Effect of ultragrain refinement on quenching and partitioning steels manufactured by a novel method

 abstract

In this study, ultrafine grained (UFG) quenching and partitioning steels was achieved by using tempered andde formed martensite as the pre-microstructure of the quenching and partitioning treatment. Compared with those manufactured through the conventional routine, superior mechanical properties were realized in UFG steels by using tempered and deformed martensite as the pre-microstructure of the quenching and partitioning treatment. The grain subdivision mechanism during deformation and the microstructure evolution during heating were investigated. Effect of carbide on the grain subdivision and its pinning effect against grain growthis highlighted. The proposed method produced UFG steels with considerable amount of retained austenite,which contributed to the enhanced mechanical properties of investigated steels .

Keywords:

Steels, Microstructure, Mechanical property

Ultrafine grain Retained austenite.

Introduction

Throughout the main strengthening mechanisms, grain refinementas the best method enhancing both strength and toughness, havebeen extensively investigated in recent years. Ultra-fine grained (UFG)steels, which have shown attractive improvement of strength with outextensive alloying, draw wide attention for replacing convention allow-alloyed high strength steels. Most existing methods for producing UFG structure in low-alloyed steels are based on severe plastic deforma-tion (SPD) [1–8], in which large accumulated plastic strains were introduced at ambient or elevated temperatures. Representative SPD methods include high pressure torsion (HPT) [2,3], equal-channel angu-lar pressing (ECAP) [4,5] and accumulative roll bonding (ARB) [6,7], etc.Although nano scaled microstructure is promisingly obtained, specific devices and processes are acquired while the mass production is difficult. One alternative technique for producing UFG microstructures could be warm rolling or temper forming [9,10], which is characterized by large strain warm deformation. UFG grains are obtained through pro-nounced recovery progress and rearrangement of dislocations during the deformation and subsequent annealings. Another effective strategy depends on low-temperature inter critical annealing of steels with ele-vated manganese or nickel content [11,12], which is characterized by a desirable ductility due to large amount of retained austenite. With a high alloy content, the inter critical region is lowered thus the grain growth is retarded and UFG duplex microstructure is obtained. Further-more, manganese or nickel as the stabilizer of austenite dramatically enhances the fraction of retained austenite and improves the ductility of the steel by significant transformation induced plasticity (TRIP) effec.

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