WO2013175745A1 - 脆性き裂伝播停止特性に優れた構造用高強度厚鋼板およびその製造方法 - Google Patents
脆性き裂伝播停止特性に優れた構造用高強度厚鋼板およびその製造方法 Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- 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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- 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
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- 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
- C21D8/0221—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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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- 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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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- 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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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- C21—METALLURGY OF IRON
- 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- C21—METALLURGY OF IRON
- 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
Definitions
- the present invention relates to a high-strength thick steel plate excellent in brittle crack propagation arrestability and a method for producing the same, and in particular, a ship using a steel plate having a thickness of 50 mm or more, an offshore structure It is related with a thing suitable for large structures, such as a thing, a low-temperature storage tank, and a construction and civil engineering structure.
- brittle crack propagation arrestability As a means to improve brittle crack propagation arrestability (brittle crack arrestability) of steel materials, a method of increasing the Ni content has been conventionally known. In a storage tank of liquefied natural gas (Liquefied Natural Gas) (LNG) 9% Ni steel is used on a commercial scale.
- LNG liquefied Natural Gas
- TMCP THERMO-MECHANICAL CONTROL PROCESS
- Graining can be achieved, low temperature toughness can be improved, and excellent brittle cracking arrestability can be imparted.
- Patent Document 1 proposes a steel material in which the microstructure of the surface layer is ultrafine crystallization in order to improve the brittle crack propagation arrestability without increasing the alloy cost.
- the steel material with excellent brittle crack propagation characteristics described in Patent Document 1 is shear lips (plastic deformation region) generated in the steel surface layer when a brittle crack propagates. Focusing on the effect of improving the brittle crack propagation arrestability, the propagation of the propagating brittle crack is reduced by refining the crystal grains in the shear lips (plastic deformation region). It is characterized by absorbing energy.
- the surface layer portion is cooled below the Ar 3 transformation point by controlled cooling after hot rolling, and then the controlled cooling is stopped and the surface layer portion is transformed.
- the process of recuperate more than the point) is repeated one or more times.
- the steel material is subjected to reduction, and it is repeatedly transformed or processed and recrystallized. ) Or bainite structure is described.
- both surface portions of the steel material have an equivalent grain size.
- the steel materials excellent in brittle crack propagation arrestability (brittle1 ⁇ 2crack arrestability) described in Patent Documents 1 and 2 are recuperate after cooling only the steel surface layer, and during recuperate. In this process, a specific structure is obtained, and control is not easy on an actual production scale. Particularly, in the case of a thick material having a plate thickness exceeding 50 mm, it is a process with a heavy load on the rolling and cooling equipment.
- Patent Document 3 focuses on not only the refinement of ferrite crystal grains but also the subgrains formed in the ferrite crystal grains, and improves the brittle crack propagation arrestability (brittle crack arrestability). The technology on the extension of (THERMO-MECHANICAL CONTROL PROCESS) is described.
- a method of improving brittle crack propagation characteristics by applying a reduction to transformed ferrite to develop a texture is also known.
- the resistance to brittle fracture is increased by generating separation on the fracture surface of the steel material in a direction parallel to the plate surface and relaxing the stress at the tip of the brittle crack.
- the (110) plane X-ray intensity ratio is set to 2 or more by controlled rolling, and coarse grains having a diameter equivalent to circles (diameter equivalent to a circle in the crystal grains) of 20 ⁇ m or more are set to 10% or less.
- coarse grains having a diameter equivalent to circles (diameter equivalent to a circle in the crystal grains) of 20 ⁇ m or more are set to 10% or less.
- Non-Patent Document 1 describes the stopping of brittle crack propagation of steel plates having a thickness of 65 mm. The characteristics (brittle crack arrestability) were evaluated, and the results of the brittle crack not stopping in the large brittle crack propagation stop test of the base metal were reported.
- the Kca value ( ⁇ 10 ° C.) at the use temperature of ⁇ 10 ° C.) is less than 3000 N / mm 3/2.
- Kca ⁇ 10 ° C.
- Patent Documents 1 to 4 mainly have a thickness of about 50 mm from manufacturing conditions and disclosed experimental data, and have a thickness exceeding 50 mm. When it is applied to meat, it is unclear whether a predetermined characteristic can be obtained, and the characteristics against crack propagation in the plate thickness direction necessary for the hull structure have not been verified at all.
- the present invention optimizes rolling conditions and achieves brittle crack propagation arrestability (brittle crack arrestability) that can be stably manufactured by an industrially simple process that controls texture in the thickness direction. It is an object of the present invention to provide an excellent high-strength steel plate having a thickness of 50 mm or more and a method for producing the same.
- the inventors of the present invention have made extensive studies to achieve the above-mentioned problems, and stably obtain a high-strength thick steel plate having excellent crack propagation stopping characteristics even in a thick steel plate and the steel plate.
- the following knowledge about the manufacturing method was obtained. 1.
- the Charpy fracture surface is 1/2 + 6 mm of the plate thickness (t). Particularly good results are obtained when the transition temperature is below -40 ° C.
- it is effective to reduce specific chemical components, particularly impurity elements Si and P.
- the rolling conditions are also important. By rolling under specific hot rolling conditions that define the temperature at the center of the plate thickness, the microstructure can be made into a structure mainly composed of processed ferrite. As a result, further improvement in toughness is achieved.
- the present invention has been made by further studying the obtained knowledge, that is, the present invention, 1.
- Steel composition is mass%, C: 0.03-0.20%, Si: 0.1% or less, Mn: 0.5-2.2%, P: 0.008% or less, S: 0.00. 01% or less, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.03%, Al: 0.005 to 0.08%, N: 0.0075% or less, the balance being Fe and inevitable
- It has a composition composed of mechanical impurities, is a structure mainly composed of ferrite whose microstructure has been processed, and has a Charpy fracture surface transition temperature of 1 ⁇ 2 + 6 mm part of the plate thickness (t) of ⁇ 40 ° C. or lower.
- high-strength thick steel plate with excellent brittle crack arrestability.
- the steel composition is further mass%, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.0%, Cr: 0.01 to 0.5%, Mo: 0.01 to 0 0.5%, V: 0.001 to 0.1%, B: 0.003% or less, Ca: 0.005% or less, REM: 0.01% or less.
- high strength steel plate high structural steel
- the steel material (slab) having the composition described in 5.1 or 2 is heated to a temperature of 1000 to 1200 ° C., and rolling is performed at a central thickness of 30% or more in the austenite recrystallization temperature range.
- a high-strength thick steel plate having an appropriately controlled texture in the thickness direction and excellent in brittle crack propagation characteristics (brittle crack arrestability) is obtained, and the thickness is 50 mm or more, preferably It is effective to apply to a steel plate having a plate thickness of more than 50 mm, more preferably a plate thickness of 55 mm or more, and even more preferably a plate thickness of 60 mm or more, because it exerts a marked advantage over the steel according to the prior art.
- it is extremely useful in the industry, for example, by contributing to improved safety of ships by applying it to hatch side combing and deck members in the structure of large decks of bulk container ships and bulk carriers.
- Base material toughness 2. chemical composition; Define the microstructure. 1. Base material toughness In order to suppress the growth of cracks, it is an important requirement that the base material toughness at the center of the plate thickness has good characteristics. In the steel plate according to the present invention, the Charpy fracture surface transition temperature at 1/2 + 6 mm part of the plate thickness (t) is specified.
- the thickness (t ) Is defined as ⁇ 40 ° C. or lower.
- the Charpy fracture surface transition temperature at 1/2 + 6 mm part of the plate thickness (t) is taken by shifting the center position of the Charpy impact test piece by 6 mm from the 1/2 part plate thickness (that is, the plate thickness center).
- C 0.03 to 0.20%
- C is an element that improves the strength of steel. In the present invention, it is necessary to contain 0.03% or more in order to ensure the desired strength. Not only is it deteriorated, it also has an adverse effect on toughness. Therefore, C is specified in the range of 0.03 to 0.20%. Preferably, the content is 0.05 to 0.15%.
- Si 0.1% or less Si is effective as a deoxidizing element and as a steel strengthening element.
- the content is made 0.1% or less in order to prevent toughness deterioration in the central part of the steel plate.
- Mn 0.5 to 2.2% Mn is contained as a strengthening element. If it is less than 0.5%, the effect is not sufficient, and if it exceeds 2.2%, the weldability deteriorates and the cost of the steel material increases, so 0.5 to 2.2%.
- P 0.008% or less
- P is an inevitable impurity in steel.
- An increase in the amount of P causes a deterioration in toughness. Therefore, in order to keep the toughness of the central portion of the steel sheet favorable, the upper limit needs to be P: 0.008% or less.
- S 0.01% or less S, like P, is an unavoidable impurity in steel. If it exceeds 0.01%, the toughness deteriorates, so 0.01% or less is desirable, and 0.005% or less is more desirable.
- Nb 0.005 to 0.05%
- Nb precipitates as NbC at the time of ferrite transformation or reheating, and contributes to the increase in strength.
- Nb has the effect of expanding the non-recrystallized region in rolling in the austenite region, and contributes to the refinement of ferrite, so it is also effective in improving toughness.
- it is necessary to contain 0.005% or more, but if it exceeds 0.05%, coarse NbC precipitates and conversely causes a decrease in toughness, so 0.005 to 0 .05%.
- Ti 0.005 to 0.03%
- Ti When Ti is contained in a trace amount, it forms nitrides, carbides, or carbonitrides, and has the effect of refining crystal grains and improving the base material toughness. The effect is obtained by containing 0.005% or more, but if it exceeds 0.03%, the toughness of the base metal and the weld heat affected zone is lowered, so 0.005 to 0.03% is set. .
- Al acts as a deoxidizing agent, and for this purpose, it is necessary to contain 0.005% or more. However, if it contains more than 0.08%, the toughness is lowered and welding is performed. Reduce the toughness of the metal part. For this reason, Al is made 0.005 to 0.08%. Preferably, the content is 0.02 to 0.04%.
- N 0.0075% or less N combines with Al in the steel, adjusts the crystal grain size at the time of rolling, and strengthens the steel. However, if it exceeds 0.0075%, the toughness deteriorates. 0075% or less.
- the above is the basic component composition of the present invention, the balance being Fe and unavoidable impurities.
- one or more of Cu, Ni, Cr, Mo, V, B, Ca, and REM are included. It is possible.
- Cu, Ni, Cr, Mo Cu, Ni, Cr, and Mo are all elements that enhance the hardenability of steel. While contributing directly to strength improvement after rolling, it can be included for functional improvement such as toughness, high-temperature strength, or weather resistance, but excessive content deteriorates toughness and weldability.
- the upper limit is preferably set to Cu: 0.5%, Ni: 1.0%, Cr: 0.5%, Mo: 0.5%, respectively.
- the content is less than 0.01%, the effect does not appear. Therefore, when it is contained, it is preferable that the content is 0.01% or more.
- V 0.001 to 0.1%
- V is an element that improves the strength of the steel by precipitation strengthening as V (CN), and may be contained in an amount of 0.001% or more, but if it exceeds 0.1%, the toughness is lowered. Therefore, when V is contained, the content is preferably 0.001 to 0.1%.
- B 0.003% or less B may be contained in a small amount as an element that enhances the hardenability of steel. However, if it exceeds 0.003%, the toughness of the welded portion is lowered. Therefore, when it is included, the content is preferably 0.003% or less.
- REM 0.01% or less Ca
- REM is necessary because it refines the structure of the weld heat-affected zone and improves toughness, and even if contained, the effect of the present invention is not impaired. You may make it contain according to it. However, if excessively contained, coarse inclusions are formed and the toughness of the base material is deteriorated. Therefore, when it is included, the upper limit of the content is 0.005% for Ca and 0.01% for REM. Is preferred.
- the carbon equivalent (Ceq) represented by the following formula is preferably 0.45% or less.
- Ceq C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (Each element symbol on the right side indicates the content (mass%) of the element.) 3.
- Microstructure Toughness is greatly affected by the microstructure as well as the chemical composition.
- a ferrite structure (ferrite structure) processed and flattened structure that is, a processed ferrite (hereinafter, simply referred to as processed ferrite) as a main component
- the structure in the thickness direction is refined to improve toughness.
- the structure mainly composed of processed ferrite refers to a structure in which the area fraction of the processed ferrite ferrite is 50% or more.
- the balance is one or more selected from pearlite, bainite, martensite, island martensite (MA), and ferrite that has not been processed after transformation from austenite. 4).
- Production Conditions As production conditions for the thick steel plate according to the present invention, the slab heating temperature, the cumulative reduction ratio in the austenite recrystallization temperature range in hot rolling, the cooling rate after rolling in the austenite recrystallization temperature range to Ar 3 points or less , Ar The cumulative rolling reduction and cooling rate at 3 points or less, the cooling stop temperature and the temper temperature are defined.
- the temperature (° C.) is the temperature at the center of the plate thickness (1/2 t portion (t is the plate thickness)) of the steel plate.
- the temperature at the center of the plate thickness of the steel plate is determined by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, the temperature at the center of the plate thickness of the steel sheet is obtained by calculating the temperature distribution in the plate thickness direction using the difference method.
- Thick materials with a thickness of 50 mm or more used for outer plates of ship's hulls such as recent container ships and bulk carriers have Kca values at -10 ° C to ensure structural safety. It is necessary to obtain brittle crack propagation stopping performance with Kca ( ⁇ 10 ° C.) of 6000 N / mm 3/2 or more.
- molten steel having the above composition is melted in a converter or the like, and is made into a steel material (slab) (slab) by continuous casting or the like.
- the obtained steel material (slab) is heated to a temperature of 1000 to 1200 ° C. and then hot-rolled.
- the heating temperature is set to 1000 to 1200 ° C. From the viewpoint of toughness, the preferred heating temperature range is 1000 to 1150 ° C, more preferably 1000 to 1050 ° C.
- hot rolling is performed such that the temperature at the center of the plate thickness is 30% or more in the austenite recrystallization temperature range. If the cumulative rolling reduction is less than 30%, the austenite is not sufficiently refined and the toughness is not improved.
- the first cooling is performed until the temperature at the central portion of the sheet thickness becomes Ar 3 points or less.
- the cooling rate up to the Ar 3 point or lower is set to 15 ° C./s or lower.
- the Ar 3 point (° C.) is obtained by the following equation.
- Ar 3 (° C.) 910-273C-74Mn-57Ni-16Cr-9Mo-5Cu
- each element symbol is the content (% by mass) in steel, and 0 if not contained.
- the rolled steel sheet is subjected to the second cooling to 600 ° C. or less at a cooling rate of 4 ° C./s or more.
- the cooling rate is lower than 4 ° C./s, the structure becomes coarse and the toughness decreases.
- the cooling stop temperature is set to 600 ° C. or less.
- the tempering temperature is set to 1 Ac or less so as not to damage the structure obtained by rolling and cooling.
- Ac 1 point (° C.) is obtained by the following equation.
- Ac 1 point 751-26.6C + 17.6Si-11.6Mn-169Al-23Cu-23Ni + 24.1Cr + 22.5Mo + 233Nb-39.7V-5.7Ti-895B
- each element symbol is the content (% by mass) in steel, and 0 if not contained.
- Molten steel (steel symbols A to P) of each composition shown in Table 1 is melted in a converter and made into a steel material (slab) (slab 280 mm thick) by a continuous casting method, with the first cooling in the middle After hot rolling to a plate thickness of 50 to 80 mm, second cooling is performed and Sample steels 1 to 22 were obtained.
- Table 2 shows hot rolling conditions and cooling conditions.
- a JIS 14A test piece having a diameter of 14 mm was taken from 1/4 part of the plate thickness (t) so that the longitudinal direction of the test piece was perpendicular to the rolling direction, a tensile test was performed, and the yield point ( Yield Strength (YS) and Tensile Strength (TS) were measured.
- the microstructure was confirmed by observing three fields of view with an optical microscope at a magnification of 400 with respect to a cross section parallel to the rolling direction from 1 ⁇ 4 part of the plate thickness.
- a JIS No. 4 impact test piece was taken so that the longitudinal axis direction of the test piece was parallel to the rolling direction so that a 1/2 + 6 mm portion of the plate thickness (t) was the center of the test piece, and a Charpy impact test was performed.
- the fracture surface transition temperature (vTrs) was determined.
- the Charpy fracture surface transition temperature at 1 ⁇ 2 + 6 mm part of the plate thickness (t) was ⁇ 40 ° C. or less within the scope of the present invention.
- Table 3 shows the results of these tests.
- Kca ⁇ 10 ° C.
- the volume fraction of the processed ferrite was 50% or more.
- test steel plates whose composition is outside the scope of the present invention production Nos. 12 to 18
- steel sheets whose production conditions are outside the scope of the present invention and whose texture does not meet the provisions of the present invention In the production Nos. 19 to 22), the value of Kca ( ⁇ 10 ° C.) was 3800 N / mm 3/2 or less, which was not equivalent to the example of the present invention.
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Abstract
Description
1.板厚50mm以上の厚鋼板において脆性き裂伝播停止特性(brittle crack arrestability)の向上には板厚中央部の靭性の向上が有効であり、板厚(t)の1/2+6mm部のシャルピー破面遷移温度が-40℃以下の場合に特に良好な結果が得られる。
2.上記靭性値の達成には、特定の化学成分、特に不純物元素であるSiおよびPの低減が有効である。
3.化学成分と並行して圧延条件も重要であり、板厚中央部の温度を規定した特定の熱間圧延条件で圧延することにより、ミクロ組織を加工されたフェライトを主体とする組織にすることができ、その結果、さらに靭性向上が達成される。
1.鋼組成が、質量%で、C:0.03~0.20%、Si:0.1%以下、Mn:0.5~2.2%、P:0.008%以下、S:0.01%以下、Nb:0.005~0.05%、Ti:0.005~0.03%、Al:0.005~0.08%、N:0.0075%以下、残部がFeおよび不可避的不純物からなる組成を有し、ミクロ組織が加工されたフェライトを主体とする組織であり、板厚(t)の1/2+6mm部のシャルピー破面遷移温度が-40℃以下であることを特徴とする脆性き裂伝播停止特性(brittle crack arrestability)に優れた構造用(for structural use)高強度厚鋼板(high-strength thick steel plate)。
2.鋼組成が、更に、質量%で、Cu:0.01~0.5%、Ni:0.01~1.0%、Cr:0.01~0.5%、Mo:0.01~0.5%、V:0.001~0.1%、B:0.003%以下、Ca:0.005%以下、REM:0.01%以下の1種または2種以上を含有することを特徴とする1記載の脆性き裂伝播停止特性(brittle crack arrestability)に優れた構造用(for structural use)高強度厚鋼板(high-strength thick steel plate)。
3.ミクロ組織における第2相として、パーライト、ベイナイト、マルテンサイト、島状マルテンサイト(MA)、及びオーステナイトから変態後、加工を受けていないフェライトの1種または2種以上を有することを特徴とする、1または2記載の脆性き裂伝播停止特性(brittle crack arrestability)に優れた構造用(for structural use)高強度厚鋼板(high-strength thick steel plate)。
4.板厚が50mm超えであることを特徴とする1~3のいずれか一つに記載の脆性き裂伝播停止特性(brittle crack arrestability)に優れた構造用(for structural use)高強度厚鋼板(high-strength thick steel plate)。
5.1または2に記載の組成を有する鋼素材(slab)を、1000~1200℃の温度に加熱し、板厚中央部がオーステナイト再結晶温度域での累積圧下率30%以上の圧延を行った後、15℃/s以下の冷却速度で板厚中央部の温度がAr3点以下となるまで第1の冷却を行い、板厚中央部の温度がAr3点以下の温度域において累積圧下率40%以上の圧延を行った後、4℃/s以上の冷却速度にて600℃以下まで第2の冷却を実施することを特徴とする脆性き裂伝播停止特性(brittle crack arrestability)に優れた構造用(for structural use)高強度厚鋼板(high-strength thick steel plate)の製造方法。
6.前記第2の冷却の後、さらに、Ac1点以下の温度に焼戻すことを特徴とする5に記載の脆性き裂伝播停止特性(brittle crack arrestability)に優れた構造用(for structural use)高強度厚鋼板(high-strength thick steel plate)の製造方法。
1.母材靭性
き裂の進展を抑制するためには、板厚中央部の母材靭性が良好な特性を有することが重要な要件となる。本発明に係る鋼板では板厚(t)の1/2+6mm部におけるシャルピー破面遷移温度について規定する。
2.化学成分
説明において%は質量%とする。
Cは鋼の強度を向上する元素であり、本発明では、所望の強度を確保するためには0.03%以上含有することを必要とするが、0.20%を超えると、溶接性が劣化するばかりか靭性にも悪影響がある。このため、Cは、0.03~0.20%の範囲に規定した。なお、好ましくは0.05~0.15%である。
Siは脱酸元素として、また、鋼の強化元素として有効であるが、含有量が過度に多くなると靭性が極端に劣化するという欠点がある。従って、鋼板中央部の靭性低下を防ぐためにも、その含有量を0.1%以下とする。
Mnは、強化元素として含有させる。0.5%より少ないとその効果が十分でなく、2.2%を超えると溶接性が劣化し、鋼材コストも上昇するため、0.5~2.2%とする。
Pは、鋼中の不可避的不純物である。P量の増加は靭性の劣化を招くので鋼板中央部の靭性を良好に保つためには、その上限をP:0.008%以下とする必要がある。
Sは、Pと同様に鋼中の不可避的不純物である。0.01%を超えると靭性が劣化するため、0.01%以下が望ましく、0.005%以下がさらに望ましい。
Nbは、NbCとしてフェライト変態時あるいは再加熱時に析出し、高強度化に寄与する。また、オーステナイト域の圧延において未再結晶域を拡大させる効果をもち、フェライトの細粒化に寄与するので、靭性の改善にも有効である。その効果を得るためには0.005%以上含有することが必要であるが0.05%を超えて含有すると、粗大なNbCが析出し逆に、靭性の低下を招くので0.005~0.05%とする。
Tiは微量を含有させることにより、窒化物、炭化物、あるいは炭窒化物を形成し、結晶粒を微細化して母材靭性を向上させる効果を有する。その効果は0.005%以上含有することによって得られるが、0.03%を超えて含有すると、母材および溶接熱影響部の靭性を低下させるので、0.005~0.03%とする。
Alは、脱酸剤として作用し、このためには0.005%以上含有することを必要とするが、0.08%を超えて含有すると、靭性を低下させるとともに、溶接した場合に、溶接金属部の靭性を低下させる。このため、Alは、0.005~0.08%とする。なお、好ましくは、0.02~0.04%である。
Nは、鋼中のAlと結合し、圧延加工時の結晶粒径を調整し、鋼を強化するが、0.0075%を超えると靭性が劣化するため、0.0075%以下とする。
Cu、Ni、Cr、Moはいずれも鋼の焼入れ性を高める元素である。圧延後の強度向上に直接寄与するとともに、靭性、高温強度、あるいは耐候性などの機能向上のために含有させることができるが、過度に含有すると靭性や溶接性を劣化させるため、含有させる場合には、それぞれ上限をCu:0.5%、Ni:1.0%、Cr:0.5%、Mo:0.5%とすることが好ましい。一方、含有量が0.01%未満であるとその効果が現れないため、含有する場合には、いずれも0.01%以上の含有量とすることが好ましい。
Vは、V(CN)として析出強化により、鋼の強度を向上する元素であり、0.001%以上含有してもよいが、0.1%を超えて含有すると、靭性を低下させる。このため、Vを含有させる場合には、0.001~0.1%とすることが好ましい。
Bは微量で鋼の焼き入れ性を高める元素として含有させてもよい。しかし、0.003%を超えて含有すると溶接部の靭性を低下させるので、含有させる場合には、0.003%以下の含有量とすることが好ましい。
Ca、REMは溶接熱影響部の組織を微細化し靭性を向上させ、含有しても本発明の効果が損なわれることはないので必要に応じて含有させてもよい。しかし、過度に含有すると、粗大な介在物を形成し母材の靭性を劣化させるので、含有させる場合には、含有量の上限をCaは0.005%、REMは0.01%とすることが好ましい。
Ceq=C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5
(右辺の各元素記号は、その元素の含有量(質量%)を示すものとする。)
3.ミクロ組織
靭性は化学成分の他、ミクロ組織にも大きな影響を受ける。本発明に係る鋼板では靭性に優れる組織として、特に、フェライト組織(ferrite structure)の中でも加工され扁平した組織、すなわち加工されたフェライト(以下、単に、加工フェライトとも称する)を主体とすることで、板厚方向の組織を細粒化させて靭性の向上を達成している。
4.製造条件
本発明に係る厚鋼板の製造条件として、スラブ加熱温度、熱間圧延におけるオーステナイト再結晶温度域での累積圧下率、オーステナイト再結晶温度域での圧延後からAr3点以下までの冷却速度、Ar3点以下での累積圧下率および冷却速度、冷却停止温度および焼戻し(temper)温度を規定する。以下の説明において温度(℃)は鋼板の板厚中央部(1/2t部(tは板厚))の温度とする。鋼板の板厚中央部の温度は、板厚、表面温度および冷却条件等から、シミュレーション計算等により求められる。例えば、差分法を用い、板厚方向の温度分布を計算することにより、鋼板の板厚中央部の温度が求められる。
Ar3(℃)=910-273C-74Mn-57Ni-16Cr-9Mo-5Cu
式において各元素記号は鋼中含有量(質量%)で、含有しない場合は0とする。
この第1の冷却を実施することにより、前記板厚の中央部の温度がオーステナイト再結晶温度域での圧延によって得られた、細粒化したオーステナイトを粗大化させることなく、次の板厚中央部の温度がAr3点以下の温度域での圧延を実施できるので、最終的に得られる組織の細粒化にも寄与する。
なお、亀裂伝播特性の向上には未再結晶域圧延よりもAr3点以下の温度域での圧延の方が効果が大きいため、できる限り有効な圧下をこの温度域に振り分ける必要がある。従って、本発明では未再結晶域圧延は行わない。
Ac1点=751-26.6C+17.6Si-11.6Mn-169Al-23Cu-23Ni+24.1Cr+22.5Mo+233Nb-39.7V-5.7Ti-895B
式において各元素記号は鋼中含有量(質量%)で、含有しない場合は0とする。
Claims (6)
- 鋼組成が、質量%で、C:0.03~0.20%、Si:0.1%以下、Mn:0.5~2.2%、P:0.008%以下、S:0.01%以下、Nb:0.005~0.05%、Ti:0.005~0.03%、Al:0.005~0.08%、N:0.0075%以下、残部がFeおよび不可避的不純物からなる組成を有し、ミクロ組織が加工されたフェライトを主体とする組織であり、板厚(t)の1/2+6mm部のシャルピー破面遷移温度が-40℃以下であることを特徴とする脆性き裂伝播停止特性(brittle crack arrestability)に優れた構造用(for structural use)高強度厚鋼板(high-strength thick steel plate)。
- 鋼組成が、更に、質量%で、Cu:0.01~0.5%、Ni:0.01~1.0%、Cr:0.01~0.5%、Mo:0.01~0.5%、V:0.001~0.1%、B:0.003%以下、Ca:0.005%以下、REM:0.01%以下の1種または2種以上を含有することを特徴とする請求項1記載の脆性き裂伝播停止特性(brittle crack arrestability)に優れた構造用(for structural use)高強度厚鋼板(high-strength thick steel plate)。
- ミクロ組織における第2相として、パーライト、ベイナイト、マルテンサイト、島状マルテンサイト(MA)、及びオーステナイトから変態後、加工を受けていないフェライトの1種または2種以上を有することを特徴とする、請求項1または2記載の脆性き裂伝播停止特性(brittle crack arrestability)に優れた構造用(for structural use)高強度厚鋼板(high-strength thick steel plate)。
- 板厚が50mm超えであることを特徴とする請求項1~3のいずれか一つに記載の脆性き裂伝播停止特性(brittle crack arrestability)に優れた構造用(for structural use)高強度厚鋼板(high-strength thick steel plate)。
- 請求項1または2に記載の組成を有する鋼素材(slab)を、1000~1200℃の温度に加熱し、板厚中央部がオーステナイト再結晶温度域での累積圧下率30%以上の圧延を行った後、15℃/s以下の冷却速度で板厚中央部の温度がAr3点以下となるまで第1の冷却を行い、板厚中央部の温度がAr3点以下の温度域において累積圧下率40%以上の圧延を行った後、4℃/s以上の冷却速度にて600℃以下まで第2の冷却を実施することを特徴とする脆性き裂伝播停止特性(brittle crack arrestability)に優れた構造用(for structural use)高強度厚鋼板(high-strength thick steel plate)の製造方法。
- 前記第2の冷却の後、さらに、Ac1点以下の温度に焼戻すことを特徴とする請求項5に記載の脆性き裂伝播停止特性(brittle crack arrestability)に優れた構造用(for structural use)高強度厚鋼板(high-strength thick steel plate)の製造方法。
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CN104334762A (zh) | 2015-02-04 |
BR112014028230A2 (pt) | 2017-06-27 |
JPWO2013175745A1 (ja) | 2016-01-12 |
JP5812193B2 (ja) | 2015-11-11 |
TW201402836A (zh) | 2014-01-16 |
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