WO2013160928A1 - High-strength steel sheet and method for manufacturing same - Google Patents
High-strength steel sheet and method for manufacturing same Download PDFInfo
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- WO2013160928A1 WO2013160928A1 PCT/JP2012/002775 JP2012002775W WO2013160928A1 WO 2013160928 A1 WO2013160928 A1 WO 2013160928A1 JP 2012002775 W JP2012002775 W JP 2012002775W WO 2013160928 A1 WO2013160928 A1 WO 2013160928A1
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
Definitions
- the present invention relates to a high-strength steel sheet for automobiles, home appliances and the like, which is used through a press forming process, and a method for manufacturing the same.
- Patent Document 1 For example, if the tensile strength (TS) of the steel sheet is up to 440 MPa, the Ti and Nb are dissolved in the solute carbon and solid solution in the steel.
- a method of adding a solid solution strengthening element such as Si, Mn and P based on a steel that has been converted to IF (Interstitial free) by adding a sufficient amount to fix nitrogen is disclosed (for example, Patent Documents) 1).
- This Patent Document 1 has a composition of C: 0.002 to 0.015%, Nb: C% ⁇ 3 to C% ⁇ 8 + 0.020%, Si: 1.2%, Mn: 0.04 to 0.8%, and P: 0.03 to 0.10%.
- a technique for obtaining a high-tensile cold-rolled steel sheet having a non-aging property with a tensile strength of 35 to 45 kg / mm 2 class (340 to 440 MPa class) and excellent formability is disclosed.
- TS tensile strength
- DP steel sheets having a two-phase structure of ferrite-martensite and TRIP steel sheets utilizing residual ⁇ are known.
- the former has a feature of high work hardening ability while having low yield strength due to residual strain around martensite.
- the latter has the feature that the uniform elongation of the steel sheet is increased by performing plasticity-induced martensitic transformation.
- the mechanical properties of high-tensile steel sheets are often evaluated with tensile properties in a specific direction, such as a direction perpendicular to the rolling direction, and in part, the in-plane anisotropy ( ⁇ r) of the r value is a problem.
- evaluation may be made based on r values in the rolling direction, 45 ° direction, and 90 ° direction.
- the formability of the press is determined by the properties in the direction of lower ductility than the direction in which the mechanical properties are evaluated, particularly the elongation value. I understood that.
- Patent Document 2 discloses a method of obtaining a cold-rolled steel sheet for an automotive outer panel component having excellent bake hardenability and small in-plane anisotropy.
- the in-plane anisotropy and dent resistance of a steel sheet are defined by defining the in-plane anisotropy of the r value, that is, ⁇ r, according to the amount of C and the cold pressure ratio. It is said that both can be achieved.
- the cooling is started within 2 seconds after the hot rolling and that the cooling is performed at a cooling rate of 70 ° C./s or more and in a temperature range of 100 ° C. or more.
- Patent Document 3 discloses a steel sheet excellent in shape freezing property. According to this, ferrite or bainite is used as the largest phase in volume fraction, and volume fraction: 1 % As a composite structure steel containing martensite in a range of 25% to 25%.
- the average value (A) of the X-ray random intensity ratio of the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups is 4.0 or more
- the average value (B) of the X-ray random intensity ratios of the three crystal orientations of ⁇ 554 ⁇ ⁇ 225>, ⁇ 111 ⁇ ⁇ 112> and ⁇ 111 ⁇ ⁇ 110> is 5.5 or less
- (Iv) ⁇ 100 ⁇ ⁇ 011> X-ray reflection random intensity ratio is equal to or greater than ⁇ 211 ⁇ ⁇ 011> X-ray random intensity ratio;
- the above conditions (i) to (iv) are all satisfied, and at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is 0.7 or less, and the anisotropy ⁇ uE1 of uniform elongation is 4 % Or less, the local elongation anisotropy
- ⁇ uE1 and ⁇ LE1 are obtained by the following equations.
- ⁇ uEl ⁇
- ⁇ LEl ⁇
- the uniform elongation in the direction parallel to the rolling direction (L direction), vertical (C direction), and 45 ° is uEl (L), uEl (C), and uEl (45 °), respectively, and the rolling direction
- the local elongations in the parallel (L direction), vertical (C direction), and 45 ° directions are LEl (L), LEl (C), and LEl (45 °), respectively.
- the present invention advantageously solves the above-mentioned problems, and proposes a high-strength steel sheet capable of reducing ductility anisotropy and suppressing cracking during press forming together with its advantageous manufacturing method. With the goal.
- the inventors have intensively studied to solve the above-mentioned problems, and by defining a reduction ratio according to the contents of Ti and Nb and developing a certain texture, ductility In particular, the in-plane anisotropy of uniform elongation was successfully reduced.
- the present invention has been completed based on the above findings.
- the gist configuration of the present invention is as follows. 1. In mass%, C: more than 0.0005% and less than 0.10%, Si: 1.5% or less, Mn: 0.1% or more and 3.0% or less, P: 0.080% or less, S: 0.03% or less, sol.Al: 0.01% or more and 0.50% or less And N: 0.005% or less, and Nb: 0.20% or less and Ti: 0.2% or less selected from one or two kinds, the balance is composed of Fe and inevitable impurities,
- the steel structure is a ferrite phase with a volume fraction of 60% or more, Intensity of ODF ⁇ 0 °, 0 °, 45 ° ⁇ when ⁇ is 0 °, ⁇ 1 is 0 °, and ⁇ 2 is 45 ° in the density function (ODF) ⁇ OD1, ⁇ , ⁇ 2 ⁇ of the three-dimensional crystal orientation Is a high-strength steel sheet in which the strength of ODF ⁇ 0 °, 35 °, 45
- the steel sheet is further mass%, V: 0.40% or less, Cr: 0.50% or less, Mo: 0.50% or less, W: 0.15% or less, Zr: 0.10% or less, Cu: 0.50% or less, Ni: 0.50%
- the following 1 containing at least one selected from B: 0.0050% or less, Sn: 0.20% or less, Sb: 0.20% or less, Ca: 0.010% or less, Ce: 0.01% or less, and La: 0.01% or less
- the steel slab having the component composition described in 1 or 2 above is hot-rolled at a finishing temperature of 820 ° C. or higher and 950 ° C. or lower, and the rolling reduction (X%) satisfies the relationship of the following formula (1):
- a method for producing a high-strength steel sheet which is subjected to cold rolling under satisfying conditions, then subjected to continuous annealing in a temperature range from the recrystallization temperature to 900 ° C., and then cooled. Record 0.30 ⁇ ⁇ 1.6 ⁇ ([% Ti] +2 ⁇ [% Nb]) + 0.004X ⁇ ⁇ 0.36 (1)
- [% A] indicates the content (mass%) of element A in steel.
- the in-plane anisotropy of uniform elongation can be effectively reduced, even if the strength and ductility in the direction perpendicular to the rolling direction are the same as in conventional steel, the occurrence of press cracks It is possible to obtain a high-strength steel sheet that further suppresses
- (a) and (b) are graphs showing the results of evaluating the influence of the rolling reduction on strength and the relationship with the in-plane anisotropy of uniform elongation.
- (C) is a graph showing the relationship between strength and uniform elongation.
- the ductility anisotropy can be reduced regardless of the accumulation of ⁇ -fiber, which is said to be related to other orientations, for example, the r value which is an index of deep drawability.
- % Representing the content of each component element means “% by mass” unless otherwise specified.
- C more than 0.0005% and less than 0.10% C is an element necessary for increasing the strength of the steel sheet while suppressing the area ratio of the second phase.
- the lower limit of the C amount is set to more than 0.0005%, which is normally possible with a melting technique.
- the C content is less than 0.10%, preferably less than 0.08%.
- Si 1.5% or less Si improves the surface quality by delaying scale formation during hot rolling, and moderately delays the alloying reaction between the iron and zinc in the plating bath or alloying process. Furthermore, since it has various effects such as an effect of increasing the work hardening ability of ferrite, the Si content is preferably 0.01% or more, more preferably 0.05% or more. However, if the Si content exceeds 1.5%, the appearance quality deteriorates and the ⁇ ⁇ ⁇ transformation point rises, hot rolling cannot be performed in the ⁇ region, and the texture changes greatly. For this reason, the in-plane anisotropy of the uniform elongation of the steel sheet cannot be controlled. Therefore, the Si content is 1.5% or less. Preferably it is 1.2% or less.
- Mn 0.1% to 3.0% Mn not only suppresses hot ductility due to FeS, but can also be used as a solid solution strengthening element. Therefore, addition of 0.1% or more is necessary. On the other hand, if the Mn content is less than 0.1%, the grain growth is improved, which is not preferable from the viewpoint of controlling the in-plane anisotropy. Note that Mn is an element effective for increasing the strength by making martensite present in the second phase because it enhances hardenability. From the viewpoint of such complex organization, addition of 1.0% or more is preferable.
- the Mn content is 3.0% or less. In order to control the in-plane anisotropy of uniform elongation to a higher degree, 2.5% or less is preferable.
- the amount of P is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.015% or more.
- the P content is 0.080% or less, preferably 0.050% or less.
- the S content is preferably 0.01% or less, and more preferably 0.002% or less.
- sol.Al 0.01% or more and 0.50% or less
- Al is useful as a deoxidizing element for steel and has the effect of fixing solid solution N to improve normal temperature aging resistance. Shall be allowed to.
- the addition exceeding 0.50% increases the manufacturing cost and further induces surface defects in the steel sheet. Therefore, Al is 0.50% or less, preferably 0.08% or less.
- N 0.005% or less N is preferably reduced as much as possible because if it is too much, the room temperature aging resistance deteriorates and a large amount of Al or Ti needs to be added. Therefore, the upper limit is made 0.005%.
- Nb is capable of controlling the in-plane anisotropy of uniform elongation after cold rolling annealing by refining the structure and suppressing recrystallization of austenite in the hot rolling process. This is an important element in the present invention. However, if added over 0.20%, not only the cost is increased, but the texture in hot rolling is excessively developed, and the in-plane anisotropy of uniform elongation cannot be controlled due to excessive increase in the recrystallization temperature. Therefore, Nb needs to be 0.20% or less. Preferably it is 0.12% or less. In order to obtain the above effect, Nb is preferably contained in an amount of 0.005% or more.
- Ti 0.20% or less Ti, like Nb, refines the structure, suppresses recrystallization of austenite in the hot rolling process, and controls the in-plane anisotropy of uniform elongation after cold rolling annealing. This is an important element in the present invention. However, if added over 0.20%, not only the cost is increased, but the texture in hot rolling is excessively developed, and the in-plane anisotropy of uniform elongation cannot be controlled due to excessive increase in the recrystallization temperature. Therefore, Ti needs to be 0.20% or less. Preferably it is 0.12% or less. In addition, when obtaining the said effect, it is preferable to contain Ti 0.005% or more.
- V 0.40% or less
- V is an element that improves hardenability, and since it hardly reduces the plating quality and corrosion resistance, it can be used as a substitute for Mn and Cr. However, if added over 0.40%, the cost will increase significantly, so it is desirable to add V at 0.40% or less.
- Cr 0.50% or less Cr, like Mn, is an element that contributes to high strength by forming a composite structure of a steel sheet. In order to acquire this effect, it is preferable to contain 0.10% or more. However, excessive addition of Cr not only saturates the above effect, but also causes high alloy costs, so the upper limit is made 0.50%.
- Mo 0.50% or less Mo is an element that improves hardenability, suppresses the formation of pearlite, and contributes to high strength. However, since Mo is an extremely expensive element, a large amount of addition leads to a significant cost increase. Therefore, the amount of Mo added is preferably 0.50% or less.
- W 0.15% or less W can be used as a hardenability improving element and a precipitation strengthening element. However, if the added amount is too large, ductility is reduced, so the added amount of W is preferably 0.15% or less.
- Zr 0.10% or less Zr can be used as a hardenability improving element and a precipitation strengthening element. However, if the added amount is too large, ductility is lowered, so it is desirable to add Zr at 0.10% or less.
- Cu 0.50% or less By allowing contamination, Cu can be used as a raw material, and manufacturing costs can be reduced. In consideration of improving corrosion resistance, it is desirable to add 0.03% or more when Cu is added. However, if the content is too large, it may cause surface defects, so the upper limit is preferably 0.50%.
- Ni 0.50% or less
- Ni is an element that improves corrosion resistance, and has the effect of reducing surface defects that are likely to occur when Cu is contained. Therefore, from the viewpoint of improving the surface quality while improving the corrosion resistance, it is desirable to add Ni by 0.02% or more. On the other hand, if the amount of Ni added is too large, scale generation in the heating furnace becomes non-uniform, causing surface defects and a significant cost increase. Therefore, the upper limit is desirably 0.50%.
- B 0.0050% or less B is an element that improves the hardenability of steel.
- B is an element that improves the hardenability of steel.
- the content exceeds 0.0050%, the effect is saturated. Therefore, when added, the content is preferably 0.0050% or less.
- Sn 0.20% or less
- Sn is preferably added from the viewpoint of suppressing nitridation and oxidation of the steel sheet surface, or decarburization and deboronization (de-B) of the steel sheet surface layer caused by oxidation. From the viewpoint of suppressing nitriding and oxidation, it is desirable to add 0.005% or more, but if it exceeds 0.20%, excessive yield strength (YP) will be increased and toughness will be deteriorated, so Sn should be contained at 0.20% or less. Is desirable.
- Sb 0.20% or less
- Sb is desirably added from the viewpoint of suppressing nitridation and oxidation of the steel sheet surface, or decarburization and de-B of the steel sheet surface layer caused by oxidation, as with Sn.
- Sb is desirable to add 0.005% or more.
- Sb is desirably contained at 0.20% or less.
- Ca 0.010% or less Ca has the effect of fixing S in steel as CaS, further increasing the pH in corrosive organisms, and improving the corrosion resistance around the hem-processed part and spot welded part. Moreover, the production
- Ce 0.01% or less Ce can be added for the purpose of fixing S in steel. However, since it is an expensive element, adding a large amount increases the cost. Therefore, it is desirable to add Ce at 0.01% or less.
- La 0.01% or less La can be added for the purpose of fixing S in steel. However, since it is an expensive element, adding a large amount increases the cost. Therefore, it is desirable to add La at 0.01% or less.
- the balance other than the above elements is Fe and inevitable impurities.
- the ferrite texture is controlled, but when confirming the texture, the X-ray diffraction method is generally used.
- the main phase of ferrite cannot be clearly distinguished from the second phase such as martensite and bainite
- the main point of the present invention is when the second phase fraction becomes high. It becomes impossible to control the anisotropy of uniform elongation by texture control.
- the second phase when the second phase increases, the second phase surrounds the periphery of the ferrite in a network shape, and the macroscopic plastic behavior of the steel sheet does not depend on the crystal orientation of the ferrite.
- the ferrite needs to be 60% or more in volume fraction.
- the volume fraction is preferably 75% or more.
- the volume fraction of the ferrite phase can be obtained on the basis of the volume fraction of the second phase by regarding the area ratio of the second phase obtained by the following procedure as the volume fraction of the second phase.
- the area ratio of the second phase is obtained from a photograph of the structure taken after polishing the L cross section (vertical cross section parallel to the rolling direction) of the steel plate, corroding with nital, observing 10 fields of view with a SEM at a magnification of 4000 times.
- the ferrite is observed as a slightly black contrast region, the region where the carbides are formed in a lamellar or dotted pattern is pearlite and bainite, and the particles with white contrast are martensite or residual ⁇ . is there.
- the fine dot-like particles having a diameter of 0.4 ⁇ m or less recognized on the SEM photograph are mainly carbides, and since their area ratio is very small, it is considered that the material is hardly affected.
- the area ratio of the second phase is obtained from the ratio of the second phase existing on the lattice using a square mesh (point count method).
- the area ratio (%) of the second phase thus determined is directly used as the volume fraction (%) of the second phase.
- the volume fraction (%) of the ferrite phase can be obtained by subtracting the volume fraction (%) of the second phase from 100%.
- ODF three-dimensional crystal orientation density function
- ODF density function
- ⁇ 1, ⁇ , ⁇ 2 ⁇ ⁇ 0 °, 35 °, 45 ° ⁇
- the rolling direction and the direction perpendicular to the rolling direction are relatively low, so press cracks occur. This is because it becomes easier.
- the strength exceeds 4.5, the uniform elongation in the D direction (a direction that forms 45 ° with the rolling direction) is relatively low. This is because the texture of the steel sheet is related to the anisotropy of the yield strength, so the yield strength and ductility are in a trade-off relationship, the direction of high strength has a low uniform elongation, and Ti and Nb are rolled.
- the azimuth strength in the present invention is determined as follows. First, the pole figures of the three surfaces (200), (211), and (110) are measured by the reflection method to obtain three incomplete pole figures. Next, these three incomplete pole figures are converted into a three-dimensional crystal orientation density function by the series expansion method, and the strength of the desired orientation is obtained.
- the steel slab used in the production method of the present invention is preferably produced by a continuous casting method in order to prevent macro segregation of components, but is not particularly limited, and is produced by an ingot-making method or a thin slab casting method. Also good.
- direct feed rolling in which the steel slab is charged without being cooled and charged in a heating furnace and hot-rolled, or a little heat retention After carrying out, energy saving processes such as direct feed rolling and direct rolling that are immediately hot rolled can be applied without any problem.
- the slab heating temperature is not particularly limited, but it is desirable that the slab heating temperature is low in order to improve the deep drawability by developing a ⁇ 111 ⁇ recrystallization texture by coarsening the precipitates. However, if the heating temperature is less than 1000 ° C., the rolling load increases and the risk of trouble during hot rolling increases, so the slab heating temperature is preferably 1000 ° C. or higher. Note that the upper limit of the slab heating temperature is preferably 1300 ° C. in view of an increase in scale loss accompanying an increase in oxidized weight.
- ⁇ Hot rolling for rough rolling and finish rolling is performed on the steel slab heated under the above conditions.
- the steel slab is made into a sheet bar by rough rolling.
- the conditions for rough rolling need not be specified, and may be performed according to a conventional method. From the viewpoint of lowering the slab heating temperature and preventing problems during hot rolling, it is an effective method to use a so-called sheet bar heater that heats the sheet bar.
- Finishing temperature 820 ° C or more and 950 ° C or less Finishing and rolling the sheet bar to make a hot-rolled sheet.
- the finishing temperature that is, the finish rolling outlet temperature (hereinafter referred to as FT) is 820 ° C. or higher and 950 ° C. or lower. This is to obtain a texture preferable for in-plane anisotropy of uniform elongation after cold rolling and recrystallization annealing.
- FT finish rolling outlet temperature
- lubrication rolling may be performed between some or all passes of finish rolling.
- Performing the lubrication rolling is effective from the viewpoint of homogenizing the shape of the steel plate and homogenizing the material.
- the coefficient of friction between the roll and the steel sheet is preferably in the range of 0.10 to 0.25.
- the coil winding temperature (CT) in the present invention is not particularly specified, but is preferably 400 ° C. or higher and 720 ° C. or lower.
- CT coil winding temperature
- the crystal grains are coarsened, resulting in a decrease in strength and an increase in r value after cold rolling annealing.
- the hot-rolled sheet is subjected to pickling and cold rolling to perform a cold rolling process to obtain a cold-rolled sheet.
- Pickling may be performed under normal conditions.
- Ti and Nb are important elements for rolling in the austenite non-recrystallized region of hot rolling. Rolling due to the development of ⁇ texture due to non-recrystallizing rolling and the variant restrictions during the subsequent transformation. Greatly involved in the texture. Further, the rolling reduction is an important condition for developing the rolling texture.
- the inventors balance the texture because the texture due to the coarsening of the ferrite grain size and the texture due to the non-recrystallized ⁇ rolling have opposite effects on the in-plane anisotropy of elongation.
- the in-plane anisotropy of uniform elongation and the strength f ( ⁇ 35 °) of ODF ⁇ 0 °, 35 °, 45 ° ⁇ were investigated for steels with varying Ti, Nb and rolling reduction. .
- the amount of Nb is the amount of Ti ([% Ti]) from the viewpoint of the recrystallization suppression effect in the solid solution state with the precipitate and the difference in atomic weight.
- the anisotropy was evaluated by uniform elongation ⁇ UEL normalized by UEL L using the following formula (2).
- the uniform elongation of the L, D, C direction of a steel plate be UEL L , UEL D , UEL C , respectively.
- the recrystallization temperature is set. This is because the ductility is reduced.
- the recrystallization temperature is obtained by subjecting the cold-rolled plate to a predetermined temperature by changing the annealing temperature, immediately after cooling (with a holding time within 1 s), and immediately quenching into cold water. The temperature may be determined as a temperature at which unrecrystallization is not observed.
- the annealing temperature may be changed at a pitch of 10 ° C. from 650 ° C., for example.
- the average cooling rate in the temperature range from the annealing temperature to 500 ° C is 5 ° C / s to 15 ° C / s. It is preferable to cool in the following range. If the average cooling rate in this temperature range is less than 5 ° C./s, martensite is hardly formed, and a ferrite single phase structure may be formed, resulting in insufficient structure strengthening.
- the average cooling rate up to 500 ° C. is equal to or higher than the critical cooling rate. In order to achieve this, 5 ° C / s or more is necessary.
- the cooling at 500 ° C. or lower is not particularly limited because the ⁇ phase is stabilized to some extent by the cooling so far, but subsequently, the temperature range up to about 300 ° C. is maintained at an average cooling rate of 5 ° C./s or more. It is preferable to cool, and when performing an overaging treatment, it is preferable that the average cooling rate is 5 ° C./s or more up to the overaging treatment temperature.
- the steel sheet can be galvanized as required.
- martensite is formed as the second phase in the hot dip galvanizing line, it is usually 450 to 500 from the annealing temperature after soaking.
- the average cooling rate to the temperature of the galvanizing bath maintained in the temperature range up to 0 ° C. is desirably 2 to 30 ° C./s.
- the cooling rate is slower than 2 ° C./s, a large amount of pearlite is generated in the temperature range of 500 to 650 ° C., and a hard second phase cannot be obtained.
- the cooling rate is higher than 30 ° C./s, the ⁇ ⁇ ⁇ transformation proceeds remarkably in the vicinity of 500 ° C. before and after being immersed in the plating bath, so that the second phase becomes finer and ductility decreases.
- the cold-rolled annealed plate and the plated steel plate can be subjected to temper rolling or leveler processing for the purpose of shape correction, surface roughness adjustment and the like.
- the total elongation of temper rolling or leveler processing is preferably in the range of 0.2 to 15%. If it is less than 0.2%, the intended purpose of shape correction and roughness adjustment cannot be achieved. On the other hand, if it exceeds 15%, the ductility is significantly reduced.
- the processing form differs between temper rolling and leveler processing, it has been confirmed that there is no significant difference between the two. Note that temper rolling and leveler processing are effective even after plating.
- the obtained cold-rolled annealed sheet was subjected to temper rolling with an elongation of 0.5%.
- the recrystallization temperature of each steel plate was 700 to 760 ° C., which was obtained by observation of the structure after heat-quenching for a short time, and was above the recrystallization temperature under all conditions.
- the No. 5 steel sheet was subjected to a cold-rolled sheet annealing process in a continuous hot dip galvanizing line, followed by in-line hot dip galvanizing (plating bath temperature: 480 ° C.) to obtain a hot dip galvanized steel sheet.
- Test pieces were collected from the obtained cold-rolled annealed sheet and hot-dip galvanized steel sheet, and the microstructure, tensile properties, and density function (ODF) of three-dimensional crystal orientation were determined by the following methods.
- the in-plane anisotropy of uniform elongation was evaluated by obtaining a value of ⁇ UEL by the following equation (2).
- the in-plane anisotropy of uniform elongation is excellent when the value of ⁇ UEL obtained by the following equation (2) is in the range of -0.020 to 0.020.
- Table 2 The obtained results are also shown in Table 2.
- ⁇ UEL ⁇ UEL L + UEL C ⁇ 2 ⁇ UEL D ⁇ / (2 ⁇ UEL L ) (2) Note that the microstructure (ferrite volume fraction) was determined based on the area ratio (volume ratio) of the second phase determined by the point counting method from the SEM photograph as described above.
- the texture is not included in the scope of the present invention, and as a result, the anisotropy of uniform elongation is large. It has become a steel plate.
Abstract
Description
しかしながら、自動車車体の軽量化と強度向上とを同時に満たすためには、剛性に影響のない範囲で部品素材を高強度化すると同時に、板厚を減して軽量化することが不可欠である。そのため、最近では、高強度鋼板が自動車部品として積極的に使用されている。 In recent years, in order to regulate CO 2 emissions from the viewpoint of protecting the global environment, it has been required to improve the fuel consumption of automobiles. In addition, in order to ensure the safety of the occupant in the event of a vehicle body collision, there is a demand for improvement in safety with a focus on improving vehicle body collision characteristics. In this way, not only the weight of the automobile body is reduced, but the strength of the automobile body is being actively improved.
However, in order to satisfy the weight reduction and strength improvement of the automobile body at the same time, it is indispensable to increase the strength of the component material within a range that does not affect the rigidity, and at the same time reduce the plate thickness and reduce the weight. For this reason, recently, high-strength steel sheets are actively used as automobile parts.
しかしながら、内板および外板用のパネル材料等の鋼板を素材とする自動車部品の多くは、プレス加工によって成形される。そのため、自動車部品用鋼板としては、優れたプレス成形性を有していることが必要とされる。これに対し、高強度鋼板の多くは、通常の軟鋼板に比べ、成形性や、延性、深絞り性等に大きく劣るため、その改善が求められていた。 The above-described measures for reducing the weight increase the effect of the steel plate to be used as the strength is increased because the plate thickness can be reduced. Accordingly, automobile manufacturers tend to use high-strength steel sheets having a tensile strength (TS) of 390 MPa or more as panel materials for inner plates and outer plates, for example.
However, many automobile parts made of steel plates such as panel materials for inner and outer plates are formed by press working. For this reason, the steel sheet for automobile parts is required to have excellent press formability. On the other hand, many high-strength steel sheets are greatly inferior in formability, ductility, deep drawability, and the like as compared with ordinary mild steel sheets, and thus improvement thereof has been demanded.
この特許文献1には、C:0.002~0.015%,Nb:C%×3~C%×8+0.020%,Si:1.2%,Mn:0.04~0.8%およびP:0.03~0.10%の組成を有し、引張強さ35~45kg/mm2級(340~440MPa級)の非時効性を具え、成形性に優れた高張力冷延鋼板を得るための技術が開示されている。 For example, if the tensile strength (TS) of the steel sheet is up to 440 MPa, the Ti and Nb are dissolved in the solute carbon and solid solution in the steel. A method of adding a solid solution strengthening element such as Si, Mn and P based on a steel that has been converted to IF (Interstitial free) by adding a sufficient amount to fix nitrogen is disclosed (for example, Patent Documents) 1).
This
(i){100}<011>~{223}<110>方位群のX線ランダム強度比の平均値(A)が4.0以上、
(ii){554}<225>、{111}<112>および{111}<110>の3つの結晶方位のX線ランダム強度比の平均値(B)が5.5以下、
(iii)(A)/(B)≧1.5、
(iv){100}<011>X線反射ランダム強度比が、{211}<011>X線ランダム強度比以上、
という(i)~(iv)の条件を全て満足し、かつ、圧延方向のr値および圧延方向と直角方向のr値のうち少なくとも1つを0.7以下とし、均一伸びの異方性ΔuElを4%以下として、局部伸びの異方性ΔLElを2%以上で、かつΔuElをΔLEl以下とするとしている。 On the other hand, regarding ductility anisotropy, Patent Document 3 discloses a steel sheet excellent in shape freezing property. According to this, ferrite or bainite is used as the largest phase in volume fraction, and volume fraction: 1 % As a composite structure steel containing martensite in a range of 25% to 25%.
(I) The average value (A) of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation groups is 4.0 or more,
(Ii) The average value (B) of the X-ray random intensity ratios of the three crystal orientations of {554} <225>, {111} <112> and {111} <110> is 5.5 or less,
(Iii) (A) / (B) ≧ 1.5,
(Iv) {100} <011> X-ray reflection random intensity ratio is equal to or greater than {211} <011> X-ray random intensity ratio;
The above conditions (i) to (iv) are all satisfied, and at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is 0.7 or less, and the anisotropy ΔuE1 of uniform elongation is 4 % Or less, the local elongation anisotropy ΔLE1 is set to 2% or more, and ΔuE1 is set to ΔLE1 or less.
ΔuEl={|uEl(L)-uEl(45°)|+|uEl(C)-uEl(45°)|}/2 ・・・(3)
ΔLEl={|LEl(L)-LEl(45°)|+|LEl(C)-LEl(45°)|}/2 ・・・(4)
但し、圧延方向と平行(L方向)、垂直(C方向)、および、45°方向の均一伸びを、それぞれ、uEl(L)、uEl(C)、および、uEl(45°)とし、圧延方向と平行(L方向)、垂直(C方向)、および、45°方向の局部伸びを、それぞれ、LEl(L)、LEl(C)、および、LEl(45°)とする。 At this time, ΔuE1 and ΔLE1 are obtained by the following equations.
ΔuEl = {| uEl (L) −uEl (45 °) | + | uEl (C) −uEl (45 °) |} / 2 (3)
ΔLEl = {| LEl (L) −LEl (45 °) | + | LEl (C) −LEl (45 °) |} / 2 (4)
However, the uniform elongation in the direction parallel to the rolling direction (L direction), vertical (C direction), and 45 ° is uEl (L), uEl (C), and uEl (45 °), respectively, and the rolling direction The local elongations in the parallel (L direction), vertical (C direction), and 45 ° directions are LEl (L), LEl (C), and LEl (45 °), respectively.
しかしながら、均一伸びの絶対値は強度レベルによって変化するため、均一伸びの異方性ΔuElを4%以下とすると限定した強度レベルとなり、また{100}<011>の集合組織の発達は絞り性を低下させるというおそれが生じる。 Moreover, in patent document 3, as a means for achieving all these conditions, optimization of hot-rolling finishing conditions and winding up below the critical temperature according to a Mn equivalent are essential.
However, since the absolute value of the uniform elongation varies depending on the strength level, the uniform elongation anisotropy ΔuE1 is set to 4% or less, and the strength level is limited. The texture development of {100} <011> There is a risk of lowering.
本発明は、上記の知見に基づき、完成されたものである。 Now, the inventors have intensively studied to solve the above-mentioned problems, and by defining a reduction ratio according to the contents of Ti and Nb and developing a certain texture, ductility In particular, the in-plane anisotropy of uniform elongation was successfully reduced.
The present invention has been completed based on the above findings.
1.質量%で、C:0.0005%超0.10%未満、Si:1.5%以下、Mn:0.1%以上3.0%以下、P:0.080%以下、S:0.03%以下、sol.Al:0.01%以上0.50%以下およびN:0.005%以下を含み、かつNb:0.20%以下およびTi:0.20%以下のうちから選んだ1種または2種を含有し、残部はFeおよび不可避不純物の組成からなり、
鋼組織を、体積分率で60%以上のフェライト相とし、
3次元結晶方位の密度関数(ODF){φ1,Φ,φ2}において、Φが0°で、φ1が0°、φ2が45°のときのODF{0°,0°,45°}の強度が3.0以下で、かつΦが35°で、φ1が0°、φ2が45°のときのODF{0°,35°,45°}の強度が2.5以上4.5以下の範囲である高強度鋼板。 That is, the gist configuration of the present invention is as follows.
1. In mass%, C: more than 0.0005% and less than 0.10%, Si: 1.5% or less, Mn: 0.1% or more and 3.0% or less, P: 0.080% or less, S: 0.03% or less, sol.Al: 0.01% or more and 0.50% or less And N: 0.005% or less, and Nb: 0.20% or less and Ti: 0.2% or less selected from one or two kinds, the balance is composed of Fe and inevitable impurities,
The steel structure is a ferrite phase with a volume fraction of 60% or more,
Intensity of ODF {0 °, 0 °, 45 °} when Φ is 0 °, φ1 is 0 °, and φ2 is 45 ° in the density function (ODF) {OD1, φ, φ2} of the three-dimensional crystal orientation Is a high-strength steel sheet in which the strength of ODF {0 °, 35 °, 45 °} is 2.5 to 4.5 when φ is 3.0 or less, Φ is 35 °, φ1 is 0 °, and φ2 is 45 °.
記
0.30≦{1.6・([%Ti]+2・[%Nb])+0.004X}≦0.36 ・・・(1)
ただし、[%A]はA元素の鋼中含有量(質量%)を示す。 3. The steel slab having the component composition described in 1 or 2 above is hot-rolled at a finishing temperature of 820 ° C. or higher and 950 ° C. or lower, and the rolling reduction (X%) satisfies the relationship of the following formula (1): A method for producing a high-strength steel sheet, which is subjected to cold rolling under satisfying conditions, then subjected to continuous annealing in a temperature range from the recrystallization temperature to 900 ° C., and then cooled.
Record
0.30 ≦ {1.6 ・ ([% Ti] +2 ・ [% Nb]) + 0.004X} ≦ 0.36 (1)
However, [% A] indicates the content (mass%) of element A in steel.
本発明は、TiとNbの含有量に応じて圧下率を規定し、後述するΦ=35のODF強度を2.5以上4.5以下の範囲に制御することによって、延性、特に均一伸びの面内異方性を小さくすることができるという新規の知見に立脚するものである。
この理由については、必ずしも明らかではないが、発明者らは次のように考えている。
一般に、冷延鋼板の集合組織は、<110>方向がRD方向に平行になるα-fiberと、<111>方向がND方向に平行になるγ-fiberとが発達するといわれており、特に後者が発達すると、鋼板のr値が高くなると言われている。しかしながら、本発明では、α-fiberのある特定方位{φ1,Φ,φ2}={0°,35°,45°}、すなわち、Φ=35°のODF強度を2.5以上4.5以下の範囲とすることで、他の方位、例えば、深絞り性の指標であるr値に関係してると言われるγ-fiberの集積には関係なく、延性の異方性を減らすことができる。 The present invention will be specifically described below.
The present invention defines the rolling reduction ratio according to the contents of Ti and Nb, and controls the ODF strength of Φ = 35, which will be described later, in the range of 2.5 to 4.5, thereby allowing for in-plane anisotropic properties of ductility, particularly uniform elongation. This is based on the new finding that it is possible to reduce the sex.
Although this reason is not necessarily clear, the inventors consider as follows.
In general, it is said that the texture of cold-rolled steel sheets develops α-fiber whose <110> direction is parallel to the RD direction and γ-fiber whose <111> direction is parallel to the ND direction. It is said that the r value of a steel plate increases when the development of. However, in the present invention, α-fiber has a specific orientation {φ1, Φ, φ2} = {0 °, 35 °, 45 °}, that is, the ODF intensity at Φ = 35 ° is in the range of 2.5 to 4.5. Thus, the ductility anisotropy can be reduced regardless of the accumulation of γ-fiber, which is said to be related to other orientations, for example, the r value which is an index of deep drawability.
上記した種々の条件を満足することによって、鋼板の均一伸びの面内異方性が小さくなり、例えば、圧延直角方向の強度と延性が同じであっても、プレス割れが発生し難い鋼板を製造できるようになる。 Further, in order to appropriately suppress the above ODF strength, it is necessary to add a predetermined amount of Ti and Nb, and further, by cold rolling by performing some rolling of non-recrystallized austenite in hot rolling, It was found that the texture after annealing became the desired structure. Therefore, it is important to control the Ti and Nb content and the rolling reduction within a predetermined range.
By satisfying the various conditions described above, the in-plane anisotropy of the uniform elongation of the steel sheet is reduced. For example, even if the strength and ductility in the direction perpendicular to the rolling direction are the same, a steel sheet that does not easily generate press cracks is manufactured. become able to.
C:0.0005%超0.10%未満
Cは、第2相の面積率を抑制しつつ、鋼板を高強度化するために必要な元素であるが、本発明では、後述するように、フェライト単相であっても、それに応じて均一伸びの面内異方性を制御できるので、C量の下限は溶製技術で通常可能な0.0005%超とする。一方、C量が0.10%以上になると、第2相の面積率が大きくなりすぎて延性の低下が起こり、また第2相がネットワークを組んでフェライト相を囲んでしまうため、フェライトの集合組織における均一伸びの面内異方性の制御が難しくなる。従って、C量は0.10%未満とし、好ましくは0.08%未満とする。 Next, the reason why the component composition of the steel sheet is limited to the above range in the high-strength steel sheet of the present invention will be described. “%” Representing the content of each component element means “% by mass” unless otherwise specified.
C: more than 0.0005% and less than 0.10% C is an element necessary for increasing the strength of the steel sheet while suppressing the area ratio of the second phase. In the present invention, as described later, Even if it exists, since the in-plane anisotropy of uniform elongation can be controlled accordingly, the lower limit of the C amount is set to more than 0.0005%, which is normally possible with a melting technique. On the other hand, when the amount of C is 0.10% or more, the area ratio of the second phase becomes too large and ductility is lowered, and the second phase forms a network and surrounds the ferrite phase. It becomes difficult to control the in-plane anisotropy of uniform elongation. Therefore, the C content is less than 0.10%, preferably less than 0.08%.
Siは、熱間圧延でのスケール生成を遅延させて表面品質を改善する効果、まためっき浴中あるいは合金化処理中の地鉄と亜鉛の合金化反応を適度に遅延させる効果、さらにはフェライトの加工硬化能を上げる効果等、種々の効果を有するので、Si量は0.01%以上とすることが好ましく、より好ましくは0.05%以上である。しかしながら、Si量が1.5%を超えると、外観品質が劣化すると共に、α→γ変態点が上昇し、熱間圧延をγ域で行うことができなくなり、集合組織が大きく変化する。そのため、鋼板の均一伸びの面内異方性が、制御できなくなってしまう。従って、Si量は1.5%以下とする。好ましくは1.2%以下である。 Si: 1.5% or less Si improves the surface quality by delaying scale formation during hot rolling, and moderately delays the alloying reaction between the iron and zinc in the plating bath or alloying process. Furthermore, since it has various effects such as an effect of increasing the work hardening ability of ferrite, the Si content is preferably 0.01% or more, more preferably 0.05% or more. However, if the Si content exceeds 1.5%, the appearance quality deteriorates and the α → γ transformation point rises, hot rolling cannot be performed in the γ region, and the texture changes greatly. For this reason, the in-plane anisotropy of the uniform elongation of the steel sheet cannot be controlled. Therefore, the Si content is 1.5% or less. Preferably it is 1.2% or less.
Mnは、FeSによる熱間延性を抑制するだけでなく、固溶強化元素としても活用することができる。そのため0.1%以上の添加が必要である。また、Mn量が0.1%に満たないと、粒成長性が良くなってしまい、面内異方性を制御する観点から好ましくない。
なお、Mnは、焼入性を高めるので、第2相にマルテンサイトを存在させて高強度化するのに有効な元素である。そのような複合組織化の観点からは1.0%以上の添加が好ましい。一方、その含有量が多すぎると、焼鈍過程におけるα→γ変態温度が低くなり、再結晶直後の微細なフェライト粒界あるいは再結晶途中の回復粒の界面にγ粒が生成してしまう。そのため、フェライト粒が展伸して不均一になると共に第2相が微細化して、延性が低下したり、均一伸びの面内異方性が制御できなくなる。従って、Mn量は3.0%以下とする。均一伸びの面内異方性を、一層高度に制御するためには、2.5%以下が好ましい。 Mn: 0.1% to 3.0% Mn not only suppresses hot ductility due to FeS, but can also be used as a solid solution strengthening element. Therefore, addition of 0.1% or more is necessary. On the other hand, if the Mn content is less than 0.1%, the grain growth is improved, which is not preferable from the viewpoint of controlling the in-plane anisotropy.
Note that Mn is an element effective for increasing the strength by making martensite present in the second phase because it enhances hardenability. From the viewpoint of such complex organization, addition of 1.0% or more is preferable. On the other hand, if the content is too large, the α → γ transformation temperature in the annealing process is lowered, and γ grains are generated at the fine ferrite grain boundary immediately after recrystallization or at the interface between recovered grains in the middle of recrystallization. For this reason, the ferrite grains expand and become non-uniform, and the second phase becomes fine, resulting in a decrease in ductility and in-plane anisotropy of uniform elongation cannot be controlled. Therefore, the Mn content is 3.0% or less. In order to control the in-plane anisotropy of uniform elongation to a higher degree, 2.5% or less is preferable.
Pは、従来より固溶強化元素として活用されており、また微量添加でも大きな焼入性の向上効果を有していることが明らかになった。このようなP添加による効果を得るには、P量を、0.005%以上とすることが好ましく、0.010%以上、さらには0.015%以上とするのがより好ましい。一方、P量が0.080%を超えると、地鉄とめっき層の合金化反応が著しく遅延して耐パウダリング性が劣化し、溶接性も劣化する。従って、P量は0.080%以下とし、好ましくは0.050%以下とする。 P: 0.080% or less It has been clarified that P has been conventionally used as a solid solution strengthening element and has a large effect of improving hardenability even when added in a small amount. In order to obtain such an effect by addition of P, the amount of P is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.015% or more. On the other hand, if the amount of P exceeds 0.080%, the alloying reaction between the base iron and the plating layer is remarkably delayed to deteriorate the powdering resistance and the weldability. Therefore, the P content is 0.080% or less, preferably 0.050% or less.
Sは、その含有量が多いと鋼中に析出するMnSが多くなり過ぎ、鋼板の伸びや伸びフランジ性といった延性を低下させて、プレス成形性を低下させる。また、スラブの熱間延性を低下させ、表面欠陥を発生させやすくする。さらには耐食性を、僅かではあるが低下させる。従って、S量は0.03%以下とする。なお、延性や耐食性を向上させる観点からは、S量を0.01%以下とすることが望ましく、0.002%以下とすることがさらに望ましい。 S: 0.03% or less If the content of S is large, too much MnS precipitates in the steel, and the ductility such as elongation and stretch flangeability of the steel sheet is lowered, and press formability is lowered. Moreover, it reduces the hot ductility of the slab and facilitates the generation of surface defects. Furthermore, the corrosion resistance is slightly reduced. Therefore, the S amount is 0.03% or less. From the viewpoint of improving ductility and corrosion resistance, the S content is preferably 0.01% or less, and more preferably 0.002% or less.
Alは、鋼の脱酸元素として有用であり、固溶Nを固定して耐常温時効性を向上させる作用があるため、sol.Al で0.01%以上含有させるものとする。一方、0.50%を超える添加は、製造のコスト高を招き、さらに鋼板の表面欠陥を誘発する。従って、Alは0.50%以下とし、好ましくは0.08%以下とする。 sol.Al: 0.01% or more and 0.50% or less Al is useful as a deoxidizing element for steel and has the effect of fixing solid solution N to improve normal temperature aging resistance. Shall be allowed to. On the other hand, the addition exceeding 0.50% increases the manufacturing cost and further induces surface defects in the steel sheet. Therefore, Al is 0.50% or less, preferably 0.08% or less.
Nは、多すぎると耐常温時効性を劣化させ、多量のAlやTiの添加が必要となるため、できるだけ低減することが好ましい。従って、上限を0.005%とする。 N: 0.005% or less N is preferably reduced as much as possible because if it is too much, the room temperature aging resistance deteriorates and a large amount of Al or Ti needs to be added. Therefore, the upper limit is made 0.005%.
Nb:0.20%以下
Nbは、組織を細粒化するとともに、熱間圧延工程でオーステナイトの再結晶を抑制して、冷延焼鈍後の均一伸びの面内異方性を制御することができるため、本発明において重要な元素である。しかしながら、0.20%を超えて添加すると、コストアップになるだけでなく熱延での集合組織が発達し過ぎること、および再結晶温度の過度な上昇によって均一伸びの面内異方性が制御できなくなるので、Nbは0.20%以下とする必要がある。好ましくは0.12%以下とする。なお、上記効果を得る上で、Nbは0.005%以上含有することが好ましい。 One or two selected from Nb: 0.20% or less and Ti: 0.20% or less
Nb: 0.20% or less Nb is capable of controlling the in-plane anisotropy of uniform elongation after cold rolling annealing by refining the structure and suppressing recrystallization of austenite in the hot rolling process. This is an important element in the present invention. However, if added over 0.20%, not only the cost is increased, but the texture in hot rolling is excessively developed, and the in-plane anisotropy of uniform elongation cannot be controlled due to excessive increase in the recrystallization temperature. Therefore, Nb needs to be 0.20% or less. Preferably it is 0.12% or less. In order to obtain the above effect, Nb is preferably contained in an amount of 0.005% or more.
Tiは、Nbと同様に、組織を細粒化し、熱間圧延工程でオーステナイトの再結晶を抑制して、冷延焼鈍後の均一伸びの面内異方性を制御するため、本発明において重要な元素である。しかしながら、0.20%を超えて添加すると、コストアップになるだけでなく熱延での集合組織が発達し過ぎること、および再結晶温度の過度な上昇によって均一伸びの面内異方性が制御できなくなるので、Tiは0.20%以下とする必要がある。好ましくは0.12%以下とする。なお、上記効果を得る上で、Tiは0.005%以上含有することが好ましい。 Ti: 0.20% or less Ti, like Nb, refines the structure, suppresses recrystallization of austenite in the hot rolling process, and controls the in-plane anisotropy of uniform elongation after cold rolling annealing. This is an important element in the present invention. However, if added over 0.20%, not only the cost is increased, but the texture in hot rolling is excessively developed, and the in-plane anisotropy of uniform elongation cannot be controlled due to excessive increase in the recrystallization temperature. Therefore, Ti needs to be 0.20% or less. Preferably it is 0.12% or less. In addition, when obtaining the said effect, it is preferable to contain Ti 0.005% or more.
Vは、焼入性を向上させる元素であり、めっき品質や耐食性を劣化させることが少ないので、MnやCrの代替として利用することができる。しかしながら、0.40%を超えて添加すると著しいコスト増になるので、Vは0.40%以下で添加することが望ましい。 V: 0.40% or less V is an element that improves hardenability, and since it hardly reduces the plating quality and corrosion resistance, it can be used as a substitute for Mn and Cr. However, if added over 0.40%, the cost will increase significantly, so it is desirable to add V at 0.40% or less.
Crは、Mnと同様に、鋼板を複合組織化して高強度化に寄与する元素である。この効果を得るためには、0.10%以上含有することが好ましい。しかしながら,過剰のCr添加は上記の効果を飽和させるだけでなく、高合金コストを招くため、上限を0.50%とする。 Cr: 0.50% or less Cr, like Mn, is an element that contributes to high strength by forming a composite structure of a steel sheet. In order to acquire this effect, it is preferable to contain 0.10% or more. However, excessive addition of Cr not only saturates the above effect, but also causes high alloy costs, so the upper limit is made 0.50%.
Moは、焼入性を向上させてパーライトの生成を抑制し、高強度化に寄与する元素である。しかしながら、Moは極めて高価な元素なので、その添加量が多いと著しいコストアップにつながる。従って、Moの添加量は0.50%以下とすることが好ましい。 Mo: 0.50% or less Mo is an element that improves hardenability, suppresses the formation of pearlite, and contributes to high strength. However, since Mo is an extremely expensive element, a large amount of addition leads to a significant cost increase. Therefore, the amount of Mo added is preferably 0.50% or less.
Wは、焼入性向上元素、析出強化元素として活用できる。しかしながら、その添加量が多すぎると延性の低下を招くので、Wの添加量は0.15%以下とすることが望ましい。 W: 0.15% or less W can be used as a hardenability improving element and a precipitation strengthening element. However, if the added amount is too large, ductility is reduced, so the added amount of W is preferably 0.15% or less.
Zrは、焼入性向上元素、析出強化元素として活用できる。しかしながら、その添加量が多すぎると延性の低下を招くので、Zrは0.10%以下で添加することが望ましい。 Zr: 0.10% or less Zr can be used as a hardenability improving element and a precipitation strengthening element. However, if the added amount is too large, ductility is lowered, so it is desirable to add Zr at 0.10% or less.
Cuは、混入を許容することでリサイクル資材を原料資材として活用することができ、製造コストを削減することができる。また耐食性向上の観点も加味すると、Cuを添加する場合は0.03%以上添加するのが望ましい。しかしながら、その含有量が多くなりすぎると表面欠陥の原因となるので、上限は0.50%とするのが望ましい。 Cu: 0.50% or less By allowing contamination, Cu can be used as a raw material, and manufacturing costs can be reduced. In consideration of improving corrosion resistance, it is desirable to add 0.03% or more when Cu is added. However, if the content is too large, it may cause surface defects, so the upper limit is preferably 0.50%.
Niは、耐食性を向上させる元素であり、Cuを含有させた場合に生じやすい表面欠陥を低減する作用がある。従って、耐食性を向上させつつ表面品質を改善する観点から、Niは0.02%以上添加するのが望ましい。一方、Niの添加量が多くなりすぎると加熱炉内でのスケール生成が不均一になり表面欠陥の原因になるとともに、著しいコスト増となる。従って、上限は0.50%とするのが望ましい。 Ni: 0.50% or less Ni is an element that improves corrosion resistance, and has the effect of reducing surface defects that are likely to occur when Cu is contained. Therefore, from the viewpoint of improving the surface quality while improving the corrosion resistance, it is desirable to add Ni by 0.02% or more. On the other hand, if the amount of Ni added is too large, scale generation in the heating furnace becomes non-uniform, causing surface defects and a significant cost increase. Therefore, the upper limit is desirably 0.50%.
Bは、鋼の焼入性を向上させる元素である。また、特にフェライト単相組織においては、二次加工脆性を抑制することができる。しかし、その含有量が0.0050%を超えるとその効果が飽和するため、添加する場合は0.0050%以下が好ましい。 B: 0.0050% or less B is an element that improves the hardenability of steel. In particular, in the ferrite single phase structure, secondary work brittleness can be suppressed. However, when the content exceeds 0.0050%, the effect is saturated. Therefore, when added, the content is preferably 0.0050% or less.
Snは、鋼板表面の窒化や酸化、あるいは酸化により生じる鋼板表層の脱炭や脱ボロン(脱B)を抑制する観点から添加するのが望ましい。窒化や酸化を抑制する観点からは0.005%以上添加することが望ましいが、0.20%を超えると、降伏強度(YP)の過度な上昇や靱性の劣化を招くのでSnは0.20%以下で含有させるのが望ましい。 Sn: 0.20% or less Sn is preferably added from the viewpoint of suppressing nitridation and oxidation of the steel sheet surface, or decarburization and deboronization (de-B) of the steel sheet surface layer caused by oxidation. From the viewpoint of suppressing nitriding and oxidation, it is desirable to add 0.005% or more, but if it exceeds 0.20%, excessive yield strength (YP) will be increased and toughness will be deteriorated, so Sn should be contained at 0.20% or less. Is desirable.
Sbは、Snと同様に、鋼板表面の窒化や酸化、あるいは酸化により生じる鋼板表層の脱炭や脱Bを抑制する観点から添加するのが望ましい。これら窒化や酸化を抑制することで、鋼板表層のマルテンサイトの生成量が減少するのを防止したり、Bの減少により焼入性が低下するのを防止したり、溶融亜鉛めっきの濡れ性を向上させてめっき外観品質を向上させたりすることができる。これら窒化や酸化を抑制する観点から、0.005%以上添加することが望ましいが、0.20%を超えるとYPの過度な上昇や靱性の劣化を招くのでSbは0.20%以下で含有させるのが望ましい。 Sb: 0.20% or less Sb is desirably added from the viewpoint of suppressing nitridation and oxidation of the steel sheet surface, or decarburization and de-B of the steel sheet surface layer caused by oxidation, as with Sn. By suppressing these nitriding and oxidation, it is possible to prevent a decrease in the amount of martensite produced on the surface layer of the steel sheet, to prevent a decrease in hardenability due to a decrease in B, and to improve the wettability of hot dip galvanizing. It is possible to improve the plating appearance quality. From the viewpoint of suppressing nitriding and oxidation, it is desirable to add 0.005% or more. However, if it exceeds 0.20%, excessive increase in YP and deterioration of toughness are caused, so Sb is desirably contained at 0.20% or less.
Caは、鋼中のSをCaSとして固定し、さらには腐食性生物中のpHを増加させ、ヘム加工部やスポット溶接部周辺の耐食性を向上させる作用がある。また、CaSが生成することにより、伸びフランジ性を低下させるMnSの生成を抑制し、伸びフランジ性を向上させる作用がある。これらの観点から、Caは0.0005%以上添加することが望ましい。しかしながら、Caは溶鋼中で酸化物として浮上分離しやすく、鋼中に多量添加することは難しい。従って、Caは0.010%以下で添加するのが望ましい。 Ca: 0.010% or less Ca has the effect of fixing S in steel as CaS, further increasing the pH in corrosive organisms, and improving the corrosion resistance around the hem-processed part and spot welded part. Moreover, the production | generation of CaS has the effect | action which suppresses the production | generation of MnS which reduces stretch flangeability and improves stretch flangeability. From these viewpoints, Ca is preferably added in an amount of 0.0005% or more. However, Ca easily floats and separates as an oxide in molten steel, and it is difficult to add a large amount to the steel. Therefore, it is desirable to add Ca at 0.010% or less.
Ceは、鋼中のSを固定する目的で添加することができる。しかし、高価な元素であるので多量添加するとコストアップになる。従って、Ceは0.01%以下で添加するのが望ましい。 Ce: 0.01% or less Ce can be added for the purpose of fixing S in steel. However, since it is an expensive element, adding a large amount increases the cost. Therefore, it is desirable to add Ce at 0.01% or less.
Laは、鋼中のSを固定する目的で添加することができる。しかし、高価な元素であるので多量添加するとコストアップになる。従って、Laは0.01%以下で添加するのが望ましい。
なお、上記した元素以外の残部は、Feおよび不可避不純物である。 La: 0.01% or less La can be added for the purpose of fixing S in steel. However, since it is an expensive element, adding a large amount increases the cost. Therefore, it is desirable to add La at 0.01% or less.
The balance other than the above elements is Fe and inevitable impurities.
鋼組織として、体積分率で60%以上のフェライト相
本発明では、フェライトの集合組織を制御しているが、集合組織を確認する場合、X線回折法が一般である。しかしながら、X線回折法では、フェライトの主相と、マルテンサイトやベイナイトなどの第2相とを明確に区別することができないために、第2相分率が高くなった場合、本発明の主眼とする集合組織制御による均一伸びの異方性を制御することができなくなる。また、第2相が増加すると、第2相がフェライトの周囲をネットワーク状に取り囲むようになり、鋼板のマクロ的な塑性挙動がフェライトの結晶方位に依存しなくなってしまう。
これらの理由から、本発明では、フェライトを体積分率で60%以上とする必要がある。さらに、75%以上の体積分率とすることが好ましい。
なお、フェライト相の体積分率は、以下の手順で求めた第2相の面積率を、第2相の体積分率とみなし、この第2相の体積分率に基づいて求めることができる。 Next, the structure of the high strength steel sheet of the present invention will be described.
Ferrite phase having a volume fraction of 60% or more as a steel structure In the present invention, the ferrite texture is controlled, but when confirming the texture, the X-ray diffraction method is generally used. However, in the X-ray diffraction method, since the main phase of ferrite cannot be clearly distinguished from the second phase such as martensite and bainite, the main point of the present invention is when the second phase fraction becomes high. It becomes impossible to control the anisotropy of uniform elongation by texture control. Further, when the second phase increases, the second phase surrounds the periphery of the ferrite in a network shape, and the macroscopic plastic behavior of the steel sheet does not depend on the crystal orientation of the ferrite.
For these reasons, in the present invention, the ferrite needs to be 60% or more in volume fraction. Furthermore, the volume fraction is preferably 75% or more.
The volume fraction of the ferrite phase can be obtained on the basis of the volume fraction of the second phase by regarding the area ratio of the second phase obtained by the following procedure as the volume fraction of the second phase.
また、第2相の面積率の求め方は、正方メッシュを用いて、その格子上に第2相が存在する割合から求める(ポイントカウント法)。かようにして求めた第2相の面積率(%)を、本発明では、そのまま第2相の体積分率(%)とする。さらに、フェライト相の体積分率(%)は、100%から、第2相の体積分率(%)を引くことで求められる。 The area ratio of the second phase is obtained from a photograph of the structure taken after polishing the L cross section (vertical cross section parallel to the rolling direction) of the steel plate, corroding with nital, observing 10 fields of view with a SEM at a magnification of 4000 times. In the micrograph, the ferrite is observed as a slightly black contrast region, the region where the carbides are formed in a lamellar or dotted pattern is pearlite and bainite, and the particles with white contrast are martensite or residual γ. is there. Note that the fine dot-like particles having a diameter of 0.4 μm or less recognized on the SEM photograph are mainly carbides, and since their area ratio is very small, it is considered that the material is hardly affected. Therefore, in the present invention, particles having a particle size of 0.4 μm or less are excluded from the area ratio evaluation.
Further, the area ratio of the second phase is obtained from the ratio of the second phase existing on the lattice using a square mesh (point count method). In the present invention, the area ratio (%) of the second phase thus determined is directly used as the volume fraction (%) of the second phase. Further, the volume fraction (%) of the ferrite phase can be obtained by subtracting the volume fraction (%) of the second phase from 100%.
均一伸びの異方性を制御する要件としては、以下のΦ=35°のODF強度が最も重要であるが、{φ1,Φ,φ2}={0°,0°,45°}の方位の強度が高いと、絞り性が低下して、プレス成形能が低下する。従って、{0°,0°,45°}の強度は3.0以下とする。 Intensity of ODF {0 °, 0 °, 45 °} when Φ is 0 °, φ1 is 0 °, and φ2 is 45 ° in the three-dimensional crystal orientation density function (ODF) {φ1, Φ, φ2} Is 3.0 or less As a requirement for controlling the anisotropy of uniform elongation, the following ODF strength of Φ = 35 ° is most important, but {φ1, Φ, φ2} = {0 °, 0 °, 45 °} If the strength of the orientation is high, the drawability is lowered, and the press forming ability is lowered. Therefore, the intensity of {0 °, 0 °, 45 °} is 3.0 or less.
前述したように、均一伸びの異方性を制御する要件として、3次元結晶方位密度関数(ODF)、{φ1,Φ,φ2}={0°,35°,45°}の方位の強度を、2.5以上4.5以下の範囲とする必要がある。というのは、その強度が2.5に満たないと、圧延方向と圧延方向に直角な方向(以下、圧延直角方向という)、特に圧延方向の均一伸びが相対的に低くなるため、プレス割れが発生しやすくなるからである。
一方、その強度が4.5を超えると、D方向(圧延方向と45°をなす方向)の均一伸びが相対的に低くなる。これは、鋼板の集合組織は降伏強度の異方性に関係するために、降伏強度と延性がトレードオフの関係となり、高強度の方向は均一伸びが低くなり、加えて、TiやNbが圧延方向に結晶粒を展伸させるなどして、組織的な延性の異方性が影響していることなどが考えられる。
なお、上記以外の他の方位、例えばγ-fiberの強度は、均一伸びの異方性に影響を与えないため、いずれの方位も特に制限はない。 Intensity of ODF {0 °, 35 °, 45 °} when Φ is 35 °, φ1 is 0 °, and φ2 is 45 ° in the density function (ODF) {φ1, Φ, φ2} of the three-dimensional crystal orientation Is 2.5 or more and 4.5 or less As described above, as a requirement for controlling the anisotropy of uniform elongation, the three-dimensional crystal orientation density function (ODF), {φ1, Φ, φ2} = {0 °, 35 °, 45 °} It is necessary to set the intensity of the orientation of the range from 2.5 to 4.5. This is because if the strength is less than 2.5, the rolling direction and the direction perpendicular to the rolling direction (hereinafter referred to as the direction perpendicular to the rolling direction), particularly the uniform elongation in the rolling direction, are relatively low, so press cracks occur. This is because it becomes easier.
On the other hand, when the strength exceeds 4.5, the uniform elongation in the D direction (a direction that forms 45 ° with the rolling direction) is relatively low. This is because the texture of the steel sheet is related to the anisotropy of the yield strength, so the yield strength and ductility are in a trade-off relationship, the direction of high strength has a low uniform elongation, and Ti and Nb are rolled. It is conceivable that the systematic ductility anisotropy is affected by extending the crystal grains in the direction.
It should be noted that other orientations other than the above, for example, the strength of γ-fiber, does not affect the anisotropy of uniform elongation, and thus any orientation is not particularly limited.
まず、(200)(211)(110)の3面の極点図を反射法により測定して、3つの不完全極点図を求める。ついで、これら3つの不完全極点図を、級数展開法により3次元結晶方位密度関数とし、それぞれ求めたい方位の強度を求める。 The azimuth strength in the present invention is determined as follows.
First, the pole figures of the three surfaces (200), (211), and (110) are measured by the reflection method to obtain three incomplete pole figures. Next, these three incomplete pole figures are converted into a three-dimensional crystal orientation density function by the series expansion method, and the strength of the desired orientation is obtained.
本発明の製造方法で使用する鋼スラブは、成分のマクロ偏析を防止すべく連続鋳造法製造することが望ましいが、特に限定されることはなく、造塊法や薄スラブ鋳造法で製造してもよい。また、鋼スラブを製造した後、いったん室温まで冷却し、その後再度加熱する従来法に加え、冷却せず温片のままで加熱炉に装入し熱間圧延する直送圧延や、わずかの保熱を行った後、直ちに熱間圧延する直送圧延・直接圧延などの省エネルギープロセスも問題なく適用することができる。 Next, the manufacturing method of the high intensity | strength hot-rolled steel plate of this invention is demonstrated.
The steel slab used in the production method of the present invention is preferably produced by a continuous casting method in order to prevent macro segregation of components, but is not particularly limited, and is produced by an ingot-making method or a thin slab casting method. Also good. In addition to the conventional method in which the steel slab is manufactured and then cooled to room temperature and then heated again, direct feed rolling in which the steel slab is charged without being cooled and charged in a heating furnace and hot-rolled, or a little heat retention After carrying out, energy saving processes such as direct feed rolling and direct rolling that are immediately hot rolled can be applied without any problem.
シートバーを仕上げ圧延して熱延板とする。仕上げ温度、すなわち仕上圧延出側温度(以下、FTという)は820℃以上950℃以下とする。これは、冷間圧延および再結晶焼鈍後に均一伸びの面内異方性に好ましい集合組織を得るためである。FTが820℃に満たない場合、圧延負荷が大きくなると共に、一部の成分系ではフェライト域の圧延となり、集合組織が大きく変化する。一方、950℃を超えた場合、組織が粗大化すると共に、十分にオーステナイト未再結晶状態で圧延できないため、冷延焼鈍後に、D方向の均一伸びが低下してしまう。 Finishing temperature: 820 ° C or more and 950 ° C or less Finishing and rolling the sheet bar to make a hot-rolled sheet. The finishing temperature, that is, the finish rolling outlet temperature (hereinafter referred to as FT) is 820 ° C. or higher and 950 ° C. or lower. This is to obtain a texture preferable for in-plane anisotropy of uniform elongation after cold rolling and recrystallization annealing. When FT is less than 820 ° C., the rolling load increases, and in some component systems, the ferrite region is rolled and the texture changes greatly. On the other hand, when the temperature exceeds 950 ° C., the structure becomes coarse and cannot be rolled sufficiently in the austenite non-recrystallized state, so that the uniform elongation in the D direction is lowered after cold rolling annealing.
本発明における冷間圧延では、圧下率(X%)を以下の(1)式の関係を満足するように行う必要がある。
0.30≦{1.6・([%Ti]+2・[%Nb])+0.004X}≦0.36 ・・・ (1)
TiおよびNbは熱間圧延のオーステナイト未再結晶域での圧延を行うのに重要な元素であり、未再結晶圧延によるγの集合組織の発達と、その後の変態時のバリアント制約のため、圧延集合組織に大きく関与する。また、圧下率は、圧延集合組織を発達させるのに重要な条件となる。そこで、発明者らは、フェライト粒径の粗大化による集合組織と、上記未再結晶γ圧延による集合組織が、伸びの面内異方性に対して反対の効果があるため、両者をバランスさせるとの考えのもと、Ti,Nbおよび圧下率を変化させた鋼で、均一伸びの面内異方性およびODF{0°,35°,45°}の強度f(φ35°)を調べた。また、得られた結果中、TiとNbについては、析出物と固溶状態での再結晶抑制効果および原子量の違いの観点から、Nb量([%Nb])はTi量([%Ti])に比して約2倍の影響を与えるものと考えて、これら、Ti,Nb量と圧下率がf(φ35°)に与える影響および均一伸びの面内異方性との関係を評価した。その結果を図1(a)、(b)に示す。また、f(φ35°)と均一伸びの関係を図1(c)に示す。
特に、図1(a)より、{1.6・([%Ti]+2・[%Nb])+0.004X}とf(φ35°)とが良好な対応関係にあること、また図1(b)、(c)より、上掲(1)式を満足することで、f(φ35°)の値を2.5以上4.5以下の範囲に制御することができ、さらに均一伸びの面内異方性を小さくできることが分かった。 Subsequently, the hot-rolled sheet is subjected to pickling and cold rolling to perform a cold rolling process to obtain a cold-rolled sheet. Pickling may be performed under normal conditions.
In the cold rolling in the present invention, it is necessary to perform the rolling reduction (X%) so as to satisfy the relationship of the following expression (1).
0.30 ≦ {1.6 ・ ([% Ti] +2 ・ [% Nb]) + 0.004X} ≦ 0.36 (1)
Ti and Nb are important elements for rolling in the austenite non-recrystallized region of hot rolling. Rolling due to the development of γ texture due to non-recrystallizing rolling and the variant restrictions during the subsequent transformation. Greatly involved in the texture. Further, the rolling reduction is an important condition for developing the rolling texture. Therefore, the inventors balance the texture because the texture due to the coarsening of the ferrite grain size and the texture due to the non-recrystallized γ rolling have opposite effects on the in-plane anisotropy of elongation. Based on this idea, the in-plane anisotropy of uniform elongation and the strength f (φ35 °) of ODF {0 °, 35 °, 45 °} were investigated for steels with varying Ti, Nb and rolling reduction. . In addition, among the obtained results, for Ti and Nb, the amount of Nb ([% Nb]) is the amount of Ti ([% Ti]) from the viewpoint of the recrystallization suppression effect in the solid solution state with the precipitate and the difference in atomic weight. The effect of Ti, Nb content and rolling reduction on f (φ35 °) and the in-plane anisotropy of uniform elongation were evaluated. . The results are shown in FIGS. 1 (a) and (b). The relationship between f (φ35 °) and uniform elongation is shown in FIG.
In particular, from Fig. 1 (a), {1.6 · ([% Ti] + 2 · [% Nb]) + 0.004X} and f (φ35 °) are in a good correspondence, and Fig. 1 (b) From (c), by satisfying the above equation (1), the value of f (φ35 °) can be controlled in the range of 2.5 to 4.5 and the in-plane anisotropy of uniform elongation can be reduced. I understood that I could do it.
ΔUEL={ (UELL/UELL)+(UELC/UELL)-2・(UELD/UELL) }/2
となるため、まとめると、
ΔUEL={ UELL+UELC-2・UELD}/(2・UELL)・・・(2)
(UELLで規格化した均一伸びで異方性を評価)
となる。 Since the in-plane anisotropy differs in the absolute value of ductility depending on the strength level of the steel sheet, the anisotropy was evaluated by uniform elongation ΔUEL normalized by UEL L using the following formula (2). In addition, let the uniform elongation of the L, D, C direction of a steel plate be UEL L , UEL D , UEL C , respectively.
ΔUEL = {(UEL L / UEL L ) + (UEL C / UEL L ) -2 ・ (UEL D / UEL L )} / 2
So, in summary,
ΔUEL = {UEL L + UEL C −2 · UEL D } / (2 · UEL L ) (2)
(Evaluation of anisotropy with uniform elongation standardized by UEL L )
It becomes.
冷延の加工歪を抑制するために再結晶温度以上にする。というのは、延性が低下してしまうからである。一方、900℃を超えると、焼鈍炉体の寿命が短くなるだけでなく、異常粒成長したり、γ分率が高くなるなどして、逆変態後の集合組織が大きく変化してしまう。ここで、再結晶温度は、冷延板を、焼鈍温度を変化させて所定の温度に到達後、直ちに(1s以内の保持時間で)冷却する短時間焼鈍を施し、直後に水に焼入れて組織観察を行い、未再結晶が認められなくなる温度として求めればよい。なお、焼鈍温度は、例えば650℃から10℃ピッチで変化せれば良い。 Next, it is annealed at an annealing temperature not lower than the recrystallization temperature and not higher than 900 ° C. and cooled.
In order to suppress cold-rolling processing strain, the recrystallization temperature is set. This is because the ductility is reduced. On the other hand, when the temperature exceeds 900 ° C., not only the life of the annealing furnace body is shortened, but also abnormal grain growth and the γ fraction increase, and the texture after reverse transformation changes greatly. Here, the recrystallization temperature is obtained by subjecting the cold-rolled plate to a predetermined temperature by changing the annealing temperature, immediately after cooling (with a holding time within 1 s), and immediately quenching into cold water. The temperature may be determined as a temperature at which unrecrystallization is not observed. Note that the annealing temperature may be changed at a pitch of 10 ° C. from 650 ° C., for example.
ここに、本発明では、マルテンサイトを含む第2相の存在が許容されることから、500℃までの平均冷却速度が臨界冷却速度以上であることが望ましく、これを達成するためには、概ね5℃/s以上が必要である。一方、15℃/sを超えると、複合組織にはなるものの、第2相分率が高くなって延性には好ましくない分布となる。このため、5℃/s以上15℃/s以下として冷却することが好ましい。
また、500℃以下の冷却については、それまでの冷却によりγ相はある程度安定化するので、特に限定はしないが、引き続き、300℃程度までの温度範囲を5℃/s以上の平均冷却速度で冷却することが好ましく、過時効処理を施す場合は、過時効処理温度までを平均冷却速度で5℃/s以上とすることが好ましい。 Although the cooling rate after the annealing is not particularly specified, when martensite is formed as the second phase, the average cooling rate in the temperature range from the annealing temperature to 500 ° C is 5 ° C / s to 15 ° C / s. It is preferable to cool in the following range. If the average cooling rate in this temperature range is less than 5 ° C./s, martensite is hardly formed, and a ferrite single phase structure may be formed, resulting in insufficient structure strengthening.
Here, in the present invention, since the presence of the second phase containing martensite is allowed, it is desirable that the average cooling rate up to 500 ° C. is equal to or higher than the critical cooling rate. In order to achieve this, 5 ° C / s or more is necessary. On the other hand, when it exceeds 15 ° C./s, although it becomes a composite structure, the second phase fraction becomes high and the distribution is not preferable for ductility. For this reason, it is preferable to cool at 5 ° C./s or more and 15 ° C./s or less.
In addition, the cooling at 500 ° C. or lower is not particularly limited because the γ phase is stabilized to some extent by the cooling so far, but subsequently, the temperature range up to about 300 ° C. is maintained at an average cooling rate of 5 ° C./s or more. It is preferable to cool, and when performing an overaging treatment, it is preferable that the average cooling rate is 5 ° C./s or more up to the overaging treatment temperature.
なお、各鋼板の再結晶温度は、前述したように、短時間加熱焼入れ後の組織観察により求めたが、700~760℃であり、全ての条件で再結晶温度以上であった。
また、No.5の鋼板は、連続溶融亜鉛めっきラインにて冷延板焼鈍工程を施し、ついで、インラインで溶融亜鉛めっき(めっき浴温:480℃)を施して溶融亜鉛めっき鋼板とした。
得られた冷延焼鈍板および溶融亜鉛めっき鋼板から試験片を採取し、以下に示す方法で、微視組織、引張特性、および3次元結晶方位の密度関数(ODF)を求めた。 Steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) by a continuous casting method. Subsequently, these steel slabs were heated to 1250 ° C. and rough-rolled to obtain sheet bars. Subsequently, it hot-rolled by finish-rolling on the conditions shown in Table 2, and was set as the hot rolled sheet. These hot-rolled sheets were pickled and then made into cold-rolled sheets by a cold rolling process with a rolling reduction (CR) shown in Table 2. Subsequently, these cold-rolled sheets were subjected to continuous annealing at the annealing temperature (AnnT) shown in Table 2 in a continuous annealing line. Further, the obtained cold-rolled annealed sheet was subjected to temper rolling with an elongation of 0.5%.
As described above, the recrystallization temperature of each steel plate was 700 to 760 ° C., which was obtained by observation of the structure after heat-quenching for a short time, and was above the recrystallization temperature under all conditions.
In addition, the No. 5 steel sheet was subjected to a cold-rolled sheet annealing process in a continuous hot dip galvanizing line, followed by in-line hot dip galvanizing (plating bath temperature: 480 ° C.) to obtain a hot dip galvanized steel sheet.
Test pieces were collected from the obtained cold-rolled annealed sheet and hot-dip galvanized steel sheet, and the microstructure, tensile properties, and density function (ODF) of three-dimensional crystal orientation were determined by the following methods.
各々得られた冷延焼鈍板から圧延方向に対して0°(L方向)、45°(D方向)、90°方向(C方向)にJIS 5号引張試験片を採取し、JIS Z 2241の規定に準拠してクロスヘッド速度:10mm/minで引張試験をおこない、降伏応力(YS)、引張強さ(TS)および各方向の均一伸び(UEL)を求めた。 ・ Tensile properties JIS No. 5 tensile test specimens were collected from each of the obtained cold-rolled annealed plates at 0 ° (L direction), 45 ° (D direction), and 90 ° direction (C direction) with respect to the rolling direction. A tensile test was performed at a crosshead speed of 10 mm / min in accordance with the provisions of Z 2241 to determine yield stress (YS), tensile strength (TS), and uniform elongation (UEL) in each direction.
前述の方法にて、{φ1,Φ,φ2}における、Φが0°で、φ1が0°、φ2が45°のときのODF{0°,0°,45°}、{φ1,Φ,φ2}における、Φが35°で、φ1が0°、φ2が45°のときのODF{0°,35°,45°}の強度をそれぞれ求めた。 -Density function of three-dimensional crystal orientation ODF {0 °, 0 °, 45 when φ is 0 °, φ1 is 0 °, φ2 is 45 ° in {φ1, Φ, φ2} °}, the intensity of ODF {0 °, 35 °, 45 °} when φ is 35 °, φ1 is 0 °, and φ2 is 45 ° in {φ1, Φ, φ2}, respectively.
均一伸びの面内異方性は、以下の(2)式でΔUELの値を求めて評価した。なお、本発明では、以下の(2)式で求められるΔUELの値が-0.020~0.020の範囲であれば均一伸びの面内異方性に優れているといえる。
得られた結果を表2に併記する。
ΔUEL={ UELL+UELC-2・UELD}/(2・UELL)・・・(2)
なお、微視組織(フェライト体積分率)は、前述したとおりに、SEM写真からのポイントカウント法により求めた第2相の面積率(体積率)に基づき求めた。 -In-plane anisotropy of uniform elongation The in-plane anisotropy of uniform elongation was evaluated by obtaining a value of ΔUEL by the following equation (2). In the present invention, it can be said that the in-plane anisotropy of uniform elongation is excellent when the value of ΔUEL obtained by the following equation (2) is in the range of -0.020 to 0.020.
The obtained results are also shown in Table 2.
ΔUEL = {UEL L + UEL C −2 · UEL D } / (2 · UEL L ) (2)
Note that the microstructure (ferrite volume fraction) was determined based on the area ratio (volume ratio) of the second phase determined by the point counting method from the SEM photograph as described above.
これに対し、鋼成分が本発明の範囲を外れたり、本発明の範囲を外れる条件で製造した比較例では、集合組織が本発明範囲の含まれず、その結果として均一伸びの異方性が大きくなった鋼板となっている。 As shown in Table 2, in all the inventive examples, the intensity of {φ1, Φ, φ2} = {0 °, 0 °, 45 °} is 3.0 or less, and {φ1, Φ, φ2} = {0 ° , 35 °, 45 °} have a strength of 2.5 to 4.5 and a ΔUEL value in the range of −0.020 to 0.020 and excellent in-plane anisotropy of uniform elongation.
On the other hand, in the comparative example manufactured under conditions where the steel component is out of the scope of the present invention or out of the scope of the present invention, the texture is not included in the scope of the present invention, and as a result, the anisotropy of uniform elongation is large. It has become a steel plate.
Claims (3)
- 質量%で、C:0.0005%超0.10%未満、Si:1.5%以下、Mn:0.1%以上3.0%以下、P: 0.080%以下、S:0.03%以下、sol.Al:0.01%以上0.50%以下およびN:0.005%以下を含み、かつNb:0.20%以下およびTi:0.20%以下のうちから選んだ1種または2種を含有し、残部はFeおよび不可避不純物の組成からなり、
鋼組織を、体積分率で60%以上のフェライト相とし、
3次元結晶方位の密度関数(ODF){φ1,Φ,φ2}において、Φが0°で、φ1が0°、φ2が45°のときのODF{0°,0°,45°}の強度が3.0以下で、かつΦが35°で、φ1が0°、φ2が45°のときのODF{0°,35°,45°}の強度が2.5以上4.5以下の範囲である高強度鋼板。 In mass%, C: more than 0.0005% and less than 0.10%, Si: 1.5% or less, Mn: 0.1% or more and 3.0% or less, P: 0.080% or less, S: 0.03% or less, sol.Al: 0.01% or more and 0.50% or less And N: 0.005% or less, and Nb: 0.20% or less and Ti: 0.2% or less selected from one or two kinds, the balance is composed of Fe and inevitable impurities,
The steel structure is a ferrite phase with a volume fraction of 60% or more,
Intensity of ODF {0 °, 0 °, 45 °} when Φ is 0 °, φ1 is 0 °, and φ2 is 45 ° in the density function (ODF) {OD1, φ, φ2} of the three-dimensional crystal orientation Is a high-strength steel sheet in which the strength of ODF {0 °, 35 °, 45 °} is 2.5 to 4.5 when φ is 3.0 or less, Φ is 35 °, φ1 is 0 °, and φ2 is 45 °. - 前記鋼板が、さらに、質量%で、V:0.40%以下、Cr:0.50%以下、Mo:0.50%以下、W:0.15%以下、Zr:0.10%以下、Cu:0.50%以下、Ni:0.50%以下、B:0.0050%以下、Sn:0.20%以下、Sb:0.20%以下、Ca:0.010%以下、Ce:0.01%以下およびLa:0.01%以下のうちから選んだ少なくとも1種を含有する請求項1に記載の高強度鋼板。 Further, the steel sheet is further mass%, V: 0.40% or less, Cr: 0.50% or less, Mo: 0.50% or less, W: 0.15% or less, Zr: 0.10% or less, Cu: 0.50% or less, Ni: 0.50% Claims containing at least one selected from B: 0.0050% or less, Sn: 0.20% or less, Sb: 0.20% or less, Ca: 0.010% or less, Ce: 0.01% or less, and La: 0.01% or less 1. A high-strength steel sheet according to 1.
- 請求項1または2に記載の成分組成からなる鋼スラブを、仕上げ温度:820℃以上950℃以下の範囲で熱間圧延を行ったのち、圧下率(X%)が下記(1)式の関係を満足する条件で冷間圧延を施し、ついで再結晶温度以上900℃以下の温度域で連続焼鈍を施し、その後冷却する高強度鋼板の製造方法。
記
0.30≦{1.6・([%Ti]+2・[%Nb])+0.004X}≦0.36 ・・・(1)
ただし、[%A]はA元素の鋼中含有量(質量%)を示す。 The steel slab having the component composition according to claim 1 or 2 is subjected to hot rolling at a finishing temperature of 820 ° C. or higher and 950 ° C. or lower, and the reduction ratio (X%) is expressed by the following formula (1): A method for producing a high-strength steel sheet, in which cold rolling is performed under conditions that satisfy the above conditions, followed by continuous annealing in a temperature range from the recrystallization temperature to 900 ° C. and then cooling.
Record
0.30 ≦ {1.6 ・ ([% Ti] +2 ・ [% Nb]) + 0.004X} ≦ 0.36 (1)
However, [% A] indicates the content (mass%) of element A in steel.
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