EP2824212B1 - Hot-working steel - Google Patents

Description

The invention relates to a hot-work tool steel with a nitrogen content of at least 0.1 wt .-%.

While nitrogen is undesirable in low alloy steels due to the formation of embrittling nitrides, the presence of nitrogen in high alloy steels has a number of advantages. Higher N contents can increase strength without restricting the toughness of the respective steel. Also, by high N contents, the corrosion resistance of the steel can be improved. In addition, high N contents contribute to increasing the heat resistance. Furthermore, in the case of austenitic steels, the presence of high N contents stabilizes or widens the temperature range in which austenitic structure is present in the steel. For steels that undergo high cold deformations, high N contents can prevent the formation of stress-induced martensite. In addition, high N contents inhibit the precipitation of intermetallic phases. Finally, Mn austenitic steels with high N contents have a significantly higher heat resistance compared to conventional CrNi austenitic steels.

In order to produce steels with high N contents, these steels must undergo special manufacturing processes, as nitrogen at higher levels can not be alloyed by conventional melt metallurgy to steels of the type in question.

An important role in the production of high nitrogen alloys is remelting, in particular the pressure electroslag remelting process ("DESU process"). In this method, in a first step with the usual means of melt metallurgy, z. B. using the ladle metallurgy under atmospheric pressure, a melt produced, which has up to the required N content, the respective intended alloying constituents in the predetermined levels. This melt is poured into a cylindrical electrode.

This electrode serves as starting material for the subsequently passed pressure remelting. In this process, the self-consuming electrode is melted in a continuous process in a pressure vessel at its lower end in the direction of gravity end by resistance heating. The melted, molten molten steel then drips through a slag bath, which serves as a heating conductor and as a refined reactant. At the same time, the slag increases the nitrogen content of the melt. For this purpose, nitrogen-containing granules are introduced into the pressure vessel via a metering device, which drops onto the slag and melts there to release the nitrogen. Alternatively, it is also possible to pass gaseous nitrogen into the pressure vessel to the desired To achieve nitriding of the steel. The atmospheric pressure in the pressure vessel is set in consideration of the temperature and the steel composition so that the nitrogen partial pressure is sufficient to introduce the desired amount of nitrogen in the molten steel.

The physical principles of the process of embroidering can be described by Sievert's square root law, according to which the theoretically achievable nitrogen solubility [% N] is a function of pressure and temperature: $$\left[\%N\right]=k\cdot \sqrt{p_{N2}}$$

In equation (1), " p _N2 " denotes the nitrogen partial pressure above the melt in bar and " k " denotes a temperature- and alloy-dependent material constant.

wobei hier mit "p_N2 " wiederum der Stickstoffpartialdruck über der Schmelze in bar, mit "[%N]_Fe-x" die Stickstofflöslichkeit in eisenbasierten Mehrstoffsystemen und mit "[%N]_Fe" die Gleichgewichtskonstante in reinem Fe bei 1.600°C und 1 bar bezeichnet sind ([%N]_Fe = 0,044 %).In real systems, the alloying elements present in each case influence the actual nitrogen solubility. To describe this effect, one uses their thermodynamic activities: $${\left[\%N\right]}_{Fe-X}=\frac{{\left[\%N\right]}_{Fe}}{f_N^X}\cdot \sqrt{p_{N2}}$$

Here again with " p _N2 " the nitrogen partial pressure above the melt in bar, with "[% N] _Fe-x " the nitrogen solubility in iron-based multi-substance systems and with "[% N] _Fe " the equilibrium constant in pure Fe at 1,600 ° C and 1 bar ( [% N] _Fe = 0.044%).

lässt sich dabei gemäß Gleichung (3) wie folgt berechnen: $$\log f_N^X=e_N^X\left[\%X\right]$$

wobei hier mit $ʺe_N^Xʺ$

der Wechselwirkungskoeffizient und mit "[%X]" die Konzentration des jeweiligen Elements in Gew.-% angegeben ist.The activity coefficient $"f_N^X"$

can be calculated according to equation (3) as follows: $$\log f_N^X=e_N^X\left[\%X\right]$$

being here with $"e_N^X"$

the interaction coefficient and with "[ % X ]" the concentration of the respective element in wt .-% is given.

It can be seen that, depending on the particular interaction coefficient, certain elements increase nitrogen solubility (eg, manganese), while other elements (eg, silicon) lower nitrogen solubility. These effects affect on the one hand on the nitriding during the remelting and on the other hand on the excretion of any intermetallic phases in the solid state ( U. Kamachi Mudali, Baldev Raj; "High Nitrogen Steels and Stainless Steels. Manufacturing, Properties and Applications."; ASM International, Narosa Publishing House, New Delhi, Chennai, Mumbai, Kolkata, 2004, New Delhi, India . Anne Satir-Kolorz, Heinrich Feichtinger, Markus Speidel: "Literature study and theoretical considerations on the dissolution behavior of nitrogen in iron, steel and cast steel melts", Foundry Research 42, 1990, No. 1, pp. 36-49 ).

A hot work tool suitable for the production of casting molds for aluminum casting is from the EP 1 696 045 A1 known. The known steel contains (in% by weight) 0.1-0.35% C, less than 0.8% Si, up to 3% Mn, 2.0-7.0% Cr, W and Mo. With the proviso that the sum of the Mo content and the half W content 0.3-5%, 0.05-0.5% N, up to 0.0100% O, up to 0.05% P, up to 0.05% Al and balance iron and unavoidable impurities. The sum of the N and C contents should be 0.2 - 0.6%, whereby the ratio C / N of the C content to the N content should apply at the same time: C / N ≤ 6. By restricting the C content is intended to avoid the formation of an intermetallic compound on the Al melt contacting surfaces of a mold formed from the known steel. In addition, by setting a low C content, the formation of coarse carbides should be suppressed. In addition to the alloying elements mentioned, the known steel may have contents of V in order to increase the strength. Tantalum can also be added to the known steel in amounts of up to 1% in order to prevent coarsening of the structure during quenching, with Ta contents of at least 0.05% being considered particularly favorable for this effect.

Against the background of the prior art discussed above, the object of the invention was to provide a hot work tool steel in which there is an optimized combination of strength and toughness when used at high temperatures.

This object has been achieved by a steel having the composition specified in claim 1.

Advantageous embodiments of the invention are specified in the dependent claims and are explained below as the general inventive concept in detail. The invention is based on the finding that, given sufficiently high C and N contents, the alloying elements Ta and V likewise added as compulsory constituents in sufficient amounts form thermally stable nitrides, carbides or carbonitrides, which also make a decisive contribution to the strength of the steel according to the invention under high operating temperatures.

• C: 0,38 - 0,45 %,
• Si: bis zu 0,8 %,
• Mn: bis zu 0,5 %,
• Cr: 4,0 - 6,0 %,
• Co: 0,3 - 0,8 %,
• Ni: 0,8 - 1,1 %,
• Mo: 2,3 - 2,8 %,
• Ta: 0,1 - 1,0 %,
• Al: bis zu 0,025 %,
• Ti: bis zu 0,03 %,
• V: 0,15 - 0,3 %,
• N: 0,1 - 0,5 %,
• Rest Fe und herstellungsbedingt unvermeidbare Verunreinigungen.

In order to make use of this effect, a hot-work steel according to the invention has the following composition (in% by weight):

• C: 0.38-0.45%,
• Si: up to 0.8%,
• Mn: up to 0.5%,
• Cr: 4.0-6.0%,
• Co: 0.3-0.8%,
• Ni: 0.8-1.1%,
• Mo: 2.3-2.8%,
• Ta: 0.1-1.0%,
• Al: up to 0.025%,
• Ti: up to 0.03%,
• V: 0.15-0.3%,
• N: 0.1-0.5%,
• Remaining Fe and inevitable impurities due to production.

As already mentioned, carbon is present in amounts of 0.38-0.45% by weight in a steel according to the invention, so that a sufficient supply of C is available in the steel for the formation of T and V carbides or carbonitrides which are also under high temperatures of for example, 600 ° C and more stable. Carbonitrides, which are more temperature-stable than pure carbides, retard grain coarsening and shift them to higher temperatures. In this way, the comparatively high C contents contribute significantly to the significantly higher heat resistance of a steel according to the invention compared to the prior art. These effects can be achieved particularly reliably if up to 0.43% by weight C are present in the steel according to the invention.

Si may be present in steels of the invention in amounts of up to 0.8% by weight in order to increase the strength of the steel of the present invention. For the introduction of Si into the steel according to the invention, in the course of increasing the N content, the given contents are reached because, for this purpose, Si ₃ N _{4 is} usually used as nitrogen carrier. In order to avoid the risk of temper embrittlement, the Si content of the steel according to the invention is limited to 0.8% by weight.

Mn in amounts of up to 0.5% by weight may increase the austenite stability of the steel of the invention and increase the solubility of N in the steel.

Cr contents of 4.0-6.0% by weight increase the strength of the steel according to the invention through the formation of Cr carbides and Cr nitrides. In addition, Cr increases the through-hardenability of thick-walled workpieces, which are made of steel according to the invention, in the contents prescribed according to the invention. They are especially safe use positive effects of Cr in the steel according to the invention, if its Cr content is at least 4.5 wt .-%.

Co is present at levels of 0.3-0.8% by weight in the steel of the present invention to improve its ductility and heat resistance. Co contributes to increasing the mixed crystal hardness and shifting the recrystallization temperature to higher temperatures. In addition, Co increases the solubility of C and N.

Ni at levels of 0.8-2 wt% also increases solid solution hardness and is an important austenite stabilizer. At the same time, the solubility of C in austenite is increased by the presence of Ni in the amounts specified by the invention and accordingly has an advantageous effect on the adjustment of the hardness of the steel according to the invention. In doing so, Ni suppresses the delta-ferrite formation and improves the ductility and forgeability in the contents prescribed according to the invention. This effect is achieved when the Ni contents do not exceed 2% by weight. Higher nickel contents limit the solubility for N. Optimum effects of Ni in the steel according to the invention can then be achieved if the Ni content of the steel according to the invention is reduced to max. 1.1 wt .-% is limited.

Mo in contents of 2.3-2.8% by weight increases the heat resistance of the steel according to the invention. Mo forms carbides and carbonitrides, which on the one hand increase the hardness, on the other hand retard grain growth. The wear resistance of dies made of steel according to the invention increases with increasing Mo content.

However, an excessively high Mo content leads to long-term embrittlement by precipitation of intermetallic Laves and Sigma phases. In addition, with too high Mo contents, the hot strength increases in such a way that shaping is made more difficult.

Ta is present in a steel of the invention at levels of 0.1-1.0 weight percent to form Ta carbides, Ta nitrides, or Ta carbonitrides, which have been found to be particularly stable even at high temperatures. The carbides, carbonitrides and nitrides formed with Ta retard grain growth at high temperatures and thus increase the strength. In addition, Ta increases the solubility for N in the steel. The positive effects of Ta in the steel according to the invention can be used with particular reliability if the Ta content of a steel according to the invention is at least 0.4% by weight, with optimum use if the Ta contents of the steel according to the invention are not more than 0 , 8 wt .-% are limited.

Al may be used in the smelting of the steel of the invention for deoxidation and is then present at levels typically up to 0.025% by weight. The Al content is such that the formation of Al nitrides is largely avoided. To achieve this, the Al content can be restricted to less than 0.015% by weight.

Ti at levels of up to 0.03 wt% also contributes to increasing the strength and forming a fine-grained texture by forming fine TiC and Ti (C, N) precipitates. Too high Ti contents should be avoided, however, to prevent the precipitation of primary nitrides out of the melt. These worsen the ductility. In order to exclude negative influences of Ti on the properties of the steel according to the invention, the Ti content can be reduced to max. 0.005 wt .-% be limited.

V in contents of 0.15-0.3% by weight also forms fine carbides and carbonitrides in the steel according to the invention and thus increases the strength and fine-grainedness of a steel according to the invention. The temperature-stable carbides, nitrides and carbonitrides formed with V shift recrystallization to higher temperatures and retard grain growth.

The provided in the steel according to the invention high contents of N from 0.1 to 0.5 wt .-% form the basis for the formation of Ta nitrides, which have proven to be particularly stable to temperature and as such contribute significantly to the high heat resistance, the one has steel according to the invention. Nitrogen increases in the inventively predetermined levels in the steel according to the invention, the recrystallization temperature and retards the dynamic recrystallization, which is noticeable by a better heat resistance. Furthermore, N forms nitrides and carbonitrides, which also increase strength and grain refining. In addition, in the presence of nitrogen, the precipitated carbides are distributed much finely dispersed. As a result, lower segregations occur, resulting in homogeneous material properties and better toughness. Especially safe can be the positive Use influences of N in a steel according to the invention, if its N content is at least 0.28 wt .-%, resulting in an optimum relation of manufacturing costs and benefits, if in a hot work tool according to the invention the N content not more than 0.4 wt. -% is.

The intended for a steel according to the invention high N contents can be produced in a manner known per se process stable by pressure-electric-slag remelting.

Practical and theoretical investigations have shown that a hot work tool steel assembled in accordance with the invention has superior mechanical properties even at high operating temperatures. Thus, a steel according to the invention has a compressive strength σ of at least 1,600 MPa at 600 ° C., values of up to 2,000 MPa being proven in practical tests.

The invention will be explained in more detail by means of exemplary embodiments.

In the attached diagram, as a result of a hot compression test, the respective compression force is plotted along the path over which the respective sample was compressed in the course of the experiment until the maximum pressing force was reached.

A steel was melted and cast into a cylindrical electrode, which then went through a DESU process. In the course of the remelting in the DESU plant, the N content of the steel has been increased, so that the steel from which the steel ingot obtained as a result of the DESU process consisted, in addition to Fe and unavoidable impurities, of the alloy contents shown in Table 1. Table 1 C Cr Co Ni Not a word Ta V N 0.40 5.00 0.50 1.00 2.50 0.60 0.20 0.30 Data in% by weight

From the block obtained after the DESU process bar samples were taken with a diameter of 8 mm each.

A first of these samples was heated from room temperature to 810 ° C. in a soft annealing test at a heating rate of 30 ° C./h and kept at this temperature for 640 minutes. Thereafter, the sample was first cooled in the oven to 300 ° C and then stored in still air. The measurement of Brinell hardness HBW 10/3000 according to DIN EN ISO 65061 showed a hardness value of 215 for the three spatial directions related to a Cartesian coordinate system.

Heat treatment tests were carried out with further samples P1 - P5 to determine the hardness or tempering curve of the steel and to determine the heat distortion behavior.

A first sample P1 has been left in the initial state.

In a first heat treatment experiment, a sample P2 was first kept at 1000 ° C for 30 minutes and then cooled in oil. Thereafter, the sample was held twice in a row for two hours in an oven at 600 ° C and cooled in the oven to room temperature. The hardness test carried out at room temperature in accordance with DIN EN ISO 65061 showed a Brinell hardness HBW 10/3000 of 44.5 in each of the three spatial directions for sample P2.

In a second heat treatment test, a sample P3 was first kept at 1020 ° C for 30 minutes and then cooled in oil. Thereafter, the sample P3 twice in a row over two hours in an oven was kept at 600 ° C and cooled in the oven to room temperature. The hardness test carried out at room temperature in accordance with DIN EN ISO 65061 gave a Brinell hardness HBW 10/3000 of 42.5, 43 and 42.5 for the sample P3 for the three spatial directions.

In a third heat treatment test, a sample P4 was first kept at 1020 ° C. for 30 minutes and then cooled at a cooling rate of 80 ° C./h. Thereafter, the samples were also twice in a row for two hours in an oven kept at 600 ° C and cooled in the oven to room temperature. The hardness test carried out at room temperature in accordance with DIN EN ISO 65061 showed a Brinell hardness HBW 10/3000 of 44.5, 43.5 and 44.5 in the three spatial directions for sample P4.

The cracking hardness of the examined samples was 56 HRC.

To determine the hot strength of the steel, another sample P5 was held twice consecutively in an oven at 800 ° C. for 240 minutes and then cooled in the oven to 300 ° C. Then it has been deposited in air and cooled to room temperature.

The untreated sample P1 and the heat-treated samples P2-P5 were each subjected to a hot compression test in which they were loaded at 600 ° C for a period of 30 minutes up to a maximum pressing force of 370 kN. The results of the hot upsetting tests are summarized in the attached diagram.

It turned out that the samples started to flow at a load of approx. 100 kN each. The untreated sample P1 and the heat-treated, respectively slowly cooled sample P5 remained without cracking until the maximum pressing force was reached. At the same time, the sample P5 could be deformed to the maximum. On the other hand, when the maximum pressing force was reached, the samples P2 and P4 had longitudinal cracks and the sample P3 longitudinal and transverse cracks, the maximum deformability of the samples P2 - P4 being approximately equal.

• Durchmesser d der Proben: 8 mm
• Presskraft F bis Fließen: 100 kN
• Querschnittsfläche A der Proben: π/4*d² = 50,3 mm² Druckfestigkeit σ = F/A = 100 * 10³/50,3 MPa = 1988 MPa.

Based on the results of the hot press tests, the theoretically achievable compressive strength σ (yield strength) of the steel with the composition given in Table 1 could be determined as follows:

• Diameter d of the samples: 8 mm
• Pressing force F to flow: 100 kN
• Cross-sectional area A of the samples: π / 4 * d ² = 50.3 mm ² Compressive strength σ = F / A = 100 * 10 ³ / 50.3 MPa = 1988 MPa .

Claims (11)

1. Hot-working steel, with the following composition (in % w/w):

   C: 0.38 - 0.45%,

   Si: up to 0.8%,

   Mn: up to 0.5%,

   Cr: 4.0 - 6.0%,

   Co: 0.3 - 0.8%,

   Ni: 0.8 - 2%,

   Mo: 2.3 - 2.8%,

   Ta: 0.1 - 1.0%,

   Al: up to 0.025%,

   Ti: up to 0.03%,

   V: 0.15 - 0.3%,

   N: 0.1 - 0.5%,

   Remainder Fe and impurities unavoidable due to manufacture.
2. Hot-working steel according to Claim 1, characterised in that its C content is at most 0.43% w/w.
3. Hot-working steel according to any one of the above claims, characterised in that its Cr content is at least 4.5% w/w.
4. Hot-working steel according to any one of the above claims, characterised in that its Ni content is at most 1.1% w/w.
5. Hot-working steel according to any one of the above claims, characterised in that its Ta content is at least 0.4% w/w.
6. Hot-working steel according to any one of the above claims, characterised in that its Ta content is at most 0.8% w/w.
7. Hot-working steel according to any one of the above claims, characterised in that its Ti content is at most 0.005% w/w.
8. Hot-working steel according to any one of the above claims, characterised in that its N content is at least 0.28% w/w.
9. Hot-working steel according to any one of the above claims, characterised in that its N content is at most 0.4% w/w.
10. Hot-working steel according to any one of the above claims, characterised in that it is produced by means of pressure electroslag remelting.
11. Hot-working steel according to any one of the above claims, characterised in that its compressive strength σ at 600°C is at least 1600 MPa.

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