AU2018430608B2 - Steel plate and method of manufacturing the same - Google Patents

Abstract

i 8$ - t'~ T] 40 ;1 J IT d- 51 (19)tl- [ J r i i ( (10) (43) P9Wm> PIHWO 00 398A 2020*2f 27H(27.02.2020) WO 2020/039485 A1 WIPO I PCT (51) ;'t &I-: -I-IN(FURUYA Hitoshi); T1008071 ANVF C22C 38/00 (2006.01) C22C38/54(2006.01) It F9 E: A ) T F1 6 * 1 -q V H 1; 4 C21D 8/02 (2006.01) AgToh Tokyo (JP). )II ai #IIE(KAWABATA (21) 2 PCT/JP2018/030676 Norimasa); T1008071 A T :7 It flJ 0) TF1 6*1 -q ftH ]A iT Tokyo (22)IiH RtJ : 2018l8) A20H(20.08.2018) (JP)_ -- A tf (MIYAKE Takumi); ~T1008071 (26) pf 0 l: H $H IMA 4 $1 Tokyo (JP). (71)t AX:H a 4% (NIPPON STEEL (74) tiXA : 44 | At, (TANAI Sumio et al.); CORPORATION) [JP/JP]; T1008071 WA#+ T 1006620 W 32TT S :7 I E T IF E Ao T H 6 * 1 - Tokyo (P). 9 2 Tokyo (P). (81)tt(0t\IR) 0t0i t (72)#HB3#:ih d$ 5k (TAKAMINE Fumitoshi); t A GMT U ~i~1008071 ~~ ~ ~ 9 fibST t EM TE6) : AE, AG, AL, AM, AO, AT, AU, AZ, T1008071 3 0:7 I 4 $X:A0 T 6 ( - BA, BB, BG, BH, BN, BR, BW, BY, BZ, CA, CH, 1 -q ft E 21tW-4PTok yoJP). tr (54) Title: STEEL SHEET AND PRODUCTION METHOD THEREFOR (54) RME P 0) -: ]t4)1 J t OYiWAtm 11 - 13 12 1mm ------------------------- ......... ... . ...... . . . .5mm co - 5rnhf 1mm 12 13 IC(57) Abstract: One embodiment of the present application pertains to a steel sheet having a chemical composition that falls within a prescribed range, wherein: the central part of the steel sheet in the thickness direction contains martensite and bainite in a total area fraction of at least 99%; in the central part in the sheet thickness direction, the average value of en. 4 prior austenite grains is less than 80 pm, Ceq equals 0.750%-0.800%, Al x N is at least 2.0 x 10- , Ti/N is at most 3.4, and 4 x f > 9.00 is satisfied; in the central part in the sheet thickness direction, the three point average of a C-direction Charpy test is not less than 47 J at -20°C; and a surface layer W O 2020/039485 A 1 ||||||||||||||||||||||||||||||||||1|1||||||||||||||||||||||||||||||||||||| CL, CN, CO, CR, CU, CZ, DE, DJ, DK, DM, DO, DZ, EC, EE, EG, ES, Fl, GB, GD, GE, GH, GM, GT, HN, HR, HU, ID, IL, IN, IR, IS, JO, JP, KE, KG, KH, KN, KP, KR, KW, KZ, LA, LC, LK, LR, LS, LU, LY, MA, MD, ME, MG, MK, MN, MW, MX, MY, MZ, NA, NG, NI, NO, NZ, OM, PA, PE, PG, PH, PL, PT, QA, RO, RS, RU, RW, SA, SC, SD, SE, SG, SK, SL, SM, ST, SV, SY, TH, TJ, TM, TN, TR, TT, TZ, UA, UG, US, UZ, VC, VN, ZA, ZM, ZW. (84) L\(0 pU) & Jgg Jg A fib): ARIPO (BW, GH, GM, KE, LR, LS, MW, MZ, NA, RW, SD, SL, ST, SZ, TZ, UG, ZM, ZW), .a-5 7 (AM, AZ, BY, KG, KZ, RU, TJ, TM), - ' (AL, AT, BE, BG, CH, CY, CZ, DE, DK, EE, ES, Fl, FR, GB, GR, HR, HU, IE, IS, IT, LT, LU, LV, MC, MK, MT, NL, NO, PL, PT, RO, RS, SE, SI, SK, SM, TR), OAPI (BF, BJ, CF, CG, CI, CM, GA, GN, GQ, GW, KM, ML, MR, NE, SN, TD, TG). and the sheet-thickness-direction central part have an HB hardness of at least 350 and a sheet thickness of over 200 nun. t /+-f 4 R &y /1Z^t f 4 ) V111 -6--t [MAR% *- 9 9 % L), - T 'J 4 M FP Ij L t, I El tJ 7 t -f 0 $), CeqbC0. 750-~0. 800%T& '), AIX N bC 2. O X 1 0 L'.),L C ), T i / N bC 3. 4.0L' -F -b, 4 X f 9. 0 0 4B4 6a&5 Tff-, E~ rPj 8: t6 - 2 0°'C T 0) C 2rdJ np J E° - 0 3 15itbC 4 7 J L),T ), -eb Y -rB 1-0 So& h f-FPb N) kb H B T 3 5 0 L),kLT M,b $ bM 2 0 0 m m M T 76.o

Description

[Document Type] Specification

[Title of the Invention] STEEL PLATE AND METHOD OF MANUFACTURING

THESAME

[Technical Field of the Invention]

[0001]

The present invention relates to a steel plate and a method of manufacturing

the same.

[Related Art]

[0002]

A huge gear (gear) is used in a rotating mechanism of a large industrial

machine represented by a rotary kiln. From the viewpoint of the fatigue resistance

and durability of the gear, the steel plate, which is to become a material, is required to

have hardness and toughness. In recent years, the steel plate which is to become the

material is required to have HB 350 or more in the surface layer and in the thickness

middle portion and satisfy vE-20°C > 47 J in the thickness middle portion. This is

because the properties of the thickness middle portion are important in order to

manufacture a gear by machining the steel to the thickness middle portion.

[0003]

Furthermore, in recent years, with the aim of increasing the size of gears,

there has been a demand for a steel plate with a plate thickness of more than 200 mm,

which has not been achieved in the related art. As the plate thickness is increased, the

cooling rate of the thickness middle portion at the time of quenching is decreased.

Therefore, with a steel plate having a plate thickness of more than 200 mm, it is

difficult to obtain the hardness of themiddle portion even after tempering. On the

other hand, composition design for the purpose of merely increasing the hardness causes a decrease in toughness. Usually, hardness and toughness are in inverse proportion to each other. Therefore, with an ultra thick material having a plate thickness of more than 200 mm, it is extremely difficult to adjust the composition balance for securing the surface layer hardness and the middle portion hardness and also securing the toughness.

[0004]

Furthermore, for the purpose of improving weldability, a demand has arisen to

cause a carbon equivalent Ceq to be 0.800% or less by the elements primarily

contained. In a case where Ceq exceeds 0.800%, an increase in load on a customer,

such as increasing the preheating temperature at the time of welding, is incurred.

Since the number of welding passes is very large in a welding operation of the ultra

thick material such as the present steel, the increase in welding load is also large. Ceq

is represented, for example, by Formula (1). The element symbol included in

Formula (1) shows the amount (mass%) of the corresponding element in the chemical

composition of the steel.

Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5: Formula (1)

[0005]

In the related art, there has been no steel having a plate thickness of more than

200 mm, which secures Ceq < 0.800% and middle portion hardness > HB 350 and

guarantees the above-mentioned low temperature toughness at -20°C. Moreover, a

steel plate, which is to become a material, has to be tempered at 500°C or more so that

the material does not change by stress relief annealing after gear processing. The

need for tempering was also a disadvantage for achieving the target hardness of the

steel.

[0006]

Patent Document 1 aims to provide a thick steel plate having a plate thickness of more than 200

mm and a small hardness difference between the surface layer and the center, as a huge gear

material used in a rotating mechanism of a large industrial machine, and a method of

manufacturing the same, and provides a thick steel plate in which a three-point average ofC

direction Charpy at -20°C is 20 J or more in the thickness middle portion, the hardness of the

surface layer is 330 or more by HB, the hardness of the thickness middle portion is 300 or more

by HB, and the hardness difference AHB between the surface layer and the thickness middle

portion is 30 or less. However, Patent Document 1 does not aim to stably cause the hardness of

the thickness middle portion to be HB 350 or more.

[Prior Art Document]

[Patent Document]

[0007]

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No.

2017-186592

[Disclosure of the Invention]

[0008]

Under such circumstances, the present invention provides a steel plate in which the

plate thickness exceeds 200 mm, Ceq shown by the following Formula is 0.800% or less, and

Ceq is 0.750% or more for the purpose of securing the hardness of the thickness middle portion,

the hardness of the surface layer and the thickness middle portion is HB 350 or more, the

absorbed energy of the thickness middle portion at -20°C is 47 J or more, and a method of

manufacturing the same.

[0009]

The gist of the present invention is as follows.

(I) According to an aspect of the present invention, a steel plate includes, as a

chemical composition, by mass%: C: 0.16% to 0.20%; Si: 0.50% to 1.00%; Mn: 0.90%

to 1.50%; P: 0.010% or less; S: 0.0020% or less; Cu: 0% to 0.40%; Ni: 0.20% to

1.00%; Cr: 0.60% to 0.99%; Mo: 0.60% to 1.00%; V: 0% to 0.050%; Al: 0.050% to

0.085%; N: 0.0020% to 0.0070%; B: 0.0005% to 0.0020%; Nb: 0% to 0.050%; Ti: 0%

to 0.020%; Ca: 0% to 0.0030%; Mg: 0% to 0.0030%; REM: 0% to 0.0030%; and a

remainder including Fe and impurities, in which a total area ratio of martensite and

bainite in a thickness middle portion is 99% or more, an average value of a prior

austenite grain size in the thickness middle portion is less than 80 m, Ceq represented

by Formula (1) is 0.750% to 0.800%, Alx N is 2.0 x 10-4 or more, Ti/N is 3.4 or less,

a value f represented by Formula (2) and a value g represented by Formula (3) satisfy 4

x f/g > 9.00, a -20°C Charpy absorbed energy measured in a C direction in the

thickness middle portion is 47 J or more, hardnesses of a surface layer and the

thickness middle portion are HB 350 or more, and a plate thickness of the steel plate is

more than 200 mm,

Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5: Formula (1)

f =4 x C + Si + 2 x Mn + Ni + 2 x Cr + 5 x Mo: Formula (2)

g =2 x Cr + 3 x Mo + 5 x V: Formula (3)

where each element symbol described in each of the Formulas means an

amount of an element corresponding to the element symbol in unit mass%.

(II) According to another aspect of the present invention, a method of

manufacturing the steel plate according to (1), includes: heating a slab; hot rolling the

slab to obtain a steel plate having a plate thickness of more than 200 mm; cooling the steel plate; performing a precipitation treatment on the steel plate; quenching the steel plate; and tempering the steel plate, in which the slab includes, as a chemical composition, by unit mass%, C: 0.16% to 0.20%, Si: 0.50% to 1.00%, Mn: 0.90% to

1.50%, P: 0.010 % or less, S: 0.0020% or less, Cu: 0% to 0.40%, Ni: 0.20% to 1.00%,

Cr: 0.60% to 0.99%, Mo: 0.60% to 1. 00%, V: 0% to 0.050%, Al: 0.050% to 0.085%,

N: 0.0020% to 0.0070%, B: 0.0005% to 0.0020%, Nb: 0% to 0.050%, Ti: 0% to

0.020%, Ca: 0% to 0.0030%, Mg: 0% to 0.0030%, REM: 0% to 0.0030%, and a

remainder including Fe and impurities, Ceq represented by Formula (1) of the slab is

0.750% to 0.800%, Alx N of the slab is 2.0 x 10-4 or more, Ti/N of the slab is 3.4 or

less, and a value f of the slab represented by Formula (2) and a value g of the slab

represented by Formula (3) satisfy 4 x f/g > 9.00, a slab heating temperature in the

heating the slab is equal to or more than an AlN solid solution temperature Ts (°C)

calculated by Formula (4), the precipitation treatment is performed on the steel plate by

heating the steel plate to a precipitation treatment temperature Tp (C) of more than

550°C and less than Acl and retaining the steel plate at this temperature for a

precipitation treatment time tp (hour), the precipitation treatment temperature Tp (C)

and the precipitation treatment time tp (hour) satisfy Formula (5), the Ac is

represented by Formula (7), the quenching is performed on the steel plate by heating

the steel plate to a quenching retention temperature Tq (C) of 900°C to 950°C,

retaining the steel plate at this temperature for a quenching retention time tq (minute)

or more represented by Formula (6), and water cooling the steel plate, and the

tempering is performed on the steel plate by heating the steel plate to a tempering

temperature of 500°C to 550°C and cooling the steel plate to 150°C or less,

Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5: Formula (1)

f = 4 x C + Si + 2 x Mn + Ni + 2 x Cr + 5 x Mo: Formula (2) g = 2 x Cr + 3 x Mo + 5 x V: Formula (3)

Ts = 7400 / (1.95 - logio(Al x N)) - 273: Formula (4)

Logio(tp) + 0.012 x Tp > 8.7: Formula (5)

tq 0.033 x (950 - Tq) 2 +(1.5 x f)2/10: Formula (6)

Acl = 750 - 25 x C + 22 x Si-40 x Mn-30 x Ni + 20 x Cr + 25 x Mo:

Formula (7)

where each element symbol described in each of the Formulas means an

amount of an element corresponding to the element symbol in unit mass%.

(III) In the method of manufacturing the steel plate according to (II), a cooling

finishing temperature in the cooling the steel plate may be 150°C or less.

[Effects of the Invention]

[0010]

According to the present invention, it is possible to provide a steel plate which

is excellent in hardness of a surface layer and a thickness middle portion and impact

absorbed energy performance of the thickness middle portion and suppresses Ceq to

0.800% or less even in a steel plate having a plate thickness of more than 200 mm, and

the steel plate is applicable to a rotating mechanism of a large industrial machine

represented by a rotary kiln.

[Brief Description of the Drawings]

[0011]

FIG. 1 is a schematic view of a cross section of a steel plate according to an

embodiment, perpendicular to a rolling direction.

FIG. 2 is a diagram showing the relationship between the amount of C and a

thickness middle portion hardness, and the relationship between the amount of C and

thickness middle portion toughness (vE-20°C).

FIG. 3 is a diagram showing the relationship between Ceq and the middle

portion hardness.

FIG. 4 is a diagram showing the relationship between 4 x f/g and the thickness

middle portion toughness.

FIG. 5 is a graph showing the relationship between a precipitation treatment

temperature Tp and a precipitation treatment time Logio (tp) tested by using

composition A4 of Examples.

FIG. 6A is a graph showing the relationship between a quenching retention

temperature Tq, a quenching retention time tq, and the middle portion hardness

obtained by conducting experiments using composition A6 of the Examples.

FIG. 6B is a view showing the relationship between the quenching retention

temperature Tq, the quenching retention time tq, and the middle portion hardness

obtained by conducting experiments using composition A2 of the Examples.

FIG. 7 is a flowchart showing a method of manufacturing the steel plate

according to the embodiment.

[Embodiments of the Invention]

[0012]

In a steel plate according to the present embodiment, the mechanical

properties of both the thickness middle portion of the steel plate (sometimes simply

referred to as "middle portion") and the surface layer of the steel plate (sometimes

simply referred to as "surface layer") are controlled. As shown in FIG. 1, a thickness

middle portion 11of a steel plate I is a region between a plane at a depth of 3/8 of a

plate thickness T of the steel plate 1 from a rolled surface 13, which is the outermost

surface of the steel plate 1, and a plane at a depth of 5/8 of the plate thickness T of the

steel plate 1 from the rolled surface 13. The center surface of the thickness middle portion 11 of the steel plate 1 and the center surface of the steel plate I are coincident with each other. The surface layer 12 of the steel plate 1 is a region between a plane at a depth of 1 mm and a plane at a depth of 5 mm from the rolled surface 13 of the steel plate 1. The region from the outermost surface of the steel plate 1 to the depth of 1 mm is excluded from the surface layer 12 of the steel plate I in the present embodiment. This is because the area corresponds to a decarburized layer and a portion to be removed during processing. In addition, in principle, test pieces for a mechanical test, microstructure observation, and the like are to be collected from portions separated from the end portions of the steel plate in the length direction and the width direction by the plate thickness or more.

[0013]

In the steel plate according to the present embodiment, the following (1) to (7)

have important meanings. As a requirement for achieving both the hardness of HB

350 grade and vE-20cC > 47 J in the thickness middle portion of a steel plate having a

chemical composition satisfying Ceq < 0.800%, Composition Parameter Formula (3)

and a precipitation treatment (5) are particularly important.

(1) Restriction of Upper and Lower Limits of Amount of C for Achieving

Both Middle Portion Hardness and Middle Portion Toughness (under Conditions

Described Later)

In general, in a case where the middle portion hardness is HB 350 or more,

and it can be secured HB 350 or more in the surface layer.

(2) Ceq Lower Limit for Securing Middle Portion Hardness

(3) Lower Limit of Parameter Formula 4 x f/g for Securing Middle Portion

Toughness

(4) Lower limit of Parameter Formula Al x N for Securing Middle Portion

Toughness

(5) Solutionizing Treatment and Precipitation Treatment (Temperature and

Time) before Quenching for Securing Middle Portion Hardness and Toughness

(6) Quenching Conditions (Temperature and Time) for Securing Middle

Portion Hardness

(7) Restriction of Upper and Lower Limits of Tempering Temperature for

Securing Hardness and Toughness of Middle Portion

The details will be described below.

[0014]

(1) Restriction of Upper and Lower Limits of Amount Of C for Achieving

Both Middle Portion Hardness and Middle Portion Toughness (under Conditions

Described Later)

As a first item, in order to increase both the hardness and toughness of the

thickness middle portion under the conditions described later, the amount of C needs to

satisfy 0.16% to 0.20% as a composition (mass%) of the steel. In order to secure both

the toughness and the hardness at the thickness middle portion of the steel plate having

a plate thickness of more than 200 mm, it is necessary to suppress the formation of

carbides, which become the brittle fracture origin. In order to suppress the formation

of carbides and to achieve vE-20°C (ave.) > 47 J at the thickness middle portion, as

shown in FIG. 2, the amount of C has to be 0.20% or less. On the other hand, a

decrease in the amount of C greatly reduces the hardness of the steel. Therefore, in

order to cause the hardness of the middle portion to be HB 350 or more after tempering

at 500°C or more, the amount of C needs to be 0.16% or more as shown in FIG. 2.

[0015]

(2) Defining of Ceq Lower Limit for Securing Middle Portion Hardness

As a second item, in order to secure the hardness of the middle portion in the

steel plate having a plate thickness of 200 mm or more, sufficient hardenability is

required. Therefore, Ceq calculated by Formula (1) needs to satisfy 0.750% or more

after a precipitation treatment, which will be described later, is performed. This is to

avoid the formation of ferrite, which is a soft structure, during quenching and to form a

structure primarily containing bainite and martensite. From the viewpoint of

achieving both the hardness and toughness of the middle portion, it is not necessary to

determine the upper limit of Ceq. However, an increase in Ceq tends to cause weld

cracking. In a case where the Ceq exceeds 0.800%, the welding operation efficiency

is significantly deteriorated because it becomes necessary to raise the preheating

temperature before welding in order to avoid weld cracking. Therefore, Ceq in the

steel plate according to the present embodiment is 0.800% or less. Ceq may be

0.790% or less, 0.785% or less, or 0.780% or less.

Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5: Formula (1)

The element symbol included in Formula (1) shows the amount (mass%) of

the corresponding element in the chemical composition of the steel plate.

[0016]

As shown in FIG. 3, the present inventors found that in the steel plate having a

plate thickness of more than 200 mm, in a case where Ceq is less than 0.750%, the

hardness of the thickness middle portion becomes less than HB 350 even if the

precipitation treatment is performed. It is considered that the reason way the hardness

of the thickness middle portion is insufficient in the case where Ceq is less than

0.750% that ferrite which is a soft structure is formed. Ceq may be 0.755% or more,

0.760% or more, or 0.770% or more. In addition, the steel in which the hardness of

the thickness middle portion is insufficient even if Ceq is 0.750% or more is plotted in

FIG 3. The reason why the hardness of the thickness middle portion in this steel is

insufficient is that the precipitation treatment is not performed.

[0017]

(3) Lower Limit of Parameter Formula "4 x f/g" for Securing Middle Portion

Toughness

As a third item, in the steel plate having a plate thickness of more than 200

mm, in order to secure the hardness of the middle portion > HB 350 while achieving

Ceq < 0.800%, and to achieve a toughness ofvE-20c > 47J at the thickness middle

portion, the parameter f defined by Formula (2) and the parameter g defined by

Formula (3) need to satisfy a relationship in which 4 x f/g is 9.00 or more.

f =4 x C + Si + 2 x Mn + Ni + 2 x Cr + 5 x Mo: Formula (2)

g =2 x Cr + 3 x Mo + 5 x V: Formula (3)

The element symbol included in Formula (2) and Formula (3) shows the

amount (mass%) of the corresponding element in the chemical composition of the steel

plate.

[0018]

As shown in FIG. 4, the present inventors found that in the steel plate in which

the plate thickness is more than 200 mm, Ceq < 0.800% is satisfied, and the hardness

of the thickness middle portion is HB 350 or more, the toughness of the thickness

middle portion cannot be secured in a case where 4 x f/g is less than 9.00. The

element related to the parameter f is an element that improves the hardenability of the

steel plate by being solutionized in the matrix during quenching. On the other hand,

the element related to the parameter g is an element that reduces the toughness of the

steel plate by forming precipitates during tempering. That is, while these elements

improve the hardenability, they reduce the toughness by the formation of precipitates during tempering. A large 4 x f/g indicates that the hardenability is increased while reducing the elements precipitated during tempering.

[0019]

In the steel plate according to the present embodiment, Cr precipitates, Mo

precipitates, and V precipitates at the time of tempering are fine to such an extent that

the precipitates cannot be observed without a transmission electron microscope.

Therefore, it is industrially unpractical to define the distribution state of the above

mentioned precipitates themselves. From this, it is possible to understand the

usefulness of controlling the precipitates by the parameter Formula 4 x f/g.

[0020]

4 x f/g may be 9.20 or more, 9.50 or more, or 9.80 or more. The upper limit

of 4 x f/g need not be particularly defined, but may be, for example, 11.00, 10.70,

10.50, 10.00, or 9.90.

[0021]

(4) Lower limit of Parameter Formula Al x N for Securing Middle Portion

Toughness

In order to secure both the hardness and low temperature toughness in the

middle portion of the steel plate having a plate thickness of more than 200 mm, the Al

content needs to be 0.050% or more, and Al x N (the product of the Al content

(mass%) and the N content (mass%) of the steel plate) needs to be 2.0 x 10-4 or more.

This is a requirement for utilizing the austenite pinning effect of AIN, which

contributes to the refinement of the structure of the steel plate. In a case where the

amount of Al is less than 0.050% or Al x N is less than 2.0 x 10- 4 , the prior austenite

grain size is coarsened, and the low temperature toughness of the middle portion of the

steel plate is deteriorated. It is considered that this is because the total amount of AIN is insufficient.

[0022]

In addition, AIN which acts as austenite pinning particles in the steel plate

according to the present is too fine to be observed. Therefore, it is industrially

unpractical to define the distribution state of AIN itself acting as the austenite pinning

particles. From this, it is possible to understand the usefulness of controlling AIN

acting as the austenite pinning particles by the parameter Al x N.

[0023]

Al x N maybe 2.2 x 10-4 or more, 2.5 x 10- 4 or more, or 3.0 x 10- 4 or more.

The upper limit of Al x N need not be particularly defined, but 5.95 x 10-4 which is the

product of the upper limits of the Al content and the N content, which will be described

later, may be used as the upper limit of Al x N. Al x N may be 5.7 x 10-4 or less, 5.5

x 10-4 or less, 5.2 x 10-4 or less, or 4.8 x 10-4 or less.

[0024]

(5) Solutionizing Treatment and Precipitation Treatment (Temperature and

Time) before Quenching for Securing Middle Portion Hardness and Toughness

As process requirements for obtaining the austenite pinning effect of AIN,

there are solutionizing and precipitation treatments. In the solutionizing treatment, a

slab is heated to the AN solid solution temperature Ts or more calculated by Formula

(4). Hot rolling is performed after the solutionizing treatment. In the precipitation

treatment, in order to cause Al and N solutionized in the matrix by the solutionizing to

finely precipitate as AIN, after hot rolling or before quenching, a hot rolled steel plate

is heated to a precipitation treatment temperature Tp, which is a temperature of more

than 550°C and less than Acl, and is retained at the precipitation treatment temperature

Tp for a precipitation treatment time tp. Here, it is necessary to perform the precipitation treatment such that the precipitation treatment temperature Tp and the precipitation treatment time tp satisfy Formula (5).

Ts = 7400 / (1.95 - logio(Al x N)) - 273: Formula (4)

Logio(tp) + 0.012 x Tp > 8.7: Formula (5)

Here, Ts in Formula (4) is the solid solution temperature (°C) of AIN, and

"Al" and "N" are the Al content and the N content, respectively. "Tp" in the Formula

(5) is the precipitation treatment temperature (C), and "tp" is the precipitation

treatment time (hour).

In addition, slight temperature fluctuations are allowed during the temperature

retention of the precipitation treatment. In addition, there are cases where

temperature fluctuations occur in actual operation. Therefore, the precipitation

treatment temperature Tp is defined as the average temperature of the steel plate of the

thickness middle portion after the temperature of the thickness middle portion of the

steel plate lastly exceeds "the maximum temperature of the thickness middle portion of

the steel plate during the precipitation treatment - 40°C" until the steel plate is

extracted from a heat treatment furnace. Specifically, the precipitation treatment

temperature Tp is a value calculated by Formula (8).

Tp = {.[tA - t]T(t)dtj / (tB-tA): Formula (8)

tA: Time at which the temperature of the thickness middle portion of the steel

plate lastly exceeds "the maximum temperature of the thickness middle portion of the

steel plate during the precipitation treatment - 40°C"

tB: Time at which the steel plate is extracted from the heat treatment furnace

T(t): Change in the temperature of the thickness middle portion of the steel

plate with respect to time (time history of temperature)

f[tA -+ tB]T(t)dt: Integral value from tA to tB of the change in the thickness middle portion of the steel plate with respect to time

In addition, the precipitation treatment time tp is defined as the time (that is,

after the temperature of the thickness middle portion of the steel plate lastly -tB-tA)

exceeds "the maximum temperature of the thickness middle portion of the steel plate

during the precipitation treatment - 40°C" until the steel plate is extracted from the

heat treatment furnace. If the precipitation treatment temperature Tp obtained by

applying the time history of the temperature during the precipitation treatment of the

thickness middle portion of the steel plate to Formula (8) described above is more than

550°C and less than Ac , and the precipitation treatment temperature Tp and the

precipitation treatment time tp satisfy Formula (5), it is determined that a suitable

precipitation treatment has been performed.

[0025]

In a case where the solutionizing treatment is not performed before hot rolling,

coarse AIN formed during casting of the steel remains in the steel, and the total amount

of AlN in the steel is decreased. Therefore, fine AIN obtained by the precipitation

treatment is reduced, and the austenite pinning effect cannot be obtained.

[0026]

The present inventors measured vE-20o of a steel plate manufactured by

applying various precipitation treatment times tp and precipitation treatment

temperatures Tp to a steel having composition A4 of the Examples described below.

The results are shown in FIG. 5. It can be seen from FIG. 5 that in order to obtain the

austenite pinning effect of AIN, it is necessary to perform the precipitation treatment at

an appropriate precipitation treatment temperature Tp for a precipitation treatment time

tp.

[0027]

Specifically, FIG. 5 plots steel plates with the horizontal axis representing the

precipitation treatment temperature Tp of each of the steel plates and the vertical axis

representing Logio(tp) of each of the steel plates. The unit of tp is time (Hr). In FIG.

, the steel plates plotted by X marks are those having a vE-20°c of less than 47 J, and

the steel plates plotted by 0 marks are those having a vE-20°c of 47 J or more. It can

be seen from FIG. 5 that the toughness cannot be secured under the processing

condition of Logio(tp) +0.012 xT< 8.7. It is presumed that this is because

precipitation of AIN is not sufficiently performed in the precipitation treatment, and the

austenite pinning effect cannot be exhibited. On the other hand, it is understood that

the toughness cannot be secured even in a case where the precipitation treatment

temperature Tp exceeds Acl. It is presumed that this is because, in a case where the

precipitation treatment temperature Tp exceeds Ac1, the precipitation treatment

becomes temperature retention in an Q-y dual phase region, so that Al and N are

concentrated in the y region and coarsening of AIN is incurred. The upper limit of the

precipitation treatment time tp is not particularly limited from the viewpoint of

mechanical properties. However, from the viewpoint of industrial production

efficiency, 5 days = 120 hours may be the upper limit of the precipitation treatment

time tp.

[0028]

(6) Quenching Conditions (Temperature and Time) for Securing Middle

Portion Hardness

As a sixth item, in order to cause the hardness of the thickness middle portion

to be HB 350 or more in the composition range of the steel plate according to the

present embodiment, it is necessary to perform quenching under predetermined

conditions after sufficient precipitation of AlN caused by the above-mentioned precipitation treatment. Specifically, it is necessary to reheat the hot rolled steel plate to a quenching retention temperature Tq of 900°C or more and 950°C or less, retain the hot rolled steel plate at this temperature for a quenching retention time tq (minute) or more represented by Formula (6), and then performing a quenching treatment by water cooling the hot rolled steel plate.

tq = 0.033 x (950 - Tq)2 +(1.5 x f)2/10: Formula (6)

In Formula (6), Tq is the quenching retention temperature (°C), and f is a

value obtained by Formula (2) described above. In addition, the quenching retention

temperature Tq indicates not the setting temperature of the heat treatment furnace but

the temperature of the thickness middle portion of the steel plate.

Slight temperature fluctuations are allowed during the temperature retention

of the quenching. In addition, temperature fluctuations may occur in actual operation.

Therefore, the quenching retention temperature Tq is defined as the average

temperature of the steel plate of the thickness middle portion after the temperature of

the thickness middle portion of the steel plate lastly exceeds "the maximum

temperature of the thickness middle portion of the steel plate during the quenching

°C" until the steel plate is extracted from a heat treatment furnace. Specifically, the

quenching retention temperature Tq is a value calculated by Formula (9).

Tq = {.I[ti -+ t 2]T(t)dt} / (t2-tl): Formula (9)

ti: Time at which the temperature of the thickness middle portion of the steel

plate lastly exceeds "the maximum temperature of the thickness middle portion of the

steel plate during the quenching - 40°C"

t2: When the steel plate is extracted from the heat treatment furnace

T(t): Change in the temperature of the thickness middle portion of the steel

plate with respect to time (time history of temperature)

I[tit-> t2]T(t)dt: Integral value from ti to t2 of the change in the thickness middle portion of the steel plate with respect to time

Hereinafter, in order to distinguish from Tq as a target value for an operation

described later, there are cases where the value calculated by Formula (8) is described

as "actual Tq". In addition, the quenching retention time of the steel plate as an

actual value is defined as the time (that is, "t2-ti") after the temperature of the

thickness middle portion of the steel plate lastly exceeds "the maximum temperature of

the thickness middle portion of the steel plate during the quenching - 40°C" until the

steel plate is extracted from the heat treatment furnace. There are cases where the

quenching retention time of the steel plate as the actual value defined as "t2-ti"below

is described as "actual tq". Moreover, there are cases where the quenching retention

time tq calculated from Formula (6) is described as "necessary tq". It is required as a

manufacturing condition of the steel plate according to the present embodiment that the

actual tq is equal to or more than the necessary tq.

The quenching retention temperature Tq may be controlled based on a value

measured by inserting a thermocouple into the vicinity of the thickness middle portion

of the steel plate, or the like, or this value may be controlled based on an estimated

value obtained by heat conduction calculation based on the furnace temperature, the

plate thickness, and the like.

An example of an actual quenching method is described below. For example,

before the quenching treatment, a quenching retention temperature (target Tq) and a

quenching retention time (target tq) as target values that satisfy Formula (6) are

determined in advance. The steel plate is inserted into the heat treatment furnace, the

steel plate is heated to a temperature range within a target Tq ±20°C, and retained at

the temperature. After retaining the temperature of the steel plate within the range of the target Tq ±20°C for at least the target tq, a cooling treatment for quenching is performed. Thereafter, the actual Tq is calculated by applying the time history T(t) of the actual temperature (measured value or estimated value) of the thickness middle portion of the steel plate to Formula (8) described above. In addition, the time elapsed from the time ti at which the temperature of the thickness middle portion of the steel plate lastly exceeds "the maximum temperature of the thickness middle portion of the steel plate during the quenching - 40°C" to the time t2 at which the steel plate is extracted from the heat treatment furnace is regarded as the actual tq. Next, the necessary tq is calculated by substituting the actual Tq into Tq of Formula (6). In a case where the actual tq is not smaller than the necessary tq (that is, in a case of the actual tq > the necessary tq), it is determined that an appropriate quenching treatment has been performed.

In addition, also in the precipitation treatment, the determination in the same

procedure is required.

[0029]

FIG. 6A shows the results of an experiment using a steel having composition

A6 of the Examples described later, and FIG. 6B shows the results of an experiment

using a steel having composition A2 of the Examples described later.

[0030]

The present inventors manufactured various steel plates by applying various

temperature retention times (the time for which the temperature of the middle portion

of the hot rolled steel plate is retained isothermally at the quenching retention

temperature Tq) and the quenching retention temperature Tq to these steels and

measured the middle portion hardness thereof. FIGS. 6A and 6B plot the steel plates

with the horizontal axis representing the quenching retention temperature Tq of each of the steel plates and the vertical axis representing the temperature retention time of each of the steel plates. In FIGS. 6A and 6B, the steel plates plotted by X marks are those having a middle portion hardness of less than 350 HB, and the steel plates plotted by 0 marks are those having a middle portion hardness of 350 HB or more.

[0031]

It can be seen from FIGS. 6A and 6B that the steel plates having a temperature

retention time shorter than the quenching retention time tq represented by Formula (6)

described above (the steel plates plotted below the curves in FIGS. 6A and 6B) have a

middle portion hardness of less than HB 350. It is considered that this is because

alloys that improve the hardenability were not sufficiently solutionized in the matrix

and the hardenability could not be secured. In addition, the quenching retention time

tq is a function of f because the larger the amount of alloys, the longer the time

necessary for such solutionizing.

[0032]

In a case where the quenching retention temperature Tq is less than 900°C,

solutionizing of alloying elements is not sufficiently performed. Therefore, the

hardenability cannot be secured, and HB 350 cannot be achieved at the middle portion

of the steel plate. On the other hand, in a case where the quenching retention

temperature Tq exceeds 950°C, AIN is partially solutionized, and liberated N is bonded

to B in steel. Accordingly, the hardenability improvement effect of B is inhibited, and

HB 350 at the middle portion of the steel plate cannot be achieved.

[0033]

(7) Restriction of Upper and Lower Limits of Tempering Temperature for

Securing Hardness and Toughness of Middle Portion

As a seventh item, in consideration of the construction requirements of a gear

(prevention of deterioration of the material in stress relief annealing), the tempering

temperature needs to be 5000 C or more. In addition, the tempering temperature needs

to be 500°C or more in order to secure the toughness of the steel plate by sufficiently

tempering the structure. On the other hand, there is concern that in the steel plate

according to the present embodiment, the hardness may be rapidly reduced due to

tempering at more than 550°C. From this, the tempering temperature needs to be

550°C or less. After this tempering, the steel plate is cooled to 150°C or less.

[0034]

Next, the structure of the steel plate according to the present embodiment will

be described. In the steel plate according to the present embodiment, the total area

ratio of martensite and bainite is 99% or more. Although the remainder of the

structure is not particularly defined, for example, ferrite, pearlite, and retained

austenite can be considered. Other structures are acceptable in an amount of less than

1 area%

[0035]

The above structure is achieved by quenching under conditions under which

ferrite is not formed and tempering at a sufficiently high temperature. Specifically,

the structure is achieved by performing quenching on the steel plate having a

composition of Ceq > 0.750% under the above conditions after the precipitation

treatment under the above conditions and performing tempering thereon under the

above conditions.

[0036]

Ferrite is a factor that reduces the hardness of steel. In particular, ferrite

tends to be formed at the thickness middle portion where the quenching cooling rate is

slow. In order to secure the middle portion hardness, the amount of ferrite has to be as low as possible.

[0037]

Although pearlite is effective in securing hardness, it becomes a brittle

fracture origin because of its hardness. Therefore, the amount of pearlite has to be as

low as possible. Pearlite is formed by the concentration of C discharged during ferrite

precipitation. Therefore, the formation of pearlite is simultaneously suppressed by

the avoidance of ferrite precipitation.

[0038]

Retained austenite is a brittle fracture origin and reduces the toughness of

steel. Therefore, the amount of retained austenite has to be as low as possible.

When tempering is performed at a tempering temperature of 500°C or more, the

formation of retained austenite is suppressed.

[0039]

As described above, it is necessary to suppress the formation of ferrite,

pearlite, retained austenite, and the like, which are harmful structures in the steel plate

according to the present embodiment, as much as possible. Also in consideration of

microsegregation and production due to operation variation, the structure which is

neither of martensite and bainite has to be reduced to less than 1%.

[0040]

Next, various composition ranges in the steel plate according to the present

embodiment will be described. The unit "%" of the amount of an alloying element

means mass%.

[0041]

C: 0.16% to 0.20%

C increases the hardness of a hardened structure and is thus an element effective for improving the hardness. Based on the experimental results shown in FIG.

2 described above, 0.16% is set to the lower limit of the C content. Ontheotherhand,

an excessive amount of C impairs the toughness of the steel plate and also becomes a

factor of the hardness difference between the surface layer and the middle portion.

Therefore, similarly based on the experimental results shown by FIG. 2 described

above, the upper limit of the C content is set to 0.20%. The C content maybe 0.17%

ormore, 0.18% or more, or 0.19% or more. The C content may be 0.19% or less,

0.18% or less, or 0.17% or less.

[0042]

Si: 0.50% to 1.00%

Si has a deoxidizing effect. Moreover, Si is an element also effective for

improving the strength of a steel plate, and can improve hardenability without raising

Ceq. Therefore, the Si content is 0.50% or more. However, a large amount of Si

promotes temper embrittlement and reduces the toughness of the steel plate.

Therefore, it is preferable to reduce the Si content, and the upper limit thereof is 1.00%.

The Si content may be 0.60% or more, 0.65% or more, or 0.70% or more. The Si

content may be 0.90% or less, 0.85% or less, or 0.80% or less.

[0043]

Mn: 0.90% to 1.50%

Mn has a deoxidizing effect. In addition, Mn is an element which improves

hardenability and is effective in improving the strength of a steel plate. Therefore, the

Mn content is 0.90% or more. On the other hand, excessive Mn promotes temper

embrittlement and lowers the toughness of the steel plate. Therefore, the upper limit

of the Mn content is 1.50%. The Mn content may be 1.00% or more, 1.05% or more,

or1.10%ormore. The Mn content maybe 1.40% or less, 1.35% or less, or 1.30% or less.

[0044]

P: 0.010% or Less

P is an impurity element contained in steel. P is a harniful element that

promotes intergranular embrittlement and reduces the toughness of the steel plate.

Therefore, the P content is preferably as small as possible. Therefore, the P content is

reduced to 0.010% or less. Since Pis not required by the steel plate according to the

present embodiment, the lower limit of the P content is 0%. However, from the

viewpoint of refining cost and productivity, the P content may be defined as 0.001% or

more. The P content maybe 0.002% or more, 0.003% or more, or 0.005% or more.

The P content may be 0.008% or less, 0.007% or less, or 0.006% or less.

[0045]

S: 0.0020% or Less

S is an impurity element contained in steel. S is an element that reduces the

toughness of a steel plate through segregation and formation of sulfides. Therefore, it

is preferable that the S content is as small as possible. Therefore, the S content is

reduced to 0.0020% or less. Since S is not required by the steel plate according to the

present embodiment, the lower limit of the S content is 0%. However, from the

viewpoint of refining cost and productivity, the S content may be 0.0004% or more.

The S content maybe 0.0005% or more, 0.0006% or more, or 0.0007% or more. The

S content may be 0.0018% or less, 0.0015% or less, or 0.0010% or less.

[0046]

Cu: 0% to 0.40%

Cu is an element that can increase the strength of steel without impairing low

temperature toughness. However, there are cases where a large amount of Cu causes a crack in a steel plate during hot working. Furthermore, there is concern that a large amount of Cu may lower the toughness of the steel plate through the precipitation of metal Cu and the like. Therefore, the upper limit of the Cu content is 0.40%.

Although Cu contributes to suppression of ferrite by raising Ceq, since Cu can be

substituted with other alloying elements, Cu is not essential for the steel plate

according to the present embodiment. Therefore, the lower limit of the Cu content is

%. However, since a reduction in Cu requires cost, from the viewpoint of the

refining cost, the lower limit of the Cu content maybe set to 0.01% or 0.02%. The

Cu content may be 0.03% or more, 0.05% or more, or 0.10% or more. TheCu

content may be 0.35% or less, 0.30% or less, or 0.20% or less.

[0047]

Ni: 0.20% to 1.00%

Ni is an element effective in improving the strength and toughness of steel.

Therefore, the Ni content is 0.20% or more. On the other hand, the effect is saturated

even if the amount of Ni is excessive, and increasing the amount of Ni, which is an

expensive alloy, causes the deterioration of the manufacturing cost. Therefore, the

upper limit of the Ni content is 1.00%. The Ni content may be 0.25% or more, 0.30%

or more, or 0.40% or more. The Ni content may be 0.90% or less, 0.80% or less, or

0.70% or less.

[0048]

Cr: 0.60% to 0.99%

Mo: 0.60% to 1.00%

Cr and Mo have a function of improving hardenability and increasing the

middle portion hardness. Moreover, Cr and Mo also have an effect of raising the

hardness of the surface layer and the middle portion by precipitation quenching.

Therefore, the amount of each of Cr and Mo is 0.60% or more. However, there is

concern that excessive amounts of Cr and Mo may lower the toughness due to the

formation of alloy carbides. Therefore, the upper limit of the Cr content is 0.99%,

and the upper limit of the Mo content is 1.00%. The Cr content may be 0.65% or

more, 0.70% or more, or 0.75% ormore. The Cr content maybe 0.95% or less,

0.90% or less, or 0.80% or less. The Mo content may be 0.65% or more, 0.70% or

more, or 0.75% or more. The Mo content may be 0.95% or less, 0.90% or less, or

0.80% or less.

[0049]

V: 0% to 0.050%

V improves the base metal strength through the formation of carbides.

However, a large amount of V causes a reduction in toughness due to the formation of

alloy carbides. Therefore, the upper limit of the V content is 0.050%. Although V

contributes to suppression of ferrite by raising Ceq, since V is an expensive alloying

element and can be substituted with another alloy, V is not essential for the steel plate

according to the present embodiment. Therefore, the lower limit of the V content is

%. However, from the viewpoint of the refining cost, the lower limit of the V

content maybe 0.003%. The V content may be 0.005% or more, 0.010% or more, or

0.015% or more. The V content may be 0.045% or less, 0.040% or less, or 0.035% or

less.

[0050]

Al: 0.050% to 0.085%,

Al is an element effective as a deoxidizing material. Furthermore, Al is

bonded to N in steel to form AlN, which contributes to the refinement of the structure.

In addition, Al forms AIN in the precipitation treatment and contributes to the decomposition of BN, thereby also having a function of stabilizing the hardenability exhibited by B. Therefore, the Al content is 0.050% or more. However, an excess of Al forms coarse AIN and causes a reduction in toughness and cracking in a cast piece. Therefore, the upper limit of the Al content is 0.085%. TheAlcontentmay be 0.055% or more, 0.060% or more, or 0.065% or more. The Al content maybe

0.080% or less, 0.075% or less, or 0.070% or less.

[0051]

N: 0.0020% to 0.0070%,

N forms nitrides and carbonitrides with alloying elements and contributes to

the refinement of the structure of a steel plate. Therefore, the lower limit of the N

content is set to 0.0020%. On the other hand, in a case where N is excessively

solutionized in the steel, and in a case where N forms coarse nitrides, carbonitrides,

and the like, the toughness of the steel plate is lowered. Therefore, the upper limit of

the N content is set to 0.0070%. The N content may be 0.0025% or more, 0.0030%

ormore, or 0.0035% or more. The N content maybe 0.0065% or less, 0.0060% or

less, or 0.0050% or less.

[0052]

As described above, Al x N (the product of the Al content and the N content)

needs to be 2.0 x 10-4 or more. The purpose is to utilize the austenite pinning effect

of AIN, which contributes to the refinement of the structure of the steel plate.

[0053]

B: 0.0005% to 0.0020%

B is an element which improves the hardenability of steel and improves the

strength. Therefore, the B content is 0.0005% or more. However, in a case where B

is excessively contained, B forms carboborides and lowers the hardenability.

Therefore, the upper limit of the B content is 0.0020%. The B content may be

0.0007% or more, 0.0008% or more, or 0.0010% or more. TheBcontentmaybe

0.0018% or less, 0.0016% or less, or 0.0015% or less.

[0054]

Further, the amounts of the following elements that affect the toughness are

specified as selective elements. However, since the following selective elements are

not essential for solving the problems by the steel plate according to the present

embodiment, the lower limit of the amount of each of the selective elements is 0%.

[0055]

Nb: 0% to 0.050%

Nb is an element that contributes to the refinement of the internal structure of

steel by forming carbonitrides, and affects the toughness. Therefore, 0.001% ormore

of Nb can be contained. However, coarse carbonitrides generated by a large amount

of Nb rather lower the toughness. Therefore, the upper limit of the Nb content is

0.050%. The Nb content may be 0.002% or more, 0.005% or more, or 0.008% or

more. The Nb content may be 0.045% or less, 0.040% or less, or 0.035% or less.

[0056]

Ti: 0% to 0.020%

Ti/N < 3.4

Ti is an element that contributes to the refinement of the structure by forming

stable nitrides and affects the toughness. Therefore, 0.001% orimore of Ti can be

contained. However, an excess of Ti causes a reduction in toughness due to coarse

nitrides. Therefore, the upper limit of the Ti content is 0.020%. The Ti content may

be 0.002% or more, 0.005% or more, or 0.008% or more. The Ti content may be

0.018% or less, 0.016% or less, or 0.012% or less.

[00571

In a case where the Ti content exceeds the stoichiometric ratio of TiN,

specifically, in a case of Ti/N> 3.4, an excess of Ti forms carbides and lowers the

toughness. Therefore, it is preferable that Ti/N < 3.4 is satisfied. Ti/N may be 3.3 or

less, 3.2 or less, or 3.0 or less. Although it is not necessary to define the lower limit

of Ti/N, since the lower limit of the Ti content is 0%, the lower limit of Ti/N may be

defined as 0%. Ti/N may be 0.2 or more, 0.5 or more, or 1.0 or more.

[0058]

Ca: 0% to 0.0030%,

Mg: 0% to 0.0030%,

REM: 0% to 0.0030%,

All of Ca, Mg, and REM are bonded to harmful impurities such as S to form

harmless inclusions. Accordingly, all of Ca, Mg, and REM can improve mechanical

properties such as the toughness of steel. Therefore, the amount of each of Ca, Mg,

and REM can be 0.0001% or more. However, when the amounts of Ca, Mg, and

REM become excessive, not only is the effect saturated, but also the erosion of

refractory materials such as casting nozzles is promoted. Therefore, the upper limit

of the amount of each of Ca, Mg, and REM is 0.0030%. The amount of each of Ca,

Mg, and REM may be 0.0002% or more, 0.0005% or more, or 0.0010% or more. The

amount of each of Ca, Mg, and REM may be 0.0025% or less, 0.0020% or less, or

0.0015% or less. The term "REM" refers to a total of 17 elements consisting of Sc, Y,

and lanthanoids, and "amount of REM" means the total amount of these 17 elements.

[0059]

The remainder of the chemical composition of the steel plate according to the

present embodiment contains iron and impurities. Impurities are components which are incorporated due to various factors of the raw material such as ore or scrap, or a manufacturing process in the industrial production of steel.

[0060]

The average value of the prior austenite grain size in the thickness middle

portion of the steel plate according to the present embodiment is less than 80 m. In

a case where the prior austenite grain size in the thickness middle portion is less than

m, the thickness middle portion has high toughness. The prior austenite grain

size of the thickness middle portion may be 76 pm or less, 73 tm or less, 70 pm or less,

or 68 tm or less. The refinement of the prior austenite grain size in the thickness

middle portion of the steel plate according to the present embodiment is achieved

mainly by the austenite pinning effect of fine AIN, as described above.

[0061]

The plate thickness of the steel plate according to the present embodiment is

more than 200 mm. Since a steel plate having a thickness of more than 200 mn can

be used as a material of a huge gear used in a rotating mechanism of a large industrial

machine represented by a rotary kiln, the steel plate has high industrial applicability.

However, the steel plate according to the present embodiment has good hardness and

low temperature toughness even when the plate thickness is 200 mm or less. The

thickness of the steel plate may be 205 nnn or more, 210 nun or more, or 220 mm or

more. The upper limit of the plate thickness of the steel plate is not particularly

limited, but the plate thickness may be 250 mm or less, 240 mm or less, or 230 mm or

less.

[0062]

The -20°C Charpy absorbed energy (vE-20°c) measured in the C direction of

the thickness middle portion of the steel plate according to the present embodiment is

47 J or more. Here, the Charpy absorbed energy is a three-point average of values

measured according to American Society for Testing and Materials (ASTM) A370

2017a. vE-20°C measured in the C direction is vE-20°c obtained using a Charpy impact

test piece collected along the C direction (direction perpendicular to the rolling

direction and the plate thickness direction). A steel plate that satisfies the above

requirements with respect to Charpy absorbed energy has high low temperature

toughness even in the thickness middle portion where it is usually difficult to control

mechanical properties. The -20°C Charpy absorbed energy measured in the C

direction at the thickness middle portion of the steel plate according to the present

embodiment may be 50 J or more, 55 J or more, or 60 J or more. Although it is not

necessary to define the upper limit of the -20°C Charpy absorbed energy measured in

the C direction at the thickness middle portion of the steel plate according to the

present embodiment, for example, the upper limit may be defined as 400 J, 380 J, or

350 J.

[0063]

The hardness of the surface layer and the thickness middle portion of the steel

plate according to the present embodiment is HB 350 or more. Here, the hardness of

the steel plate according to the present embodiment is a five-point average of HBW

/3000 (indenter diameter 10 mm, test force 3000 kgf) defined in JIS Z 2243-1:2018.

The steel plate that satisfies the above requirements in terms of hardness does not have

excessive hardness at the surface layer while having high hardness in the thickness

middle portion where it is usually difficult to secure high hardness, and is thus highly

applicable as steel for machine structural use. The hardness of the surface layer of the

steel plate according to the present embodiment may be HB 360 or more, HB 370 or

more, or HB 380 or more. The hardness of the thickness middle portion of the steel plate according to the present embodiment may be HB 360 or more, HB 370 or more, or HB 380 or more. The upper limit of the hardness of the surface layer of the steel plate according to the present embodiment need not be defined, but may be defined as, for example, HB 450, HB 420, or HB 400. The upper limit of the hardness of the thickness middle portion of the steel plate according to the present embodiment need not be defined, but may be defined as, for example, HB 450, HB 420 or HB 400.

[0064]

Next, a method of measuring each of the constituent elements of the steel

plate according to the present embodiment will be described below.

[0065]

The composition of the steel plate is measured according to a known method

at a 1/4T portion of the steel plate (a position at a depth of 1/4 of the thickness T of the

steel plate from the rolled surface of the steel plate and the vicinity thereof) in order to

exclude the effects of surface layer decarburization and segregation. Based on the

measured values, Ceq, Al x N, Ti/N, and 4 x f/g of the steel plate are calculated. If

the molten steel analysis value of a slab which is the material of the steel plate is

known, the analysis value may be regarded as the chemical composition of the steel

plate.

[0066]

The -20°C Charpy absorbed energy (vE-20) measured in the C direction of

the thickness middle portion is measured according to ASTM A370-2017a. Test

pieces are V-notch test pieces. Three test pieces are collected from the thickness

middle portion of the steel plate. During the collection of the test pieces, the

longitudinal direction of the test pieces and the C direction of the steel plate (the

direction perpendicular to the rolling direction and the plate thickness direction) are caused to be coincident with each other. The average value of vE-20°C of these three test pieces is taken as the -20°C Charpy absorbed energy measured in the C direction at the thickness middle portion of the steel plate.

[0067]

The hardness of the surface layer of the steel plate and the hardness of the

thickness middle portion of the steel plate are measured based on JIS Z 2243-1:2018.

HBW 10/3000 is obtained by setting the indenter diameter to 10 mm and the test force

to 3000 kgf. The measurement of the surface layer hardness is performed by pressing

an indenter against the surface formed by removing the area from the rolled surface of

the steel plate to a depth of at least 1 mm. The average value of the measurement

results of the surface layer hardness at five points is taken the hardness of the surface

layer of the steel plate. The hardness of the thickness middle portion of the steel plate

is measured by pressing an indenter against a portion corresponding to the thickness

middle portion in a section formed by cutting the steel plate parallel to the rolled

surface. The average value of the measurement results of the hardness of the

thickness middle portion at five points is taken as the hardness of the thickness middle

portion of the steel plate.

[0068]

A method of measuring the area ratios of martensite and bainite in the

thickness middle portion is as follows. An observed section is a surface parallel to

the rolling direction of the steel plate, and is subjected to polishing and nital etching.

The observed section is observed with an optical microscope at a magnification of 500

fold. Based on the optical micrograph, the sum of the area ratios of martensite and

bainite can be measured. The total area of the observed visual field is 0.300 mm 2 or

more.

[00691

A method of measuring the average value of the prior austenite particle size in

the thickness middle portion is as follows. An observed section is a surface parallel

to the rolling direction of the steel plate, and is subjected to polishing and picric acid

etching. The average section length is measured by a section method, and the average

section length is taken as the average priory particle size. However, the section

length at the time of measurement is 1000 pm (1 mm) or more. Although it is not

necessary to particularly determine the upper limit of the section length, it is not

necessary to measure a section with a length of more than 2000 m (2 mm), and the

upper limit thereof may be 2000 m (2 mm).

[0070]

Next, a preferable method of manufacturing the steel plate according to the

present embodiment will be described. According to the knowledge of the present

inventors, the steel plate according to the present embodiment can be obtained

according to the manufacturing conditions described below. However, even a steel

plate obtained by conditions other than the manufacturing conditions described below

corresponds to the steel plate according to the present embodiment as long as the

above-described requirements are satisfied.

[00711

The method of manufacturing the steel plate according to the present

embodiment includes, as shown in FIG.7, a step S Iof heating a slab, a step S2 of hot

rolling the slab to obtain a steel plate, a step S3 of cooling the steel plate, and a step S4

of performing a precipitation treatment on the steel plate, a step S5 of performing a

quenching treatment on the steel plate, and a step S6 of tempering the steel plate. The

manufacturing conditions in these steps are as shown in the following table.

0~ L)

+

- C CD

Cl

4- - P u 17

tflQ 0.0 to

ZC~Cn C/ '-'i

[00731

In the step S of heating the slab, after casting a slab having the composition

of the steel plate according to the present embodiment described above, the slab is

heated to a temperature equal to or more than the AlN solid solution temperature Ts

calculated by Formula (4) described above. The technical significance of the AIN

solid solution temperature Ts is as described above.

For the composition of the slab, not only be the upper and lower limits of each

of the alloying elements satisfied, but also it is necessary, as in the steel plate, Ceq is

0.750% to 0.800%, Alx N is 2.0 x 10-4 or more, Ti/N is 3.4 or less, and 4 x f/g is 9.00

or more. Preferable numerical ranges of the amount of each alloying element, Ceq,

Al x N, Ti/N, and 4 x f/g are the same as those of the steel plate. In a case where the

molten steel analysis value of the slab is known, that value may be regarded as the

chemical composition of the slab.

[0074]

The step S2 of hot rolling the heated slab is not particularly limited. Since

the present embodiment aims to manufacture a steel plate having a thickness of more

than 200 num, the thickness of the steel plate (hot-rolled steel plate) obtained by the

hot rolling is more than 200 mm.

[0075]

In the step S3 of cooling the steel plate, it is preferable to complete the

transformation from austenite in the structure of the steel plate to other structures by

cooling the steel plate to 500°C or less, and preferably 150°C or less.

[0076]

In step S4 of performing the precipitation treatment on the steel plate, the steel

plate is heated to the precipitation treatment temperature Tp, and the temperature is retained at this temperature T. The precipitation treatment temperature Tp is a temperature of more than 550°C and less than Ac1, and is regarded as satisfying

Formula (5) described above. The precipitation treatment time tp is also regarded as

satisfying Formula (5) described above. The technical significance of the

precipitation treatment conditions is as described above. After the temperature

retention in the step S4 of performing the precipitation treatment, the steel plate may

be cooled to 500°C or less, and preferably 150°C or less (for example, room

temperature), or may be heated as it is for subsequent quenching.

[0077]

In the step S5 of performing quenching treatment on the steel plate, the steel

plate is heated to the quenching retention temperature Tq (C) of 900°C or more and

950 0C or less, is subjected to temperature retention for the quenching retention time tq

(minute) or more represented by Formula (6), and is then subjected to water cooling.

The technical significance of the quenching retention temperature Tq and the

quenching retention time tq is as described above. In the quenching treatment, means

for cooling the steel plate after the completion of the temperature retention is water

cooling or one that can achieve a cooling rate equivalent to this. The quenching

finishing temperature is, for example, 150°C or less.

[0078]

In the step S6 of tempering the steel plate, it is preferable that the steel plate is

tempered at a tempering temperature of 500°C or more and 550°C or less and is then

cooled to 150°C or less. The technical significance of the tempering temperature is as

described above.

[Examples]

[0079]

Slabs obtained by melting steels of Al to A10 and B Ito B24 having the

chemical composition shown in Table 2-1 were heated, rolled, and heat-treated under

respective conditions of Present Invention Steels Nos. I to 10 shown in Tables 3-1 to 3

3 and Comparative Examples Nos. 11 to 43, whereby steel plates having a plate

thickness of 210mm to 230mm were manufactured. Manufacturing conditions

which are not described in the tables are as follows. All the chemical compositions in

Table 2-1 are molten steel analysis values.

Cooling finishing temperature after hot rolling: 150°C or less in all the

Cxamples and Comparative Examples

Cooling means in quenching: water cooling (cooling to 150°C or less)

Cooling finishing temperature in tempering: 150°C or less in all the Examples

and Comparative Examples

[0080]

The hardness of the surface layer of the steel plate and the hardness of the

thickness middle portion of the steel plate were measured based on JIS Z 2243-1:2018.

HBW 10/3000 was obtained by setting the indenter diameter to 10 mm and the test

force to 3000 kgf. The measurement of the surface layer hardness was performed by

pressing the indenter against the surface formed by removing the area from the rolled

surface of the steel plate to a depth of at least 1 mm. The average value of the

measurement results of the surface layer hardness at five points was taken as the

hardness of the surface layer of the steel plate (Table 4 "HB surface layer"). The

hardness of the thickness middle portion of the steel plate was measured by pressing

the indenter against a portion corresponding to the thickness middle portion in a

surface formed by cutting the steel plate parallel to the rolled surface. The average

value of the measurement results of the thickness middle portion hardness at five points was taken as the hardness of the thickness middle portion of the steel plate

(Table 4 "HB middle portion"). In addition, the test pieces of the steel plate were

collected from portions separated from the end portions of the steel plate in the length

direction and the width direction by the plate thickness or more. The -20°C Charpy

absorbed energy (vE-2 0 °C) measured in the C direction of the thickness middle portion

was measured according to ASTM A370-2017a. Three test pieces were collected

from the thickness middle portion of the steel plate. During the collection of the test

pieces, the longitudinal direction of the test pieces and the C direction of the steel plate

(the direction perpendicular to the rolling direction and the plate thickness direction)

were caused to be coincident with each other. The average value of vE-20°c of these

three test pieces was taken as the -20°C Charpy absorbed energy measured in the C

direction at the thickness middle portion of the steel plate (Table 4 "vE-20°c").

[0081 ]

A method of measuring the area ratios of martensite and bainite in the

thickness middle portion is as follows. An observed section was a surface parallel to

the rolling direction of the steel plate, and was subjected to polishing and nital etching.

The observed section is observed with an optical microscope at a magnification of 500

fold. Based on the optical micrograph, the sum of the area ratios of martensite and

bainite was measured. The total area of the observed visual field was 0.300 mm2 or

more.

A method of measuring the average value of the prior-austenite particle size

in the thickness middle portion was as follows. An observed section was a surface

parallel to the rolling direction of the steel plate, and was subjected to polishing and

picric acid etching. The average section length was measured by a section method

(section length: 1000 pm or more and 2000 m or less), and the average section length was taken as the average value of the prior austenite grain size of the thickness middle portion (Table 4 "priory grain size").

[0082]

Hereinafter, compositions are shown in Tables 2-1 and 2-2, manufacturing

methods are shown in Tables 3-1 to 3-3, and materials, evaluation and the like are

shown in Table 4. The "precipitation treatment temperature Tp" described in the

tables is a value obtained by applying the thermal history at the time of the

precipitation treatment to Formula (8). The "precipitation treatment time tp"

described in the tables is the time after the temperature of the thickness middle portion

of the steel plate lastly exceeds "the maximum temperature of the thickness middle

portion of the steel plate during the precipitation treatment - 40°C" until the steel plate

is extracted from the heat treatment furnace. The "quenching retention temperature

Tq" described in the tables is a value obtained by applying the thermal history at the

time of the quenching treatment to Formula (9). The "actual quenching retention

time" described in the tables means is the time after the temperature of the thickness

middle portion of the steel plate lastly exceeds "the maximum temperature of the

thickness middle portion of the steel plate during the quenching - 40°C" until the steel

plate is extracted from the heat treatment furnace (that is, actual tq). The "quenching

retention time tq" described in the tables is a value obtained by substituting the

quenching retention temperature Tq and f described in the tables into the Formula (6)

described above. However, in the Al x N column of Table 2-2, for example, 2.2E-04

means 2.2 x 10-4. In addition, in Table 3-2, it is determined whether or not the

precipitation treatment temperature Tp and the precipitation treatment time tp satisfy

Formula (5) using a precipitation treatment time threshold obtained using Formula (5')

below. When the precipitation treatment time tp is equal to or more than the precipitation treatment time threshold, the precipitation treatment temperature Tp and the precipitation treatment time tp satisfy Formula (5).

(Precipitation treatment time threshold) = 10(-0.012 x Tp + 8.7): Formula (5')

C'4

OE

cc

Sc C', CC

<I=- -I Cl = ON ON ON

ONI ON ON m1

ON ON ON, 6 C) 6 - C C -) C

z I I

ON~O M-CtON C-, CC~lClC ON l--O - COp-ON-l l-ONC

oC 00 r-Cl (C r ON-O r- 0l r~ ' C -O 'C Is -- (CON-lCC OC r- C1 :c OC (C 0

CC~ ~ ~ ~~~~o ClC N( C-t~* l0C f C lC C7 ON ON ON ON ON ON ON ONO NO N NO NO ON ONl ON ON O ON ON ON ONO NO N NO NO - 10 c~ oO 11C mNC NC tO CC CC l'

Cl ' O C O C C (m C C-C CCl -- 'C IC~ C C ONC Ct Cl ONL N ' 4,'CCl(C'

ON-O -- OCO O --------- N-N----------------r~ -O ON ON ON ONi ON! ON ON ON ONO NO NO NO N NO NO NO NO NO NO NO NO

MOMOMOCIANONMNONM C 66 6T ON1 ON' ON ON ONl ON ON ON ON ON ON O- ONO'NOI; NO NO NO N NO NO

(K 6CC ~C l( C( C / C( C' f~C fC f' l' ~( lC C' CC

ON ON ON ON ONO NO NO NO NO NO NO NO NO NO NO NO ON~~~ ON ON ON .N ON ON ON ON .N ONO.NO NO NO NO NO NONO NONO NO NO NO NO ON ON ON ON6 O N O O 6 O NO6 z NO NO N6 zN 6

ON~l'C~--(C~CC (C~-ON~'C~f)'C l~lfCO l~lC (Cll=I

[00841

[Table 2-2] Composition 'riN g 4x/g AlxN Ceq ____________(Formula (1)) Al 10.06 4.08 9.87 2.2E-04 0.765 A2 11.1 4.49 9.89 2.41,-04 0.798 A3 2.4 11.07 4.61 9.62 3.3E-04 0.798 A4 10.05 4.02 10.00 3.I1E-04 0.778 A5 9.96 3.81 10.47 4.2E-04 0.763 A6 9.93 4.26 9.33 2.OE-04 0.767 A7 10.85 4.50 9.64 2.6E-04 0.798 A8 10.76 4.04 10.65 2.I1E-04 0.788 A9 10.46 4.52 9.27 2.6E-04 0.787 A10 10.83 4.74 9.14 5.7E-04 0.798 B31 10.64 4.14 10.29 4.OE-04 0.752 B2 10.04 3.57 11.25 2.3E-04 0.782 B3 10.22 4.26 9.60 2.7E-04 0.797 B4 9.92 3.94 10.07 4. 1E-04 0.751 B35 10.50 4.60 9.14 4.9E-04 0.754 B6 9.85 3.39 11.64 3.9E-04 0.769 B7 10.98 4.64 9.48 3.2E-04 0.769 B8 9.68 3.90J 9.93 4.OE-04 0.756 B39 9.21 3.47 10.63 3.OE-04 0.769 1310 9.69 3.91 9.93 3.OE-04 0.761 B11 10.60 3.98 10.67 2.2E-04 0.751 B 12 9.53 4.21t 9.07 2.9E-04 0.783 1313 8.96 3.06 11.73 3.9E-04 0.751 1B14 11.42 5.02 9.11 3.3E-04 0.793 B315 10.36 4.25 9.75 3.7E-04 0.794 1316 10.82 4.09 10.58 2.1 E-04 0.798 1317 10.63 4.45 9.57 3.4E-04 0.766 1318 10.19 4.47 9.12 9.4E-05 0.756 1319 9.33 3.90 9.57 6.1IE-04 0.751 1320 9.92 4.20 9.45 3.7E-04 0-757 1321 10.99 4.03 10.91 2.2E-04 0.785 1322 9.42 3.57 10.55 4.5E-04 0.737 1323 10.31 4.80 8.59 2.5E-04 0.775 1324 10.24 4.64 8.84 3.1IE-04 0.760

Al xN indicates an index based on JIS X0210-1986.

[00851

[Table 3-1] Slab heating temperature AIN solid solution Fonnula (4) evaluation Plate thickness Composition No. (°C') temperature Ts (°C) (mm) Al 1 1150 1045 SUCCESS 210 A2 2 1150 1058 SUCCESS 220 A3 3 1200 1090 SUCCESS 230 A4 4 1200 1082 SUCCESS 210 A5 5 1250 1116 SUCCESS 210 A6 6 1100 1038 SUCCESS 210 A7 7 1150 1062 SUCCESS 220 A8 8 1150 1040 SUCCESS 210 A9 9 1200 1065 SUCCESS 220 A1O 10 1250 1152 SUCCESS 210 B1 11 1250 1111 SUCCESS 210 B2 12 1100 1051 SUCCESS 210 B3 13 1200 1068 SUCCESS 210 B4 14 1150 1113 SUCCESS 210 B5 15 1150 1135 SUCCESS 210 B6 16 1150 1107 SUCCESS 210 B7 17 1150 1085 SUCCESS 210 B8 18 1150 1109 SUCCESS 210 B9 19 1150 1078 SUCCESS 210 B10 20 1150 1079 SUCCESS 210 B11 21 1200 1048 SUCCESS 210 B12 22 1200 1077 SUCCESS 210 B13 23 1150 1108 SUCCESS 210 B14 24 1200 1088 SUCCESS 210 B15 25 1200 1102 SUCCESS 210 B16 26 1150 1044 SUCCESS 210 B17 27 1200 1093 SUCCESS 210 B18 28 1050 965 SUCCESS 210 B19 29 1200 1159 SUCCESS 210 B20 30 1150 1101 SUCCESS 210 B21 31 1200 1048 SUCCESS 210 B22 32 1150 1125 SUCCESS 210 B23 33 1150 1061 SUCCESS 210 B24 34 1200 1084 SUCCESS 210 A1O 35 1100 1152 FAILURE 210 A3 36 1200 1090 SUCCESS 230 A4 37 1200 1082 SUCCESS 210 Al 38 1150 1045 SUCCESS 210 A6 39 1100 1038 SUCCESS 210 A7 40 1150 1062 SUCCESS 220 A8 41 1150 1040 SUCCESS 210 A5 42 1250 1116 SUCCESS 210 A9 43 1200 1065 SUCCESS 220

[00861

[Table 3-2] Precipitation treatment Precipitation treatment time Formula (5) No. Ac l Precipitation treatment 0 (°C) temperature Tp ( C) time tp (hour) threshold by Formula (5') (hour) evaluation 1 694 625 20 16 SUCCESS 2 709 600 60 32 SUCCESS 3 696 650 10 8 SUCCESS 4 686 600 50 32 SUCCESS 5 703 625 30 16 SUCCESS 6 707 650 10 8 SUCCESS 7 694 575 80 63 SUCCESS 8 694 600 60 32 SUCCESS 9 706 575 100 63 SUCCESS 10 705 625 80 16 SUCCESS 11 701 575 80 63 SUCCESS 12 693 625 40 16 SUCCESS 13 691 625 40 16 SUCCESS 14 706 650 20 8 SUCCESS 15 715 600 60 32 SUCCESS 16 681 600 60 32 SUCCESS 17 708 650 40 8 SUCCESS 18 695 625 20 16 SUCCESS 19 692 650 10 8 SUCCESS 20 703 650 20 8 SUCCESS 21 696 625 20 16 SUCCESS 22 702 625 40 16 SUCCESS 23 670 575 80 63 SUCCESS 24 718 625 40 16 SUCCESS 25 697 625 20 16 SUCCESS 26 679 600 60 32 SUCCESS 27 703 625 20 16 SUCCESS 28 720 600 40 32 SUCCESS 29 699 600 100 32 SUCCESS 30 693 600 80 32 SUCCESS 31 694 625 100 16 SUCCESS 32 695 625 20 16 SUCCESS 33 721 600 60 32 SUCCESS 34 712 575 80 63 SUCCESS 35 706 625 80 16 SUCCESS 36 696 550 130 126 SUCCESS 37 686 700 30 2 SUCCESS 38 694 600 20 32 FAILURE 39 707 650 10 8 SUCCESS 40 694 575 80 63 SUCCESS 41 694 600 60 32 SUCCESS 42 703 625 30 16 SUCCESS 43 706 575 100 63 SUCCESS

Precipitation treatment time threshold is a value obtained by0 (8.7-0.012x1p) 1

[00871

[Table 3-3] Nt. Quenching retention Actual quenching Quenching retention timeetq Formula (6) Tempering N. temperature Tq ('C) retentiontime (nin) by Formula (6) (min) evaluation tepertr (10 1920 60 52 SUCCESS 520 2 930 60 41 SUCCESS 520 3 910 120 80 SUCCESS 550 4 930 60 36 -SUCCESS 510 910 120 75 SUCCESS 500 6 900 t20 105 SUCCESS 550 7 950 40 26 SUCCESS 500 8 940 40 29 SUCCESS 530 9 900 120 107 -SUCCESS 500 930 40 40 SUCCESS 520 11 940 40 29 SUCCESS 500 12 910 s0 75 SUCCESS 550 13 910 80 76 SUCCESS 500 14 940 40 25 SUCCESS 550 I5 930 40 38 SUCCESS 500 16 900 120 104 SUCCESS 550 17 930 120 40 SUCCESS 550 18 950 40 2t SUCCESS 550 19 900 120 102 SUCCESS 550 930 40 34 SUCCESS 550 21 930 80 38 SUCCESS 500 22 930 40 34 SUCCESS 550 23 910 80 71 SUCCESS 500 24 950 100 29 SUCCESS 550 910 80 77 SUCCESS 550 26 910 80 79 SUCCESS 500 27 950 40 25 SUCCESS 550 28 930 80 37 SUCCESS 550 29 930 40 33 SUCCESS 550 930 60 35 SUCCESS 500 31 930 60 40 SUCCESS 550 32 950 40 20 -SUCCESS 500 33 930 40 37 SUCCESS 550 34 910 100 76 -SUCCESS 550 930 60 40 SUCCESS 520 36 910 120 80 SUCCESS 520 37 930 60 36 SUCCESS 530 38 920 60 52 SUCCESS 500 39 890 160 141 SUCCESS 540 960 40 30 SUCCESS 520 41 910 40 79 FAILlURE 530 42 910 120 75 SUCCESS 480 43 900 120 107 SUCCESS 560

[00881

[Table 4] Prior ygrain 1113surface I413 middle E2c No. Martensite +Bainite (area%) size layer portion (J)~U Note (P M) (141) (HB) 1____ 100 62 371 355 81 2 100 66 383 369 48 3 100 51 358 353 102 4 1100 71 363 352 65 100 73 360 356 49 Ivention 6 100 72 368 354 80 Examples 7 100 71 361 352 79 8 100 56 368 363 67 9 100 59 370 361 50 100 62 375 362 53 1I 100 69 348 341 75 12 100 72 378 355 39 13 100 67 359 348 81t 14 100 59 365 356 41t 100 68 363 346 48 16 100 70 361 358 21t 17 100 64 363 356 39 18 100 73 371 353 45 19 100 71 357 352 45 100 72 359 355 37 21 99 60 369 348 19 22 100 54 358 353 45 23 99 63 361 346 42 24 100 62 359 357 39 100 71 366 359 39 26 98 90 375 348 32 .naatv 27 100 56 360 353 46 Eape 28 100 91 355 350 31 Eape 29 100 77 357 352 45 95 56 372 349 21 31 100 78 360 353 41 32 97 75 367 346 31 33 100 68 358 354 41 34 100 59 359 356 39 100 85 361 352 39 36 100 88 358 351 45 37100 61 357 350 44 38 100 89 371 350 46 39 97 65 361 348 33 100 86 354 350 42 41t 99 55 360 346 43 42 100 74 372 365 33 43 100 51 351 343 71 _______

[0089]

Test Nos. 1 to 10 satisfy the chemical composition range of the present

invention and suitable manufacturing conditions. The total area ratio of martensite

and bainite is 99% or more in all the steels, the average value of the prior austenite

grain size in the middle portion is 80 m or less, and the surface layer hardness, the

middle portion hardness, the middle portion impact absorbed energy satisfy the targets.

[0090]

In Test Nos. 11 and 12, C deviates from the chemical composition range of the

present invention. In Test No. 11, C is insufficient, and the hardness at the time of

quenching is insufficient, so that the hardness cannot satisfy the target value even after

tempering. On the other hand, Test No. 12 is an example in which C is excessive, and

the impact absorbed energy is low due to the influence of precipitation of hard carbides

which are fracture origins.

[0091]

In Test Nos. 13 and 14, Si deviates from the chemical composition range of

the present invention. In Test No. 13, Si is insufficient, and the hardenability cannot

be secured, so that the middle portion hardness cannot satisfy the target value. On the

other hand, Test No. 14 is an example in which Si is excessive, and although the

hardness is sufficient, the impact absorbed energy does not satisfy the target due to the

promotion of temper embrittlement by Si.

[0092]

In Test Nos. 15 and 16, Mn deviates from the chemical composition range of

the present invention. In Test No. 15, Mn is insufficient, and the hardness at the time

of quenching is insufficient, so that the middle portion hardness cannot satisfy the

target value even after tempering. On the other hand, Test No. 16 is an example in which Mn is excessive, and the impact absorbed energy does not satisfy the target value due to the promotion of temper embrittlement.

[0093]

In Test No. 17, the P content is high outside the chemical composition range

of the present invention, and although the hardness is sufficient, the impact absorbed

energy does not satisfy the target due to the embrittlement caused by P.

[0094]

In Test No. 18, the S content is high outside the chemical composition range

of the present invention. Therefore, in Test No. 18, the impact absorbed energy

cannot satisfy the target due to the formation of MnS, which is a stretched inclusion.

[0095]

In Test No. 19, the Cu content was high outside the chemical composition

range of the present invention, and the precipitated metal Cu became the brittle fracture

origin. Therefore, in Test No. 19, the impact absorbed energy does not satisfy the

target.

[0096]

In Test No. 20, the Ni content is low outside the chemical composition range

of the present invention, and does not satisfy the amount that improves the toughness.

Therefore, in Test No. 20, the impact absorbed energy does not satisfy the target.

[0097]

Test Nos. 21 and 22 are examples in which Cr deviates from the chemical

composition range of the present invention. In test No. 21, Cris insufficient, and

sufficient hardenability and precipitation strengthening action are not obtained. From

this, in the test No. 21, the middle portion hardness does not satisfy the target, and

furthermore, the impact absorbed energy does not achieve the target. On the other hand, in Test No. 22, Cr was excessive, and the influence of precipitation of coarse Cr carbides was significant. As a result, in Test No. 22, the impact absorbed energy does not satisfy the target.

[0098]

Test Nos. 23 and 24 are examples in which Mo deviates from the chemical

composition range of the present invention. In Test No. 23, Mo is insufficient, and

sufficient hardenability and precipitation strengthening action are not obtained. From

this, in Test No. 23, the middle portion hardness does not satisfy the target, and the

impact absorbed energy does not achieve the target. On the other hand, in Test No.

24, Mo was excessive, and the influence of precipitation of coarse Mo carbides was

significant. Accordingly, in Test No. 24, the impact absorbed energy does not satisfy

the target value.

[0099]

In Test No. 25, V was high outside the chemical composition range of the

present invention, and coarse carbides and nitrides of V and the like became the brittle

fracture origin. From this, in Test No. 25, the impact absorbed energy does not satisfy

the target.

[0100]

Test Nos. 26 and 27 are examples in which Al deviates from the chemical

composition range of the present invention. Test No. 26 is an example in which Al is

insufficient, AIN effective for austenite pinning could not be secured, and an excess of

N was bonded B, so that the effect of improving the hardenability was reduced. For

this reason, in Test No. 26, the structures other than martensite and bainite were

excessive, and the grain size of retained austenite was coarsened. As a result, in Test

No. 26, the middle portion hardness and the impact absorbed energy cannot satisfy the targets. On the other hand, Test No. 27 is an example in which Al is excessive, and

AIN was excessively coarsened and became a brittle fracture origin. Therefore, in

Test No. 27, the impact absorbed energy cannot satisfy the target.

[0101]

Test Nos. 28 and 29 are examples in which N deviates from the chemical

composition range of the present invention. Test No. 28 is an example in which N is

insufficient and Al x N is less than a predetermined range, and the amount of nitrides,

carbonitrides, and the like formed was insufficient, so that the austenite pinning effect

was weak and grain coarsening had occurred. As a result, in Test No. 28, the impact

absorbed energy cannot satisfy the target. On the other hand, Test No. 29 is an

example in which N is excessive, and excessive coarsening of nitrides, carbonitrides,

and the like occurs. As a result, in Test No. 29, the impact absorbed energy cannot

satisfy the target.

[0102]

In Test Nos. 30 and 31, B deviates from the chemical composition range of

the present invention. Test No. 30 is an example in which B was insufficient, and the

amount of solute B necessary for hardenability could not be secured. As a result, in

Test No. 30, the structures other than martensite and bainite are excessive, and the

middle portion hardness and impact absorbed energy cannot satisfy the targets. On

the other hand, Test No. 31 is an example in which B is excessively contained, and

carboborides of metal elements are precipitated, so that the impact absorbed energy

cannot satisfy the target.

[0103]

In Test No. 32, although the composition range of each alloying element is

within the range of the present invention, Ceq is low outside the suitable scope of the present invention. In Test No. 32, as a result of the formation of ferrite in the structure due to the decrease in hardenability, the middle portion hardness and impact absorbed energy cannot satisfy the targets.

[0104]

In Test Nos. 33 and 34, although the composition range of each alloying

element and Ceq are within the ranges of the present invention, the parameter formula

4 x f/g is low outside the suitable range of the present invention. In Test Nos. 33 and

34, the quenching effect of the precipitated elements was larger than the improvement

of the hardenability. Therefore, in Test Nos. 33 and 34, the impact absorbed energy

cannot satisfy the target.

[0105]

In Test No. 35, although various indexes derived from the composition range

of each alloying element and the chemical composition are within the ranges of the

present invention, the heating temperature before rolling is less than the solid solution

temperature Ts. In Test No. 35, undissolved coarse AIN remained and became a

brittle fracture origin. Therefore, in Test No. 35, the prior austenite grain size is

coarsened, and the absorbed energy cannot satisfy the target.

[0106]

In Test Nos. 36 and 37, although various indexes derived from the

composition range of each alloying element and the chemical composition are within

the ranges of the present invention, the precipitation treatment temperature deviates

from the suitable range of the present invention. Test No. 36 is an example in which

the precipitation treatment temperature was low, and AlN was not sufficiently

precipitated, so that AIN effective for austenite pinning could not be secured. From

this, in Test No. 36, the prior austenite grain size is coarsened, and the absorbed energy cannot satisfy the target. On the other hand, Test No. 37 is an example in which the precipitation treatment temperature exceeded Ac1, and the coarsening of AIN had occurred locally due to the retention in the a-y dual region. Therefore, in Test No. 37, the absorbed energy cannot satisfy the target.

[0107]

Test No. 38 is an example in which, although various indexes derived from

the composition range of each alloying element and the chemical composition were

within the ranges of the present invention, the temperature and time of the precipitation

treatment did not satisfy Formula (5), which is a suitable range of the present invention.

In Test No. 38, AIN was not sufficiently precipitated, so that AIN effective for austenite

pinning could not be secured. From this, in Test No. 38, the prior austenite grain size

is coarsened, and the absorbed energy cannot satisfy the target.

[0108]

Test No. 39 is an example in which, although various indexes derived from

the composition range of each alloying element and the chemical composition are

within the ranges of the present invention, the quenching temperature is less than the

suitable range of the present invention. In Test No. 39, solutionizing of the alloying

elements was not sufficiently performed, so that the hardenability was low and ferrite

was excessively formed. Accordingly, in Test No. 39, the middle portion hardness

and the absorbed energy cannot satisfy the targets.

[0109]

Test No. 40 is an example in which, although various indexes derived from

the composition range of each alloying element and the chemical composition are

within the ranges of the present invention, the quenching retention temperature Tq

exceeds the suitable range of the present invention. Test No. 40 resulted in excessive coarsening of grains. From this, in Test No. 40, the prior austenite grain size is coarsened, and the absorbed energy cannot satisfy the target.

[0110]

Test No. 41 is an example in which, although various indexes derived from

the composition range of each alloying element and the chemical composition are

within the ranges of the present invention, the actual quenching retention time is less

than the quenching retention time tq, which is the suitable range of the present

invention, and solutionizing of the alloying elements was not sufficiently performed.

From this, in Test No. 41, the hardenability was low and ferrite was excessively formed.

As a result, in Test No. 41, the middle portion hardness and the absorbed energy cannot

satisfy the targets.

[0111]

Test No. 42 is an example in which, although various indexes derived from

the composition range of each alloying element and the chemical composition are

within the ranges of the present invention, the tempering temperature is less than the

suitable range. In Test No. 42, temper embrittlement had occurred. From this, in

Test No. 42, the absorbed energy cannot satisfy the target.

[0112]

Test No. 43 is an example in which, although various indexes derived from

the composition range of each alloying element and the chemical composition are

within the ranges of the present invention, the tempering temperature is more than the

suitable range. In Test No. 43, the precipitation quenching effect of the alloy carbides

was reduced. Therefore, in Test No. 43, the middle portion hardness does not satisfy

the target.

[Brief Description of the Reference Symbols]

[0113]

1 steel plate

11 thickness middle portion

12 surface layer

13 rolled surface

[Document Type] CLAIMS

1. A steel plate comprising, as a chemical composition, by mass%:

C: 0.16% to 0.20%;

Si: 0.50% to 1.00%;

Mn: 0.90% to 1.50%;

P: 0.010% or less;

S: 0.0020% or less;

Cu: 0% to 0.40%;

Ni: 0.20% to 1.00%;

Cr: 0.60% to 0.99%;

Mo: 0.60% to 1.00%;

V: 0% to 0.050%;

Al: 0.050% to 0.085%;

N: 0.0020% to 0.0070%;

B: 0.0005% to 0.0020%;

Nb: 0% to 0.050%;

Ti: 0% to 0.020%;

Ca: 0% to 0.0030%;

Mg: 0% to 0.0030%;

REM: 0% to 0.0030%; and

a remainder including Fe and impurities,

wherein a total area ratio ofmartensite and bainite in a thickness middle

portion is 99% or more,

an average value of a prior austenite grain size in the thickness middle portion

is less than 80 tm,

Ceq represented by Formula (1) is 0.750% to 0.800%,

Al x N is 2.0 x 10-4 or more,

Ti/N is 3.4 or less,

a value f represented by Formula (2) and a value g represented by Formula (3)

satisfy 4 x f/g > 9.00,

a -20°C Charpy absorbed energy measured in a C direction in the thickness

middle portion is 47 J or more,

hardnesses of a surface layer and the thickness middle portion are HB 350 or

more, and

a plate thickness of the steel plate is more than 200 mm,

Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5: Formula (1)

f =4 x C + Si + 2 x Mn + Ni + 2 x Cr + 5 x Mo: Formula (2)

g 2 x Cr + 3 x Mo + 5 x V: Formula (3)

where each element symbol described in each of the formulas means an

amount of an element corresponding to the element symbol in unit mass%.

2. A method of manufacturing the steel plate according to claim 1, the

method comprising:

heating a slab;

hot rolling the slab to obtain a steel plate having a plate thickness of more

than 200 mm;

cooling the steel plate;

performing a precipitation treatment on the steel plate;

quenching the steel plate; and

tempering the steel plate, wherein the slab includes, as a chemical composition, by unit mass%, C:

0.16% to 0.20%, Si: 0.50% to 1.00%, Mn: 0.90% to 1.50%, P: 0.010 % or less, S:

0.0020% or less, Cu: 0% to 0.40%, Ni: 0.20% to 1.00%, Cr: 0.60% to 0.99%, Mo:

0.60% to 1. 00%, V: 0% to 0.050%, Al: 0.050% to 0.085%, N: 0.0020% to 0.0070%,

B: 0.0005% to 0.0020%, Nb: 0% to 0.050%, Ti: 0% to 0.020%, Ca: 0% to 0.0030%,

Mg: 0% to 0.0030%, REM: 0% to 0.0030%, and a remainder including Fe and

impurities, Ceq represented by Formula (1) of the slab is 0.750% to 0.800%, Al x N of

the slab is 2.0 x 10-4 or more, Ti/N of the slab is 3.4 or less, and a value f of the slab

represented by Formula (2) and a value g of the slab represented by Formula (3) satisfy

4 x f/g > 9.00,

a slab heating temperature in the heating the slab is equal to or more than an

AIN solid solution temperature Ts (C) calculated by Formula (4),

the precipitation treatment is performed on the steel plate by heating the steel

plate to a precipitation treatment temperature Tp (C) of more than 550°C and less than

Ac and retaining the steel plate at this temperature for a precipitation treatment time

tp (hour), the precipitation treatment temperature Tp (C) and the precipitation

treatment time tp (hour) satisfy Formula (5), and the Ac is represented by Formula (7),

the quenching is performed on the steel plate by heating the steel plate to a

quenching retention temperature Tq (°C) of 900°C to 950°C, retaining the steel plate at

this temperature for a quenching retention time tq (minute) or more represented by

Formula (6), and water cooling the steel plate, and

the tempering is performed on the steel plate by heating the steel plate to a

tempering temperature of 500°C to 550°C and cooling the steel plate to 150°C or less,

Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5: Formula (1)

f = 4 x C + Si + 2 x Mn + Ni + 2 x Cr + 5 x Mo: Formula (2)

Claims (1)

1. g = 2 x Cr + 3 x Mo + 5 x V: Formula (3)

   Ts = 7400 / (1.95 - logio(Al x N)) - 273: Formula (4)

   Logio(tp) + 0.012 x Tp > 8.7: Formula (5)

   tq 0.033 x (950 - Tq) 2 +(1.5 x f)2/10: Formula (6)

   Acl = 750 - 25 x C + 22 x Si - 40 x Mn-30 x Ni + 20 x Cr + 25 x Mo:

   Formula (7)

   where an each element symbol described in each of the formulas means an

   amount of an element corresponding to the element symbol in unit mass%.

   3. The method of manufacturing the steel plate according to claim 2,

   wherein, a cooling finishing temperature in the cooling the steel plate is

   150 0C or less.