Our first-quarter adjusted EBITDA of $513 million represented a 79% increase over last quarter, reflecting our first full quarter of results from the former AM USA assets, as well as stronger steel pricing, offset by reduced third-party pellet sales due to the annual maintenance of the Great Lake flocks.
In the Steelmaking segment, we sold 4.1 million net tons of steel products, which included 28% hot rolled, 18% cold-rolled, and 33% coated, with the remaining 21% consisting of stainless, electrical, plate, slab, and rail.
Our aggregate average selling price of $900 per ton in Q1 is certainly the low point for the year in our forecast, and is lower than our Q4 2020 average, solely because of the different mix associated with the former AM USA plants.
DD&A was $217 million for the quarter, and we expect about $840 million on a full-year basis now that purchase price accounting has been further refined.
This short-term anomaly had a negative impact on our first quarter of approximately $50 million and will be a $40 million headwind in Q2.
After that, the impact will be negligible, creating nearly a $100 million EBITDA tailwind going forward in comparison to the first half of 2021.
As for synergies, we have already identified and set in motion $100 million in cost synergies from the AM USA acquisition, some of which will take effect later this year.
We are well-positioned to reach our target of $150 million of annual run-rate savings by the end of this year for a total of $310 million from the two combined acquisitions.
As was contemplated in the acquisition of AM USA, we had a significant investment in working capital of nearly $650 million during the quarter, due, first, to the completion of the unwind of the ArcelorMittal AR factoring agreement, as well as other acquisition-related cash impacts.
Also, we made our deferred pension contribution related to the CARES Act of $118 million in January.
With the passage of the most recent stimulus bill and the extended amortization feature, future cash pension contributions will be reduced by an average of $40 million per year over the next seven years.
Sustaining capex will be approximately $525 million annually.
And by the end of March, we had enough pricing visibility to disclose a $1.2 billion adjusted EBITDA guide for Q2.
On the liquidity side, we currently have $200 million in cash and $1.6 billion of availability under our current credit facility.
Our ABL debt balance is currently $1.6 billion, and we expect to have this paid off by the end of the year.
Based on what we are seeing in the market, we believe our estimates supporting $4 billion of adjusted EBITDA for the year are conservative relative to today's forward curve.
Our attitude toward commercial and steel pricing is the main reason behind the massive numbers we are showing for the quarter and guiding for the balance of the year, including $513 million of EBITDA in Q1, $1.2 billion EBITDA next quarter, and $4 billion EBITDA for 2021.
For the record, from our proximate years, the median yearly pay of our 25,000 Cleveland-Cliffs employees is $102,000.
Since December 9, 2020, we have already added 710 new employees to our workforce.
Of all the world CO2 emissions from the steel industry, the U.S. comprises just 2%, while China is responsible for 64%.
EAFs make up more than 70% of steel production in our country.
Meanwhile, we at Cleveland-Cliffs, will continue to enjoy the steady cost structure of our iron feedstock, our own 100% internally sourced pellets, with decades of iron ore reserves ahead, and our in-house production of HBI, fed by our online and pellet plant.
We produced 120,000 tons of recast in the month of March and expect to reach our annual run rate of 1.9 million tons this quarter.
Particularly at our EAFs, HBI currently makes up between 20% and 30% of their melt.
We completed the blast furnace repair in less than 14 days.
Under our latest forecast, we expect to generate a record level of free cash flow in the last nine months of 2021, which will put us at a figure of less than 1 times EBITDA leverage by the end of the year.

Summary:
Based on what we are seeing in the market, we believe our estimates supporting $4 billion of adjusted EBITDA for the year are conservative relative to today's forward curve.
Our attitude toward commercial and steel pricing is the main reason behind the massive numbers we are showing for the quarter and guiding for the balance of the year, including $513 million of EBITDA in Q1, $1.2 billion EBITDA next quarter, and $4 billion EBITDA for 2021.