Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-09-24
2003-11-25
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
Reexamination Certificate
active
06652671
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a high carbon steel sheet having chemical composition specified by JIS G 4051 (Carbon steels for machine structural use), JIS G 4401 (Carbon tool steels) or JIS G 4802 (Cold-rolled steel strips for springs), and in particular to a high carbon steel sheet having excellent hardenability and toughness, and workability with a high dimensional precision, and a method of producing the same.
BACKGROUND ART
High carbon steel sheets having chemical compositions specified by JIS G 4051, JIS G 4401 or JIS G 4802 have conventionally much often been applied to parts for machine structural use such as washers, chains or the like. Such high carbon steel sheets have accordingly been demanded to have good hardenability, and recently not only the good hardenability after quenching treatment but also low temperature—short time of quenching treatment for cost down and high toughness after quenching treatment for safety during services. In addition, since the high carbon steel sheets have large planar anisotropy of mechanical properties caused by production process such as hot rolling, annealing and cold rolling, it has been difficult to apply the high carbon steel sheets to parts as gears which are conventionally produced by casting or forging, and demanded to have workability with a high dimensional precision.
Therefore, for improving the hardenability and the toughness of the high carbon steel sheets, and reducing their planar anisotropy of mechanical properties, the following methods have been proposed.
(1) JP-A-5-9588, (the term “JP-A” referred to herein signifies “Unexamined Japanese Patent Publication”) (Prior Art 1): hot rolling, cooling down to 20 to 500° C. at a rate of 10° C./sec or higher, reheating for a short time, and coiling so as to accelerate spheroidization of carbides for improving the hardenability.
(2) JP-A-5-98388 (Prior Art 2): adding Nb and Ti to high carbon steels containing 0.30 to 0.70% of C so as to form carbonitrides for restraining austenite grain growth and improving the toughness.
(3) “Material and Process”, vol.1 (1988), p.1729 (Prior Art 3): hot rolling a high carbon steel containing 0.65% of C, cold rolling at a reduction rate of 50%, batch annealing at 650° C. for 24 hr, subjecting to secondary cold rolling at a reduction rate of 65%, and secondary batch annealing at 680° C. for 24 hr for improving the workability; otherwise adjusting the chemical composition of a high carbon steel containing 0.65% of C, repeating the rolling and the annealing as above mentioned so as to graphitize cementites for improving the workability and reducing the planar anisotropy of r-value.
(4) JP-A-10-152757 (Prior Art 4): adjusting contents of C, Si, Mn, P, Cr, Ni, Mo, V, Ti and Al, decreasing S content below 0.002 wt %, so that 6 &mgr;m or less is the average length of sulfide based non metallic inclusions narrowly elongated in the rolling direction, and 80% or more of all the inclusions are the inclusions whose length in the rolling direction is 4 &mgr;m or less, whereby the planar anisotropy of toughness and ductility is made small.
(5) JP-A-6-271935 (Prior Art 5): hot rolling, at Ar3 transformation point or higher, a steel whose contents of C, Si, Mn, Cr, Mo, Ni, B and Al were adjusted, cooling at a rate of 30° C./sec or higher, coiling at 550 to 700° C., descaling, primarily annealing at 600 to 680° C., cold rolling at a reduction rate of 40% or more, secondarily annealing at 600 to 680° C., and temper rolling so as to reduce the planar shape anisotropy caused by quenching treatment.
However, there are following problems in the above mentioned prior arts.
Prior Art 1: Although reheating for a short time, followed by coiling, a treating time for spheroidizing carbides is very short, and the spheroidization of carbides is insufficient so that the good hardenability might not be probably sometimes provided. Further, for reheating for a short time until coiling after cooling, a rapidly heating apparatus such as an electrically conductive heater is needed, resulting in an increase of production cost.
Prior Art 2: Because of adding expensive Nb and Ti, the production cost is increased.
Prior Art 3: &Dgr;r=(r0+r90−2×r45)/4 is −0.47, which is a parameter of planar anisotropy of r-value (r0, r45, and r90 shows a r-value of the directions of 0° (L), 45° (S) and 90° (C) with respect to the rolling direction respectively). &Dgr;max of r-value being a difference between the maximum value and the minimum value among r0, r45, and r90 is 1.17. Since the &Dgr;r and the &Dgr;max of r-value are high, it is difficult to carry out a forming with a high dimensional precision.
Besides, by graphitizing the cementites, the &Dgr;r decreases to 0.34 and the &Dgr;max of r-value decreases to 0.85, but the forming could not be carried out with a high dimensional precision. In case graphitizing, since a dissolving speed of graphites into austenite phase is slow, the hardenability is remarkably degraded.
Prior Art 4: The planar anisotropy caused by inclusions is decreased, but the forming could not be always carried out with a high dimensional precision.
Prior Art 5: Poor shaping caused by quenching treatment could be improved, but the forming could not be always carried out with a high dimensional precision.
DISCLOSURE OF THE INVENTION
The present invention has been realized to solve above these problems, and it is an object of the invention to provide a high carbon steel sheet having excellent hardenability and toughness, and workability with a high dimensional precision, and a method of producing the same.
The present object could be accomplished by a high carbon steel sheet having chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, in which the ratio of number of carbides having a diameter of 0.6 &mgr;m or less with respect to all the carbides is 80% or more, more than 50 carbides having a diameter of 1.5 &mgr;m or larger exist in 2500 &mgr;m
2
of observation field area of electron microscope, and the &Dgr;r being a parameter of planar anisotropy of r-value is more than −0.15 to less than 0.15.
The above mentioned high carbon steel sheet can be produced by a method comprising the steps of: hot rolling a steel having chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, coiling the hot rolled steel sheet at 520 to 600° C., descaling the coiled steel sheet, primarily annealing the descaled steel sheet at 640 to 690° C. for 20 hr or longer, cold rolling the annealed steel sheet at a reduction rate of 50% or more, and secondarily annealing the cold rolled steel sheet at 620 to 680° C.
The JIS G standards JIS G 4051 (1979), JIS G 4401:2000 and JIS G 4802:1999 and particularly the section of each disclosing the chemical composition, are hereby incorporated by reference.
REFERENCES:
patent: 5-9588 (1993-01-01), None
patent: 5-98388 (1993-04-01), None
patent: 6-271935 (1994-09-01), None
patent: 52-47512 (1997-04-01), None
patent: 10-152757 (1998-06-01), None
patent: 2000-328172 (2000-11-01), None
Fukui et al, “Formable High Carbon Cold Rolled Steel Sheet Utilizing Graphitization of Cementite”, Report of the ISIJ Meeting of Current Advances in Materials and Processes, vol. 1 (1988), p. 1729, Published by The Iron and Steel Institute of Japan (with English language translation).
JIS G 4051 (1979).
JIS G 4401 (1983).
JIS G 4802 (1983).
JIS G 4802:1999 (1999).
JIS G 4401:2000 (2000).
English language version only.
Fujita Takeshi
Ito Katsutoshi
Nakamura Nobuyuki
Takada Yasuyuki
NKK Corporation
Yee Deborah
LandOfFree
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