Development of Nb Microalloying Technology in China During 11^(th) Five-Year Period

The paper shows the development of Nb-bearing steels in China in 11th five-year period. The output of Nb-bearing steels was more than 40 million tons by the end of 2010 and FeNb consumption also increased greatly during 11th five-year period. Nb microalloyed technology is expanding to other application such as low C-high Nb technology, high C steels, forging steels.

Continuous Cooling Transformation Behavior and Microstructure Control of Nb Microalloyed Reinforcing Bars

In this paper,CCT(continuous cooling transformation)diagrams are determined for Nb-containing reinforcing bars with different Nb,Mn additions,and initial austenite grain sizes by simulating industrial conditions via dilatometry tests.It was found that coarse austenite grain size,high Mn content and Nb remaining in solution all increase hardenability of Nb-containing rebars,namely lower Ar3 for acicular phase transformation products,which leads to continuous yielding during tensile deformation when the volume fraction of acicular ferrite or bainite microstructure reaches a certain volume fraction.By coupling with actual cooling rates for different size rebars,it can be explained why bainitic structure is prone to form in the center of rebars,especially for small size rebars.In order to achieve required ferrite-pearlite microstructure,cooling strategy is optimized for industrial production.

Solution and Precipitation Behavior of Nb and Its Role in Medium Carbon Long Product Steels

Controlling of the solution and precipitation for microalloying additions of Nb,V,Ti and their combination is the basic building block of microalloyed steels.Among three microalloying elements,Nb has been regarded the most effective microalloying ingredient used for low carbon flat products due to marked strengthening effect of grain refinement and relatively weak precipitation strengthening effect by matching proper thermomechanical processing(TMP).However,Nb was not viewed as attractive as V for medium carbon long products in the past because of limited solubility during reheating process.What is more,hot working is usually carried out at high temperatures in the recrystallization regime,so it is difficult to exert conventional controlled rolling to obtain pancaked austenite,which has further affected the research and application of Nb in medium carbon long products.Because of these factors,studies of Nb in medium carbon long products were incomplete,and even some recognitions and conclusions are subjected to debate.In order to clarify the strengthening effects of Nb in medium carbon long products,the reinforcing bars have been chosen as experimental steels to clarify the role of Nb on microstructural changes along the whole hot working,cooling processes.In addition,qualitative and quantitative analyses of Nb’s state and distribution in reheating,rolling and cooling had been carried out to illustrate some singularities.

Mathematical Modelling of Carbonitride Precipitation during Hot Working in Nb Microalloyed Steels

Based on thermodynamics and kinetics, precipitation behavior of microalloyed steels was analyzed. Deformation greatly promotes isothermal carbonitride precipitation and makes C-curve shift leftwards. The position and shape of C-curve also depend on the content of Nb and N. C-curve shifts leftwards a little when N content increases and the nose temperature is raised with increasing Nb content. Deformation shortened precipitation start time during continuous cooling, raised precipitation start temperature, accelerated precipitation kinetics of carbonitrides. With decreasing the finishing temperature and coiling temperature, the precipitates volume fraction increases and strength increment is raised during hot rolling. The simulated results are in agreement with experiment results.

Effects of Deformation Conditions on Dynamic Recrystallization of a Nb Microalloyed Anti-seismic Rebar

The hot deformation behavior of a Nb microalloyed anti-seismic rebar was investigated at deformation temperatures of 950-1 100 ℃ and strain rates of 0. 01-0. 1 s- 1 on a Gleeble-3800 thermo-mechnical simulator. The flow stress-strain curves show the typical dynamic recrystallization with a peak,before reaching the steady state flow at higher deformation temperatures and lower strain rates. The constitutive equation governing the dynamic recrystallization( DRX) was obtained and the average activation energy of deformation was calculated as Q = 389. 5 kJ / mol by the regression analysis. The DRX grain size was also found to decrease with the increasing strain rate and the decreasing deformation temperature. The austenite grain size was refined from 118. 0 μm to 15. 07-40. 01 μm by DRX. The DRX grain size under diverse deformation conditions predicted by mathematical model agrees well with experimental results.

Unexpected Effect of Nb Addition as a Microalloying Element on Mechanical Properties of δ-TRIP Steels

The concept of microalloying was applied to the δ-TRIP （transformation-induced plasticity） steel to inves- tigate the feasibility of increasing the mechanical properties and understanding the effect of microalloying on the morphology and structure of the steel. A hot rolled δ-TRIP steel with three different contents of Nb （0, 0.03, 0.07 mass%） was subjected to the microstructural and mechanical examination. The high Al and Si concentration in these steels guaranteed the presence of the considerable δ-ferrite phase in the microstructure after the casting and the subsequent hot rolling. The obtained results showed that Nb dramatically affects the microstructure the dynamic re- covery and recrystallization behavior, as well as the grain shape and thus the stability of austenite after the thermo- mechanical process of hot rolling. The results also revealed an unexpected effect of Nb on the mechanical properties. The addition of Nb to the δ-TRIP steel led to a significant decrease in the ultimate strength （from 1144 to 917 MPa） and an increase in ductility （from 24% to 28%）. These unconventional results could be explained by the change in the steel microstructure. The work-hardening＇behaviors of all samples exhibit three stages of the work-hardening rate evolution. At the stage 2, the work-hardening rate of the studied steels increased, being attributed to the TRIP effect and the transformation of austenite to martensite.

Effect of Nb addition on microstructure and mechanical properties of 25CrNiMoV(DZ2)steel for high-speed railway axles

The microstructure,precipitates and properties of 25CrNiMoV(DZ2)steel for high-speed railway axles with different Nb contents were investigated by means of optical microscopy,scanning electron microscopy,electron back-scattering diffraction,transmission electron microscopy and physicochemical phase analysis.The results show that the grain size of the original austenite of the test steels decreases from 20.5 to 14.2 and 10.8μm after adding 0.026 and 0.039 wt.%Nb to a 25CrNiMoV steel,respectively.Moreover,the block width of the tempered martensite in the test steels is refined from 1.91 to 1.72 and 1.60µm,respectively.MC-type precipitates in 25CrNiMoV steel are mainly VC,while(Nb,V)C gradually precipitates when Nb is microalloyed,and the amount of precipitates increases with increasing Nb content.Through strengthening mechanism analysis,it is found that grain refinement strengthening is the primary way to increase the strength.The improvement in the yield strength with increasing Nb content is attributed to a significant increase in precipitation strengthening,grain refinement strengthening and dislocation strengthening.

Influence of Nb on Microstructure and Property of Low-Carbon Mn-Series Air-Cooled Bainitic Steel

The effect of Nb on the microstructure evolution of low-carbon Mn-series air cooled bainitic steels has been studied by using thermomeehanical simulation machine, SEM, and TEM observation. The results show that the amount of ferrites is relatively higher in the steel without Nb than that of Nb-bearing steel under same deformation conditions, and the ferrites in Nb-bearing steel are more finer. The mechanical properties of the Nb-bearing steel are higher than those of the steel without Nb at the same finishing rolling temperature （FRT）, and the toughness of the Nb- bearing steel is about 100J higher than that of the steel without Nh at the finishing rolling temperature 750℃. The toughness of Nb microalloyed steel will be improved more effectively than that of the steel without Nb with the decrease of FRT.

Effect of Cooling Patterns on Microstructure and Mechanical Properties of Hot-Rolled Nb Microalloyed Multiphase Steel Plates

The effect of the run-out table cooling patterns on the microstructure and mechanical properties of Nb microalloyed steel plates was investigated by hot rolling experiment. The results showed that the mixed microstrueture containing ferrite, bainite and significant amounts of retained austenite can be obtained through three kinds of cooling patterns on the run-out table under the same hot rolling condition. Three kinds of cooling patterns possess different austenite transformation kinetics, which leads to variations in microconstituent characteristics. The yield strength increases, the tensile strength decreases and the total elongation tends to increase as the cooling patterns Ⅰ , Ⅱ and Ⅲ were applied respectively. The yield strength, the total elongation and the product of tensile strength and ductility reach the maximum values （547 MPa, 37. 2% and 28 384 MPa% respectively） for the steel plate processed by cooling pattern Ⅲ.

Quantitative Research on Dissolving of Nb in High Nb Microalloyed Steels during Reheating

The accurate prediction of soluble Nb content during reheating is helpful for the design of chemical compo sition and reheating parameters for Nb-containing steels. The dissolution behavior of Nb in high Nb mieroailoyed steels was investigated. The results show that Nh does not entirely dissolve for high Nb microalloyed steels containing Ti after holding at 1 300 ℃ for 3 h. The soluble Nb content increases with the decrease of C content and/or the increase of Nb content in steels. Moreover, an expression has been established to describe the amount evolution of soluble Nb in high Nb microalloyed steels during reheating and the validity of this expression has also been verified by experiment.

Microstructures and Properties of X60 Grade Pipeline Strip Steel in CSP Plant

The microstructures and properties of X60 grade pipeline strip steel were researched.With Nb microalloying and TMCP,the X60 grade pipeline strip steel was developed in the Baotou CSP Plant.The mechanical properties meet the demands on machining and using of pipeline manufacture.The X60 strip steels had been used extensively,which had fine and uniform structures,good performance and excellent toughness.