Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-05-26
2001-03-27
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S320000, C148S579000, C148S660000, C420S041000, C420S089000, C420S108000, C420S129000
Reexamination Certificate
active
06206983
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to medium carbon steels and low alloy steels and methods of making same. More specifically, the present invention relates to medium carbon steels and low alloy steels having a concentration of tin and/or antimony at the ferrite grain boundaries of the steel which enhances the machinability of the steel. The present invention also relates to processes for producing such steels.
b) Description of the Related Art
“Medium carbon steel” is a term used in the steelmaking art to refer to grades of steel that have carbon contents in the range of about 0.2 to about 0.4 weight percent. Likewise, the term “low alloy steel” refers to grades of steel having a carbon content in the range of about 0.1 to about 0.2 weight carbon and have alloy contents in the range of about 1to about 4 weight percent. These steels are often used in forgings, gears, and other parts for automotive and structural applications. Many of the parts made from these steels are machined at least once during their manufacture. Thus, the machinability of these steels is of great industrial importance.
Machinability, however, is a complex and not fully understood property. A full understanding of machinability would require taking into account a multitude of factors, including the effect of the steel composition, the elastic strain, plastic flow, and fracture mechanics of the metal workpiece, and the cutting dynamics that occur when steel is machined by cutting tools in such operations as turning, forming, milling, drilling, reaming, boring, shaving, and threading. Due to the complexities of the cutting process and the inherent difficulties in making real time observations at a microscopic level, knowledge of the extent of the range of mechanisms that affect machinability is also incomplete.
Lead is used to enhance the machinability of some grades of steel. However, the use of lead has serious drawbacks. Lead and lead oxides are hazardous. Caution must be taken during steelmaking and any other processing steps involving high temperatures. Such process steps produce lead and/or lead oxide fumes. Atmosphere control procedures must be incorporated into high temperature processing of lead-bearing steels. Disposal of the machining chips from lead-bearing free-machining steels is also problematic due to the lead content of the chips. Another serious disadvantage is that lead is not uniformly distributed throughout conventional steel products. This is because lead is not soluble in the steel and, due to its high density, it settles out during the teeming and solidification processes, resulting in segregation or non-uniform distribution within the steel. Furthermore, lead can envelope manganese sulfide inclusions in the steel thereby decreasing the steel's mechanical properties and service life through reduced resistance to impact and fatigue stresses.
Steels have also been developed in which machinability is enhanced through the use of inclusions in the steel. Manganese sulfide inclusions are often employed to promote machinability. Sometimes a soft phase, such as a low melting metal like bismuth or a plastic oxide, such as a complex oxide containing calcium, selenium or tellurium, is used to surround manganese sulfide inclusions to further enhance machinability. However, strategies that improve machinability through the use of inclusions have the disadvantage that the inclusions remain in the steel's microstructure after machining. The presence of inclusions in the steel may lower the steel's mechanical properties and lower its service life through reduced resistance to impact and fatigue stresses. Thus, although inclusions may be used as agents to improve machinability in some grades of steel, a price is paid in terms of reduced mechanical properties and service life.
SUMMARY OF THE INVENTION
The present invention improves the machinability of medium carbon steels and low alloy steels by concentrating tin and/or antimony at the ferrite grain boundaries of the steel in a manner in which the effects on the mechanical properties and service life of the steel caused by the agent used to improve machinability may be substantially removed after machining. For convenience sake, the term “machinability enhancing agent” or “MEA” will be used herein to refer to tin or antimony, either alone or in combination with each other, which is added to the steel to enhance the steel's machinability. The present invention permits the machinability enhancing effects of the MEA to be amplified through the use of thermal practices which act to concentrate the MEA at ferrite grain boundaries of the steel. Thus, the present invention uses only very low MEA bulk contents in the steel and thereby avoids the deleterious effects, such as hot tearing, associated with higher MEA bulk contents.
Concentration of MEAs at the ferrite grain boundaries in accordance with the present invention improves the machinability of medium carbon steels and low alloy steels by reducing the ferrite grain boundary strength and thereby reducing the energy required for metal removal during machining operations. However, the present invention also optionally allows any deleterious embrittling effects resulting from the MEA ferrite grain boundary concentrations to be substantially reversed through the use of subsequent thermal practices which act to redistribute the MEA more homogeneously throughout the steel. Such embrittling effects may manifest themselves in the form of reductions in ductility or impact toughness. Thus, the present invention permits the machinability of a low alloy steel to be improved through a first thermal practice and then through the use of an optional second thermal practice to controllably and substantially remove any deleterious embrittling effects resulting from the MEA ferrite grain boundary concentrations by redistributing the MEA more homogeneously throughout the steel. In other words, the present invention controllably enhances the machinability of low allow steel by reversibly concentrating MEA at the ferrite grain boundaries of the steel.
Thus, it is an object of the present invention to improve the machinability of medium carbon steels and low alloy steels.
It is another object of the present invention to improve the machinability of medium carbon steels and low alloy steels without the need to rely on the use of lead for enhancing machinability and thereby avoid disadvantages that accompany the use of lead.
It is another object of the present invention to provide a low alloy steel without the need to rely on the formation of a a soft phase surrounding sulfide inclusions, such as a low melting metal like lead or bismuth or a plastic oxide, such as a complex oxide containing calcium, selenium or tellurium to improve the machinability of medium carbon steels and low alloy steels.
Another object of the present invention is to provide medium carbon steels and low alloy steels in which enhanced machinability can be controllably induced into the steel prior to machining and then the enhancement can be controllably and substantially removed from the steel after machining.
Another object of the present invention is to controllably enhance the machinability of medium carbon steels and low allow steels containing a small bulk content of MEA by reversibly concentrating MEA at ferrite grain boundaries of the steel.
It is another object of the present invention to provide medium carbon steels and low alloy steels which can be machined into parts which are useful as machined steel parts.
It is another object of the present invention to provide processes of making medium carbon steels and low alloy steels which accomplish the foregoing objects. A further object of the present invention is to provide products produced by those processes.
The present invention accomplishes the foregoing objects by providing medium carbon steels and low alloy steels compositions having a concentration of MEA at ferrite grain boundaries to improve the machinability of the steel
DeArdo Anthony J.
Garcia C. Isaac
Lizzi Thomas
Metz Schermer & Lewis, LLC
Schermer Leland P.
University of Pittsburgh - of the Commonwealth System of Higher
Yee Deborah
LandOfFree
Medium carbon steels and low alloy steels with enhanced... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Medium carbon steels and low alloy steels with enhanced..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Medium carbon steels and low alloy steels with enhanced... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2493259