High strenght steel sheet having superior ductility and method for manufacturing the same

Abstract

The invention discloses a high strength steel sheet and a method for manufacturing the same are proposed, the high strength steel sheet having superior phosphatability properties and hot-dip galvannealed properties besides a tensile strength of 950 MPa or more and a high ductility, and also having a small variation in mechanical properties with the change in annealing conditions. The high strength steel sheet described above has a component composition which includes 0.05 to 0.20 mass percent of C, 0.5 mass percent or less of Si, 1.5 to 3.0 mass percent of Mn, 0.06 mass percent or less of P, 0.01 mass percent or less of S, 0.3 to 1.5 mass percent of Al, 0.02 mass percent or less of N, 0.01 to 0.1 mass percent of Ti, and 0.0005 to 0.0030 mass percent of B; at least one of 0.1 to 1.5 mass percent of Cr and 0.01 to 2.0 mass percent of Mo; and the balance being Fe and inevitable impurities, and in addition, the high strength steel sheet is composed of a microstructure including ferrite and martensite and has a tensile strength of 950 MPa or more.

Description

technical field [0001] The present invention relates to a high-strength steel sheet having high strength and excellent formability (ductility) mainly suitable for automobile bodies, particularly for automobile structural members, and a method for manufacturing the same; excellent Phosphatability and Zn plating performance; little change in mechanical properties with changes in annealing conditions during manufacture; and tensile strength above 950MPa. In this case, the above "Small change in mechanical properties with changes in annealing conditions" indicates the difference between the maximum and minimum tensile strength ΔTS in the soaking temperature range of 780-860°C in the annealing step Below 100MPa. Background technique [0002] In recent years, due to global environmental protection, there has been a strong demand to improve the fuel efficiency of automobiles. Therefore, increasing the strength, reducing the thickness, and reducing the weight of the material formi...

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Problems solved by technology

Therefore, it has been pointed out that at the above positions, hydrogen is accumulated and, as a result, delayed fracture is disadvantageously prone to occur

[0008] On the other hand, although transformation hardening type DP steel composed of ferrite and martensite is known as a steel plate with low yield stress and high ductility, in order to achieve high strength and high ductility, it is still necessary to add a large amount of Si, as a result, the problem of deterioration of phosphating treatment performance and / or alloying hot-dip galvanizing performance occurs

Therefore, in Patent Documents 6 and 7, in order to ensure alloying hot-dip galvanizing properties, a steel sheet has been disclosed in which the amount of Si is reduced and Al is added; however, it cannot be said that sufficient ductility is achieved

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