No abstract

A low alloy steel with the nominal chemical composition of C 0.3 %, Mn 1 %, Cr 4 %, Mo 1 %, V 0.4 % with lower amount of P and S is prepared by electroslag refining (ESR). The result of the mechanical properties of this steel after tempering confirms that this steel may be placed in the family of ultrahigh strength steel. The tensile, yield and impact strength of this steel are 1 650 MPa, 1 450 MPa and 300 kJ/m 2 respectively, coupled with good ductility and hardness. Three more steels are prepared by inoculation of titanium during the ESR process similar to above composition. The analysis values of titanium in these steels are 0.07 %, 0.20 % and 0.4 %. The addition of titanium resulted a sharp change in both microstructures and mechanical properties. It is noticed that 0.07 % titanium results a sharp increase of tensile properties compared to un-inoculated steel. The tensile and yield strength of the 0.07 % titanium steel are 1 730 MPa and 1 512 MPa, respectively. Further increase of titanium to 0.2 % and 0.4 % result a significant drop of mechanical properties compared to steel with no titanium. The optical, SEM, TEM and TEM-carbon replica studies confirms that at lower concentration of titanium (0.07 %), finer Ti(C, N) particles are precipitated which are able to restrict the austenite grain sizes. The refinement of austenite grains and consequent finer lath martensite may be the possible reason for strenghthening. Higher titanium (Ն0.2 %) led to larger carbonitride particles which has no role on grain refinement.KEY WORDS: ESR process; mechanical properties; microstructures; precipitation hardening; grain boundary pinning effect; carbon replica.ISIJ International, Vol. 46 (2006), No. 9, pp. 1361No. 9, pp. -1370 Experimental DetailsSteels with the nominal chemical composition of C 0.3 %, Mn 1 %, Cr 4 %, Mo 1 %, V 0.4 %, S 0.2 % max. and P 0.2 % max. containing different percentage of titanium had been prepared by induction melting followed by electroslag refining. Electrodes for ESR processes were prepared in batches of 20 kg air induction melting by suitable addition of scrap of low alloy steel and ferroalloys. 0.2 % Al was added as deoxidizer. It was then cast into 40 mm diameter rod and used as the electrode for ESR processes.The electrodes, which were prepared by induction melting, were remelted using the prefused slag (70 CaF 2 : 30 Al 2 O 3 ). About 800 g slag was used. It was preheated in muffle furnace at 800°C temperature for 5-6 h to eliminate free and combined moisture.10) Electrodes were remelted in a water-cooled steel mould of 80 mm diameter connecting with DC positive (ϩve) power. The current and voltage during this process were 730 amps and 25Ϯ2 V, respectively with mould water flow rate of 30 L/min and base plate water flow rate of 20 L/min. ESR ingots were approximately 150 mm long, 75-80 mm diameter and 6 kg in weight. Inoculation of titanium was carried out by attaching a thin walled steel tube of 8 mm ODϫ6 mm ID filled with calculated amount of ferroalloys (ferrotitanium) during ESR. Af...

No abstract

No abstract

In this investigation, attempts are made to develop special steel by electroslag refining (ESR) process. The base alloy is prepared with a chemical composition of 0.28 % C, 1.0 % Mn, 1.0 % Mo, 0.35 % V, 4.2 % Cr with trace amount of S and P. This alloy displays yield strength of about 1 450 MPa in quenched-and-tempered condition. It is further attempted to increase the strength and toughness of the base alloy by addition of 1 to 3 % of nickel. As a result there is a substantial improvement of mechanical properties. 3 % nickel steel displays tensile strength of 1 758 MPa and yield strength of 1 542 MPa. The austenite grains are marginally reduced in nickel alloys. With the chemical composition of nickel alloys when about 0.07 % titanium is inoculated during ESR process, there is a deterioration of the tensile properties. In 3 % nickel alloy the effect is more prominent, which displays yield strength of 1 455 MPa along with the lowest value of elongation. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis (EPMA) studies indicate that the microstructures of un-inoculated nickel steel predominantly consist of tempered lath martensites. In titanium inoculated alloys the microstructures are mixture of retain austenite, bainite and martensites. Strengths are deteriorated possibly due to the inhomogenous distribution of carbon because of the formation of different phases and depletion of nickel due to the precipitation of the corresponding intermetallics of (Ni, Ti).KEY WORDS: electroslag refining; effect of alloying; ultrahigh strength steel; mechanical properties; microstructures.