| As China is in the industrialization process,steel industry plays an important role in promoting the national economy.Nevertheless,steel industry is faced with issues including increasingly severe shortage of resources and energy,environmental degradation in China.Thus,production reduction and energy saving in production are of great importance in promoting the sustainable development of economy.Achieving high-strength iron and steel materials with low cost micro alloy system combined with controlled rolling and controlled cooling is the major method for saving energy and resources,protecting environment,and reduction.Additionally,it is the development trend of the new generation of alloy steel.In the paper,pig iron containing boron was used as boron adding agent,and properties of C-Mn steel(Q235 steel and 45 steel)were improved by boron micro-alloyed that combined with controlled rolling,controlled cooling,quenching and tempering process.The influence boron and boron nitrogen compounds exerted on the austenite grains of C-Mn steel is studied through different chemical compositions,and high temperature deformation behavior of austenitic and phase transition behavior of the continuous cooling process were investigated.Based on the simulated rolling,Q235 steel with boron micro-alloyed was experimented after controlling rolling and cooling,and No.45 steel with boron imicro-alloyed was experimented through quenching and tempering under different temperatures.Also,preliminary exploration was conducted to study the mechanical properties of phosphorus bearing steel and copper bearing steel added boron.The main content and innovations in this paper are as follows.(1)Effect of different amounts of boron and addition of titanium on the original austenite grains was studied through thermodynamic calculation and analysis of the formation law of boron,titanium and aluminum nitride in the zone of austenite by FactSage7.0.It was found that BN was precipitated before solidification,and thus,an appropriate amount of titanium should be added into steel to protect the boron from eliminating the interaction between boron and nitride.Meanwhile,it was discovered that very little sulfur was added in steel by adding pig iron containing boron,which can be ignored.When the titanium was not added,the original austenite grains became bigger with the increase of the amount of boron addition.The phenomenon was not obvious in the range of 1050-1100 ?,but it was coarsening obviously when the temperature was higher than 1125?.Ti-B steel shows smaller austenite grains and lower growing tendency than B steel under every experimental temperature.(2)With the thermal simulation technology,high temperature deformation behavior for experimental austenitic steel was studied through experimenting single channel compression with different parameters.The effects of process parameters,boron content,and addition of titanium on the deformation resistance and dynamic recrystallization behavior were analyzed,and regression analysis was conducted to analyze the dynamic recrystallization model.The results showed that peak stress decreased with the increase of the boron content in the curve of dynamic recrystallization,while the addition of titanium had the same effect;deformation activation energy decreased with the increase of boron content and the addition of titanium.Critical strain of dynamic recrystallization started to increase with the increase of boron content when the strain rate was 0.1s-1.When the strain rate was Is-1,peak stress and critical stress decreased with the increase of boron content.Nevertheless,no obvious change was observed on the peak strain and critical strain.Peak stress,critical stress and critical strain of dynamic recrystallization started to decrease with the addition of titanium.(3)Effect of boron,titanium,deformation and cooling conditions on the transformati- on of microstructure of experiment steel were analyzed,and effect of carbon content on boron hardenability were also analyzed through simulation experiment of continuous cooling.It was noticed that the stability of austenite in boron steel improved and the transformation of ferrite and pearlite was delayed with the addition of titanium in the low carbon steel.Thus,the bainite structure could be obtained in a wider range of cold speed.Due to the deformation,the range of cold speed for ferrite and pearlite became wider,and the CCT curve of experiment steel shifted to the left.Stability of austenite in experiment steel improved and the transformation of ferrite and pearlite delayed with the addition of titanium in the low carbon steel.Stability of austenite in experiment steel improved,transformation of ferrite and pearlite was inhibited and the formation of bainite and martensite was facilitated with the addition of boron content in the medium carbon steel.Boron addition exerted no influence on the microstructure and hardness of different zonesin the high carbon steel after quenching.The effect of boron on improving the hardenabilityof steel decreased when the experiment steel had more carbon.(4)The effect of hot rolling parameters on microstructure was studied through thermalsimulation experiment,and then the laboratory controlled rolling and controlled coolingexperiments were carried out.The boron,which contained a small amount of pig iron and titanium,were added in the original Q235 steel components,and the yield strength of Q235 steel would increase to 427MPa with the integrated use of fine grain and phase transformat ion strengthening.Moreover,these processes made use of the control rolling and control cooling technology under laboratory conditions.Similarly,the experiment steel has good plasticity and impact toughness,and thus it achieves the same performance requirements.The upgrading and rolling of Q235 steel is realized without the technology of "lowtemperature and large deformation" and upgrading of the mill equipment.(5)The martensite morphology of boron micro-alloyed steel 45 was studied,and the effects of different quenching and tempering temperatures on the mechanical properties were analyzed.With the increase of quenching temperature,the martensite lath became thicker and longer,and the yield strength and hardness increased as well,which firstly increased and then decreased with the maximum value 840.Also,the proportion of large angle grain boundaries and the size of the block increased with the increase of quenching temperature.Meanwhile,the yield strength and hardness decreased significantly with the increase of tempering temperature.The elongation and impact energy increased with the increase of the tempering temperature.With the increase of tempering temperature,the aggregation of carbon atoms is significant with wide range and high concentration.The carbon atoms were aggregated at the lath interface and the original austenite grain boundaries.Considering that different application fields emphasize different mechanical properties of the steel in,the appropriate microstructure and mechanical properties in different fields can be obtained by adjusting the quenching and tempering temperature.(6)A preliminary study was carried out on the effect of boron on austenite grain and low-temperature impact toughness of high-phosphorus steels,and the the effect of different boron content on the tensile strength and ductility of copper containing steel was researched.The results showed that the austenite grain size of phosphorus-containing steel decreased with the increase of boron contents;boron restrained the austenite grain from coarsening by phosphorus.In addition,the addition of boron could not improve the impact toughness of phosphorus-containing steel at a low temperature.Lastly,the two kinds of boron-containing copper steel(0.5%Cu)with different amount of boron both have high thermoplasticity,and they are not affected by copper,and the addition of boron can increase the tensile strength at a high temperature. |
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