METHOD FOR PRODUCING HIGH-STRENGTH STEEL SHEET

摘要

The present invention provides a method for manufacturing a high strength steel sheet, comprising the steps of: heating a steel sheet containing a predetermined content of carbon to either temperature in the austenite single phase region or temperature in the (austenite + ferrite) two-phase region; cooling the steel sheet to a cooling temperature region ranging from Ms to (Ms - 150°C) to allow a portion of non-transformed austenite to proceed to martensitic transformation; and heating the sheet temperature to temper said martensite, characterized in that the method further comprises retaining the coldest part in the sheet widthwise direction of the steel sheet at temperature in a temperature range from the cooling stop temperature as the target temperature to (the cooling stop temperature + 15°C) for a period ranging from 15 seconds to 100 seconds (inclusive of 15 seconds and 100 seconds), wherein "Ms" represents martensitic transformation start temperature and said cooling temperature region is exclusive of Ms and inclusive of (Ms - 150°C). As a result, there can be obtained a high strength steel sheet being excellent in formability, exhibiting good stability in mechanical properties and having tensile strength (TS) of at least 980 MPa.