.

Claims (15)

Martensitic stainless steel including: a - from 0.5 to 0.85% by weight of carbon + nitrogen b - from 0.15 to 0.24% by weight of nitrogen c - from 0 to 0.50% by weight of silicon d - from 0 to 0.70% by weight of manganese e - from 0 to 0.50% by weight of nickel f - from 14 to 17% by weight of chromium g - from 1 to 2.5% by weight of molybdenum h - from 0.10 to 0.70% by weight of vanadium the remaining fraction consisting of iron and residues and the respective contents of each of these elements being determined so as to limit the residual austenite content to a value less than 10% by volume and to obtain a Rockwell C hardness greater than or equal to 58 HRC after heat treatment. Steel according to claim 1, characterized in that the nitrogen content is between 0.18 and 0.23% by weight. Steel according to one of the preceding claims, characterized in that the carbon content is between 0.30 and 0.60% by weight. Steel according to one of the preceding claims, characterized in that the carbon + nitrogen content is between 0.55 and 0.80% by weight. Steel according to one of the preceding claims, characterized in that the size of the carbides is less than or equal to 40 µm. A method of manufacturing a martensitic stainless steel with nitrogen and low carbon content comprising in particular steps of preparing a metal bath, refining this steel bath in the liquid state, pouring the steel in ingots and thermomechanical transformation of solidified ingots, characterized in that the carbon content in the liquid steel is lowered to values between 0.30 and 0.60% of the total weight of the steel and an amount of nitrogen is added to it such that the solubility limit thereof is not exceeded in the liquid steel. Process according to claim 6, characterized in that the total content of carbon and nitrogen is adjusted to a value between 0.50 and 0.85% of the weight of the steel. Method according to claim 6 or 7, characterized in that the residual austenity content is limited to a value less than 10% by volume. Method according to one of claims 6 to 8, characterized in that nitrogen is introduced in gaseous form into the liquid steel by bubbling. Method according to one of claims 8 to 8, characterized in that nitrogen is introduced in solid form by nitrided charges. Method according to one of claims 6 to 10, characterized in that the partial pressure of nitrogen above the liquid steel is maintained at a value less than or equal to 1 bar. Method according to one of Claims 6 to 11, characterized in that, after thermomechanical transformation, a heat treatment is carried out comprising a heating step prior to an austenitization temperature, between 1020 ° C and 1070 ° C followed by quenching and then an annealing cycle between 130 ° C and 180 ° C for 2 hours. Method according to one of claims 6 to 11, characterized in that a heat treatment is carried out after thermomechanical transformation comprising a step of heating prior to an austenitization temperature between 1070 ° C and 1120 ° C followed by quenching then a double tempering cycle between 500 ° C and 525 ° C for 2 hours. Method according to one of claims 12 or 13, characterized in that one performs, after quenching and before tempering, a cryogenic treatment at -80 ° C. Use of a steel according to one of claims 1 to 5, for producing mechanical parts, ball bearings, surgical tools, or intended for the chemical and food industries for the transformation of plastics.

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