JP5522084B2 - Thick steel plate manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a thick steel plate, and more particularly, to a method for producing a very thick steel plate for a welded structure having high rolling productivity and excellent low temperature toughness.

  Steel plates used for welded structures such as ships, buildings, tanks, offshore structures, line pipes, wind power towers, etc. are required to have low temperature toughness in order to suppress brittle fracture of the structures. In particular, large structures often use extra-thick steel plates having a yield stress of 315 to 460 MPa and a plate thickness of 60 to 100 mm.

  In general, in order to improve toughness, in the rolling process, rolling with a large cumulative rolling reduction is performed at a low temperature of about 750 to 850 ° C. called a γ non-recrystallization temperature range. As a result, the dislocation density is increased, and the crystal grains can be made fine by generating a large number of ferrites using the dislocations as transformation nuclei. And toughness can be improved by this crystal grain refinement | miniaturization.

  Conventionally, various proposals have been made on methods for improving the toughness of a thick steel plate. For example, there are techniques disclosed in Patent Documents 1 to 5.

  Patent Document 1 describes a steel plate excellent in the arrestability of a brittle crack having a thickness of 40 mm or more. Patent Document 2 describes a method for producing a thick high-strength steel plate having excellent toughness with a thickness of 50 mm or more. In Patent Document 3, the time between passes from the completion of the final five passes in finish rolling to the start before the final four passes is 30 seconds or more, and the time between each pass from the last four passes to the final pass is 15 seconds or less. A method of manufacturing a steel material with little material variation is described.

  In Patent Document 4, rolling conditions are set so as to satisfy a predetermined relationship between rolling temperature and rolling reduction in each rolling pass, and the effect of refinement of recrystallized γ grains and non-recrystallized rolling can be fully enjoyed. A method for producing a steel sheet having a finer final structure and having excellent strength and toughness is described. In Patent Document 5, tandem rolling is performed within 5 seconds between passes by using two rolling mills, the recrystallization is promoted, and the cumulative reduction ratio in the non-recrystallized region is set to 70% or more. A method for producing a steel sheet having excellent toughness is described.

JP 2007-302993 A JP 2007-46096 A JP 2002-249822 A JP 2004-269924 A JP-A-11-181519

  However, Patent Documents 1 to 5 have the following problems.

  The manufacturing method described in Patent Document 1 requires low-temperature rolling (CR) where the plate thickness is large, that is, where the cumulative rolling reduction is large. When low temperature rolling is performed, the crystal grains can be made finer and the toughness is improved. However, low temperature rolling in a place where the plate thickness is thick requires a long time to wait for the temperature to drop, and the rolling productivity is significantly reduced.

  The manufacturing method described in Patent Document 2 requires hot rolling at a place where the plate thickness is 800 to 900 ° C. and is 2.5 times thicker than the product plate thickness. Although this method can be expected to improve toughness, it takes a long time to wait for the temperature to drop, and the rolling productivity is significantly reduced.

  The manufacturing method described in Patent Document 3 is intended for a steel plate having a thin product plate thickness, and does not consider the reduction ratio required for recrystallization rolling or non-recrystallization rolling. It cannot be applied to a manufacturing method for improving rolling productivity and toughness.

  Since the manufacturing method described in Patent Document 4 manages the rolling temperature by the surface temperature, the material variation is large, and furthermore, it is necessary to wait for the temperature of recrystallization rolling in a place where the plate thickness after heat extraction is thick. In addition, the rolling productivity of extra-thick steel plates is reduced.

  Like the manufacturing method described in Patent Document 5, tandem rolling using two rolling mills is not practical because of great restrictions on facilities.

  Therefore, the present invention improves the low productivity due to the need for low temperature rolling in the prior art where the plate thickness is thick, and the yield stress is 315 to 460 MPa, and the plate thickness is 60 to 100 mm. It is an object of the present invention to provide a method for producing an extremely thick steel sheet for welded structures that can be applied to an extremely thick steel sheet, does not require special equipment, has small material variations, and has excellent low-temperature toughness. Specifically, by refining γ grains by recrystallization rolling at high temperature and introducing dislocations in γ by non-recrystallization zone rolling, further specifying the appropriate temperature, reduction rate, and time, production It is an object of the present invention to provide a method for producing a very thick steel plate that has high properties, can refine the structure, and can improve toughness.

  The present inventors diligently studied a method for producing a thick steel plate. As a result, in finish rolling after rough rolling, γ is refined by recrystallization by rolling at a high temperature exceeding 900 to 1020 ° C., which is called a γ recrystallization temperature range, and γ unrecrystallized in a place where the plate thickness is thin. We have found manufacturing conditions that can refine the final structure by rolling at a low temperature of 780 to 900 ° C., which is called a temperature range, and have realized a method for manufacturing a thick steel plate that can achieve both rolling productivity and low temperature toughness.

  Specifically, rolling is performed from a thickness of 1.8 to 2.2 times the product thickness in the γ recrystallization temperature range of the first stage of hot finish rolling (hereinafter also referred to as “primary finish rolling”). . As a result, the recrystallized γ grains can be refined efficiently. Further, rolling is performed from a plate thickness 1.2 to 1.6 times the product plate thickness in the γ non-recrystallization temperature range after the hot finish rolling (hereinafter also referred to as “secondary finish rolling”). As a result, dislocation recovery and recrystallization can be suppressed, the dislocation density can be increased efficiently, and the final structure can be refined. In each finish rolling, the sheet thickness at the start of rolling is limited to be thin as long as the effect of refining the structure is possible, so that the temperature waiting time is short and the productivity can be improved.

  Conventionally, recrystallized γ grains have been refined by rough rolling, and finish rolling at low temperatures has been performed for a long time, waiting for a temperature drop at a sheet thickness of about twice or more the product sheet thickness. For extra-thick steel sheets, if the temperature waiting time is long, the γ grains become extremely coarse, or even if the plate surface is cold, the inside of the plate is hot, and the dislocations introduced in finish rolling are recovered or recrystallized. As a result, there is a problem that the final structure cannot be refined and the productivity is remarkably lowered.

  However, according to the study by the present inventors, it is possible to refine the final structure without impairing the productivity by setting the appropriate temperature range and thickness for both the primary finish rolling and the secondary finish rolling. I found out.

  The present invention has been made on the basis of the above-described findings and further takes into consideration the component composition of steel excellent in productivity and low temperature toughness, and the gist thereof is as follows.

(1) In mass%,
C: 0.04 to 0.16%,
Si: 0.01 to 0.5%,
Mn: 0.2 to 2.5%
P: 0.03% or less,
S: 0.02% or less,
Al: 0.001 to 0.10%,
Nb: 0.003 to 0.02%,
Ti: 0.003 to 0.05%,
N: 0.001 to 0.008%
And a steel slab consisting of the following formula (A) having a carbon equivalent Ceq. Of 0.3 to 0.5%, the balance being Fe and unavoidable impurities is heated to 1000 to 1200 ° C., Rough rolling is performed at a thickness center temperature of 950 to 1200 ° C. to a thickness of 1.8 to 2.2 times the product plate thickness, and then in the austenite recrystallization region at a thickness center temperature of over 900 to 1020 ° C. The primary finish rolling is performed to a thickness of 1.2 to 1.6 times the plate thickness, and then the secondary finish rolling is performed to the product thickness in the austenite non-recrystallized region at a thickness center temperature of 780 to 900 ° C. Subsequently, accelerated cooling is performed from a sheet thickness center temperature of 720 ° C. or more to a temperature of 550 ° C. or less at a sheet thickness center cooling rate of 1 to 10 ° C./s, a sheet thickness of 60 to 100 mm, and a yield stress of 315 to 315 ° C. 460 MPa, microstructure is ferrite and bainnai Or a thick steel plate having a mixed structure of ferrite, pearlite, and bainite and having an average crystal grain size of 5 to 20 μm at the center of the plate thickness.
Carbon equivalent Ceq. = C + Mn / 6 (A)

(2) The steel slab is further mass%, Cu: 0.03-1.5%, Ni: 0.03-2.0%, Cr: 0.03-1.5%, Mo: 0 0.01-1.0%, V: 0.003-0.2%, B: 0.0002-0.005%, Ca: 0.0005-0.01%, Mg: 0.0005-0.01 %, REM: 0.0005 to 0.01% of one type or two or more types, and a carbon equivalent Ceq. Is 0.3-0.5%, The manufacturing method of the thick steel plate as described in said (1) characterized by the above-mentioned.
Carbon equivalent Ceq. = C + Mn / 6 + (Cu + Ni) / 15
+ (Cr + Mo + V) / 5 (B)

  (3) The method for producing a thick steel plate according to (1) or (2) above, wherein a cumulative rolling reduction of the rough rolling is 30 to 80%.

  (4) The rolling reduction in each pass of the finishing primary rolling is 5 to 25%, the rolling reduction in each pass of the finishing secondary rolling is 5 to 25%, and the time between passes is 1 to 60 seconds. The manufacturing method of the thick steel plate in any one of said (1)-(3) characterized by the above-mentioned.

  (5) The method for producing a thick steel plate according to any one of (1) to (4), wherein tempering is performed at 300 to 650 ° C. after the accelerated cooling is completed.

  The method for manufacturing a steel plate for welded structure according to the present invention has a short temperature waiting time and a high rolling productivity because the plate thickness during finish rolling is small. Furthermore, according to the manufacturing method of the present invention, by utilizing the refinement by recrystallization of γ and the increase of dislocation density in γ, the final structure is refined, thereby achieving an extremely thick welded structure with excellent low-temperature toughness. Steel sheets can be manufactured.

  Hereinafter, preferred embodiments of the present invention will be described.

  First, the preferable manufacturing method of the thick steel plate of this invention is demonstrated.

  First, molten steel adjusted to a desired component composition is melted by a known melting method using a converter or the like, and is made into a steel slab by a known casting method such as continuous casting.

  During or after cooling during casting, the steel slab is heated to a temperature of 1000 to 1200 ° C. When the heating temperature of the steel slab is less than 1000 ° C., solutionization becomes insufficient. When the heating temperature exceeds 1200 ° C., the heated γ grains become coarse, and it becomes difficult to refine in the subsequent rolling process, and furthermore, there is a time to wait for the temperature to drop before the start of high temperature rolling, Productivity is reduced. A preferable heating temperature range is 1050 to 1150 ° C.

  Next, rough rolling is performed at a plate thickness center temperature of 950 to 1200 ° C. to a plate thickness of 1.8 to 2.2 times the product plate thickness.

  When the sheet thickness center temperature exceeds 1200 ° C., the recrystallized γ grains cannot be made fine even in the subsequent finish rolling. When the plate thickness center temperature is less than 950 ° C., the productivity is lowered. A preferable plate thickness center temperature is 1000 to 1150 ° C.

  In rough rolling up to a sheet thickness of less than 1.8 times the product sheet thickness, the reduction ratio in the subsequent finish rolling is insufficient, so that the structure cannot be refined. In the rough rolling up to a sheet thickness of more than 2.2 times the product sheet thickness, the temperature waiting time in the subsequent finish rolling becomes longer, so the productivity is lowered. A preferable plate thickness is 1.9 to 2.1 times the product plate thickness.

  The cumulative rolling reduction of rough rolling is particularly preferably 30 to 80%. When the cumulative rolling reduction is less than 30%, porosity remains, and internal cracks, ductility, and toughness may be deteriorated. When the cumulative rolling reduction exceeds 80%, the number of passes increases and the productivity tends to decrease. A preferred cumulative rolling reduction is 35 to 75%.

  Next, primary finish rolling is performed to a thickness of 1.2 to 1.6 times the product thickness in an austenite (γ) recrystallization region having a thickness center temperature exceeding 900 to 1020 ° C.

  When the plate thickness center temperature exceeds 1020 ° C., the recrystallized γ grains cannot be made fine. When the plate thickness center temperature is 900 ° C. or less, the productivity decreases. A preferable thickness center temperature is 920 to 1000 ° C.

  In the primary finish rolling up to a thickness less than 1.2 times the product plate thickness, the reduction ratio in the subsequent secondary finish rolling is insufficient, so the structure cannot be refined. In the primary finish rolling up to a thickness of 1.6 times the product plate thickness, the temperature waiting time in the subsequent secondary finish rolling becomes longer, so the productivity is lowered. The preferred plate thickness is 1.25 to 1.55 times the product plate thickness.

  The rolling reduction per pass of the primary finish rolling is preferably 5 to 25%. If the rolling reduction per pass is less than 5%, it is difficult to refine the recrystallized γ grains, and the number of passes increases, so the productivity tends to decrease. If the rolling reduction per pass exceeds 25%, the burden on the rolling mill becomes very large, which is difficult to realize. The rolling reduction per pass of preferable primary finish rolling is 7 to 23%.

  Next, secondary finish rolling is performed at a plate thickness center temperature of 780 to 900 ° C.

  When the plate thickness center temperature exceeds 900 ° C., it does not sufficiently enter the non-recrystallized region, the increase in dislocation is suppressed, and the structure cannot be made fine. When the plate thickness center temperature is less than 780 ° C., the productivity is lowered. A preferable thickness center temperature is 800 to 880 ° C.

The rolling reduction per pass of the secondary finish rolling is preferably 5 to 25%. When the rolling reduction per pass is less than 5%, dislocation recovery tends to occur, and it is difficult to refine the structure, and the number of passes increases, so the productivity tends to decrease.
If the rolling reduction per pass exceeds 25%, the burden on the rolling mill increases, which is not preferable. Moreover, the rolling reduction per pass of this secondary finish rolling is preferably 7 to 23%.

  Furthermore, the time between passes is preferably 1 to 60 seconds. If the time between passes exceeds 60 seconds, dislocation recovery and recrystallization are likely to occur, so that it becomes difficult to refine the structure and the productivity tends to decrease. The shorter the time between passes, the better. However, if the time is too short, the temperature change due to recuperation increases, making temperature control difficult, so the lower limit is set to 1 second. A preferred time between passes is 2 to 28 seconds. More preferably, it is 4 to 20 seconds.

  Subsequent to the above hot rolling, accelerated cooling is performed from a sheet thickness center temperature of 720 ° C. to a temperature of 550 ° C. or less at a sheet thickness center cooling rate of 1 to 10 ° C./s.

  When the plate thickness center temperature at the start of cooling is less than 720 ° C., the ferrite transformation proceeds, so that it is difficult to obtain a ferrite fine grain structure. When the sheet thickness center cooling rate is less than 1 ° C./s, it becomes difficult to obtain a fine structure. The plate thickness center cooling rate exceeding 10 ° C./s cannot be realized with a steel plate having a plate thickness of 60 mm or more.

  When the cooling stop temperature of accelerated cooling exceeds 550 ° C., it becomes difficult to obtain a fine structure and secure strength.

  Preferred accelerated cooling conditions are a plate thickness center temperature at the start of cooling of 740 ° C. or higher, a cooling rate of 2 to 8 ° C./s, and a cooling stop temperature of 500 ° C. or lower.

  In addition, it is the characteristics of the manufacturing method of the steel plate of this invention to control manufacture using the plate | board thickness center temperature of a steel plate. By using the plate thickness center temperature, it is possible to properly control the manufacturing conditions even when the plate thickness changes compared to the case where the surface temperature of the steel plate is used. Can be manufactured efficiently.

  In the rolling process, usually the temperature distribution inside the steel sheet is calculated while measuring the surface temperature of the steel sheet from heating to rolling, and the rolling control is performed while predicting the rolling reaction force from the calculation result of the temperature distribution. It is carried out. Thus, the steel plate center temperature can be easily obtained during rolling. In the case of performing accelerated cooling, the accelerated cooling is controlled while predicting the temperature distribution inside the plate thickness.

  After performing accelerated cooling, you may temper at 300-650 degreeC as needed.

  When tempering at less than 300 ° C., the effect of tempering is difficult to obtain. When the tempering temperature exceeds 650 ° C., the amount of softening increases and it becomes difficult to ensure the strength. A preferable tempering temperature is 400-600 degreeC.

  The production method of the present invention can be applied to the production of a steel plate having a plate thickness of 60 to 100 mm and a yield stress of 315 to 460 MPa. In particular, the present invention is applicable to the production of yield stress 315 MPa class, 355 MPa class or 390 MPa class steel sheets for hulls and offshore structures.

  For a steel plate having a thickness of less than 60 mm and a steel plate having a yield stress of less than 315 MPa, it is easy to produce by a generally known production method, and it is not necessary to apply the present invention. Productivity and toughness cannot be achieved for steel plates with a plate thickness exceeding 100 mm and steel plates with a yield stress exceeding 550 MPa.

  According to the above production conditions, the final structure can be refined by refining γ by recrystallization in the primary finish rolling and further increasing the dislocation density in γ by the secondary finish rolling. Furthermore, the manufacturing method of the present invention is a manufacturing method in which the temperature waiting time is short and the rolling productivity is excellent by reducing the plate thickness during each finish rolling.

  The component composition of the thick steel plate to which the production method of the present invention is applied is as follows in consideration of strength, toughness, weld heat affected zone (HAZ) toughness, weldability, and the like.

  C is added by 0.04% or more in order to ensure the strength and toughness of the base material. If the C content exceeds 0.16%, it becomes difficult to ensure good HAZ toughness, so the C content is set to 0.16% or less. Preferably, it is 0.05 to 0.15%, more preferably 0.06 to 0.14%.

  Since Si is effective as a deoxidizing element and a strengthening element, 0.01% or more is added. If the Si content exceeds 0.5%, the HAZ toughness is greatly deteriorated, so the Si content is 0.5% or less. Preferably, it is 0.05 to 0.4%, more preferably 0.1 to 0.35%.

  Mn is added in an amount of 0.2% or more in order to ensure the strength and toughness of the base material. If the Mn content exceeds 2.5%, the center segregation becomes prominent, and the toughness of the base material and the HAZ where the center segregation has occurred deteriorates, so the Mn content is 2.5% or less. . Preferably, it is 0.6 to 1.8%, more preferably 0.8 to 1.6%.

  P is an impurity element. In order to ensure the HAZ toughness stably, it is necessary to reduce the P content to 0.03% or less. Preferably, it is 0.02% or less, more preferably 0.015% or less.

  S is an impurity element. In order to stably ensure the characteristics of the base material and the HAZ toughness, the S content needs to be reduced to 0.02% or less. Preferably, it is 0.01% or less, More preferably, it is 0.008% or less.

  Al is an element necessary for deoxidation and reducing O which is an impurity element. In addition to Al, Mn and Si also contribute to deoxidation. However, even when Mn or Si is added, if the Al content is less than 0.001%, O cannot be stably reduced. However, if the Al content exceeds 0.10%, alumina-based coarse oxides and clusters thereof are generated, and the base material and the HAZ toughness are impaired. Therefore, the Al content is 0.10% or less. . Preferably, it is 0.01 to 0.08%, and more preferably 0.015 to 0.06%.

  Nb contributes to the improvement of the strength and toughness of the base material by adding 0.003% or more. However, if the Nb content exceeds 0.02%, the HAZ toughness and weldability deteriorate, so the Nb content is set to 0.02% or less. Preferably, it is 0.005-0.025%, More preferably, it is 0.008-0.018%.

  When Ti is added, TiN is formed and suppresses an increase in the austenite grain size when the steel slab is heated. As the austenite grain size increases, the crystal grain size after transformation also increases and the toughness decreases. In order to obtain a crystal grain size of a size necessary for preventing the toughness from being lowered, it is necessary to add 0.003% or more of Ti. However, if the Ti content exceeds 0.05%, TiC is formed and the HAZ toughness decreases, so the Ti content is set to 0.05% or less. Preferably, it is 0.005 to 0.02%, and more preferably 0.007 to 0.016%.

  N forms TiN and suppresses an increase in the austenite grain size when the steel slab is heated, so 0.001% or more is added. If the N content exceeds 0.008%, the steel material becomes brittle, so the N content is set to 0.008% or less. Preferably, it is 0.0015 to 0.065%, more preferably 0.002 to 0.006%.

  In addition to the above-described additive elements, as optional elements that can be added as necessary, in mass%, Cu: 0.03-1.5%, Ni: 0.03-2.0%, Cr: 0 0.03 to 1.5%, Mo: 0.01 to 1.0%, V: 0.03 to 0.2%, and B: 0.0002 to 0.005%. May be. By adding these elements, the strength and toughness of the base material can be improved. However, if the content of these elements is too large, the HAZ toughness and weldability deteriorate, so the upper limit of the content is specified as described above. Further, if the content of these elements is too small, sufficient HAZ toughness and weldability cannot be obtained, so it is preferable to define the lower limit as described above.

  If necessary, the upper limit of the Cu content is 1.0%, 0.5% or 0.3%, and the upper limit of the Ni content is 1.0%, 0.5% or 0.3%. %, The upper limit of Cr content is 1.0%, 0.5% or 0.3%, the upper limit of Mo content is 0.3%, 0.2% or 0.1%, V The upper limit of the B content may be limited to 0.1%, 0.07% or 0.05%, and the upper limit of the B content may be limited to 0.003%, 0.002 or 0.001%.

  Furthermore, as other selective elements, one or two of Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, and REM: 0.0005 to 0.01% by mass%. You may contain the above. By adding more than the lower limit of these elements, the HAZ toughness is improved. However, even if added exceeding the upper limit, the effect is saturated, and HAZ toughness and weldability are deteriorated.

  These selective elements can be intentionally added to improve the strength and toughness of the base material. However, it is not necessary to add any of these selective elements in order to reduce alloy costs. Even if these elements are not intentionally added, Cu: 0.05% or less, Ni: 0.05% or less, Cr: 0.05% or less, Mo: 0.03 as inevitable impurities % Or less, V: 0.01% or less, B: 0.0004% or less, Ca: 0.0008% or less, Mg: 0.0008% or less: REM: 0.0008% or less can be contained in the steel. . Even when these elements are contained as inevitable impurities in the steel, they do not affect the method for producing a thick steel plate of the present invention.

The steel plate manufactured with the manufacturing method of the thick steel plate for welded structures of this invention makes the carbon equivalent calculated | required by the following (A) and (B) formula to 0.3 to 0.5%. When the selected element is contained as an unavoidable impurity, the carbon equivalent is obtained by substituting its content.
Carbon equivalent Ceq. = C + Mn / 6 (A)
Carbon equivalent Ceq. = C + Mn / 6 + (Cu + Ni) / 15
+ (Cr + Mo + V) / 5 (B)
Carbon equivalent Ceq. If it is less than 0.3%, the strength required for the steel sheet produced by the production method of the present invention cannot be satisfied. If the carbon equivalent exceeds 0.5%, the toughness and weldability required for the steel sheet produced by the production method of the present invention cannot be satisfied. Preferably, the carbon equivalent Ceq. Is 0.31 to 0.44%, more preferably 0.32 to 0.42%, and 0.33 to 0.40%.

  The microstructure of the steel sheet produced by the method for producing a thick steel sheet of the present invention is ferrite and bainite, or a mixed structure of ferrite, pearlite, and bainite. By having such a structure, the strength and toughness required for the steel sheet produced by the production method of the present invention are ensured.

  The average crystal grain size in the central part of the thickness of the steel sheet produced by the method for producing a thick steel sheet of the present invention is 5 to 20 μm. As a result, the strength and toughness required for the steel sheet produced by the method for producing a thick steel sheet of the present invention are satisfied.

  Hereinafter, it demonstrates still in detail based on an Example.

  The component composition of the molten steel was adjusted in the steel making process, and then a steel slab was produced by continuous casting.

  Subsequently, this steel slab was reheated and further subjected to thick plate rolling to obtain a thick steel plate having a thickness of 60 to 100 mm, and then the thick steel plate was water-cooled. Test No. In the steel plate 31, air cooling was performed instead of water cooling (comparative example). Thereafter, heat treatment was performed as necessary.

  Table 1 shows the component composition of each thick steel plate. The underline in Table 1 indicates that the content is outside the scope of the present invention, and the parentheses indicate the analytical value of the amount contained as an unavoidable impurity.

  About each manufactured steel plate, the microstructure phase fraction, the average crystal grain size, and the mechanical property were measured.

  The microstructure phase fraction was defined as the average value of the area ratios of the respective phases with respect to the entire visual field region obtained by imaging the microstructure at a plate thickness center position at a magnification of 500 times with an optical microscope.

  The average crystal grain size is determined by measuring an area of 500 μm × 500 μm at a pitch of 1 μm by an EBSP (Electron Back Scattering Pattern) method, and defining a boundary where the crystal orientation difference from adjacent grains is 15 ° or more as a grain boundary. It was set as the average value of the crystal grain size.

  Among the mechanical properties, the yield stress and Charpy fracture surface transition temperature (vTrs) were tested using test pieces taken from the center of the plate thickness, and the results were used as representative values for each steel plate.

  The tensile test was carried out in accordance with “Metal Material Tensile Test Method” of JIS Z 2241 (1998), and two of them were tested and measured, and the average value was obtained. The tensile test piece was a No. 4 test piece of JIS Z 2201 (1998).

  The Charpy fracture surface transition temperature (vTrs) is based on JIS Z 2242 (2005) “Charpy impact test method for metal materials” using 2 mm V notch Charpy impact test specimens. The temperature at the time of 50% brittle fracture surface ratio was measured.

  These measurement results of each thick steel plate are shown in Tables 2 to 4 together with the manufacturing method. The temperature and cooling rate in the manufacturing method are values at the center position of the plate thickness, and were obtained from the measured surface temperature by heat conduction analysis using a known differential method.

  In this example, a fracture surface transition temperature of −40 ° C. or lower and a rolling time of less than 1000 s were defined as good. Underlines in Tables 2 to 4 indicate that the conditions are outside the scope of the present invention, or that the characteristics and productivity of the steel sheet are outside the values defined as good.

  Test No. 1-No. No. 20 is an example of the present invention that satisfies all the conditions of the present invention, and is good in strength, toughness, and productivity.

  Of these, test no. 16-No. 20 is an example of the present invention. Since it deviates from the preferable conditions of the present invention, the strength, toughness, and productivity are as shown in Test No. 1-No. Compared to 15, it was slightly inferior.

  Test No. 21-No. 40 is a comparative example in which the underlined condition deviates from the scope of the present invention. Among the tests of these comparative examples, the test No. Nos. 21 to 25 had low toughness because the component range was out of the range of the present invention. Test No. In No. 26, since the slab heating temperature was too high, the average crystal grain size was large, the toughness was low, and the rolling time was long and the productivity was low. Test No. In No. 27, since the plate thickness at the start of the primary finish rolling was too large, the rolling time was long and the productivity was low. Test No. In No. 28, since the plate thickness at the start of primary finish rolling was too small, the average crystal grain size was large and the toughness was low. Test No. In No. 29, the primary finish rolling temperature was too high, so the average crystal grain size was large and the toughness was low. Test No. In No. 30, the primary finish rolling temperature was too low, so the rolling time was long and the productivity was low.

  Test No. Since 31 was cooled by air cooling, the average crystal grain size was large, and the strength and toughness were low. Test No. No. 32 was too thick at the start of secondary finish rolling, so the rolling time was long and the productivity was low. Test No. In No. 33, since the plate thickness at the start of secondary finish rolling was too small, the average crystal grain size was large and the toughness was low.

  Test No. Since the secondary finish rolling temperature was too high, No. 34 had a large average crystal grain size and low toughness. Test No. No. 35, because the secondary finish rolling temperature was too low, the rolling time was long and the productivity was low. Test No. In No. 36, since the cooling start temperature was too low, the average crystal grain size was large, and the strength and toughness were low. Test No. Since the cooling rate of No. 37 was too small, the average crystal grain size was large, and the strength and toughness were low.

  Test No. In No. 38, the cooling end temperature was too high, so the average crystal grain size was large, and the strength and toughness were low. Test No. In No. 39, since the plate thickness at the start of the primary finish rolling and the secondary finish rolling was too large, the rolling time was long and the productivity was low. Test No. In No. 40, since the plate thickness at the start of the primary finish rolling and the secondary finish rolling was too small, the rolling time was long, the average crystal grain size was large, and the toughness was low.

  From the test results of the above examples, according to the production method of the present invention, γ is refined by recrystallization in the primary finish rolling, and further, the dislocation density in γ is increased by secondary finish rolling. It was confirmed that by refining the final structure, a thick steel plate having excellent low-temperature toughness can be obtained.

  In addition, this invention is not limited to embodiment mentioned above. Various modifications can be made without departing from the spirit of the present invention.

  The method for producing a thick steel plate according to the present invention can refine the final structure by refining γ by recrystallization in the primary finish rolling, and further increasing the dislocation density in γ by secondary finish rolling. The thickness can be used for welded structures such as shipbuilding, construction, tanks, offshore structures, line pipes, wind power towers, etc. It can be applied to the production of steel sheets and has great industrial applicability.

Claims (5)

% By mass
C: 0.04 to 0.16%,
Si: 0.01 to 0.5%,
Mn: 0.2 to 2.5%
P: 0.03% or less,
S: 0.02% or less,
Al: 0.001 to 0.10%,
Nb: 0.003 to 0.02%,
Ti: 0.003 to 0.05%,
N: 0.001 to 0.008%
And a steel slab consisting of the following formula (A) having a carbon equivalent Ceq. Of 0.3 to 0.5%, the balance being Fe and unavoidable impurities is heated to 1000 to 1200 ° C., Rough rolling is performed at a thickness center temperature of 950 to 1200 ° C. to a thickness of 1.8 to 2.2 times the product plate thickness, and then in the austenite recrystallization region at a thickness center temperature of over 900 to 1020 ° C. The primary finish rolling is performed to a thickness of 1.2 to 1.6 times the plate thickness, and then the secondary finish rolling is performed to the product thickness in the austenite non-recrystallized region at a thickness center temperature of 780 to 900 ° C. Subsequently, accelerated cooling is performed from a sheet thickness center temperature of 720 ° C. or more to a temperature of 550 ° C. or less at a sheet thickness center cooling rate of 1 to 10 ° C./s, a sheet thickness of 60 to 100 mm, and a yield stress of 315 to 315 ° C. 460 MPa, microstructure is ferrite and bainnai Or a thick steel plate having a mixed structure of ferrite, pearlite, and bainite and having an average crystal grain size of 5 to 20 μm at the center of the plate thickness.
Carbon equivalent Ceq. = C + Mn / 6 (A) The steel slab is further mass%, Cu: 0.03-1.5%, Ni: 0.03-2.0%, Cr: 0.03-1.5%, Mo: 0.01- 1.0%, V: 0.003-0.2%, B: 0.0002-0.005%, Ca: 0.0005-0.01%, Mg: 0.0005-0.01%, REM : 0.0005 to 0.01% of one or more of carbon equivalents of the following formula (B) Ceq. The method for producing a thick steel plate according to claim 1, wherein the ratio is 0.3 to 0.5%.
Carbon equivalent Ceq. = C + Mn / 6 + (Cu + Ni) / 15
+ (Cr + Mo + V) / 5 (B)   The method for producing a thick steel plate according to claim 1 or 2, wherein a cumulative rolling reduction of the rough rolling is 30 to 80%.   The rolling reduction in each pass of the finishing primary rolling is 5 to 25%, the rolling reduction in each pass of the finishing secondary rolling is 5 to 25%, and the time between passes is 1 to 60 seconds. The manufacturing method of the thick steel plate in any one of Claims 1-3 characterized by the above-mentioned.   The method for producing a thick steel plate according to any one of claims 1 to 4, wherein after the accelerated cooling is completed, tempering is performed at 300 to 650 ° C.