The mechanical properties and microstructure of Mn-series bainitic steel as 5Mn2SiNbTi after air cooling and tempering have been investigated in this paper. The results show that the investigated steel show high strength (tensile strength up to 671MPa), low toughness (20J at -30°C) after hot rolling (final rolling at 1050°C) and air cooling. Medium temperature tempering can improve the toughness, without sacrificing strength of the steel. After tempering at 450°C for 1h, the steel obtained 632MPa tensile strength, 467MPa yield strength, 140J impact energy at -30°C, remaining 25.0% elongation. It is indicated that the tested steel can gain excellent combination of strength and toughness without precious alloy adding, control rolling and control cooling.

The effect of 0.06%Nb on the microstructure and mechanical properties of grain boundary allotriomorphic ferrite (F-GBA) / granular bainite (B-g) air-cooling bainitic steels has been investigated in this paper. The results indicate that the steel acquires superior mechanical properties by adding 0.06%Nb. Compared with Non-Nb steel, the addition of 0.06%Nb increases the tensile strength and yield strength about 37.1% (From 780MPa to 1070MPa)and 26.6%(From 557MPa to 705MPa) respectively, remaining 18.3% elongation and 97J toughness. The addition of 0.06%Nb not only promotes the nucleation of intragranular ferrite but also refines the allotriomorphic ferrite grain, both of which in turn contribute to the refinement of granular bainite cluster including its ferrite platelets and M-A islands. Under the synthetic roles of the microstructure refinement and precipitation strengthening, 148MPa yield strength improvement has been acquired in the low carbon air-cooling bainitic steel by the adding of 0.06%Nb.

The alloying design idea,strengthening-toughening mechanism,microstructure,mechanical performances,development and application in China of new type Mn-series bainitic steels are introduced.Mn-series air-cooling bainitic steels including granular bainitic steels,FGBA /BG duplex steels,CFB/M duplex steels,medium carbon bainite/martensite steels,cast bainitic steels are presented.The invented idea mechanical performances,development and application of second generation of Mn-series bainitic steels,i.e.water quenching Mn-series bainitic steels invented by the authors newly are introduced.The water quenching Mn-series bainitic steels cover severe series steels containing ultra-low carbon,low-low carbon,medium-low carbon,and high-low carbon content etc,which can reduce the amount of alloying content,increase hardening capability and improve weldability.It should be pointed out that the application of both air cooling and water quenching Mn-series bainitic steels are complementary and mutually reinforcing,and the new type Mn-series bainitic steels can meet the performance requirements of most steels used in engineering structure.Some newest technologies of Mn-series bainitic steels in China are discussed in this paper.It is suggested that the significance of the development of the Mn-series bainitic steels can be summarized as:significantly reducing costs of both raw materials and production;good combination of strength and toughness;excellent weldability;simple procedure;large savings in energy resources and reduced environmental pollution.

The effect of rare earth (Re) and titanium (Ti) multi-modification on the impact wear behavior of Mn-B high-Si bainitic cast steel was investigated systematically. The experiments show that the impact wear resistance can be improved greatly with the addition of Re and Ti. Its wear loss is only about 1/3-1/2 as large as that of the unmodified bainitic cast steel. By the Re/Ti modification, coarse dendrite grains and bainitic/martensite duplex structure have been refined effectively, and the impact toughness of the bainitic cast steel is nearly tripled (10 mm×10 mm×55 mm, with unnotched sample). Consequently, the modified bainitic cast steel possesses good wear resistance under high impact. For both modified and unmodified bainitic cast steels, high hardness white layer and deformed zone are developed beneath the worn surface under the high impact wear, but the formation and propagation of cracks are different for these bainitic casting steels. Different models for the formation and propagation of cracks for both modified and unmodified bainitic cast steels under high impact wear are proposed.

The effect of different Nb additions on the mechanical properties and microstructure evolution of grain boundary allotriomorphic ferrite (FGBA) / granular bainite (BG) air cooling bainitic steels was investigated. The results indicate that the tensile strength and yield strength increase by 157 and 97 MPa, respectively with the addition of 0.02wt% Nb. The steel acquires superior strength and toughness with the addition of 0.06wt% Nb. The results of scanning electron microscopy and transmission electron microscopy reveal that the addition of Nb not only refines the size of granular bainite but also increases the volume fraction of granular bainite in FGBA/BG steels. The refinement effect of granular bainite is improved with the increase of Nb content.

The origin and development of air-cooled Mn-series bainite steels are introduced. The invented idea, strengthening-toughening mechanism, mechanical performances, development and application of this kind of steel including granular bainitic steels, FGBA / BG duplex steels, CFB/M duplex steels, medium carbon bainite/martensite steels, cast bainitic steels are presented. The invented idea mechanical performances, development and application of second generation of Mn-series bainitic steels, i.e. water-quenched Mn-series bainitic steels invented by the authors newly are introduced. The water quenched Mn-series bainitic steels can meet the performance requirements of most steels used in engineering structure, reduce the amount of alloying content, increase harden capability and improve weldability. It should be pointed out that the application of both air-cold and water- quenched Mn-series bainitic steels are complementary and mutually reinforcing. Some newest technology of Mn-series bainitic steels in China are discussed in this paper. It is suggested that the significance of the development of the Mn-series bainitic steels can be summarized as: significantly reducing costs of both raw materials and production; good combination of strength and toughness; excellent weldability; simple procedure; large savings in energy resources and environmental pollution is reduced.

A new hot rolled low-carbon air-cooled bainitic steel containing Nb of 0.02% has been developed based on alloying design of the grain boundary allotriomorphic ferrite (Fgba)/granular bainite (Bg) duplex steel. The as-rolled microstructure and mechanical properties of bainitic steel containing Nb of 0.02% were investigated by tensile test, optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that adding 0.02% of Nb obviously improves the strength without sacrificing toughness of the Fgba/Bg steel. Adding 0.02% of Nb not only refines the grain boundary allotriomorphic ferrite grains but also promotes the refinement of granular bainite including its bainitic ferrite and M/A island. Any Nb(C,N) has been hardly observed in the steel containing Nb of 0.02%. It is suggested that the strengthening mechanism of Nb of 0.02% can be mainly attributed to the effect of Nb in solution (solute drag-like effect) on the phase transformation rather than the precipitation strengthening of Nb(C,N) particles.

In order to produce Mn-series low carbon steels using alloying elements with low cost and following the conventional production line without controlled cooling facilities and not needing special heat-treatment, Mu-series low carbon air cooling grain boundary allotriomorphic ferrite (F-GBA)/granular bainite (B-G) steels, alloying with Si-Mn-Cr, have been developed. The effect of 0.02%Nb on the phase transformation and strength-toughness of a F-GBA/B-G steel have been investigated by using Gleeble-1500D machine and hot rolling test in this paper. Two specimens with and without Nb addition were deformed by 21% at 850 degrees C and then air cooling to room temperature. For Nb free steel, the average size of F-GBA is 25 mu m in length and 10 mu m in width. The average size of intragranular ferrite is about 6-8 mu m. For F-GBA/B-G steel with 0.02%Nb, the average size of F-GBA is 10 mu m in length and 4 mu m in width. The average size of intragranular ferrite is less than 4 tan. Compared with Nb free steel, the size of bainitic ferrite and martensite/austenite (M-A) island decrease and their corresponding volume fractions increase in the FGBA/BG steel with 0.02%Nb. The experimental results indicate that the addition of 0.02%Nb increases the hardenability of the F-GBA/B-G steel, suppresses the transformation of gamma -> alpha, refines the size of grain boundary allotriomorphic ferrite, promotes the granular bainitic transformation, lowers the bainitic start temperature (B-s), and refines the granular bainite including its bainitic ferrite and M-A islands. it is suggested that the effects of structure refinement and increase of strengthening phases improve the strength of the F-GBA/B-G steel with 0.02%Nb. Compared with typical Nb free F-GBA/B-G steel after hot rolling and air cooling, it is found that the tensile strength and yield strength of the F-GBA/B-G steel with 0.02Nb% rise to 157 MPa and 97 MPa respectively. Moreover, some possible reasons of structure refinement and increase of strengthening phases induced by 0.02%Nb addition have been disscussed in this paper.

The development and mechanical performances of new type air-cooled Mn series bainitie steels including granular bainitie steels, FGBA/BG duplex steels, CFB/M duplex steels, medium carbon bainite/martensite steels, cast bainitie steels invented by the authors are summarized. The novel series of bainitie steels are alloyed with Mn, and several series bainitie duplex microstructures can be easily obtained under the condition of air cooling through unique composition design. The invented idea, the principle of alloying design, the strengthening mechanism, and the evolution of the microstructure of new type air-cooled Mn series bainitie steels are presented. Furthermore, the applications in different fields of these Mn series air-cooled bainitie steels with different strength level are also introduced. It is suggested that the significance of the development of the air-cooled Mn series bainitie steel can be summarized as follows: reducing costs of both raw materials and production; good combination of strength and toughness; self-hardening with high bainitie hardenability by air cooling from hot working without additional quenching-tempering treatment or quenching procedure; large savings in energy resources; and reduced environmental pollution.

The invention idea, following development ,principle of alloying design, strengthening mechanism ,evolution of the microstructure, and the unique advantage of new type air-cooled Mn-series bainitic steels are pre-sented briefly in this paper.

The development and its mechanical performances of new type air-cooled Mn-series bainitic steels including granular bainitic steels, F-GBA/B-G duplex steels, CFB/M duplex steels, medium carbon bainite/martensite steels, cast bainitic steels invented by the authors are summarized. The novel series of bainitic steels are alloyed with Mn and several series bainitic duplex microstructures can be easily obtained under the condition of air cooling through unique composition design.The invented idea, principle of alloying design, strengthening mechanism and the evolution of the microstructure of new type air-cooled Mn-series bainitic steels are presented in this paper. Furthermore, the application in different fields of these Mn-series air-cooled bainitic steels with different strength level are also introduced.It is suggested that the significance of the development of the air cooled Mn-series bainite steel can be summarized as: reducing costs of both raw materials and production; good combination of strength and toughness; self-hardened with high bainitic hardenability by air cooling from hot working without additional quenching-tempering treatment or quenching procedure; large savings in energy resources; environmental pollution is reduced.

By using three groups of experiment,it is shown that the lower bainitic carbides can nucleate in theγside ofα/γinterface or between two bainitic ferrite sub-units and grow towards austenite.Morphologically,the carbides can exist inside the a or pass across the interface ofα/γorα/α;The carbides grow competitively with the ferrite during the whole transformation process,and the carbides are precipitated from austenite side of theα/γ,interface,meanwhile the growing rate of ferrite is higher than that of carbides,as a result,the carbides are surrounded by ferrite,resulting in a false inpression that the lower bainitic carbides are precipitated from bainitic ferrite;The nucleation and growth model of lower bainitic carbides is proprosed on the basics of thermodynamics and the ledge-wise growth theory;It is suggested that the lower bainitic carbides precipitated from carbon- enriched retained austenite rather than supersaturated ferrite,which directly concerns the mechanism of bainitic transformation.

清华大学后勤改革已经历了20余年的探索与实践,学校后勤面貌发生了翻天覆地的变化.

保护环境,走可持续发展之路,是当前世界各国面临的共同课题.中国作为一个发展中的大国,经过改革开放后20年的建设与发展,国家的综合实力空前增强,人民生活水平显著提高.然而,粗放型的经济增长方式使我们为此付出了巨大的生态环境代价,造成严重的环境污染和生态破坏.

The high stress-wear resistance and mechanism for bainitic cast steels with different Si content (0.7 %～2.4 %) have been studied systematically.The experiments have shown that the wear loss of high Si bainitic cast steels is only about 1/2 times as that of low Si baini tic cast steels, showing better wear resistance of the former under high stress impact.The reason is that the impact wear mechanism is different for low and hi gh Si bainitic cast steels.Low Si bainitic cast steel has poor wear resistance under high stress impact because of its low toughness, coarse dendrite and micro -segregation, etc.So the damage cracks are easily formed in the wear surface b efore formation of white layer, and propagate inside the deformation zone and ma trix, exhibiting mechanism of deformation zone and matrix delamination.The imp act toughness of high Si bainitic cast steels are improved apparently for the re ason that the brittle cementite is replaced by ductile retained austentite film, resulting in the brittle delamination of white layer.

A new high Si and low carbon aircooled bainite steel (HSLCACBS) for coalexcavator pickers was designed to overcome the shortcomings of 35CrMnSi pickers, such as high raw material price, complex heat treatment, huge energy expense, serious environmental pollution and short lifespan. The composition (mass fraction, 100× w ) of the new material is C 0.19~0.27, Mn 2.0~4.0, B 0.0015 ~0.003, Si 0.7~1.6. Higher Si content increases the hardening ability and the toughness after aircooling. Lower carbon content greatly increases the toughness and ability to resistant fracture of the pickers. Cooling in air after austernization at 1163K produces the bainite or bainite/martensite composite microstructure with both high hardness and high toughness. All the HSLCACBS picker properties reach or exceed the regulations. The aircooling pickers have several advances, such as simple processing, no saltbath quenching, energy savings, reduced cost (10%) and reduced pollution. The results obtained in the testing mine show that 124 HSLCACBS pickers are needed for every 10000 tons of coal, 8.8% less than for the original 35CrMnSi pickers, with no bending and breaking of the picker handles.