| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a thick steel plate having an excellent resistance property in fatigue crack propagation which is suitable as a structure material used in a ship hull, a bridge or the like. <P>SOLUTION: In the method, a cast slab having a rationalized chemical composition is heated at 1,000°C-1,300°C and hot-rolled so that a rolling finishing temperature reaches an Ar<SB POS="POST">3</SB>transformation point or higher. After that, the cast slab is heated again at a temperature range of the Ac<SB POS="POST">3</SB>transformation point or higher, and after being held for a short time, the cast slab is cooled by air or cooled so as to be accelerated. Furthermore, the cast slab is heated again at a temperature range of an Ac<SB POS="POST">1</SB>transformation point plus 10°C to an Ac<SB POS="POST">3</SB>transformation point minus 10°C, and after being held for a short time, the cast slab is hardened at an average cooling rate of 5°C/s or higher, and tempered at 400°C or higher and an Ac<SB POS="POST">1</SB>point or lower. Alternatively, the cast slab is heated again at 1,000°C-1,300°C, and hot-rolled so that a rolling finishing temperature reaches the Ar<SB POS="POST">3</SB>transformation point or higher. Then after being cooled, the cast slab is heated again at the temperature range of the Ac<SB POS="POST">3</SB>transformation point or higher, and after keeping this temperature, the cast slab is cooled by air or cooled so as to be accelerated. Furthermore, the cast slab is heated again at two phase temperature ranges of the Ac<SB POS="POST">1</SB>transformation point plus 10°C to the Ac<SB POS="POST">3</SB>transformation point minus 10°C and after being held for a short time, the cast slab is hardened at an average cooling rate of 5°C/s or higher, and tempered at 400°C or higher and the Ac<SB POS="POST">1</SB>transformation point or lower. <P>COPYRIGHT: (C)2012,JPO&INPIT |
