High yield ratio and high-strength hot-dip galvanized steel sheet excellent in workability and production method thereof

Abstract

A high-strength hot-dip galvanized steel sheet excellent in workability according to the present invention: contains C, Si, Mn and other elements; has a dual phase structure containing ferrite and martensite as the metallographic structure; and, in the ferrite structure, satisfies the expression 0.2<=(Lb / La)<=1.5 when the length per unit area of the grain boundaries of crystal grains the crystal orientation differences of which are 10 degrees or more is defined as La and the length per unit area of the grain boundaries of crystal grains the crystal orientation differences of which are less than 10 degrees is defined as Lb and further satisfies the requirements that the average value of D is 25 [mu]m or less and the area ratio of crystal grains satisfying the expression D<=30 [mu]m in the ferrite grains surrounded by the grain boundaries of crystal grains the crystal orientation differences of which are 10 degrees or more is 50% or more when the circle equivalent diameter of each of ferrite grains surrounded by the grain boundaries of crystal grains the crystal orientation differences of which are 10 degrees or more is defined as D; and has a tensile strength of 980 MPa or more.

Description

technical field [0001] The present invention relates to a high-strength hot-dip galvanized steel sheet (also includes high-strength alloyed hot-dip galvanized steel sheet, the same below) exhibiting a high yield ratio and having a high elongation of 980 MPa or more, which is suitable for steel sheets for automobiles. And a useful manufacturing method for making such high-strength hot-dip galvanized steel sheets. Background technique [0002] In recent years, due to the increasing awareness of global environmental issues, various automobile manufacturers have reduced the weight of vehicle bodies for the purpose of improving fuel efficiency. In addition, from the viewpoint of the safety of passengers, crash safety standards for automobiles have been strengthened, and the durability of components against impacts is also required. Therefore, in recent vehicles, the use ratio of high-strength steel sheets has further increased, among which high-strength hot-dip galvanized steel ...

AI Technical Summary

Problems solved by technology

[0004] However, in the techniques of Japanese Patent Application No. 55-122820 and Japanese Patent Application No. 2001-220641, martensite is generated by cooling after galvanizing or subsequent alloying treatment. Movable dislocations are introduced into ferrite, so it becomes a low yield strength

In addition, Japanese Patent Application No. 2002-322539, which has improved the yield strength, utilizes nano-scale precipitated particles, but when annealing is performed after hot rolling and cold rolling, it is difficult to finely disperse the precipitated particles, and it is difficult to achieve high yield strength. with high ductility

[0005] In addition, Japanese Patent Application No. 2006-274378 discloses a high-strength hot-dip galvanized steel sheet having both spot weldability and high yield ratio and its manufacturing method. Stretched grains, inhomogeneous in structure, so it may not be called good processability

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