1. Field of the Invention
The present invention relates to a ferritic stainless steel having good workability with less anisotropy useful as material worked to sheets for an automobile and other parts.
2. Description of Related Art
Ferritic stainless steels having improved heat- and corrosion-resistance by stabilization of C and N with Nb or Ti have been used in broad industrial fields. For instance, such ferritic stainless steel is used as a member of an exhaust system for an automobile. A steel material such as SUS409L, SUS436L or SUS436J1L, which contains Nb or Ti to suppress sensitization and to improve intergranular corrosion-resistance, is used as a center pipe or muffler having good corrosion-resistance. A steel material such as SUS430LX, SUS430J1L or SUS444, which contains Nb or Ti more than a stoichiometric ratio of C and N contents to improve high-temperature strength due to dissolution of surplus Nb or Ti in a steel matrix is used as an exhaust manifold or front pipe having good heat-resistance.
In addition, there is the tendency that a member of an exhaust system is designed in a more and more complicated shape for space-saving and for improvement in exhaust efficiency. Due to such complicated shapes, ferritic stainless steel should possess superior workability without occurrence of defects even after severe deformation.
Demand for improvement of workability is not only for use as an exhaust system but also for other uses. That is, ferritic stainless steel shall be deformed with heavier duty as more complicated shape of a product in order to improve function and/or design of the product.
There are various proposals for improvement of ferritic stainless steel in workability. These proposals are basically classified to proper control of composition and proper control of manufacturing conditions.
An alloying design proposed by JP 51-29694B and JP 51-35369B is to reduce C and N contents together with addition of carbonitride-forming elements such as Ti or Nb at a relatively great ratio. Addition of Ti and/or Nb to ferritic stainless steel for use as a member for an exhaust system is meaningful in improvement of workability and performance for system requirements, since the additives Ti and Nb improve the workability of the steel as well as corrosion- and heat-resistance necessary for a member for an exhaust system.
A value {overscore (r)} representing deep drawability is surely improved by the addition of Ti and/or Nb, but the additives Ti and Nb unfavorably enlarge in-plane anisotropy Δr of the value {overscore (r)}. In this sense, mere addition of such alloying elements is not enough to bestow ferritic stainless steel with sufficient workability, which meets requirements for severe deformation.
Addition of one or more of Al, B and Cu is also known for improvement of workability.
There have also been proposed various methods on proper control of manufacturing conditions from a steel-making step to a cold-rolling or finish-annealing step. For instance, reformation of an as-cast slab to tesseral crystalline structure in a steel-making step, and lowering of an initial temperature, soaking a steel strip at a proper temperature, lowering of a finish temperature and lowering of a coiling temperature in a hot-rolling step. These temperature controls are often carried out in combination with control of a reduction ratio. Control of a friction coefficient between a steel strip and a work roll during hot-rolling is also effective for improvement of workability. All of these methods aim at destruction of an as-cast structure, which puts harmful influences on re-crystallization.
Even in steps succeeding to the hot-rolling step, increase of a cold-rolling ratio is also effective for improvement of a value {overscore (r)} with less in-plane anisotropy Δr, as reported in “Stainless Steel Handbook” (edited by Stainless Steel Society in Japan and issued by Nikkan Kogyo Shimbun Co. in 1995) p.935. A cold-rolling ratio of Ti-alloyed steel is necessarily determined at a value more than 60% (preferably 70–90%) for the purpose. Twice cold rolling-twice annealing in various combinations of cold rolling conditions with annealing conditions or with a bigger work roll is also effective for improvement of workability. For instance, a steel material based on SUS430 composition, to which alloying elements are alloyed at small ratios, or a steel material based on SUS430 compositions, to which Al and Ti are alloyed, are those steels improved in workability by manufacturing conditions.
However, there are only a few reports on investigation of manufacturing conditions of Ti- or Nb-alloyed ferritic stainless steel for corrosion- or heat-resistance use, with extension referring to knowledge represented by “one or two of Ti and Nb”, as described in JP 6-17519B and JP 8-311542A. These methods proposed so far need additional means in a conventional manufacturing process or inevitably change a manufacturing process itself, resulting in rising of a manufacturing cost and a product cost in the end.
Effects of manufacturing conditions on workability have been researched for a ferritic stainless steel sheet of 0.7–0.8 mm in thickness, but such effects on workability of a ferritic stainless steel sheet thicker than 1.0 mm are not clarified yet. Taking into account actual use, a thicker steel sheet of 2 mm or so in thickness has been broadly used as a member of an exhaust system for an automobile. When the above-mentioned method is applied to a process of manufacturing such a thick stainless steel sheet, a hot-rolled steel strip is necessarily thicker than 6 mm in order to realize a cold-rolling ratio of more than 70%. As a result, a hot-rolled steel sheet shall be cold-rolled with a heavy duty while stabilizing its traveling influenced by low-temperature toughness and bendability, so that rising of a manufacturing cost is unavoidable.
In short, there is a strong need for a Ti- or Nb-alloyed ferritic stainless steel having good workability without the necessity of additional means or rising of manufacturing cost, even when the ferritic stainless steel is rolled to a strip thicker than 1.0 mm.