IIRC, M1A1(HA)/M1A2 armor scheme is as follows:

• Hull Glacis - RHA/Ceramic Composite (Chobham variant)
• Hull Sides - RHA Steel with side skirts to pre-detonate HEAT rounds
• Hull Rear - RHA Steel
• Turret Front - Ceramic Composite (Chobham variant) with DU inserts
• Turret Sides - Ceramic Composite (Chobham variant)
• Turret Rear - RHA Steel
• Turret Roof - RHA Steel

As one can see, priorities were placed on the turret.

So, there is some minute differences between the Chally 2 and M1A2 with respect to the type of steel used. IIRC, the US uses a steel with less yield strength than the British tank overall. But this is mainly because of the lessened importance of steel in the overall protective scheme.

Major differences in Ceramic composites:

• M1 has Chobham
• IPM1 variant has improved Chobham
• M1A1 has improved ceramic composites
• M1A1(HA) further improvements in ceramic composites + Gen I DU Armor
• M1A1(HC) ceramic improvements + Gen II DU Armor (same as early M1A2 models)
• M1A2 ceramic improvements + Gen II DU Armor
• M1A2 SEP further ceramic improvements + Gen III DU Armor

By "improvements" in ceramic armor, I'm referring to improved fabrication and cladding processes to improve the ballistic properties of the armor when struck (holding the ceramic together longer) to keep it from breaking into pieces, as well as generally preventing wear and tear on the ceramic components through use (damage from vibration when driving over rough terrain and other impacts).

The Army incorporated these improvements each time they upgraded the Abrams series.

The current standard is M1A2SEPV2