JP2017186614A - Thick steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thick steel sheet excellent in arrest property and having sheet thickness over 70 mm and a manufacturing method therefor.SOLUTION: There is provided a thick steel sheet having sheet thickness of over 70 mm, a chemical composition of 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 to 1.50%, Nb:0.005 to 0.050%, Ti:0.005 to 0.050%, sol.Al:0.005 to 0.060%, N:0.001 to 0.010%, Cr, Mo, V, B, Ca, Mg and REM as optional addition elements and the balance:Fe with impurities and having Ceq. of 0.40 to 0.52, (a) average aspect ratio of a surface layer structure of 1.5 or more, (b) a micro structure inside of the steel sheet having a composite structure of ferrite and bainite, ferrite fraction of 1/4 t part:15.0 to 40.0% and 1/2 t part:10.0 to 40.0%, a structure other than ferrite and bainite of total less than 5% including 0% at each sheet thickness position, (c) average bainite particle diameter of 1/4 t part:25.0 μm or less and 1/2 t part:35.0 μm or less and (d) average effective crystal particle diameter of 1/4 t part:22.0 μm or less and 1/2 t part:32.0 μm or less.SELECTED DRAWING: None

Description

  The present invention relates to a thick steel plate and a method for manufacturing the same. In particular, the present invention relates to a thick steel plate having excellent brittle crack propagation stopping characteristics (hereinafter also referred to as arrestability) and a 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 a brittle fracture occurs, the fracture proceeds at a high speed and over a long range, which can have a great impact on the environment and economy.

  In recent years, in the case of container ships, demand for large container ships exceeding 10,000 TEU is 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 thickness. In particular, for steel materials used for upper decks or hatch side combing, which are important members of container ships, even if brittle fracture occurs, high-strength thick wall with arrest characteristics that stop the progress of brittle fracture The application of materials is progressing. Generally, as the strength increases and the plate thickness increases, the resistance to the occurrence and propagation of brittle fracture decreases. For this reason, it is necessary to develop a steel material that is more sophisticated than steel materials that have been conventionally used.

  For example, Patent Document 1 defines an area ratio of each structure and a grain boundary density corresponding to a crystal grain size, and further a structure fraction having a {100} plane of 1/4 t part, {1/2 t part { A high-strength thick steel plate that ensures good arrest characteristics by defining a texture fraction having a 110} plane is disclosed.

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

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

  Patent Document 4 discloses a thick steel plate that regulates the microstructure and the grain size at the center of the plate thickness and ensures good arrest characteristics. When manufacturing a thick steel plate, an optimum reduction is given while controlling the temperature at the center of the plate thickness.

  Patent Document 5 discloses a thick high-strength steel sheet that defines 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 on the surface layer are suppressed, the size of cementite is controlled, and the maximum effective crystal grain size necessary for satisfying the arrest characteristics is determined from the Ni amount and the plate thickness. A thick high-strength steel sheet that ensures good arrest properties by calculation is disclosed. Similarly, when a thick steel plate is manufactured, a necessary rolling temperature is defined from the Ni amount and the plate thickness.

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

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

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

  In Patent Document 10, rolling is performed at a relatively low temperature, and further, the surface temperature of the steel sheet after finish rolling is controlled to refine the crystal grains of the surface layer, thereby producing a thick high-strength steel sheet that ensures good arrest characteristics. A method is disclosed.

International Publication No. 13/150687 JP 2013-221190 A JP2013-221189A JP 2012-172258 A JP 2011-214116 A JP 2008-248382 A JP 2011-52244 A JP 2011-52243 A JP 2008-261030 A Japanese Patent Laying-Open No. 2015-25205

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

  Regarding securing of arrest properties, it is generally desirable to promote the refinement of crystal grains. However, when the plate thickness exceeds 70 mm, it is difficult to ensure arrestability simply by adjusting the components and production conditions in order to realize the conventionally known crystal grain size, and the surface layer reaches the inside of the plate thickness. It was necessary to precisely control the microstructure.

  In view of such a current situation, the present invention has an object to provide a thick steel plate having excellent arrestability and a thickness exceeding 70 mm and a method for manufacturing the same.

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

  First, the components were reviewed in order to increase the strength and thickness of the steel sheet.

  Based on steel materials used for offshore structures, the effect of increasing the strength and toughness was studied using steel materials with a plate thickness of 100 mm. As a result of investigating the relationship between the addition amount and toughness of Ni, Mo, and V, Ni did not show a decrease in toughness regardless of the addition amount, but Mo and V significantly decreased toughness as the addition 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 was found that Mo and V can not only increase the strength but also ensure a certain toughness if the addition amount is limited. In addition, when vTrs is allowed to be −80 ° C., the upper limit value of the V content is 0.15%, and the upper limit value of the Mo content is 0.35%.

  Next, regarding the relationship between the carbon equivalent (Ceq.) Necessary for satisfying the strength standard of EH47 and the strength of the central portion of each plate thickness, using steel materials to which various components having a plate thickness of 70 to 100 mm are added. Investigated by changing rolling conditions and tempering conditions. As a result, the strength standard of EH47 can be satisfied by setting Ceq. To 0.40 or more when the plate thickness is 70 mm and setting Ceq. To 0.47 or more when the plate thickness is 100 mm. I understood. Moreover, when Ceq. Was 0.52 or more, it turned out that intensity | strength becomes higher than necessary and toughness falls.

  Furthermore, the present inventors paid attention to the structure and the grain size in order to ensure arrest characteristics in a thick steel plate having a plate thickness exceeding 70 mm. Generally, it is said that the arrest characteristic is improved if the crystal grain size is reduced. Moreover, in the case of a thick steel plate having a plate thickness exceeding 70 mm, it is difficult to rapidly cool the inside of the steel plate, so that a ferrite structure always appears in the steel plate. Then, 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 with a 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 by increasing the ferrite fraction. In order to obtain good arrest characteristics, it is effective to increase the ferrite fraction. However, an excessive increase in ferrite fraction results in a decrease in strength. For this reason, in a thick high-strength material, it is not effective to refine 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 is refined by refinement of the bainite grain size. Therefore, it was considered that the arrest characteristics could be remarkably improved if a certain amount of ferrite fraction satisfying the steel plate strength was secured and the bainite grain size could be refined. Then, about refinement | miniaturization of a bainite structure, CR ratio was examined using the material of slab thickness 305mm of the same composition. As a result, the bainite particle size was not significantly refined with an increase in CR rate. In order to secure the CR ratio, it is necessary to secure the reduction amount of the primary rolling performed in the recrystallization region. However, since the reduction amount was not ensured, the austenite grains were not refined, and the CR This is probably because the rate increase effect was not achieved. Therefore, the influence of heating temperature was examined using the same slab as before. FIG. 3 is a graph showing the relationship between the heating temperature and the average grain size of bainite and ferrite in the center of the plate thickness. As can be seen from FIG. 3, when the heating temperature was changed to produce an 80 mm thick steel plate, the ferrite grain size was almost constant, but the average bainite grain size was remarkably refined as the heating temperature decreased.

Also, using slabs of the same composition, producing thick steel plates with a thickness of 80 mm under different manufacturing conditions, and conducting an ESSO test, the ferrite fraction, ferrite grain size, and effective crystal grain size remain unchanged. Nevertheless, the arrest characteristics may differ greatly. For example, the ferrite fraction and the ferrite grain size remained almost the same for the thick steel plate that was slab heated at 1050 ° C. and finish-rolled at 730 ° C. or 780 ° C. Also, the effective crystal grain size is 19.0 μm for a thick steel plate finish-rolled at 780 ° C. (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). 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. Therefore, when these steel plates were examined in detail, it was found that the structure of the surface layer portion of the steel plates was greatly different. In the 730 ° C finish-rolled thick steel plate with high arrest properties, the structure is a flat structure along the rolling direction, and this structure prevents the thick steel sheet from cracking in the initial stage. it is conceivable that.

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

  The present invention has been made on the basis of the above-mentioned knowledge, and the gist thereof resides in the thick steel plate and the manufacturing method thereof shown below.

(1) A thick steel plate having a thickness exceeding 70 mm,
Chemical composition is 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 to 0.060%,
N: 0.001 to 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: 0-0.0050%, and
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) The 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 the ¼ t portion of the plate thickness is 15.0 to 40.0%, and the ferrite portion of the ½ t portion of the plate thickness The ratio is 10.0 to 40.0%, and at each plate thickness position, the structure other than ferrite and bainite is less than 5% (including 0%) in total in area%.
(C) The average bainite particle size of ¼ t part of the plate thickness is 25.0 μm or less, and the average bainite particle size of ½ t part of the plate thickness is 35.0 μm or less.
(D) The average effective crystal grain size at ¼ t part of the plate thickness is 22.0 μm or less, and the average effective crystal grain size at ½ 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 mass%,
Cr: 0.05 to 0.50%,
Mo: 0.05 to 0.35%, and
V: 0.005 to 0.15%,
The thick steel plate according to (1), which contains one or more selected from the above.

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

(4) The chemical composition is 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 above.

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

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

  ADVANTAGE OF THE INVENTION According to this invention, the thick steel plate which is excellent in arrestability and plate | board thickness exceeds 70 mm and its manufacturing method can be provided.

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 center of the plate thickness.

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

(A) About chemical composition The effect of each element and the reason for limiting the content are as follows. In the following description, “%” for the content means “% by mass”.

C: 0.04 to 0.12%
C is an element that increases the strength of the steel material. When the C content is less than 0.04%, this effect cannot be obtained. On the other hand, if the C content exceeds 0.12%, the toughness deteriorates due to the increase in strength, the weldability deteriorates, and the toughness of the heat affected zone (HAZ) deteriorates. In addition, the arrest characteristic is degraded. 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, if the Si content exceeds 0.50%, the toughness is reduced due to the hardening of the HAZ. 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 increases the hardenability of steel and increases the strength and toughness of the steel material. If the Mn content is less than 1.30%, these effects cannot be obtained. On the other hand, if the Mn content exceeds 2.20%, the center segregation becomes remarkable and the toughness of the center portion of the plate thickness is remarkably lowered. In addition, the arrest characteristic is degraded. Therefore, the Mn content is 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, which lowers the mechanical properties of the steel material, in particular, lowers the low temperature toughness. Therefore, the P content is 0.020% or less. The P content is preferably 0.015% or less, and more preferably as low as possible.

S: 0.010% or less S is an impurity element, which combines with Mn to form MnS, thereby reducing 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, and more preferably as low as possible.

Cu: 0.05-1.00%
Cu is an element that can be dissolved in steel to increase strength without impairing toughness and improve arrest properties. 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 arrest characteristics are deteriorated due to an increase in precipitates, and further, minute cracks are generated on the surface during hot working. Therefore, the Cu content is set to 0.05 to 1.00%. The Cu content is preferably 0.20% or more, and preferably 0.50% or less.

Ni: 0.05 to 1.50%
Ni is an element that improves the arrest property by dissolving in steel and increasing the strength without impairing toughness. 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 to 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 small amount, and contributes to improvement of strength and arrest characteristics by refining the structure. Furthermore, 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, if the Nb content exceeds 0.050%, coarse precipitates are generated, and not only the arrest properties are deteriorated, but also the HAZ toughness is remarkably deteriorated. Therefore, the Nb content is 0.005 to 0.050%. The Nb content is preferably 0.007% or more, and preferably 0.020% or less.

Ti: 0.005 to 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 when the steel slab is heated. As the austenite grain size increases, the grain size of the bainite after transformation also increases. Therefore, in order to obtain a desired bainite particle size, the Ti content is set to 0.005% or more. When Ti content exceeds 0.050%, TiC will produce | generate and toughness will fall. Therefore, the Ti content is set to 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 deoxidation of steel. For deoxidation, sol. The Al content is required to be 0.005% or more. On the other hand, sol. When the Al content exceeds 0.060%, not only coarse inclusions increase and the arrest properties deteriorate, but also the HAZ toughness decreases. Therefore, sol. Al content shall be 0.005-0.060%. sol. The Al content is preferably 0.010% or more, and preferably 0.050% or less.

N: 0.001 to 0.010%
N is an element having an action of binding to Ti to form TiN and suppressing austenite grain coarsening. 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 exists as an impurity, leading to a decrease in toughness. As a result, the arrest characteristic is degraded. Therefore, the N content is 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 increases the strength of the steel material, it may be contained as necessary. When the Cr content exceeds 0.50%, a decrease in toughness accompanying an increase in the strength of the steel material becomes significant. 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%
Since Mo is an element that increases the strength of the steel material, it may be contained as necessary. When the Mo content exceeds 0.35%, a decrease in toughness accompanying an increase in strength of the steel material becomes significant. In addition, the arrest characteristic is degraded. Therefore, the Mo content is set to 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 has a function of forming carbonitride and precipitating and strengthening the steel material. Therefore, V may be contained as necessary. When the V content exceeds 0.15%, a decrease in toughness accompanying precipitation strengthening becomes significant. 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 by adding a small amount, and may be contained if necessary. When the B content exceeds 0.0030%, the effect is saturated and the toughness of the HAZ is lowered. 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 as 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 improved stably. 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 as necessary. If Mg exceeds 0.0050%, the 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 HAZ toughness, and may be contained as necessary. When REM exceeds 0.0050%, HAZ toughness and weldability deteriorate. Therefore, the REM content is set to 0.0050% or less. On the other hand, if the REM content is less than 0.0005%, the HAZ toughness may not be improved stably. Therefore, the REM content is preferably 0.0005% or more.

  The thick steel plate of the present invention contains the above-described elements, and the balance has a chemical composition that is Fe and impurities. "Impurity" is a component that is mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when steel is industrially manufactured, and is allowed 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. If it is less than 0.40, the center of the plate thickness is not baked, and a high strength of yield strength of 460 MPa or more cannot be obtained. In addition, toughness may be reduced. On the other hand, Ceq. If it exceeds 0.52, the required strength can be easily obtained, but the toughness and weldability are lowered and the cost is also increased. In addition, the arrest characteristic is degraded. Therefore, Ceq. Is set to 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 structure rules (a) to (d) is not satisfied, good strength and arrest characteristics cannot be obtained.

  (A) The 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 that is the ratio of the minor axis to the major axis of the structure formed in the L cross section within 5 mm of the steel sheet surface layer is 1.5 or more. Making the aspect ratio of the structure formed in the L cross section within 5 mm of the steel sheet surface layer to be 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 characteristic is remarkably deteriorated. When the aspect ratio is 1.5 or more, shear lip formation is good and the arrest characteristics are remarkably improved. Since the steel plate of the present invention has a plate thickness exceeding 70 mm, the surface layer structure tends to be different from the internal structure. Although not particularly defined, 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 the ¼ t portion of the plate thickness is 15.0 to 40.0%, and the ferrite portion of the ½ t portion of the plate thickness The ratio is 10.0 to 40.0%, and at each plate thickness position, the structure other than ferrite and bainite is less than 5% (including 0%) in total in area%.

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

  The tissue fraction also affects the arrest characteristics. The bainite structure was evaluated by superimposing an image quality image and a grain boundary image defining a boundary having an orientation difference of 15 ° or more as a grain boundary using EBSD. As a result, the structure corresponding to bainite is coarser than the ferrite structure. It was found to exhibit crystal grains. For this reason, the structural fraction of ferrite and bainite is one factor that affects the effective crystal grain size of the steel sheet, which will be described later. In order to impart strength and appropriately control the effective crystal grain size to improve the arrest characteristics, the ferrite fraction of the 1/4 t 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 fraction was defined for each of the 1/4 t part of the plate thickness and the 1/2 t part of the plate thickness. When the plate thickness exceeded 70 mm, the austenite grain size during rolling and the accumulated strain amount were This is because the ¼ t portion of the thickness and the ½ t portion of the plate thickness are greatly different.

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

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

  (C) The average bainite particle size of ¼ t part of the plate thickness is 25.0 μm or less, and the average bainite particle size of ½ t part of the plate thickness is 35.0 μm or less.

  The average bainite particle 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. When the bainite particle size exceeds the above value, the effective crystal particle size is not remarkably reduced, and good arrest characteristics cannot be obtained. The finer the bainite particle size, the better the arrest characteristics. However, in the case of a plate thickness exceeding 70 mm, in order to achieve a finer bainite grain size, rolling at a low temperature and a high rolling reduction must be performed, which places a burden on the rolling equipment and makes manufacturing difficult. Not industrial. The lower limit value of the bainite particle size is not specified, but 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, 15 0.0 μm or more. The reason why the average bainite grain size is specified in the ¼ t part of the plate thickness and the ½ t part of the plate thickness is that when the plate thickness exceeds 70 mm, the austenite grain size during rolling and the accumulated strain amount are This is because the ¼ t portion of the plate thickness and the ½ t portion of the plate thickness are greatly different.

  (D) The average effective crystal grain size at ¼ t part of the plate thickness is 22.0 μm or less, and the average effective crystal grain size at ½ t part of the plate thickness is 32.0 μm or less.

  The average value of the effective crystal grain size of the 1/4 t part and 1/2 t part 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 finer, 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.

  In the case of a plate thickness exceeding 70 mm, in order to refine the effective crystal grain size, rolling at a low temperature and a high rolling reduction must be performed, which imposes a burden on the rolling apparatus and makes manufacturing difficult, which is not industrial. The lower limit value of the average effective crystal grain size is not specified, but when manufactured by the manufacturing method of the present invention, the average effective crystal grain size of 1/4 t part and 1/2 t part is substantially 5.0 μm or more, respectively. 10.0 μm or more. The reason why the average effective crystal grain size is specified for each of 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 grain size during rolling and the accumulated strain amount This is because the ¼ t portion of the plate thickness and the ½ t portion of the plate thickness are greatly different.

(C) the production method heating _temperature: Ac 3 to 1000 ° C. lower than the heating temperature, the _Ac 3 below to 1000 ° C. The temperature of the center of plate thickness as a reference. In the case of a thick material having a plate thickness exceeding 70 mm, a temperature difference occurs between the surface of the steel plate and the center portion of the plate thickness, so if the control temperature is set to the surface, the temperature at the center portion of the plate thickness is not lowered. Nevertheless, it proceeds to the next process. As a result, the required strength, toughness and arrest properties may not be satisfied. Therefore, the heating temperature is based on the temperature at the center of the plate thickness. When 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 properties. On the other hand, when the heating temperature is less than Ac ₃ ° C., the solution is not completely solutionized, so that the final arrest characteristics may be deteriorated. Moreover, since the load in rolling becomes too high, rolling with an optimal rolling load becomes difficult. The heating temperature is preferably 900 ° C. or higher, and preferably 980 ° C. or lower.

Rough rolling: In the temperature range where the center of the plate thickness is Ac _{3 to} less than 1000 ° C., the cumulative rolling reduction is 15.0 to 60.0%, and the average rolling reduction of each pass is 3.5% or more rough rolling (primary rolling) The temperature range is set to a range of Ac _{3 to} less than 1000 ° C. based on the temperature at the center of the plate thickness. The reason why the temperature at the central portion of the plate thickness is used as a reference is the same as the reason why the temperature at the central portion 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, 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 characteristic may be deteriorated because it is not completely solutionized. Moreover, since the load in rolling becomes too high, rolling at an optimum rolling load becomes difficult, and the influence of internal defects such as porosity generated during casting may not be reduced. The cumulative rolling reduction is 15.0% or more. If the cumulative rolling reduction is less than 15.0%, not only the influence of internal defects such as porosity generated during casting cannot be reduced, but also the width of the steel material necessary for the product cannot be obtained. If the cumulative rolling reduction ratio in rough rolling exceeds 60.0%, recrystallized austenite will be refined and rolling strain will be introduced into some austenite. However, the cumulative rolling reduction in the subsequent finish rolling can be secured. Disappear. Therefore, the cumulative rolling reduction in rough rolling is set to 60.0% or less. Moreover, the average rolling reduction of each pass shall be 3.5% or more. If the average rolling reduction rate of each pass is less than 3.5%, not only the influence of internal defects such as porosity generated during casting cannot be reduced, but also the productivity decreases due to an increase in the number of passes. There is no particular upper limit for the average rolling reduction in each pass, but if the average rolling reduction in each pass exceeds 10.0%, recrystallized austenite may become coarse. Therefore, it is preferable that the average rolling reduction of each pass is 10.0% or less. Moreover, it is preferable that the average rolling reduction of each pass is 4.0% or more.

Finish rolling: In the temperature range where the sheet thickness center is Ar _{3 to} 950 ° C., the cumulative rolling reduction is 40% or more, the average rolling reduction of each pass is 5.0% or more, and the temperature range of finishing rolling (secondary rolling) is Ar _{3 to} 950 ° C. based on the temperature at the center of the plate thickness. The reason why the temperature at the central portion of the plate thickness is used as a reference is the same as the reason why the temperature at the central portion of the plate thickness is used as the reference at the heating temperature. If the temperature range of finish rolling exceeds 950 ° C., austenite flatness cannot be obtained, and a fine structure cannot be obtained after cooling. On the other hand, if the temperature range of the finish rolling is less than Ar ₃ point, it becomes a two-phase rolling of ferrite and austenite, and a large number of coarse ferrites are generated in the surface layer and 1/4 t portion, and the arrest characteristics cannot be obtained. 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 there is no particular upper limit value for the cumulative rolling reduction, if the plate thickness exceeds 70 mm, the cumulative rolling reduction in rough rolling cannot be secured if an attempt is made to secure a cumulative rolling reduction exceeding 65% by finish rolling. Therefore, the cumulative rolling reduction is preferably 65% or less. Moreover, the average rolling reduction of each pass shall be 5.0% or more. If the average rolling reduction of each pass is less than 5.0%, not only rolling distortion is introduced to the inside of the steel sheet and a fine structure is not obtained, but also the number of passes increases and productivity decreases.

Final pass start temperature of finish rolling: Ar ₃ −20 ° C. to Ar ₃ + 30 ° C. at the plate thickness surface
The final pass start temperature of finish rolling is Ar ₃ −20 ° C. to Ar ₃ + 30 ° C. in terms of the surface temperature of the plate. 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 a plate thickness surface of 5 mm. 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 in a region exceeding the plate thickness surface of 5 mm, and the arrest characteristics are deteriorated.

Accelerated cooling: Accelerated cooling is started after completion of finish rolling, and accelerated cooling to a surface temperature of 550 ° C. or lower. Accelerated cooling starts accelerated cooling after completion of finish rolling. From the viewpoint of improving strength and toughness, accelerated cooling is preferably started before the temperature is lowered by 20 ° C. or more after completion of rolling. In a thick steel plate having a plate thickness exceeding 70 mm, the heat transfer is delayed, so the cooling rate at the central portion of the plate thickness is only about 1 to 10 ° C./s. However, in the present invention, it is preferable to set the cooling rate of the surface temperature to 50 ° C./s or more in order to control the structure of the steel sheet surface. The surface temperature of the accelerated cooling stop temperature is 550 ° C. or lower. When the stop temperature of accelerated cooling exceeds 550 ° C., cooling at the center of the plate thickness becomes insufficient, and strength and toughness are reduced. 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 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 accelerated cooling, the tempering temperature is set to 350 to 650 ° C. When the tempering temperature is less than 350 ° C., the effect of tempering is insufficient. Further, if the tempering temperature is less than 350 ° C., it is not industrial because a long heat treatment is required to obtain the same effect as that obtained when the tempering temperature is 350 ° C. or higher. 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 exceeding 70 mm. The thick steel plate of the present invention has good arrest characteristics even when the plate thickness exceeds 70 mm. The upper limit of the plate thickness is not particularly provided, but 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 the brittle fracture surface is 50% in the Charpy impact test at −40 ° C. or less. Fulfill. The thick steel plate of the present invention satisfies a Kca value at −10 ° C., which is an evaluation index of arrest properties, of 6000 N / mm ^1.5 or more.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

<Manufacture of thick steel plates>
By producing steel types a to n having the chemical composition shown in Table 1 under the conditions shown in Table 2, test No. 1 to 24 thick steel plates were obtained. Table 3 shows the thickness of each steel plate.

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

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

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

<Arest characteristic evaluation>
The arrest characteristic was performed by calculating a Kca value at −10 ° C. The Kca value was calculated by performing a temperature gradient type ESSO test. Specifically, a temperature gradient type ESSO test is performed with the load stress being at least three conditions or more, a Kca value obtained from the load stress and the brittle crack length is plotted at a temperature at which the brittle crack is stopped, 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 set to 6000 N / mm ^1.5 or more.

  Test No. Since the thick steel plates 1 to 7 satisfy all the requirements defined in the present invention, good characteristics were obtained.

  Test No. In the thick steel plates 8 and 14, the arrest performance deteriorated because the C and Mo components deviated from the optimum ranges, respectively.

  Test No. As for the thick steel plate of No. 9, since Mn was not within the optimum component range, the arrest performance was lowered due to the center segregation and the influence of MnS generated at the center.

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

  Test No. In the thick steel plates 11 and 12, Nb and Ti were not within the optimum component ranges, respectively, so that the toughness decreased due to the increase in precipitates, and as a result, the arrest performance decreased.

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

  Test No. Since the heating temperature of the thick steel plate No. 15 deviated from the optimum range, the grain size of bainite became coarse. As a result, the effective crystal grain size was not significantly reduced, and the arrest performance was lowered.

  Test No. No. 16 thick steel plates were not heated because the heating temperature was too low. Therefore, the ferrite fraction, strength, toughness, and arrest performance were lowered.

  Test No. No. 17 thick steel plate could not secure the cumulative reduction ratio in the secondary rolling sufficiently because the cumulative reduction ratio in the primary rolling was too large. Therefore, as a result of insufficient accumulation of strain in unrecrystallized austenite, the amount of ferrite produced was insufficient and refinement of the effective crystal grain size was not achieved. As a result, arrest performance decreased.

  Test No. The 18 thick steel plates could not reduce internal defects because of the low average rolling reduction in primary rolling. As a result, arrest performance decreased.

  Test No. No. 19 thick steel plate had a high rolling start temperature in the secondary rolling, so that the rolling in the substantially non-recrystallized region was not performed at the beginning of rolling, and the accumulation of strain inside the austenite did not progress. Therefore, the refinement of the effective crystal grain size was not achieved, and as a result, the arrest performance was lowered.

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

  Test No. The thick steel plate No. 21 had a low average rolling reduction, so that the accumulation of strain inside the austenite did not progress. Therefore, the refinement of the effective crystal grain size was not achieved, and as a result, the arrest performance was lowered.

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

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

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

  ADVANTAGE OF THE INVENTION According to this invention, the thick steel plate which is excellent in arrestability and plate | board thickness exceeds 70 mm and its manufacturing method can be provided. Therefore, the steel plate of the present invention can be suitably used for a steel plate having a thickness exceeding 70 mm, such as an upper deck or hatch side combing, which is an important member of a container ship.

Claims (6)

A steel plate having a thickness exceeding 70 mm,
Chemical composition is 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 to 0.060%,
N: 0.001 to 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: 0-0.0050%, and
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) The 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 the ¼ t portion of the plate thickness is 15.0 to 40.0%, and the ferrite portion of the ½ t portion of the plate thickness The ratio is 10.0 to 40.0%, and at each plate thickness position, the structure other than ferrite and bainite is less than 5% (including 0%) in total in area%.
(C) The average bainite particle size of ¼ t part of the plate thickness is 25.0 μm or less, and the average bainite particle size of ½ t part of the plate thickness is 35.0 μm or less.
(D) The average effective crystal grain size at ¼ t part of the plate thickness is 22.0 μm or less, and the average effective crystal grain size at ½ 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 mass%,
Cr: 0.05 to 0.50%,
Mo: 0.05 to 0.35%, and
V: 0.005 to 0.15%,
The thick steel plate according to claim 1, comprising at least one selected from the group consisting of: The chemical composition is mass%,
B: 0.0003 to 0.0030%,
The thick steel plate according to claim 1 or 2, comprising: The chemical composition is 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, which contains one or more selected from the above. A method for producing the thick steel plate according to any one of claims 1 to 4,
The steel piece having the composition according to any one of claims 1 to 4 is heated so that the thickness center portion is less than Ac _{3 to} 1000 ° C.
In the temperature range where the sheet thickness center is Ac _{3 to} less than 1000 ° C., after rolling roughly 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 sheet thickness center is Ar _{3 to} 950 ° C., the finish rolling is performed at a cumulative rolling reduction of 40% or more and the average rolling reduction of each pass is 5.0% or more,
Furthermore, the final pass start temperature of this finish rolling is set to Ar ₃ −20 ° C. to Ar ₃ + 30 ° C. on the plate thickness surface, and the rolling is completed.
Next, a method for producing a thick steel plate, in which accelerated cooling is started and accelerated cooling is performed to a surface temperature of 550 ° C. or lower. The manufacturing method of the thick steel plate of Claim 5 which performs a tempering process at the temperature of 350-650 degreeC after the completion | finish of the said accelerated cooling.

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