Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Patent
1994-09-14
1996-02-20
Yee, Deborah
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
148602, 148609, C21D 802
Patent
active
054925754
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
In recent years, a technology of casting a thin cast strip having a strip thickness of 10 mm or less directly from a steel melt was developed and tested on an industrial scale. This new technology provides a process of producing a cold-rolled thin sheet product, in which a hot rolling step is either simplified or omitted, and thereby attracts considerable attention and is expected from the view point of saving energy and cost.
Processes of producing a thin sheet, including the above process, will hereinafter be referred to as "STC processes" (Strip Casting Process). In contrast, processes of producing a cold-rolled thin sheet product which includes the steps of continuous-casting a 100 mm or more thick steel slab, hot-rolling the slab to a several millimeters thick hot-rolled strip, and cold-rolling the hot-rolled strip, will be referred to as a conventional process.
The present invention relates to a process of producing a thin cast strip having a high toughness, particularly a thin cast strip of a Cr-stainless steel containing Nb, Ti, Al, etc., by an STC process.
BACKGROUND ART
Conventionally, Cr-stainless steels were produced by a conventional hot rolling-process which included casting a slab and hot-rolling the slab. This process had a problem that ridging (or roping) occurs in the cold-rolled thin sheet products due to a texture established during hot rolling. Then, trials were conducted in which the STC process was used to cast a thin cast strip for producing a thin sheet product in which no ridging occurs. For example, Japanese Unexamined Patent Publication (Kokai) No. 62-176649 disclosed "Process of Producing Ferritic Stainless Steel Thin Strip Having No Roping". This publication, however, did not describe the reduction in toughness which occurs in Cr-stainless steels having a single phase structure and containing Nb, Ti, Al, and V in an amount of from 0.05 to 1.0 wt % in total. Therefore, there remains a problem that a cast strip of a Cr-stainless steel containing Nb, Ti, Al and V in the above-recited total amount has too low a toughness to be cold-rolled in a subsequent step.
Japanese Unexamined Patent Publication (Kokai) No. 64-4458 entitled "Rapid-Cooled Strip of Ferritic Stainless Steel Having High Toughness" disclosed that a cast strip having a high toughness can be produced by controlling its columnar crystal content to 70% or more, but did not consider the technological significance of the relationship between the toughness of and the precipitates in the cast strips of Cr-stainless steels containing NB, Ti, Al, and V.
The present inventors have been developing a technology of producing Cr-stainless steel thin sheet by using an STC process. As a result, it became apparent that cast strips have a poor toughness which causes cracking to occur during cold rolling of SUS 430 or other steel systems in which a .gamma.-phase is precipitated during cooling after solidification to room temperature and a martensite phase transformed from the .gamma.-phase remains at room temperature.
To prevent the precipitation of .gamma.-phase during cooling after solidification to room temperature, the present inventors produced a thin cast strip of a Cr-stainless steel with a controlled chemical composition having a .gamma.p value of 0% or less. The term .gamma.p is a parameter predicting the precipitate amount of .gamma.-phase based on the chemical composition. However, even when a Cr-stainless steel has a .gamma.p of 0% or less, there remains a problem that a cast strip has a poor toughness and is broken during cold rolling when it contains one or more of Nb, Ti, Al and V in an amount of 0.05 wt % or more in total.
The present inventors made a study and found that thin cast strips of Cr-stainless steels containing such elements and exhibiting a poor toughness contain fine precipitates with a size of 0.1 .mu.m or less. It is known that such fine precipitates harden the steel matrix and thus deteriorates the toughness.
A thin cast strip cast by an STC process contains fine precipitates o
Mizunuma Takehisa
Nakazawa Takanori
Satoh Yuichi
Teraoka Shin-ichi
Nippon Steel Corporation
Yee Deborah
LandOfFree
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