JP6682967B2 - Steel plate and method of manufacturing the same - Google Patents

Description

  The present invention relates to a thick steel plate and a manufacturing method thereof. In particular, the present invention relates to a thick steel plate having a brittle crack propagation stopping property (hereinafter, also referred to as arrestability) and a plate thickness of more than 70 mm, and a manufacturing method thereof.

  For ships, offshore structures, cold storage tanks, and large structures represented by construction and civil engineering structures, it is necessary to ensure safety against destruction. In particular, once brittle fracture occurs, the fracture progresses at high speed and over a long range, which can have a great impact on the environment and economy.

  In recent years, with respect to container ships, the demand for large container ships exceeding 10,000 TEU has been increasing for the purpose of improving the efficiency of marine transportation. Therefore, steel materials used for container ships are required to have high strength and thick wall. Especially for steel materials used for upper deck or hatchside coaming on the upper hull, which is an important member of container vessels, even if brittle fracture occurs, high-strength thick-walled materials with arrest characteristics are added to stop the progress of brittle fracture. The application of wood is progressing. Generally, as strength increases and plate thickness increases, resistance to the occurrence and propagation of brittle fracture decreases. Therefore, it is necessary to develop a steel material that is more sophisticated than the steel materials that have been conventionally used.

  For example, in Patent Document 1, the area ratio of each structure and the crystal grain boundary density corresponding to the crystal grain size are defined, and the structure fraction having the {100} plane of ¼t part and the {t of 1 / 2t part} A high-strength thick steel sheet that secures good arrest characteristics by defining a structural fraction having a 110} plane is disclosed.

  In Patent Document 2, the grain size and hardness of the microstructure and the texture formed in the surface layer, and the grain size of the central portion are defined, and further, the area ratio of the (100) plane at each plate thickness position is defined. Therefore, a high-strength thick steel plate ensuring good arrest characteristics is disclosed.

  In Patent Document 3, the existence ratio of coarse grains formed in the microstructure and the surface layer and the grain size of the central portion are defined, and further, the area ratio of the (100) plane at each plate thickness position is defined. , A high-strength steel plate with good arrest characteristics is disclosed.

  Patent Document 4 discloses a thick steel plate that defines a microstructure and a grain size at the center of the plate thickness and ensures good arrest characteristics. At the time of manufacturing a thick steel plate, it is characterized by giving an optimum reduction while controlling the temperature of the central portion of the plate thickness.

  Patent Document 5 discloses a thick high-strength steel sheet that regulates the crystal grain size of the surface layer and the central portion of the plate thickness and the texture and ensures good arrest characteristics.

  In Patent Document 6, the microstructure and the coarse ferrite in the surface layer are suppressed, the size of cementite is further controlled, and the maximum value of the effective crystal grain size necessary to satisfy the arrest characteristics is determined from the Ni content and the plate thickness. A thick high-strength steel sheet that ensures good arrest characteristics by calculation is disclosed. Similarly, at the time of manufacturing a thick steel plate, the required rolling temperature is specified from the amount of Ni and the plate thickness.

  Patent Document 7 discloses a method for manufacturing a thick high-strength steel sheet that ensures good arrest characteristics by defining the temperature rising rate during tempering, the surface and internal temperature states, and the tempering temperature. .

  Patent Document 8 is a method for producing a thick high-strength steel sheet that is industrially stable and can be efficiently produced, and by regulating the time from accelerated cooling to tempering, good arrest characteristics are ensured. There is disclosed a method for producing the thick and high strength steel sheet.

  Patent Document 9 discloses a method for manufacturing a thick high-strength steel sheet that ensures good arrest characteristics by performing optimal reduction at an optimal temperature in primary finish rolling and secondary finish rolling.

  In Patent Document 10, rolling at a relatively low temperature, and further, by controlling the surface temperature of the steel sheet after finish rolling to refine the crystal grains in the surface layer, production of a thick high-strength steel sheet ensuring good arrest characteristics. A method is disclosed.

International Publication No. 13/150687 JP, 2013-221190, A JP, 2013-221189, A JP, 2012-172258, A JP, 2011-214116, A JP, 2008-248382, A JP, 2011-52244, A JP, 2011-52243, A JP, 2008-261030, A JP, 2005-25205, A

  Generally, as the plate thickness increases, the hardenability of the central part of the plate thickness decreases, so that the required strength cannot be obtained. In the region where the plate thickness is 70 mm or less, the insufficient hardenability can be compensated by increasing the cooling rate. However, when the plate thickness exceeds 70 mm, the cooling rate in the central part of the plate thickness is determined depending on the plate thickness. Therefore, it has been necessary to find a microstructure and heating and rolling conditions for imparting better arrest characteristics while ensuring strength by optimizing the components.

  In order to ensure the arrest characteristics, it is generally considered desirable to promote the refinement of crystal grains. However, when the plate thickness exceeds 70 mm, it is difficult to secure the arrestability only by adjusting the components and manufacturing conditions in order to realize the conventionally known crystal grain size, and it is difficult to secure the arrestability from the surface layer to the inside of the plate thickness. It was necessary to precisely control the microstructure up to.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a thick steel plate having excellent arrestability and a plate thickness of more than 70 mm, and a manufacturing method thereof.

  As a result of intensive studies to solve the above problems, the present inventors have obtained the following findings.

  First, the components were reviewed to increase the strength and thickness of steel sheets.

  Based on the steel materials used for offshore structures, the influence of the components that increase the strength and toughness were examined using steel materials with a plate thickness of 100 mm. When the relationship between the added amount and toughness of Ni, Mo and V was investigated, Ni did not show a decrease in toughness regardless of the added amount, but Mo and V showed a marked decrease in toughness when the added amount increased. From this, it was found that Ni is the most suitable element for improving the strength and toughness of the extremely thick steel material. However, it has been found that Mo and V can not only increase the strength but also ensure a certain toughness by limiting the addition amount. When vTrs has an allowable value up to −80 ° C., the upper limit value of V content is 0.15% and the upper limit value of Mo content is 0.35%.

  Next, regarding the relationship between the carbon equivalent (Ceq.) Required to satisfy the strength standard of EH47 and the strength of the central portion of each plate thickness, using a steel material having a plate thickness of 70 to 100 mm to which various components have been added The rolling conditions and the tempering conditions were changed and investigated. As a result, the strength standard of EH47 can be satisfied by setting the Ceq. To 0.40 or more when the plate thickness is 70 mm and setting the Ceq. To 0.47 or more when the plate thickness is 100 mm. Do you get it. It was also found that when Ceq. Is 0.52 or more, the strength becomes unnecessarily high and the toughness decreases.

  Further, the inventors of the present invention focused on the structure and the grain size thereof in order to secure arrest characteristics in a thick steel plate having a plate thickness of more than 70 mm. Generally, it is said that the arrest property is improved by decreasing the crystal grain size. Further, in a thick steel plate having a plate thickness of more than 70 mm, it is difficult to rapidly cool the inside of the steel plate, and therefore, a ferrite structure is necessarily developed inside the steel plate. Therefore, the relationship between the effective crystal grain size, the ferrite fraction and the bainite grain size was investigated for a thick steel plate having a ferrite-bainite structure and a plate thickness exceeding 70 mm.

  FIG. 1 is a graph showing the relationship between the ferrite fraction and the average effective crystal grain size. As can be seen from FIG. 1, the effective crystal grain size is refined due to the increase in the ferrite fraction. Increasing the ferrite fraction is effective for obtaining good arrest characteristics. However, an excessive increase in the ferrite fraction causes a decrease in strength. Therefore, in the thick and high strength material, it is not effective to reduce the effective crystal grain size by increasing the ferrite fraction. Therefore, attention was paid to the relationship between the bainite grain size and the effective crystal grain size. FIG. 2 is a graph showing the relationship between the bainite grain size and the average effective crystal grain size. As can be seen from FIG. 2, the effective crystal grain size becomes finer as the bainite grain size becomes finer. Therefore, it was considered that the arrest characteristics could be remarkably improved if a certain amount of ferrite satisfying the steel plate strength was secured and the bainite grain size was made fine. Therefore, with respect to the refinement of the bainite structure, the CR rate was examined using a material having the same composition and a slab thickness of 305 mm. As a result, the bainite grain size was not significantly refined as the CR rate increased. This is because, in order to secure the CR rate, it is necessary to secure the reduction amount of the primary rolling performed in the recrystallization region, but since the reduction amount could not be secured, the austenite grains did not become fine, and the CR This is probably because the effect of increasing the rate was not obtained. Therefore, using the same slab as before, we examined the effect of heating temperature. FIG. 3 is a graph showing the relationship between the heating temperature and the average grain size of bainite and ferrite in the central portion of the plate thickness. As can be seen from FIG. 3, when a heating steel plate having a thickness of 80 mm was produced by changing the heating temperature, the ferrite grain size was almost constant, but the average bainite grain size was remarkably refined as the heating temperature was lowered.

Further, using a slab having the same composition, a thick steel plate having a plate thickness of 80 mm was manufactured under different manufacturing conditions, and an ESSO test was carried out. As a result, the ferrite fraction, the ferrite grain size, and the effective crystal grain size did not change. Nevertheless, there were cases where the arrest characteristics were significantly different. For example, with respect to the thick steel plate which was slab heated at 1050 ° C. and finish-rolled at 730 ° C. or 780 ° C., the ferrite fraction and the ferrite grain size were almost unchanged. Also, regarding the effective crystal grain size, in a thick steel plate subjected to finish rolling at 780 ° C., 19.0 μm (1/4 t part when the plate thickness is t, hereinafter simply referred to as “1/4 t”), 24. 0 μm (1/2 t part when the plate thickness is t, hereinafter simply referred to as “1/2 t”), 17.6 μm (1/4 t), 22.8 μm (1 / 2t) was not so different. However, for the arrest characteristics, Kca value respectively ^596N / mm 1.5 at -10 ° C., was 7007N ^{/ mm 1.5,} became one digit different results. Then, when these thick steel plates were examined in detail, it was found that the structures of the surface layers of the thick steel plates were significantly different. In the thick steel plate of 730 ° C finish rolling with high arrest characteristics, the structure has a flat structure along the rolling direction, and this structure prevents the thick steel plate from cracking at the initial stage. it is conceivable that.

  Therefore, in order to obtain a thick steel sheet having a high strength and high arrestability and a thickness exceeding 70 mm, in addition to defining the ferrite fraction, bainite grain size and effective crystal grain size, the surface layer portion of the thick steel plate It was found necessary to specify.

  The present invention has been made on the basis of the above findings, and its gist resides in the thick steel plate and the manufacturing method thereof described below.

(1) A thick steel plate having a thickness of more than 70 mm,
The chemical composition is% by mass,
C: 0.04 to 0.12%,
Si: 0.05 to 0.50%,
Mn: 1.30 to 2.20%,
P: 0.020% or less,
S: 0.010% or less,
Cu: 0.05 to 1.00%,
Ni: 0.05-1.50%,
Nb: 0.005 to 0.050%,
Ti: 0.005 to 0.050%,
sol. Al: 0.005-0.060%,
N: 0.001-0.010%,
Cr: 0 to 0.50%,
Mo: 0 to 0.35%,
V: 0 to 0.15%,
B: 0 to 0.0030%,
Ca: 0 to 0.010%,
Mg: 0 to 0.0050%,
REM: 0 to 0.0050%, and
The balance: Fe and impurities,
Ceq. Represented by the following formula (i). Is 0.40 to 0.52, and
A thick steel plate that satisfies the following (a) to (d).
(A) A structure within 5 mm of the surface layer forms a structure elongated in the rolling direction, and the average aspect ratio of this structure is 1.5 or more.
(B) The microstructure inside the steel sheet has a composite structure of ferrite and bainite, the ferrite fraction of 1/4 t part of the plate thickness is 15.0 to 40.0%, and the ferrite content of 1/2 t part of the plate thickness. The ratio is 10.0 to 40.0%, and the structure other than ferrite and bainite has a total of less than 5% (including 0%) in area% at each plate thickness position.
(C) The average bainite particle size of 1/4 t part of the plate thickness is 25.0 μm or less, and the average bainite particle size of 1/2 t part of the plate thickness is 35.0 μm or less.
(D) The average effective crystal grain size of 1/4 t part of the plate thickness is 22.0 μm or less, and the average effective crystal grain size of 1/2 t part of the plate thickness is 32.0 μm or less.
Ceq. = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (i)

(2) The chemical composition is represented by mass%
Cr: 0.05 to 0.50%,
Mo: 0.05 to 0.35%, and
V: 0.005-0.15%,
The thick steel plate according to (1) above, containing one or more selected from the following.

(3) the chemical composition is expressed in mass%;
B: 0.0003 to 0.0030%,
The thick steel plate according to (1) or (2) above, which comprises:

(4) The chemical composition is represented by mass%,
Ca: 0.0005 to 0.010%,
Mg: 0.0005 to 0.0050%, and
REM: 0.0005 to 0.0050%,
The thick steel plate according to any one of (1) to (3), which contains one or more selected from the following.

(5) A method for manufacturing the thick steel sheet according to any one of (1) to (4) above,
Heating a steel slab having the composition according to any one of (1) to (4) so that the center part of the plate thickness is Ac _{3 to} less than 1000 ° C.,
In the temperature range in which the sheet thickness center portion is Ac _{3 to} less than 1000 ° C., after performing rough rolling at a cumulative reduction ratio of 15.0 to 60.0% and an average reduction ratio of each pass of 3.5% or more,
In the temperature range where the plate thickness center portion is Ar _{3 to} 950 ° C., finish rolling is performed with a cumulative reduction of 40% or more and an average reduction of 5.0% or more in each pass.
Furthermore, the final pass start temperature of this finish rolling is set to Ar ₃ −20 ° C. to Ar ₃ + 30 ° C. on the surface of the plate thickness to complete the rolling,
Next, a method for manufacturing a thick steel sheet, in which accelerated cooling is started and the surface temperature is accelerated cooled to 550 ° C or lower.

  (6) The method for manufacturing a thick steel sheet according to (5), wherein after the accelerated cooling, tempering treatment is performed at a temperature of 350 to 650 ° C.

  According to the present invention, it is possible to provide a thick steel plate having excellent arrestability and a plate thickness of more than 70 mm, and a manufacturing method thereof.

FIG. 1 is a graph showing the relationship between the ferrite fraction and the average effective crystal grain size. FIG. 2 is a graph showing the relationship between the bainite grain size and the average effective crystal grain size. FIG. 3 is a graph showing the relationship between the heating temperature and the average grain size of bainite and ferrite in the central portion of the plate thickness.

  Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition The effects of each element and the reasons for limiting the content are as follows. In addition, in the following description, "%" regarding the content means "mass%".

C: 0.04 to 0.12%
C is an element that enhances the strength of the steel material. If the C content is less than 0.04%, this effect cannot be obtained. On the other hand, when the C content exceeds 0.12%, the toughness decreases due to the increase in strength, the weldability deteriorates, and the toughness of the heat affected zone (HAZ: Heat Affected Zone) deteriorates. Moreover, the arrest characteristics are deteriorated. Therefore, the C content is 0.04 to 0.12%. The C content is preferably 0.05% or more, and preferably 0.09% or less.

Si: 0.05 to 0.50%
Si is a deoxidizing element and an element effective for strength. If the Si content is less than 0.05%, these effects cannot be obtained. On the other hand, when the Si content exceeds 0.50%, the HAZ is hardened and the toughness is reduced. Therefore, the Si content is set to 0.05 to 0.50%. The Si content is preferably 0.10% or more, and preferably 0.30% or less.

Mn: 1.30 to 2.20%
Mn is an element that enhances the hardenability of steel and enhances the strength and toughness of steel. If the Mn content is less than 1.30%, these effects cannot be obtained. On the other hand, when the Mn content exceeds 2.20%, the center segregation becomes remarkable and the toughness at the center part of the plate thickness remarkably decreases. Moreover, the arrest characteristics are deteriorated. Therefore, the Mn content is set to 1.30 to 2.20%. The Mn content is preferably 1.60% or more, and preferably 2.00% or less.

P: 0.020% or less P is an impurity element and deteriorates the mechanical properties of the steel material, and particularly deteriorates the low temperature toughness. Therefore, the P content is 0.020% or less. The P content is preferably 0.015% or less, more preferably as low as possible.

S: 0.010% or less S is an impurity element and combines with Mn to form MnS, which lowers the low temperature toughness of the steel material. Therefore, the S content is 0.010% or less. The S content is preferably 0.005% or less, more preferably as low as possible.

Cu: 0.05-1.00%
Cu is an element capable of forming a solid solution in steel to increase strength without impairing toughness and improving arrest characteristics. If the Cu content is less than 0.05%, these effects cannot be obtained. On the other hand, if the Cu content exceeds 1.00%, the precipitation characteristics increase due to an increase in precipitates, and further, minute cracks occur on the surface during hot working. Therefore, the Cu content is 0.05 to 1.00%. The Cu content is preferably 0.20% or more, and preferably 0.50% or less.

Ni: 0.05-1.50%
Ni is an element capable of forming a solid solution in steel to enhance strength without impairing toughness and improving arrest characteristics. If the Ni content is less than 0.05%, these effects cannot be obtained. On the other hand, Ni is an expensive element, and excessive addition causes an increase in cost. Therefore, the Ni content is 0.05 to 1.50%. The Ni content is preferably 0.30% or more, and preferably 1.10% or less.

Nb: 0.005-0.050%
Nb is an important element in the steel sheet of the present invention. Nb expands the non-recrystallized austenite region by adding a trace amount, and contributes to the improvement of strength and arrest properties due to the refinement of the structure. Further, it contributes to transformation strengthening and precipitation strengthening. If the Nb content is less than 0.005%, the above effect cannot be obtained. On the other hand, when the Nb content exceeds 0.050%, coarse precipitates are formed, and not only the arrest characteristics are deteriorated but also the HAZ toughness is significantly deteriorated. Therefore, the Nb content is set to 0.005 to 0.050%. The Nb content is preferably 0.007% or more, and preferably 0.020% or less.

Ti: 0.005-0.050%
Ti is an important element in the steel sheet of the present invention. Ti is an element that forms TiN and suppresses an increase in the austenite grain size during heating of the steel slab. The larger the austenite grain size, the larger the grain size of bainite after transformation. Therefore, in order to obtain the desired bainite grain size, the Ti content is set to 0.005% or more. When the Ti content exceeds 0.050%, TiC is generated and the toughness is reduced. Therefore, the Ti content is 0.050% or less. The Ti content is preferably 0.007% or more, and preferably 0.020% or less.

sol. Al: 0.005-0.060%
sol. Al is an element necessary for deoxidizing steel. For deoxidation, sol. The Al content is required to be 0.005% or more. On the other hand, sol. If the Al content exceeds 0.060%, not only coarse inclusions increase and the arrest characteristics deteriorate, but also the toughness of the HAZ decreases. Therefore, sol. The Al content is 0.005-0.060%. sol. The Al content is preferably 0.010% or more, and preferably 0.050% or less.

N: 0.001-0.010%
N is an element that combines with Ti to form TiN and has an action of suppressing coarsening of austenite grains. If the N content is less than 0.001%, this effect cannot be obtained. On the other hand, if the N content exceeds 0.010%, it is present as an impurity, resulting in a decrease in toughness. As a result, the arrest characteristics are deteriorated. Therefore, the N content is set to 0.001 to 0.010%. The N content is preferably 0.002% or more, and preferably 0.006% or less.

Cr: 0 to 0.50%
Since Cr is an element that enhances the strength of the steel material, it may be contained if necessary. When the Cr content exceeds 0.50%, the toughness is significantly reduced as the strength of the steel material is increased. Therefore, the Cr content is 0.50% or less. On the other hand, if the Cr content is less than 0.05%, the strength of the steel material may not be sufficiently increased. Therefore, the Cr content is preferably 0.05% or more.

Mo: 0 to 0.35%
Mo is an element that enhances the strength of the steel material, and thus may be contained if necessary. When the Mo content exceeds 0.35%, the toughness is significantly reduced as the strength of the steel material is increased. Moreover, the arrest characteristics are deteriorated. Therefore, the Mo content is 0.35% or less. On the other hand, if the Mo content is less than 0.05%, the strength of the steel material may not be sufficiently increased. Therefore, the Mo content is preferably 0.05% or more.

V: 0 to 0.15%
V forms a carbonitride and has a function of precipitating and strengthening the steel material, so V may be contained if necessary. When the V content exceeds 0.15%, the toughness is significantly reduced due to precipitation strengthening. Therefore, the V content is 0.15% or less. On the other hand, if the V content is less than 0.005%, the steel material may not be sufficiently precipitation strengthened. Therefore, the V content is preferably 0.005% or more.

B: 0 to 0.0030%
B is an element that enhances the hardenability with a small amount of addition, and may be contained if necessary. If the B content exceeds 0.0030%, the effect is saturated and the HAZ toughness is reduced. Therefore, the B content is 0.0030% or less. On the other hand, if the B content is less than 0.0003%, the hardenability may not be stably improved. Therefore, the B content is preferably 0.0003% or more.

Ca: 0 to 0.010%
Ca is an element that improves the HAZ toughness and may be contained if necessary. If the Ca content exceeds 0.010%, the HAZ toughness and weldability deteriorate. Therefore, the Ca content is 0.010% or less. On the other hand, if the Ca content is less than 0.0005%, the HAZ toughness may not be stably improved. Therefore, the Ca content is preferably 0.0005% or more.

Mg: 0 to 0.0050%
Mg is an element that improves the HAZ toughness, and may be contained if necessary. If Mg exceeds 0.0050%, HAZ toughness and weldability deteriorate. Therefore, the Mg content is 0.0050% or less. On the other hand, if the Mg content is less than 0.0005%, the HAZ toughness may not be stably improved. Therefore, the Mg content is preferably 0.0005% or more.

REM: 0 to 0.0050%
REM is an element that improves the HAZ toughness, and may be contained if necessary. When REM exceeds 0.0050%, HAZ toughness and weldability deteriorate. Therefore, the REM content is 0.0050% or less. On the other hand, if the REM content is less than 0.0005%, it may not be possible to stably improve the HAZ toughness. Therefore, the REM content is preferably 0.0005% or more.

  The thick steel plate of the present invention contains the above elements, and the balance has a chemical composition of Fe and impurities. The "impurity" is a component that is mixed by ores, raw materials such as scrap, and various factors of the manufacturing process when industrially manufacturing steel, and is acceptable within a range that does not adversely affect the present invention. Means

Ceq. : 0.40 to 0.52
Carbon equivalent Ceq. Is represented by the following formula (i). Ceq. Is less than 0.40, quenching does not occur up to the center of the plate thickness, and high strength of yield strength 460 MPa or more cannot be obtained. Further, the toughness may decrease. On the other hand, Ceq. When the value exceeds 0.52, the required strength can be easily obtained, but the toughness and weldability deteriorate, and the cost also increases. Moreover, the arrest characteristics are deteriorated. Therefore, Ceq. Is 0.40 to 0.52.
Ceq. = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (i)

(B) Microstructure If any one of the following structural rules (a) to (d) is not satisfied, good strength and arrest properties cannot be obtained.

  (A) A structure within 5 mm of the surface layer forms a structure elongated in the rolling direction, and the average aspect ratio of this structure is 1.5 or more.

  The average aspect ratio, which is the ratio of the short axis to the long axis of the structure formed in the L section within 5 mm of the steel sheet surface layer, is set to 1.5 or more. Setting the aspect ratio of the structure formed in the L section within 5 mm of the steel sheet surface layer to 1.5 or more is one of the most important factors for obtaining good arrest characteristics in the present invention. When the aspect ratio is less than 1.5, the arrest characteristics are significantly deteriorated. When the aspect ratio is 1.5 or more, the formation of shear lips becomes good and the arrest characteristics are remarkably improved. Since the plate thickness of the steel sheet of the present invention exceeds 70 mm, the surface layer structure tends to be different from the internal structure. Although not particularly specified, the surface layer structure is preferably formed of any one of ferrite, bainite, martensite, tempered bainite and tempered martensite, or a mixed structure thereof.

  (B) The microstructure inside the steel sheet has a composite structure of ferrite and bainite, the ferrite fraction of 1/4 t part of the plate thickness is 15.0 to 40.0%, and the ferrite content of 1/2 t part of the plate thickness. The ratio is 10.0 to 40.0%, and the structure other than ferrite and bainite has a total of less than 5% (including 0%) in area% at each plate thickness position.

  In the present invention, it is necessary to adjust the structural fraction of ferrite and bainite in order to impart good strength to the steel sheet. The steel sheet of the present invention basically has a composite structure of ferrite and bainite. In the production of the steel sheet of the present invention, tempering may be performed, but bainite and tempered bainite, martensite and tempered martensite may be treated without distinction.

  Further, the tissue fraction also affects arrest characteristics. The bainite structure was evaluated using EBSD by superimposing the image quality image and the grain boundary image in which the boundary having a misorientation of 15 ° or more was defined as a grain boundary, and the bainite structure was coarser than the ferrite structure. It was found to exhibit crystal grains. From this, the structural fraction of ferrite and bainite is one of the factors that influence the effective crystal grain size of the steel sheet described later. In order to impart strength and appropriately control the effective crystal grain size to improve the arrest characteristics, the ferrite fraction in the 1 / 4t portion of the plate thickness is 15.0 to 40.0%, and the plate thickness is 1%. The ferrite fraction of the / 2t part needs to be 10.0 to 40.0%. The ferrite structure fractions are defined respectively for the 1/4 t portion of the sheet thickness and the 1/2 t portion of the sheet thickness, because when the sheet thickness exceeds 70 mm, the austenite grain size during rolling and the accumulated strain amount are This is because the ¼t portion of the thickness and the ½t portion of the plate thickness are greatly different.

  Note that the microstructure inside the steel sheet has a total of less than 5% (including 0%) in area% of the structures other than ferrite and bainite at each plate thickness position. Examples of structures other than ferrite and bainite include pearlite, martensite, and island martensite (MA).

  Although the tissue fraction has been shown above, the strength and arrest characteristics also depend on the state of each tissue, and therefore it is necessary for the tissue to further satisfy the following requirements.

  (C) The average bainite particle size of 1/4 t part of the plate thickness is 25.0 μm or less, and the average bainite particle size of 1/2 t part of the plate thickness is 35.0 μm or less.

  The average bainite grain size of 1/4 t part and 1/2 t part of the plate thickness is 25.0 μm or less and 35.0 μm or less, respectively. If the bainite grain size exceeds the above value, the effective crystal grain size is not significantly refined, and good arrest characteristics cannot be obtained. The finer the grain size of bainite, the better the arrest characteristics. However, when the plate thickness exceeds 70 mm, in order to achieve the refinement of the bainite grain size, it is necessary to perform rolling at a low temperature with a high reduction ratio, which imposes a burden on the rolling apparatus and makes manufacturing difficult. Not industrial. Although the lower limit of the bainite particle size is not specified, when manufactured by the manufacturing method of the present invention, the average bainite particle size of 1/4 t part and 1/2 t part is substantially 7.5 μm or more and 15 μm or more, respectively. It becomes 0.0 μm or more. The reason for defining the average bainite grain size in the 1/4 t portion of the sheet thickness and the 1/2 t portion of the sheet thickness is that when the sheet thickness exceeds 70 mm, the austenite grain size during rolling and the accumulated strain amount are increased. This is because there is a large difference between the 1/4 t portion of the plate thickness and the 1/2 t portion of the plate thickness.

  (D) The average effective crystal grain size of 1/4 t part of the plate thickness is 22.0 μm or less, and the average effective crystal grain size of 1/2 t part of the plate thickness is 32.0 μm or less.

  The average values of the effective crystal grain sizes at the 1/4 t portion and the 1/2 t portion of the plate thickness must be 22.0 μm or less and 32.0 μm or less, respectively. The effective crystal grain size is the most important factor for obtaining good arrest characteristics, and the finer the grain, the better the arrest characteristics. Therefore, when the average effective crystal grain size at each plate thickness position exceeds the above value, good arrest characteristics cannot be obtained.

  When the plate thickness is more than 70 mm, in order to reduce the effective crystal grain size, it is necessary to perform rolling at a low temperature with a high reduction ratio, which imposes a burden on the rolling apparatus and makes manufacturing difficult, which is not industrial. Although the lower limit of the average effective crystal grain size is not specified, in the case of production by the production method of the present invention, the average effective crystal grain sizes of 1/4 t part and 1/2 t part are each substantially 5.0 μm or more. It becomes 10.0 μm or more. The reason for defining the average effective crystal grain size in the 1/4 t part of the plate thickness and the 1/2 t part of the plate thickness is that when the plate thickness exceeds 70 mm, the austenite particle size during rolling and the accumulated strain amount. However, this is because the ¼t portion of the plate thickness and the ½t portion of the plate thickness are significantly different.

(C) Manufacturing method Heating temperature: Ac _{3 to} less than 1000 ° C. The heating temperature is set to Ac _{3 to} less than 1000 ° C. with reference to the temperature at the center of the plate thickness. In the case of a thick material having a plate thickness of more than 70 mm, a temperature difference occurs between the surface of the steel plate and the central part of the plate thickness. Nevertheless, it proceeds to the next process. As a result, necessary strength, toughness and arrest characteristics may not be satisfied. Therefore, the heating temperature is based on the temperature at the center of the plate thickness. If the heating temperature is 1000 ° C. or higher, the heated γ grains become coarse, and it becomes difficult to obtain a fine structure for obtaining arrest characteristics. On the other hand, if the heating temperature is less than Ac ₃ ° C., the solution may not be completely solutionized, so that the final arrest characteristics may deteriorate. Further, since the load in rolling becomes too high, it becomes difficult to carry out rolling with an optimum rolling load. The heating temperature is preferably 900 ° C. or higher, and preferably 980 ° C. or lower.

Rough rolling: In the temperature range in which the center part of the plate thickness is Ac _{3 to} less than 1000 ° C., the cumulative rolling reduction is 15.0 to 60.0%, the average rolling reduction of each pass is 3.5% or more. Rough rolling (primary rolling) The temperature range of is set to a range of Ac _{3 to} less than 1000 ° C. based on the temperature of the central portion of the plate thickness. The reason why the temperature at the center of the plate thickness is used as the reference is the same as the reason that the temperature at the center of the plate thickness is used as the reference at the heating temperature. If the temperature range of rough rolling is 1000 ° C. or higher, the austenite grains may become large. On the other hand, if the temperature range of the rough rolling is less than Ac ₃ ° C, the final arrest characteristics may be deteriorated because the solution is not completely solutionized. Further, since the load in rolling becomes too high, it becomes difficult to perform the rolling under the optimum rolling load, and the influence of internal defects such as porosity generated during casting may not be reduced. Further, the cumulative rolling reduction is 15.0% or more. If the cumulative rolling reduction is less than 15.0%, not only the effect of internal defects such as porosity generated during casting cannot be reduced, but also the width of the steel material required for the product cannot be obtained. If the cumulative reduction rate in rough rolling exceeds 60.0%, the refinement of recrystallized austenite and the introduction of rolling strain into some austenite proceed, but the cumulative reduction amount in the subsequent finish rolling can be secured. Disappear. Therefore, the cumulative reduction rate in rough rolling is 60.0% or less. The average rolling reduction of each pass is 3.5% or more. If the average rolling reduction of each pass is less than 3.5%, not only the effect of internal defects such as porosity generated during casting cannot be reduced, but also the number of passes increases, which reduces productivity. Although the upper limit of the average rolling reduction of each pass is not particularly set, if the average rolling reduction of each pass exceeds 10.0%, the recrystallized austenite may become coarse. Therefore, the average rolling reduction of each pass is preferably 10.0% or less. The average rolling reduction of each pass is preferably 4.0% or more.

Finish rolling: In the temperature range where the plate thickness center portion is Ar _{3 to} 950 ° C., the cumulative reduction rate is 40% or more, the average reduction rate of each pass is 5.0% or more. The temperature range of finish rolling (secondary rolling) is Ar _{3 to} 950 ° C. is set with the temperature at the center of the plate thickness as a reference. The reason why the temperature at the center of the plate thickness is used as the reference is the same as the reason that the temperature at the center of the plate thickness is used as the reference at the heating temperature. When the temperature range of finish rolling exceeds 950 ° C., flattening of austenite cannot be obtained and a fine structure cannot be obtained after cooling. On the other hand, if the temperature range of finish rolling is less than the Ar ₃ point, two-phase region rolling of ferrite and austenite occurs, and a large amount of coarse ferrite is generated in the surface layer and 1/4 t portion, and arrest characteristics cannot be obtained. Further, if the cumulative rolling reduction is less than 40%, the effect of CR (controlled rolling) becomes insufficient and a fine structure cannot be obtained. Therefore, the cumulative rolling reduction is 40% or more. Although the upper limit of the cumulative reduction ratio is not particularly set, if the plate thickness exceeds 70 mm, if the cumulative reduction ratio exceeding 65% is to be ensured in finish rolling, the cumulative reduction ratio in rough rolling cannot be ensured. Therefore, the cumulative rolling reduction is preferably 65% or less. The average rolling reduction of each pass is 5.0% or more. If the average rolling reduction of each pass is less than 5.0%, rolling strain is not introduced into the steel sheet, a fine structure cannot be obtained, and the number of passes increases and productivity decreases.

Final pass start temperature of finish rolling: Ar ₃ −20 ° C. to Ar ₃ + 30 ° C. on plate surface
The final pass start temperature of finish rolling is set to Ar ₃ −20 ° C. to Ar ₃ + 30 ° C. at the temperature of the plate thickness surface. When the final pass start temperature of finish rolling exceeds Ar ₃ + 30 ° C., a structure having an aspect ratio of 1.5 or more is not formed within 5 mm of the plate thickness surface. On the other hand, when the final pass start temperature of finish rolling is less than Ar ₃ −20 ° C., a structure having an aspect ratio of 1.5 or more is formed even in a region where the plate thickness surface exceeds 5 mm, and the arrest characteristics deteriorate.

Accelerated cooling: Accelerated cooling is started after finishing rolling is completed and the surface temperature is accelerated to 550 ° C or lower. In accelerated cooling, accelerated cooling is started after finishing rolling is completed. From the viewpoint of improving strength and toughness, accelerated cooling is preferably started after the completion of rolling and before the temperature decreases by 20 ° C. or more. With thick steel plates having a plate thickness of more than 70 mm, the heat transfer is delayed, so that the cooling rate at the central part of the plate thickness is only about 1 to 10 ° C./s. However, in the present invention, the cooling rate of the surface temperature is preferably 50 ° C./s or more in order to control the microstructure state of the steel sheet surface. The surface temperature of the accelerated cooling stop temperature is 550 ° C. or less. When the stop temperature of the accelerated cooling exceeds 550 ° C, the cooling of the central portion of the plate thickness becomes insufficient and the strength and toughness deteriorate. Therefore, it is desirable to cool to room temperature. However, in actual production, it is necessary to consider dehydrogenation of the steel sheet. Therefore, the stop temperature of the accelerated cooling is preferably 300 ° C. or higher, and preferably 400 ° C. or lower.

Tempering temperature: 350-650 ° C
When tempering is performed after the completion of accelerated cooling, the tempering temperature is 350 to 650 ° C. If the tempering temperature is lower than 350 ° C, the tempering effect becomes insufficient. Further, if the tempering temperature is lower than 350 ° C., long-time heat treatment is required to obtain the same effect as that obtained when the tempering temperature is 350 ° C. or higher, which is not industrial. On the other hand, when the tempering temperature exceeds 650 ° C., the strength is remarkably lowered, and sufficient strength cannot be obtained. In addition, the formation of fine precipitates may harden the structure and reduce toughness. The tempering temperature is preferably 400 ° C. or higher, and preferably 550 ° C. or lower.

  The thick steel plate of the present invention obtained by the above steps has a plate thickness of more than 70 mm. The thick steel plate of the present invention has good arrest characteristics even when the plate thickness exceeds 70 mm. Although the upper limit of the plate thickness is not particularly set, the thick steel plate of the present invention can ensure good arrest characteristics even when the plate thickness is 100 mm.

The thick steel plate of the present invention has a yield strength (YS) of 460 MPa or more, a tensile strength (TS) of 570 to 720 MPa, and a temperature (vTrs) at which a brittle fracture surface becomes 50% in a Charpy impact test of −40 ° C. or less. Fulfill. Further, the thick steel sheet of the present invention has a Kca value at -10 ° C, which is an evaluation index of arrest characteristics, of 6000 N / mm ^1.5 or more.

  Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to these Examples.

<Manufacture of thick steel plate>
Test Nos. A to n having the chemical compositions shown in Table 1 were manufactured under the conditions shown in Table 2. 1 to 24 thick steel plates were obtained. Table 3 shows the plate thickness of each thick steel plate.

<Tissue measurement method>
A sample of L cross section was cut out from each plate thickness position of each thick steel plate before heat treatment, and after mirror polishing, sample adjustment was performed with colloidal silica, and the ferrite fraction, bainite grain size and effective crystal grain size were determined by EBSD. Was measured using. The effective crystal grain size was measured by setting the magnification to 90 times, measuring a range of 1 mm × 2 mm at a pitch of 2 μm, and determining a 15 ° tilt angle as a grain boundary. Further, the ferrite fraction and the bainite grain size were calculated by setting the magnification to 400 times, measuring a range of 200 μm × 300 μm with a 0.25 μm pitch, and then setting the GAM threshold value to 0.5, The ferrite fraction and the bainite grain size were determined by separating 5 or less as a ferrite structure and exceeding 0.5 as a bainite structure and separating them. The aspect ratio within 5 mm of the surface layer was determined by photographing the Nital-corroded structure of the L cross section at a magnification of 500 times with an optical microscope. The results are shown in Table 3.

<Yield strength and tensile strength test>
No. 4 test piece specified in JIS Z 2241 (2011) was sampled in the direction parallel to the rolling direction from the 1 / 4t part and the 1 / 2t part of each thick steel plate, and the yield strength (YS) and tensile strength were measured. (TS) was measured. The results are shown in Table 3. The target value of yield strength was 460 MPa or more, and the target value of tensile strength was 570 to 720 MPa.

<Charpy impact test>
A V-notch test piece specified in JIS Z 2242: 2005 was sampled in the direction parallel to the rolling direction from the surface of each thick steel plate, the 1 / 4t part and the 1 / 2t part, respectively, and a Charpy impact test was performed to determine the brittleness. The temperature (vTrs) at which the fracture surface reaches 50% was measured. The results are shown in Table 3. The target value of vTrs was set to -40 ° C or lower.

<Arrest characteristic evaluation>
The arrest characteristics were determined by calculating the Kca value at -10 ° C. The Kca value was calculated by carrying out a temperature gradient type ESSO test. Specifically, a temperature gradient type ESSO test is performed under load stress of at least 3 conditions, and a Kca value obtained from the load stress and brittle crack length is plotted at a temperature at a position where the brittle crack stops, The Kca value at −10 ° C. was calculated from the logarithmic approximation. The results are shown in Table 3. The target value of the Kca value at −10 ° C. was 6000 N / mm ^1.5 or more.

  Test No. The thick steel plates of 1 to 7 satisfied all the requirements specified in the present invention, and therefore, good characteristics were obtained.

  Test No. In the thick steel plates of Nos. 8 and 14, the C and Mo components were out of the optimum ranges, respectively, and thus the arrestability was deteriorated.

  Test No. In the thick steel plate of No. 9, Mn was not within the optimum component range, so the arrest performance was deteriorated due to the influence of central segregation and MnS generated in the central portion.

  Test No. In the thick steel plate of No. 10, since Cu was not within the optimum component range, the toughness of the surface was deteriorated due to Cu checking, and the toughness inside was deteriorated due to Cu precipitation. As a result, arrest performance deteriorated.

  Test No. In the thick steel sheets 11 and 12, Nb and Ti were not within the optimum component ranges, respectively, so the toughness was lowered due to the increase of precipitates, and as a result, the arrest performance was lowered.

  Test No. The thick steel plate of No. 13 is Ceq. Is out of the optimum range, the strength is remarkably increased, and the arrest performance is deteriorated.

  Test No. In the thick steel plate of No. 15, the heating temperature was out of the optimum range, so that the bainite grain size became coarse. As a result, the effective crystal grain size was not remarkably reduced, and the arrest performance was deteriorated.

  Test No. The 16 thick steel plates did not all solution because the heating temperature was too low. Therefore, the ferrite fraction, strength, toughness, and arrest performance decreased.

  Test No. The thick steel sheet of No. 17 could not sufficiently secure the cumulative reduction rate in the secondary rolling because the cumulative reduction rate in the primary rolling was too large. Therefore, as a result of insufficient accumulation of strain in the unrecrystallized austenite, the amount of ferrite produced was insufficient, and the effective grain size was not refined. As a result, arrest performance deteriorated.

  Test No. Since the 18th thick steel plate had a low average rolling reduction in primary rolling, internal defects could not be reduced. As a result, arrest performance deteriorated.

  Test No. Since the thick steel plate of No. 19 had a high rolling start temperature in the secondary rolling, the rolling in the substantially unrecrystallized region was not performed at the initial stage of rolling, and the accumulation of strain inside the austenite did not proceed. Therefore, the effective crystal grain size was not reduced, and as a result, the arrest performance was deteriorated.

  Test No. In the thick steel plate of No. 20, the rolling end temperature was lowered throughout the inside, so that a large amount of coarse ferrite was generated. As a result, not only the strength was lowered, but also the arrest performance was lowered.

  Test No. In the thick steel plate of No. 21, the average rolling reduction was low, and thus the accumulation of strain inside the austenite did not proceed. Therefore, the effective crystal grain size was not reduced, and as a result, the arrest performance was deteriorated.

  Test No. The thick steel plate of No. 22 had a high final pass temperature at the time of finishing, so that the surface elongation structure was undeveloped, and as a result, the arrest performance deteriorated.

  Test No. Since the thick steel plate of No. 23 had a high cooling stop temperature, the internal structure was not quenched, and as a result, the strength was reduced.

  Test No. In the thick steel plate of No. 24, the tempering temperature was out of the optimum range, so that the YS was lowered due to the generation of MA, and the arrest performance was further lowered.

  According to the present invention, it is possible to provide a thick steel plate having excellent arrestability and a plate thickness of more than 70 mm, and a manufacturing method thereof. Therefore, the thick steel plate of the present invention can be suitably used for a thick steel plate having a plate thickness of more than 70 mm, such as an upper deck of the upper hull or a hatchside combing, which is an important member of a container ship.

Claims (6)

A thick steel plate having a plate thickness of more than 70 mm,
The chemical composition is% by mass,
C: 0.04 to 0.12%,
Si: 0.05 to 0.50%,
Mn: 1.30 to 2.20%,
P: 0.020% or less,
S: 0.010% or less,
Cu: 0.05 to 1.00%,
Ni: 0.05-1.50%,
Nb: 0.005 to 0.050%,
Ti: 0.005 to 0.050%,
sol. Al: 0.005-0.060%,
N: 0.001-0.010%,
Cr: 0 to 0.50%,
Mo: 0 to 0.35%,
V: 0 to 0.15%,
B: 0 to 0.0030%,
Ca: 0 to 0.010%,
Mg: 0 to 0.0050%,
REM: 0 to 0.0050%, and
The balance: Fe and impurities,
Ceq. Represented by the following formula (i). Is more than 0.40 to 0.52, and
A thick steel plate that satisfies the following (a) to (d) and has a Kca value at -10 ° C, which is an evaluation index of arrest characteristics, of 6000 N / mm ^1.5 or more .
(A) A structure within 5 mm of the surface layer forms a structure elongated in the rolling direction, and the average aspect ratio of this structure is 1.5 or more.
(B) The microstructure inside the steel sheet has a composite structure of ferrite and bainite, the ferrite fraction of 1/4 t part of the plate thickness is 15.0 to 40.0%, and the ferrite content of 1/2 t part of the plate thickness. The ratio is 10.0 to 40.0%, and the structure other than ferrite and bainite has a total of less than 5% (including 0%) in area% at each plate thickness position.
(C) The average bainite particle size of 1/4 t part of the plate thickness is 25.0 μm or less, and the average bainite particle size of 1/2 t part of the plate thickness is 35.0 μm or less.
(D) The average effective crystal grain size of 1/4 t part of the plate thickness is 22.0 μm or less, and the average effective crystal grain size of 1/2 t part of the plate thickness is 32.0 μm or less.
Ceq. = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (i) The chemical composition is% by mass,
Cr: 0.05 to 0.50%,
Mo: 0.05 to 0.35%, and
V: 0.005-0.15%,
The thick steel plate according to claim 1, containing at least one selected from the group consisting of: The chemical composition is% by mass,
B: 0.0003 to 0.0030%,
The thick steel plate according to claim 1 or 2, which comprises: The chemical composition is% by mass,
Ca: 0.0005 to 0.010%,
Mg: 0.0005 to 0.0050%, and
REM: 0.0005 to 0.0050%,
The thick steel plate according to any one of claims 1 to 3, containing at least one selected from the group consisting of: A method for manufacturing the thick steel plate according to claim 1,
A steel slab having the composition according to any one of claims 1 to 4 is heated so that the center part of the plate thickness is Ac _{3 to} less than 1000 ° C,
In the temperature range in which the sheet thickness center portion is Ac _{3 to} less than 1000 ° C., after performing rough rolling at a cumulative reduction ratio of 15.0 to 60.0% and an average reduction ratio of each pass of 3.5% or more,
In the temperature range where the plate thickness center portion is Ar _{3 to} 950 ° C., finish rolling is performed with a cumulative reduction of 40% or more and an average reduction of 5.0% or more in each pass.
Furthermore, the final pass start temperature of this finish rolling is set to Ar ₃ −20 ° C. to Ar ₃ + 30 ° C. on the surface of the plate thickness to complete the rolling,
Next, a method for manufacturing a thick steel sheet, in which accelerated cooling is started and the surface temperature is accelerated cooled to 550 ° C or lower.   The method for manufacturing a thick steel sheet according to claim 5, wherein tempering treatment is performed at a temperature of 350 to 650 ° C. after completion of the accelerated cooling.

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