CN115917031A

CN115917031A - Hot-working tool steel

Info

Publication number: CN115917031A
Application number: CN202180042183.1A
Authority: CN
Inventors: S.艾纳马克; A.克瓦内德; R.奥利弗
Original assignee: Uddeholms AB
Current assignee: Uddeholms AB
Priority date: 2020-06-12
Filing date: 2021-06-11
Publication date: 2023-04-04
Also published as: WO2021251892A1; KR20230024334A; CA3182089A1; EP4165224A1; TW202206620A; SE544123C2; US20240011135A1; BR112022025309A2; MX2022015305A; JP2023530420A; SE2050705A1

Abstract

The present invention relates to a matrix type hot work tool steel having improved wear resistance in demanding applications in use. It is particularly suitable for applications in hot forging, die casting or hot extrusion. It should also be suitable for press hardening, in particular for Advanced High Strength Steels (AHSS), and have a high hot wear resistance. The hot-work tool steel according to the invention has a composition as defined by the independent claim.

Description

Hot-working tool steel

Technical Field

The present invention relates to matrix type hot work tool steel.

Background

Vanadium alloy matrix tool steels have been on the market for decades and have attracted considerable interest due to the fact that they combine high wear resistance with excellent dimensional stability and good toughness. The matrix tool steel is a steel which does not contain any primary carbides (primary carbides) or contains only a very low content of small primary carbides and has a matrix consisting of tempered martensite.

US 3117863 may be the first patent for base steel. The basic idea of US 3117863 is to produce a steel with the composition of a known High Speed Steel (HSS) matrix. The structure of this type of steel was developed to improve the toughness and fatigue strength of the steel by refining the microstructure.

WO 03/106727 A1 by the applicant discloses a hot-worked base steel having excellent toughness and ductility as well as good hot strength and wear resistance. The material is marketed under the name

And are known.

EP1 300 482 A1 discloses another base steel with high hardness and wear resistance and very high toughness and is therefore particularly suitable for tools which are stressed at high temperatures, such as tools for hot and warm forming. The steel is marketed under the name W360

Is known and has a nominal composition of 0.50% C, 0.20% Si, 0.25% Mn, 4.5% Cr, 3.00% Mo and 0.60% V.

Base steels are typically produced by Vacuum Arc Remelting (VAR) or electroslag remelting (ESR) to improve chemical homogeneity and micro-cleanliness.

Other examples of hot-working tool base steels are given in JP2003226939A, EP3050986A1, US2004/0187972A1 and US2005/0161125A 1.

Modern base steels are being developed with the aid of software for calculating phase diagrams and equilibrium phase balances as a function of temperature. Themo-

(TC) is a user-friendly and frequently used software for this purpose, in order to find compositions that result in large austenite single phase areas (areas) at soaking temperatures, because of the fact that the dissolution of possibly present MC carbides, formed by segregation during casting, is of the utmost importance.

Hot-working base steels have a wide range of applications such as die casting and forging. Steel is typically produced by conventional metallurgy followed by electroslag remelting (ESR). However, a disadvantage of the known steel is the limited wear resistance. In particular, wear resistance may limit the life of known steels in harsh hot working operations, such as hot forging, extrusion, and press hardening. These tools are expensive and often require welding to effect repairs. Therefore, solderability is important. However, the weldability of tool steels with a high carbon content is generally considered to be poor and requires special measures, such as a high preheating temperature. It would therefore be useful if the steel could be welded with standard welding consumables (preferably without preheating).

Disclosure of Invention

It is an object of the present invention to provide a matrix type hot work tool steel with improved wear resistance in use in demanding applications. In particular, the steel should be suitable for use in hot forging, die casting or hot extrusion applications. It should also be suitable for press hardening, in particular for Advanced High Strength Steels (AHSS). For these applications, the thermal abrasion resistance must be high.

Tempering resistance is an important property because in use, steel can experience high temperatures for long periods of time. Therefore, it is preferable that the steel not only have high hardness after hardening but also have small decrease in hardness. Other important properties include high ductility and toughness, which means that the steel should have a high cleanliness with respect to micro-slag, be completely free of grain boundary carbides and a uniform hardness for thicknesses up to 300 mm.

It should be possible to adjust the hardness over a large interval in order to optimize the steel for the intended use. It should also be possible to obtain high tensile strength and yield strength in combination with sufficient ductility.

The foregoing objects, as well as additional advantages, are achieved in large part by providing a hot-work tool steel having the composition set forth in the claims.

The invention is defined in the claims.

Detailed Description

The importance of the individual elements and their interaction with each other and the limitations of the chemical composition of the claimed alloys are briefly explained in the following. All percentages of the chemical composition of the steel are given in weight% (wt.%) throughout the specification. The amount of hard phase is given in vol.%. The upper and lower limits of the individual elements may be freely combined within the limits set forth in the claims. The arithmetic precision of the values can be increased by one or two digits. Thus, a value given as e.g. 0.1% may also be expressed as 0.10% or 0.100%.

Carbon (0.5-0.9%)

Present in a minimum content of 0.5%, preferably at least 0.55, 0.60, 0.66, 0.67 or 0.68%. The upper limit of carbon is 0.9%, and may be set to 0.85, 0.80, 0.75, 0.74, 0.73, or 0.72%. Preferred ranges are 0.6-0.8% and 0.65-0.75%. In any case, the amount of carbon should be controlled so as to limit M in the steel ₂₃ C ₆ 、M ₇ C ₃ And M ₆ The amount of primary carbides of type C, preferably the steel is free of such primary carbides.

Silicon (0.03-0.8%)

Silicon is used for deoxidation. Si is present in dissolved form in the steel. Si is a strong ferrite former and increases the carbon activity and thus the risk of formation of undesired carbides, which adversely affects the impact strength. Silicon also tends to segregate at interfaces, which can lead to reduced toughness and thermal fatigue resistance. Therefore, si is limited to 0.8%. The upper limit may be 0.7, 0.6, 0.5, 0.40, 0.35, 0.30, 0.28, 0.27, 0.26, 0.25, 0.24, 0.23, and 0.22%. The lower limit may be 0.05, 0.10, 0.11, 0.12, 0.13, 0.14 or 0.15%.

Manganese (0.1-1.8%)

Manganese contributes to improving the hardenability (hardenability) of the steel and, together with manganese sulphide, to improving machinability. Thus, manganese should be present at a minimum level of 0.1%, preferably at least 0.2, 0.3, 0.35 or 0.4%. At higher sulphur contents, manganese prevents red brittleness in the steel. Mn also causes undesirable micro-segregation, resulting in a ribbon-like structure. The steel should contain a maximum of 1.8%, preferably a maximum of 0.8, 0.75, 0.7, 0.6, 0.55 or 0.5%.

Chromium (4.0-6.6%)

Chromium will be present in a content of at least 4% in order to provide good hardenability in a larger cross section during heat treatment. If the chromium content is too high, this can lead to the formation of high temperature ferrites, which reduce hot workability. The lower limit may be 4.5, 4.6, 4.7, 4.8 or 4.9%. The upper limit may be 6.0, 5.9, 5.8, 5.7, 5.6, 5.5, 5.4, 5.3, 5.2, or 5.1%.

Molybdenum (1.8-3.5%)

Mo is known to have a very favourable effect on hardenability. Molybdenum is critical to achieving a good secondary hardening response. The minimum content is 1.8%, and may be set to 1.9, 2.0, 2.1, 2.15, or 2.2%. Molybdenum is a strong carbide former and also a strong ferrite former. The maximum content of molybdenum is therefore 3.5%. Mo may be limited to 2.9, 2.7, 2.6, 2.5, 2.4 or 2.3%.

Tungsten (W is less than or equal to 0.5%)

Tungsten is not an essential element in the present invention. The upper limit is 0.5%, and may be set to 0.4, 0.3, 0.2, or 0.1%.

Nickel (less than or equal to 1%)

Nickel is not an essential element in the present invention. The upper limit may be set to 0.5, 0.4, 0.3, or 0.25%.

Vanadium (1.3-2.3%)

In the matrix of steel, vanadium forms uniformly distributed primary precipitated carbides and carbonitrides of the VC and V type (C, N). These carbides and carbonitrides may also be represented as MX, where M is primarily V, but Cr and Mo may be present, and X is one or more of C, N and B. However, hereinafter, VC is used only in the same sense as MX. Vanadium is used to form a controlled amount of relatively large VC and should therefore be present in an amount of 1.3% -2.3%. The lower limit may be set to 1.35, 1.4, 1.45, 1.5, or 1.55%. The upper limit may be set to 2.2, 2.1, 2.0, 1.9, 1.8, 1.7, or 1.65%.

Aluminum (less than or equal to 0.1%)

Aluminum may be 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 deoxygenation. The upper limit is limited to 0.1% to avoid precipitation of undesirable phases such as AlN. The upper limit may be 0.05, 0.04 or 0.3%.

Nitrogen (less than or equal to 0.12%)

Nitrogen is an optional element. N is limited to 0.12% to avoid too high amounts of hard phases, in particular V (C, N). However, the nitrogen content may be balanced with the vanadium content to form primary precipitated vanadium-rich carbon nitrides. These will partially dissolve during the austenitizing step and then precipitate as nano-sized particles during the tempering step. The thermal stability of vanadium carbo-nitride is considered to be better than that of vanadium carbide, so that tempering resistance of the tool steel can be improved and grain growth resistance at high austenitizing temperature can be enhanced. If the nitrogen content is intentionally controlled for the above reason, the lower limit may be set to 0.006, 0.007, 0.08, 0.09, 0.01, 0.012, 0.013, 0.014, or 0.015%. The upper limit may be 0.11, 0.10, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04, or 0.03%.

Copper (1% or less)

Cu is an optional element that may help increase the hardness and corrosion resistance of the steel. However, once copper is added, it is not possible to extract copper from the steel. This greatly makes waste disposal more difficult. For this reason, copper is usually not added intentionally. The upper limit may be limited to 0.5, 0.4, 0.3, 0.2, or 0.15%.

Cobalt (less than or equal to 5 percent)

Co is an optional element. Co causes the solidus temperature (solidus temperature) to rise and thus provides an opportunity to raise the hardening temperature. Thus, during austenitization, a greater portion of the carbides may be dissolved, thereby enhancing hardenability. However, co is expensive, and a large amount of Co may also cause a decrease in toughness and wear resistance. Therefore, the maximum amount is 5%. However, intentional addition of Co is not usually performed. The maximum content may be set to 2, 1, 0.5 or 0.2%.

Niobium (less than or equal to 0.1%)

Niobium is similar to vanadium in that it forms carbonitrides of the M (N, C) type. However, nb results in a more angular shape of M (N, C) and at high contents can reduce hardenability. Therefore, the maximum amount is 0.1%, preferably 0.05%. Nb precipitates are more stable than V precipitates and therefore can be used for grain refinement because the fine dispersion of NbC acts as pinning (pining) grain boundaries, resulting in grain refinement and improved toughness and improved softening resistance at high temperatures. For this purpose, nb is an optional element and may be present in an amount of 0.1% or less. The upper limit may be set to 0.06, 0.05, 0.04, 0.03, 0.01, or 0.005%. The lower limit may be set to 0.005, 0.006, 0.007, 0.008, 0.009, or 0.01%.

Ti, zr and Ta

These elements are carbide forming elements and may be present in the alloy in the claimed range for changing the composition of the hard phase. However, these elements are not usually added. The amount of each element is preferably 0.5% or less, 0.1% or less, or 0.05% or less, preferably 0.01% or 0.005%.

Boron (less than or equal to 0.01%)

B may be used to further increase the hardness of the steel. The amount is limited to 0.01%, preferably 0.006% or less, more preferably 0.005%.

Ca. Mg and REM (rare earth metals)

These elements may be added to the steel in the claimed amounts for modifying the non-metallic inclusions and/or for further improving the machinability, hot workability and/or weldability. The amounts of Ca and Mg are preferably 0.01% or less, preferably 0.005% or less. The amount of REM is preferably ≦ 0.2%, preferably ≦ 0.1%, or even 0.05%.

Impurity element

During the manufacture of steel, impurity elements cannot be avoided. Thus, impurity elements are included with the balance, the level of which is not essential to the definition of the invention.

P, S and O are the major impurities that generally negatively impact the mechanical properties of the steel. These elements are unavoidable and may be present in the steel in the usual impurity contents. However, since these elements may have a negative effect on the properties of the steel, their impurity content may be further limited. Preferred limitations are as follows. P may be limited to 0.1%, 0.05 or 0.03%. S may be limited to 0.5, 0.1, 0.05, 0.0015, 0.0010, 0.0008, 0.0005, or even 0.0001%. O may be limited to 0.01, 0.003, 0.0015, 0.0012, 0.0010, 0.0008, 0.0006, or 0.0005%.

Steel production

Tool steels with the claimed chemical composition can be produced by conventional metallurgy including smelting (melting) in an Electric Arc Furnace (EAF) and further refining in a ladle (ladle), optionally followed by vacuum treatment before casting. The ingot (ingot) may also be subjected to electroslag remelting (ESR) to further improve the cleanliness and microstructure uniformity of the ingot. In addition, the steel may also be subjected to Vacuum Induction Melting (VIM) and/or Vacuum Arc Remelting (VAR). An alternative processing route to the claimed steel is gas atomization followed by Hot Isostatic Pressing (HIP) to form a hot isostatic pressed (HIPed) ingot, which may also be used under hot isostatic pressed (as-HIPed) conditions. The ingot may be subjected to further hot working to final dimensions and soft annealed to a Brinell hardness of 360HBW or less, preferably 300HBW or less. Brinell hardness is measured with a 10mm diameter tungsten carbide ball and a load of 3000kgf (29400N), and may also be expressed as HBW _10/3000 . Prior to use, the steel may undergo hardening and tempering.

The steel is typically delivered to the customer under soft annealing conditions that result in a ferritic matrix with carbides uniformly distributed therein. For large dimensions and according to a preferred embodiment, the soft annealed steel also has uniform properties, the uniformity of hardness should have an average hardness of ≦ 360HBW, and for a thickness of at least 100mm, and a maximum deviation from the average brinell hardness value in the thickness direction measured according to ASTM E10-01 of less than 10%, preferably less than 5%, and wherein the minimum distance of the center of the indentation (indentation) from the edge of the sample or the edge of another indentation should be at least 2.5 times the diameter of the indentation, and the maximum distance should not exceed 4 times the diameter of the indentation.

Atomized powders may also be used for additive manufacturing.

Herein, the present invention will be described in more detail.

The hot-worked steel according to the invention consists of, in weight% (% wt.%):

the balance being iron and impurities.

Preferably, the hot-work tool steel meets at least one of the following requirements:

more preferably, the composition of the steel meets one or more of the following requirements:

preferably, the steel meets at least one of the following requirements:

in a particularly preferred embodiment, all these requirements are met.

In order to enhance the wear resistance, the composition may be adjusted so that the steel in hardened and tempered condition contains a small and controlled amount of vanadium carbides having a size greater than or equal to 1 μm. The dimensions are given in terms of Equivalent Circular Diameter (ECD) calculated from the image area (a) obtained in the image analysis. The ECD has the same projected area as the particle and it is equal to 2 √ (a/π).

The steel should preferably contain 0.2-4 vol%, preferably 0.5-3 vol% and more preferably 1.5-2.3 vol% VC.

M ₆ C and M ₇ C ₃ The amount of (b) should be limited to 2 vol%, preferably 0.5 vol%, and more preferably 0.1 vol% each.

By selecting the austenitizing time and temperature, the cooling rate (t) expressed as the cooling time in the temperature interval of 800 ℃ to 500 ℃ _5/8 ) And the tempering temperature, the hardness of the steel can be adjusted. Typically, the steel is tempered twice for two hours (2 x2 h) in order to reduce the amount of retained austenite to less than 2 vol%.

After hardening and tempering to a hardness of 55-57HRC, the mechanical properties of the steel should preferably meet at least one of the following requirements:

yield strength (rp0.2): 1700MPa or more, preferably 1725MPa or more, and more preferably 1750MPa or more.

Tensile strength (Rm): 1950MPa or more, preferably 2050MPa or more, more preferably 2050MPa or more, most preferably 2100MPa or more.

Elongation (A5): 3% or more, preferably 4% or more, more preferably 5% or more, most preferably 6% or more.

Reduction of area (Z): 5% or more, preferably 10% or more, more preferably 15% or more, most preferably 20% or more.

Example 1

Table 1 discloses hardness in Rockwell C (HRC) as a function of hardening parameters austenitizing time and temperature. It can be seen that the hardness can be easily adjusted in the range of 49 to 61 HRC. The composition of the ESR ingot was as follows: 0.71 percent of C, 0.22 percent of Si, 0.46 percent of Mn, 5.01 percent of Cr, 2.24 percent of Mo, 1.62 percent of V, and 0.007 percent of Al.

TABLE 1 hardness in hardened and tempered conditions (HRC). For all samples, cooling and tempering in vacuum at t8/5=300s for 2x2h.

The tempering resistance of the steels austenitized at 1130 ℃ and tempered at 580 ℃ and 600 ℃, respectively, was examined. The steel samples were subjected to heating at 600 ℃ for 10 hours. In the first case, the hardness was reduced from 58.4 to 53.6HRC, and for the second sample, the hardness was reduced from 55.9 to 52.8HRC. Thus, the hardness loss was 4.8HRC and 3.1HRC, respectively.

These values can be compared with the steels mentioned at the outset

Are compared. Samples of the steel were prepared with a nominal composition of C0.5%, si 0.2%, mn 0.5%, cr 5.0%, mo 2.3% and V0.5%. By austenitizing at 1050 ℃ for 30 minutes, at t _8/5 =300s and tempered 2x2h at 540 ℃, the steel was hardened to 57.8HRC. The initial hardness was 57.8HRC and the hardness after 10 hours at 600 ℃ was 49.4HRC. Therefore, the hardness loss of steel is known to be 8.4HRC. It can therefore be concluded that the steel according to the invention has a better tempering resistance than the known steels.

The cleanliness of the steels according to the invention with respect to the micro-slags was tested according to ASTM E45-97, method A, panel I-r and the results are given in Table 2.

A

B

C

D

T

H

T

H

T

H

T

H

0.5

0

0.5

1.0

0

0.5

1.0

TABLE 2 cleanliness of plaques I-r according to ASTM E45-97, method A.

Example 2

The ESR ingot of example 1 was hot rolled to a diameter of 196mm, from which three samples were taken in the LC2 direction and examined for mechanical properties. By austenitizing at 1050 ℃ for 30 minutes, t _8/5 Cooling in vacuum for 300 seconds followed by tempering twice at 560 ℃ for 2 hours hardened the steel sample to a hardness of 56HRC. The following average values for the examinations are given below:

yield strength (rp0.2): 1761MPa

Tensile strength (Rm): 2117MPa

Elongation (A5): 7 percent of

Reduction of area (Z): 26 percent of

Example 3

In this example, the inventive steel is compared to a standard base steel or forging tool used.

The alloy has the following composition (in weight%):

the balance being iron and impurities.

The alloy is subjected to standard heat treatment, forging and soft annealing to a hardness of about 300 HBW. Both steels were subjected to hardening and tempering by heating to 1100 ℃ for 30 minutes, quenched and tempered twice (2 x2 h) at 540 ℃ during two hours. The hardness of the inventive steel was 57HRC, and the hardness of the comparative steel was 56HRC. The wear resistance of the steel was checked by the Pin-on-Disk (Pin on Disk) method using 800 mesh alumina paper from the same batch. The wear loss of the inventive steel was found to be 178mg/min and the wear loss of the comparative steel was found to be 219mg/min.

Other samples of the inventive steel were prepared to obtain the same hardness as the comparative steel. This was achieved by heating to 1100 ℃ for 30 minutes and tempering at 540 ℃ for 2x2h. The hardness was 56HRC. As expected, the wear loss of this sample was slightly higher (189 mg/min) compared to the steel with a hardness of 57HRC, but significantly lower than the comparative steel with the same hardness.

Example 4

Steel samples of the same composition as in example 1 were prepared for the weld test. A solid block of steel was milled to a sharp 90 ° internal angle and the sample was subjected to two different hardening treatments. The first heat treatment comprises austenitizing at 1050 ℃ for 30 minutes, t _8/5 Cooling in vacuum for 300 seconds, followed by 5 secondsTempering was carried out twice at 60 ℃ for 2 hours. The second heat treatment is different in that austenitizing is performed at 1130 c for 10 minutes.

The samples were then TIG welded at Room Temperature (RT), 80 ℃, 225 ℃ and 325 ℃ using 1.6mm diameter rods with three different standard welding consumables. Applicants own Caldie TIG and QRO TIG and UTP A696 TIG from Schweissmaterial GmbH.

At all temperatures, the consumable Caldie TIG experienced cracking. Surprisingly, however, it was found that two other consumables could be used to produce crack-free welds also at room temperature, without cracks. Thus, the steel of the invention has surprisingly good weldability.

Industrial applicability

The steel of the present invention is useful for hot working applications where the tool is subjected to wear. In particular, the steel is suitable as a tool for hot forging, press hardening, die casting, high-pressure die casting or hot extrusion.

The claims (modification according to treaty clause 19)

1. Hot work tool steel for hot forging, press hardening, die casting or hot extrusion consisting of by weight (wt.%):

the balance being iron and impurities.

2. The hot work tool steel according to claim 1, wherein the steel fulfils at least one of the following requirements:

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3. the hot work tool steel according to claim 1 or 2, wherein the steel fulfils at least one of the following requirements:

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4. the hot work tool steel according to any one of the preceding claims, wherein the steel comprises carbides having a size ≧ 1 μm, and at least one of the following requirements is satisfied with respect to the amount of carbides in volume%:

VC 0.2–4

M ₆ C ≤2

M ₇ C ₃ ≤2

5. the hot work tool steel according to claim 4, wherein the steel contains carbides with a size ≧ 1 μm, and at least one of the following requirements is satisfied with respect to the amount of carbides in volume%:

VC 0.5–3

M ₆ C ≤0.5

M ₇ C ₃ ≤0.5

6. the hot work tool steel according to claim 5, wherein the steel contains carbides with a size ≧ 1 μm, and at least one of the following requirements is satisfied with respect to the amount of the carbides in volume%:

VC 1.5–2.3

M ₆ C ≤0.1

M ₇ C ₃ ≤0.1

7. the hot work tool steel according to any of the preceding claims, wherein the steel after hardening and tempering has a hardness of 55-57HRC, and wherein the steel fulfils at least one of the following requirements:

according to ASTM E45-97, method A, panels I-r, cleanliness meets the following maximum requirements with respect to micro-slags:

A

B

C

D

T

H

T

H

T

H

T

H

1.0

0

1.5

1.0

0

1.5

1.0

8. the hot work tool steel according to any of the preceding claims, wherein the steel fulfils at least one of the following requirements:

9. the hot work tool steel according to any of the preceding claims, wherein the steel fulfils at least one of the following requirements:

10. the steel according to any one of claims 1-6 having an average hardness of ≦ 360HBW, wherein the steel has a thickness of at least 100mm and a maximum deviation from the average Brinell hardness value in the thickness direction measured according to ASTM E10-01 of less than 10%, preferably less than 5%, and wherein the minimum distance of the center of a score from the edge of a sample or the edge of another score should be at least 2.5 times the diameter of the score and the maximum distance should not exceed 4 times the diameter of the score.

11. Use of the steel according to any of the preceding claims as a tool for hot forging, press hardening, die casting, high pressure die casting or hot extrusion.

Claims

the balance being iron and impurities.

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VC 0.2–4

M ₆ C ≤2

M ₇ C ₃ ≤2

VC 0.5–3

M ₆ C ≤0.5

M ₇ C ₃ ≤0.5

VC 1.5–2.3

M ₆ C ≤0.1

M ₇ C ₃ ≤0.1

A A B B C C D D T H T H T H T H 1.0 0 1.5 1.0 0 0 1.5 1.0

9. the steel according to any one of claims 1-3, wherein the steel is soft annealed and has an average hardness of ≦ 360HBW, and wherein the steel has a thickness of at least 100mm and a maximum deviation from the average Brinell hardness value in the thickness direction measured according to ASTM E10-01 of less than 10%, preferably less than 5%, and wherein the minimum distance of the center of an indentation from the edge of a sample or the edge of another indentation should be at least 2.5 times the diameter of the indentation and the maximum distance should not exceed 4 times the diameter of the indentation.

10. Use of the steel according to any of the preceding claims as a tool for hot forging, press hardening, die casting, high pressure die casting or hot extrusion.