CA2969200C

CA2969200C - Thick-walled high-toughness high-strength steel plate and method for manufacturing the same

Info

Publication number: CA2969200C
Application number: CA2969200A
Authority: CA
Inventors: Shigeki Kitsuya; Katsuyuki Ichimiya; Kazukuni Hase
Original assignee: JFE Steel Corp
Priority date: 2015-01-16
Filing date: 2016-01-15
Publication date: 2020-06-02
Anticipated expiration: 2036-01-15
Also published as: EP3246426A4; JPWO2016114146A1; CN107208212B; EP3246426A1; JP6048626B1; CA2969200A1; SG11201704242TA; CN107208212A; US20170369958A1; KR20170095307A; EP3246426B1; WO2016114146A1; KR101994784B1

Abstract

Provided is a technique whereby internal strength and toughness in a steel sheet and toughness of the surface thereof are both obtained at the same time. A thick high-toughness high-strength steel sheet having a specific composition and excellent toughness of the steel sheet surface and internal strength and toughness in the steel sheet, the steel sheet being manufactured from a steel raw material cast under conditions of a cooling rate of 1°C/s during solidification of the surface thereof, and the toughness (vE 40) of the steel sheet surface being 70 J or greater and the sheet thickness being 100 mm or greater.

Description

.2015.S. 0(123 ell DESCRIPTION

Title of Invention: THICK-WALLED HIGH-TOUGHNESS HIGH-STRENGTH STEEL PLATE AND METHOD FOR MANUFACTURING THE SAME

Technical Field [0001]

The present invention relates to a thick-walled high- toughness high-strength steel plate for use in steel structures in construction, bridges, shipbuilding, offshore structures, construction and industrial machinery, tanks, penstocks, and the like and to a method for manufacturing the thick-walled high-toughness high-strength steel plate.

In particular, the surface of the steel plate has high toughness, and the inner part of the steel plate has high strength and toughness. The steel plate has a thickness of 100 mm or more and a yield strength of 620 MPa or more.

Background Art [0002]

In general, steel for use in construction, bridges, shipbuilding, offshore structures, construction and industrial machinery, tanks, penstocks, and other fields is welded to have a desired shape. In recent years, with significantly increasing in size of steel structures, the strength and thickness of steel to be used have also been greatly increased. [0003]

Despite trying to manufacture a thick-walled high- strength steel plate having a thickness of 100 mm or more and having high strength and toughness in a half-thickness portion (the central portion in the thickness direction), a structure having relatively low strength, such as ferrite, tends to be formed in the half-thickness portion due to a decreased cooling rate. Thus, the addition of large amounts of alloying elements is required to reduce the formation of such a structure. [0004]

In particular, in order to achieve high strength and toughness of a half-thickness portion of a thick-walled material (a thick-walled steel plate having a thickness of 100 mm or more), it is important to form bainite or a mixed structure of bainite and martensite in the half-thickness portion during quenching. This requires the addition of large amounts of alloying elements, such as Mn, Ni, Cr, and/or Mo. [0005]

The cooling rate is higher on the surface of a steel plate than in the half-thickness portion. Thus, a martensite structure having low toughness is formed on the surface of the steel plate. Thus, a high-strength steel plate having a thickness of 100 mm or more rarely has both high surface toughness and high strength and toughness of CA02969200 2017-05-29 - 3 - the inner part thereof. [0006]

Asteel plate related to the present invention is described in the following two pieces of non-patent literature, for example. Non Patent Literature 1 describes a material having a thickness of 210 mm, and Non Patent Literature 2 describes a material having a thickness of 180 mm.

Citation List Non Patent Literature [0007]

NPL1: Nippon Steel Technical Report, 348 (1993), 10-16 NPL2: Nippon Kokan Technical Report, 107 (1985), 21-30 Summary of Invention Technical Problem [0008]

These pieces of non-patent literature describe high strength and toughness of the half-thickness portion.

However, these pieces of non-patent literature do not describe the toughness (Charpy impact characteristics) of the surface of a steel plate. In general, thick-walled materials are manufactured by a quenching and tempering process. The formation of a martensite structure on the surface of a steel plate, on which the cooling rate is higher than in the half-thickness portion, deteriorates the CA02969200 2017-05-29 - 4 - toughness (Charpy impact characteristics) of the surface of the steel plate. However, these pieces of non-patent literature do not describe the manufacture of a steel plate consistently having a tough surface. [0009]

The present invention has been made to solve such problems and aims to provide a thick-walled high-toughness high-strength steel plate that has high surface toughness and high strength and toughness of the inner part thereof and a method for manufacturing the thick-walled high- toughness high-strength steel plate.

Solution to Problem [0010]

In order to solve the problems, the present inventors have extensively studied the microstructure control factors that satisfy high toughness of the surface of a thick-walled steel plate having a yield strength of 620 MPa or more and a thickness of 100 mm or more and also satisfy high strength and toughness of the half-thickness portion of the thick- walled steel plate, and have found the following. [0011] 1. When the cooling rate during the solidification of a raw material steel exceeds 1 C/s, microsegregation competes with the solidification reaction. This reduces microsegregation. In the manufacture of a large piece of CA02969200 2017-05-29 - 5 - steel, the cooling rate during the solidification of the steel decreases to 1 C/s or less, and consequently microsegregation becomes noticeable. Even in such a case, in order to achieve high toughness of the surface of a steel plate, on which a martensite structure is formed during quenching, it is important to reduce the P content and microsegregation during solidification. When primary crystals during solidification form a 6 phase, and the percentage of the 6 phase at the beginning of the formation of a y phase is 30% or more, this results in reduced microsegregation and improved toughness. The percentage % described above refers to % by volume. [0012] 2. In order to achieve high strength and toughness of the half-thickness portion, in which the cooling rate is much lower than on the surface of a steel plate during cooling after hot working, it is important to appropriately select the steel composition (components) so as to form a martensite and/or bainite microstructure even at a low cooling rate. To this end, the alloy components should be appropriately selected, and in particular the carbon equivalent (Ceq) should be 0.65% or more. In addition to the appropriate component design, it is also important to form the desired structure by hot working and heat treatment. [0013] -6- 3. Refinement of prior y grain size is effective in improving toughness. Refinement of prior y grain size before heat treatment, that is, refinement of prior y grain size just after hot working is important for refinement of prior y grain size after heat treatment.

Thus, it is important to select appropriate hot working conditions and rolling conditions. [0014]

As a result of further investigation of these findings, the present invention provides the following. [0015] [1] A thick-walled steel plate having a thickness of 100 mm or more, comprising, on a mass percent basis, C: 0.08% to 0.20%, Si: 0.40% or less, Mn: 0.5% to 5.0%, P: 0.010% or less, S: 0.0050% or less, Cr: 1.44% or less, Ni: 0.1% to 5.0%, Al: 0.010% to 0.080%, N: 0.0070% or less, 0: 0.0025% or less, Cu: 0.50% or less, Mo: 1.50% or less, V: 0.400% or less, B: 0.0030% or less, and optionally Ti: 0.005% to 0.020% following formulae (1) and (2) being satisfied, a remainder being Fe and incidental impurities, and having a toughness (vE-40) of 70 J or more at a surface portion of the steel plate;

Cegliw = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 0.65 (1) CA2969200 2019-10-28 - 7 - (CL - C)/CL x 100 30 (2) wherein CL is defined by the following formula:

CL= 0.2 - (-0.1 x (0.2 - Si) - 0.03 x (1.1 - Mn) - 0.12 x (0.2 - Cu) - 0.11 x (3 - Ni) + 0.025 x (1.2 - Cr) + 0.1 x (0.5 - Mo) + 0.2 x (0.04 - V) - 0.05 x (0.06 - Al)) (3) wherein element symbols in the formulae denote the respective alloy component contents in % by mass, and in the absence of an element the element symbol is denoted by 0; and wherein a reduction of area in a thickness direction at half the thickness of the plate, which is the percentage of the difference between the minimum cross-sectional area after a round bar tensile test specimen including the half-thickness portion of the steel plate in the thickness direction is broken in a tensile test and the original cross-sectional area relative to the original cross- sectional area, is 40% or more.

The surface of the steel plate has high toughness, and the inner part of the steel plate has high strength and toughness. [0016] [2] The thick-walled high-toughness high-strength steel plate according to [1], further containing, on a mass percent basis, Nb: 0.100% or less. [0017] [3] The thick-walled high-toughness high-strength steel plate according to [1] or [2], further containing, on a mass percent basis, at least one of Mg: 0.0001% to 0.0050%, Ta: 0.01% to 0.20%, Zr: 0.005% to 0.1%, Y: 0.001% to 0.01%, Ca: 0.0005% to 0.0050%, and REM: 0.0005% to 0.0100%. [0018] [4] The thick-walled high-toughness high-strength steel plate according to any one of [1] to [3], wherein the thick-walled high- toughness high-strength steel plate has a yield CA02969200 2017-05-29 - 8 - strength of 620 MPa or more. [0019] [5] The thick-walled high-toughness high-strength steel plate according to any one of [1] to [4], wherein the reduction of area in the thickness direction at half the thickness of the plate is 40% or more. [0020] [6] The method for manufacturing the thick-walled high- toughness high-strength steel plate according to any one of [1] to [5], heating steel to 1200 C to 1350 C, hot-forging the steel at a cumulative rolling reduction of 25% or more, heating the steel to an Ac3 temperature or more and 1200 C or less, hot-rolling the steel at a cumulative rolling reduction of 40% or more, leaving the steel to cool, reheating the steel to the Ac3 temperature or more and 1050 C or less, rapidly cooling the steel from the Ac3 temperature or more to a lower temperature selected from 350 C or less and an Ar3 temperature or less, and tempering the steel at a temperature in the range of 450 C to 700 C. [0021] [7] The method for manufacturing the thick-walled high- toughness high-strength steel plate according to any one of [1] to [5], heating steel to 1200 C to 1350 C, hot-forging the steel CA02969200 2017-05-29 - 9 - at a cumulative rolling reduction of 25% or more, heating the steel to an Ac3 temperature or more and 1200 C or less, hot-rolling the steel at a cumulative rolling reduction of 40% or more, rapidly cooling the steel from the Ar3 temperature or more to a lower temperature selected from 350 C or less and an Ar3 temperature or less, and tempering the steel at a temperature in the range of 450 C to 700 C. [0022] [8] The method for manufacturing the thick-walled high- toughness high-strength steel plate according to any one of [1] to [5], heating steel to 1200 C to 1350 C, slabbing the steel at a cumulative rolling reduction of 40% or more, heating the steel to an Ac3 temperature or more and 1200 C or less, hot- rolling the steel at a cumulative rolling reduction of 40% or more, leaving the steel to cool, reheating the steel to the Ac3 temperature or more and 1050 C or less, rapidly cooling the steel from the Ac3 temperature or more to a lower temperature selected from 350 C or less and an Ar3 temperature or less, and tempering the steel at a temperature in the range of 450 C to 700 C. [0023] [9] The method for manufacturing the thick-walled high- toughness high-strength steel plate according to any one of [1] to [5], CA02969200 2017-05-29 - 10 - heating steel to 1200 C to 1350 C, slabbing the steel at a cumulative rolling reduction of 40% or more, heating the steel to an Ac3 temperature or more and 1200 C or less, hot- rolling the steel at a cumulative rolling reduction of 40% or more, rapidly cooling the steel from the Ar3 temperature or more to a lower temperature selected from 350 C or less and an Ar3 temperature or less, and tempering the steel at a temperature in the range of 450 C to 700 C.

Advantageous Effects of Invention [0024]

The present invention provides a thick-walled high- toughness high-strength steel plate having a thickness of 100 mm or more and having a yield strength of 620 MPa or more and high toughness. The thick-walled high-toughness high-strength steel plate can be used to manufacture steel structures having high safety.

Description of Embodiments [0025]

Embodiments of the present invention will be described below. The present invention is not limited to these embodiments. [0026] <Thick-Walled High-Toughness High-Strength Steel Plate>

Athick-walled high-toughness high-strength steel plate according to the present invention has a composition CA02969200 2017-05-29 - 11 - containing, on a mass percent basis, C: 0.08% to 0.20%, Si: 0.40% or less (including 0%), Mn: 0.5% to 5.0%, P: 0.010% or less (including 0%), S: 0.0050% or less (including 0%), Cr: 3.0% or less (including 0%), Ni: 0.1% to 5.0%, Al: 0.010% to 0.080%, N: 0.0070% or less (including 0%), and 0: 0.0025% or less (including 0%). Each of the components will be described below. The symbol "%" in the component content refers to "% by mass". [0027]

C: 0.08% to 0.20%

Cis an element useful for achieving the strength necessary for structural steel at low cost. This effect requires a C content of 0.08% or more. In a steel structure manufactured from a thick-walled high-toughness high- strength steel plate by welding, however, a C content of more than 0.20% significantly deteriorates toughness of the base metal and weld. Thus, the C content has an upper limit of 0.20%. The C content preferably ranges from 0.08% to 0.14%. [0028]

Si: 0.40% or less Si is added for deoxidation. When another element is added for deoxidation, however, a steel plate according to the present invention does not necessarily contain Si. In a steel structure manufactured from a thick-walled high-CA02969200 2017-05-29 - 12 - toughness high-strength steel plate by welding, a Si content of more than 0.40% significantly deteriorates toughness of the base metal and heat-affected zone. Thus, the Si content is 0.40% or less, preferably 0.05% to 0.3%, more preferably 0.1% to 0.3%. [0029]

Mn: 0.5% to 5.0%

Mn is added to ensure high strength of the base metal.

This effect is insufficient at a Mn content of less than 0.5%. A Mn content of more than 5.0% promotes center segregation, results in a larger casting defect of the slab, and deteriorates mechanical properties of the base metal in a steel structure manufactured from a thick-walled high- toughness high-strength steel plate by welding. Thus, the Mn content has an upper limit of 5.0%. The Mn content preferably ranges from 0.6% to 2%, more preferably 0.6% to 1.6%. [0030]

P: 0.010% or less In a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding, a P content of more than 0.010% significantly deteriorates toughness of the base metal and heat-affected zone. Thus, the P content is preferably minimized (may be zero) and is limited to 0.010% or less.

CA02969200 2017-05-29 - 13 - [0031]

S: 0.0050% or less In a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding, a S content of more than 0.0050% significantly deteriorates toughness of the base metal and heat-affected zone. Thus, the S content is preferably minimized (may be zero) and is 0.0050% or less. [0032]

Cr: 3.0% or less Cr is an element effective in strengthening the base metal. However, an excessively high Cr content deteriorates weldability. Thus, the Cr content is 3.0% or less, preferably 0.1% to 2%, more preferably 0.7% to 1.7%. The Cr content may be 0%. [0033]

Ni: 0.1% to 5.0%

Ni is an element useful for improving the strength of steel and the toughness of the heat-affected zone. This effect requires a Ni content of 0.1% or more. However, a Ni content of more than 5.0% significantly deteriorates economic efficiency. Thus, the Ni content has an upper limit of 5.0%. The Ni content preferably ranges from 0.4% to 4%, more preferably 0.8% to 3.8%. [0034]

Al: 0.010% to 0.080%

Al is added for sufficient deoxidation of molten steel.

An Al content of less than 0.010% is insufficient for the effect. On the other hand, an Al content of more than 0.080% deteriorates toughness of the base metal due to an increased dissolved Al content in the base metal in a steel structure manufactured from a thick-walled high-toughness high-strength steel plate by welding. Thus, the Al content is 0.080% or less, preferably 0.030% to 0.080%, more preferably 0.030% to 0.070%. [0035]

N: 0.0070% or less N, together with Ti, forms a nitride and thereby performs refinement of the structure and improves the toughness of the base metal and heat-affected zone in a steel structure manufactured from a thick-walled high- toughness high-strength steel plate by welding. The toughness can be improved by a constituent other than N.

Thus, a steel plate according to the present invention does not necessarily contain N. When trying to produce this effect with N, the N content is preferably 0.0015% or more.

In a steel structure manufactured from a thick-walled high- toughness high-strength steel plate by welding, however, a N content of more than 0.0070% deteriorates toughness of the base metal due to an increased dissolved N content in the CA02969200 2017-05-29 - 15 - base metal and deteriorates toughness of the heat-affected zone due to the formation of coarse carbonitride. Thus, the Ncontent is 0.0070% or less, preferably 0.006% or less, more preferably 0.005% or less. [0036] 0: 0.0025% or less An 0 content of more than 0.0025% significantly deteriorates toughness due to the formation of a hard oxide in steel. Thus, the 0 content is preferably minimized (may be zero) and is 0.0025% or less. [0037]

In addition to these elements, a thick-walled high- toughness high-strength steel plate according to the present invention can contain at least one of Cu, Mo, V, Nb, and Ti in order to further improve strength and/or toughness. [0038]

Cu: 0.50% or less Cu can improve the strength of steel without reducing toughness. A Cu content of more than 0.50% may cause a crack on the surface of a steel plate during hot working.

Thus, the Cu content, if any, is 0.50% or less. [0039]

Mo: 1.50% or less Mo contributes to high strength of the base metal in a steel structure manufactured from a thick-walled high-CA02969200 2017-05-29 - 16 - toughness high-strength steel plate by welding. However, a Mo content of more than 1.50% results in increased hardness and deteriorates toughness due to the precipitation of alloy carbide. Thus, the Mo content, if any, has an upper limit of 1.50%. The Mo content preferably ranges from 0.2% to 0.8%. [0040]

V: 0.400% or less Vcontributes to improved strength and toughness of the base metal in a steel structure manufactured from a thick- walled high-toughness high-strength steel plate by welding.

Vprecipitates as VN and is effective in decreasing the amount of dissolved N. However, a V content of more than 0.400% deteriorates toughness due to the precipitation of hard VC. Thus, the V content, if any, is preferably 0.400% or less, more preferably 0.01% to 0.1%. [0041]

Nb: 0.100% or less Nb is effective in improving the strength of the base metal. A Nb content of more than 0.100% deteriorates toughness of the base metal. Thus, the Nb content has an upper limit of 0.100%. The Nb content is preferably 0.025% or less. [0042]

Ti: 0.005% to 0.020%

Ti forms TiN during heating and effectively suppresses the coarsening of austenite. In a steel structure manufactured from a thick-walled high-toughness high- strength steel plate by welding, Ti improves the toughness of the base metal and heat-affected zone. However, a Ti content of more than 0.020% results in coarsening of Ti nitride and deteriorates toughness of the base metal. Thus, the Ti content, if any, ranges from 0.005% to 0.020%, preferably 0.008% to 0.015%. [0043]

In addition to these components, a thick-walled high- toughness high-strength steel plate according to the present invention can further contain at least one of Mg, Ta, Zr, Y, B, Ca, and REM to improve the material quality. [0044]

Mg: 0.0001% to 0.0050%

Mg forms a stable oxide at high temperatures, effectively suppresses the coarsening of prior y grains in the heat-affected zone, and is effective in improving the toughness of the weld. These effects require a Mg content of 0.0001% or more. However, a Mg content of more than 0.0050% results in an increased number of inclusions and deteriorates toughness. Thus, the Mg content, if any, is preferably 0.0050% or less, more preferably 0.0001% to 0.015%.

CA02969200 2017-05-29 - 18 - [0045]

Ta: 0.01% to 0.20%

The addition of an adequate amount of Ta is effective in improving strength. More specifically, a Ta content of 0.01% or more is effective. However, a Ta content of more than 0.20% deteriorates toughness due to formation of precipitates. Thus, the Ta content, if any, ranges from 0.01% to 0.20%. [0046]

Zr: 0.005% to 0.1%

Zr is an element effective in improving strength. A Zr content of 0.005% or more is effective in producing this effect. However, a Zr content of more than 0.1% deteriorates toughness due to the formation of a coarse precipitate. Thus, the Zr content, if any, ranges from 0.005% to 0.1%. [0047]

Y: 0.001% to 0.01%

Yforms a stable oxide at high temperatures, effectively suppresses the coarsening of prior 7 grains in the heat-affected zone, and is effective in improving the toughness of the weld. An Y content of 0.001% or more is effective in producing these effects. However, an Y content of more than 0.01% results in an increased number of inclusions and deteriorates toughness. Thus, the Y content, CA02969200 2017-05-29 - 19 - if any, ranges from 0.001% to 0.01%. [0048]

B: 0.0030% or less Bsegregates at austenite grain boundaries, suppresses ferrite transformation from the grain boundaries, and improves hardenability. However, a B content of more than 0.0030% deteriorates hardenability and toughness due to the precipitation of B as a carbonitride. Thus, the B content is 0.0030% or less. The B content, if any, preferably ranges from 0.0003% to 0.0030%, more preferably 0.0005% to 0.002%. [0049]

Ca: 0.0005% to 0.0050%

Ca is an element useful for the morphology control of a sulfide inclusion. This effect requires a Ca content of 0.0005% or more. However, a Ca content of more than 0.0050% deteriorates cleanliness and toughness. Thus, the Ca content, if any, is preferably 0.0050% or less, more preferably 0.0005% to 0.0025%. [0050]

REM: 0.0005% to 0.0100%

Like Ca, REM forms an oxide and a sulfide in steel and is effective in improving the material quality. This effect requires a REM content of 0.0005% or more. However, the effect levels off at a REM content of 0.0100% or more. Thus, CA02969200 2017-05-29 - 20 - the REM content, if any, is 0.0100% or less, preferably 0.0005% to 0.005%. [0051]

These optional elements in amounts below the lower limits do not reduce the advantages of the present invention.

Thus, the optional elements in amounts below the lower limits are considered to be contained as incidental impurities. [0052]

Ceqn-w 0.65%

In the present invention, an appropriate alloy component needs to be added to ensure that a half-thickness portion of a thick-walled high-toughness high-strength steel plate having a thickness of 100 mm or more has a yield strength of 620 MPa or more and high toughness. More specifically, as represented by the following formula (1), alloying element contents need to be adjusted such that the carbon equivalent (Cecil) is 0.65% or more.

CeciIII4 = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 L. 0.65 (1) The element symbols in the formula denote the corresponding element contents (% by mass). In the absence of an element, the element symbol is denoted by 0. [0053] (CL - C)/CL x 100 L 30 (2) As described later, the present invention provides a steel plate having desirable characteristics even when the steel plate is manufactured from steel casted under conditions where the cooling rate of a slab surface during solidification is 100/s or less. In the present invention, microsegregation needs to be reduced to achieve high toughness (vE-40 70 J) of the surface of a thick-walled high-toughness high-strength steel plate having a thickness of 100 mm or more, particularly manufactured from steel casted under conditions where the cooling rate of a slab surface during solidification is 1 C/s or less. To this end, primary crystals during solidification need to form a 8 phase, and the percentage ((CL - C)/CL x 100) of the 8 phase at the beginning of the formation of a y phase needs to be 30% or more.

CL= 0.2 - (-0.1 x (0.2 - Si) - 0.03 x (1.1 - Mn) - 0.12 x (0.2 - Cu) - 0.11 x (3 - Ni) + 0.025 x (1.2 - Cr) + 0.1 x (0.5 - No) + 0.2 x (0.04 - V) - 0.05 x (0.06 - Al)) (3) In the formula (3), the element symbols denote the respective alloy component contents (% by mass), and in the absence of an element, the element symbol is denoted by 0. [0054]

In order to form a 5 phase, the C content needs to be specified depending on each component other than C, such as Si or Mn. The effects of an alloying element on the C solid CA02969200 2017-05-29 - 22 - solubility limit (CL) of the 8 phase were calculated using thermodynamic calculation software "Thermo-Calc". The result was used to determine the factor. For example, the factor "-0.1" for "Si" means that 1% Si decreases the C solid solubility limit of the 6 phase by 0.1%, and the C content of the base metal needs to be decreased to achieve the required percentage of the 6 phase. In the present invention, the calculation of CL was based on the component of C: 0.12%, Si: 0.2%, Mn: 1.1%, Cu: 0.2%, Cr: 1.2%, Ni: 3%, Mo: 0.5%, V: 0.04% and Al: 0.06%, and the factors for the calculation of CL were determined by calculating a variation from the dissolved C content caused by a variation in each alloying element content. When the percentage (CL - C)/CL x 100 of C to be added relative to the C solid solubility limit in the 8 phase thus calculated is 30% or more, the percentage of the 5 phase at the beginning of the formation of the y phase can be 30% or more. [0055]

In the present invention, in order to ensure the safety of steel during use, the reduction of area in the thickness direction at half the thickness of the plate is preferably 40% or more when measured by a method described in the example. [0056] <Method for Manufacturing Thick-Walled High-Toughness High-Strength Steel Plate>

The manufacturing conditions in the present invention will be described below. In the description, the temperature " C" refers to the temperature in the half- thickness portion except for the quenching temperature in the case of quenching without leaving to cool after rolling.

The quenching temperature in the case of quenching without leaving to cool after rolling is the surface temperature of the steel plate. This is because the temperature distribution of the steel plate in the thickness direction increases during rolling, and a decrease in the surface temperature of the steel plate needs to be considered. The temperature of the half-thickness portion is determined, for example, by simulation calculation from the thickness, surface temperature, and cooling conditions. For example, the temperature of the half-thickness portion is determined by calculating the temperature distribution in the thickness direction using finite difference methods. [0057]

Steel Amolten steel having the composition described above is produced by a conventional method, such as with a converter, an electric furnace, or a vacuum melting furnace, and is formed into a piece of steel, such as a slab or billet, by a conventional casting method, such as a CA02969200 2017-05-29 - 24 - continuous casting process or an ingot casting process. The cooling rate during solidification is determined by direct measurement with a thermocouple or by simulation calculation, such as heat-transfer calculation. As described above, in the present invention, steel manufactured under conditions where the cooling rate of a surface during solidification is 1 C/s or less can preferably be used. [0058]

When the loads of a forging machine and a rolling mill and so on are restricted, the thickness of the material may be reduced by slabbing. [0059]

Hot-Forging Conditions for Steel Acast bloom or steel bloom having the composition described above is heated to a temperature in the range of 1200 C to 1350 C. A reheating temperature of less than 1200 C results in not only an insufficient rolling reduction due to an increased load to achieve a predetermined cumulative rolling reduction in hot working but also low production efficiency due to additional heating as required during working. Thus, the reheating temperature is 1200 C or more. A large amount of additive alloying element as steel having a carbon equivalent of 0.65% or more according to the present invention results in a casting defect such as a center porosity or porous shrinkage cavity, having a much =

CA02969200 2017-05-29 - 25 - increased size in steel. In order to make them harmless by pressure bonding, the cumulative rolling reduction needs to be 25% or more. On the other hand, a reheating temperature of more than 1350 C results in excessive energy consumption, increased likelihood of occurrence of surface flaws due to scales during heating, and increased repair load after hot forging. Thus, the upper limit is 1350 C. [0060]

Slabbing Conditions for Steel Acast bloom or steel bloom having the composition described above is heated to a temperature in the range of 1200 C to 1350 C. A reheating temperature of less than 1200 C results in not only an insufficient rolling reduction due to an increased load to achieve a predetermined cumulative rolling reduction in hot working but also low production efficiency due to additional heating as required during working. Thus, the reheating temperature is 1200 C or more. In order to make casting defects harmless by pressure bonding and to provide the advantages of the present invention, the cumulative rolling reduction is 30% or more, preferably 40% or more in terms of good reduction of area (RA). On the other hand, a reheating temperature of more than 1350 C results in excessive energy consumption, increased likelihood of surface flaws due to scales during heating, and increased repair load after hot forging. Thus, CA02969200 2017-05-29 - 26 - the upper limit is 1350 C. [0061]

Reheating of Steel after Forging or after Slabbing Steel after forging is heated to an Ac3 transformation temperature or more and 1200 C or less in order that the steel may have a uniform austenite structure alone. The heating temperature preferably ranges from 1000 C to 1200 C.

The Ac3 transformation temperature is calculated using the following formula (4).

Ac3 - 937.2 - 476.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti + 198.4A1 + 3315B (4) The element symbols in the formula (4) denote the respective alloy component contents (% by mass). [0062]

Hot Rolling Conditions Steel is hot-rolled to form a plate having a desired thickness. In order to ensure desirable mechanical properties of a half-thickness portion of a thick-walled steel plate having a thickness of 100 mm or more, it is necessary to adjust the steel in the rolling step in order to sufficiently elicit an effect of adjusting and refining of the prior y grain size. More specifically, rolling at a cumulative rolling reduction of 40% or more can adjust the grain size in the rolling step even in the half-thickness portion in which recrystallization rarely occurs in CA02969200 2017-05-29 - 27 - processing. [0063]

Heat-Treatment Conditions In order to achieve high strength and toughness of a half-thickness portion, in the present invention, a steel plate is left to cool (for example, air cooling) after hot rolling or is rapidly cooled from the Ar3 temperature or more to 350 C or less without leaving to cool after hot rolling. When the steel plate is left to cool, the steel plate is reheated to the Ac3 temperature to 1050 C and is rapidly cooled from the Ac3 temperature or more to 350 C or less. The reason for the reheating temperature of 1050 C or less is that reheating at a high temperature of more than 1050 C results in coarsening of austenite grains and significantly deteriorates toughness of the base metal in a steel structure manufactured from a thick-walled high- toughness high-strength steel plate by welding. The reheating temperature is the Ac3 temperature or more in order that the steel plate may entirely have an austenite structure. The quenching temperature is the Ac3 temperature or more because the desirable characteristics are not obtained at a temperature below the Ac3 temperature due to the formation of a nonuniform structure composed of ferrite and austenite. In the case of rapid cooling without leaving to cool, the quenching temperature is the Ar3 temperature or CA02969200 2017-05-29 - 28 - more for quenching from the austenite single phase region.

The rapid cooling stop temperature is a lower temperature selected from 350 C or less and the Ar3 temperature or less in order to ensure that the steel plate entirely has a transformed structure. The stop temperature should be the Ar3 temperature or less and 350 C or less.

The Ar3 transformation temperature is calculated using the following formula (5).

Ar3 = 910 - 310C - 80Mn - 200u - 15Cr - 55Ni - 80Mo (5) The element symbols in the formula (5) denote the respective alloy component contents (% by mass). [0064]

In general, the rapid cooling method is industrially water cooling. It is desirable that the cooling rate be as high as possible. Thus, the cooling method is not necessarily water cooling and may be gas cooling, for example. [0065]

Tempering Conditions The reason for tempering at a temperature in the range of 450 C to 700 C after rapid cooling is described below.

Residual stress is not effectively relieved at less than 450 C. On the other hand, a temperature of more than 700 C results in precipitation of various carbides and coarsens the structure of the base metal and deteriorates strength CA02969200 2017-05-29 - 29 - and toughness of the base metal in a steel structure manufactured from a thick-walled high-toughness high- strength steel plate by welding. [0066]

Industrially, quenching is sometimes repeated to strengthen steel. Although quenching may be repeated also in the present invention, final quenching requires rapid cooling to 350 C or less after heating to the Ac3 temperature to 1050 C and requires subsequent tempering at 450 C to 700 C.

EXAMPLES [0067]

Steel plate samples No. 1 to No. 38 were manufactured by melting and casting steel No. 1 to No. 30 listed in Table 1 under the conditions listed in Table 2, performing hot forging (except for the samples No. 5, No. 6, and No. 41) or slabbing (the samples No. 5, No. 6, and No. 41), hot-rolling the steel to form a steel plate having a thickness listed in Table 2, and subjecting the steel plate to water quenching and tempering. The steel plate samples No. 1 to No. 38 were subjected to the following tests. In reheating and quenching in this example, the reheating temperature corresponds to the quenching temperature. [0068]

The percentage of the 6 phase is calculated using the CA02969200 2017-05-29 - 30 - formula (2) from CL calculated using the formula (3) with each base metal component and the C content of the base metal. [0069]

The cooling rate during solidification in the manufacture of steel is determined by heat-transfer calculation from the mold surface temperature data measured with a radiation thermometer. [0070]

Tensile Test Around bar tensile test piece (012.5 mm, GL 50 mm) was taken from the half-thickness portion of each steel plate in the direction perpendicular to the rolling direction and was measured in terms of yield strength (YS) and tensile strength (TS). [0071]

Charpy Impact Test Three 2-mm V-notched Charpy impact test specimens were taken from each surface and half-thickness portion of the steel plates. The rolling direction was the longitudinal direction. The absorbed energies of the test specimens were measured at a test temperature of -40 C in a Charpy impact test and were averaged (the average value for the test specimens taken from the half-thickness portion and the average value for the test specimens taken from the surface).

CA02969200 2017-05-29 - 31 - [0072]

Tensile Test in Thickness Direction Around bar tensile test piece (4)10 mm) was taken from a region including the half-thickness portion of each steel plate in the thickness direction and was measured in terms of reduction of area (RA). The reduction of area is the percentage of the difference between the minimum cross- sectional area after the test specimen was broken and the original cross-sectional area relative to the original cross-sectional area. [0073]

Table 2 shows the test results. The results showed that the steel plates of the examples having a steel composition according to the present invention (samples No. 1 to No. 21 and No. 41) had YS of 620 MPa or more, TS of 720 MPa or more, and toughness (vE-40) of 70 J or more at -40 C in the surface and half-thickness portion of the base metal, showing high strength and toughness of the base metal. A comparison between Nos. 5 and 6 and No. 41 showed that reduction of area (RA) was also satisfactory under particular slabbing conditions. [0074]

In contrast, in the steel plates according to comparative examples having a composition outside the scope of the present invention (samples No. 22 to No. 32), the CA02969200 2017-05-29 - 32 - base metal had at least one of YS of less than 620 MPa, TS of less than 720 MPa, and toughness (vE-40) of less than 70 J, thus deteriorating characteristics. [0075]

As in samples No. 33 to No. 40, even if steel plates had a steel composition according to the present invention, steel plates manufactured under the conditions outside the scope of the present invention had at least one deterioration in YS, TS, and toughness (vE-40) (No. 41, which had a cumulative rolling reduction of 30% and satisfied the minimum conditions required to provide the advantages of the present invention, is not outside the scope of the present invention). - 33 - .. [0076] = [Table 1]

Steel 5 Chemical composition (mass) Ac3 Ar3 Category ingot iti % fraction No. C Si Mn P S Cr Ni Ti Al , N I 13 Cu Mo V 0 Nb Mg Ta Zr Y Ca REM Cecillw C C% 1 0.085 0.18 1.61 0.006 0.0012 0.66 0.88 0.063 0.0041 0.0013 0.25 0.45 0.035 0.0021 0.0017 0.658 883 656 78 2 0.095 0.22 1.15 0.005 0.0006 1.80 1.95 0.045 0.0022 0.0015 , 0.777 830 654 68 3 0.128 0.21 0.98 0.005 0.0011 0.81 1.01 0.010 0.045 0.0040 0.0011 0.28 0.54 0.045 0.0018 0.0019 0.0021 0.656 873 675 69 4 0.120 0.22 1.14 0.006 0.0006 0.94 1.94 0.062 0.0049 0.0011 0.18 0.49 0.039 0.0016 0.0017 0.745 850 618 61 0.126 0.20 1.15 0.003 0.0008 0.95 2.05 0.010 0.043 0.0028 0.0009 0.22 0.51 0.042 0.0018 0.0015 0.769 840 607 58 6 0.123 0.20 1.18 0.004 0.0007 0.87 2.44 0.066 0.0050 0.0012 0.19 0.52 0.041 0.0019 0.0020 0.781 836 585 52 7 0.109 0.17 0.85 0.003 0.0004 1.13 3.56 0.071 0.0049 0.0010 0.08 0.53 0.040 0.0022 0.0012 0.833 819 551 34 8 0.121 0.22 0.88 0.006 0.0011 1.49 2.93 0.068 0.0046 0.0008 0.21 0.55 0.038 0.0021 0.0019 0.893 828 570 46 9 0.142 0.21 0.69 0.006 0.0009 1.51 2.87 0.071 0.0038, 0.0013 0.20 0.50 0.037 0.0019 0.0018 0.871 823 586 39 Example 10 0.113 0.11 0.81 0.006 0.0005 1.53 3.61 0.069 0.00520.0010 0.02 0.56 0.040 0.0023 0.0016 0.916 813 543 39 R 11 0.178 0.09 0.60 0.006 0.0006 1.14 2.84 0.078 0.0054 0.0009 0.11 0.55 0.041 0,0024 0.821 807 587 31 12 0.196 0.06 0.53 0.009 0.0007 1.35 3.65 0.062 0.0046 1.43 0.059 0.0021 1.095 807 471 31 13 0.095 0.23 2.21 0.006 0.0004 0.74 0.035 0.0039 0.0011 0.20 0.65 0.0022 0.0022 . 0.656 874 607 76 0 14 0.118 0.21 1.13 0.006 0.0008 1.44 3.49 0.065 0.0045 0.12 0.57 0.045 0.0021 0.0013, 0.958 809 521 30 .., 0.142 0.05 0.57 0.009 0.0012 2.41 0.15 0.009 0.026 0.0050 0.0009 0.14 1.07 0.328 0.0022 0.031 1.018 934 688 80 Li 16 0.161 0.22 1.03 0.005 0.0013 1.55 0.98 0.011 0.045 0.0033 0.0011 0.32 0.043 0.0009 0.0013 0.0019 0.738 833 694 56 17 0.103 0.20 0.98 0.006 0.0018 0.56 2.21 0.049 0.0063 0.0013 0.18 0.55 0.036 0.0022 0.048 0.0022 0.655 855 622 63 18 0.132 0.13 1.01 0.005 0.0006 0.86 , 1.44 0.012 0.065 0.0041 0.0009 0.22 0.44 0.028 0.0019 0.015 0.0018 0.677 853 657 64 19 0.162 0.16 0.59 0.006 0.0019 1.41 3.98 0.063 0.0038 0.0018 0.16 1.47 0.115 0.0018 0.005 0.0016 1.135 831 452 31 20 0.113 0.22 1.14 0.006 0.0006 0.81 3.57 0.070 0.0050 0.0012 0.20 0.51 0.040 0.0020 0.0016 0.826 813 530 9 21 0.083 0.16 1.45 0.007 0.0011 0.67 0.45 0.006 0.0043 0.0014 0.21 0.45 0.038 0.0022 0.0018 0.600 887 693 82 22 0.221 0.55 1.02 0.006 0.0006 0.91 0.89 0.010 0.044 0.0028 0.0011 0.12 0.46 0.0013 0.732 844 658 43 23 0.058 0.39 1.41 0.010 0.0018 1.23 1.96 0.045 0.0056 0.0012 0.23 0.41 0.041 0.0018, 0.0023 0.775 875 616 79 24 0.129 0.63 1.33 0.009 0.0009 0.98 0.65 0.058 0.0049 0.0009 0.21 0.55 0.039 0.0021 0.0016 0.722 900 665 68 Comparative 25 0.153 0.18 6.11 0.009 0.0009 1.04 1.21 0.066 0.0051 0.0009 0.21 0.44 0.041 0.0016 0.0014 1.570 751 252 36 example 26 0.124 0.22 1.41 0.018 0.0007 0.91 2.11 0.056 0.0048 0.0007 0.19 0.38 0.0026 0.0016 0.770 827 595 52 _ 27 0.135 0.21 1.14 0.005 0.0069 1.21 1.48 0.0650.0044 0.0009 0.48 0.026 0.0019 0.767 854 639 65 28 0.124 0.26 0.65 0.005 0.0006 4.10 1.38 0.056 0.00380.0007 0.25 0.0016 0.0026 1.194 846 662 72 29 0.131 0.24 1.15 0.009 0.0008 1.05 1.95 0.006 0.098 0.0069 0.0009 0.29 _ 0.48 0.0016 0.778 845 610 54 30 0.133 0.26 1.13 0.009 0.0018 0.96 1.99 0.063 0.0123 0.0008 0.22 0.43 0.0022 0.0018 0.747 836 616 53 Cecillw, Ac3, Ar3, and 8. fraction were calculated using the formulae (1), (2), (4), and (5), respectively. = - 34 - [00-77] [Table 2]

Toughness of Cooling rate Hot forging or slabbing Hot rolling Final heat treatment conditions Mechanical properties of base metal (1/20 base metal Sample S.teel Thickness Thickness of during (surface) Category ingot of material . .. . product Type of heat treatment No. solidification Cumulative Cumulative Quenching = Tensile RA

Heating Heating Cooling stop Tempering YS TS vE-40 vE-40 rollIng i rn rolling reduction (mm) temperature (.c) in thickness ( C) (MPa) (MPa) (,J) NIreduction (%) ''''''' (%) ( C) direction RA (%) (J) 1 1 1000 0.32 1270 80 1130 43 100 Direct quenching 850 150 630 695 786 108 73 98 2 2 500 0.64 1230 65 1130 43 100 Reheating quenching 930 100 650 698 795 167 63 105 3 3 300 0.9C 1200 30 1130 50 100 Reheating quenching 930 100 630 725 793 102 70 116 4 4 1000 0.33 1270 70 1160 43 150 Reheating quenching 900 100 630 785 856 205 65 221 , 4 1000 0.33 1320 55 1200 53 210 Reheating quenching 900 100 630 743 829 145 71 124 6 4 1000 0.33 1300 55 1180 67 150 Direct quenching 860 100 630 751 832 126 58 159 7 5 500 0.61 1270 45 1160 43 150 Reheating quenching , 900 100 630 725 821 153 63 205 8 6 1000 0.32 1270 55 1130 60 180 Reheating quenching HO 100 63C 705 816 238 61 211 9 7 1000 0.31 1270 55 1130 53 210 Reheating quenching 880 100 650 756 864 216 63 163 8 1000 0.31 1300 55 1136 53 210 Reheating quenching 880 100 650 741 836 193 60 121 Example 11 9 1000 0.30 1270 55 1130 53 210 Reheating quenchina 830 100 650 759 857 186 62 101 12 10 1200 0.25 1270 55 1160 50 250 Reheating quenching , 900 100 630 748 852 221 59 146 R 13 11 1000 0.29 1270 70 1160 43 150 Reheating quenching 880., 100 630 728 839 169 71 101 2 14 12 1300 0.19 1270 60 1160 50 250 Reheating quenching 830., 100 660 793 912 221 68 145 13 1200 0.32 1270 85 1130 43 100 Reheating ouenching 930 100 650 665 756 103 73 126 ,t 16 14 1000 0.31 1270 55 1160 53 210 Reheating quenching , 830 100 650 723 814 203 69 163 g 17 15 1000 0.32 1270 55 1160 53 210 Reheating quenching 1050 150 670 641 738 145 70 101 18 16 500 0.64 1230 65 1130 43 100 Reheating quenching 930 100 650 732 84 167 77 83 t, 19 17 500 0.65 1230 65 1160 43 100 Reheating quenching 900 100 650 703 789 196 73 113 18 300 0.91 1200 40 1160 43 100 Direct quenchina 830 150 650 752 846 138 65 106 T 21 19 1000 0.29 1350 50 1160 50 250 Reheating Quenching 900, 100 650 746 863 231 62 r, 93 . 22 20 1000 0.29 1270 55 1160 53 210 Reheating quenching 880 100 640 716 815 196 61 43 23 21 300 0.92 1200 40 1160 43 100 Reheating quenching 930 100 650 589 695 22 69 121 24 22 1000 0.31 1270 70 1160 43 150 Reheating quenching 900 100 630 845 956 19 62 96 23 1000 0.29 1270 70 1160 43 150 Reheating quenching 930 100 630 547 663 21 68 124 26 24 1000 0.29 1270 55 1130 53 210 Reheating quenching 930 100 650 728 823 41 64 39 27 25 1200 0.24 1270 55 1160 50 250 Reheating quenchina 880 100 630 802 915 23 51 96 28 26 1000 0.28 1270 70 1160 43 150 Reheating quenching 830 100 630 731 829 32 57 23 29 27 1000 0.31 1270 55 1130 53 210 Reheating quenching 900 100 650 741 854 35 63 31 Comparative 30 28 1000 0.29 1270 55 1130 53 210 Reheating quenching 900 100 650 796 921 43 61 89 31 29 1000 0.32 1270 55 1130 53 210 Reheating quenching 900 100 650 732 816 48 72 56 example 32 30 1000 0.31 1270 70 1160 43 150 Reheating quenching 900 100 630 698 784 61 70, 52 33 3 300 0.91 1200 15 1130 60 100 Reheating quenching 930 100 630 716 804 43 23 115 34 3 300 0.89 1200 55 1130 20 100 Reheating quenching 930 100 630 722 816 45 35 96 3 , 300 0.90 1200 30 1130, 50 100 Reheating quenching 1100 100 630 735 824 36 53 49 36 3 300 0.92 1200 30 1130 50 100 Reheating quenching 840 100 630 536 646 46 49 52 37 3 300 0.90 1200 30 1130 50 100 Direct quenching 623 100 630 596 683 52 51 61 38 3 300 0.91 1200 30 1130 50 100 Reheating quenching 930 400 630 503 641 32 64 103 39 , 3 300 0.90 1200 30 1130 50 100 Reheatina auenchinq 930 100 720 569 657 153 68 146 3 300 0.91 1200 30 1130 50 100 Reheating quenching 930 100 400 793 902 41 63 53 Example 41 5 500 0.6' 1300 30 1180 40 210 Reheatina auenchina 900 100 600 683 766 105 29 192

Claims

- 35 - CLAIMS

[Claim 1] A thick-walled steel plate having a thickness of 100 mm or more, comprising, on a mass percent basis, C: 0.08% to 0.20%, Si: 0.40% or less, Mn: 0.5% to 5.0%, P: 0.010% or less, S: 0.0050% or less, Cr: 1.44% or less, Ni: 0.1% to 5.0%, Al: 0.010% to 0.080%, N: 0.0070% or less, O: 0.0025% or less, Cu: 0.50% or less, Mo: 1.50% or less, V: 0.400% or less, B: 0.0030% or less, and optionally Ti: 0.005% to 0.020% following formulae (1) and (2) being satisfied, a remainder being Fe and incidental impurities, and having a toughness (vE-40) of 70 J or more at a surface portion of the steel plate; Ceq IIw - C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 .gtoreq. 0.65 (1) (CL - C)/CL x 100 .gtoreq. 30 (2) wherein CL is defined by the following formula: CL = 0.2 - (-0.1 x (0.2 - Si) - 0.03 x (1.1 - Mn) - 0.12 - 36 - x (0.2 - Cu) - 0.11 x (3 - Ni) + 0.025 x (1.2 - Cr) + 0.1 x (0.5 - Mo) + 0.2 x (0.04 - V) - 0.05 x (0.06 - Al)) (3) wherein element symbols in the formulae denote the respective alloy component contents in % by mass, and in the absence of an element the element symbol is denoted by 0; and wherein a reduction of area in a thickness direction at half the thickness of the plate, which is the percentage of the difference between the minimum cross-sectional area after a round bar tensile test specimen including the half-thickness portion of the steel plate in the thickness direction is broken in a tensile test and the original cross-sectional area relative to the original cross-sectional area, is 40% or more.
[Claim 2] The thick-walled steel plate according to Claim 1, further comprising, on a mass percent basis, Nb: 0.100% or less.
[Claim 3] The thick-walled steel plate according to Claim 1 or 2, further comprising, on a mass percent basis, at least one of Mg: 0.0001% to 0.0050%, Ta: 0.01% to 0.20%, Zr: 0.005% to 0.1%, Y: 0.001% to 0.01%, Ca: 0.0005% to 0.0050%, and REM: 0.0005% to 0.0100%. - 37 -
[Claim 4] The thick-walled steel plate having a yield strength of 620 MPa or more according to any one of Claims 1 to 3.
[Claim 5] A method for manufacturing the thick-walled steel plate according to any one of Claims 1 to 4, comprising: heating steel to 1200.degree.C to 1350.degree.C, hot-forging the steel at a cumulative rolling reduction of 25% or more, heating the steel to an Ac3 temperature or more and 1200.degree.C or less, hot-rolling the steel at a cumulative rolling reduction of 40% or more, leaving the steel to cool by air cooling, reheating the steel to the Ac3 temperature or more and 1050.degree.C or less, accelerated cooling of the steel by quenching from the Ac3 temperature or more to a lower temperature being both 350.degree.C or less and an Ar3 temperature or less, and tempering the steel at a temperature in the range of 450.degree.C to 700.degree.C; the Ac3 temperature being calculated using the following formula (4): Ac3 = 937.2 - 476.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti + 198.4A1 + 3315B (4), and the Ar3 temperature being calculated using the - 38 - following formula (5): Ar3 = 910 - 310C - 80Mn - 20Cu - 15Cr - 55Ni - 80Mo (5); wherein in the above formulas (4) and (5), the element symbols denote the respective alloy component contents in % by mass.
[Claim 6] A method for manufacturing the thick-walled steel plate according to any one of Claims 1 to 4, comprising: heating steel to 1200.degree.C to 1350.degree.C, hot-forging the steel at a cumulative rolling reduction of 25% or more, heating the steel to an Ac3 temperature or more and 1200.degree.C or less, hot-rolling the steel at a cumulative rolling reduction of 40% or more, accelerated cooling of the steel by quenching from an Ar3 temperature or more to a lower temperature being both 350.degree.C or less and the Ar3 temperature or less, and tempering the steel at a temperature in the range of 450.degree.C to 700.degree.C the Ac3 temperature being calculated using the following formula (4): Ac3 = 937.2 - 476.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti + 198.4Al + 3315B (4), and the Ar3 temperature being calculated using the - 39 - following formula (5): Ar3 = 910 - 310C - 80Mn - 20Cu - 15Cr - 55Ni - 80Mo (5); wherein in the above formulas (4) and (5), the element symbols denote the respective alloy component contents in % by mass.
[Claim 7] A method for manufacturing the thick-walled steel plate according to any one of Claims 1 to 4, comprising: heating steel to 1200.degree.C to 1350.degree.C, blooming the steel at a cumulative rolling reduction of 40% or more, heating the steel to an Ac3 temperature or more and 1200.degree.C or less, hot-rolling the steel at a cumulative rolling reduction of 40% or more, leaving the steel to cool by air cooling, reheating the steel to the Ac3 temperature or more and 1050.degree.C or less, accelerated cooling of the steel by quenching from the Ac3 temperature or more to a lower temperature being both 350.degree.C or less and an Ar3 temperature or less, and tempering the steel at a temperature in the range of 450.degree.C to 700.degree.C; the Ac3 temperature being calculated using the following formula (4): Ac3 = 937.2 - 476.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti + 198.4Al + 3315B (4), - 40 - and the Ar3 temperature being calculated using the following formula (5): Ar3 = 910 - 310C - 80Mn - 20Cu - 15Cr - 55Ni - 80Mo (5); wherein in the above formulas (4) and (5), the element symbols denote the respective alloy component contents in % by mass.
[Claim 8] A method for manufacturing the thick-walled steel plate according to any one of Claims 1 to 4, comprising: heating steel to 1200.degree.C to 1350.degree.C, blooming the steel at a cumulative rolling reduction of 40% or more, heating the steel to an Ac3 temperature or more and 1200.degree.C or less, hot-rolling the steel at a cumulative rolling reduction of 40% or more, accelerated cooling of the steel by quenching from an Ar3 temperature or more to a lower temperature being both 350.degree.C or less and an Ar3 temperature or less, and tempering the steel at a temperature in the range of 450.degree.C to 700.degree.C the Ac3 temperature being calculated using the following formula (4): Ac3 = 937.2 - 476.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti + 198.4Al + 3315B (4), - 41 - and the Ar3 temperature being calculated using the following formula (5): Ar3 = 910 - 310C - 80Mn - 20Cu - 15Cr - 55Ni - 80Mo (5); wherein in the above formulas (4) and (5), the element symbols denote the respective alloy component contents in % by mass.