"Have at least 600 scrap in your ship storage."
Accumulate a total of 600 scrap or more at one time. Note that simply earning and spending 600 scrap in total does not unlock this achievement; you must have 600 scrap in your storage at one point in time.
This is a ship achievement exclusively for the Lanius Cruiser.
As are all in-game achievements, this is most expediently achieved on 'easy' difficulty, mostly on account of increased scrap rewards. Every Scrap Recovery Arm augmentation you equip stacks another 10% increase on rewards from all sources as well. Instead of rushing, it is advisable to explore every sector as thoroughly as possible, to maximize your profits from the multiplicity of beacons. Boarding ships rather than destroying them also tends to reap greater rewards. As you progress, each sector yields higher rewards, so make sure to capitalize on that.
Where practical, saving your scrap is just as important as earning more of it. Once you've reached a manageable shields and weapons level, good enough to come out victorious and unscathed in most battles, stop investing until it is absolutely necessary again. In stores, buy only what you need: crew, weapons, and fuel should be considered your lowest priorities, as all of those can, at some point, be harvested from random events. If you by chance happen upon any Zoltan crew members, they can even supplement power for your systems, thus alleviating the need for reactor upgrades.
Upon acquiring this achievement, there is no explicit reason to hold onto your scrap, so you could use your scrap with no consequence.
Alternatively, if you are close to the 600 scrap mark, liquefying your entire inventory would be enough to reach the amount needed for the achievement, provided that you are okay with quickselling all the weapons, drones, and augments that you own.