WO2012161248A1 - 熱延鋼板及びその製造方法 - Google Patents
熱延鋼板及びその製造方法 Download PDFInfo
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- C21D2211/00—Microstructure comprising significant phases
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- Y10T428/12771—Transition metal-base component
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Definitions
- the present invention is a high-strength hot-rolled steel sheet excellent in both uniform deformability that contributes to stretch workability and drawability, and local deformability that contributes to bendability, stretch flangeability, burring workability, and the like. It relates to the manufacturing method.
- the present invention relates to a steel sheet having a DP (Dual Phase) structure.
- Non-Patent Document 1 discloses that by increasing the strength of a steel sheet, the uniform elongation important for drawing and overhanging decreases.
- Non-Patent Document 2 discloses a method of ensuring uniform elongation even with the same strength by compounding the metal structure of a steel plate.
- Non-Patent Document 3 describes a metal structure in which local ductility represented by bendability, hole expansibility and burring workability is improved by inclusion control, single structure formation, and reduction in hardness difference between structures.
- a control method is disclosed. This is to improve the local deformability that contributes to hole expansibility and the like by making the steel sheet into a single structure by structure control.
- heat treatment from an austenite single phase is the basis of the manufacturing method.
- Non-Patent Document 4 the strength of the steel sheet is obtained by obtaining preferable forms of precipitates and transformation structures and appropriate fractions of ferrite and bainite by controlling the metal structure by cooling control after hot rolling. And a technology that achieves both ductility and the ductility are disclosed.
- any of the above techniques is a method for improving local deformability that relies on tissue control, and is greatly influenced by the formation of the base structure.
- Non-Patent Document 5 discloses that a steel plate is made by refining the crystal grains of ferrite, which is the main phase of the product, by performing large pressure reduction in the lowest temperature region within the austenite region and transforming from unrecrystallized austenite to ferrite. A technique for increasing the strength and toughness of the steel is disclosed. However, in Non-Patent Document 5, no consideration is given to means for improving the local deformability that the present invention intends to solve.
- an object is to provide a hot-rolled steel sheet with less formability orientation dependency (anisotropic) and a method for producing the same.
- “strength” mainly means tensile strength
- “high strength” means a tensile strength of 440 MPa or more.
- high strength and excellent in uniform deformability and local deformability include tensile strength (TS), uniform elongation (u-EL), hole expansion ratio ( ⁇ ), and plate thickness d.
- TS ⁇ 440 (unit: MPa), TS ⁇ u-EL ⁇ 7000 (unit: MPa ⁇ %), TS ⁇ ⁇ ⁇ 30000 using the characteristic value of d / RmC, which is the ratio to the minimum C-direction bending radius RmC (Unit: MPa ⁇ %) and d / RmC ⁇ 1 (no unit) all the conditions are satisfied simultaneously.
- d / RmC which is the ratio to the minimum C-direction bending radius RmC (Unit: MPa ⁇ %) and d / RmC ⁇ 1 (no unit) all the conditions are satisfied simultaneously.
- the improvement of local deformability that contributes to hole expandability and bendability is the inclusion control, precipitate refinement, structure homogenization, single structure, and between structures This was done by reducing the hardness difference.
- these technologies alone must limit the main organizational structure.
- the anisotropy becomes extremely large when Nb, Ti, or the like, which is a representative element that greatly contributes to an increase in strength, is added to increase the strength. Therefore, other formability factors must be sacrificed or the direction of blank removal before molding must be limited, and the application is limited.
- the uniform deformability can be improved by dispersing a hard structure such as martensite in the metal structure.
- the present inventors have newly added a metal of the steel plate.
- a metal of the steel plate In addition to controlling the fraction and form of the structure, we focused on the influence of the texture of the steel sheet, and investigated and studied its effects in detail. As a result, by controlling the chemical composition of the steel sheet, the metal structure, and the texture represented by the extreme density of each orientation of a specific crystal orientation group, the strength is high and the rolling direction and the rolling direction are 90 °.
- the gist of the present invention is as follows.
- the chemical composition of the steel sheet is mass%, C: 0.01% or more and 0.4% or less, Si: 0.001% or more, and 2.5 %: Mn: 0.001% or more and 4.0% or less, Al: 0.001% or more and 2.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.01% or less, O: 0.01% or less, with the balance being iron and unavoidable impurities; center of thickness in the range of 5/8 to 3/8 thickness from the surface of the steel plate Part is represented by an arithmetic average of polar densities of each crystal orientation of ⁇ 100 ⁇ ⁇ 011>, ⁇ 116 ⁇ ⁇ 110>, ⁇ 114 ⁇ ⁇ 110>, ⁇ 112 ⁇ ⁇ 110>, ⁇ 223 ⁇ ⁇ 110>.
- the average pole density of the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups which is the pole density to be applied, is 1.0 to 5.0 And ⁇ 332 ⁇ ⁇ 113> crystal orientation pole density is 1.0 or more and 4.0 or less;
- the metal structure of the steel sheet has a plurality of crystal grains, and the metal structure has an area
- the ratio of ferrite and bainite is 30% or more and 99% or less, martensite is 1% or more and 70% or less; the martensite area ratio is fM in unit area%, and the average size of martensite is
- the unit ⁇ m is dia
- the average distance between the martensites is dis in the unit ⁇ m
- the tensile strength of the steel sheet is TS in the unit of MPa
- the chemical composition of the steel sheet further includes, in mass%, Mo: 0.001% or more and 1.0% or less, Cr: 0.001% or more and 2.0% or less, Ni: 0.001% to 2.0%, Cu: 0.001% to 2.0%, B: 0.0001% to 0.005%, Nb: 0.001% to 0.2%, Ti: 0.001% to 0.2%, V: 0.001% to 1.0%, W: 0.001% to 1.
- a volume average diameter of the crystal grains may be 5 ⁇ m or more and 30 ⁇ m or less.
- the average pole density of the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups is 1.0 or more and 4.0 or less.
- the pole density of the crystal orientation of ⁇ 332 ⁇ ⁇ 113> may be 1.0 or more and 3.0 or less.
- the area ratio may be 50% or more and 100% or less with respect to the martensite area ratio fM.
- the metal structure may include the ferrite in an area ratio of 30% to 99%.
- the metal structure may include the bainite in an area ratio of 5% or more and 80% or more.
- the martensite may contain tempered martensite.
- the area ratio of coarse crystal grains having a grain size exceeding 35 ⁇ m among the crystal grains in the metal structure of the steel sheet May be 0% or more and 10% or less.
- the hardness H of the ferrite may satisfy the following formula 4. H ⁇ 200 + 30 ⁇ [Si] + 21 ⁇ [Mn] + 270 ⁇ [P] + 78 ⁇ [Nb] 1/2 + 108 ⁇ [Ti] 1/2 (Formula 4) (11)
- a value obtained by dividing the standard deviation of the hardness by the average value of the hardness may be 0.2 or less.
- the method for producing a hot-rolled steel sheet according to an aspect of the present invention is, in mass%, C: 0.01% or more and 0.4% or less, Si: 0.001% or more and 2.5% or less, Mn: 0.001% or more and 4.0% or less, Al: 0.001% or more and 2.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0 .01% or less, O: limited to 0.01% or less, with a balance of 40% or more in a temperature range of 1000 ° C. or more and 1200 ° C. or less with respect to a steel having a chemical composition consisting of iron and inevitable impurities.
- the first hot rolling including at least one pass of the rolling reduction is performed, the average austenite grain size of the steel is set to 200 ⁇ m or less; the temperature calculated by the following formula 5 is set to T1 in the unit ° C., and the following formula 6
- T1 + 30 comprise ° C. or more and T1 + 200 ° C. temperature range below 30% or higher reduction ratio of the large reduction path, and the cumulative rolling reduction in the temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less 50% or more, Ar 3 or more and
- the steel is subjected to a second hot rolling in which the cumulative rolling reduction in the temperature range below T1 + 30 ° C.
- the steel temperature at the start of cooling is subjected to primary cooling in which the change in cooling temperature, which is the difference from the steel temperature at the end of cooling, is 40 ° C. or higher and 140 ° C. or lower, and the steel temperature at the end of cooling is T1 + 100 ° C. or lower; Second hot rolling After completion, the steel is secondarily cooled to a temperature range of 600 ° C. to 800 ° C.
- Tf is the temperature in degrees Celsius of the steel at the completion of the final pass
- P1 is a percentage of the rolling reduction in the final pass.
- the steel further has, as the chemical composition, mass%, Mo: 0.001% to 1.0%, Cr: 0 0.001% to 2.0%, Ni: 0.001% to 2.0%, Cu: 0.001% to 2.0%, B: 0.0001% to 0.005 %: Nb: 0.001% to 0.2%, Ti: 0.001% to 0.2%, V: 0.001% to 1.0%, W: 0.001 % To 1.0%, Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01%, Zr: 0.0001% to 0.2% , Rare Earth Metal: 0.0001% or more and 0.1% or less, As: 0.
- the temperature calculated by 9 may be T1.
- T1 850 + 10 ⁇ ([C] + [N]) ⁇ [Mn] + 350 ⁇ [Nb] + 250 ⁇ [Ti] + 40 ⁇ [B] + 10 ⁇ [Cr] + 100 ⁇ [Mo] + 100 ⁇ [V]
- [C], [N], [Mn], [Nb], [Ti], [B], [Cr], [Mo] and [V] are C, N, Mn, Nb, It is a mass percentage of Ti, B, Cr, Mo and V.
- the waiting time t may further satisfy the following formula 10.
- the waiting time t may further satisfy the following formula 11. t1 ⁇ t ⁇ t1 ⁇ 2.5 (Expression 11)
- the first hot rolling is performed at least twice or more at a reduction rate of 40% or more.
- the average austenite particle size may be 100 ⁇ m or less.
- the secondary cooling is started within 3 seconds after the end of the second hot rolling. May be.
- the temperature increase of the steel between each pass is set to 18 ° C. or less in the second hot rolling. Also good.
- a hot-rolled steel sheet that has little influence on anisotropy even when elements such as Nb and Ti are added, has high strength, and is excellent in local deformability and uniform deformability. Obtainable.
- Average pole density of crystal orientation D1 1.0 or more and 5.0 or less
- Polar density of crystal orientation D2 1.0 or more and 4.0 or less
- poles of two kinds of crystal orientations A plate having a density range of 5/8 to 3/8 as a density (range of 5/8 to 3/8 of the plate thickness in the plate thickness direction (depth direction) of the steel plate from the surface of the steel plate)
- the average pole density D1 is a feature point (orientation accumulation degree, texture development degree) of a particularly important texture (crystal orientation of crystal grains in the metal structure).
- the average pole density D1 is the pole density of each crystal orientation of ⁇ 100 ⁇ ⁇ 011>, ⁇ 116 ⁇ ⁇ 110>, ⁇ 114 ⁇ ⁇ 110>, ⁇ 112 ⁇ ⁇ 110>, ⁇ 223 ⁇ ⁇ 110>. It is a pole density expressed by an arithmetic mean.
- EBSD Electro Back Scattering Diffraction
- X-ray diffraction is performed on the above-mentioned cross section in the central portion of the plate thickness which is a plate thickness range of 5/8 to 3/8, and the electron diffraction intensity or X-ray of each direction with respect to a random sample
- the intensity ratio of the diffraction intensities is obtained, and the average pole density D1 of the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups can be obtained from the intensity ratios.
- the d / RmC plate that is the minimum required for processing the undercarriage parts and the skeleton parts
- the index obtained by dividing the thickness d by the minimum bending radius RmC (C direction bending) can satisfy 1.0 or more.
- the tensile strength TS, the hole expansion ratio ⁇ , and the total elongation EL are two conditions required for the underbody member of the automobile body, namely TS ⁇ ⁇ ⁇ 30000 and TS ⁇ EL ⁇ 14000. It is also a condition for satisfying the above.
- the average pole density D1 is 4.0 or less, the minimum bending radius Rm45 of 45 ° direction bending with respect to the minimum bending radius RmC of C direction bending, which is an index of orientation dependency (isotropy) of formability, The ratio (Rm45 / RmC) decreases, and high local deformability independent of the bending direction can be ensured.
- the average pole density D1 is preferably 5.0 or less, and preferably 4.0 or less. When better hole expansibility and small critical bending properties are required, the average pole density D1 is more desirably less than 3.5, and even more desirably less than 3.0.
- the average pole density D1 is 1.0 or more.
- the pole density D2 of the crystal orientation of ⁇ 332 ⁇ ⁇ 113> in the central portion of the plate thickness that is a plate thickness range of 5/8 to 3/8 is set to 4.0 or less.
- This condition is one condition in which the steel sheet satisfies d / RmC ⁇ 1.0, and in particular, the tensile strength TS, the hole expansion ratio ⁇ , and the total elongation EL are required for the suspension member 2 It is also a condition for preferably satisfying two conditions, namely TS ⁇ ⁇ ⁇ 30000 and TS ⁇ EL ⁇ 14000.
- the pole density D2 is desirably 2.5 or less, and more desirably 2.0 or less. If the pole density D2 is more than 4.0, the anisotropy of the mechanical properties of the steel sheet becomes extremely strong. As a result, the local deformability only in a specific direction is improved, but the local deformability in a direction different from that direction is significantly reduced. Therefore, in this case, the steel sheet cannot sufficiently satisfy d / RmC ⁇ 1.0.
- the polar density D2 of the crystal orientation of ⁇ 332 ⁇ ⁇ 113> is 1.0 or more.
- the pole density is synonymous with the X-ray random intensity ratio.
- the X-ray random intensity ratio is obtained by measuring the diffraction intensity (X-rays and electrons) of a standard sample that does not accumulate in a specific orientation and the diffraction intensity of the specimen by the X-ray diffraction method under the same conditions. It is a numerical value obtained by dividing the diffraction intensity of the obtained specimen by the diffraction intensity of the standard sample. This extreme density can be measured using X-ray diffraction, EBSD (Electron Back Scattering Diffraction), or ECP (Electron-Channeling-Pattern).
- the average pole density D1 of the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups is among the ⁇ 110 ⁇ , ⁇ 100 ⁇ , ⁇ 211 ⁇ , ⁇ 310 ⁇ pole figures measured by these methods.
- ODF Orientation Distribution Functions
- the steel sheet is reduced to a predetermined thickness by mechanical polishing, and then the strain is removed by chemical polishing, electrolytic polishing, etc., and at the same time, the thickness is reduced to 5 / 8-3.
- What is necessary is just to measure a pole density according to the above-mentioned method, adjusting a sample so that the suitable surface containing the range of / 8 may become a measurement surface.
- the steel plate satisfies the above-mentioned pole density, so that the local deformability is further improved.
- the material at the central portion of the plate thickness generally represents the material characteristics of the entire steel plate. Therefore, the average pole density D1 of the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation group and the pole density D2 of the crystal orientation of ⁇ 332 ⁇ ⁇ 113> in the central portion of the thickness of 5/8 to 3/8. It stipulates.
- ⁇ hkl ⁇ ⁇ uvw> indicates that the normal direction of the plate surface is parallel to ⁇ hkl> and the rolling direction is parallel to ⁇ uvw> when the sample is collected by the above method.
- the crystal orientation is usually expressed as (hkl) or ⁇ hkl ⁇ in the direction perpendicular to the plate surface and [uvw] or ⁇ uvw> in the direction parallel to the rolling direction.
- ⁇ Hkl ⁇ ⁇ uvw> is a general term for equivalent planes, and (hkl) [uvw] refers to individual crystal planes.
- the body-centered cubic structure (bcc structure) is targeted, for example, (111), ( ⁇ 111), (1-11), (11-1), ( ⁇ 1-11) ), (-11-1), (1-1-1), and (-1-1-1) are equivalent and cannot be distinguished. In such a case, these orientations are collectively referred to as ⁇ 111 ⁇ planes. Since the ODF display is also used for displaying the orientation of other crystal structures with low symmetry, in the ODF display, the individual orientation is generally displayed as (hkl) [uvw]. , ⁇ Hkl ⁇ ⁇ uvw> and (hkl) [uvw] are synonymous.
- the basic metal structure of the hot-rolled steel sheet according to the present embodiment is a DP (Dual Phase) structure including a plurality of crystal grains, having ferrite and / or bainite as a main phase, and martensite as a second phase.
- DP Dual Phase
- the improvement of the uniform deformability is attributed to an increase in the work hardening rate of the steel sheet due to the fine dispersion of martensite, which is a hard structure, in the metal structure.
- the ferrite and bainite mentioned here include polygonal ferrite and bainetic ferrite.
- the hot-rolled steel sheet according to the present embodiment includes retained austenite, pearlite, cementite, and a plurality of inclusions as structures other than ferrite, bainite, and martensite. It is preferable to limit the structures other than ferrite, bainite, and martensite to 0% or more and 10% or less in terms of area ratio. Further, if austenite remains in the structure, the secondary work brittleness and delayed fracture characteristics deteriorate. Therefore, it is preferable that substantially no residual austenite is contained other than the residual austenite having an area ratio of about 5%.
- Area ratio of ferrite and bainite as main phases 30% or more and less than 99% Ferrite and bainite as main phases are relatively soft and have high deformability.
- the area ratio of ferrite and bainite is 30% or more, both the uniform deformability and the local deformability of the hot-rolled steel sheet according to this embodiment are satisfied.
- the total area ratio of ferrite and bainite is 50% or more.
- the combined area ratio of ferrite and bainite is 99% or more, the strength and uniform deformability of the steel sheet are lowered.
- the area ratio of ferrite is 30% or more and 99% or less.
- ductility (deformability) can be more preferably increased in the balance between strength and ductility (deformability) of the steel sheet.
- ferrite contributes to improvement of uniform deformability.
- the area ratio of bainite may be 5% or more and 80% or less.
- the strength can be more preferably increased in the balance between the strength and ductility (deformability) of the steel plate.
- the area ratio of bainite which is harder than ferrite, the strength of the steel sheet is improved.
- bainite having a hardness difference from martensite smaller than ferrite suppresses the generation of voids at the interface between the soft phase and the hard phase, and improves the hole expandability.
- Martensite area ratio fM 1% or more and 70% or less
- the martensite which is a hard structure as the second phase, is dispersed in the metal structure, whereby the strength and the uniform deformability can be increased.
- the area ratio of martensite is less than 1%, there is little dispersion
- the area ratio of martensite is 3% or more.
- the area ratio of martensite may be 50% or less depending on the balance between strength and deformability.
- the area ratio of martensite may be 30% or less. More preferably, the martensite area ratio may be 20% or less.
- Average size dia of martensite crystal grains 13 ⁇ m or less
- the average size of martensite exceeds 13 ⁇ m, the uniform deformability of the steel sheet decreases and the local deformability also decreases. This is because if the average size of martensite is coarse, the contribution to work hardening will be small and the uniform elongation will be low, and voids will easily occur around the coarse martensite and local deformability will be low. Conceivable.
- the average size of martensite is 10 ⁇ m or less. More preferably, the average martensite size is 7 ⁇ m or less.
- TS / fM ⁇ dis / dia relationship 500 or more
- the tensile strength is unit MPa
- TS Torsile Strength
- the martensite area ratio is unit%
- fM fraction of martensite
- Equation 1 When the relationship of TS / fM ⁇ dis / dia is smaller than 500, the uniform deformability of the steel sheet may be greatly reduced.
- the physical meaning of Equation 1 is not clear. However, it is considered that this is because the smaller the average distance dis between the martensite crystal grains and the larger the average size dia of the martensite crystal grains, the more work hardening occurs.
- there is no particular upper limit in the relationship of TS / fM ⁇ dis / dia In actual operation, the relationship of TS / fM ⁇ dis / dia is rarely over 10,000, so the upper limit is made 10,000 or less.
- Ratio of martensite whose major axis / minor axis ratio is 5.0 or less 50% or more
- the major axis of the martensite crystal grains is La in the unit ⁇ m and the minor axis is Lb in the unit ⁇ m
- the martensite crystal grains satisfying Equation 2 are 50% or more and 100% or less in terms of area ratio with respect to the martensite area ratio fM, it is preferable because local deformability is improved.
- the martensite crystal grains having La / Lb of 3.0 or less have an area ratio of 50% or more with respect to fM. More preferably, the martensite crystal grains having La / Lb of 2.0 or less have an area ratio of 50% or more with respect to fM. Further, if the ratio of equiaxed martensite is less than 50% with respect to fM, local deformability may be deteriorated.
- the lower limit value of Equation 2 is 1.0.
- part or all of the martensite may be tempered martensite.
- tempered martensite By using tempered martensite, the strength of the steel sheet is reduced, but the hardness difference between the main phase and the second phase is reduced, and the hole expandability of the steel sheet is improved. What is necessary is just to control the area ratio of the tempered martensite with respect to the martensite area ratio fM according to the balance between the required strength and deformability.
- the above-described metal structures such as ferrite, bainite, and martensite have field emission type scanning electrons within a thickness range of 1/8 to 3/8 (that is, a thickness range centered on a 1/4 thickness position). It can be observed with a microscope (FE-SEM: Field Emission Scanning Electron Microscope). The characteristic value can be determined from the image obtained by this observation. Alternatively, it can be determined by EBSD described later. In this FE-SEM observation, a sample was taken so that a cross section of the plate thickness parallel to the rolling direction of the steel plate (the normal direction is the plate thickness direction) was the observation surface, and polishing and nital etching were performed on this observation surface. It is carried out.
- FE-SEM Field Emission Scanning Electron Microscope
- the metal structure (component) of the steel sheet may be significantly different from other parts due to decarburization and Mn segregation, respectively. For this reason, in the present embodiment, the metal structure is observed based on the 1 ⁇ 4 thickness position.
- volume average diameter of crystal grains 5 ⁇ m or more and 30 ⁇ m or less
- the size of crystal grains in the metal structure particularly the volume average diameter, may be refined. Furthermore, by reducing the volume average diameter, the fatigue characteristics (fatigue limit ratio) required for automobile steel sheets and the like are also improved. Since the influence of the number of coarse grains on the deformability is higher than that of fine grains, the deformability is more strongly correlated with the volume average diameter calculated by the weighted average of the volume than the number average diameter.
- the volume average diameter is 5 ⁇ m or more and 30 ⁇ m or less, desirably 5 ⁇ m or more and 20 ⁇ m or less, and more desirably 5 ⁇ m or more and 10 ⁇ m or less.
- the volume average diameter when the volume average diameter is reduced, local strain concentration occurring at the micro order is suppressed, strain at the time of local deformation can be dispersed, and elongation, particularly uniform elongation, is improved.
- the grain boundary that becomes a barrier to dislocation motion can be controlled appropriately, and this grain boundary acts on repeated plastic deformation (fatigue phenomenon) caused by the dislocation motion, thereby improving fatigue characteristics. .
- each crystal grain can be determined as follows.
- the pearlite is specified by observing the structure with an optical microscope.
- the grain units of ferrite, austenite, bainite, and martensite are specified by EBSD. If the crystal structure of the region determined by EBSD is a face-centered cubic structure (fcc structure), this region is determined to be austenite. Further, if the crystal structure of the region determined by EBSD is a body-centered cubic structure (bcc structure), this region is determined as one of ferrite, bainite, and martensite.
- Ferrite, bainite, and martensite can be identified using the KAM (Kernel Average Missoration) method equipped in EBSP-OIM (registered trademark, Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy).
- KAM Kernel Average Missoration
- EBSP-OIM Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy
- the second approximation using all 12 pixels (19 pixels in total), or the third approximation using all 18 pixels outside these 12 pixels (total 37 pixels) the orientation difference between each pixel And the average value obtained is determined as the value of the center pixel, and such an operation is performed on the entire pixel.
- a map expressing the orientation change in the grain can be created. This map represents a strain distribution based on local orientation changes in the grains.
- the azimuth difference between adjacent pixels is calculated by the third approximation.
- the grain size of ferrite, bainite, martensite, and austenite is measured, for example, by performing the above-mentioned orientation measurement at a measurement step of 0.5 ⁇ m or less at a magnification of 1500 times, and at a position where the orientation difference between adjacent measurement points exceeds 15 °. It is obtained by defining a boundary (this grain boundary is not necessarily a general crystal grain boundary) and calculating the equivalent circle diameter.
- the crystal grain size of pearlite can be calculated by applying an image processing method such as binarization or cutting to the image obtained by the optical microscope. it can.
- the equivalent circle radius (half the equivalent circle diameter) in the case of the r the volume of individual grains is obtained by 4 ⁇ ⁇ ⁇ r 3/3 , this The volume average diameter can be obtained by weighted average of the volumes.
- the area ratio of the following coarse grain can be obtained by dividing the area ratio of the coarse grain obtained by this method by the area to be measured.
- the average size dia of the above-described martensite crystal grains uses the above-mentioned equivalent circle diameter or the crystal grain diameter obtained by the binarization process and the cutting method.
- the average distance dis between the above-mentioned martensite crystal grains is not limited to the above-mentioned FE-SEM observation method, but is obtained by this EBSD method (however, FE-SEM capable of EBSD). It can also be determined using the boundary between the grains.
- the particle size is 35 ⁇ m per unit area for all the components of the metal structure. It is preferable to limit the ratio of the area (coarse grain area ratio) occupied by grains exceeding 60% (coarse grains) to 0% or more and 10% or less. As the number of large grains increases, the tensile strength decreases and the local deformability also decreases. Therefore, it is preferable to make the crystal grains as fine as possible. In addition, since all the crystal grains are uniformly and equivalently strained, the local deformability is improved. Therefore, by limiting the amount of coarse grains, local crystal grain distortion can be suppressed.
- Standard deviation of average distance dis between crystal grains of martensite 5 ⁇ m or less
- martens which is a hard structure It is preferable that the sites are dispersed in the metal structure. Therefore, the standard deviation of the average distance dis between the martensite crystal grains is preferably 0 ⁇ m or more and 5 ⁇ m or less. In this case, for at least 100 martensite crystal grains, the distance between the crystal grains may be measured to obtain an average distance dis and its standard deviation.
- Hardness H of ferrite It is preferable to satisfy the following formula 3. Soft ferrite, which is the main phase, contributes to improving the deformability of the steel sheet. Therefore, it is desirable that the average value of the hardness H of the ferrite satisfies the following formula 3. If hard ferrite exists in the following formula 3 or more, there is a possibility that the effect of improving the deformability of the steel sheet cannot be obtained.
- the average value of the hardness H of the ferrite is determined by measuring 100 or more points of the hardness of the ferrite with a load of 1 mN using a nanoindenter.
- Standard deviation / average value of hardness of ferrite or bainite 0.2 or less
- the present inventors have found that the main phase has high homogeneity. It has been found that the balance between uniform deformability and local deformability can be preferably improved for a tissue. Specifically, it is preferable that the value obtained by dividing the standard deviation of the hardness of the ferrite by the average value of the hardness of the ferrite is 0.2 or less because the above effect can be obtained.
- the value which divided the standard deviation of the hardness of bainite by the average value of the hardness of bainite is 0.2 or less, since the above-mentioned effect is acquired, it is preferred.
- This homogeneity can be defined by measuring the hardness of 100 or more points of ferrite or bainite as a main phase with a nanoindenter at a load of 1 mN and using the average value and the standard deviation thereof. That is, the lower the standard value of hardness / the average value of hardness, the higher the homogeneity, and the effect is obtained when the hardness is 0.2 or less.
- a nanoindenter for example, UMIS-2000 manufactured by CSIRO
- the hardness of a single crystal grain that does not include a grain boundary can be measured by using an indenter smaller than the crystal grain size.
- C 0.01% or more and 0.4% or less
- C (carbon) is an element that increases the strength of the steel sheet, and is an essential element for securing the area ratio of martensite.
- the reason why the lower limit of the C content is set to 0.01% is to obtain martensite in an area ratio of 1% or more.
- the C content exceeds 0.40%, the deformability of the steel sheet decreases, and the weldability of the steel sheet also deteriorates.
- the C content is 0.30% or less.
- Si 0.001% or more and 2.5% or less
- Si is a deoxidizing element of steel, and is an element effective for increasing the mechanical strength of a steel sheet.
- Si is an element that stabilizes ferrite during temperature control after hot rolling and suppresses cementite precipitation during bainite transformation.
- the Si content exceeds 2.5%, the deformability of the steel sheet decreases, and surface flaws tend to occur on the steel sheet.
- the Si content is less than 0.001%, it is difficult to obtain the above effects.
- Mn 0.001% or more and 4.0% or less
- Mn manganese
- Mn is an element effective for increasing the mechanical strength of the steel sheet.
- the Mn content is 3.5% or less. More preferably, the Mn content is 3.0% or less.
- Mn is also an element that prevents cracking during hot rolling by fixing S (sulfur) in steel.
- S sulfur
- Al 0.001% or more and 2.0% or less
- Al is a deoxidizing element of steel.
- Al is an element that stabilizes ferrite during temperature control after hot rolling and suppresses cementite precipitation during bainite transformation.
- the Al content is set to 0.001% or more.
- the Al content exceeds 2.0%, the weldability becomes poor.
- Al is an element that remarkably increases the temperature Ar 3 at which transformation starts from ⁇ (austenite) to ⁇ (ferrite) during steel cooling. Therefore, the Al content may be controlled Ar 3 of the steel.
- the hot-rolled steel sheet according to this embodiment contains inevitable impurities in addition to the basic components described above.
- the inevitable impurities mean secondary materials such as scrap and elements such as P, S, N, O, Cd, Zn, and Sb that are inevitably mixed from the manufacturing process.
- P, S, N, and O are limited as follows in order to preferably exhibit the above effects.
- the limit range of the impurity content includes 0%, but it is difficult to achieve 0% stably industrially.
- the described% is mass%.
- P 0.15% or less
- P phosphorus
- the P content is limited to 0.15% or less.
- the P content is limited to 0.05% or less.
- the lower limit of the P content may be 0%. In consideration of current general refining (including secondary refining), the lower limit of the P content may be 0.0005%.
- S 0.03% or less S (sulfur) is an impurity, and when excessively contained in steel, MnS stretched by hot rolling is generated and is an element that lowers the deformability of the steel sheet. Therefore, the S content is limited to 0.03% or less.
- the lower limit of the S content may be 0%.
- the lower limit of the P content may be 0.0005%.
- N 0.01% or less
- N nitrogen
- the lower limit of the N content may be 0%.
- the lower limit of the N content may be 0.0005%.
- O 0.01% or less
- O (oxygen) is an impurity and is an element that lowers the deformability of the steel sheet. Therefore, the O content is limited to 0.01% or less.
- the lower limit of the O content may be 0%. In consideration of current general refining (including secondary refining), the lower limit of the O content may be 0.0005%.
- the above chemical elements are the basic components (basic elements) of the steel in the present embodiment, the basic elements are controlled (contained or restricted), and the chemical composition consisting of iron and unavoidable impurities as the balance is Basic composition.
- the following chemical elements may be further contained in the steel as necessary.
- these selection elements are inevitably mixed in the steel (for example, an amount less than the lower limit of the amount of each selection element), the effect in the present embodiment is not impaired.
- the hot-rolled steel sheet according to the present embodiment has Mo, Cr, Ni, Cu, B, Nb, Ti, V, W, Ca, Mg as optional components in addition to the basic components and impurity elements described above.
- Zr, REM, As, Co, Sn, Pb, Y, Hf may be contained.
- the numerical limitation range of the selected component and the reason for limitation will be described.
- the described% is mass%.
- Ti 0.001% or more and 0.2% or less
- Nb 0.001% or more and 0.2% or less
- B 0.0001% or more and 0.005% or less
- Ti (titanium), Nb (niobium), B (Boron) is a selective element that brings about effects such as precipitation strengthening, structure control, and fine grain strengthening in steel because carbon and nitrogen in steel are fixed to produce fine carbonitrides. Therefore, if necessary, one or more of Ti, Nb, and B may be added to the steel. In order to obtain the above effects, it is desirable that the Ti content is 0.001% or more, the Nb content is 0.001% or more, and the B content is 0.0001% or more.
- the Ti content is 0.2% or less
- the Nb content is 0.2% or less
- the B content is 0.005% or less.
- the lower limit of the content of these selective elements is 0%.
- Mg 0.0001% or more and 0.01% or less REM: 0.0001% or more and 0.1% or less Ca: 0.0001% or more and 0.01% or less Mg (magnesium), REM (Rare Earth Metal) , Ca (calcium) is an important selection element for controlling inclusions in a harmless form and improving the local deformability of the steel sheet. Therefore, as needed, you may add any 1 or more types in Mg, REM, and Ca in steel. In order to obtain the above effects, it is desirable that the Mg content is 0.0001% or more, the REM content is 0.0001% or more, and the Ca content is 0.0001% or more.
- the Mg content is 0.01% or less
- the REM content is 0.1% or less
- the Ca content is 0.01% or less.
- the lower limit of the content of these selective elements is 0%.
- REM is a collective term for a total of 16 elements including 15 elements from lanthanum with atomic number 57 to lutesium with 71 and scandium with atomic number 21. Usually, it is supplied in the form of misch metal, which is a mixture of these elements, and added to the steel.
- Mo 0.001% to 1.0% Cr: 0.001% to 2.0% Ni: 0.001% to 2.0% W: 0.001% to 1.0% % Or less Zr: 0.0001% or more and 0.2% or less As: 0.0001% or more and 0.5% or less Mo (molybdenum), Cr (chromium), Ni (nickel), W (tungsten), Zr ( Zirconium) and As (arsenic) are selective elements that increase the mechanical strength of the steel sheet. Therefore, if necessary, one or more of Mo, Cr, Ni, W, Zr, and As may be added to the steel.
- the Mo content is 0.001% or more, the Cr content is 0.001% or more, the Ni content is 0.001% or more, the W content is 0.001% or more, and the Zr content. Is preferably 0.0001% or more, and the As content is preferably 0.0001% or more.
- Mo content is 1.0% or less, Cr content is 2.0% or less, Ni content is 2.0% or less, W content is 1.0% or less, Zr content is 0.2%.
- the As content is preferably 0.5% or less.
- the lower limit of the content of these selective elements is 0%.
- V 0.001% or more and 1.0% or less
- Cu 0.001% or more and 2.0% or less
- V (vanadium) and Cu (copper) have the effect of precipitation strengthening, like Nb and Ti. It is a selective element. Further, the addition of V and Cu has a lower degree of decrease compared to the decrease in local deformability caused by the addition of Nb, Ti and the like. Therefore, it is a selective element that is more effective than Nb or Ti when it is desired to enhance the local deformation ability such as hole expandability and bendability with high strength. Therefore, as needed, you may add any 1 or more types of V and Cu in steel. In order to acquire the said effect, it is preferable that V content is 0.001% or less and Cu content is 0.001% or less.
- the V content is 1.0% or less and the Cu content is 2.0% or less.
- the lower limit of the content of these selective elements is 0%.
- Co 0.0001% or more and 1.0% or less
- Co (cobalt) is difficult to show the effect quantitatively, but the temperature Ar 3 at which transformation starts from ⁇ (austenite) to ⁇ (ferrite) during steel cooling Is a selective element that remarkably increases. Therefore, the Co content may control the Ar 3 of the steel.
- Co is a selective element that improves the strength of the steel sheet.
- the Co content is preferably 0.0001% or more.
- the Co content is preferably 1.0% or less.
- the effect in this embodiment is not impaired.
- the lower limit of the content of this selective element is 0%.
- Sn 0.0001% or more and 0.2% or less
- Pb 0.0001% or more and 0.2% or less
- Sn (tin) and Pb (lead) improve plating wettability and plating adhesion. It is an effective selective element. Therefore, you may add any 1 or more types in Sn and Pb in steel as needed. In order to obtain the above effects, it is preferable that the Sn content is 0.0001% or more and the Pb content is 0.0001% or more. However, when these selective elements are excessively added to the steel, hot embrittlement occurs, cracks occur during hot working, and surface flaws are likely to occur in the steel sheet. Therefore, it is preferable that the Sn content is 0.2% or less and the Pb content is 0.2% or less.
- Y 0.0001% or more and 0.2% or less
- Hf 0.0001% or more and 0.2% or less
- Y (yttrium) and Hf (hafnium) are effective selection elements for improving the corrosion resistance of the steel sheet. is there. Therefore, you may add any 1 or more types of Y and Hf in steel as needed.
- the Y content is 0.0001% or more and the Hf content is 0.0001% or more.
- the Y content is 0.20% or less and the Hf content is 0.20% or less.
- Y has an effect of forming an oxide in steel and adsorbing hydrogen in the steel. For this reason, the diffusible hydrogen in steel is reduced, and it can also be expected to improve the hydrogen embrittlement resistance of the steel sheet. This effect can also be obtained within the range of the Y content described above. In addition, even if these selective elements are contained in the steel in an amount less than the lower limit, the effects in this embodiment are not impaired. Moreover, since it is not necessary to intentionally add these selective elements to the steel in order to reduce the alloy cost, the lower limit of the content of these selective elements is 0%.
- the hot-rolled steel sheet according to the present embodiment includes the above-described basic element, and the balance is selected from the chemical composition composed of Fe and inevitable impurities, or the above-described basic element and the above-described selective element. It has at least one kind, and the balance has a chemical composition consisting of iron and inevitable impurities.
- the hot-rolled steel sheet according to this embodiment may be surface-treated.
- surface treatment such as electroplating, hot dipping, vapor deposition plating, alloying treatment after plating, organic film formation, film lamination, organic and inorganic salt treatment, non-chromate treatment (non-chromate treatment)
- the rolled steel sheet may be provided with various coatings (film or coating).
- the hot-rolled steel sheet may have a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on its surface. Even if the hot-rolled steel sheet is provided with the above-described coating film, it is possible to sufficiently maintain high strength and uniform deformability and local deformability.
- the thickness of the hot-rolled steel sheet is not particularly limited, but may be, for example, 1.5 to 10 mm, or 2.0 to 10 mm.
- the strength of the hot-rolled steel sheet is not particularly limited, and for example, the tensile strength may be 440 to 1500 MPa.
- the hot-rolled steel sheet according to this embodiment can be applied to all uses of high-strength steel sheets, has excellent uniform deformability, and dramatically improves local deformability such as bending workability and hole expansibility of high-strength steel sheets. ing.
- the direction of bending the hot-rolled steel sheet is not particularly limited because it varies depending on the processed parts.
- the same characteristics are obtained in any bending direction, and the hot-rolled steel sheet can be applied to composite forming including processing modes such as bending, stretching, and drawing.
- the production method preceding hot rolling is not particularly limited.
- various secondary refining can be performed subsequent to smelting and refining in a blast furnace, electric furnace, converter, etc., and steel satisfying the above chemical composition can be melted to obtain steel (molten steel).
- the steel can be cast by a casting method such as a normal continuous casting method, an ingot method, or a thin slab casting method.
- the steel may be once cooled to a low temperature (for example, room temperature) and reheated, and then the steel may be hot-rolled, or the steel immediately after casting (cast slab) may be continuously It may be hot rolled.
- 1st hot rolling process As a 1st hot rolling process, 40% or more in the temperature range of 1000 degreeC or more and 1200 degrees C or less (preferably 1150 degrees C or less) using the said ingot made by melting and casting A rolling pass with a reduction ratio of at least once is performed.
- the average austenite grain size of the steel sheet after the first hot rolling process becomes 200 ⁇ m or less, and the uniform deformability and local deformation of the finally obtained hot rolled steel sheet Contributes to the improvement of performance.
- the average austenite grain size of the steel sheet is 100 ⁇ m or less by performing rolling in which the rolling reduction rate of one pass is 40% or more twice (two passes) in the first hot rolling step.
- the reduction rate of one pass is limited to 70% or less, or the number of reductions (number of passes) is limited to 10 times or less, thereby reducing the steel sheet temperature and excessive scale. Generation concerns can be reduced. Therefore, in rough rolling, the rolling reduction of one pass may be 70% or less, and the number of rolling (number of passes) may be 10 or less.
- the austenite grains after the first hot rolling process fine, the austenite grains can be made finer in the subsequent process, and the ferrite, bainite, transformed from the austenite in the subsequent process, And martensite is preferable because it can be dispersed finely and uniformly.
- the texture can be controlled, so that the anisotropy and local deformability of the steel sheet can be improved, and the metal structure can be refined, so that the uniform deformability and local deformability of the steel sheet can be improved ( In particular, the uniform deformability is improved.
- the austenite grain boundaries refined by the first hot rolling step during the second hot rolling step, which is a subsequent step function as one of the recrystallization nuclei.
- the steel plate after the first hot rolling step it is desirable to rapidly cool the steel plate after the first hot rolling step at a cooling rate as large as possible.
- the steel sheet is cooled at an average cooling rate of 10 ° C./second or more.
- the cross section of the plate piece collected from the steel plate obtained by cooling is etched to make the austenite grain boundary in the microstructure stand up and measured with an optical microscope.
- the austenite grain size was measured by image analysis or a cutting method, and the austenite grain size measured in each field of view was averaged to obtain an average austenite grain size. Get.
- the sheet bar may be joined and the second hot rolling step, which is a subsequent step, may be continuously performed.
- the coarse bar may be wound once in a coil shape, stored in a cover having a heat retaining function as necessary, and rewound again before joining.
- Second Hot Rolling Step when the temperature calculated by the following equation 4 is T1 in the unit of ° C. on the steel plate after the first hot rolling step, T1 + 30 ° C. or more and Includes a large reduction pass with a reduction rate of 30% or more in the temperature range of T1 + 200 ° C or less, the cumulative reduction rate in the temperature range of T1 + 30 ° C or more and T1 + 200 ° C or less is 50%, Ar 3 ° C or more and less than T1 + 30 ° C Rolling is performed such that the cumulative rolling reduction in the temperature range is limited to 30% or less and the rolling end temperature is Ar 3 ° C or higher.
- a temperature T1 (as shown in the following formula 4 depending on the chemical composition (unit: mass%) of the steel) The rolling is controlled based on the unit (° C).
- T1 850 + 10 ⁇ ([C] + [N]) ⁇ [Mn] + 350 ⁇ [Nb] + 250 ⁇ [Ti] + 40 ⁇ [B] + 10 ⁇ [Cr] + 100 ⁇ [Mo] + 100 ⁇ [V] (Formula 4)
- [C], [N], [Mn], [Nb], [Ti], [B], [Cr], [Mo], and [V] are C, N, It is the mass percentage of Mn, Nb, Ti, B, Cr, Mo and V.
- a temperature range of T1 + 30 ° C. or higher and T1 + 200 ° C. or lower (preferably T1 + 50 ° C. or higher and T1 + 100 ° C. or lower) based on the temperature T1 (unit: ° C) obtained by the above formula 4 or formula 5.
- T1 unit: ° C
- a large reduction ratio is secured, and the reduction ratio is limited to a small range (including 0%) in a temperature range of Ar 3 ° C or higher and lower than T1 + 30 ° C.
- This temperature T1 itself has been determined empirically.
- the present inventors have empirically found through experiments that the temperature range in which recrystallization in the austenite region of each steel can be promoted can be determined based on the temperature T1.
- T1 + 30 ° C. or more and T1 + 200 ° C. A plurality of passes are rolled in the following temperature range, and the cumulative reduction ratio is set to 50% or more.
- this cumulative rolling reduction is desirably 70% or more from the viewpoint of promoting recrystallization due to strain accumulation. Further, by limiting the upper limit of the cumulative rolling reduction, the rolling temperature can be secured more sufficiently and the rolling load can be further suppressed. Therefore, the cumulative rolling reduction may be 90% or less.
- a dynamic recrystallized structure accumulates strain received during processing in the crystal, and a recrystallized region and a non-recrystallized region are locally mixed. Therefore, the texture is relatively developed and anisotropic.
- the metal structure may be mixed.
- the method for producing a hot-rolled steel sheet according to the present embodiment is characterized in that austenite is recrystallized by static recrystallization. Therefore, the recrystallized austenite structure is uniform, fine, equiaxed, and suppresses the development of texture. Can be obtained.
- the rolling reduction in one pass is 30% or more in the temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less.
- the second hot rolling is controlled so as to include at least one large reduction pass. In this way, in the second hot rolling, at a temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less, the reduction at a reduction rate of 30% or more in one pass is performed at least once.
- the rolling reduction of the final pass in this temperature range is preferably 25% or more, and more preferably 30% or more.
- the final pass in this temperature range is a large reduction pass (a rolling pass with a reduction rate of 30% or more).
- the rolling reduction ratios of the first half pass are all less than 30%, and the rolling reduction ratios of the final two passes are each 30% or more.
- a large reduction pass with a reduction rate of 40% or more in one pass is preferably performed.
- a large rolling pass with a rolling reduction rate in one pass of 70% or less is used.
- the temperature rise of the steel plate between each pass of rolling can be suppressed to, for example, 18 ° C. or less to obtain more uniform recrystallized austenite. it can.
- 0% is more desirable. That is, in the temperature range of Ar 3 ° C. or higher and lower than T1 + 30 ° C., the reduction does not have to be performed, and even when the reduction is performed, the cumulative reduction rate is set to 30% or less.
- austenite can be recrystallized uniformly, finely and equiaxially, and the uniform structure and local deformability can be improved by controlling the texture, metal structure and anisotropy of the steel sheet. it can.
- the final hot-rolled steel sheet has a major axis / minor axis ratio of martensite, an average size of martensite, and an average distance between martensites. Etc. can be controlled.
- the Ar 3 ° C. or more and the cumulative rolling reduction at a temperature range of less than T1 + 30 ° C. is too large, austenite texture Develop.
- the finally obtained hot-rolled steel sheet has an average pole density D1 in the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups of 1.0 or more and 5.0 or less at the center of the plate thickness. Or at least one of the conditions of ⁇ 332 ⁇ ⁇ 113> in which the pole density D2 of the crystal orientation is 1.0 or more and 4.0 or less.
- the pole density D2 of the crystal orientation is 1.0 or more and 4.0 or less.
- the cumulative rolling reduction in the temperature range of T1 + 30 ° C. or higher and T1 + 200 ° C. or lower is too small, uniform and fine Recrystallization does not occur, and the metal structure includes coarse grains or mixed grains, or the metal structure becomes mixed grains. Therefore, the area ratio and volume average diameter of crystal grains exceeding 35 ⁇ m increase.
- the second hot rolling Ar 3 when completed in less than a temperature, Ar 3 (Unit: ° C.) at less and rolling end temperature or temperature range, the two-phase of austenite and ferrite Steel is rolled in the region (two-phase temperature region). Therefore, the texture of the steel plate develops, and the anisotropy and local deformability of the steel plate are significantly deteriorated.
- the rolling end temperature of the second hot rolling when the rolling end temperature of the second hot rolling is equal to or higher than T1, the amount of strain in the temperature range below T1 can be reduced to further reduce the anisotropy, and as a result, the local deformability can be further increased. Can do. Therefore, the rolling end temperature of the second hot rolling may be T1 or higher.
- the rolling reduction can be obtained by actual results or calculation from measurement of rolling load or sheet thickness.
- the rolling temperature for example, each of the above temperature ranges
- the rolling temperature can be measured by an inter-stand thermometer, or can be calculated by a calculation simulation considering processing heat generation from line speed, rolling reduction, etc. (both actual measurement and calculation) It can be obtained by performing.
- the above-described reduction ratio in one pass is the amount of reduction in one pass relative to the inlet plate thickness before passing through the rolling stand (difference between the inlet plate thickness before passing through the rolling stand and the outlet plate thickness after passing through the rolling stand). The percentage.
- the cumulative reduction ratio is based on the inlet plate thickness before the first pass in rolling in each of the above temperature ranges, and the cumulative reduction amount relative to this reference (the inlet plate thickness before the first pass in rolling in each of the above temperature ranges and the above mentioned It is a percentage of the difference between the outlet plate thickness after the final pass in rolling in each temperature range.
- Ar 3 which is the ferrite transformation temperature from austenite during cooling, is determined by the following formula 6 in units of ° C. As described above, although it is difficult to show an effect quantitatively, Al and Co also affect Ar 3 .
- Ar 3 879.4 ⁇ 516.1 ⁇ [C] ⁇ 65.7 ⁇ [Mn] + 38.0 ⁇ [Si] + 274.7 ⁇ [P] (Formula 6)
- [C], [Mn], [Si], and [P] are mass percentages of C, Mn, Si, and P, respectively.
- Tf in Equation 8 is the temperature (unit: ° C.) of the steel sheet at the time of completion of the final pass in the large reduction pass
- P1 is the reduction rate (unit:%) in the final pass of the large reduction pass. is there.
- the austenite crystal grains can be controlled to have a metal structure that is equiaxed and has few coarse grains (having a uniform size). Therefore, the finally obtained hot-rolled steel sheet also has a metal structure that is equiaxed and has few coarse grains (uniform size), and the ratio of the major axis to the minor axis of martensite, the average size of martensite, and between martensites The average distance can be preferably controlled.
- the value on the right side of Formula 7 (2.5 ⁇ t1) means the time when the recrystallization of austenite is almost completed.
- the waiting time t exceeds the value on the right side of Formula 7 (2.5 ⁇ t1), the recrystallized crystal grains grow significantly and the crystal grain size increases. Therefore, the strength, uniform deformability and local deformability, fatigue characteristics, and the like of the steel plate are reduced. Accordingly, the waiting time t is 2.5 ⁇ t1 seconds or less.
- This primary cooling may be performed between rolling stands in consideration of operability (for example, control of shape correction and secondary cooling). Note that the lower limit of the waiting time t is 0 second or longer.
- the volume average diameter of the finally obtained hot-rolled steel sheet can be controlled to 30 ⁇ m or less. As a result, even if the recrystallization of austenite does not proceed sufficiently, the characteristics of the steel sheet, particularly the uniform deformability and fatigue characteristics can be preferably improved.
- the development of the texture can be suppressed by limiting the waiting time t to t1 seconds or more and 2.5 ⁇ t1 seconds or less so that t1 ⁇ t ⁇ 2.5 ⁇ t1.
- the waiting time is longer than the case where the waiting time t is less than t1 seconds, the volume average diameter increases, but the recrystallization of austenite proceeds sufficiently to randomize the crystal orientation.
- the anisotropy and local deformability of the steel sheet can be preferably improved.
- the primary cooling described above can be performed between rolling stands in the temperature range of T1 + 30 ° C. or higher and T1 + 200 ° C. or after the last rolling stand in this temperature range. That is, if the waiting time t satisfies the above condition, one pass reduction is performed in the temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less after the completion of the final pass of the large reduction pass to the start of primary cooling. Rolling at a rate of 30% or less may be further performed. Further, after the primary cooling, if the rolling reduction in one pass is 30% or less, rolling may be further performed in a temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less.
- the change in cooling temperature which is the difference between the steel plate temperature at the start of cooling (steel temperature) and the steel plate temperature at the end of cooling (steel temperature), is desirably 40 ° C. or higher and 140 ° C. or lower. If this cooling temperature change is 40 ° C. or higher, the grain growth of recrystallized austenite grains can be further suppressed. If the change in cooling temperature is 140 ° C. or less, recrystallization can proceed more sufficiently, and the extreme density can be preferably improved. Moreover, by limiting the cooling temperature change to 140 ° C.
- the temperature of the steel sheet not only can the temperature of the steel sheet be controlled relatively easily, but also the variant selection (variant limitation) can be controlled more effectively, and the development of the recrystallized texture is preferable. It can also be suppressed. Therefore, in this case, the isotropic property can be further increased, and the orientation dependency of the formability can be further reduced. If the change in cooling temperature exceeds 140 ° C., the progress of recrystallization becomes insufficient, the desired texture cannot be obtained, the ferrite becomes difficult to obtain, and the hardness of the obtained ferrite becomes high. There is a possibility that the uniform deformability and the local deformability are lowered.
- the steel plate temperature T2 at the end of the primary cooling is T1 + 100 ° C. or less.
- the steel plate temperature T2 at the end of the primary cooling is T1 + 100 ° C. or less.
- the average cooling rate in the primary cooling is 50 ° C./second or more.
- the average cooling rate in the primary cooling is 50 ° C./second or more, the grain growth of the recrystallized austenite grains can be further suppressed.
- the upper limit of the average cooling rate is not particularly required, but the average cooling rate may be 200 ° C./second or less from the viewpoint of the steel plate shape.
- Secondary cooling step As the secondary cooling step, after the second hot rolling and after the primary cooling step, at an average cooling rate of 15 ° C / second or more and 300 ° C / second or less, 600 ° C or more and 800 ° C. It is preferable to cool the steel sheet to the following temperature range. During this secondary cooling step, when the steel sheet is cooled and the temperature of the steel sheet becomes Ar 3 or less, austenite begins to transform into ferrite. By setting the average cooling rate to 15 ° C./second or more, coarsening of austenite crystal grains can be preferably suppressed. The upper limit of the average cooling rate is not particularly required, but the average cooling rate may be 300 ° C./second or less from the viewpoint of the steel plate shape. Further, it is preferable to start secondary cooling within 3 seconds after the second hot rolling and after the primary cooling step. When the start of secondary cooling exceeds 3 seconds, austenite may be coarsened.
- a steel plate is hold
- the transformation from austenite to ferrite proceeds, and the ferrite area ratio of the steel sheet can be increased.
- a steel plate is hold
- the holding time is set to 1 second or more in order to advance the ferrite transformation. However. If it exceeds 15 seconds, the ferrite crystal grains become coarse and cementite may be precipitated.
- the holding time is preferably 3 seconds or more and 15 seconds or less.
- the steel sheet is cooled to a temperature range of room temperature to 350 ° C. at an average cooling rate of 50 ° C./second to 300 ° C./second.
- austenite that has not been transformed into ferrite even after the holding step is transformed into bainite and martensite.
- the bainite transformation proceeds excessively because the temperature is too high, and finally, martensite cannot be obtained in an area ratio of 1% or more.
- the minimum in particular of the cooling stop temperature of a tertiary cooling process when water cooling is assumed, what is necessary is just room temperature or more. Further, when cooling at an average cooling rate of less than 50 ° C./second, pearlite transformation may occur during cooling.
- the upper limit of the average cooling rate in the tertiary cooling step is not particularly required, but may be 300 ° C. or less from the viewpoint of operation.
- the bainite area ratio can be increased by reducing the average cooling rate.
- the martensite area ratio can be increased.
- the crystal grain size of bainite and martensite is also fine.
- the area ratio of ferrite and bainite as the main phase and martensite as the second phase may be controlled.
- ferrite can be controlled mainly in the holding process
- bainite and martensite can be controlled mainly in the tertiary cooling process.
- the crystal grain size and shape of these main phases, ferrite and bainite, and martensite, which is the second phase largely depend on the grain size and shape of austenite, which is a structure before transformation. It also depends on the holding process and the tertiary cooling process.
- the value of TS / fM ⁇ dis / dia which is the relationship between the martensite area ratio fM, the martensite average size dia, the martensite average distance dis, and the tensile strength TS of the steel sheet, It can be satisfied by controlling the above manufacturing process in a complex manner.
- Winding process As the winding process, after the tertiary cooling process, winding of the steel sheet is started at a temperature not lower than room temperature and not higher than 350 ° C., which is the cooling stop temperature of the tertiary cooling, and then air-cooled.
- the hot-rolled steel sheet according to the present embodiment can be manufactured.
- This skin pass rolling it is possible to prevent stretcher strain generated during processing and to correct the steel plate shape.
- surface treatment such as electroplating, hot dipping, vapor deposition, alloying after plating, organic film formation, film lamination, organic / inorganic salt treatment, non-chromic treatment, etc. is applied to the obtained hot-rolled steel sheet.
- a hot-dip galvanized layer or an alloyed hot-dip galvanized layer may be formed on the surface of the hot-rolled steel sheet. Even if the above surface treatment is performed, the uniform deformability and the local deformability can be sufficiently maintained.
- a tempering treatment or an aging treatment may be performed as the reheating treatment.
- Nb, Ti, Zr, V, W, Mo, or the like dissolved in the steel may be precipitated as carbides, or martensite may be softened as tempered martensite.
- martensite may be softened as tempered martensite.
- the effect of this reheating treatment can also be obtained by heating for the above-described hot dipping or alloying treatment.
- the conditions in the present embodiment are one condition example adopted for confirming the feasibility and effects of the present invention, and the present invention is not limited to this one condition example.
- the present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
- Tables 15 to 22 show characteristic points such as metal structure, texture, and mechanical properties.
- the average pole density of the ⁇ 100 ⁇ ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> orientation groups is denoted by D1
- the pole density of the ⁇ 332 ⁇ ⁇ 113> crystal orientation is denoted by D2.
- the area fractions of ferrite, bainite, martensite, pearlite, and retained austenite are indicated as F, B, fM, P, and ⁇ , respectively.
- the average martensite size is denoted by dia
- the average distance between martensites is denoted by dis.
- the standard deviation ratio of hardness means a value obtained by dividing the standard deviation of hardness by the average value of the hardness with respect to the higher area fraction of ferrite or bainite.
- the hole expansion rate ⁇ of the final product and the critical bending radius (d / RmC) by 90 ° V-bending were used.
- the bending test was C direction bending.
- the tensile test (measurement of TS, u-EL, and EL), the bending test, and the hole expansion test were compliant with JIS Z 2241, JIS Z 2248 (V block 90 ° bending test), and the iron linkage standard JFS T1001, respectively.
- JIS Z 2241 JIS Z 2241
- JIS Z 2248 V block 90 ° bending test
- JFS T1001 iron linkage standard
- the pole density was measured at a measurement step of 0.5 ⁇ m with respect to the central part.
- the r value (Rankford value) in each direction was measured in accordance with JIS Z 2254 (2008) (ISO 10113 (2006)).
- surface shows that it is a value which does not satisfy
- TS ⁇ 440 (unit: MPa), TS ⁇ u ⁇ EL ⁇ 7000 (unit: MPa ⁇ %), TS ⁇ ⁇ ⁇ 30000 (unit: MPa ⁇ %), and d / RmC ⁇ 1 ( It can be said that this is a hot-rolled steel sheet that satisfies all the conditions of (no unit) at the same time, has high strength, and is excellent in uniform deformability and local deformability.
- P3-P6, P8, P9, P12, P15, P20, P22, P28, P32, P35, P42-P47, and P78-P140 are comparative examples that did not satisfy the conditions of the present invention.
- TS ⁇ 440 unit: MPa
- TS ⁇ u ⁇ EL ⁇ 7000 unit: MPa ⁇ %)
- TS ⁇ ⁇ ⁇ 30000 unit: MPa ⁇ %)
- d / RmC ⁇ 1 The unit is not satisfied.
- FIG. 1 and FIG. 2 are graphs showing the relationship between D1 and D2 and d / RmC with respect to the above example and the comparative example. As shown in FIG. 1 and FIG. 2, d / RmC ⁇ 1 is satisfied when D1 is 5.0 or less and when D2 is 4.0 or less.
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Abstract
Description
本願は、2011年5月25日に、日本に出願された特願2011-117432号に基づき優先権を主張し、その内容をここに援用する。
(1)本発明の一態様に係る熱延鋼板は、鋼板の化学組成が、質量%で、C:0.01%以上かつ0.4%以下、Si:0.001%以上かつ2.5%以下、Mn:0.001%以上かつ4.0%以下、Al:0.001%以上かつ2.0%以下を含有し、P:0.15%以下、S:0.03%以下、N:0.01%以下、O:0.01%以下に制限し、残部が鉄および不可避的不純物からなり;前記鋼板の表面から5/8~3/8の板厚範囲である板厚中央部では、{100}<011>、{116}<110>、{114}<110>、{112}<110>、{223}<110>の各結晶方位の極密度の相加平均で表される極密度である{100}<011>~{223}<110>方位群の平均極密度が1.0以上かつ5.0以下であり、かつ、{332}<113>の結晶方位の極密度が1.0以上かつ4.0以下であり;前記鋼板の金属組織に、複数の結晶粒が存在し、この金属組織が、面積率で、フェライトとベイナイトとを合わせて30%以上かつ99%以下、マルテンサイトを1%以上かつ70%以下含み;前記マルテンサイトの面積率を単位面積%でfM、前記マルテンサイトの平均サイズを単位μmでdia、前記マルテンサイト間の平均距離を単位μmでdis、前記鋼板の引張強度を単位MPaでTSとしたとき、下記の式1及び式2を満たす。
dia≦13μm ・・・(式1)
TS/fM×dis/dia≧500 ・・・(式2)
(2)上記(1)に記載の熱延鋼板では、前記鋼板の化学組成では、更に、質量%で、Mo:0.001%以上かつ1.0%以下、Cr:0.001%以上かつ2.0%以下、Ni:0.001%以上かつ2.0%以下、Cu:0.001%以上かつ2.0%以下、B :0.0001%以上かつ0.005%以下、Nb:0.001%以上かつ0.2%以下、Ti:0.001%以上かつ0.2%以下、V:0.001%以上かつ1.0%以下、W:0.001%以上かつ1.0%以下、Ca:0.0001%以上かつ0.01%以下、Mg:0.0001%以上かつ0.01%以下、Zr:0.0001%以上かつ0.2%以下、Rare Earth Metal:0.0001%以上かつ0.1%以下、As:0.0001%以上かつ0.5%以下、Co:0.0001%以上かつ1.0%以下、Sn:0.0001%以上かつ0.2%以下、Pb:0.0001%以上かつ0.2%以下、Y:0.0001%以上かつ0.2%以下、Hf:0.0001%以上かつ0.2%以下の1種以上を含有してもよい。
(3)上記(1)又は(2)に記載の熱延鋼板では、前記結晶粒の体積平均径が5μm以上かつ30μm以下であってもよい。
(4)上記(1)又は(2)に記載の熱延鋼板では、前記{100}<011>~{223}<110>方位群の平均極密度が1.0以上かつ4.0以下であり、前記{332}<113>の結晶方位の極密度が1.0以上かつ3.0以下であってもよい。
(5)上記(1)~(4)の何れか一項に記載の熱延鋼板では、前記マルテンサイトの長軸をLa及び短軸をLbとしたとき、下記の式3を満たす前記マルテンサイトの面積率が、前記マルテンサイト面積率fMに対して50%以上かつ100%以下であってもよい。
La/Lb≦5.0 ・・・(式3)
(6)上記(1)~(5)の何れか一項に記載の熱延鋼板では、前記金属組織が、面積率で、前記フェライトを30%以上かつ99%以下含んでもよい。
(7)上記(1)~(6)の何れか一項に記載の熱延鋼板では、前記金属組織が、面積率で、前記ベイナイトを5%以上かつ80%以上含んでもよい。
(8)上記(1)~(7)の何れか一項に記載の熱延鋼板では、前記マルテンサイトに焼き戻しマルテンサイトが含まれてもよい。
(9)上記(1)~(8)の何れか一項に記載の熱延鋼板では、前記鋼板の前記金属組織中の前記結晶粒のうち、粒径が35μmを超える粗大結晶粒の面積率が0%以上10%以下であってもよい。
(10)上記(1)~(9)の何れか一項に記載の熱延鋼板では、前記フェライトの硬さHが下記の式4を満たしてもよい。
H<200+30×[Si]+21×[Mn]+270×[P]+78×[Nb]1/2+108×[Ti]1/2 ・・・(式4)
(11)上記(1)~(10)の何れか一項に記載の熱延鋼板では、主相である前記フェライトまたは前記ベイナイトに対して100点以上の点について硬さの測定を行った場合に、前記硬さの標準偏差を前記硬さの平均値で除した値が0.2以下であってもよい。
(12)本発明の一態様に係る熱延鋼板の製造方法は、質量%で、C:0.01%以上かつ0.4%以下、Si:0.001%以上かつ2.5%以下、Mn:0.001%以上かつ4.0%以下、Al:0.001%以上、2.0%以下を含有し、P:0.15%以下、S:0.03%以下、 N:0.01%以下、O:0.01%以下に制限し、残部が鉄および不可避的不純物からなる化学組成を有する鋼に対して、1000℃以上かつ1200℃以下の温度範囲で、40%以上の圧下率のパスを少なくとも1回以上含む第1の熱間圧延を行い、前記鋼の平均オーステナイト粒径を200μm以下とし;下記の式5により算出される温度を単位℃でT1とし、下記式6により算出されるフェライト変態温度を単位℃でAr3とした場合、T1+30℃以上かつT1+200℃以下の温度範囲に30%以上の圧下率の大圧下パスを含み、T1+30℃以上かつT1+200℃以下の温度範囲での累積圧下率が50%以上であり、Ar3以上かつT1+30℃未満の温度範囲での累積圧下率が30%以下に制限され、圧延終了温度がAr3以上である第2の熱間圧延を前記鋼に対して行い;前記大圧下パスのうちの最終パスの完了から冷却開始までの待ち時間を単位秒でtとしたとき、この待ち時間tが下記の式7を満たし、平均冷却速度が50℃/秒以上であり、冷却開始時の鋼温度と冷却終了時の鋼温度との差である冷却温度変化が40℃以上かつ140℃以下であり、前記冷却終了時の鋼温度がT1+100℃以下である一次冷却を、前記鋼に対して行い;前記第2の熱間圧延の終了後に、15℃/秒以上かつ300℃/秒以下の平均冷却速度で、600℃以上かつ800℃以下の温度範囲まで、前記鋼を二次冷却し;600℃以上かつ800℃の温度範囲内で1秒以上かつ15秒以下前記鋼を保持し;前記保持後に、50℃/秒以上かつ300℃/秒以下の平均冷却速度で、室温以上かつ350℃以下の温度範囲まで、前記鋼を三次冷却し;室温以上かつ350℃以下の温度範囲で前記鋼を巻き取る。
T1=850+10×([C]+[N])×[Mn] ・・・(式5)
ここで、[C]、[N]及び[Mn]は、それぞれ、C、N及びMnの質量百分率である。
Ar3=879.4-516.1×[C]-65.7×[Mn]+38.0×[Si]+274.7×[P] ・・・(式6)
なお、この式6で、[C]、[Mn]、[Si]、及び[P]は、それぞれ、C、Mn、Si及びPの質量百分率である。
t≦2.5×t1 ・・・(式7)
ここで、t1は下記の式8で表される。
t1=0.001×((Tf-T1)×P1/100)2-0.109×((Tf-T1)×P1/100)+3.1 ・・・(式8)
ここで、Tfは前記最終パス完了時の前記鋼の摂氏温度であり、P1は前記最終パスでの圧下率の百分率である。
(13)上記(12)に記載の熱延鋼板の製造方法では、前記鋼は、前記化学組成として、更に、質量%で、Mo:0.001%以上かつ1.0%以下、Cr:0.001%以上かつ2.0%以下、Ni:0.001%以上かつ2.0%以下、Cu:0.001%以上かつ2.0%以下、B:0.0001%以上かつ0.005%以下、Nb:0.001%以上かつ0.2%以下、Ti:0.001%以上かつ0.2%以下、V:0.001%以上かつ1.0%以下、W:0.001%以上かつ1.0%以下、Ca:0.0001%以上かつ0.01%以下、Mg:0.0001%以上かつ0.01%以下、Zr:0.0001%以上かつ0.2%以下、Rare Earth Metal:0.0001%以上かつ0.1%以下、As:0.0001%以上かつ0.5%以下、Co:0.0001%以上かつ1.0%以下、Sn:0.0001%以上かつ0.2%以下、Pb:0.0001%以上かつ0.2%以下、Y:0.0001%以上かつ0.2%以下、Hf:0.0001%以上かつ0.2%以下の1種以上を含有し、前記式5により算出される温度の代わりに下記式9により算出される温度を前記T1としてもよい。
T1=850+10×([C]+[N])×[Mn]+350×[Nb]+250×[Ti]+40×[B]+10×[Cr]+100×[Mo]+100×[V] ・・・(式9)
ここで、[C]、[N]、[Mn]、[Nb]、[Ti]、[B]、[Cr]、[Mo]及び[V]は、それぞれ、C、N、Mn、Nb、Ti、B、Cr、Mo及びVの質量百分率である。
(14)上記(12)又は(13)に記載の熱延鋼板の製造方法では、前記待ち時間tが、さらに下記式10を満たしてもよい。
0≦t<t1 ・・・(式10)
(15)上記(12)又は(13)に記載の熱延鋼板の製造方法では、前記待ち時間tが、さらに下記式11を満たしてもよい。
t1≦t≦t1×2.5 ・・・(式11)
(16)上記(12)~(15)の何れか一項に記載の熱延鋼板の製造方法では、前記第1の熱間圧延で、40%以上の圧下率である圧下を少なくとも2回以上行い、前記平均オーステナイト粒径を100μm以下としてもよい。
(17)上記(12)~(16)の何れか一項に記載の熱延鋼板の製造方法では、前記第2の熱間圧延の終了後、3秒以内に、前記二次冷却を開始してもよい。
(18)上記(12)~(17)の何れか一項に記載の熱延鋼板の製造方法では、前記第2の熱間圧延で、各パス間の前記鋼の温度上昇を18℃以下としてもよい。
(19)上記(12)~(18)の何れか一項に記載の熱延鋼板の製造方法では、T1+30℃以上かつT1+200℃以下の温度範囲での圧延の最終パスが前記大圧下パスであってもよい。
(20)上記(12)~(19)の何れか一項に記載の熱延鋼板の製造方法では、前記保持が、600℃以上かつ680℃以下の温度範囲内で、3秒以上かつ15秒以下の保持であってもよい。
(21)上記(12)~(20)の何れか一項に記載の熱延鋼板の製造方法では、前記一次冷却を圧延スタンド間で行ってもよい。
結晶方位の極密度D2:1.0以上かつ4.0以下
本実施形態に係る熱延鋼板では、2種類の結晶方位の極密度として、5/8~3/8の板厚範囲(鋼板の表面から鋼板の板厚方向(深さ方向)に板厚の5/8~3/8の距離だけ離れた範囲)である板厚中央部における圧延方向に平行な(板厚方向を法線とする)板厚断面に対して、{100}<011>~{223}<110>方位群の平均極密度D1(以下では、平均極密度と省略する場合がある)と、{332}<113>の結晶方位の極密度D2とを制御している。
主相であるフェライト及びベイナイトは、比較的軟質であり高い変形能を有する。フェライトとベイナイトとを合わせて面積率が30%以上である場合に、本実施形態に係る熱延鋼板の均一変形能と局部変形能との両方の特性が満足される。より好ましくは、フェライトとベイナイトとを合わせて面積率で50%以上とする。一方、フェライトとベイナイトとを合わせた面積率が99%以上であると、鋼板の強度と均一変形能とが低下する。
第二相として硬質組織であるマルテンサイトが金属組織中に分散することで、強度と、均一変形能とを高めることが可能となる。マルテンサイトの面積率が1%未満の場合、硬質組織の分散が少なく、加工硬化率が低くなり、均一変形能が低下する。好ましくは、マルテンサイトの面積率が3%以上である。一方、面積率で70%を超えるマルテンサイトを含む場合には、硬質組織の面積率が高すぎるために、鋼板の変形能が大幅に減少する。強度と変形能とのバランスに応じて、マルテンサイトの面積率を50%以下としてもよい。好ましくは、マルテンサイトの面積率が30%以下であってもよい。より好ましくは、マルテンサイトの面積率が20%以下であってもよい。
マルテンサイトの平均サイズが13μmを超える場合、鋼板の均一変形能が低くなり、また、局部変形能も低くなる。これは、マルテンサイトの平均サイズが粗大であると、加工硬化に対する寄与が小さくなるため均一伸びが低くなり、また、粗大なマルテンサイトの周囲でボイドが発生しやすくなるため局部変形能が低くなると考えられる。好ましくは、マルテンサイトの平均サイズが10μm以下である。より好ましくは、マルテンサイトの平均サイズが7μm以下である。
また、本発明者らが鋭意検討した結果、引張強度を単位MPaでTS(Tensile Strength)、マルテンサイトの面積率を単位%でfM(fraction of Martensite)、マルテンサイトの結晶粒間の平均距離を単位μmでdis(distance)、マルテンサイトの結晶粒の平均サイズを単位μmでdia(diameter)としたとき、TS、fM、dis、diaの関係が下記の式1を満たす場合に、鋼板の均一変形能が向上することが明らかになった。
TS/fM×dis/dia≧500 ・・・(式1)
更に、マルテンサイトの結晶粒の長軸を単位μmでLaとし、短軸を単位μmでLbとしたとき、下記の式2を満たすマルテンサイトの結晶粒が、上記マルテンサイト面積率fMに対して、面積率で50%以上かつ100%以下である場合に、局部変形能が向上するので好ましい。
La/Lb≦5.0 ・・・(式2)
加えて、さらに変形能を向上させる場合には、金属組織中の結晶粒のサイズ、特に、体積平均径を微細化するとよい。さらに、体積平均径を微細化することで、自動車用鋼板などで求められる疲労特性(疲労限度比)も向上する。細粒に比べると粗大粒の数が変形能へ与える影響度が高いため、変形能は、個数平均径よりも体積の重み付け平均で算出される体積平均径と強く相関する。そのため、上記の効果を得る場合には、体積平均径が、5μm以上かつ30μm以下、望ましくは、5μm以上かつ20μm以下、さらに望ましくは、5μm以上かつ10μm以下であるとよい。
更に、局部変形能をより改善する場合には、金属組織の全構成要素について、単位面積当たりに粒径が35μmを超える粒(粗大粒)が占める面積の割合(粗大粒の面積率)を0%以上かつ10%以下に制限するとよい。粒径の大きな粒が増えると、引張強度が小さくなり、局部変形能も低下する。したがって、なるべく結晶粒を細粒にすることが好ましい。加えて、全ての結晶粒が均一かつ等価に歪を受けることにより局部変形能が改善されるため、粗大粒の量を制限することにより、局部的な結晶粒の歪を抑制することができる。
また、曲げ性、伸びフランジ性、バーリング加工性、穴拡げ性などの局部変形能をさらに向上させるためには、硬質組織であるマルテンサイトが金属組織中で分散しているほうが好ましい。そのため、マルテンサイトの結晶粒間の平均距離disの標準偏差が、0μm以上かつ5μm以下であると好ましい。この場合、少なくとも100個のマルテンサイトの結晶粒について、結晶粒間の距離を測定し、平均距離disとその標準偏差とを得るとよい。
主相である軟質なフェライトは、鋼板の変形能向上に寄与する。よって、フェライトの硬さHの平均値が、下記の式3を満たすことが望ましい。下記の式3以上に硬質なフェライトが存在すると、鋼板の変形能向上効果が得られない虞がある。なお、フェライトの硬さHの平均値は、ナノインデンターにて1mNの荷重にてフェライトの硬さを100点以上測定して求めることとする。
H<200+30×[Si]+21×[Mn]+270×[P]+78×[Nb]1/2+108×[Ti]1/2 ・・・(式3)
ここで、[Si]、[Mn]、[P]、[Nb]、及び[Ti]は、それぞれ、Si、Mn、P、Nb、及びTiの質量百分率である。
本発明者らは、主相であるフェライトまたはベイナイトの均質性に着目した検討を行った結果、この主相の均質性が高い組織であると、均一変形能と局部変形能とのバランスを好ましく改善できることを見出した。具体的には、フェライトの硬さの標準偏差を、フェライトの硬さの平均値で割った値が0.2以下であると、上記効果が得られるので好ましい。または、ベイナイトの硬さの標準偏差を、ベイナイトの硬さの平均値で割った値が0.2以下であると、上記効果が得られるので好ましい。この均質性は、主相であるフェライトまたはベイナイトについてナノインデンターにて1mNの荷重にて硬さを100点以上測定し、その平均値とその標準偏差とを用いることで定義できる。すなわち、硬さの標準偏差/硬さの平均値の値が低いほど均質性は高く、0.2以下の時にその効果が得られる。ナノインデンター(例えばCSIRO社製 UMIS-2000)では、結晶粒径よりも小さな圧子を使用することで、結晶粒界を含まない単一の結晶粒の硬さを測定することができる。
C(炭素)は、鋼板の強度を高める元素であり、また、マルテンサイトの面積率を確保するために必須な元素である。C含有量の下限を0.01%としたのは、マルテンサイトを面積率で1%以上得るためである。一方、C含有量が0.40%超になると鋼板の変形能が低下し、また、鋼板の溶接性も悪化する。好ましくは、C含有量を0.30%以下とする。
Si(ケイ素)は、鋼の脱酸元素であり、鋼板の機械的強度を高めるのに有効な元素である。また、Siは、熱間圧延後の温度制御時にフェライトを安定化させ、かつ、ベイナイト変態時のセメンタイト析出を抑制する元素である。しかし、Si含有量が、2.5%超となると、鋼板の変形能が低下し、また、鋼板に表面疵が発生しやすくなる。一方、Si含有量が0.001%未満では、上記効果を得ることが困難である。
Mn(マンガン)は、鋼板の機械的強度を高めるのに有効な元素である。しかし、Mn含有量が、4.0%超となると、鋼板の変形能が低下する。好ましくは、Mn含有量を3.5%以下とする。更に好ましくは、Mn含有量を3.0%以下とする。一方、Mn含有量が、0.001%未満では、上記効果を得ることが困難である。また、Mnは、鋼中のS(硫黄)を固定化することにより、熱間圧延時の割れを防ぐ元素でもある。Mn以外に、Sによる熱間圧延時の割れの発生を抑制するTiなどの元素が十分に添加されない場合には、Mn含有量とS含有量とが、質量%で、Mn/S≧20を満足することが望ましい。
Al(アルミニウム)は、鋼の脱酸元素である。また、Alは、熱間圧延後の温度制御時にフェライトを安定化させ、かつ、ベイナイト変態時のセメンタイト析出を抑制する元素である。この効果を得るために、Al含有量を0.001%以上とする。しかし、Al含有量が2.0%超では、溶接性が劣悪となる。また、定量的に効果を示すことが難しいが、Alは、鋼冷却時にγ(オーステナイト)からα(フェライト)へ変態が開始する温度Ar3を、顕著に上昇させる元素である。従って、Al含有量によって、鋼のAr3を制御してもよい。
P(リン)は不純物であり、過剰に鋼中に含有すると、熱間圧延または冷間圧延時の割れを助長する元素であり、また、鋼板の延性や溶接性を損なう元素である。したがって、P含有量を0.15%以下に制限する。好ましくは、P含有量を0.05%以下に制限する。なお、Pは固溶強化元素として作用し、また不可避的に鋼中に含まれるので、P含有量の下限を特に制限する必要がない。P含有量の下限は0%でもよい。また、現行の一般的な精錬(二次精錬を含む)を考慮すると、P含有量の下限は0.0005%であってもよい。
S(硫黄)は、不純物であり、過剰に鋼中に含有すると、熱間圧延によって伸張したMnSが生成され、鋼板の変形能を低下させる元素である。したがって、S含有量を0.03%以下に制限する。なお、Sは不可避的に鋼中に含まれるので、S含有量の下限を特に制限する必要がない。S含有量の下限は0%でもよい。また、現行の一般的な精錬(二次精錬を含む)を考慮すると、P含有量の下限は0.0005%であってもよい。
N(窒素)は、不純物であり、鋼板の変形能を低下させる元素である。したがって、N含有量を0.01%以下に制限する。なお、Nは不可避的に鋼中に含まれるので、N含有量の下限を特に制限する必要がない。N含有量の下限は0%でもよい。また、現行の一般的な精錬(二次精錬を含む)を考慮すると、N含有量の下限は0.0005%であってもよい。
O(酸素)は、不純物であり、鋼板の変形能を低下させる元素である。したがって、O含有量を0.01%以下に制限する。なお、Oは不可避的に鋼中に含まれるので、O含有量の下限を特に制限する必要がない。O含有量の下限は0%でもよい。また、現行の一般的な精錬(二次精錬を含む)を考慮すると、O含有量の下限は0.0005%であってもよい。
Nb:0.001%以上かつ0.2%以下
B:0.0001%以上かつ0.005%以下
Ti(チタニウム)、Nb(ニオブ)、B(ホウ素)は、鋼中の炭素及び窒素を固定して微細な炭窒化物を生成するので、鋼に析出強化、組織制御、細粒強化など効果をもたらす選択元素である。そのため、必要に応じて、Ti、Nb、Bのうちのいずれか1種以上を鋼中に添加してもよい。上記効果を得るために、Ti含有量を0.001%以上、Nb含有量を0.001%以上、B含有量を0.0001%以上とすることが望ましい。しかし、これらの選択元素を過度に鋼中に添加しても、上記飽和してしまうことに加え、熱延後の再結晶が抑制されて結晶方位の制御が困難になり、鋼板の加工性(変形能)を劣化させる虞がある。よって、Ti含有量を0.2%以下、Nb含有量を0.2%以下、B含有量を0.005%以下とすることが好ましい。なお、下限未満の量のこれらの選択元素が鋼中に含有されても、本実施形態における効果を損なわない。また、合金コストの低減のためには、これらの選択元素を意図的に鋼中に添加する必要がないので、これらの選択元素含有量の下限は、いずれも0%である。
REM:0.0001%以上かつ0.1%以下
Ca:0.0001%以上かつ0.01%以下
Mg(マグネシウム)、REM(Rare Earth Metal)、Ca(カルシウム)は、介在物を無害な形態に制御し、鋼板の局部変形能を向上させるために重要な選択元素である。そのため、必要に応じて、Mg、REM、Caのうちのいずれか1種以上を鋼中に添加してもよい。上記効果を得るために、Mg含有量を0.0001%以上、REM含有量を0.0001%以上、Ca含有量を0.0001%以上とすることが望ましい。一方、これらの選択元素を過剰に鋼中に添加すると、延伸した形状の介在物が形成され、鋼板の変形能を低下させる虞がある。よって、Mg含有量を0.01%以下、REM含有量を0.1%以下、Ca含有量を0.01%以下とすることが好ましい。なお、下限未満の量のこれらの選択元素が鋼中に含有されても、本実施形態における効果を損なわない。また、合金コストの低減のためには、これらの選択元素を意図的に鋼中に添加する必要がないので、これらの選択元素含有量の下限は、いずれも0%である。
Cr:0.001%以上かつ2.0%以下
Ni:0.001%以上かつ2.0%以下
W:0.001%以上かつ1.0%以下
Zr:0.0001%以上かつ0.2%以下
As:0.0001%以上かつ0.5%以下
Mo(モリブデン)、Cr(クロミウム)、Ni(ニッケル)、W(タングステン)、Zr(ジルコニウム)、As(ヒ素)は、鋼板の機械的強度を高める選択元素である。そのため、必要に応じて、Mo、Cr、Ni、W、Zr、Asのうちのいずれか1種以上を鋼中に添加してもよい。上記効果を得るために、Mo含有量を0.001%以上、Cr含有量を0.001%以上、Ni含有量を0.001%以上、W含有量を0.001%以上、Zr含有量を0.0001%以上、As含有量を0.0001%以上とすることが望ましい。しかし、これらの選択元素を過度に鋼中に添加すると、鋼板の変形能を低下させる虞がある。よって、Mo含有量を1.0%以下、Cr含有量を2.0%以下、Ni含有量を2.0%以下、W含有量を1.0%以下、Zr含有量を0.2%以下、As含有量を0.5%以下とすることが好ましい。なお、下限未満の量のこれらの選択元素が鋼中に含有されても、本実施形態における効果を損なわない。また、合金コストの低減のためには、これらの選択元素を意図的に鋼中に添加する必要がないので、これらの選択元素含有量の下限は、いずれも0%である。
Cu:0.001%以上かつ2.0%以下
V(バナジウム)及びCu(銅)は、Nb及びTi等と同様に、析出強化の効果を有する選択元素である。また、V及びCuの添加は、Nb及びTi等の添加により生じる局部変形能の低下と比較して、その低下の度合いが小さい。よって、高強度でかつ、穴拡げ性や曲げ性などの局部変形能をより高めたい場合には、NbやTiなどよりも効果的な選択元素である。そのため、必要に応じて、V及びCuのうちのいずれか1種以上を鋼中に添加してもよい。上記効果を得るために、V含有量を0.001%以下、Cu含有量を0.001%以下とすることが好ましい。しかし、これらの選択元素を過剰に鋼中に添加すると、鋼板の変形能を低下させる虞がある。よって、V含有量を1.0%以下、Cu含有量を2.0%以下とすることが好ましい。なお、下限未満の量のこれらの選択元素が鋼中に含有されても、本実施形態における効果を損なわない。また、合金コストの低減のためには、これらの選択元素を意図的に鋼中に添加する必要がないので、これらの選択元素含有量の下限は、いずれも0%である。
Co(コバルト)は、定量的に効果を示すことが難しいが、鋼冷却時にγ(オーステナイト)からα(フェライト)へ変態が開始する温度Ar3を、顕著に上昇させる選択元素である。従って、Co含有量によって、鋼のAr3を制御してもよい。また、Coは、鋼板の強度を向上させる選択元素である。上記効果を得るために、Co含有量を0.0001%以上とすることが好ましい。しかし、Coを過剰に鋼中に添加すると、鋼板の溶接性が劣化し、また鋼板の変形能を低下させる虞がある。よって、Co含有量を1.0%以下とすることが好ましい。なお、下限未満の量のこの選択元素が鋼中に含有されても、本実施形態における効果を損なわない。また、合金コストの低減のためには、この選択元素を意図的に鋼中に添加する必要がないので、この選択元素含有量の下限は、0%である。
Pb:0.0001%以上かつ0.2%以下
Sn(スズ)及びPb(鉛)は、めっき濡れ性とめっき密着性とを向上させるのに有効な選択元素である。そのため、必要に応じて、Sn及びPbのうちのいずれか1種以上を鋼中に添加してもよい。上記効果を得るために、Sn含有量を0.0001%以上、Pb含有量を0.0001%以上とすることが好ましい。しかし、これらの選択元素を過度に鋼中に添加すると、熱間での脆化が起こり熱間加工で割れが生じ、鋼板に表面疵が発生しやすくなる虞がある。よって、Sn含有量を0.2%以下、Pb含有量を0.2%以下とすることが好ましい。なお、下限未満の量のこれらの選択元素が鋼中に含有されても、本実施形態における効果を損なわない。また、合金コストの低減のためには、これらの選択元素を意図的に鋼中に添加する必要がないので、これらの選択元素含有量の下限は、0%である。
Hf:0.0001%以上かつ0.2%以下
Y(イットリウム)及びHf(ハフニウム)は、鋼板の耐食性を向上させるのに有効な選択元素である。そのため、必要に応じて、Y及びHfのうちのいずれか1種以上を鋼中に添加してもよい。上記効果を得るために、Y含有量を0.0001%以上、Hf含有量を0.0001%以上とすることが好ましい。しかし、これらの選択元素を過度に鋼中に添加すると、穴拡げ性などの局部変形能が低下する虞がある。よって、Y含有量を0.20%以下、Hf含有量を0.20%以下とすることが好ましい。また、Yは、鋼中で酸化物を形成し、鋼中の水素を吸着する効果を有する。このため鋼中の拡散性水素が低減され、鋼板の耐水素脆化特性を向上させることも期待できる。この効果も上記したY含有量の範囲内で得ることが出来る。なお、下限未満の量のこれらの選択元素が鋼中に含有されても、本実施形態における効果を損なわない。また、合金コストの低減のためには、これらの選択元素を意図的に鋼中に添加する必要がないので、これらの選択元素含有量の下限は、0%である。
第1の熱間圧延工程として、上記溶製及び鋳造した鋼塊を用いて、1000℃以上かつ1200℃以下(好ましくは1150℃以下)の温度範囲で、40%以上の圧下率の圧延パスを少なくとも1回以上行う。これらの条件で第1の熱間圧延を行うことで、第1の熱間圧延工程後の鋼板の平均オーステナイト粒径が200μm以下となり、最終的に得られる熱延鋼板の均一変形能と局部変形能との向上に寄与する。
第2の熱間圧延工程として、第1の熱間圧延工程後の鋼板に、下記の式4により算出される温度を単位℃でT1としたとき、T1+30℃以上かつT1+200℃以下の温度範囲に30%以上の圧下率の大圧下パスを含み、T1+30℃以上かつT1+200℃以下の温度範囲での累積圧下率が50%であり、Ar3℃以上かつT1+30℃未満の温度範囲での累積圧下率が30%以下に制限され、圧延終了温度がAr3℃以上である圧延を行う。
T1=850+10×([C]+[N])×[Mn]+350×[Nb]+250×[Ti]+40×[B]+10×[Cr]+100×[Mo]+100×[V] ・・・(式4)
なお、この式4で、[C]、[N]、[Mn]、[Nb]、[Ti]、[B]、[Cr]、[Mo]及び[V]は、それぞれ、C、N、Mn、Nb、Ti、B、Cr、Mo及びVの質量百分率である。
T1=850+10×([C]+[N])×[Mn] ・・・(式5)
また、鋼が上記の選択元素を含む化学組成の場合には、式5により算出される温度の代わりに、式4により算出される温度をT1(単位:℃)とする必要がある。
Ar3=879.4-516.1×[C]-65.7×[Mn]+38.0×[Si]+274.7×[P] ・・・(式6)
なお、この式6で、[C]、[Mn]、[Si]、及び[P]は、それぞれ、C、Mn、Si及びPの質量百分率である。
一次冷却工程として、上記したT1+30℃以上かつT1+200℃以下の温度範囲における1パスの圧下率が30%以上である大圧下パスのうちの最終パスの完了後、この最終パスの完了から冷却開始までの待ち時間を単位秒でtとしたとき、この待ち時間tが下記の式7を満たすように、鋼板に対して冷却を行う。ここで、式7中のt1は、下記の式8により求めることができる。式8中のTfは、大圧下パスのうちの最終パス完了時の鋼板の温度(単位:℃)であり、P1は、大圧下パスのうちの最終パスでの圧下率(単位:%)である。
t≦2.5×t1 ・・・(式7)
t1=0.001×((Tf-T1)×P1/100)2-0.109×((Tf-T1)×P1/100)+3.1 ・・・(式8)
二次冷却工程として、上記第2の熱間圧延後、及び、上記一次冷却工程後、15℃/秒以上かつ300℃/秒以下の平均冷却速度で、600℃以上かつ800℃以下の温度範囲まで、鋼板を冷却することが好ましい。この二次冷却工程中、鋼板の冷却が進行して鋼板の温度がAr3以下となると、オーステナイトがフェライトへ変態し始める。15℃/秒以上の平均冷却速度とすることで、オーステナイトの結晶粒の粗大化を好ましく抑制することができる。この平均冷却速度の上限を特に定める必要はないが、鋼板形状の観点から平均冷却速度が300℃/秒以下であればよい。また、上記第2の熱間圧延後、及び、上記一次冷却工程後から、3秒以内に二次冷却を開始することが好ましい。二次冷却開始が3秒を超えると、オーステナイトの粗大化を招く虞がある。
保持工程として、二次冷却工程後、600℃以上かつ800℃以下の温度範囲内で、1秒以上かつ15秒以下の間、鋼板を保持する。この温度域での保持によって、オーステナイトからフェライトへの変態が進み、鋼板のフェライト面積率を高めることができる。さらに好ましくは、600℃以上かつ680℃以下の温度範囲内で鋼板を保持する。このような比較的低温域でフェライト変態させることによって、フェライト組織を微細かつ均一に制御することができる。これに伴い、後工程で形成されるベイナイト及びマルテンサイトも、金属組織内で微細かつ均一に制御することができる。また、保持時間は、フェライト変態を進めるために1秒以上とする。しかし。15秒を超えると、フェライトの結晶粒が粗大になり、セメンタイトが析出する虞もある。600以上かつ680℃以下の比較的低温で保持する場合には、保持時間を3秒以上かつ15秒以下とすることが好ましい。
三次冷却工程として、保持工程後、50℃/秒以上かつ300℃/秒以下の平均冷却速度で、室温以上かつ350℃以下の温度範囲まで、鋼板を冷却する。この三次冷却工程中に、保持工程後でもフェライトに変態していないオーステナイトが、ベイナイトおよびマルテンサイトへ変態する。350℃超の温度で三次冷却を停止すると、温度が高すぎるためにベイナイト変態が過度に進行して、最終的にマルテンサイトを面積率で1%以上得ることが出来ない。なお、三次冷却工程の冷却停止温度の下限を特に定める必要はないが、水冷を前提とした場合、室温以上であればよい。また、50℃/秒未満の平均冷却速度で冷却すると、冷却中、パーライト変態が生じる可能性がある。なお、三次冷却工程の平均冷却速度の上限を特に定める必要はないが、操業上の観点から300℃以下であればよい。この平均冷却速度の上記範囲内で、平均冷却速度を遅くするとベイナイト面積率を高めることができる。一方、この平均冷却速度の上記範囲内で、平均冷却速度を速くするとマルテンサイト面積率を高めることができる。また、ベイナイト及びマルテンサイトの結晶粒径も微細となる。
巻き取り工程として、三次冷却工程後、三次冷却の冷却停止温度である室温以上かつ350℃以下の温度で、鋼板の巻き取りを開始し、そして空冷する。このようにして本実施形態に係る熱延鋼板を製造することができる。
Claims (21)
- 鋼板の化学組成が、質量%で、
C:0.01%以上かつ0.4%以下、
Si:0.001%以上かつ2.5%以下、
Mn:0.001%以上かつ4.0%以下、
Al:0.001%以上かつ2.0%以下
を含有し、
P :0.15%以下、
S :0.03%以下、
N :0.01%以下、
O :0.01%以下
に制限し、残部が鉄および不可避的不純物からなり;
前記鋼板の表面から5/8~3/8の板厚範囲である板厚中央部では、{100}<011>、{116}<110>、{114}<110>、{112}<110>、{223}<110>の各結晶方位の極密度の相加平均で表される極密度である{100}<011>~{223}<110>方位群の平均極密度が1.0以上かつ5.0以下であり、かつ、{332}<113>の結晶方位の極密度が1.0以上かつ4.0以下であり;
前記鋼板の金属組織に、複数の結晶粒が存在し、この金属組織が、面積率で、フェライトとベイナイトとを合わせて30%以上かつ99%以下、マルテンサイトを1%以上かつ70%以下含み;
前記マルテンサイトの面積率を単位面積%でfM、前記マルテンサイトの平均サイズを単位μmでdia、前記マルテンサイト間の平均距離を単位μmでdis、前記鋼板の引張強度を単位MPaでTSとしたとき、下記の式1及び式2を満たす;
ことを特徴とする熱延鋼板。
dia≦13μm ・・・(式1)
TS/fM×dis/dia≧500 ・・・(式2) - 前記鋼板の化学組成では、更に、質量%で、
Mo:0.001%以上かつ1.0%以下、
Cr:0.001%以上かつ2.0%以下、
Ni:0.001%以上かつ2.0%以下、
Cu:0.001%以上かつ2.0%以下、
B :0.0001%以上かつ0.005%以下、
Nb:0.001%以上かつ0.2%以下、
Ti:0.001%以上かつ0.2%以下、
V:0.001%以上かつ1.0%以下、
W:0.001%以上かつ1.0%以下、
Ca:0.0001%以上かつ0.01%以下、
Mg:0.0001%以上かつ0.01%以下、
Zr:0.0001%以上かつ0.2%以下、
Rare Earth Metal:0.0001%以上かつ0.1%以下、
As:0.0001%以上かつ0.5%以下、
Co:0.0001%以上かつ1.0%以下、
Sn:0.0001%以上かつ0.2%以下、
Pb:0.0001%以上かつ0.2%以下、
Y:0.0001%以上かつ0.2%以下、
Hf:0.0001%以上かつ0.2%以下
の1種以上を含有することを特徴とする請求項1に記載の熱延鋼板。 - 前記結晶粒の体積平均径が5μm以上かつ30μm以下であることを特徴とする請求項1または2に記載の熱延鋼板。
- 前記{100}<011>~{223}<110>方位群の平均極密度が1.0以上かつ4.0以下であり、前記{332}<113>の結晶方位の極密度が1.0以上かつ3.0以下であることを特徴とする請求項1または2に記載の熱延鋼板。
- 前記マルテンサイトの長軸をLa及び短軸をLbとしたとき、下記の式3を満たす前記マルテンサイトの面積率が、前記マルテンサイト面積率fMに対して50%以上かつ100%以下であることを特徴とする請求項1または2に記載の熱延鋼板。
La/Lb≦5.0 ・・・(式3) - 前記金属組織が、面積率で、前記フェライトを30%以上かつ99%以下含むことを特徴とする請求項1または2に記載の熱延鋼板。
- 前記金属組織が、面積率で、前記ベイナイトを5%以上かつ80%以上含むことを特徴とする請求項1または2に記載の熱延鋼板。
- 前記マルテンサイトに焼き戻しマルテンサイトが含まれることを特徴とする請求項1または2に記載の熱延鋼板。
- 前記鋼板の前記金属組織中の前記結晶粒のうち、粒径が35μmを超える粗大結晶粒の面積率が0%以上10%以下であることを特徴とする請求項1または2に記載の熱延鋼板。
- 前記フェライトの硬さHが下記の式4を満たすことを特徴とする請求項1または2に記載の熱延鋼板。
H<200+30×[Si]+21×[Mn]+270×[P]+78×[Nb]1/2+108×[Ti]1/2 ・・・(式4) - 主相である前記フェライトまたは前記ベイナイトに対して100点以上の点について硬さの測定を行った場合に、前記硬さの標準偏差を前記硬さの平均値で除した値が0.2以下であることを特徴とする請求項1または2に記載の熱延鋼板。
- 質量%で、
C:0.01%以上かつ0.4%以下、
Si:0.001%以上かつ2.5%以下、
Mn:0.001%以上かつ4.0%以下、
Al:0.001%以上、2.0%以下
を含有し、
P:0.15%以下、
S:0.03%以下、
N:0.01%以下、
O:0.01%以下
に制限し、残部が鉄および不可避的不純物からなる化学組成を有する鋼に対して、1000℃以上かつ1200℃以下の温度範囲で、40%以上の圧下率のパスを少なくとも1回以上含む第1の熱間圧延を行い、前記鋼の平均オーステナイト粒径を200μm以下とし;
下記の式5により算出される温度を単位℃でT1とし、下記式6により算出されるフェライト変態温度を単位℃でAr3とした場合、T1+30℃以上かつT1+200℃以下の温度範囲に30%以上の圧下率の大圧下パスを含み、T1+30℃以上かつT1+200℃以下の温度範囲での累積圧下率が50%以上であり、Ar3以上かつT1+30℃未満の温度範囲での累積圧下率が30%以下に制限され、圧延終了温度がAr3以上である第2の熱間圧延を前記鋼に対して行い;
前記大圧下パスのうちの最終パスの完了から冷却開始までの待ち時間を単位秒でtとしたとき、この待ち時間tが下記の式7を満たし、平均冷却速度が50℃/秒以上であり、冷却開始時の鋼温度と冷却終了時の鋼温度との差である冷却温度変化が40℃以上かつ140℃以下であり、前記冷却終了時の鋼温度がT1+100℃以下である一次冷却を、前記鋼に対して行い;
前記第2の熱間圧延の終了後に、15℃/秒以上かつ300℃/秒以下の平均冷却速度で、600℃以上かつ800℃以下の温度範囲まで、前記鋼を二次冷却し;
600℃以上かつ800℃の温度範囲内で1秒以上かつ15秒以下前記鋼を保持し;
前記保持後に、50℃/秒以上かつ300℃/秒以下の平均冷却速度で、室温℃以上かつ350℃以下の温度範囲まで、前記鋼を三次冷却し;
室温℃以上かつ350℃以下の温度範囲で前記鋼を巻き取る
ことを特徴とする熱延鋼板の製造方法。
T1=850+10×([C]+[N])×[Mn] ・・・(式5)
ここで、[C]、[N]及び[Mn]は、それぞれ、C、N及びMnの質量百分率である。
Ar3=879.4-516.1×[C]-65.7×[Mn]+38.0×[Si]+274.7×[P] ・・・(式6)
なお、この式6で、[C]、[Mn]、[Si]、及び[P]は、それぞれ、C、Mn、Si及びPの質量百分率である。
t≦2.5×t1 ・・・(式7)
ここで、t1は下記の式8で表される。
t1=0.001×((Tf-T1)×P1/100)2-0.109×((Tf-T1)×P1/100)+3.1 ・・・(式8)
ここで、Tfは前記最終パス完了時の前記鋼の摂氏温度であり、P1は前記最終パスでの圧下率の百分率である。 - 前記鋼は、前記化学組成として、更に、質量%で、
Mo:0.001%以上かつ1.0%以下、
Cr:0.001%以上かつ2.0%以下、
Ni:0.001%以上かつ2.0%以下、
Cu:0.001%以上かつ2.0%以下、
B:0.0001%以上かつ0.005%以下、
Nb:0.001%以上かつ0.2%以下、
Ti:0.001%以上かつ0.2%以下、
V:0.001%以上かつ1.0%以下、
W:0.001%以上かつ1.0%以下、
Ca:0.0001%以上かつ0.01%以下、
Mg:0.0001%以上かつ0.01%以下、
Zr:0.0001%以上かつ0.2%以下、
Rare Earth Metal:0.0001%以上かつ0.1%以下、
As:0.0001%以上かつ0.5%以下、
Co:0.0001%以上かつ1.0%以下、
Sn:0.0001%以上かつ0.2%以下、
Pb:0.0001%以上かつ0.2%以下、
Y:0.0001%以上かつ0.2%以下、
Hf:0.0001%以上かつ0.2%以下
の1種以上を含有し、前記式5により算出される温度の代わりに下記式9により算出される温度を前記T1とすることを特徴とする請求項12に記載の熱延鋼板の製造方法。
T1=850+10×([C]+[N])×[Mn]+350×[Nb]+250×[Ti]+40×[B]+10×[Cr]+100×[Mo]+100×[V] ・・・(式9)
ここで、[C]、[N]、[Mn]、[Nb]、[Ti]、[B]、[Cr]、[Mo]及び[V]は、それぞれ、C、N、Mn、Nb、Ti、B、Cr、Mo及びVの質量百分率である。 - 前記待ち時間tが、さらに下記式10を満たすことを特徴とする請求項12または13に記載の熱延鋼板の製造方法。
0≦t<t1 ・・・(式10) - 前記待ち時間tが、さらに下記式11を満たすことを特徴とする請求項12または13に記載の熱延鋼板の製造方法。
t1≦t≦t1×2.5 ・・・(式11) - 前記第1の熱間圧延で、40%以上の圧下率である圧下を少なくとも2回以上行い、前記平均オーステナイト粒径を100μm以下とすることを特徴とする請求項12または13に記載の熱延鋼板の製造方法。
- 前記第2の熱間圧延の終了後、3秒以内に、前記二次冷却を開始することを特徴とする請求項12または13に記載の熱延鋼板の製造方法。
- 前記第2の熱間圧延で、各パス間の前記鋼の温度上昇を18℃以下とすることを特徴とする請求項12または13に記載の熱延鋼板の製造方法。
- T1+30℃以上かつT1+200℃以下の温度範囲での圧延の最終パスが前記大圧下パスであることを特徴とする請求項12または13に記載の熱延鋼板の製造方法。
- 前記保持が、600℃以上かつ680℃以下の温度範囲内で、3秒以上かつ15秒以下の保持であることを特徴とする請求項12または13に記載の熱延鋼板の製造方法。
- 前記一次冷却を圧延スタンド間で行うことを特徴とする請求項12または13に記載の熱延鋼板の製造方法。
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