The steel industry's legal assault on steel imports accelerated yesterday as National Steel Corp. charged six European countries with dumping, or illegally selling below cost, sheet steel and asked theTresury to assess special duties on the imports.

Meanwhile, Commerce Secretary Juanita Kreps suggested that the governmnet should consider tax breaks, simplifying environmental regulations and revising antitrust laws to help the U.S. steel industry become healthy again.

She said that the industry's problems go well beyond import inroads by Europe and Japan.

As part of the administration's program to help the trouble-plagued industry, however, President Carter promised a vigorous enforcement of the nation's anti-dumping laws to insure "fair" competition from abroad, something he said earlier administrations had not done.

Treasury Secretary W. Micheal Blumenthal, who administers the anti-dumping lsws, invited steel makers to be more aggressive in filing dumping charges and said his agency would expedite processing of dumping complaints.

Wednesday the Treasury said it found cause to pursue an investigation of a complaint filed last month by U.S. Steel Corp., alleging that Japanese steel makers dumped $1.2 billion of steel in the United States market.

National chairman George A. Stinson siad his company's complaint against six major European producers was a "direct response" to the Carter invitation to file more dumping suits, although such complaints are lengthy and expensive and takes months, not a weeks, to produce.

The National complaint cites cold-rolled and galvenized (zinc-coated) sheets from Belgium, France, Italy, West Germany and the Netherlands, and cold-rolled sheets only from the United Kingdom.

These imports have totaled $236.8 million so far this year, Stinson said. For all of last year the six countries shipped $222.4 million of the named products, he said.

Sheet steel is used in products such as cars and appliances. The sheets are coated with zinc to retard corrision. National is the nation's third biggest producer in terms of shipments and is fifth in sales. It is a major producers of sheet steel.

In the complaint filed with the Treasury - which the governmnet must decide whether to pursue within 30 days - National charges that the countries in question are undervaluing their products by as much as 47 per cent on galvanized sheets and 43.5 per cent on cold-rolled sheets.

Japan has argued that because of the way the U.S. laws are written, anti-dumping laws do not ensure fair trade, but are in fact protectionist. When deciding whether a company is selling its product in the U.S. below the prices it sells in the home market, the Treasury is forced to disregard any sales in a company's market that is below cost.

That artificially inflates the price of the average dometic prices of steel Japan argues. Japan has complained about the anti-dumping provisions of the 1974 U.S. trade laws, which it considers to be illegal under international agreements.

Stinson appeared to break ranks with the industry position against import quotas that was taken last week at a special White House conference. At that conference U.S. Steel chairman Edgar B. Speer said quotas are not the answer and approved of the administration's decision to enforce anti-dumping laws.

Stinson said yesterday that National would continue to seek more immediate relief through quotas - a full-scale dumping investigation can take more than a year before injury is found and duties formally levied.

"Quotas are certainly not an ideal solution. They have weaknesses", the Associated Press quoted Stinson. "But anti-dumping is also not ideal. It's lengthy and uncertain."

Kreps, speaking to the Pennsylvania Chamber of Commerce meeting in Pittsburgh, said the government should consider whether to permit steel companies to engage in joint ventures.

She said such joint ventures "might be one way of strengthening the steel industry and may in fact be one way of ensuring that smaller and medium-sized producers survive." The government must seek "ways, within our general antitrust policies to allow these sensible arrangements to occur," Kreps said.

She said the tax reform proposal should consider permitting steel makers to write off quickly the costs of installing non-productive investments, such as pollution control devices.

In an appearance in Washington Kreps said the government stands ready to help communites hit by steel mill closings and worker layoffs. Since mid-year nearly 20,000 blue and white collar steel workers have been laid off, many of them permanently.