Effects of Mn on Microstructures and Properties of Hot Rolled Low Carbon Bainitic Steels

Two kinds of Mn-Si-Mo low carbon steels were designed to study the effects of Mn on the microstructures and properties of hot rolled low carbon bainitic steels.To reduce the production cost,a very low Mo content of 0.13%was added in both steels.After hot rolling,the mechanical properties of samples were tested.Microstructure was observed and analyzed by optical microscope and transmission electron microscope.The results show that the strength of tested steels increases with the increase in Mn content,while the elongation decreases.When Mn content increases,the bainite microstructure increases.The results can provide a theoretical basis for composition design and industrial production of low cost low carbon bainitic steels.

Continuous Cooling Transformation Behavior and Kinetic Models of Transformations for an Ultra-Low Carbon Bainitic Steel

The aim was to investigate transformation behavior and transformation kinetics of an ultra-low carbon bai- nitic steel during continuous cooling. Continuous cooling transformation （CCT） curves of tested steel were measured by thermal dilatometer and metallographic structures at room temperature were observed by optical microscope. Then transformation kinetic equation of austenite to ferrite as well as austenite to bainite was established by analyzing the relationship of lnln]-l/（1--f）] and lnt in the kinetic equation on the basis of processed experimental data. Finally, the measured and calculated kinetic behaviors of the steel during continuous cooling were compared and growth pat- terns of transformed ferrite and bainite were analyzed. Results showed that calculated result was in reasonable agree- ment with the experimental data. It could be concluded that the growth modes of transformed ferrite and bainite were mainly one dimension as the Avrami exponents were between 1 and 2.

Effect of boron addition on the microstructures and mechanical properties of thermomechanically processed and tempered low carbon bainitic steels

Thermomechanical process and tempering heat treatment were employed to produce the experimental steel plates. The effect of boron addition on the microstructure and mechanical properties of low carbon bainitic steels was studied in this paper. Microstructure observation and crystallographic features were conducted by using optical microscopy, SEM, TEM and electron back scattering diffraction （EBSD） analysis. The results showed that under the same rolling processes and heat treatment conditions, a substantial increase in strength is obtained by addition of boron into steel, but accompanied by an obvious drop in toughness. New martensite phase forms along the grain boundaries on tempering at 650℃ mainly due to boron segregation, which can further deteriorate impact toughness of the boron bearing steel. The EBSD analysis showed that high angle grain boundary, is not responsible for the deteriorated toughness of the boron bearing steel because it has relatively higher percentage of high angle grain boundary than the boron free steel. The low toughness of the boron bearing steel is mainly attributed to the coarse boride precipitated particles according to the results of fractograph observation.

Study on Microstructure Refinement of Vanadium and Nitrogen Microalloyed Low Carbon Bainitic Steel

The effect of relaxation treatment after finish rolling on microstructure and mechanical properties has been investigated for a vanadium and nitrogen microalloyed low carbon bainitic steel.Finer lath bainite microstructure can be obtained in the plate with relaxation.The results of quantitative statistics show that in the plate without relaxation,80% of the total bainite lath bundles are in the range 5-15μm in length and 3-13μm in width,while in the plate with relaxation 80% of the total bundles are in the range 3-9μm in length and 1-7μm in width.The mechanical properties show that the plate with relaxation has higher impact energy,yield strength and hardness than the plate without relaxation,also the comprehensive performance after tempered at 650℃ is superior to the plate without relaxation.

Evolution of Microstructures of a Low Carbon Bainitic Steel Held at High Service Temperature

Low carbon bainitic steel derives the high strength mainly from high density of dislocations rather than carbon and alloy element content, so it tends to evolve into equilibrium microstructure with low density of dislocations under thermal disturbance. In the present investigation, granular bainite and lath-like bainitic ferrite were produced respectively in Mo-free low-carbon steels by changing cooling rate;. It has been found that granular bainite possesses a lower strength at room temperature than bainitic ferrite, but it exhibits a slower decrease of strength with temperature increasing. Dislocation density in both granular bainite and bainitic ferrite decreases via recovery and recrystallization at high temperature. However, when reheating of bainite is carded out at temperature below 600 ℃, a long time will be needed for incubation of recrystallization, during which the hardness of bainite maintains stable. The property makes bainite, especially granular bainite, become a potential microstructure for matrix of high strength fire-resistant steel.

Effect of zirconium addition on the austenite grain coarsening behavior and mechanical properties of 900 MPa low carbon bainite steel

The ultra-fine bainitic microstructure of a 900 MPa low carbon bainitic Cu-Ni-Mo-B steel was obtained by a newly developed relaxation precipitation control （RPC） phase transformation processing. In a pan-cake like prior-austenite grain, the micro- structure consisted of lath bainite, a little of abnormal granular bainite, and acicular ferrite. The effect of zirconium carbonitrides on the austenite grain coarsening behavior was studied by transmission electron microscopy （TEM）. The results show that, the lath is narrower with increasing cooling rate. The ratio of all kinds of bainitic microstructure is proper with the intermediate cooling rate; and Zr-containing precipitates distribute uniformly, which restrains austenite grain growing in heat-affected welding zone.

Effect of Tempering Temperature on Microstructure and Properties of E690 Offshore Plate Steel

A new kind of Mn Mo Nb Cu B bainite steel which satisfied mechanical demands of E690 offshore plate steel was designed. The effect of two processes thermomechanical control process （TMCP）＋ tempering （T） and thermomechanical control process （TMCP） ＋ reheating and quenching （RQ） ＋ tempering （T） on microstructure and mechanical properties were studied by means of scan electron microscope （SEM）, transmission electron micro- scope （TEM） and electron back scatter diffraction （EBSD）. The results showed that optimal mechanical properties were available when tempering at 550 ℃ for both processes. The microstructure of the TMCP＋T treated sample tempering in the range of 450 to 550 ℃ for 1 h did not change dramatically yet the lath in the TMCP＋RQ＋T trea- ted sample merged together and transformed into polygonal ferrite. At the same time, the sub-structure of grain bainite transformed from lath to cell shape to refine grains with tempering temperature mounting. Lots of sub-grain boundaries were located within bainite and adjacent bainite grain boundaries were high angle.