Research On Cold Upsetting Crack Of Low Carbon Cold Heading Steel

Cold heading steel is mainly used for cold heading molding process of various mechanical basic parts,its main role is to connect and fastening.The fastener is small,but its importance is self-evident.If the fasteners are not qualified,the loss is often disastrous,making use of cold heading steel fasteners,a main index is the cold heading cracking rate.Shaoguan Steel with reference to the Japanese standard has developed SWRCH6A,SWRCH8A,SWRCH18A,SWRCH22A and other low carbon aluminum calm cold heading steel.The main process routes taken are:hot metal desulphurization→converter smelting→RH vacuum→LF refining→billet continuous casting→high speed wire rod rolling.Product sampling test 1/3 cold heading cracking rate reached 12%,1/4 Cold heading cracking rate of 46%,the product grain size fluctuations in the 7.5 to 9.0 level,unable to meet the processing needs of high-end products.At the same time according to user quality objection feedback,cold heading cracking accounted for 62.4%of opposition feedback.In this paper,low carbon cold heading steel cold heading for the main work carried out as follows:(1)According to the field long track finishing,the cracking of low carbon cold heading steels in Shaoguan Steel has three kinds of morphologies:45°oblique cracking,symmetrical double line or single thread cracking,dense microcracks.The back of the two cracks as the main crack morphology.(2)Through field sampling and laboratory detection methods,the effects of chemical composition,mechanical properties,surface quality,microstructure,inclusion control and rolling process control of cold heading steel cold heading performance,identify the impact of low carbon cold heading steel main cracking reason.(3)The factors affecting the chemical composition,metallographic structure and surface and shallow surface quality of low carbon cold heading steel are analyzed through field tracking and experimental comparison.The results show that the reasonable calcium treatment process is selected in the process of smelting and continuous casting.Aluminum addition and electromagnetic stirring process can effectively improve the chemical composition uniformity and shallow surface quality of the billet.The rolling transformation of the overpass channel can effectively reduce the scratch of the rolled piece,and the reasonable controlled rolling and cooling control process can effectively improve the uniformity of metallographic structure,thus improving the cold heading performance of the cold heading steel.(4)Through the research,the pass and part of guide wheel structure of steel rolling line have been improved,the shear,looper and 18#rolling mill and finishing mill passage have been revamped,the controlled rolling and controlled cooling process has been optimized,and the rolling tank has used tonnage and cooling water route.As a result,the cracking rate of low carbon cold heading steel was reduced to 0.21%.