STEEL PLATE FOR PRESSURE VESSEL WITH EXCELLENT CRYOGENIC TOUGHNESS AND EXCELLENT DUCTILITY AND MANUFACTURING METHOD THEREOF

Abstract

Provided are a steel plate for a pressure vessel with excellent cryogenic toughness and elongation resistance, and a manufacturing method thereof. The steel plate for a pressure vessel of the present invention comprises, in weight %, 0.05 to 0.15% of C; 0.20 to 0.40% of Si; 0.3 to 0.6% of Mn; 0.001 to 0.05% of Al; 0.012% or less of P; 0.015% or less of S; 4.0 to 5.0% of Ni; 0.001 to 0.10% of In; and the balance being Fe and unavoidable impurities, wherein a steel microstructure consists of 15 to 80 area % of tempered bainite and the balance being tempered martensite.

Claims

1. A method of manufacturing a steel plate for a low temperature pressure vessel having excellent cryogenic toughness and ductility, the method comprising: reheating a steel slab at 1050 to 1250 C., the steel slab containing, in weight %, 0.05 to 0.15% of C, 0.20 to 0.40% of Si, 0.3 to 0.6% of Mn, 0.001 to 0.05% of Al, 0.012% or less of P, 0.015% or less of S, 4.0 to 5.0% of Ni, 0.001 to 0.10% of In, a balance of Fe, and unavoidable impurities; hot rolling the reheated steel plate at a reduction ratio of 5 to 30% per pass, and terminating rolling at a temperature of 800 C. or higher; primary cooling the hot-rolled steel plate at a cooling rate of 2.5 to 50 C./sec within 30 seconds after hot rolling; performing an intermediate heat treatment on the cooled steel plate for {2.4t+(1030)} minutes at a temperature of 690 to 760 C., where t is a thickness (mm) of a steel plate, and then, secondary cooling the steel plate at a cooling rate of 2.5 to 50 C./sec; and tempering the secondary cooled steel plate for {2.4t+(1030)} minutes at a temperature of 600 to 670 C., where t is a thickness (mm) of a steel plate.

2. The method of manufacturing a steel plate for a low temperature pressure vessel having excellent cryogenic toughness and ductility, of claim 1, wherein a steel microstructure obtained by the tempering is comprised of 15 to 80 area % of tempered bainite and a remainder of tempered martensite.

3. The method of manufacturing a steel plate for a low temperature pressure vessel having excellent cryogenic toughness and ductility, of claim 1. wherein the In is contained in an amount of 0.05 to 0.08%.

Description

EXAMPLE

[0049] After preparing the respective steel slabs having the composition components illustrated in Table 1, these steel slabs were respectively reheated at a temperature ranging from 1050 to 1250 C. In addition, each of these reheated steel plates was hot-rolled at a reduction ratio of 5 to 30% per pass, and at this time, the hot rolling end temperature was controlled as illustrated in Table 2. Then, each of the hot-rolled steel plates was primarily cooled under the conditions of Table 2 within 30 seconds after hot-rolling, and then, was subjected to heat treatment under the conditions of Table 2. Subsequently, the heat-treated hot-rolled steel plate was secondarily cooled to room temperature, and then, the secondary cooled steel plate was tempered under the conditions illustrated in Table 2.

[0050] As described above, the yield strength, tensile strength, and low-temperature toughness were evaluated for the manufactured steel plates, and the results are also illustrated in Table 2 below. On the other hand, in Table 2 below, the low-temperature toughness is a result of evaluating with the Charpy impact energy value obtained by performing a Charpy impact test on a specimen having a V notch at 150 C. In addition, tensile tests for measuring tensile strength and yield strength were conducted in accordance with ASTM A20, A370 and E8.

TABLE-US-00001 TABLE 1 Steel Composition Component (weight %) Grade C Mn Si Al P S Ni In Inventive 0.10 0.52 0.29 0.032 0.009 0.0012 4.49 0.08 Steel a Inventive 0.09 0.55 0.27 0.029 0.008 0.0010 4.45 0.05 Steel b Inventive 0.10 0.50 0.28 0.033 0.010 0.0011 4.85 0.07 Steel c Comparative 0.11 0.50 0.29 0.030 0.012 0.0012 4.20 Steel d

TABLE-US-00002 TABLE 2 Steel YS TS EL Classification Grade A* B* C* D* E* F* G* (MPa) (MPa) (%) H* IE 1 a 850 15.0 730 50 630 1.5 65 658 718 38 256 IE 2 860 8.5 740 90 640 2.0 60 657 722 36 251 IE 3 b 850 15.0 750 50 630 1.5 68 658 720 35 227 IE 4 860 8.5 730 90 640 2.0 57 657 715 37 233 IE 5 c 850 15.0 720 50 630 1.5 68 660 725 38 230 IE 6 850 8.5 730 90 640 2.0 65 651 730 38 215 CE 1 a 850 Air 630 1.5 0 565 633 22 54 cooling CE 2 860 Air 640 2.0 0 562 621 21 35 cooling CE 3 d 860 10.0 720 50 630 1.5 35 540 655 24 85 CE 4 860 7.5 730 90 640 2.0 30 543 648 26 86 CE 5 850 Air 630 1.5 0 528 623 25 55 cooling CE 6 850 Air 640 2.0 0 522 616 23 48 cooling [0051] In Table 2, IE is Inventive Example, CE is Comparative Example, A* is the hot rolling end temperature ( C.), B* is the primary cooling (water cooling) rate ( C./s), C* is the heat treatment temperature ( C.), D* is the heat treatment time (min.), E* is the tempering temperature ( C.), F* is the tempering time (hr), G* is the tempered bainite fraction (%), and H* is the 150 C. impact toughness value (J).

[0052] As illustrated in Tables 1 and 2, in the case of Inventive Examples 1-6 in which the steel composition components and manufacturing process conditions satisfy the scope of the present disclosure, after tempering treatment, 15-80 area % of tempered bainite and the remainder tempered martensite structure may be obtained, and thus, it can be seen that the yield strength and tensile strength are excellent by about 100 MPa and 80 MPa, respectively, compared to Comparative Examples 1-6, and further, the elongation is excellent by 10% or more, and the 150 C. low-temperature toughness is also excellent by 100 J or more, compared to Comparative Examples 1-6.

[0053] Meanwhile, in Comparative Examples 1 and 2 in which the steel composition component range proposed in the present disclosure is satisfied, but the manufacturing process conditions are out of the scope of the present disclosure, and in Comparative Examples 2 to 4 in which steel manufacturing process conditions are within the scope of the present disclosure, but the steel composition component is outside the scope of the present disclosure; it can be seen that it is difficult to secure a required microstructure and secure required physical properties.

[0054] In addition, in Comparative Examples 5 and 6, in which not only the steel composition components but also the manufacturing process conditions are outside the scope of the present disclosure, it can be confirmed that it is difficult to secure a required microstructure and secure required physical properties.

[0055] As described above, in the detailed description of the present disclosure, exemplary embodiments of the present disclosure have been described, but various modifications may be made by those of ordinary skill in the art to which the present disclosure pertains without departing from the scope of the present disclosure. Therefore, the scope of the present disclosure is not limited to and should not be determined by the 10 described embodiments, and should be determined by the claims to be described later, as well as those equivalent thereto.