Metal treatment – Stock – Ferrous
Reexamination Certificate
1998-09-21
2001-04-24
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S649000, C148S654000
Reexamination Certificate
active
06221178
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to ultra-fine grain steel and a method for producing it. More precisely, the invention relates to ultra-fine grain steel which is useful for high-strength steel for construction and even those for ordinary weld constructions, and also relates to a method for producing the steel.
BACKGROUND OF THE INVENTION
Conventional steel reinforcement includes solid solution reinforcement, reinforcement with second phases with martensite or the like, precipitation reinforcement, and formation of fine grains. Above all, the method of forming fine grains in steel is the best for increasing both the strength and the toughness of steel and form improving the strength-ductility balance in steel. This method does not require any expensive elements such as Ni, Cr or the like, and it is said that high-strength steel can be produced according to the method at low production costs. From the viewpoint of forming fine grains in steel, it is expected that when the size of fine ferrite grains constituting steel could be reduced to 3 &mgr;m or smaller, the strength of the steel could be greatly increased.
At present, however, it is impossible to much more increase the strength of steel obtainable in the current ordinary working and heat-treatment, in which the grains have a size of around 5 &mgr;m, or so, even though the steel of that type could have relatively high strength.
Steel comprising finer ferrite grains could have higher yield strength and higher tensile strength, but is problematic in that its uniform elongation is greatly lowered and that the increase in its yield strength is larger than that in its tensile strength. In other words, the yield ratio of the steel is high. This means the decrease in the n value (the work-hardening coefficient) of the steel. The same shall apply to ultra-fine, single ferrite phase steel having a ferrite grain size of not larger than 4 &mgr;m. That is, the strength of the steel could be increased but the elongation is greatly lowered.
Given that situation, it has heretofore been said that, in order to increase the strength of ferrite steel and to improve the strength-ductility balance thereof, needed is any other technique that is quite different from the conventional technique for much more fining the ferrite grains constituting the steel.
Heretofore, the conventional controlled rolling-accelerated cooling technique has been one effective means for forming fine ferrite grains that contribute to the increase in the strength of low-alloy steel. According to the technique, both the cumulative reduction ratio in the unrecrystallized austenite region in low-alloy steel in the rolling step and the cooling rate for the steel in the next step are controlled to thereby make the steel have finer structure. Even in this, however, the ferrite grains formed could have a grain size of at least 10 microns in Si—Mn steel and a grain size of at least 5 microns in Nb steel. Thus, the technique is still limitative. On the other hand, as in Japanese Patent Publication (JP-B) 62-39228 and 62-7247, formed are ferrite grains having a grain size of around 3~4 microns or so by rolling steel under pressure to attain a total reduction ration of 75% or more at a temperature falling within a range of (Ar
1
to Ar
3
+100° C.) including the two-phase range, followed by cooling it at a cooling rate of not lower than 20 K/s. As in JP-B Hei-5-65564, an extremely great reduction ration and a cooling rate of not lower than 40 K/s are needed for forming finer ferrite grains having a grain size of smaller than 3 microns. However, the rapid cooling at a rate of 20 K/s or larger is acceptable only in the production of steel sheets, but could not widely in the production of ordinary steel for weld constructions.
Given that situation in forming finer ferrite grains capable of contributing to the increase in the strength of steel, it is extremely difficult in the prior art to form finer ferrite grains having a grain size of smaller than 3 microns. In fact, no effective technique has heretofore been realized for forming such finer ferrite grains.
In addition, the increase in the reduction ratio in the unrecrystallized region in the controlled rolling causes another problem. For example, as in
FIG. 11
(from “Iron and Steel”, 65 (1979), 1747-1755), the increase in the working ratio results in the increase in the density of specific orientations (332) <113> and (113) <110> of ferrite grains, whereby the proportion of the small angle grain boundaries is increased. Even if fine grains having a grain size of 3 microns or so could be formed in steel, the strength and even the fatigue strength of the steel could not be increased so much to the level of the expected degree corresponding to the fined size of the grains. In addition, in that case, since there is a great probability that the ferrite grains formed all have the same orientation, large aggregates of the ferrite grains will grow. If so, it is essentially difficult to form fine ferrite grains. From this viewpoint, in the conventional technique of forming fine ferrite grains, the lowermost limit of the grain size is at least 5 &mgr;m.
In the prior art, no technique was know at all for controlling the orientation of ferrite grains formed. Therefore, it was impossible to form fine ferrite grains while randomizing the orientation of the grains formed.
SUMMARY OF THE INVENTION
Given the situation, the present invention is to overcome the limits in the prior art noted above, and to realize a novel technique for forming ultra-fine ferrite grains surrounded by large angle grain boundaries, while randomizing the orientation of the grains. Accordingly, the subject matter of the invention is to provide ferrite matrix steel with a good weldability having increased strength and improved strength-ductility balance, which is novel ultra-fine grain steel useful in ordinary weld constructions, and to provide a method for producing the steel.
In order to solve the problems noted above, the invention provides ultra-fine grain steel in which the mother phase comprises ferrite grains having a mean grain size of not larger than 3 &mgr;m and in which the grains are surrounded by large angle grain boundaries having misorientation angle not smaller than 15°.
The invention further provides the following:
Ultra-fine grain steel of which the carbon (C) content is not larger than 0.3% by weight;
Ultra-fine grain steel of which the composition comprises C, Si, Mn, Al, P, S and N, and a balance of Fe and inevitable impurities;
Ultra-fine grain steel which contains pearlite in an amount of not smaller than 3% by mass; and
Ultra-fine grain steel which contains ferrite grains having a mean grain size of not larger than 3.0 &mgr;m and surrounded by large angle grain boundaries having misorientation of not smaller than 15°, in an amount of not smaller than 60% by volues fraction, and in which the density of specific orientations of the ferrite grains is not larger than 4.
The invention also provide the following methods for producing the ultra-fine grain steel in which the mother phase comprised ferrite grains having a mean grain size of not larger than 3 &mgr;m and in which the grains are surrounded by large angle grain boundaries having misorientation angle not smaller than 15°;
A method for producing ultra-fine grain steel by processing austenite, which comprises compressing the starting steel to a reduction ratio of not smaller than 30% at a non-recrystallized temperature of the austenite, followed by cooling it at a rate of not lower than 3 K/s;
A method for producing ultra-fine grain steel which has ferrite grains having a mean grain size of not larger than 3 &mgr;m in its mother phase, the method comprising heating starting steel at a temperature not lower than its Ac
3
point to thereby austenitizing it, then compressing it with anvils at a temperature not lower than its Ar
3
point to a reduction ratio of not smaller than 50%, and thereafter cooling it;
The method for producing ultra-fine grain steel in which t
Nagai Kotobu
Torizuka Shiro
Tsuzaki Kaneaki
Umezawa Osamu
National Research Institute for Metals
Wenderoth , Lind & Ponack, L.L.P.
Yee Deborah
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