WO2014002941A1 - 高強度熱延鋼板及びその製造方法 - Google Patents
高強度熱延鋼板及びその製造方法 Download PDFInfo
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
- C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment following hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
- C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2201/00—Treatment for obtaining particular effects
- C21D2201/05—Grain orientation
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
Definitions
- the present invention relates to a hot-rolled steel sheet suitable for high-strength structural parts such as automobiles that are subjected to burring and stretch-flange processing, and to a method of manufacturing the hot-rolled steel sheet that is less likely to cause damage to the end face when the steel sheet is punched.
- Patent Documents 5 and 6 in order to suppress breakage at the grain boundaries during processing, high strength hot-rolled steel sheet that suppresses the occurrence of damage to the punched end face by adding B or limiting the amount of P added. has been developed (see Patent Documents 5 and 6). Furthermore, by controlling the amount of segregation of C and C and B at the large-angle grain boundaries of ferrite, high-strength heat that can prevent the occurrence of damage to the punched end face even when punching is performed under extremely severe conditions A rolled steel sheet was developed (see Patent Documents 7 and 8). However, the steel sheets of Patent Documents 5 to 8 have a structure mainly composed of a ferrite phase, and it has been difficult to achieve a high strength of 850 MPa or more.
- the present invention has been made in order to solve the above-described problems, and has both excellent stretch flangeability and ductility, and particularly has a high strength of a tensile strength of 850 MPa or more, under extremely severe conditions.
- An object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in punching workability that can prevent end face damage even when punching is performed.
- the inventors of the present invention have examined the correlation between the frequency of occurrence of damage on the punched end face and the segregation element type and segregation amount at the grain boundaries, with the clearance of the punching process being the most severe condition.
- the ratio of the large-angle crystal grain boundary where the bainite structure is mainly used and the grain boundary angle of the steel sheet is 15 ° or more and the small-angle crystal grain boundary where the grain boundary angle is 5 ° or more and less than 15 ° is appropriately set. It was found that the damage of the punched end face is reduced by segregating an appropriate amount of C and B at the large-angle grain boundaries.
- This invention is made
- a method for producing a high-strength hot-rolled steel sheet which is air-cooled and then secondarily cooled to 200 ° C. or lower at a cooling rate of 15 ° C./s or higher and wound.
- the steel slab is mass%, P: 0.02% or less, The manufacturing method of the high intensity
- the balance between stretch flangeability and ductility is good, the tensile strength is particularly high, at least 850 MPa, and the occurrence of damage to the end face is suppressed regardless of the punching clearance conditions.
- a hot-rolled high-strength steel sheet having excellent punchability is provided.
- the present invention has a significant industrial contribution.
- the present inventors performed punching at various clearances using a high-strength hot-rolled steel sheet having a tensile strength of 850 MPa or more excellent in ductility and hole expansibility, and quantitatively investigated the end face properties.
- a 10 mm diameter hole was punched by changing the clearance using the hole expansion test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996, and damage was visually observed in the entire circumference of the end face punched into a circle.
- the angles in the obtained range were measured and summed, and the value was divided by 360 ° to determine the damage occurrence ratio (referred to as the punched end face damage occurrence ratio) on the entire circumference of the punched end face.
- the clearance is increased, peeling or damage that cannot be confirmed when punching with a clearance of around 12.5% recommended in the normal hole expansion test method will occur, and 16%
- the clearance was found to be the most severe condition. Therefore, in the following, the investigation was carried out using a clearance of 16%.
- the large-angle crystal grain boundary is defined as a grain boundary where the difference in crystal orientation between adjacent crystal grains is 15 ° or more.
- the small-angle crystal grain boundary is defined as a grain boundary where the difference in crystal orientation between adjacent crystal grains is 5 ° or more and less than 15 °.
- the segregation amounts of B, C, and P at five or more large-angle grain boundaries in each steel material were measured, and an average value was obtained.
- small angle crystal grain boundaries having an angle of less than 15 ° are included in addition to large angle crystal grain boundaries.
- the angle of the crystal orientation was determined by analyzing the Kikuchi pattern obtained from observation of the sample with a transmission electron microscope.
- the structure mainly composed of bainite in the present invention preferably contains bainite of more than 50% in area ratio when the cross section is observed, and may contain less than 50% of ferrite and second phase.
- FIGS. 1 (a) and 1 (b) An example of observation of crystal grain boundaries and an example of ladder chart analysis are shown in FIGS. 1 (a) and 1 (b), respectively.
- the amount of segregation of each atom was evaluated using an excess amount that is segregated, that is, the number of atoms added from the solid solution amount per unit grain interface area. This evaluation was conducted by Takahashi et al., “Quantitative Observation of Grain Boundary Segregation Carbon Content of Paint Baking Hardened Steel Sheet”, Nippon Steel Technical Report, No. 381, October 2004, p. See 26-30.
- the crystal grain boundary is originally a plane, but in the present invention, the length evaluated as follows was used as an index.
- the sample cut out so as to obtain a cross section parallel to the rolling direction and the thickness direction of the steel plate was polished and further electropolished.
- EBSP-OIM TM Electro Back Scatter Diffraction Pattern-Orientation Imaging Microscopy
- the EBSP-OIM TM method uses a high-sensitivity camera to shoot a Kikuchi pattern formed by irradiating an electron beam onto a highly tilted sample in a scanning electron microscope (SEM) and backscattering it, and processing the computer image By doing so, it is composed of an apparatus and software for measuring the crystal orientation of the irradiation point in a short time.
- the crystal orientation on the surface of the bulk sample can be quantitatively analyzed, and the analysis area is an area that can be observed by SEM.
- the measurement can be performed over several hours, and tens of thousands of points can be mapped in an equally spaced grid to analyze the crystal orientation distribution in the sample.
- FIG. 2 shows the relationship between the total amount of segregation of C and B, the ratio of the length of the large-angle grain boundary to the length of the small-angle grain boundary, and the punched end face damage occurrence ratio of the steel material. As shown in FIG. 2, many segregations of C and B were observed at the large-angle grain boundaries of the steel sheet having a small punching end face damage occurrence ratio.
- carbides of Ti, Nb, V, and Mo are partially dispersed and precipitated in the crystal grains to ensure solid solution C in the crystal grains, and nitrides of Ti, Nb, and V are precipitated to form BN.
- the total amount of segregation of C and B at the grain boundaries can be within an appropriate range by suppressing the precipitation of slag and leaving the solid solution B in the crystal grains.
- the damage resistance of the end surface at the time of punching a steel plate can be maintained favorably.
- the reason why the end face damage resistance of the steel sheet is thus improved is that the segregated C and B reinforce the crystal grain boundaries and suppress the crack growth at the grain boundaries during the punching process.
- FIG. 3 shows the relationship between the amount of segregation of P and the punched end face damage occurrence ratio. As shown in FIG. 3, it is understood that the punching damage occurrence ratio increases when the segregation amount of C and B is set to a certain level or more at the grain boundary, P is intentionally added, and the P segregation amount is increased. It was.
- the sum of the segregation amount of C and the segregation amount of B at the large-angle grain boundaries is 4 atoms / nm 2 or more, punching is performed when the steel sheet is punched under the strictest clearance conditions.
- the end face damage occurrence ratio can be within 0.3.
- the grain boundary strengthening amount is insufficient, and punching end face damage becomes remarkable.
- the upper limit of the amount that can be substantially segregated in the steel sheet of the present invention was considered to be about 20 atoms / nm 2 .
- a more preferable range of the total amount of C segregation and B segregation at the grain boundary is 6 to 15 atoms / nm 2 at which damage to the punched end face hardly occurs.
- the grain boundary segregation amount of C is reduced, and after achieving a predetermined segregation by cooling after hot rolling.
- the total of the segregation amount of C and the segregation amount of B can be 4 to 20 atoms / nm 2 .
- the amount of segregation is small.
- the reason for this is considered that P has an effect of embrittlement of grain boundaries.
- the amount of segregation of P increases, the progress of the crack at the time of stamping is promoted, and the occurrence rate of damage is increased.
- the amount of segregation of P is preferably 1 atoms / nm 2 or less. In order to reduce the amount of segregation of P to 1 atoms / nm 2 or less, the P content may be limited to 0.02% or less.
- the sum of the segregation amount of C and the segregation amount of B is 4 to 20 atoms / nm 2
- the length ratio of the large-angle crystal grain boundary to the length of the small-angle crystal grain boundary is 1 or more and 4 or less. If there is, the punching end face damage occurrence ratio when the steel sheet is punched under the strictest clearance conditions can be within 0.3. If the length ratio of the large-angle grain boundaries to the length of the small-angle grain boundaries is smaller than 1, the block grain size of bainite tends to increase, and the toughness deteriorates, and the ratio of the occurrence of punching edge damage increases. It is done.
- the punched end face damage generation ratio can be suppressed low, but the structure is mainly composed of ferrite, so that the strength is reduced and the tensile strength is 850 MPa or more.
- the steel plate of the present invention is not satisfied.
- the steel sheet structure has the above-mentioned grain boundary segregation amount and the length ratio of the large angle grain boundaries to the small angle grain boundaries, the steel sheet has an elongation of 15% or more, a hole expansion ratio of 25% or more, and a tensile strength of 850 MPa.
- the component composition of the steel sheet is preferably defined as follows. The “%” shown below means “mass%” unless otherwise specified.
- the basic components described below are sufficiently effective for the purpose of the present invention, but in the range that does not impair the steel sheet properties of the present invention, it is possible to contain other components It is acceptable.
- it may contain less than 0.2% Cr and less than 0.15% Cu.
- C is an element that contributes to the improvement of strength, and in order to obtain a structure mainly composed of bainite of the present invention and to ensure a sufficient amount of C segregation to the grain boundary, the content of C is 0.050% or more. is necessary. On the other hand, when the C content exceeds 0.200%, formation of cementite and formation of a transformation structure such as pearlite and martensite are promoted more than necessary, and elongation and hole expansibility decrease. Therefore, the C content is set to 0.050 to 0.200%.
- B is an important element in the present invention, and even when the segregation of C at the grain boundary is insufficient, the segregation of B prevents the punched end face from being damaged. In order to acquire this effect, it is necessary to contain B 0.0002% or more. On the other hand, when B exceeds 0.0030%, workability such as ductility is lowered. Therefore, the B content is 0.0002 to 0.0030%.
- Si is effective as a solid solution strengthening element for increasing the strength, and it is necessary to contain 0.01% or more to obtain the effect. On the other hand, if the Si content exceeds 1.5%, the workability deteriorates. Therefore, the Si content is in the range of 0.01 to 1.5%.
- Mn is necessary for deoxidation and desulfurization, and is also effective as a solid solution strengthening element. Moreover, in order to stabilize austenite and to easily obtain a bainite structure, the Mn content needs to be 1.0% or more. On the other hand, when the Mn content exceeds 3.0%, segregation is likely to occur and the workability is deteriorated. Therefore, the Mn content needs to be 1.0 to 3.0%.
- Ti is an element that precipitates carbides and nitrides in ferrite and bainite crystal grains and increases the strength of the steel sheet by precipitation strengthening. In order to sufficiently generate carbide and nitride, the Ti content is set to 0.03% or more. On the other hand, if the Ti content exceeds 0.20%, carbides and nitrides may become coarse. Therefore, the Ti content is set to 0.03 to 0.20%.
- P is an impurity, and it is necessary to limit the P content to 0.05% or less. In order to suppress segregation of P to grain boundaries and prevent grain boundary cracking, it is preferable to limit to 0.02% or less.
- one or more of V, Nb, and Mo may be contained as carbide precipitation elements in the crystal grains.
- V and Nb, which are nitride precipitation elements are nitride precipitation elements, to suppress the precipitation of BN.
- V, Nb V and Nb are elements that precipitate carbide and nitride in ferrite and bainite crystal grains and increase the strength of the steel sheet by precipitation strengthening.
- the contents of V and Nb it is preferable to set the contents of V and Nb to 0.01% or more, respectively.
- the contents of V and Nb are preferably set to 0.01 to 0.20%, respectively.
- Mo is a carbide forming element, and can be contained for the purpose of precipitating carbide in crystal grains and contributing to precipitation strengthening. In order to sufficiently generate carbide, it is preferable to contain 0.01% or more of Mo. On the other hand, when the addition amount of Mo exceeds 0.20%, coarse carbides may be generated. Therefore, the Mo content is preferably 0.01 to 0.20%.
- N forms nitrides and lowers the workability of the steel sheet, so the content is preferably limited to 0.009% or less.
- S S is preferably contained as inclusions such as MnS and deteriorates the stretch flangeability, and further causes cracking during hot rolling, so that it is preferably reduced as much as possible.
- S content in order to prevent cracking during hot rolling and improve workability, it is preferable to limit the S content to 0.005% or less.
- Al Since Al forms precipitates such as nitrides and impairs the workability of the steel sheet, it is preferably limited to 0.5% or less. In addition, it is preferable to add 0.002% or more for molten steel deoxidation.
- W may be added as a solid solution strengthening element for the purpose of improving the strength of the steel sheet.
- the steel slab is preferably manufactured by continuous casting equipment from the viewpoint of productivity.
- the heating temperature of the hot rolling is set to 1200 ° C. or higher in order to sufficiently decompose and dissolve the carbide forming element and carbon in the steel material. Since it is not economically preferable to make the heating temperature excessively high, the upper limit of the heating temperature is preferably 1300 ° C. or less.
- the steel slab may be cooled and rolling may be started at a temperature of 1200 ° C. or higher. When heating a steel piece cooled to 1200 ° C. or lower, it is preferable to hold for at least 1 hour.
- the finishing temperature of finish rolling in hot rolling needs to be 910 ° C. or higher in order to suppress the formation of coarse carbides.
- the upper limit of the finishing temperature of finish rolling is not particularly required to obtain the effect of the present invention, but is preferably 1000 ° C. or less because there is a possibility that scale flaws may occur in operation.
- the rolling reduction is 60% or more in total from the last stand to 3 stands. The rolling reduction is preferably as high as possible, but 95% is a practical upper limit from the viewpoint of productivity and equipment load.
- the cooling rate of primary cooling is set to 40 ° C./s or more, and the end temperature of primary cooling is set to 550 ° C. or less and 450 ° C. or more. It is necessary to. If the cooling rate of the primary cooling is less than 40 ° C./s, coarse carbide precipitates during the cooling and segregates at the grain boundaries, and the punched end face may be damaged. Although the upper limit of the cooling rate of primary cooling is not particularly defined, an appropriate cooling rate is 300 ° C./s or less because of the capacity of the cooling facility.
- the bainite transformation it is necessary to hold or air cool at a temperature not higher than the primary cooling stop temperature and not lower than 450 ° C. for 7.5 seconds or longer. If it is less than 7.5 seconds, the bainite transformation becomes insufficient, and a large amount of martensite is generated by subsequent cooling, resulting in deterioration of workability. Preferably it is 10 seconds or more, More preferably, it is 15 seconds or more. Air cooling is preferable from the viewpoint of productivity, and the upper limit is 30 seconds.
- secondary cooling is performed at a temperature of 15 ° C./s or higher to a temperature of 200 ° C. or lower.
- the reason for this is that if it is kept at a temperature higher than 200 ° C. after bainite transformation, carbides such as cementite precipitate and C to be segregated becomes insufficient, and it becomes difficult to obtain the grain boundary segregation amount of C of the present invention. Because.
- the upper limit of the cooling rate of the secondary cooling is not particularly defined, but 200 ° C./s or less is a reasonable cooling rate due to the capacity of the cooling facility. By cooling to 200 ° C.
- Hot rolling was performed under the production conditions shown in Table 2 to produce a hot-rolled steel sheet.
- Primary cooling is cooling immediately after the end of hot rolling, and secondary cooling is cooling before winding.
- the hole expansion test was evaluated according to the test method described in Japan Iron and Steel Federation Standard JFS T 1001-1996.
- the damage occurrence ratio of the punched end face damage occurrence ratio is the range in which damage is recognized among the end faces punched out in a circular shape by punching a 10 mm diameter hole as in the hole expansion test and visually observing the end face shape. By measuring the angle, the punching end face damage occurrence ratio was determined.
- the hole expansion rate was tested in accordance with the hole expansion test method for metallic materials described in JIS Z 2256, and the hole expansion rate was evaluated as passing 25% or more.
- a columnar sample having a size of 0.3 mm ⁇ 0.3 mm ⁇ 10 mm was cut out from the steel plate, and the target grain boundary portion was made into a sharp needle shape by electrolytic polishing or a focused ion beam processing method, and three-dimensional atom probe measurement was performed.
- a rectangular parallelepiped was cut out from the atomic distribution image including the grain boundary perpendicular to the grain boundary to obtain a ladder chart. From the ladder chart analysis, the segregation amount of each atom was evaluated using the Excess amount. In each steel material, the segregation amount of each element was examined for five or more grain boundaries, and the average value was defined as each element segregation amount of each steel material.
- the sample cut out so as to obtain a cross section parallel to the rolling direction and the thickness direction of the steel plate was polished, further electropolished, and using the above-mentioned EBSP-OIM TM method, the magnification was 2000 times, the area of 40 ⁇ m ⁇ 80 ⁇ m, Measurement Step EBSP measurement was performed under measurement conditions of 0.1 ⁇ m. From the measurement results of each steel material, the region where the crystal grain orientation difference is 15 ° or more is recognized as a large-angle crystal grain boundary, and the region where the crystal grain orientation difference is 5 ° or more and less than 15 ° is recognized as a small-angle crystal grain boundary. I asked for the length above.
- Test No. 2, 4, 7, 9, and 10 are examples in which the components and production conditions of the steel sheet are within the scope of the present invention, and have high strength, good hole expansibility, and a small damage ratio of the punched end face.
- no. No. 1 has a slow primary cooling rate.
- No. 6 is an example in which the coiling temperature is high, the total amount of segregation of grain boundaries of C and B is insufficient, and the punched end face is damaged.
- No. No. 5 is an example in which the end temperature of primary cooling is low, a large amount of martensite is generated, and the hole expansion rate is lowered.
- No. No. 3 has a short air cooling time after hot rolling.
- No. 8 has a high primary cooling end temperature.
- No. 14 is an example in which the C content is insufficient and the strength is lowered.
- No. No. 11 is an example in which the end temperature of the primary cooling is slightly high, the ratio of the large-angle grain boundaries is reduced, and the punched end face is damaged.
- No. No. 13 is an example in which the B content is insufficient, the grain boundary segregation amount cannot be achieved, and end face damage occurs during punching.
- No. No. 12 is an example in which the P content is large and
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Abstract
Description
しかし、熱延鋼板の高強度化に伴い、鋼板を打ち抜き加工して形成された穴の端面にハガレやメクレ上の欠陥が発生することが問題となっている。これらの欠陥は製品端面の意匠性を著しく損なうばかりか、応力集中部となって疲労強度などにも影響を及ぼす危険性がある。
本発明は、このような知見に基づいてなされたものであり、その要旨とするところは、以下の通りである。
C:0.050~0.200%、
Si:0.01~1.5%、
Mn:
1.0~3.0%、
B:0.0002~0.0030%、
Ti:0.03~0.20%、
を含有し、
P:0.05%以下、
S:0.005%以下、
Al:0.5%以下、
N:0.009%以下
に制限され、
Nb:0.01~0.20%、
V:0.01~0.20%、
Mo:0.01~0.20%
のうち1種または2種以上を含有し、残部がFeおよび不可避的不純物からなり、
結晶方位角5°以上15°未満の界面である小角結晶粒界の長さと結晶方位角15°以上の界面である大角結晶粒界の長さとの比率が1:1~1:4であり、前記大角結晶粒界へのCの偏析量とBの偏析量との合計が4~20atoms/nm2であり、引張強度が850MPa以上であり、穴広げ率が25%以上である、高強度熱延鋼板。
(2)質量%で、
P:0.02%以下
P:0.02%以下、
に制限され、前記大角結晶粒界へのPの偏析量が1atoms/nm2以下である、(1)に記載の高強度熱延鋼板。
C:0.050~0.200%、
Si:0.01~1.5%、
Mn:
1.0~3.0%、
B:0.0002~0.0030%、
Ti:0.03~0.20%、
を含有し、
P:0.05%以下、
S:0.005%以下、
Al:0.5%以下、
N:0.009%以下
に制限され、
Nb:0.01~0.20%、
V:0.01~0.20%、
Mo:0.01~0.20%
のうち1種または2種以上を含有し、残部がFeおよび不可避的不純物からなる鋼片を1200℃以上に加熱し、910℃以上の温度で仕上圧延を完了し、前記仕上圧延終了後に0.5~7秒の空冷を行い、40℃/s以上の冷却速度で550~450℃まで一次冷却し、前記一次冷却の停止温度以下、450℃以上の温度で7.5~30秒間、保持または空冷し、続いて15℃/s以上の冷却速度で200℃以下まで二次冷却し、巻取る、高強度熱延鋼板の製造方法。
(4)前記鋼片は、質量%で、
P:0.02%以下、
に制限されている、(3)に記載の高強度熱延鋼板の製造方法。
その結果、クリアランスを増加させると、通常の穴拡げ試験方法で推奨されている12.5%前後のクリアランスで打ち抜いた場合には確認できないハガレやメクレ状の損傷が発生するようになり、16%のクリアランスが最も厳しい条件であることが判った。
そこで、以下では16%のクリアランスを用いて調査を進めた。
これらの鋼板から、JIS Z 2201の5号試験片を採取し、JIS Z 2241に準拠して引張特性を評価した。また、日本鉄鋼連盟規格JFS T 1001-1996に記載の試験方法に従って穴拡げ試験を行い、鋼板の伸びフランジ性を評価した。なお、打ち抜き加工後、穴拡げ試験前に、打ち抜き端面損傷発生比率を評価した。
本発明の鋼板においては、ベイナイトを積極的に活用するため、大角結晶粒界に加えて角度が15°未満の小角結晶粒界も含まれる。小角結晶粒界では、偏析元素のトラップサイト数等の違いから大角粒界と比べ偏析量が減少する傾向を示した。しかし、大角結晶粒界の偏析量との相関が認められたため、ここでは大角粒界での偏析量を測定した。結晶方位の角度は、試料の透過型電子顕微鏡観察から得られる菊池図形を解析することにより求めた。
本発明におけるベイナイトを主体とする組織は、断面観察したときの面積率で50%超のベイナイトを含んでいることが望ましく、50%未満のフェライトや第二相を含んでいても良い。
三次元アトムプローブ測定では、積算されたデータを再構築して実空間での実際の原子の分布像として求めることができる。粒界位置は原子面が不連続となることからこれを粒界面と認識することができ、また種々の元素が偏析している様子が視覚的に観察できる。
ラダーチャート解析から、各原子の偏析量を、偏析している、つまり固溶量からの上乗せ分の原子個数を単位粒界面積当たりで表すExcess量を用いて評価した。この評価は、高橋らによる、「塗装焼付硬化型鋼板の粒界偏析炭素量の定量観察」、新日鉄技報、第381号、2004年10月、p.26-30を参照にした。
鋼板の圧延方向および板厚方向に平行な断面が得られるように切り出した試料を研磨し、さらに電解研磨した。続いてEBSP-OIMTM(Electron Back Scatter Diffraction Pattern-Orientation Imaging
Microscopy)法を用いて、倍率2000倍、40μm×80μmエリア、測定ステップ0.1μmの測定条件でEBSP測定を実施した。
EBSP測定ではバルク試料表面の結晶方位の定量的解析ができ、分析エリアはSEMで観察できる領域である。数時間かけて測定し、分析したい領域を等間隔のグリッド状に数万点マッピングして行い、試料内の結晶方位分布を知ることができる。
測定結果より、結晶粒の方位差が15°以上となる領域が線上に現れ、これを大角結晶粒界と認識し、ソフトウェア上で大角結晶粒界の長さを求めた。同様に結晶粒の方位差が5°以上15°未満となる領域を小角結晶粒界と認識し、ソフトウェア上で小角結晶粒界の長さを求めた。
図2に示されるように、打ち抜き端面損傷発生比率が小さい鋼板の大角結晶粒界にはC及びBの偏析が多く認められた。
このように鋼板の耐端面損傷性が向上する理由として、偏析したC及びBにより結晶粒界が強化され、打ち抜き加工時に粒界におけるき裂の進展が抑制されることが考えられる。
その結果、結晶粒内への炭化物及びBNの析出を抑制すると、打ち抜き端面の損傷が抑制されることを見出した。一方、C及びBとは異なり、粒界に偏析すると粒界強化量を低下させる元素があることも見出した。
(偏析量)
最も厳しい条件であるクリアランスでの打ち抜き端面損傷発生比率が0.3以内であれば実用鋼として許容できる範囲である。本発明の検討では、16%のクリアランスが最も厳しい条件であったが、これは、鋼板の材質、工具により変化するため、クリアランスを12.5~25%の間で変化させて打ち抜き加工を行って、端面の性状を確認し、最も厳しいクリアランスの条件を確認する必要がある。最も厳しいクリアランスの条件で鋼板の打ち抜き加工を行った際の端面損傷を0.3以内とするためには、以下のように結晶粒界の粒界偏析元素量を適正化することが必要である。
一方、好ましい結晶粒界のCの偏析量とBの偏析量の合計の上限はないが、本発明の鋼板において実質的に偏析できる量の上限は20atoms/nm2程度と考えられた。結晶粒界のCの偏析量とBの偏析量の合計の更に好ましい範囲は打ち抜き端面損傷がほとんど発生しなくなる6~15atoms/nm2である。
図2に示すように、Cの偏析量とBの偏析量の合計が4~20atoms/nm2となり、更に小角結晶粒界の長さに対する大角結晶粒界の長さ比率が1以上4以下であれば、最も厳しいクリアランスの条件で鋼板の打ち抜き加工を行った際の打ち抜き端面損傷発生比率を0.3以内にすることができる。小角結晶粒界の長さに対する大角結晶粒界の長さ比率が1より小さいとベイナイトのブロック粒径が大きくなる傾向となり、靭性が劣化することと関係し、打ち抜き端面損傷発生比率が増加すると考えられる。また、小角結晶粒界に対する大角結晶粒界の長さ比率が4より大きいと、打ち抜き端面損傷発生比率は低く抑えられるものの、フェライトを主体とする組織となるため、強度が低下し引張強度850MPa以上の本発明の鋼板を満たさなくなる。
本発明において、鋼板組織として上記粒界偏析量および小角結晶粒界に対する大角結晶粒界の長さ比率を有し、鋼板の伸びを15%以上、穴拡げ率を25%以上、引張強度を850MPa以上とし、最も厳しいクリアランスの条件で鋼板の打ち抜き加工を行った際の打ち抜き端面損傷発生比率を0.3以内とするためには、鋼板の成分組成を以下のように規定することが好ましい。なお、以下に示す「%」は特に説明がない限り、「質量%」を意味するものとする。
また、以下に説明する基本成分により本発明の目的とする効果は十分に発揮されるものであるが、本発明の目的とする上記鋼板特性を阻害しない範囲で、その他の成分を含有することは許容されるものである。例えば、0.2%未満のCr、0.15%未満のCuを含有してもよい。
N:Nは窒化物を形成し、鋼板の加工性を低下させるため、含有量を0.009%以下に制限することが好ましい。
S:Sは、MnSなどの介在物として伸びフランジ性を劣化させ、更に熱間圧延時に割れを引き起こすので極力低下させるのが好ましい。特に、熱間圧延時に割れを防止し、加工性を良好にするためには、S含有量を0.005%以下に制限することが好ましい。
Al:Alは、窒化物などの析出物を形成して鋼板の加工性を損なうため、0.5%以下に制限することが好ましい。なお、溶鋼脱酸のためには、0.002%以上を添加することが好ましい。
上記成分組成を有する鋼を常法によって溶製、鋳造し、得られた鋼片を熱間圧延する。鋼片は、生産性の観点から、連続鋳造設備で製造することが好ましい。熱間圧延の加熱温度は、炭化物形成元素と炭素を十分に鋼材中に分解溶解させるため、1200℃以上とする。加熱温度を過度に高温にすることは、経済上好ましくないため、加熱温度の上限は1300℃以下とすることが好ましい。鋳造後、鋼片を冷却して、1200℃以上の温度で圧延を開始しても良い。1200℃以下に冷却された鋼片を加熱する場合は、1時間以上の保持を行うことが好ましい。
なお、仕上圧延ではオーステナイトの結晶粒径を微細化するために、最終スタンドから3スタンドの合計で60%以上の圧下率とすることが好ましい。圧下率はできるだけ高いことが好ましいが、生産性や設備負荷の観点から95%が実質的な上限である。
一次冷却の冷却速度が40℃/s未満であると、冷却途中に粗大な炭化物が析出し粒界に偏析するCが減少して打ち抜き端面の損傷が増加する恐れがある。一次冷却の冷却速度の上限は特に定めないが、冷却設備の能力上300℃/s以下が妥当な冷却速度である。また一次冷却の終了温度が550℃超であると、高温でのベイナイトが生成し大角結晶粒界の長さの比率が低下し、さらに600℃超であるとフェライト変態が促進されて強度が低下したり、パーライトの生成により穴広げ率が低下したりする。一方で、450℃より低いとマルテンサイトが多量に生成し穴広げ率が低下する。
表1に示す成分組成(残部はFe及び不可避的不純物)を有する材料を種々溶解した。表の成分値は化学分析値であり、単位は質量%である。表1の「-」は、意図的に添加していないことを意味する。
試験No.2、4、7、9、10は、鋼板の成分及び製造条件を本発明の範囲内とした例であり、高強度で、穴広げ性が良好であり、打ち抜き端面の損傷比率も小さい。
No.5は一次冷却の終了温度が低く、マルテンサイトが多量に発生し穴広げ率が低下した例である。
No.3は熱間圧延後の空冷時間が短く、No.8は一次冷却の終了温度が高く、No.14はCの含有量が不足しており、強度が低下した例である。
No.11は一次冷却の終了温度がやや高く、大角結晶粒界の比率が低下し、打ち抜き端面の損傷が発生した例である。
No.13は、Bの含有量が不足しており、粒界偏析量を達成することができず、打ち抜き時の端面損傷が発生した例である。
No.12は、Pの含有量が多く、打ち抜き端面の損傷が発生した例である。
Claims (4)
- 質量%で、
C:0.050~0.200%、
Si:0.01~1.5%、
Mn:
1.0~3.0%、
B:0.0002~0.0030%、
Ti:0.03~0.20%、
を含有し、
P:0.05%以下、
S:0.005%以下、
Al:0.5%以下、
N:0.009%以下
に制限され、
Nb:0.01~0.20%、
V:0.01~0.20%、
Mo:0.01~0.20%
のうち1種または2種以上を含有し、残部がFeおよび不可避的不純物からなり、
結晶方位角5°以上15°未満の界面である小角結晶粒界の長さと結晶方位角15°以上の界面である大角結晶粒界の長さとの比率が1:1~1:4であり、前記大角結晶粒界へのCの偏析量とBの偏析量との合計が4~20atoms/nm2であり、引張強度が850MPa以上であり、穴広げ率が25%以上である、高強度熱延鋼板。 - 質量%で、
P:0.02%以下、
に制限され、前記大角結晶粒界へのPの偏析量が1atoms/nm2以下である、請求項1に記載の高強度熱延鋼板。 - 質量%で、
C:0.050~0.200%、
Si:0.01~1.5%、
Mn:
1.0~3.0%、
B:0.0002~0.0030%、
Ti:0.03~0.20%、
を含有し、
P:0.05%以下、
S:0.005%以下、
Al:0.5%以下、
N:0.009%以下
に制限され、
Nb:0.01~0.20%、
V:0.01~0.20%、
Mo:0.01~0.20%
のうち1種または2種以上を含有し、残部がFeおよび不可避的不純物からなる鋼片を1200℃以上に加熱し、910℃以上の温度で仕上圧延を完了し、前記仕上圧延終了後に0.5~7秒の空冷を行い、40℃/s以上の冷却速度で550~450℃まで一次冷却し、前記一次冷却の停止温度以下、450℃以上の温度で7.5~30秒間、保持または空冷し、続いて15℃/s以上の冷却速度で200℃以下まで二次冷却し、巻取る、高強度熱延鋼板の製造方法。 - 前記鋼片は、質量%で、
P:0.02%以下、
に制限されている、請求項3に記載の高強度熱延鋼板の製造方法。
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WO2018026016A1 (ja) * | 2016-08-05 | 2018-02-08 | 新日鐵住金株式会社 | 鋼板及びめっき鋼板 |
JP6354917B2 (ja) * | 2016-08-05 | 2018-07-11 | 新日鐵住金株式会社 | 鋼板及びめっき鋼板 |
JPWO2018026016A1 (ja) * | 2016-08-05 | 2018-08-02 | 新日鐵住金株式会社 | 鋼板及びめっき鋼板 |
US11230755B2 (en) | 2016-08-05 | 2022-01-25 | Nippon Steel Corporation | Steel sheet and plated steel sheet |
Also Published As
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MX2014015218A (es) | 2015-03-05 |
KR101706478B1 (ko) | 2017-02-13 |
EP2865778A4 (en) | 2016-03-16 |
EP2865778B1 (en) | 2018-01-31 |
MX353735B (es) | 2018-01-26 |
US20150159244A1 (en) | 2015-06-11 |
TWI471426B (zh) | 2015-02-01 |
CN104395490A (zh) | 2015-03-04 |
JPWO2014002941A1 (ja) | 2016-05-30 |
US9803266B2 (en) | 2017-10-31 |
BR112014031739B1 (pt) | 2019-05-28 |
PL2865778T3 (pl) | 2018-06-29 |
ES2663995T3 (es) | 2018-04-17 |
JP6019117B2 (ja) | 2016-11-02 |
BR112014031739A2 (pt) | 2017-06-27 |
IN2014DN11227A (ja) | 2015-10-02 |
EP2865778A1 (en) | 2015-04-29 |
CN104395490B (zh) | 2017-03-08 |
TW201410880A (zh) | 2014-03-16 |
KR20150023699A (ko) | 2015-03-05 |
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