Effect of Mo content on the thermal stability of Ti-Mo-bearing ferritic steel

The effects of tempering holding time at 700℃on the morphology,mechanical properties,and behavior of nanoparticles in Ti-Mo ferritic steel with different Mo contents were analyzed using scanning electron microscopy and transmission electron microscopy.The equilibrium solid solution amounts of Mo,Ti,and C in ferritic steel at various temperatures were calculated,and changes in the sizes of nanoparticles over time at different Mo contents were analyzed.The experimental results and theoretical calculations were in good agreement with each other and showed that the size of nanoparticles in middle Mo content nano-ferrite(MNF)steel changed the least during aging.High Mo contents inhibited the maturation and growth of nanoparticles,but no obvious inhibitory effect was observed when the Mo content exceeded 0.37wt%.The tensile strength and yield strength continuously decreased with the tempering time.Analysis of the strengthening and toughening mechanisms showed that the different mechanical properties among the three different Mo content experiment steels were mainly determined by grain refinement strengthening(the difference range was 30-40 MPa)and precipitation strengthening(the difference range was 78-127 MPa).MNF steel displayed an ideal chemical ratio and the highest thermodynamic stability,whereas low Mo content nano-ferrite(LNF)steel and high Mo content nano-ferrite(HNF)steel displayed relatively similar thermodynamic stabilities.

Carbide Precipitation in Ferrite in Nb–V-Bearing Low-Carbon Steel During Isothermal Quenching Process

The precipitation behavior of nanometer-sized carbides in ferrite in Nb-V-bearing low-carbon steel was studied by electron microscopy and nanoindentation hardness measurements. The results indicated that interphase precipitation and random precipitation could occur simultaneously for the specimen isothermally treated at 700 ℃ for 60 min, while in other specimens, only random precipitation was observed. This phenomenon might be explained by mass balance criterion during the diffusional phase transformation. Nanohardness result indicated that the average hardness of the specimens isothermally held at 600 ℃ for 20 min was 3.87 GPa. For the specimen isothermally holding at 650 ℃ for 20 min, the average hardness was 4.10 GPa and the distribution of the nanohardness was in a narrower range compared with that of the specimen isothermal holding at 600 ℃ for 20 min. These implied that the carbides in the specimens isothermal treated at 650℃ were more uniformly dispersed, and the number density of the carbides was greater than that treated at 600 ℃. Using Ashby-Orowan model, the contribution of precipitation strengthening to yield strength was estimated to be 110 MPa for the specimen isothermally treated at the temperature of 650 ℃ for 20 min.

Precipitates in Steels With Ti Additive Produced by CSP Process

Hot strips of low carbon steels with Ti additive [-contain C 0.04 % -0. 07 % , Si≤0.6%, Mn≤0.6%, Ti 0. 060/00- 0.14% （mass percent）] prodvced by EAF-CSP （Electric Arc Furnaces-Compact Strip Production） process were examined by TEM, HREM and XRD. Carbonitrides with different N/C ratio were found in the sam- ples. The varying composition of the Ti-carbonitrides resulted from the supersaturation of Ti and temperature at which the compound was formed. In the tested steel, total mass fraction of the precipitates including cementite, carbonitride and a small quantity of Fe3O4, AO2O3 , Ti2 CS and A1N was about 0. 305 %. XRD results showed that about a quarter of the powder extracted by electrolysis was titanium nitrides, carbonitrides and carbides. Particle arrays formed by interphase precipitation could be observed either in slabs or in hot strips. The dominant reaction mecha- nisms were discussed. Compared with the conventional cold charge process, small amount of Ti addition would be more effective for orecipitation of fine orecioitates in the steels oroduced by CSP process.

Influence of Finishing Cooling Temperature and Holding Time on Nanometer-size Carbide of Nb-Ti Microalloyed Steel

The nanometer-size carbides formed in ferrite matrix of Nb-Ti microalloyed steel at different finishing cooling temperatures and holding time have been investigated. The characteristics of nanometer-size carbides in ferrite were observed by transmission electron microscopy, and mechanical properties of ferrite were detected by a nano-hardness tester. The results showed that interphase precipitation and diffusion precipitation were observed at different finishing cooling temperatures, and the interphase precipitation was planar and curved. Sheet spacing of inter-phase precipitation increased with the increase of finishing cooling temperature and changed a little when holding for 50--1000 s. Interphase precipitation shows higher nano-hardness at 640℃ compared with diffusion precipitation at 600℃, and the contribution of interphase precipitation to the mechanical properties of ferrite was larger than that of diffusion precipitation.

Advanced HSLA Steels for Automotive Use

Characteristic HSLA steels for automotive use are introduced in which both of precipitation and microstructure is controlled to obtain suitable mechanical properties.For outer panels such as fender,the combination of low yield strength,high tensile strength and deep-drawability were realized by controlling the distribution of NbC and precipitation free zone.The other steel,developed for chassis parts such as lower arm,utilizes extremely fine interphase precipitation to obtain high yield strength and excellent hole expansionability.Both steels have contributed to the reduction of weight in car body.