Nickel-containing steel for low temperature

Abstract

A nickel-containing steel for low temperature according to an aspect of the present invention has a chemical composition within a predetermined range, in which a metallographic structure of a thickness middle portion contains 2.0 vol % to 20.0 vol % of an austenite phase, an average grain size of prior austenite grains is 3.0 μm to 15.0 μm, an average aspect ratio of the prior austenite grains is 1.0 to 2.4, a plate thickness is 4.5 mm to 30 mm, the chemical composition and the average grain size of the prior austenite grains are further limited depending on the plate thickness, a yield stress at room temperature is 460 MPa to 710 MPa, and a tensile strength at the room temperature is 560 MPa to 810 MPa.

Claims

1. A nickel-containing steel comprising, as a chemical composition, by mass %: C: 0.030% to 0.070%; Si: 0.03% to 0.30%; Mn: 0.20% to 0.80%; Ni: 10.5% to 12.4%; Al: 0.010% to 0.060%; N: 0.0015% to 0.0060%; O: 0.0007% to 0.0030%; Cu: 0% to 0.50%; Cr: 0% to 0.50%; Mo: 0% to 0.40%; Nb: 0% to 0.020%; V: 0% to 0.080%; Ti: 0% to 0.020%; B: 0% to 0.0020%; Ca: 0% to 0.0040%; REM: 0% to 0.0050%; P: 0.0080% or less; S: 0.0040% or less; and a remainder: Fe and impurities, wherein a metallographic structure of a thickness middle plane contains 2.0 vol % to 20.0 vol % of an austenite phase, an average grain size of prior austenite grains, measured in accordance with JIS G 0551, measured in a section of the thickness plane parallel to a rolling direction and a thickness direction is 3.0 μm to 15.0 μm, an average aspect ratio of the prior austenite grains, wherein the aspect ratio of the prior austenite grains is defined as: length of the prior austenite grains in a rolling direction/thickness of the prior austenite grains in the thickness direction, measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 1.0 to 2.4, a plate thickness is 4.5 mm to 20 mm, when the nickel-containing steel contains Ni: less than 11.5%, the nickel-containing steel contains C: 0.060% or less, Si: 0.19% or less, Mn: 0.30% to 0.50%, A1: 0.050% or less, N: 0.0050% or less, Cr: 0.35% or less, Nb: 0.015% or less, V: 0.060% or less, Ti: 0.015% or less, P: 0.0060% or less, and S: 0.0030% or less, and the average grain size of the prior austenite grains, measured in accordance with JIS G 0551, is 8.0 μm or less, a yield stress at room temperature, measured in accordance with JIS Z 2241, is 460 MPa to 710 MPa, and a tensile strength at the room temperature, measured in accordance with JIS Z 2241, is 560 MPa to 810 MPa.

2. The nickel-containing steel according to claim 1 comprising, as the chemical composition, by mass %: Ni: 11.5% or more, and Mn: 0.50% or less.

3. The nickel-containing steel according to claim 1 comprising, as the chemical composition, by mass %: Ni: 11.5% or more, wherein the average grain size of the prior austenite grains, measured in accordance with JIS G 0551, is 9.0 μm or less.

4. The nickel-containing steel according to claim 1, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 8.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.

5. The nickel-containing steel according to claim 1, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 5.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.

6. The nickel-containing steel according to claim 2 comprising, as the chemical composition, by mass %: Ni: 11.5% or more, wherein the average grain size of the prior austenite grains, measured in accordance with JIS G 0551, is 9.0 μm or less.

7. The nickel-containing steel according to claim 2, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 8.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.

8. The nickel-containing steel according to claim 3, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 8.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.

9. The nickel-containing steel according to claim 6, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 8.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.

10. The nickel-containing steel according to claim 2, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 5.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.

11. The nickel-containing steel according to claim 3, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 5.0 μm, and wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.

12. The nickel-containing steel according to claim 6, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 5.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.

Description

EXAMPLES

(1) Examples of the present invention will be described below. However, the following examples are examples of the present invention, and the present invention is not limited to the examples described below.

Example 1: Ni Steel Having Ni Content of 11.5% or More

(2) Steel was melted by a converter and slabs having a thickness of 150 mm to 300 mm were manufactured by continuous casting. Tables 1 and 2 show the chemical compositions of Kinds of steel A1 to A24. These slabs were heated, subjected to controlled rolling, directly subjected to water cooling or air cooling, and subjected to heat treatments including reheating hardening, an intermediate heat treatment, and tempering, whereby steel plates were manufactured. The retention time of the heating of the hot rolling was set to 30 minutes to 120 minutes, and the retention time of the heat treatments including the reheating hardening, the intermediate heat treatment, and the tempering was set to 20 minutes to 60 minutes. In a case of performing water cooling after the hot rolling, water cooling to 200° C. or lower was performed. Means for cooling in the heat treatments including the reheating hardening, the intermediate heat treatment, and the tempering was water cooling, and water cooling to 200° C. or lower from the treatment temperature of each of the heat treatments was performed. Samples were taken from the steel plates, and the metallographic structure, tensile properties, and toughness thereof were evaluated.

(3) TABLE-US-00001 TABLE 1 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material C Si Mn P S Cu Ni Cr Mo Al Nb Ti A1 0.030 0.19 0.25 0.0060 0.0010 0.05 11.8 0.03 0.03 0.025 A2 0.070 0.08 0.31 0.0040 0.0022 11.9 0.024 A3 0.033 0.30 0.36 0.0030 0.0009 12.3 0.02 0.028 0.013 A4 0.038 0.08 0.20 0.0030 0.0007 11.5 0.025 A5 0.032 0.25 0.80 0.0040 0.0008 0.04 12.2 0.017 0.006 A6 0.035 0.03 0.22 0.0020 0.0040 12.0 0.50 0.40 0.025 A7 0.063 0.11 0.45 0.0030 0.0011 12.0 0.11 0.20 0.060 A8 0.058 0.20 0.35 0.0080 0.0013 12.1 0.020 0.020 0.020 A9 0.067 0.05 0.75 0.0060 0.0013 0.50 12.4 0.010 A10 0.031 0.27 0.21 0.0030 0.0005 0.02 11.6 0.35 0.015 0.015 A11 0.052 0.07 0.60 0.0030 0.0031 11.7 0.32 0.15 0.040 0.008 A12 0.050 0.12 0.41 0.0040 0.0007 0.24 11.8 0.050 A13 0.046 0.14 0.40 0.0050 0.0007 0.43 11.7 0.033 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material V B Ca REM N O Note A1 0.0025 0.0009 Present A2 0.0025 0.0015 Invention A3 0.061 0.0010 0.0026 0.0013 Example A4 0.020 0.0025 0.0012 A5 0.0005 0.0015 0.0035 0.0016 A6 0.0008 0.0040 0.0012 A7 0.0020 0.0053 0.0008 A8 0.080 0.0028 0.0018 0.0027 A9 0.0020 0.0027 0.0011 A10 0.0040 0.0040 0.0013 A11 0.045 0.0050 0.0015 0.0016 A12 0.0060 0.0007 A13 0.0020 0.0030 Blank means that no element is intentionally added.

(4) TABLE-US-00002 TABLE 2 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material C Si Mn P S Cu Ni Cr Mo Al Nb Ti A14 0.026 0.24 0.24 0.0030 0.0015 12.0 0.030 A15 0.077 0.26 0.26 0.0030 0.0015 12.0 0.030 A16 0.055 0.34 0.26 0.0030 0.0014 12.1 0.032 A17 0.054 0.27 0.15 0.0030 0.0010 12.0 0.031 A18 0.048 0.21 0.89 0.0030 0.0005 12.0 0.030 A19 0.049 0.22 0.21 0.0090 0.0032 0.10 12.2 0.15 0.031 A20 0.060 0.22 0.26 0.0050 0.0046 11.9 0.13 0.08 0.020 A21 0.063 0.21 0.72 0.0050 0.0009 12.0 0.61 0.020 A22 0.064 0.05 0.72 0.0050 0.0012 0.10 12.0 0.066 A23 0.036 0.05 0.71 0.0030 0.0006 12.0 0.020 0.024 A24 0.035 0.05 0.71 0.0030 0.0008 11.7 0.020 0.026 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material V B Ca REM N O Note A14 0.0030 0.0014 Comparative A15 0.0030 0.0014 Example A16 0.0030 0.0014 A17 0.003 0.0029 0.0013 A18 0.0003 0.0023 0.0031 0.0015 A19 0.0045 0.0015 A20 0.0031 0.0010 A21 0.0035 0.0020 0.0015 A22 0.0025 0.0013 A23 0.0036 0.0030 0.0015 A24 0.0050 0.0074 0.0012 Blank means that no element is intentionally added. Underline means outside the range of the present invention.

(5) The average grain size of prior austenite grains (the average grain size of prior γ) to be measured in a section of a thickness middle portion parallel to a rolling direction and a thickness direction was measured in a section (L-section) of a thickness middle portion parallel to a rolling direction and a thickness direction as an observed section. The average grain size of the prior austenite grains was measured according to JIS G 0551. First, the observed section of the sample was corroded with a saturated aqueous solution of picric acid to reveal the prior austenite grain boundaries, and thereafter five or more visual fields were photographed with a scanning electron microscope at a magnification of 1,000-fold or 2,000-fold. After identifying the prior austenite grain boundaries using the structural photographs, the circle equivalent grain sizes (diameters) of at least 20 prior austenite grains were obtained by image processing, and the average value thereof was determined as the average grain size of the prior austenite grains.

(6) In addition, in the steel of the present invention, the refinement of the prior austenite grain size, suppression of the P content, and the like are carried out so that fracture is less likely to occur at the prior austenite grain boundaries. Therefore, it may be difficult to identify the prior austenite grain boundaries by corrosion. In such a case, after heating the sample to 450° C. to 490° C., a heat treatment of temperature retention for one hour or longer was performed, and then the average grain size of the prior austenite grains was measured by the method described above.

(7) In a case where identification of the prior austenite grain boundaries was difficult even if the heat treatment at 450° C. to 490° C. was performed, a Charpy test piece was taken from the heat-treated sample, and the sample subjected to an impact test at −196° C. and fractured at the prior austenite grain boundaries was used. In this case, a cross section of a fracture surface at the section (L-section) parallel to the rolling direction and the thickness direction was created and corroded, and thereafter, the prior austenite grain sizes were measured by identifying the prior austenite grain boundaries of the cross section of the fracture surface of the thickness middle portion with the scanning electron microscope. When the prior austenite grain boundaries are embrittled by a heat treatment, minute cracks are generated at the prior austenite grain boundaries due to an impact load during the Charpy test, so that the prior austenite grain boundaries are easily identified.

(8) The average aspect ratio of the prior austenite grains (average aspect ratio of prior γ grains) measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction was obtained as a ratio between the maximum value (length in the rolling direction) and the minimum value (thickness in the thickness direction) of the length of a region (prior austenite grain) surrounded by the prior austenite grain boundary identified as described above. The average aspect ratios of at least 20 prior austenite grains were measured, and the average value thereof was calculated to obtain the average aspect ratio.

(9) The volume fraction of the austenite phase (volume fraction of γ phase) contained in the metallographic structure of the thickness middle portion was measured by taking a sample parallel to the plate surface from the thickness middle portion and performing an X-ray diffraction method thereon. The volume fraction of the austenite phase was determined from the ratio between the integrated intensities of austenite (face-centered cubic structure) and tempered martensite (body-centered cubic structure) of X-ray peaks.

(10) The average effective grain size measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction was measured by using an EBSD analyzer attached to the scanning electron microscope, with the section (L-section) of the thickness middle portion parallel to the rolling direction and the thickness direction as an observed section. Observation of five or more visual fields was performed at a magnification of 2,000-fold, a boundary of a metallographic structure having an orientation difference of 15° or more was regarded as a grain boundary, grains surrounded by the grain boundaries were regarded as effective grains, the circle equivalent grain sizes (diameters) were obtained from the areas by image processing, and the average value of the circle equivalent grain sizes were determined as an average effective grain size.

(11) By taking a 1A full-thickness tensile test piece specified in JIS Z 2241 whose longitudinal direction was parallel to the rolling direction (L direction), strength (yield stress and tensile strength) was measured at room temperature by the method specified in JIS Z 2241. The target value of the yield stress is 460 MPa to 580 MPa, and the target value of the tensile strength is 560 MPa to 680 MPa. The yield stress was a lower yield stress. However, there were many cases where no clear lower yield stress was observed, and in that case, the 0.2% proof stress was taken.

(12) Regarding the extremely low temperature toughness, a CT test piece of full thickness with front and rear surfaces each ground 0.5 mm was taken in a direction (C direction) perpendicular to the rolling direction. A J-R curve was created according to the unloading compliance method specified in ASTM standard E1820-13 in liquid hydrogen (−253° C.), and a J value was converted into a Kw value. The target value of the extremely low temperature toughness is 150 MPa.Math.√m or more.

(13) Tables 3 and 4 show the plate thickness, manufacturing method, base metal properties, and metallographic structure of steel materials (Steel materials Nos. 1 to 32) manufactured using slabs having the chemical compositions of Kinds of steel A1 to A24.

(14) TABLE-US-00003 TABLE 3 Heating, rolling, and heat treatment conditions Cumulative Inter- Heating rolling Water mediate temperature reduction Rolling cooling start Reheating heat Plate during at 950° C. finishing temperature hardening treatment Tempering Manufacturing Steel thickness rolling or lower temperature after rolling temperature temperature temperature condition No. material (mm) (° C.) (%) (° C.) (° C.) (° C.) (° C.) (° C.) a1 A1 12 1160 95 800 750 850 630 560 a2 A2 20 1100 80 830 Air cooling 830 640 550 after rolling a3 A3 20 970 92 720 680 760 630 530 a4 A4 12 980 95 790 Air cooling 720 620 530 after rolling a5 A5 30 1030 80 770 730 780 640 550 a6 A6 30 1000 91 800 Air cooling 770 620 550 after rolling a7 A7 30 1160 88 800 810 880 640 560 a8 A8 30 970 90 700 660 760 620 540 a9 A9 30 1070 85 840 810 820 640 540 a10 A9 30 1070 85 830 800 820 640 540 a11 A9 30 1070 85 830 800 820 640 540 a12 A9 30 1080 85 830 810 820 640 540 a13 A10 30 1050 88 790 760 800 640 570 a14 A11 30 1060 88 840 Air cooling 760 650 540 after rolling a15 A12 30 960 90 670 630 750 630 530 a16 A13 30 950 90 650 610 750 630 540 Metallographic structure Average Base metal properties grain Average Volume Average Extremely size of aspect fraction effective low prior γ ratio of of γ grain Yield Tensile temperature Manufacturing Steel grains prior γ phase size stress strength toughness* condition No. material (μm) grains (%) (μm) (MPa) (MPa) (MPa .Math. √m) Note a1 A1 11.0 1.7 12.0 4.5 558 623 156 Present a2 A2 9.6 1.6 9.1 7.0 499 581 152 Invention a3 A3 4.3 2.1 7.5 2.8 527 598 165 Example a4 A4 5.2 1.8 8.5 3.2 534 624 163 a5 A5 6.8 1.9 6.3 3.0 483 570 164 a6 A6 6.3 1.7 6.8 4.7 555 663 155 a7 A7 15.0 1.0 14.0 8.6 573 673 150 a8 A8 3.5 2.2 2.4 2.6 525 613 189 a9 A9 8.2 1.4 7.6 3.8 546 645 160 a10 A9 8.3 1.4 2.0 3.7 569 665 151 a11 A9 10.5 1.2 2.1 5.6 534 641 152 a12 A9 8.2 1.4 15.0 3.8 555 645 152 a13 A10 7.5 1.8 8.0 3.5 510 602 162 a14 A11 9.3 1.5 6.7 5.1 485 580 154 a15 A12 3.4 2.3 4.5 2.3 495 580 195 a16 A13 3.3 2.4 5.6 2.0 480 567 210 *Extremely low temperature toughness is the K.sub.IC value (converted from J value) in liquid hydrogen (−253° C.), the unit is MPa .Math. √m.

(15) TABLE-US-00004 TABLE 4 Heating, rolling, and heat treatment conditions Cumulative Water Inter- Heating rolling cooling mediate temperature reduction Rolling start Reheating heat Plate during at 950° C. finishing temperature hardening treatment Tempering Manufacturing Steel thickness rolling or lower temperature after rolling temperature temperature temperature condition No. material (mm) (° C.) (%) (° C.) (° C.) (° C.) (° C.) (° C.) a17 A14 30 1100 80 800 770 800 640 560 a18 A15 30 1080 85 780 750 760 620 550 a19 A16 30 1060 90 770 740 770 650 540 a20 A17 30 1050 90 760 720 750 630 550 a21 A18 30 1050 85 800 760 830 650 540 a22 A19 30 1080 85 780 750 760 630 550 a23 A20 30 1070 80 800 750 800 630 550 a24 A21 30 1070 80 790 760 780 640 560 a25 A22 30 1070 80 770 740 780 640 530 a26 A23 30 1070 80 780 740 820 630 530 a27 A24 30 1080 80 780 750 830 630 540 a28 A8 30 930 85 640 610 860 630 550 a29 A7 30 1180 88 830 800 860 640 550 a30 A7 30 1160 50 840 810 850 640 550 a31 A7 30 1130 88 880 850 840 650 560 a32 A9 30 1080 85 840 800 910 640 530 Metallographic structure Average Base metal properties grain Average Volume Average Extremely size of aspect fraction effective low prior γ ratio of of γ grain Yield Tensile temperature Manufacturing Steel grains prior γ phase size stress strength toughness* condition No. material (μm) grains (%) (μm) (MPa) (MPa) (MPa .Math. √m) Note a17 A14 7.3 1.2 1.2 4.0 453 552 66 Comparative a18 A15 6.6 1.4 5.5 3.5 506 602 67 Example a19 A16 6.8 1.4 11.4  3.6 486 578 58 a20 A17 6.1 1.5 0.8 3.3 455 563 75 a21 A18 7.6 1.1 10.5  4.0 545 647 64 a22 A19 6.5 1.2 5.2 3.4 496 590 80 a23 A20 7.2 1.1 4.3 3.9 526 618 65 a24 A21 7.1 1.3 14.0  3.5 560 657 62 a25 A22 7.0 1.4 12.5  3.4 543 639 59 a26 A23 7.4 2.5 4.5 4.1 503 594 67 a27 A24 7.5 1.0 3.6 4.2 499 585 70 a28 A8 3.6 2.5 7.2 2.8 713 817 69 a29 A7 15.7  1.0 7.2 8.7 570 667 125 a30 A7 15.6  1.1 7.0 8.8 565 661 120 a31 A7 16.2  1.0 7.2 9.1 564 664 118 a32 A9 16.3  1.2 5.6 9.3 571 678 123 Underline means outside the range of the present invention. *Extremely low temperature toughness is the K.sub.IC value (converted from J value) in liquid hydrogen (−253° C.). the unit is MPa .Math. √m.

(16) As is apparent from Tables 3 and 4, in Manufacturing Nos. a1 to a16, the tensile strength and the yield stress at room temperature, and the extremely low temperature toughness satisfied the target values.

(17) On the other hand, in Manufacturing No. a17, the C content was small, and in Manufacturing No. a20, the Mn content was small, so that the tensile strength was low and the extremely low temperature toughness had also decreased. In each of Manufacturing Nos. a18, a19, and a21 to a25, the C content, Si content, Mn content, P content, S content, Cr content, and Al content were large, and the extremely low temperature toughness had decreased. In Manufacturing No. a26, the Nb content and the B content were too large, and the extremely low temperature toughness had decreased. In Steel material No. a27, the Ti content and the N content were large, and the extremely low temperature toughness had decreased.

(18) Manufacturing Nos. a28 to a32 are examples in which manufacturing conditions that deviated from preferable ranges are adopted. In Manufacturing No. a28, the heating temperature during rolling was low and the rolling finishing temperature was also low, so that the tensile strength had increased excessively and the extremely low temperature toughness had decreased. In each of Manufacturing Nos. a29, a31, and a32, the heating temperature during rolling, the rolling finishing temperature, and the reheating hardening temperature were high, and the prior austenite grain size had increased, so that the extremely low temperature toughness had decreased. In Manufacturing No. a30, the cumulative rolling reduction at 950° C. or lower was small, and the prior austenite grain size had increased, so that the extremely low temperature toughness had decreased.

Example 2: Ni Steel Having Ni Content of Less than 11.5%

(19) Steel was melted by a converter and slabs having a thickness of 100 mm to 300 mm were manufactured by continuous casting. Tables 5 and 6 show the chemical compositions of Kinds of steel B1 to B24. These slabs were heated, subjected to controlled rolling, directly subjected to water cooling, and subjected to heat treatments including reheating hardening, an intermediate heat treatment, and tempering, whereby steel plates were manufactured. The retention time of the heating of the hot rolling was set to 30 minutes to 120 minutes, and the retention time of the heat treatments including the reheating hardening, the intermediate heat treatment, and the tempering was set to 20 minutes to 60 minutes. Water cooling to 200° C. or lower was performed after the hot rolling. Means for cooling in the heat treatments including the reheating hardening, the intermediate heat treatment, and the tempering was water cooling, and water cooling to 200° C. or lower from the treatment temperature of each of the heat treatments was performed. Samples were taken from the steel plates, and the metallographic structure, tensile properties, and toughness thereof were evaluated.

(20) TABLE-US-00005 TABLE 5 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material C Si Mn P S Cu Ni Cr Mo Al Nb Ti B1 0.030 0.18 0.40 0.0040 0.0015 10.7 0.024 0.007 B2 0.060 0.06 0.44 0.0030 0.0017 10.8 0.03 0.01 0.023 B3 0.032 0.19 0.33 0.0020 0.0010 11.0 0.02 0.04 0.011 B4 0.035 0.10 0.30 0.0050 0.0009 0.05 11.4 0.020 0.001 B5 0.042 0.19 0.50 0.0040 0.0007 0.02 11.3 0.04 0.019 B6 0.036 0.14 0.41 0.0040 0.0030 11.2 0.35 0.40 0.017 0.009 B7 0.050 0.10 0.42 0.0030 0.0015 11.2 0.15 0.20 0.050 0.009 B8 0.056 0.19 0.38 0.0060 0.0012 11.3 0.05 0.025 0.015 0.015 B9 0.045 0.16 0.40 0.0050 0.0010 0.50 10.5 0.06 0.010 B10 0.035 0.17 0.41 0.0040 0.0005 10.6 0.038 0.002 B11 0.049 0.06 0.39 0.0030 0.0025 0.03 10.9 0.03 0.045 B12 0.048 0.08 0.43 0.0030 0.0006 0.20 11.1 0.04 0.040 0.012 B13 0.045 0.16 0.44 0.0050 0.0007 0.40 11.0 0.25 0.016 0.010 0.005 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material V B Ca REM N O Note B1 0.0002 0.0020 0.0010 Present B2 0.030 0.0021 0.0007 Invention B3 0.0002 0.0017 0.0024 0.0012 Example B4 0.012 0.0030 0.0014 B5 0.0002 0.0025 0.0012 B6 0.0025 0.0016 B7 0.041 0.0010 0.0005 0.0030 0.0027 0.0009 B8 0.060 0.0016 0.0018 0.0016 B9 0.0020 0.0025 0.0032 0.0020 B10 0.0040 0.0044 0.0011 B11 0.0002 0.0050 0.0015 0.0015 B12 0.0050 0.0010 B13 0.010 0.0026 0.0030 Blank means that no element is intentionally added.

(21) TABLE-US-00006 TABLE 6 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material C Si Mn P S Cu Ni Cr Mo Al Nb Ti B14 0.024 0.18 0.48 0.0040 0.0025 11.2 0.14 0.020 0.012 0.012 B15 0.070 0.18 0.45 0.0050 0.0016 11.3 0.030 B16 0.055 0.21 0.44 0.0050 0.0010 11.1 0.015 B17 0.033 0.17 0.20 0.0050 0.0014 10.6 0.030 0.010 0.010 B18 0.052 0.17 0.63 0.0060 0.0025 0.02 11.2 0.20 0.043 0.012 B19 0.055 0.16 0.49 0.0070 0.0020 11.2 0.30 0.038 0.009 B20 0.056 0.16 0.48 0.0030 0.0044 10.8 0.28 0.040 0.010 B21 0.054 0.05 0.38 0.0050 0.0024 11.0 0.43 0.015 0.012 0.010 B22 0.058 0.08 0.48 0.0050 0.0020 0.40 10.8 0.06 0.15 0.060 B23 0.056 0.18 0.48 0.0050 0.0027 0.25 11.2 0.30 0.15 0.040 0.019 B24 0.055 0.19 0.47 0.0050 0.0027 0.15 10.8 0.14 0.15 0.013 0.018 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material V B Ca REM N O Note B14 0.045 0.0002 0.0032 0.0012 Comparative B15 0.0030 0.0014 Example B16 0.0025 0.0034 0.0020 B17 0.035 0.0033 0.0016 B18 0.0002 0.0036 0.0014 B19 0.0039 0.0019 B20 0.035 0.0012 0.0037 0.0015 B21 0.044 0.0015 0.0047 0.0021 B22 0.0048 0.0020 B23 0.0032 0.0046 0.0016 B24 0.0002 0.0032 0.0060 0.0025 Blank means that no element is intentionally added. Underline means outside the range of the present invention.

(22) A method of identifying the metallographic structure of the sample, a method of evaluating the mechanical properties, and acceptance criteria for the mechanical properties were the same as those for the samples disclosed in Tables 1 to 4. Tables 3 and 4 show the plate thickness, manufacturing method, base metal properties, and metallographic structure of steel materials (Manufacturing Nos. b1 to b30) manufactured using slabs having the chemical compositions of Kinds of steel B1 to B24.

(23) TABLE-US-00007 TABLE 7 Heating, rolling, and heat treatment conditions Cumulative Water Inter- Heating rolling cooling mediate temperature reduction Rolling start Reheating heat Plate during at 950° C. finishing temperature hardening treatment Tempering Manufacturing Steel thickness rolling or lower temperature after rolling temperature temperature temperature condition No. material (mm) (° C.) (%) (° C.) (° C.) (° C.) (° C.) (° C.) b1 B1 12 1050 95 800 710 760 650 560 b2 B2 18 1070 82 780 680 750 630 540 b3 B3 14 1050 90 800 710 770 640 550 b4 B4 12 1060 88 770 680 740 640 530 b5 B5 18 1030 90 790 720 760 630 530 b6 B6 20 1050 83 650 550 770 630 540 b7 B7 20 1100 80 680 590 780 630 540 b8 B8 20 990 90 770 690 760 630 540 b9 B8 20 1030 83 780 710 760 640 540 b10 B9 18 950 82 780 740 760 630 540 b11 B10 14 970 92 760 700 780 640 540 b12 B11 16 1020 87 770 680 750 630 530 b13 B12 18 980 85 720 630 750 620 530 b14 B13 20 1000 87 700 590 750 630 540 Metallographic structure Average Base metal properties grain Average Volume Average Extremely size of aspect fraction effective low prior γ ratio of of γ grain Yield Tensile temperature Manufacturing Steel grains prior γ phase size stress strength toughness* condition No. material (μm) grains (%) (μm) (MPa) (MPa) (MPa .Math. √m) Note b1 B1 8.0 1.1 2.4 5.0 473 568 220 Present b2 B2 6.8 1.2 4.9 2.5 524 633 179 Invention b3 B3 7.7 1.0 3.5 3.6 486 596 196 Example b4 B4 7.2 1.5 2.3 4.9 460 560 215 b5 B5 7.8 1.1 4.0 3.4 521 635 183 b6 B6 3.0 2.4 3.6 2.3 580 675 150 b7 B7 4.2 1.7 4.6 2.4 566 670 155 b8 B8 6.7 1.9 4.7 3.0 550 659 157 b9 B8 7.5 1.6 2.0 5.5 486 586 151 b10 B9 7.1 1.3 4.0 3.3 507 615 185 b11 B10 4.8 1.7 2.5 4.7 478 577 212 b12 B11 6.7 1.4 4.8 4.4 485 580 204 b13 B12 5.5 2.2 5.0 4.0 537 634 170 b14 B13 4.3 2.3 4.2 2.0 564 650 166 *Extremely low temperature toughness is the K.sub.IC value (converted from J value) in liquid hydrogen (−253° C.), the unit is MPa .Math. √m.

(24) TABLE-US-00008 TABLE 8 Heating, rolling, and heat treatment conditions Cumulative Water Inter- Heating rolling cooling mediate temperature reduction Rolling start Reheating heat Plate during at 950° C. finishing temperature hardening treatment Tempering Manufacturing Steel thickness rolling or lower temperature after rolling temperature temperature temperature condition No. material (mm) (° C.) (%) (° C.) (° C.) (° C.) (° C.) (° C.) b15 B14 20 1040 83 790 720 780 640 540 b16 B15 20 1050 83 780 710 750 630 540 b17 B16 20 1080 83 800 740 770 640 550 b18 B17 20 1080 83 790 720 780 650 560 b19 B18 20 1100 83 800 740 770 630 550 b20 B19 20 1050 83 780 710 770 630 550 b21 B20 20 1080 83 790 700 770 640 550 b22 B21 20 1070 83 790 730 760 630 550 b23 B22 20 1080 83 770 710 770 630 550 b24 B23 20 1050 83 760 690 770 630 550 b25 B24 20 1060 83 770 690 770 640 550 b26 B12 18 930 85 630 600 750 620 530 b27 B1 12 1130 90 800 750 760 650 560 b28 B5 20 1090 75 790 730 760 630 530 b29 B6 20 1080 83 830 770 770 630 540 b30 B7 20 1080 80 700 630 800 630 540 Metallographic structure Average Base metal properties grain Average Volume Average Extremely size of aspect fraction effective low prior γ ratio of of γ grain Yield Tensile temperature Manufacturing Steel grains prior γ phase size stress strength toughness* condition No. material (pm) grains (%) (μm) (MPa) (MPa) (MPa .Math. √m) Note b15 B14 7.2 1.5 1.2 4.7 470 564 98 Comparative b16 B15 6.4 1.2 4.6 4.3 480 573 116 Example b17 B16 7.1 1.0 3.5 4.6 470 566 113 b18 B17 6.5 1.1 1.6 4.8 465 563 115 b19 B18 6.8 1.5 4.7 3.2 526 627 120 b20 B19 6.7 1.4 3.4 3.4 522 625 112 b21 B20 6.8 1.2 3.2 3.4 530 638 105 b22 B21 6.9 1.6 4.2 3.3 532 643 124 b23 B22 4.0 1.3 3.6 3.1 550 650 130 b24 B23 6.7 2.5 3.4 2.3 570 674 116 b25 B24 3.7 1.2 3.5 2.8 560 667 82 b26 B12 3.4 2.5 4.3 2.1 590 695 80 b27 B1 9.2 1.0 2.6 5.6 485 578 105 b28 B5 8.6 1.1 4.1 5.0 535 640 124 b29 B6 8.5 1.4 3.9 5.5 580 678 115 b30 B7 8.7 1.5 4.5 5.3 570 675 120 Underline means outside the range of the present invention. *Extremely low temperature toughness is the K.sub.IC value (converted from J value) in liquid hydrogen (−253° C.), the unit is MPa .Math. √m.

(25) As is apparent from Tables 7 and 8, in Manufacturing Nos. b1 to b14, the tensile strength and the yield stress at room temperature, and the extremely low temperature toughness satisfied the target values.

(26) On the other hand, in Manufacturing No. b15, the C content was small, and in Manufacturing No. b18, the Mn content was small, so that the extremely low temperature toughness had decreased. In each of Manufacturing Nos. b16, b17, and b19 to b23, the C content, Si content, Mn content, P content, S content, Cr content, and Al content were large, and the extremely low temperature toughness had decreased. In Manufacturing No. b24, the Nb content and the B content were too large, and the extremely low temperature toughness had decreased. In Manufacturing No. b25, the Ti content and the N content were large, and the extremely low temperature toughness had decreased.

(27) Manufacturing Nos. b26 to b30 are examples in which manufacturing conditions that deviated from preferable ranges are adopted. In Manufacturing No. b26, the heating temperature during rolling was low and the rolling finishing temperature was also low, so that the tensile strength had increased excessively and the extremely low temperature toughness had decreased. In each of Manufacturing Nos. b27, b29, and b30, the heating temperature during rolling, the rolling finishing temperature, and the reheating hardening temperature were high, the prior austenite grain size had increased, and the effective grain size had also increased, so that the extremely low temperature toughness had decreased. In Manufacturing No. b28, the cumulative rolling reduction at 950° C. or lower was small, and the prior austenite grain size had increased, so that the extremely low temperature toughness had decreased.