Metal treatment – Stock – Ferrous
Reexamination Certificate
1998-12-30
2001-12-18
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S593000
Reexamination Certificate
active
06331216
ABSTRACT:
This application is a 371 of PC/JP98/01924 filed Apr. 27, 1998.
1. Technical Field
The present invention relates to a steel product which has high strength and high ductility and is superior in toughness and resistance to collision and impact, particularly a steel product, such as steel pipe, wire rod, steel bar, steel section, steel plate, and steel strip, having fine crystal grains, and also to a process for production thereof.
2. Background Art
Common practice to increase the strength of a steel product is to add an alloying element such as Mn and Si, to perform heat treatment such as controlled rolling, controlled cooling, quenching, and tempering, and to add a precipitation hardening element such as Nb and V. However, what is required of steel products is not only high strength but also high ductility and high toughness. There has been a demand for a steel product which has well-balanced strength and ductility/toughness.
Making grains finer is one of a few important means to improve both strength and ductility/toughness. This is accomplished by performing austenite-ferrite transformation from fine austenite while preventing austenite grains from becoming coarse, thereby giving fine ferrite grains, by working which makes austenite grains finer, thereby giving fine ferrite grains, or by utilizing martensite and lower bainite that result from quenching and tempering.
One of these methods in general use for steel production is controlled rolling which consists of strengthening in the austenite region and its ensuing austenite-ferrite transformation to give rise to fine ferrite grains. Another way in practice is to add a trace amount of Nb which suppresses the recrystallization of austenite grains, thereby yielding finer ferrite grains. Working at a temperature at which austenite does not yet recrystallize permits austenite grains to grow, giving rise to the deformation zone in grains, and finer ferrite grains occur from this deformation zone. A recent practice to obtain finer ferrite grains is controlled cooling that is carried out during or after working.
The above-mentioned methods, however, need rebuilding of the existing facilities and considerable remodeling of the current process in the production of steel products, such as steel pipes, having improved collision and impact resistance required for better automotive safety, an ever increasing demand. Therefore, they are economically unfeasible.
In the meantime, steel products for line pipe need resistance to stress corrosion cracking by sulfides, and this object is achieved by hardness control through the reduction of impurities or the adjustment of alloying elements. In addition, conventional practices to improve fatigue resistance include heat treatment, such as thermal refining, induction hardening, and carburizing, and addition of a large amount of expensive alloying elements such as Ni, Cr, and Mo. The disadvantage of these methods is poor weldability and high production cost.
Steel pipes of small to medium diameter are produced mainly by electric resistance welding with high frequency current. The process for their production consists of continuously feeding a flat strip steel, making it into a pipe stock using a forming roll, heating the opposing edges of the pipe stock to a temperature above the melting point of steel by means of high frequency current, and butt-welding the heated edges by means of squeeze rolls.
This process, however, has a disadvantage of requiring rolls that conform to the dimensions of the desired steel pipe; therefore, it is not suitable for multi-product production in small lots.
In order to address this problem, there has been proposed a new process in, for example, Japanese Patent Publication No. 24606/1990. This process consists of heating a flat strip steel in a preheating furnace and a heating furnace, making the strip steel into a pipe by electric resistance welding, heating the pipe to a temperature above the A
3
transformation point, and rolling the heated pipe by a reducing mill so that it has a predetermined outside diameter.
This process, however, poses problems due to heating above A
3
point. Heating gives rise to scale which is bitten during rolling. Heating also makes crystal grains coarse, aggravating the ductility, strength, and toughness of the resulting steel pipe.
A cold sizing process has been proposed in Japanese Patent Laid-open No. 33105/1988. This process is designed to reduce the outside diameter of a hollow pipe stock, such as seamless steel pipes and electric welded pipes, in the cold state by using a series of reducing mills, each consisting of three rolls. The disadvantage of this process is the necessity of a large-scale mill to withstand high loads due to cold rolling and the necessity of a lubricating facility to prevent rolls from seizing. In addition, cold rolling gives rise to working strain, which aggravates ductility and toughness.
It is an object of the present invention, which was completed to address the above-mentioned problems, to provide a steel product and a process for production thereof, said steel product being superior in ductility, strength, toughness, and resistance to collision and impact owing to fine ferrite crystal grains.
DISCLOSURE OF THE INVENTION
The present inventors carried out extensive studies on a process for efficient production of high-strength steel pipes superior in ductility, which led to the finding that it is possible to produce desired steel pipes with balanced ductility and strength by reducing steel pipes of specific composition at a temperature of ferrite recrystallization.
The present invention is based on the experimental results explained below.
The experiment was carried out on electric welded steel pipes (42.7 mm in dia. and 2.9 mm thick) containing 0.09 wt % C, 0.40 wt % Si, 0.80 wt % Mn, and 0.04 wt % Al. After heating at various temperatures ranging from 750 to 400° C., they were passed through a reducing mill at a rolling speed of 200 m/min so that their outside diameter was reduced variously to 33.2-15.0 mm. The rolled pipes were tested for tensile strength (TS) and elongation (El). The relation between elongation and tensile strength is shown in
FIG. 1
(black dots). Incidentally, white dots in
FIG. 1
represent the relation between elongation and tensile strength of electric welded pipes in various sizes without reducing. Elongation (El) is expressed in terms of values calculated from
El=El
0
×({square root over ( )}(a
0
/a))0.4
(where El
0
is the actually measured elongation, a
0
is 292 mm
2
, and a is the sectional area (mm
2
) of the specimen.) This converted value was used in consideration of the size effect of the specimen.
It is noted from
FIG. 1
that the specimens obtained by reducing after heating at 750-400° C. exhibit higher elongation for the same strength than electric welded pipes without reducing. In other words, the present inventors found that it is possible to produce high-strength steel pipes with balanced ductility and strength by reducing steel pipes of specific composition at a temperature ranging from 750° C. to 400° C.
Moreover, it was found that the above-mentioned steel pipe has fine ferrite grains not greater than 3 &mgr;m,. In order to examine resistance to collision and impact, the present inventors established the relation between tensile strength (TS) and ferrite grain size, with the strain rate greatly changed over a broad range (2000 s
−1
). The results are shown in FIG.
2
. It is noted from
FIG. 2
that the tensile strength remarkably increases with the decreasing ferrite grain size not more than 3 &mgr;m, preferably not more than 1 &mgr;m, and this tendency is significant in the case of high strain rate as is experienced in deformation by collision and impact. In other words, it was found that steel pipes with fine ferrite grains are superior in ductile-strength balance and have greatly improved resistance to collision and impact.
The present invention is based on the above-mentioned findings.
The present invention covers a steel product with h
Furukimi Osamu
Hashimoto Yuji
Hira Takaaki
Itadani Motoaki
Kanayama Taro
Kawasaki Steel Corporation
Yee Deborah
Young & Thompson
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