A hot-dip galvanized steel sheet has been frequently used for automobiles, home electric appliances, building materials, and the like. Extremely high formability is required for an automotive steel sheet which is pressed into complicated shapes. In addition, since a demand for rust-proof performance of the automobile increases in recent years, the hot-dip galvanized steel sheet comes to be often used for the automotive steel sheet.
In recent years, there are increasing needs for a high-strength steel sheet excellent in strength and ductility in view of reduction in vehicle body weight. For example, Patent Document 1 discloses a steel sheet having a steel sheet structure where a ferrite phase, a bainite phase, and an austenite phase are mixed. In addition, it is disclosed that this steel sheet is a steel sheet making use of transformation-induced plasticity exerting high ductility by the transformation of retained austenite into martensite at a forming time.
This type of steel sheet contains, for example, 0.05 to 0.4 mass % of C, 0.2 to 3.0 mass % of Si, and 0.1 to 2.5 mass % of Mn, and forms a complex structure by controlling a temperature pattern during a cooling process after annealing at a two-phase region. There is a characteristic that desired properties can be secured without using expensive alloying elements. In recent years, there are increasing needs for a high-strength hot-dip galvanized steel sheet where a steel sheet base material surface is subjected to hot-dip galvanized plating in order to secure the rust-proof performance even for the aforementioned high-strength steel sheet.
There are increasing cases where the high-strength steel sheet is used not only as a strengthening member for an inner plate use but also as an outer surface member which may receive impact from flying stones or obstacles at a vehicle body driving time. When the high-strength steel sheet is applied to a complicated shaped member, high processability is required. The high-strength hot-dip galvanized steel sheet is required to secure high plating adhesiveness evaluated by severe evaluation methods such as a ball impact test and a draw bead test in addition to a normal 60° V bending test assuming cases when impacts from flying stones or obstacles are received during driving and the plating adhesiveness at a hard working time.
Since cracks occur at plating and a base iron regarding a portion receiving extremely severe working such as a 180° bending work vertex part, corrosion is likely to occur from the portion even after it is subjected to a conversion treatment and an electrodeposition coating. Even a slight corrosion may cause hydrogen penetration from the corroded part to thereby increase a possibility of hydrogen embrittlement cracking particularly when the base material is the high-strength steel sheet.
When the high-strength steel sheet is subjected to galvanized plating at a continuous hot-dip galvanizing facility, plating wettability largely decreases when an Si amount of the steel sheet exceeds 0.3 mass %. There is therefore a problem that unplating occurs to deteriorate an appearance quality in case of a Sendzimir method using a normal Al containing plating bath. In addition, the plating adhesiveness at the impact time or hard working time was difficult to be secured because the plating adhesiveness is simultaneously largely lowered.
It is said that this is because an external oxide film which contains oxides containing Si and Mn having low wettability with respect to molten Zn is generated at a steel sheet surface at a reduction annealing time.
As a means to solve this problem, Patent Document 2 proposes a method where the steel sheet is previously heated in an atmosphere at an air ratio of 0.9 to 1.2 to generate Fe oxide, and then plating is performed in a bath where Mn and Al are added after a thickness of the oxide is set to 500 Å or less in a reduction zone containing H2. However, there have been problems that it is difficult to accurately control the thickness of the oxide, and a manufacturing conditional range in an actual machine is narrow because various steel sheets containing various additive elements are passed through an actual line. Further, though an effect of improving wettability and plating adhesiveness at a normal working time could be expected, an effect of improving the plating adhesiveness at the impact time or hard working time was small.
Patent Document 3 discloses a method improving the platability by supplying specific plating to a lower layer as another unplating suppression means. However, this method requires to newly provide a plating facility at a previous stage of an annealing furnace in a hot-dip plating line, or previously perform a plating treatment in an electroplating line. Both cases cause serious increase in manufacturing cost. Further, the effects of improving the plating adhesiveness at the impact time or hard working time and the worked portion corrosion resistance were small.
Meanwhile, Patent Document 4 discloses a method manufacturing a hot-dip galvanized steel sheet without oxidizing Fe in the steel sheet by adjusting an oxygen potential in an annealing atmosphere at the annealing time. In this method, easily oxidizable elements such as Si and Mn in the steel are internally oxidized by controlling the oxygen potential of the atmosphere to suppress formation of an external oxide film, to thereby improve the platability. Although sufficient adhesiveness can be secured at the normal working time by applying this method, the effects of improving the plating adhesiveness at the impact time or hard working time and the worked portion corrosion resistance could not be expected.