1300 MPa High Strength Steel for Bolt with Superior Delayed Fracture Resistance

By the increase in Mo content,the addition of microalloying elements V and Nb and by reducing the contents of Mn,P and S based on the composition of steel 42 CrMo,we have developed a 1 300 MPa-grade high strength steel(ADF1)for bolts.The sustained load bending test,sustained load tensile test and stress corrosion cracking test have been carried out to evaluate the delayed fracture resistance of steel ADFl and commercial steel 42 CrMo.The results showed that steel ADF1 has superior delayed fracture resistance to that of 42 CrMo steel.It's concluded that the superior delayed fracture resistance of ADF1 is mainly due to the increase of tempering temperature,fine homogeneously distributed MC carbide and fine prior austenite grain size.

汽车螺旋悬挂弹簧用钢的发展动向

介绍了轿车螺旋悬挂弹簧的减重要求和弹簧设计应力的变化 ,提出了弹簧钢高应力化的主要途径 ,并介绍了国内外高强度弹簧钢的研究和开发情况。与工业发达国家不断推出新工艺和新钢种相比 。

Surface Ferrite Grain Refinement and Mechanical Properties of Low Carbon Steel Plates

Considering the specialities of the steel plate production, the TMCP study has been carried out with Gleeble 2000 tester to explore the possibility of fine grained ferrite in the low carbon steel plates with the chemical composition of C 0.13--0.18, Si 0.12-0.18, Mn 0.50-0. 65, P 0. 010-0. 025, and S 0. 005-0. 028. The plates with thickness of 8. 7 mm in which the ferrite grain size is smaller than 8μm have been produced by special de- formation process in the laboratory. Furthermore, the trial production of special plain carbon steel plates of 16-25 mm in thickness and 2 000- 2 800 mm in width with fine grained ferrite has been successfully carried out in the Shougang Steel Plate Rolling Plant. The ferrite grain size is 5.5-7μm in the surface layers and 9.5-15μm in the central layer respectively. The yield strength is 320- 360 MPa, tensile strength is 440-520 MPa and the elongation is 25%- 34 %. It is very important for the rolling plants to improve the low carbon steel plates＇ mechanical properties. The results show that the ferrite grains in the surface layer can be refined effectively by the appropriate rolling process, and the strength can be also increased.

Effect of Heat Treatment on Delayed Fracture Resistance of High Strength Steel 30CrMnSi2NiNb

The steel specimens of nominal composition 0.3C-1.0Cr-1.0Mn-2.0Si-1.0Ni-0.04 Nb were quenched and tempered or isothermally quenched from various temperatures.It is found that the steel quenched and tempered with a tensile strength of 1 500-1 600 MPa has a KISCC（critical stress intensity factor） value below 15.0 MPa · m1/2.The steel isothermally quenched with a tensile strength of 1 350-1 750 MPa has a KISCC value about 20.0 MPa·m1/2.In addition,with increase of isothermal quenching temperature,the tensile strength decreases greatly and KISCC value does not pronouncedly change.The microstructure of isothermally quenched specimens is composed of bainite and retained austenite.The delayed fracture resistance is dependent on the stability of austenite,which is in turn related to the retained austenite volume fraction and carbon content in austenite.

High-cycle Fatigue Fracture Behavior of Ultrahigh Strength Steels

The fatigue fracture behavior of four ultrahigh strength steels with different melting processes and therefore different inclusion sizes were studied by using a rotating bar two-point bending fatigue machine in the high-cycle regime up to 107 cycles of loading. The fracture surfaces were observed by field emission scanning electron microscopy （FESEM）. It was found that the size of inclusion has significant effect on the fatigue behavior. For AtSI 4340 steel in which the inclusion size is smaller than 5.5 μm, all the fatigue cracks except one did not initiated from inclusion but from specimen surface and conventional S-N curve exists. For 65Si2MnWE and Aermet 100 steels in which the average inclusion sizes are 12.2 and 14.9 μm, respectively, fatigue cracks initiated from inclusions at lower stress amplitudes and stepwise S-N curves were observed. The S-N curve displays a continuous decline and fatigue failures originated from large oxide inclusion for 60Si2CrVA steel in which the average inclusion size is 44.4 pro. In the case of internal inclusion-induced fractures at cycles beyond about 1×10^6 for 65Si2MnWE and 60Si2CrVA steels, inclusion was always found inside the fish-eye and a granular bright facet （GBF） was observed in the vicinity around the inclusion. The GBF sizes increase with increasing the number of cycles to failure Nf in the long-life regime. The values of stress intensity factor range at crack initiation site for the GBF are almost constant with Nf, and are almost equal to that for the surface inclusion and the internal inclusion at cycles lower than about 1×10^6. Neither fish-eye nor GBF was observed for Aermet 100 steel in the present study.

Effect of Boron on Delayed Fracture Resistance of Medium-Carbon High Strength Spring Steel

The delayed fracture behavior of medium carbon high strength spring steel containing different amounts of boron （0. 000 5%, 0. 001 6 %） was studied using sustained load delayed fracture test. The results show that delayed fracture resistance of boron containing steels is higher than that of conventional steel 60Si2MnA at the same strength level and it increases with the increase of boron eontent from 0. 000 5% to 0. 001 6%. The delayed fracture mode is mainly intergranular in the boron containing steels tempered at 350℃, which indicates that the addition of boron does not change the fracture character. However, the increase of boron content enlarges the size of the crack initia tion area. Further study of phase analysis indicates that most boron is in solid solution, and only a very small quantity of boron is in the M3 （C, B） phase.

Microstructure of Ni-Based Self-Fluxing Alloy Coating(PartⅠ)

The delayed fracture behaviors of CrMo-type high strength steels containing different amount of titanium(0to 0.10%)were studied.The steels were quenched at 880℃ and tempered from 400℃ to 650℃,and a wide range of tensile strength was obtained.The sustained load tensile test was carried out by using notched tensile specimens in Walpole solution.The experimental results showed that with higher strength,the Ti-microalloyed steels show higher resistance to delayed fracture compared with non-microalloyed steel due to titanium beneficial role and microstructure changes.The undissolved TiC is uniformly distributed as strong hydrogen traps,retarding or preventing the diffusion and accumulation of hydrogen to lower-interaction energy sites,such as prior austenite and martensite lath boundaries in stress concentration area.Meanwhile,the grain refining effect of titanium is also an important factor to improve the delayed fracture resistance of Ti-microalloyed steels.The characteristics of delayed fracture remain nearly the same with titanium addition.

Present Situation and Thoughts of Education and Training in China's Refractories Industry

In recent years, China has fully developed in output, quality, and technical advancement of refractories, and become the biggest country in refractories production and consumption in the world. Meanwhile, education and training in China＇s refractories industry have developed rapidly. This paper introduces the present situations of undergraduate education, postgraduate education, academic education, and vocational and other training of China＇s refractories industry＇, including the student recruiting situation, subject arrangement, teachers .force, and R ＆ D equipment level, and also puts forward some suggestions for China＇s refractories education and training.

Microstructures and mechanical properties of the third generation automobile steels fabricated by ART-annealing

The mechanical properties dependence on the microstructure was reviewed and analyzed,and the ultrafine grained duplex microstructure of BCC matrix and large fractioned austenite was given as one of the optimum structures to develop the third generation steel with high strength and high ductility.The medium-Mn steels with different carbon contents processed by austenite reverted transformation(ART-annealing) were studied thoroughly to fabricate the ultrafine duplex steels with large fractioned austenite.The lamellar typed ultrafine structure,the granular typed ultrafine duplex structure and the corresponding mechanical properties of the medium-Mn steels processed by ART-annealing were demonstrated in this paper.It was revealed that the duplex structure with large fraction of austenite and ultrafine grain structure is capable of producing steels with excellent combination of strength and ductility,i.e.,Rm A about 30-50 GPa%,which is about two times of that of the conventional automobile steels and close to that of the TWIP steels.It was concluded that the ART-annealing of the medium-Mn steels would be at least one of the promising ways to fabricate the third generation automobile steels in the near future.

Mechanical Properties of Steels Treated by Q-P-T Process Incorporating Carbide-free-bainite/martensite Multiphase Microstructure

Quenching partitioning tempering （QP-T） process were applied to a quenched carbide-free bainite/mar- tensite （CFB/M） multiphase steel 40Mn2Si2Cr and its effect on the mechanical properties was studied. The results showed that the partitioning time of Q P T treatment can he increased greatly to 90 min or even longer. The tensile strength, elongation and product of strength and elongation （PSE） of the experimental steel can achieve 1650 MPa, about 20% and over 33 GPa ·%, respectively. The significant increase of partitioning time and excellent mechanical properties could be related to the CFB microstrueture introduced in the quenching process during the Q-P-T treat- ment.

Third generation high strength low alloy steels with improved toughness

On-line thermo mechanical controlled processing(TMCP) was conducted to develop the third generation high strength low alloy(HSLA) steel with high toughness economically.The ultra-low carbon content ensured a high level of upper shelf energy while ultrafine lath martensitic structure transformed from super-thin pancaked austenite during controlled rolling and cooling.The reduction of martensite block size decreased ductile-to-brittle transition temperature(DBTT) and compensated the strength loss due to carbon reduction.Consequently,the excellent balance of strength and toughness values was obtained as 950-1060 MPa for yield strength,180 J for Charpy V-notch impact absorbed energy at 30℃,which is much superior to that of traditional martensitic steel.Two mechanisms for the refinement of lath martensite block were proposed:One is the austenite grain refinement in the direction of thickness,and the other is the reduction in the fraction of sub-block boundaries with small misorientation and the increase in the fraction of block boundaries with large misorientation,possibly due to austenite hardening.

Hot Working of High Nitrogen Austenitic Stainless Steel

With hot rolling in laboratory and Gleeble thermal simulator, the hot working of a high nitrogen austenitic stainless steel （HNASS） was researched. The results showed that dynamic recovery （DRV） and dynamic recrystalli- zation （DRX） in HNASS occurred during hot working, and both of them had well-defined stress peaks in flow curves under different conditions. During hot rolling experiment at temperature from 950 to 1 050 ℃, recrystallization phe- nomenon does not take place in test material until the deformation ratio is up to 40%. Recrystallization influences remarkably the strength and ductility of material, and the test HNASS possesses better combination of strength with ductility. According to the curve of θ--α （strain hardening rate-steady state stress）, the DRX critical strain of test material was determined. Also, the activation energy of hot working was calculated to be 746.5 kJ/mol and the equation of hot working was obtained.

Thermodynamic study and syntheses of β-SiAlON ceramics

Based on the thermodynamic study of β-SiAlON and one section of Si-Al-O-N phase diagram for Si/Al=9 at 1773 K, the pure β-SiAlON (Si6-ZAlZOZN8-Z) ceramics with different Z values (viz. 1.4, 1.8, 2.2, 2.6, 3.0) were synthesized at different temperatures using the pressure-less technique. The relationship of the experimental-saturated Z value (the maximum Z value at a constant temperature) with temperature was then discussed. The experimental results indicated that there is a good linear relationship between Z value and temperature, which is Z=_10.76+0.0071T (R=0.9985). The β-SiAlON stability regions at different temperatures were exhibited in a sketch phase diagram of Si3N4-SiO2-Al2O3-AlN system. In addi- tion, the relationship between experimental-saturated Z value and the lattice parameters was also characterized, and the linear regression results a=7.604+0.02872Z and c=2.908+0.02496Z were obtained, respectively.

Conductivity properties of β-SiAlON ceramics

The thermal diffusivity and heat capacity of β-SiAION ceramics with different Z values （0.6, 1.5, 3.0） were measured by laser flash technique and differential scanning calorimetry （DSC）, respectively. The thermal conductivity of β-SiAION was calculated from thermal diffusivity, heat capacity and density and it decreased and then kept almost constant with increasing of temperature. Two different models representing effective thermal conductivity were established and the various parameters of these models were simulated based on the results from the present work. The fitted regression lines were in good agreement with the experimental values.