WO2011039885A1 - Tôle d'acier laminée à froid - Google Patents

Tôle d'acier laminée à froid Download PDF

Info

Publication number
WO2011039885A1
WO2011039885A1 PCT/JP2009/067171 JP2009067171W WO2011039885A1 WO 2011039885 A1 WO2011039885 A1 WO 2011039885A1 JP 2009067171 W JP2009067171 W JP 2009067171W WO 2011039885 A1 WO2011039885 A1 WO 2011039885A1
Authority
WO
WIPO (PCT)
Prior art keywords
mass
less
steel sheet
bainite
cold
Prior art date
Application number
PCT/JP2009/067171
Other languages
English (en)
Japanese (ja)
Inventor
俊夫 村上
朗 伊庭野
Original Assignee
株式会社神戸製鋼所
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by 株式会社神戸製鋼所 filed Critical 株式会社神戸製鋼所
Priority to PCT/JP2009/067171 priority Critical patent/WO2011039885A1/fr
Publication of WO2011039885A1 publication Critical patent/WO2011039885A1/fr

Links

Images

Classifications

    • 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
    • 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
    • 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
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • 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
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • 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

Definitions

  • the present invention relates to a cold-rolled steel sheet (cold-rolled thin steel sheet) suitable for automobile parts and the like, and in particular, a high-strength cold-rolled steel sheet having excellent workability, or excellent hydrogen embrittlement resistance in addition to workability. It relates to a high-strength cold-rolled steel sheet.
  • cold-rolled steel sheets used for automobile frame parts and the like are required to have high strength for the purpose of achieving both collision safety and fuel efficiency reduction by reducing the weight of the vehicle body, and excellent for processing into complex frame parts Also, moldability is required. For this reason, it has been desired to provide a high-strength steel sheet having both increased elongation (total elongation; El) and stretch flangeability (hole expansion ratio; ⁇ ). There is a demand for a steel sheet having a hole expansion ratio of 90% or more at 10% or more.
  • Patent Document 1 discloses a high-tensile cold-rolled steel sheet containing 1.6 to 2.5% by mass in total of at least one of Mn, Cr, and Mo and substantially comprising a martensite single-phase structure. Has been. In this steel sheet, the hole expansion ratio (stretch flangeability) is 90% or more, but the elongation does not reach 10%.
  • Patent Document 2 discloses a high-tensile steel plate having a two-phase structure of ferrite having an area ratio of 65 to 85% and the balance being tempered martensite.
  • Patent Document 3 discloses a high-tensile steel plate having a two-phase structure in which the average crystal grain sizes of ferrite and martensite are both 2 ⁇ m or less and the volume ratio of martensite is 20% or more and less than 60%. . Although both the high-tensile steel sheets disclosed in Patent Document 2 and Patent Document 3 ensure an elongation of 10% or more, the hole expansion ratio (stretch flangeability) does not reach 90%.
  • Patent Document 4 1.0 to 2.0% by mass of Si is added, and the space factor is 5% or more of retained austenite and 60% or more of bainitic ferrite.
  • a high-strength cold-rolled steel sheet having a phase structure is disclosed. Although this steel sheet has high elongation and relatively high stretch flangeability, it is mainly used to improve elongation. Although elongation of 10% or more is obtained, stretch flangeability is up to about 60%. It has only been obtained.
  • Patent Document 5 1.0 to 2.0% by mass of Si is added, and the Baini is composed of a structure including a space factor of 5 to 20% of retained austenite and 50% or more of bainitic ferrite.
  • a high-strength cold-rolled steel sheet composed of a multiphase structure mainly composed of tick ferrite is disclosed. Although this steel sheet exhibits very excellent elongation of 20% or more, stretch flangeability is only obtained up to about 80%.
  • Non-Patent Document 1 describes that if a metal structure is mainly tempered martensite and an element (Cr, Mo, V, etc.) exhibiting temper softening resistance is added, it is effective in improving delayed fracture resistance. ing. This is a technology that suppresses fracture by precipitating alloy carbides and utilizing them as hydrogen trap sites to shift the delayed fracture mode from grain boundaries to intragranular fracture.
  • these findings are for application to medium carbon steel, and cannot be used as it is for a thin steel sheet having a low carbon content that requires weldability and workability.
  • the present applicants developed an ultra-high strength thin steel sheet having excellent hydrogen embrittlement resistance, satisfying the carbon content C: more than 0.25 to 0.60% and the balance being iron and inevitable impurities ( Patent Document 6).
  • the metal structure after tensile processing with a processing rate of 3% satisfies the area ratio of the entire structure, and the residual austenite structure: 1% or more, bainitic ferrite and martensite: 80% or more in total
  • the average axial ratio (major axis / minor axis) of the residual austenite crystal grains satisfies 5 or more.
  • the thin steel sheet described in Patent Document 6 exhibits excellent strength, elongation, and hydrogen embrittlement resistance. However, even in the thin steel sheet described in Patent Document 6, retained austenite becomes a starting point of fracture and becomes a factor of reducing stretch flangeability. As described above, the stretch flangeability, which is becoming increasingly important in recent years, is assured of a desired level (at least TS ⁇ ⁇ : 60000 (MPa ⁇ %) [unit of Ts: MPa], preferably ⁇ : 100% or more). It was difficult to achieve.
  • an object of the present invention is to provide a cold-rolled steel sheet (cold-rolled thin steel sheet) with improved workability (specifically, stretch flangeability). More specifically, one object of the present invention is to provide a high-strength cold-rolled steel sheet excellent in workability, in which both elongation and stretch flangeability are enhanced. Another object of the present invention is to provide a high-strength cold-rolled steel sheet with improved stretch flangeability while ensuring excellent hydrogen embrittlement resistance.
  • the cold-rolled steel sheet of the present invention that has achieved the above-mentioned object is C: 0.03 mass% or more, less than 0.30 mass%, Si: 2.0 mass% or less (including 0 mass%), Mn: 0.1 to 2.8% by mass, P: 0.1% by mass or less, S: 0.005% by mass or less, N: 0.01% by mass or less, and Al: 0.01 to 1.00% by mass,
  • the area ratio of tempered bainite is 50% or more (including 100%)
  • the total area ratio of tempered martensite and retained austenite is less than 3% (including 0%)
  • the balance is ferrite.
  • a distribution state of precipitates in the tempered bainite and / or a distribution state of cementite particles in the tempered bainite are controlled.
  • the above-described cold-rolled steel sheet contains Si: 1.0% by mass or less (0 Mn: 0.5 to 2.4% by mass, and Al: 0.01% by mass or more and less than 0.10% by mass, and the area ratio of the tempered bainite is 70% or more (100%). It is recommended that the number of cementite particles having an equivalent circle diameter of 0.1 ⁇ m or more be 3 or less per 1 ⁇ m 2 of the tempered bainite.
  • V 0.001 to 1.00 mass 20% or more of precipitates having an equivalent circle diameter of 1 to 10 nm per 1 ⁇ m 2 of the tempered bainite, and 10 precipitates or less of equivalent circle diameter of 20 nm or more containing V per 1 ⁇ m 2 of the tempered bainite. It is recommended that there be.
  • the number of cementite particles having an equivalent circle diameter of 0.1 ⁇ m or more is 3 or less per 1 ⁇ m 2 of the tempered bainite.
  • the above-described cold-rolled steel sheet of the present invention preferably contains Cr: 0.1 to 3.0% by mass.
  • the cold-rolled steel sheet of the present invention described above contains B: 0.0002 to 0.0050 mass%, and Nb and / or Ti is ((N) ⁇ 0.003) / 12 ⁇ [Nb] / It is preferable to include so as to satisfy the relationship of 96+ [Ti] / 48 ⁇ ([N] +0.01) / 12 (however, [] means the content (% by mass) of each element).
  • the cold-rolled steel sheet of the present invention described above has Mo: 0.01 to 1.0% by mass, Cu: 0.05 to 1.0% by mass, and Ni: 0.05 to 1.0% by mass. It is also preferable that it contains 1 or more types.
  • the cold-rolled steel sheet of the present invention described above has Ca: 0.0005 to 0.01% by mass, Mg: 0.0005 to 0.01% by mass, and REM: 0.0004 to 0.01% by mass. It is also preferable that it contains 1 or more types.
  • the ratio of tempered martensite and retained austenite which is the main cause of fracture in a tempered bainite single phase structure or a multiphase structure mainly composed of ferrite and tempered bainite, is reduced as much as possible.
  • the present invention can provide a high-strength cold-rolled steel sheet excellent in workability or a high-strength cold-rolled steel sheet excellent in workability and hydrogen embrittlement resistance.
  • the present inventors have a high structure composed mainly of tempered bainite (hereinafter simply referred to as “bainite”), which has a higher deformability than tempered martensite (hereinafter sometimes simply referred to as “martensite”).
  • bainite tempered bainite
  • martensite tempered martensite
  • the present invention includes C: 0.03% by mass or more and less than 0.30% by mass, Si: 2.0% by mass or less (including 0% by mass), Mn: 0.1 to 2.8
  • a cold-rolled steel sheet containing, by mass, P: 0.1% by mass or less, S: 0.005% by mass or less, N: 0.01% by mass or less, and Al: 0.01 to 1.00% by mass
  • the area ratio of tempered bainite is 50% or more (including 100%)
  • the total area ratio of tempered martensite and residual austenite is less than 3% (including 0%) and the balance has a structure made of ferrite
  • a cold-rolled steel sheet characterized by controlling the distribution of precipitates in the tempered bainite and / or the distribution of cementite particles in the tempered bainite (hereinafter, this invention is referred to as a basic invention). is there).
  • the steel sheet of the present invention is based on a tempered bainite single phase or a multiphase structure mainly composed of ferrite and tempered bainite.
  • the present invention particularly reduces the tempered martensite structure and residual austenite structure as much as possible, and controls the distribution of precipitates in the tempered bainite and / or the distribution of cementite particles present in the tempered bainite. It has the characteristics.
  • Bainite is a homogeneous structure having high strength and excellent plasticity.
  • stretch flangeability can be improved while securing tensile strength and elongation.
  • the area ratio of tempered bainite is 50% or more, preferably 70% or more, in order to effectively exhibit the above-described action.
  • it is 90% or more (including 100%).
  • ferrite since ferrite has high ductility but low strength, increasing the proportion of ferrite improves the elongation but decreases the strength.
  • the balance of the steel sheet of the present invention is ferrite, but the ferrite area ratio is smaller than that of tempered bainite.
  • the total area ratio of tempered martensite and retained austenite be limited to less than 3%, further 2% or less, particularly 1% or less.
  • each measuring method of each area ratio of tempered bainite, ferrite, tempered martensite and retained austenite will be described.
  • the area ratio of ferrite first, each test steel sheet was mirror-polished and corroded with a 3% nital solution to reveal a metal structure. Thereafter, five fields of view were observed with a scanning electron microscope (SEM) at a magnification of 2000 times, and the equiaxed region not containing cementite was determined to be ferrite by image analysis, and the area ratio of ferrite was determined from the area ratio of the ferrite region to the entire structure. Calculated.
  • SEM scanning electron microscope
  • each test steel sheet was mirror-polished and corroded with a repeller corrosive solution to reveal a metal structure. Thereafter, five fields of view were observed with a scanning electron microscope (SEM) at a magnification of 2000 times, and the area that appeared white from the image contrast by image analysis was made tempered martensite and retained austenite. The total area ratio of sites and retained austenite was calculated.
  • SEM scanning electron microscope
  • Component composition of the basic invention (C: 0.03 mass% or more, less than 0.30 mass%) C is an important element that affects the area ratio of bainite and affects strength and stretch flangeability. If the C content is less than 0.03% by mass, the bainite area ratio is insufficient, so that the strength cannot be ensured. On the other hand, if the C content exceeds 0.30% by mass, the hardenability becomes too high, the transformation to bainite is too suppressed, the ratio of martensite and austenite increases, and stretch flangeability cannot be ensured.
  • the range of the C content is preferably 0.05 to 0.25% by mass, more preferably 0.07 to 0.20% by mass.
  • Si 2.0 mass% or less (including 0 mass%)
  • Si is a useful element that can increase strength without deteriorating elongation as a solid solution strengthening element. If the Si content exceeds 2.0% by mass, the formation of austenite during heating is inhibited, so that the area ratio of bainite cannot be ensured and stretch flangeability cannot be ensured. In this case, the formation of cementite is inhibited, and retained austenite (residual ⁇ ) and martensite are likely to remain.
  • the range of the Si content is preferably 1.8% by mass or less, more preferably 1.5% by mass or less (including 0% by mass).
  • Mn 0.1 to 2.8% by mass
  • Mn is a useful element having an effect of enhancing the strength and stretch flangeability by increasing the hardenability and securing the bainite area ratio during rapid cooling after heating during annealing. If the Mn content is less than 0.1% by mass, the bainite area ratio is insufficient, so that the strength and stretch flangeability cannot be ensured. On the other hand, if the Mn content exceeds 2.8% by mass, bainite transformation is suppressed, martensite and austenite remain, and stretch flangeability is deteriorated.
  • the range of the Mn content is preferably 0.30 to 2.5% by mass, more preferably 0.50 to 2.2% by mass.
  • P 0.1% by mass or less
  • P content is 0.1% by mass or less. P content becomes like this. Preferably it is 0.05 mass% or less, More preferably, it is 0.03 mass% or less.
  • S (S: 0.005 mass% or less) S is also unavoidably present as an impurity element, forms MnS inclusions, and becomes a starting point of a crack when the hole is expanded, thereby reducing stretch flangeability. Therefore, the P content is 0.005% by mass or less, more preferably 0.003% by mass or less.
  • N 0.01% by mass or less
  • N is inevitably present as an impurity element, and the elongation and stretch flangeability are lowered by strain aging.
  • the N content is preferably as low as 0.01% by mass or less.
  • Al 0.01 to 1.00% by mass
  • Al combines with N to form AlN and reduces the solid solution N that contributes to the occurrence of strain aging, thereby preventing the stretch flangeability from deteriorating and contributing to the strength improvement by solid solution strengthening.
  • the Al content is less than 0.01% by mass, solid solution N remains in the steel, strain aging occurs, and elongation and stretch flangeability cannot be ensured.
  • the Al content exceeds 1.00% by mass, the formation of austenite during heating is inhibited, so that the area ratio of bainite cannot be ensured and stretch flangeability cannot be ensured.
  • the inventors of the present invention ensured elongation by using a tempered bainite structure having a higher deformability than tempered martensite as a main structure and introducing a ferrite structure into the bainite structure as necessary.
  • the present inventors reduced the ratio of tempered martensite and retained austenite (hereinafter, sometimes referred to as “residual ⁇ ”), which is the main cause of destruction, and other main causes of destruction.
  • the hole expansion ratio is 90% at a total elongation of 10% or more with respect to the steel sheet having the desired level, that is, the tensile strength of 800 MPa class or more. It has been found that stretch flangeability of at least% can be secured.
  • a high-strength cold-rolled steel sheet having both improved elongation and flangeability and excellent workability is based on the basic invention described above, Si: 1.0 mass% or less (including 0 mass%), Mn : 0.5 to 2.4% by mass and Al: 0.01% by mass or more and less than 0.10% by mass, the area ratio of the tempered bainite is 70% or more (including 100%),
  • the steel sheet of the present invention is based on a tempered bainite single phase or a multiphase structure mainly composed of ferrite and tempered bainite.
  • the tempered bainite It is characterized in that the number of coarse cementite particles precipitated therein is controlled.
  • Bainite is a homogeneous structure having high strength and excellent plasticity.
  • the steel sheet of the present invention can improve stretch flangeability while securing tensile strength and elongation by using a bainite-based structure having such properties.
  • the area ratio of tempered bainite is 70% or more, preferably 80% or more. Preferably, it is 90% or more (including 100%).
  • the cementite particles having an equivalent circle diameter of 0.1 ⁇ m or more present in the tempered bainite are 3 or less, preferably 2.4 or less, more preferably 1 per 1 ⁇ m 2 of the tempered bainite. 6 or less.
  • each specimen steel plate was mirror-polished and corroded with 3% nital to reveal the metal structure, and then a scanning electron microscope with a magnification of 10,000 times with respect to the field of view of 100 ⁇ m 2 so that the region inside the bainite could be analyzed. (SEM) images were observed. Then, the white portion is marked and marked as cementite particles from the contrast of the image, and with the image analysis software, the equivalent circle diameter is calculated from the area of each marked cementite particle, and a predetermined size existing per unit area The number of cementite particles was determined.
  • Si 1.0% by mass or less (including 0% by mass)
  • Si as a solid solution strengthening element, has the effect of increasing strength without deteriorating elongation and suppressing the coarsening of cementite particles present in bainite during tempering. Si also has the effect of improving stretch flangeability by preventing the formation of such coarse cementite particles.
  • the range of Si content is preferably 0.5% by mass or less, more preferably 0.2% by mass or less (including 0% by mass).
  • Mn 0.5 to 2.4% by mass
  • Mn is a solid solution strengthening element similar to Si, increasing strength without deteriorating elongation, increasing hardenability and contributing to securing the bainite area ratio, and also having the effect of improving strength and stretch flangeability Element. If the Mn content is less than 0.5% by mass, the hardenability is insufficient, the bainite area ratio cannot be secured, and the strength cannot be secured. On the other hand, if the Mn content is more than 2.4% by mass, the hardenability becomes too high, and martensite is excessively formed, and the stretch flangeability deteriorates.
  • the range of the Mn content is preferably 0.8 to 3.0% by mass, more preferably 1.0 to 2.2% by mass.
  • Al 0.01% by mass or more and less than 0.10% by mass
  • Al combines with N to form AlN and reduces the solid solution N that contributes to the occurrence of strain aging, thereby preventing the stretch flangeability from deteriorating and contributing to the strength improvement by solid solution strengthening. If the Al content is less than 0.01% by mass, solid solution N remains in the steel, strain aging occurs, and elongation and stretch flangeability cannot be ensured.
  • the Al content is 0.10% by mass or more, the formation of cementite is inhibited, and the total area ratio of tempered martensite and retained austenite becomes excessive, so that stretch flangeability deteriorates.
  • the preferable manufacturing method for obtaining the cold rolled steel sheet of 1st invention is demonstrated below.
  • the hot rolling conditions steel having the above composition is melted and slab is formed by ingot forming or continuous casting and then hot-rolled.
  • the finish rolling finish temperature is set to Ar 3 or higher, and after cooling appropriately, winding is performed in the range of 450 to 700 ° C.
  • pickling is performed and then cold rolling is performed.
  • the cold rolling rate is preferably about 30% or more. And after the said cold rolling, although it anneals continuously, you may further temper as needed.
  • annealing heating temperature Ac 3 to 1000 ° C.
  • annealing holding time held for 3600 seconds or less
  • annealing temperature to 400 to 550 ° C. (cooling end temperature) 10 to 200 ° C./second
  • the cooling end temperature is maintained for 10 to 600 seconds, and then cooled to room temperature.
  • ⁇ Annealing heating temperature Ac 3 to 1000 ° C.>
  • the reason why the annealing heating temperature is set to Ac 3 or more and 1000 ° C. or less is to ensure an area ratio of bainite that is sufficiently transformed into austenite during annealing and is transformed from austenite during subsequent cooling. It is less than the annealing heating temperature Ac 3, due to the lack of transformation of the austenite at the time of annealing heating, the amount of bainite to transformation product from austenite during subsequent cooling can not be ensured by the area rate of 70% or more reduced.
  • the annealing heating temperature exceeds 1000 ° C.
  • the austenite structure becomes coarse and the bendability and toughness of the steel sheet deteriorate, and the annealing equipment deteriorates.
  • the annealing holding time exceeds 3600 seconds, productivity is extremely deteriorated, which is not preferable.
  • the heat history during holding in the cooling end temperature range (400 to 550 ° C.) is not particularly limited, and may be any heat history in an isothermal holding state, a cooling state, or a re-temperature raising state.
  • V-containing precipitates V carbides and carbonitrides
  • the main structure is a tempered bainite structure having a higher deformability than tempered martensite, and if necessary, a certain degree of elongation is ensured by introducing a ferrite structure into the bainite structure.
  • residual ⁇ certain tempered martensite and retained austenite
  • V 0.001 to 1.00% by mass
  • the steel sheet of the present invention is based on a tempered bainite single phase or a multiphase structure mainly composed of ferrite and tempered bainite, and particularly reduces the tempered martensite structure and residual austenite structure as much as possible.
  • it is characterized in that the distribution state of the V-containing precipitates in the tempered bainite is controlled.
  • the number of fine precipitates having an equivalent circle diameter of 1 to 10 nm is 20 or more, preferably 50 or more, more preferably 100 or more per 1 ⁇ m 2 of tempered bainite.
  • a preferable range of the size (equivalent circle diameter) of the fine precipitate is 1 to 8 nm, and a more preferable range is 1 to 6 nm.
  • Precipitates containing V such as VC have extremely high rigidity and critical shear stress compared to the parent phase, and the precipitates themselves are not easily deformed even if the periphery of the precipitates is deformed. Therefore, when the precipitate containing V has a size of 20 nm or more, a large strain is generated at the interface between the parent phase and the precipitate, and breakage occurs. For this reason, if there are a large amount of coarse precipitates containing V of 20 nm or more, stretch flangeability deteriorates. Therefore, stretch flangeability can be improved by restricting the density of coarse V-containing precipitates.
  • coarse precipitates containing V having an equivalent circle diameter of 20 nm or more are limited to 10 or less, preferably 5 or less, more preferably 3 or less, per 1 ⁇ m 2 of tempered bainite.
  • a steel having bainite as a main structure such as a conventional high-strength cold-rolled steel sheet
  • tempered martensite and retained austenite are easily formed, and these structures are the starting points of fracture. Therefore, the dispersion state of the cementite particles precipitated in the bainite during tempering does not significantly affect the stretch flangeability.
  • the number of cementite particles having an equivalent circle diameter of 0.1 ⁇ m or more present in the tempered bainite is limited to 3 or less, further 2.5 or less, especially 2 or less per 1 ⁇ m 2 of the tempered bainite. It is recommended that you do this.
  • a thin film sample is prepared by a thin film method or an extraction replica method, and an area of 2 ⁇ m 2 or more is observed with this sample using a field emission transmission electron microscope (FE-TEM) at a magnification of 100,000 to 300,000 times. . Then, mark the dark part from the contrast of the image as a precipitate, and with the image analysis software, calculate the equivalent circle diameter from the area of each marked precipitate, and the precipitate of a predetermined size present per unit area The number was determined.
  • FE-TEM field emission transmission electron microscope
  • V Component composition of the second invention
  • V promotes the production of ⁇ -FeOOH, which is iron oxide, which is said to be thermodynamically stable and protective among rust produced in the atmosphere.
  • V is an important element for improving hydrogen embrittlement resistance because it functions as a hydrogen trap site by being present in steel as fine carbides and carbonitrides.
  • the V content is less than 0.01% by mass, the effect of improving the hydrogen embrittlement resistance cannot be sufficiently obtained.
  • the V content exceeds 1.00% by mass, the stretch flangeability deteriorates because V carbide or V carbonitride that grows coarsely and exists in the steel in an insoluble state during heating during annealing.
  • the range of V content is preferably 0.01% by mass or more and less than 0.50% by mass, and more preferably 0.02% by mass or more and less than 0.30% by mass.
  • Hot rolling conditions it is recommended that the hot rolling heating temperature is set to 900 ° C. or higher, the hot rolling finish rolling temperature is set to 800 ° C. or higher, and after appropriate cooling, winding is performed at a temperature of 450 ° C. or lower.
  • V is completely dissolved in the heating stage, suppressing precipitation during hot rolling and precipitation of V carbide and carbonitride during winding, and annealing. During heating, coarse V carbides and carbonitrides can be prevented from remaining.
  • cold rolling rate is preferably about 30% or more.
  • annealing heating temperature Ta (° C.): [ ⁇ 9500 / ⁇ log ([% C] ⁇ [% V]) ⁇ 6.72 ⁇ ⁇ 273] ° C. or higher, and Ac 3 or higher and 1000 ° C. or lower And hold annealing time: 20 to 3600 seconds. Then, after quenching from the annealing heating temperature to the cooling end temperature: 300 ° C. to [Bs-100] ° C. at a cooling rate of 10 to 200 ° C./second, the cooling end temperature is held for 10 to 600 seconds. Good.
  • the annealing heating temperature Ta (° C.) ⁇ Ac 3 when the annealing heating temperature Ta (° C.) ⁇ Ac 3 , the amount of transformation to austenite is insufficient during annealing heating, so that the amount of bainite transformed from austenite during subsequent cooling is reduced, and the area ratio is 50% or more. This is not preferable because it cannot be secured.
  • an annealing heating temperature Ta (° C.)> 1000 ° C. is not preferable because the austenite structure becomes coarse and the bendability and toughness of the steel sheet deteriorate and the annealing equipment deteriorates.
  • the annealing holding time is less than 20 seconds, it is not preferable because V carbides cannot be completely dissolved. On the other hand, if the annealing holding time exceeds 3600 seconds, productivity is extremely deteriorated.
  • the holding time at the cooling end temperature is less than 10 seconds, the bainite transformation does not proceed sufficiently and the elongation and stretch flangeability cannot be secured. On the other hand, if it exceeds 600 seconds, the productivity is extremely deteriorated.
  • Pg exp [ ⁇ 13123 / (Tt + 273)] ⁇ t is based on the grain growth model of the precipitate described in Koichi Sugimoto et al., Material Histology, Asakura Shoten Publishing, p106 formula (4.18). It is a parameter that defines the size of the precipitate, with variables set and simplified.
  • the tempering holding time t (seconds) be Pg ⁇ 0.20 ⁇ 10 ⁇ 5 , whereby the stretch flangeability can be further improved by preventing the growth of cementite.
  • the cold-rolled steel sheet of the present invention basically contains the aforementioned components, with the balance being substantially iron and impurities.
  • the following allowable components can be added as long as the effects of the present invention are not impaired.
  • the upper bainite which is mainly targeted by the steel of the present invention, is (1) formation of bainitic ferrite ⁇ (2) spout of carbon from bainitic ferrite to austenite ⁇ (3) cementite from austenite It is formed by a transformation phenomenon that proceeds in the flow of formation. In this flow, the formation of cementite from austenite is delayed by the addition of an alloy element such as Si, so that residual austenite and tempered martensite are easily formed.
  • Cr is an element that increases the driving force for nucleation of cementite, and promotes the formation of cementite, thereby suppressing the formation of retained austenite and tempered martensite.
  • the cementite formed is usually coarsened due to the diffusion rate-determining of the carbon having a high diffusion rate, and thus is coarsened.
  • the coarsening proceeds due to the diffusion-limited rate of Cr having a low diffusion rate. Therefore, cementite coarsening can be suppressed.
  • the range of the Cr content is preferably 0.3 to 2.5% by mass, more preferably 0.6 to 2.0% by mass.
  • B is an element useful for improving the hardenability and increasing the area ratio of bainite by being present in the austenite grain boundary in a solid solution state in the steel. If the addition amount of B is less than 0.0002% by mass, the above-described effects cannot be exhibited effectively. On the other hand, if the addition amount of B exceeds 0.0050% by mass, Fe 23 (CB) 6 is formed and the solid solution B is reduced, so that the effect of improving hardenability is diminished.
  • Nb and / or Ti ([N] ⁇ 0.003) / 12 ⁇ [Nb] / 96 + [Ti] / 48 ⁇ ([N] +0.01) / 12 ([] is the content of each element ( Mass%)))
  • N forms BN and consumes B
  • Ti and Nb are elements useful for exerting a hardenability improving effect by B because N is strongly fixed as TiN or Nb (CN) and the formation of BN is suppressed. If the addition amount of these elements is insufficient, the above BN formation inhibiting action is not effectively exhibited.
  • the addition amount of these elements becomes excessive, the formation of cementite is inhibited, the ratio of tempered martensite and retained austenite increases, and stretch flangeability deteriorates.
  • Mo 0.01 to 1.0 mass%, Cu: 0.05 to 1.0 mass%, Ni: 0.05 to 1.0 mass%
  • Mo forms alloy carbides and carbonitrides that can become hydrogen trap sites during tempering, and Cu and Ni, like V, promote the formation of ⁇ -FeOOH, thereby improving hydrogen embrittlement resistance. Also has the effect of improving. If the addition amount of each element is less than each of the above lower limit values, the above effects cannot be exhibited effectively. On the other hand, when the addition amount of each element exceeds 1.0 mass%, austenite remains at the time of quenching, and stretch flangeability is deteriorated.
  • REM refers to a rare earth element, that is, a group 3A element in the periodic table.
  • Example 1 Example according to the first invention
  • Steels having the components shown in Table 1 were melted to produce 120 mm thick ingots. This was hot rolled to a thickness of 25 mm, and then hot rolled again to a thickness of 3.2 mm. After pickling this, it cold-rolled to 1.6 mm in thickness to make a test material, and heat-treated on the conditions shown in Table 2.
  • the area ratios of tempered bainite, ferrite, tempered martensite and retained austenite, and the size and number of cementite particles were measured by the measurement method described above.
  • the tensile strength TS, the elongation El, and the stretch flangeability ⁇ were measured for each of the above steel plates.
  • the tensile strength TS and elongation El were measured in accordance with JIS Z 2241 by preparing a No. 5 test piece described in JIS Z 2201 with the long axis in the direction perpendicular to the rolling direction.
  • stretch flangeability (lambda) performed the hole expansion test according to the iron continuous standard JFST1001, and measured the hole expansion rate, and made this the stretch flangeability.
  • steel no. 1 to 6, 9 to 11, 14 to 16, 18, and 21 to 25 all have a tensile strength TS of 800 MPa or more, an elongation El of 10% or more, and a stretch flangeability (hole expansion ratio) ⁇ of 90. %, And a high-strength cold-rolled steel sheet having both elongation and stretch flangeability that satisfies the above-mentioned required level was obtained.
  • steel No. which is a comparative example. 7, 8, 12, 13, 17, 19, 20, 26 to 28 are inferior in any of the characteristics.
  • steel No. No. 7 is inferior in stretch flangeability because the total area ratio of martensite and retained austenite becomes excessive because the Si content is too high.
  • Steel No. No. 8 is inferior in tensile strength because the amount of cementite in bainite is insufficient due to the C content being too low.
  • FIG. 1 As a result of arranging the degree of influence of the number of cementite particles on the stretch flangeability (hole expansion ratio) ⁇ , FIG. 1 was obtained.
  • the stretch flangeability (hole expansion ratio) ⁇ decreases almost linearly as the number of coarse cementite particles having an equivalent circle diameter of 0.1 ⁇ m or more increases.
  • FIG. 1 shows that the number of coarse cementite particles needs to be 3 / ⁇ m 2 or less in order to ensure ⁇ ⁇ 90% above the desired level.
  • Example 2 Example according to the second invention
  • Steels having the components shown in Table 4 were melted to prepare an ingot having a thickness of 120 mm. After this was hot rolled to a thickness of 25 mm, it was again hot rolled to a thickness of 3 mm. After pickling this, it cold-rolled to thickness 1.2mm to make a test material, and heat-treated on the conditions shown in Table 5.
  • each area ratio of tempered bainite, ferrite, tempered martensite and retained austenite, the size and number of precipitates (existence density), and the size of cementite particles and The existence number (existence density) was measured.
  • tensile strength TS, elongation El, stretch flangeability ⁇ are measured to evaluate mechanical properties, and hydrogen embrittlement risk index is measured to evaluate hydrogen embrittlement resistance. did.
  • the tensile strength TS and elongation El were measured in accordance with JIS Z 2241 by preparing a No. 5 test piece described in JIS Z 2201 with the long axis perpendicular to the rolling direction.
  • the stretch flangeability ⁇ was measured according to the iron standard JFST1001, the hole expansion rate was measured, and the hole expansion rate was measured.
  • the hydrogen embrittlement risk index was determined by performing a low strain rate technique (SSRT) with a strain rate of 1 ⁇ 10 ⁇ 4 / s using a flat plate test piece having a thickness of 1.2 mm.
  • the hydrogen embrittlement risk index was calculated from the definition formula.
  • Hydrogen embrittlement risk index (%) 100 ⁇ (1 ⁇ E 1 / E 0 )
  • E 0 indicates the elongation at break of a test piece substantially free of hydrogen in steel
  • E 1 indicates a steel material (test piece) electrochemically charged with hydrogen in sulfuric acid. Elongation at break is shown.
  • the hydrogen charge is performed by immersing a steel material (test piece) in a mixed solution of H 2 SO 4 (0.5 mol / L) and KSCN (0.01 mol / L) at room temperature and a constant current (100 A / m 2). ).
  • Table 6 shows the measurement results of the mechanical properties and hydrogen embrittlement resistance.
  • invention steels (steel Nos. 30, 31, 38, 39, 42, 44, 45, 48, 49) satisfying the essential constituent requirements of the present invention (the above-mentioned component composition rules and the above-mentioned essential structure rules).
  • , 54, 56, 60 to 65; all of the circles) have an tensile strength TS of 780 MPa or more, and an index TS ⁇ ⁇ for evaluating the balance between the tensile strength TS and stretch flangeability (hole expansion ratio) ⁇ .
  • comparative steel lacking at least one of the essential constituent elements of the present invention (steel Nos. 29, 32 to 37, 40, 41, 43, 46, 47, 50 to 53, 55; ) Is inferior in any of the mechanical properties and hydrogen embrittlement resistance properties.
  • the recommended steels (steel Nos. 57 to 59; marked with ⁇ ) that satisfy the above recommended structure provision (a) are all tensile.
  • the strength TS is 980 MPa or more
  • the stretch flangeability (hole expansion ratio) ⁇ is 100% or more
  • the hydrogen embrittlement risk index is 15% or less, which is superior in strength and workability to the steel of the invention. It was found that a high-strength cold-rolled steel sheet can be obtained.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

La présente invention se rapporte à une tôle d'acier laminée à froid présentant une meilleure aptitude au traitement et une meilleure résistance à la fragilisation par l'hydrogène. La tôle d'acier laminée à froid comprend une quantité de carbone (C) supérieure ou égale à 0,03 % en masse et inférieure à 0,30 % en masse, une quantité de silicium (Si) inférieure ou égale à 2,0 % en masse (y compris 0 % en masse), une quantité de manganèse (Mn) comprise entre 0,1 et 2,8 % en masse, une quantité de phosphore (P) inférieure ou égale à 0,1 % en masse, une quantité de soufre (S) inférieure ou égale à 0,005 % en masse, une quantité d'azote (N) inférieure ou égale à 0,01 % en masse et une quantité d'aluminium (Al) comprise entre 0,01 et 1,00 % en masse. Dans la tôle d'acier laminée à froid, le rapport d'aire de la bainite revenue est égal ou supérieur à 50 % (y compris 100 %), le rapport d'aire total de la martensite revenue et de l'austénite restante est inférieur à 3 % (y compris 0 %) et une structure comprenant de la ferrite constitue le reste. Dans la tôle d'acier laminée à froid, la répartition des précipités dans la bainite revenue et la répartition des particules de cémentite dans la bainite revenue sont régulées.
PCT/JP2009/067171 2009-10-01 2009-10-01 Tôle d'acier laminée à froid WO2011039885A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
PCT/JP2009/067171 WO2011039885A1 (fr) 2009-10-01 2009-10-01 Tôle d'acier laminée à froid

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
PCT/JP2009/067171 WO2011039885A1 (fr) 2009-10-01 2009-10-01 Tôle d'acier laminée à froid

Publications (1)

Publication Number Publication Date
WO2011039885A1 true WO2011039885A1 (fr) 2011-04-07

Family

ID=43825739

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/JP2009/067171 WO2011039885A1 (fr) 2009-10-01 2009-10-01 Tôle d'acier laminée à froid

Country Status (1)

Country Link
WO (1) WO2011039885A1 (fr)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2015097354A1 (fr) * 2013-12-24 2015-07-02 Arcelormittal Wire France Fil laminé à froid en acier à haute résistance à la fatigue et à la fragilisation par l'hydrogène et renfort de conduites flexibles l'incorporant

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0657375A (ja) * 1992-08-07 1994-03-01 Sumitomo Metal Ind Ltd 超高張力冷延鋼板およびその製造方法
JP2003073773A (ja) * 2001-08-31 2003-03-12 Kobe Steel Ltd 加工性及び疲労特性に優れた高強度鋼板およびその製造方法
JP2005220440A (ja) * 2004-01-09 2005-08-18 Kobe Steel Ltd 耐水素脆化特性に優れた超高強度鋼板及びその製造方法
JP2005298956A (ja) * 2004-04-16 2005-10-27 Sumitomo Metal Ind Ltd 熱延鋼板およびその製造方法

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0657375A (ja) * 1992-08-07 1994-03-01 Sumitomo Metal Ind Ltd 超高張力冷延鋼板およびその製造方法
JP2003073773A (ja) * 2001-08-31 2003-03-12 Kobe Steel Ltd 加工性及び疲労特性に優れた高強度鋼板およびその製造方法
JP2005220440A (ja) * 2004-01-09 2005-08-18 Kobe Steel Ltd 耐水素脆化特性に優れた超高強度鋼板及びその製造方法
JP2005298956A (ja) * 2004-04-16 2005-10-27 Sumitomo Metal Ind Ltd 熱延鋼板およびその製造方法

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2015097354A1 (fr) * 2013-12-24 2015-07-02 Arcelormittal Wire France Fil laminé à froid en acier à haute résistance à la fatigue et à la fragilisation par l'hydrogène et renfort de conduites flexibles l'incorporant
WO2015097349A1 (fr) * 2013-12-24 2015-07-02 Arcelormittal Wire France Fil laminé à froid en acier à haute résistance à la fatigue et à la fragilisation par l'hydrogène et renfort de conduites flexibles l'incorporant
US10550448B2 (en) 2013-12-24 2020-02-04 Arcelormittal Wire France Cold rolled steel wire, method and reinforcement of flexible conduits
EP3960884A1 (fr) * 2013-12-24 2022-03-02 Arcelormittal Wire France Fil laminé à froid en acier à haute résistance à la fatigue et à la fragilisation par l'hydrogène et renfort de conduites flexibles l'incorporant
US11408049B2 (en) 2013-12-24 2022-08-09 Arcelormittal Wire France Cold rolled steel wire, method and reinforcement of flexible conduits

Similar Documents

Publication Publication Date Title
JP4712882B2 (ja) 耐水素脆化特性および加工性に優れた高強度冷延鋼板
JP5093422B2 (ja) 高強度鋼板及びその製造方法
KR101126953B1 (ko) 고강도 냉연 강판
JP4712838B2 (ja) 耐水素脆化特性および加工性に優れた高強度冷延鋼板
WO2009110607A1 (fr) Tôles d'acier laminées à froid
CN109072371B (zh) 温加工用高强度钢板及其制造方法
JP4324225B1 (ja) 伸びフランジ性に優れた高強度冷延鋼板
JP5466552B2 (ja) 伸び、伸びフランジ性および溶接性を兼備した高強度冷延鋼板
EP2765211B1 (fr) Tôle en acier laminée à chaud de force de traction élevée, et procédé de fabrication de celle-ci
JP5363922B2 (ja) 伸びと伸びフランジ性のバランスに優れた高強度冷延鋼板
JP2006118000A (ja) 延性に優れた軽量高強度鋼とその製造方法
WO2012036309A1 (fr) Feuille d'acier laminée à chaud à haute résistance possédant une excellente aptitude au cintrage et procédé de production
JP2014159610A (ja) 曲げ性に優れた高強度冷延鋼板
JP2005336526A (ja) 加工性に優れた高強度鋼板及びその製造方法
JP2010285636A (ja) 伸び、伸びフランジ性および溶接性を兼備した高強度冷延鋼板
JP2005298956A (ja) 熱延鋼板およびその製造方法
JP5302840B2 (ja) 伸びと伸びフランジ性のバランスに優れた高強度冷延鋼板
JP2009215572A (ja) 降伏応力と伸びと伸びフランジ性に優れた高強度冷延鋼板
JP5080215B2 (ja) 等方性と伸びおよび伸びフランジ性に優れた高強度冷延鋼板
JP4712842B2 (ja) 伸びおよび伸びフランジ性に優れた高強度冷延鋼板
JP5189959B2 (ja) 伸びおよび伸びフランジ性に優れた高強度冷延鋼板
JP5483562B2 (ja) 伸びと伸びフランジ性のバランスに優れた高強度冷延鋼板
JP2005273004A (ja) 延性に優れた低比重鋼板およびその製造方法
JP4712839B2 (ja) 耐水素脆化特性および加工性に優れた高強度冷延鋼板
JP2008266792A (ja) 熱延鋼板

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 09850075

Country of ref document: EP

Kind code of ref document: A1

NENP Non-entry into the national phase

Ref country code: DE

122 Ep: pct application non-entry in european phase

Ref document number: 09850075

Country of ref document: EP

Kind code of ref document: A1

NENP Non-entry into the national phase

Ref country code: JP