Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2002-08-16
2004-01-06
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S651000
Reexamination Certificate
active
06673171
ABSTRACT:
TECHNICAL FIELD
This invention relates to a process for producing medium carbon steel sheet and strip with a spheroidized microstructure having enhanced uniform elongation suitable for deep drawing applications. In one embodiment for producing the steel in hot rolled and annealed form, the steel is hot rolled with a lower than normal finishing temperature and subsequently annealed at a temperature below the A
1
to provide an unexpected increase in uniform elongation. In a second embodiment for producing the steel in cold rolled and annealed form, increased lower yield strength levels are unexpectedly achieved at the higher end of the carbon range without significant decrease in uniform elongation by interaction of Mn and Si at various levels with the higher carbon.
BACKGROUND ART
There is a need for steel products having high strength and uniform elongation. High uniform elongation is particularly useful where the steel is to be formed into parts for automotive applications. In order to decrease the weight of cars and other vehicles, high strength along with high uniform elongation is particularly desired so that thinner gauge steels may be used. Dual phase steels developed for automotive applications typically have a proeutectoid ferrite microstructure with a significant fraction of low carbon martensite and/or lower bainite to provide high strength and high formability. Dual-phase steels generally require the addition of expensive alloying elements to increase hardenability and special cooling practices on a continuous annealing line or a hot strip mill to control the microstructure. Continuous annealing lines having controlled cooling capability are very expensive and require a substantial outlay of capital funds. There is a need therefore for steel producers that do not have continuous annealing lines with controlled cooling capability to be able to produce high strength steels having high uniform elongation using conventional batch or box annealing facilities.
A spheroidizing anneal below the A
1
temperature is disclosed in U.S. Pat. No. 5,108,518 to Fukui et al. The reference discloses a method of manufacturing a steel sheet having high strength and excellent resistance to hydrogen embrittlement after heat treatments performed subsequent to the spheroidizing anneal. The steel is used for articles such as chain elements, gear members, clutch members, buckles for seat belts and washers. The steel consists essentially of C 0.30/0.70, Si 0.10/0.70, Mn 0.05/1.00, P not greater than 0.030, S not greater than 0.020, Cr 0.50/2.00, Mo 0.10/0.50, Ti 0.005/0.10, Nb 0.005/0.10, sol. Al not greater than 0.10, N greater than 0.002 but not greater than 0.015, optionally B 0.0005/0.002, balance iron and incidental impurities. The method includes hot rolling the steel with a finishing temperature of 800° C. or higher, cooling at a rate of 5-40° C./second to a temperature range of 500-700° C. The hot rolled steel may optionally be cold rolled 20-80% and box annealed at a temperature of (A
c1
−50) to (A
c1
+30)° C. for one hour or longer to spheroidize the cementites. Thin steel sheet produced by this method is formed and shaped by the customer and then subjected to heat treatment to provide sufficient hardness of the final product. The reference does not teach a hot rolled product that is spheroidize annealed, nor that lower hot roll finishing temperature will increase the uniform elongation of hot rolled steel strip after a spheroidizing anneal. The reference also does not teach the interaction of C, Mn, and Si for achieving various higher strength levels of cold rolled and spheroidize annealed strip product without substantial decrease in uniform elongation.
Several other references disclose spheroidization annealing of carbon steels below the A
1
temperature. JP 11 264049 discloses a method of producing a high carbon steel strip free from shape defects such as sagging. The steel contains 0.2/0.8 C, up to 0.3 Si, 0.6/1.6 Mn, 0.01/0.1 sol. Al, 0.002/0.01 N, sol. Al/N: 5 to 10 and 0/0.01 Ca, the balance Fe with inevitable impurities. The steel is hot rolled, finish rolling at 850° C. (from the example), coiled at 550/680° C., cold rolled 10/80% annealed at 650/725° C. and secondary cold rolled 5/25%. The spheroidizing ratio is regulated to up to 80%. Tensile strength (TS) is regulated between 600 to 700 N/mm2 and the TS (N/cm2)×(100−El %): 50×103 to 65×103. This reference does not disclose steel having high uniform elongation, nor controlling the hot roll finishing temperature to obtain high uniform elongation. The reference also does not disclose the interaction of C, Mn and Si on lower yield strength without significant decrease in uniform elongation after a spheroidizing anneal in cold rolled form. JP 57 134457 discloses improving the rate of spheroidization of the entire part of a hot rolled steel strip. The middle to high carbon steel is hot rolled, coiled and reheated after which it is again coiled. It is then mechanically descaled, pickled and spheroidization annealed. Applicants' invention does not require reheating after hot rolling prior to a spheroidization anneal. JP 10 060540 discloses prevention of seizing flaws in steel strip. The steel is hot rolled to strip, pickled, descaled and repeatedly subjected to soaking and slow cooling below and above the A1 temperature. The spheroidizing rate improves and the production of a soft hot rolled steel strip is made possible. The strip is then cold rolled 20/85% and final annealed at a temperature between 630° C. to the A
c1
temperature. This reference discloses additional reheating after hot rolling before cold rolling and spheroidize annealing which is unnecessary in Applicants' invention. JP 61 076619 discloses a high carbon cold rolled steel strip having superior ductility. The steel contains 0.27/0.90 C, 0.15/0.30 Si, 0.60/0.90 Mn, up to 0.030 P, up to 0.035 S, and the balance Fe and inevitable impurities. The hot rolled strip is annealed for 15 hours at a temperature within the range 680/720° C., cold rolled 20/45% and then annealed for 10 hours at a temperature within the same temperature range as the hot roll anneal. This reference requires a spheroidization anneal after both hot rolling and cold rolling which is not required in Applicants' invention. Also this reference does not disclose hot rolling with a lower than normal finishing temperature to obtain enhanced uniform elongation. JP 31 22216 discloses a process for producing a cold rolled steel strip under the following conditions when the carbon content of the steel is less than 0.6%. The steel is hot rolled, coiled at a temperature within the range of 460-600° C., cooled after hot rolling at a velocity regulated to 30-45° C., cold rolled 50-85% and spheroidize annealed at a temperature between 680° C. and the Ac3 temperature. This reference requires control cooling of the coil after hot rolling which is not required in Applicants' invention. The reference also does not restrict the spheroidization temperature to a temperature below the A1 temperature.
A reference in which a sub-critical anneal is used to graphitize 50% or more of the cementite is disclosed in U.S. Pat. No. 5,454,887 to Fukui. The steel consists essentially of C 0.20/0.70, Si 0.20/2.00, Mn 0.05/0.50, P not more than 0.020, S not more than 0.010, sol. Al 0.01/1.00, B 0.0003/0.005, N 0.002/0.010, B/N 0.2/0.8, Cu 0/1.00, Ni 0/2.00, Ca 0/0.010, balance iron and incidental impurities. The steel is hot rolled with a finishing temperature of 700-900° C., cooled at a rate of 5-50° C./second and coiled at 400-650° C. The steel is optionally cold rolled 20-85% and annealed at a temperature of 600° C. to the A
c1
temperature for 1 hour or longer. The upper limit of 0.50% Mn is essential to ensure formation of graphite during the sub-critical anneal. To obtain graphitization, the chemical composition of the steel must be such that cementite is unstable at the temperature and time of the sub-critical anneal so as to permit breakdown of cementite into its co
Hlady Craig O.
Osman Todd M.
Ip Sikyin
Jones, Jr. E. H.
Riesmeyer, III W. F.
United States Steel Corporation
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