Improving Cleanliness of 95CrMo Drill Rod Steel by Slag Refining
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DRILL rods, which are extensively used in quarries, open pit mines, and construction sites, are made from hollow steel. They have to bear not only high-frequency peak-value impulse loads but also torsion, bend stress, abrasion, and corrosion from working media such like rock powder.[1] The cleanliness of the molten steel is one of the most important factors influencing the fatigue life and service performance.[2] Stress concentration at the interface between inclusions and the steel substrate, which is affected by the size, morphology, and expansion coefficient difference between the inclusion and steel substrate, is the main cause of fatigue failure.[3] Large, hard, and high melting point inclusions easily initiate micro-voids and cracks at the inclusion/steel interface during hot rolling that can cause fatigue fractures.[4] Strict control of steel cleanliness, including oxygen content and inclusions, therefore, has a very significant influence on improving drill rod quality.[5] Slag refining is one of the most important and effective ways to evaluate and control steel cleanliness.[6] Slag refining is important to study because of its effects on deoxidation, absorption, and removal of inclusions.[7–9] Low oxygen content and good inclusions can be obtained by improving the physicochemical properties and chemistry of refining slags, such as the value of the pctCaO/pctAl2O3 ratio, slag basicity, fluidity, LINZHU WANG, Doctoral Student, SHUFENG YANG, Associate Professor, JINGSHE LI, Professor, and TUO WU and WEI LIU, Master’s Students, are with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China. Contact e-mail: [email protected] JIAZE XIONG, Undergraduate Student, is with Shougang Guiyang Special Steel Co., Ltd., Guiyang 550005, Guizhou, China. Manuscript submitted November 24, 2014. Article published online October 20, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
inclusion/slag interfacial tension, and component activity.[6,10,11] Many researchers have studied the reactions between slag, inclusions, and molten steel and tried to understand the effect of slag on the evolution of inclusions.[4,12] High melting point spinel and high Al2O3 content inclusions are common with the use of Al as a deoxidizer and in the application of MgO-based refractory materials. These inclusions have a strong impact on the performance and fatigue life of the products.[13] Kim et al.[14] proposed that the weight fraction of MgO in the slag is one of the major factors affecting MgOÆAl2O3 spinel crystallization. Nishi, Okuyam and Park[15] investigated the equilibrium between Fe-16Cr melts and CaO-Al2O3-MgO slags and found that the relationship between the log of the ratio of mole fraction (XMgO/ XAl2O3) of inclusions and the log of the ratio of activities (aMgO/aAl2O3) of the slags exhibited a linear correlation. The absorption ability of slag for Al2O3 was studied by Yoon et al.[9] and Bao et al.[16] The total oxygen content decreased to 7 to 10 ppm and the absorption ability
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