DE2304553A1 - STEEL ALLOY - Google Patents

Description

Dipl.-Ing. H. Sauerland · Dr.-Ing. R. König ■ Dipl.-Ing. K. Bergen

Patent Attorneys · 4qdo Düsseldorf ao · Cecilienallee 76 ■ Telephone 433732

January 29, 1973 28 273 K.

International Nickel Limited, Thames House Millbank London SW1 / England

"Steel alloy"

The invention relates to a steel alloy, which is particularly suitable for use at sub-zero temperatures suitable.

The use, treatment and storage of liquid gas requires materials that have both high strength and good toughness at very low temperatures. For example, for the storage of liquid nitrogen, a material with good technological properties is up to one. Temperature of -196 ° C required. At the moment, steel alloys containing 8% or 9% nickel in accordance with ASTM standard A 553 / 70a are used at very low temperatures. The nickel content of this steel alloy on the one hand causes a high price and on the other hand is an essential prerequisite for the use of the material at sub-zero temperatures, as can be seen from the fact that a steel containing 9% nickel can be used up to a temperature of -196 ° C, while

309635/0852

-z-

can be used during an 8 $ nickel-containing steel only to -171 ⁰ C. Numerous attempts have already been made to improve the low temperature properties of the steel alloys containing 6 to 8% nickel. Attempts have been made, inter alia, to use a heat treatment consisting of austenitizing and two-stage tempering. However, there is a risk of impairment of the technological properties of the heat-affected zones of welded joints, so that the aforementioned heat treatment is not suitable for welded structures to be used at very low temperatures.

The invention is now based on the object of creating a comparatively inexpensive steel alloy with high strength and sufficient toughness both in the rolling direction and transversely to the rolling direction for use at sub-zero temperatures. The solution to this problem is a steel alloy with 5 to 7 »nickel, 0.4 to 2.8% manganese, 0.01 to 0.5% molybdenum, 0.05 to 0.2% carbon, up to 0.1% Niobium and 0.005 to 0.1% of at least one of the elements calcium, magnesium, barium, strontium, zirconium and rare earths, the remainder including iron impurities caused by the smelting process, the nickel and manganese contents of which lie within the ACDEA polygon of the diagram in the accompanying drawing the nickel content of the steel alloy is plotted against the manganese content, and with a nickel content of 5% at least 0.12% molybdenum, with a nickel content of 5.5% at least 0.08% molybdenum, with a nickel content of 6% at least 0.05 % Molybdenum and with a nickel content of 7%

309838/0682

contains at least 0.01% molybdenum and an annealing structure with 3 to 30 vol .-% austenite in finely dispersed distribution in a martensitic basic structure.

The contents of nickel and angan are preferably within the polygon BCDEB of the diagram of the drawing.

The steel alloy 5 ₉ preferably contains 5 to 6.5% nickel, 1.75 to 2.5% manganese, 0.05 to 0.12% molybdenum, 0.01 to 0.05% calcium, 0.06 to 0, 14% carbon and up to 0.1% niobium, the remainder including iron impurities caused by the melting process and has a tempering structure with at least 10% by volume of finely dispersed austenite in a martensitic basic structure.

It has been determined through experiments that a steel alloy with the preferred content limits has a yield strength over 60 cb and a tensile strength over 70 cb

with a notched impact strength of at least 0.35 J / cm both in the longitudinal and in the transverse direction at -196 ° C. Normally, the notched impact strength is such a steel alloy in the longitudinal direction

r / c

0.43 and often even more than 0.52 J / cm and is at least 0.43 J / cm * in the transverse direction

The nickel content of the steel alloy plays an important role in that the nickel is too high Contributes to strength and toughness and a reduction in the A ^ curve and stabilization of the austenite causes. However, nickel contents above 7> 5% are not required and only lead to an increase in costs; rather, experiments have shown that a nickel content from 5.5 to 6.5%, for example from 5.75%, is quite sufficient.

309835/0852

Manganese, like nickel, shifts the A ^ curve downwards, so that a structure with a finely dispersed austenite can be set in a martensitic basic structure by tempering. The manganese also improves toughness in both the longitudinal and transverse directions; in detail, however, this depends on the respective nickel content. For this reason, optimal mechanical properties can only be achieved if the contents of manganese and nickel are within the ACDEA polygon of the diagram in the drawing, preferably within the BCDEB polygon. Satisfactory results have been obtained with a manganese content of 1.75 to 2.5 with a nickel content of 5.5 to 6.5% .

The finely divided austenite is obviously the cause of the excellent cold toughness of the steel alloy. In addition, however, the manganese itself may also have a slight effect on the transverse toughness of the alloy in an order of magnitude of 0.4 to 2.8%. However, a manganese content of Λ% or more seems to impair the transverse toughness, since the manganese forms sulfidic inclusions with the sulfur contained as impurity and these inclusions are linear or elongated or streaky. Such a linear structure, however, results in poor transverse toughness. It is known that the transverse toughness can be improved by vigorous skew rolling. Although the steel alloy in question can also be skew-rolled, this is a very complex way of improving the transverse toughness. An equally costly way to improve transverse toughness is

309835 / 08B2

stands for the use of high-purity raw materials to prevent the introduction of sulfur and other impurities to avoid and to suppress the lines.

In the case of the proposed alloy, however, the problem of insufficient transverse toughness does not exist, since this alloy Contains elements that spherulitize any inclusions such as sulphides and thus create lines counteract. These elements include above all calcium, which up to a content of 0.1% die Improved transverse properties, with a residual calcium content of 0.015 to 0.025% in the steel being very favorable, although residual contents of 0.005 to 0.05%, for example 0.01% calcium also give satisfactory results early. Instead of calcium, magnesium, barium, strontium, zirconium and rare earths, in particular, can also be used Cerium, occur in a total amount not exceeding 0.1%. Because the elements counteracting a row The steel alloy can reduce the disadvantages emanating from any inclusions up to 0.04% sulfur do not contain sulfur contents of 0.015% or 0.02% for reasons of better weldability should be exceeded.

The molybdenum improves toughness and strength. A molybdenum content of only 0.1% already improves noticeably the notched impact strength both in the longitudinal and in the transverse direction and increases the strength. The molybdenum content however, it should be matched to the nickel content of the alloy so that it is at least 5% nickel 0.12% molybdenum, with 5.5% nickel at least 0.08% molybdenum, with 6% nickel at least 0.05% molybdenum and beLnfndestais 7% Contains at least 0.01% molybdenum. The one between the

309835/0352

The contents lying above the aforementioned values can be easily determined by interpolation. With a nickel content from 5.5 "to 6.5% has a molybdenum content of 0.08% Ms 0.15% very well proven.

The carbon has a significant effect on the toughness. In contrast to the common view that the toughness of such steel alloys increases with decreasing carbon content, it has been found that with a carbon content of 0.05 to 0.14%, an optimum toughness is found at around 0.08% molybdenum or 0.07 to Yields 0.09% molybdenum. The toughness falls gradually with 0.08% carbon content in excess of fermentation and quite sharply when the carbon content is reduced below 0.08%. With regard to sufficient toughness, the carbon content is therefore above 0.05%, for example at least 0.06%, preferably 0.08%. Although the steel alloy can also contain 0.2% carbon, the upper limit for the carbon content is 0.14% ^X for reasons of better weldability. The steel alloy thus preferably contains 0.06 to 0.14% carbon.

The residual iron content also includes the usual impurities caused by the melting, such as deoxidation and refining elements and other impurities that do not impair the properties of the steel alloy. The steel alloy may also comprise up 0.5%, for example up to 0.3% silicon and in a diligent step ^F usual phosphorus contents, for example, contain up to 0.25% phosphorus.

The steel alloy can be produced by the usual melting, casting and hot rolling. It should

309836/0862

The deoxidation with a fine-grained structure resulting deoxidizer, "for example, be 0.15% aluminum. Special measures are otherwise not be required. Preferably, the steel alloy is an hour," austenitized at about 816 ⁰ C, quenched in water, then about 2 hours annealed at 607-621 ⁰ C and quenched in water again. The austenitizing can at 774-871 ° C and the tempering temperature be at 593 to 649 ⁰ C. It is also possible to cool in air. A two-stage normalization and tempering, for example, as used in conventional steel alloys with 9% nickel, are suitable as further heat treatment. It is always crucial that a structure with a relatively finely dispersed and evenly distributed austenite is formed in a basic structure of tempered martensite and that the austenite content is at least 3 _{% by volume} . -% is preferably at least 5% by volume, better still at least 10% by volume. However, the austenite content need not exceed 25% by volume or more than 30% by volume.

The invention is explained in more detail below on the basis of exemplary embodiments.

Various steel alloys were melted in a conventional induction furnace and then cast into blocks. The melts were built from high-purity raw materials such as electrolyte nickel, iron, ferromanganese and ferroSilicon and contaminated with small amounts of ferrophosphorus and ferrous sulfide in order to simulate the degree of purity of a conventional steel alloy with 9% nickel. The melts were then deoxidized first with carbon and silica-manganese and then with aluminum. The calcium

309835 / Ü8S2

was followed by aluminum deoxidation by immersion a calcium-silicon master alloy brought into the melt. All alloys were at 1038 ° C rolled out in one direction down to a sheet metal 1.6 cm thick. The panels were then kept at 816 ° C for one hour austenitized, quenched in water, tempered for two hours at 613 ° C and quenched again in water.

Unnotched tensile specimens were then machined lengthwise from each sheet; in addition, both transverse and longitudinal samples were taken for the impact test. The tensile strengths were determined at 21 ⁰ C, and the notched impact strength at -196 ° C in the usual manner.

The test alloys contained 0.10% molybdenum, 0.23% silicon, 0.12% carbon, 0/3) 8% phosphorus, 0.015% sulfur, 0.06% aluminum, 0.05% niobium and 0.015% calcium and nickel and manganese in the amounts shown in Table I below. This table also provides information about the results of the tensile and notch impact and toughness tests as well as the volume proportions of the austenite

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Table I.

Le- Ni Mn Auste- Stretch- Tensile- Deh- Notched Impact Strength & -

gienitan limit gradation at -1.96 C gradation (J / cm ² ) '

(%) (%) (%) (cb) (cb) (%) lengthways across

1 5.57 1.56 8

2 5.60 1.93 5

3 5.58 2.20 9

4 5.65 2.45 19

5 6.12 1.59 15

6 6.15 1.93 16

7 6.15 2.15 18

8 6.13 2.45 13

9 6.65 1.60 17

6.65 1.95 11

6.65 2.15 16

6.65 2.55 21

75.7 85.0 25th 0.48 0.33 84.2 90.1 20th 0.47 - 0.36 80.5 88.8 21 0.48 0.40 77.1 90.5 25th 0.50 0.43 69.5 89.8 27 0.54 0.47 72.9 90.6 25th 0.57 0.47 78.9 90.9 26th 0.52 0.45 84.7 93.0 23 0.48 0.40 72.9 89.1 27 0.61 0.47 84.4 92.3 22nd 0.57 0.43 83.4 92.5 22nd 0.59 0.42 82.0 94.1 24 · 0.52 0.45

309835/08 52

The need to coordinate the nickel and manganese contents illustrates the fact that that the transverse toughness of alloy 1 is not so great is like the transverse toughness of "alloys 3 and 4, the have essentially the same nickel content but higher manganese contents. The alloy 1 has in the Did a composite that lies within the polygon ABEA of the diagram of the drawing. In contrast to the particularly preferred steel alloys 2 to 12 lie within the polygon BCDEB. Supplementary in this context it should be noted that the toughness at higher nickel contents is due to the superposition because the nickel is not sensitive to changes in the manganese content. The austenite formers, especially nickel and margins, should be used carefully be matched to the ferrite former of the alloy in order to set the desired structure. This can be done in in the usual way, for example on the basis of the well-known Schaeffler diagram.

Although the alloys of Table I are nominally 0.05% Contained niobium, this is of no essential importance, so that the alloy is preferably niobium-free or at most contains less than 0.01% niobium. A similar to alloy 7 according to Table I, but free of niobium Steel alloy had a toughness of 0.71 J / cm in the longitudinal direction and 0.66 J / cm in the transverse direction. These Values stand out in front of the corresponding notched impact strengths of alloy 7 in the amount of 0.52 J / cm and 0.45 J / cm. However, the tensile strength of the niobium-free steel alloy was lower than the tensile strength of the Steel alloy 7, without that being of decisive importance.

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To illustrate the unexpected effect of carbon on toughness, four steel alloy grades 13 Ms 16 containing 6.1% nickel, 2.2% manganese, 0.1% molybdenum, 0.25% silicon, 0.012% phosphorus, 0.016% Sulfur, 0.07% niobium and 0.015% calcium and the carbon contents shown in Table II below, in addition to the relevant mechanical properties, were melted. The data in Table II show that alloy 14, which was 0.079% carbon, had optimum toughness in both the longitudinal and transverse directions. The toughness of the alloys up to 16 would have been even higher if they had been niobium-free.

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Table II

Le- C stretch- tensile strength- deformation limit ■ ability tion (%) <cbl (cb) (%)

Notched impact strength

at -196 ° C / cm ^) lengthways across

13th 0.050 64.8 95.6 22nd 0.36 0.40 14th 0.079 69.2 94.0 25th 0.68 0.54 15th 0.120 '87, 1 94.5 21 ' 0.55 0.50 16 0.180 87.2 97.7 21 0.43 0.38

The noticeable improvement in transverse toughness in the presence of elements that counteract a possible linearity, is shown in two alloys 17 and 18 falling under the invention with 6.1% nickel, 2.1% manganese, 0.09% molybdenum, 0.2% silicon, 0.13% carbon, 0.06% niobium, 0.006% phosphorus and 0.013% sulfur as well Calcium or cerium in an amount resulting from the following Table III in comparison to an equivalent, but no alloy A containing any elements that counteract any streaking.

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Table III

additive
OO Stretch
border
(cb) Zugfe
stig
speed
(cb Deh
tion
(.%) Notch impact
at -19C
along ^ Tenacity
across Le
gie
tion 0.014 approx
0.03 Ce 82.1
83.0
78.8 89.9
92.3
89.6 21
21
24 0.61
0.62
0.62 0.36
0.50
0.50 A.
17th
18th

The proposed alloy can be easily welded using welding wires such as those used when welding a steel alloy containing 9% nickel. In any case, welding tests have shown that the welded joints, including the heat-affected zones, have sufficient toughness up to a temperature of -196 ° C.

The steel alloy in question is suitable as a material for the manufacture of objects which, like structural parts must have high cold toughness and be used at temperatures up to t196 ° C. For this tanks, containers, reservoirs, boilers, heat exchangers and similar equipment for the manufacture, treatment, Storage and / or distribution of liquid gas including fittings such as valves, pumps, pipes, lines and reinforcement elements for boilers, for example. In particular, the steel alloy is suitable as a material for liquid gas such as hydrocarbons including

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23Ü4553

Containing natural gas, methane, propane, butane or other petroleum gases, liquid nitrogen and liquid absorbent Boiler.

309835 / Q8S2

Claims (7)

International Nickel Limited, Thames House Millbank London S.W.1 / England Patent claims ι 1. steel alloy with 5 to 7.5% nickel, 0.4 to 2.8% manganese, 0.01 to 0.5% molybdenum, 0.05 to 0.2% carbon, up to 0.1% niobium and 0.005 to 0.1% at least one of the elements calcium, magnesium, barium, strontium, zirconium and rare earths, the remainder including smelting, is caused by impurities in iron, their contents of nickel and manganese are within the ACDEA polygonal line of the diagram in the drawing, and those at 5% nickel at least 0.12% molybdenum, with 5.5% nickel at least 0.08% molybdenum, with 6% nickel at least 0.05% molybdenum and with 7% nickel contains at least 0.01% molybdenum and an annealing structure of 3 to 30% by volume finely dispersed distributed austenite in a martensitic basic structure owns. 2. Steel alloy according to claim 1, the contents of nickel and manganese of which, however, lie within the polygon BCDEB. 3. Steel alloy according to claim 1 or 2, but which contains 5.5 to 6.5% nickel and 0.08 to 0.15% molybdenum. 4. Steel alloy according to one or more of the claims 1 to 3, but containing 0.05 to 0.14% carbon. 30983B / 0fib2 5. Steel alloy according to one or more of claims 1 Ms 4, but containing 5.5 to 6.5% nickel, 1.75 to 2.5% manganese, 0.05 to 0.12% molybdenum, 0.01 to 0.05% calcium, 0.06 to 0.14% carbon and up to 0.1% niobium, the remainder including impurities caused by the smelting iron contains and an annealing structure with at least 10 vol .-% finely dispersed austenite in a martensitic Has basic structure. 6. Steel alloy according to one or more of claims 1 to 5, but which contains 0.08% carbon. 7. Use of a steel alloy according to claims 1 to 6 as a material for articles which must have high tensile strength, yield strength and impact strength of the core at temperatures down to -196 ⁰ C. 309835/0852