A70-2 is the AFNOR designation for a high-strength, low-alloy structural steel corresponding to the German DIN 17100 grade St70-2 (also cataloged as Fe690-2), one of the strongest grades in the older DIN structural steel series before EN 10025 renamed and reorganized these classifications. Unlike the low-strength Fe330 family, St70-2 gets its strength from a combination of controlled carbon content, fine grain practice, and typically some manganese and microalloying rather than from a single dominant alloying element, which lets it deliver roughly double the tensile strength of basic mild steel while still being weldable with reasonable precautions.

This grade is used where a structural steel needs to carry meaningfully higher loads than mild steel without moving up to a quenched-and-tempered plate product — heavy equipment frames, crane and lifting structures, high-stress fasteners and pins, and fabricated components in mobile and off-highway equipment. Because higher-strength structural steels like this one still rely primarily on grain refinement and moderate alloying rather than martensitic hardening, they retain useful ductility and notch toughness at their strength level, which matters in structures exposed to shock loading or cold service temperatures.

St70-2 / A70-2 is defined by DIN 17100 primarily through guaranteed mechanical properties and grain-refining practice rather than a fixed chemical analysis published for general reference. It is understood to be a low-carbon, fine-grained structural steel with modest manganese content and standard limits on phosphorus and sulfur. A verified composition range is not published here rather than present an unreliable figure.

A70-2's higher strength changes the machining picture noticeably compared with mild structural grades. Cutting forces run higher, and the finer, more work-hardened grain structure that gives this steel its strength also makes it somewhat more abrasive on the cutting edge than a low-carbon equivalent. Built-up edge is less of a concern than on soft mild steel, since the material shears more cleanly rather than smearing, but flank wear becomes the more relevant limiting factor as strength increases.

Chip formation tends to be more favorable than in dead-soft steel — the higher strength promotes cleaner shear and better natural chip breaking, which reduces (though doesn't eliminate) the reliance on chipbreaker geometry to control chip length. Heat generation at the tool-chip interface runs higher than with mild steel, so coated carbide grades with good hot hardness hold an advantage over uncoated tooling in anything beyond light finishing cuts.

Overall, A70-2 sits in a middle ground: still a plain low-alloy steel rather than a hardened or highly alloyed grade, but strong enough that cutting parameters should be pulled back from mild-steel defaults. Rigid setups and a wear-resistant coating make the biggest difference in tool life at this strength level.