Of Q235 Hot Rolled Strip Transverse Fold Defect And Governance

The break marks is a kind of strip marks that is vertical to rolling direction, occurring at some locations of hot-rolled steel strip during uncoiling. In some cases it extends all over the width of the steel strip, the surface of which touches uneven. The break marks become dense after straightening, so it is detrimental to the surface quality of sheets. The break marks are frequently noticed on hot-rolled low carbon steel strip after uncoiling. Products either home made or imported exhibit break marks more or less, bringing a lot of trouble as well as cost increase to consumers.Mechanism for generating the break marks has not been well justified, but so far, it is basically believed that the spike yield is the fundamental cause. To eliminate the break marks, researches and experiments have been done by steelmakers at home and abroad, but the results were undesirable. Therefore, temper rolling is generally adopted to eliminate the break marks, resulting in the rising of production cost. In addition, the effect is limited.In paper, the method of eliminating the break marks on hot rolled Q235 steel strip is to reduce the spike yield height by controlling the morphology of microstructures.The hot rolled Q235 steel strip from Pangang was used as test specimens. The undeformed specimens were used for conducting tensile tests to obtain the spike yield height. A dilatometer was used to map out CCT diagram and to conduct thermal simulation test. Under the optical microscope, microstructures of these specimens were observed. Results show that, (1)there exists a critical value for the spike yield height which causes the break marks. The 'quasi acicular ferrite' has a high primitive mobile dislocation density, which can reduce the spike yield height to a level below the critical value so that the hot rolled Q235 steel strip is free of the break marks. More importantly, as the phase transformation temperature of the 'quasi acicular ferrite' is between that of the polygon ferrite and the acicular ferrite, it is much easier to be obtained during industrial production;(2)Factors such as higher carbon content, coarseraustenite grain, faster water cooling rate, lower starting/ending temperature of water cooling can increase the stability of austenite to promote the formation of the 'quasi-acicular ferrite' microstructure. Optimal starting and ending temperature of water cooling are the temperatures at which pro-eutectoid ferrite and pearlite start to form respectively.During industrial production, it is found that coiling temperature is one of the key parameters, which is the most difficult to be established. Mathematical modeling is used to calculate the cooling rate in different sectors of the laminar cooling line, through which coiling temperature for hot-rolled Q235 steel strip is worked out and hence production process of hot-rolled Q235A/B steel strip is established as follows. Finishing rolling temperature must be higher than 820 °C, and later sector cooling pattern (cooling water in the front sectors is screened) is used. In the case of carbon content of 0.13%~0.14%, coiling temperature is 590"C;In the case of carbon content of 0.15%~ 0.18%, coiling temperature is 620°C. The above methods have been applied effectively to eliminate the break marks on commercially-produced hot-rolled Q235 steel strip in Pangang.