Patent Grant
7727342
The present invention relates generally to the metallurgy of steel and, more particularly, to low carbon microalloyed steel compositions. It is common practice to use conventional microalloyed steels in various applications for bars and tubular products. However, there are needs for stronger and tougher microalloyed steels in a number of different applications such as, for example, in communication towers and hub assemblies.
The steels of the present invention are much more weldable and tougher than conventional microalloyed steels. The present invention is directed to an alloy broadly comprising in wt. %, about 0.05-0.30 C; up to 1.5 Mn; 1.0 max Si; 0.5-2 Ni; 0.05-0.3 V; up to 2 Cu; 10-250 ppm N; balance Fe and other minor additions and impurities.
The family of microalloyed steels of the present invention provide better weldability and much higher impact toughness and tensile ductility than conventional microalloyed steels. A critical factor in the design of microalloyed ferrite pearlite steels is the extent to which precipitation strengthening supplements the base strength provided by solid solution and grain refinement. It is known that this precipitation strengthening factor, referred to as ΔYSp, is controlled by the ferrite transformation temperature, Ar3, all other things being equal. As the Ar3 temperature is lowered, ΔYSp increases up to a maximum and then decreases as a result of precipitation suppression through the usual kinetic limitations at lower temperatures. This relationship is graphically depicted in
Consideration of these requirements shows that out of all the common alloying elements, Ni is the most effective. The results of a study to examine the effect of Ni on ΔYSp showed that 45 ksi minimum yield strength and as high as 80 ksi was possible for large bars or tubular products. The results are given in the following tables.
One presently preferred alloy composition according to the present invention contains in % by weight: 0.05-0.30 C, 0.5-1.5 Mn; 1.0 max Si; 0.5-2.0 Ni; 0.05-0.30 V; 0-2.0 Cu; 0.0050-0.0250 N; balance Fe and other minor additions and impurities. The S level is 0.04 wt. % max and preferably about 0.035 wt. % max. The P content is 0.025 wt. % max and preferably about 0.02 wt. % max. In the above composition, the C and N contents may preferably be 0.05-0.15 wt. % C and 0.0010-0.0250 wt. % N.
A further presently preferred embodiment of the present invention includes the alloy composition set forth above, also containing about 0.25-2.0 wt. % Cu. Copper in these microalloyed steels will form as ε-copper particles by both interphase precipitation and the normal nucleation and growth process, thus increasing strength by increasing ΔYSp, and maintaining high levels of toughness and tensile ductility as seen in Tables III and IV.
The alloy may also contain additional constituents such as Cr, Mo, Cb and Al, for example, 0.05-0.3 Cr, up to about 0.15 Mo, up to about 0.2 Cb, up to about 0.05 Al, and more preferably about 0.01-0.03 Al.
The “C.E.” or carbon equivalent values reported in Table I may broadly range between 0.3 and 0.65 but, more preferably, are controlled within a range of 0.3 to 0.55 and, still more preferably, controlled within a range of 0.4-0.5 to ensure superior physical properties. The C.E. value of an alloy is calculated using the following formula:
Various alloy compositions of the present invention are set forth in Table I which also includes the calculated C.E. values for each. Table II describes the rolling schedule for each of the steel alloy heats made from the compositions of Table I. It will be noted that the billets were either air cooled after completion of rolling or they were sand cooled to simulate the mid-radius position of a large diameter bar of, for example, a 6-inch diameter bar. These sand cooled rolled heats have an “S” designation in Tables III and IV while the rolled heats that, were air cooled have no letter designation in the tables.
While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. The presently preferred embodiments described herein are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US03/04090 | 2/12/2003 | WO | 00 | 3/18/2005 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO03/069008 | 8/21/2003 | WO | A |
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| Number | Date | Country | |
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| 20050178479 A1 | Aug 2005 | US |
