WO2018179389A1 - 熱間圧延鋼板および鋼製鍛造部品ならびにそれらの製造方法 - Google Patents
熱間圧延鋼板および鋼製鍛造部品ならびにそれらの製造方法 Download PDFInfo
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Definitions
- the present invention relates to a hot-rolled steel plate, a steel forged part, and a method for producing them.
- Steel sheets used in automobile body structures are required to have high strength and high press workability from the viewpoint of improving safety and reducing weight.
- a high-strength steel sheet excellent in hole expansibility (high burring property) better than before has been proposed.
- a steel sheet excellent in hole expansibility ( ⁇ value) a steel sheet of a ferrite main phase that is precipitation strengthened by fine precipitates such as Ti and Nb and a manufacturing method thereof have been reported.
- Patent Document 1 discloses a hot-rolled steel sheet having high strength and excellent stretch flangeability.
- Patent Document 2 discloses a high-formability, high-tensile hot-rolled steel sheet having excellent material uniformity.
- Patent Document 3 discloses a high-tensile hot-rolled steel sheet having excellent elongation and stretch flangeability.
- the processing of steel sheets for automobiles is not limited to conventional press processing elements, but new processing is applied to conventional press processing elements such as plate forging. Elements have been combined in a complex way.
- Conventional press working elements include, for example, deep drawing, hole expansion, stretch forming, bending, and ironing.
- plate forging is a press work having a composite working element including a working element peculiar to forging, in addition to a working element when pressing a conventional steel plate.
- the plate thickness of the steel plate remains the original plate thickness, or the steel plate is deformed while being reduced (thinned) by conventional press processing, while the part is being molded,
- the thickness of the steel sheet is increased (thickening) so that it can be efficiently deformed so that it has the thickness of the steel sheet necessary for its function. The strength of the parts can be ensured.
- Patent Documents 1 to 3 do not mention any processing including a composite processing element typified by plate forging. Moreover, the winding conditions for manufacturing the hot-rolled steel sheet described in Patent Document 1 are very strict and unrealistic. Furthermore, since the hot-rolled steel sheets described in Patent Documents 2 and 3 contain 0.07% or more of Mo, which is an expensive alloy element, there is a problem that the manufacturing cost is high.
- High burring steel is known to exhibit good formability in conventional pressing.
- plate forging which is a forming method that includes elements of forging in the conventional press working, cracks may occur in the steel sheet even when the degree of processing is small.
- press cracks occur in the areas where sheet thickness constriction (reduction in sheet thickness) occurs, but even in processes that do not involve sheet thickness constriction, such as sheet forging, the material cracks. It has been found that the product may not be obtained due to breakage.
- the present invention has been made in order to solve the above-mentioned problems, and while maintaining the basic function as a high burring steel, the crack limit of the portion subjected to forging by applying a partial compression force is achieved. It aims at providing the hot rolled steel plate excellent in the plate forgeability which can be improved.
- the present invention has been made in order to solve the above-mentioned problems, and provides the following hot-rolled steel sheet and forged steel parts and methods for producing them.
- the chemical composition of the steel sheet is mass%, C: 0.020 to 0.070%, Si: 0.05 to 1.70%, Mn: 0.60 to 2.50%, Al: 0.010 to 1.000% N: more than 0% to 0.0030% or less, P: 0.050% or less, S: 0.005% or less, Ti: 0.015 to 0.170%, Nb: 0 to 0.100%, V: 0 to 0.300%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Cr: 0 to 2.00% Mo: 0 to 1.00%, B: 0 to 0.0100%, Mg: 0 to 0.0100%, Ca: 0 to 0.0100%, REM: 0 to 0.1000%, Zr: 0 to 1.000%, Co: 0 to 1.000% Zn: 0 to 1.000%, W: 0 to 1.000% Sn: 0 to 0.050%, and Balance: Fe and impurities, In the cross section perpendicular to the rolling direction of the steel sheet, when the
- the average equivalent circle diameter of the precipitate containing Ti is 1.00 to 3.00 nm.
- Tensile strength is 780 MPa or more
- the product of uniform elongation and tensile strength is 7000 MPa ⁇ % or more
- the product of the hole expansion ratio and the tensile strength is 50000 MPa ⁇ % or more
- a method for producing a hot-rolled steel sheet according to any one of (1) to (3) above For the slab having the chemical composition described in (1) above, a heating process, a continuous hot rolling process, a first cooling process, a second cooling process, and a winding process are performed in order, In the heating step, the slab is heated to a temperature of SRTmin ° C. or higher and 1260 ° C. or lower represented by the following formula (i):
- the continuous hot rolling step includes rough rolling and multi-stage finish rolling of three or more stages, The end temperature of the rough rolling is 1100 ° C.
- the cumulative strain in the final three stages of rolling in the multistage finish rolling is 0.01 to 0.10
- the rolling end temperature of the multistage finish rolling is a temperature of Ar 3 + 30 ° C. or higher obtained by the following formula (ii): In the first cooling step, after the multistage finish rolling is completed, cooling is started after 1.00 to 5.00 s, and the temperature is 10 ° C./s or more from the rolling end temperature to a temperature range of 650 to 750 ° C.
- Cool at average cooling rate then hold in air for 1-10s
- cooling is performed at an average cooling rate of 10 ° C./s or more from a temperature range of 600 to 740 ° C.
- winding is performed at a winding temperature of 450 to 650 ° C. Manufacturing method of hot rolled steel sheet.
- At least forging processing is performed on the hot-rolled steel sheet according to any one of (1) to (3) above. Manufacturing method of steel forged parts.
- FIG.1 (a) is a figure which shows the test piece of a simple shear test.
- FIG.1 (b) is a figure which shows the test piece after a simple shear test.
- the inventors of the present invention conducted intensive studies to solve the above problems and obtained the following knowledge.
- (A) Equivalent plastic strain Plate forging includes deformation in a strain range (high strain range) exceeding the fracture strain in the conventional tensile test. Moreover, since plate forging is a complex process, it cannot be evaluated simply by tensile test and shear test data. Therefore, the present inventors introduced “equivalent plastic strain” as an index, and established a new evaluation method.
- Equivalent plastic strain converts the relationship between the shear stress ⁇ s and the shear plastic strain ⁇ sp in the simple shear test into the relationship between the tensile stress ⁇ and the tensile strain ⁇ in the uniaxial tensile test with different deformation modes. . Then, assuming the relationship between the isotropic hardening rule and the plastic work conjugate, the conversion can be performed as shown in the following equation by using a constant conversion coefficient ( ⁇ ). After calculating the conversion coefficient ( ⁇ ) by the method described later, the equivalent plastic strain is derived.
- the shear test is performed in multiple stages, and after each stage of the shear test, the starting point of the crack of the test piece generated in the part holding the test piece is machined to crack the test piece.
- the test results were evaluated by connecting these shear test results in series.
- conventional tensile testing methods can be applied to tensile stress and tensile strain.
- a JIS No. 5 test piece based on JIS Z2241 (2011) can be used.
- TiN a structure mainly composed of ferrite (precipitation strengthened ferrite) that is precipitation strengthened by fine precipitates such as Ti and Nb is used in order to obtain excellent hole expansibility.
- ferrite precipitation strengthened ferrite
- fine precipitates such as Ti and Nb
- TiN coarse TiN precipitates unless a special manufacturing method is used (hereinafter, the deposited TiN is also simply referred to as “TiN”).
- TiN is a compound in which TiN is thermodynamically very stable and preferential to other compounds at high temperatures such as casting during the steel sheet manufacturing process, hot rolling heating, or rough rolling initial stage. This is because of crystallization or precipitation.
- TiN is hard enough to be used as a coating for cutting tools, machine parts, plastic molding dies, sports equipment, ornaments, etc., and its hardness is known to be about Hv 2000-2300, and it is a very hard precipitate. It is a thing. Therefore, when subjected to deformation in a high strain region such as plate forging, voids are likely to occur at the interface due to the difference in deformability from the matrix structure.
- the equivalent plastic strain at break is 0.90 (90%) or more.
- the equivalent plastic strain at the time of fracture becomes 0.90 (90%) or more, and a certain workability can be obtained even in complex machining such as plate forging. Confirmed that it is possible to secure.
- the effective cumulative strain is an index that takes into account the temperature during rolling, the recovery of crystal grains due to the rolling reduction of the steel sheet by rolling, recrystallization, and grain growth. Therefore, when obtaining the effective cumulative strain, a constitutive law expressing a static recovery phenomenon over time after rolling was used. Considering that the grains recover statically over time after rolling, the release of energy accumulated as strain in the grains after rolling is due to static recovery due to the disappearance of dislocations in the thermal grains. Because it happens. The disappearance of this thermal dislocation is influenced by the rolling temperature and the elapsed time after rolling. Therefore, taking this static recovery into account, we introduced an index that describes the temperature during rolling, the rolling reduction (logarithmic strain) of the steel sheet due to rolling, and the elapsed time after rolling as parameters, and this is called “effective cumulative strain”. Defined.
- the desired microstructure can be obtained and the variation in nano hardness is reduced, so by suppressing the generation of voids at the interface between the hard tissue and the soft tissue, Since cracking does not occur even after plate forging, a steel plate excellent in plate forgeability can be obtained.
- C 0.020 to 0.070%
- C combines with Nb, Ti, etc. to form precipitates in the steel sheet, and contributes to strength improvement by precipitation strengthening.
- the C content is less than 0.020%, the effect by the above action cannot be sufficiently obtained.
- the C content exceeds 0.070%, the iron-based carbide that becomes the starting point of cracking during hole expansion processing increases, and the hole expansion value deteriorates. Therefore, the C content is 0.020 to 0.070%.
- the C content is preferably 0.025% or more, and more preferably 0.030% or more.
- it is preferable that C content is 0.060% or less, and it is more preferable that it is 0.050% or less.
- Si 0.05 to 1.70%
- Si has a deoxidizing effect and an effect of suppressing precipitation of iron-based carbides such as cementite in the material structure and contributing to improvement of ductility and hole expandability.
- carbides containing Ti are likely to precipitate at high temperatures. Precipitation of carbides at high temperatures tends to cause variations in the amount of precipitation, resulting in material variations such as strength and hole expandability. Therefore, the Si content is set to 0.05 to 1.70%.
- the Si content is preferably 0.06% or more, more preferably 0.08% or more, from the viewpoint of suppressing the occurrence of scale defects such as scales and spindle scales. Moreover, it is preferable that Si content is 1.50% or less, and it is more preferable that it is 1.00% or less from a viewpoint of improving chemical conversion property and corrosion resistance after coating.
- Mn 0.60 to 2.50%
- Mn is an element that contributes to strengthening ferrite and improving hardenability.
- the Mn content is set to 0.60 to 2.50%.
- the Mn content is preferably 1.00% or more, and more preferably 1.50% or more.
- Mn content is 2.00% or less, and it is more preferable that it is 1.80% or less.
- Al 0.010 to 1.000%
- Al like Si, has a deoxidizing effect and an effect of generating ferrite.
- the Al content is set to 0.010 to 1.000%.
- the Al content is preferably 0.015% or more or 0.020% or more, and more preferably 0.025% or more or 0.030% or more.
- the Al content is preferably 0.800% or less, 0.700% or less or 0.600% or less, more preferably 0.500% or less or 0.400% or less.
- N More than 0% to 0.0030% or less If N is contained in a large amount, not only does solid-solution nitrogen remain and ductility decreases, but TiN precipitates and decreases hole expansibility. Therefore, the N content is 0.0030% or less. The N content is preferably 0.0025% or less.
- P 0.050% or less
- P is an impurity contained in the hot metal, and since it segregates at the grain boundaries, it degrades local ductility and weldability. Therefore, the P content is limited to 0.050% or less.
- the P content is preferably 0.030% or less or 0.020% or less.
- the lower limit is 0%. However, excessively reducing the content increases the cost during refining, so the lower limit may be made 0.001%.
- S 0.005% or less
- S is also an impurity contained in the hot metal, and forms MnS to deteriorate local ductility and weldability. Therefore, the S content is limited to 0.005% or less.
- the S content may be 0.003% or less or 0.002% or less.
- the lower limit is 0%. However, excessively reducing the content increases the cost during refining, so the lower limit may be made 0.0005%.
- Ti 0.015 to 0.170%
- Ti has the effect that carbonitride or solute Ti delays grain growth during hot rolling, thereby reducing the grain size of the hot-rolled sheet and improving low-temperature toughness. Further, by finely dispersing in the ferrite as TiC, it contributes to increasing the strength of the steel sheet through precipitation strengthening. However, when the content is excessive, in addition to saturation of the effect, TiN that is a hard precipitate is likely to be precipitated. Therefore, the Ti content is set to 0.015 to 0.170%.
- the Ti content is preferably 0.030% or more, 0.045% or more, or 0.060% or more, 0.070% or more, 0.080% or more, 0.090% or more, or 0.100% or more. It is more preferable that The Ti content is preferably 0.160% or less, 0.150% or less, 0.140% or less, 0.130% or less, or 0.120% or less.
- Nb 0 to 0.100%
- Nb has the effect of reducing the grain size of the hot-rolled sheet and improving low-temperature toughness by delaying grain growth during hot rolling by carbonitride or solute Nb.
- NbC by existing as NbC, it contributes to the strengthening of a steel plate through precipitation strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Nb content is 0.100% or less. If necessary, the Nb content may be 0.080% or less, 0.060% or less, or 0.050% or less.
- the lower limit is 0%, but the lower limit may be 0.001% or 0.010% in order to sufficiently obtain the above effect.
- V 0 to 0.300%
- V is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the V content is set to 0.300% or less. If necessary, the V content may be 0.200% or less, 0.100% or less, or 0.060% or less. The lower limit is 0%, but the lower limit may be 0.001% or 0.010% in order to sufficiently obtain the above effect.
- Cu 0 to 2.00%
- Cu is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Cu content is 2.00% or less. In addition, if the Cu content is large, scratches due to scale may occur on the surface of the steel sheet. Therefore, the Cu content may be 1.20% or less, 0.80% or less, 0.50% or less, or 0.25% or less.
- the lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Cu content may be 0.01%.
- Ni 0 to 2.00%
- Ni is an element having an effect of improving the strength of the steel sheet by solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Ni content is 2.00% or less. Moreover, when Ni content is contained abundantly, there exists a possibility that ductility may deteriorate. Therefore, the Ni content may be 0.60% or less, 0.35% or less, or 0.20% or less. The lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Ni content may be 0.01%.
- Cr 0 to 2.00% Cr is an element having an effect of improving the strength of the steel sheet by solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Cr content is 2.00% or less.
- the upper limit may be set to 1.00%, 0.60%, or 0.30%.
- the lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Cr content may be 0.01%.
- Mo 0 to 1.00%
- Mo is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Mo content is set to 1.00% or less. In order to further improve economy, the upper limit may be set to 0.60%, 0.30%, or 0.10%. The lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Mo content may be 0.005% or 0.01%.
- B 0 to 0.0100% B segregates at the grain boundaries and improves the low temperature toughness by increasing the grain boundary strength. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the B content is 0.0100% or less. Further, B is a strong quenching element, and if its content is large, ferrite transformation does not proceed sufficiently during cooling, and sufficient retained austenite may not be obtained. Therefore, the B content may be 0.0050% or less, 0.0020% or less, or 0.0015%. The lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the B content may be 0.0001% or 0.0002%.
- Mg 0 to 0.0100%
- Mg is an element that improves the workability by controlling the form of non-metallic inclusions that become the starting point of fracture and cause the workability to deteriorate. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Mg content is 0.0100% or less.
- the lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Mg content may be 0.0001% or 0.0005%.
- Ca 0 to 0.0100% Ca is an element that improves the workability by controlling the form of non-metallic inclusions that become the starting point of fracture and cause the workability to deteriorate. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Ca content is 0.0100% or less.
- the lower limit is 0%, but in order to sufficiently obtain the above effects, the Ca content is preferably 0.0005% or more.
- REM 0 to 0.1000% REM (rare earth element) is an element that improves the workability by controlling the form of non-metallic inclusions that become the starting point of destruction and cause the workability to deteriorate. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the REM content is 0.1000% or less. If necessary, the upper limit may be 0.0100% or 0.0060%. The lower limit is 0%, but the lower limit of the REM content may be 0.0001% or 0.0005% in order to sufficiently obtain the above effect.
- REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total content of these elements.
- the lanthanoid is industrially added in the form of misch metal.
- Zr 0 to 1.000% Co: 0 to 1.000% Zn: 0 to 1.000% W: 0 to 1.000% It has been confirmed that even if Zr, Co, Zn, and W are each in the range of 1.000% or less, the effects of the present invention are not impaired. These upper limits may be set to 0.300% or 0.10%.
- the total content of Zr, Co, Zn and W is preferably 1.000% or less or 0.100%. These contents are not essential, and the lower limit is 0%, but the lower limit may be 0.0001% if necessary.
- Sn 0 to 0.050% It has been confirmed that the effect of the present invention is not impaired even if Sn is contained in a small amount. However, if it exceeds 0.05%, wrinkles may occur during hot rolling. Therefore, the Sn content is 0.050% or less.
- the content of Sn is not essential, and the lower limit is 0%, but the lower limit may be 0.001% if necessary.
- the balance is Fe and impurities.
- impurities are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when industrially manufacturing steel sheets, and are permitted within a range that does not adversely affect the present invention. Means something.
- (B) Metal structure The metal structure of the steel plate of this invention is demonstrated.
- the metallographic structure is 1/4 W or 3/4 W from the end face of the steel sheet when the width and thickness of the steel sheet are W and t, respectively, in a cross section perpendicular to the rolling direction of the steel sheet, and The structure at a position of 1/4 t or 3/4 t from the surface of the steel sheet.
- “%” means “area%”.
- Precipitation strengthened ferrite 5 to 70% Supersaturation of Ti carbides when fine precipitates containing Ti (such as finely precipitated Ti carbides, hereinafter also referred to as “fine Ti precipitates”) undergo a ⁇ ⁇ ⁇ transformation during cooling after rolling. With the degree of driving force, Ti carbide is precipitated at the phase interface or homogeneously nucleated in the ferrite, and the pro-eutectoid ferrite in which the Ti carbide is finely dispersed is precipitation strengthened ferrite (hereinafter also referred to as “precipitation strengthened ferrite”). ). Precipitation strengthened ferrite is a structure necessary for achieving both excellent uniform elongation and strength.
- the area ratio of the precipitation strengthened ferrite is set to 5 to 70%.
- the area ratio of precipitation strengthened ferrite is preferably 7% or more, and more preferably 10% or more. Further, the area ratio of the precipitation strengthened ferrite is preferably 65% or less, and more preferably 60% or less.
- the precipitation strengthened ferrite means a ferrite in which the number density of fine Ti precipitates contained in the grains is 1.0 ⁇ 10 16 to 50.0 ⁇ 10 16 pieces / cm 3. . If the number density of fine Ti precipitates contained in the ferrite grains is less than 1.0 ⁇ 10 16 pieces / cm 3 , the effect of precipitation strengthening cannot be sufficiently obtained. On the other hand, when the number density of fine Ti precipitates exceeds 50.0 ⁇ 10 16 pieces / cm 3 , not only the strength is saturated but also the ductility is lowered.
- the area ratio of precipitation strengthened ferrite is 5 to 70%.
- the area ratio of ferrite is 5 to 70% and the number density of fine Ti precipitates contained in the ferrite grains is 1.0. It means that it is ⁇ 10 16 to 50.0 ⁇ 10 16 pieces / cm 3 .
- the average equivalent circle diameter of the fine Ti precipitates contained in the grains of the precipitation strengthened ferrite is preferably 1.00 to 3.00 nm. If the average equivalent circle diameter of the fine Ti precipitate is less than 1.00 nm, it is difficult to obtain the effect of precipitation strengthening. On the other hand, if the average equivalent circle diameter is more than 3.00 nm due to coarse grains, a sufficient amount of fine Ti precipitate is precipitated. This is because things cannot be secured.
- Bainite 30-95% Bainite is a structure necessary for obtaining a balance between strength and local ductility, and has an effect of suppressing crack propagation. However, if the amount of bainite increases too much, ferrite decreases, and the local elongation is excellent, but the uniform elongation is significantly deteriorated. Therefore, the area ratio of bainite is set to 30 to 95%. The area ratio of bainite is preferably 80% or less, and more preferably 70% or less when the uniform elongation is important.
- Residual austenite 2% or less
- the high burring steel is characterized by ensuring high strength and ensuring both strength and workability while ensuring workability due to the presence of precipitation strengthened ferrite and bainite.
- the presence of thermodynamically stable retained austenite that did not cause martensitic transformation in the steel sheet means that the C concentration of the retained austenite is high, and the retained austenite is formed by work-induced transformation during sheet forging. The hardness of the site becomes too high and promotes the generation of voids. Therefore, the retained austenite is preferably as small as possible, and the area ratio is 2% or less.
- the area ratio of retained austenite is preferably 1.5% or less, 1% or less, or 0.5% or less. In particular, there is no need to define a lower limit, and the lower limit is 0%, and 0% is most preferable.
- Martensite 2% or less High burring steel is characterized by ensuring both high strength and workability while ensuring workability due to the presence of precipitation strengthened ferrite and bainite.
- the area ratio of martensite which is a hard structure, exceeds 2%, voids are likely to be generated at the boundary between martensite and ferrite with an increase in distortion of the steel sheet due to plate forging, and breakage is likely to occur. Therefore, the area ratio of martensite is 2% or less.
- the area ratio of martensite is preferably 1.5% or less, 1% or less, or 0.5% or less. In particular, it is not necessary to specify a lower limit, and the lower limit is 0%.
- Pearlite 1% or less Since pearlite is the starting point of fracture during hole expansion molding, its area ratio is 1% or less. The area ratio of pearlite is preferably 0.5% or less. The area ratio of pearlite is preferably reduced as much as possible, and is preferably 0%.
- Total of precipitation strengthening ferrite and bainite 95% or more High burring steel has precipitation strengthening ferrite that achieves both excellent uniform elongation and strength, and bainite that satisfies both strength and local ductility. This provides excellent strength, uniform elongation and local ductility. If the total area ratio of precipitation strengthened ferrite and bainite is less than 95%, these characteristics deteriorate. Therefore, the total area ratio of precipitation strengthened ferrite and bainite is 95% or more. The total area ratio is preferably 97% or more, and more preferably 98% or more.
- the area ratio of the metal structure is obtained as follows. As described above, first, a sample is taken from a position of 1/4 W or 3/4 W from the end surface of the steel plate and from a position of 1/4 t or 3/4 t from the surface of the steel plate. And the rolling direction cross section (what is called L direction cross section) of this sample is observed.
- the sample is subjected to nital etching, and after etching, observation is performed in a 300 ⁇ m ⁇ 300 ⁇ m visual field using an optical microscope. Then, by performing image analysis on the obtained structure photograph, the area ratio A of ferrite, the area ratio B of pearlite, and the total area ratio C of bainite, martensite and retained austenite are obtained.
- the nital-etched portion is repeller-etched and observed with a 300 ⁇ m ⁇ 300 ⁇ m field of view using an optical microscope.
- the total area ratio D of a retained austenite and a martensite is computed by performing image analysis with respect to the obtained structure
- the volume fraction of retained austenite is obtained by X-ray diffraction measurement using a sample that is chamfered from the normal direction of the rolling surface to 1 ⁇ 4 depth of the plate thickness. Since the volume ratio is substantially equal to the area ratio, the volume ratio is defined as the area ratio E of retained austenite.
- the area ratio of bainite is determined from the difference between the area ratio C and the area ratio D, and the area ratio of martensite is determined from the difference between the area ratio E and the area ratio D.
- the area ratio of precipitation strengthened ferrite can be determined by the Kernel Average Misoration (KAM) method equipped in EBSP-OIMTM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy).
- KAM Kernel Average Misoration
- the analysis conditions for precipitation strengthened ferrite in the present invention are as follows.
- EBSP-OIMTM the average orientation difference between adjacent pixels is calculated by the third approximation, and the portion where this orientation difference is calculated to be 1 ° or less is precipitation strengthened. Ferrite was used.
- the formation temperature range of the precipitation-strengthened ferrite of the present invention is that the Ti carbide precipitates at the phase interface or homogeneously nucleates in the ferrite with the driving force as the supersaturation degree of the Ti carbide during ⁇ ⁇ ⁇ transformation during cooling after rolling. It matches the temperature range.
- Polygonal pro-eutectoid ferrite transformed at high temperature is formed by diffusion transformation, so that the dislocation density is small and the intra-granular strain is small, so the intra-granular difference in crystal orientation is small. Therefore, the crystal orientation difference of the precipitation strengthened ferrite is similarly reduced.
- the polygonal ferrite area ratio obtained by optical microscope observation and the area where the azimuth difference in the third approximation measured by the KAM method is 1 ° or less can be obtained. This is because the area ratios are almost the same.
- the measurement of the area fraction of precipitation strengthened ferrite was carried out in detail as follows.
- a sample collected in the same manner as described in the structure observation was polished with a colloidal silica abrasive for 30 to 60 minutes, and EBSP measurement was performed under measurement conditions of a magnification of 400 times, an area of 160 ⁇ m ⁇ 256 ⁇ m, and a measurement step of 0.5 ⁇ m.
- the EBSP-OIMTM method irradiates an electron beam onto a highly inclined sample in a scanning electron microscope (SEM), images the Kikuchi pattern formed by backscattering with a high-sensitivity camera, and processes the computer image. It consists of a device and software that measure the crystal orientation of the glass in a short time.
- the EBSP method allows quantitative analysis of the microstructure and crystal orientation of the bulk sample surface, and the analysis area is an area that can be observed with an SEM. Depending on the resolution of the SEM, the analysis can be performed with a resolution of a minimum of 20 nm. The analysis takes several hours and is performed by mapping tens of thousands of points to be analyzed in a grid at equal intervals. In a polycrystalline material, the crystal orientation distribution and crystal grain size in the sample can be seen.
- a needle-like sample is prepared from a sample to be measured by cutting and electrolytic polishing using a focused ion beam processing method in combination with an electrolytic polishing method as necessary.
- the accumulated data can be reconstructed and obtained as an actual distribution image of atoms in real space.
- the size of the fine Ti precipitate is an equivalent circle diameter calculated by assuming that the fine Ti precipitate is spherical from the number of Ti atoms constituting the fine Ti precipitate and the lattice constant of the fine Ti precipitate.
- a method for obtaining the equivalent circle diameter (diameter) R of the precipitate using the number of Ti atoms of the fine Ti precipitate obtained by the three-dimensional atom probe measurement method is shown below.
- the equivalent circle diameter is calculated to be approximately 1 nm.
- the equivalent circle diameter (diameter) of 30 or more fine Ti precipitates is arbitrarily measured, and the average value is obtained.
- the number density of fine Ti precipitates is determined using the measurement field of view as the denominator and the number of fine Ti precipitates as the numerator.
- 5 or more fields of view of 10 nm (plate thickness direction t) ⁇ 40 nm (plate width direction W) ⁇ 60 nm (plate longitudinal direction L) were measured, and the number density (pieces / cm 3 ) The average value was obtained.
- TiN In the present invention, the existence state of TiN is also defined as follows.
- the average equivalent circle diameter of TiN 1.0-10.0 ⁇ m
- the average equivalent circle diameter of TiN is set to 10.0 ⁇ m or less.
- the average equivalent circle diameter of TiN is preferably 8.0 ⁇ m or less, and more preferably 5.0 ⁇ m or less.
- TiN is preferably as small as possible, it is not necessary to originally provide a lower limit for the average equivalent circle diameter of TiN. However, in the TiN observation method described later, if the equivalent circle diameter of TiN is less than 1.0 ⁇ m, it is difficult to determine whether it is TiN. Therefore, in the present invention, only those having an equivalent circle diameter of 1.0 ⁇ m or more are set as TiN as measurement targets. Therefore, the average equivalent circle diameter of TiN is 1.0 ⁇ m or more.
- the average equivalent circle diameter (diameter) of TiN is obtained as follows. As described above, first, a sample is taken from a position of 1/4 W or 3/4 W from the end surface of the steel plate and from a position of 1/4 t or 3/4 t from the surface of the steel plate. Then, the rolling direction cross section (so-called L direction cross section) of the sample is polished and observed without being etched. Specifically, a microstructure photograph is taken at a magnification of 1000 times using an optical microscope, and the microstructure photograph is observed visually or with an image processing apparatus or the like.
- the equivalent circle diameter (diameter) is obtained for those that can be identified as TiN, and only those having an equivalent circle diameter (diameter) of 1.0 ⁇ m or more are defined as TiN. Then, the visual field of 60 ⁇ m (rolling direction L) ⁇ 40 ⁇ m (sheet thickness direction t) was observed for 20 or more visual fields, and the average of the equivalent circle diameter (diameter) of TiN was averaged. Diameter).
- Average value of the shortest distance between adjacent TiN 10.0 ⁇ m or more
- the distance between TiN It is necessary to secure a certain amount. Therefore, the average value of the distance between adjacent TiNs is set to 10.0 ⁇ m or more.
- the average value is preferably 15.0 ⁇ m or more, and more preferably 20.0 ⁇ m or more.
- the upper limit is not particularly set, it is inevitable that TiN is deposited to some extent. Therefore, the average value of the shortest distance between adjacent TiNs is preferably set to 1000 ⁇ m or less.
- the average value of the shortest distance between adjacent TiNs is obtained as follows. Twenty arbitrary TiNs are selected, the distance to the nearest TiN is measured, and the average value is calculated. In addition, the measurement of the shortest distance between TiN is calculated
- Nano hardness can be measured using, for example, TriscopeScope / TriboIndenter manufactured by Hystron.
- the nano hardness of 100 points or more can be arbitrarily measured at a load of 1 mN, and the standard deviation of the nano hardness can be calculated from the result.
- the standard deviation of the nano hardness should be as small as 1.0 GPa or less.
- the standard deviation of nano hardness is preferably 0.8 GPa or less.
- Tensile strength 780 MPa or more
- the steel sheet according to the present invention preferably has a tensile strength of 780 MPa or more equivalent to that of conventional high burring steel.
- the upper limit of the tensile strength is not particularly required, but may be 1200 MPa, 1150 MPa, or 1000 MPa. However, the tensile strength indicates the tensile strength of JIS Z 2241 (2011).
- the uniform elongation is a nominal value at which the value obtained when the nominal stress ⁇ n is differentiated by the nominal strain ⁇ n is zero in the relationship between the nominal stress ⁇ n and the nominal strain ⁇ n in the test specified by JIS Z 2241 (2011).
- the strain is ⁇ n0, it is expressed by the following formula.
- Uniform elongation (u-EL) ln ( ⁇ n0 + 1)
- the hole expansion rate ( ⁇ ) represents ( ⁇ ) of the hole expansion rate according to a test method based on JIS Z 2256 (2010).
- Equivalent plastic strain 0.9 or more Equivalent plastic strain is the relationship between the shear stress ⁇ s and the shear plastic strain ⁇ sp in the simple shear test, and the tensile stress ⁇ and tensile strain ⁇ in the uniaxial tensile test with different deformation modes. Assuming the relationship between the isotropic hardening rule and the plastic work conjugate, the relationship is converted using a constant conversion coefficient ( ⁇ ).
- the isotropic hardening law is a work hardening law that assumes that the shape of the yield curve does not change even when strain progresses (that is, expands to a similar shape).
- the relation of plastic work conjugation is a relation that work hardening is described as a function of only plastic work, and shows the same work hardening amount when given the same plastic work ( ⁇ ⁇ ⁇ ) regardless of the deformation form.
- the conversion coefficient ⁇ is determined so that the relationship between shear stress and shear plastic strain is similar to the relationship between tensile stress and tensile strain.
- the conversion coefficient ⁇ can be obtained by the following procedure. First, the relationship between tensile strain ⁇ (actual value) and tensile stress ⁇ (actual value) in a uniaxial tensile test is obtained. Subsequently, the relationship between the shear strain ⁇ s (actual value) and the shear stress ⁇ s (actual value) in the uniaxial shear test is obtained.
- the tensile strain ⁇ (conversion) obtained from the shear strain ⁇ s (actual value) and the tensile stress ⁇ (conversion) obtained from the shear stress ⁇ s (actual value) are obtained in advance.
- the tensile stress ⁇ (conversion) is determined when the strain ⁇ (conversion) is between 0.2% and uniform elongation (u-EL).
- u-EL uniform elongation
- the equivalent plastic strain ⁇ eq is defined as a value obtained by converting the shear plastic strain ⁇ sp (rupture) at the time of rupture in the simple shear test into the tensile strain ⁇ in the simple tensile test using the obtained ⁇ .
- the steel plate according to the present invention is characterized by good processing characteristics in a high strain region represented by plate forging, and the equivalent plastic strain ⁇ eq satisfies 0.50 or more. Since the equivalent plastic strain of the conventional TRIP steel is at most about 0.30, it was confirmed that the plate forgeability of the steel sheet according to the present invention is good.
- the steel plate according to the present invention is mainly used for automobiles and the like, and its thickness range is mainly 1.0 to 4.0 mm. For this reason, the plate thickness range may be 1.0 to 4.0 mm.
- the lower limit is 1.2 mm, 1.4 mm, or 1.6 mm
- the upper limit is 3.6 mm, 3.2 mm, or 2. It may be 8 mm.
- the manufacturing method preceding hot rolling is not particularly limited. That is, it adjusts so that it may become the component composition mentioned above by performing various secondary smelting following melting by a blast furnace or an electric furnace. Then, what is necessary is just to manufacture a slab by methods, such as normal continuous casting and thin slab casting. At that time, scrap or the like may be used as a raw material as long as it can be controlled within the component range of the present invention.
- (B) Heating process The manufactured slab is hot-rolled to obtain a hot-rolled steel sheet.
- the slab is heated.
- the slab is heated to a temperature of SRTmin ° C. or higher and 1260 ° C. or lower represented by the following formula (i).
- SRTmin means the solution temperature of TiC.
- SRTmin 7000 / ⁇ 2.75-log (Ti ⁇ C) ⁇ -273 (i)
- the element symbol in the said formula represents content (mass%) in the hot-rolled steel plate of each element, and shall substitute 0 when not containing.
- the end temperature of rough rolling is 1100 ° C. or higher so that precipitates containing Ti do not precipitate.
- the multistage finish rolling is performed by continuous rolling of three or more stages (for example, six stages or seven stages). Then, multi-stage finish rolling is performed so that the cumulative strain (effective cumulative strain) in the final three-stage rolling is 0.01 to 0.10.
- the effective cumulative strain is the change in crystal grain size due to rolling temperature, rolling reduction of the steel sheet due to rolling, and change in crystal grain size where the crystal grains recover statically over time after rolling. It is an index that takes into account.
- the effective cumulative strain ( ⁇ eff) can be obtained by the following equation.
- Effective cumulative strain ( ⁇ eff) ⁇ i (ti, Ti) (iii)
- ⁇ i is expressed by the following equation.
- ⁇ i (ti, Ti) ei / exp ((ti / ⁇ R) 2/3 )
- ti Time from the last i-th rolling to the start of primary cooling after the last rolling (s)
- Q Q:
- the desired microstructure can be obtained and the variation in nano hardness can be reduced.
- it suppresses the growth of voids generated at the interface between the hard and soft tissues, makes it difficult to bond even when voids grow, and does not generate cracks even after plate forging. Steel plate can be obtained.
- the end temperature of multi-stage finish rolling may be set to Ar 3 (° C.) + 30 ° C. or higher using Ar 3 obtained by the following equation (ii). This is because the intended precipitation-strengthened ferrite and bainite are obtained in the present invention.
- Ar 3 970-325 ⁇ C + 33 ⁇ Si + 287 ⁇ P + 40 ⁇ Al ⁇ 92 ⁇ (Mn + Mo + Cu) ⁇ 46 ⁇ (Cr + Ni) (ii)
- the element symbol in the said formula represents content (mass%) in the hot-rolled steel plate of each element, and shall substitute 0 when not containing.
- the average cooling rate in the first cooling step is less than 10 ° C./s, pearlite is easily generated.
- the upper limit of the cooling rate is not particularly limited, but may be 300 ° C./s or less in order to avoid overcooling.
- the holding temperature in the atmosphere is lower than 650 ° C., bainite is easily generated, and the bainite area ratio is increased.
- the holding temperature in the atmosphere exceeds 750 ° C., pearlite is easily generated.
- holding in the atmosphere includes that the hot-rolled steel sheet is limited to air cooling or cooling in the cooling facility to a minimum, and the lower limit of the cooling rate at this time is ideally 0 ° C./s.
- the upper limit is 8 ° C./s.
- (E) Second (accelerated) cooling step After holding in the air, cooling is performed at an average cooling rate of 10 ° C./s or more from a temperature range of 600 to 740 ° C.
- the cooling start temperature is less than 600 ° C.
- the ferrite transformation does not proceed sufficiently and the precipitation of fine Ti precipitates becomes insufficient.
- the cooling start temperature exceeds 740 ° C.
- the ferrite transformation proceeds excessively, and pearlite is generated, which may deteriorate the hole expandability.
- the average cooling rate is less than 10 ° C./s, pearlite is generated and the hole expandability may be deteriorated.
- the upper limit of the average cooling rate is not particularly limited, but it may be 1000 ° C./s or less because there is a concern that the steel sheet warps due to thermal strain due to thermal deviation.
- (F) Winding step Thereafter, the cooled hot-rolled steel sheet is wound at a winding temperature of 450 to 650 ° C.
- the conditions after the winding process are not particularly limited.
- Table 1 Steel having the chemical composition shown in Table 1 was melted to produce a slab.
- the slab was hot-rolled under the conditions shown in Table 2 and then cooled and wound to produce a hot-rolled steel sheet.
- Table 3 shows the thickness of the obtained hot-rolled steel sheet.
- Metal structure The metal structure of the obtained hot rolled steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, in the cross section perpendicular to the rolling direction of the steel sheet, when the width and thickness of the steel sheet are W and t, respectively, 1/4 W from the end face of the steel sheet and 1 from the surface of the steel sheet A specimen for observing the metal structure was cut out from the position of / 4t.
- the rolling direction cross section (so-called L direction cross section) of the above test piece was subjected to nital etching, and after etching, observation was performed in a 300 ⁇ m ⁇ 300 ⁇ m visual field using an optical microscope. Then, by performing image analysis on the obtained structure photograph, the area ratio A of ferrite, the area ratio B of pearlite, and the total area ratio C of bainite, martensite and retained austenite were obtained.
- the nital-etched portion was repeller-etched and observed with a 300 ⁇ m ⁇ 300 ⁇ m field of view using an optical microscope.
- the total area rate D of a retained austenite and a martensite was computed by performing image analysis with respect to the obtained structure
- the volume ratio of the retained austenite was calculated
- the area ratio of bainite was determined from the difference between the area ratio C and the area ratio D, and the area ratio of martensite was determined from the difference between the area ratio E and the area ratio D. By this method, the area ratios of ferrite, bainite, martensite, retained austenite, and pearlite were determined.
- the area ratio of precipitation-strengthened ferrite was measured by EBSP after polishing the test piece with a colloidal silica abrasive and measuring a field of view of 160 ⁇ 256 ⁇ m at a magnification of 400 ⁇ under a measurement step of 0.5 ⁇ m. I asked for.
- the fine Ti precipitate was also subjected to electrolytic polishing of the test piece and measured by a three-dimensional atom probe measurement method, and the equivalent circle diameter and number density were obtained.
- the test specimen was observed at 20 magnifications at a magnification of 1000 times and a 60 ⁇ 40 ⁇ m visual field, and the average equivalent circular diameter of TiN was determined by image processing. Further, the shortest distance between TiNs was obtained by observing the same part as that in the structure investigation with a 500 times metal microscope.
- the tensile strength properties (tensile strength (TS), uniform elongation (u-EL), hole expansion rate ( ⁇ )) are 1 / (2) from one end of the plate to the plate width direction when the plate width is W.
- TS tensile strength
- u-EL uniform elongation
- ⁇ hole expansion rate
- the test piece of the simple shear test is a direction (width direction) orthogonal to the rolling direction at a position of 1/4 W or 3/4 W from one end of the plate to the plate width direction when the plate width of the steel plate is W. Is taken as the longitudinal direction.
- An example of a test piece is shown to Fig.1 (a).
- the test piece of the simple shear test shown in FIG. 1 has a rectangular thickness of 23 mm in the width direction of the steel plate and 38 mm in the rolling direction of the steel plate so that both sides are evenly ground so that the thickness is 2.0 mm. It processed so that it might become a test piece.
- the chucking part 2 on both sides is chucked by 10 mm toward the long piece side (rolling direction) of the test piece in the short piece direction (width direction), and a shear width of 3 mm (shear deformation generating part 1) is formed at the center of the test piece. It was made to provide. In addition, when the plate thickness was less than 2.0 mm, the plate thickness was tested as it was without grinding. Moreover, the center of the test piece was marked with a straight line with a pen or the like in the short piece direction (width direction).
- FIG. 1B shows an example of a test piece subjected to shear deformation.
- shear strain ⁇ s tan ( ⁇ )
- the simple shear test In the simple shear test, a simple shear tester (maximum displacement 8 mm) was used. Therefore, there is a limit on the stroke (displacement) of the testing machine. In addition, due to the occurrence of cracks at the end of the test piece or at the chuck part, in one shear test, the test may not be performed until the test piece breaks. Therefore, as described above, the “multi-stage shear test method” is adopted, which repeats a series of operations such as loading of the shear test load, unloading of the load, cutting off the end of the chuck part of the test piece in a straight line, and reloading of the load. did.
- Shear modulus is taken into account from the shear strain ( ⁇ s) obtained in each stage of the shear test.
- the shear plastic strain ( ⁇ sp) obtained by subtracting the shear elastic strain ( ⁇ se) was determined as follows, and the shear plastic strain ( ⁇ s) at each stage was combined and joined together.
- Shear plastic strain ⁇ sp Shear strain ⁇ s-Shear elastic strain ⁇ se
- Shear elastic strain ⁇ se ⁇ s / G ⁇ s: Shear stress
- G Shear elastic modulus
- G Shear elastic modulus
- G E / 2 (1 + ⁇ ) ⁇ 78000 (MPa).
- the test is performed until the specimen breaks. In this way, the relationship between the shear stress ⁇ s and the shear plastic strain ⁇ sp can be traced.
- the shear plastic strain when the test piece breaks is ⁇ spf.
- the standard deviation of nano hardness was measured.
- the specimen for observing the metallographic structure was ground again, and at a load of 1 mN (loading 10 s, unloading 10 s), a 1/4 depth position (1 / 4t part), a measurement area of 25 ⁇ m ⁇ 25 ⁇ m was measured at intervals of 5 ⁇ m. From the results, the average value of nano hardness and the standard deviation of nano hardness were calculated.
- the measurement of nano hardness was carried out using a Triscope or TriboIndenter manufactured by Hystron.
- the hot rolled steel sheet according to the present invention has a tensile strength (TS) of 780 MPa or more, a product of uniform elongation u-EL and tensile strength TS (TS ⁇ u-EL ) Is 7000 MPa ⁇ % or more, the product of the hole expansion ratio ⁇ and the tensile strength TS (TS ⁇ ⁇ ) is 50000 MPa ⁇ % or more, and a hot-rolled steel sheet having well-balanced characteristics is obtained. Moreover, it was confirmed that the hot-rolled steel sheet according to the present invention is a steel sheet that has an equivalent plastic strain exceeding 0.90 (90%) and can withstand high strain region processing such as plate forging.
- TS tensile strength
- the hot-rolled steel sheet according to the present invention can be widely used for machine parts and the like.
- the remarkable effect can be obtained.
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Abstract
Description
C:0.020~0.070%、
Si:0.05~1.70%、
Mn:0.60~2.50%、
Al:0.010~1.000%、
N:0%超~0.0030%以下、
P:0.050%以下、
S:0.005%以下、
Ti:0.015~0.170%、
Nb:0~0.100%、
V:0~0.300%、
Cu:0~2.00%、
Ni:0~2.00%、
Cr:0~2.00%、
Mo:0~1.00%、
B:0~0.0100%、
Mg:0~0.0100%、
Ca:0~0.0100%、
REM:0~0.1000%、
Zr:0~1.000%、
Co:0~1.000%、
Zn:0~1.000%、
W:0~1.000%、
Sn:0~0.050%、および、
残部:Feおよび不純物であり、
前記鋼板の圧延方向と垂直な断面において、前記鋼板の幅および厚さをそれぞれWおよびtとしたときに、前記鋼板の端面から1/4Wまたは3/4Wで、かつ、前記鋼板の表面から1/4tまたは3/4tの位置における金属組織が、面積%で、
フェライト:5~70%、
ベイナイト:30~95%、
残留オーステナイト:2%以下、
マルテンサイト:2%以下、および、
パーライト:1%以下、であり、かつ、
フェライトおよびベイナイトの合計:95%以上であり、
前記フェライトは、粒内にTiを含む析出物を有し、
前記Tiを含む析出物の個数密度が、1.0×1016~50.0×1016個/cm3であり、
前記鋼板中にTiN析出物が含まれ、
前記TiN析出物の平均円相当径が1.0~10.0μmであり、
隣接する前記TiN析出物間の最短距離の平均値が10.0μm以上であり、
ナノ硬度の標準偏差が1.00GPa以下である、
熱間圧延鋼板。
上記(1)に記載の熱間圧延鋼板。
均一伸びと引張強さとの積が7000MPa・%以上であり、
穴広げ率と引張強さとの積が50000MPa・%以上である、
上記(1)または(2)に記載の熱間圧延鋼板。
上記(1)に記載の化学組成を有するスラブに対して、加熱工程、連続熱延工程、第1冷却工程、第2冷却工程および巻取工程を順に施し、
前記加熱工程において、前記スラブを下記(i)式で表わされるSRTmin℃以上、1260℃以下の温度に加熱し、
前記連続熱延工程は、粗圧延と3段以上の多段仕上圧延とを含み、
前記粗圧延の終了温度が1100℃以上であり、
前記多段仕上圧延における最終3段の圧延における累積歪みが、0.01~0.10であり、
前記多段仕上圧延の圧延終了温度が、下記(ii)式で求められるAr3+30℃以上の温度であり、
前記第1冷却工程では、前記多段仕上圧延が終了した後、1.00~5.00s後に冷却を開始し、前記圧延終了温度から、650~750℃の温度範囲まで、10℃/s以上の平均冷却速度で冷却し、その後、大気中で1~10s保持し、
前記第2冷却工程では、前記大気中での保持後に、600~740℃の温度範囲から、10℃/s以上の平均冷却速度で冷却し、
前記巻取工程では、450~650℃の巻取り温度で巻取る、
熱間圧延鋼板の製造方法。
SRTmin=7000/{2.75-log(Ti×C)}-273 ・・・(i)
Ar3=970-325×C+33×Si+287×P+40×Al-92×(Mn+Mo+Cu)-46×(Cr+Ni) ・・・(ii)
但し、上記式中の元素記号は、各元素の熱間圧延鋼板中の含有量(質量%)を表し、含有されない場合は0を代入するものとする。
鋼製鍛造部品。
鋼製鍛造部品の製造方法。
板鍛造は、従来の引張試験での破断歪みを超える歪み域(高歪み域)での変形を含んでいる。また、板鍛造は複合的加工のため、単純に引張試験およびせん断試験データだけでは評価できない。そこで、本発明者らは、「相当塑性歪み」を指標として導入し、新たな評価法を確立した。
単軸引張試験での引張応力σ=単純せん断試験でのせん断応力σs×κ
単軸引張試験での引張歪みε=単純せん断試験でのせん断塑性歪みεsp/κ
相当塑性歪みを求めるためには、引張試験による引張応力および引張歪みの関係と、せん断試験によるせん断応力およびせん断歪みの関係を取得する必要がある。しかし、板鍛造は、高歪み域での変形を含んでいる。そのため、通常使用されているせん断試験装置を用いて1回で試験を行うと、試験片を保持している部分から試験片に亀裂が進行してしまう。その結果、高歪み域までの変形を試験することができない場合が多い。したがって、板鍛造のような鋼板の板厚の減厚(減肉およびくびれ)が生じない加工を再現する方法が必要となる。
上述の多段せん断試験と、相当塑性歪みを用いた評価法と、板鍛造の前後における鋼板のミクロ調査とを採用することにより、亀裂の発生メカニズムについて、以下の知見を得た。
すなわち、ボイドは硬質析出物のTiNの粒界に発生するため、TiNの平均径を限定することで、ボイドの発生が低減できる。
すわなち、ボイドはTiNの粒界に発生するため、TiN同士を離して配置することにより、ボイドが成長しても結合しにくくすることができる。
すなわち、硬質組織と軟質組織の硬度差をできるだけ低減することにより、ボイドの発生が低減できる。
前記の(i)~(iii)の条件を満足することにより、破断時の相当塑性歪みが0.90(90%)以上となり、板鍛造のような複合的加工においても、一定の加工性を担保することが可能であることを確認した。
上記(i)~(iv)の組織を得るために、熱間圧延における3段以上の多段(例えば6段または7段)の連続圧延で行われる多段仕上圧延において、最終3段の圧延における累積歪(以下「有効累積歪み」と記述する場合がある)が0.01~0.10になるように、最終仕上圧延を行なうことが必要である。
各元素の限定理由は下記のとおりである。なお、以下の説明において含有量についての「%」は、「質量%」を意味する。
Cは、Nb、Ti等と結合して鋼板中で析出物を形成し、析出強化により強度向上に寄与する。C含有量が0.020%未満では、上記作用による効果が十分には得られない。一方、C含有量が0.070%を超えると、穴広げ加工時の割れの起点となる鉄系炭化物が増加し、穴広げ値が劣化する。そのため、C含有量は0.020~0.070%とする。C含有量は0.025%以上であるのが好ましく、0.030%以上であるのがより好ましい。また、C含有量は0.060%以下であるのが好ましく、0.050%以下であるのがより好ましい。
Siは、脱酸効果、ならびに材料組織中におけるセメンタイト等の鉄系炭化物の析出を抑制し、延性および穴広げ性の向上に寄与する効果を有する。しかし、その含有量が過剰な場合、高温域でフェライト変態が生じやすくなり、これに伴い高温域でTiを含む炭化物が析出しやすくなる。高温域での炭化物の析出は、析出量のばらつきを生じやすく、結果として強度や穴広げ性等の材質変動をもたらす。そのため、Si含有量は0.05~1.70%とする。
Mnは、フェライトの強化および焼入れ性の向上に寄与する元素である。一方、多量に含有させると、焼入れ性が必要以上に高まりフェライトを十分に確保できず、また、鋳造時にスラブ割れが発生する。そのため、Mn含有量は0.60~2.50%とする。Mn含有量は1.00%以上であるのが好ましく、1.50%以上であるのがより好ましい。また、Mn含有量は2.00%以下であるのが好ましく、1.80%以下であるのがより好ましい。
Alは、Siと同様に脱酸効果とフェライトを生成する効果を有する。一方、その含有量が過剰であると脆化を招くとともに、鋳造時にタンディッシュノズルを閉塞し易くする。そのため、Al含有量は0.010~1.000%とする。Al含有量は0.015%以上または0.020%以上が好ましく、0.025%以上または0.030%以上がより好ましい。また、Al含有量は0.800%以下、0.700%以下または0.600%以下が好ましく、0.500%以下または0.400%以下がより好ましい。
Nは、多く含有すると、固溶窒素が残存して延性が低下するだけでなく、TiNが析出し穴広げ性を低下させる。そのため、N含有量は0.0030%以下とする。N含有量は0.0025%以下であるのが好ましい。
Pは溶銑に含まれる不純物であり、粒界偏析するため局部延性を劣化させるとともに、溶接性を劣化させるので、できるだけ少ない方がよい。そのため、P含有量は0.050%以下に制限する。P含有量は0.030%以下または0.020%以下が好ましい。特に下限を規定する必要はなく、下限は0%である。しかし、過度に含有量を低下させることは精錬時のコスト増になるため、下限を0.001%としてもよい。
Sも溶銑に含まれる不純物であり、MnSを形成して局部延性および溶接性を劣化させるので、できるだけ少ない方がよい。そのため、S含有量は0.005%以下に制限する。延性または溶接性の向上のため、S含有量を0.003%以下または0.002%以下としてもよい。特に下限を規定する必要はなく、下限は0%である。しかし、過度に含有量を低下させることは精錬時のコスト増になるため、下限を0.0005%としてもよい。
Tiは、炭窒化物、または固溶Tiが熱間圧延時の粒成長を遅延させることで、熱延板の粒径を微細化し、低温靭性を向上させる効果を有する。また、TiCとしてフェライト中に微細分散することで、析出強化を通じて鋼板の高強度化に寄与する。しかし、その含有量が過剰であると、効果が飽和することに加えて、硬質析出物であるTiNを析出し易くなる。そのため、Ti含有量は0.015~0.170%とする。Ti含有量は0.030%以上、0.045%以上または0.060%以上であるのが好ましく、0.070%以上、0.080%以上、0.090%以上または0.100%以上であるのがより好ましい。また、Ti含有量は0.160%以下、0.150%以下、0.140%以下、0.130%以下または0.120%以下であるのが好ましい。
Nbは、炭窒化物、または固溶Nbが熱間圧延時の粒成長を遅延させることで、熱延板の粒径を微細化し、低温靭性を向上させる効果を有する。また、NbCとして存在することで、析出強化を通じて鋼板の高強度化に寄与する。したがって、必要に応じて含有させてもよい。しかし、その含有量が過剰であると、効果が飽和して経済性が低下する。そのため、Nb含有量は0.100%以下とする。必要に応じて、Nb含有量を0.080%以下、0.060%以下または0.050%以下としてもよい。その下限は0%であるが、上記効果を十分に得るために、下限を0.001%または0.010%としてもよい。
Vは、析出強化または固溶強化により鋼板の強度を向上させる効果がある元素である。したがって、必要に応じて含有させてもよい。しかし、その含有量が過剰であると、効果が飽和して経済性が低下する。そのため、V含有量は0.300%以下とする。必要に応じて、V含有量を0.200%以下、0.100%以下または0.060%以下としてもよい。その下限は0%であるが、上記効果を十分に得るために、下限を0.001%または0.010%としてもよい。
Cuは、析出強化または固溶強化により鋼板の強度を向上させる効果がある元素である。したがって、必要に応じて含有させてもよい。しかし、その含有量が過剰であると、効果が飽和して経済性が低下する。そのため、Cu含有量は2.00%以下とする。また、Cu含有量が多量に含まれると鋼板の表面にスケール起因の傷が発生することがある。そのため、Cu含有量は1.20%以下、0.80%以下、0.50%以下または0.25%以下としてもよい。その下限は0%であるが、上記効果を十分に得るために、Cu含有量の下限を0.01%としてもよい。
Niは、固溶強化により鋼板の強度を向上させる効果がある元素である。したがって、必要に応じて含有させてもよい。しかし、その含有量が過剰であると、効果が飽和して経済性が低下する。そのため、Ni含有量は2.00%以下とする。また、Ni含有量が多量に含まれると延性が劣化するおそれがある。そのため、Ni含有量を0.60%以下、0.35%以下または0.20%以下としてもよい。その下限は0%であるが、上記効果を十分に得るために、Ni含有量の下限を0.01%としてもよい。
Crは、固溶強化により鋼板の強度を向上させる効果がある元素である。したがって、必要に応じて含有させてもよい。しかし、その含有量が過剰であると、効果が飽和して経済性が低下する。そのため、Cr含有量は2.00%以下とする。より経済性を高めるため、その上限を1.00%、0.60%または0.30%としてもよい。その下限は0%であるが、上記効果を十分に得るために、Cr含有量の下限を0.01%としてもよい。
Moは、析出強化または固溶強化により鋼板の強度を向上させる効果がある元素である。したがって、必要に応じて含有させてもよい。しかし、その含有量が過剰であると、効果が飽和して経済性が低下する。そのため、Mo含有量は1.00%以下とする。より経済性を高めるため、その上限を0.60%、0.30%または0.10%としてもよい。その下限は0%であるが、上記効果を十分に得るために、Mo含有量の下限を0.005%または0.01%としてもよい。
Bは粒界に偏析し、粒界強度を高めることで低温靭性を向上させる。したがって、必要に応じて含有させてもよい。しかし、その含有量が過剰であると、効果が飽和して経済性が低下する。そのため、B含有量は0.0100%以下とする。また、Bは強力な焼き入れ元素であり、その含有量が多量に含まれると冷却中にフェライト変態が十分に進行せず、十分な残留オーステナイトが得られないことがある。そのため、B含有量を0.0050%以下、0.0020%以下または0.0015%としてもよい。その下限は0%であるが、上記効果を十分に得るために、B含有量の下限を0.0001%または0.0002%としてもよい。
Mgは、破壊の起点となり、加工性を劣化させる原因となる非金属介在物の形態を制御し、加工性を向上させる元素である。したがって、必要に応じて含有させてもよい。しかし、その含有量が過剰であると、効果が飽和して経済性が低下する。そのため、Mg含有量は0.0100%以下とする。その下限は0%であるが、上記効果を十分に得るために、Mg含有量の下限を0.0001%または0.0005%としてもよい。
Caは、破壊の起点となり、加工性を劣化させる原因となる非金属介在物の形態を制御し、加工性を向上させる元素である。したがって、必要に応じて含有させてもよい。しかし、その含有量が過剰であると、効果が飽和して経済性が低下する。そのため、Ca含有量は0.0100%以下とする。その下限は0%であるが、上記効果を十分に得るためには、Ca含有量は0.0005%以上であるのが好ましい。
REM(希土類元素)は、破壊の起点となり、加工性を劣化させる原因となる非金属介在物の形態を制御し、加工性を向上させる元素である。したがって、必要に応じて含有させてもよい。しかし、その含有量が過剰であると、効果が飽和して経済性が低下する。そのため、REM含有量は0.1000%以下とする。必要に応じて、その上限を0.0100%または0.0060%としてもよい。その下限は0%であるが、上記効果を十分に得るために、REM含有量の下限を0.0001%または0.0005%としてもよい。
Co:0~1.000%
Zn:0~1.000%
W:0~1.000%
Zr、Co、ZnおよびWは、それぞれ1.000%以下の範囲であれば含有しても本発明の効果は損なわれないことを確認している。これらの上限を0.300%または0.10%としてもよい。Zr、Co、ZnおよびWの合計含有量が1.000%以下または0.100%であることが好ましい。これらの含有は必須でなく、下限は0%であるが、必要に応じて、下限を0.0001%としてもよい。
Snは、少量であれば含有しても本発明の効果は損なわれないことを確認している。しかし、0.05%を超えると熱間圧延時に疵が発生するおそれがある。そのため、Sn含有量は0.050%以下とする。Snの含有は必須でなく、下限は0%であるが、必要に応じて、下限を0.001%としてもよい。
本発明の鋼板の金属組織について説明する。なお、本発明において金属組織は、鋼板の圧延方向と垂直な断面において、鋼板の幅および厚さをそれぞれWおよびtとしたときに、該鋼板の端面から1/4Wまたは3/4Wで、かつ、該鋼板の表面から1/4tまたは3/4tの位置における組織をいうものとする。また、以下の説明において「%」は、「面積%」を意味する。
Tiを含有する微細な析出物(微細析出したTiの炭化物などであり、以下、「微細Ti析出物」ともいう。)が圧延後の冷却中にγ→α変態する際のTiの炭化物の過飽和度を駆動力としてTiの炭化物がフェライト中に相界面析出または均質核生成してTiの炭化物が微細に分散した初析フェライトが析出強化したフェライトである(以下、「析出強化フェライト」ともいう。)。析出強化フェライトは、優れた均一伸びと強度とを両立するために必要な組織である。
ベイナイトは、強度と局部延性とのバランスを得るために必要な組織であり、亀裂の伝搬を抑制する効果がある。しかし、ベイナイトが多くなりすぎると、フェライトが減少し、局部延性は優れるものの均一伸びが著しく劣化してしまう。そのため、ベイナイトの面積率は30~95%とする。ベイナイトの面積率は80%以下であることが好ましく、さらに均一伸びを重視する場合は70%以下にすることがより好ましい。
高バーリング鋼は、析出強化フェライトおよびベイナイトの存在により加工性を確保しつつ、高強度も確保し、強度と加工性とを両立することが特徴である。しかしながら、鋼板中にマルテンサイト変態を起こさなかった熱力学的に安定な残留オーステナイトが存在するということは、その残留オーステナイトのC濃度は高く、残留オーステナイトが板鍛造時に加工誘起変態して生成するマルテンサイトの硬度が高くなりすぎて、ボイドの発生を助長してしまう。そのため、残留オーステナイトはできるだけ少ない方がよく、その面積率は2%以下とする。残留オーステナイトの面積率は1.5%以下、1%以下または0.5%以下が好ましい。特に下限を規定する必要はなく、下限は0%であり、0%が最も好ましい。
高バーリング鋼は、析出強化フェライトおよびベイナイトの存在により加工性を確保しつつ、高強度も確保し、強度と加工性とを両立することが特徴である。しかしながら、硬質組織であるマルテンサイトの面積率が2%を超えると、板鍛造による鋼板の歪み増加に伴い、マルテンサイトとフェライトとの境界にボイドが発生し易くなり、破断しやすくなる。そのため、マルテンサイトの面積率は2%以下とする。マルテンサイトの面積率は1.5%以下、1%以下または0.5%以下であるのが好ましい。特に下限を規定する必要はなく、下限は0%である。
パーライトは、穴広げ成形の際に破壊の起点となるため、その面積率は1%以下とする。パーライトの面積率は0.5%以下であるのが好ましい。パーライトの面積率は極力低減することが好ましく、0%であることが好ましい。
高バーリング鋼は、優れた均一伸びと強度とを両立する析出強化フェライト、および強度と局部延性とを両立するベイナイトを有する。これにより優れた強度、均一伸びおよび局部延性が得られる。析出強化フェライトとベイナイトとの合計面積率が95%未満であると、これら特性が劣化してしまう。したがって、析出強化フェライトおよびベイナイトの合計面積率は95%以上とする。該合計面積率は97%以上であるのが好ましく、98%以上であるのがより好ましい。
円相当径(直径)R={(6/8)・(1/π)・N・a3}(1/3)
TiNが大きいと、板鍛造による鋼板の歪み増加に伴い、粒界に存在するボイドが結合し易くなることから、TiNの平均円相当径は10.0μm以下とする。これらの効果をより確実に確保するため、TiNの平均円相当径は8.0μm以下であるのが好ましく、5.0μm以下であることがより好ましい。
TiNとフェライトとの界面に発生したボイドが成長し、ボイド同士が結合してさらに大きなボイドとならないようにするため、TiN間の距離を一定量確保する必要がある。そのため、隣接するTiN間の距離の平均値を10.0μm以上とする。
ナノ硬度の標準偏差:1.0GPa以下
硬質組織と軟質組織との変形能の差を小さくすることにより両組織の界面に発生するボイドを少なくし、さらにボイド間隔をあけることにより、ボイドが結合して亀裂に成長することを抑制することが可能になる。そこで、硬質組織と軟質組織との変形能の差に対応するナノ硬度差をできるだけ低減することにより、ボイドの発生が抑制できる。本発明においては、軟質組織と硬質組織との硬度差の指標として、試料断面におけるナノ硬度の標準偏差を採用する。
本発明に係る鋼板は、従来の高バーリング鋼と同等の780MPa以上の引張強さを有することが好ましい。引張強さの上限を特に定める必要はないが、1200MPa、1150MPaまたは1000MPaとしてもよい。ただし、引張強さは、JIS Z 2241(2011)の引張強さを示す。
均一伸びが小さいとプレス成型時にネッキングによる板厚減少が起こり易く、プレス割れの原因となる。プレス成形性を確保するため、均一伸び(u-EL)と引張強さ(TS)との積:TS×u-EL≧7000MPa%を満たすことが好ましい。ただし、均一伸びは、JIS Z 2241(2011)で規定する試験において、公称応力σnと公称歪みεnとの関係で、公称応力σnを公称歪みεnで微分したときの値がゼロとなる点の公称歪みをεn0とした時、以下の式で表される。
均一伸び(u-EL)=ln(εn0+1)
穴広げ性が悪いと、伸びフランジ加工をした際に材料流れ性が悪く割れを生じる可能性がある。そのため、穴広げ性を確保するため、穴広げ率(λ)と引張強さ(TS)との積:(TS)×(λ)≧50000MPa%を満たすことが好ましい。ただし、穴広げ率(λ)は、JIS Z 2256(2010)に準拠した試験方法による穴広げ率の(λ)を表す。
相当塑性歪みは、単純せん断試験でのせん断応力σsとせん断塑性歪みεspとの関係を、変形形態の異なる、単軸引張試験での引張応力σと引張歪みεとの関係に変換するものであり、等方硬化則と塑性仕事共役との関係を仮定して、定数である変換係数(κ)を用いて変換したものである。
単軸引張試験での引張応力σ(変換)=単純せん断試験でのせん断応力σs×κ
単軸引張試験での引張歪みε(変換)=単純せん断試験でのせん断塑性歪みεsp/κ
板厚:1.0~4.0mm
本発明に係る鋼板は、主に自動車などが主な用途であり、その板厚範囲は主に1.0~4.0mmである。このため、板厚範囲を1.0~4.0mmとしてもよい、必要に応じて、下限を1.2mm、1.4mmまたは1.6mmに、上限を3.6mm、3.2mmまたは2.8mmとしてもよい。
発明者らは、これまでの研究により、下記に示す(a)から(f)までの製造工程を順に行うことにより、本発明の熱間圧延鋼板を製造することができることを確認している。以下、各製造工程について詳しく説明する。
熱間圧延に先行する製造方法は特に限定するものではない。すなわち、高炉または電炉等による溶製に引き続き各種の2次製錬を行って上述した成分組成となるように調整する。次いで、通常の連続鋳造、薄スラブ鋳造などの方法でスラブを製造すればよい。その際、本発明の成分範囲に制御できるのであれば、原料にはスクラップ等を使用しても構わない。
製造されたスラブに熱間圧延を施し、熱間圧延鋼板とする。熱間圧延を行うに際しては、まず、スラブを加熱する。加熱工程においては、スラブを下記(i)式で表わされるSRTmin℃以上、1260℃以下の温度に加熱する。連続鋳造の場合には一度低温まで冷却した後、再度加熱してもよいし、特に冷却することなく連続鋳造に引き続いて加熱してもよい。ここで、SRTminは、TiCの溶体化温度を意味する。
SRTmin=7000/{2.75-log(Ti×C)}-273 ・・・(i)
但し、上記式中の元素記号は、各元素の熱間圧延鋼板中の含有量(質量%)を表し、含有されない場合は0を代入するものとする。
加熱後は、加熱炉より抽出したスラブに対して粗圧延およびその後の多段仕上圧延を施す。Tiを含む析出物が析出しないように、粗圧延の終了温度は1100℃以上とする。また、前述したように、多段仕上圧延は、3段以上の多段(例えば6段または7段)の連続圧延で行われる。そして、最終3段の圧延における累積歪(有効累積歪み)が、0.01~0.10になるように多段仕上圧延を行なう。
上式(iii)中のΣは、i=1~3についての総和を示す。
但し、i=1は、多段仕上圧延において最後から1段目の圧延(つまり、最終段圧延)を、i=2は最後から2段目の圧延、i=3は最後から3段目の圧延を、それぞれ示す。
εi(ti,Ti)=ei/exp((ti/τR)2/3) ・・・(iv)
ti:最後からi段目の圧延から最終段圧延後の一次冷却開始までの時間(s)
Ti:最後からi段目の圧延の圧延温度(K)
ei:最後からi段目の圧延で圧下したときの対数歪み
ei=|ln{1-(i段目の入側板厚-i段目の出側板厚)/(i段目の入側板厚)}|
=|ln{(i段目の出側板厚)/(i段目の入側板厚)}| ・・・(v)
τR=τ0・exp(Q/(R・Ti)) ・・・(vi)
τ0=8.46×10-9(s)
Q:Feの転位の移動に関する活性化エネルギーの定数=183200(J/mol)
R:ガス定数=8.314(J/(K・mol))
Ar3=970-325×C+33×Si+287×P+40×Al-92×(Mn+Mo+Cu)-46×(Cr+Ni) ・・・(ii)
但し、上記式中の元素記号は、各元素の熱間圧延鋼板中の含有量(質量%)を表し、含有されない場合は0を代入するものとする。
多段仕上圧延終了後、1.00~5.00s後に得られた熱間圧延鋼板の冷却を開始する。そして、圧延終了温度から、650~750℃の温度まで10℃/s以上の平均冷却速度で冷却し、その後、大気中で1~10s保持する。
大気中での保持後に、600~740℃の温度範囲から、10℃/s以上の平均冷却速度で冷却する。冷却開始温度が600℃未満では、フェライト変態が十分に進行せず、微細Ti析出物の析出も不十分となる。一方、冷却開始温度が740℃を超えると、フェライト変態が過度に進行するとともに、パーライトが生成して穴広げ性が劣化するおそれがある。また、微細Ti析出物が粗大化して強度が低下するおそれがある。
その後、冷却された熱間圧延鋼板を、450~650℃の巻取り温度で巻き取る。巻取工程後の条件は、特に限定されない。
上記のようにして得られた熱延鋼板は、板鍛造性に優れているため、当該熱延鋼板を板鍛造などの鍛造加工することにより、従来ではなし得なかった高強度を要する複雑形状の鍛造部品を得ることができる。
得られた熱間圧延鋼板の金属組織観察を行い、各組織の面積率の測定を行った。具体的には、まず鋼板の圧延方向と垂直な断面において、鋼板の幅および厚さをそれぞれWおよびtとしたときに、該鋼板の端面から1/4Wで、かつ、該鋼板の表面から1/4tの位置から金属組織観察用の試験片を切り出した。
機械特性のうち引張強度特性(引張強さ(TS)、均一伸び(u-EL)、穴広げ率(λ))は、板幅をWとした時に、板の片端から板幅方向に1/4Wもしくは3/4Wのいずれかの位置において、圧延方向に直行する方向(幅方向)を長手方向として採取したJIS Z 2241(2011)の5号試験片を用いて、JIS Z 2241(2011)に準拠して評価した。穴広げ率は、引張試験片採取位置と同様の位置から試験片を採取し、JIS Z 2256 2010記載の試験方法に準拠して評価した。
せん断応力σs=せん断力/(鋼板の圧延方向の試験片の長さ×試験片の板厚)
せん断ひずみεs=tan(θ)
せん断塑性ひずみεsp=せん断ひずみεs-せん断弾性ひずみεse
せん断弾性ひずみεse=σs/G
σs:せん断応力
G:せん断弾性率
ここで、G=E/2(1+ν)≒78000(MPa)とした。
E(ヤング率(縦弾性係数))=206000(MPa)
ポアソン比(ν)=0.3
2 チャッキング部
Claims (6)
- 鋼板の化学組成が、質量%で、
C:0.020~0.070%、
Si:0.05~1.70%、
Mn:0.60~2.50%、
Al:0.010~1.000%、
N:0%超~0.0030%以下、
P:0.050%以下、
S:0.005%以下、
Ti:0.015~0.170%、
Nb:0~0.100%、
V:0~0.300%、
Cu:0~2.00%、
Ni:0~2.00%、
Cr:0~2.00%、
Mo:0~1.00%、
B:0~0.0100%、
Mg:0~0.0100%、
Ca:0~0.0100%、
REM:0~0.1000%、
Zr:0~1.000%、
Co:0~1.000%、
Zn:0~1.000%、
W:0~1.000%、
Sn:0~0.050%、および、
残部:Feおよび不純物であり、
前記鋼板の圧延方向と垂直な断面において、前記鋼板の幅および厚さをそれぞれWおよびtとしたときに、前記鋼板の端面から1/4Wまたは3/4Wで、かつ、前記鋼板の表面から1/4tまたは3/4tの位置における金属組織が、面積%で、
フェライト:5~70%、
ベイナイト:30~95%、
残留オーステナイト:2%以下、
マルテンサイト:2%以下、および、
パーライト:1%以下、であり、かつ、
フェライトおよびベイナイトの合計:95%以上であり、
前記フェライトは、粒内にTiを含む析出物を有し、
前記Tiを含む析出物の個数密度が、1.0×1016~50.0×1016個/cm3であり、
前記鋼板中にTiN析出物が含まれ、
前記TiN析出物の平均円相当径が1.0~10.0μmであり、
隣接する前記TiN析出物間の最短距離の平均値が10.0μm以上であり、
ナノ硬度の標準偏差が1.00GPa以下である、
熱間圧延鋼板。 - 前記Tiを含む析出物の平均円相当径が1.00~3.00nmである、
請求項1に記載の熱間圧延鋼板。 - 引張強さが780MPa以上であり、
均一伸びと引張強さとの積が7000MPa・%以上であり、
穴広げ率と引張強さとの積が50000MPa・%以上である、
請求項1または請求項2に記載の熱間圧延鋼板。 - 請求項1から請求項3までのいずれかに記載の熱間圧延鋼板を製造する方法であって、
請求項1に記載の化学組成を有するスラブに対して、加熱工程、連続熱延工程、第1冷却工程、第2冷却工程および巻取工程を順に施し、
前記加熱工程において、前記スラブを下記(i)式で表わされるSRTmin℃以上、1260℃以下の温度に加熱し、
前記連続熱延工程は、粗圧延と3段以上の多段仕上圧延とを含み、
前記粗圧延の終了温度が1100℃以上であり、
前記多段仕上圧延における最終3段の圧延における累積歪みが、0.01~0.10であり、
前記多段仕上圧延の圧延終了温度が、下記(ii)式で求められるAr3+30℃以上の温度であり、
前記第1冷却工程では、前記多段仕上圧延が終了した後、1.00~5.00s後に冷却を開始し、前記圧延終了温度から、650~750℃の温度範囲まで、10℃/s以上の平均冷却速度で冷却し、その後、大気中で1~10s保持し、
前記第2冷却工程では、前記大気中での保持後に、600~740℃の温度範囲から、10℃/s以上の平均冷却速度で冷却し、
前記巻取工程では、450~650℃の巻取り温度で巻取る、
熱間圧延鋼板の製造方法。
SRTmin=7000/{2.75-log(Ti×C)}-273 ・・・(i)
Ar3=970-325×C+33×Si+287×P+40×Al-92×(Mn+Mo+Cu)-46×(Cr+Ni) ・・・(ii)
但し、上記式中の元素記号は、各元素の熱間圧延鋼板中の含有量(質量%)を表し、含有されない場合は0を代入するものとする。 - 請求項1から請求項3までのいずれかに記載の熱間圧延鋼板から得られる、
鋼製鍛造部品。 - 請求項1から請求項3までのいずれかに記載の熱間圧延鋼板に対して、少なくとも鍛造加工を施す、
鋼製鍛造部品の製造方法。
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