Tool steel having low adhesive wear during workpiece machining

Abstract

According to an aspect of the disclosure a tool steel consists of, in % weight: C: 0.8 – 1.5%, Si: 1.1 – 2.3%, Cr: 6 – 10%, Mo: 0.5 - 4%, Al: 1.1 – 2.3%, Ni: 0.6 – 2.3%, and optionally one or more of Co: < 3%, Mn: < 0.8%, V: < 0.8%, Ti: < 0.3%, Nb: < 0.3%, Cu: < 0.3%, W: < 0.3%, the balance being Fe and incidental impurities.

Description

TOOL STEEL HAVING LOW ADHESIVE WEAR DURING WORKPIECE MACHINING Technical Field

[0001] This disclosure relates to a tool steel, and in particular to a tool steel usable for a metalworking tool for machining a metal workpiece and a method of manufacturing such tool steel and / or metalworking tool.Background

[0002] Metalworking processes are widely used in the industry due to their suitability for the mass production of metallic workpieces. For example, blanking and / or punching and cold forming are well known metalworking processes. These processes are used, inter alia, as manufacturing processes for automotive parts, household appliances, electronic components and nearly every large-scale metal production good.

[0003] One challenge in metalworking is the occurrence of adhesive tool wear formation, which is the result of galling, which is also known as adhesion formation. Adhesive wear results from interactions of the workpiece with the tool material on a micro- and nanoscopic scale. Adhesive wear, which is caused by a material transfer from the workpiece to an active element of the metalworking tool, entails negative effects like workpiece quality reduction especially at the formed or cut surface, thereby raising the number of defective workpieces (product parts) . Furthermore, adhesive wear impairs cycle time and tool service live requirements. For these and other reasons, it is important to reduce adhesive wear during metalworking, inparticular for metalworking on aluminum based workpieces, titanium based workpieces and / or stainless steel workpieces.

[0004] Conventional approaches to reducing adhesive wear are tool coatings and / or the use of lubricants. However, these approaches are not always applicable and may have disadvantages depending on the specific metalworking tool - workpiece combination .

[0005] EP 2 679 69 7A1 describes a method for manufacturing a cold-work die by hot working a specific cold-work tool steel, quenching and tempering it to a hardness of 58-62 HRC, and then machining it into a die shape. The tool steel's composition includes carbon, silicon, manganese, sulfur, chromium, molybdenum and / or tungsten, aluminum, and iron with unavoidable impurities. A key aspect is the addition of aluminum and sulfur to form a complex lubricating protective film (AI2O3 and MnS) on the cutting tool during machining, improving machinability. The method may also include annealing before quenching or direct quenching after hot working. Based on this, there is no indication of the positive effect of the Seebeck coefficient between a workpiece to be formed and the tool steel.Summary

[0006] According to an aspect of the disclosure a tool steel consists of, in % in weight: C: 0.8 - 1.5%, Si: 1.1 - 2.3%, Cr: 6 - 10%, Mo: 0.5 - 4%, Al: 1.1 - 2.3%, Ni: 0.6 - 2.3%, and optionally one or more of Co: < 3%, Mn: < 0.8%, V: < 0.8%, Ti : < 0.3%, Nb : < 0.3%, Cu : < 0.3%, W: < 0.3%, the balance Fe and incidental impurities.

[0007] According to another aspect of the disclosure an active element of a metalworking tool comprises the tool steel as recited above, wherein a process of manufacturing the active element comprises machining the tool steel.

[0008] According to another aspect of the disclosure a method of manufacturing a tool steel comprises: casting molten metal of a composition of, in % in weight: C: 0.8 - 1.5%, Si: 1.1 - 2.3%, Cr: 6 - 10%, Mo: 0.5 - 4%, Al: 1.1 - 2.3%, Ni: 0.6 - 2.3%, and optionally one or more of Co: < 3%, Mn: < 0.8%, V: < 0.8%,Ti : < 0.3%, Nb : < 0.3%, Cu : < 0.3%, W: < 0.3%, the balance Fe and incidental impurities, into a preliminary product; hot-forming the preliminary product into a hot-formed preliminary product; and annealing the hot-formed preliminary product at an annealing temperature in an austenite phase region.

[0009] According to another aspect of the disclosure a method of manufacturing an active element of a metalworking tool, wherein the active element comprises the tool steel as recited above, comprises machining the tool steel.Brief description of the drawings

[0010] Figure 1 is a schematic sectional view of a metalworking tool for machining a workpiece and a circuitry for measuring thermoelectric currents during the process of punching.

[0011] Figure 2 is a diagram illustrating a measurement setup for measuring a thermoelectric voltage Uth caused by the Seebeck effect .

[0012] Figure 3 is a diagram illustrating a dependency of the thermoelectric voltage Uth from temperature T for conventional cold working steel materials and a conventional workpiece material .

[0013] Figure 4 is a diagram illustrating a dependency of the thermoelectric voltage Uth from temperature T of various examples of tool steel products according to the disclosure and a conventional workpiece material.

[0014] Figure 5 is a diagram illustrating a dependency of the thermoelectric voltage Uth from temperature T of various examples of tool steel products according to the disclosure and a conventional workpiece material.

[0015] Figure 6 is a diagram illustrating (at an enlarged scale compared to Figures 3 to 5) a dependency of the thermoelectric voltage Uth from temperature T of various examples of tool steel products according to the disclosure and a conventional workpiece material .

[0016] Figure 7 is a schematic sectional view of a punch tool - workpiece combination and a circuitry for determining thermoelectric properties.

[0017] Figure 8 is a diagram illustrating a dependency of the thermoelectric current Ith (right hand) and a dependency of the punch force F (left hand) from the punch travel of various examples of tool steel products according to the disclosure as measured in the punch tool - workpiece combination of Figure 7.

[0018] Figure 9 is a diagram illustrating a dependency of the thermoelectric voltage Uth from temperature T of various examples of tool steel products according to the disclosure and a conventional workpiece material as measured in the punch tool - workpiece combination of Figure 7.

[0019] Figure 10 is a diagram illustrating an average height H of adhesive material on an active element of the punch tool of Figure 7 for various examples of tool steel products according to the disclosure.

[0020] Figure 11 is a diagram illustrating a relative change of the punch force F during the retraction movement between the first stroke and the 9thstroke in percent of the punch tool of Figure 7.

[0021] Figure 12 illustrates exemplary stages of a method of manufacturing a tool steel and an active element of a metalworking tool according to the disclosure.Detailed description

[0022] The tool steel disclosed herein exhibits low tendency for adhesive wear when used as a material of an active element of a metalworking tool for metal processing. For example, such metalworking tool may include a punch and / or a die as an active element for shaping (e.g. , forming and / or cutting) a metal workpiece such as, e.g. , a metal strip, a metal rod, a metal sheet, a metal blank, a metal block, a metal bar, etc.

[0023] The tool steel disclosed herein is based on the concept that thermoelectricity is a significant factor that influencesmaterial transfer from a workpiece to a tool and thus adhesive wear during metalworking. Thermoelectricity, which arises in every metalworking tool, is due to the Seebeck effect.

[0024] The basis for the Seebeck effect is a displacement (diffusion) of electrons in a metallic conductor caused by a temperature gradient in the conductor. Without such temperature gradient, the velocity of electrons is homogeneous inside the conductor. If one end of the conductor is heated, the electron velocity at this end raises and results in an average velocity vector pointing toward the cold end of the conductor. A thermodiffusion is provoked until moving electrons develop an electric field that counteracts and finally stops the thermodiffusion .

[0025] A material-specific constant, the Seebeck coefficient, determines the potential difference in relation to temperature. It describes the mobility and number of electrons and mainly depends on the material composition of the conductor.

[0026] If two different conductors with different temperatures are connected, a thermoelectric voltage Uth occurs . The thermoelectric voltage Uth depends on the difference between the Seebeck coefficients of both materials of the conductors and the temperature gradient. In a (short) circuit, the thermoelectric voltage leads to a measurable (circular) thermoelectric current Ith.

[0027] According to the disclosure, the amount of adhesive wear increases with higher thermoelectric currents Ith flowing between the active element of the metalworking tool and the workpiece.Hence, the chemical composition of the tool steel according tothe disclosure is set to provide for low or minimum thermoelectric currents Ith during metalworking tool operation.

[0028] In other words, if the tool steel and the workpiece material exhibit similar Seebeck coefficients, the amount of adhesive wear is significantly reduced. Accordingly, it was investigated if and how it is possible to systematically modify the Seebeck coefficient of a tool steel by changing the chemical composition thereof, and whether it is possible to systematically approximate the Seebeck coefficient of the tool steel to the Seebeck coefficient of the workpiece material.

[0029] The outcome of these investigations, which are described in more detail further below, is a tool steel composition as set out in Table 1. This tool steel composition shows significantly reduced adhesive wear in comparison with conventional tool steels used in metalworking tools.Table 1: Chemical composition of tool steel (in weight percent)Table 1: Continuation

[0030] All upper limits or preferred upper limits quoted inTable 1 are meant to read less than, i.e. <. All lower limits orpreferred lower limits quoted in Table 1 are meant to read more than, i . e . > .

[0031] Figure 1 illustrates an exemplary metalworking tool 100 according to the disclosure. Further, Figure 1 illustrates a metalworking tool setup for determining thermoelectric and mechanical parameters during shear cutting (e.g. punching, blanking) .

[0032] The exemplary metalworking tool 100 includes an active element 110, which is, in this specific example, a punch. The active element 110 includes or consists of the tool steel disclosed herein.

[0033] A workpiece 120, e.g. a metal sheet or blank, is processed by the active element 110. The workpiece 120 may, e.g. , be an aluminum based workpiece, a titanium based workpiece and / or a stainless steel workpiece. Other metal materials having a similar Seebeck coefficient than one or more of these materials are also possible.

[0034] The metalworking tool 100 may further include a workpiece holder 130 (e.g. , blank holder) and a workpiece support 140 (e.g. , die) . Further, the metalworking tool 100 may optionally include a counter punch 180.

[0035] The metalworking tool 100 may be suitable for various different metalworking operations, e.g. forming operations, in which the workpiece 120 is reshaped without adding or removing material, and / or cutting operations, in which the workpiece 120 is reshaped or brought to a specified geometry by removing material .

[0036] In areas 150, the active element 110 of the metalworking tool 100 engages or mechanically interacts with the workpiece 120. At least in these areas 150, the active element 110 includes or is of the tool steel disclosed herein.

[0037] In general, a metalworking tool according to the disclosure does not need to be designed as shown in the specific example of Figure 1. A metalworking tool according to the disclosure may be designed to be suitable for performing forming operations, such as bending, deep drawing, rolling, etc. , and / or cutting operations, such as cutting, shearing, punching, blanking, fine blanking, piercing, trimming, etc.

[0038] More specifically, applications of the metalworking steel (tool) may include:Tooling : trimming blades for sheet metal / strips, slitting rolls, extrusion dies, saw bands, shears for sheet metal processing, deep drawing dies, etc.Cold forming processes that can be classified according to the load : compression forming, tensile compression forming, tensile forming and bending forming: Punching, pressing, cold rolling, roll forming, driving, pressing, (cold) extrusion, stretch forming, bending, stretching, flanging, rolling-in, tumbling, etc .Non-tooling applications: guide rollers, bolts, bearings, sliding elements in general, etc.

[0039] Active elements of such metalworking tools may be, in general, all elements which engage with the workpiece during metalworking, e.g. , during forming or cutting operations. For example, the workpiece support 140 may also be referred to as anactive element, since adhesive wear in areas 150 not only occurs at the punch (active element 110) but also at the die (workpiece support 140) .

[0040] That is, even if the metalworking tool is described as an active element in the following for purpose of explanation, the metalworking steel (metalworking tool) can also take on a "passive" role when it comes into contact with the workpiece (e.g. , aluminum / aus teni te ) . That is, the "workpiece" to be machined (e.g. , aluminum / aus tenite ) may take over the movement and the steel (i.e. the metalworking tool) may stand still.

[0041] In other words, the technical concept disclosed herein is generally useful in applications where adhesive wear is reduced by reducing the difference in the Seebeck coefficients of a cooperating (engaging) material pairing.

[0042] Returning to Figure 1, for thermoelectric measurement purposes, the workpiece 120 may be electrically insulated from the workpiece holder 130 and the workpiece support 140 by insulating layers 160. A thermoelectric current Ith can be measured by a current measurement unit 170. For example, the thermoelectric current Ith may be measured via an induced voltage V by a voltage measurement unit 172. Other devices for current measurement may also be used. The arrow F denotes the direction of the punch force F, and the arrow C denotes the direction of the thermoelectric current Ith flowing between the active element 110 and the workpiece 120 during tool operation.

[0043] As mentioned above, according to the disclosure, the amount of adhesive wear increases with higher thermoelectric currents Ith. Hence, the chemical composition of the tool steelaccording to the disclosure (see Table 1) is set to provide for low or minimum thermoelectric currents during metalworking tool operation .

[0044] To this end, Seebeck coefficient measurements were conducted on conventional tool steel materials, conventional workpiece materials, various pure metals usable for alloying cold working steels, and examples of tool steel materials having a composition set in accordance with the disclosure.

[0045] Figure 2 schematically illustrates a measurement setup 200 for Seebeck measurements. In this example, the thermoelectric voltage Uth caused by the Seebeck effect is measured as a function of the temperature T.

[0046] The thermoelectric voltage Uth can be measured if at least two conductors are in contact. The Seebeck coefficients of the two conductors and the temperature gradient are significant for the thermoelectric voltage Uth that occurs.

[0047] More specifically, a specimen 210 ("first conductor") of the material on which the Seebeck measurement is carried out is subjected to a temperature gradient AT (see arrow at AT) . For example, one end of the specimen 210 is gradually heated up to temperature T (e.g. , 500°C) by applying heat H, while the other end is kept at a reference temperature Tref of, e.g. , 0°C (in an iced bath, for example) . A wire 220 ("second conductor") serving as reference material (e.g. , of platinum) connects both ends of the specimen 210.

[0048] The thermoelectric voltage Uth may be measured by the voltage measurement unit 270 of Figure 2. The prevailing temperatures at both ends of the specimen 210 may be measured by high precision thermocouples (not shown) , for example. For a given AT = T - Tref (which may, e.g. , be equal to T ifTref = 0°C as used in this example) and reference material (e.g. , platinum of wires 220) , the relative Seebeck coefficient of the material of the specimen 210 can be calculated directly from the slope of the thermoelectric voltage Uth versus temperature T.

[0049] In Figure 3, the thermoelectric voltage Uth as measured by the measurement setup 200 of Figure 2 versus temperature T is shown for examples of exemplary workpieces Wl and W2 (compositions see Table 2) and an example of a conventional cold working tool steel material T2 (composition see Table 3) .Table 2: Chemical composition (in weight percent) of examples Wl, W2 of a workpiece

[0050] The exemplary workpiece Wl, also referred to asEN AW 5083 ( AlMg4.5MnO 7 ) in the art, had a thickness of 4 mm and tensile strength of 270 MPa. Aluminum-based workpieces such as, e.g. , workpiece Wl are known to have a high tendency to adhesive tool wear if processed by metalworking tools having an active element made of cold working steel. As known in the art, whenprocessing aluminum, adhesive wear may be the main determinant of cost efficiency in metalworking.

[0051] Workpiece W2 is made of a conventional Cr-Ni austenitic stainless steel (X5CrNil8-10 ) with a hardness of 23 HRC, Young's modulus of 200 GPa and good corrosion resistance. This steel is also referred to as 1.4301 in the art. It is not a tool steel and does not have sufficient hardness, wear resistance, etc. to be used for tools.

[0052] T2 is a conventional cold working steel which is often used for metalworking tools in the art. The composition of T2 is given in Table 3.

[0053] It is apparent that the curve of the thermoelectric voltage Uth of the conventional cold working steel T2 substantially deviates from the curves of the thermoelectric voltage Uth of the workpieces W1 and W2. The curve of the thermoelectric voltage Uth of workpiece W2 is close to the curve of the thermoelectric voltage Uth of workpiece W1. As a consequence of the deviation of the thermoelectric voltages Uth between tool steel T2 and workpieces W1 and W2, in both cases (workpiece W1 and workpiece W2) , adhesive wear occurs during metalworking tool operation.Thermoelectricity investigations

[0054] According to the disclosure, it was investigated if and how it is possible to systematically modify the Seebeck coefficient of a tool steel by changing the chemical composition thereof .A) Thermoelectricity investigations of pure elements

[0055] As a starting point, thermoelectric properties of specimens 210 of pure metals which are usable for alloying cold working steels were measured by the measurement setup 200 of Figure 2. It was found that pure Al, Cr, Fe, Mn, Mo, Nb, V, W, Cu, Ti each has a positive effect on the Seebeck coefficient (i.e. , the thermoelectric voltage Uth increases with increasing temperature) while Ni, Co and Si each has a negative effect on the Seebeck coefficient (i.e. , the thermoelectric voltage Uth lowers with increasing temperature) . It is to be noted that these measured effects relate to pure metals, i.e. do not take into account synergistic effects due to interactions between different alloying elements.B) Thermoelectricity investigations of tool steel compositions

[0056] Examples of tool steel compositions are set out in Table 3. The manufacturing processes of these examples will be described further below. The effects of individual alloying elements regarding thermoelectric properties of the tool steel examples are illustrated in Figures 4 to 6.Table 3: Chemical composition (in weight percent) of tool steel examples (conventional tool steels T1-T2 and tool steels as investigated according to the present disclosure T3-T15)Table 3: Continuation

[0057] In Table 3, composition values that fall outside the ranges of Table 1 are in bold. Examples T1 to T7, T9 and T12 relate to tool steel compositions outside the ranges of Table 1.

[0058] Referring to Figure 4, it is apparent that by raising the Al-content (T5: Al = 1.1%; T4: Al = 1.3; T3 : Al = 1.6%) , the curves of the thermoelectric voltage Uth of tool steels T5, T4, T3 gradually approach the curve of the thermoelectric voltage Uth of the workpiece W1 (or, differently put, the Seebeck- coefficients of tool steels T5, T4, T3 approach the Seebeck- coefficient of workpiece Wl) .

[0059] Further examinations revealed that also Si and / or Ni have a significant effect on the thermoelectric properties of the tool steel and are suitable for "Seebeck coefficient trimming" in tool steels. Inter alia, tool steel examples T6, T7, T8 were examined to clarify upper and lower range values for Si, Al and Ni . It was found that- Al = 0.2% (T6) is too low and Al = 3.2% (T7) is too high;- Si = 2.9% (T6) is too high and Si = 0.2% (T7) is too low; and- Ni may compensate lower Al and / or Si contents, wherein about Ni = 1.5% (T8) appears to be preferred for compensation.

[0060] Further, it was found that Al and Si have a synergistic effect with regard to thermoelectricity properties: The effect of Al to approach the Seebeck-coef f icient of a tool steel composition to the Seebeck-coef f icient of workpiece Wl (which is similar to the Seebeck-coef f icient of workpiece W2) only occurs if the Si-content is not too low. For example, for tool steel T7 with low Si = 0.2% and high Al = 3.2%, the deviation of the Seebeck-coef f icient from the Seebeck-coef f icient of the workpieceW1 was greater than for tool steel T9, which has a lower Al content (Al = 2.6%) but more Si (Si = 1.0%) . In other words, it was found that a minimum Si content of approximatelySi = 1.1% is required to make use the effect caused by Al.

[0061] Figure 5 illustrates the measured curves of the thermoelectric voltage Uth of tool steels T10, Til and T12. These curves closely approximate the curves of the thermoelectric voltage Uth of the workpieces W1 and W2.

[0062] Figure 6 shows (in an enlarged view) the measured curves of the thermoelectric voltage Uth of the tool steels T13, T14 and T15 at lower temperatures than in Figures 3 to 5. It can be seen that T14 provides the best approximation to the thermoelectric voltage curves of W1 and W2, followed by T15 and T13 in that order .Measurement of tool steel characteristics during operation

[0063] Examples of tool steels have been investigated by measuring their metalworking performance in operation. An (exemplary) metalworking tool 100 (e.g. , blanking tool) as shown in Figure 7 was used. The metalworking tool 100 of Figure 7 may be identical to the metalworking tool 100 of Figure 1, and reference is made to the above description to avoid reiteration.

[0064] The workpiece 120 (sheet metal of composition W1(EN AW 5083) ) had a thickness of 4 mm. During all measurements, 50 pm die clearance was used, which corresponds to 1,25% of the workpiece thickness. A circular punch 110 with a diameter of 15 mm and cutting edge radii of 50 pm was used. A punch impact speed of 50 mm / s was chosen. The downward punch movement stoppedat the bottom dead centre and then the return stroke began. No lubricants were applied.

[0065] The punch force F was measured by piezoelectric load cells (not shown) with a measuring range up to 90 kN in a force shunt configuration able to record tensile forces during the return stroke. A measurement unit MEAS was used to measure the thermoelectric current Ith and / or the temperature T at the workpiece 120. The temperature T may, e.g. , be measured based on the thermoelectric voltage Uth as described in EP 4 122 619 Al.

[0066] Figure 8 is a diagram illustrating a dependency of the thermoelectric current Ith (right Y-axis) and a dependency of the punch force F (left X-axis) of conventional tool steel examples Tl, T2 and tool steel examples T13, T14 and T15 according to the disclosure from the punch travel. Negative values of the punch travel characterize the downward movement. Workpiece separation occurs at reference sign 810. The bottom dead centre (reversal point) is at 0 of the punch travel and positive values of the punch travel characterize the upward movement.

[0067] The maximum downward movement force F is 35.8 kN and the maximum retraction force F is 1.5 kN. These forces are similar for all tool steel examples (solid bold line applies to all tool steel examples Tl, T2, T13, T14, T15) .

[0068] However, the thermoelectric current Ith of the tool steel examples Tl, T2 and T13, T14, T15 is quite different. While the conventional tool steel examples Tl, T2 exhibit large thermoelectric currents Ith both in downward and retraction direction, the thermoelectric currents Ith of the tool steelexamples T13, T14, T15 are negligible in both punch movement directions .

[0069] Temperature measurements revealed that the same (maximum) temperature of operation (about 57°C) was reached by all tool steel examples Tl, T2, T13, T14, T15 when the same workpiece (here: Wl) was used. That is, the workpiece determines the temperature of operation (for different tool steels used with the same operational parameters) . Figure 9 illustrates the thermoelectric voltages Uth for the above-mentioned different tool steels at the (common) temperature of operation. As the temperature of operation was always the same, temperature effects can be ruled out as an influencing factor on the differences in adhesive wear formation.Measurements of adhesive wear

[0070] Figures 10 and 11 illustrate measurements on the amount of transferred material observed at the surface of the active element 110 (i.e. , the punch) after 9 strokes. In Figure 10 an average height H in pm of transferred material on the active element 110 of the punch tool 100 for the tool steel examples Tl, T2, T13, T14, T15 is shown. It can be seen that material transfer decreases by 96% from tool steel example Tl to tool steel example T15. In fact, adhesive wear (which is a consequence of the material transfer from workpiece to tool steel) was hardly detectable for tool steel example T15 under the experimental conditions describe above.

[0071] In Figure 11, a relative change in percent of the punch force F (as measured during the retraction movement) between the first stroke and the 9thstroke is shown for the tool steel examples Tl, T2, T13, T14, T15. While the punch force F increasesby 122% for tool steel example Tl, the tool steel examples T14, T15 showed the lowest increase in the punch force F during operation .

[0072] In summary, it has been shown that it is possible to systematically change the Seebeck coefficient of a tool steel to a targeted Seebeck coefficient by changing the composition of the tool steel. That is, targeted adjustment ("trimming") of the Seebeck coefficient is possible (proof through alloy variation, see adhesion quantity, thermoelectric current, temperaturevoltage curve) . The effects of the alloying elements Al and Si revealed synergistic effects to influence the Seebeck coefficient, while Ni appears to be suitable to compensate lower Al and / or Si contents. Adhesive wear-optimized active elements for metalworking were produced. Verification of the theory by means of active elements used in blanking test were carried out and theory proven.Manufacturing process

[0073] In Figure 12, exemplary process steps of a method of manufacturing a tool steel according to the disclosure are shown. The process steps explained below are exemplary and can be partly replaced or supplemented by other or similar process steps. In particular, additional processes which are not discussed in this disclosure may be provided between the process steps described below .

[0074] The starting point for tool steel production is a furnace process 1210 in which molten metal is provided. This can be done, for example, in an EAF (Electro Arc Furnace) or in an induction furnace (e.g. , by VID (Vacuum Induction Degassing) ) . These processes are typically primary metallurgy processes.

[0075] In a following optional secondary metallurgy process (not shown) , the final chemical composition of the tool steel may be set .

[0076] At 1220, the molten tool steel is cast to a preliminary product. For example, ingot casting using, e.g. , consumable electrodes may be carried out to produce ingots.

[0077] Optionally, the preliminary product is then remelted (not shown in Figure 12) . Remelting may be carried out by ESR (Electro Slag Remelting) , PESR (Pressure Electro Slag Remelting) or VAR (Vacuum Arc Remelting) , for example. During remelting, the preliminary product is again melted. Remelting homogenizes the preliminary product. For example, segregations are removed and carbides are more uniformly distributed in the preliminary product after remelting.

[0078] At 1230, the preliminary product may be hot-formed (e.g. , forged and / or hot-rolled) . Hot-forming may be carried out at temperatures of about 1100°C - 1160°C. The hot-formed preliminary product may be, for example, a strip (hot strip) , a sheet, a blank, a block, a bar, etc.

[0079] For example, if the preliminary product is forged (at 1230_l) , a plurality of forging steps with intermediate heating phases to the above-mentioned temperatures may be performed. If the preliminary product is hot-rolled (at 1230_2) , hot-rolling may include, for example, rough rolling in a rough rolling mill and / or (optionally) further forming steps, such as multi-line rolling in a multi-line rolling mill.

[0080] After hot-forming, the preliminary product may be annealed at 1240. Annealing may start at a temperature significantly below the hot-forming temperature, e.g. at about 500°C. Annealing is performed by heating the solid, hot-formed preliminary product to a temperature in the austenite phase region, i.e. above a temperature at which a transformation of ferrite in austenite ends. To this end, the hot-formed tool steel product is heated to 850°C to 950°C at a holding time of 0.5 hours to 15 hours. For example, the hot-formed product may be heated to about 900°C at a holding time of, e.g. , about 1 hour. Annealing of the hot-formed tool steel is carried out to improve machinability of the hot-formed preliminary product. This kind of annealing is also referred to as "soft annealing" in the art.

[0081] ( Soft) annealing at 1240 may include heating the hot- formed tool steel product into the austenite area, followed by slowly cooling it through the austenite and ferrite two-phase region (note that there are also carbides distributed in the steel matrix as additional phases; however, the term "two-phase region" only relates to the austenite and ferrite steel matrix phases and disregards any additional carbide phases) . The annealed tool steel may be cooled down from the austenite phase region through the austenite and ferrite phase region by a slow cooling process at a cooling rate of, e.g. , about 10°C / h. The cooling should pass slowly through this two-phase region until this transformation is complete (may be about 770°C, for example) .

[0082] Aft er annealing, the annealed tool steel is cooled down to room temperature. In some examples, ambient air cooling may be used .

[0083] More specifically, the annealed tool steel may be cooled down to, e.g. , 830 °C in the annealing furnace, followed by a slow cooling process at a cooling rate of, e.g. , about 10°C / h to an intermediate temperature (e.g. , about 770°C) . Thereafter, it may be cooled slowly down to room temperature.

[0084] Subsequently, the active element may be manufactured from the annealed tool steel. The active element may, for example, be produced by a customer of the tool steel producer (in this case, the tool steel may be delivered to a customer at the stage of the dashed line in Figure 12) or by the tool steel producer itself.

[0085] Manufacturing the active element 110 may include machining the tool steel by, e.g. , metal-cutting processes such as, e.g. , turning, milling and / or drilling, etc. to produce the active element of a metalworking tool.

[0086] Optionally, a heat treatment (at 1250) may be performed on the active element 110, i.e. on the machined tool steel.

[0087] The (optional) heat treatment at 1250 may include hardening (at 1250_l) the machined tool steel of the active element 110. Hardening may comprise heating the machined tool steel of the active element to a temperature between, e.g. , 1000°C and 1180°C followed by rapid cooling to produce an active element 110 of hardened tool steel. For example, the heating time may be between 2 minutes and 1 hour. For example, the machined tool steel of the active element 110 may be heated to a temperature of about 1050°C for, e.g. , 0.5 h. The heated tool steal may subsequently be quenched (e.g. , by N2 having a pressure of equal to or greater than 4, 5 or 6 bar) .

[0088] The (optional) heat treatment at 1250 may further include a subsequent tempering (at 1250_2) of the hardened preliminary metalworking tool (active element 110) at a temperature equal to or above, e.g. , 500°C and / or equal to or below 620°C or 600°C or 580 °C for one or more hours, for example. For example, the preliminary metalworking tool may be tempered at 500 to 600°C, e.g. , at about 550°C for, e.g. , a plurality of hours (e.g. , for 1, 2 or 3 cycles of 20 minutes to 6 hours each) .

[0089] It was found that the heat treatment at 1250 slightly changes the thermoelectric properties of the active element 110. More specifically, the curve of the thermoelectric voltage Uth of a heat-treated active element 110 is slightly shifted downwards to lower voltages. This (small) offset, which can be adjusted by the heat treatment on the otherwise finished active element 110, can be used to further approximate the Seebeck coefficient of the active element 110 to the Seebeck coefficient of the workpiece 120.Manufacturing processes used to manufacture the tool steel examples T3 to T15:

[0090] In general, all tool steels and active elements as disclosed herein, including Examples T3 to T15, may be manufactured by using, e.g. , the above-described manufacturing process. In the following, without loss of generality, exemplary manufacturing processes used for tool steel examples T3 to T15 are described. Features described in connection with these processes can generally be applied to any manufacturing process.

[0091] Examples T3 to T12 were manufactured as laboratory samples by the following process:1. Inductive melting at 1210 of the tool steel composition.2. Block casting at 1220. Then, cooling the sample blocks down to room temperature.3. Heating the casted samples to a temperature between 1100°C to 1160°C in a furnace and forging at 1230_l the samples to preliminary products of 12 to 14 mm diameter.4. Soft annealing at 1240 to improve machinability by heating the forged preliminary products to 900°C in a furnace into the austenite region with a holding time of 1 h. Then, cooling the preliminary products to 830°C in the furnace. Thereafter, slow cooling the preliminary products at 10°C / h to 770°C.Subsequently, cooling the preliminary products in air or otherwise cooling them to room temperature after the furnace has been switched off.5. Turning the samples to a diameter of 8 to 10 mm to produce the finalized active elements (punches) 110.

[0092] Examples T13 to T15 were manufactured on an industrial scale, using similar process parameters as for the examples T1 to T12 :1. Melting at 1210 and casting (ingot casting) at 1220 were performed in an Ar atmosphere. The ingots were cooled down to room temperature.2. Heating the ingots to 1150°C for 3 h. Forging at 1230_l was performed as follows: First forging process to reduce the thickness of the ingots to 205 mm. Intermediate heating to 1150°C for 1 to 2 hours. Second forging process to reduce the thickness of the ingots to 170 mm. Intermediate heating to 750°C for at least 6 h. Intermediate grinding. Heating the ingots to 1130°C for 1 to 2 h. Third forging process to reduce the thickness of the ingots to 110 mm. Then, hot delivery to heat treatment plant.3. Annealing at 1240 was performed as follows: Material transfer at 500°C to the annealing furnace. Soft annealing by heating theforged preliminary products up to the austenite region and slow cooling down through the two-phase region.

[0093] The measurement results disclosed in Figures 3 to 6 were obtained by tool steel examples produced as described above. The measurement results disclosed in Figures 8 to 11 were obtained by active elements 110 made of the tool steel examples after subjecting them to a subsequent heat treatment at 1250. The heat treatment included heating the active elements 110 in a vacuum furnace for hardening them at 1050°C for 30 min, followed by quenching with N2 at a pressure of 6 bar. Subsequently, the hardened active elements 110 were tempered at 550°C for 3 cycles of 2 hours each.

[0094] The specific process parameters described above for manufacturing the tool steel examples T3 to T15 and / or the active elements 110 are each applicable separately or in combination to any manufacturing process disclosed herein.Experimental data for active elements of tool steel example T15 produced by different manufacturing processes

[0095] Table 4 shows measured data of Rockwell hardness, impact bending strength and retained austenite content of examples of active elements for hardening temperatures (at 1250_l) of 1030°C and 1070°C and various tempering temperatures (at 1250_2) for tool steel example T15 (chemical composition 1.2% C, 1.5% Si, 8.2% Cr, 2.0% Mo, 1.5% Al, 1.4% Ni, 0.4% Mn, 0.5% V) .

[0096] Hardness values are given in Rockwell C (HRC) measured according to ASTM E18-17, wherein the average value from 3 measurements is given.

[0097] Impact bending test values are measured according to SEP1314 and are given in unit joule [J] . The absorbed energy indicates the material's toughness.

[0098] Retained austenite values are determined using XRD (X-Ray Diffraction) measurement and the Rietveld method.

[0099] The composition of the microstructure was determined. All values are in vol . % . The microstructure contained M7C3, MeC and (Nb,Mo)C carbides and a martensite steel matrix (M denotes a metal element of the steel) . The following composition was determined: M7C3 : 10.39 ± 1.77 vol.%; MeC: 0.36 ± 0.11 vol.%; (Nb,Mo)C: 0.18 ± 0.01 vol.%; rest: martensite (and, optionally, retained austenite) .Table 4: Mechanical properties of examples of active elements of composition T15 manufactured by different manufacturing processes

[0100] For T15_l, T15_2, T15_8, and T15_9, the tempering temperature is below 500°C, with high contents of retained austenite. The results in Table 4 show that a target temperature for tempering may be around 520-530°C, as this is where the combination of hardness (HRC) and impact bending work (J) is almost at its maximum. At the same time, the retained austenite content is also very good, as it is as low as possible.

[0101] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and / or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.

Claims

Claims1. A tool steel consisting of, in % in weight:C: 0.8 - 1.5%,Si: 1.1 - 2.3%,Cr: 6 - 10%,Mo : 0.5 - 4 % ,Al: 1.1 - 2.3%,Ni : 0.6 - 2.3%, and optionally one or more ofCo: < 3%,Mn : < 0.8%,V: < 0.8%,Ti : < 0.3%,Nb : < 0.3%,Cu: < 0.3%,W: < 0.3%, the balance Fe and incidental impurities.

2. The tool steel of claim 1, wherein Al: 1.2 - 2.0%, in particular 1.2 - 1.8%.

3. The tool steel of claim 1 or 2, wherein Si: 1.2 - 2.0%, in particular 1.2 - 1.8%.

4. The tool steel of any of the preceding claims, wherein Ni : 1.1 - 2.0%, in particular 1.2 - 1.8%.

5. The tool steel of any of the preceding claims, wherein Cr : 7 - 9% .

6. The tool steel of any of the preceding claims, wherein29Co: < 1.0% or Co: < 0.5%.

7. A tool steel obtainable by a process comprising: casting molten metal of the composition of, in % in weight:C: 0.8 - 1.5%,Si: 1.1 - 2.3%,Cr: 6 - 10%,Mo : 0.5 - 4 % ,Al: 1.1 - 2.3%,Ni : 0.6 - 2.3%, and optionally one or more ofCo: < 3%,Mn : < 0.8%,V: < 0.8%,Ti : < 0.3%,Nb : < 0.3%,Cu: < 0.3%,W: < 0.3%, the balance Fe and incidental impurities, into a preliminary product; hot-forming the preliminary product into a hot-formed preliminary product; and annealing the hot-formed preliminary product at an annealing temperature in an austenite phase region.

8. An active element of a metalworking tool, wherein an active element is an element which engages with a workpiece during metalworking, the active element comprising the tool steel of any of the preceding claims, wherein a process of manufacturing the active element comprises machining the tool steel.

309. The active element of claim 8, wherein the process of manufacturing the active element further comprises performing a heat treatment on the machined tool steel.

10. The active element of claim 9, wherein the heat treatment comprises : hardening the machined tool steel by heating it to a temperature equal to or above 1000°C followed by rapid cooling; and tempering the hardened tool steel at a temperature equal to or above 500°C.

11. A method of manufacturing a tool steel, the method comprising : casting molten metal of a composition of, in % in weight:C: 0.8 - 1.5%,Si: 1.1 - 2.3%,Cr: 6 - 10%,Mo : 0.5 - 4 % ,Al: 1.1 - 2.3%,Ni : 0.6 - 2.3%, and optionally one or more ofCo: < 3%,Mn : < 0.8%,V: < 0.8%,Ti : < 0.3%,Nb : < 0.3%,Cu: < 0.3%,W: < 0.3%, the balance Fe and incidental impurities, into a preliminary product;hot-forming the preliminary product into a hot-formed preliminary product; and annealing the hot-formed preliminary product at an annealing temperature in an austenite phase region.

12. A method of manufacturing an active element of a metalworking tool, wherein an active element is an element which engages with a workpiece during metalworking, wherein the active element comprises the tool steel manufactured according to claim 11, the method comprising machining the tool steel.

13. The method of claim 12, further comprising performing a heat treatment on the machined tool steel.

14. The method of claim 13, wherein the heat treatment comprises : hardening the machined tool steel by heating it to a temperature equal to or above 1000°C followed by rapid cooling; and tempering the hardened tool steel at a temperature equal to or above 500°C.

15. The method of claim 14, wherein hardening is carried out at a temperature between 1000°C and 1180°C for a time between 2 minutes and 1 hour; and / or tempering is carried out at a temperature between 500°C and 620°C for a cycle time between 20 minutes and 6 hour, wherein one, two or three cycles are performed.

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