KR102012950B1 - Hot-work tool steel and a process for making a hot-work tool steel - Google Patents

Abstract

The present invention, in weight%, C: 0.08-0.40, N: 0.015-0.30, C + N: 0.30-0.50, Cr: 1-4, Mo: 1.5-3, V: 0.8-1.3, Mn: 0.5- 2, Si: 0.1-0.5, optionally less than Ni: 3, Co: 5 or less, B: less than 0.01, and Fe: balance, excluding impurities, and hot chrome hot-work tool steels; A method for producing low chrome hot worked tool steel products with increased tempering resistance.

Description

HOT-WORK TOOL STEEL AND A PROCESS FOR MAKING A HOT-WORK TOOL STEEL}

The present invention relates to a method for manufacturing low chrome hot worked tool steels and low chrome hot worked tool steel products.

The term "hot-work tools" refers to a number of different types of tools, for example dies, inserts and cores, for working or forming metals at relatively high temperatures. Tools for die casting, such as inlet parts, nozzles, ejector elements, pistons, pressure chambers, and the like; Tools for extrusion tooling such as dies, die holders, liners, pressure pads and stems, spindles, and the like; Tools for hot pressing, such as tools for hot pressing of aluminum, magnesium, copper, copper alloys and steel; Molds for plastics such as molds for injection molding, compression molding and extrusion; This applies to a variety of other types of tools such as hot shear, shrink-rings / collars and tools for wear parts intended for use in working at high temperatures. Low alloy hot work tool steels are used in small to medium sized tools in applications where the demands on tempering resistance and thermal fatigue are high. Tempering resistance is the ability of a hot worked tool steel to maintain the hardness of the steel at elevated temperatures for a sustained period of time. Hot work tool steels are developed for strength and hardness during sustained exposure to elevated temperatures and generally use a significant amount of carbide forming alloys.

Other types of tool steels are high speed steels, which are used for cutting tools whose strength and hardness must be maintained at temperatures below or above 760 ° C. In order to reduce the required amounts of tungsten and chromium, such as 18% by weight and 4% by weight, respectively, variants using molybdenum (5-10% by weight) have been developed. High speed tool steels differ from hot tool steels in composition and price and cannot be used as a replacement for hot tool steels.

It is an object of the present invention to provide a low chrome hot worked tool steel having an improved property profile, in particular an improved tempering resistance.

The steels of the invention are particularly suitable for small tools that do not require steel compositions with high hardenability for their fabrication.

The object is to provide a low chrome hot working tool steel as defined in claim 1, ie

In weight percent,

C: 0.08-0.40

* N: 0.015-0.30

C + N: 0.30-0.50

Cr: 1-4

Mo: 1.5-3

V: 0.8-1.3

Mn: 0.5-2

Si: 0.1-0.5,

Optionally,

Ni: less than 3

Co: 5 or less

B: less than 0.01, and

Fe: obtained by providing a steel composed of a balance excluding impurities.

Further objects can be achieved by the low chrome hot working tool steel according to the invention, which satisfies one or more of the following conditions:

In weight percent,

C: 0.20-0.38, preferably 0.30-0.35

N: 0.03-0.30, preferably 0.03-0.10

C + N: 0.30-0.50, preferably 0.36-0.44

Cr: 1-3, preferably, 1.2-2.6

Mo: 1.9-2.9, preferably 2.2-2.8

V: 1.0-1.3, preferably 1.15-1.25

Mn: 1-2, preferably 1.1-1.9

Si: 0.1-0.5, preferably 0.2-0.4

Ni: less than 1, preferably less than 0.25

Co: less than 4, preferably less than 0.20

B: 0.001-0.01, preferably 0.001-0.005.

Preferred embodiments of low chrome hot worked tool steels may satisfy one or more of the following conditions:

In weight percent,

C: 0.25 to 0.35, preferably 0.27-0.34

N: 0.04-0.30, preferably 0.04-0.10

C + N: 0.38-0.42

Cr: 1.3-2.5, preferably 1.4-2.3.

More preferred embodiments of low chrome hot worked tool steels may satisfy one or more of the following conditions:

In weight percent,

N: 0.042-0.15, preferably 0.045-0.12

C + N: 0.39-0.41

Cr: 1.3-2.3, preferably, 1.4-2.1

Even more preferred embodiments of low chrome hot worked tool steels may satisfy one or more of the following conditions:

In weight percent,

C: 0.20-0.35, preferably 0.30-0.34

N: 0.042-0.12, preferably 0.045-0.12

C + N: 0.39-0.41

Cr: 1.4-1.9, preferably, 1.5-1.7

Mo / V: 1.8-2.3, preferably 1.9-2.1

Cr / V: less than 2, preferably less than 1.8

According to the inventive concept, the low chrome hot worked tool steel may have a composition (in weight%) according to the following examples:

C: 0.20-0.40

N: 0.03-0.30

C + N: 0.30-0.50

Cr: 1.2-2.3

Mo: 1-3

V: 0.8-1.3

Mn: 1-2

Si: 0.1-0.4

Ni: less than 1,

Optionally,

Co: 3 to 5

B: 0.001-0.01

Mo / V: 1.8-2.3

Cr / V: less than 2

Fe: balance excluding impurities, or

C: 0.20-0.40

N: 0.03-0.30

C + N: 0.30-0.50

* Cr: 1.2-2.3

Mo: 1.5-3

V: 0.8-1.3

Mn: 1-2

Si: 0.1-0.4

Ni: less than 1

Optionally,

Co: 3 to 5

B: 0.001-0.01

Mo / V: 1.8-2.3

Cr / V: less than 2

Fe: balance excluding impurities, or

C: 0.20-0.40

N: 0.04-0.30

C + N: 0.30-0.50

Cr: 1.2-2.3

Mo: 1-3

V: 0.8-1.3

Mn: 1-2

Si: 0.1-0.4

Ni: less than 1

Co <0.2

Optionally,

B: 0.001-0.01

Mo / V: 1.8-2.3

Cr / V: less than 2

Fe: balance excluding impurities, or

C: 0.20-0.38

N: 0.04-0.30

C + N 0.36-0.44

Cr: 1.2-2.3

Mo: 1.9-2.9

V: 0.8-1.3

Mn: 1-2

Si: 0.1-0.4

Ni: less than 0.25

Co: less than 0.20

Optionally,

B: 0.001-0.01

Mo / V: 1.8-2.3

Cr / V: less than 2

Fe: balance excluding impurities, or

C: 0.30-0.34

N: 0.04-0.09

C + N: 0.37-0.43

* Cr: 1.4-1.9

Mo: 2.2-2.8

V: 1.0-1.3

Mn: 1-2

Si: 0.2-0.4

Ni: less than 0.25

Co: less than 0.20

Optionally,

B: 0.001-0.005

Mo / V: 1.8-2.3

Cr / V: less than 2

Fe: balance excluding impurities.

Another object is to provide a low chrome hot worked tool steel product with an improved property profile, in particular an improved tempering resistance.

According to the invention, this object is achieved by a method as defined in claim 11, namely

a) providing a low chrome hot worked tool steel as defined in any of the claims,

b) forming a steel product from the steel composition,

c) austenitizing the steel product obtained in step b) to a temperature of at most 1200 ° C. for a period of approximately 30 minutes followed by quenching, and

d) tempering the quenched steel product for at least two hours at a temperature between 500 ° C and 700 ° C.

Preferred embodiments of this method are described in the dependent claims 12 to 15.

In creep resistant steel having a high chromium content, ie 9-12% by weight, it is possible to dissolve vanadium carbonitrides at relatively low temperatures, ie 1020-1050 ° C. However, if the chromium content is low, less than about 4-5% by weight, primary vanadium carbonitrides will form in the melt, which are virtually impossible to dissolve later.

In the steel of the present invention, the total amount of carbon and nitrogen should be adjusted to 0.30 ≦ (C + N) ≦ 0.50, preferably 0,36 ≦ (C + N) ≦ 0.44. The nominal content should be around 0.40% by weight. At the same time, the nitrogen content is advantageously adjusted to 0.015 to 0.30 N, preferably 0.015 to 0.15 N, and more preferably 0.015 to 0.10 N, and the carbon may preferably be adjusted to at least 0.20% by weight. have. Preferred ranges are described in the product claims.

When the nitrogen content is balanced at about 0.05 to 0.10 wt%, vanadium carbonitrides will form, which will dissolve in part during the austenitization step and then precipitate during the tempering step as nanometer sized particles. The thermal stability of vanadium carbides is better than the thermal stability of vanadium carbides, and as a result, the tempering resistance of low chrome hot worked tool steel products will be much improved. In addition, by two or more times of tempering, the tempering curve (showing hardness with tempering temperature) will have a higher secondary peak.

In the most preferred embodiment of the invention, the nitrogen content is preferably on the order of 0.05% by weight. This value gives better performance than higher values. Nitrogen content on the order of 0.05% by weight gives higher potential for secondary hardening in quenching at higher contents, thereby giving the steel a high hardness. However, an amount on the order of 0.10 wt% appears to impart a shift of the secondary cure peak to positive somewhat higher tempering temperatures. Preferred ranges are described in the product claims. In addition, the tests and modeling operations performed indicate that increased austenitization temperature is required in connection with increased nitrogen contents.

Chromium enhances the hardenability and corrosion resistance of the steels. At too low contents, it will adversely affect the corrosion resistance. Therefore, the minimum chromium content in the steel is set at 1% by weight. Maximum content of chromium rich carbides / carbonitrides, such as M ₂₃ C ₆ It is set at 4% by weight to avoid undesired formation. The chromium content will preferably not exceed 3% by weight and even more preferably will not exceed 2.6% by weight. In one embodiment of the invention, the chromium content is 1.5-1.7 weight percent. Preferred ranges are described in the product claims. The low chromium content delays the precipitation of chromium carbides in the microstructure in favor of a more thermally stable vanadium rich carbonitride. This slows recovery in the material and improves the tamper resistance.

The steel will comprise vanadium in an amount of at least 0.8% by weight to provide sufficient precipitation potential, thereby providing adequate tempering resistance and desired high temperature strength characteristics. The upper limit of vanadium is to avoid excessive formation of M (C, N) precipitates that can increase the risk of large undissolved precipitates remaining in the base after heat treatment and further concern of depletion of carbon and nitrogen in the base. 1.3 weight%. Preferably, vanadium is 1.0 to 1.3 weight percent. Preferred ranges are described in the product claims.

The ratio Cr / V should preferably be less than 2, more preferably less than 1.8 in order to obtain the desired MC phase. The reason is that Cr can be considered as a poison to the MC phase.

Silicon will be present in the steel in an amount between 0.1-0.5% by weight, preferably between 0.2-0.4% by weight. By keeping the content of silicon low, it is possible to obtain initial precipitation of metastable M ₃ C carbides. These carbides will act as a carbon reservoir for subsequent precipitation of the desired M (C, N) particles. In addition, precipitation of undesirable chromium rich M ₂₃ C ₆ particles at grain boundaries and lattice boundaries is avoided. Preferred ranges are described in the product claims.

Manganese is present to impart adequate hardenability to the steel, in particular given to the steel a relatively low content of chromium and molybdenum. The content of manganese in the steel is 0.5 to 2% by weight, preferably 1.0 to 2.0% by weight. Preferred ranges are described in the product claims.

Molybdenum will be present in the steel in an amount of 1.5 to 3% by weight, preferably 2.2 to 2.8% by weight, in order to provide secondary hardening during tempering and to contribute to hardenability. Preferred ranges are described in the product claims. Some of the molybdenum may be substituted with tungsten in a manner known per se, but the steel will preferably not contain any intentionally added amount of tungsten, ie any disadvantage associated with the presence of this element Because of this, it will not contain tungsten in amounts above the impurity level.

The ratio Mo / V is preferably in the range of 1.8-2.3, more preferably 1.9-2.1, in order to obtain the desired precipitation sequence and precipitation potential of the secondary carbides. Mo stabilizes M ₂ C and by adjusting the contents of Mo and V to be within the range of 1.8-2.3, molybdenum-rich M ₂ C can also be formed, which phase is higher coarsened compared to the vanadium-rich MC phase It is known to have a coarsening rate.

Nickel and cobalt are elements that can be included in steels in amounts up to 3 weight percent and 5 weight percent, respectively. Cobalt may increase hardness at high temperatures that may be beneficial for some applications of steel. If cobalt is added, the effective amount is about 4% by weight. Nickel can increase the corrosion resistance, hardenability and toughness of the steel. Preferred ranges are described in the product claims.

In principle, austenitization can be carried out at a temperature between a softening annealing temperature of 820 ° C. and a maximum austenitizing temperature of 1200 ° C., but preferably austenitization of steel products is preferably between about 1050-1150 ° C. Preferably at a temperature of 1080-1150 ° C, typically 1100 ° C. In-house tests show that higher austenitization temperatures shift the tempering hardness to higher temperatures, ie the secondary cure peak can be shifted to higher temperatures, which means that the desired hardness It will be reached at a higher tempering onset temperature. By this, the material will obtain improved tempering resistance and the working temperature of the tools will be raised.

Preferably, tempering of the quenched steel product is carried out two or more times with a retention time of two hours at a temperature of 500 to 700 ° C, preferably 550 to 680 ° C. In the most preferred embodiment of the steel composition, tempering is carried out at a temperature of 600 to 650 ° C, preferably 625 to 650 ° C.

Nitrogen contents in the range of 0.05-0.10% by weight can be obtained by including nitrogen by conventional casting methods to form the melt, casting the melt to form an ingot, and homogenizing the ingot by heat treatment. Nitrogen additives will result in large primary vanadium rich M (C, N) precipitates, thus imparting non-uniform hardness to the material. However, if the nitrogen content is lowered and a homogeneous heat treatment is present before subsequent forging, no large primary carbonitrides will occur.

In a variant of the steel, higher nitrogen contents than shown for the preferred embodiments would also be possible. In such variations, the nitrogen may be in an amount up to 0.30% by weight. In order to obtain higher nitrogen contents, conventional casting methods are insufficient. Instead, nitrogen first produces a steel powder of essentially desired composition, excluding nitrogen, and then nitrifies this powder in the solid phase with a nitrogen-containing fluid, for example nitrogen gas, and then forms an ingot. It can be included by isostatically hot pressing the powder at a temperature on the order of 1150 ° C. and a pressure on the order of 76 MPa. By fabricating tool steel by powder metallurgy, the problem of large primary carbide generation is avoided.

The ingot is preferably forged to a temperature of about 1270 ° C., then soft annealed to a temperature of about 820 ° C., and ready for cooling and austenitization at a rate of 10 ° C. per hour at a temperature of 650 ° C. Followed by free cooling in air.

The steel of the present invention has a much improved tempering resistance that allows for long life of the product in hot-work applications. As already indicated above, the nitrogen content is preferably on the order of 0.05% by weight and the chromium content is preferably less than 3% by weight, ie 1.2-2.6% by weight or 1.3-2.3% by weight.

The steel product of the present invention will preferably also meet some of the following requirements:

-Good tempering resistance,

Good high temperature strength,

Good thermal conductivity,

-Unacceptably high coefficient of thermal expansion.

The present invention will now be described in more detail with reference to preferred embodiments and the accompanying drawings.

1 is a diagram showing the hardness versus tempering temperature of an exemplary prior art low chrome hot worked tool steel that does not contain nitrogen.
FIG. 2 shows the hardness of prior art steels with Cr: 15, Mo: 1, C: 0.6 and Cr: 15, Mo: 1, C: 0.29, N: 0.35 (content of weight percent) at different tempering temperatures This is a diagram.
3 is a diagram illustrating the effect of low chromium content on the stability of M (C, N) in austenite.
4 is a diagram showing the mole fraction of M ₆ C, M (C, N) and bcc matrix with temperature (residual phase: austenite matrix).
5 is a diagram showing the amount of M (C, N) phase and metastable M ₂ C with temperature (residual phase: ferrite).
FIG. 6 is a diagram showing hardness versus tempering temperature curves for test alloys N0.05, N0.10 and N0.30.
FIG. 7 is a back-scattered SEM image showing small undissolved M (C, N) precipitates and globular mixed oxide-sulfide particles in NO.05.
FIG. 8 is a backscatter SEM image revealing primary M (C, N) undissolved at the former austenite grain boundaries in alloy NO.10.
9 is a backscattered SEM image showing primary particles in soft annealed NO.10.
FIG. 10 is a backscatter SEM image revealing a uniform distribution of undissolved M (C, N) particles at NO.30.
FIG. 11 is a backscatter SEM image revealing some clusters of undissolved M (C, N) found at N0.30.

Hot-work tool steels alloyed with molybdenum and vanadium media have good resistance to thermal fatigue, softening and high-temperature creep. Exemplary nominal chemical compositions of such prior art steels are provided in Table 1 (% by weight).

Each numerical value described in Table 1 is rounded off to three decimal places. Table 1 suggests that the steel of Table 1 has its high temperature characteristics for the precipitation of nanometer sized vanadium carbides during tempering. Such hard carbides of type 2900 HV impart secondary hardening of the material. 1 provides a tempering curve (hardness versus tempering temperature) for an exemplary prior art tool steel. Samples were austenitized at 1030 ° C. and then tempered twice to different temperatures; 200 ° C. to 700 ° C. for a tempering time of 2 hours + 2 hours. As can be seen, at intervals of 500 ° C. to 650 ° C., there is a secondary cure peak protruded at 550 ° C. Later work also shows that there is significant precipitation of metastable molybdenum rich M ₂ C in the exemplary prior art tool steel during tempering at 625 ° C., which contributes to the secondary curing effect do.

The hardness of the hot worked tool steel at elevated temperatures for a sustained period of time, the ability of the hot worked tool steel to maintain tempering resistance, can usually be linked to the tempering initiation temperature; If the material is kept at a temperature much lower than the tempering initiation temperature, the material will not be softened. Softening will be more pronounced at holding temperatures closer to or above the tempering initiation temperature.

If the secondary cure peak can be shifted to higher temperatures, this will mean that the desired hardness (eg, 44-46 HRC) can be reached at a higher tempering initiation temperature. Accordingly, the material will have improved tempering resistance and the working temperature of the tools will be raised.

Early work in high chrome steels suggests that when nitrogen is added to the steel, higher hardness can be obtained during tempering. Samples of Cr: 15, Mo: 1, C: 0.6 and Cr: 15, Mo: 1, C: 0.29, N: 0.35 were solutions treated at 1050 ° C, with water quenching and liquid nitrogen Following cooling, these samples were then tempered at different temperatures for 2 hours. As can be seen in FIG. 2, the peak hardness was significantly higher when adding nitrogen. The starting hardness of martensite is lower than that of nitrogen containing steel, but during tempering, this steel obtains higher hardness than that containing no nitrogen.

The explanation is that nitrogen causes the chromium to be more homogeneously distributed in the matrix due to the increased solubility of chromium in the austenite phase. After quenching, the martensite phase inherits a uniformly distributed chromium from austenite, and during tempering, a very finely distributed precipitation of chromium nitrides occurs, thus imparting a stronger curing effect to the material.

In addition, the substitution of nitrogen for portions of carbon is used to obtain higher hardness of the martensitic steel matrix. Nitrogen addition initially results in a large amount of retained austenite. However, this austenite can then be transformed into martensite by cold working, which makes it possible to obtain hardness as high as 68 HRC in this way.

It is shown that the low chromium content can have a positive effect on tempering resistance. A comparison of two different hot worked tool steels with 1.5% by mass and 5.0% by mass of chromium is that lower chromium content delays the precipitation of chromium carbides in the microstructure for a more thermally stable vanadium rich MC. Shown. This results in slow recovery in the material and improved tempering resistance.

However, studies on typical creep resistance of 9% to 12% by weight of chromium steel containing 0.06% by weight of N indicate that the low chromium contents dramatically stabilize MX (X is C + N) particles. (See Figure 3). If austenitization was performed at 1100 ° C., then all of the M (C, N) particles would be able to dissolve in the steel containing 10% by weight chromium. If the chromium content was lowered to 2.5% by weight (see example low chromium tool steel in FIG. 1), then large amounts of M (C, N) would still be present as austenite. Clearly, the result of the low chromium content is that only small amounts of interstitial will dissolve into austenite during the austenitization treatment.

According to the invention, a low chrome hot worked tool steel product with increased tempering resistance is made by performing the following method steps:

a) incorporating nitrogen in the low chrome hot working tool steel melt composition thereby providing a steel composition as defined in any of the preceding claims,

b) forming a steel product from the steel composition,

c) austenitizing the steel product obtained in step b) to a temperature of at most 1200 ° C. for a period of approximately 30 minutes followed by quenching, and

d) tempering the quenched steel product at least twice for a time of approximately 2 hours at a temperature between 500 ° C and 700 ° C.

Given the prior understanding in the art, these results predominantly teach that lowering the chromium content will result in reduced hardenability and difficulties for dissolving primary M (C, N) particles. Because, it is amazing.

In creep resistant steel having a high chromium content, i.e. 9 to 12% by weight, it is possible to dissolve vanadium carbo-nitrides at relatively low temperatures, i.e. 1020-1050 ° C. have. However, if the chromium content is low, less than about 4-5% by weight, primary vanadium carbonitrides will form in the melt, which are virtually impossible to dissolve later.

The inventors have found that when the nitrogen content is balanced at about 0.015 to 0.30 wt% in low chromium steel, vanadium carbonitrides will form, which will dissolve in part during the austenitization step and then temper as nanometer sized particles. It was found that it would precipitate out during the step. The particles are on the order of about 1 μm to about 10 μm. In some cases where the nitrogen content is low, typically 0.05% by weight, the average size of the particles is less than 1 μm. The thermal stability of vanadium carbides is better than the thermal stability of vanadium carbides, and as a result, the tempering resistance of low chrome hot worked tool steel products will be much improved. In addition, by two or more temperings, the tempering curve (showing hardness with tempering temperature) will have a higher secondary peak.

In a preferred embodiment of the steel, the nitrogen content is preferably on the order of 0.05% by weight. This value gives better performance than higher values. Nitrogen content on the order of 0.05% by weight imparts higher potential for secondary hardening in the quenching at higher contents.

In a preferred embodiment, the chromium content is preferably 1.5-1.7 weight percent. The low chromium content delays the precipitation of chromium carbides in the microstructure for a more thermally stable vanadium rich carbonitride. This slows recovery in the material and improves the tempering resistance.

In principle, austenitization can be carried out at a temperature between a softening annealing temperature of 820 ° C. and a maximum austenitization temperature of 1200 ° C. In a preferred embodiment, ie in a composition with a nitrogen content on the order of 0.05% by weight and a chromium content on the order of 1.5 to 1.7% by weight, the austenitization of the steel product is preferably between 1050-1150 ° C, preferably 1100 ° C Is run at a temperature of. In-house tests show that higher austenitization temperatures shift the tempering hardness to higher temperatures, ie the secondary cure peak can be shifted to higher temperatures, which means that the desired hardness It will be reached at a higher tempering onset temperature. By this, the material will obtain improved tempering resistance and the working temperature of the tools will be raised.

Preferably, tempering of the quenched steel product is carried out two or more times with a retention time of two hours at a temperature of 500 to 700 ° C, preferably 550 to 680 ° C. In the most preferred embodiment of the steel composition, tempering is carried out at a temperature of 600 to 650 ° C, preferably 625 to 650 ° C.

Nitrogen contents within 0.05-0.10 weight% can be obtained by including nitrogen by conventional casting methods to form the melt, casting the melt to form an ingot, and homogenizing the ingot by heat treatment. Nitrogen additives will result in large primary vanadium rich M (C, N) precipitates, thus imparting non-uniform hardness to the material. However, if the nitrogen content is lowered and a homogeneous heat treatment is present before subsequent forging, no large primary carbonitrides will occur.

In a preferred embodiment of the invention, the nitrogen content is preferably on the order of 0.05% by weight. This value gives better performance than higher values. Nitrogen content on the order of 0.05% by weight imparts a higher potential for secondary hardening in quenching than is done at higher contents, thereby imparting high hardness to the steel. However, an amount on the order of 0.10 wt% is shown to impart a shift of the secondary cure peak for positive and somewhat higher tempering temperatures. In addition, the tests and modeling calculations performed indicate that increased austenitization temperature is required in connection with increased nitrogen contents.

In a variant of the steel, a preferred embodiment with higher nitrogen contents than shown will also be possible. In such a variant, the nitrogen may contain up to 0.30% by weight. In order to obtain higher nitrogen contents, conventional casting methods are insufficient. Instead, nitrogen is then included, preferably steel powder of essentially desired composition, excluding nitrogen, first followed by nitriding of this solid powder with nitrogen gas and then forming an ingot at 1150 ° C. By hot pressing the powder isostatically to a temperature of about 76 MPa and a pressure of about 76 MPa. By fabricating the tool steel by powder metallurgy, the problem of primary carbide generation is avoided.

The ingot is preferably forged to a temperature of about 1270 ° C., then soft annealed to a temperature of about 820 ° C., and ready for cooling and austenitization at a rate of 10 ° C. per hour at a temperature of 650 ° C. Followed by free cooling in air.

Example  One

In Table 2 below, the chemical compositions of three different alloys (NO.05, NO.10 and NO.30) are provided in weight percent. NO.05 represents a material having a nitrogen content such as 0.05% by weight. Note that these are the actual compositions of the test ingots.

The purpose was to keep the level of all alloying elements except carbon and nitrogen constant. Compared to the standard low chrome hot worked tool steels in Table 1, chromium was also slightly reduced. Molybdenum content was slightly decreased and manganese content was increased. For carbon and nitrogen, the aim was to have these elements have a constant sum of approximately 0.40% by weight, which was achieved relatively well.

Each numerical value shown in Table 2 is rounded off to three decimal places. The tempering stage mainly takes into account meta-stable phases, and previous electron microscopy work has shown that these metastable phases are within the standard low chrome hot working tool steel at tempering temperature intervals, i.e. 400 to 700 ° C. Show what exists. These carbide phases are mainly vanadium rich MC (FCC) and molybdenum rich M ₂ C (HCP). In addition, some amount of chromium rich M ₇ C ₃ It is found in this standard low chrome hot working tool steel.

The following operations are performed to determine whether these nitrogen containing alloys are hardenable, i.e. whether sufficient alloying elements can be dissolved into the austenite matrix at the austenitization temperature so that martensite can be formed during quenching. Made. Thus, the temperature interval of interest was between 820 ° C., a softening annealing temperature, and 1200 ° C., the maximum austenitization temperature actually set.

The results of these equilibrium calculations are provided in FIG. 4. Here, the mole fractions of M ₆ C, M (C, N) and bcc known are plotted with temperature. The balance phase is austenite. Full curves represent NO.05; The dashed curves represent NO.10 and the dashed curves represent NO.30. Note that the high content of M (C, N) in the NO.30 alloy is uniform up to 1200 ° C. As expected, the bcc phase is unstable above 850 ° C. It is interesting to show that the slope of the equilibrium curve representing the amount of M (C, N) decreases with increasing nitrogen content. This means that it is more difficult to dissolve M (C, N) in NO.30 than in NO.05. As such, the amounts of carbon, nitrogen and vanadium are expected to be lowered at the NO.30 site after austenitization at 1100 ° C. than at the N0.05 base.

Since the molybdenum rich M ₆ C phase only dissolves carbon carbon and does not dissolve nitrogen, the carbon content is lower in NO. 10 and NO. 30, thereby reducing the amount of M ₆ C as the carbon content decreases. do. It should also be noted that all M ₆ C is dissolved at the austenitization temperature used.

Operations performed in the tempering temperature region were performed only to estimate the potential for secondary precipitation at NO.05, NO.10 and NO.30. The found equilibria can best represent what phases exist in the material after a sufficiently long time. The previous work has actually shown that there is some auto-tempering in standard low chrome hot working tool steels. This means that M ₃ C (cementite) will precipitate after the austenitization process.

Results from operations in the tempering temperature zone are provided in FIG. 5. Full curves represent NO.05, dashed line curves represent NO.10, and dashed line curves represent NO.30. Secondary curing usually occurs between 500 ° C. and 650 ° C., and at this temperature interval there is no significant difference between NO.05 and NO.10 with respect to the amount of M (C, N), and On the one hand, NO. 30 has a high and almost constant amount of M (C, N), possibly due to high vanadium and nitrogen contents.

Higher carbon content in NO.05 produces more M ₂ C phases in equilibrium with NO.10. In NO.30 there is much less M ₂ C.

Based on previous calculations, it should be possible to estimate the potential for secondary precipitation in these alloys after austenitization at a given temperature. This potential depends on the difference in the amounts of the M (C, N) phase and the M ₂ C phase between the equilibrium at the austenitization temperature and the metastable equilibrium at the tempering temperature. In Table 3, these differences are provided as secondary precipitation potentials for three different alloys. These values are given in mole percent.

Each numerical value shown in Table 3 is rounded off to three decimal places. The results provided in Table 3 show that NO.05 has the best hardening response due to the low amount of M (C, N) phase present at 1100 ° C., ie many alloying elements have austenite matrix It can be dissolved into. In addition, NO.05 indicates that it has an optimum potential for good secondary curing during tempering at 625 ° C.

Example  2

Two alloys (NO.05 and NO.10) were cast in a conventional manner as small ingots of 50 kg. NO.10 was the first test and there was no homogeneous treatment performed on this ingot prior to the forging process. In the second test, NO.05, a homogeneous treatment was applied at 1300 ° C. for 15 hours before forging. The third alloy, NO.30, had a nitrogen content too high to be produced by conventional casting. Thus, these alloys were made using powder metallurgy. First, a steel powder was produced, which was then nitrided into a solid state by pressurized N ₂ gas. The powder was then hot isostatically pressed (HIP) at 1150 ° C. at a pressure of 76 MPa.

All three ingots were forged at 1270 ° C., and then samples were cut out by dimensions of 15 × 15 × 8 mm. The samples were first soft annealed at 820 ° C. and the sequence for cooling after annealing was 650 ° C. at 10 ° C. per hour and then free air cooled. After soft annealing, NO.05 was austenitized at 1100 ° C. for 30 minutes. To compensate for the lower potential for precipitation, NO. 10 was austenized at 1150 ° C. for 30 minutes and NO. 30 was austenitized at 1200 ° C. for 30 minutes. Nine samples from each of the three alloys were tempered at the following temperatures: 450 ° C, 525 ° C, 550 ° C, 575 ° C, 600 ° C, 625 ° C, 650 ° C, 675 ° C and 700 ° C. The soaking time was 2 hours, which was double tempering, ie the total tempering time was 4 hours. After the heat treatment, the hardness of the samples was measured. Scanning transcriptional microscopy (SEM) was performed to further investigate the morphology, distribution and size of the undissolved particles in the samples. The SEM tool used was FEI Quanta 600 F.

Hardness measurements

Results from the hardness measurements are provided in FIG. 6. As can be seen, all three alloys have secondary curing peaks at temperature intervals of 500 ° C. to 650 ° C. All tempering was done for 2 hours + 2 hours. NO.05 had the highest hardness under quenching (as-quenched) condition (53FIRC), while NO.10 and NO.30 had somewhat lower hardness. However, all three alloys are considered hardenable. The hardening curve of NO.05 is very similar to the curve of standard low chrome hot worked tool steel, which is maximum at approximately 54 HRC as shown in FIG. 1.

The secondary curing peak of NO.10 will shift somewhat to higher temperatures with peak hardness at 600 ° C. Peak hardness for both NO.05 and NO.30 was at 550 ° C.

Scanning electron microscopy

The undissolved M (C, N) particles, the alloy with the lowest nitrogen content, in conventionally cast NO.05 have an average size of less than 1 μm. This is comparable to ordinary undissolved carbides in the river. Another phase that is easily found in NO.05 is a mixture of aluminum oxide and manganese-sulphide (see FIG. 7), and FIG. 7 shows spherical mixed oxide-sulfide particles (1) in NO.05. And SEM image (backscattering) showing small undissolved M (C, N) precipitates (2). Samples were austenitized at 1100 ° C. for 30 minutes and tempered at 625 ° C. for 2 hours + 2 hours.

The reason for the large number of nonmetallic inclusions in NO.05 (and NO.10) is that all test ingots were manufactured and cast in an open atmosphere.

The most common size of M (C, N) particles in NO.10 is a circle equivalent diameter of 5 μm to 10 μm after being austenitized at 1150 ° C. for 30 minutes and tempered at 625 ° C. for 2 hours + 2 hours. Equivalent Circle Diameter (ECD). Larger primary carbides 3 (precipitated in the melt) are frequently found in the former austenite grain boundaries (see FIG. 8), and FIG. 8 shows cosmetic at the former austenite grain boundaries in alloy NO.10. Solution, backscattered SEM image revealing primary M (C, N). Samples are austenitized at 1150 ° C. for 30 minutes and tempered at 625 ° C. for 2 hours + 2 hours.

9 is a detailed SEM micrograph of the primary M (C, N) particles 4 in NO.10. These particles were automatically discovered in the SEM using the INCA Feature software from Oxford Instruments. The sharp edges of the particles indicated that these particles had precipitated out of the melt. White areas in the image are molybdenum rich M ₆ C particles 5. Note that in this case, the sample was soft annealed NO.10.

In powder metallurgy NO.30, the undissolved M (C, N) particles 6 had a size distribution (ECD) between 1 and 5 μm, the most common being 2 μm, thereby Even with a high nitrogen content, the particles were small. The particles were distributed homogeneously in the microstructure (see FIG. 10). However, as shown in FIG. 11, some clusters 7 of M (C, N) have been found.

The chemical composition of the undissolved particles on M (C, N) in all three alloys was measured by EDS and the results are given in Table 4, which shows the alloys (NO.05, NO.10 and Chemical composition of M (C, N) particles in NO. These values are given in atomic percent. Although the accuracy in EDS for light elements such as carbon and nitrogen is not so high, the balance of carbon and nitrogen on M (C, N) can be expected based on nominal compositions It will be appreciated. The ± values given in the table are those given in the INCA program (Oxford instruments). Some of the iron recorded is probably derived from the surrounding base, in particular for alloy NO.05.

[Industry availability]

The method and low chrome hot working tool steels of the present invention are applicable where it is desired to obtain hot working tool steels that can be utilized at increased temperatures for an extended period of time.

Claims (15)

As a low chrome hot working tool steel tool for working metal, forming metal, or for processing and forming metal at high temperatures,
In weight percent,
C: 0.08-0.40
N: 0.015-0.30
C + N: 0.30-0.50
Cr: 1-4
Mo: 1.5-3
V: 0.8-1.3
Mn: 0.5-2
Si: 0.1-0.5,
Optionally,
Ni: less than 3
Co: 5 or less
B: less than 0.01, and
Fe: consists of balance except impurities,
Vanadium carbonitride has a size of 10 μm or less,
Low chrome hot work tool steel tool.
The method of claim 1,
In weight percent,
C: 0.20-0.38;
N: 0.03-0.30;
C + N: 0.30-0.50;
Cr: 1-3;
Mo: 1.9-2.9;
V: 1.0-1.3;
Mn: 1-2;
Si: 0.1-0.5;
Ni: less than 1;
Co: less than 4; And
B: 0.001-0.01; satisfies one or more conditions selected from the conditions consisting of,
Low chrome hot work tool steel tool. The method according to claim 1 or 2,
In weight percent,
C: 0.25-0.35;
N: 0.04-0.30;
C + N: 0.38-0.42; And
Cr: 1.3 to 2.5; satisfies one or more conditions selected from the conditions consisting of,
Low chrome hot work tool steel tool. The method according to claim 1 or 2,
In weight percent,
N: 0.042-0.15;
C + N: 0.39-0.41; And
Cr: satisfies one or more conditions selected from the conditions consisting of 1.3-2.3;
Low chrome hot work tool steel tool. The method according to claim 1 or 2,
In weight percent,
C: 0.20-0.35;
N: 0.042-0.12;
C + N: 0.39-0.41;
Cr: 1.4-1.9;
Mo / V: 1.8-2.3; And
Cr / V: less than 2; satisfies one or more conditions selected from the conditions consisting of
Low chrome hot work tool steel tool. The method of claim 1,
In weight percent,
C: 0.20-0.40
N: 0.03-0.30
C + N: 0.30-0.50
Cr: 1.2-2.3
Mo: 1-3
V: 0.8-1.3
Mn: 1-2
Si: 0.1-0.4
Ni: less than 1,
Optionally,
Co: 3-5
B: 0.001-0.01
Mo / V: 1.8-2.3
Cr / V: less than 2
Fe: composed of balance excluding impurities,
Low chrome hot work tool steel tool. The method of claim 1,
In weight percent,
C: 0.20-0.40
N: 0.03-0.30
C + N: 0.30-0.50
Cr: 1.2-2.3
Mo: 1.5-3
V: 0.8-1.3
Mn: 1-2
Si: 0.1-0.4
Ni: less than 1,
Optionally,
Co: 3-5
B: 0.001-0.01
Mo / V: 1.8-2.3
Cr / V: less than 2
Fe: composed of balance excluding impurities,
Low chrome hot work tool steel tool. The method of claim 1,
In weight percent,
C: 0.20-0.40
N: 0.04-0.30
C + N: 0.30-0.50
Cr: 1.2-2.3
Mo: 1-3
V: 0.8-1.3
Mn: 1-2
Si: 0.1-0.4
Ni: less than 1
Co: less than 0.2,
Optionally,
B: 0.001-0.01
Mo / V: 1.8-2.3
Cr / V: less than 2
Fe: composed of balance excluding impurities,
Low chrome hot work tool steel tool. The method of claim 1,
In weight percent,
C: 0.20-0.38
N: 0.04-0.30
C + N 0.36-0.44
Cr: 1.2-2.3
Mo: 1.9-2.9
V: 0.8-1.3
Mn: 1-2
Si: 0.1-0.4
Ni: less than 0.25
Co: less than 0.20,
Optionally,
B: 0.001-0.01
Mo / V: 1.8-2.3
Cr / V: less than 2
Fe: composed of balance excluding impurities,
Low chrome hot work tool steel tool. The method of claim 1,
In weight percent,
C: 0.30-0.34
N: 0.04-0.09
C + N: 0.37-0.43
Cr: 1.4-1.9
Mo: 2.2-2.8
V: 1.0-1.3
Mn: 1-2
Si: 0.2-0.4
Ni: less than 0.25
Co: less than 0.20,
Optionally,
B: 0.001-0.005
Mo / V: 1.8-2.3
Cr / V: less than 2
Fe: composed of balance excluding impurities,
Low chrome hot work tool steel tool. delete delete delete delete delete

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