TWI638054B - Corrosion and wear resistant cold work tool steel - Google Patents

Abstract

The invention relates to a corrosion-resistant and wear-resistant cold-work tool steel. This steel includes the following main components (in% by weight):

Balance optional elements, iron and impurities.

Description

Corrosion-resistant and wear-resistant cold-work tool steel

The invention relates to a corrosion-resistant and wear-resistant cold-work tool steel and a method for making cold-work tool steel and using the cold-work tool steel.

The nitrogen-containing alloy martensitic tool steel has recently been introduced to the market, and it has attracted considerable attention because of its combination of high wear resistance and excellent corrosion resistance. These steels have a wide range of applications, such as for the molding of aggressive plastics, knives or other components for food processing, and for reducing corrosion caused by pollution in the pharmaceutical industry.

The steel is usually made of powder metallurgy. The basic steel components are first atomized and then nitridized to introduce the required amount of nitrogen into the powder. After that, the powder is filled into capsules and hot isostatic pressing (HIP) is performed to produce isotropic steel.

The carbon content is reduced to a very low level compared to conventional tool steels. By replacing most carbon with nitrogen, it is possible to replace chromium-rich carbides of M ₇ C ₃ type and M ₂₃ C ₆ type with very stable hard particles of MN-nitride type.

Two important effects have to be achieved. First, M ₇ C ₃ -carbide ( 1700HV) The relatively soft and anisotropic phase is made of MN type ( 2800HV) is small and evenly distributed, very hard and stable phase replacement. Therefore, at the same volume ratio of the hard phase, the wear resistance is improved. Secondly, at the hardening temperature, very much chromium, molybdenum and nitrogen are added to the solid solution because less chromium is limited to the hard phase and because the carbides of the M ₂₃ C ₆ and M ₇ C ₃ types are not Has any solubility in nitrogen. Therefore, more chromium is left in the solid solution and strengthens the thin passivated chromium-rich surface film, which leads to an increase in general corrosion resistance and pitting corrosion resistance.

In order to obtain good corrosion resistance, the carbon content is therefore limited to less than 0.3% C, preferably less than 0.1% C in the application of DE 42 31 695 A1 and to the limit of WO 2005/054531 A1 0.12%.

The object of the present invention is to provide a nitrogen-containing alloy cold work tool steel alloy made of powder metallurgy, which has improved properties, especially good corrosion resistance and high hardness.

A specific objective is to provide a nitrogen-containing alloy Ma Tian loose iron cold work tool steel with improved corrosion resistance at a fixed chromium content.

A further object is to provide a method for producing the above materials.

The above-mentioned objects and other advantages can be achieved by providing significant means of cold work tool steel with the composition of the alloy as defined in the scope of the patent application.

The invention is defined in the scope of patent application.

Figure 1 reveals an anode polarization curve and the information obtained from the curve.

Figure 2 reveals that the number of hard phases is a function of the C / N ratio, and it can be seen from the figure that M ₂ X decreases rapidly as the C / N ratio increases.

Figure 3 discloses the calculated PRE-value as a function of the C / N ratio, and the highest value obtained for the steel according to the invention can be seen from the figure.

FIG. 4 reveals that the number of hard phases is a function of the C / N ratio, and it can be seen from the figure that the number of M ₂ X decreases rapidly as the C / N ratio increases.

Figure 5 reveals the calculated PRE-value as a function of the C / N ratio, and again from the figure we can see the highest value obtained for the steel according to the invention.

The individual elements and their interaction with each other and the limitations of the chemical composition of the alloy as defined in the patent application will be briefly described below. Throughout the text, all proportions of the chemical composition of steel are given in weight percent (wt.%).

Carbon (0.3-0.8%)

The minimum content of carbon given is 0.3%, preferably at least 0.35%. At high carbon content, carbides of M ₂₃ C ₆ and M ₇ C ₃ will form in the steel. Therefore, the carbon content should not exceed 0.8%. The upper limit of carbon can be set to 0.7% or 0.6%. Preferably, the carbon content is limited to 0.5%. The better ranges are 0.32-0.48%, 0.35-0.45%, 0.37-0.44% and 0.38-0.42%. In either case, the carbon content should be controlled so that the amount of carbides of the type M ₂₃ C ₆ and M ₇ C ₃ in the steel is limited to 10 vol.%, Preferably the steel does not contain the carbides.

Nitrogen (1.0-2.2%)

Relative to carbon, nitrogen is not included in M ₇ C ₃ . Therefore, the nitrogen content should be higher than the carbon content to avoid precipitation of M ₇ C ₃ -carbides. In order to obtain the desired type and content of the hard phase, the nitrogen content is balanced with respect to the content of strong carbide forming elements, especially vanadium. The nitrogen content is limited to 1.0-2.2%, preferably 1.1-1.8% or 1.3-1.7%.

(Carbon + nitrogen) (1.3-2.2%)

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

Carbon / Nitrogen (0.17-0.50)

The proper balance of carbon and nitrogen is an essential feature of the present invention. By controlling the carbon and nitrogen content, the type and quantity of the hard phase can be controlled. In particular, the number of hexagonal crystal phases M ₂ X will decrease after hardening. The carbon / nitrogen ratio should therefore be 0.17-0.50. The lower ratio is 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, or 0.25. The higher ratio is 0.5, 0.48, 0.46, 0.45, 0.44, 0.42, 0.40, 0.38, 0.36 or 0.34. The upper limit can be freely combined with the lower limit. The preferred ranges are 0.20-0.46 and 0.22-0.45.

Chromium (13-30%)

When chromium appears at a decomposition amount of at least 11%, chromium will cause A passivation film is formed on it. In order to give steel a good hardenability and oxidation and corrosion resistance, chromium is present in the steel between 13 and 30%. Preferably, in order to ensure a good pitting corrosion resistance, the chromium content is greater than 16%. The lower limit is set according to the intended application and can be 17%, 18%, 19%, 20%, 21% or 22%. However, chromium is a strong ferrite forming element and its content needs to be controlled in order to avoid ferrous salts after hardening. For practical reasons, the upper limit can be reduced to 26%, 24%, or even 22%. Better ranges include 16-26%, 18-24%, 19-21%, 20-22% and 21-23%.

Molybdenum (0.5-3.0%)

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

Tungsten (Tungsten) ( 1%)

In principle, molybdenum can be replaced by twice the amount of tungsten. However, tungsten is expensive and complicates the processing of metal scrap. Therefore, the maximum content is limited to 1%, preferably 0.2%, and more preferably, it may not be added.

Vanadium (2.0-5.0%)

In the matrix of steel, vanadium will form a uniformly distributed mainly precipitated M (N, C) type of nitrocarbide (nitrocarbide). In current steels, M is mainly vanadium, but chromium and molybdenum can also be present in significant amounts. Vanadium should therefore be present in a content of 2-5. The upper limit can be set to 4.8%, 4.6%, 4.4%, 4.2% or 4.0%. The lower limit can be 2.2%, 2.4%, 2.5%, 2.6%, 2.7%, 2,8%, 2.8% or 2.9%. Within the scope defined in item 1 of the patent application scope, the upper and lower limits can be freely combined. The better range includes 2-4%.

Niobium (Niobium) ( 2.0%)

Niobium is similar to vanadium in forming M (N, C) type nitrogen-containing carbides, and can be used in principle to replace vanadium, but the amount of niobium needs to be twice that of vanadium. Therefore, the maximum amount of niobium added is 2.0%. The combined content of (V + Nb / 2) should be 2.0-5.0%. However, niobium causes M (N, C) to form a polygonal shape. Therefore, the preferred maximum content is 0.5%. Preferably, no niobium is added.

Silicon (Silicon) ( 1.0%)

Silicon is used for deoxidation. Silicon exists in steel in decomposed form. Silicon is a strong ferrous salt-forming element and should therefore be limited to 1.0%.

Manganese (0.2-2.0%)

Manganese helps improve the hardenability of steel, and sulfur and manganese together can help improve machinability by forming manganese sulfides. Manganese should therefore be present in a minimum content of 0.2%, preferably at least 0.3%. At higher sulfur content, manganese prevents red brittleness in steel. The steel should contain a maximum content of 2.0% manganese, preferably a maximum content of 1.0% manganese. The better ranges are 0.2-0.5%, 0.2-0.4%, 0.3-0.5% and 0.3-0.4%.

Nickel (Nickel) ( 5.0%)

Nickel is selective and is present at levels up to 5%. It gives steel one A good hardenability and ductility. Due to price considerations, the nickel content of steel should be limited as much as possible. Accordingly, the nickel content is limited to 1%, preferably 0.25%.

copper( 3.0%)

Copper is a selective element, which helps increase the hardness and corrosion resistance of steel. If copper is used, the preferred range is 0.02-2% and the more preferred range is 0.04-1.6%. However, once copper is added to the steel, it is impossible to extract copper from the steel. This greatly makes waste disposal more difficult. For this reason, copper is usually not deliberately added.

Cobalt (Cobalt) ( 10.0%)

Cobalt is a selective element. Helps increase the hardness of Ma Tian loose iron. The maximum content is 10%, and if added, the effective content is about 4 to 6%. However, for practical reasons, such as waste disposal, cobalt is not intentionally added. The preferred maximum content is 0.2%.

Sulphur (Sulphur) 0.5%)

Sulfur helps improve the machinability of steel. At higher sulfur content, there is a risk of red embrittlement. In addition, high sulfur content can have an adverse effect on the fatigue properties of steel. Steel should therefore contain content 0.5% sulfur, preferably 0.035%.

Beryllium (Be), Bismuth (Bi), Selenium (Se), Magnesium (Mg) and Rare Earth Metals (REM)

These elements can be added to the steel as defined in the patent application to further improve machinability, hot workability and / or weldability.

Boron (Boron) ( 0.01%)

Boron can be used to further increase the hardness of the steel. The content of boron is limited to 0.01%, preferably 0.004%.

Titanium (Ti), zirconium (Zr), aluminum (Al) and tantalum (Ta)

These elements are carbide forming elements and exist in the alloy within the range defined by the patent application range to change the composition of the hard phase. However, usually these elements are not added.

Hard phases

The total content of hard phases MX, M ₂ X, M ₂₃ C ₆ and M ₇ C ₃ will not exceed 50 vol.%, Where M is one or more of the above specified metals, especially vanadium, molybdenum and / or Chromium, and X is carbon, nitrogen, and / or boron, and the composition of the hard phase satisfies the following conditions (vol.%): MX 3-25, preferably 5-20; M ₂ X 10, preferably 5; M ₂₃ C ₆ + M ₇ C ₃ 10, preferably 5.

More preferably, the content of MX is 5-15 vol.%, And the content of M ₂ X is vol.3%, and the content of M ₂₃ C ₆ + M ₇ C ₃ is 3vol.%. More preferably, the steel has no M ₇ C ₃ component.

Resistance to pitting corrosion (PRE)

The pitting resistance equivalent (PRE) is often used to quantify the pitting corrosion resistance of stainless steel. A higher value indicates a higher resistance to pitting corrosion. For high-nitrogen hemp loose stainless steel, the following formula can be used: PRE =% Cr + 3.3% Mo + 30% N

Among them,% Cr,% Mo and% N are calculated equilibrium contents decomposed in the matrix of the Vostian iron temperature (TA), of which the chromium content in the Vostian iron decomposition is at least 13%. For the actual Vostian iron temperature (TA) and / or the measurement in the steel after quenching, the decomposition content can be calculated by Thermo-Calc.

The range of the Vostian ferrite temperature (TA) is between 950-1200 ° C, typically 1080-1150 ° C.

From the above deduction, at the ferrosity temperature of the field, the field iron composition has a considerable effect on the pitting corrosion resistance of the steel. The lower limit of the calculated PRE-value can be 25, 26, 27, 28, 29, 30, 31, 32 or 33.

High nitrogen stainless steel is based on nitrogen instead of carbon. By replacing most of the carbon with nitrogen, it is possible to replace chromium-rich carbides of M ₇ C ₃ type and M ₂₃ C ₆ type with very stable hard particles of MN-nitride type. At the hardening temperature, the content of chromium, molybdenum and nitrogen in the solid solution increases very much, because less chromium is limited to the hard phase and because the carbides of the type M ₂₃ C ₆ and M ₇ C ₃ do not have any Solubility to nitrogen. Therefore, more chromium is left in the solid solution and strengthens the thin passivated chromium-rich surface film, which leads to an increase in general corrosion resistance and pitting resistance. From this, it can be expected that if carbon replaces part of the nitrogen, the pitting corrosion resistance will be reduced. Therefore, high-nitrogen stainless steels known in the art have a low carbon content.

However, the inventor of the present application has surprisingly found that by increasing the carbon content discussed in the subsequent examples to more than 0.3%, it is possible to increase the corrosion resistance.

Steel production

Tool steels with the chemical composition required by the patent application scope can be produced by conventional gas atomization and subsequent powder nitridation before the hot pressure equalization is performed. The nitrogen content in steel after gas atomization is usually less than 0.2%. The remaining nitrogen is therefore added during the powder nitriding process. After solidification, the steel can be used in a form such as being heat-equalized or formed into a desired shape. Normally, steel is hardened and tempered before being used.

Vossian ferrization can be performed via tempering at a Vossian ferrization temperature (TA) of 950-1200 ° C, typically 1080-1150 ° C. A typical treatment is tempering at 1080 ° C for 30 minutes. The steel can be hardened by quenching in a vacuum furnace, deep cooling in liquid nitrogen, and then tempering twice at 200 ° C for 2 hours (2x2h).

example 1

In this example, the steel according to the invention is compared with steel having a low carbon composition and a different balance between carbon and nitrogen. The aforementioned two steels are made by powder metallurgy.

The basic steel components are melted and gas atomized. Next, the obtained powder is subjected to nitridation treatment to introduce the required amount of nitrogen into the powder. The nitrogen content increased from approximately 0.1% to the respective content.

After that, by performing conventional hot isostatic pressing (HIP) at 1100 ° C for 2 hours, the nitrided powder is converted into an isotropic solid steel body. The applied pressure is 100 MPa.

The steel thus obtained has the following composition (wt.%):

Balance iron and impurities.

The steel is tempered at 1080 ° C for 30 minutes, and hardened by quenching in a vacuum furnace, by deep cooling in liquid nitrogen and then tempering at 200 ° C twice for 2 hours (2x2h). The steel of the present invention has a hardness of 60 HRC, while the comparative steel has a hardness of 58 HRC.

The alloy microstructure is composed of tempered hemp iron and hard phase. The two steel microstructures identified two different hard phases: MX and M ₂ X.

In the comparative steels, the hexagonal M ₂ X is the majority phase, while the face-centered cubic MX-phase is the minority phase. However, in the steel of the present invention, MX is the majority phase, and M ₂ X is the minority phase.

The sensitivity of the materials used for pitting corrosion is experimentally checked via anodic polarization scanning. Electrochemical cells with a saturated silver / silver chloride reference electrode and a carbon mesh counter electrode were used for cyclic polarization measurement. 500 mesh ground samples are recorded with the first open circuit potential (OCP) of 0.1M sodium chloride solution to ensure that a stable potential is reached. Next, the cyclic polarization measurement was performed at a scanning speed of 10 millivolts per minute (mV / min). The starting potential is -0.2V relative to OCP, and the final potential is set to OCP. By selecting a setting in the software, when the anode current density reaches 0.1 milliamps per square centimeter (mA / cm ² ), the upward potential sweep can be automatically reversed.

Figure 1 reveals an anode polarization curve and the information obtained from the curve. The forward scan provides initial information about pitting corrosion, while the reverse scan provides information about the repassivation properties of the alloy. Eb is the potential value of click through. Above the aforementioned potential value, new pitting corrosion will start and the existing pitting corrosion will spread. When the potential decreases in the reverse scan, the current density will decrease. The alloy is repassivated when the reverse scan intersects the forward scan. Ep is the repassivation potential, or protection potential, that is, the potential below which no pitting occurs. The difference between Eb and Ep is related to the sensitivity of pitting corrosion and crack corrosion. The greater the difference, the greater the sensitivity.

Table 1 reveals that the steel of the present invention with increased carbon content has less tendency to resist local corrosion, and also reveals that the steel of the present invention is easier to re-passivate than other steels in comparison. Accordingly, the steel of the present invention is less sensitive to pitting corrosion and crack corrosion.

These results are completely unpredictable because the steel of the present invention has a lower content of chromium, molybdenum and nitrogen than the steel of the comparison. Therefore, these reasons are not yet fully understood. However, the inventors have noticed that the above differences may be related to the type and content of the hard phase remaining in the steel after austenizing and quenching.

Example 2

For steels with variable carbon and nitrogen content and weight% as follows: chromium: 19.8, molybdenum: 2.5, vanadium: 2.75; silicon: 0.3, manganese: 0.3, iron balanced steel, different hard phases in the steel The influence of the relative content of carbon and nitrogen on the molding can be calculated by Thermo-Calc.

Figure 2 reveals that the number of hard phases is a function of the C / N ratio, and it can be seen from the figure that M ₂ X decreases rapidly as the C / N ratio increases. However, M ₂₃ C ₆ started to form at a C / N ratio of about 0.25.

Figure 3 discloses the calculated PRE-value as a function of the C / N ratio, and the highest value obtained for the steel according to the invention can be seen from the figure.

Example 3

For steels with variable carbon and nitrogen content and basic composition expressed in weight% as follows: chromium: 18.2, molybdenum: 1.04, vanadium: 3.47; silicon: 0.3, manganese: 0.3, iron balance, different hard phases in steel The influence of the relative content of carbon and nitrogen on the molding can be calculated by Thermo-Calc.

Figure 4 reveals that the number of hard phases is a function of the C / N ratio, and it can be seen that the number of M ₂ X decreases rapidly as the C / N ratio increases. It can also be seen from this that M ₂₃ C ₆ starts to be formed when the C / N ratio is about 0.3.

Figure 5 reveals the calculated PRE-value as a function of the C / N ratio, and again from this we can see the highest value obtained for the steel according to the invention.

These results prove that an appropriate balance of carbon and nitrogen is a necessary feature of the present invention. The increase in carbon content can be carefully controlled without causing problems with carbides of the type M ₂₃ C ₆ and M ₇ C ₃ in the steel. These results show that if the carbon and nitrogen content can be controlled as defined in the patent application, then the amount of hexagonal phase M ₂ X will decrease after hardening. This phase is mainly related to, for example, dichromium nitride (Cr ₂ N), but it can also include a significant amount of molybdenum. During Vossification, the reduction in the number of M ₂ X is the result of decomposition. Although under certain conditions, part of these elements can be found in the increased part of MX (Figure 2), but with the corresponding increase in PRE-number until a certain degree, it shows that the decomposition of M ₂ X leads to decomposition The amount of chromium, molybdenum and nitrogen increased. Afterwards, due to the formation of M ₂₃ C ₆ , the PRE-value will decrease because the phase described is rich in chromium and molybdenum.

Another mechanism that helps to improve the corrosion resistance disclosed in Table 1 and Figure 1 may be the boundary zone around the hard phase M ₂ X, which may be in chromium and molybdenum due to the formation of chromium and molybdenum rich M ₂ X loss.

Another possible mechanism that can affect corrosion resistance is that the increased carbon content in the hard phase MX can lead to a lower solubility of chromium in this phase. This leads to a reduced volume ratio of MX and more chromium remains in the solid solution, which helps to improve corrosion resistance.

Accordingly, the present invention provides a nitrogen-containing alloy cold tool steel made of powder metallurgy, which has improved corrosion resistance and high hardness.

Industrial applicability

The cold tool steel of the present invention is particularly useful in applications requiring high wear resistance and high pitting corrosion resistance.

Claims (11)

A steel made of powder metallurgy, which consists of the following elements (in% by weight):

Balance iron and impurities. The steel made by powder metallurgy as described in claim 1, wherein the upper limit content of the vanadium is 4.8%, 4.6%, 4.4%, 4.2% or 4.0%. The steel made by powder metallurgy as described in claim 1, wherein the steel satisfies at least one of the following conditions (in% by weight):

The steel made by powder metallurgy according to claim 1, wherein the steel satisfies at least one of the following conditions (in weight percent): carbon 0.35-0.45

The steel made by powder metallurgy as described in claim 1, wherein the steel satisfies at least one of the following conditions (in weight percent):

Except for the addition of cobalt as defined in claim 4

The steel made of powder metallurgy as described in claim 1, wherein the microstructure includes tempered hemp iron and a hard phase composed of one or more MX, M ₂ X, M ₂₃ C ₆ and M ₇ C ₃ , And wherein the steel has a hardness of 58-64 HRC. The steel made by powder metallurgy as described in claim 6, wherein the components (in% of capacity) of the hard phases MX, M ₂ X, M ₂₃ C ₆ and M ₇ C ₃ satisfy the following conditions:

Among them, M is one or more of vanadium, molybdenum and chromium, X is one or more of carbon, nitrogen or boron. The requested item in powder metallurgy of steel, wherein the PRE has been calculated at 1080 deg.] C austenite temperature (T _A) of the said steel 1

18, where PRE = Cr + 3.3 Mo + 30 N, and chromium, molybdenum and nitrogen in the matrix T _A as calculated equilibrium content of decomposition, in which the austenite decomposed chromium content of at least 13%. The requested item in powder metallurgy of steel, wherein the PRE has been calculated at 1080 deg.] C austenite temperature (T _A) of the said steel 1

20, where PRE = Cr + 3.3 Mo + 30 N, and chromium, molybdenum and nitrogen in the matrix T _A as calculated equilibrium content of decomposition, in which the austenite decomposed chromium content of at least 16%. The requested item in powder metallurgy of steel, wherein the PRE has been calculated at 1080 deg.] C austenite temperature (T _A) of the said steel 1

22, where PRE = Cr + 3.3 Mo + 30 N, and chromium, molybdenum and nitrogen in the matrix T _A as the calculated equilibrium content of decomposition. The requested item in powder metallurgy of steel, wherein the PRE has been calculated at 1080 deg.] C austenite temperature (T _A) of the said steel 1

25, where PRE = Cr + 3.3 Mo + 30 N, and chromium, molybdenum and nitrogen in the matrix T _A as the calculated equilibrium content of decomposition.