This example in done with FreeCAD 0.19, it will be a little different if done in 0.18
Step by step instructions for Part WB Boolean Cut, to get the desired cross section with face view.
1) to import STEP, just open FreeCAD, go to Part WB, then "File" menu ...open, select your STEP file
comments, STEP import has been improved in FreeCAD 0.19, you can import an assembly as different items grouped in a "part" object. You can also import colours. FreeCAD 0.18 will import all the solids in a STEP file as one object. There are various options in the preferences.
2) go to the Cube solid on the tool bar in Part WB, click on that Cube, you get a default Part Cube which is only 10mm, now just change its placement and dimension properties so that it is larger than the model and intersects it half way through (which it will by default).
In this first screen shot, I have selected the Cube (either in the top part of Combi view, the panel on the left, or by clicking on the cube in the 3d window). Then lower in Combi view you have the property editor, I expanded the Placement properties, then expanded the Position properties that were under Placement, then adjust the Y and Z into negative numbers so that the cube moves forward and down so that it is in front of your model and covers the half that you want to hide. Then Adjust the size properties of the cube, Length and height to make the cube the size you see in the picture or bigger. Only after that do you adjust the Width property as that will expand the cube along the full length of you STEP file model, after which you will have a Cube like the second picture below.
I changed he View orientation in this second picture just so that you can see what it looks like, you are not effecting the model by doing that, you are just looking at it from a different direction. You do not need to do this, I just did it for demonstration purposes.
3) Select the yellow "Part" group object, and then hold Ctrl key down on you keyboard while clicking on Cube, then click on Part WB Cut number 3 in this next picture.
and you should end up with this
OS: Ubuntu 18.04.4 LTS (KDE/plasma)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Build type: Release
Python version: 3.6.9
Qt version: 5.9.5
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/Australia (en_AU)