EP2470681B1 - Stainless steel martensitic steel - Google Patents

Abstract

The invention relates to stainless, martensitic chromium steel comprising 0.40 to 0.80% of carbon, 0.20 to 1.50% of silicon, 0.15 to 1.00% of nickel, 0.30 to 1.00% of manganese, 0.015 to 0.035% of sulphur, 16 to 18% of chromium, 1.25 to 1.50% of molybdenum, at most 0.8% of tungsten, 0.04 to 0.08% of nitrogen, 0.15 to 0.20% of vanadium, up to 0.05% each of titanium and niobium, 0.001 to 0.03% of aluminium, 0.02 to 0.5% of copper, no more than 0.5% of cobalt and no more than 0.004% of boron, the remainder being iron, including impurities resulting from the steel-making process, said steel being especially suited as a corrosion-resistant, in particular pitting-resistant material for articles subject to frictional wear due to the wettability of the steel with lubricants.

Description

The invention relates to a stainless martensitic chromium steel and its use.

Such steels are known in large numbers and are suitable depending on their composition for a very different range of uses.

For example, the German patent specification describes 100 27 049 B4 a martensitic chromium steel with 0.4 to 0.75% carbon, up to 0.7% silicon, up to 0.2% nickel, 0.4 to 1.6% manganese, 0.02 to 0.15% sulfur, 12 to 19% chromium, 0.5 to 1.5% molybdenum, up to 1.5% tungsten, up to 0.1% nitrogen and 0.05 to 0.3% vanadium, titanium and niobium singly or side by side and up to 0.008% boron. This steel has good processability, corrosion resistance and low plastic deformability and high wear and abrasion resistance; it is therefore suitable without a galvanic coating as a material for industrial needles and in particular allows a high sewing speed.

However, the material is not very suitable for use, the characteristic feature of which is a sliding contact metal / metal in the presence of a lubricant film. This applies in particular to components that are fuel-based, in particular biofuels, where it is very important in addition to other material properties on a good film formation or adhesion, their life therefore of the material abrasion in a metal / metal frictional contact as in the case Of valve and dispensing needles and scraper rings of compressors is very important.

However, such a frictional wear reducing film does not require a lubricant such as oil and grease in all uses. higher molecular weight hydrocarbon compounds, but the parts may also be lubricated by the resource itself, such as fuel, as in injection systems or oil control rings. The decisive factor is always the emergence of a deterrent film. In any case, the practice uses a number of partly expensive, partly ecologically questionable additives such as EP additives, detergents, HD additives, lead compounds and chlorinated diphenyls for influencing, in particular for stabilizing and fixing the wear-inhibiting layer.

Crucial here is especially in the case of systems for conveying or compressing fuels or in piston rings and wiper seals and dosing or valve needles for chemical or pharmaceutical mixtures, the stability and the adhesion of the wear-reducing surface layer. However, stabilization is not possible in various systems, such as with new methanol and ethanol containing fuels.

In addition, in many cases, the legislature prohibits the use of excipients and additives such as lead-containing compounds for environmental reasons.

Extensive research has shown that the abrasion of martensitic chromium steels is strongly influenced by the wettability of the surface. Such a steel is subject to increased wear even at high strength, which can be attributed to insufficient wetting of the steel surface by the lubricant. Under the action of force, local displacement of the lubricant can occur, and this danger is particularly great where microscopically small surface elevations are under a correspondingly high specific pressure. The reason for this is a displacement of the lubricant molecules at such elevations and tips, which are separated and torn as a result of the dynamic stress. The consequence of such adhesive wear is an increased roughness of the metal surface and thereby in turn increased wear.

The invention is therefore based on the problem of finding a stainless martensitic chromium steel, which has better wettability due to its chemical affinity and strong adhesion forces and forms a stable lubricant film, which is much more difficult to disturb or displace than conventional steels of this type and consequently causes less wear.

The solution to this problem consists in a martensitic chromium steel with 0.40 up to 0.80% carbon 0.20 up to 1.50% silicon 0.15 up to 1.00% nickel 0.30 up to 1.00% manganese 0,015 up to 0.035% sulfur 16 up to 18% Chrome, 1.25 up to 1.50% molybdenum up to 0.8% tungsten 0.04 up to 0.08% nitrogen 0.15 up to 0.20% vanadium, up to 0.05% titanium up to 0.05% niobium 0.001 up to 0.03% aluminum 0.02 up to 0.5% copper up to 0.5% cobalt up to 0.004% Boron, Remaining iron, including impurities caused by melting.

Preferably, the steel contains each individually or side by side 0.55 up to 0.75% Carbon, up to 0.65% Silicon, up to 0.8% manganese at least 0.001% Tungsten.

Practice has hitherto been limited to dry wear tests, i. oriented on the results of lubricant-free tests. It has therefore been found that the frictional wear in a contact was also significant using a lubricant.

The carbon is austenite and therefore stabilizes the austenitic crystal lattice. At the same time, however, carbon, together with the carbide formers, also contributes to carbide precipitations, which increase the hardness and abrasion resistance of the steel. In order to avoid a coarse and line-like separation of chromium carbides, it is recommended to pre-precipitate other carbides in austenitic Condition that cause better carbide homogeneity. The steel therefore contains 0.40 to 0.80%, preferably 0.55 to 0.75% carbon.

Silicon serves as a deoxidizer. However, higher contents can lead to the formation of intermetallic phases. On the other hand, silicon is also a ferrite former. The steel therefore contains 0.2 to 1.5% silicon, preferably up to 0.65% silicon.

Nickel belongs to the austenite blidners, but the austenite part in the microstructure carries the risk of a deterioration of the wear properties. On the other hand, nickel is also an advantage as part of the crystal lattice with its influence on the c / a ratio of the martensite. The steel therefore contains 0.15 to 1% nickel.

Manganese stabilizes the austenite and advantageously shifts the martensite formation to lower temperatures. The maximum level of manganese is therefore 1%, but a minimum content of 0.30% should not be undercut, because manganese simultaneously alters the c / a ratio of the newly formed martensite and advantageously influences the precipitation behavior of the fine precipitates in the austenitic region.

The sulfur content is limited to a maximum of 0.035%, since at higher sulfur levels, troublesome sulfidic precipitates may occur.

Chromium is required to ensure the corrosion resistance of the steel in combination with its molybdenum content of 1.25 to 1.50%, in particular adequate resistance to pitting corrosion. The chromium content is therefore at least 16%. However, in view of the ferritizing effect of chromium, its content is limited to 18%. The synergistic effect of chromium and molybdenum on pitting resistance is particularly ensured when the contents of chromium, molybdenum and tungsten satisfy the following equation: $$\left(\%\mathrm{Cr}\right)+\mathrm{3}\left(\%\mathrm{Not\; a\; word}\right)+\left(\%\mathrm{W}\right)=\mathrm{19}.\mathrm{7}\mathrm{to}\mathrm{23}.\mathrm{Third}$$

The steel contains from 0.001 to 0.8% tungsten, preferably at least 0.001% tungsten, which together with the iron and molybdenum forms mixed carbides, which contribute significantly to the hot strength of the steel and lead to secondary carbide secondary precipitation during tempering.

The nitrogen together with the carbon forms carbonitrides, but also worsens the wettability of the steel to hydrocarbon lubricants; the upper content limit for nitrogen is therefore 0.08%.

The steel contains vanadium, niobium and titanium as carbide formers with the advantage that they form nucleation nuclei for the formation of chromium carbides even at very high temperatures in view of their high affinity for carbon. The vanadium content is therefore 0.15 to 0.20% at titanium and niobium contents of up to 0.05%. The following effective sum of the carbide formers is particularly advantageous: $$\begin{array}{ll}\mathrm{K}\mathrm{1}& =\left(\%\mathrm{Nb}\right)+\left(\%\mathrm{Ti}\right)+\left(\%\mathrm{V}\right)\\ & =\mathrm{0}.\mathrm{15}\mathrm{to}\mathrm{0}.\mathrm{25th}\end{array}$$

The steel also contains 0.001 to 0.03% aluminum as a deoxidizer, but not more, because higher aluminum contents have an embrittling effect.

The maximum copper content is 0.5% and, especially during tempering, leads to fine-grained secondary precipitations, which together with other precipitates improve the wettability of the steel for oils or hydrocarbons.

Finally, cobalt promotes the formation of ε-carbides and other fine precipitates; it improves the hot strength of the steel in this way. For cost reasons, however, a cobalt content of 0.5% should not be exceeded.

Since the decisive for the material properties of chromium carbides on cooling under the influence of crystallization nuclei form 90% in the temperature range of 1100 to 900 ° C, the cooling rate should not exceed 50 ° C / sec (heat treatment A), so as not to affect the carbide formation and to prevent part of the reactants from remaining metastable at interstitial sites when the carbides are formed. Upon further cooling, martensite spontaneously forms below the MS temperature from the cubic body-centered crystal lattice, with the result that the previously formed carbides are incorporated in the martensite, but the stress state of the matrix is lower, the more finely distributed the carbides are. This condition greatly enhances the lubricant or oil wetting of the surface of the steel.

The martensite from the austenite transformation has a tetragonal distorted crystal lattice with a ratio of the crystal axes a / c over 1. The martensite formed during the conversion can be influenced by a heat treatment in the temperature range below 550 ° C. following martensite formation in such a way that the martensite Crystal axis ratio s / c reduced, which has an extremely beneficial effect on the material properties. This advantage arises in particular if the total content of niobium, titanium and vanadium satisfies the following condition: $$\begin{array}{ll}{\mathrm{K}}_1& =\left(\%\mathrm{Nb}\right)+\left(\%\mathrm{Ti}\right)+\left(\%\mathrm{V}\right)\\ & =\mathrm{0}.\mathrm{15}\mathrm{to}\mathrm{0}.\mathrm{25th}\end{array}$$

Below the value of 0.15, the nucleation is less favorable, so that the chromium carbide can be subject to crystallization inhibition and crystallized later. Associated with this is a coarser grained chromium carbide with an unfavorable distribution in the matrix. When the upper limit is exceeded, coarse MC carbides of niobium, titanium and vanadium may be formed instead of fine primary carbides. Thus, the effect of the aforementioned elements is lost as carbide crystallization nuclei for the chromium carbides.

abgestimmten Matrix die Schmierstoffbenetzung vorteilhaft beeinflussen.The heat treatment B consists in a starter annealing at 100 to 550 ° C, preferably at least 200 ° C instead and leads to the formation of fine precipitates in the stabilized in the previous heat treatment A in the temperature range of 1100 to 900 ° C martensite. In-solution atoms such as copper and cobalt and the carbide-forming elements play an important role, since they go into the fine precipitates, in particular with the according to $$\begin{array}{ll}\mathrm{K}2& =100x\left(\%\mathrm{N}-0.03\right)x\left(\%\mathrm{N}\right)/\mathrm{C}\\ & =\mathrm{0}.053\mathrm{to}\mathrm{0}.730\end{array}$$

coordinated matrix affect the lubricant wetting advantageous.

Another advantage is compliance with the condition $$\begin{array}{ll}\mathrm{K}3& =\frac{\left(\%\mathrm{Ni}\right)+\left(\%\mathrm{Co}\right)}{\left(\%\mathrm{Mn}\right)}\\ & =\mathrm{0}.40\mathrm{to}3.\mathrm{33rd}\end{array}$$

Fig. 1

in schematischer Darstellung einen Öltropfen auf einer Stahloberfläche,

Fig. 2

eine Vorrichtung zum Bestimmen des Abriebverschleißes in schematischer Darstellung und

Fig. 3

die Spurbreite R einer Kugelschleifbahn nach Fig. 2 als Maß für die Verschleißfestigkeit.

The invention is explained below with reference to embodiments and the drawings of the closer. In the drawing show:

Fig. 1

a schematic representation of an oil drop on a steel surface,

Fig. 2

a device for determining the abrasion wear in a schematic representation and

Fig. 3

the track width R of a ball track after Fig. 2 as a measure of wear resistance.

Table I below shows the analyzes of five conventional comparative steels V1 to V5 and three steels E1 to E3 which fall under the invention. <b> Table I </ b> Leg. % C % N % Ni % Si % Mn % Cr %Not a word % W % V % Cu % Co V1 0.80 0.12 0.06 0.54 0.34 14,80 0.75 track 0.12 0.25 nn V2 0.58 0.09 0.08 0.71 0.45 15.40 0.45 nn 0.10 0.31 nn V3 0.79 0.25 0.95 0.65 0.65 19.25 0.95 0.25 0.08 0.30 nn V4 0.71 0.18 0.25 0.61 0.70 20.20 1.05 0.80 0.05 0.28 nn V5 0.49 0.22 0.30 0.55 1.20 22.40 2.10 0.24 0.30 0.34 nn E1 0.56 0.06 0.25 0.35 0.44 16.50 1.32 0.12 0.17 0.32 0.01 E2 0.60 0.05 0.29 0.47 0.45 17,20 1.35 0.40 0.16 0.25 0.08 E3 0.65 0.07 0.38 0.54 0.37 17.80 1.45 0.72 0.19 0.42 0.12 nn not detectable

Table II below shows the cumulative values for K1 to K3 resulting from the analyzes. <b> Table II </ b> K1 K2 K3 0.12 1.35 0.18 0.10 0.93 0.18 0.18 6.96 1.46 0.13 3.80 0.36 0.36 8.53 0.25 0.19 0.32 0.59 0.17 0.17 0.82 0.19 0.43 1.35

Eight of the prior art samples 1 to 8 and nine falling under the invention samples 9 to 17 with the compositions resulting from Table I were melted in a medium frequency oven under inert gas and poured into test tubes in a mold and 30 min. outsourced at 1200 ° C. Thereafter, the samples were forged into rods, freed of their scale layer and turned off by means of carbide cutting plates to cylindrical test bars. The test bars had a diameter of 15 mm and had different austenitizing temperatures (A) and tempering temperatures (B) to finally determine the quality of oil wetting and abrasion wear.

Austenitizing annealing indicated by A in Table III took place at 1020 ° C or 1050 ° C, followed by rapid cooling at a cooling rate of at least 50 ° C / sec to 800 ° C followed by cooling within 5 min. to 300 ° C and a slow cooling to room temperature.

The samples were finally heated according to the test series B to a temperature of 100 to 530 ° C and cooled at a rate of 100 ° C / h to room temperature.

To determine the wettability, the samples were then ground and polished, cleaned in an aqueous ultrasonic bath at 50 ° C, with hot distilled water under the action of ultrasound for another 20 min. freed of detergent residues and then dried. In order to determine the wettability index B, 10 μl of paraffin oil were then applied to each sample by means of microdosing, and the oil droplets then forming were measured with respect to their width B, as shown schematically in FIG Fig. 1 results. The measurement results found are recorded together with the respective austenitizing temperature in the following Table III. <b> Table III </ b> Verse no. Leg. heat treatment wetting abrasion corrosion test A B B R 1 V1 1050 - 2.10 205 5 2 V2 1050 - 2.30 195 5 3 V3 1050 - 1.98 200 3 4 V3 1020 530 2.00 205 3 5 V4 1050 530 2.12 190 2 6 V5 1050 480 2.00 196 1 7 V5 1050 530 2.21 200 1-2 8th V1 1050 550 2.50 180 5 9 E1 1050 530 3.40 132 1 10 E1 1020 550 3.10 135 1 11 E1 1050 500 3.50 130 0-1 12 E2 1050 480 3.70 125 0-1 13 E2 1050 500 3.40 134 1 14 E2 1050 530 3.35 130 1 15 E3 1050 450 3.60 135 1 16 E3 1050 430 4.00 115 1 17 E3 1050 400 3.70 125 1

The abrasion and wear resistance was determined by means of a modified "pin on disk" test *, whereby the cylindrical samples were first ground flat, then cleaned, clamped in a holder and then correspondingly Fig. 2 Under a rotating steel shaft with an eccentric carbide ball under load and spring preload dynamically loaded. During the experiment, the contact zone between the sliding hard metal ball and the sample surface was steadily lubricated by the dripping of lubricating oil. At the end of the test period, the average width R of the sliding or wear track was then measured under a microscope at four points offset by ninety degrees from each other, and from these four measured values in each case the mean value R (see Table IV). Fig. 3 ) educated. In this case, a wide wear track or a large R value indicates that the steel ball has penetrated deeper and accordingly with greater width into the samples and that the sample material accordingly has a lower seal strength than those samples with a small wear track width R.

The results are clear; the samples 9 to 17 covered by the invention have a significantly better wear resistance than the samples 1 to 8 from conventional steels. The significance of the K2 value for a further improvement of the wear resistance results from a comparison of the data from the following Table IV with the corresponding data of column 6 of Table III . <b> Table IV </ b> K2 1.35 0.93 6.96 6.96 3.80 8.53 8.53 1.35 0.32 0.32 0.32 0.17 0.17 0.17 0.43 0.43 0.43

Since many small chromium carbide precipitates are more effective in corrosion resistance than a few coarse precipitates, the result of a salt spray test can serve as an indicator of the size and distribution of the chromium carbide precipitates. The samples were therefore subjected to 120 hours of corrosion testing as part of a modified salt spray test with a 3% NaCl solution and 5% alcohol.

The test results are listed in column 7 of Table III .

It is well known that martensitic chromium steels are subject to severe pitting corrosion, depending on the size and density of the chromium carbide precipitates in chloride solutions. The data of column 7 of Table III confirm that for the conventional comparative steels 1 to 8 in comparison with the steels 9 to 17 according to the invention.

Depending on the width of the zone of a chromium depletion, pitting corrosion occurs, which leads to the finding that a large number of small chromium carbide precipitations is more favorable in terms of pitting corrosion than a smaller number of large precipitates. In this respect, the results of the salt spray test are an indicator of the size and distribution of chromium carbide precipitates.

The results of the corrosion tests according to Table III last column were evaluated according to a quality scale of 0 to 5, where 0 stands for no rusting and 5 indicates at least five rust spots. The results of the salt spray test are summarized in Table III , last column.

Overall, the test results show that the wetting behavior of the inventive martensitic chromium steels for lubricants is significantly better than that of the comparative steels. The good wettability results in less adhesive wear in the presence of lubrication. Not only the chemical composition of the steel is of crucial importance. A significant influence on the wettability is also exerted by a heat treatment of the samples. This is shown by the larger C values and the smaller R values of the samples according to the invention of experiments 9 to 17.

Decisive for the better wear resistance R of the samples according to the invention should be primarily the composition of the steel, where the two-stage heat treatment for influencing the precipitates is due insofar not only the special composition of the steel, but also its precipitates in the structure the material properties. It should be noted that the wettability of the steel for a lubricant can be improved especially by the heat treatments A and B. This suggests that the carbonaceous fine precipitates in martensite are more favorable for lubricant wetting than nitrogen-modified carbonitrides and a nitrogen-containing matrix. The decisive factor is thus the low nitrogen content of the alloy as well as the factor K2. This is demonstrated in particular by the comparison steels V3 with 0.25% nitrogen and also V5 with 0.22% nitrogen, in contrast to the steels E1 to E3 covered by the invention with only 0.05 to 0.07% nitrogen.

Finally, the factors K1 and K3 also show that the favorable test results are based on a more favorable precipitation of the carbides and other phases as well as the basic structure.

Claims (13)

1. Stainless martensitic chromium steel with: 0.4 to 0.8% carbon 0.2 to 1.5% silicon 0.15 to 1.0% nickel 0.3 to 1.00% manganese 0.015 to 0.035% sulphur 16 to 18% chromium 1.25 to 1.50% molybdenum up to 0.8% tungsten 0.04 to 0.08% nitrogen 0.15 to 0.20% vanadium up to 0.05% titanium up to 0.05% niobium 0.001 to 0.03% aluminium 0.02 to 0.5% copper up to 0.5% cobalt up to 0.040% boron, the remainder iron, including melt-induced impurities.
2. Chromium steel according to claim 1, which contains individually or together: 0.55 to 0.75% carbon up to 0.65% silicon up to 0.8% manganese at least 0.001 % tungsten
3. Chromium steel according to claim 1 or 2 with a total content of chromium, molybdenum and tungsten of $$\left(\%\mathrm{Cr}\right)+\mathrm{3}\left(\%\mathrm{Mo}\right)+\left(\%\mathrm{W}\right)=\mathrm{19.7}\mathrm{to}\mathrm{23.3}$$

   
4. Chromium steel according to any one of claims 1 to 3 which observes the balancing rule: $$\begin{array}{ll}\mathrm{K}\mathrm{1}& =\left(\%\mathrm{Nb}\right)+\left(\%\mathrm{Ti}\right)+\left(\%\mathrm{V}\right)\\ & =\mathrm{0.15}\mathrm{to}\mathrm{0.25}\%\end{array}$$

   
5. Chromium steel according to any of claims 1 to 4, characterised in that it observes the balancing rule: $$\begin{array}{ll}\mathrm{K}2& =100x\left(\%\mathrm{N}-0.03\right)\%x\left(\%\right)\mathrm{N}/\left(\%\mathrm{C}\right)\\ & =0.053\mathrm{to}0.730\end{array}$$

   
6. Chromium steel according to any one of claims 1 to 5, characterised in that it observes the balancing rule: $$\begin{array}{ll}\mathrm{K}3& =\left[\left(\%\mathrm{Ni}\right)+\left(\%\mathrm{Co}\right)\right]/\left(\%\mathrm{Mn}\right)\\ & =\mathrm{0},40\mathrm{to}3.33\%\end{array}$$

   
7. Chromium steel according to any one of claims 1 to 6, characterised in that it is annealed austenitising at 1020 to 1050°C, then cooled rapidly with a cooling rate of at least 50°C/sec to 800°C, and then subjected to a five-minute cooling to 300°C with subsequent air cooling to room temperature.
8. Chromium steel according to claim 7 which is reheated to 100 to 530°C and then cooled slowly to room temperature at a rate of 100°C/h.
9. Use of a chromium steel according to any one of claims 1 to 8 as a pitting-resistant material.
10. Use of a chromium steel according to any one of claims 1 to 8 as a material resistant to abrasion.
11. Use according to claim 9 or 10 with a lubricant film.
12. Use according to claim 11 in a metal/metal slip contact with a lubricant intermediate layer.
13. Use of a chromium steel according to any one of claims 1 to 7 as a material for the production of valve stems, control and metering needles, guide sleeves, function components of fuel injection systems, piston rings for plungers and engines, and sealing and scraping rings for compressors.

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