Method for producing a tool steel and steel thus obtained

Abstract

The present invention relates to a method for producing a tool steel characterised in that it comprises the steps of: atomising molten steel with gas or water, wherein the composition of the molten metal is: C: 1.2 - 1.7 wt.-%, Si: 0.25 - 0.35 wt.-%, Mn: 0.1 - 0.3 wt.-%, Cr: 5 - 9 wt.-%, Mo: 0.3 - 0.9 wt.-%, V: 7 - 10 wt.-% and balance Fe and unavoidable impurities; sieving the powder particles obtained in the atomisation step; shaping the powder particles selected in the sieving step; and consolidating the product obtained in the shaping step by hot sintering, wherein the consolidation is performed at a temperature between 1290°C and 1340°C for at least 1 hour, in an atmosphere with a controlled partial pressure of N2.

Description

[0001] DESCRIPTION

[0002] PROCEDURE FOR OBTAINING A TOOL STEEL AND THE STEEL THUS OBTAINED

[0003] Technology sector

[0004] The invention relates to a process for obtaining steel, specifically a process for obtaining tool steel suitable for cold working, as well as the tool steel thus obtained.

[0005] State of the art

[0006] Today, we know of cold-work tool steels, which are steels designed to be used for working a material in its cold state. They are used, for example, for stamping, punching, cutting, rolling, cold forging, coining, and other similar applications.

[0007] Typically, cold work tool steels are used in industrial applications up to about 200 °C where certain properties such as high hardness, good wear resistance, high toughness and compressive strength are required.

[0008] An example of cold work tool steels are D2 type steels (also known as ASTM A68, DIN EN ISO 4957, JIS G4404, 1.2379 / X153CrMo12 or SKD11), which have a high relative toughness compared to others with greater wear resistance.

[0009] Specifically, D2 type steels have the following chemical composition in % by weight:

[0010] C: 1.4 - 1.6% by weight

[0011] Yes: 0.10 - 0.60% by weight

[0012] Mn: 0.1 - 0.6% by weight

[0013] Cr: 11 - 13% by weight

[0014] Mo: 0.7 - 1.2% by weight

[0015] V: 0.5 - 1.1% by weight

[0016] Equilibrium iron and unavoidable impurities; where D2 steel comprises carbides of the M7C3 and M23C6 types, rich in Cr, which are embedded in a martensitic matrix reinforced with secondary carbide precipitation. This results in D2-type steels having limited wear resistance compared to other tool steels such as high-speed steels, in which the carbides are harder.

[0017] Likewise, if steel is obtained by hot consolidation through sintering, it is required that it be carried out in a temperature range between 1390 °C and 1395 °C, this range being quite narrow which increases the possibility of obtaining low density or over-sintered steels.

[0018] Therefore, a process is needed to obtain a new tool steel suitable for cold working that has greater wear resistance than D2 type steel, as well as greater toughness compared to other high-speed steels.

[0019] Object of the invention

[0020] In order to achieve this objective and solve the technical problems discussed so far, as well as provide additional advantages that may arise later, the present invention provides a process for obtaining a cold-work tool steel comprising the following steps:

[0021] Atomization of a steel molten metal broth with gas or water, where the composition of the molten metal broth is:

[0022] C: 1.2 - 1.7% by weight

[0023] Yes: 0.25 - 0.35% by weight

[0024] Mn: 0.1 - 0.3% by weight

[0025] Cr: 5 - 9% by weight

[0026] Mo: 0.3 - 0.9% by weight

[0027] V: 7 - 10% by weight

[0028] Faith of balance and inevitable impurities;

[0029] where the gas atomization conditions are:

[0030] or the temperature of the metal broth before atomization between 1550 °C and 1700 °C, or the pressure of the atomizing gas between 20 bar and 50 bar,

[0031] where the atomization conditions with water are:

[0032] or temperature of the metal broth before atomization between 1350 °C and 1600 °C, or pressure of the atomizing water between 60 bar and 1000 bar, - sieving of the powder particles obtained in the atomization stage, shaping of the powder particles selected in the sieving stage, hot consolidation by sintering of the product obtained in the shaping stage, wherein the consolidation is carried out at a temperature between 1290 °C and 1340 °C for at least 1 hour, with an atmosphere with a controlled partial pressure of N2.

[0033] In this way, after hot consolidation by sintering, vanadium carbonitrides are formed which are embedded in the steel matrix, which has a martensitic microstructure with retained austenite.

[0034] The high vanadium content in the powder and its interaction with the sintering atmosphere causes vanadium carbonitrides to be present in a large quantity in the steel matrix, which allows the steel to have greater wear resistance than other steels on the market, such as D2 type steel.

[0035] Furthermore, given the composition of the new steel, the temperature range for hot consolidation by sintering is between 1290 °C and 1340 °C, which is a wider temperature range than the temperature range for D2 steel (1390 °C to 1395 °C), thus reducing the possibility of obtaining low-density or over-sintered steels, resulting in a more robust process.

[0036] Furthermore, since the temperatures for hot consolidation by sintering in the new steel are lower than the sintering temperatures for D2 type steel, the present process entails lower energy expenditure, resulting in a longer lifespan of the heating elements used in the process and a lower cost of the process of the present invention compared to the process of obtaining D2 type steel.

[0037] Additionally, the use of an atmosphere with a controlled partial pressure of N2 in the hot consolidation stage by sintering allows the N2 required for the formation of vanadium carbonitrides to be supplied, which increases the wear resistance of the steel.

[0038] Preferably, the temperature of the molten metal before atomization in the gas atomization stage is between 1600 °C and 1700 °C, preferably 1650 °C, resulting in powder particles that allow for a better final steel. Preferably, the gas atomization pressure is between 20 bar and 35 bar, preferably 20 bar, which allows for a higher percentage of alloy particles within the desired size range for the sieving stage.

[0039] Preferably the temperature of the molten metal before atomizing in the water atomization stage is between 1350 °C and 1550 °C, preferably 1400 °C, which also produces powder particles that allow for a better final steel.

[0040] Preferably the pressure of the water atomization gas is between 150 MPa and 600 MPa, which also allows obtaining a higher percentage of alloy particles that are within the desired size range for the sieving stage.

[0041] Preferably, when atomization is carried out with gas, the atomizing gas is pure N2 with a purity of at least 99%, thus providing between 0.15% and 0.25% by weight of N2 to the powder particles obtained in the atomization stage.

[0042] Preferably, the atmosphere with a controlled partial pressure of N2 in the hot consolidation stage by sintering comprises a composition of between 25% and 75% N2 and the remainder H2, preferably 25% N2 and 75% H2, thereby providing between 0.95% and 1.05% by weight of nitrogen to the chemical composition of the part obtained in the forming stage.

[0043] Preferably, hot consolidation by sintering is carried out at a temperature between 1290 °C and 1320 °C, preferably 1300 °C, which results in better N2 absorption and the production of a steel with better properties.

[0044] Preferably, the forming is done by means of printing (BJ), encapsulation (HIP) or compaction.

[0045] Preferably, the D90 of the selected powder particles in the sieving stage is less than 200 pm, thus achieving an optimal powder particle size range for encapsulation (HIP) or compaction forming. Preferably, the D90 is less than 50 pm, thus achieving an optimal powder particle size range for impression forming (BJ).

[0046] Preferably, after the hot consolidation stage, an austenitizing heat treatment is carried out at a temperature between 1000 °C and 1075 °C, preferably 1050 °C, for at least 15 minutes in an argon or vacuum atmosphere, resulting in an austenite matrix in the steel. The steel is then quenched with sufficiently rapid cooling to transform most of the austenite into martensite.

[0047] More preferably, after austenitizing, at least three tempering heat treatments are carried out at a temperature between 475 °C and 550 °C, preferably 500 °C for at least 1 hour, with intermediate coolings to room temperature, thereby transforming a large part of the retained austenite into martensite, and all the martensite into tempered martensite, in addition to promoting the precipitation of secondary nanometric carbides and adjusting the hardness of the steel.

[0048] Another object of the present invention is a cold-work tool steel comprising the following chemical composition in % by weight:

[0049] C: 1.3-1.5% by weight

[0050] Yes: 0.15-0.3% by weight

[0051] Mn: 0.2-0.3% by weight

[0052] Cr: 6, 9-7, 6% by weight

[0053] Mo: 0.6-0.9% by weight

[0054] V: 7.1-8.5% by weight

[0055] N: 1.1-1.5% by weight

[0056] Faith of balance and inevitable impurities;

[0057] and a microstructure that, after heat treatment of the steel, contains vanadium-rich MCN-type carbonitrides embedded in a martensitic matrix reinforced with secondary carbide precipitation, where C is carbon, N is nitrogen, and M is vanadium.

[0058] Vanadium-rich MCN-type carbides and / or carbonitrides are fine, hard, and anchor the austenite grain boundaries, preventing their growth. These MCN-type carbonitrides provide wear resistance and hardness to steel.

[0059] Preferably, the microstructure of the steel after heat treatment contains between 13% and 15% by volume of MCN-type carbonitrides, preferably 14%.

[0060] As mentioned previously, the presence of vanadium carbonitrides in the steel matrix allows the steel to have greater wear resistance than other steels on the market, for example, D2 type steels.

[0061] Description of the figures

[0062] Figure 1 shows the microstructure of a D2 type steel.

[0063] Figure 2 shows the microstructure of a steel obtained according to the process of the present invention.

[0064] Figures 3 and 4 show examples of the powder particles obtained after the sieving stage of the present invention.

[0065] Figures 5 and 6 show the microstructure of the product obtained after the hot consolidation stage by sintering of the present invention.

[0066] Figure 7 shows an X-ray diffraction graph, where the transformation of all the austenite retained in the piece after hot consolidation by sintering, the austenitizing heat treatment and the tempering heat treatments of the present invention can be observed.

[0067] Figures 8 and 9 show the microstructure of the product of Figures 5 and 6 respectively, after the austenitizing and multiple tempering heat treatments of the present invention.

[0068] Detailed description of the invention

[0069] The different stages of the process of obtaining a cold-working tool steel of the present invention will then be explained in more detail.

[0070] Atomization Stage: The atomization stage of the process of the invention allows for obtaining alloy particles with the desired composition. The atomization conditions can be adjusted to obtain a higher percentage of alloy particles that fall within the desired size range for the subsequent stages.

[0071] The manufacture of alloy particles (commonly called powders) is primarily carried out by atomization with gas or water. The raw material (in elemental form, as master alloys, and / or pre-alloyed) in the appropriate proportions is loaded into a furnace, where it melts, resulting in a metallic molten material with a homogeneous composition. This molten material is conveyed through a feed tube to the atomization chamber. At the tube's outlet, the stream of molten material is impacted by a flow of gas or water, causing it to fragment into microscopic droplets, which then solidify into particles.

[0072] The atomization conditions that can be adjusted to obtain the desired powder particle size are:

[0073] atomizing gas when atomization is carried out with gas (in the present invention, preferably the atomizing gas is pure N2 with a purity of at least 99%, although argon, helium, etc. can also be used),

[0074] atomizing gas pressure (in the present invention, preferably between 20 bar and 30 bar, more preferably 20 bar, but it is also possible between 20 bar and 50 bar) or atomizing water pressure (in the present invention, preferably between 150 MPa and 600 MPa),

[0075] temperature of the alloy before atomizing with gas (in the present invention, preferably between 1600 °C and 1700 °C, more preferably at 1650 °C, but it can also be used between 1550 °C and 1700 °C) or with water (in the present invention, preferably between 1350 °C and 1550 °C, more preferably at 1400 °C, but it can also be used between 1350 °C and 1600 °C).

[0076] As mentioned previously, it is preferable for the atomizing gas to be pure nitrogen with a purity greater than 99%. This gas will contribute between 0.15% and 0.25% nitrogen to the final composition of the resulting powder particles. However, the present invention also works with argon as the atomizing gas, requiring the steel to absorb more nitrogen during the subsequent hot consolidation stage by sintering. It should also be noted that performing the atomization stage with argon is more expensive than with nitrogen, so using pure N2 with a purity of at least 99% as the atomizing gas is more economical.

[0077] Sieving stage

[0078] After the atomization stage, the resulting powder particles are classified by sieving and the desired powder particle size distribution is selected for the next stage of material shaping.

[0079] A preferred technique for measuring the particle size distribution of powder is laser diffraction, where a laser diffraction instrument produces a histogram of volume percentage versus powder particle size. From this data, there are many different ways to describe this distribution.

[0080] In the present invention, the 90th percentile (D90) is used as a reference. These values ​​indicate the width of the distribution (e.g., D90-D10). Although other ways of describing a statistical distribution of powder particle sizes are applicable to the process of the invention, such as using the median.

[0081] Therefore, depending on the method used for the next forming stage, the desired powder particle size is selected. If it is done by encapsulation (HIP) or compaction, the D90 of the powder particles selected in the sieving stage would be less than 200 pm, while if it is done by printing (BJ), the D90 of the powder particles selected in the sieving stage would be less than 50 pm.

[0082] Forming stage

[0083] In the forming stage, a piece is shaped, in the case of the present invention using the powder particles obtained after the sieving stage, where the forming is done by means of printing (BJ), encapsulation (HIP) or compaction, although other forming methods are not ruled out.

[0084] The Binder Jetting (BJ) 3D printing technique consists of depositing a binding agent in specific locations of thin layers of powder material, in the present invention said powder material being the powder particles obtained after the sieving stage.

[0085] During the Binder Jetting (BJ) printing process, the 3D print head deposits droplets of binder onto the build platform. When a layer is complete, the powder bed moves down, and the printer spreads a new layer of powder over the build area. The process continues layer by layer until the desired part is complete.

[0086] In the HIP (Hot Isostatic Pressing) technique of encapsulated powder, the powder particles obtained in the sieving stage fill a capsule that gives them shape and are densified by hot isostatic pressing after a degassing process.

[0087] In the compaction technique, the obtained alloy powder particles are compressed using mechanical or hydraulic presses in a die, resulting in the desired part. The most commonly used presses are uniaxial, in which pressure is applied to the powder in a single direction. Uniaxial compaction allows for the production of parts with precise dimensions and finishes.

[0088] Hot consolidation stage by sintering

[0089] Once the piece is obtained in the forming stage, a hot consolidation is carried out by sintering, which consists of heating the piece in a controlled atmosphere.

[0090] Specifically, hot consolidation by sintering is carried out at a temperature between 1290 °C and 1340 °C (preferably 1290 °C and 1320 °C, and more preferably 1300 °C) for at least 1 hour (preferably between 1 hour and 3 hours), with an atmosphere with a controlled partial pressure of N2.

[0091] Likewise, the controlled partial N2 atmosphere of hot consolidation by sintering comprises a composition of between 25% and 75% N2 and the remainder H2, preferably 75% H2 and 25% N2, thereby providing between 0.95% and 1.05% by weight of nitrogen to the chemical composition of the part obtained in the forming stage.

[0092] Subsequent Heat Treatments: Heat treatments allow modification of the microstructure of a steel in order to adapt its properties for its final use. Specifically, for a high-speed steel (such as the present invention), the steel is required to have high toughness and wear resistance.

[0093] Therefore, it is considered that the most suitable heat treatment would include at least one austenitizing treatment, followed preferably by a multiple tempering treatment.

[0094] Austenitization is a heat treatment process that involves heating steel to a specific temperature to transform it into austenite. The austenitizing temperature influences the microstructure of the steel, which in turn affects its mechanical properties, such as hardness, toughness, and wear resistance.

[0095] In the present invention, an austenitizing heat treatment is preferably carried out at a temperature between 1000 °C and 1075 °C, preferably 1050 °C, for at least 15 minutes with an argon or vacuum atmosphere.

[0096] In the present invention, a multiple tempering process is preferably carried out (more preferably three tempering treatments), where each tempering treatment is carried out at a temperature between 475 °C and 550 °C, preferably 500 °C for at least 1 hour, with intermediate coolings to room temperature, thereby transforming a large part of the retained austenite into martensite, and all the martensite into tempered martensite, in addition to promoting the precipitation of secondary nanometric carbides and adjusting the hardness of the steel.

[0097] Cooling is preferably done in air, since this type of steel has a high hardenability, so a more severe cooling medium is not necessary.

[0098] Characteristics of the steel obtained

[0099] This results in a high-speed steel with improved wear resistance and greater toughness compared to other D2-type cold-work tool steels or M2-type high-speed steel. For reference, Table 1 shows a comparison of the chemical composition (weight percent) between D2-type steel and a steel according to the present invention.

[0100] Table 1

[0101]

[0102] Comparatively, this steel has a higher toughness than D2 type steels, as can be seen in Table 2:

[0103] Table 2

[0104]

[0105] This tenacity has been measured using the Barker test.

[0106] The carbides present in D2-type steel are Cr-rich carbides of the M7C3 and M23C6 types, which are embedded in a martensitic matrix reinforced by secondary carbide precipitation. Figure 1 shows an example of the microstructure of D2-type steels.

[0107] Furthermore, the newly developed steel exhibits MCN-type carbides and / or carbonitrides in its microstructure, rich in V, which are also embedded in a martensitic matrix reinforced with secondary carbide precipitation. Figure 2 shows an example of the microstructure of the new steel obtained in the present invention.

[0108] As can be seen in Table 3, compared to the new steel, which is rich in V, the wear resistance is much greater, since the volume fraction and its size are greater than for D2, and the hardness of MCN is much higher than that of Cr7C3 and Cr23C6 carbide.

[0109] Table 3

[0110]

[0111] The wear tests were performed on a “pin-on-disc” type tribometer.

[0112] As mentioned above, given the composition of the new steel, the temperature range for hot consolidation by sintering is between 1290 °C and 1340 °C, which means that the sintering temperature is lower and the sintering temperature range is wider than for D2 type steel (1390 °C to 1395 °C), which implies lower energy expenditure, greater robustness of the process and therefore a lower cost of the present invention with respect to the process of obtaining D2 type steel.

[0113] Example of implementation

[0114] Next, a non-limiting example of the embodiment of the present invention is disclosed.

[0115] First, the raw material comprising the alloying elements in elemental form and / or high purity metal alloy is selected (e.g., in the form of bars, blocks, briquettes, etc.).

[0116] This raw material is weighed and added in the proportions necessary to achieve the target composition, which comprises:

[0117] C: 1.50% by weight

[0118] Yes: 0.30% by weight

[0119] Mn: 0.20% by weight

[0120] Cr: 7% by weight

[0121] Mo: 0.6% by weight

[0122] V: 8.5% by weight

[0123] Faith of balance and inevitable impurities.

[0124] The raw material is then heated to a temperature of 1650 °C until a homogeneous metallic broth is obtained. This metallic broth is then atomized with gas of 99% purity at a pressure of 20 bar, and the composition of the resulting powder particles comprises:

[0125] C: 1.50% by weight

[0126] Yes: 0.30% by weight

[0127] Mn: 0.20% by weight

[0128] Cr: 7% by weight

[0129] Mo: 0.6% by weight

[0130] V: 8.5% by weight

[0131] N: 0.25% by weight

[0132] Faith of balance and inevitable impurities.

[0133] The microstructure of the powder particles obtained is homogeneous (because the atomized powder is pre-alloyed) and they do not have internal porosity, as can be seen in figures 3 and 4, since the powder particles are small and the nitrogen dissolves in the steel.

[0134] After this, the obtained powder particles are sieved, seeking to obtain a powder particle size of 0.1 pm to 200 pm.

[0135] After the sieving step, the sieved powder particles are shaped by compaction.

[0136] Next, the product obtained in the forming stage is hot-consolidated by sintering at a temperature of 1300 °C for 1 hour, in an atmosphere with a controlled partial pressure of 75% H2 and 25% N2.

[0137] At this point in the process, the remaining percentage by weight of nitrogen is added to the steel part, resulting in the following composition of the steel:

[0138] C: 1.40% by weight

[0139] Yes: 0.30% by weight

[0140] Mn: 0.20% by weight

[0141] Cr: 7% by weight

[0142] Mo: 0.6% by weight

[0143] V: 8.5% by weight

[0144] N: 1.20% by weight of equilibrium Fe and unavoidable impurities.

[0145] As mentioned previously, during hot consolidation by sintering, vanadium carbonitrides are formed and embedded in the steel matrix. The steel matrix has a martensitic microstructure with retained austenite, as can be seen in Figures 5 and 6.

[0146] Subsequently, the piece obtained after hot consolidation by sintering undergoes an austenitizing heat treatment at a temperature of 1050 °C for 15 minutes in an argon atmosphere, after which the piece is tempered by allowing it to cool in air.

[0147] Once the piece has cooled, three tempering heat treatments are performed, specifically each tempering heat treatment is carried out at a temperature of 500 °C for 1 hour, after each treatment the piece is left to cool in air to room temperature.

[0148] Heat treatments transform the retained austenite into martensite, transforming all the martensite into tempered martensite, in addition to promoting the precipitation of vanadium carbonitrides and other secondary nanometric carbides and adjusting the hardness of the material, as can be seen in the graph in Figure 7.

[0149] Specifically, Figure 7 compares X-ray diffraction patterns after sintering, austenitizing, and multiple tempering processes. After these tempering processes, the transformation of all the retained austenite in the part can be observed. The lower line represents the diffractogram after hot consolidation by sintering, the middle line after the austenitizing heat treatment, and the upper line after the tempering heat treatments. Number 1 represents austenite peaks, number 2 represents vanadium carbonitrides, and number 3 represents martensite.

[0150] After these multiple austenitizing and tempering treatments, the microstructure of the steel in Figures 5 and 6 would be as shown in Figures 8 and 9, respectively. The microstructure of the steel after heat treatment contains 14% by volume of MCN-type carbonitrides.

Claims

CLAIMS 1. Process for obtaining a tool steel characterized in that it comprises the following steps: - atomization of a steel molten metal broth with gas or water, where the composition of the molten metal broth is: C: 1.2 - 1.7% by weight Yes: 0.25 - 0.35% by weight Mn: 0.1 - 0.3% by weight Cr: 5 - 9% by weight Mo: 0.3 - 0.9% by weight V: 7 - 10% by weight Faith of balance and inevitable impurities; where the gas atomization conditions are: or the temperature of the metal broth before atomization between 1550 °C and 1700 °C, or the pressure of the atomizing gas between 20 bar and 50 bar, where the atomization conditions with water are: or the temperature of the metal broth before atomization between 1350 °C and 1600 °C, or the pressure of the atomizing gas between 60 bar and 1000 bar, - sieving of the powder particles obtained in the atomization stage, - shaping of the selected powder particles in the sieving stage, - hot consolidation by sintering of the product obtained in the forming stage, where consolidation is carried out at a temperature between 1290 °C and 1340 °C for at least 1 hour, with an atmosphere with a controlled partial pressure of N2.

2. The process, according to the preceding claim, wherein the temperature of the metal broth before atomization of the gas atomization stage is between 1600 °C and 1700 °C.

3. The process, according to any one of the preceding claims, wherein the gas atomization pressure is between 20 bar and 35 bar, preferably 20 bar.

4. The process according to any one of the preceding claims, wherein the atomizing gas is pure N2 with a purity of at least 99%.

5. The process according to claim 1, wherein the temperature of the metal broth before atomizing the water atomization stage is between 1350 °C and 1550 °C.

6. The process, according to any one of claims 1 or 5, wherein the water atomization gas pressure is between 150 MPa and 600 MPa 7. The process, according to any one of the preceding claims, wherein the atmosphere with a controlled partial pressure of N2 of the hot consolidation stage by sintering comprises a composition of between 25% and 75% N2 and the remainder H2.

8. The process, according to any one of the preceding claims, wherein the hot consolidation by sintering is carried out at a temperature between 1290 °C and 1320 °C, preferably 1300 °C.

9. The process, according to any one of the preceding claims, wherein the shaping is carried out by means of printing (BJ), encapsulation (HIP) or compaction.

10. The process, according to any one of the preceding claims, wherein the D90 of the selected powder particles in the sieving stage is less than 200 pm.

11. The process, according to the preceding claim, wherein D90 is less than 50 pm.

12. The process, according to any one of the preceding claims, wherein after the hot consolidation stage, an austenitizing heat treatment is carried out at a temperature between 1000 °C and 1075 °C for at least 15 minutes with an argon atmosphere or in a vacuum.

13. The process, according to the preceding claim, wherein after austenitizing at least three tempering heat treatments are carried out at a temperature between 475 °C and 550 °C for at least 1 hour, with intermediate coolings to room temperature.

14. Tool steel, obtained according to any one of claims 1 to 10, characterized in that it comprises the following chemical composition in % by weight C: 1.3-1.5% by weight Si: 0.15-0.3% by weight Mn: 0.2-0.3% by weight Cr: 6, 9-7, 6% by weight Mo: 0.6-0.9% by weight V: 7.1-8.5% by weight N: 1.1-1.5% by weight Faith of balance and inevitable impurities; and a microstructure of the steel after quenching and tempering containing vanadium-rich MCN-type carbonitrides embedded in a martensitic matrix reinforced with secondary precipitation of carbides, where C is carbon, N is nitrogen, and M is vanadium.

15. Steel, according to the preceding claim, wherein the microstructure of the steel after heat treatment contains between 13% and 15% by volume of MCN-type carbonitrides.

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