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In addition, how to improve the productivity of the site is also a big issue. In addition to improving the hardware and soft conditions related to production technology, it also needs steel with less deformation and easy processing. For this reason, in recent years, with the advancement of steelmaking and rolling technology in Japan, Japan has developed high-performance shipbuilding steel materials with high strength reliability and good processability, and industrialized them.
Cracked high stop steel
The cracked high-stop steel sheet is a super-fine grain steel sheet (SUF for short). In the very moments of collision, reefing, etc., in order to avoid large-scale serious damage to the hull, the shipboard must have high brittle crack stopping performance. The Japan Maritime Association regards this as an important indicator, and defines the steel plate with this performance as "high stop." Steel plate".
Usually, when the ship is shipping, the stress is relieved after the crack is generated, the steel has a certain stopping performance for cracking, and the structural discontinuity (such as the intersection of the outer plate and the skeleton material) may cause welding defects and fatigue. The brittle crack that propagates as a starting point for cracking stops before causing fatal damage.
However, from the analysis of the collision state of the "very time" full-load tanker, it is known that the side outer portion of the collision ship is damaged and the internal structural material is damaged, and the steel near the crack produces a plastic strain of up to about 10%. . Therefore, it is often impossible to expect the crack stopping performance of the steel under normal conditions at "very time".
The SUF steel plate is a steel plate having a superfine grain ferrite structure of 1-2 vm in the range from the surface layer to about 1/6 of the plate thickness. When the brittle crack propagates, the shear lip formed by the SUF portion on both sides of the steel plate has Stop effect.
The Japan Maritime Association classifies the state of stress generated by the hull as "very time" for collisions and reefs, and "normal shipping time" under normal conditions, requiring the brittle fracture stop characteristic value (hereinafter referred to as Kca) on the brittle fracture stop material. Normally, "Kca 4000N/mm1.5 is required, and Kca~>6000N/mm1.5 is required at "very time". .
After applying 5% and 10% plastic strain to the general hull with EH grade KE36 steel plate and SUF steel plate, the results of the temperature gradient type ESSO (large fracture model) test show that the KE36 ship plate is at the lowest use temperature of -10 °C. "Unusually, even under the condition of 5% lower plastic strain, the Kca value falls below the required value of the crack stop performance; on the contrary, even if a high plastic strain of 10% is applied to the SUF steel plate, it is sufficient. The high Kca value, that is, by this ESSO test, confirmed that the SUF steel sheet had a crack stop performance at "very time".
The SUF steel plate was originally used for the 78000 m3 low temperature liquefied petroleum gas carrier (LPG) with the goal of not causing large-scale fractures at "very time". To this end, an SUF steel plate is disposed on the ship-side outer plate portion covering the entire area of ​​the Cargo tank. Since then, a large number of cargo ships and mining vessels have also used SUF steel plates to increase their use.
In addition, the SUF steel plate not only provides "very time", but also provides an effective guarantee for improving the safety of the hull structure in the low-temperature sea navigation vessel and the low-temperature cargo tank. It can be expected that the scope of use of SUF steel plates will continue to expand in the context of the concern of the entire society.
Fatigue strength steel plate
The high fatigue strength steel sheet is a fatigue crack growth inhibition steel sheet (referred to as FCA). In the past, prevention of fatigue damage was carried out in accordance with the viewpoint of how to suppress the generated stress at a specific portion.
This knowledge is acceptable if the occurrence of fatigue cracks in the welded portion and the propagation characteristics are independent of the strength and organization of the steel. However, by using the structural control of the steel, it is possible to develop a steel sheet which can suppress fatigue crack growth and obtain higher fatigue strength reliability.
The expansion inhibition mechanism of the FCA steel sheet is that the ferrite + bainite composite structure composed of an appropriate ratio has the following three kinds of inhibitory effects on the fatigue crack: the phase boundary suppresses the crack propagation speed, and the alternating softening affects the crack opening. Inhibition, inhibition of ductile fracture resistance after alternating softening pre-strain.
The crack propagation rate of the FCA steel sheet was confirmed to be about 1/2 or less of the original steel sheet on the small CT sample. In order to confirm the effect of improving the fatigue life of the actual steel structure, an analog hull structural model fatigue test was performed.
From the results, it is understood that the life from the fatigue crack to the surface fracture caused by the FCA steel plate model reinforcing plate corner welded portion is about twice that of the original steel plate, which shows that even in the structure, there is a remarkable fatigue life improving effect.
The FCA steel plate was originally used as a double-layered bottom material for the LPG ship. The purpose of this is to improve the fatigue life of the full-length through-weld portion. From the analysis of the fatigue crack propagation of the FCA steel plate and the original steel plate, the fatigue life of the FCA steel plate is known. It is about 2.5 times that of the original steel plate.
Preventing fatigue fracture is an important technology to ensure structural safety, and it can also reduce maintenance costs. In the future, the importance of FCA high fatigue strength steel sheets in shipbuilding projects will become more prominent.
Deformation suppression steel plate
The thermal deformation suppression steel sheet is a residual stress control steel sheet. In the manufacturing process of steel structures including shipbuilding, it is difficult to avoid deformation of the steel sheet caused by heat input by cutting and welding, thereby reducing construction efficiency and increasing the number of correction man-hours, which is a mechanization with high-precision assembly of components and improvement of cutting (welding) precision. The goal of automated field operations runs counter to the direction of development.
The causes of steel deformation during cutting and welding are input heat and its fluctuations, poor shape of the steel after rolling, and internal residual stress. In particular, because uneven load stress is difficult to predict, it is even more necessary to try to reduce it. To this end, based on the original TM-CP (ie, controlled rolling and cooling) process technology, the steel shape and residual stress control technology of the system was developed.
On the one hand, in the process of heating, rolling and rapid cooling after rolling, the flatness is poor and the residual stress is suppressed. At the same time, the online residual stress prediction technology is used to evaluate the steel and its shape after cutting. High-precision residual stress control is used as an effective means to solve the problem of poor shape and residual stress of steel after rolling.
The residual stress control steel plate produced by the above technology can be used for the hull portion; when the parallel base of the double-shell of the assembled ship (the size is 12m in width × 20m in length and 3m in height), the longitudinal beam and the panel are welded, different steel plates are detected. Production error and working time.
The results show that the longitudinal beam gap of the steel plate is controlled within the allowable error range by using the residual stress control; otherwise, the original steel plate error often exceeds the allowable range. Since the former can be constructed without adjustment, the latter can save a lot of man-hours and improve construction efficiency.
Improve security
In order to ensure the safety of the hull and reduce the life cycle cost with the extension of life, how to suppress the corrosion of the hull is the key. The Japan Shipbuilding Association actually investigated the corrosion status of the ship's oil tanks. Based on the identification of the causes and mechanisms of corrosion, the developed ship's corrosion resistant ship plates not only inhibit the pitting of the bottom plate and the corrosion of the inner surface of the upper deck, but also have the original The TMCP type ship plate has the same weldability and workability. This type of steel sheet is being used continuously in the construction of oil tankers and has begun to show the effects of delaying corrosion, extending life and preventing damage accidents (and polluting the marine environment).
On the other hand, the upper deck of the large container is made of YP390N/mm2 high-strength steel plate with a thickness of 70-80mm. Recent research reports indicate that the brittle crack propagation test of ultra-thick plates shows that brittle cracking should be stopped at the minimum shipping temperature (-10 ° C) and certain stress conditions, whether in the high heat input weld or the base metal. It is all difficult.
Therefore, how to ensure the weld toughness of the super-thick steel plate, the brittle fracture of the base metal, and the detection and elimination of welding defects. This ensures that the reliability of this super-thick plate structure is an important issue. As an example of solving these problems, a YP460 N/mm2 high-strength steel sheet for large heat input welding has been developed.
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