1. Field of the Invention
The present invention concerns a method of manufacturing hot-rolled steel sheets, cold-rolled steel sheets, hot dip galvanized hot-rolled steel sheets, hot dip galvanized cold-rolled steel sheets, etc. and, in particular, it relates to a method of manufacturing various kinds of steel sheets as described having excellent resistance to cold-work embrittlement or provided with bake-hardening property (BH property).
2. Description of the Prior Art
Steel sheets used for automobile parts or outer panels of electric equipments have been required to be light in weight, free from rusting and having excellent cold workability in recent years.
For such requirements, component steels, a so-called IF (Interstitial Free) steels, in which carbo-nitride forming elements such as Ti or Nb are added alone or in combination to ultra-low carbon steels for stabilizing C and N in the steel have generally been used.
However, ultra-low carbon steels in which C and N in the steels are sufficiently stabilized by the addition of carbo-nitride forming elements such as Ti and/or Nb involve a problem that cracking due to brittle fracture occurs in the cold-work after press forming. This is attributable to that solid-solute C and N are not present in the steels and, accordingly, C and N are no more segregated into the grain boundary to weaken the grain boundary.
Further, P-added steels involve a problem that P is segregated to the grain boundary to promote brittleness or hot dip galvanized steels involve a problem that zinc intrudes into the grain boundary upon hot dip galvanizing treatment to further reduce the strength of the grain boundary. Furthermore, since the baking hardening (BH) property is obtained under the effect of solid-solute C and N in the steels, the property can not be provided in such IF steels.
It has, accordingly, attempted, for improving the resistance to cold-work embrittlement or providing the BH property, to melt the steels while previously controlling the addition amount of Ti and Nb such that solid-solute C and N in the steels may be left. In this method, however, even if component steels having residual solid-solute C and N can be prepared, remarkable reduction is inevitable for the press formability since the solid-solute C and N generally deteriorate the r-value and the ductility of the steels. That is, the press formability and the resistance to the cold-work embrittlement or the BH property can not be compatible with each other. Furthermore, such a slight amount of solid-solute C and N can not be left in the steels in view of the steel making technology.
In view of the above, although the proposals as described below have been made so far, it is difficult to attain both excellent press formability and the resistance to cold-work embrittlement or the BH property together.
For instance, there has been proposed a method of adding Ti and Nb to stabilize C in the steels applying carburization upon open coil annealing after cold rolling thereby forming a carburized layer at the surface of steel sheets with an aim of improving the resistance to cold-work embrittlement of steel sheets used for deep drawing (Japanese Patent Laid-Open Sho 563-38556). In this method, however, since carburization is applied upon batch annealing conducted over a long period of time, it involves problems that a steel sheet has a difference in the composition and the microstructure are in the direction of the sheet thickness, such as a carburized layer at high concentration (average amount of C: 0.02 to 0.10%) is formed only at the surface layer of the steel sheet and a difference is caused in the ferrite grain size between the surface layer and the central portion. Furthermore, such a batch annealing naturally has low productivity, as well as results in a disadvantage that the material tends to be inhomogenous in the direction of the length and width of the sheet.
Also, as a method of manufacturing a steel sheet for use in deep drawing by the addition of Ti and Nb, there has also been proposed a method of applying recrystallization annealing after cold rolling and then further applying carburization (Japanese Patent Laid-Open Hei 1-96330). However, this method intends to improve the strength mainly by the precipitation of a great amount of carbides or nitrides and no consideration is taken for the resistance to cold-work embrittlement and the BH property. In addition, since carburization is applied batch-wise for a long period of time after annealing, the amount of carburization tends to become excessive and inhomogenous, as well as the productivity is low and the steps are complicate.