Classifications

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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
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  • C22C—ALLOYS
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  • C22C—ALLOYS
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  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a steel plate used for steel structures such as buildings, bridges, shipbuilding, offshore structures, construction machines, tanks, penstocks, etc., and more particularly to a thick steel plate having a thickness of 100 mm or more and a method for producing the same.
• Non-Patent Document 1 reports a very thick steel plate of 210 mm in thickness developed for a jack-up rig rack.
• Non-Patent Document 1 describes component compositions and manufacturing conditions for securing the toughness of the central portion of the thick steel plate.
• high strength steel plates having a thickness of 100 mm or more are manufactured by giving high toughness and high toughness in addition to high strength by applying quenching and tempering after hot rolling.
• the cooling rate in the quenching step after hot rolling is lower inside the steel plate on the inner side of the surface layer than in the surface layer of the steel plate. Is likely to be formed.
• the surface layer of the steel plate refers to each region on the front surface side and the back surface side bordering on the position of 1 ⁇ 4 t (t represents the plate thickness) in the thickness direction from the front and back surfaces of the steel plate respectively.
• the inner side (including 1 ⁇ 4 t) is the inside of the steel plate.
• bainite or a mixed structure of bainite and martensite inside the steel sheet at the time of quenching in order to satisfy the strength and toughness inside the thick steel sheet, and alloying elements such as Mn, Ni, Cr, Mo, etc. Needs to be added in large amounts.
• Non-Patent Document 1 does not touch.
• the present invention has been made in view of the above circumstances, and aims to stably manufacture a high strength steel plate excellent in toughness, not only for the inside of the steel plate but also for the surface of the steel plate.
• the present inventors target a thick steel plate having a yield strength of 620 MPa or more and a plate thickness of 100 mm or more and a microstructural control factor for suppressing the reduction in toughness in the steel plate surface and the strength in the steel plate.
• the following findings I to III were obtained.
• the microstructure In order to obtain high strength while maintaining good toughness in the steel plate inside where the cooling rate is significantly reduced compared to the surface of the steel sheet during quenching, even if the quenching is a low cooling rate, the microstructure has a martensite and / or bainite microstructure. In order to do so, it is necessary to appropriately select the component composition and to make the carbon equivalent 0.57% or more.
• the surface layer of the steel sheet and the inside of the steel sheet are in a temperature range of (Ar3 transformation point + 50) ° C or more (Ar3 transformation point -20) ° C or less
• the average cooling rate is important to form bainite at a predetermined ratio or more on the surface of the steel sheet.
• the component composition is further In mass%, Cu: 0.50% or less, Mo: 1.50% or less, Nb: not more than 0.100%, V: 0.200% or less and Ti: 0.005% or more and 0.020% or less selected from one or more selected from the above formula (1) in a range satisfying the following formula (2),
• the component composition is further In mass%, Mg: 0.0005% or more and 0.0100% or less, Ta: 0.010% or more and 0.200% or less, Zr: 0.0050% to 0.1000%, Y: 0.001% or more and 0.010% or less,
• the component composition is further In mass%, Cu: 0.50% or less, Mo: 1.50% or less, Nb: not more than 0.100%, V: 0.200% or less and Ti: 0.005% or more and 0.020% or less
• the component composition is further In mass%, Mg: 0.0005% or more and 0.0100% or less, Ta: 0.010% or more and 0.200% or less, Zr: 0.0050% to 0.1000%, Y: 0.001% or more and 0.010% or less,
• the manufacturing method of the steel plate of said 4 or 5 containing 1 type, or 2 or more types chosen from Ca: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0200% or less.
• C 0.080% or more and 0.200% or less C is a useful element to obtain the strength required for the structural steel at low cost, and addition of 0.080% or more is necessary to obtain the effect.
• the content exceeds 0.200%, the toughness of the base material and the weld zone is significantly deteriorated, so the upper limit is made 0.200%.
• it is set to 0.080% or more and 0.140% or less.
• Si 0.40% or less Si is added preferably at 0.05% or more for deoxidation, but if it is added in excess of 0.40%, the toughness of the base material and the weld heat affected zone is significantly reduced, so the amount of Si is 0.40 % Or less. Preferably, it is 0.05% or more and 0.30% or less. More preferably, it is 0.05% or more and 0.25% or less.
• Mn 0.50% or more and 5.00% or less Mn is added from the viewpoint of securing the base material strength, but the addition of less than 0.50% does not have sufficient effect.
• it is added in excess of 5.00%, not only the toughness of the base material is deteriorated but also the upper limit is made 5.00% in order to promote center segregation.
• it is 0.60% or more and 2.00% or less. More preferably, it is 0.60% or more and 1.60% or less.
• P 0.015% or less
• the toughness of the base material and the weld heat affected zone is significantly reduced. Therefore, it limits to 0.015% or less. Preferably, it is 0.010% or less.
• it since it is difficult in manufacture on industrial scale to make it less than 0.001%, containing 0.001% or more is accept
• S 0.0050% or less
• S is contained in excess of 0.0050%, the toughness of the base material and the weld heat affected zone is significantly reduced. Therefore, S is made 0.0050% or less. Preferably, it is 0.0010% or less.
• it is difficult in manufacture of industrial scale to make less than 0.0001%, containing 0.0001% or more is accept
• Cr 3.00% or less
• Cr is an element effective for increasing the strength of the base material, and is preferably added at 0.10% or more, but if added in large amounts, it reduces weldability. Therefore, Cr is made 3.00% or less. Preferably, it is 0.10% or more and 2.00% or less.
• Ni 5.00% or less
• Ni is a beneficial element to improve the strength of the steel and the toughness of the weld heat affected zone, and is preferably added at 0.50% or more, but if it is added above 5.00%, the economic efficiency is significantly reduced Do. Therefore, the Ni content is 5.00% or less. Preferably, it is 0.50% or more and 4.00% or less.
• Al 0.080% or less Al is added to sufficiently deoxidize molten steel, but when it is added in excess of 0.080%, the amount of Al solid-solved in the base material increases, and the base material toughness is lowered. Therefore, Al is made 0.080% or less. Preferably, it is 0.030% or more and 0.080% or less. More preferably, it is 0.030% or more and 0.060% or less.
• N has the effect of refining the structure by forming a nitride with Al or the like and improving the toughness of the base material and the weld heat affected zone, so 0.0020% or more of N is preferably added. May be However, when it is added in excess of 0.0070%, the amount of nitrides precipitated in the base material increases, the base material toughness is significantly reduced, and a coarse carbonitride is formed in the heat affected zone to lower the toughness. Therefore, N is 0.0070% or less. Preferably, it is 0.0050% or less, more preferably 0.0040% or less. Note that N may be 0%.
• B 0.0030% or less
• B has the effect of enhancing the hardenability by segregating at austenite grain boundaries, thereby suppressing ferrite transformation from the grain boundaries, and therefore, B is preferably added at 0.0003% or more.
• B is made 0.0030% or less.
• it is 0.0003% or more and 0.0030% or less. More preferably, it is 0.0005% to 0.0020%.
• Carbon equivalent CeqIIW in order to secure a strength of 620 MPa or more and good toughness in yield strength particularly in a steel plate having a plate thickness of 100 mm or more, it is necessary to design an appropriate component composition. It is necessary to adjust the component composition to a range satisfying the above. This is because, if the carbon equivalent does not satisfy the following formula (1), ferrite or the like having low strength is easily formed, and it becomes difficult to stably secure a desired strength. [C] + [Mn] / 6 + [Ni] / 15 + [Cr] /15 ⁇ 0.57 (1) here, [] Is the content (% by mass) of the element in the [].
• the basic components of the present invention have been described above.
• the balance other than the above components is Fe and unavoidable impurities, but in the present invention, other elements can be appropriately contained as needed.
• Cu 0.50% or less Cu can improve the strength of the steel without losing the toughness, but if it is added more than 0.50%, cracking occurs in the surface layer of the steel sheet during hot working. Therefore, when it contains Cu, it makes it 0.50% or less. Preferably, it is 0.03% or more and 0.40% or less.
• Mo 1.50% or less Mo is an element effective for strengthening the base material, but when it is added in excess of 1.50%, precipitation of alloy carbides increases hardness and lowers toughness. Therefore, when it contains Mo, it is 1.50% or less. Preferably, it is 0.02% or more and 0.80% or less.
• Nb not more than 0.100% Nb is effective because it is effective in improving the strength of the base material, but addition exceeding 0.100% significantly reduces the toughness of the base material. Therefore, when Nb is contained, the upper limit is made 0.100%. Preferably, it is 0.025% or less. In addition, since the improvement effect of a characteristic will not be acquired if it shall be less than 0.003%, when adding, it shall be 0.003% or more.
• V 0.200% or less
• V is effective in improving the strength and toughness of the base material, and is effective in reducing solid solution N by precipitating as VN, but when added in excess of 0.200%, it is hard VC Toughness decreases due to precipitation of Therefore, when V is contained, it is 0.200% or less. Preferably, it is 0.010% or more and 0.100% or less.
• Ti 0.005% or more and 0.020% or less Ti forms TiN at the time of heating, effectively suppresses austenite coarsening, and improves the toughness of the base material and the weld heat affected zone. However, if it is added in excess of 0.020%, the Ti nitride is coarsened to lower the toughness of the base material. Therefore, when it contains Ti, it is referred to as 0.005% or more and 0.020% or less. Preferably, it is 0.008% or more and 0.015% or less.
• Mg 0.0005% to 0.0100%
• Ta 0.010% to 0.200%
• Zr 0.0050% to 0.1000%
• Y 0.001% to 0.010%
• Ca 0.0005% It can be made to contain 1 type or 2 types or more chosen from more than 0.0050% and REM: 0.0005% or more and 0.0200% or less.
• Mg 0.0005% or more and 0.0100% or less Mg forms a stable oxide at a high temperature, effectively suppresses the coarsening of the old ⁇ grain in the weld heat affected zone, and is effective for improving the toughness of the weld zone It is an element.
• the addition amount is less than 0.0005%, a clear effect is not obtained, and when the addition amount exceeds 0.0100%, the amount of inclusions increases and the toughness decreases. Therefore, when it contains Mg, it is referred to as 0.0005% or more and 0.0100% or less. Preferably, it is 0.0005% or more and 0.0050% or less.
• Ta 0.010% or more and 0.200% or less Ta is effective in improving the strength. However, when the addition amount is less than 0.010%, a clear effect can not be obtained, and when it exceeds 0.200%, toughness is reduced due to the formation of precipitates. Therefore, in the case of containing Ta, the content is made 0.010% or more and 0.200% or less.
• Zr 0.0050% or more and 0.1000% or less
• Zr is an element effective for increasing the strength, but a remarkable effect is not obtained when the addition amount is less than 0.0050%, and coarse precipitation occurs when it exceeds 0.1000%.
• Y 0.001% or more and 0.010% or less Y forms a stable oxide at a high temperature, effectively suppresses the coarsening of old ⁇ grains in the heat affected zone of the welding, and is effective for improving the toughness of the welded portion It is an element.
• the addition of less than 0.001% is not effective, and the addition of more than 0.010% increases the amount of inclusions and lowers the toughness. Therefore, when Y is contained, the content is set to 0.001% or more and 0.010% or less.
• Ca 0.0005% or more and 0.0050% or less
• Ca is an element useful for controlling the form of sulfide inclusions, and in order to exert its effect, addition of 0.0005% or more is necessary. However, if it is added in excess of 0.0050%, the cleanliness will be reduced and the toughness will be degraded. Therefore, when it contains Ca, it is referred to as 0.0005% or more and 0.0050% or less. Preferably, they are 0.0005% to 0.0025%.
• REM 0.0005% or more and 0.0200% or less REM (rare earth metal) also has the effect of forming oxides and sulfides in the steel to improve the quality of the material as Ca, and in order to obtain the effect, the addition of 0.0005% or more is necessary. However, even if it is added over 0.0200%, the effect is saturated. Therefore, when REM is contained, the content is made 0.0005% or more and 0.0200% or less. Preferably, it is 0.0005% or more and 0.0050% or less.
• the bainite area fraction in the surface layer of the steel sheet it is important to set to 10% or more.
• the bainite area fraction of the surface layer of the steel sheet is preferably 20% or more.
• the balance is tempered martensite, ferrite or the like.
• the bainite area fraction in not only a steel plate surface layer but the inside of a steel plate 10% or more. Since the inside of the steel plate also has such a structure, it is possible to obtain a steel plate with a small difference in characteristics between the surface of the steel plate and the inside of the steel plate.
• the bainite area fraction inside the steel plate is more preferably 20% or more.
• evaluation of the area fraction of the structure of the steel plate surface layer and the inside of the steel plate is performed by taking a sample of the cross section of the as-quenched steel in the rolling direction, revealing the structure with nital corrosive liquid, and 5 fields of view with a 200 ⁇ optical microscope It can carry out by observing above and calculating
• the surface layer of the steel plate a sample of a cross section in the rolling direction with a thickness of 15 mm is taken centering on the position of 1/8 t of the plate thickness.
• the sample of a 15-mm-thick rolling direction cross section is extract
• the average cooling rate is 0.2 to 10 ° C./s in the temperature range of (Ar 3 transformation point + 50) ° C. or more (Ar 3 transformation point -20) ° C. It is necessary to apply some cooling treatment to cool to 350 ° C. or less. As for the temperature conditions specified here, it is important that both the surface layer of the hot rolled steel sheet and the inside of the steel sheet be satisfied. Details will be described later.
• the toughness of the steel sheet surface has not been paid attention so far, it has been required to have the same level as the inside of the steel sheet in response to the increasing demand for improving the safety of the structure.
• the difference in toughness between the surface layer surface of the steel plate and the inside of the steel plate is evaluated by the ductile-brittle fracture transition temperature (vTrs)
• the difference in vTrs is preferably 20 ° C. or less.
• vTrs was evaluated by the method described in JIS Z2242.
• vTrs The difference between vTrs is 20 ° C or less, because even if the evaluation of toughness by vTrs is the same toughness level, the value may be up to about 20 ° C due to the measurement error of the brittle surface fracture rate. It was within 20 ° C. which is considered to be substantially equivalent.
• the yield strength inside the steel plate is 620 MPa or more. The reason is that a yield strength of 620 MPa or more is required to contribute to the enlargement of the structure.
• the temperature in the following description shall mean the temperature in a plate thickness center part (1 / 2t).
• the molten steel having the above composition is melted by a usual method such as a converter, an electric furnace, a vacuum melting furnace or the like, and made into a steel material such as a slab or a billet by a usual casting method such as a continuous casting method or a block forming method.
• the steel material may be further forged or rolled to reduce the thickness of the steel material.
• the steel plate after the hot rolling is air cooled or accelerated cooled.
• accelerated cooling is effective to improve toughness. Accelerated cooling shortens the residence time in a high temperature range as compared with air cooling, and can suppress refinement of the crystal grain size and coarsening of precipitates. Therefore, in the case of accelerated cooling, the Ar content is less than 3 points.
• the cooling at the time of accelerated cooling is performed by water cooling or air blast, and in either case, it is preferable to set a cooling rate of 0.1 ° C./s or more on the steel sheet surface.
• Heating temperature after hot rolling Ac 3 transformation point or more and 1050 ° C. or less
• the hot-rolled steel sheet after cooling is heated to the Ac 3 transformation point or more and 1050 ° C. or less.
• the reason for heating above the Ac 3 transformation point is to homogenize the steel into an austenite single phase.
• the reason for setting the reheating temperature to 1050 ° C. or lower is that the reduction in base material toughness due to the coarsening of austenite grains is significantly reduced in high temperature reheating exceeding 1050 ° C.
• the temperature is higher than the Ac 3 transformation point and lower than or equal to 1000 ° C.
• the Ac 3 transformation point or more and 950 ° C. or less are more preferable.
• bainite By performing such cooling, a structure having a bainite area fraction of 10% or more can be formed on the surface of the steel sheet, and the toughness of the surface of the steel sheet can be remarkably improved. Similarly, in the inside of the steel sheet, bainite can form a structure of 10% or more.
• the control of the cooling rate can be performed by a method of adjusting the flow rate of water, performing intermittent cooling, or performing cooling by a blast. Specifically, control of the average cooling rate in the steel sheet surface layer and the inside of the steel sheet is performed by deriving a cooling method, water amount adjustment, intermittent conditions by simulation or the like so as to achieve a desired cooling rate.
• the temperature of the steel plate surface layer and the inside of the steel plate can be determined by simulation calculation or the like from the plate thickness, the surface temperature, the cooling condition and the like.
• the temperature from the surface of the steel plate to the inside of the steel plate can be determined by calculating the temperature distribution in the thickness direction using the difference method.
• Ar 3 transformation point a value that is calculated by the following equation (4).
• Ar 3 910-310 [C] -80 [Mn] -20 [Cu] -15 [Cr] -55 [Ni] -80 [Mo] (4)
• each elemental symbol in Formula (4) shows content (mass%) in the steel raw material of each component composition, and what is not contained is calculated as zero.
• the cooling stop temperature is set to 350 ° C. or less. If cooling to 350 ° C. or less, transformation is completed in the entire steel plate and a uniform structure is obtained.
• a method of cooling it is general to use water cooling industrially, but the method of cooling may be other than water cooling, for example, there is also a method such as gas cooling.
• tempering is performed in a temperature range of 450 ° C. to 700 ° C. as necessary. If it is less than 450 ° C., the removal effect of residual stress is small, while if it exceeds 700 ° C., various carbides precipitate and the structure of the base material becomes coarse, and the strength and the toughness decrease significantly.
• the steel is repeatedly quenched for the purpose of toughening the steel, but in the present invention, it may be quenched repeatedly.
• the average cooling rate in the temperature range of (Ar 3 transformation point + 50) ° C or more (Ar 3 transformation point-20) ° C or less is 0.2 ° C / s or more and 10 ° C / It is preferable to perform cooling which is s or less, then to 350 ° C. or less, and temper at 450 ° C. or more and 700 ° C. or less.
• the steels No. 1 to 31 shown in Table 1 are melted and made into slabs, then made into steel plates with a thickness of 100 mm or more and 240 mm or less according to the manufacturing conditions shown in Table 2. Thereafter, cooling treatment and tempering treatment The sample No. 1 to 37 thick steel plates were manufactured and subjected to the following test.
• YS inside the steel plate is less than 620 MPa, TS is less than 720 MPa, or surface of steel plate and 1 / 4t part Has a toughness (vTrs) of -30 ° C. or higher, or a difference of vTrs exceeds 20 ° C., and any of the above-mentioned properties is inferior.
• the present invention it is possible to obtain a thick steel plate of 100 mm or more excellent in toughness of the surface layer of the steel plate, strength and toughness inside the steel plate, and manufacturing stability while the yield strength of the base material is 620 MPa or more. It greatly contributes to the enlargement of the steel structure and the improvement of the safety of the steel structure.

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