Zinc hot dip galvanized composite high strength steel plate excellent in formability and bore-expanding characteristics and method for production thereof

Abstract

The present invention provides a hot dip galvanized composite high strength steel sheet excellent in shapeability and hole enlargement ability and a method of production of the same, that is, a hot dip galvanized composite high strength steel sheet excellent in shapeability and hole enlargement ability containing C: 0.01 to 0.3%, Si: 0.005 to 0.6%, Mn: 0.1 to 3.3%, P: 0.001 to 0.06%, S: 0.001 to 0.01%, Al: 0.01 to 1.8%, and N: 0.0005 to 0.01% and having a metal structure of ferrite and, by area rate, 5% to 60% of tempered martensite and a method of production of the same comprising hot rolling, then cold rolling a slab including the above ingredients, heating the sheet in the hot dip galvanization heating process to Ac 1 to Ac 3 +100°C, holding it there for 30 seconds to 30 minutes, then cooling it by a 1°C / s or higher cooling rate to 450 to 600°C, hot dip galvanizing it at that temperature, then cooling it at a 1°C / s or higher cooling rate to the martensite transformation point or lower in temperature, holding it there at 200°C to 500°C for 1 second to 5 minutes, then cooling it at a 5°C / s or higher cooling rate to 100°C or less.

Description

technical field [0001] The present invention relates to a hot-dip galvanized composite high-strength steel sheet excellent in formability and hole expandability and a manufacturing method thereof. Background technique [0002] In recent years, due to the increase in automobile fuel costs, there has been an increasing demand for weight reduction of vehicle bodies, and in order to reduce weight, the demand for high-strength steel sheets has increased. However, forming of high-strength steel sheets becomes difficult with an increase in strength, and the elongation of steel materials in particular decreases. In response to this, recently, TRIP steel (High Retained Austenitic Steel) having both high strength and elongation is gradually being used for frame parts of automobiles. [0003] However, since conventional TRIP steels contain more than 1% Si, it is difficult for plating to adhere uniformly, and the parts to which it can be applied are limited. In addition, in retained a...

AI Technical Summary

Problems solved by technology

In response to this requirement, in the ferrite + martensite DP steel proposed in Patent Document 2, the strength difference between martensite and ferrite is large, and there is a problem of poor hole expandability.