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mechanism of atom movements during bainite formation is still b and the understanding of relationships between mechanical properties remains limited. The applied steel design philosophy uses the micro relations that govern the impact strength transition temperature and fracture to identify a suitable microstructure and then employs the preferred tactic was to manipukte the microstructure into the appropriate form. characteristic of resistance to brittle fracture, since high strength is a desir The present study summarizes a portion of the results of the research pro at The Technical University of Cz@ochowa, Poland, on the d ship vessels and other constructions and The Basic Metals Processing University of Pittsburgh, USA, on ship and submarine heavy plate steels Dr A.K.Lis there. &o,

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The ULCB steels chemical compositions rely on s&icient alloying to obtain carbon, low temperature transformation products, mainly bainite and/or m of quenching operations. Microalloying with titanium and niobium or van mechanical processing is used for enhanced microstructural refjnement and increased low temperature toughness [2,3-6,1 l-141. The chemical compositions of the investigated steels are roperties of the shown in Table 1. Since, it is well known that both the structural integrity HA.2 zone can be improved with a reduction in base plate carbon c these new steels involves a relatively low carbon content less than 0.08% wt susceptibility to heat affected zone cracking of the new steels is greatly reduce safe conditions of welding according to Figure 1. The steels are divided into There are manganese ULCB steels in the first group. continuous cooling conditions of the representative HSLAlOOand 3S%Ni steel diagrams on Figure 2. The 3.5%Ni steel is a reference steel for the next two groups. The second group is created from high strength low alloyed copper bearing plate steels. Their mechanical and impact strength and fkactur properties afier water quenching from 9OO”C/1 hour and then tempering during one hour at temperature in the range 400°C up to 900°C are presented in the paper. The ULCB steels with carbon content less than 0.03% alloyed mainly with 3S%Ni-Mo-Mn-Cr and having very low sulfur content belong to the last group. They were thermomechanically control processed (TAKP) in two stages. After reheating to 1250°C they were rough rolled with total deformation 53% between 1200 to 1100°C then air cooled and reheated at 950°C to 1050°C. The finishhg rolling with total reduction 82% in several passes was conducted between 850°C and 775*C to the fIna plate thickness 25mm. The yield strength of the investigated steels was far greater then 69OMPa thus the effect of annealing treatments at 575”Whour and 675”C/lhour on the mechanical properties of the as-hot rolled ULCB steels was also investigated. The influence of reheating temperatures of UL steels in the range 7 10°C to 1050°C and thermomechanical processing via extrusion from diameter 72mm into 16mm and rolling practice ofblooms down into the y + a region to the &MIthickness of 12mm on the microstructural refinement, tensile properties and impact toughness obtained from Charpy Vnotch specimens in the range -120°C up to 20°C were investigated. Finally fracture toughness KICvalues determined from compact and three point bend tests for HSLA 100 steels containing

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259

1.6wt_% and 2wt.% Cu as a fimction of yield strengthat given aging temperaturehave been compared. The influence of the carbon content and carbon equivalent CE values on the Klc IIf the investigated steels have been shown.

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A.K. Lts et al. /Journal of Idateriah Processing Technology 61 (I 997) 255-266

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5acl 600 7 YIELD STRENGTH o Figure 3. Impact transition temperature-ITT affected by proc strength of the conventional and ULCB and HSLAlOO-130steels.

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AK. Lis etal. /Journal of Materials Processrng TecCrnolo,qy64 (1997) 255-36

Figure 7. This represents an improvement of -90°C in low to microstructural refinement of austenite and produtis 0 TlMGp is finished at higher temperatures the heterogenous recrystallized austenite. In effect the ITT at 5OJrises to about observed for ULCBlOO steeL The ductile bainitic and microstructures have been achieved in control of alloying (Figure 2) as well as ULCB steels, which have been rolled below the recrysta by the prior austenite grain size and its mo~holo~ effective inter-facialarea of grains per unit volume, the The steel cleanliness is related to the nucleation of cleavage era growth of cleavage cracks. Hence, high resistance to cleavage excellent austenite conditioning through appropriate therm It is also shown on the Figure 7, that aging of as TMCP moderate improvement in lowering ITT. The IJLCB steels tempering treatment when tempered simultaneously the impact upper shelf the ITT about 40°C (Figure 7). The increase in t four factors synergistic effect: nucleation of the microstructure leading ‘to further grain refin purity ferrite matrix and new disper suppression of temper embrittlement associated with grater volume interphase boundaries.

In ULCB steels, grain refinement, solid solution stren

were utilked to obtain high strength. The I-ISLAlOOsteels, on the other somehow combine these mechanisms and particle str their high strength. It has been found that the strength highly -dislocated,aged martensitic-bainitic structures and the pree which retards the sofiening associated during the aging process [6]. impact toughness at -84OCresponse to aging LAlOO steel in temperature range 400°C to 900°C is shown in Figure 8. Copper, when in form of precipitates, at aging temperatures will act to retard the recovery and recrystallization of the as-quenched matrix. This will also lead to higher strength levels at all aging conditions. At the aging temperatures 450” to 558 Cu_clusters were identied with Moire patterns, later on becoming spherical in shape diameters less than 1Onm. At higher temperatures of aging they were d to have kc. crystal structure and possess a kudiumov- Sachs orientation relationship the b.c.c. matrk of the recovered marten&c ferrite. The extraordinary improvement in toughness found at aging temperatures 640°C to 665°C and at 705°C to 728°C is associated with the formation of a highly alloyed, thermally stable austenite at the lath boundaries. The new austenite is found to be rich in alloying elements such as Ni, Cu, Mn, and Cr Figure 9. The positive elect of the fIr.telydispersed stable austenite on the impact energy is quite apparent which is shown in Figure 8. with the temperature increase beyond 685”G705”C, the content of stable austenite

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Figure 9. Element ysis by L JQEL2OOOFX STEM from steel aged at 705W1 hour.

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AK. LiS eta!. !hu’na~

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64 (‘3997) 25.5-266

decreases again. It becomes poorer in alloying which transforms on cooling to bainite or mart heating

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s temperature of 420°C is rea maximum value for the glO°C. This temperature is roughly equivalent to the end of CX+y Of particular interest in this work was the contrii microstructure and properties of quenched and aged @u-be in fracture toughness expressed by apparent KICvalues dete temperature and yield strength ior HSLAIQO in Figure 10. The toughness of the 2%Cu material followe Figure 8 for HSLAlOO steel. Both steels possesed the best CO toughness in overage conditions suggesting that the presence of into allows for easier formation of new stable austenite. The new aust austenite, can provide a strong bar:ier to polygon&d matrix. With precise setting of treatment an optimum compromise, betwe strength is produced, making maximum use of the fo tempering austenite. The bainitic or low carbon soft martensitic con microstructure together with Etempering. However, the mech affect impact toughness in these steels steel may have yield strength over 89OMPa in the overage condition excellent toughness. It is assumed that key role in maint bainitic steel is associated with the presence in their micr or lathes with very low carbon metastable austenites with varying ca

topography Corn the work of Gerberich, Hemmings and Zackay [15] were co obtained for the investigated steels. The carbon equivalent CE cc0 because it was difficult to establish r austenites. It is quite clear that the 1 ahoy design point for low temper cleavage fracture at low temperatures may result Corn enhanced ductility imparted by the strai&nduced phase transformation y a a’ and energy dissipation in y phase at crack For the ultra-low carbon material the fracture morphology is ductile rupture and the ductile-brittle transition temperature lowers with Ni additions what explains high Krc values for HSLA Cu containing steels. For steels with high CE, if the brittlefracture mode is transgranular the alloy may be toughened by decreasing its effective grain size and meafl free path or facet size for cleavage fracture [2, lo].

We conclude from the work reported here that an alloy design strategy toward suppressing the ductile-brittle transition temperature at the high strength level of the investigated steels should base on a control of the a’-+ y -3 (a’+a) reversion cycle and further stabilization of the recovered and polygonised a’ martensite-bainite microstructure by Ti(CN), Nb(CN)

-/

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AX. Lis et al. /Journal of A4aterid.sProcessing Technology 64 (199?) 255-266

precipitates and on E-Cu particles age hardening. wt pet in chemical composition of steel causes an increase in nickel and copper content up to about 6% additional increase in alloy strength at low temperature without 1 shown, that a thermal treatment which combines grain refinem stable retained austenite during intercritical aging or tempe promising combination of strength and toughne steels.

1. A. Lk, L. Jeziorslci and J.Lis, Arch. of Material Science, 4 (1983) 67. 2. A. Lis, Archives ofMetallurgy, Vol.3 1 NO. 3 (1986) 379. 3. C.I. Garcia, AK. Lis and A J. DeArdo, 3 1st Mechanical Co& Proceedings, Chicago (1989), Vol.XXVII, Prtbl.

Warrendale, PA, US, (1990) 505. 4. Steel Plate, Sheet or and HSLA-loo), US 5. M.R Blicharski.,C.I. Garcia, S. and Properties of HSLA Steels, The Warrendale, PA, US, (1988) 3 17. 6. AX. Lis, M. Mujahid, C.I. Ga Microscopy of Solid Materials 7. E.J. Czyrca, Development o Ship Construction, David T (1990). 8. C.I. Garcia, A.K. Lis, S Vol. 13 (1492) 97 and I& 9. AK. Lis, M. Muj 34th Mechanical Wo Publ. The Iron and 10. A.K. Lis, The effect of structure on toughness of tbermomechanicalIy treated l~w-~arl)o~ high manganese steels, Ph.D. thesis, Technical University of Czestochowa, ( 1980). 11. A.Lis, J. Lis, Wiadomok Hutnicze No.2 (1985) 35 12. A. Lis, L. Jeziorski and J. Lis, Procedur of Extrusion or Rolling for Stiength and Toughness, Polish Patent No.234538, u&tin of Polish Office fo enting No.14 (1983). 13. A Lis, J. Lis and L. JeziorsQ 7th Conf Proc.

AT TREATMENT’94, Ferrous Metallurgy Institute and Committee of Materials Science PAN, Gliwicc-Ustron, (I 994) 77 14. C.I. Garcia and A.J. DeArdo, ConfProc. World Materials Congress, Microalloyed HULA International., USA, (1988) 29 Steels, Chicago, A 15. W.W. Gerberkh, P.L. Hemmings and V.F. Zacbay, Metall. Trans.A, 24A (1971) 2243

?is paper wasprelented daring the 14th InternationalScientific Conference ‘Advanced MU&& and Technologies TW’ in Zakopane, land, 17-21 May 1995.