TWI490345B - Stainless steel product, use of the product and method of its manufacture - Google Patents

Description

Stainless steel product, use thereof and manufacturing method

The present invention relates to a casting made of stainless steel having a duplex ferrite-austenite (Fermented Iron-Worth Iron) microstructure and having high structural stability and improved combination of properties, particularly mechanical A combination of processability and weldability. The invention further relates to the use of the article and to a method of making the casting.

A ferrite-austenitic or duplex stainless steel casting is generally defined as an alloy having a nearly equal proportion of a mixture of ferrite and austenite, compared to an austenite containing at least 10-15% ferrite. Body castings. For dual phase castings according to the ASTM A890 standard, the ferrite content is not specified, but the listed alloys will develop a range of about 30% to 60% ferrite with the remainder being austenite. With a two-phase structure, interesting properties can be designed. The development of the first duplex stainless steel occurred almost 80 years ago, most likely by austenitic castings, and the microstructure of the austenitic castings proved to be preferred by the inclusion of some amount of ferrite. In fact, the two-phase composition generally exhibits superior casting properties than the austenitic composition. Other excellent properties of the two-phase material are high mechanical strength, excellent fatigue strength, good wear resistance and good corrosion resistance. It has been found that both cast and forged products have a variety of attractive uses. Several duplex alloy compositions have been described in a number of best cases. In many cases, castings are also included in the subject matter of the two-phase composition patent. In recent years, with the original materials This increase is particularly focused on reducing the nickel and molybdenum content of the alloy while still maintaining the proper properties.

The phase balance of the 30% to 70% ferrite and austenite ranges can achieve the advantageous properties of duplex stainless steel. The interaction of the main alloying elements, especially chromium, nitrogen, nickel, and molybdenum, is quite complex. In order to achieve a stable two-phase structure that responds well to processing and manufacturing, care must be taken to correct the content of each element. In addition to phase equilibration, the second major problem with duplex stainless steel is the formation of a harmful intermediate phase at elevated temperatures. The σ phase and the χ phase are formed in high chromium, high molybdenum stainless steel, and precipitate preferentially in ferrite. The addition of nitrogen advantageously changes the phase equilibrium to avoid the formation of such phases.

U.S. Patent No. 4,500,351 is related to a cast duplex stainless steel in which the microstructure in the casting comprises a ferrite matrix, treated at 1200 ° C and rapidly cooled using water cooling to avoid at least about 30% after formation of the sigma phase. Austenite. The castings contained 0.02% carbon, 24% chromium, about 9.5% nickel, about 6% molybdenum, about 0.5% manganese, about 0.2% rhodium, and about 0.25% nitrogen, by weight percent. The casting of U.S. Patent No. 4,500,351 can be used for pump components such as vanes and cabinets as well as for valve components such as valve seats and gate valves.

A duplex stainless steel having a good combination of properties in an as-cast condition and resistant to heat conversion to martensite (Mita Iron) is described in U.S. Patent 4,828,630. The stainless steel contains up to 0.07% carbon, 17% to 21.5% chromium, 1% to 4% nickel, 4% to 8% manganese, 0.05% to 0.15% nitrogen, less than 2% bismuth, less than 2% molybdenum and Less than 1.5% copper. The special The stainless steel of the case contains 30% to 60% ferrite, which is particularly suitable for thin-wall castings for automotive underbody components. Stainless steel has cast properties including 10% minimum elongation, 0.2% relief strength greater than 50 ksi (350 Newtons per square millimeter), at least 20 inch-pounds (30 Newton meters) of toughness and nitrogen-free porosity at 0 °C.

U.S. Patent 6,033,497 relates to a pitting resistant duplex stainless steel having improved machinability, containing less than 0.1% carbon, 25-27% chromium, 5-7.5% nickel, and less than iron, in addition to iron. 0.5% molybdenum, less than 0.15% nitrogen, less than 1.5% bismuth, less than 2.0% manganese, 1.5-3.5% copper. The prior art of this U.S. patent states that the machinability of austenitic stainless steel can be achieved by the addition of alloying elements such as sulfur and selenium which reduce corrosion efficiency. Further, it is said that the addition of copper without molybdenum allows the duplex stainless steel alloy to be cooled very slowly in a controlled manner in a closed heat treatment oven, thereby reducing harmful tensile residual stress while maintaining excellent ductility and corrosion resistance.

According to U.S. Patent 6,033,497, an independent and slow heat treatment step is not used, and the steel grade after casting is treated by accelerated in-mold heat treatment. The steel grade of this patent is particularly useful for hollow cylindrical centrifugal castings, for example for paper machine suction roll shell applications. The in-mold heat treatment comprises controlling the casting cooling rate to a temperature range of from about 260 ° C to about 1090 ° C and maintaining the alloy temperature in the mold within about 450 ° C of the outside temperature of the mold. When the accelerated heat treatment is carried out in the mold after casting, the same alloy composition is slowly controlled in the closed heat treatment oven, and the former steel grade has improved machinability. The alloy that has not been treated in the mold has a standard of 24 MPa. The inner diameter tensile residual stress is referred to, and the value of the alloy treated in the mold after casting is 52 MPa.

EP Patent 1,327,008 describes a ferrite-austenitic stainless steel having a microstructure mainly comprising 35-65 vol% ferrite and 35-65 vol% austenite. The steel grade composition contains, by weight percent, 0.02-0.07% carbon, 19-23% chromium, 1.1-1.7% nickel, 0.15-0.30% nitrogen, 3-8% manganese, optionally less than 1% molybdenum. And / or copper, as the main alloying component. The steel of this EP patent is manufactured by Outokumpu under the registered trademark LDX 2101®, and forged products have gained high commercial interest.

In general, duplex stainless steel castings exhibit good casting properties. However, since the solubility of nitrogen in the ferrite phase which is usually solidified in a steel melt composed of a duplex stainless steel alloy is limited, there is a risk of forming nitrogen pores during solidification. It is generally said that most stainless steel castings are subjected to a variety of machining operations to fit into the system in which the casting will be used. In this regard, duplex stainless steel is considered to be more difficult to machine, for example, austenitic stainless steel. The higher strength of the aforementioned categories of steel may indicate this performance. The addition of both carbon and nitrogen increases the strength and the degree of strain hardening of the steel, so both carbon and nitrogen must be maintained at low levels to achieve good machinability. However, modern duplex stainless steels are alloyed with high nitrogen content for good weldability and sacrificial machinability for optimum weldability.

One application for casting or forging duplex stainless steel is for the steel casing of a paper machine suction roll. One of the important material properties of this application is also machinability. The final suction roll is manufactured because the cast or forged steel casing is subjected to substantial machining. One way to improve machinability is to add sulfur or selenium as described in U.S. Patent No. 6,033,497, but these elements reduce corrosion performance.

WO Publication No. 2006/041344 describes a steel casing for a paper machine suction roll in which the forged steel grade LDX 2101® of EP Patent 1,327,008 is used without any addition of sulfur. Further, no treatment for improved machinability has been performed, and compared to U.S. Patent No. 6,033,497, copper and molybdenum are added as needed significantly less.

Schramm et al. at the 2007 Stainless Steel World Conference, Maastricht, November 6-8, 2007, "Diffuse Duplex Stainless Steel for Pump Applications", reveals the results of a study on poor duplex materials for special pump applications. An alloy "cast 2101" is made of a cast rod having a composition of 0.028% carbon, 0.97% ruthenium, 5.04% manganese, 0.011% phosphorus, 0.004% sulfur, 20.73% chromium, 0.31 by weight percent. % molybdenum, 1.73% nickel, 0.20% nitrogen and 0.30% copper. As a result of annealing and water quenching of the alloy "cast 2101" at a temperature of 1050 ° C, Schramm et al. describe, for example, a 0.2% guaranteed strength of 473 MPa and an A5 elongation of 37.3%. As for corrosion resistance, Schramm et al. state that alloy "cast 2101" has a pitting tendency lower than alloy 2304, and alloy 2304 has a composition of 0.024% carbon, 0.64% bismuth, 1.32% manganese, 0.015% phosphorus, 0.001% by weight. Sulfur, 22.50% chromium, 0.28% molybdenum, 4.92% nickel, 0.09% nitrogen and 0.26% copper. However, Schramm et al. did not mention the alloy "cast 2101" for the intended application.

The object of the present invention is to eliminate the disadvantages of the prior art and to achieve the casting of duplex stainless steel. In the manufacturing method, the casting member is sufficiently stable for the formation of harmful precipitates such as intermetallic phases, has high strength and good corrosion resistance, and is well cast. A combination of properties and good machinability. The main features of the present invention are set forth in the accompanying patent application.

The present invention relates to a stainless steel article, preferably for a duplex stainless steel casting having high mechanical workability, containing up to 0.07% carbon, up to 2% bismuth, more than 3% to 8% manganese, and more than 19% by weight. % to 23% chromium, greater than 0.5% to 1.7% nickel and greater than 0.15% to 0.30% nitrogen. Alloys for the manufacture of duplex stainless steel castings having the aforementioned ranges contain small amounts of other elements or impurities and optionally used elements, such as up to 1% copper, less than 1% according to the formula (Mo+1/2W) Up to a total of 1% molybdenum and / or tungsten, the balance is iron and incidental impurities. The microstructure of the duplex stainless steel casting of the present invention contains 30-70 vol% ferrite and 30-70 vol% austenite. The invention also relates to a casting method for making a casting and the use of the casting.

In the manufacture of large stainless steel castings, it is necessary to have sufficient stability against the formation of harmful deposits such as intermetallic phases, because of the adverse effects of the properties of the metal relative to the casting. For this reason, it is desirable that the casting of the present invention has a lean balanced two-phase composition. Preferably, the duplex stainless steel microstructure of the present invention contains 50 vol% ferrite and 50 vol% austenite.

Another important property of steel castings is the ease of repair welding. Except In addition to good castability, the cast parts of the present invention are generally quite resistant to thermal cracking during welding. If the fusion is to be repaired, most of the welding needs to be followed by heat treatment because the welded metal and heat-affected sections are easily exposed to rapid cooling because the small fusion pool is surrounded by large casting sections. This may result in a microstructure having a high ferrite content which is sensitive to cracking and degradation of properties, which is why subsequent heat treatment is required. For this reason, it is desirable that the duplex stainless steel composition of the present invention has high austenite recombination during rapid heating cycles such as welding. In order to obtain such characteristics, the duplex stainless steel casting of the present invention is proposed to have a high nitrogen content.

The duplex stainless steel casting of the present invention preferably comprises, by weight, preferably up to 0.05% carbon and more preferably up to 0.03% carbon, preferably up to 1% hydrazine, preferably more than 4% to 6% manganese, preferably more than 21% by weight. % to 22% chromium, preferably greater than 1.1% to 1.7% nickel and more preferably greater than 1.35% to 1.7% nickel and preferably greater than 0.20% to 0.26% nitrogen, and optionally up to 1% copper, according to formula (Mo+1/2W) less than 1% up to a total of 1% molybdenum and/or tungsten, the balance being iron and incidental impurities.

The duplex stainless steel casting of the present invention is tested for machinability and weldability, particularly for weld repair.

In order to test the processability of the machine, a casting having the following chemical composition was produced, and Table 1 shows the chemical composition in weight percent:

Cast slabs with a 140 mm square section were subjected to different tests in the as-cast condition without any prior heat treatment. The properties of the castings are shown in Table 2 below:

The strength level is much higher than the strength of the austenitic casting, which typically has a relief strength of about 200 MPa and a final tensile strength of about 500 MPa. The machinability test was carried out using a rotary test of a cylindrical test piece, and the results are shown in Fig. 1. This figure illustrates the allowable cutting speed for tool life for 15 minutes of turning. Tool inserts are classified as cemented carbides. The machinability of the casting according to the present invention is superior to that of the 304L austenitic steel. This point is inconsistent with the desired result, and austenitic steel is considered to have better machinability.

Further testing was carried out using the casting according to the present invention, and the casting of the present invention was produced in the following chemical composition and is shown in Table 3 in weight percent:

From the profile of the 140 mm thick casting, a square specimen with a thickness of 30 mm was taken out, and the specimen was subjected to simulated repair welding using shielded metal arc welding. Basic gold The genus is in the cast state. A trench is formed in the sample and subsequently welded using a fill material suitable for use in the alloy. The arc energy is 0.7 to 0.8 kJ/mm. The resulting welds are free of cracks and exhibit a normal microstructure, as well as in the heat affected zone. An example is illustrated in Figure 2.

Castings according to the present invention can be cast by various casting methods such as centrifugal casting, cold drawn casting, compression molding, overmolding, pressure casting, permanent casting, sand casting, and vacuum casting. Good castability, showing no tendency to crack or form voids despite high nitrogen content. The reason is that the manganese content of steel is high, up to 3-8% and it is preferred to use manganese in the range of 4-6%. The cast article is preferably annealed at a temperature of 1020 ° C to 1100 ° C, followed by rapid cooling. However, a thin profile can be used in the as-cast condition. Although the microstructure is not a property and is difficult to measure correctly, the present invention contains roughly equal amounts of austenite and ferrite, and the allowable phase ranges from 30% to 70%. In addition, the microstructure is extremely resistant to precipitation of the intermetallic phase, which in turn provides low sensitivity to embrittlement. The cast piece of the present invention exhibits excellent machinability in a cast state and under solution annealing conditions.

The duplex castor of the present invention provides a desirable and cost-effective alternative to austenitic cast materials due to its high machinability, high strength and good weldability. The casting of the present invention can be used particularly advantageously for a variety of solutions and components of pumps, valves, vanes, or for other castings as needed in the as-cast state or in several further processes such as solution annealing and cold shock conditions. A combination of high machinability, high strength and good weldability.

Fig. 1 shows the results of testing the machinability of the casting of the present invention with prior art austenitic stainless steel, and Fig. 2 shows the microstructure of the simulated fusion repair of the casting of the present invention.

Claims (16)

A stainless steel product, in particular, a duplex stainless steel casting having high machinability in a cast state, the microstructure of the casting comprising 30-70 vol% ferrite and 30-70 vol% austenite, the casting is weighted The percentage meter contains: up to 0.07% carbon, up to 2% bismuth, 4-6% manganese, 19-23% chromium, 0.5-1.7% nickel, in accordance with the formula (Mo+1/2W) less than 1% up to 1% molybdenum And/or tungsten, up to 1% copper and 0.20-0.26% nitrogen, the balance being iron and incidental impurities. A stainless steel article as claimed in claim 1, wherein the article contains up to 0.05% carbon. A stainless steel article as claimed in claim 2, wherein the article contains up to 0.03% carbon. A stainless steel article according to any one of claims 1 to 3, wherein the article contains 21-22% chromium. A stainless steel article according to any one of claims 1 to 3, wherein the article contains 1.1 to 1.7% nickel. A use of a stainless steel article according to any one of claims 1 to 5, which is for use in a pump. A use of a stainless steel article according to any one of claims 1 to 5 for a valve. A use of a stainless steel article according to any one of claims 1 to 5, which is for use in a bucket. A stainless steel used in the manufacture of any one of claims 1 to 5 A method of steel product, wherein the product is manufactured by centrifugal casting. A method for producing a stainless steel article according to any one of claims 1 to 5, wherein the article is produced by cold-stretch casting. A method for producing a stainless steel article according to any one of claims 1 to 5, wherein the article is manufactured by die casting. A method for producing a stainless steel article according to any one of claims 1 to 5, wherein the article is manufactured by overmolding. A method for producing a stainless steel article according to any one of claims 1 to 5, wherein the article is manufactured by pressure casting. A method for producing a stainless steel article according to any one of claims 1 to 5, wherein the article is manufactured by permanent casting. A method for producing a stainless steel article according to any one of claims 1 to 5, wherein the article is manufactured by sand casting. A method for producing a stainless steel article according to any one of claims 1 to 5, wherein the article is manufactured by vacuum casting.