CH626120A5 - Steel alloy having a low carbon content - Google Patents

Description

The invention relates to heat-resistant steel alloys with a low carbon content.

Commercially available low carbon steel alloys of the type which are said to generally contain 1.25% Cr, 0.5% Mo are listed on page 467 of the ASM 45

"Metals Handbook", 8th edition, volume 1.1961 mentioned. The manual specifies the nominal composition for the alloy in the following form: maximum 0.15% C; 0.45% Mn; 0.75% Si; 1.25% Cr; 0.5% Mo; Rest of Fe. A steel alloy with 1.25 Cr -0.5 Mo has been used on a large scale in the past 50 years as a material for pressure vessels, tubes, pipes and steam turbine housings. Although commercial quality alloys show good high temperature properties, there is a potential limit of use for this alloy at temperatures above about 538 ° C because of the creep problem. Heat treatment processes that can achieve higher strength values unfortunately result in a loss of ductility and make the steel susceptible to notching. Improvements in creep and rupture strength would not only allow 60 to use this alloy at higher temperatures, but would also reduce thickness requirements, resulting in weight and material savings. Commercial steel alloys with 1.25 Cr - 0.5 Mo contain numerous embrittling elements, such as phosphorus, antimony and tin, which at high temperatures tend to collect at the grain boundaries and embrittlement and subsequent weakening at the Create limits. The high temperature properties of the alloy could be improved if these impurities are removed, but it is not economically feasible to remove the embrittling impurity elements to the very low required levels in commercial steel melting practice.

It has already been proposed to add boron, titanium, vanadium, copper or columbium to an alloy of the general type mentioned in the introduction in order to improve the impact resistance at low temperatures and the welding resistance, see e.g. B. US Pat. Nos. 3,251,682 and 3,288,600. The object of the present invention, namely to eliminate the embrittlement of the 1.25 Cr - 0.5 Mo alloy at high temperatures, is achieved in that controlled additions of titanium or of titanium and boron to improve both the high temperature properties and the room temperature properties of the alloy, even in the presence of the embrittling impurities phosphorus, antimony and tin in the steel.

The present invention therefore lies in the creation of a steel alloy with a low carbon content, which is characterized by high creep and fracture strength and by high fracture ductility at 538 ° C. According to the invention, the object is achieved by the features of claim 1, that is to say in that the alloy as essential constituents 0.10 to 0.20% by weight carbon, 0.30 to 0.80% by weight manganese, 1.00 up to 1.50% by weight chromium, 0.45 to 0.65% by weight molybdenum, 0.40 to 0.75% by weight silicon, 0.01 to 0.05% by weight titanium, phosphorus and / or antimony and / or tin in a total amount of 0.01 to 0.05% by weight and the rest at least for the most part contains iron.

The invention also encompasses a method for producing a heat-resistant steel with a low carbon content, which is characterized by high creep and fracture strengths and by high fracture ductility at 538 ° C. The method consists in initially providing a steel with a high degree of impurity - phosphorus, antimony and / or tin in a total amount of 0.01 to 0.05% by weight - which is also 0.10 to 0.20 % Carbon, 0.30 to 0.80% manganese, 1.00 to 1.50% chromium, 0.45 to 0.65% molybdenum, 0.40 to 0 , 75 wt .-% silicon, the rest iron, and that then 0.01 to 0.05 wt .-% titanium is added to the alloy before casting.

The alloys preferably contain 0.004 to 0.008% by weight of boron.

According to a preferred embodiment of the invention, carbon, manganese, chromium, molybdenum, silicon, titanium and boron are present in the following approximate amounts by weight: 0.15; 0.60; 1.25; 0.50; 0.45; 0.03; or 0.004 to 0.006.

The invention provides an alloy that exhibits greater hardness and tensile strength at room temperature without deterioration in percent elongation and percent reduction area. The addition of titanium and boron creates an alloy that exhibits significantly improved breaking strength at 538 ° C (1000 ° F). The alloy according to the invention, which contains titanium or titanium and boron, results in higher fracture ductility in stress fracture tests which were carried out at 538 ° C. for periods of up to 1000 hours. The advantageous effects of additions to titanium or titanium and boron occur regardless of the amounts of the embrittling impurities, phosphorus, antimony and tin present in the alloy.

Further advantages and details of the invention will become apparent from the accompanying description of an example in conjunction with the accompanying drawings. Show it:

1 shows the breaking load in ksi (ksi = kilopounds per square inch; 1 ksi = 70.3 kp / cm2) as a function of time

626 120

in hours (logarithmic scale);

2 shows a representation of the change in the percentage area reduction at break for the alloys as a function of the logarithmically plotted time in hours at 538 ° C. and

Fig. 3 shows the logarithmic minimum creep rate in percent per hour for different materials depending on the load in ksi.

example

In order to be able to check the high-temperature properties, in particular the creep-fracture ductility of 1.25 Cr - 0.5 Mo steel alloys with low carbon content of commercial quality, 8 batches were produced by means of vacuum induction melting. The chemical analysis of these batches is shown in Table 1 below. Batch VM 1706 was a high-purity contro-alloy that received low amounts of the embrittlement elements phosphorus, antimony and tin. VM 1711 and 1712 contained titanium or titanium and boron, with antimony and tin as basic impurities. VM 1701 contained only boron, with tin as the basic impurity. VM 1713 and 1717 provide a comparison between a titanium additive and an additive made of titanium plus boron, with phosphorus as the basic impurity. VM 1715 contained no titanium or boron and showed high levels of phosphorus, antimony and tin. VM 1716 also contained high amounts of embrittling impurities plus a boron additive.

The batches melted under vacuum by induction were cast into bars of 5 × 5 cm in size and then forged at 1093 ° C. to form a square lumen head with an edge length of 1.6 cm. Specimens from the lumen head were normalized at 925 ° C for one hour and annealed at 675 ° C for one hour and then quenched in water. A commercial 1,25-

Cr-0.5-Mo steel alloy tested and was in the form of a square longitudinal beam block casting with an edge length of 5 cm. The commercial alloy was normalized at 925 ° C and subsequently annealed at 704 ° C. The lower tempering temperature for the laboratory steels was mainly chosen to produce higher strength levels and thereby accelerate any possible embrittlement effects in short-term tests. After the last heat treatment, the test pieces were removed from the test pieces by machine treatment. The sample pieces had a measuring and groove diameter of 0.907 cm. The groove had a root radius of 0.0254 cm and a larger diameter of 1.27 cm. This geometry produced a theoretical load concentration factor of 4. Creep-15 branches were carried out in air with lever arm type machines at 538 ° C. The tensile test data was obtained at room temperature.

The results from the tensile strength and hardness tests at room temperature for the different alloys are listed further in Table 2 below. Among the experimental batches, VM 1712, 1713 and 1714 showed significantly increased strength at room temperature without a noticeable reduction in ductility, which indicates the advantageous effects due to the addition of titanium or titanium plus boron. The 25 batch VM 1711 is an exception to this and shows slightly lower strength values at room temperature compared to the control steel VM 1706 % compared to the control batch VM 1706, and approximately 35% compared to commercial steel alloy. All tensile tests failed in the soft backing area, indicating that there was no notch sensitivity at room temperature.

35

table

Chemical composition of the experimental steels

stole

PPM

C.

Mn

Cr

Mon

Si

Ti

B

P

Sb

Sn

S

N

O

VM 1706

0.15

0.58

1.25

0.49

0.45

20th

4th

20th

30th

5

25th

(Control)

VM 1707 *

0.16

0.58

1.22

0.49

0.45

-

45

20th

5

320

32

20th

23

VM 1711

0.16

0.59

1.17

0.49

0.45

300

-

20th

100

330

25th

10th

15

VM 1712

0.16

0.56

1.24

0.49

0.45

300

60

10th

93

280

27th

6

23

VM 1713

0.16

0.55

1.24

0.49

0.45

300

-

300

3rd

21st

26

14

37

VM 1714

0.16

0.57

1.24

0.50

0.45

300

55

320

2nd

23

26

8th

25th

VM 1715 *

0.17

0.55

1.25

0.50

0.45

-

-

270

94

350

22

6

50

VM 1716 *

0.16

0.56

1.25

0.49

0.45

-

40

300

97

325

26

7

33

* Comparative tests t

626 120 4

Table 2

Results of the hardness and tensile strength tests at room temperature on experimental steels

Steel hardness, Rb Eventually 0.2% elongation area

Tensile strength, yield% reduction ksi strength, ksi%

VM 1706

94.5

92.3

74.4

26.9

73.9

VM 1707

94.0

92.3

74.4

25.7

70.9

VM 1711

91.5

88.0

69.4

19.0

71.6

VM 1712

97.0

102.7

88.9

24.9

64.8

VM 1713

97.0

105.7

90.6

22.0

63.4

VM 1714

98.6

108.4

93.7

20.4

65.5

VM 1715

94.6

92.7

71.8

25.3

69.8

VM 1716

92.5

94.0

85.8

26.3

69.2

commercial

-

77.5

54.0

29.0

71.9

1.25 Cr-0.5 Mo steel alloy

As can be seen from the tables above, Charge VM 1714 was the strongest alloy at room temperature. VM 1714 contained 0.16% carbon; 0.57% manganese; 1.24% chromium; 0.5% molbydane; 0.45% silicon; 300 ppm or 0.03% titanium; 55 ppm or 0.0055% boron. The major impurity was phosphorus in an amount of 320 ppm (ppm = parts per million parts) or 0.032%, while the rest was essentially iron with trace impurities of antimony, tin, sulfur, nitrogen and Was oxygen. It appears that phosphorus and tin affect the strength at room temperature. VM 1714, which contained both titanium and boron with a high phosphorus and low tin content, achieved higher strengths than the batch VM 1712, which had similar amounts of titanium and boron and a low phosphorus and high tin content. This obvious connection is further illustrated by a comparison of VM 1713 with VM 1711, which contained titanium and no boron. VM 1713 with high phosphorus and low tin and low antimony content was much stronger than VM 1711, which contained little phosphorus and moderate amounts of antimony and high amounts of tin.

The improved breaking strength properties at elevated temperatures for the alloys according to the invention can be explained with reference to FIG. 1. The stress fracture tests were performed at 538 ° C on a VM 1706 contro alloy and on two alloys containing titanium and boron additives, VM 1712 and VM 1714. The tests were carried out using four levels of stress, namely 35 ksi, 38 ksi, 44 ksi and 50 ksi (1 ksi = 70.3 kp / cm2). The VM 1712 alloy with the high tin-antimony content showed the best stress-breaking properties in this test. VM 1714, which had a high phosphorus content, also showed good stress-breaking properties, which were above that of the high-purity contro alloy VM 1706. It can be seen from FIG. 1 that the addition of titanium and boron leads to a substantial improvement in the breaking strength, even in the presence of embrittling impurities. The 10,000 hour breaking strengths of batches VM 1712 and 1714 are believed to be at least 20% higher compared to control steel alloy VM 1706.

Figure 2 illustrates the observed variations in percent area reduction at break for the alloys as a function of logarithmic time to break at 538 ° C, the time being given in hours.

These tests show the improved fracture ductility as a result

25 of the titanium or titanium plus boron additives. The curve for the control steel alloy VM 1706 is characterized by an initial range of constant ductility, followed by a range of decreasing ductility when tr exceeds approximately 100 hours. For the alloys according to the invention, the ductility values are in a narrow range from 80 to 90% and show no tendency to decrease at times which are over 1000 hours. For example, the batch VM 1712 had an area reduction of 81% even at 2300 hours. It is noteworthy that alloys VM 1707 and 35 1716, which contained only boron with high levels of impurity,

showed a very rapid drop in ductility after 100 hours, a faster drop than the control lot VM 1706. VM 1715 was similar in composition to the control lot but contained higher levels of phosphorus, anti-40 mon and tin, which was obviously the faster drop which caused ductility after 100 hours.

It is believed that the impurity elements collect at the grain boundaries at high temperatures and weaken the grain boundaries, reducing the creep fracture ductility of the alloy. This effect is eliminated by the addition of titanium and boron, which reduce the sensitivity to fractures occurring between the grain boundaries, even in the presence of these grain boundary brittleness.

The change in the logarithmically plotted minimum creep rate is plotted in FIG. 3 as a function of the load for a temperature of 538 ° C. for several batches. The minimum creep rates are uniformly smaller by at least a factor of 3 for both VM 1712 and VM 1714 55, compared to the contro-allocation VM 1706,

indicating significant improvements due to the addition of titanium and boron. The effect of the titanium additive alone - in the absence of boron - is not yet completely understandable and changes with the type and amount of the other impurities present. For example, VM 1711 showed that titanium had fundamentally lower creep resistance in the presence of antimony and tin, while VM 1713, which contained titanium and had a high phosphorus content, consistently had improved creep resistance compared to the control 65 VM 1706 alloy.

The low carbon 1,25-Cr-0.5-Mo steel alloys are generally used at high temperature and high stress. Such an application is

5

626 120

for the casing of steam turbines. These housings will be ben. The heat treatment and test procedures that were cast and subjected to use under the high pressures of the samples cast on them were superheated steam and high temperatures in the range identical to those previously performed on the forged samples 538-593 ° C. To determine if the properties of the cast were carried out. Table 3 below shows the alloy is comparable to the forged samples, the results of the creep test were carried out at 538 ° C and 35 ksi on forged additional tests on cast samples. The detached and cast samples again, namely for the materials used for this determination, consisted of batches VM 1706, 1713, 1712 and 1714, together with the semi-bars, which correspond to batches of the previously commercial alloy.

analysis of forged samples

Table 3

Creep test results at 538 ° C and 35 ksi

stole

Status

Final

Creep rate tr

% Areas

hardness

(%/Hour)

(Hour)

reduction

(VHN)

in the event of breakage

VM 1706

forged

202

0.0033

1111

63

(Control)

poured

198

0.003

664

79

VM 1713

forged

246

0.0013

1156

83

(Ti)

poured

224

0.0042

571

83

VM 1712

forged

233

0.00045

2306

81

(Ti + B)

poured

218

0.0024

1030

83

VM 1714

forged

251

0.0011

1301

84

(Ti + B)

poured

237

0.0009

1511

73

commercial

poured

-

-

1000

88

target

The results presented in Table 3 show that 1712 and 1714 show significant improvement in strength in both the forged and cast condition of the steel alloy 35 due to the addition of titanium and boron, titanium and boron containing steels, VM 1712 and 1714, regardless of the amount of embrittling present compared to the control batch VM 1706 and the commercial target elements phosphorus, antimony and tin. VM 1713, which is superior to high len alloys in terms of creep resistance, titanium with a high phosphorus content, also shows a lockable tensile strength, breaking strength and ductility. better strength compared to VM 1706 (control). VM 1711,

Based on the above results, it can be seen that 40 has the high impurity content of antimony and tin in low carbon 1.25-Cr-0.5-Mo steel alloys, on the other hand, has lower room temperature content by the addition of controlled amounts of titanium strength as the contro Alloy VM 1706.

or improvements of both titanium and boron. The results show that the alloy according to the invention

with regard to the room temperature strength as well as with regard to improved breaking strength at 538 ° C and improved stress-breaking strength at 538 ° C 45 serte room temperature tensile strength can be achieved with none. The data show that increases in tensile strength and concomitant loss of ductility are related to the combined addition of breaking strength. Addition of titanium and boron. Furthermore, additions of titanium to the creep strength also lead to the stress-fracture strength or titanium and boron to significant improvements in the relationship, the three parameters under the stress-fracture ductility at 538 ° C despite the presence

The term “firmness” can be summarized in order to facilitate the discus- so of large amounts of embrittling impurities such as the results. The test results of VM antimony, phosphorus and tin in the steel.

G

2 sheets of drawings

Claims (7)

626 120 2nd PATENT CLAIMS 1. Steel alloy with low carbon content with high creep and fracture strength and high fracture ductility 538 ° C, characterized in that the alloy as essential components 0.10 to 0.20 wt .-% carbon; 0.30 to 0.80 5 wt% manganese; 1.00 to 1.50 wt% chromium; 0.45 to 0.65 wt% molybdenum; 0.40 to 0.75 wt% silicon; 0.01 to 0.05 wt% titanium; Contains phosphorus and / or antimony and / or tin in a total amount of 0.01 to 0.05% by weight and the rest at least for the most part iron. ok 2. Steel alloy according to claim 1, characterized in that the silicon content is approximately 0.45% by weight. 3. Steel alloy according to claim 1 or 2, characterized in that it also contains 0.004 to 0.008 wt .-% boron. 4. Steel alloy according to claim 3, characterized in that it contains approximately 0.15% by weight of carbon, approximately 0.60% by weight of manganese, approximately 1.25% by weight of chromium, approximately 0.50% by weight % Molybdenum, about 0.03% titanium and 0.004-0.006% boron. 5. Steel alloy according to one of claims 1 to 4, 20 characterized in that it contains approximately 0.03% by weight of phosphorus and a combined antimony and tin content of less than 0.003% by weight. 6. A method for producing a heat-resistant steel alloy according to claim 1, characterized in that 25 an alloy which has a high impurity content of 0.01 to 0.05 wt .-% phosphorus and / or antimony and / or tin and 0.10 to 0.20% by weight carbon, 0.30 to 0.80% by weight manganese, 1.00 to 1.50% by weight chromium, 0.45 to 0.65% by weight Molybdenum, 0.40 to 0.75% by weight silicon and the balance 30 at least for the most part contains iron, before casting, add 0.01 to 0.05% by weight titanium. 7. The method according to claim 6, characterized in that that 0.004 to 0.008% by weight boron is also added to the alloy before casting. 35 40