RESEARCH ON THE APPLICATION OF RARE EARTHS IN IRON AND STEEL IN RECENT YEARS

China has the biggest rare earths resource in the world,no wonder she pays much at-tention to the application of rare earth metals (REM).The application of REM in iron andsteel in China began at the end of 1950’s.The production of REM-treated iron and steelhad been made steady progress in the period of 1980’s.The production of REM-treatediron and steel in 1989 was 1.4 million tons and 250,000 tons respectively.The interest oftreating steel with REM in China keeps growing even in these years,It comes from the nat-

Effect of Rare Earth Elements on Low Frequency Fatigue Cyclic Softening of Hot Rolled Steel 60CrMnMo at High Temperature

The effect of rare earth (RE) on cyclic softening of low frequency fatigue of the hot rolled steel 60CrMnMo at high temperature was investigated. The hardness of specimens before thermal fatigue test and being cycled 180 times from room temperature to 650℃ was compared. The results show that by adding a certain amount of RE in steel 60CrMnMo, the cyclic softening during low frequency fatigue test at high temperature can be restrained considerly. The carbide particles were refined during tempering treatment and the agglomeration and growth of carbide particles can be obstructed during thermal cycle.

RARE EARTH DISSOLVED IN SOLID SOLUTION OF STEEL AND ITS EFFECT ON MICROSTRUCTURE

The amounts of rare earth in the solid solution in steel 16Mn were determined by means of inductive coupling plasma(ICP)spectroscopy.While the RE/S ratio was less than 1.9,the amounts of rare earth in solid solution were not more than 8 ppm,which rised slightly with the increase of the rare earth content in the steel.While the RE/S was more than 1.9,MnS disappeared completely in the steel and the amounts of rare earth in solid solution increased rapidly with the increasing of the rare earth content.The solubility of cerium in steel 16 Mn(St 52)is less than 0.011 wt% at room temperature.The results also indicate that rare earth in solid solution can reduce the amount of pearlite and increase that of ferrite and its mierohardness.The rela- tionship between microhardness(Hv)and the amount of rare earth in solid solution can be expressed by the equation of Hv=117+7 RE(ppm).

EFFECTS OF RARE EARTH METAL ON HYDROGEN BEHAVIOUR IN STEEL 16Mn

The slow tensile tests,dynamic hydrogen charging tensile tests and hydrogen evolution tests after hydrogen charging were used to study the effects of rare earth metal(REM)on hydrogen behaviour in a steel 16Mn(St.52).The ratios of RE/S were chosen as 0,0.7,2.2 and 7.7,respectively.It was shown that the steel with RE/S = 2.2 give a lower hydrogen embrittlement susceptibility than others.The steels without REM can adsorb much more amount of hydrogen than that with REM under the same hydrogen charging conditions.And the amount of adsorbed hydrogen for the foriner can be evolved easier than that for the latter at room temperature,50℃ and 80℃,respectively.The experimental results were explained by the trap theory of hydrogen,the short-circuit diffusion paths in the interfaces between the elongated MnS inclusions and the matrix,and strong ability of REM to adsorb hydrogen.

NUMBER 3 RARE EARTH FACTORY OF BAOTOU IRON—STEEL AND RARE EARTH CORPORATION,FORGING AHEAD IN THE MARCH OF REFORM

The Baotou iron-Steel and Rare Earth Co. is located in Baotou of the AutonomousRegion of Inner Mongolia, where China has her largest mineral reserves of rare earths, par-ticularly that at Baiyunebo. The rare earth reserves in this region amount to as high as 80%of that of the whole Nation. The No. 3 Rare Earth Factory was established in 196l and itwas then a pilot plant of the Baotou Steel Works. In 1970 it was reconstructed as a formalplant. Now, it becomes one of the biggest rare earth production bases in China.

Reaction mechanism between Al_(2)O_(3)–MgO refractory materials and rare earth high-carbon heavy rail steel

Submerged entry nozzle(SEN)clogging is a major problem affecting the production quality of rare earth steel,and finding a suitable refractory outlet can significantly reduce production costs.To explore the relationship between refractory composition and interface interaction,unprotected coated Al_(2)O_(3)–MgO refractories and SiO2-coated Al_(2)O_(3)–MgO refractories were added to rare earth high-carbon heavy rail steel under laboratory conditions,and the Al_(2)O_(3)–MgO refractory was found to be more suitable.The results show that,from the epoxy resin side to the refractory side,the contour of the refractory interface reaction layer can be divided into two main layers:an iron-rich reaction layer and an iron-poor reaction layer.Calculations based on the spherical model suggest that the adhesion force is proportional to the size of the refractory particles and inclusions,and the same result applies to the surface tension.Controlling the inclusions at a smaller size has a specific effect on alleviating the erosion of refractories.Combined with the erosion mechanism of Al_(2)O_(3)–MgO refractories,the interface reaction mechanism between Al_(2)O_(3)–MgO refractories and molten steel was proposed,which provides ideas for solving SEN clogging.

Internal Friction and Profile of Property and Structure of PD_3 and PD_3RE Rail Steels

The profile of both hardness and microstructure of continuous casting blanks and rails of PD3 heavy rail steel and PD3RE steel containing small amount of rare earth （RE） added from tundish were studied. It is found that the addition of RE makes the fluctuation of the hardness value decrease, and inhibits effectively and/or even removes lower hardness value（zone） in the centric area of the blank with a cross section of 280 mm × 380 mm, and that the surface layer in the rail head of PD3RE steel is obviously hardened and the reason causing this hardening is due to the thinning of the pearlite structure. The metallographic observation shows that there exists a Fe-P（C） eutectic structure in the blank center of the PD3 steel containing only 0. 016% P, while the addition of RE can fully inhibit this structure.

Effect of rare earth cerium on brittleness of simulated welding heat-affected zones in a reactor pressure vessel steel

The welding coarse-grained heat-affected zones（CGHAZs） in the undoped and Ce-doped samples of SA508CL-3 reactor pressure vessel steel were simulated using a Gleeble 1500 D thermomechanical simulator with a peak temperature of 1320 oC at the heat inputs of 30, 50 and 100 kJ /cm, respectively. The ductile-to-brittle transition temperature（DBTT） of the simulated CGHAZs was evaluated along with microstructural and microchemical characterizations. The results indicated that Ce could substantially lower the DBTT of the CGHAZs by its microstructural and microchemical effects. After the thermal cycling of welding, the microstructure in the Ce-doped samples was apparently finer than that in the undoped samples, regardless of the lath bainite obtained at the heat inputs of 30 and 50 kJ /cm or the granular bainite acquired at the heat input of 100 kJ /cm, leading to lower DBTTs for the Ce-doped samples. Moreover, grain boundary segregation of Ce occurred apparently in the Ce-doped samples and exhibited a non-equilibrium characteristic. The segregation of Ce could play an important role in lowering the DBTT of CGHAZs or toughening the CGHAZs.

Effect of rare earth cerium and impurity tin on the hot ductility of a Cr-Mo low alloy steel

The specimens of 1Cr-0.5Mo low alloy steel, undoped, Sn-doped and Ce＋Sn-doped, were austenitized at 1300 oC and then cooled down to different temperatures in the range of 700–1050 oC, followed by tensile tests with the aid of a Gleeble machine. The reduction of area（RA） obtained from the test was employed to evaluate the hot ductility of the steel. The tested specimens were characterized using different techniques. Minor Sn could considerably reduce the RA values of the steel in the whole temperature range, and the hot ductility curve could be widened and deepened. Nevertheless, minor Ce could improve the hot ductility of the Ce＋Sn-doped steel by fully suppressing the Sn-induced hot ductility deterioration. FEGSTEM microanalysis showed that the Sn or Ce and Sn atoms segregated to austenite grain boundaries in the Sn-doped or Ce＋Sn doped specimens. The detrimental effect of Sn on the hot ductility could be attributed mainly to the segregation of Sn as it could decrease the grain boundary cohesion and in turn enhanced the grain boundary sliding and cracking. However, this detrimental effect of Sn could be counteracted by the segregation of Ce which could increase the grain boundary cohesion and in turn restrained the grain boundary sliding and cracking. Accordingly, a minor addition of rare earth Ce could be an effective method of suppressing the detrimental effect of impurity elements on the hot ductility of a Cr-Mo low alloy steel.

Effect of rare earth alloying on creep rupture of economical 21Cr-11Ni-N heat-resistant austenitic steel at 650 ℃

The effect of rare earth（RE） on creep rupture of economical 21Cr-11Ni-N heat-resistant austenitic steel was investigated at 650 °C under different stress levels. It was found that RE could increase the time to creep rupture, especially at long-term creep duration. The logarithm of the time to creep rupture（lgtr） was a linear function of the applied stress（σ）. RE addition was favorable to generating a high fraction of low-coincidence site lattice（CSL） boundaries which was a possible cause for improving the creep rupture resistance. The fracture surface of RE-added steel exhibited less intergranular cracks suggesting the alteration on the nature of grain boundaries due to the presence of RE. RE addition changed the morphology of the intergranular chromium carbides from continuous network shape to fragmentary distribution which was another cause for longer creep duration. These results strongly suggested that the effect of RE alloying played a crucial role in improving the creep rupture resistance.

A Coupled Thermodynamic Model for Prediction of Inclusions Precipitation during Solidification of Heat-resistant Steel Containing Cerium

A coupled thermodynamic model of inclusions precipitation both in liquid and solid phase and microseg- regation of solute elements during solidification of heat-resistant steel containing cerium was established. Then the model was validated by the SEM analysis of the industrial products. The type and amount of inclusions in solidifica- tion structure of 253MA heat-resistant steel were predicted by the model, and the valuable results for the inclusions controlling in 253MA steel were obtained. When the cerium addition increases, the types of inclusions transform from SiO2 and MnS to Ce2 O3 and Ce2O2 S in 253MA steel and the precipitation temperature of SiO2 and MnS decrea- ses. The inclusions CeS and CeN convert to Ce2 O3 and Ce2 O2 S as the oxygen content increases and Ce2 O3 and CeN convert to Ce2 O2 S, Ce3 S4, and MnS as the sulfur content increases. The formation temperature of SiO2 increases when the oxygen content increases and the MnS precipitation temperature increases when the sulfur content increa ses. There is only a small quantity of inclusions containing cerium in 253MA steel with high cleanliness, i. e. , low oxygen and sulfur contents. By contrast, a mass of SiO2 , MnS and Ce2 O2 S are formed in steel when the oxygen and sulfur contents are high enough. The condition that MnS precipitates in 253MA steel is 1.2wEo[O] ＋W[s]〉0. 01% and SiO2 precipitates when 2w[O] ＋wrs[S]〉0. 017% （W[S]0. 005%） and w[O]〉0. 006% （w[S]〉0. 005%）.

Viscous properties of new mould flux based on aluminate system with CeO_2 for continuous casting of RE alloyed heat resistant steel

The conventional mould fluxes can not be applied to the continuous casting of RE alloyed heat resistant steel, because severe slag-metal interface reactions occur generally in the mold. To restrain the interface reaction and improve conditions for continuous casting, a new mould flux based on aluminate system was devised. The viscous properties were investigated. Scanning electron microscopy and X-ray diffraction were applied to detect and characterize the crystalline phases in the continuous cooling process. The results showed that appropriate addition of CeOcould avoid the precipitation of CaO and decrease the viscosity of the mould flux. Increasing the mass ratio of CaO /AlO, especially to a value exceeding 1, could worsen the stability of the mould flux. With a content of less than 14 wt.%, LiO could reduce the viscosity and breaking temperature, but its effect could be weakened for the promoted precipitation of LiAlO. To obtain a mould flux with stable viscous properties, such as viscosity and breaking temperature, appropriate contents of CeOand LiO should be controlled to around 10 wt.% and 14 wt.%, while the mass ratio of CaO /AlOshould not be more than 1.

Self-healing effect of ceria electrodeposited thin films on stainless steel in aggressive 0.5 mol/L NaCl aqueous solution

The self-healing effect of electrochemically deposited CeO2-Ce2O3 films on stainless steel OC404（SS） in 0.5 mol/L NaCl solution was studied. It was established that the corrosion potential of the steel, after covering it with CeO2-Ce2O3 layer and thermal treatment（i.e. potential of the system CeO2-Ce2O3/SSt.t.）, was shifted sharply to a considerably more positive value, while the corrosion current was reduced noticeably. The X-ray photoelectron spectroscopy（XPS）, energy dispersive spectroscopy（EDS） and scanning electron microscopy（SEM） analyses on the observed scratched surface area of the system CeO2-Ce2O3/SSt.t., after exposure of investigated specimens to 0.5 mol/L NaCl corrosion media, showed partial accumulation of ceria, as well as remarkable increase in the concentrations of oxides of Al, Cr and Fe, on the mechanically revealed steel surface. On the basis of the obtained results one could conclude that the secondary passive oxide/hydroxide films, formed after a definite time interval of exposure to corrosion media, acted as barriers, hindering the corrosion processes in active zones. A hypothesis was put forward about the mechanism of self-repairing oxide layers on the steel surface and their corrosion protection effect respectively.