Arthur Ajemyan
Analyst · KeyBanc Capital Markets
Thanks, Steve, and good morning, everyone. Our first quarter 2024 non-GAAP diluted earnings per share of $5.30 were at the low end of our guided range as our tons sold seasonally improved, but pricing softened more than we anticipated. We successfully outperformed industry shipment levels across nearly all products, which bolstered our quarterly earnings despite the challenging pricing environment. Our 31% gross profit margin for the first quarter was attributable in part to higher-than-anticipated LIFO, benefit, better alignment of costs on hand and replacement costs and gross profit margin stability from our value-added processing capabilities.
We recorded LIFO income of $50 million in the first quarter compared to our guidance of $20 million. As prices declined more than we anticipated, we have increased our 2024 annual LIFO income estimate from $80 million to $200 million. Accordingly, we currently expect to record approximately $50 million of LIFO income in the second quarter of 2024.
On a FIFO basis, which is how we monitor our day-to-day operating performance, and which excludes the effect of our LIFO inventory valuation method. Our gross profit margin improved by roughly 80 basis points compared to the prior quarter to 29.6% as we saw improved alignment of inventory cost on hand with replacement costs. This trend began to reverse in March and has continued in the second quarter. While most carbon product prices began to stabilize in April, we'll continue to see short-term gross profit margin pressures throughout the second quarter as we get better alignment of costs on hand with replacement costs. As of the end of the first quarter, the LIFO reserve on our balance sheet was $529 million, which will generate LIFO income and benefit future period operating results to mitigate the impact of potential further declines in metal prices.
Moving along to expenses. On a year-over-year basis, same-store non-GAAP SG&A expenses increased $7.1 million or 1.1%, primarily due to the increased head count to accommodate organic growth, which was partially offset by lower incentive-based compensation resulting from lower profitability. As a reminder, our model normalizes expenses by rightsizing incentives as profits trend down.
I'll now move on to discuss balance sheet and cash flow. For the first quarter, we generated $126.3 million in operating cash flow, which helps fund $108.7 million in capital expenditures, $53.7 million for an acquisition and the return of $65.3 million to our stockholders through dividends. While we did not have any share repurchases in the first quarter of 2024, we have $1.44 billion remaining under our share repurchase authorization and ample liquidity for opportunistic repurchases.
Turning now to our second quarter outlook. Overall, we expect a better-than-normal seasonal recovery in demand in the second quarter of 2024 despite prevailing macroeconomic uncertainty and geopolitical matters. We also expect shipping volumes to increase 2.5% to 4.5% sequentially in the second quarter with approximately 2% of the sequential growth coming from recently completed acquisitions.
On the pricing side, we expect our average selling price per ton sold for the second quarter to be down 1% to 3% compared to the first quarter, which will generate some short-term pressure on our gross profit margin as we work through higher cost inventory on hand. Based on these expectations, we anticipate non-GAAP earnings per diluted share in the range of $4.70 to $4.90 for the second quarter of 2024.
This concludes our prepared remarks. Thank you for your participation. And at this time, we'll now open the call up to questions. Operator?