Method for producing a steel material, and steel material

Abstract

The invention relates to a method for producing a steel material, particularly a corrosion-resistant steel material for pumps and similar, in which a steel corresponding to the following analysis (in wt. %) is smelted: C<0.050; Si<0.70; Mn<1.00; P<0.030; S<0.010; Cr=14-15.50; Mo=0.30-0.60; Ni=4.50-5.50; V<0.20; W<0.20; Cu=2.50-4.00; Co<0.30; Ti<0.05; Al<0.05; Nb<0.05; Ta<0.05; N<0.05.

Claims

1. A method for producing a corrosion-resistant steel material for pumps, comprising: melting components of the steel material conventionally or using electro-slag remelting or vacuum arc remelting; forming the steel material at 800° C. to 1250° C.; heat treating the steel material by solution annealing at 850° C. to 1050° C.; cooling the steel material; tempering the steel material at 450° C. to 600° C., wherein the steel material is composed of martensite with at most 1 vol. % delta ferrite and at most 8 vol. % tempered austenite and is free of primary hard phases based on niobium, tantalum, titanium, or vanadium, wherein the steel material is further composed of (in wt %):
C<0.050;
Si<0.70;
Mn<1.00;
P<0.030;
S<0.010;
Cr=14-15.50;
Mo=0.30-0.60;
Ni=4.50-5.50;
V<0.20;
W<0.20;
Cu=2.50-4.00;
Co<0.30;
Ti<0.05;
Al<0.05;
Nb<0.05;
Ta<0.05;
N<0.05; and the remainder iron and melting-related impurities.

2. The method according to claim 1, wherein the steel material is composed of (in wt %):
C<0.030;
Si<0.40;
Mn<0.60;
P<0.025;
S<0.005;
Cr=14.20-14.60;
Mo=0.30-0.45;
Ni=4.80-5.20;
V<0.10;
W<0.10;
Cu=3.00-3.70;
Co<0.15;
Ti<0.010;
Al<0.030;
Nb<0.02;
Ta<0.02;
N<0.02; and the remainder iron and melting-related impurities.

3. The method according to claim 1, characterized in that at a tempering temperature of 485° C., the material achieves a yield strength of 1100 MPa with a toughness of over 60 J at −40° C.

4. The method according to claim 1, wherein the tempering the steel material occurs at from 450° C. to 520° C.

5. The method according to claim 1, wherein the tempering the steel material occurs at from 485° C. to 520° C.

Description

(1) In the drawings: Table 1 shows 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 shows the mechanical properties of the material according to the invention in the transverse direction with a tempering at 520° C.; Table 3 shows the mechanical properties of the material according to the invention in the transverse direction with a tempering at 485° C.; Table 4 shows the mechanical properties of a standard material that is not according to the invention in the transverse direction; Table 5 shows the mechanical properties of another standard material in the transverse direction; Table 6 shows the mechanical properties of another standard material in the transverse direction; Table 7 shows the mechanical properties of the material according to the invention in the transverse direction with a tempering at 450° C.; Table 8 shows the resistance to erosion corrosion based on tensile test parameters of the samples tested and a comparison of the mass loss of standard materials to that of the material according to the invention.

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

(3) The tempering temperatures were 485° in one case and 520° C. in the other case.

(4) After the heat treatment, the bars are cut in the middle and then undergo complete mechanical testing in the zones of the bottom, the middle, and the cropped region.

(5) The mechanical testing in this case is composed of a tensile test at room temperature, a notched bar impact test (Charpy V notch) at room temperature, and a notched bar impact test (Charpy V notch) at −40° C.

(6) The analysis according to Table 1 shows that in the desired state of the steel material according to the invention, in particular the manganese content and phosphorus content have been removed, in particular also including removal of the sulfur content. The chromium content is between that of the materials DIN 1.4313 and DIN 1.4418 and finally, the nitrogen content is particularly low and copper is also present.

(7) The mechanical properties in the two tempered states are shown in Tables 2 and 3 and demonstrate that the strength differs by approx. 100 MPa and with the specified heat treatments, yield strengths of approx. 1000 and 1100 MPa, respectively, can be achieved. The exceptional feature of the material according to the invention, however, is a strikingly high toughness level, even at low temperatures.

(8) This outstanding combination of properties is based on the insight according to the invention that by and large, delta ferrite can be avoided through an appropriate analysis configuration. In addition, with the invention, the maximum quantity of niobium is sharply limited so that a niobium stabilization has to be ruled out and the niobium content is so low that toughness-reducing hard phases are avoided.

(9) For the sake of comparison, comparison data of the materials D 1.4313 and D 1.4418 are shown in Table 4 and Table 5; these, too, have been determined based on forged bars in the same dimensional range.

(10) In this case, the steel material according to the invention has the best combination of strength and toughness.

(11) Table 6 shows the results of a smaller DIN 1.4542 forged bar with the dimensions 520×280, which achieves only a fraction of the toughness at the same strength.

(12) In the context of the development of the material according to the invention 15-5MOD, the maximum strength potential that could be achieved with the specified analysis was studied. It turned out that through a reduction of the tempering temperature to 450° C., a further strength increase to a yield strength of approx. 1177-1190 MPa can be achieved. In this extremely strong state, the toughness determined by means of the notched bar impact test at −40° C. is naturally reduced relative to a tempering at 485° C., although at 20 J to 78 J (Table 7), the material exhibits a notched bar impact work level that is still several times higher than that of the material DIN 1.4542 at a yield strength that is more than 100 MPa higher so that even this WBH state must be considered to be extremely relevant from a practical standpoint despite the lower low-temperature toughness.

(13) Since the material, in addition to having a high strength and an accompanying high toughness, must also have a sufficient corrosion resistance, additional corrosion tests were also conducted.

(14) The mass loss due to erosion corrosion was determined in 20% ethanoic acid, which was acidified to pH—1.6 with sulfuric acid. The test lasted for 24 hours. The results (Table 8) show that the materials DIN 1.4418, DIN 1.4542, and the material according to the invention exhibit hardly any erosion and their corrosion resistances under these conditions can also be considered to be equivalent. As expected, the material 1.4313 exhibits a significant material loss due to its lower alloy content. In this case, it is particularly apparent that the material according to the invention is able to improve both the strength and the toughness even further while retaining the same level of corrosion resistance.

(15) With the method according to the invention, the material is conventionally melted into large block formats weighing up to >10 t with an analysis corresponding to Table 1.

(16) Then, the material is shaped in the range from 800 to 1250° C., followed by a heat treatment.

(17) The heat treatment is comprised of a solution annealing at 850 to 1050° C., a subsequent hardening, a subsequent cooling, and tempering at 450 to 600° C.; the temperature range of 450 to 520° C. is preferable for the sake of achieving a maximum of strength.

(18) The structure of the material according to the invention is then composed of martensite with a maximum of 1% delta ferrite; it is free of primary hard phases (mainly based on niobium, tantalum, titanium, vanadium); and the tempered austenite content is at most 8%.

(19) 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.

(20) According to the invention, with increased demands on fatigue strength, particularly in subassemblies that are subjected to highly dynamic loads or in the case of safety-critical structural components in the aviation and aerospace industry, the material can also be produced in the form of a high-purity remelting product in accordance with the ESU or VLBO method. The purity grade improvement associated with the remelting yields the sufficiently well-known improvements in fatigue properties due to a reduction in the defect sizes in the material.

(21) With the invention, it is advantageous that through a very precise analysis management on the one hand and through an implementation of the analysis and the reduction of the delta ferrite and primary hard phases, a material is produced, which achieves very high strength, corrosion resistance, and toughness in a way that could not previously be combined with one another.