Control Abnormal Behavior Of Surface Microstructures On Cold Heading Steel 35K

Cold heading steel utilizes the plasticity of metal, uses technics of cold heading molding and machining, and finally produces steel for standard parts which have higher interchangeability. With increasing export, and the speedy development of motorcar, petroleum and machinery industries, the requirement of fastener and fittings material increase continuousely. Therefore, market puts forward higher requirement about property and reliability of cold heading steel.Based on the cold heading cracking phenomenon on cold heading steel 35K in Maanshan Iron and Steel Co. Ltd. in machining process, analysis and research was done about actual steel quality and microstructures of strand in this paper. The results showed that the cold heading cracking was due to abnormal surface microstructures of steel. The abnormal surface microstructures have three kinds of behavior. The first kind is that ferrite and pearlite growing up synchronously, the second is larger than two grades of widmanstantten microstructure and the third is a spot of pearlite but ferrite growing largely. Three kinds of abnormal surface microstructure are both called widmanstantten microstructure after a mass of microstructure observed and contrastted with standard microstructure pictures of steel. Nevertheless, the second kind of microstructure is the primary reason about cold heading cracking because of the worst harmness for cold heading property.The reason about second kind of microstructure on wire rod is that large size of austenite grain on surface of wire rod, microstructure carburization of strand and nonuniform microstructure component which because of large size of widmanstantten microstructure on the surface strand.Therefore, in order to research the second kind of abnormal microstructure with cold heading steel 35K having determinate carbon content, the method about laboratory research associating with industrial trial are used. The continuous casting and rolling process are regarded as a system and the key of research is the second kind of abnormal microstructure. The research showed that the primary reason about surface widmanstantten microstructure of steel is large austenite grain on the surface of steel and nonuniform microcosmic component which is induced by large widmanstantten microstructure on the surface of strand. In order to prevent the large widmanstantten microstructure from producing and avoid cold heading cracking, a series of researchs were conducted in this paper. The research contained as follows: oscillation parameters of mold was analysed, electromagnetic stirring parameters in mold and lubrication property of mold flux were optimized, physical modeling between casting speed and inserting depths of nozzle was studied, heating condition of haeting furnace in laboratory was researched, STELMOR cooling rate in rolling process was analysed and the way to controll rolling process temperatures was researched.After several aspects were researched hereinbefore, control measures were put forward in this paper. In order to avoid microstructure carburization of strand which becase of the involvement about enriched carbon layers of mould fulx, logical inserting depths of nozzle was 80~120 mm when casting speed was 2.3 m.min-1, level fluctuation of mould was less than 6 mm and the liquid slag layer of mould fulx adjusted to 8~12 mm. In order to avoid nonuniform microcosmic component which was induced by large widmanstantten microstructure on the surface of strand, physical and chemical parameters of mould fulx was optimized and the lubricity of mould fulx was improved. In order to control austenite grain on the surface of wire rod, heating time and soaking temperature in heating furnace, finishing rolling temperatur and laying temperature in rolling process were respectively controlled at 80~90 min, 1160~1220℃, 820~840℃and 800~820℃. The results showed that microstructures of cold heading steel was uniform, the grade of surface widmanstantten microstructure was less than 1 and cold heading cracking was effectively avioded.