TW201814067A - Hot work tool steel - Google Patents

Abstract

The invention relates hot work tool steel. The steel comprises the following main components (in wt. %): C 0.26 - 0.38 Si 0.1 - 0.3 Mn 0.1 - 0.8 Cr 1.4 - 3.9 Mo 2.0 - 3.0 W 0.8 - 1.5 Ni 0.6 - 1.7 V 0.1 - 0.4 balance optional elements, iron and impurities.

Description

 Hot working tool steel  

This invention relates to a hot working tool steel.

Vanadium alloy matrix tool steels have been on the market for decades and have received considerable attention due to the fact that they combine high wear resistance with excellent dimensional stability and because of their good toughness. These steels have a wide range of applications, such as die casting and forging. These steels are usually manufactured by conventional metallurgy followed by Electroslag Remelting (ESR).

Uddeholm DIEVAR ^® is a high performance chrome molybdenum vanadium steel, as described in WO 99/50468 A1, which contains a balanced carbon and vanadium content.

Low chromium tool steels alloyed with nitrogen and vanadium are also known for hot working, especially for small tools that do not require high hardenability but are highly resistant to tempering and thermal fatigue. The tempering resistance is the ability of the hot working tool steel to maintain its hardness at high temperatures for a long time. Such steel is disclosed in WO 2012/119925 A1.

Although vanadium alloy tool steels manufactured by ESR have better properties in terms of thermal cracking, total cracking, thermal attrition, and plastic deformation than conventionally manufactured tool steels, further improvements are needed to reduce hot work tool failures, such as in high pressure molds. Risk of thermal cracking and total cracking in die casting. In addition, it may be advantageous to further improve the thermal strength and tempering resistance of the hot worked tool steel.

It is an object of the present invention to provide a hot worked tool steel having an improved profile which results in an increased tool life.

Another object of the present invention is to provide a steel which has improved tempering resistance and high toughness and good hardenability, thereby allowing the manufacture of large materials having good properties. It also wants to improve hot cracking while maintaining good heat and abrasion resistance and good resistance to total cracking. A further object is still to provide a steel composition in powder form which is suitable for additive manufacturing (AM), in particular for the manufacture or repair of injection moulding tools and moulds.

The above objects and additional advantages are achieved to a significant extent by providing hot work tool steel having the composition as described in the scope of the patent application.

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

【Detailed description】

The following is a brief description of the importance of each component and its interactions, as well as the chemical composition of the alloy being ordered. Throughout the specification, all percentages of the chemical composition of steel are provided in weight percent (wt.%). The amount of hard phase is provided in volume % (vol. %). The upper and lower limits of individual components can be freely combined within the limits stated in the scope of the patent application.

Carbon (0.26-0.38%)

It is present at a minimum level of 0.26%, preferably at least 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33 or 0.34%. The upper limit of carbon is 0.38% and can be set to 0.37, 0.36 or 0.35%. The preferred range is from 0.30 to 0.38% and from 0.33 to 0.37%. In any case, the amount of carbon should be controlled so that the amount of one of the M ₂₃ C ₆ , M ₇ C ₃ and M ₆ C type carbides in the steel is limited. Preferably, the steel does not contain such primary carbide.

矽 (0.1-0.3%)

The lanthanide is used for deoxidation. The Si system is present in the steel in dissolved form. Si is a ferrite former and increases carbon activity, thus increasing the risk of forming undesired carbides, which has a negative impact on impact strength. Tantalum is also prone to interfacial separation, which may result in reduced toughness and thermal fatigue resistance. Therefore, Si is limited to 0.3%. The upper limits can be 0.29, 0.28, 0.27, 0.26, 0.25, 0.24, 0.23, and 0.22%. The lower limit can be 0.12, 0.14, 0.16, 0.18, and 0.20%. A preferred range is from 0.10 to 0.25% and from 0.15 to 0.24%.

Manganese (0.1-0.8%)

Manganese contributes to the improvement of the hardenability of steel and, together with sulphur manganese, contributes to improved machinability by forming manganese sulphide. Therefore, manganese should be present in a minimum amount of 0.1%, preferably at least 0.2, 0.3, 0.35, 0.4, 0.45 or 0.5%. At higher sulfur levels, manganese prevents red brittleness in steel. Mn may also cause undesired microseparation resulting in a banded structure. The steel should contain a maximum of 0.8%, preferably a maximum of 0.7, 0.6, 0.55 or 0.5%.

Chromium (1.4-3.9%)

The chromium system is present in an amount of at least 1.6% to provide good hardenability in a larger cross section during heat treatment. If the chromium content is too high, high temperature ferrite may be formed, which reduces hot workability. The lower limit can be 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5 or 2.6%. The upper limit can be 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7 or 3.8%.

Molybdenum (2.0-3.0%)

Mo is known to have a very advantageous effect on hardenability. Molybdenum is necessary for obtaining a good secondary hardening reaction. The minimum content is 2.0% and can be set to 2.1, 2.2 or 2.3%. Molybdenum is a strong carbide forming element and is also a strong ferrite forming body. Therefore, the maximum content of molybdenum is 3.0%. Mo can be limited to 2.9, 2.8, 2.7, 2.6, 2.5 or 2.4%.

Tungsten (0.8-1.5%)

Tungsten is an essential element of the invention. If the steel is subjected to nitrogen atomization, W contributes to secondary hardening and does not react with nitrogen to form a nitride. During secondary hardening, tungsten can form carbides of the M ₂ C type. It is believed that the large radius of the tungsten atom makes it slow to diffuse, thus positively contributing to improved tempering resistance.

Nickel (0.6-1.7%)

Nickel should be present in an amount of from 0.6 to 1.7% to provide good hardenability and toughness of the steel. Furthermore, Ni in combination with Mo seems to reduce the amount of retained austenite in the steel of the desired type. However, due to cost, the nickel content of the steel should be limited. Therefore, the upper limit can be set to 1.6, 1.5, 1.5, 1.4 or 1.3%. The lower limit can be set to 0.7, 0.8, 0.9, 1.0 or 1.1%.

Vanadium (0.1-0.4%)

Vanadium forms uniformly distributed V(N,C) type first stage precipitated carbides and carbonitrides in the steel matrix. This hard phase can also be expressed as MX, where M is primarily V but Cr and Mo can be present, and X is one or more of C, N, and B. Therefore, vanadium should be present in an amount of from 0.1 to 0.4%. The upper limit can be set to 0.39, 0.38, 0.37, 0.36, 0.35, 0.34, 0.33, 0.32, 0.31 or 0.30%. The lower limit can be 0.15, 0.16, 0.17, 0.18, 0.19 or 0.20%.

Aluminum (0.001-0.06%)

Aluminum is used in combination with Si and Mn for deoxidation. The lower limit is set to 0.001, 0.003, 0.005 or 0.007% to ensure good deoxidation. The upper limit is limited to 0.06% to avoid precipitation of undesired phases (such as AIN). The upper limit can be 0.05, 0.04, 0.03, 0.02 or 0.015%.

Nitrogen (0.01-0.12%)

Nitrogen is an element selected as appropriate. N can be limited to 0.01-0.12% to obtain the desired type and amount of hard phase (especially V(C, N)). When the nitrogen content is properly balanced for the vanadium content, a vanadium-rich carbonitride V(C,N) is formed. These will partially dissolve during the austenitizing step and then precipitate as nano-sized particles during the tempering step. The thermal stability of vanadium carbonitrides is considered to be superior to the thermal stability of vanadium carbides, thus improving the tempering resistance of tool steels and increasing grain growth resistance at high austenitizing temperatures. The lower limit can be 0.011, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017, 0.018, 0.019 or 0.02%. The upper limit can be 0.11, 0.10, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04 or 0.03%.

Copper (0.02-2.0%)

Cu is an element selected as appropriate, which can contribute to increase the hardness and corrosion resistance of the steel. If used, the preferred range is from 0.02 to 1%. However, once copper is added, it is impossible to extract copper from steel. This makes waste disposal more difficult. For this reason, copper is not intentionally added.

cobalt( 8%)

Co is an element chosen as appropriate. Co causes the solidus temperature to rise, thus providing an opportunity to increase the hardening temperature, which can be 15-30 ° C higher than without Co. Therefore, a greater portion of the carbides can be dissolved during austenitization, thereby increasing the hardenability. Co also increases the M _s temperature. However, a large amount of Co may cause a decrease in toughness and wear resistance. If added, the maximum amount is 8%, and the effective amount can be 2-6%, especially 4% to 5%. However, for practical reasons (such as waste disposal), Co is usually not intentionally added. The maximum impurity content can then be set to 1%, 0.5%, 0.3%, 0.2% or 0.1%.

niobium( 0.1%)

Niobium is similar to vanadium because it forms a carbonitride of the M(N,C) type. However, Nb causes M(N, C) to have a more angular shape and may reduce hardenability at high contents. Therefore, the maximum amount is 0.1%, preferably 0.05%. The Nb precipitate is more stable than the V precipitate and can therefore be used for grain refinement because the fine dispersion of NbC acts as a pinning grain boundary, resulting in grain refinement and improved toughness and improved softening resistance at high temperatures. . For this reason, Nb may be present in an amount of 0.1%, preferably 0.01% to 0.05%.

Ti, Zr and Ta

These elements are carbide formers and may be present in the alloy in a desired range to alter the composition of the hard phase. However, these elements are usually not added.

boron( 0.01%)

B can be used to further increase the hardness of the steel. The amount is limited to 0.01%, preferably 0.005%. The preferred range for the addition of B is from 0.001 to 0.004%.

Ca, Mg and REM (Rare Earth Metals)

These elements may be added to the steel in the amount requested to modify non-metallic inclusions and/or to further improve machinability, hot workability and/or weldability.

Impurity element

P, S and O are the main impurities which usually have a negative effect on the mechanical properties of the steel. Therefore, P can be limited to 0.03%, preferably 0.01%. S can be limited to 0.0015, 0.0010, 0.0008, 0.0005 or even 0.0001%. O can be limited to 0.0015, 0.0012, 0.0010, 0.0008, 0.0006 or 0.0005%. However, S may be selected in the range of 0.005-0.5%, especially 0.05-0.3%, to improve the machinability of the steel.

Steel manufacturing

Tool steels having the desired chemical composition can be made by conventional metallurgy, including melting in an electric arc furnace (EAF) and further refining in a ladle, optionally before casting. Vacuum treatment. The ingot can also be subjected to electroslag remelting (ESR) to further improve the cleanliness and microstructure uniformity of the ingot. However, the preferred processing route for the steel requested is gas atomization followed by hot isostatic pressing (HIP). The steel thus produced can be used as a HIP or subjected to further processing such as forging and rolling.

Usually steel is subjected to hardening and tempering before being used.

Austenitization can be carried out at an austenitizing temperature (T _A ) in the range of from 1000 to 1070 ° C, preferably from 1030 to 050 ° C. A typical T _A is 1040 ° C with a hold time of 30 minutes followed by rapid quenching. The tempering temperature is selected according to the hardness conditions, and is carried out at 600-650 ° C for at least two times (2 × 2 h) for 2 hours, followed by cooling in air.

In this embodiment, high-alloy steel Uddeholm Dievar ^® comparing the two according to the invention.

The alloy has the following composition (in wt.%):  

The rest are iron and impurities.

All steels were subjected to austenitizing treatment at 1020 ° C in a vacuum furnace, and then gas quenching was carried out in the interval of 800-500 ° C for 100 seconds (t _8/5 = 100 s). Steel 1 and Uddeholm Dievar ^® were tempered at 615 ° C for 2 hours 2 times (2 x 2). Steel 2 was subjected to tempering at 615 ° C for 2 hours 3 times (3 x 2).

The tempering resistance of the alloy was examined at a temperature of °C. While the steel of the invention has a lower initial hardness at the start of the test, it can be seen from FIG. 1, Steel and Comparative Steel Uddeholm Dievar ^® compared to the present invention had significantly better tempering resistance.

Industrial utilization

The tool steel of the present invention has a mold for requiring good hardenability and good temper resistance. The atomized powder of the alloy can be used to make a HIP product having excellent structural uniformity. The alloy powder can be used in the manufacture or repair of molds, especially by additive manufacturing methods.

Claims (9)

A tool steel for thermal processing consisting of the following components in wt% (wt.%): C 0.26-0.38, Si 0.1-0.3, Mn 0.1-0.8, Cr 1.4-3.9, Mo 2.0- 3.0, W 0.8-1.5, Ni 0.6-1.7, V 0.1-0.4, depending on the situation, choose one or more of the following: Al 0.001-0.06, N 0.01-0.12, Cu 0.02-2, Co 8, Nb 0.1, Ti 0.05, Zr 0.05, Ta 0.05, B 0.01,Ca 0.00005-0.009, Mg 0.01, REM 0.2, S 0.005-0.5, except Fe, the rest is Fe.  A steel according to the first aspect of the patent application, wherein the steel is produced by gas atomization followed by thermal pressure equalization, and wherein at least 80% of the carbides, nitrides and/or carbonitrides in the microstructure have an equivalent circle The Equivalent Circle Diameter (ECD) is less than 5 μm, preferably less than 2.5 μm, wherein the ECD is 2√A/π, where A is the surface of the carbide particles in the study portion.    According to the steel of item 1 or 2 of the patent application, it satisfies at least one of the following conditions: C 0.26-0.37, Mn 0.2-0.8, Cr 1.6-3.7, Mo 2.0-3.0, W 0.8-1.5, Ni 0.6-1.6, V 0.15-0.40, Al 0.001-0.06, N 0.01-0.10, Nb 0.01-0.05.    A steel according to any one of the preceding claims, which satisfies at least one of the following conditions: C 0.28-0.35, Mn 0.2-0.7, Cr 1.7-3.5, Mo 2.1-2.9, W 0.8-1.3, Ni 0.8- 1.5, V 0.20-0.35, Al 0.005-0.05, N 0.01-0.07.    A steel according to any one of the preceding claims, which satisfies the following conditions: C 0.28-0.35, Cr 1.7-3.5, Mo 2.1-2.9, W 0.8-1.3, Ni 0.8-1.5, N 0.01-0.07.    A steel powder having the composition according to any one of items 1 and 3 to 5 of the patent application.   a steel powder according to item 6 of the patent application, wherein the size of the powder is 500 μm.  A use of a steel powder according to item 6 or item 7 of the patent application, which is used for laser coating, additive manufacturing or metal injection molding.    A mold comprising at least a portion of an alloy as defined in any one of claims 1 and 3 to 5.