Improvement of cracking of hot rolled strip steel
The production process of hot rolled strip steel:
Liquid iron pretreatmentA top and bottom double-blown converter a deoxidation alloying a blown argon a slab continuous casta casting billet testa heating a roughing high-pressure water descaling a vertical roller rolling a reversible roughing a middle rolling a hot coil a finishing rolling a high pressure water descaling a finishing rolling a laminar-flow cooling a coil a storage.
The cracking phenomenon in the process of production inspection and user use of hot rolled strip steel was analyzed by chemical composition, low power and metallographic inspection. The results show that the chemical composition meets the requirements. The cracking of the strip is mainly caused by the presence of subdermic bubbles in the casting billet, non-metallic inclusions and free cementite in the strip.
For the above situation to give you targeted improvement suggestions, specific as follows:
1, subcutaneous bubble and inclusion control, the focus is to control the oxygen content within 60×10-4%(mass fraction), there should be enough deoxidation components in the steel (such as acid-soluble aluminum content should be controlled above 0.015). Minimize oxygen in steel in these two forms: one is solidly dissolved in the matrix, the other is in the form of bubbles or inclusions.
2, the control of free cementite, in order to reduce free cementite precipitation, should control the coiling temperature (below 660℃), and dispersed stacking is also conducive to reducing the three cementite.
(1) There is a problem of high strength for steel plates, such as steel with a composition in line with Q195, the tensile strength after rolling can reach more than 430MPa. By metallographic examination, its structure is ferrite and pearlite, but the grain is fine (up to 12.5). The grain size can be improved by adjusting the controlled rolling process and optimizing the grain size. This helps to reduce tensile strength and hardness. If the laminar cooling method is adjusted from the front end cooling to the back end cooling, the strength of the steel is reduced to 40MPa, and the grain size is 9 ~ 9.5, the effect is significant. (2) The dimensional accuracy of the steel plate has a great impact on the quality of stamping and punching parts and the consumption of the mold. If the thickness of the steel plate exceeds the allowable tolerance, for example, the steel plate is too thin, the surface of the part is wrinkled due to insufficient compression, or the correct geometry is not obtained due to springback; The thickness of the steel plate will lead to the concave and convex mold clearance is too small, the fit is too tight, and even damage the mold or cause cracking of the parts. Therefore, ensuring the dimensional accuracy of the steel plate is another measure to improve its stamping performance. (3) Because the rolling direction of the steel plate is also related to the stamping performance, the longitudinal and transverse properties of the continuous rolled plate are different, so the user's press bending direction should be as vertical as possible with the rolling direction.
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