JP5837945B2 - Hot working tool steel articles - Google Patents

Description

  The present invention relates to a low chromium hot work tool steel and a method for producing a low chromium hot work tool steel article.

  The term “hot working tool” is used for many different types of tools that process or form metal at relatively high temperatures, for example, dies, inserts and cores, inlet parts, nozzles, ejector components, Tools for die casting, such as pistons and pressure chambers; Tools for extrusion tools such as dies, die holders, liners, pressure pads and stems, spindles; Hot pressing aluminum, magnesium, copper, copper alloys, steel Hot press tools, such as tools for injection; Molds for plastics such as injection molding, pressure molding, extrusion molds; Hot shearing tools, reinforcement rings / collars, wear for high temperature processing Used for various other types of tools such as parts.

  Low alloy hot work tool steels are used for small to medium size tools in applications where demands on tempering resistance and thermal fatigue are high. Tempering resistance is the ability of hot work tool steel to maintain hardness at high temperatures for long periods of time. Hot work tool steels have been developed for strength and hardness during long-term exposure to high temperatures and generally use significant amounts of carbide forming alloys.

  Another type of tool steel is a high speed steel used in cutting tools where strength and hardness must be maintained at high temperatures up to and above 760 ° C. In order to reduce the amount of tungsten and chromium required (eg 18 and 4% by weight, respectively), variations with molybdenum (5-10% by weight) were developed. High speed steel, unlike hot work steel in composition and price, cannot be used as an alternative to hot work steel.

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

  The steel of the present invention is particularly suitable for small tools that do not require a steel composition with high hardenability for production.

The object is to achieve the low chromium hot-worked steel as defined in claim 1, i.e. by weight%,
C 0.08-0.40
N 0.015-0.30
C + N 0.30 to 0.50
Cr 1-4
Mo 1.5-3
V 0.8-1.3
Mn 0.5-2
Si 0.1-0.5
In some cases, Ni <3
Co ≦ 5
B <0.01
The balance is achieved by providing steel made of iron, with the exception of impurities.

Further objects can be achieved by a low chromium hot work tool steel according to the invention that satisfies one or more of the following conditions (in weight percent):
C 0.20 to 0.38, preferably 0.30 to 0.35
N 0.03-0.30, preferably 0.03-0.10
C + N 0.30 to 0.50, preferably 0.36 to 0.44
Cr 1-3, preferably 1.2-2.6
Mo 1.9 to 2.9, preferably 2.2 to 2.8
V 1.0 to 1.3, preferably 1.15 to 1.25
Mn 1-2, preferably 1.1-1.9
Si 0.1-0.5, preferably 0.2-0.4
Ni <1, preferably <0.25
Co <4, preferably <0.20
B 0.001-0.01, preferably 0.001-0.005.

A preferred form of low chromium hot work tool steel can satisfy one or more of the following conditions (in weight percent):
C 0.25 to 0.35, preferably 0.27 to 0.34
N 0.04-0.30, preferably 0.04-0.10
C + N 0.38 to 0.42
Cr 1.3-2.5, preferably 1.4-2.3.

A more preferred form of low chromium hot work tool steel can satisfy one or more of the following conditions (in weight percent):
N 0.042 to 0.15, preferably 0.045 to 0.12
C + N 0.39 to 0.41
Cr 1.3-2.3, preferably 1.4-2.1.

A more preferred form of low chromium hot work tool steel can satisfy one or more of the following conditions (in weight percent):
C 0.20 to 0.35, preferably 0.30 to 0.34
N 0.042 to 0.12, preferably 0.045 to 0.12
C + N 0.39 to 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 <2, preferably <1.8
According to the concept of the present invention, the low chromium hot work tool steel may have a composition (in% by weight) according to the following example.
C 0.20-0.40
N 0.03-0.30
C + N 0.30 to 0.50
Cr 1.2-2.3
Mo 1-3
V 0.8-1.3
Mn 1-2
Si 0.1-0.4
Ni <1
In some cases Co 3-5
B 0.001-0.01
Mo / V 1.8-2.3
Cr / V <2
Remaining Fe apart from impurities, or Fe
C 0.20-0.40
N 0.03-0.30
C + N 0.30 to 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 <1
In some cases Co 3-5
B 0.001-0.01
Mo / V 1.8-2.3
Cr / V <2
Remaining Fe apart from impurities, or Fe
C 0.20-0.40
N 0.04-0.30
C + N 0.30 to 0.50
Cr 1.2-2.3
Mo 1-3
V 0.8-1.3
Mn 1-2
Si 0.1-0.4
Ni <1
Co <0.2
In some cases
B 0.001-0.01
Mo / V 1.8-2.3
Cr / V <2
Remaining Fe apart from impurities, or Fe
C 0.20 to 0.38
N 0.04-0.30
C + N 0.36 to 0.44
Cr 1.2-2.3
Mo 1.9 to 2.9
V 0.8-1.3
Mn 1-2
Si 0.1-0.4
Ni <0.25
Co <0.20
In some cases B 0.001-0.01
Mo / V 1.8-2.3
Cr / V <2
Remaining Fe apart from impurities, or Fe
C 0.30 to 0.34
N 0.04-0.09
C + N 0.37 to 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 <0.25
Co <0.20
In some cases
B 0.001-0.005
Mo / V 1.8-2.3
Cr / V <2
The remainder Fe except for impurities.

  Another object of the present invention is to provide a low chromium hot work tool steel article having an improved property profile, particularly improved tempering resistance.

According to the invention, this object is achieved by the method as defined in claim 11, ie a) providing a low chromium hot work tool steel as defined in any claim,
b) forming a steel article from the steel composition;
c) a step of austenitizing the steel article obtained in step b) at a temperature of 1200 ° C. or less for about half an hour, followed by quenching; and d) quenching the steel article at 500 ° C. and 700 ° C. It is achieved by a method comprising a step of tempering at least twice at a temperature between 0 ° C. and a time of about 2 hours.

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

  In a creep resistant steel having a high chromium content (ie 9-12% by weight), vanadium carbonitride can already be dissolved at a relatively low temperature (ie 1020-1050 ° C.). However, if the chromium content is low, if it is less than 4 to 5% by weight, primary vanadium carbonitride is formed in the molten steel, which cannot practically be dissolved thereafter.

  In the steel of the present invention, the total amount of carbon and nitrogen will be adjusted to 0.30 ≦ (C + N) ≦ 0.50, preferably 0.36 ≦ (C + N) ≦ 0.44. The nominal content will be around 0.40% by weight. At the same time, the nitrogen content can be adjusted to N between 0.015 and 0.30, preferably between 0.015 and 0.15, more preferably between 0.015 and 0.10. Advantageously, the carbon can preferably be adjusted to at least 0.20% by weight. Preferred ranges are given in the product claims.

  When the nitrogen content is balanced from about 0.05 to 0.10% by weight, vanadium carbonitrides are formed, which partially dissolve during the austenitizing process and then during the tempering process. Will deposit as nanometer-sized particles. The thermal stability of vanadium carbonitride is better than that of vanadium carbide, thus greatly improving the tempering resistance of low chromium hot work tool steel articles. Furthermore, by tempering at least twice, the tempering performance curve (indicating hardness as a function of tempering temperature) will have a higher secondary peak.

  In the most preferred embodiment of the present invention, the nitrogen content is preferably on the order of 0.05% by weight. This value gives better performance than a higher value. A nitrogen content on the order of 0.05% by weight gives a higher potential for secondary hardening during quenching compared to a higher content, thus giving the steel a higher hardness. However, quantities on the order of 0.10% by weight have been shown to shift the secondary cure peak to a somewhat higher tempering temperature, which is positive. Preferred ranges are given in the product claims. In addition, tests and modeling calculations performed indicate that an increased austenitizing temperature is required in conjunction with increased nitrogen content.

Chromium promotes the hardenability and corrosion resistance of steel. At too low a content, the corrosion resistance will be adversely affected. Therefore, the minimum chromium content in the steel is set to 1% by weight. The maximum content is set to 4% by weight in order to avoid unwanted formation of chromium-rich carbide / carbonitrides such as M ₂₃ C ₆ . 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% by weight. Preferred ranges are given in the product claims. The low chromium content delays chromium carbide precipitation in the microstructure, in favor of a thermally more stable vanadium rich carbonitride. Accordingly, recovery in the material is slowed and tempering resistance is improved.

  In order to provide sufficient precipitation potential and thus suitable tempering resistance and desirable high temperature strength properties, the steel will contain vanadium in an amount of at least 0.8 wt%. An excess of M (C, N) precipitates (which increases the risk of large, non-solid precipitates remaining in the matrix after heat treatment, and further depletion of carbon and nitrogen in the matrix). In order to avoid excessive formation, the upper limit of vanadium is 1.3% by weight. Preferably, the vanadium is between 1.0 and 1.3% by weight. Preferred ranges are given in the product claims.

  The Cr / V ration 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 a poison for the MC phase.

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

  Manganese is present to provide adequate hardenability to the steel, especially when the chromium and molybdenum content in the steel is relatively low. The manganese content in the steel is between 0.5 and 2% by weight, preferably between 1.0 and 2.0% by weight. Preferred ranges are given in the product claims.

Molybdenum provides secondary hardening during tempering and in an amount between 1.5 and 3 wt%, preferably 2.2 to 2.8 wt%, in order to provide a hardenability contribution, Will be present in steel. Preferred ranges are given in the product claims.
Some of the molybdenum may be replaced with tungsten as is known per se, but preferably the steel contains an intentionally added amount of tungsten, i.e. an amount above the impurity level. There will be no. This is due to certain disadvantages associated with the presence of this element.

The Mo / V ratio should preferably be in the range of 1.8 to 2.3, more preferably in the range of 1.9 to 2.1 in order to obtain the precipitation potential of the secondary carbides and the desired precipitation sequence. is there. Mo is known to stabilize the M ₂ C phase, and molybdenum rich M ₂ C is also formed by adjusting the contents of Mo and V to be in the range of 1.8-2.3. (This phase has a higher coarsening rate compared to the vanadium-rich MC phase).

  Nickel and cobalt are elements that can be included in the steel in amounts of up to 3% and 5% by weight, respectively. Cobalt can increase the high temperature hardness which can be advantageous for some steel applications. When 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 given in the product claims.

  In principle, austenitization can be carried out at a temperature between a soft annealing temperature of 820 ° C. and a maximum austenitizing temperature of 1200 ° C. Preferably, however, the austenitization of the steel article is carried out at about 1050-1150 ° C, preferably 1080-1150 ° C, typically 1100 ° C. In-house tests have shown that a higher austenitizing temperature shifts the tempering hardness to a higher temperature, i.e., the peak of the secondary cure is shifted to a higher temperature. This means that the desired hardness is reached at higher initial tempering temperatures. Thus, the material will obtain improved tempering resistance and the tool processing temperature will be increased.

  Preferably, tempering of the quenched steel article is carried out at least twice at a temperature between 500 and 700 ° C, preferably between 550 and 680 ° C, with a holding time of 2 hours. In the most preferred embodiment of the steel composition, tempering is performed at a temperature between 600 and 650 ° C, preferably between 625 and 650 ° C.

  Nitrogen content in the range of 0.05 to 0.10% by weight forms the molten steel, casts the molten steel to form an ingot, and homogenizes the ingot by heat treatment. Can be obtained by incorporating. The addition of nitrogen will produce large primary vanadium rich M (C, N) precipitates which in turn will give the material uneven hardness. However, if the nitrogen content is low and there is a homogenization heat treatment prior to subsequent stepping, large primary carbonitrides will not be generated.

  For steel variants, higher nitrogen contents are also conceivable than those shown for the preferred embodiment. In this variant, nitrogen can reach 0.30% by weight. In order to obtain a higher nitrogen content, conventional casting methods are insufficient. Instead, first a steel powder of essentially the desired composition other than nitrogen is produced, then this powder is nitrided in a solid state with a nitrogen-containing fluid, for example nitrogen gas, after which the powder is heated to a temperature of the order of 1150 ° C. Nitrogen can be incorporated by forming an ingot by hot isostatic pressing at a pressure of about 76 MPa. By producing tool steel by powder metallurgy, the problem of the generation of large primary carbides is avoided.

  Preferably, the ingot is forged at a temperature on the order of 1270 ° C., then soft annealed at a temperature on the order of 820 ° C., subsequently cooled to a temperature of 650 ° C. at a rate of 10 ° C./h and then allowed to cool in air (free cooling) and ready to austenite it.

  The steel of the present invention has a greatly improved tempering resistance that allows for a longer life of the article in hot working applications. As already mentioned above, the nitrogen content is preferably around 0.05% by weight and the chromium content is preferably less than 3% by weight, ie 1.2-2.6 or 1.3-2.3.

The steel article of the present invention will preferably meet some of the following requirements:
-Good tempering resistance,
-Good high temperature strength,
-Good thermal conductivity,
-It does not have an unacceptably large coefficient of thermal expansion;

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

FIG. 2 shows hardness versus tempering temperature of a typical prior art low chromium hot work tool steel containing no nitrogen. FIG. 3 shows the hardness at various tempering temperatures of 15Cr, 1Mo, 0.6C and 15Cr, 1Mo, 0.29C, 0.35N (wt% content) prior art steel. It is a figure which shows the effect of low chromium content on the stability of M (C, N) in austenite. FIG. 5 shows the molar fraction of M ₆ C, M (C, N) and bcc matrix as a function of temperature (balance phase: austenite matrix). As a function of temperature, a diagram showing the amount of M (C, N) phase and the metastable M _{2 C} (Balance Phase: ferrite). It is a figure which shows a hardness versus tempering temperature curve about the test alloy of N0.05, N0.10 and N0.30. FIG. 6 is a backscattered SEM image showing small undissolved M (C, N) precipitates and spherical mixed oxide-sulfide particles at N 0.05. These are back-scattered SEM images representing the primary M (C, N) not dissolved in the former austenite grain boundaries in alloy N0.10. It is a backscattering SEM image showing the primary particle of N 0.10 which carried out softening annealing. It is a backscattering SEM image which shows the uniform distribution of the M (C, N) particle | grains which are not dissolved in N0.30. FIG. 6 is a backscattered SEM image showing several clusters of M (C, N) that are not dissolved in N 0.30. FIG.

  Molybdenum and vanadium medium alloy hot work tool steels have good resistance to thermal fatigue, softening and high temperature creep. A typical nominal chemical composition of such prior art steel is shown in Table 1 (wt%).

In addition, it is suggested that the high temperature characteristic of the steel of Table 1 originates in the precipitation of nanometer size vanadium carbide during tempering. These MC type hard carbides (2900 HV) provide secondary hardening of the material. FIG. 1 shows the tempering curve (hardness vs. tempering temperature) for a typical prior art tool steel. The sample was austenitized at 1030 ° C. and then tempered twice at different temperatures; from 200 ° C. to 700 ° C. for a tempering time of 2 + 2 hours. As can be seen, there is a significant secondary cure peak at 550 ° C. in the 500 to 650 ° C. interval. Subsequent studies have shown that there is significant precipitation of metastable molybdenum-rich M ₂ C in typical prior art tool steels during tempering at 625 ° C., which contributes to the secondary hardening effect. It contributes.

  The ability of hot work tool steel to maintain its hardness at high temperatures for extended periods of time, tempering resistance, can usually be combined with the initial tempering temperature; if the material is held at a temperature well below the initial tempering temperature, it softens do not do. Softening becomes more pronounced when held at a temperature closer to or higher than the initial tempering temperature.

  If the secondary cure peak can be shifted to higher temperatures, it means that the desired hardness (eg 44-46 HRC) can be reached at higher initial tempering temperatures. Thus, the material will have improved tempering resistance and the tooling temperature will be increased.

  Previous studies on high chromium steels suggest that higher hardness can be achieved during tempering when nitrogen is added to the steel. Samples of 15Cr, 1Mo, 0.6C and 15Cr, 1Mo, 0.29C, 0.35N were solution treated at 1050 ° C., followed by water quenching and cooling to liquid nitrogen. And they were tempered at different temperatures for 2 hours. As can be seen from FIG. 2, the peak hardness increased significantly when nitrogen was added. The initial hardness of martensite is lower in steels containing nitrogen, but during tempering this steel achieves a higher hardness than steels that do not contain nitrogen.

  The explanation for this is that as a result of the increased solubility of chromium in the austenite phase, nitrogen distributes chromium more uniformly in the matrix. After quenching, the uniformly dispersed chromium is inherited by the martensite phase from austenite, resulting in very finely dispersed precipitation of chromium nitride during tempering, thus giving the material a stronger hardening effect.

  Furthermore, replacing part of the carbon with nitrogen is used to achieve a higher hardness of the martensitic steel matrix. By adding nitrogen, the amount of retained austenite initially increases. However, this austenite can later be converted to martensite by cold working, and in this way a high hardness of 68HRC can be achieved.

  The low chromium content appears to have a positive effect on tempering resistance. A comparison of two different hot work tool steels of 1.5 and 5.0 wt% chromium shows that the lower chromium content favors the more thermally stable vanadium-rich MC and that the chromium in the microstructure It has been shown to delay the precipitation of carbides. Therefore, recovery in the material is slowed and tempering resistance is improved.

  However, a study on a typical creep resistant 9-12 wt% chromium steel containing 0.06 wt% N dramatically stabilizes MX (X is C + N) particles with low chromium content Is shown (see FIG. 3). If austenitization takes place at 1100 ° C., all of the M (C, N) particles will be dissolved in steel containing 10% by weight chromium. If the chromium content is reduced to 2.5% by weight (see: exemplary low chromium tool steel of FIG. 1), large amounts of M (C, N) will still be present in the austenite. As a result of the low chromium content, only a small amount of interstitial atoms appear to dissolve in the austenite during the austenitizing treatment.

In accordance with the present invention, a low chromium hot work tool steel article having increased tempering resistance is made by performing the following steps:
a) incorporating nitrogen into the molten steel composition of the low chromium hot work tool steel, thereby providing a steel composition as defined in any of the process claims;
b) forming a steel article from the steel composition;
c) The steel article obtained in step b) is austenitized at a temperature of 1200 ° C. or lower for about half an hour, followed by rapid cooling,
d) tempering the quenched steel article at least twice at a temperature between 500 and 700 ° C. for a period of about 2 hours.

  In view of the prior understanding in this technical field, the teaching that the reduction in chromium content will lead to a decrease in hardenability and difficulty in solid solution of primary M (C, N) particles is widely accepted. These results are surprising.

  In a creep resistant steel with a high chromium content, ie 9-12% by weight, the vanadium carbonitride can already be dissolved at a relatively low temperature, ie 1020-1050 ° C. However, if the chromium content is low, less than about 4-5%, it will be virtually impossible to form primary vanadium carbonitrides in the molten steel and then dissolve them.

  We have found that in low chromium steels, when the nitrogen content is balanced to about 0.015 to 0.30 wt%, vanadium carbonitrides form, which are partially solidified during the austenitizing process. It was found that it melts and then precipitates as nanometer-sized particles during the tempering step. The particles are about 1 μm to about 10 μm. In some cases, the average size of the particles is less than 1 μm when the nitrogen content is low, typically 0.05% by weight. The thermal stability of vanadium carbonitrides is better than vanadium carbides, and therefore the tempering resistance of low chromium hot work tool steel articles will be greatly improved. Furthermore, by tempering at least twice, the tempering curve (indicating hardness as a function of 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 weight percent. This value gives better performance than a higher value. A nitrogen content on the order of 0.05 weight percent provides a higher potential for secondary cure during quenching than a higher content.

  In a preferred embodiment, the chromium content is preferably 1.5 to 1.7 weight percent. A low chromium content delays chromium carbide precipitation in the microstructure, in favor of a more thermally stable vanadium-rich carbonitride. Therefore, recovery in the material is slowed and tempering resistance is improved.

  In principle, austenitization can be performed at a temperature between a softening annealing temperature of 820 ° C and a maximum austenitizing temperature of 1200 ° C. In a preferred embodiment, i.e. in compositions having a nitrogen content on the order of 0.05 weight percent and a chromium content on the order of 1.5 to 1.7 weight percent, the austenitization of the steel article is preferably between 1050 and 1150. The reaction is performed at a temperature of about 1 ° C, preferably 1100 ° C. In-house testing has shown that higher austenitizing temperatures will shift tempering hardness to higher temperatures, i.e., the peak of secondary curing will shift to higher temperatures, which is the desired hardness. Means that it will be achieved at higher initial tempering temperatures. Thus, the material will obtain improved tempering resistance and the machining temperature of the tool will be increased.

  Tempering of the quenched steel article is carried out at least twice, with a holding time of 2 hours, at a temperature between 500 and 700 ° C, preferably between 550 and 680 ° C. In the most preferred embodiment of the steel composition, tempering is performed at a temperature between 600 and 650 ° C, preferably between 625 and 650 ° C.

  Nitrogen content in the range of 0.05-0.10% by weight incorporates nitrogen by conventional casting methods, forming molten steel, casting molten steel to form ingot, and homogenizing ingot by heat treatment Can be obtained by: Nitrogen addition will produce large primary vanadium-rich M (C, N) precipitates, which then give the material uneven hardness. However, if the nitrogen content is low and there is a homogenization heat treatment prior to subsequent stepping, large primary carbonitrides will not be generated.

  In a preferred embodiment of the present invention, the nitrogen content is preferably on the order of 0.05% by weight. This value gives better properties than the higher value. A nitrogen content on the order of 0.05% by weight gives a higher potential for secondary hardening during quenching than a higher content, thus giving the steel a higher hardness. However, an amount on the order of 0.10% by weight has been shown to shift the secondary cure peak to a somewhat higher tempering temperature, which is practical. In addition, tests and modeling calculations performed indicate that an increased austenitizing temperature is required in conjunction with increased nitrogen content.

  In steel variants, higher nitrogen contents are also conceivable compared to those shown for the preferred embodiment. In this variant, nitrogen may reach 0.30% by weight. In order to obtain a higher nitrogen content, conventional casting methods are insufficient. Instead, it is preferable to first produce a steel powder with essentially the desired composition except for nitrogen, and then to nitride this powder in a solid state with nitrogen gas, after which the powder is at a temperature of about 1150 ° C. and a pressure of about 76 MPa. Incorporate nitrogen by forming an ingot by hot isostatic pressing at. By producing tool steel by powder metallurgy, the problem of primary carbide generation is avoided.

  Preferably, the ingot is forged at a temperature of about 1270 ° C., then soft annealed at a temperature of about 820 ° C., subsequently cooled to a temperature of 650 ° C. at a rate of 10 ° C./h, and then allowed to cool in air. Ready to austenite it.

Example 1
Table 2 below lists the chemical composition by weight percent of three different alloys, N 0.05; N 0.10 and N 0.30. N 0.05 refers to a material with a nitrogen content of 0.05% by weight, and so on. Note that these are the actual composition of the test ingot.

  The aim was to keep the level of all alloying elements except carbon and nitrogen constant. Compared to the standard low chromium hot work tool steel of Table 1, chromium was also slightly reduced. There was a small decrease in molybdenum content and an increase in manganese content. For carbon and nitrogen, the goal was to keep the sum of these elements constant at approximately 0.40% by weight, which was relatively well achieved.

The tempering stage is mainly related to the metastable phase, and these phases are present in standard low chromium hot work tool steels in the tempering temperature interval, ie 400 to 700 ° C., as studied by previous electron microscopy. It has been shown. These carbide phases are mainly vanadium rich MC (FCC) and molybdenum rich M ₂ C (HCP). Some amount of chromium rich M ₇ C ₃ has also been found in standard low chromium hot work tool steels.

  To determine whether these nitrogen-containing alloys can be hardened, i.e., whether sufficient alloying elements can be dissolved in the austenitic matrix at the austenitizing temperature so that martensite is formed during quenching. The following calculation was performed. The interesting temperature interval was between the soft annealing temperature 820 ° C. and the set practical maximum austenitizing temperature 1200 ° C.

The results of these equilibrium calculations are shown in FIG. Here the molar fractions of M ₆ C, M (C, N) and bcc matrix are shown as a function of temperature. The remaining phase is austenite. The solid curve represents N 0.05, the dashed curve represents N 0.10, and the dotted curve represents N 0.30. Note the high content of M (C, N) in alloy N 0.30 up to 1200 ° C. As expected, the bcc phase is unstable above 850 ° C. It is interesting to see that the slope of the equilibrium curve showing the amount of M (C, N) decreases as the nitrogen content increases. This means that it is difficult to dissolve M (C, N) in N 0.30 compared to N 0.05. Therefore, it is expected that the content of carbon, nitrogen and vanadium after austenitization at 1100 ° C. is lower in the N 0.30 matrix than in the N 0.05 matrix.

Since the molybdenum-rich M ₆ C phase only dissolves carbon and not nitrogen, it suffers from the lower carbon content at N 0.10 and N 0.30, and thus with decreasing carbon content, M ₆ The amount of C decreases. It should also be noted that all M ₆ C is in solution at the austenitizing temperature used.

The calculations performed in the tempering temperature region were performed only to estimate the secondary precipitation potential at N 0.05, N 0.10, and N 0.30. The equilibrium found is at best and can indicate which phase is present in the material after a sufficiently long time. Previous studies have shown that there is actually some auto-tempering in standard low chromium hot work tool steels. This means that M ₃ C (cementite) will precipitate after the austenitization process.

The result of the calculation in the tempering temperature region is shown in FIG. The solid curve represents N 0.05, the dashed curve represents N 0.10, and the dotted curve represents N 0.30.
Secondary curing usually occurs between 500 and 650 ° C., and there is no significant difference between N 0.05 and N 0.10 with respect to the amount of M (C, N) in this temperature interval. On the other hand, N 0.30 has a high and nearly constant M (C, N) content, possibly as a result of the high content of vanadium and nitrogen.

A higher carbon content at N 0.05 produces more M ₂ C phase in equilibrium with the matrix compared to N 0.10. At N 0.30 there is much less M ₂ C.

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

  The results shown in Table 3 show that N 0.05 will have the best hardening response as a result of the small amount of M (C, N) phase present at 1100 ° C., ie many These alloy elements can be dissolved in the austenite matrix. It also shows that N 0.05 has the best potential for good secondary cure during tempering at 625 ° C.

Example 2
Two alloys of N 0.05 and N 0.10 were conventionally cast into a 50 kg small ingot. NO. No. 10 is the first test, and the ingot was not homogenized before the forging process. For the second test, N 0.05, a homogenization treatment at 1300 ° C. for 15 hours was applied before forging. The third alloy, N 0.30, had a nitrogen content that was too high to be produced by conventional casting. Therefore, this alloy was produced using powder metallurgy. First, a steel powder was prepared, and then this powder was nitrided in a solid state with pressurized N ₂ gas. The powder was hot isostatically pressed (HIP) at 1150 ° C. and a pressure of 76 MPa.

  All three ingots were forged at 1270 ° C., after which samples were cut with dimensions of 15 × 15 × 8 mm. The sample was heat-treated by an initial soft annealing at 820 ° C .; the cooling sequence after annealing was 10 ° C./h to 650 ° C. and then allowed to cool in air. After soft annealing, N 0.05 was austenitized at 1100 ° C. for 30 minutes. In order to compensate for the inferior precipitation potential, N0.10 was austenitized at 1150 ° C for 30 minutes and N0.30 was austenitized at 1200 ° C for 30 minutes. Nine samples from each of the three alloys were tempered at the following temperatures: 450, 525, 550, 575, 600, 625, 650, 675 and 700 ° C. The soaking time was 2 hours, which was double tempering, ie the total time for tempering was 4 hours. After the heat treatment, the hardness of the sample was measured. Further, a scanning electron microscope (SEM) method was performed in order to examine the size, dispersion, and morphology of particles not dissolved in the sample. The SEM device used was an FEI Auanta 600F.

Hardness measurement The results of the hardness measurement are shown in FIG. As can be seen, all three alloys have a secondary hardening peak in the temperature range 500 to 650 ° C. All tempers were performed for 2 + 2 hours. N 0.05 had the highest hardness (53HRC) while still quenched, and N 0.10 and N 0.30 had slightly lower hardness. However, all three alloys are considered hardenable. The hardness curve of N 0.05 is very similar to that of a standard low chromium hot work tool steel with a maximum of about 54 HRC as shown in FIG.

  The N 0.10 secondary cure peak appears to have a peak hardness at 600 ° C. and somewhat shifted to higher temperatures. The peak hardness for both N 0.05 and N 0.30 was at 550 ° C.

Scanning Electron Microscopy Non-solid solution M (C, N) particles in conventionally cast N 0.05 (alloy with the lowest nitrogen content) have an average size of less than 1 μm. This is comparable to the usual undissolved carbides of steel. Another phase that is easily found in N 0.05 is a mixture of aluminum oxide and manganese sulfide (see FIG. 7). FIG. 7 is an SEM image (backscattering) showing a small undissolved M (C, N) precipitate 2 and spherical mixed oxide-sulfide particles 1 in N 0.05. The sample was austenitized at 1100 ° C. for 30 minutes and tempered at 625 ° C. for 2 + 2 hours.

  The reason for the many non-metallic inclusions in N 0.05 (and N 0.10) is that all test ingots were manufactured and cast in an open atmosphere.

  The most common size of M (C, N) particles at N 0.10 is the equivalent circle diameter between 5 and 10 μm after austenitization at 1150 ° C. for 30 minutes and tempering at 625 ° C. for 2 + 2 hours ( ECD). Larger primary carbides 3 (precipitated in the molten steel) are often found at prior austenite grain boundaries (see FIG. 8). FIG. 8 is a backscattered SEM image showing the primary M (C, N) not dissolved in the prior austenite grain boundaries in alloy N0.10. The sample was austenitized at 1150 ° C. for 30 minutes and tempered at 625 ° C. for 2 + 2 hours.

FIG. 9 is a detailed SEM micrograph of primary M (C, N) particles 4 in N0.10.
They were automatically discovered in the SEM using INCA Feature software from Oxford Instruments. Their sharp edges indicate that they were precipitated from the molten steel. White portions in the image are molybdenum rich M ₆ C particles 5. Note that in this case, the sample was soft annealed N0.10.

  In N 0.30 produced by powder metallurgy, undissolved M (C, N) particles 6 have a particle size distribution (ECD) of 1 to 5 μm, the most common size is 2 μm, Thus, the particles were small despite the high nitrogen content. The particles were uniformly dispersed in the microstructure (see FIG. 10). However, as shown in FIG. 11, some M (C, N) clusters 7 were discovered.

  The chemical composition of the non-solid particles of the M (C, N) phase in all three alloys was measured by EDS and the results are shown in Table 4. Table 4 shows the chemical composition of M (C, N) particles in alloys N 0.05, N 0.10 and N 0.30. The value is given in atomic percent. Note that even if the accuracy of EDS for light elements such as carbon and nitrogen is not very high, the balance of carbon and nitrogen in the M (C, N) phase can be expected based on the nominal composition. I want to be. The ± values in the table are those given by the INCA program (Oxford Instruments). Part of the recorded iron probably comes from the surrounding matrix, especially for alloy N 0.05.

INDUSTRIAL APPLICABILITY The method of the present invention and the low chromium hot work tool steel can be applied where it is desired to obtain a hot work steel tool that can be used at high temperatures for a long time.

Claims (10)

% By weight
C 0.08-0.35
N 0.015-0.30
C + N 0.30 to 0.50
Cr 1-3
Mo 2.2 ~3
V 0.8-1.3
Mn 0.5-2
Si 0.1-0.5
In some cases, Ni <3
Co ≦ 5
B <0.01
The remainder Fe, apart from impurities
A low chromium hot work tool steel article for at least one of metal processing and forming at high temperatures . In weight%, the following condition C 0.20 to 0.35
N 0.03-0.30
C + N 0.30 to 0.50
Cr 1-3
Mo 2.2 to 2.9
V 1.0-1.3
Mn 1-2
Si 0.1-0.5
Ni <1
Co <4
B 0.001-0.01
The low chromium hot work tool steel article of claim 1 satisfying one or more of the following: In weight%, the following condition C 0.25-0.35
N 0.04-0.30
C + N 0.38 to 0.42
Cr 1.3-2.5
The low chromium hot work tool steel article according to claim 1 or 2, satisfying one or more of the following: In weight%, the following condition N 0.042-0.15
C + N 0.39 to 0.41
Cr 1.3-2.3
The low chromium hot work tool steel article according to any one of claims 1 to 3, satisfying at least one of the following. In weight%, the following condition C 0.20 to 0.35
N 0.042-0.12
C + N 0.39 to 0.41
Cr 1.4-1.9
Mo / V 1.8-2.3
Cr / V <2
The low chromium hot work tool steel article according to any one of claims 1 to 4, which satisfies one or more of the following. % By weight
C 0.20 to 0.35
N 0.03-0.30
C + N 0.30 to 0.50
Cr 1.2-2.3
Mo 2.2 ~3
V 0.8-1.3
Mn 1-2
Si 0.1-0.4
Ni <1
In some cases Co 3-5
B 0.001-0.01
Mo / V 1.8-2.3
Cr / V <2
The remainder Fe, apart from impurities
The low chromium hot work tool steel article according to claim 1, comprising: % By weight
C 0.20 to 0.35
N 0.03-0.30
C + N 0.30 to 0.50
Cr 1.2-2.3
Mo 2.2 ~3
V 0.8-1.3
Mn 1-2
Si 0.1-0.4
Ni <1
In some cases Co 3-5
B 0.001-0.01
Mo / V 1.8-2.3
Cr / V <2
The remainder Fe, apart from impurities
The low chromium hot work tool steel article according to claim 1, comprising: % By weight
C 0.20 to 0.35
N 0.04-0.30
C + N 0.30 to 0.50
Cr 1.2-2.3
Mo 2.2 ~3
V 0.8-1.3
Mn 1-2
Si 0.1-0.4
Ni <1
Co <0.2
In some cases B 0.001-0.01
Mo / V 1.8-2.3
Cr / V <2
The remainder Fe, apart from impurities
The low chromium hot work tool steel article according to claim 1, comprising: % By weight
C 0.20 to 0.35
N 0.04-0.30
C + N 0.36 to 0.44
Cr 1.2-2.3
Mo 2.2 to 2.9
V 0.8-1.3
Mn 1-2
Si 0.1-0.4
Ni <0.25
Co <0.20
In some cases B 0.001-0.01
Mo / V 1.8-2.3
Cr / V <2
The remainder Fe, apart from impurities
The low chromium hot work tool steel article according to claim 1, comprising: % By weight
C 0.30 to 0.34
N 0.04-0.09
C + N 0.37 to 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 <0.25
Co <0.20
In some cases B 0.001-0.005
Mo / V 1.8-2.3
Cr / V <2
The remainder Fe, apart from impurities
The low chromium hot work tool steel article according to claim 1, comprising:

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