ES2797953T3 - Stainless steel product, use of the product and method of its manufacture - Google Patents

Abstract

A duplex stainless steel casting with high machinability, characterized in that the microstructure of the casting contains 30-70 volume percent ferrite and 30-70 volume percent austenite and the casting consists of a weight percent : up to 0.07% carbon, up to 2% silicon, 4-6% manganese, 19-23% chromium, 0.5-1.7% nickel, optionally up to 1% molybdenum and / or less 1% tungsten with the formula (Mo + ½ W), optionally up to 1% copper and 0.20-0.26% nitrogen, the rest is iron and incidental impurities.

Description

DESCRIPTION

Stainless steel product, use of the product and method of its manufacture

The invention relates to a casting produced from stainless steel, which has a ferrite-austenite duplex microstructure and which has a high structural stability and an improved combination of properties, particularly machinability and weldability. The invention further relates to a use of the product and to a method of manufacturing castings.

Ferritic-austenitic or duplex stainless steel castings are generally defined as alloys with a mixture of nearly equal proportions of ferrite and austenite in contrast to austenitic castings that contain primarily 10-15% ferrite. For duplex castings in accordance with ASTM A890, ferrite levels are not specified, but the alloys listed will develop a range of approximately 30 to 60% ferrite with austenite balance. Interesting property profiles can be designed with the two-phase structure. The first duplex stainless steels were developed almost 80 years ago and most likely arose from austenitic castings where certain amounts of ferrite in the microstructure proved to be advantageous. In fact, duplex compositions generally show better meltability than austenitics. Other favorable properties of duplex materials are high mechanical strength, superior fatigue resistance, good wear resistance, and good corrosion resistance. Therefore, both cast and forged products have found many attractive applications. Various duplex alloy compositions have been described with various optimizations. In many cases, cast articles have also been included as articles in patents for duplex compositions. In recent years, with the considerable increase in raw material costs, special attention has been paid to reducing the levels of nickel and molybdenum in the alloys and still maintaining the appropriate properties.

The favorable properties of duplex stainless steels can be achieved for phase equilibria in the range of 30 to 70% ferrite and austenite. The interactions of the main alloying elements, particularly chromium, nitrogen, nickel, and molybdenum, are quite complex. To achieve a stable duplex structure that responds well to processing and manufacturing, care must be taken to obtain the correct level of each of these elements. In addition to phase equilibrium, the formation of harmful intermetallic phases at elevated temperatures is the second main concern with duplex stainless steels. The sigma and chi phases are formed in high-chromium, high-molybdenum stainless steels and preferentially precipitate on ferrite. The addition of nitrogen changes the phase equilibrium favorably to avoid the formation of such phases.

US Patent 4,500,351 relates to a cast duplex stainless steel, in which the microstructure in a casting comprises a ferritic matrix containing at least about 30% austenite after treatment with solution at 1200 ° C and cooling. rapid with rapid cooling of water to avoid the formation of the sigma phase. The casting contains by weight percent about 0.02 percent carbon, 24 percent chromium, about 9.5 percent nickel, about 6 percent molybdenum, about 0.5 percent manganese, about 0. 2 percent silicon and about 0.25 percent nitrogen. The castings of US Patent 4,500,351 are useful in pump parts such as impellers and housings and in valve parts such as seats and gates.

Publication WO 2008/000347 relates to a steel piston which is cast by the low pressure casting method as a single component of a steel alloy. One of those steels in the publication has a composition in% by weight of 0.01-0.03% carbon, 0.3-1% silicon, 3-9% manganese, 15-27% chromium, 1 -3% nickel, 0.2-1% copper and 0.05-0.17% nitrogen. By having the nitrogen content essentially low, there is a risk of formation of intermetallic phases. Furthermore, the range for chromium is very large and therefore the balance between the austenite phase and the ferrite phase is not clear.

Publication WO 2008/018242 describes a duplex stainless steel containing inter alia titanium (Ti) making possible the fnxTixN feature, also mentioned in claim 1 of the publication. The molten ingots are heat treated, hot rolled and heat treated again. In these tests, due at least to rolling, the material is no longer in cast condition. A duplex stainless steel having a good combination of properties in the cast condition and resistance to thermal transformation to martensite is described in US Patent 4,828,630. Steel contains in weight percent up to 0.07 percent carbon, 17 to 21.5 percent chromium, 1 to 4 percent nickel, 4 to 8 percent manganese, 0.05 to 0.15 percent. percent nitrogen, less than 2 percent silicon, less than 2 percent molybdenum, and less than 1.5 percent copper. The steel in this patent contains 30 to 60% ferrite and is particularly suitable for thin-walled castings for automotive underside components. Steel has casting properties including a minimum elongation of 10%, a 0.2% yield strength greater than 50 ksi (350 N / mm2), a toughness of at least 30 Nm (20 ft-lb) at 0 ° C and no nitrogen porosity.

Publication WO 03/038136 describes a duplex stainless steel with many embodiments having different compositions. Carbon, manganese, silicon, molybdenum, copper, and cobalt are even said to be optional components in these compositions (page 7, lines 2-4) and therefore the only components required for steel Document D4 are chromium, nickel, nitrogen, and in one embodiment, boron. Despite the optionality Whether or not, a duplex stainless steel with only chromium, nickel, nitrogen and possibly boron is not stable and therefore the steel needs at least some of those optionally named components.

US Patent 6,033,497 relates to a pitting resistant duplex steel alloy with improved machinability that contains in addition to iron in less than 0.1 weight percent carbon, 25-27 percent carbon. chromium, 5-7.5 percent nickel, less than 0.5 percent molybdenum, less than 0.15 nitrogen, less than 1.5 percent silicon, less than 2.0 percent manganese, 1 , 5-3.5 percent copper. In the prior art of this US patent it is said that the machinability of austenitic stainless steels can be improved by adding alloying elements such as sulfur and selenium which can reduce the corrosion performance. In addition, the addition of moly-free copper is said to allow the duplex stainless steel alloy to cool very slowly in a hermetically sealed heat treating furnace, so that residual damaging residual stresses are minimized while maintaining excellent ductility and strength. corrosion.

According to US Patent 6,033,497, the grade of the steel is treated by an accelerated in-mold heat treatment after casting without using a separate slow heat treatment step. The steel grade of the patent is particularly for a hollow cylindrical centrifugal casting and is used, for example, for paper machine suction roll casing applications. In-mold heat treatment comprises controlling the cooling rate of the mold in the temperature range from about 260 ° C to about 1090 ° C and keeping the temperature of the alloy in the mold within about 450 ° C of the temperature outside the mold. . The grade of the steel has improved machinability when treated in the mold after casting by accelerated heat treatment compared to the same alloy composition that is slowly cooled in a hermetically sealed heat treatment furnace. The alloy without the in-mold treatment has a nominal internal diameter tensile residual stress of 24 MPa, while the respective value for the alloy treated in the mold after casting is 52 MPa.

Patent EP 1,327,008 describes a ferritic-austenitic stainless steel having a microstructure, essentially containing 35-65% by volume of ferrite and 35-65% by volume of austenite. The composition of this steel grade contains as main alloy components in weight percentage 0.02-0.07 percent carbon, 19-23 percent chromium, 1.1-1.7 percent nickel, 0 , 15-0.30 percent nitrogen, 3-8 percent manganese, optionally molybdenum and / or copper less than 1 percent. The steel of this EP patent is produced by Outokumpu under the trademark LDX 2101® and the forged products have been received with great commercial interest.

Duplex stainless steel castings generally show good meltability. However, there is a risk of nitrogen gas pore formation during solidification due to the limited solubility of nitrogen in the ferrite phase that normally solidifies from a steel melt with a composition of a duplex stainless steel alloy. . In general, it can be stated that most stainless steel castings are subjected to various machining operations to be installed in the system in which the castings will be used. In this regard, duplex stainless steels are considered more difficult to machine than, for example, austenitic stainless steels. The higher resistance levels of the old type of steel explain this behavior. Carbon and nitrogen additions increase the strength and degree of strain hardening of the steel and therefore must be kept low to achieve good machinability. However, modern duplex stainless steels are alloyed with high nitrogen content for good weldability and the best weldability properties at the sacrifice of machinability.

One application where cast or forged duplex stainless steels are used is a steel casing for a paper machine suction roll. An important material property for this application is machinability as well, because cast or forged steel housings are subjected to substantial machining to produce the final suction roll. As stated in connection with US Patent 6,033,497, one way to improve machinability is to add sulfur or selenium, the elements of which, however, reduce corrosion performance.

publication WO 2006/041344 describes a steel casing for a paper machine suction roll, in which the forged steel grade LDX 2101® from EP 1,327,008 is used without any addition of sulfur. Also, no machinability enhancing treatment is performed, as the optional copper and moly additions are notably smaller compared to US Patent 6,033,497.

Schramm et al published in the "Lean Duplex Stainless Steels for Pump Applications" presentation at the World Stainless Steel Conference 2007, Maastricht, 6-8 November 2007, results of studies on lean duplex materials for specific pump applications. A "cast 2101" alloy was made from cast bars having a weight percent composition of 0.028 percent carbon, 0.97 percent silicon, 5.04 percent manganese, 0.011 percent phosphorus, 0.004 percent. sulfur percent, chromium 20.73 percent, molybdenum 0.31 percent, nickel 1.73 percent, nitrogen 0.20 percent and copper 0.30 percent. As results for this "cast 2101" alloy after annealing the solution at a temperature of 1050 ° C and rapid cooling with water, Schramm et al mention, for example, the values of 473 MPa for a test resistance of 0.2% and an A5 elongation of 37.3%. Regarding corrosion properties, Schramm et al say that the "cast2101" alloy has a lower pitting potential than the 2304 alloy which has a composition in weight percent of 0.024 percent carbon, 0.64 percent silicon, 1.32 percent manganese, 0.015 percent phosphorus, 0.001 percent sulfur, 22.50 percent chromium, 0, 28 percent molybdenum, 4.92 percent nickel, 0.09 percent nitrogen, and 0.26 percent copper. However, Schramm et al do not mention any information on the applicability of this "cast 2101" alloy for the desired applications.

The object of the present invention is to eliminate some drawbacks of the prior art and achieve a casting of a duplex stainless steel, which in the method of manufacturing a cast iron is sufficiently stable against the formation of harmful precipitates, such as the intermetallic phase and which has as properties, a combination of high strength and good corrosion resistance, good meltability and high machinability. The essential features of the invention are listed in the appended claims.

The present invention relates to a high machinability duplex stainless steel casting comprising, in weight percent, up to 0.07 percent carbon, up to 2 percent silicon, 4 to 6 percent manganese, 19 to 23 percent chromium, 0.5 to 1.7 percent nickel, and 0.20 to 0.26 percent nitrogen. The alloys to be used in the production of duplex stainless steel castings with the aforementioned range may contain small amounts of other elements or impurities and, optionally, elements such as up to 1 percent copper, up to 1 percent molybdenum, and / or less than 1 percent tungsten according to the formula (Mo 1/2 W), the rest is iron and incidental impurities. The microstructure of duplex stainless steel castings of the invention contains 30-70 volume percent ferrite and 30-70 volume percent austenite. The invention also relates to a casting method for producing the castings, as well as to the use of the castings.

In the manufacture of large stainless steel castings, it is important to have a microstructure that is sufficiently stable against the formation of harmful precipitates, such as the intermetallic phase, since such phases have adverse effects on properties. For this, a balanced and thin duplex composition of the casting of the invention is desirable. Preferably, the microstructure of the duplex stainless steel of the invention contains 50 volume percent ferrite and 50 volume percent austenite.

Another important property for steel castings is ease of repair welds. In addition to its good meltability, the casting of the invention is generally quite resistant to hot cracking during welding. If repair of the weld is required, post weld heat treatment is required in most cases as the weld metal and heat affected zone will be easily exposed to rapid cooling due to a small weld pool surrounded by a large cast iron section. This can result in a high ferrite content microstructure that is sensitive to cracking and reduced properties for which heat treatment must follow. For this reason, it is desirable that the duplex stainless steel composition of the invention have high austenite reforming during rapid thermal cycling such as in welding. To obtain such a characteristic, a high nitrogen content is advisable in the duplex stainless steel casting of the invention.

The duplex stainless steel casting of the invention consists of a weight percent, preferably up to 0.05 percent carbon and more preferably up to 0.03 percent carbon, preferably up to 1 percent silicon, preferably more than 4 up to 6 percent manganese, preferably greater than 21 to 22 percent chromium, preferably greater than 1.1 to 1.7 percent nickel, and more preferably greater than 1.35 to 1.7 percent nickel, and preferably greater than 0.20 and up to 0.26 percent nitrogen, and optionally elements up to 1 percent copper, up to fully 1 percent molybdenum and / or less than 1 percent tungsten according to the formula (Mo / W), the rest is iron and incidental impurities.

The invention is described in more detail below with reference to the drawings in which

Figure 1 shows the results of the test when comparing the machinability of the casting of the invention with the austenite stainless steel of the prior art.

Figure 2 shows the microstructure of a simulated weld repair in a foundry of the invention.

The duplex stainless steel casting of the present invention was tested for machinability and welding, especially in weld repair.

To test machinability, a casting was produced that has the following chemical composition in weight percent in Table 1:

Table 1

A cast billet with a square section of 140 mm was subjected to different tests in the cast condition without any prior heat treatment. The mechanical properties of the cast iron were as follows in Table 2:

Table 2

The strength level is well above that of austenitic castings, which typically have a yield strength of approximately 200 MPa and a final strength of approximately 500 MPa. The machinability test was performed with turning of cylindrical test pieces and the results are shown in Figure 1. The figure illustrates the allowable cutting speed for a tool life of 15 minutes in turning. The insert of the tool was of the cemented carbide type. The machinability of the casting according to the invention is higher than that of an austenitic steel of type 304L. This is in contradiction to the expected result when austenitic steel is considered to have better machinability.

Additional tests were performed with a casting in accordance with the present invention that was produced with the following chemical composition, in percent by weight in Table 3:

Table 3

From a 140 mm thick casting section, 30 mm thick square samples were drawn and the samples were subjected to simulated repair welding using shielded metal arc welding. The base metal was in cast condition. Grooves were made in the sample and then filled by welding using a filler material suitable for this alloy. The arc energy was 0.7 to 0.8 kJ / mm. The resulting welds were free of cracks and exhibited normal microstructure, also in the heat affected zone. This is illustrated in Figure 2.

The castings according to the present invention can be cast by different casting processes, such as centrifugal casting, cold casting, high pressure casting, investment casting, die casting, permanent mold casting, sand casting and casting. to vacuum. The meltability is good and does not show a tendency to crack or pore despite the high nitrogen content. This is because the high level range of 4-6% manganese can be used. The molten articles are preferably collected in solution at a temperature of 1020 to 1100 ° C followed by rapid cooling. However, the thinner sections can be used in casting conditions. Although microstructure is not a property and can be difficult to measure correctly, the present invention will contain approximately equal amounts of austenite and ferrite, with the allowed phase range being 30 to 70%. In addition, the microstructure is highly resistant to precipitation of intermetallic phases, which in turn provides low sensitivity to brittleness. The castings of the present invention exhibit superior machinability under both cast and solution annealed conditions.

Therefore, the duplex castings of the present invention offer desirable and economical cost alternatives to austenitic cast materials due to their high machinability, high strength, and good weldability. The castings of the present invention may be especially favorable for use in various solutions and parts for pumps, valves, impellers or for use in other solutions where a combination of high machinability, high strength and good weldability is needed in one casting as a casting condition or after some additional treatment, such as solution annealing and quenching.

Claims (7)

1. A duplex stainless steel casting with high machinability, characterized in that the microstructure of the casting contains 30-70 volume percent ferrite and 30-70 volume percent austenite and the casting consists of a percentage by weight: up to 0.07% carbon, up to 2% silicon, 4-6% manganese, 19-23% chromium, 0.5-1.7% nickel, optionally up to 1% molybdenum and / or less than 1% tungsten with the formula (Mo 1V W), optionally up to 1% copper and 0.20-0.26% nitrogen, the rest is iron and incidental impurities. 2. The duplex stainless steel casting of claim 1, characterized in that it contains up to 0.05% carbon. 3. The duplex stainless steel casting of claim 2, characterized in that it contains up to 0.03% carbon. 4. The duplex stainless steel casting of any of the preceding claims, characterized in that it contains 21-22% chromium. 5. The duplex stainless steel casting of any of the preceding claims, characterized in that it contains 1.1-1.7% nickel. The use of the high machinability duplex stainless steel casting of any of claims 1-5 in pumps, valves or impellers. 7. Method for producing the high machinability duplex stainless steel casting of any of claims 1-5, characterized in that the casting is produced by centrifugal casting, cold casting, high pressure casting, investment casting, casting die casting, permanent mold casting, sand casting or vacuum casting.