WO2011043287A1 - Steel for linepipe having good strength and malleability, and method for producing the same - Google Patents
Steel for linepipe having good strength and malleability, and method for producing the same Download PDFInfo
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- WO2011043287A1 WO2011043287A1 PCT/JP2010/067351 JP2010067351W WO2011043287A1 WO 2011043287 A1 WO2011043287 A1 WO 2011043287A1 JP 2010067351 W JP2010067351 W JP 2010067351W WO 2011043287 A1 WO2011043287 A1 WO 2011043287A1
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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
- C21D8/0205—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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- 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
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
- C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
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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
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
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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
- C22C38/001—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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- 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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing 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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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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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- 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
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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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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- 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 toughness, high strength, high ductility steel having a sufficient strength as a welded structural steel and having excellent ductility characteristics and excellent low temperature toughness, and a method for producing the same.
- the present invention relates to a steel for line pipes having strength and ductility excellent in elongation that requires low temperature toughness in a cold region and a method for producing the same.
- Patent Document 1 presents steel aimed at obtaining high uniform elongation in order to suppress ductile fracture.
- a mixed structure in which an appropriate amount of a hardened phase is introduced into ferrite is mixed by quenching, two-phase treatment, and tempering treatment (QLT treatment) to achieve high ductility.
- QLT treatment tempering treatment
- high ductility is aimed at by optimization and accelerated cooling of a steel component and quenching hardenability (Di).
- Patent Document 3 presents a steel sheet having excellent HIC resistance.
- a mixed structure composed of ferrite-pearlite-bainite is manufactured without accelerated cooling of a steel plate of 16 mm or less and X56 or less to ensure HIC resistance.
- Patent Document 4 presents a steel plate of 570 MPa or more. After accelerated cooling, the formation of island martensite (MA) is suppressed by induction heating, and the hardness of the surface layer is suppressed. High strength and toughness are achieved by suppressing variations in hardness in the thickness direction.
- MA island martensite
- JP 2003-253331 A JP 2003-288512 A JP 2001-158936 A JP 2008-121036 A
- the present inventor has found that the prior art has the following problems.
- Steel for large diameter line pipes is required to reduce variations in properties such as strength and ductility in the plate in order to manage ductility after pipe making such as UOE and JCOE.
- a technique for reducing the in-plate variation in steel characteristics by forming a uniform structure by QLT (Quenching-Lamellarizing-Tempering) processing is adopted.
- QLT Quadenching-Lamellarizing-Tempering
- the QLT process is expensive because it is heat-treated at least three times at a high temperature.
- the local elongation can be improved by reducing the hardness of the surface layer by induction heating after accelerated cooling as in Patent Document 4 above.
- a structure manufactured by accelerated cooling and called a mixed structure of bainite and ferrite as shown in FIG. 2 of the publication, no band structure is formed, and general ferrite is not recognized.
- the local elongation is improved, but the uniform elongation is significantly reduced, and the total elongation is reduced.
- the present invention provides an inexpensive high-strength thick steel plate having good toughness and ductility in line pipe steel, a manufacturing method thereof, and a quality control method thereof.
- the present inventors have made the steel structure a two-phase structure of ferrite and pearlite from the viewpoint of strength and ductility balance. And it discovered that the fall of local elongation was prevented by suppressing the production
- the gist of the present invention is as follows.
- the steel for line pipes with good strength and ductility according to one embodiment of the present invention is C: 0.07 to 0.15%, Si: 0.05 to 0.60%, Mn: 0.80 to 1.80%, P: 0.010 or less, S: 0.007% or less, V: 0.05 to 0.12%, Nb: 0.005 to 0.070%, Al: 0.00.
- Ti 0.005 to 0.030%
- Ca 0.0005 to 0.0035%
- N 0.0020 to 0.0060%
- O 0.0030% or less
- the balance is composed of iron and inevitable impurities
- the structure is a two-phase structure of ferrite and pearlite
- the area fraction of island martensite is less than 1.5%
- the plate thickness is 18 mm or more
- the steel for line pipes of (1) above is further, in mass%, Cu: 0.05 to 0.70%, Ni: 0.05 to 0.70%, Cr: 0.80% or less, One or more of Mo: 0.30% or less, B: 0.0003 to 0.0030%, Mg: 0.0003 to 0.0030%, REM: 0.0005 to 0.0050% are contained. Also good.
- the bainite structure may not be detected under an optical microscope.
- the segregation degree of Mn may be 1.7 or less.
- the segregation degrees of Si and P may be 1.5 or less and 8.0 or less, respectively.
- a method for producing a steel for a line pipe with good strength and ductility is obtained by adding a slab having a chemical component of the steel for a line pipe according to (1) or (2) to 1250 ° C. After heating to a temperature of 850 ° C., hot rolling is performed at a cumulative reduction ratio of 40% or higher in a temperature range of 850 ° C. or higher, and hot rolling is terminated in a temperature range of 700 to 800 ° C., followed by air cooling.
- the steel sheet may be tempered in the temperature range of 500 to 300 ° C. after the air cooling.
- a method for quality control of steel for line pipes with good strength and ductility measures the amount of island martensite in a steel plate produced by hot rolling, Ductility is improved by controlling the production conditions so that the area fraction is less than 1.5%.
- the ductility may be improved by setting the cooling method after hot rolling to air cooling.
- the ductility may be improved by setting the segregation degree of Mn at the steel plate stage to 1.7 or less.
- the ductility may be improved by setting the bainite structure fraction at the steel sheet stage to 1% or less.
- the present inventors have conducted detailed studies on the influence of the structure on the ductility, investigated the influence of the hardened structure such as MA on the ductility and the segregation that promotes the formation of MA, and the following is necessary. It revealed that. 1. From the viewpoint of strength and ductility balance, it is necessary to have a two-phase structure of ferrite and pearlite. 2. The production of MA has little influence on the uniform elongation, but as the MA fraction increases, the local elongation deteriorates significantly. Also, bainite of about 400 Hv or more shows the same behavior as MA, and its generation reduces local elongation. 3. In order to suppress the formation of MA, it is important to control the manufacturing process, but it is also important to reduce segregation. In particular, as for segregation, when the degree of segregation increases, band-like MA is formed and the local elongation is remarkably reduced.
- the ductility of a material which is generally increased in strength for line pipes is low.
- a bainite single phase structure is formed by using accelerated cooling, it is easy to secure a strength of about 450 MPa class in terms of YS (yield strength).
- YS yield strength
- the ductility (uniform elongation) decreases with increasing strength.
- local elongation is remarkably reduced, and it is difficult to ensure a balance between strength and ductility.
- a ferrite single phase it is possible to increase ductility, but it is difficult to ensure strength.
- the ductility is ensured despite the high-strength steel by controlling the two-phase structure of ferrite and pearlite and controlling the MA and segregation, while being inexpensive.
- the content% means mass%.
- C 0.07 to 0.15% C is an element necessary for ensuring strength, and it is necessary to add 0.07% or more. Since a large amount of addition may cause a decrease in ductility and low temperature toughness, the upper limit is set to 0.15%. Desirably, 0.12% or less is good.
- Si 0.05 to 0.60%
- Si is an element effective as a deoxidizing element and increases the strength of the steel by solid solution strengthening. However, when its content is less than 0.05%, these effects are not recognized. Further, if added over 0.60%, a large amount of MA is generated in the structure, so that the toughness is deteriorated. Therefore, the addition amount of Si is set to 0.20 to 0.60%. Note that since MA (or retained austenite) starts to increase at 0.45% or more, it is preferably less than 0.45%.
- Mn 0.80 to 1.80% Mn is an effective element for increasing strength because it increases the strength of steel. For that purpose, addition of 0.80% or more is necessary. However, if it exceeds 1.80%, the degree of segregation such as center segregation and microsegregation increases, so that the formation of MA is promoted and the local elongation is lowered. Therefore, the appropriate range of Mn addition amount is set to 0.80 to 1.80%. In order to increase the strength, it is desirable that the lower limit of Mn is 1.0%, 1.2% or 1.3%.
- P 0.010% or less
- P exceeds 0.010% it segregates at the grain boundaries and significantly deteriorates the toughness of the steel.
- concentration of P in the segregation zone is increased to promote the formation of MA.
- the upper limit of the addition amount was set to 0.010%.
- S 0.007% or less S is present in steel by forming MnS, and has the effect of refining the structure after rolling and cooling. However, if it exceeds 0.007%, the toughness of the base metal and the welded portion is reduced. Deteriorate. For this reason, S was made into 0.007% or less.
- Nb 0.005 to 0.070%
- the cooling is air cooling for the structure control
- Nb is an important element for securing the strength.
- the heated austenite at the time of slab reheating or quenching becomes finer and the strength of the steel is increased. Therefore, it is necessary to add 0.005% or more.
- the upper limit of the Nb addition amount is set to 0.070%. In order to improve the toughness of the base material, the upper limit of Nb may be limited to 0.050% or 0.35%.
- V 0.05 to 0.12% V has almost the same function as Nb, but its effect is smaller than that of Nb.
- the effect similar to Nb is less effective at less than 0.05%.
- the appropriate range of the amount of V added is set to 0.05 to 0.12%.
- Al 0.005 to 0.08%
- Al needs to be added in an amount of 0.005% or more. If it is less than 0.005%, the formation of MA is suppressed, but it becomes weak deoxidation and the generation probability of coarse oxides is increased, so the local elongation is lowered. On the other hand, excessive addition exceeding 0.08% reduces weldability. This problem is particularly remarkable in SAW (Submerged Arc Welding) using flux, etc., which deteriorates the toughness of the weld metal and decreases the HAZ (Heart Affected Zone) toughness. For this reason, the upper limit of Al was made 0.08%. In order to improve weldability, it is preferable to limit the upper limit of Al to 0.06 or 0.04%.
- Ti 0.005 to 0.030% Addition of 0.005% or more is desirable for Ti to combine with N to form TiN effective for high strength and high ductility in steel. However, if Ti is added in an amount exceeding 0.030%, TiN may be coarsened and the ductility of the base material may be reduced. Therefore, Ti is set in the range of 0.005 to 0.030%.
- Ca 0.0005 to 0.0035%, When Ca is added in an amount of 0.0005% or more, it has an effect of controlling the form of sulfide (MnS), increasing the absorbed energy of Charpy and improving the low temperature toughness. However, if it exceeds 0.0035%, a large amount of coarse CaO and CaS is generated, which adversely affects the ductility and toughness of the steel. For this reason, 0.0035% was limited to the upper limit of the Ca content.
- N 0.0020 to 0.0060% N needs to be added in an amount of 0.0020% or more in order to form TiN effective in increasing the strength and ductility in steel by bonding with Ti.
- the upper limit of N is set to 0.0060% so that the effect of TiN can be maximized without greatly affecting the ductility.
- O 0.0030% or less In the case of high-strength steel, if O exceeds 0.0030%, the cleanliness and toughness of the steel are deteriorated. Therefore, the upper limit value is set to 0.0030%.
- the basic components in the present invention are as described above and can sufficiently achieve the target value, but in order to further improve the characteristics, one or more of the following elements are added as selective elements as necessary. Also good.
- Cu 0.05 to 0.70%
- Cu is an element advantageous for increasing the strength.
- addition of 0.05% or more is desirable.
- excessive addition increases the hardness of the base material and decreases the ductility, so the upper limit was made 0.70%.
- Ni 0.05-0.70% Ni improves strength and toughness without adversely affecting weldability and the like, and is also effective in preventing Cu cracking. In order to obtain these effects, addition of 0.05% or more is necessary. However, since Ni is expensive, if it is made 0.70% or more, it becomes impossible to manufacture steel at a low cost, so the addition amount is limited to 0.70% or less. In order to improve ductility, it is desirable to add less Ni, and the content may be limited to 0.50% or less, 0.30% or less, or 0.20% or less.
- Cr 0.80% or less Cr is an element that increases the strength of the base material. However, if it exceeds 0.80%, the hardness of the base material is increased and the ductility is deteriorated. Therefore, the upper limit is set to 0.80%. In order to improve ductility, it is preferable to limit Cr to 0.50% or less, 0.30% or less, or 0.10% or less.
- Mo 0.30% or less Cr, like Mo, is an element that increases the strength of the base material. However, if it exceeds 0.30%, the hardness of the base material is increased and the ductility is deteriorated. Therefore, the upper limit is set to 0.30%. In order to improve ductility, it is preferable to limit Mo to 0.20% or less or 0.10% or less.
- B 0.0003 to 0.0030%
- B is an element that dissolves in steel to increase the hardenability and increase the strength. In order to obtain this effect, addition of 0.0003% or more is necessary. However, if B is added excessively, the base material toughness is lowered. For this reason, the upper limit was made 0.0030%.
- Mg 0.0003 to 0.0030% Mg also suppresses the growth of austenite grains, has the effect of maintaining fine grains, and improves toughness. In order to enjoy this effect, addition of at least 0.0003% or more is essential, and this amount is set as the lower limit. On the other hand, if the amount added is increased more than necessary, not only does the effect margin for the amount added become small, but Mg does not necessarily have a high steelmaking yield, so the economy is lost. In view of these, the upper limit is limited to 0.0030% in the present invention.
- REM 0.0005 to 0.0050% REM, like Mg, also suppresses the growth of austenite grains, keeps them fine, and improves toughness. In order to enjoy this effect, at least 0.0005% or more must be added, and this amount is set as the lower limit. On the other hand, if the amount added is increased more than necessary, not only does the effect margin for the amount added become small, but Mg does not necessarily have a high steelmaking yield, so the economy is lost. In view of these, the upper limit is limited to 0.0050% in the present invention.
- UOE and JCOE steel pipes having high strength and high ductility mainly as steel materials for line pipe welding.
- One feature of the steel according to the present invention is that the composite properties of strength, toughness and ductility required for the line pipe are ensured mainly by a two-phase structure of ferrite (having a ferrite band structure) and pearlite.
- the ferrite structure in the present invention has a so-called ferrite band structure along the hot rolling direction in the structure.
- the MA fraction is 1.5% or more, a large amount of voids are generated in the vicinity of the MA during the tensile test, and the shear elongation is promoted by the plastic flow, and the local elongation is remarkably reduced.
- MA having a Vickers hardness of 400 to 700 Hv causes the occurrence of voids particularly frequently and causes a significant deterioration in local elongation. For this reason, if the MA of 400 to 700 Hv is suppressed, deterioration of local elongation can be avoided.
- the segregation degree of each element in the steel sheet is not necessarily regulated in the present invention.
- the segregation degree of the Mn steel plate is preferably 1.7 or less. More preferably, the segregation degrees of the Si and P steel sheets are 1.5 and 8.0 or less, respectively. When the Mn segregation degree of the steel sheet exceeds 1.7, the Si segregation degree exceeds 1.5, or the P segregation degree exceeds 8.0, the formation of MA becomes significant.
- the Mn segregation degree of the steel sheet after rolling is specified, but in order to ensure that the Mn segregation degree is 1.7 or less when the sheet is formed, the Mn segregation degree in the slab stage is 1.1 or less.
- the manufacturing method of a slab is not limited.
- the degree of segregation means component analysis in a 1 mm 2 region in the plate thickness direction 1/2, and the peak concentration of Mn, Si or P at these positions divided by the average concentration of each element.
- component analysis for example, EPMA (Electron Probe Micro Analyzer) or CMA (Computer-aided Micro Analyzer) can be used.
- the Mn segregation degree of the slab is preferably 1.1 or more. If a steel plate is manufactured from such a slab, the segregation degree of Mn of a steel plate can be reliably made 1.7 or less. As described above, when it exceeds 1.7, the generation of MA tends to be remarkable. In the present invention, only the segregation degree of Mn is defined for the slab. Although segregation of Si and P is important, it is not specified because the influence of the manufacturing process is larger than that of Mn. If allowed by the manufacturing process, the segregation of Si and P is preferably 1.5 or 8.0 or less.
- a widely known method such as light reduction (soft reduction), electromagnetic stirring during continuous casting, diffusion heat treatment of the segregation element by high-temperature heat treatment to the slab, and the like should be used. Can do. In the manufacturing process of the slab used in the present invention, light reduction was performed.
- the heating temperature exceeds 1250 ° C.
- the crystal grain size becomes very coarse, and a large amount of scale is generated on the steel surface, and the quality of the surface is significantly lowered.
- the upper limit of reheating temperature was 1250 degreeC. It is necessary to perform hot rolling with a cumulative rolling reduction of 40% or more in a temperature range of 850 ° C. or higher.
- the increase in the amount of reduction in this temperature range contributes to the refinement of the austenite grains during rolling, and as a result, has the effect of refining the ferrite grains and improving the mechanical properties.
- the cumulative rolling reduction needs to be 40% or more in the temperature range of 850 ° C. or higher.
- the cumulative rolling reduction in the non-recrystallized region needs to be 40% or more. For this reason, the cumulative reduction amount in the non-recrystallized region is limited to 40% or more.
- the steel slab needs to be air-cooled after hot rolling is completed in the temperature range of 800 to 700 ° C. In this case, it is desirable to cool slowly at a cooling rate of less than 5 ° C./s.
- rolling is completed at a two-phase region temperature of 800 to 700 ° C. to produce a mixed structure of ferrite and pearlite.
- base material toughness such as DWTT, high strength, and high ductility are achieved.
- it is more desirable that the hot rolling is in a temperature range of 780 to 720 ° C.
- the cooling method is defined only as air cooling, and the cooling rate is determined in relation to the plate thickness.
- the cooling rate exceeds 5 ° C./s, MA and bainite are likely to be formed, and the toughness and ductility are reduced. For this reason, slow cooling at a cooling rate of less than 5 ° C./s is desirable. More preferably, it is 2 ° C./s or less.
- the thermal tempering treatment in the temperature range of 500 to 300 ° C. improves local elongation from the viewpoint of dehydrogenation.
- the structure needs to be a two-phase structure of ferrite and pearlite as described above.
- the structure fraction is not necessarily specified, but preferably the ferrite fraction is about 60 to 95%.
- bainite is not detected when measured with an optical microscope.
- the present invention steel has a two-phase structure of ferrite and pearlite.
- a bainite structure is confirmed at the electron microscope level.
- the tissue fraction bainite is desirably 1% or less.
- the plate thickness is 18 mm or more, and the yield strength is 450 MPa or more.
- the plate thickness is less than 18 mm, it is easy to ensure the yield strength.
- the cooling rate increases, so that a large amount of a hardened structure such as bainite is generated and it is difficult to ensure the elongation.
- the elongation improves as the yield strength decreases. If it is less than 540 MPa, it is possible to easily obtain a total elongation of 20% or more in the GOST tensile test and 40% or more in the API tensile test without controlling the chemical composition, structure, manufacturing method and the like.
- this invention exhibits an especially suitable effect in the steel plate whose yield strength is 450 Mpa or more. More preferably, the steel sheet may have a yield strength of 540 MPa or more.
- the toughness of the steel sheet is preferably 70% or more at a ductile fracture surface ratio of ⁇ 20 ° C. by the DWTT test.
- a slab obtained by casting molten steel having the chemical components shown in Table 1 was hot-rolled under the conditions shown in Table 2 to obtain a steel plate.
- the “cumulative reduction amount” column indicates the cumulative reduction rate in a temperature range of 850 ° C. or higher. This was followed by a test to evaluate the mechanical properties.
- the cooling method column of Table 2 the comparative steels r and s were accelerated and cooled at the rate indicated in parentheses (° C./s). The steel plates other than the above were cooled by air cooling.
- Table 3 shows a summary of the mechanical properties of each steel.
- Steels a to o are examples of the present invention. As is apparent from Tables 1 and 2, these steel sheets satisfy the requirements of chemical components and production conditions, and as shown in Table 3, the base metal strength, ductility and toughness are good.
- the steel structure was a ferrite + pearlite two-phase structure (under an optical microscope), and MA was 1.5% or less.
- steels p to ad are comparative steels that depart from the scope of the present invention.
- the production conditions for steels p to v and ab to ad are different from those of the present invention, and the chemical components of steels w to aa are outside the scope of the present invention.
- the steels p to ad were inferior to the steel of the present invention in one or more of the mechanical properties of the base material.
- the “structure” column in Table 3 indicates the structure in the steel sheet, where F is ferrite, P is pearlite, and B is bainite.
- steel p had low toughness due to low cumulative reduction.
- Steel q had a high rolling end temperature and was unable to control the structure, so the toughness decreased.
- steel r and steel s were accelerated and cooled, a large amount of bainite and MA were produced in the process, resulting in a decrease in ductility.
- MA is decomposed by tempering, but because there is bainite, the toughness is recovered, but the recovery of elongation is insufficient.
- Steels t ab to ad intentionally produced a large amount of MA by increasing the segregation degree of the plate by omitting the light reduction treatment in the slab manufacturing stage. For this reason, the elongation is reduced.
- steel u Since steel u has a small plate thickness and a high cooling rate, MA is primarily generated before tempering. Steel u is subsequently tempered, but since the temperature is low, MA is not decomposed and elongation and toughness cannot be obtained.
- Steel v has a high tempering treatment temperature, and the amount of MA is reduced, but the yield strength is low. Since the steel C has a low C content, the strength of the base material has decreased. Moreover, since steel x has a high Si content, MA increased and ductility decreased. Since the steel y has a high Mn content, the increase in MA and the MA itself harden, so that predetermined elongation characteristics and toughness cannot be obtained.
- Steel z is weakly deoxidized because of its low Al content. Steel aa has a high Ca content. For this reason, a relatively coarse oxide is produced in steels z and aa, and sufficient elongation cannot be obtained.
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Abstract
Description
本願は、2009年10月5日に、日本に出願された特願2009-231799号に基づき優先権を主張し、その内容をここに援用する。 The present invention relates to a high toughness, high strength, high ductility steel having a sufficient strength as a welded structural steel and having excellent ductility characteristics and excellent low temperature toughness, and a method for producing the same. In particular, the present invention relates to a steel for line pipes having strength and ductility excellent in elongation that requires low temperature toughness in a cold region and a method for producing the same.
This application claims priority based on Japanese Patent Application No. 2009-231799 filed in Japan on October 5, 2009, the contents of which are incorporated herein by reference.
特許文献4では、570MPa以上の鋼板を提示している。加速冷却後、誘導加熱により島状マルテンサイト(MA、Martensite-Austenite-Constituent)の生成を抑え、表層の硬さを抑えている。板厚方向の硬さのバラツキを抑えることにより高強度高靭性をはかっている。 Patent Document 3 presents a steel sheet having excellent HIC resistance. A mixed structure composed of ferrite-pearlite-bainite is manufactured without accelerated cooling of a steel plate of 16 mm or less and X56 or less to ensure HIC resistance.
Patent Document 4 presents a steel plate of 570 MPa or more. After accelerated cooling, the formation of island martensite (MA) is suppressed by induction heating, and the hardness of the surface layer is suppressed. High strength and toughness are achieved by suppressing variations in hardness in the thickness direction.
大径ラインパイプ用鋼ではUOE、JCOEなど造管後の延性を管理するために板内での強度や延性などの特性のバラツキの低減が要求されている。そのため、たとえばQLT(Quenching-Lamellarizing-Tempering)処理による均一な組織の形成により鋼特性の板内バラツキを小さくする技術が採用されている。しかし、QLT処理は少なくとも高温で3回以上の熱処理をするため高コストとなる。また、二相域熱処理に相当する加速冷却によって高強度、高延性をはかることは可能であるが、加速冷却において板内の冷却を均一にすることは極めて困難である。 The present inventor has found that the prior art has the following problems.
Steel for large diameter line pipes is required to reduce variations in properties such as strength and ductility in the plate in order to manage ductility after pipe making such as UOE and JCOE. For this reason, for example, a technique for reducing the in-plate variation in steel characteristics by forming a uniform structure by QLT (Quenching-Lamellarizing-Tempering) processing is adopted. However, the QLT process is expensive because it is heat-treated at least three times at a high temperature. Further, it is possible to achieve high strength and high ductility by accelerated cooling corresponding to the two-phase region heat treatment, but it is extremely difficult to make the cooling in the plate uniform in accelerated cooling.
(1)本発明の一態様にかかる強度および延性の良好なラインパイプ用鋼は、質量%で、C:0.07~0.15%、Si:0.05~0.60%、Mn:0.80~1.80%、P:0.010以下、S:0.007%以下、V:0.05~0.12%、Nb: 0.005~0.070%、Al:0.005~0.08%、Ti:0.005~0.030%、Ca:0.0005~0.0035%、N:0.0020~0.0060%、O:0.0030%以下を含有し、残部が鉄および不可避的不純物からなり、組織がフェライトとパーライトの二相組織であり、島状マルテンサイトの面積分率が1.5%未満であり、板厚が18mm以上であり、降伏強度が450MPa以上である。
(2)上記(1)のラインパイプ用鋼は、更に、質量%で、Cu:0.05~0.70%、Ni:0.05~0.70%、Cr:0.80%以下、Mo:0.30%以下、B:0.0003~0.0030%、Mg:0.0003~0.0030%、REM:0.0005~0.0050%の一種または二種以上を含有してもよい。
(3)上記(1)または(2)のラインパイプ用鋼において、光学顕微鏡下でベイナイト組織が検出されなくてもよい。
(4)上記(1)または(2)のラインパイプ用鋼において、Mnの偏析度が1.7以下であってもよい。
(5)上記(4)のラインパイプ用鋼において、Si,Pの偏析度がそれぞれ1.5以下、8.0以下であってもよい。
(6)本発明の別の一態様にかかる強度および延性の良好なラインパイプ用鋼の製造方法は、上記(1)または(2)のラインパイプ用鋼の化学成分を持つスラブを1250℃以下の温度に加熱後、850℃以上の温度域において累積圧下率で40%以上の熱間圧延をし、700~800℃の温度領域で熱間圧延を終了させた後、空冷する。
(7)上記(6)の製造方法において、前記空冷後、鋼板に500~300℃の温度領域で焼戻し処理を施してもよい。
(8)本発明の別の一態様にかかる強度および延性の良好なラインパイプ用鋼の品質管理方法は、熱間圧延によって製造された鋼板中の島状マルテンサイト量を測定し、島状マルテンサイトの面積分率が1.5%未満になるように製造条件を制御することによって延性を改善させる。
(9)上記(8)の品質管理方法では、熱間圧延後の冷却方法を空冷とすることによって延性を改善させてもよい。
(10)上記(8)または(9)の品質管理方法では、鋼板段階のMnの偏析度を1.7以下とすることによって延性を改善させてもよい。
(11)上記(8)または(9)の品質管理方法では、鋼板段階のベイナイト組織分率を1%以下にすることによって延性を改善させてもよい。 The gist of the present invention is as follows.
(1) The steel for line pipes with good strength and ductility according to one embodiment of the present invention is C: 0.07 to 0.15%, Si: 0.05 to 0.60%, Mn: 0.80 to 1.80%, P: 0.010 or less, S: 0.007% or less, V: 0.05 to 0.12%, Nb: 0.005 to 0.070%, Al: 0.00. 005 to 0.08%, Ti: 0.005 to 0.030%, Ca: 0.0005 to 0.0035%, N: 0.0020 to 0.0060%, O: 0.0030% or less The balance is composed of iron and inevitable impurities, the structure is a two-phase structure of ferrite and pearlite, the area fraction of island martensite is less than 1.5%, the plate thickness is 18 mm or more, and the yield strength Is 450 MPa or more.
(2) The steel for line pipes of (1) above is further, in mass%, Cu: 0.05 to 0.70%, Ni: 0.05 to 0.70%, Cr: 0.80% or less, One or more of Mo: 0.30% or less, B: 0.0003 to 0.0030%, Mg: 0.0003 to 0.0030%, REM: 0.0005 to 0.0050% are contained. Also good.
(3) In the steel for line pipes according to (1) or (2), the bainite structure may not be detected under an optical microscope.
(4) In the steel for line pipes of (1) or (2) above, the segregation degree of Mn may be 1.7 or less.
(5) In the steel for line pipes of (4) above, the segregation degrees of Si and P may be 1.5 or less and 8.0 or less, respectively.
(6) According to another aspect of the present invention, a method for producing a steel for a line pipe with good strength and ductility is obtained by adding a slab having a chemical component of the steel for a line pipe according to (1) or (2) to 1250 ° C. After heating to a temperature of 850 ° C., hot rolling is performed at a cumulative reduction ratio of 40% or higher in a temperature range of 850 ° C. or higher, and hot rolling is terminated in a temperature range of 700 to 800 ° C., followed by air cooling.
(7) In the manufacturing method of (6), the steel sheet may be tempered in the temperature range of 500 to 300 ° C. after the air cooling.
(8) According to another aspect of the present invention, a method for quality control of steel for line pipes with good strength and ductility measures the amount of island martensite in a steel plate produced by hot rolling, Ductility is improved by controlling the production conditions so that the area fraction is less than 1.5%.
(9) In the quality control method of (8) above, the ductility may be improved by setting the cooling method after hot rolling to air cooling.
(10) In the quality control method of (8) or (9) above, the ductility may be improved by setting the segregation degree of Mn at the steel plate stage to 1.7 or less.
(11) In the quality control method of the above (8) or (9), the ductility may be improved by setting the bainite structure fraction at the steel sheet stage to 1% or less.
1. 強度、延性バランスの観点からフェライトとパーライトの二相組織とする必要がある。
2. MAの生成は一様伸びに及ぼす影響は少ないが、MA分率が上がると局部伸びが著しく劣化する。また、約400Hv以上のベイナイトもMAと同様の挙動を示し、その生成は局部伸びを低下させる。
3. MA生成を抑制するためには、製造プロセスを制御することも重要であるが、偏析の低減も重要である。特に偏析については偏析度が高くなるとバンド状のMAを形成し著しく局部伸びを低下させる。 Therefore, the present inventors have conducted detailed studies on the influence of the structure on the ductility, investigated the influence of the hardened structure such as MA on the ductility and the segregation that promotes the formation of MA, and the following is necessary. It revealed that.
1. From the viewpoint of strength and ductility balance, it is necessary to have a two-phase structure of ferrite and pearlite.
2. The production of MA has little influence on the uniform elongation, but as the MA fraction increases, the local elongation deteriorates significantly. Also, bainite of about 400 Hv or more shows the same behavior as MA, and its generation reduces local elongation.
3. In order to suppress the formation of MA, it is important to control the manufacturing process, but it is also important to reduce segregation. In particular, as for segregation, when the degree of segregation increases, band-like MA is formed and the local elongation is remarkably reduced.
このため、高延性化をはかるためのフェライトと強度を確保するためのパーライトの二相組織が必要となる。 As described above, the ductility of a material which is generally increased in strength for line pipes is low. For example, when a bainite single phase structure is formed by using accelerated cooling, it is easy to secure a strength of about 450 MPa class in terms of YS (yield strength). However, the ductility (uniform elongation) decreases with increasing strength. In the case of a single-phase structure, particularly with respect to ductility, local elongation is remarkably reduced, and it is difficult to ensure a balance between strength and ductility. Further, when a ferrite single phase is used, it is possible to increase ductility, but it is difficult to ensure strength. Further, if there is ferrite and a hardened bainitic structure generated by accelerated cooling, local elongation is lowered.
For this reason, a two-phase structure of ferrite for ensuring high ductility and pearlite for ensuring strength is required.
Cは強度を確保するために必要な元素であり、0.07%以上の添加が必要である。多量の添加は延性や低温靭性の低下を招くおそれがあるために、その上限値を0.15%とする。望ましくは0.12%以下が良い。 C: 0.07 to 0.15%
C is an element necessary for ensuring strength, and it is necessary to add 0.07% or more. Since a large amount of addition may cause a decrease in ductility and low temperature toughness, the upper limit is set to 0.15%. Desirably, 0.12% or less is good.
Siは脱酸元素として、また固溶強化により鋼の強度を増加させるのに有効な元素であるが、0.05%未満の添加ではそれらの効果が認められない。また、0.60%を超えて添加すると、組織内にMAが多量に生成するため靭性を劣化させる。このため、Siの添加量は0.20~0.60%とした。なお、0.45%以上になるとMA(または残留オーステナイト)が増加し始めるため、望ましくは0.45%未満が良い。 Si: 0.05 to 0.60%
Si is an element effective as a deoxidizing element and increases the strength of the steel by solid solution strengthening. However, when its content is less than 0.05%, these effects are not recognized. Further, if added over 0.60%, a large amount of MA is generated in the structure, so that the toughness is deteriorated. Therefore, the addition amount of Si is set to 0.20 to 0.60%. Note that since MA (or retained austenite) starts to increase at 0.45% or more, it is preferably less than 0.45%.
Mnは、鋼の強度を増加するため高強度化には有効な元素である。そのためには、0.80%以上の添加が必要である。しかし、1.80%を超えると、中心偏析やミクロ偏析等の偏析度が増加するためMAの生成を助長するため局部伸びを低下させる。このため、Mnの添加量の適正範囲を0.80~1.80%とした。高強度化のため、Mnの下限を1.0%、1.2%または1.3%とすることが望ましい。 Mn: 0.80 to 1.80%
Mn is an effective element for increasing strength because it increases the strength of steel. For that purpose, addition of 0.80% or more is necessary. However, if it exceeds 1.80%, the degree of segregation such as center segregation and microsegregation increases, so that the formation of MA is promoted and the local elongation is lowered. Therefore, the appropriate range of Mn addition amount is set to 0.80 to 1.80%. In order to increase the strength, it is desirable that the lower limit of Mn is 1.0%, 1.2% or 1.3%.
Pは、0.010%超となると粒界に偏析して鋼の靱性を著しく劣化させる。また、偏析帯のPの濃度が増加しMAの生成を助長する。このため添加量の上限を0.010%とした。なお、延性や靭性値の低下を抑制する観点からはできるだけ低減することが望ましい。 P: 0.010% or less When P exceeds 0.010%, it segregates at the grain boundaries and significantly deteriorates the toughness of the steel. In addition, the concentration of P in the segregation zone is increased to promote the formation of MA. For this reason, the upper limit of the addition amount was set to 0.010%. In addition, it is desirable to reduce as much as possible from a viewpoint of suppressing the fall of a ductility and a toughness value.
Sは、MnSを形成して鋼中に存在し、圧延冷却後の組織を微細にする作用を有するが、0.007%を超えると母材および溶接部の靭性を劣化させる。このため、Sは0.007%以下とした。 S: 0.007% or less S is present in steel by forming MnS, and has the effect of refining the structure after rolling and cooling. However, if it exceeds 0.007%, the toughness of the base metal and the welded portion is reduced. Deteriorate. For this reason, S was made into 0.007% or less.
本発明では組織制御のため冷却を空冷としたため、強度を確保するためにNbが重要な元素である。また、Nb添加によって、スラブ再加熱時や焼入れ時の加熱オーステナイトが細粒化し、鋼の高強度化がはかれる。そのためには0.005%以上添加する必要がある。しかしながら、過量なNb添加は母材の延性を低下させるため、Nb添加量の上限値を0.070%とした。母材の靭性向上のために、Nbの上限を0.050%または0.35%に制限してもよい。 Nb: 0.005 to 0.070%
In the present invention, since the cooling is air cooling for the structure control, Nb is an important element for securing the strength. Moreover, by adding Nb, the heated austenite at the time of slab reheating or quenching becomes finer and the strength of the steel is increased. Therefore, it is necessary to add 0.005% or more. However, since excessive Nb addition reduces the ductility of the base material, the upper limit of the Nb addition amount is set to 0.070%. In order to improve the toughness of the base material, the upper limit of Nb may be limited to 0.050% or 0.35%.
Vは、Nbとほぼ同様の作用を有するが、Nbに比べてその効果は小さい。Nbと同様の効果は0.05%未満では効果が少ない。しかし、0.12%を超えると延性が劣化する。このため、Vの添加量の適正範囲を0.05~0.12%とした。 V: 0.05 to 0.12%
V has almost the same function as Nb, but its effect is smaller than that of Nb. The effect similar to Nb is less effective at less than 0.05%. However, if it exceeds 0.12%, the ductility deteriorates. For this reason, the appropriate range of the amount of V added is set to 0.05 to 0.12%.
脱酸の目的で、Alは0.005%以上添加する必要がある。0.005%未満とするとMAの生成は抑制されるが、弱脱酸となり粗大な酸化物の発生確率が高くなるため局部伸びを低下させる。一方、0.08%を超える過度の添加は溶接性を低下させる。この問題は、特にフラックスを使用するSAW(Submerged Arc Welding)等で顕著であり、溶接金属の靭性を劣化させ、HAZ(Heart Affected Zone)靱性も低下する。このため、Alの上限を0.08%とした。溶接性の向上のために、Alの上限を0.06または0.04%に制限することが好ましい。 Al: 0.005 to 0.08%
For the purpose of deoxidation, Al needs to be added in an amount of 0.005% or more. If it is less than 0.005%, the formation of MA is suppressed, but it becomes weak deoxidation and the generation probability of coarse oxides is increased, so the local elongation is lowered. On the other hand, excessive addition exceeding 0.08% reduces weldability. This problem is particularly remarkable in SAW (Submerged Arc Welding) using flux, etc., which deteriorates the toughness of the weld metal and decreases the HAZ (Heart Affected Zone) toughness. For this reason, the upper limit of Al was made 0.08%. In order to improve weldability, it is preferable to limit the upper limit of Al to 0.06 or 0.04%.
Tiは、Nと結合して鋼中に高強度、高延性化に有効なTiNを形成させるために、0.005%以上の添加が望まれる。ただし、0.030%を超えてTiを添加すると、TiNを粗大化させ、母材の延性を低下させるおそれがある。このため、Tiは0.005~0.030%の範囲とした。
Ca: 0.0005~0.0035%、
Caは、0.0005%以上添加した場合、硫化物(MnS)の形態を制御し、シャルピーの吸収エネルギーを増大させ低温靭性を向上させる効果がある。ただし、0.0035%を超えると粗大なCaOやCaSが多量に発生するため鋼の延性や靱性に悪影響を及ぼす。このため、ため、0.0035%をCa量の上限と限定した。 Ti: 0.005 to 0.030%
Addition of 0.005% or more is desirable for Ti to combine with N to form TiN effective for high strength and high ductility in steel. However, if Ti is added in an amount exceeding 0.030%, TiN may be coarsened and the ductility of the base material may be reduced. Therefore, Ti is set in the range of 0.005 to 0.030%.
Ca: 0.0005 to 0.0035%,
When Ca is added in an amount of 0.0005% or more, it has an effect of controlling the form of sulfide (MnS), increasing the absorbed energy of Charpy and improving the low temperature toughness. However, if it exceeds 0.0035%, a large amount of coarse CaO and CaS is generated, which adversely affects the ductility and toughness of the steel. For this reason, 0.0035% was limited to the upper limit of the Ca content.
Nは、Tiと結合して鋼中に高強度、高延性化に有効なTiNを形成させるために、0.0020%以上の添加が必要である。ただし、Nは固溶強化元素としても非常に大きな効果があるため、多量に添加すると延性を劣化するおそれが考えられる。そのため、延性に大きな影響を与えずTiNの効果を最大限に得られるように、Nの上限を0.0060%とした。 N: 0.0020 to 0.0060%
N needs to be added in an amount of 0.0020% or more in order to form TiN effective in increasing the strength and ductility in steel by bonding with Ti. However, since N has a very large effect as a solid solution strengthening element, adding a large amount may cause deterioration of ductility. Therefore, the upper limit of N is set to 0.0060% so that the effect of TiN can be maximized without greatly affecting the ductility.
高強度鋼の場合、Oが0.0030%を超えると鋼の清浄度、靱性劣化を招く。このため上限値を0.0030%とした。 O: 0.0030% or less In the case of high-strength steel, if O exceeds 0.0030%, the cleanliness and toughness of the steel are deteriorated. Therefore, the upper limit value is set to 0.0030%.
Cuは高強度化をはかるために有利な元素である。Cuによる析出効果を確保するためには0.05%以上の添加が望ましい。しかし過剰な添加は母材の硬さを上昇させ延性を低下させるためその上限を0.70%とした。延性向上のため、Cuを0.50%以下、0.30%以下または0.20%以下に制限することが好ましい。 Cu: 0.05 to 0.70%
Cu is an element advantageous for increasing the strength. In order to ensure the precipitation effect by Cu, addition of 0.05% or more is desirable. However, excessive addition increases the hardness of the base material and decreases the ductility, so the upper limit was made 0.70%. In order to improve ductility, it is preferable to limit Cu to 0.50% or less, 0.30% or less, or 0.20% or less.
Niは溶接性等に悪影響をおよぼすことなく、強度、靭性を向上させるほか、Cu割れの防止にも効果がある。これらの効果を得るためには0.05%以上の添加が必要である。しかし、Niは高価であるため0.70%以上とすると廉価に鋼を製造できなくなるため添加量を0.70%以下に制限する。延性向上のためにはNi添加は少ない方が望ましく、0.50%以下、0.30%以下または0.20%以下に制限してもよい。 Ni: 0.05-0.70%
Ni improves strength and toughness without adversely affecting weldability and the like, and is also effective in preventing Cu cracking. In order to obtain these effects, addition of 0.05% or more is necessary. However, since Ni is expensive, if it is made 0.70% or more, it becomes impossible to manufacture steel at a low cost, so the addition amount is limited to 0.70% or less. In order to improve ductility, it is desirable to add less Ni, and the content may be limited to 0.50% or less, 0.30% or less, or 0.20% or less.
Crは母材の強度を高める元素である。しかし、0.80%を超えると母材の硬さを上昇させ延性を劣化させる。そのため上限値を0.80%とした。延性向上のため、Crを0.50%以下、0.30%以下または0.10%以下に制限することが好ましい。 Cr: 0.80% or less Cr is an element that increases the strength of the base material. However, if it exceeds 0.80%, the hardness of the base material is increased and the ductility is deteriorated. Therefore, the upper limit is set to 0.80%. In order to improve ductility, it is preferable to limit Cr to 0.50% or less, 0.30% or less, or 0.10% or less.
CrもMoと同様、母材の強度を高める元素である。しかし、0.30%を超えると母材の硬さを上昇させ延性を劣化させる。そのため上限値を0.30%とした。延性向上のため、Moを0.20%以下または0.10%以下に制限することが好ましい。 Mo: 0.30% or less Cr, like Mo, is an element that increases the strength of the base material. However, if it exceeds 0.30%, the hardness of the base material is increased and the ductility is deteriorated. Therefore, the upper limit is set to 0.30%. In order to improve ductility, it is preferable to limit Mo to 0.20% or less or 0.10% or less.
Bは鋼中に固溶して焼入れ性を高め強度を上昇させる元素である。この効果を得るためには0.0003%以上の添加が必要である。しかし、Bを過多に添加すると母材靭性を低下させる。このためその上限値を0.0030%とした。 B: 0.0003 to 0.0030%
B is an element that dissolves in steel to increase the hardenability and increase the strength. In order to obtain this effect, addition of 0.0003% or more is necessary. However, if B is added excessively, the base material toughness is lowered. For this reason, the upper limit was made 0.0030%.
Mgは、オーステナイト粒の成長をも抑制し、細粒に保つ作用があり、靭性を向上させる。この効果を享受するためには、少なくとも0.0003%以上の添加が必須であり、この量を下限とした。一方、必要以上に添加量が増えても添加量に対する効果代が小さくなるばかりでなく、Mgは製鋼歩留まりが必ずしも高くないため、経済性も失することになる。これらを鑑み、本願発明においては上限を0.0030%に限定した。 Mg: 0.0003 to 0.0030%
Mg also suppresses the growth of austenite grains, has the effect of maintaining fine grains, and improves toughness. In order to enjoy this effect, addition of at least 0.0003% or more is essential, and this amount is set as the lower limit. On the other hand, if the amount added is increased more than necessary, not only does the effect margin for the amount added become small, but Mg does not necessarily have a high steelmaking yield, so the economy is lost. In view of these, the upper limit is limited to 0.0030% in the present invention.
REMもMgと同様、オーステナイト粒の成長をも抑制し、細粒に保つ作用があり、靭性を向上させる。この効果を享受するためには、少なくとも0.0005%以上の添加が必須であり、この量を下限とした。一方、必要以上に添加量が増えても添加量に対する効果代が小さくなるばかりでなく、Mgは製鋼歩留まりが必ずしも高くないため、経済性も失することになる。これらを鑑み、本願発明においては上限を0.0050%に限定した。 REM: 0.0005 to 0.0050%
REM, like Mg, also suppresses the growth of austenite grains, keeps them fine, and improves toughness. In order to enjoy this effect, at least 0.0005% or more must be added, and this amount is set as the lower limit. On the other hand, if the amount added is increased more than necessary, not only does the effect margin for the amount added become small, but Mg does not necessarily have a high steelmaking yield, so the economy is lost. In view of these, the upper limit is limited to 0.0050% in the present invention.
鋼板のMn偏析度が1.7超、またはSi偏析度が1.5超、またはP偏析度が8.0超になるとMAの生成が顕著となる。本発明では圧延後の鋼板の元素偏析度のみを規定するが、板とした時にMnの偏析度が1.7以下を確保するためには、スラブ段階のMn偏析度は1.1以下にする必要がある。また、本発明では最終的な鋼板のMn偏析を制御するためスラブの製造方法については限定しない。ただし、MAの分布を制御するためには中心偏析などのマクロ偏析だけではなくミクロ偏析も低減する必要がある。 Further, at this time, if the MA fraction is 1.5% or more, a large amount of voids are generated in the vicinity of the MA during the tensile test, and the shear elongation is promoted by the plastic flow, and the local elongation is remarkably reduced. In order to suppress the segregation-induced MA that deteriorates the local elongation and to make the MA less than 1.5%, it is important not to cool rapidly. More specifically, MA having a Vickers hardness of 400 to 700 Hv causes the occurrence of voids particularly frequently and causes a significant deterioration in local elongation. For this reason, if the MA of 400 to 700 Hv is suppressed, deterioration of local elongation can be avoided. As long as the MA fraction is in the above range, the segregation degree of each element in the steel sheet is not necessarily regulated in the present invention. However, the segregation degree of the Mn steel plate is preferably 1.7 or less. More preferably, the segregation degrees of the Si and P steel sheets are 1.5 and 8.0 or less, respectively.
When the Mn segregation degree of the steel sheet exceeds 1.7, the Si segregation degree exceeds 1.5, or the P segregation degree exceeds 8.0, the formation of MA becomes significant. In the present invention, only the element segregation degree of the steel sheet after rolling is specified, but in order to ensure that the Mn segregation degree is 1.7 or less when the sheet is formed, the Mn segregation degree in the slab stage is 1.1 or less. There is a need. Moreover, in this invention, in order to control Mn segregation of the final steel plate, the manufacturing method of a slab is not limited. However, in order to control the MA distribution, it is necessary to reduce not only macro segregation such as center segregation but also micro segregation.
850℃以上の温度域において累積圧下率で40%以上の熱間圧延を行う必要がある。この温度域における圧下量の増加は、圧延中のオーステナイト粒の微細化に寄与し、結果としてフェライト粒を微細化し機械的性質を向上させる効果がある。このような効果を得るためには、850℃以上の温度域において累積圧下率が40%以上必要である。 When the heating temperature exceeds 1250 ° C., the crystal grain size becomes very coarse, and a large amount of scale is generated on the steel surface, and the quality of the surface is significantly lowered. For this reason, the upper limit of reheating temperature was 1250 degreeC.
It is necessary to perform hot rolling with a cumulative rolling reduction of 40% or more in a temperature range of 850 ° C. or higher. The increase in the amount of reduction in this temperature range contributes to the refinement of the austenite grains during rolling, and as a result, has the effect of refining the ferrite grains and improving the mechanical properties. In order to obtain such an effect, the cumulative rolling reduction needs to be 40% or more in the temperature range of 850 ° C. or higher.
本発明では冷却方法を空冷とのみ規定し、冷却速度は板厚との関係において定める。一般的な板厚では、冷却速度5℃/sを越すとMAやベイナイトが生成しやすくなり、靭性や延性を低下させる。このため、冷却速度5℃/s未満での緩冷却が望ましい。更に望ましくは2℃/s以下がよい。空冷を行うことによって容易に上記のような冷却速度を得る事ができる。 When the rolling end temperature exceeds 800 ° C., a band-like pearlite structure is not formed, and ductility and base material toughness are lowered. Moreover, when it becomes less than 700 degreeC, the amount of work ferrite will increase and ductility (uniform elongation) will fall remarkably.
In the present invention, the cooling method is defined only as air cooling, and the cooling rate is determined in relation to the plate thickness. With a general plate thickness, when the cooling rate exceeds 5 ° C./s, MA and bainite are likely to be formed, and the toughness and ductility are reduced. For this reason, slow cooling at a cooling rate of less than 5 ° C./s is desirable. More preferably, it is 2 ° C./s or less. By performing air cooling, the above cooling rate can be easily obtained.
この後、機械的性質を評価するために試験を実施した。表2の冷却方法欄において、比較鋼r、sでは括弧に示す速度(℃/s)で加速冷却した。上記以外の鋼板は空冷で冷却された。 A slab obtained by casting molten steel having the chemical components shown in Table 1 was hot-rolled under the conditions shown in Table 2 to obtain a steel plate. In Table 2, the “cumulative reduction amount” column indicates the cumulative reduction rate in a temperature range of 850 ° C. or higher.
This was followed by a test to evaluate the mechanical properties. In the cooling method column of Table 2, the comparative steels r and s were accelerated and cooled at the rate indicated in parentheses (° C./s). The steel plates other than the above were cooled by air cooling.
鋼wはC量が低いため母材強度が低下した。また、鋼xはSi量が高いためMAが増加し延性が低下した。鋼yはMn量が高いため、MAの増加とMAそのものが硬化するため所定の伸び特性、靭性が得られない。鋼zはAl量が少ないため弱脱酸である。また鋼aaはCa量が高い。このため、鋼z、aaでは比較的粗大な酸化物が生成し、十分な伸びが得られない。 As can be seen with reference to Tables 1 to 3, steel p had low toughness due to low cumulative reduction. Steel q had a high rolling end temperature and was unable to control the structure, so the toughness decreased. Since the steel r and steel s were accelerated and cooled, a large amount of bainite and MA were produced in the process, resulting in a decrease in ductility. In steel r, MA is decomposed by tempering, but because there is bainite, the toughness is recovered, but the recovery of elongation is insufficient. Steels t, ab to ad intentionally produced a large amount of MA by increasing the segregation degree of the plate by omitting the light reduction treatment in the slab manufacturing stage. For this reason, the elongation is reduced. Since steel u has a small plate thickness and a high cooling rate, MA is primarily generated before tempering. Steel u is subsequently tempered, but since the temperature is low, MA is not decomposed and elongation and toughness cannot be obtained. Steel v has a high tempering treatment temperature, and the amount of MA is reduced, but the yield strength is low.
Since the steel C has a low C content, the strength of the base material has decreased. Moreover, since steel x has a high Si content, MA increased and ductility decreased. Since the steel y has a high Mn content, the increase in MA and the MA itself harden, so that predetermined elongation characteristics and toughness cannot be obtained. Steel z is weakly deoxidized because of its low Al content. Steel aa has a high Ca content. For this reason, a relatively coarse oxide is produced in steels z and aa, and sufficient elongation cannot be obtained.
Claims (11)
- 質量%で、
C:0.07~0.15%、
Si:0.05~0.60%、
Mn:0.80~1.80%、
P:0.010以下、
S:0.007%以下、
V:0.05~0.12%、
Nb: 0.005~0.070%、
Al:0.005~0.08%、
Ti:0.005~0.030%、
Ca:0.0005~0.0035%、
N:0.0020~0.0060%、
O:0.0030%以下を含有し、残部が鉄および不可避的不純物からなり、
組織がフェライトとパーライトの二相組織であり、
島状マルテンサイトの面積分率が1.5%未満であり、
板厚が18mm以上であり、降伏強度が450MPa以上である
ことを特徴とする、強度および延性の良好なラインパイプ用鋼。 % By mass
C: 0.07 to 0.15%,
Si: 0.05 to 0.60%,
Mn: 0.80 to 1.80%,
P: 0.010 or less,
S: 0.007% or less,
V: 0.05 to 0.12%,
Nb: 0.005 to 0.070%,
Al: 0.005 to 0.08%,
Ti: 0.005 to 0.030%,
Ca: 0.0005 to 0.0035%,
N: 0.0020 to 0.0060%,
O: contains 0.0030% or less, the balance consists of iron and inevitable impurities,
The structure is a two-phase structure of ferrite and pearlite,
The area fraction of island martensite is less than 1.5%,
A steel for line pipes with good strength and ductility, characterized in that the plate thickness is 18 mm or more and the yield strength is 450 MPa or more. - 更に、質量%で、
Cu:0.05~0.70%、
Ni:0.05~0.70%、
Cr:0.80%以下、
Mo:0.30%以下、
B:0.0003~0.0030%、
Mg:0.0003~0.0030%、
REM:0.0005~0.0050%の一種または二種以上を含有したことを特徴とする請求項1に記載の強度および延性の良好なラインパイプ用鋼。 Furthermore, in mass%,
Cu: 0.05 to 0.70%,
Ni: 0.05 to 0.70%,
Cr: 0.80% or less,
Mo: 0.30% or less,
B: 0.0003 to 0.0030%,
Mg: 0.0003 to 0.0030%,
The steel for line pipes with good strength and ductility according to claim 1, characterized by containing one or more of REM: 0.0005 to 0.0050%. - 光学顕微鏡下でベイナイト組織が検出されないことを特徴とする請求項1または2に記載の強度および延性の良好なラインパイプ用鋼。 3. Steel for line pipes with good strength and ductility according to claim 1 or 2, wherein a bainite structure is not detected under an optical microscope.
- Mnの偏析度が1.7以下であることを特徴とする請求項1または2に記載の強度および延性の良好なラインパイプ用鋼。 3. The steel for line pipes with good strength and ductility according to claim 1 or 2, wherein the segregation degree of Mn is 1.7 or less.
- Si,Pの偏析度がそれぞれ1.5以下、8.0以下であることを特徴とする請求項4に記載の強度および延性の良好なラインパイプ用鋼。 The segregation degree of Si and P is 1.5 or less and 8.0 or less, respectively, and the steel for line pipes having good strength and ductility according to claim 4.
- 請求項1または2に記載の化学成分の鋳片を1250℃以下の温度に加熱後、850℃以上の温度域において累積圧下率で40%以上の熱間圧延をし、700~800℃の温度領域で熱間圧延を終了させた後、空冷することを特徴とする強度および延性の良好なラインパイプ用鋼の製造方法。 3. The slab of the chemical component according to claim 1 or 2 is heated to a temperature of 1250 ° C. or lower, and then hot-rolled at a cumulative reduction ratio of 40% or higher in a temperature range of 850 ° C. or higher to a temperature of 700 to 800 ° C. A method for producing steel for line pipes with good strength and ductility, characterized by air cooling after finishing hot rolling in a region.
- 前記空冷後、鋼板に500~300℃の温度領域で焼戻し処理を施すことを特徴とする請求項6に記載の強度および延性の良好なラインパイプ用鋼の製造方法。 The method for producing steel for a line pipe with good strength and ductility according to claim 6, wherein after the air cooling, the steel sheet is tempered in a temperature range of 500 to 300 ° C.
- 熱間圧延によって製造された鋼板中の島状マルテンサイト量を測定し、
島状マルテンサイトの面積分率が1.5%未満になるように製造条件を制御することによって延性を改善させることを特徴とする強度および延性の良好なラインパイプ用鋼の品質管理方法。 Measure the amount of island martensite in the steel sheet manufactured by hot rolling,
A quality control method for steel for line pipes having good strength and ductility, wherein ductility is improved by controlling production conditions so that the area fraction of island martensite is less than 1.5%. - 熱間圧延後の冷却方法を空冷とすることによって延性を改善させることを特徴とする請求項8に記載の強度および延性の良好なラインパイプ用鋼の品質管理方法。 The quality control method for steel for line pipes with good strength and ductility according to claim 8, wherein the ductility is improved by air cooling as a cooling method after hot rolling.
- 鋼板段階のMnの偏析度を1.7以下とすることによって延性を改善させることを特徴とする請求項8または9に記載の強度および延性の良好なラインパイプ用鋼の品質管理方法。 The method for quality control of steel for line pipes with good strength and ductility according to claim 8 or 9, wherein the ductility is improved by setting the segregation degree of Mn at a steel plate stage to 1.7 or less.
- 鋼板段階のベイナイト組織分率を1%以下にすることによって延性を改善させることを特徴とする請求項8または9に記載の強度および延性の良好なラインパイプ用鋼の品質管理方法。 The method for quality control of steel for line pipes with good strength and ductility according to claim 8 or 9, wherein the ductility is improved by setting the bainite structure fraction in the steel plate stage to 1% or less.
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JP2013019014A (en) * | 2011-07-11 | 2013-01-31 | Jfe Steel Corp | Steel for weld structure superior in ctod property at large heat input weld heat affected zone, and production method thereof |
CN113025885A (en) * | 2021-02-08 | 2021-06-25 | 江阴兴澄特种钢铁有限公司 | Low-yield-ratio high-strength pipeline steel plate with good HIC (hydrogen induced cracking) resistance and manufacturing method thereof |
CN114737120A (en) * | 2022-04-02 | 2022-07-12 | 鞍钢股份有限公司 | Steel for large-diameter tube bundle outer bearing tube and preparation method thereof |
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JP2013019014A (en) * | 2011-07-11 | 2013-01-31 | Jfe Steel Corp | Steel for weld structure superior in ctod property at large heat input weld heat affected zone, and production method thereof |
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CN114737120B (en) * | 2022-04-02 | 2022-11-18 | 鞍钢股份有限公司 | Steel for large-diameter tube bundle outer bearing tube and preparation method thereof |
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