Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1999-03-15
2001-04-17
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S653000
Reexamination Certificate
active
06217680
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention relates to cold-rolled sheet steel favorable to use for compressor covers, oil pans for vehicles and others, in particular, to that with good deep drawability having a thickness of not smaller than 1.2 mm, and also to a method for producing it.
2. Background Art
Many parts of compressor covers, oil pans for vehicles and others are produced through deep drawing of thick sheet steel, and sheet steel for those applications is desired to have a high r value. Thick sheet steel having a thickness of not smaller than 1.2 mm and having an r value of about 2.0 is produced in an ordinary hot rolling-cold rolling process. The amount of sheet steel to be shaped into articles is increasing, and steel articles are desired to have complicated shapes, for which sheet steel is desired to have a much higher r value.
For producing cold-rolled sheet steel having a high r value, known is a method comprising hot-rolling steel under a lubricative condition at a finishing delivery temperature falling within a range not higher than the Ar
3
transformation point of the steel (lubricative warm-rolling), for example, as in Japanese Patent Application Laid-Open (JP-A) Sho-61-119621, Hei-3-150316, etc. In JP-A Hei-3-150916, they say that the sheet steel produced has an r value of about 2.9.
However, in order to obtain sheet steel having such a high r value according to the known method, steel must be subjected to lubricative warm-rolling to a reduction ratio of higher than 90%, and then to cold rolling to a reduction ratio of 75% or higher. For example, in the method disclosed in JP-A Sho-61-119621, where steel is subjected to lubricative warm-rolling to a reduction ratio of not higher than 90% and then to cold rolling to a reduction ratio of lower than 75%, the r value of the resulting sheet steel could be at most about 2.0.
This is because, in the process of rolling steel through lubricative warm-rolling followed by cold rolling to such a low reduction ratio, the steel could not satisfactorily get the effect of lubricative warm-rolling. Therefore, in the prior art, it was extremely difficult to increase the r value of cold-rolled thick sheet steel, for which the reduction ratio could not be lowered to a satisfactory degree.
Specifically, in the conventional rolling process, the thickness of the slab to be rolled must be at most about 200 mm or so, and the reduction ratio in the rough hot-rolling step must be at least 85% in order that the steel grains could be sufficiently fined in the lubricative warm-rolling step prior to the final rolling step for finishing. For these reasons, therefore, in the actual production line for the conventional rolling process, the thickness of the sheet bar to be rolled shall be at most about 30 mm or so. In continuous rolling in which one sheet bar is joined to another, the thickness of the sheet bars to be rolled shall be at most about 30 mm or so in view of the coiling ability of the sheet bar coiler to be used therein.
As mentioned above, the thickness of the sheet bars capable of being rolled in the conventional process could be at most about 30 mm or so. Therefore, according to the conventional rolling process, it is extremely difficult to obtain cold-rolled sheet steel having a thickness of not smaller than 1.2 mm, while satisfying the combination of the reduction ratio in the lubricative warm-rolling step of being not lower than 90% and the reduction ratio in the cold rolling step of being not lower than 75%. Even if the reduction ratio in the lubricative warm-rolling step could be at most 86% and that in the cold rolling step be at most 75% under various conditions, the r value of the actually rolled sheets could be at most about 2.6 or so.
Given that situation, one object of the present invention is to provide cold-rolled thick sheet steel having a thickness of not smaller than 1.2 mm and having an r value of not lower than 2.9.
Another object of the invention is to provide a practicable method for producing cold-rolled thick sheet steel having a thickness of not smaller than 1.2 mm and having an r value of not lower than 2.9.
DISCLOSURE OF THE INVENTION
Despite of the problems noted above, we, the present inventors still believed that the combination of lubricative warm rolling and cold rolling would be the best for producing the intended, thick cold-rolled sheet steel, in view of its effect for improving the mechanical properties of the sheet steel produced and from the economical viewpoint of it. In that situation, we have assiduously studied in order to solve the problems in the prior art noted above and to obtain good, thick cold-rolled sheet steel, and, as a result, have completed the present invention. The constitution of the invention is described hereinunder.
Specifically, the invention provides the following:
(1) Thick cold-rolled sheet steel with excellent deep drawability, which has a thickness of not smaller than 1.2 mm and has an r value to be defined by the following equation (1) of not smaller than 2.9:
r=(r
0
+2r
45
+r
90
)/4 (1)
wherein r
0
, r
45
and r
90
each indicate the Lankford value of the sheet steel in the rolling direction, in the direction at an angle of 45° relative to the rolling direction, and in the direction at an angle of 90° relative to the rolling direction, respectively.
(2) A method for producing thick cold-rolled sheet steel from a steel slab having a composition that comprises at most 0.008% by weight of C, at most 0.5% by weight of Si, at most 1.0% by weight of Mn, at most 0.15% by weight of P, at most 0.02% by weight of S, from 0.01 to 0.10% by weight of Al, at most 0.008% by weight of N, from 0.035 to 0.20% by weight of Ti, and from 0.001 to 0.015% by weight of Nb, with a balance of Fe and inevitable impurities, in which those C, S, N, Ti and Nb satisfy the following condition (2):
1.2(C/12+N/14+S/32)<(Ti/48+Nb/93) (2),
the method comprising subjecting said steel slab to rough hot-rolling to a reduction ratio of not lower than 85%, at a temperature falling between the Ar
3
transformation point of the steel and 950° C., then subjecting it to lubricative warm-rolling for finishing hot-rolling to a reduction ratio of not lower than 65%, at a temperature falling between 600° C. and the Ar
3
transformation point of the steel, while lubricating it, to thereby make it have a mean shear strain of not larger than 0.06, then pickling it, pre-annealing it at a temperature falling between 700 and 920° C., cold-rolling it to a reduction ratio of not lower than 65%, and thereafter further annealing it for recrystallization at a temperature falling between 700 and 920° C.
(3) The method for producing thick cold-rolled sheet steel as in (2), wherein the thickness of the hot-rolled sheet after the finishing hot-rolling step is not smaller than 5 mm.
(4) The method for producing thick cold-rolled sheet steel as in (2) or (3), wherein the steel composition additionally contains B in an amount of from 0.0001 to 0.01% by weight.
(5) The method for producing thick cold-rolled sheet steel as in any one of (2) to (4), wherein the steel composition additionally contains any one or more of from 0.001 to 0.05% by weight of Sb, from 0.001 to 0.05% by weight of Bi and from 0.001 to 0.05% by weight of Se.
(6) The method for producing thick cold-rolled sheet steel as in (2), wherein the reduction ratio for the sheet in the lubricative warm-rolling step to be effected at a temperature falling between 600° C. and the Ar
3
transformation point of the steel is lower than 85% when the reduction ratio for the cold-rolled sheet is lower than 96.6% relative to the sheet bar.
REFERENCES:
patent: 4504326 (1985-03-01), Tokunaga et al.
patent: 5041166 (1991-08-01), Matsuoka et al.
patent: 5360493 (1994-11-01), Matsuoka et al.
patent: 5587027 (1996-12-01), Tosaka et al.
patent: 5846343 (1998-12-01), Yasuhara et al.
patent: 5855696 (1999-01-01), Tezuka et al.
patent: 61-119621 (1986-06-01), None
patent: 63-100134 (1988-05-01), None
patent: 63-230828
Kawabata Yoshikazu
Obara Takashi
Ogino Atsushi
Okuda Kaneharu
Sakata Kei
Kawasaki Steel Corporation
Oliff & Berridg,e PLC
Wyszomierski George
LandOfFree
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