SHELL ELEMENTS

Help! I am trying to mesh a cube on Python. My code is the following:

gmsh.open(directory+'\Cube1.STEP') 
gmsh.model.mesh.optimize()
refine=0
""""
while refine<50:
    gmsh.model.mesh.refine()  
    refine+=1
gmsh.option.setNumber("Mesh.MeshSizeMin", 3)
gmsh.option.setNumber("Mesh.MeshSizeMax", 3)
"""

gmsh.model.mesh.generate(3)
gmsh.write('mesh'+str(iteration)+'.msh')
gmsh.write('mesh'+str(iteration)+'.key')

However, when I check the .key file generated, I notice that there are some shell elements which I do not not where they come from. How can I make sure there are no shell elements in my key file ? I need it to run simulations on LS-DYNA. Thank you !