Microstructure And Mechanical Property Of Ultrafine-Grained Martensite Steels Transformed From Metastable Prior Austenite Via Heavily Warm-Rolling

Ultrahigh-strength martensitic steels exhibit high strength and great toughness,which plays an important role in the area of aerospace,automotive industry,safety protection and national equipments.At present,the most commonly used process for fabricating ultrafine ultrahigh-strength martensitic steels in industrial production is heavily warm-rolling(HWR).There is still a lack of researches about the effects of metastable prior austenite(MPA)grain size on the martensite variants,microstructures and mechanical properties.In this paper,low carbon high alloy steel(S1: Fe-0.1C-11.0Cr-3.0W-3.0Co-0.3Si-0.2Mn-0.2V)is used as a model steel,which is subjected to 93% HWR,70% cold rolling and cyclic heat treatment.Microstructure mechanical and variant of martensite transformed from the ultrafine lamellar structured(UFLS)MPA and ultrafine grained MPA were studied in depth.At the same time,relevant research results are extended and applied to medium carbon low alloy steel(C1: Fe-0.38C-0.39Si-1.03Mn-0.16Cr-0.07Ni-0.02V-0.02Al-0.02Mo-0.02Cu-0.02N).The purpose is to improve the comprehensive mechanical properties of C1 martensitic steels through adjusting the martensitic microstructure using 85% HWR and tempering treatment,and aims to provide a theoretical basis for the development of ultrahigh strength medium carbon low alloy steels.The conclusions of this study are as follows:The results show that the heavily warm-rolled S1 low carbon steel is characterized by the UFLS lath martensites,and the heavily warm-rolled C1 medium carbon steel is characterized by the UFLS lath martensites and twin martensites.Twin martensites transformed from the UFLS-MPA exhibit two specific morphologies: one is at a 40°～80° angle with the interface of the MPA grain boundary,the other is parallel to the interface of the MPA grain boundary along the rolling direction.Besides,ultrafine blocky retained austenites and high-density nanocarbides were distributed within the HWR C1 medium carbon martensite steels.The width of the UFLS-MPA has a great influence on the martensite microstructures.When the width of the warm-rolled UFLS-MPA is less than 200 nm,The microstructure is characterized by equiaxial martensite laths along the rolling direction without martensite packets and blocks.When the width of the warm-rolled UFLS-MPA is between 200 nm and 600 nm,The microstructure is characterized by equiaxial and lamellar martensite laths within martensite blocks without martensite packets.When the width of the UFLS-MPA is greater than 600 nm,one martensite packet is formed perpendicular to the rolling direction,which contains sevral martensite blocks.The block is characterized by conventional martensite laths and parallel lamellar martensite laths elongated to the directions 30°～60° away from the rolling direction.The coarse equiaxed MPA of S1 low carbon steels and C1 medium carbon steels were transformed into UFLS MPA along the rolling direction after HWR process,the average widths of the UFLS austenite lamellars are 461 nm for S1 low carbon steels and 780 nm for C1 medium carbon steels,respectively.The numbers and types of martensitic variants after HWR are limited by the width of the UFLS austenite lamellar: there are at least 4 martensitic packets transformed from the quenched coarse equiaxed austenite grain,and each martensitic packet contains 6 varients,24 different martensitic variants(V1～V24)were observed from the quenched coarse equiaxed austenite grain.When the width of the warm-rolled UFLS-MPA is greater than 600 nm,only one martensitic packet containing 6 martensitic variants transformed from the UFLS-MPA grain.When the width of the warm-rolled UFLS-MPA is in the range of200-600 nm,only several martensitic blocks containing 2～5 martensitic variants transformed from the UFLS-MPA grain.When the width of the warm-rolled UFLS-MPA is less than 200 nm,only one martensitic lath containing a martensitic variant transformed from the UFLS-MPA grain.The heavily warm-rolled S1 low carbon steels and C1 medium carbon steels were tempered at 300 ℃～700 ℃ for 1 h.The results show that the strength of the warm-rolled S1 low carbon steels increases firstly and then decreases with increase of tempering temperature.After tempering at 400 ℃,the warm-rolled S1 low carbon steel exhibits the best comprehensive mechanical properties,yield strength of 1805 MPa,tensile strength of 2062 MPa and elongation of 9.0%.The yield strength and elongation of the WQ-C1 medium carbon steel are 2148 MPa and 1.6%.However,the yield strength,tensile strength and total elongation of the warm-rolled C1 medium carbon steel are 2149 MPa,2932 MPa and 5.2%,respectively.During the process of tempering treatment,the strength of warm-rolled C1 medium carbon steel decreases continuously with the increase of tempering temperature,and the plasticity increases with the increase of tempering temperature.The increase of strength is mainy attributed to grain boundary strengthening,precipitation strengthening and dislocation strengthening,while the increase of ducility is mainly attributed to the refinement of martensite and existence of residual austenite.In this paper,70% cold rolling was carried out on the heavily warm-rolled S1 low carbon steels,followed by cycles heat treatment.The results show that the cold-rolled UFLS MPA were transformed into ultrafine equiaxed austenite.The largest prior austenite grain size is greater than 10 μm,and the minimum prior austenite grain size is less than 200 nm.The transformed martensite exhibits two morphologies: one equiaxed martensite lath is transformed from the MPA with grain size less than 200 nm,which not contains traditional martensite multi-level layered microstructures;ultrafine martensitic laths with unidirectional parallel distribution are transformed from the MPA with grain size more than 200 nm.The width of martensitic lath is not uniform,the narrower is about ten nanometers,and the wider is tens of nanometers.The numbers and types of martensite variants transformed from the ultrafine equiaxed MPA are significantly reduced.When the grain size of MPA is less than 200 nm,there is one only single martensite variant.