Critical precipitation behavior of MnTe inclusions in resulfurized steels during solidification

Te treatment is an effective method for modifying sulfide inclusions,and MnTe precipitation has an important effect on thermal brittleness and steel corrosion resistance.In most actual industrial applications of Te treatment,MnTe precipitation is unexpected.The critical precipitation behavior of MnTe inclusions was investigated through scanning electron microscopy,transmission electron microscopy,machine learning,and first-principles calculation.MnTe preferentially precipitated at the container mouth for sphere-like sulfides and at the interface between MnS grain boundaries and steel matrix for rod-like sulfides.The MnS/MnTe interface was semicoherent.A composition transition zone with a rock-salt structure exhibiting periodic changes existed to maintain the semicoherent interface.The critical precipitation behavior of MnTe inclusions in resulfurized steels involved three stages at varying temperatures.First,Mn(S,Te)precipitated during solidification.Second,MnTe with a rock-salt structure precipitated from Mn(S,Te).Third,MnTe with a hexagonal NiAs structure transformed from the rock-salt structure.The solubility of Te in MnS decreased with decreasing temperature.The critical precipitation behavior of MnTe inclusions in resulfurized steels was related to the MnS precipitation temperature.With the increase in MnS precipitation temperature,the critical Te/S weight ratio decreased.In consideration of the cost-effectiveness of Te addition for industrial production,the Te content in resulfurized steels should be controlled in accordance with MnS precipitation temperature and S content.

Investigations of Inclusion Modification in High Grade Pipeline Steel

SiCa line was injected into liquid pipeline steel at the end of LF refining as calcium treatment, and samples were taken from liquid steel in ladles before and after calcium treatment, liquid steel in mould, and slabs. Results show that Ca content in steel decreased obviously after calcium treatment with production, and the compositions, morphologies and sizes of inclusions in steel also varied with time; Al2O3 inclusions turned to be fine irregular CaS-CaO-Al2O3 compound inclusions after calcium treatment, and then to be fine globular CaO-Al2O3 inclusions in mould, and finally changed to be a few larger irregular CaS-CaO-Al2O3 complex inclusions in slabs; reoxidation of molten steel treated by Ca deteriorates effect of calcium treatment. Thermodynamic study revealed that inclusion variations were related with preferential reactions among Ca, Al2O3 and S and precipitation of S in high sulfur capacity CaO-Al2O3 inclusions. New calcium treatment controlling rules were put forward according to the inclusion variations and requirements of pipeline on inclusion controlling.

Effect of tin addition on the microstructure and properties of ferritic stainless steel

This article reports the effects of Sn on the inclusions as well as the mechanical properties and hot workability of ferritic stainless steel. Precipitation phases and inclusions in Sn-bearing ferritic stainless steel were observed, and the relationship between the workability and the microstructure of the steel was established. Energy-dispersive X-ray spectroscopic analysis of the steel reveals that an almost pure Sn phase forms and MnS-Sn compound inclusions appear in the steel with a higher Sn content. Little Sn segregation was observed in grain boundaries and in the areas around sulfide inclusions;however, the presence of Sn does not adversely affect the workability of the steel con-taining 0.4wt%Sn. When the Sn content is 0.1wt%-0.4wt%, Sn improves the tensile strength and the plastic strain ratio and also improves the plasticity with increasing temperature. A mechanism of improving the workability of ferritic stainless steel induced by Sn addition was discussed：the presence of Sn lowers the defect concentration in the ultra-pure ferritic lattice and the good distribution of tin in the lattice overcomes the problem of hot brittleness that occurs in low-carbon steel as a result of Sn segregation.

Study on the modification of inclusions by Ca treatment in GCr18Mo bearing steel

The effect of Ca treatment on the modification of inclusions in Al-killed GCr18Mo bearing steel was studied and a thermodynamic and kinetic model was established.The experimental results showed that oxygen content in the steel could be reduced from 120ug/g to the range 5.5-21.6ug/g by Al-Ca complex deoxidization at 1873 K.An appropriate increase in Ca/Al mass ratio is beneficial to reduce the total oxygen content in steel.When the content of dissolved aluminum was in the range of 0.03%-0.3%,w让h the increase of Ca/Al mass ratio from 0.8-1.6 to 2.4-3.2,the number of inclusions per unit area significantly reduced.In addition,the main types of inclusions were modified from large-sized Al2O3 clusters to plastic or semiplastic liquid phase calcium aluminates.The experimental results matched well with the thermodynamic analysis.

Inclusion engineering and metallurgy of calcium treatment

Although a great deal of research has been done in the area of inclusion control during steelmaking, a complete description of the process of inclusion modification and shape control cannot be found in literatures. As a result, the terms ＂inclusion modification＂ and ＂shape control＂ are often used loosely in the technical community. To utilize the full potential offered by a calcium treatment, it is important to understand why an inclusion whose composition has been modified may not necessarily have its shape modified. With the purpose of decoupling the process of inclusion phase change from inclusion shape control, and to provide a complete description of the metallurgy of calcium treatment, a detailed literature survey has been performed on the subject. By coupling the survey with a new concept developed to tailor the cleanliness of steels to the intended applications, the hope is to provide the reader with valuable information on calcium treatment for inclusion modification and shape control.

Pitting corrosion behavior of cerium treated HSLA steel induced by sulfide inclusions in 3.5 wt% NaCl solution

Pitting corrosion behavior of Ce treated HSLA steels induced by sulfide inclusions in 3.5 wt% NaCl solution was investigated with potentiodynamic polarization and immersion corrosion test.The results show that Ce added steels exhibit better pitting corrosion resistance with lower corrosion current density and bigger pitting potential compared with steel without Ce,which is mainly attributed to optimized characteristics of spherical Ce-oxysulfide inclusions with less number density,smaller average size and lower pitting corrosion susceptibility.The spherical Ce_(2)O_(2) S inclusions precipitated on the surface of CeAlO_(3) inclusions in the Ce added steels induce pits with bigger opening mouth and shallow depth,reducing their tendency of extension to go deeper due to occlusion corrosion battery in the pitting holes.Furthermore,corrosion inhibitor Ce(OH)_(3) generated by Ce_(2)O_(2) S hydration can weaken electrochemical corrosion of the matrix micro-region around the pits.To avoid harmful bigger inclusions,Ce content in steels should be regulated within reasonable range,0.015 wt% Ce in present steels effectively modified inclusions to acquire the best pitting corrosion resistance of the steels.

Experimental and DFT study on cerium inclusions in clean steels

To provide insights into deforming Ce-O-S-Al inclusions in steels and improving the mechanical properties,the evolution process of such harmful inclusions in clean steels was investigated by thermodynamic calculation,metallographic examination and first-principles calculation in this paper.For the tested IF steel,the thermodynamic analysis results are consistent with the calculated formation enthalpy.After Ce addition,the inclusions are transformed from Al_(2)O_(3)and TiN-Al_(2)TiO_(5)-Al_(2)O_(3)to Ce2O_(3),Ce_(2)O_(2)S,CeAlO_(3),TiN-Al_(2)TiO_(5)-Ce_(2)O_(3)and TiN-Al_(2)TiO_(5)-Ce_(2)O_(2)S composite inclusions,which can be confirmed by metallographic examination.The elastic constants were calculated,and the bulk modulus,Young's modulus,shear modulus and Poisson's ratio were evaluated by the Voigt-Reuss-Hill(VRH)approximation.All inclusions except Ce_(2)O_(3)show apparent brittleness.TiN,Al_(2)O_(3),Al_(2)TiO_(5)and CeAlO_(3)present much higher hardness than iron matrix,while the hardness of Ce2O_(3)or Ce_(2)O_(2)S is close to that of iron matrix.The thermal expansion coefficients of Ce_(2)O_(3)and CeAlO_(3)are close to that of iron matrix,whereas,Ce_(2)O_(2)S inclusion has largely different thermal expansion coefficient from iron matrix and may deteriorate the steel performance at higher temperatures.The relatively small differences between Ce inclusions and iron matrix in terms of hardness,toughness,brittleness,and thermal expansion coefficient can explain the improvement of the mechanical properties of the tested steel.

Effect of Zr-deoxidation on microstructure and mechanical behavior of microalloyed heavy plates with low impurity content

The significance of different deoxidation practises on the ductility and impact toughness of next generation of microalloyed heavy plates was elucidated to explore the best deoxidation practice in obtaining required mechanical properties,which was judged by the combined effects of composition,size and number density of inclusions on the ductility of the experimental high-strength low alloy steel.The impurity contents,i.e.,total O+N+S contents,of 82×10^(-6)(Al-killed)and 118×10^(-6)(Zr-killed)have been induced to characterize both the steels.Ductility was characterized using tensile and Charpy V-notch testing.The number,size and composition of the inclusions were characterized using a field emission scanning electron microscope with an energy dispersive spectrometer.In the Al-killed steel,the inclusion structure consisted of titanium nitrides,stringer calcium aluminates and elongated manganese sulfides,whereas in the Zr-killed steel,the inclusion structure consisted of mainly fine spherical oxide inclusions with sulphide shells.The impurity content did not have a significant effect on the number density of inclusions,as with higher and lower impurity content,the number of inclusions was 83.7 and 78.8 mm^(-2),respectively.However,the size distribution of the inclusions,especially the coarse inclusions with their longest length greater than 8μm,differsmuch from each other.The number density of coarse inclusions differs from 0.8 to 1.1 mm^(-2) with processing,and in Al-killed steel,55.5% of the coarse inclusions were titanium nitrides or manganese sulfides,whereas in Zr-killed steel,only 22.5% of the coarse inclusions were titanium nitrides and manganese sulfides.Coarse titanium nitrides were especially detrimental to the impact toughness.The number density of them should be below 0.33 mm-2 in order to guarantee the best possible toughness in the steel in question.The average crystallographic grain size detected by electron backscattered diffraction of Zr-killed steel(4.28±2.70μm)was smaller than that of Al-killed steel(6.00±4.80μm).As a result from the grain refinement and sulphide shape control,Zr-killed steel exhibited superior impact toughness(223±70 J)at -80℃ as compared with Al-killed steel(153±68 J).Thus,Zr-killed steel was observed to provide good performance in terms of mechanical properties as compared with Al-killed steel.

Thermodynamic Calculation on Calcium Treatment for 26CrMo4S/2 Steel

Because CaSi core wire was not fed in external refining process for 26CrMo4S/2 steel making, it was found that the molar ratio of calcium versus alumina was very low and subsequently resulted in generation of much more non-metallic inclusions. Hence, it was reasonable to sugguest feeding appropriate amount of Ca core wire. Before the performance, the thermodynamic calculation had been carried out to obtain the theoretical amount of Ca wire to be fed. According to the practical data from steel plant and the thermodynamic data, it was calculated that only when 5 2/34.97 10 [%Al]T-≥×4 2/31.38 10 [%Al] [%Ca]T T-×≥≥in molten steel the Al2O3 inclusions could be properly modified.