Ví dụ về việc sử dụng Center segregation trong Tiếng anh và bản dịch của chúng sang Tiếng việt
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Carbonitride in the center segregation area.
Observed with an electron microscope, and the maximum length of the Nb carbonitride grains in the fracture surface was measured,and this is the length of the Nb carbonitride in the center segregation area.
The CP value is 0.95 or less,then the hardness of the center segregation area could be sufficiently small preferably HV.
Shows one example of the result of a HIC test(the test method is the same as in Examples given below) of a steel plate having MnS orNb carbonitride formed in the center segregation area thereof.
However, when the alloy element concentration is too high,then the hardness of the center segregation area may also increase and the HIC resistance may deteriorate.
However, when the hardness of the center segregation area is higher than HV 250, the cracking may readily propagate and, in particular, the cracks generated in the Nb carbonitride readily propagate.
The steel plate and the steel pipe preferablysatisfy the following conditions in regard to the hardness of the center segregation area and the Nb carbonitride to be an initiation point of HIC.
The Nb carbonitride formed in the center segregation area is a hydrogen accumulation point in the HIC test, and cracks may occur initiating from the point.
In the case where the length of the Nb carbonitride is 20 jimor less,then the cracks maybe prevented from propagating when the hardness of the center segregation area is not more than HV 250.
Further, when the CP value is smaller,then the hardness of the center segregation area is lower and the HIC resistance increases and, therefore, the lowermost limit of the CP value is not defined.
When the size of the Nb carbonitride is larger, then the cracks may readily propagate and,even though the hardness of the center segregation area is not more than HV 250, the cracks may propagate.
A graph showing the relationship between the hardness of the center segregation area and the crack area ratio in a HIC test of a steel plate having MnS orNb carbo nitride formed in the center segregation area thereof.
The method of reducing the size of an Nb-containing car bonitride to an extremely small size of 5 jimor smaller, as in JP-A 2006-63351,maybe effective for preventing the occur rence of HIC in the center segregation area.
According to this, it is known that,in the case where MnS exists in the center segregation area, the crack area ratio increases even the hardness is low and, therefore, controlling the growth of MnS is extremely important.
We investigated in detail the occurrence of cracking and propagation behavior thereof in a HIC test from the viewpoint of the initiation of cracking andthe microstructure of the center segregation area and, as a result, have obtained the following findings.
The steel plate for line pipes of the above 1 or 2, wherein the hardness of the center segregation area is HV 250 or lower,and the length of the Nb carbonitride in the center segregation area is at most 20 m or less.
Using the segregation coeffi- cient obtained through the above-mentioned thermodynamic analysis, the CP value is obtained,corresponding to the car bon equivalent formula in the center segregation area repre sented by the following formula.
First, for preventing cracking in the center segregation area, a appropriate material property of the center segregation area is necessary in accordance with the type of the inclusion that is to be the initiation point of cracking FIG.
To solve this problem, it is necessary to strictly control the chemical compositions of the steel plate andcontrol the hard ness of the center segregation area to be not higher than a predetermined level(preferably at most HV 250).
In the case where the hardness of the center segregation area is HV 250 or lower, and even when small Nb carbonitride may remain in the center segre gation area, the cracking would hardly propagate and, there fore, the crack area ratio in the HIC test may be reduced.
We ther modynamically analyzed the distribution behavior(or incras- sate behavior)of the chemical composition in the center segregation area and have derived thesegregation coefficient of the individual alloy elements.
The size of the Nb carbonitride in the center segregation area is influenced by the Nb amount added and, therefore, when the uppermost limit of the Nb amount to be added is defined to be at most 0.035%, then the size may be controlled to be at most 20 jim.
We found that, when the CP value is controlled to be not larger than a predetermined level,then the hardness of the center segregation area can be thereby controlled to be not larger than the critical hardness to cause cracking FIG.
Accordingly, the hardness of the center segregation area is preferably HV 250 or lower and, in the case where severe HIC resistance is required,the hardness of the center segregation area must be ilirther reduced and, in such a case, the hardness of the center segregation area is preferably HV 230 or lower.
Accordingly, formation of an island martensite(M-A constituent)to be a initiation point of cracking in the center segregation area, as well as formation of a hardened structure such as martensite or the like to be a propagation path of cracking can be prevented.
This is because, in general, when the plate thickness(pipe wall thickness) is less than 20 mm, then the amount of the alloying element added is small and, therefore,the hard ness of the center segregation area could be low and, in such a case, the steel plate could readily have a good HIC resis tance.
And for the above-mentioned severer request for HIC resistance,the material of the center segregation zone must be extremely strictly controlled for preventing initiation of HIC and for preventing the propagation of cracking from the Nb carboni- tride that may form at some frequency.
However, even when the formation of MnS could be prevented,in the case where the center segregation area contains an Nb carbonitride and when the hardness thereof is over a predetermined level(in this, Vickers hardness, HV 250), then cracking occurs in the HIC test.
In that case, even when the MnS formation is prevented orthe micro structure of the center segregation area is improved according to the above-mentioned prior-art methods, the hardness of the center segregation area may increase and HIC may occur from Nb carbonitride.
The steel plate is defined in point of the chemical compo sition,the hardness of the center segregation area and the size of the Nb carbonitride as above, and further its microstructure is defined to be a structure of mainly bainite and, accordingly, the steel plate can have an excellent-HIC resistance even when its plate thickness is large.