TW201522664A - Corrosion and wear resistant cold work tool steel - Google Patents

Abstract

The invention relates to a corrosion and wear resistance cold work tool steel. The steel comprises the following main components (in wt. %): balance optional elements, iron and impurities.

Description

Corrosion and wear resistant cold work tool steel

This invention relates to a corrosion resistant and wear resistant cold work tool steel and a method of making cold work tool steel and using cold work tool steel.

Nitrogen-containing alloys Martensitic tool steels have recently been introduced to the market and have attracted considerable attention due to their combination of high wear resistance and excellent corrosion resistance. These steels have a wide range of applications, such as for the formation of aggressive plastics, cutting tools or other components for food processing, and for reducing corrosion caused by pollution in the pharmaceutical industry.

This steel is usually made of powder metallurgy. The basic steel component is first atomized and subsequently nitrided 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) to produce an isotropic steel.

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

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

In order to obtain good corrosion resistance, the carbon content is 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 application of WO 2005/054531 A1. 0.12%.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a nitrogen-containing alloy cold work tool steel alloy which is improved by powder metallurgy and which has improved properties, particularly good corrosion resistance and high hardness.

A specific object is to provide a nitrogen-containing alloy, a granulated iron cold work tool steel, which has improved corrosion resistance at a fixed chromium content.

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

The above objects and other advantages can be attained by providing a significant means of cold working tool steel having the composition of the alloy as defined in the patent application.

The invention is defined in the scope of the patent application.

Figure 1 discloses an anodic polarization curve and 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 that the calculated PRE-value is a function of the C/N ratio and the highest value obtained for the steel according to the invention is known from the figures.

Figure 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 discloses that the calculated PRE-value is a function of the C/N ratio, and again the figure shows the highest value obtained for the steel according to the invention.

The individual elements and their interaction with each other and the chemical composition of the alloys defined in the scope of the patent application are 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 given by carbon 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%. A more preferred range is 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 the M ₂₃ C ₆ and M ₇ C ₃ type carbides in the steel is limited to 10 vol.%, preferably the steel does not contain the carbide.

Nitrogen (1.0-2.2%)

Nitrogen is not included in M ₇ C ₃ relative to carbon. Therefore, the nitrogen content should be higher than the carbon content to avoid precipitation of M ₇ C ₃ -carbide. In order to obtain the desired type and content of the hard phase, the nitrogen content is balanced for the content of the strong carbide forming element, particularly 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 to 2.2%, preferably in the range of 1.7 to 2.1% or 1.8 to 2.0%.

Carbon/nitrogen (0.17-0.50)

A suitable balance of carbon and nitrogen is an essential feature of the invention. By controlling the carbon and nitrogen content, the type and amount of hard phase is controllable. In particular, the amount of hexagonal phase 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 ratios are 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 range is 0.20-0.46 and 0.22-0.45.

Chromium (13-30%)

When chromium is present in at least 11% of the decomposition, chromium will result in the steel surface A passivation film is formed thereon. In order to give the steel a good hardenability and oxidation and corrosion resistance, chromium is present in the steel at a level between 13 and 30%. Preferably, in order to ensure a good pitting resistance, the chromium content is greater than 16%. The lower limit is set according to the intended application and may be 17%, 18%, 19%, 20%, 21% or 22%. However, chromium is a 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%. A better range includes 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 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 is also a strong ferrous salt forming element. The maximum content of molybdenum is therefore also 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 handling of metal scrap. Therefore, the maximum content is limited to 1%, preferably 0.2%, and more preferably may not be added.

Vanadium (2.0-5.0%)

In the matrix of steel, vanadium forms a uniformly distributed main precipitated M(N,C) type of nitrogenous carbide. In current steels, M is primarily vanadium, but chromium and molybdenum can also be present in significant amounts. Vanadium should therefore be present at a level 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%. The upper and lower limits can be freely combined within the scope defined in the first claim. A better range includes 2-4%.

Niobium ( 2.0%)

Niobium is similar to vanadium in the formation of nitrogen-containing carbides of the M(N,C) type and can be used in principle to replace vanadium, but the amount of niobium is required to be twice as large as vanadium. Therefore, the maximum addition amount of hydrazine is 2.0%. The binding content of (V+Nb/2) should be 2.0-5.0%. However, 铌 causes M(N, C) to form a polygonal shape. Therefore, the preferred maximum content is 0.5%. Preferably, no defects are added.

矽 (Silicon) 1.0%)

Radon is used for deoxidation.矽 is present in steel in decomposed form.矽 is a strong ferrous salt forming element and should therefore be limited to 1.0%.

Manganese (0.2-2.0%)

Manganese contributes to the improvement of the hardenability of steel, and together with manganese, sulfur can contribute to the improvement of machinability by forming manganese sulfide. Manganese should therefore be present at a minimum of 0.2%, preferably at least 0.3%. At higher sulfur levels, manganese prevents red brittleness in steel. The steel should contain a maximum of 2.0% manganese, preferably a maximum content of 1.0% manganese. A more preferred range is 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 in amounts up to 5%. It gives steel one Good hardenability and ductility. Due to price considerations, the nickel content of the 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 that helps to 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 the copper from the steel. This greatly makes waste disposal more difficult. For this reason, copper is usually not intentionally added.

Cobalt ( 10.0%)

Cobalt is a selective element. Helps increase the hardness of the granulated 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. A preferred maximum content is 0.2%.

Sulphur 0.5%)

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

Be (Be), Bi (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 0.01%)

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

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

These elements are carbide forming elements and are present in the alloy to modify the composition of the hard phase within the scope defined by the scope of the patent application. However, usually these elements will not be added.

Hard phases

The total content of hard phases MX, M ₂ X, M ₂₃ C ₆ and M ₇ C ₃ does 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 wherein the composition of the hard phase satisfies the following condition (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.

Pitting resistance equivalent (PRE)

Pitting resistance equivalent (PRE) is often used to quantify the pitting resistance of stainless steel. Higher values indicate higher resistance to pitting corrosion. For high nitrogen gamma loose iron 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 Worthing Titanization Temperature (TA), wherein the chromium content in the Worth Iron is at least 13%. For actual Worthfield ironization temperature (TA) and/or measurement in steel after quenching, the decomposition content can be calculated by Thermo-Calc.

The Worcester Tilding Temperature (TA) ranges from 950 to 1200 °C, typically 1080-1150 °C.

From the above inference, the Worthfield iron composition has a considerable effect on the pitting resistance of steel at the Wolsfield ironification temperature. The lower limit of the calculated PRE-value may be 25, 26, 27, 28, 29, 30, 31, 32 or 33.

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

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

Steel production

Tool steels having the chemical composition required by the claimed scope can be made by conventional gas atomization and subsequent powder nitridation prior to heat ramping. The nitrogen content in the steel after gas atomization is usually less than 0.2%. The remaining nitrogen is therefore added during the nitriding treatment of the powder. After curing, the steel can be used in the form of being hot pressed or formed into a desired shape. Typically, steel will harden and temper before being used.

The Worthfield ironation can be carried out via tempering at a Worthing ironization 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 (2 x 2 h).

example 1

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

The base steel component is melted and gas atomized. Next, the obtained powder is subjected to a nitriding treatment to introduce a desired amount of nitrogen into the powder. The nitrogen content is increased from about 0.1% to the respective content.

Thereafter, by performing a 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 was ironed at Vostian for 30 minutes at 1080 ° C and hardened by quenching in a vacuum furnace, by deep cooling in liquid nitrogen and then tempering twice at 200 ° C for 2 hours (2 x 2 h). 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 consists of tempered granules and hard phases. The microstructure of the two steels identified two different hard phases: MX and M ₂ X.

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

The material sensitivity for pitting corrosion was experimentally examined via an anodic polarization scan. An electrochemical cell with a saturated silver/silver chloride reference electrode and a carbon mesh counter electrode was used for cyclic polarization measurements. The 500 mesh ground samples were recorded with a first open circuit potential (OCP) of 0.1 M sodium chloride solution to ensure a stable potential. Next, the cyclic polarization measurement was performed at a scanning speed of 10 mV/min (mV/min). The starting potential is -0.2 V with respect to OCP, and the last potential is set to OCP. By setting one of the selection software, when the anode current density reaches 0.1 mA/cm ² , the up-potential scanning can be automatically reversed.

Figure 1 discloses an anodic polarization curve and information obtained from the curve. The forward scan gives information on the onset of pitting corrosion, while the reverse scan provides information on the repassivation properties of the alloy. Eb is the value of the click through potential. Above the aforementioned potential value, new pitting corrosion will begin and the existing pitting corrosion will spread. When the potential is lowered on the reverse scan, the current density is lowered. The alloy is repassivated when the reverse scan intersects the forward scan. Ep is a re-passivation potential, or a protection potential, that is, a 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 discloses that the present invention has a tendency to have less local corrosion resistance for steels having increased carbon content, and also discloses that the steel of the present invention is easier to repass passivate with other comparative steels. 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 chromium, molybdenum and nitrogen content than the comparative steel. Therefore, these reasons are not fully understood now. However, the inventors have perceived 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 basic components with variable carbon and nitrogen contents and expressed in weight% as follows: chromium: 19.8, molybdenum: 2.5, vanadium: 2.75; 矽: 0.3, manganese: 0.3, iron-balanced steel, different hard phases in steel The effect of the relative amounts 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 begins to form at a C/N ratio of about 0.25.

Figure 3 discloses that the calculated PRE-value is a function of the C/N ratio and the highest value obtained for the steel according to the invention is known from the figures.

Example 3

For steels with variable carbon and nitrogen contents and basic components expressed as weight % as follows: chromium: 18.2, molybdenum: 1.04, vanadium: 3.47; 矽: 0.3, manganese: 0.3, iron balance, different hard phases in steel The effect of the relative amounts 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 thus it is known that the number of M ₂ X decreases rapidly as the C/N ratio increases. It is also known from this that M ₂₃ C _{6 is formed} at a C/N ratio of about 0.3.

Figure 5 discloses that the calculated PRE-value is a function of the C/N ratio, and again the highest value obtained for the steel according to the invention is thus known.

These results demonstrate that an appropriate balance of carbon and nitrogen is an essential feature of the present invention. The increase in carbon content can be carefully controlled without causing problems with the M ₂₃ C ₆ and M ₇ C ₃ type carbides in the steel. These results show that if the carbon and nitrogen contents can be controlled as defined in the scope of the patent application, then the amount of hexagonal phase M ₂ X will decrease after hardening. This phase is primarily concerned with chromium dichromide (Cr ₂ N), but it can also include significant amounts of molybdenum. During Vostian, the reduction in the number of M ₂ X is the result of decomposition. Although under certain conditions, a part of these elements can be found in the increased part of MX (Fig. 2), as the PRE-number increases correspondingly to some extent, it shows that the decomposition of M ₂ X leads to decomposition in the matrix. The amount of chromium, molybdenum and nitrogen is increased. Thereafter, due to the formation of M ₂₃ C ₆ , the PRE-value will be reduced because the phase is rich in chromium and molybdenum.

Another mechanism that helps to improve the corrosion resistance disclosed in Table 1 and Figure 1 can be the boundary zone around the hard phase M ₂ X, which can 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 result in a lower solubility of chromium in this phase. This results in a reduced volume ratio of MX and more chromium remaining in the solid solution, which helps to improve corrosion resistance.

Accordingly, the present invention provides a cold tool steel made of a nitrogen-containing alloy by 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 resistance.

Claims (14)

A steel made by powder metallurgy consisting of the following elements (% by weight): Balance iron and impurities. A steel produced by powder metallurgy according to claim 1, wherein the upper limit content of the vanadium is 4.8%, 4.6%, 4.4%, 4.2% or 4.0%. A steel produced by powder metallurgy according to any of the preceding claims, wherein the steel satisfies the following conditions (in % by weight) at least one of: A steel produced by powder metallurgy according to any of the preceding claims, wherein the steel satisfies the following conditions (in weight percent) at least one of: carbon 0.35-0.45 A steel produced by powder metallurgy according to any of the preceding claims, wherein the steel satisfies the following conditions (in % by weight) at least one of: Except for the case where cobalt is added as defined in claim 4 A steel produced by powder metallurgy according to any of the preceding claims, wherein the microstructure comprises tempered granulated iron and consists of one or more of MX, M ₂ X, M ₂₃ C ₆ and M ₇ C ₃ The hard phase, and wherein the steel has a hardness of 58-64 HRC, preferably 58-64 HRC. A steel produced by powder metallurgy according to any one of the preceding claims, wherein the components of the hard phase MX, M ₂ X, M ₂₃ C ₆ and M ₇ C ₃ (% by volume) satisfy the following conditions: MX 5 -25, preferably 5-20, more preferably 5-15; M ₂ X 10, preferably 5, more preferably 1; M ₂₃ C ₆ +M ₇ C ₃ 10, preferably 5 more preferably 1; wherein M is one or more of vanadium, molybdenum and chromium, and X is one or more of carbon, nitrogen or boron. A steel produced by powder metallurgy as claimed in any of the preceding claims, wherein the steel at the Wolsfield ironization temperature (T _A ) at 1080 ° C has a calculated PRE 18, wherein PRE = chromium + 3.3 molybdenum + 30 nitrogen and chromium, molybdenum and nitrogen are calculated equilibrium contents decomposed in the matrix of T _A , wherein the chromium content in the Worth iron is at least 13%. A steel produced by powder metallurgy as claimed in any of the preceding claims, wherein the steel at the Wolsfield ironization temperature (T _A ) at 1080 ° C has a calculated PRE 20, wherein PRE = chromium + 3.3 molybdenum + 30 nitrogen and chromium, molybdenum and nitrogen are calculated equilibrium contents decomposed in the matrix of T _A , wherein the chromium content in the Worth iron is at least 16%. A steel produced by powder metallurgy as claimed in any of the preceding claims, wherein the steel at the Wolsfield ironization temperature (T _A ) at 1080 ° C has a calculated PRE 22, wherein PRE = chromium + 3.3 molybdenum + 30 nitrogen and chromium, molybdenum and nitrogen are calculated equilibrium contents decomposed in the matrix of T _A . A steel produced by powder metallurgy as claimed in any of the preceding claims, wherein the steel at the Wolsfield ironization temperature (T _A ) at 1080 ° C has a calculated PRE 25, wherein PRE = chromium + 3.3 molybdenum + 30 nitrogen and chromium, molybdenum and nitrogen are calculated equilibrium contents decomposed in the matrix of T _A . A method for producing steel having a composition as defined in the preceding claims, comprising the steps of: atomizing a steel alloy having a chemical composition other than the nitrogen content according to any one of the preceding claims, Performing a nitriding treatment to adjust the nitrogen content of the alloy to the content described in any one of the preceding claims, filling the powder into a container and subjecting the container to a heat equalizing treatment to form the obtained steel. And the steel is hardened and tempered. The method for producing steel according to claim 12, which comprises hardening at 950-1200 ° C, preferably at 1080-1150 ° C for 30 minutes, deep cooling the hardened steel in liquid nitrogen and at 180-250 ° C, Preferably, it is tempered twice at 200 ± 10 ° C for 2 hours. The method for producing steel according to claim 12, which comprises hardening at 950-1200 ° C, preferably at 1080-1150 ° C for 30 minutes, deep cooling the hardened steel in liquid nitrogen and 450-550 ° C, preferably at 500 ± 10 ° C, tempered twice for 2 hours.