Still after the recession of '57-58. where Steel demand was lower. But the main problem of the existing Steel plants themselves.
The Steel Companies didn't want to replace the 1910-1920s era Bessemer Plants with the new basic oxygen process plants that were developed after WWII.
After all, they had won both World Wars with those plants, why upgrade? New plants cost money.
Now West Germany and Japan, they weren't shackled with the old technology plants. They were destroyed in the War
They had to start fresh, and there was no reason to build the old style plants, when the new basic Oxygen process plants was far more efficient, fewer workers but made cheaper, and higher quality steel.
By time the 1970s roll around, they just can't compete, so the Steel Unions and Steel Plant Owners, just delayed the inevitable by requesting tariffs and outright limits on the import of higher quality, lower cost steel . Now South Korean Steel was starting to do to Japan, even cheaper and just as good, with China just around the corner, with improved relations after Mao was gone
So even without the Strike, still run into the problem that US Steel costs too much, and much was of lower quality and not the newer more desired alloy types. So the old plants are shuttered, Unionmen laid off.
But at this point, new US Plants were opening using the new methods, so actually US steel production didn't totally drop off, but still was more expensive, so imports continued to take market share, as US demand for Steel kept increasing after the 1970s Economic shocks