DOI 10.1007/s11015-016-0271-1 Metallurgist, Vol. 60, Nos. 1–2, May, 2016 (Russian Original Nos. 1–2, January–February, 2016)

HIGH-STRENGTH SHIPBUILDING STEELS AND ALLOYS E. A. Chernyshov, A. D. Romanov, and E. A. Romanova

UDC 669.14.018.293

High-strength steels used in domestic and overseas shipbuilding are reviewed retrospectively. The composition and properties of these steels are provided. Keywords: steel, alloy, physicomechanical properties, alloying, heat treatment, shipbuilding, strong body, submarine.

Materials for manufacturing ship and vessel hulls, in particular strong submarine (SM) hulls and other structures, should exhibit good specific strength, and the main properties of the steels used for vessel hulls are yield strength and elasticity modulus. The better material specific strength, the less is material weight to achieve, for example, a considerable submersion depth [1]. Expansion of the range of operation is connected with greater specific loads, and cyclic loading. A broad operating temperature range and other reasons give rise to a requirement for finding new high-strength materials. Use of titanium alloys as a material for an SM hull is appreciably reflected in cost indices. However, titanium exhibits significantly better specific strength, and, for example, with approximately the same yield strength (588 MPa) alloy 48-OT3 (132.1 MPa·m3/ton) is significantly better with respect to specific strength than steel АК-25 (74.9 MPa·m3/ton) [2]. An experimental SM Aluminaut has also been constructed with a strong hull of aluminum alloy. Nonetheless, in domestic and overseas shipbuilding the majority of existing SM hulls and structures will be steel. Use of the first domestic hulls of welded steels SKhL-4 (10KhSND) and SKhL-45 with an ultimate strength corresponding to 400 and 450 MPa, created in the post-war years, was accompanied with a number of problems, in particular during manufacture of welded hulls there was often formation of significant cracks. Scientific research and test design work carried out at TsNII KM Prometei made it possible to establish that the basic brittleness of these steels was caused by the formation of a coarse ferrite-pearlite structure in hot-rolled and normalized conditions [2]. Preliminary design of an pr. 627 atomic SM hull with a submersion depth limit of 300 m, made at SKB-143 at the design stage, showed that steels SKhL-4 and MS-1 that were then used significantly increased hull weight and vessel water displacement. Therefore, the aim of subsequent research was creation of a stronger steel for the hulls of the first atomic SM and other vessels. Steel AK-45 was created, which exhibits not only better strength and good weldability, but also markedly better brittle failure resistance, and significantly better fatigue and corrosion-mechanical strength. Questions were also resolved for the strength of welded joints of steel AK-25 using EA-395/9 electrodes and welding wire EI-395 within the composition of hull structures, and also forgings and castings with increased strength (AK-25PK and AL-9) [3, 4]. There was also limited use of low-magnetic steels of the alloy systems С–Мn, С–Мn–Ni, Сr–Ni, Cr–Ni–Mn–N. Results are given in [4] for comparative studies of the corrosion resistance and corrosion-mechanical strength of low-magnetic shipbuilding steels prepared using different strengthening mechanisms. In particular, steel 45G13Yu3 with ultimate strength of 400 MPa was used for manufacturing a light hull and a number of other structures of the first five pr.651 submarines; for the rest of the pr.651 SM these structures were manufactured from steel SKhL. Shipbuilding plants encountered production difficulties in straightening and cutting steels. This is explained by the fact that low-magnetic steels have increased distortion caused by an increase in linear thermal expansion coefficient and low thermal conductivity. All of this led to a considerable

Nizhnii Novgorod State Technical University, Nizhnii Novgorod, Russia; e-mail: [email protected]. Translated from Metallurg, No. 2, pp. 59–63, February, 2016. Original article submitted January 13, 2016.

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TABLE 1. Chemical Composition of Steels Used in USA Submarine Building [1, 6–8] Chemical element content, wt.% Steel C

Mn

P

S

Si

Ni

Cr

Mo

Ti

V

Cu

HY-80

0.18–0.2

0.55– 0.75

0.02

0.015

0.15– 0.35

2.5–3.25

1.0–1.8

0.3–0.6

0.02

0.25

0.25

HY-100

0.2–0.22

0.11–0.5

0.02

0.015

0.15– 0.35

2.75–3.5

1.35– 1.85

0.3–0.6

0.02

0.03

0.25

HY-130

0.12

0.6–0.9

0.01

0.01

0.3–0.45

5.0–5.5

0.4–0.7

0.3–0.65

0.02

0.05–0.1

0.25

HY-150

0.15–0.2

0.4–0.5

0.01

0.01

0.25

3.5–4.0

1.2–1.7

0.3–0.5

–

0.07– 0.12

–

Note. Total P and S content in steels HY-80 and HY-100 should not exceed 0.045%.

TABLE 2. Mechanical Properties of USA and Russian Steels [1, 6, etc.] Steel

σu, MPa

σ0.2, MPa

δ, %

Ψ, %

HY-80

750

618

24

69

HY-100

895

>700

20

70

HY-130

>910

1050

16

60

AB2-1

>637

588–686

18

50

AB2PKM

>735

>690

18

50

AB2L

–

600–720

18

50

amount of work for straightening and aligning welded structures, and consequently an increase in residual stresses within them. SM with light hulls of steel 45G17Yu3 started to operate from 1962, but in 1966 for one of the SM considerable damage of light hull casing in the form of through and blind cracks of different extent in the area of the main ballast tank detected. A set of measures was developed for improving the corrosion and mechanical strength of light hulls, and this work was normally completed with regular repair and modernization of SM of this type [5]. In the USA, steel HY-80 was created as a replacement for the high-strength steel HTS used previously with yield strength of 329 MPa. This steel was the first of the HY series, with yield strength of 560 MPa [6, 7], and a subsequent developments were steels HY-100 and HY-130/150. The chemical composition and mechanical properties of a number of domestic and overseas steels are given in Tables 1 and 2. A combination of high strength and ductility for steels of the HY series is achieved not only due to a set of alloying elements, but also a reduced harmful impurity content. Normally for their production melting in a basic arc furnace is used with double-slag pouring and special metal treatment in order to reduce hydrogen and sulfur content. Several steel degassing methods are used: melting in a vacuum, blowing with argon, degassing metal in a ladle, and pouring in a vacuum. In particular, Linde Union Carbide Corp. has patented an argon/oxygen decarburization (AOD) method for degassing melt, i.e., blowing a melt with a mixture of argon and oxygen. Initially, the method was intended for producing steel of the type Ultra High Strength Steels (UHSS), although this method was extended to production of HY steels. Electroslag remelting is also used for producing steel of the HY series, and significant attention is devoted to grain refinement of the steel obtained [6, 7, 9–12]. In the production process for manufacturing domestic high-strength steels, one of the important tasks is preparation of metal with a very low gas content, i.e., hydrogen, oxygen, and nitrogen, whose increased content has an unfavorable effect on steel and alloy physicomechanical properties [13]. The impossibility of controlling metal degassing during melting in open furnaces by existing methods makes the process from the point of view of action on the gas content in steel uncontrollable. 187

Fig. 1. Values of crack resistance factor during welding [14]. The following principles were implemented in the course of developing Russian metallurgical technology: reduction in the degree of segregation in a cast slab; increased steel cleanliness with respect to harmful impurities, gas content, and nonmetallic inclusions with a reduction in carbon content; globularization of sulfide inclusions; and microalloying with the aim of bonding carbon in finely dispersed carbides. During creation of steels not only the component ratio was considered, but also their morphological similarity, which facilitates the formation of structural elements with a quasi-uniform structure [14]. A fundamental improvement of the production and physicomechanical properties of steels was achieved at the start of the 1970s after development and introduction of electroslag remelting (ESR). Use of ESR and simultaneous transfer to rolling in quadro instead of duo mills made it possible to increase considerably steel quality and its production qualities, mainly weldability. Simultaneously, research was carried out for evaluating the correlation of alloying with structure and mechanical properties, which made it possible to formulate principles for steel alloying, i.e., to minimize carbon content and alloying elements to amounts providing through hardenability and weakening temper brittleness [14, 15]. Melting steel in special vacuum furnaces in view of the high cost of equipment has not so far served as the main method for mass production of high-quality steel. Attempts to reduce the hydrogen content of steel, lowering the moisture content of slag-forming and alloying additives for increasing the intensity of carbon oxidation, does not give perceptible results. In view this, there is more extensive use of extra-furnace vacuum treatment of molten metal [15, 16]. Currently under conditions of development of high quality metallurgy specialists of Prometey Central Research Institute of Structural Materials (TsNII KM Prometey) together with Severstal and OMZ-Spetsstal have developed production technology, including extra-furnace refining treatment and degassing, making it possible to prepare metal no worse in quality than ESR steel [17]. Creation in the 1970–1980s at TsNII KM Prometei of cold-resistant steels 10GNB, AB-1, and AB-2R with the use of heat treatment (quenching and tempering) required an increase in the level of steel alloying. However, the possibility of improving the properties of structural steels only as a result of alloying is almost exhausted, and currently this path is not expedient economically [18]. Apart from chemical composition, heat treatment regimes made it possible to improve steel physicomechanical properties. After heat treatment (quenching and high-temperature tempering), creation of a structure is provided represented by very fine sorbite with effective solid solution strengthening of a ferritic matrix. This structure makes it possible to avoid during welding weakening of a heat-affected zone, significant overheating close to the fusion limit, and correspondingly a reduction in ductility and toughness, development of significant tensile stresses in a welded joint, and other factors reducing the structure’s operating capacity [15, 17, 19]. The next step in development was examination of the creation of elements of the nanostructure in massive billets due to severe limitation of the chemical composition and thermomechanical treatment (TMT) regimes, providing steel structure refinement with elements of a nanosize level [20–22]. High austenite dislocation density is inherited, and this provides an increase in steel strength properties, and an ultrafine grain structure makes it possible to retain good brittle failure resistance at low temperatures. For production of large rolled product of steel AB2-1 TMT, a technology has been developed followed by high-temperature tempering. An important feature of high-temperature treatment (quenching from rolling heating) is inheritance of a dislocation structure created during hot plastic deformation of steel in an austenitic condition, with polymorphic transformation of steel during subsequent accelerated cooling. 188

In order to prevent corrosive wear of materials from the action of sea water, two-layer AB2P and three-layer AB2T clad steels have been created, within which the cladding layer used is corrosion resistant steels 08Kh19N10G2B or 08Kh18N10Т [23]. However, as noted in [17], in spite of their unique properties, high-strength hull steels used extensively in shipbuilding are not without certain disadvantages, which under contemporary conditions may concern: 1) steel high cost in view of a high level of alloying with expensive and scarce elements; 2) a high value of crack resistance factor during welding (Рr = 0.3–0.37%), which requires warm-up during welding (see Fig. 1); and 3) relatively low level of ductility. In particular, welding of American steels of the HY series requires special technology, for example, heating a welding zone in order to reduce thermal stresses and degassing of a welded joint (heat-affected zone, HAZ). For this in order to provide the optimum toughness in the heat-affected zone prior heating is required before welding and between passes within the following limits in relation to sheet thickness: 12.7 mm – from 93 to 150°С; 12.7–28.6 mm – from 93 to 176.7°С; more than 28.6 mm – from 121 to 176.7°С. High Strength Low Alloy (HSLA) series steel developed has made it possible to reduce requirements for technology of making welded joints, and their changed microstructure is more suitable for welding conditions [6, 9–12]. Conclusion. The contemporary level of development of metallurgical rolling technology makes it possible to achieve a high level of properties due to the use of alternative approaches, and in particular use of melting in converters and special thermoplastic effects [14–17, 24–26]. Thus, it is possible to achieve a marked reduction in manufacturing cycle and a reduction in alloying element content within steels, thereby increasing the economic efficiency of their manufacture. A very promising area is considered to be creation of nitrogen-containing steels within which nitrogen is used instead of carbon as an alloying element [25, 26]. The concept of creating high-strength steels consists of maximum use of contemporary scientific knowledge about plastic deformation, and microadditions of modifying and alloying elements on the structure and properties of low-carbon steel [24, 25]. The principle of a single basic composition is realized as a result of adding microalloying elements and formation of a fine grain structure using less energy-consuming hot plastic deformation technology, combined with quenching or accelerated cooling, followed by tempering in contrast to traditionally used quenching and tempering for which a high alloying level is typical. On the whole, the new generation of high-strength hull steels differs from those developed previously by 10–20% lower level of alloying combined with use of microalloying with vanadium or niobium [16, 20].

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