TECHNICAL FIELD

The invention relates to corrosion and wear resistant cold work tool steel and a method of making the cold work steel and use of the cold work tool steel.

BACKGROUND OF THE INVENTION

Nitrogen alloyed martensitic tool steels have recently been introduced on the market and attained a considerable interest, because they combine a high wear resistance with an excellent corrosion resistance. These steels have a wide rang of applications such as for moulding of aggressive plastics, for knives and other components in food processing and for reducing corrosion induced contamination in the medical industry.

The steels are generally produced by powder metallurgy. The basic steel composition is firstly atomized and subsequently subjected to a nitrogenation treatment in order to introduce the desired amount of nitrogen into the powder. Thereafter the powder is filled into a capsule and subjected to hot isostatic pressing (HIP) in order to produce an isotropic steel.

The amount of carbon is generally reduced to a very low level as compared to conventional tool steels. By substituting most of the carbon with nitrogen it is possible to substitute the chromium rich carbides of the type M7C ₃ and M2 ₃ C ₆ with very stable hard particles of the type MN-nitrides.

Two important effects are achieved. Firstly, the relative soft and anisotropic phase of M ₇ C ₃ -carbide (-1700HV) is replaced by the very hard and stable phase of small and evenly distributed hard phase of the type MN («2800HV). Thereby, the wear resistance is improved at the same volume fraction of hard phase. Secondly, the amount of Cr, Mo and N in solid solution at the hardening temperature is very much increased, because less chromium is bound in the hard phase and because the carbides of the type M2 ₃ C5 and M7C ₃ do not have any solubility for nitrogen. Thereby, more chromium is left in solid solution and the thin passive chromium rich surface film is strengthened, which leads to an increased resistance to general corrosion and pitting corrosion. Hence, in order to obtain good corrosion properties the carbon content has been limited to less than 0.3 %C, preferably less than 0.1 %C in DE 42 31 695 A 1 and to < 0.12 % C in WO 2005/054531 Al . DISCLOSURE OF THE INVENTION

The general object of the present invention is to provide a powder metallurgy (PM) produced nitrogen alloyed cold work tools steel alloy having improved properties, in particular a good corrosion resistance in combination with a high hardness. A particular object is to provide a nitrogen alloyed martensitic cold work tools steel alloy having improved corrosion resistance at a fixed chromium content.

A further object is to provide a method of producing said material. The foregoing objects, as well as additional advantages are achieved to a significant measure by providing a cold work tool steel having a composition as set out in the alloy claims.

The invention is defined in the claims.

DETAILED DESCRIPTION

The importance of the separate elements and their interaction with each other as well as the limitations of the chemical ingredients of the claimed alloy are briefly explained in the following. All percentages for the chemical composition of the steel are given in weight % (wt. %) throughout the description.

Carbon (0.3 - 0.8 %)

is to be present in a minimum content of 0.3%, preferably at least 0.35%. At high carbon contents carbides of the type M2 ₃ C ₆ and M7C ₃ will be formed in the steel. The carbon content shall therefore not exceed 0 8%. The upper limit for carbon may be set to 0.7%) or 0.6%. Preferably, the carbon content is limited to 0.5%. Preferred ranges are 0.32 - 0.48%, 0.35 - 0.45%, 0.37 - 0.44% and 0.38 - 0.42%. In any case, the amount of carbon should be controlled such that the amount of carbides of the type M2 ₃ C ₆ and M7C ₃ in the steel is limited to 10 vol. %, preferably the steel is free from said carbides. Nitrogen (1.0 - 2.2%)

Contrary to carbon, nitrogen cannot be included in M7C ₃ . The nitrogen content should therefore be much higher than the carbon content in order to avoid the precipitation of MjCs-carbides. In order to get the desired type and amount of hard phases the nitrogen content is balanced against the contents of the strong carbide formers, in particular vanadium. The nitrogen content is limited to 1.0 - 2.2%, preferably 1.1 - 1.8% or 1.3 - 1.7%. (C+ ) (1.3 - 2.2%)

The total amount of carbon and nitrogen is an essential feature of the present invention. The combined amount of (C + N) should be in the range of 1.3 - 2.2%, preferably 1.7 - 2.1% or 1.8 - 2.0%. C/N (0.17 - 0.50)

A proper balance of carbon and nitrogen is an essential feature of the present invention. By controlling the carbon and nitrogen contents the type and amounts of the hard phases can be controlled. In particular, the amount of the hexagonal phase M ₂ X will be reduced after hardening. The C N ratio should therefore be 0.17 - 0.50. The lower ratio may be 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24 or 0.25. The upper ratio may be 0.5, 0.48, 0.46, 0.45, 0.44, 0.42, 0.40, 0.38, 0.36 or 0.34. The upper limit may be freely combined with the lower limit. Preferred ranges are 0.20 - 0.46 and 0. 22 - 0.45.

Chromium (13 - 30%)

When it is present in a dissolved amount of at least 1 1%, chromium results in the formation of a passive film on the steel surface. Chromium shall be present in the steel in an amount between 13 and 30 % in order to give the steel a good hardenability and oxidation and corrosion resistance. Preferably, Cr is present in an amount of more than 16% in order to safeguard a good pitting corrosion resistance. The lower limit is set in accordance to the intended application and may be 17%, 18%, 19%, 20%, 21% or 22% However, Cr is a strong ferrite former and in order to avoid ferrite after hardening the amount need to be controlled. For practical reasons the upper limit may be reduced to 26%, 24% or even 22%. Preferred ranges include 16 - 26%, 18 - 24%, 19 - 21%, 20 - 22% and 21 - 23%. Molybdenum (0.5 - 3.0%)

Mo is known to have a very favourable effect on the hardenability. It is also known to improve the pitting corrosion resistance. The minimum content is 0.5%, and may be set to 0.6%, 0.7%, 0.8% or 1.0%. Molybdenum is a strong carbide forming element and also a strong ferrite former. The maximum content of molybdenum is therefore 3.0%. Preferably Mo is limited to 2.0 %, 1.7% or even 1.5%.

Tungsten (< 1%)

In principle, molybdenum may be replaced by twice as much tungsten. However, tungsten is expensive and it also complicates the handling of scrap metal. The maximum amount is therefore limited to 1%, preferably 0.2% and most preferably no additions are made. Vanadium (2 0 5.0%)

Vanadium forms evenly distributed primary precipitated nitrocarbides of the type M(N,C) in the matrix of the steel. In the present steels M is mainly vanadium but significant amounts of Cr and Mo may be present. Vanadium shall therefore be present in an amount of 2-5. The upper limit may be set to 4.8%, 4.6%, 4.4%, 4.2% or 4.0%. The lower limit may be 2.2%, 2.4%, 2.5%, 2.6%, 2.7%, 2,8%, 2.8% and 2.9%. The upper and lower limits may be freely combined within the limits set out in claim 1. Preferred ranges includes 2 - 4%.

Niobium (<2.0%)

Niobium is similar to vanadium in that it forms nitrocabides of the type M(N,C) and may in principle be used to replace vanadium but that requires the double amount of niobium as compared to vanadium. Hence, the maximum addition of Nb is 2.0%. The combined amount of (V + Nb/2) should be 2.0 - 5.0%. However, Nb results in a more angular shape of the M(N,C). The preferred maximum amount is therefore 0.5%.

Preferably, no niobium is added.

Silicon (<1.0%)

Silicon is used for deoxidation. Si is present in the steel in a dissolved form. Si is a strong ferrite former and should therefore be limited to <1.0%. Manganese (0.2 - 2.0%)

Manganese contributes to improving the hardenability of the steel and together with sulphur manganese contributes to improving the machinability by forming manganese sulphides. Manganese shall therefore be present in a minimum content of 0.2%, preferably at least 0.3%. At higher sulphur contents, manganese prevents red brittleness in the steel. The steel shall contain max.2.0%, preferably max. 1.0 %Mn. Preferred ranges are 0.2 - 0.5%, 0.2 - 0.4%, 0.3 - 0.5% and 0.3 - 0.4%. Nickel ( < 5.0%)

Nickel is optional and may be present in an amount of up to 5%. It gives the steel a good hardenability and toughness. Because of the expense, the nickel content of the steel should be limited as far as possible. Accordingly, the Ni content is limited to 1%, preferably 0.25%.

Copper (< 3.0%)

Cu is an optional element, which may contribute to increasing the hardness and the corrosion resistance of the steel. If used, a preferred range is 0.02 - 2% and a most preferred range is 0.04 - 1.6%. However, it is not possible to extract copper from the steel once it has been added. This drastically makes the scrap handling more difficult. For this reason, copper is normally not deliberately added.

Cobalt (< 10.0%)

Co is an optional element. It contributes to increase the hardness of the martensite. The maximum amount is 10 % and, if added, an effective amount is about 4 to 6 %.

However, for practical reasons such as scrap handling there is no deliberate addition of Co. A preferred maximum content is 0.2%.

Sulphur (< 0.5%)

S contributes to improving the machinability of the steel. At higher sulphur contents there is a risk for red brittleness. Moreover, a high sulphur content may have a negative effect on the fatigue properties of the steel. The steel shall therefore contain < 0.5 %, preferably < 0.035%. Be, Bi, Se, Mg and REM (Rare Earth Metals)

These elements may be added to the steel in the claimed amounts in order to further improve the machinability, hot workability and/or weldability. Boron (< 0.01%)

B may be used in order to further increase the hardness of the steel. The amount is limited to 0.01%, preferably <0.004%.

Ti, Zr, Al and Ta

These elements are carbide formers and may be present in the alloy in the claimed ranges for altering the composition of the hard phases. However, normally none of these elements are added.

Hard phases

The total content of the hard phases MX, M ₂ X, M23C6 and M7C3 shall not exceed 50 vol. %, wherein M is one or more of the above specified metals, in particular V, Mo and/or Cr and X is C, N and/or B and wherein the contents of said hard phases fulfil the following requirements (in vol. %):

MX 3-25 preferably 5-20

M ₂ X < 10 preferably < 5

M ₂₃ C ₆ + M7C3 < 10 preferably < 5

More preferably the content of MX is 5 -15 vol. %, the content of M2X is < vol. 3% and the content of M23C6 + M7C3 is < 3 vol. %. Most preferably the steel is free from the component M7C3. PRE

The pitting resistance equivalent (PRE) is often used to quantify pitting corrosion resistance of stainless steels. A higher value indicates a higher resistance to pitting corrosion. For high nitrogen martensitic stainless steels the following expression may be used

PRE= %Cr +3.3 %Mo + 30 %N wherein %Cr, %Mo and %N are the calculated equilibrium contents dissolved in the matrix at the austenitising temperature (TA), wherein the chromium content dissolved in the austenite is at least 13 %.. The dissolved contents can be calculated with

Thermo-Calc for the actual austenitising temperature (TA) and/or measured in the steel after quenching. The austenitising temperature (TA) is in the range of 950 - 1200 °C, typically 1080- 1 150 °C.

It follows from the above reasoning that the austenite composition at austenizing temperature may have a considerable effect on the pitting corrosion resistance of the steel. The lower limit for the calculated PRE-value may be 25, 26, 27, 28, 29, 30, 31, 32 or 33.

High nitrogen stainless steels are based on a replacement of carbon with nitrogen. By substituting most of the carbon with nitrogen it is possible to substitute the chromium rich carbides of the type M7C ₃ and M2 ₃ C ₆ with very stable hard particles of the type MN-nitrides. The amount of Cr, Mo and N in solid solution at the hardening temperature is therefore very much increased, because less chromium is bound in the hard phase and because the carbides of the type M23C6 and M7C3 do not have any solubility for nitrogen. Thereby, more chromium is left in solid solution and the thin passive chromium rich surface film is strengthened, which leads to an increased resistance to general corrosion and pitting corrosion. Accordingly, it is to be expected that the pitting corrosion resistance would decrease if carbon replaces part of the nitrogen. High nitrogen stainless steels known in the art therefore have a low carbon content.

However, the present inventors have surprisingly found that it is possible to increase the corrosion resistance by increasing carbon content to above 0.3% as will be discussed in relation to the examples.

Steel production

The tool steel having the claimed chemical composition can be produced by conventional gas atomizing followed by nitrogenation of the powder before HIP-ing. The nitrogen content in the steel after gas atomizing is normally less than 0.2%. The remaining nitrogen is thus added during the nitrogenation treatment of the powder.

After consolidation the steel may be used in the as HIP-ed form or formed into a desired shape. Normally the steel is subjected to hardening and tempering before being used. Austenitising may be performed by annealing at an austenitising temperature (TA) in the range of 950 - 1200 °C, typically 1080- 1 150 °C. A typical treatment is annealing at 1080 °C for 30 minutes. The steel may be hardened by quenching in a vacuum furnace by deep cooling in liquid nitrogen, and then tempered at 200 °C for 2 times at 2 hours (2x2h). EXAMPLE 1

In this example a steel according to the invention is compared to a steel having lower carbon content and a different balance between carbon and nitrogen. Both steels were produced by powder metallurgy.

The basic steel compositions were melted and subjected to gas atomization.

Subsequently the obtained powders were subjected to a nitrogenation treatment in order to introduce the desired amount of nitrogen into the powders. The nitrogen content was increased from about 0.1% to the respective content.

Thereafter the nitrogenated powders were transformed to isotropic solid steel bodies by conventional hot isostatic pressing (HIP) at 1100 °C for 2 hours. The applied pressure was 100 MPa.

The steels thus obtained had the following compositions (in wt. %):

Inventive steel Comparative steel

C 0.35 0.18

N 1.5 1.9

(C+N) 1.85 2.08

C/N 0.23 0.09

Si 0.3 0.3

Mn 0.3 0.3

Cr 18.2 19.8

Mo 1.04 2.5

V 3.47 2.75 balance iron and impurities.

The steels were austenitised at 1080 °C for 30 minutes and hardened by quenching by deep cooling in liquid nitrogen in a vacuum furnace followed by tempering at 200 °C for 2 times at 2 hours (2x2h). The inventive steel had a hardness of 60 HRC and the comparative steel a hardness of 58 HRC.

The alloy micro structure consisted of tempered martensite and hard phases. Two distinct hard phases were identified in the micro structure of both steels: MX and M ₂ X. In the comparative steel the hexagonal M2X was the majority phase and the face centred cubic MX-phase was the minority phase. However, in the inventive steel MX was the majority phase and M ₂ X was the minority phase. The materials susceptibility for pitting corrosion was experimentally examined by anodic polarisation sweep. An electrochemical cell with a saturated Ag/AgCl reference electrode and a carbon mesh counter electrode, were used for cyclic polarization measurements. The 500 mesh grounded sample was first open circuit potential (OCP) recorded with a 0.1 M NaCl solution to ensure a stable potential was reached. Next, the cyclic polarization measurements were performed with a scan rate of 10 mV/min. Start potential was -0.2 V vs. OCP, and the final potential was set to the OCP. By choosing a setting in the software, the upward potential scan was automatically reversed when the anodic current density reached 0.1 mA/cm ² . Fig. 1 discloses a schematic anodic polarization curve and the information that can be obtained from the curve. The forward scan gives information about the initiation of pitting and the reverse scan provides information about the alloys repassivation behavior. Eb is the value of the potential for pitting breakdown above which new pits will initiate and existing pits will propagate. As the potential is decreased on the reverse scan , there is a decrease in current density. The alloy is repassivated where the reverse scan crosses the forward scan. Ep is the repassivation potential, or protection potential i.e. the potential below which no pitting occur. The difference between Eb and Ep is related to the susceptibility to pitting and crevice corrosion. The greater the difference the greater the susceptibility.

Table 1. Result of the anodic polarisation.

Table 1 discloses that the inventive steel with the increased carbon content has the less tendency to suffer localised corrosion and also that the inventive steel also repassivate more easily than the comparative steel. Accordingly, the inventive steel is much less sensitive to pitting and crevice corrosion. These results were totally unexpected because the inventive steel had lower contents of Cr, Mo and N than the comparative steel. The reasons therefore are presently not fully understood. However, the present inventors suspected that the differences may be related to the type and amount of hard phases remaining in the steel after austenizing and quenching.

EXAMPLE 2

The influence of the relative amounts of carbon and nitrogen on the formation of the different hard phases in the steel was calculated in Thermo-Calc for a steel having variable C and N contents and the following basic composition in weight %: Cr: 19.8, Mo: 2.5, V: 2.75; Si: 0.3, Mn: 0.3, Fe balance.

Table 2. Results of Example 2 at 1080 °C. Elemental concentrations in wt. %. Hard phases in vol. %. Cr, Mo andJNf denotes the calculated dissolved contents of the elements in the matrix at 1080 °C. PRE is calculated from the dissolved contents.

Fig. 2 discloses the amount of hard phases as a function of the ratio C/N and it can be seen that amount of M ₂ X decreases rapidly with increasing ratio C/N. However, M2 ₃ C ₆ starts to form already at a C/N ratio of about 0.25.

Fig. 3 discloses calculated PRE-values as a function of the ratio C/N and it can be seen that the highest values are obtained for the steels according to the invention. EXAMPLE 3

The influence of the relative amounts of carbon and nitrogen on the formation of the different hard phases in the steel was calculated in Thermo-Calc for a steel having variable C and N contents and the following basic composition in weight %: Cr: 18.2, Mo: 1.04, V: 3.47; Si: 0.3, Mn: 0.3, Fe balance. C N C/N MX M2X M23C6 Cr Mo N PRE

Comp. 0.1 2.05 0.05 7.0 7.4 0 14.0 1.15 0.23 24.7

Comp. 0.2 1.9 0.1 1 6.8 6.1 0 14.5 1.16 0.24 25.5

Inv. 0.3 1.75 0.17 6.7 4.7 0 15.5 1.16 0.26 27.1

Inv. 0.4 1.6 0.25 6.6 3.1 0 16.5 1.16 0.27 28.4

Inv. 0.5 1.45 0.34 6.8 1.2 1.6 16.8 1.1 0.27 28.5

Inv. 0.6 1.3 0.46 6.8 0 3.5 16.8 1.0 0.25 27.6

Comp. 0.7 1.15 0.60 6.3 0 5.2 16.4 0.9 0.21 25.7

Table 3. Results of Example 3 at 1080 °C. Elemental concentrations in wt. %. Hard phases in vol. %. Cr, Mo andjN denotes the calculated dissolved contents of the elements in the matrix at 1080 °C. PRE is calculated from the dissolved contents.

Fig. 4 discloses the amount of hard phases as a function of the ratio C/N and it can be seen that amount of M ₂ X decreases very rapidly with increasing ratio C/N. It can also be seen that M2 ₃ C ₆ starts to form at a C/N ratio of about 0.3.

Fig. 5 discloses calculated PRE-values as a function of the ratio C N and again it can be seen that the highest values are obtained for the steels according to the invention.

These results verify that a proper balance of carbon and nitrogen is an essential features of the present invention. A carefully controlled increase of the carbon content can be made without obtaining problems with carbides of the type M2 ₃ C ₆ and M7C ₃ in the steel. These results also reveals that if the carbon and nitrogen contents are controlled as defined in the claims, then the amount of the hexagonal phase M2X will be reduced after hardening. This phase is mainly referred to as Cr ₂ N but it may also include a substantial amount of Mo. The reduction of the amount of M ₂ X is a result of dissolution during the austenizing. Although a part of these elements under certain circumstances may be found in the increased fraction of MX (Fig. 2) it would appear that the dissolution of M ₂ X results in increased amounts of Cr, Mo and N dissolved in the matrix with a corresponding increase of the PRE-number until a certain limit. Thereafter the PRE- value will decrease as a result of the formation of M2 ₃ C5, because said phase is rich in Cr and Mo. Another mechanism, which may contribute to the improved corrosion resistance disclosed in Table 1 and Fig. 1, may be that the boundary regions surrounding the hard phase M ₂ X may be depleted in Cr and Mo due to the formation of Cr and Mo rich M ₂ X. Another possibility mechanism that may influence the corrosion resistance is that the increased carbon content in the hard phase MX may result in a lower solubility of Cr in this phase. This would result in a reduced volume fraction of MX and more chromium is retained in solid solution, which helps to improve the corrosion resistance. Accordingly, the present invention provides a to provide a powder metallurgy (PM) produced nitrogen alloyed cold work tools steel having an improved corrosion resistance in combination with a high hardness.

INDUSTRIAL APPLICABILITY

The cold work tool steel of the present invention is particular useful

in applications requiring good wear resistance in combination with a high resistance to pitting corrosion.