WO2014162680A1 - 熱延鋼板およびその製造方法 - Google Patents
熱延鋼板およびその製造方法 Download PDFInfo
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—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
Definitions
- the present invention is a steel pipe used for pipe lines, oil country pipes (Oil Country Tubular Goods), civil engineering and construction, especially API (American Petroleum Institute) standard X80 grade steel pipe
- the present invention relates to a hot-rolled steel sheet suitable for high strength and excellent in low-temperature toughness and ductility, and a method for producing the hot-rolled steel sheet.
- the steel sheet for the linepipe material has high strength, It is also required to be excellent in low-temperature toughness.
- ERW steel pipes and spiral steel pipes have been widely used for automotive members and steel pipe piles, and are generally made of hot-rolled steel sheets with relatively thin thickness. It is said.
- a hot-rolled steel plate having a thicker thickness than the conventional steel material.
- the processing conditions in the surface layer area of the steel plates become particularly severe, and long-distance line pipes are subject to forced deformation due to crustal changes such as earthquakes.
- the hot-rolled steel sheet as a line pipe material has not only the desired strength and low-temperature toughness, but also has elongation characteristics at the full thickness that can withstand the processing and deformation as described above. Necessary.
- the composition of the hot-rolled steel sheet is C: 0.02 to 0.08% by mass, Si: 0.01 to 0.50%, Mn: 0.5 to 1.8%, P: 0.025% or less, S: 0.005% or less, Al : 0.005 to 0.10%, Nb: 0.01 to 0.10%, Ti: 0.001 to 0.05%, and C, Ti and Nb are ([% Ti] + ([% Nb] / 2)) / [% C] ⁇ 4 and including the balance Fe and inevitable impurities, the structure of the hot-rolled steel sheet, the average grain size of the ferrite phase as the main phase at a position of 1 mm from the steel sheet surface to the sheet thickness direction and the steel sheet
- the difference ⁇ D from the average grain size of the ferrite phase, which is the main phase at the center of the plate thickness is 2 ⁇ m or less, and the structural fraction (volume%) of the second phase at a position 1 mm from the steel plate surface in the plate thickness direction.
- the difference ⁇ V from the structure fraction (volume%) of the second phase at the plate thickness center position of the steel sheet is 2% or less, and the bainite phase or tempered martenser at a position 1 mm from the steel sheet surface in the plate thickness direction.
- the composition of a hot-rolled steel sheet is C: 0.03 to 0.06%, Si: 1.0% or less, Mn: 1 to 2%, Al: 0.1% or less, Nb: 0.05 to 0.08%, V in mass%. : 0.05 to 0.15%, Mo: 0.10 to 0.30%, with the balance consisting of Fe and unavoidable impurities.
- the structure of the hot-rolled steel sheet is a single bainite phase, and Nb and V carbonitride in the bainite phase.
- tensile strength TS strength of 760 MPa or more and fracture surface transition temperature (fracture transition temperature)
- vTrs toughness of -100 °C or less
- Patent Document 4 discloses that the composition of the steel plates is C: 0.06 to 0.12% by mass%, Si: 0.01 to 1.0%, Mn: 1.2 to 3.0%, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005 to 0.07%, Ti: 0.005 to 0.025%, N: 0.010% or less, O: 0.005% or less, the balance Fe and
- the composition consists of inevitable impurities, and the steel sheet has a two-phase structure of bainite and island martensite (MA Constituent).
- the island martensite has an area fraction of 3 to 20% and an equivalent circle diameter.
- Patent Document 5 by mass, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.5 to 1.8 %, P: 0.025% or less, S: 0.005% or less, Al: 0.005-0.10%, Nb: 0.01-0.10%, Ti: 0.001-0.05%, and C, Ti, Nb ([% Ti] + ([% Nb] / 2)) / [% C] ⁇ 4 is included, the steel material having the composition including the balance Fe and unavoidable impurities is heated, and hot between rough rolling and finish rolling.
- accelerated cooling consisting of primary accelerated cooling and secondary accelerated cooling
- the primary accelerated cooling has an average cooling rate of 10 ° C / s or more at the center position of the plate thickness, and Cooling with a cooling rate difference of less than 80 ° C / s between the average cooling rate at the center of the plate thickness and the average cooling rate at the position of 1 mm from the surface in the plate thickness direction, is performed at the position of 1 mm from the surface in the plate thickness direction.
- the cooling is performed up to the primary cooling stop temperature at which the temperature is 650 ° C. or lower and the temperature is 500 ° C.
- BFS0 (° C) 770-300C-70Mn-70Cr-170Mo-40Cu-40Ni
- a technology for manufacturing a thick, high-tensile hot-rolled steel sheet with excellent strength and ductility balance by winding at a coiling temperature below Has been.
- any of the above prior arts is a hot-rolled steel sheet suitable as a material for line pipes, that is, high strength and excellent low-temperature toughness. Further, forced processing due to severe processing conditions during pipe making and crustal deformation after laying, etc. It is extremely difficult to obtain a thick hot-rolled steel sheet that has sufficient ductility to withstand mechanical deformation.
- the desired hot-rolled steel strip structure (Baini, the main phase) is controlled by controlling the cooling rate after hot rolling to 20 ° C./s or less. Therefore, there is a problem that the lath in the bainitic ferrite is likely to be coarsened and the strength (particularly tensile strength) is likely to be reduced. Moreover, in the technique proposed by patent document 1, in order to ensure hardenability, addition of 1 or more types in any one of Cu, Ni, and Mo is essential. However, since these elements are rare elements and hinder future stable production, they are not preferable as essential elements.
- the average cooling rate at a position of 1 mm in the sheet thickness direction from the steel sheet surface is 100 ° C./s or more, and It is necessary to perform cooling so that the average cooling rate at the center of the plate thickness is 10 ° C / s or more.
- the cooling rate on the plate surface becomes too fast, resulting in excessively high surface layer hardness and a decrease in elongation at the total thickness.
- the temperature is 550 to 650 ° C at an average cooling rate of 20 ° C / s or more at the center of the sheet thickness. It needs to be cooled.
- the technique proposed in Patent Document 3 is a technique for hot-rolled steel sheets with an extremely high strength of TS: 760 MPa or more, when the sheet thickness increases, the hardness especially in the surface area of the sheet Rises, and there is a problem that the elongation characteristics of the entire thickness are easily deteriorated.
- the technique proposed in Patent Document 4 has a uniform elongation characteristic (uniform-elongation-property) of a high-strength steel sheet by forming a structure in which island-like martensite is uniformly finely dispersed in the bainite phase. Secured. However, in the technique proposed in Patent Document 4, it is essential to include 3% or more of island martensite, and there is a problem that toughness (especially, DWTT characteristics (particularly drop weight weight test weight property)) is liable to occur. . Further, in order to secure the above structure, after hot rolling, the steel sheet is cooled to an average temperature of 500 to 680 ° C., and then immediately reheated to 550 ° C. to a cooling start temperature. However, in order to raise the average temperature of the steel sheet, there is a problem that a reheating facility or the like is substantially required and a manufacturing process is complicated.
- the average cooling rate at the center of the thickness and the position 1 mm from the surface to the thickness direction In order to cool to a predetermined temperature while controlling the difference in cooling rate from the average cooling rate at 80 ° C / s or less, a large number of cooling banks (coolingsbanks) are provided, or the steel plate transport speed (transportation) There is a problem that it is necessary to lengthen the cooling time, such as slowing the velocity), lowering the production efficiency and adding new equipment.
- the present invention solves the above-mentioned problems of the prior art, and is suitable as a material for X80 class electric resistance welded steel pipes or a material for X80 class spiral steel pipes, and has excellent strength, toughness and elongation characteristics at the entire thickness. It aims at providing a rolled steel plate and its manufacturing method.
- the present inventors for example, for a thick hot-rolled steel sheet having a thickness of 12 mm or more, without adding rare elements such as Cu, Ni, and Mo as much as possible, while ensuring high strength and high toughness,
- the means for improving the elongation characteristics of the steel were studied earnestly.
- the inventors focused on ferrite, tempered martensite and tempered bainite, which are excellent in toughness and ductility, and made these structures the main phase of the hot-rolled steel sheet, and added reinforcing elements such as Cu, Ni, and Mo.
- the means for securing the hot-rolled steel sheet strength was investigated.
- the ferrite lath structure cannot be observed with an optical microscope (optical microscope), but can be observed with a transmission electron microscope (TEM) or a scanning electron microscope (SEM) (SEM). (Magnification: 5000-20000 times). Such a lath structure is observed with acicular ferrite, bainitic ferrite, and the like, but is not observed with polygonal ferrite.
- optical microscope optical microscope
- SEM scanning electron microscope
- the strength of the hot-rolled steel sheet increases as the lath interval of the lath structure decreases.
- the lath interval becomes extremely narrow, the low temperature toughness and elongation characteristics of the hot rolled steel sheet deteriorate. Therefore, it is difficult to increase the strength of a hot-rolled steel sheet while maintaining high toughness and excellent elongation characteristics only by narrowing the lath spacing between ferrite having a lath structure, tempered martensite, and tempered bainite.
- the present inventors examined a means for ensuring a desired hot-rolled steel sheet strength without extremely narrowing the lath spacing of ferrite having a lath structure, tempered martensite, and tempered bainite.
- precipitation strengthening in addition to the above-described transformation strengthening, it is an extremely effective means to achieve both precipitation strengthening and transformation strengthening.
- the controlling factor of precipitation strengthening is mainly due to the precipitation of Nb, and the lath spacing and Nb precipitation ratio of ferrite having a lath structure, tempered martensite and tempered bainite are adjusted.
- the present inventors specify the cooling / reheating conditions and finish rolling conditions for the slab when hot rolling steel sheets are produced by hot rolling a continuous cast slab having a predetermined composition, and further finishing.
- the cooling rate at the center of the plate thickness is specified, and the cooling and recuperation conditions at the plate thickness surface layer are specified, so that hot rolling having the desired lath spacing and Nb precipitation rate as described above is achieved. It has been found that steel sheets can be manufactured.
- a high toughness, high ductility, high strength hot-rolled steel sheet characterized by having a structure in which a volume fraction of ferrite of 0.2 ⁇ m or more and 1.6 ⁇ m or less is 95% or more.
- Pcm [% C] + [% Si] / 30 + ([% Mn] + [% Cu] + [% Cr]) / 20 + [% Ni] / 60 + [% V] / 10 + [% Mo] / 7 + 5 ⁇ [% B] ⁇ 0.25
- Px 701 ⁇ [% C] + 85 ⁇ [% Mn] ⁇ 181
- [% C], [% Si], [% Mn], [% Cu], [% Cr], [% Ni], [% V], [% % Mo] and [% B] are the contents of each element (% by mass).
- Ca 0.0001% to 0.005% by mass in addition to the composition.
- Cu 0.001% to 0.5%
- Ni 0.001% to 0.5%
- Mo 0.001% or more in mass%
- cooling is performed at the center position of the plate thickness at an average cooling rate of 5 ° C / s to 50 ° C in the temperature range of 750 ° C to 650 ° C.
- the cooling stop temperature in the temperature range of 300 ° C or more and 600 ° C or less at the 1mm position of the plate thickness surface layer at 1mm / s or less, it is reheated to the temperature range of 550 ° C or more and the cooling start temperature or less over 1s, and again 300 ° C
- the cooling to the temperature range of 600 ° C or lower is performed once or more, and winding is performed in the temperature range of 350 ° C or higher and 650 ° C or lower.
- Pcm [% C] + [% Si] / 30 + ([% Mn] + [% Cu] + [% Cr]) / 20 + [% Ni] / 60 + [% V] / 10 + [% Mo] / 7 + 5 ⁇ [% B] ⁇ 0.25
- Px 701 ⁇ [% C] + 85 ⁇ [% Mn] ⁇ 181
- [% C], [% Si], [% Mn], [% Cu], [% Cr], [% Ni], [% V], [% % Mo] and [% B] are the contents of each element (% by mass).
- [7] Production of high toughness, high ductility, high strength hot-rolled steel sheet according to [5] or [6], further containing Ca: 0.0001% to 0.005% by mass% in addition to the above composition Method.
- Cu 0.001% to 0.5%
- Ni 0.001% to 0.5%
- Mo 0.001% or more in mass% 0.5% or less
- Cr 0.001% or more and 0.5% or less
- B One type or two or more types selected from 0.0001% or more and 0.004% or less Production method.
- a thin- to thick-walled hot-rolled steel sheet excellent in strength, toughness, and elongation characteristics at the entire thickness suitable for line pipes, oil well pipes, and steel pipes for civil engineering and construction can be used as rare elements and It is obtained while maintaining a high production efficiency without requiring the installation of a new reheating facility or the like, and is extremely useful industrially.
- FIG. 1 is a diagram showing a temperature history (plate thickness center position and plate thickness surface layer 1 mm position) in the cooling process after finishing rolling in the present invention.
- Fig.2 (a) is the structure
- FIG. 2B is a structural photograph (magnification: 20000 times) of a hot-rolled steel sheet No. 2A (invention example) of the example by a transmission electron microscope (TEM).
- TEM transmission electron microscope
- C 0.04% or more and 0.15% or less C increases the strength of hot-rolled steel sheets by reducing the lath spacing of ferrite, tempered martensite, and tempered bainite having a lath structure, and forming carbides with Nb, V, and Ti.
- the C content needs to be 0.04% or more.
- the lath spacing of tempered martensite and / or tempered bainite which is the main phase in the surface layer portion of the plate thickness, becomes extremely narrow and excessive increase in precipitates causes hot rolled steel sheet.
- the toughness and elongation properties at the entire thickness deteriorate.
- the carbon equivalent becomes high, and when such a hot-rolled steel sheet is piped and welded, the toughness of the welded portion deteriorates. Therefore, the C content is 0.04% or more and 0.15% or less. More preferably, it is 0.04 to 0.10%.
- the upper limit of Si content is 0.55%.
- the lower limit of the Si content is set to 0.01% from the deoxidation effect and the steelmaking technology limit. More preferably, it is 0.10 to 0.45%.
- Mn 1.0% or more and 3.0% or less
- Mn is an element necessary to suppress the formation of polygonal ferrite and to ensure strength and toughness
- Mn content needs to be 1.0% or more to exert its effect. There is.
- the Mn content exceeds 3.0%, variations in mechanical characteristics associated with segregation are likely to occur.
- the strength becomes too high, adverse effects such as a decrease in elongation characteristic appear, and the toughness of the welded portion may deteriorate as the carbon equivalent increases. Therefore, the Mn content is 1.0% or more and 3.0% or less.
- P 0.03% or less
- S 0.01% or less
- N 0.006% or less
- P is an element that exists as an impurity in the steel and easily segregates, and causes deterioration of the toughness of the steel. Therefore, the upper limit of the P content is 0.03%. More preferably, it is 0.02% or less.
- the S content has an upper limit of 0.01% and the N content has an upper limit of 0.006%. More preferably, S is 0.005% or less.
- Al 0.003% or more and 0.1% or less
- Al is useful as a deoxidizing agent for steel, and the Al content is set to 0.003% or more where a deoxidation effect is exhibited.
- the Al content is 0.003% or more and 0.1% or less. More preferably, it is 0.003 to 0.06%.
- Nb 0.035% or more and 0.1% or less
- Nb is a precipitation strengthening element that is effective for refining crystal grains, and in order to ensure X80 grade steel pipe strength, the Nb content should be 0.035% or more. There is a need to.
- the Nb content is 0.035% or more and 0.1% or less. More preferably, it is 0.035 to 0.08%.
- V 0.001% or more and 0.1% or less
- V is a precipitation strengthening element.
- the V content needs to be 0.001% or more.
- the V content is excessive, during the production of hot-rolled steel sheet, excessive precipitation occurs in the coiling temperature range (350 ° C or higher and 650 ° C or lower), which will be described later, and the toughness and elongation characteristics decrease, and weldability is reduced. Deteriorate. Therefore, the V content is 0.001% or more and 0.1% or less.
- Ti 0.001% or more and 0.1% or less Ti is effective for refining crystal grains and is a precipitation strengthening element, and the Ti content needs to be 0.001% or more in order to exhibit the effect.
- Ti content when the Ti content is excessive, during the production of hot-rolled steel sheets, excessive precipitation occurs in the coiling temperature range (350 ° C or higher and 650 ° C or lower), which will be described later, and the toughness and elongation characteristics decrease and weldability deteriorates. Let Therefore, Ti content shall be 0.001% or more and 0.1% or less. More preferably, it is 0.001 to 0.05%.
- the high toughness and high ductility high-strength hot-rolled steel sheet of the present invention preferably further contains Ca: 0.0001% or more and 0.005% or less in addition to the above component composition.
- Ca 0.0001% or more and 0.005% or less Ca has an effect of improving toughness by fixing S and suppressing the formation of MnS.
- the Ca content is preferably 0.0001% or more.
- the Ca content is preferably 0.005% or less. More preferably, it is 0.001 to 0.0035%.
- the high toughness and high ductility high strength hot-rolled steel sheet of the present invention is further Cu: 0.001% to 0.5%, Ni: 0.001% to 0.5%, Mo: 0.001% or more 0.5% or less, Cr: 0.001% or more and 0.5% or less, B: 0.0001% or more and 0.004% or less may be included.
- Cu 0.001% or more and 0.5% or less
- Cu is an element effective in controlling the transformation of steel and improving the strength of the hot-rolled steel sheet. In order to exhibit such an effect, it is preferable to make Cu content 0.001% or more.
- Cu has strong hardenability, and when its content exceeds 0.5%, the lath interval of tempered martensite and / or tempered bainite, which is the main phase particularly in the plate thickness surface layer portion, is extremely narrow, There is a possibility that the elongation characteristics at the total thickness are deteriorated and the hot workability is lowered. Therefore, the Cu content is preferably 0.001% or more and 0.5% or less.
- Ni 0.001% or more and 0.5% or less
- Ni is an element effective in controlling the transformation of steel and improving the strength of the hot-rolled steel sheet.
- the Ni content is preferably 0.001% or more.
- Ni has a strong hardenability, and when its content exceeds 0.5%, the lath interval of tempered martensite and / or tempered bainite, which is the main phase in the plate thickness surface layer portion, is extremely narrow, There is a possibility that the elongation characteristics at the total thickness are deteriorated and the hot workability is lowered. Therefore, the Ni content is preferably 0.001% or more and 0.5% or less.
- Mo 0.001% or more and 0.5% or less
- Mo is an element effective in controlling the transformation of steel and improving the strength of the hot-rolled steel sheet.
- the Mo content is preferably 0.001% or more.
- Mo has strong hardenability, and when its content exceeds 0.5%, the lath interval of tempered martensite and / or tempered bainite, which is the main phase particularly in the plate thickness surface layer portion, is extremely narrowed, and toughness is increased.
- the Mo content is preferably 0.001% or more and 0.5% or less.
- Cr 0.001% or more and 0.5% or less
- Cr has a delay effect of pearlite transformation and a reduction effect of grain boundary cementite.
- the amount is preferably 0.001% or more.
- the Cr content is excessive, the lath interval of tempered martensite and / or tempered bainite, which is the main phase particularly in the surface layer portion of the plate thickness, is extremely narrowed, and the toughness and the elongation characteristics at the entire thickness are deteriorated.
- the Cr content is preferably 0.001% or more and 0.5% or less.
- Cu, Ni, Mo, and Cr are all rare metals, and are difficult to ensure stably and are expensive elements. Therefore, from the viewpoints of securing the stability of raw materials, production costs, etc., it is preferable to avoid the addition of these elements as much as possible, and the respective contents are preferably set to 0.1% or less.
- B 0.0001% or more and 0.004% or less B is effective in suppressing ferrite transformation at high temperature and preventing ferrite hardness from being lowered during the cooling process after finishing rolling during the production of hot-rolled steel sheets.
- the B content is preferably 0.0001% or more.
- the B content is preferably 0.0001% or more and 0.004% or less. More preferably, the content is 0.0001 to 0.003%.
- the high toughness and high ductility high-strength hot-rolled steel sheet of the present invention preferably has a composition that satisfies the component indices shown in the following formulas (1) and (2).
- Pcm [% C] + [% Si] / 30 + ([% Mn] + [% Cu] + [% Cr]) / 20 + [% Ni] / 60 + [% V] / 10 + [% Mo] / 7 + 5 ⁇ [% B] ⁇ 0.25 (1)
- Px 701 ⁇ [% C] + 85 ⁇ [% Mn] ⁇ 181 (2)
- [% C], [% Si], [% Mn], [% Cu], [% Cr], [% Ni], [% V], [% % Mo] and [% B] are the contents (% by mass) of each element.
- Pcm value shall be calculated by setting [% Cu] in Formula (1) to zero. The same applies to [% Cr], [% Ni], [% V], [% Mo], and [% B].
- the Pcm shown in equation (1) is an index of hardenability.
- the Pcm value exceeds a certain value, the lath spacing of tempered martensite and / or tempered bainite, which is the main phase in the surface layer of the plate thickness, becomes extremely narrow, and the toughness and elongation characteristics of the hot rolled steel sheet deteriorate. Tend to. Therefore, the Pcm value is preferably 0.25 or less. More preferably, it is 0.23 or less.
- welding heat affected zone (HAZ) softening will occur in welding during pipe making or line pipe laying, and there is a concern that joint tensile properties will deteriorate. It is preferable.
- Px shown in the formula (2) is a lath interval of ferrite having a lath structure, tempered martensite, and tempered bainite in the range of a coiling temperature (350 ° C. or higher and 650 ° C. or lower), which will be described later, during the production of a hot rolled steel sheet. It is an index to control. In order to narrow the lath interval to such an extent that X80 grade steel pipe strength is ensured, it is preferable to set the Px value to 181 or more.
- the Px value becomes excessively high, the lath spacing of the tempered martensite and / or tempered bainite, which is the main phase in the surface layer portion of the plate thickness, becomes extremely narrow, and the toughness of the hot-rolled steel sheet and the elongation characteristics at the total thickness are reduced. Since there is concern about deterioration, it is preferably set to 300 or less.
- components other than the above are Fe and unavoidable impurities.
- inevitable impurities include Co, W, Pb, and Sn.
- the ratio of precipitated Nb to the total Nb content is 35% or more and 80% or less.
- the volume fraction of tempered martensite and / or tempered bainite with a lath spacing of 0.2 ⁇ m or more and 1.6 ⁇ m or less at the plate thickness surface layer of 1.0 mm is 95% or more, and the volume fraction is 5% or less as the balance. Ferrite, pearlite, martensite, and retained austenite.
- the ferrite has a structure in which the lath interval is 0.2 ⁇ m or more and 1.6 ⁇ m or less and the volume fraction of ferrite is 95% or more.
- the balance may include tempered martensite, tempered bainite, pearlite, martensite, residual austenite, and the like having a volume fraction of 5% or less.
- the martensite in the plate thickness surface layer position of 1.0 mm and the plate thickness center position does not include island martensite.
- Ferrite means polygonal ferrite.
- the ferrite having a lath structure includes acicular ferrite, bainitic ferrite, Woodman-Stuschten ferrite, and acicular ferrite.
- Ratio of precipitated Nb with respect to total Nb amount 35% or more and 80% or less If the precipitation ratio is less than 35%, insufficient strength is likely to occur, and variations in mechanical properties after pipe forming increase. On the other hand, if it exceeds 80%, the hardness of ferrite, tempered martensite and tempered bainite increases, and the hot rolled steel sheet toughness and elongation characteristics deteriorate, so the upper limit is made 80%.
- the ratio (mass ratio) of Nb precipitated in the steel sheet was determined by measuring the mass of Nb precipitated in the steel sheet by extraction residue analysis, and the ratio (mass%) of this measured value to the total Nb content.
- the steel sheet was dissolved in 10% acetylacetone-1% tetramethylammonium-methanol by constant-current electrolysis (approximately 20 mA / cm 2 ).
- the residue can be collected with a membrane filter (pore size: 0.2 ⁇ m ⁇ ), melted using a mixed flux of sulfuric acid, nitric acid and perchloric acid, and the amount of precipitation can be quantified by ICP emission spectrometry. .
- Main phase of hot-rolled steel sheet When manufacturing thick-walled hot-rolled steel sheets with a plate thickness of 12 mm or more, for example, the cooling rate is adjusted so that ferrite with a lath structure is generated at the center of the sheet thickness after hot rolling is completed. Then, the cooling rate in the plate thickness surface layer portion becomes extremely large. Therefore, in the case of a thick hot-rolled steel sheet, it is extremely difficult to obtain a ferrite main phase structure having a lath structure over the entire plate thickness.
- the main phase of the plate thickness surface layer portion (the surface layer portion from the steel plate surface to the plate thickness direction 1.0 mm) is tempered martensite and / or tempered bainite having a desired lath interval
- the main phase in the other region is a ferrite having a lath structure and a desired lath interval.
- a ferrite having a lath structure is defined as a ferrite transformed at a temperature lower than the temperature at which polygonal ferrite forms, and a specimen taken from the center of the thickness of the hot-rolled steel sheet is magnified 5000 to 20000 times. Means a ferrite in which a lath structure is observed in TEM observation or SEM observation. Further, the ferrite having a lath structure includes acicular ferrite, bainitic ferrite, Woodman-Stuschten ferrite, and acicular ferrite.
- Lath spacing 0.2 ⁇ m or more and 1.6 ⁇ m or less
- the lath spacing of ferrite having a lath structure, tempered martensite, and tempered bainite is a factor responsible for the strength of the hot-rolled steel sheet, and therefore needs to be somewhat fine.
- the lath spacing is less than 0.2 ⁇ m, even if Nb or the like does not precipitate, the hardness of ferrite, tempered martensite, and tempered bainite becomes excessive, and the toughness and elongation characteristics of the hot rolled steel sheet deteriorate. To do.
- the lath interval exceeds 1.6 ⁇ m, sufficient strength of the hot-rolled steel sheet cannot be ensured even when Nb or the like is sufficiently precipitated, and the X80 grade steel pipe strength cannot be satisfied. Therefore, the lath interval is 0.2 ⁇ m or more and 1.6 ⁇ m or less.
- Tempered martensite and / or tempering with desired lath spacing (0.2 ⁇ m or more and 1.6 ⁇ m or less) at 1mm position of plate thickness surface layer (position of plate thickness direction 1.0mm from steel plate surface)
- desired lath spacing 0.2 ⁇ m or more and 1.6 ⁇ m or less
- desired lath spacing 0.2 ⁇ m or more and 1.6 ⁇ m or less
- the volume fraction of ferrite having a desired lath interval (0.2 ⁇ m or more and 1.6 ⁇ m or less) is less than 95% at the center position of the plate thickness
- the low temperature toughness of the region other than the plate thickness surface layer portion is greatly reduced. Therefore, in the present invention, the volume fraction of the main phase at each position is set to 95% or more.
- the high toughness, high ductility, high strength hot-rolled steel sheet of the present invention is a slab (slab) having the above composition obtained by continuous casting, once cooled or allowed to cool to 600 ° C. or less, and after reheating, rough rolling and finishing After rolling, it can be manufactured by performing accelerated cooling under a predetermined condition and winding at a predetermined temperature.
- Cooling temperature of continuous cast slab 600 ° C. or less
- the slab (continuous cast slab) cooling temperature is set to 600 ° C. or less at which the ferrite transformation is sufficiently completed.
- Reheating temperature of continuous cast slab 1000 ° C or higher and 1250 ° C or lower If the slab heating temperature (reheating temperature of continuous cast slab) is lower than 1000 ° C, the precipitation strengthening elements Nb, V and Ti do not dissolve sufficiently. , X80 grade steel pipe strength cannot be secured. On the other hand, when the temperature exceeds 1250 ° C., austenite grains become coarse, and Nb excessively precipitates in the cooling and winding process after finish rolling, so that the toughness and elongation characteristics of the hot-rolled steel sheet deteriorate. Therefore, the reheating temperature of the continuous cast slab is 1000 ° C. or more and 1250 ° C. or less.
- the slab (continuous cast slab) after reheating is subjected to rough rolling and finish rolling and adjusted to an arbitrary plate thickness.
- the conditions for rough rolling are not particularly limited.
- Rolling ratio in no-recrystallization temperature range during finish rolling 20% or more and 85% or less Finish rolling in non-recrystallization temperature range (about 940 ° C or less in the case of the steel composition of the present invention) As a result, the recrystallization of the austenite phase is delayed and the strain accumulates, and the ferrite is refined during the ⁇ / ⁇ transformation ( ⁇ ⁇ ⁇ transformation) to improve the strength and toughness.
- the rolling reduction in the non-recrystallization temperature region during finish rolling is less than 20%, these effects are not sufficiently exhibited.
- the rolling reduction exceeds 85%, deformation resistance increases and hinders rolling. Therefore, in the present invention, the rolling reduction is set to 20% or more and 85% or less. Preferably they are 35% or more and 75% or less.
- Finishing rolling finish temperature (Ar 3 -50 ° C) or more (Ar 3 + 100 ° C) or less
- the finish rolling finish temperature is set to (Ar 3 -50 ° C) or more.
- the finish rolling finish temperature is lower than (Ar 3 -50 ° C.)
- ferrite transformation occurs inside the steel plate during finish rolling, the structure becomes non-uniform, and desired characteristics cannot be obtained.
- the finish rolling finish temperature exceeds (Ar 3 + 100 ° C.)
- the finish rolling finish temperature is set within the range of (Ar 3 ⁇ 50 ° C.) to (Ar 3 + 100 ° C.).
- the finish rolling end temperature is a measured temperature value of the steel sheet surface on the exit side of the finish rolling mill.
- FIG. 1 is a schematic diagram of a temperature history (temperature history from finish rolling finish temperature to winding temperature) after finish rolling in the present invention.
- the center position of the plate thickness is cooled to the winding temperature at a predetermined cooling rate.
- cooling and reheat treatment are performed once or more, and then cooled to the coiling temperature.
- Average cooling rate in the temperature range of 750 ° C or lower and 650 ° C or higher at the center of the plate thickness 5 ° C / s or higher and 50 ° C / s or lower
- the formation of pearlite transformation and polygonal ferrite is suppressed, and the plate
- 750 ° C. or less at the center of the plate thickness is 650 ° C. or more.
- the average cooling rate in the temperature range of 5 ° C./s or more is required.
- the upper limit is 50 ° C / s need to be.
- the total volume fraction of tempered martensite and / or tempered bainite having a desired lath interval (0.2 ⁇ m or more and 1.6 ⁇ m or less) at the plate thickness surface layer of 1.0 mm is obtained.
- This treatment is 550 over 1 second (primary recuperation time) after cooling at an arbitrary cooling rate from the accelerated cooling start temperature to the cooling stop temperature (primary cooling stop temperature) in the temperature range of 300 ° C to 600 ° C.
- the cooling stop temperature when this process is performed n times is the n-th cooling stop temperature
- the recuperation time is the n-th recuperation time
- the recuperation temperature is the n-th recuperation temperature.
- n-th cooling stop temperature 300 ° C. or more and 600 ° C. or less
- This treatment is once made into a low temperature transformation structure (martensite structure and / or bainite structure) in the surface layer part (sheet thickness surface layer region) from the surface to the sheet thickness direction 1.0 mm,
- the purpose is to temper it by reheating. Thereby, the lath interval of tempered martensite and / or tempered bainite in the surface thickness portion of the plate can be adjusted, and the surface layer hardness and further the elongation characteristics at the total thickness can be improved.
- the cooling stop temperature exceeds 600 ° C.
- the low-temperature transformation structure is not sufficiently formed, so that the plate thickness surface layer portion cannot be a tempered structure, and the elongation characteristic at the entire thickness is deteriorated.
- the n-th cooling stop temperature is less than 300 ° C.
- the target recuperation temperature cannot be reached, so that it cannot be tempered sufficiently and the elongation characteristics at the entire thickness are lowered.
- n-order recuperation temperature 550 ° C or more and cooling start temperature or less If the recuperation temperature is less than 550 ° C, the structure cannot be tempered sufficiently, and the hardness at the surface layer of the plate thickness increases, Elongation characteristics decrease. On the other hand, when the reheating (reheating) temperature exceeds the cooling start temperature (usually the finish rolling finish temperature –20 ° C to the finish rolling finish temperature), the reverse transformation from ferrite to austenite at the plate thickness surface part. When this occurs, a quenched structure is formed when cooling again. As a result, there arises a problem that the hardness at the plate thickness surface layer portion is increased and the elongation characteristics at the entire thickness are decreased. Therefore, the recuperation temperature is set to a temperature range of 550 ° C. or more and the cooling start temperature or less.
- n-order recuperation time 1 second or more
- the recuperation time is 1 second or longer.
- the recuperation time is preferably 5 seconds or less.
- the reheating process is repeated in a predetermined cycle, and then the winding is performed. Cool to the take-off temperature.
- intermittent cooling can be used as a means for performing desired cooling / reheating treatment on the 1 mm thick surface layer position while keeping the cooling rate at the central thickness position within the above range.
- an induction heating facility is provided between the cooling banks, and a means such as heating the surface layer to a predetermined recuperation temperature using this can be exemplified.
- Winding temperature 350 ° C. or higher and 650 ° C. or lower
- the winding temperature is preferably 400 ° C. or higher.
- the strength decreases due to coarsening of precipitates and expansion of the lath interval of ferrite having a lath structure, tempered martensite, and tempered bainite.
- the coiling temperature exceeds 650 ° C, coarse pearlite is generated and the toughness deteriorates, so the upper limit is set to 650 ° C.
- they are 400 degreeC or more and 650 degrees C or less.
- the coiling temperature is the temperature on the steel sheet surface. However, this is approximately equal to the temperature at the 1 mm position of the plate thickness surface layer.
- EMS electro-magnetic stirrer
- IBSR light pressure casting
- equiaxed crystals can be formed at the center of the plate thickness, and segregation can be reduced.
- light pressure casting is performed, segregation at the central portion of the plate thickness can be reduced by preventing the flow of molten steel in the unsolidified portion of the continuous cast slab.
- Test pieces were collected from the obtained hot-rolled steel sheet, and subjected to structure observation, extraction residue analysis, tensile test, impact test, DWTT test, and hardness test by the following methods.
- a thin-film sample was taken from the center of the thickness of the obtained hot-rolled steel sheet and the surface layer of 1 mm, and a transmission electron microscope (magnification: 20000 times) was used to determine the lath boundary for each sheet thickness position. Three or more visual fields were observed and imaged at locations where 4 or more were aligned in parallel. Then, by measuring all the lath intervals observed from each of the obtained photographs, and determining the average value of all the measured lath intervals, the lath interval of the ferrite at the plate thickness center position, and the surface layer 1 mm position The lath spacing of tempered martensite and tempered bainite was determined.
- the steel plate (test piece) was subjected to constant current electrolysis (about 20 mA / cm 2 ) in 10% acetylacetone-1% tetramethylammonium-methanol, and the dissolved residue was filtered with a membrane filter (pore size: 0.2 ⁇ m ⁇ ). It is collected, melted with a mixed flux of sulfuric acid, nitric acid and perchloric acid, diluted to a constant volume with water, and the Nb precipitation rate is quantified by ICP emission spectrometry.
- the yield strength was 550 MPa or more
- the tensile strength was 650 MPa or more
- the total elongation was 20% or more was evaluated as “good tensile properties”.
- the yield strength is 690 MPa or less and the tensile strength is 760 MPa or less.
- Charpy impact test A V-notched test bar (length 55 mm x height 10 mm x) so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction from the center position of the thickness of the obtained hot-rolled steel sheet 10mm width), Charpy impact test was conducted in accordance with JIS Z 2242, test temperature: Absorbed energy (J) at -60 °C and ductile-brittle fracture surface transition temperature (ductile) -brittle fracture surface transition temperature (° C).
- the number of specimens was three, and the arithmetic average of the obtained absorption energy value and ductility-brittle fracture surface transition temperature was obtained, and the absorbed energy value (vE -60 ) and ductility-brittle fracture surface transition temperature (vTrs ).
- vE- 60 was 100 J or more and vTrs was ⁇ 80 ° C. or less was evaluated as “good toughness”.
- DWTT test From the obtained hot-rolled steel sheet, a DWTT test piece (size: full thickness x width 3 in. X length 12 in.) So that the direction perpendicular to the rolling direction (direction C) is the longitudinal direction.
- Tables 3 and 4 show the results of (1) to (6) above.
- the hot-rolled steel sheets of the inventive examples did not show excessive surface layer hardening, and both the tensile properties (strength and ductility) and toughness (low-temperature toughness) were good.
- the hot-rolled steel sheet of the comparative example could not obtain sufficient characteristics in either one or both of tensile properties and toughness (low temperature toughness).
- FIG. 2A and 2 (b) are the results of observing the structure of the same specimen taken from the center of the thickness of the hot-rolled steel sheet (steel sheet: 2A) of the inventive examples described in Tables 2 to 4.
- FIG. 2A is a structure photograph obtained by observation with an optical microscope (magnification: 1000 times)
- FIG. 2B is a structure photograph obtained by TEM observation (magnification: 20000 times).
- FIG. 2A the lath structure of ferrite, tempered martensite and tempered bainite is not observed.
- FIG. 2B shows the lath structure of ferrite, tempered martensite and tempered bainite (this photo is ferrite) can be confirmed.
- the arrow in FIG.2 (b) shows a lath space
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Abstract
Description
本願は、2013年4月4日に、日本に出願された特願2013-078395号に基づき優先権を主張し、その内容をここに援用する。
%、P:0.025%以下、S:0.005%以下、Al:0.005~0.10%、Nb:0.01~0.10%、Ti:0.001~0.05%を含み、かつC、Ti、Nbを([%Ti]+([%Nb]/2))/[%C]<4を満足するように含有し、残部Feおよび不可避的不純物からなる組成の鋼素材を加熱し、粗圧延と仕上圧延とからなる熱間圧延を施して熱延鋼板とするにあたり、一次加速冷却と二次加速冷却とからなる加速冷却を実施し、一次加速冷却を、板厚中心位置の平均冷却速度が10℃/s以上で、かつ板厚中心位置の平均冷却速度と表面から板厚方向に1mmの位置での平均冷却速度との冷却速度差が、80℃/s未満である冷却を、表面から板厚方向に1mmの位置での温度が650℃以下500℃以上の温度域の温度となる一次冷却停止温度まで行う冷却とし、前記二次加速冷却を、板厚中心位置の平均冷却速度が10℃/s以上で、板厚中心位置の平均冷却速度と表面から板厚方向に1mmの位置での平均冷却速度との冷却速度差が、80℃/s以上である冷却を、板厚中心位置の温度がBFS(℃)=770-300C-70Mn-70Cr-170Mo-40Cu-40Ni-1.5CR(CR:冷却速度(℃/s))以下の二次冷却停止温度まで行う冷却とし、該二次加速冷却後に、板厚中心位置の温度でBFS0(℃)=770-300C-70Mn-70Cr-170Mo-40Cu-40Ni以下の巻取温度で巻き取ることにより、強度・延性バランスに優れた厚肉高張力熱延鋼板を製造方法する技術が提案されている。
[1] 質量%で、
C :0.04%以上0.15%以下、 Si:0.01%以上0.55%以下、
Mn:1.0%以上3.0%以下、 P :0.03%以下、
S :0.01%以下、 Al:0.003%以上0.1%以下、
N :0.006%以下、 Nb:0.035%以上0.1%以下、
V :0.001%以上0.1%以下、 Ti:0.001%以上0.1%以下
を含有し、残部がFeおよび不可避的不純物からなる組成を有し、全Nb量に対する析出Nbの割合が35%以上80%以下であり、板厚表層1.0mm位置において、ラス間隔が0.2μm以上1.6μm以下である焼戻しマルテンサイトおよび/または焼戻しベイナイトの体積分率が95%以上であり、板厚中央位置において、ラス間隔が0.2μm以上1.6μm以下であるフェライトの体積分率が95%以上である組織を有することを特徴とする高靭性高延性高強度熱延鋼板。
[2] 前記[1]において、前記組成が、下記(1)式および(2)式を満足することを特徴とする高靭性高延性高強度熱延鋼板。
Pcm=[%C]+[%Si]/30+([%Mn]+[%Cu]+[%Cr])/20
+[%Ni]/60+[%V]/10+[%Mo]/7+5×[%B]≦0.25 ・・・ (1)
Px=701×[%C]+85×[%Mn]≧181 ・・・ (2)
ここで、(1)式および(2)式において、[%C]、[%Si]、[%Mn]、[%Cu]、[%Cr]、[%Ni]、[%V]、[%Mo]、[%B]は各元素の含有量(質量%)。
[3] 前記[1]または[2]において、前記組成に加えて更に、質量%でCa:0.0001%以上0.005%以下を含有することを特徴とする高靭性高延性高強度熱延鋼板。
[4] 前記[1]ないし[3]のいずれかにおいて、前記組成に加えて更に、質量%で、Cu:0.001%以上0.5%以下、Ni:0.001%以上0.5%以下、Mo:0.001%以上0.5%以下、Cr:0.001%以上0.5%以下、B:0.0001%以上0.004%以下のうちから選ばれる1種または2種以上を含有することを特徴とする高靭性高延性高強度熱延鋼板。
[5] 質量%で、
C :0.04%以上0.15%以下、 Si:0.01%以上0.55%以下、
Mn:1.0%以上3.0%以下、 P :0.03%以下、
S :0.01%以下、 Al:0.003%以上0.1%以下、
N :0.006%以下、 Nb:0.035%以上0.1%以下、
V :0.001%以上0.1%以下、 Ti:0.001%以上0.1%以下
を含有し、残部がFeおよび不可避的不純物からなる組成の連続鋳造鋳片を、600℃以下に冷却した後、1000℃以上1250℃以下の温度域に再加熱し、粗圧延および該粗圧延に続き未再結晶温度域での圧下率を20%以上85%以下、仕上げ圧延終了温度を(Ar3-50℃)以上(Ar3+100℃)以下の温度域とする仕上げ圧延を施し、該仕上げ圧延終了後、冷却を、板厚中央位置では750℃以下650℃以上の温度域における平均冷却速度が5℃/s以上50℃/s以下で、板厚表層1mm位置では300℃以上600℃以下の温度域の冷却停止温度まで冷却後、1s以上かけて550℃以上冷却開始温度以下の温度域まで復熱させ、再度300℃以上600℃以下の温度域まで冷却する処理を1回以上実施する冷却とし、350℃以上650℃以下の温度域で巻取ることを特徴とする高靭性高延性高強度熱延鋼板の製造方法。
[6] 前記[5]において、前記組成が、下記(1)式および(2)式を満足することを特徴とする高靭性高延性高強度熱延鋼板の製造方法。
Pcm=[%C]+[%Si]/30+([%Mn]+[%Cu]+[%Cr])/20
+[%Ni]/60+[%V]/10+[%Mo]/7+5×[%B]≦0.25 ・・・ (1)
Px=701×[%C]+85×[%Mn]≧181 ・・・ (2)
ここで、(1)式および(2)式において、[%C]、[%Si]、[%Mn]、[%Cu]、[%Cr]、[%Ni]、[%V]、[%Mo]、[%B]は各元素の含有量(質量%)。
[7] 前記[5]または[6]において、前記組成に加えて更に、質量%でCa:0.0001%以上0.005%以下を含有することを特徴とする高靭性高延性高強度熱延鋼板の製造方法。
[8] 前記[5]ないし[7]のいずれかにおいて、前記組成に加えて更に、質量%で、Cu:0.001%以上0.5%以下、Ni:0.001%以上0.5%以下、Mo:0.001%以上0.5%以下、Cr:0.001%以上0.5%以下、B :0.0001%以上0.004%以下のうちから選ばれる1種または2種以上を含有することを特徴とする高靭性高延性高強度熱延鋼板の製造方法。
Cは、ラス構造を有するフェライト、焼戻しマルテンサイトおよび焼戻しベイナイトのラス間隔を小さくし、かつNb、VおよびTiと炭化物を形成することで熱延鋼板の強度を確保するために重要な元素であり、所望の強度を満足するためには、C含有量を0.04%以上とする必要がある。一方、C含有量が0.15%を超えると、板厚表層部での主相である焼戻しマルテンサイトおよび/または焼戻しベイナイトのラス間隔が極端に狭くなるとともに析出物の過剰な増加により、熱延鋼板の靭性および全厚での伸び特性が劣化する。同時に、炭素当量が高くなり、このような熱延鋼板を造管・溶接すると、溶接部の靭性が劣化する。したがって、C含有量は0.04%以上0.15%以下とする。より好ましくは0.04~0.10%である。
Siの含有量が増加すると、Mn-Si系の非金属介在物を形成して溶接部靭性を悪化させる原因となる。したがって、Si含有量は0.55%を上限とする。一方、Si含有量の下限は、脱酸効果と製鋼技術限界から0.01%に定める。より好ましくは0.10~0.45%である。
Mnは、ポリゴナルフェライトの生成を抑制し、強度と靭性を確保するために必要な元素であり、その効果の発揮にはMn含有量を1.0%以上とする必要がある。一方、Mn含有量が3.0%を超えると、偏析(segregation)に伴う機械的特性(mechanical characteristic)のバラツキ(variation)が発生し易くなる。また、強度が高くなり過ぎることで、伸び特性(elongation characteristic)が低下する等の悪影響が現れるとともに、炭素当量(carbon equivalent)の増加に伴い溶接部の靭性が劣化する可能性がある。したがって、Mn含有量は1.0%以上3.0%以下とする。
S :0.01%以下、
N :0.006%以下
Pは、鋼中に不純物として存在し、偏析し易い元素で鋼の靭性の劣化をもたらす。したがって、P含有量は0.03%を上限とする。より好ましくは0.02%以下である。
Alは、鋼の脱酸剤(deoxidizing agent)として有用であり、Al含有量は脱酸効果(deoxidation effect)の発現する0.003%以上とする。但し、Al含有量が過剰になると、アルミナ系介在物が生成し、溶接部の欠陥の原因となる。したがって、Al含有量は0.003%以上0.1%以下とする。より好ましくは0.003~0.06%である。
Nbは、結晶粒の微細化に有効でかつ析出強化元素(precipitation strengthening element)であり、X80級の鋼管強度を確保するためにはNb含有量を0.035%以上とする必要がある。一方、Nb含有量が過剰になると、熱延鋼板の製造時、後述する巻取り温度域(350℃以上650℃以下)で過剰に析出が生じて靭性と伸び特性が低下するとともに、溶接性を劣化させる。したがって、Nb含有量は0.035%以上0.1%以下とする。より好ましくは0.035~0.08%である。
Vは、析出強化元素であり、これを有効に作用させるためにはV含有量を0.001%以上とする必要がある。一方、V含有量が過剰になると、熱延鋼板の製造時、後述する巻取り温度域(350℃以上650℃以下)で過剰に析出が生じて靭性と伸び特性が低下するとともに、溶接性を劣化させる。したがって、V含有量は0.001%以上0.1%以下とする。
Tiは、結晶粒の微細化に有効でかつ析出強化元素であり、その効果の発現にはTi含有量を0.001%以上とする必要がある。一方、Ti含有量が過剰になると熱延鋼板の製造時、後述する巻取り温度域(350℃以上650℃以下)で過剰に析出が生じて靭性と伸び特性が低下するとともに、溶接性を劣化させる。したがって、Ti含有量は0.001%以上0.1%以下とする。より好ましくは0.001~0.05%である。
Caは、Sを固定し、MnSの生成を抑制することで靭性を向上させる効果がある。このような効果を発現させるためには、Ca含有量を0.0001%以上とすることが好ましい。一方、Ca含有量が過剰になると、Ca系酸化物の形成により靭性が低下するため、Ca含有量は0.005%以下とすることが好ましい。より好ましくは0.001~0.0035%である。
Cuは、鋼の変態を制御するとともに、熱延鋼板の強度向上に有効な元素である。このような効果を発現させるためには、Cu含有量を0.001%以上とすることが好ましい。但し、Cuは、焼入れ性が強く、その含有量が0.5%を超えると、特に板厚表層部での主相である焼戻しマルテンサイトおよび/または焼戻しベイナイトのラス間隔を極度に狭くし、靭性と全厚での伸び特性を劣化させるとともに、熱間加工性(hot workability)を低下させるおそれがある。したがって、Cu含有量は0.001%以上0.5%以下とすることが好ましい。
Niは、鋼の変態を制御するとともに、熱延鋼板の強度向上に有効な元素である。このような効果を発現させるためには、Ni含有量を0.001%以上とすることが好ましい。但し、Niは、焼入れ性が強く、その含有量が0.5%を超えると、特に板厚表層部での主相である焼戻しマルテンサイトおよび/または焼戻しベイナイトのラス間隔を極度に狭くし、靭性と全厚での伸び特性を劣化させるとともに、熱間加工性を低下させるおそれがある。したがって、Ni含有量は0.001%以上0.5%以下とすることが好ましい。
Moは、鋼の変態を制御するとともに、熱延鋼板の強度向上に有効な元素である。このような効果を発現させるためには、Mo含有量を0.001%以上とすることが好ましい。但し、Moは、焼入れ性が強く、その含有量が0.5%を超えると、特に板厚表層部での主相である焼戻しマルテンサイトおよび/または焼戻しベイナイトのラス間隔を極度に狭くし、靭性と全厚での伸び特性を劣化させるとともに、マルテンサイトの生成を促進して靭性を低下させるおそれがある。したがって、Mo含有量は0.001%以上0.5%以下とすることが好ましい。
Crは、パーライト変態(pearlite transformation)の遅延効果(delay effect)と粒界セメンタイト(grain boundary cementite)の低減効果があり、これらの効果を発現させるためにはCr含有量を0.001%以上とすることが好ましい。一方、Cr含有量が過剰になると、特に板厚表層部での主相である焼戻しマルテンサイトおよび/または焼戻しベイナイトのラス間隔を極度に狭くし、靭性と全厚での伸び特性を劣化させる。また、Cr含有量が過剰になると、熱延鋼板を造管・溶接する際、溶接部に焼き入れ組織を形成して溶接部靭性の劣化を招くおそれがある。したがって、Cr含有量は0.001%以上0.5%以下とすることが好ましい。
Bは、熱延鋼板の製造時、仕上げ圧延終了後の冷却過程において高温でのフェライト変態(ferrite transformation)を抑制し、フェライトの硬度低下を防止する効果がある。このような効果を発現させるためには、B含有量を0.0001%以上とすることが好ましい。一方、B含有量が過剰になると、溶接部に焼入れ組織(hardened microstructure)を形成するおそれがある。したがって、B含有量は0.0001%以上0.004%以下とすることが好ましい。より好ましくは0.0001~0.003%である。
+[%V]/10+[%Mo]/7+5×[%B]≦0.25 ・・・ (1)
Px=701×[%C]+85×[%Mn]≧181 ・・・ (2)
ここで、(1)式および(2)式において、[%C]、[%Si]、[%Mn]、[%Cu]、[%Cr]、[%Ni]、[%V]、[%Mo]、[%B]は各元素の含有量(質量%)である。また、鋼板がCuを含有しない場合、(1)式中の[%Cu]をゼロとしてPcm値を算出するものとする。[%Cr]、[%Ni]、[%V]、[%Mo]、[%B]についても同様である。
析出割合が35%未満では、強度不足が発生し易いうえ、造管後の機械的特性のバラツキが大きくなる。一方、80%超ではフェライト、焼戻しマルテンサイトおよび焼戻しベイナイトの硬度が上昇し、熱延鋼板靭性と伸び特性が劣化するため、上限を80%とする。
鋼板中に析出したNbの割合(質量比)は、抽出残渣分析により鋼板中に析出したNbの質量を測定し、この測定値の全Nb含有量に対する割合(質量%)として求めることができる。なお、抽出残渣分析では、鋼板を10%アセチルアセトン(acetylacetone)-1%テトラメチルアンモニウム(tetramethylammonium)-メタノール(methanol)中で定電流電解(constant-current electrolysis)(約20mA/cm2)し、溶解残渣をメンブレンフィルター(membrane filter)(孔径:0.2μmφ)で捕集し、硫酸、硝酸および過塩素酸の混合融剤を用いて融解し、ICP発光分析法により析出量を定量化することができる。
板厚が例えば12mm以上である厚肉熱延鋼板を製造する場合において、熱間圧延終了後、板厚中央位置でラス構造を有するフェライトが生成するように冷却速度を調整すると、板厚表層部での冷却速度が極端に大きくなる。したがって、厚肉熱延鋼板の場合、板厚全域に亘りラス構造を有するフェライト主相組織とすることは極めて困難である。
ラス構造を有するフェライト、焼戻しマルテンサイトおよび焼戻しベイナイトのラス間隔は、熱延鋼板の強度を担っている一因であるため、ある程度細かい必要がある。しかしながら、ラス間隔が0.2μm未満になると、Nb等の析出が起こらない場合でもフェライト、焼戻しマルテンサイトおよび焼戻しベイナイトの硬度上昇が過剰になり、熱延鋼板の靭性と全厚での伸び特性が劣化する。一方、ラス間隔が1.6μmを超えると、Nb等が十分に析出する場合でも十分な熱延鋼板強度を確保することができず、X80級の鋼管強度を満足できなくなる。したがって、ラス間隔は0.2μm以上1.6μm以下とする。
板厚表層1mm位置(鋼板表面から板厚方向1.0mmの位置)において、所望のラス間隔(0.2μm以上1.6μm以下)を有する焼戻しマルテンサイトおよび/または焼戻しベイナイトの体積分率の合計が95%未満になると、板厚表層部の低温靭性が大きく低下する。また、板厚中央位置において、所望のラス間隔(0.2μm以上1.6μm以下)を有するフェライトの体積分率が95%未満になる場合、板厚表層部以外の領域の低温靭性が大きく低下する。したがって、本発明では、各々の位置における主相の体積分率を95%以上とする。
スラブ(連続鋳造鋳片)の冷却が不十分な場合、スラブ表層域でフェライト変態が十分に完了せず、未変態のオーステナイトが残存したままになる。このように未変態のオーステナイトが残存すると、鋳造時にオーステナイト粒界で生じた粒界酸化が助長され、得られる熱延鋼板の表面凹凸が大きくなり、荷重負荷時に不均一変形によって全厚での伸び特性が低下する。したがって、本発明では、スラブ(連続鋳造鋳片)冷却温度をフェライト変態が十分に完了する600℃以下とする。
スラブ加熱温度(連続鋳造鋳片の再加熱温度)が1000℃未満では、析出強化元素であるNb、VおよびTiが十分固溶せず、X80級の鋼管強度が確保できない。一方、1250℃を超えると、オーステナイト粒が粗大化するとともに、仕上げ圧延終了後の冷却および巻取り過程においてNbが過剰に析出し、熱延鋼板の靭性と伸び特性が劣化する。したがって、連続鋳造鋳片の再加熱温度は1000℃以上1250℃以下とする。
未再結晶温度域(本発明の鋼組成の場合、約940℃以下)で仕上げ圧延を行うことにより、オーステナイト相の再結晶が遅延して歪が蓄積し、γ/α変態(γ→α transformation)時にフェライトが微細化して強度及び靭性が向上する。ここで、仕上げ圧延時における未再結晶温度域での圧下率が20%未満では、これらの効果が十分に発現しない。一方、上記圧下率が85%を超えると、変形抵抗(deformation resistance)が増大して圧延に支障をきたす。したがって、本発明では上記圧下率を20%以上85%以下とする。好ましくは35%以上75%以下である。
均質な粒径および組織で圧延を終了するためには、仕上げ圧延終了温度を(Ar3-50℃)以上とする必要がある。仕上げ圧延終了温度が(Ar3-50℃)を下回ると、仕上げ圧延中に鋼板内部でフェライト変態が生じ、組織が不均一になって、所望の特性が得られない。一方、仕上げ圧延終了温度が(Ar3+100℃)を超えると、結晶粒が粗大化し、靱性が劣化する。従って、仕上げ圧延終了温度を(Ar3-50℃)以上(Ar3+100℃)以下の範囲内とする。
板厚表層部以外の領域においてパーライト変態及びポリゴナルフェライトの生成を抑制し、板厚中央位置でラス構造(ラス間隔:0.2μm以上1.6μm以下)を有するフェライトの体積分率を95%以上とし、靱性を確保するためには、板厚中央位置での750℃以下650℃以上の温度域における平均冷却速度を5℃/s以上とすることが必要である。但し、この板厚中央位置での冷速が大きくなり過ぎると、ラス構造を有するフェライト、焼戻しマルテンサイトおよび焼戻しベイナイトのラス間隔が極度に小さくなり、伸び特性が劣化するため、上限は50℃/sとする必要がある。
本発明では、板厚表層1.0mm位置において所望のラス間隔(0.2μm以上1.6μm以下)を有する焼戻しマルテンサイトおよび/または焼戻しベイナイトの体積分率を合計で95%以上に制御するために、板厚中央位置での冷却速度を上記範囲に収めたまま、板厚表層1mm位置においては次の処理を実施する必要がある。この処理とは、加速冷却開始温度から300℃以上600℃以下の温度域の冷却停止温度(一次冷却停止温度)まで任意の冷却速度で冷却後、1秒以上(一次復熱時間)かけて550℃以上冷却開始温度以下の温度域(一次復熱温度)まで復熱させ、再度、300℃以上600℃以下の温度域まで冷却する処理であり、巻き取りまでにこの処理を1回以上実施することが必要である。ここで、n回この処理を実施した時の冷却停止温度をn次冷却停止温度、復熱時間をn次復熱時間、復熱温度をn次復熱温度とする。各制御因子の規定理由は以下のとおりである。
本処理は、表面から板厚方向1.0mmまでの表層部(板厚表層領域)において一旦低温変態組織(マルテンサイト組織および/またはベイナイト組織)とし、復熱によりこれを焼戻すことを目的としている。これにより、板厚表層部での焼戻しマルテンサイトおよび/または焼戻しベイナイトのラス間隔を調節し、表層硬度さらには全厚での伸び特性を向上することができる。冷却停止温度が600℃を超える場合には、低温変態組織が十分に生成しないため、板厚表層部を焼戻し組織とすることができず、全厚での伸び特性が低下する。一方、n次冷却停止温度が300℃未満である場合には、狙いの復熱温度まで到達できないため、十分に焼戻すことができず、全厚での伸び特性が低下する。
復熱温度が550℃未満である場合、十分に組織を焼戻すことができず、板厚表層部での硬度が上昇し、全厚での伸び特性が低下する。一方、復熱(再加熱)温度が冷却開始温度(通常、仕上げ圧延終了温度-20℃~仕上げ圧延終了温度)を超えると、板厚表層部でフェライトからオーストナイトへの逆変態(reverse transformation)が起こり、再度冷却する際に、焼入れ組織が形成されてしまう。その結果、板厚表層部での硬度が上昇し、全厚での伸び特性が低下するといった問題が生じる。したがって、復熱温度は550℃以上冷却開始温度以下の温度域とする。
復熱時間が1秒未満である場合、十分に組織を焼戻すことができず、板厚表層部での硬度が上昇し、全厚での伸び特性が低下する。したがって、復熱時間は1秒以上とする。但し、復熱時間が長くなり過ぎると、結果として復熱温度が高くなることから、板厚表層部でフェライトからオーストナイトへの逆変態が起こり、再度冷却する際に、焼入れ組織が形成されてしまう。したがって、板厚表層部での硬度が上昇し、全厚での伸び特性が低下するとともに製造能率が大きく低下することが懸念される。このような観点から、復熱時間は5秒以下とすることが好ましい。
Nb、V、Ti等の析出物による析出強化を活用するためには、巻取り温度を350℃以上とすることが必要である。上記析出物を特に効果的に析出させるためには、巻取り温度を400℃以上とすることが好ましい。一方、巻取り温度が650℃を超えると、析出物の粗大化や、ラス構造を有するフェライト、焼戻しマルテンサイトおよび焼戻しベイナイトのラス間隔の拡大により、強度が低下する。また、巻取り温度が650℃を超えると、粗大なパーライトが生成して靭性が劣化するため、上限を650℃とする。好ましくは、400℃以上650℃以下である。なお、巻取り温度は鋼板表面の温度である。しかしながら、これは板厚表層1mm位置の温度にほぼ等しい。
(1)組織観察
得られた熱延鋼板から、板厚方向全ての位置が観察できるようなブロック状試験片(blockish test specimen)を採取し、走査型電子顕微鏡(倍率:2000~5000倍)を用いて、L断面観察(熱延鋼板幅方向が観察面に垂直)を実施した。組織の平均的な情報を得るため、板厚1/2(中央)位置、板厚表層1mm位置について板厚位置毎に3視野以上観察、撮影した。このように3視野以上観察、撮影することにより得られた組織写真を用いて、観察視野面積に対し各構成組織(ラス構造を有するフェライト、焼戻しマルテンサイトおよび焼戻しベイナイト)が占める面積の割合を画像解析(image analysis)により求め、これらの平均値を各構成組織の体積分率とした。
(2)抽出残渣分析(析出Nb割合の測定方法)
得られた熱延鋼板の板厚中央位置および表層1mm位置のそれぞれの位置より試験片を採取し、抽出残渣分析により鋼板(試験片)中に析出したNbの質量を測定した。なお、抽出残渣分析では、鋼板(試験片)を10%アセチルアセトン-1%テトラメチルアンモニウム-メタノール中で定電流電解(約20mA/cm2)し、溶解残渣をメンブレンフィルター(孔径:0.2μmφ)で捕集し、硫酸、硝酸および過塩素酸の混合融剤を用いて融解し、水で一定量に希釈してICP発光分析法でNb析出割合を定量化するものとする。Nb析出割合が、板厚中央位置および表層1mm位置ともに35%以上80%以下の範囲内である場合を「強度、靭性、伸び特性に好ましいNb析出割合」と評価した。
(3)引張試験
得られた熱延鋼板から、圧延方向に直交する方向(C方向)が長手方向となるように、平板状の全厚引張試験片(板厚:全厚、平行部長さ:60mm、ゲージ間距離:50mm、ゲージ部幅:38mm)を採取し、ASTM E8M-04の規定に準拠して、室温で引張試験を実施し、降伏強度YS、引張強さTS、全伸びELを求めた。降伏強度が550MPa以上、引張強さが650MPa以上、全伸びが20%以上である場合を、「引張特性が良好である」と評価した。ただし、強度が高くなり過ぎると伸び特性が低下するため、降伏強度は690MPa以下、引張強さは760MPa以下であることが望ましい。
(4)シャルピー衝撃試験(Charpy impact test)
得られた熱延鋼板の板厚中央位置から、圧延方向に直交する方向(C方向)が長手方向となるようにVノッチ試験片(V-notched test bar)(長さ55mm×高さ10mm×幅10mm)を採取し、JIS Z 2242の規定に準拠してシャルピー衝撃試験を実施し、試験温度:-60℃での吸収エネルギー(absorbed energy)(J)と延性-脆性破面遷移温度(ductile-brittle fracture surface transition temperature)(℃)を求めた。なお、試験片は3本とし、得られた吸収エネルギー値と延性-脆性破面遷移温度の算術平均を求め、その鋼板の吸収エネルギー値(vE-60)と延性-脆性破面遷移温度(vTrs)とした。vE-60が100J以上、vTrsが-80℃以下である場合を「靭性が良好である」と評価した。
(5)DWTT試験
得られた熱延鋼板から、圧延方向に直交する方向(C方向)が長手方向となるようにDWTT試験片(大きさ:板厚全厚×幅3in.×長さ12in.)を採取し、ASTM E 436の規定に準拠して、DWTT試験を行い、延性破面率(shear fracture percentage)が85%となる最低温度(DWTT)を求めた。DWTTが、-30℃以下の場合を「優れたDWTT特性」を有すると評価した。
(6)硬さ試験
得られた熱延鋼板から、硬度測定用のブロック状試験片(大きさ:板厚全厚×幅10mm×長さ10mm)を採取し、ビッカース硬度試験機を用い、荷重1.0kgにて、板厚表層1mm位置での硬度を測定した。
Claims (8)
- 質量%で、
C :0.04%以上0.15%以下、 Si:0.01%以上0.55%以下、
Mn:1.0%以上3.0%以下、 P :0.03%以下、
S :0.01%以下、 Al:0.003%以上0.1%以下、
N :0.006%以下、 Nb:0.035%以上0.1%以下、
V :0.001%以上0.1%以下、 Ti:0.001%以上0.1%以下
を含有し、残部がFeおよび不可避的不純物からなる組成を有し、全Nb量に対する析出Nbの割合が35%以上80%以下であり、板厚表層1.0mm位置において、ラス間隔が0.2μm以上1.6μm以下である焼戻しマルテンサイトおよび/または焼戻しベイナイトの体積分率が95%以上であり、板厚中央位置において、ラス間隔が0.2μm以上1.6μm以下であるフェライトの体積分率が95%以上である組織を有することを特徴とする熱延鋼板。 - 前記組成が、下記(1)式および(2)式を満足することを特徴とする請求項1に記載の熱延鋼板。
記
Pcm=[%C]+[%Si]/30+([%Mn]+[%Cu]+[%Cr])/20
+[%Ni]/60+[%V]/10+[%Mo]/7+5×[%B]≦0.25 ・・・ (1)
Px=701×[%C]+85×[%Mn]≧181 ・・・ (2)
ここで、(1)式および(2)式において、[%C]、[%Si]、[%Mn]、[%Cu]、[%Cr]、[%Ni]、[%V]、[%Mo]、[%B]は各元素の含有量(質量%)。 - 前記組成に加えて更に、質量%でCa:0.0001%以上0.005%以下を含有することを特徴とする請求項1または2に記載の熱延鋼板。
- 前記組成に加えて更に、質量%で、Cu:0.001%以上0.5%以下、Ni:0.001%以上0.5%以下、Mo:0.001%以上0.5%以下、Cr:0.001%以上0.5%以下、B :0.0001%以上0.004%以下のうちから選ばれる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ないし3のいずれかに記載の熱延鋼板。
- 質量%で、
C :0.04%以上0.15%以下、 Si:0.01%以上0.55%以下、
Mn:1.0%以上3.0%以下、 P :0.03%以下、
S :0.01%以下、 Al:0.003%以上0.1%以下、
N :0.006%以下、 Nb:0.035%以上0.1%以下、
V :0.001%以上0.1%以下、 Ti:0.001%以上0.1%以下
を含有し、残部がFeおよび不可避的不純物からなる組成の連続鋳造鋳片を、600℃以下に冷却した後、1000℃以上1250℃以下の温度域に再加熱し、粗圧延および該粗圧延に続き未再結晶温度域での圧下率を20%以上85%以下、仕上げ圧延終了温度を(Ar3-50℃)以上(Ar3+100℃)以下の温度域とする仕上げ圧延を施し、該仕上げ圧延終了後、冷却を、板厚中央位置では750℃以下650℃以上の温度域における平均冷却速度が5℃/s以上50℃/s以下で、板厚表層1mm位置では300℃以上600℃以下の温度域の冷却停止温度まで冷却後、1s以上かけて550℃以上冷却開始温度以下の温度域まで復熱させ、再度300℃以上600℃以下の温度域まで冷却する処理を1回以上実施する冷却とし、350℃以上650℃以下の温度域で巻取ることを特徴とする熱延鋼板の製造方法。 - 前記組成が、下記(1)式および(2)式を満足することを特徴とする請求項5に記載の熱延鋼板の製造方法。
記
Pcm=[%C]+[%Si]/30+([%Mn]+[%Cu]+[%Cr])/20
+[%Ni]/60+[%V]/10+[%Mo]/7+5×[%B]≦0.25 ・・・ (1)
Px=701×[%C]+85×[%Mn]≧181 ・・・ (2)
ここで、(1)式および(2)式において、[%C]、[%Si]、[%Mn]、[%Cu]、[%Cr]、[%Ni]、[%V]、[%Mo]、[%B]は各元素の含有量(質量%)。 - 前記組成に加えて更に、質量%でCa:0.0001%以上0.005%以下を含有することを特徴とする請求項5または6に記載の熱延鋼板の製造方法。
- 前記組成に加えて更に、質量%で、Cu:0.001%以上0.5%以下、Ni:0.001%以上0.5%以下、Mo:0.001%以上0.5%以下、Cr:0.001%以上0.5%以下、B :0.0001%以上0.004%以下のうちから選ばれる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5ないし7のいずれかに記載の熱延鋼板の製造方法。
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CN105121684B (zh) | 2017-03-15 |
KR101728789B1 (ko) | 2017-04-20 |
BR112015023632B1 (pt) | 2020-04-28 |
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EP2949772A4 (en) | 2016-06-01 |
EP2949772A1 (en) | 2015-12-02 |
US20160017466A1 (en) | 2016-01-21 |
CN105121684A (zh) | 2015-12-02 |
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EP2949772B1 (en) | 2019-06-19 |
US10287661B2 (en) | 2019-05-14 |
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BR112015023632A2 (pt) | 2017-07-18 |
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