How to reduce the causes of Q235B steel plate column?

1. Mould powder
For Q235B steel plate and Q345B steels with high longitudinal crack rate, the mold flux should have higher crystallization temperature and crystallization rate, higher alkalinity and viscosity; at the same time, it is also required that the mold flux has good stability and slag separation. Therefore, the basicity of mold powder is controlled as 1.261, 300 ℃ and viscosity is 0.97Pas.
2. Soft blowing time
The purity of molten steel can obviously affect the longitudinal crack of plate. In order to reduce inclusions and improve the purity of molten steel. It is required to ensure the soft mixing time of refining furnace, and the soft mixing time of each furnace of molten steel shall not be less than 10 min, so as to make the Al2O3 float from the molten steel as much as possible.

3. Continuous casting process
1) Stable casting speed, avoid excessive fluctuation of liquid steel level in the mold, ensure that the liquid level fluctuation is less than ± 5mm, and ensure that the thickness of primary shell is uniform when the slag film with uniform thickness is formed. For Q235B steel plate and Q345B steels, the casting speed should be controlled at about 1.0m/min.
2) The mould taper should be selected properly, especially the narrow face taper, to keep the mould taper about 9.0 mm.
3) The superheat of molten steel should be controlled at 20-25 ℃ to reduce the development of columnar grains caused by high temperature, which will lead to the decrease of grain boundary strength and crack.
4) In order to improve the accuracy of arc alignment of caster and ensure the alignment of submerged nozzle, the insertion depth and outlet inclination angle should be appropriate to prevent serious corner erosion or liquid surface turbulence caused by too large or too small inclination angle.
5) In order to avoid straightening in the second brittle zone (700-900 ℃), the slab surface temperature should be uniform and small fluctuation, so as to ensure the internal and external quality of the slab.

4. Billet quality and rolling process
Before the slab is put into the furnace, the billet inspection shall be strengthened. If the defective slab is found, it shall be taken off the production line in time, and the surface defect shall be cleaned with flame before entering into the furnace again, so as to prevent the batch defects from appearing.
In order to avoid mixed grain rolling in two-phase region, the heating speed of low-temperature section should be controlled to ensure uniform heating, proper pass reduction and proper final rolling temperature should be selected.