Tensile and impact behaviour of a microalloyed medium carbon steel: Effect of the cooling condition and corresponding microstructure

Effect of cooling rate on the microstructure and mechanical properties of microalloyed forging steel

Effect of Cooling Rate and Vanadium Content on the Microstructure and Hardness of Medium Carbon Forging Steel

Conventional quenched and tempered steels have long been used to fabricate mechanical components, such as plates, shafts, and axles. All of these components are mainly fabricated employing medium carbon hot forged steels in the quenched and tempered condition. In the last decade, thermomechanical forging at intermediate (warm) temperatures of microalloyed medium carbon steels is increasingly adopted to reduce or eliminate quenching and tempering heat treatments in hot forged components. However, the use of these steels is still limited, being difficult to achieve a toughness comparable to that of fully heat treated ones. In an effort to improve toughness, an experimental campaign was performed to assess strength and toughness properties of three selected microalloyed medium carbon steels after thermomechanical forging simulations at warm temperatures, performed on a thermal and mechanical Gleeble 3800 ® apparatus. Microstructures and mechanical properties of these steels are shown to be highly dependent upon steel composition and thermomechanical schedule. The experimental results suggest that thermomechanical schedules can be designed to produce microalloyed medium carbon steels with refined microstructures, and therefore to realize improved strength and machinability; yet, toughness is still the most critical property as compared to that of quenched and tempered steels.

Thermomechanical warm forging of Ti–V, Ti–Nb, and Ti–B microalloyed medium carbon steels

A new kind of V–Ti–N high strength microalloyed medium carbon steel has been developed, which is used for fracture splitting connecting rod. In this article, the characteristics of this carbon steel and its production process were studied. The microstructure, precipitated phases and their effects on mechanical properties were investigated by optical microscope, SEM, and TEM. The results showed that the steel was constituted of ferrite and pearlite. By reducing the finish rolling temperature and accelerating the cooling rate after rolling, microstructure with fine grain ferrite and narrow lamellar space pearlite could be obtained in V–Ti–N microalloyed medium carbon, and a large number of precipitated phases distributed over ferrite. These led the tensile strength to be more than 1000 MPa, yield strength (YS) more than 750 MPa. The impact fractograph showed typically brittle fracture characteristic.

Microstructure and mechanical properties of V–Ti–N microalloyed steel used for fracture splitting connecting rod

Microalloyed forging steels have been developed to improve the competitiveness of wrought steel components, especially in the automotive sector, by achieving the desired properties in the as-forged condition, thus eliminating the need to subsequently heat treat, straighten and stress relieve the previously specified low alloy steels. Significant cost reductions are realised by adopting microalloyed steels. This paper reviews the metallurgical principles on which microalloyed forging steels are based, including the relationships between steel composition, thermomechanical processing, microstructure and the resulting properties, highlighting the various strengthening mechanisms that are invoked. The properties, characteristics and applications of the initial development grade, 49MnVS3, are described. Research and development then focussed on increasing the strength and/or the toughness of this steel to improve its appeal to the market, especially for safety critical applications. the various metallurgical options are described and discussed. Attention has also been placed on maximising the machinability of these steels by controlled additions of sulphur, the adoption of inclusion modification techniques and other free machining additives. The fatigue properties and toughness of microalloyed steel forgings have been demonstrated to be fit for purpose, but compared with heat treated low alloy steels their fracture toughness is lower, albeit still significantly superior to castings. A wide range of forged automotive applications had been successfully converted to air cooled microalloyed steels over the past 25 years, with a large proportion of crankshaft and connecting rods now being made by this route. Future challenges have been identified to further extend the attainable properties and to improve the combination of strength and toughness, to broaden the market applications and the product range to include bar and rod. The use of warm near net shape forming processes for microalloyed steel is also anticipated. Greater exploitation of computer aided modelling and design techniques is encouraged to facilitate rapid prototyping, in order to improve further the competitiveness of forged engineering steels.

D.J. Naylor

Papers

Microalloyed Forging Steels