High-strength steel sheet excellent in workability and manufacturing method thereof
Inactive Publication Date: 2014-02-13
KOBE STEEL LTD
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[0043]The present invention can provide a first high-strength steel sheet, which has elongation and local formability at satisfactory levels and exhibits superior workability even having a high strength of 780 MPa or more. This is achieved by forming, particularly as bainite, both two different bainitic microstructures, i.e., a high-temperature-formed bainite and a low-temperature-formed bainite, and forming polygonal ferrite in a predetermined amount. The high-temperature-formed bainite and the low-temperature-formed bainite differ from each other in existence form of retained austenite and carbides. The high-temperature-formed bainite is formed at a high temperature in the range of from 400° C. to 540° C.; whereas the low-temperature-formed bainite is formed at a low temperature in the range of from 200° C. to lower than 400° C. The present invention can also provide a method of manufacturing the first high-strength steel sheet having a strength and workability both at high levels.
[0044]The present invention ca
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However, a steel sheet generally exhibits inferior workability with an increasing strength.
However,
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experimental example 1
[0238]Experimental slabs were prepared by subjecting steels having chemical compositions given in Tables 1 and 2 below (with the remainder consisting of iron and inevitable impurities other than P, S, N, and O) to vacuum ingot making. The rare-earth elements (REM) in Tables 1 and 2 employed a misch metal containing about 50% of La and about 30% of Ce.
[0239]The Ac1 points and the Ac3 points of the steels (slabs) were calculated based on the chemical compositions given in Tables 1 and 2 and on Expression (a) and Expression (b), respectively. The results are indicated in Tables 3 to 5 below.
[0240]The prepared experimental slabs were sequentially subjected to hot rolling, cold rolling, and continuous annealing and yielded specimens. Specific conditions are as follows.
[0241]The experimental slabs were heated to 1250° C. and held at this temperature for 30 minutes, hot-rolled to a rolling reduction of about 90% at a finishing mill delivery temperature of 920° C., and cooled from this temp...
experimental example 2
[0308]Experimental slabs were prepared by subjecting steels having chemical compositions given in Table 9 below (with the remainder consisting of iron and inevitable impurities other than P, S, N, and O) to vacuum ingot making. The rare-earth elements (REM) in Table 9 employed a misch metal containing about 50% of La and about 30% of Ce.
[0309]The Ac1 points and the Ac3 points of the steels (slabs) were calculated based on the chemical compositions given in Table 9 and on Expression (a) and Expression (b), respectively. The results are indicated in Tables 10 and 11 below.
[0310]The prepared experimental slabs were sequentially subjected to hot rolling, cold rolling, and continuous annealing and yielded specimens. Specific conditions are as follows.
[0311]The experimental slabs were heated to 1250° C. and held at this temperature for 30 minutes, hot-rolled to a rolling reduction of about 90% at a finishing mill delivery temperature of 920° C., and cooled from this temperature down to a ...
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Abstract
Provided are: a high-strength steel sheet which is improved in both elongation and local formability and thus exhibits excellent workability; and a manufacturing method thereof. The high-strength steel sheet contains C, Si, Mn, Al, P and S with the remainder including iron and unavoidable impurities, and has a metal structure which includes polygonal ferrite, bainite, tempered martensite, and retained austenite. In the metal structure, (1) the bainite has a composite microstructure including both a high-temperature-formed bainite having an average distance between adjacent regions of retained austenite and/or carbide of 1 μm or more and a low-temperature-formed bainite having an average distance between adjacent regions of retained austenite and/or carbide of less than 1 μm each identified upon observation with a scanning electron microscope; and (2) the retained austenite is present in a volume percentage of 5% or more of the entire metal structure as determined by a saturation magnetization measurement.
Description
TECHNICAL FIELD[0001]The present invention relates to high-strength steel sheets having excellent workability and having a tensile strength of 780 MPa or more or 590 MPa or more; and methods of manufacturing the steel sheets.BACKGROUND ART[0002]The automobile industry is facing an urgent need of responding to global environmental issues such as CO2 emission control. On the other hand, from the viewpoint of securing safety of passengers, the safety standards for collision of automobiles have been made stricter, and structural designing sufficiently securing safety in automobile cabins has been progressing. In order to meet both of these requirements, it is effective to use a high-strength steel sheet having a tensile strength of 780 MPa or more as an automotive structural member and to make the thickness further smaller to reduce the body weight. However, a steel sheet generally exhibits inferior workability with an increasing strength. Application of high-strength steel sheets to au...
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