Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1999-02-26
2001-09-18
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S648000, C148S654000
Reexamination Certificate
active
06290789
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a steel pipe containing super-fine crystal grains, which has excellent strength, toughness and ductility and superior collision impact resistance and a method for producing the same.
BACKGROUND ART
The strength of steel materials have been increased heretofore by adding alloying elements such as Mn and Si, and by utilizing, for instance, controlled rolling, controlled cooling, thermal treatments such as quenching and tempering, or by adding precipitation hardening elements such as Nb and V. In the case of a steel material, however, not only strength but also high ductility and toughness are required. Hence, a steel material with balanced strength and ductility as well as toughness has been demanded.
The reduction in crystal size is important in that it is one of the few means for increasing not only strength, but also both of ductility and toughness at the same time. Crystal grains sufficiently reduced in size can be realized by, for example, a method which comprises preventing coarsening of austenite grains and obtaining fine ferritic crystal grains from fine austenite grains by utilizing the austenite-ferrite transformation; a method which comprises obtaining fine ferrite grains from fine austenite grains realized by working; or a method which comprises utilizing martensite or lower bainite resulting from quenching and tempering.
In particular, controlled rolling comprising intense working in the austenitic region and reducing size of ferrite grains by using the subsequent austenite - ferrite transformation is widely utilized for the production of steel materials. Furthermore, a method for further reducing the size of ferrite grains by adding a trace amount of Nb and thereby suppressing the recrystallization of austenite grains is also known in the art. By working in a temperature in the non-recrystallizing temperature region, austenite grains grow as to form a transgranular deformation band, and ferrite grains generate from the deformation band as to further reduce the size of the ferrite grains. Furthermore, controlled cooling which comprises cooling during or after working is also employed.
However, the fine grains available by the methods above have lower limits in the grain size of about 4 to 5 &mgr;m. Furthermore, the methods are too complicated to be applied to the production of steel pipes. In the light of such circumstances, a method comprising simple process steps and yet capable of further reducing the grain size of ferrite crystals for improving the toughness and ductility of steel pipes has been required. Moreover, concerning the recent increasing demand for steel pipes having superior collision impact resistances to achieve the object of improving safety of automobiles, limits in cutting cost has been found so long as the methods enumerated above are employed, because they required considerable modification in process steps inclusive of replacing the equipment and the like.
Furthermore, the improvement in resistances against sulfide stress corrosion cracks of steel pipes for use in line pipes, at present, hardness control is performed to lower the concentration of impurities and control the concentration of alloy elements.
Conventionally, fatigue resistance has been improved by employing thermal treatments such as quench hardening and tempering, induction hardening, and carburizing, or by adding expensive alloy elements such as Ni, Cr, Mo, etc. in large amounts. However, these methods has problems of impairing the weldability, and furthermore, of increasing the cost.
A high strength steel pipe having a tensile strength of over 600 MPa is produced by using a carbon-rich material containing carbon (C) at a concentration of 0.30% or more, or by a material containing C at a high concentration and other alloy elements added at large quantities. In the case of high strength steel pipes thus increased in strength by methods above, however, the elongation properties tend to be impaired. Thus, in general, the application of intense working is avoided; in case intense working is necessary, intermediate annealing is performed during working, and further thermal treatments such as normalizing, quenching and tempering, etc., is applied. However, the application of additional thermal treatment such as intermediate annealing makes the process complicated.
In the light of the circumstances above, a method which allows intense working of high strength steel pipe without applying intermediate annealing is demanded, and also, further reduction in crystal grains is desired for the improvement in workability of high strength steel pipes.
An object of the present invention is to advantageously solve the problems above, and to provide a steel pipe improved in ductility and collision impact resistance without incorporating considerable change in production process. Another object of the present invention is to provide a method for producing the same steel. Further, another object of the present invention is to provide a steel pipe and a method for producing the same, said steel pipe containing super fine grains having excellent toughness and ductility which are ferrite grains 3 &mgr;m or less in size, preferably, 2 &mgr;m, and more preferably, 1 &mgr;m or less in size.
A still another object of the present invention is to provide a high strength steel pipe containing superfine crystal grains, which is improved in workability and having a tensile strength of 600 MPa or more, and to a method for producing the same.
DISCLOSURE OF THE INVENTION
The present inventors extensively and intensively performed studies on a method of producing high strength steel pipes having excellent ductility, yet at a high production speed. As a result, it has been found that a highly ductile high strength steel pipe having well-balanced strength and ductility properties can be produced by applying reducing to a steel pipe having a specified composition in a temperature range of ferrite recovery or recrystallization.
First, the experimental results from which the present invention is derived are described below.
A seam welded steel pipe (&phgr;42.7 mm D×2.9 mm t) having a composition of 0.09 wt % C—0.40 wt % Si—0.80 wt % Mn—0.04 wt % Al was heated to each of the temperatures in a range of from 750 to 550° C., and reducing was performed by using a reducing mill to obtain product pipes differing in outer diameter in a range of &phgr; 33.2 to 15.0 mm while setting the output speed of drawing to 200 m/min. After rolling, the tensile strength (TS) and elongation (El) were measured on each of the product pipes, and the relation between elongation and strength was shown graphically as is shown in
FIG. 1
(plotted by solid circles in the figure). In the figure, the open circles show the relation between elongation and strength of seam welded steel pipes of differing size which were obtained by welding but without applying rolling.
For the values of elongation (El), a reduced value obtained by the following equation:
El=El0×({square root over ( )}(a0/a))
0.4
(where, El0 represents the observed elongation, a0 is a value equivalent to 292 mm
2
, and a represents the cross section area of the specimen (mm
2
))
Referring to
FIG. 1
, it can be seen that higher elongation can be obtained if the base steel pipe is subjected to reducing in the temperature range of from 750 to 550° C. as compared with the elongation of an as-welded seam welded steel pipe at the same strength. That is, the present inventors have been found that a high strength steel pipe having good balance in ductility and strength can be obtained by heating a base steel pipe having a specified composition to a temperature range of 750 to 400° C. and applying reducing.
Furthermore, it has been found that the steel pipe produced by the production method above contain fine ferrite grains 3 &mgr;m or less in size. To investigate the collision impact resistance properties, the present inventors further obtained the relation between the tensile strength (TS) and the grain size o
Furukimi Osamu
Hashimoto Yuji
Hira Takaaki
Itadani Motoaki
Kanayama Taro
Combs-Morillo Janelle
Kawasaki Steel Corporation
Wyszomierski George
Young & Thompson
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