In January 2019, Hyundai Heavy Industries, the biggest shipbuilding group globally, signed a conditional deal with Korea Development Bank (KDB), the biggest shareholder of DSME, on buying the company.
This would make the HHI to control more than 20% of the global market, according to Reuters.
From the beginning, the plans rose a wave of protests from the workers of both companies, who demanded job guarantees in light of potential layoffs.
The unionized workers started their four-hour strike at the company's headquarters in Ulsan, some 400 km southeast of Seoul.
In their latest express of opposition, they are planning to stage four-hour strikes until Tuesday before pushing ahead with an eight-hour strike on Wednesday, Yonhap news agency reported.
HHI shareholders are set to approve the proposed split on May 31.
Hyundai Heavy swung to black in the first quarter of this year with a net profit of 17.3 billion won from a loss of 132 billion won a year earlier. Last year, Hyundai Heavy suffered a net loss of 633 billion won.
Hyundai Heavy said it may take legal action against the union if the latest strike turns out to be illegal.