EP3458623B1 - Method for producing a steel material, and steel material - Google Patents

Description

Steels are known for the production of pumps and the like that are subject to severe corrosive loads, from which the corresponding blocks for the pumps are made, from which the pumps and pump parts are then frequently produced by cutting.

The steels for this are standardized in particular and mainly the steels DIN 1.4542, DIN 1.4418, but also DIN 1.4313 are used for such aggregates.

On the one hand, these steels are melted conventionally due to the very low price level, but on the other hand due to the very high demand on the world market.

Materials that are produced with the appropriate remelting process (ESR or VLBO) cannot be used across the board for reasons of the low price level and global demand.

In order to manufacture pump blocks, very large block formats are required, so that the casting weights are often greater than 10 t. This means that a suitable material must be designed in such a way that the product properties that are as uniform as possible can be achieved even when using conventional block formats and conventional melting due to a low tendency to segregate. Segregations are fundamentally undesirable here, since segregations can be the starting point for mechanical inhomogeneities and possibly cracks. In addition, there may be deviating corrosion resistance properties in the area of segregation.

The DIN 1.4418 steel has a high yield strength (Rp _0.2 %) of around 1000 MPa, whereby the DIN 1.4418 steel can achieve a very high low-temperature toughness, which is typically between 50 and 150J (Charpy V-Notch) impact work at -40°C amounts to. This high level of toughness is required because of the cavitation that occurs in pumps.

The material DIN 1.4542 cannot even come close to achieving this level of toughness with the same yield point and usually only has single-digit notched bar impact work values at - 40°C.

Steel DIN 1.4313 is also used for pump blocks, but due to its lower alloy content compared to DIN 1.4418, when tempered to its maximum strength level, it can only achieve yield strengths of between 900 and 1000 MPa. When using this material in the highest strength level, however, only a low level of toughness can be achieved at low temperatures, with the corrosion resistance due to the alloy being significantly lower compared to the other two steels. The materials DIN 1.4313 and DIN 1.4418 are nickel-martensitic secondary hardening alloys, while the material DIN 1.4542 is a nickel-martensitic copper-hardening material.

From the EP 0649 915 A1 a martensitic steel material is known which contains no more than 10% delta ferrite and fine copper precipitations in the matrix. The steel material is characterized by good anti-stress corrosion properties and high hardness.

The object of the invention is to create a material that has improved strength even with very high casting weights at a very high level of toughness, with corrosion resistance also being increased.

The object is achieved with a method for producing a steel material with the features of claim 2.

A further object is to create a steel material which has correspondingly similar or higher strengths than known steels, but has a higher level of toughness and better corrosion resistance.

The object is achieved with a steel material having the features of claim 1.

The inventors have set themselves the goal of developing a material that has the same or higher strength than DIN 1.4418 or DIN 1.4542, which in themselves already have very high strength, but also the very high level of toughness of DIN 1.4418 equals or exceeds it, but surpasses the corrosion resistance of the significantly less strong DIN 1.4313.

However, the additional goal is that these product properties are achieved with conventional melting, but the analysis is designed in such a way that a high-purity remelting variant (ESR or VLBO) can also be achieved. Due to its significantly lower content of oxidic inclusions of smaller size, such a high-purity remelting variant has special advantages with regard to the fatigue properties for special applications in machine or apparatus construction with high dynamic loads, such as in compressors or centrifuges is the case. By remelting in a vacuum arc furnace (VLBO), which is the usual remelting technology for highly stressed components in aviation applications, the fatigue strength of the material according to the invention can be increased by reducing the defect sizes in the material. This effect is of great importance above all when using the material according to the invention in high strength for aerospace applications.

In order to produce such material properties according to the invention, the nickel-martensitic secondary hardening method on the one hand and the nickel-martensitic copper-hardening method on the other hand must be abandoned and a new path taken.

According to the invention, copper is used for hardening in the new steel material. The inventors have recognized that delta ferrite as a structural component reduces toughness, this phase being minimized as much as possible by an optimal ratio of austenite to ferrite stabilizing elements and, due to production, everything is done to reduce the delta ferrite phase using suitable casting technology and deformation to be kept low at an optimized temperature.

A niobium stabilization, as used for example in DIN 1.4542, is completely avoided, so that according to the invention no coarse primary carbides are formed.

The inventors have recognized that material concepts such as DIN 1.4542 come from a time when plant technology in smelting metallurgy did not yet make it possible to reliably reduce the carbon content of melts with a high chromium content.

For this reason, the path has often been taken to bind the carbon, which is harmful to corrosion resistance, with strong carbide formers such as titanium or niobium through the formation of monocarbides and to prevent the formation of chromium carbides. This alloying technique was used for both austenitic materials and martensitic materials such as DIN 1.4542, and is still prescribed for this material in the international standards today.

The conscious step of forgoing stabilization in this alloy system is one of the essential measures according to the invention, which makes it possible to realize a material with the property profile according to the invention and with the manufacturing options mentioned.

The invention thus relates to a steel material for the production of pumps or the like, the steel material having the analysis according to claim 1, the structure of the steel material consists of martensite with a maximum of 1% delta ferrite, the structure being free of primary hard phases, in particular based on niobium, tantalum, titanium or vanadium, and the tempering austenite content being a maximum of 8%.

It is intended that the material will reach a yield strength of approx. 1000 MPa at a hardening temperature of 520°C with a toughness of over 70 J at -40°C and a yield strength of approx. 1100 MPa at a hardening temperature of 485°C a toughness at -40°C of more than 60 J, the values of the mechanical properties being based on measurements in the transverse direction.

In addition, the invention relates to a method for producing a steel material for pumps and the like, wherein a steel material is melted according to the analysis according to claim 2, wherein the steel material is melted conventionally or in electroslag remelting or in vacuum arc processes and is formed at 800°C to 1250°C, with a heat treatment following with a solution annealing at 850°C to 1050°C, followed by hardening, cooling and curing at 450°C to 520°C depending on the required mechanical properties.

The invention is explained by way of example with reference to a drawing.

Tabelle 1

die chemische Analyse der Normwerkstoffe, basierend auf EN 10088-3 im Vergleich zum erfindungsgemäßen Werkstoff (15-5MOD);

Tabelle 2

die mechanischen Eigenschaften des erfindungsgemäßen Werkstoffs in Querrichtung bei einer Aushärtung bei 520°C;

Tabelle 3

die mechanischen Eigenschaften des erfindungsgemäßen Werkstoffs in Querrichtung bei einer Aushärtung bei 485°C;

Tabelle 4

die mechanischen Eigenschaften eines nicht erfindungsgemäßen Normwerkstoffs in Querrichtung;

Tabelle 5

die mechanischen Eigenschaften eines weiteren Normwerkstoffs in Querrichtung;

Tabelle 6

die mechanischen Eigenschaften eines weiteren Normwerkstoffs in Querrichtung;

Tabelle 7

die mechanischen Eigenschaften des erfindungsgemäßen Werkstoffs in Querrichtung bei einer Aushärtung bei 450°C;

Tabelle 8

die Beständigkeit gegen abtragende Korrosion anhand von Zugversuchskennwerten der untersuchten Proben und der Massenverlust der Normwerkstoffe und des erfindungsgemäßen Werkstoffs im Vergleich.

They show:

Table 1

the chemical analysis of the standard materials, based on EN 10088-3 in comparison to the material according to the invention (15-5MOD);

Table 2

the mechanical properties of the material according to the invention in the transverse direction when hardened at 520° C.;

Table 3

the mechanical properties of the material according to the invention in the transverse direction when cured at 485° C.;

Table 4

the mechanical properties of a standard material not according to the invention in the transverse direction;

Table 5

the mechanical properties of another standard material in the transverse direction;

Table 6

the mechanical properties of another standard material in the transverse direction;

Table 7

the mechanical properties of the material according to the invention in the transverse direction when cured at 450° C.;

Table 8

the resistance to erosive corrosion based on tensile test parameters of the samples examined and the mass loss of the standard materials and the material according to the invention in comparison.

Table 1 shows a comparison of all the materials mentioned in comparison to the material according to the invention (15-5MOD). The material according to the invention was melted conventionally and several flat bars measuring 640×540 mm were produced by forging. After forging, the material is solution annealed at 950°C, hardened and then hardened.

The curing temperatures are 485°C in one case and 520°C in the other case.

After heat treatment, the bars are divided in the middle and completely mechanically tested in the transverse direction in the bottom, middle and crown zones.

The mechanical testing consists of a tensile test at room temperature and a notch impact test (Charpy V-Notch). Room temperature and an impact test (Charpy V-Notch) at -40°C.

The analysis according to Table 1 shows that in the target state of the steel material according to the invention, the manganese and phosphorus contents in particular are reduced, in particular also the sulfur content. The chromium content is between that of the materials DIN 1.4313 and DIN 1.4418, although the nitrogen content is particularly low and copper is also present.

The mechanical properties in the two hardening states are shown in Tables 2 and 3 and show that the strength differs by approx. 100 MPa and that a yield strength of approx. 1000 and 1100 MPa can be achieved with the specified heat treatments. The special feature of the material according to the invention, however, is an impressively high level of toughness even at low temperatures.

This excellent combination of properties is due to the finding according to the invention that delta ferrite can be largely avoided by appropriate analysis design. Furthermore, in the case of the invention, the maximum amount of niobium is severely restricted, so that niobium stabilization is ruled out and the niobium contents are so low that toughness-reducing hard phases are avoided.

For comparison, Table 4 and Table 5 list comparative data for the materials DIN 1.4313 and DIN 1.4418, which were also determined from forged bars in the same dimensional range.

The steel material according to the invention has the best combination of strength and toughness.

Table 6 shows the results of a smaller DIN 1.4542 forged rod with the dimensions 520 x 280, which only achieves a fraction of the toughness with the same strength.

As part of the development of the material 15-5MOD according to the invention, the maximum achievable strength potential with the defined analysis was also examined. It was shown that by lowering the hardening temperature to 450°C, a further increase in strength to a yield point of approx. 1177 - 1190 MPa can be achieved. In this high-strength state, the toughness determined by means of a notched bar impact test at -40°C is naturally reduced compared to hardening at 485°C, but the material with 20J to 78J (Table 7) still shows a notched bar impact work level that is many times higher than the material DIN 1.4542 with a yield point that is more than 100MPa higher, so that this WBH state can also be regarded as extremely relevant in practice, despite the lower low-temperature toughness.

Since the material has to have sufficient corrosion resistance in addition to high strength and the associated high toughness, additional corrosion tests were also carried out.

The loss of mass during erosive corrosion was determined in 20% acetic acid which was acidified to pH 1.6 with sulfuric acid. The test duration is 24 hours. The results (Table 8) show that the materials DIN 1.4418, DIN 1.4542 and the material according to the invention show hardly any wear and the corrosion resistance under these conditions can be classified as equivalent. As expected, the material DIN 1.4313 shows a significant loss of mass due to its lower alloy content. Here it becomes particularly clear that the material according to the invention is able to improve both the strength and the toughness again while maintaining the same corrosion resistance.

The method according to the invention provides for the material to be conventionally melted into large ingot formats of up to >10 t using an analysis corresponding to the first line of Table 1.

Then the material is formed in the range of 800 to 1250°C, followed by a heat treatment.

The heat treatment consists of solution annealing at 850 to 1050° C., subsequent hardening, subsequent cooling and hardening at 450 to 600° C., the temperature range of 450 to 520° C. being preferred when aiming for maximum strength.

The structure of the material according to the invention then consists of martensite with a maximum of 1% delta ferrite, being free of primary hard phases (primarily based on niobium, tantalum, titanium, vanadium), with the tempering austenite content being a maximum of 8%.

The material according to the invention is primarily used for corrosion-resistant pump blocks, but can also be used in general machine and apparatus construction.

According to the invention, the material can also be produced as a high-purity remelting quality according to the ESR or VLBO process if there are increased requirements for fatigue strength, especially for units that are dynamically heavily loaded or for safety-critical structural parts in the aerospace industry. The improvement in purity associated with the remelting results in the well-known improvements in fatigue properties by reducing the defect sizes in the material.

The advantage of the invention is that, on the one hand, a very precise analysis procedure and, on the other hand, a conversion of the analysis and the reduction of the delta ferrite and primary hard phases create a material that achieves very high strength, corrosion resistance and toughness in a way that previously could not be combined.

Claims (2)

1. Steel material for the manufacture of pumps or the like, characterised in that the steel material features the following analysis in wt%: $$\mathrm{C}\le 0.030;$$

   

   $$\mathrm{Si}\le 0.40;$$

   

   $$\mathrm{Mn}\le 0.60;$$

   

   $$\mathrm{P}\le 0.025;$$

   

   $$\mathrm{S}\le 0.005;$$

   

   $$\mathrm{Cr}=14.20-14.60;$$

   

   $$\mathrm{Mo}=0.30-0.45;$$

   

   $$\mathrm{Ni}=4.80-5.20;$$

   

   $$\mathrm{V}<0.10;$$

   

   $$\mathrm{W}<0.10;$$

   

   $$\mathrm{Cu}=3.00-3.70;$$

   

   $$\mathrm{Co}<0.15;$$

   

   $$\mathrm{Ti}<0.010;$$

   

   $$\mathrm{Al}<0.030;$$

   

   $$\mathrm{Nb}<0.02;$$

   

   $$\mathrm{Ta}<0.02;$$

   

   $$\mathrm{N}<0.02;$$

   

   remainder iron and unavoidable impurities, wherein the structure of the steel material consists of martensite with maximum 1% delta ferrite, wherein the structure is free of primary hard phases, in particular with a niobium, tantalum, titanium or vanadium base, and the temper austenite content is maximum 8%, wherein the steel material at a precipitation hardening temperature of 520°C attains a yield strength of approx. 1000 MPa with a toughness of more than 70 J at -40°C and at a precipitation hardening temperature of 485°C a yield strength of approx. 1100 MPa with a toughness of more than 60 J at -40°C, wherein the values of the mechanical properties are related to measurements in the transverse direction.
2. Method for manufacturing a steel material for pumps and the like according to claim 1, wherein a steel material corresponding to the following analysis in wt% is melted: $$\mathrm{C}\le \mathrm{0}\mathrm{.030;}$$

   

   $$\mathrm{Si}\le \mathrm{0}\mathrm{.40;}$$

   

   $$\mathrm{Mn}\le \mathrm{0}\mathrm{.60;}$$

   

   $$\mathrm{P}\le \mathrm{0}\mathrm{.025;}$$

   

   $$\mathrm{S}\le \mathrm{0}\mathrm{.005;}$$

   

   $$\mathrm{Cr}=14.20-14.60$$

   

   $$\mathrm{Mo}=0.30-0.45;$$

   

   $$\mathrm{Ni}=4.80-5.20;$$

   

   $$\mathrm{V}<0.10;$$

   

   $$\mathrm{W}<0.10;$$

   

   $$\mathrm{Cu}=3.00-3.70;$$

   

   $$\mathrm{Co}<0.15;$$

   

   $$\mathrm{Ti}<0.010;$$

   

   $$\mathrm{Al}<0.030;$$

   

   $$\mathrm{Nb}<0.02;$$

   

   $$\mathrm{Ta}<0.02;$$

   

   $$\mathrm{N}<0.02;$$

   

   remainder iron and impurities due to melting, characterised in that the steel material is melted conventionally or by the electroslag remelting process or the vacuum arc process and recast at 800°C to 1250°C, wherein heat treatment follows with solution annealing at 850°C to 1050°C, followed by hardening, cooling and precipitation hardening at 450°C to 520°C, depending on the required respective mechanical properties.

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