WO2011148490A1 - 鋼板およびその製造方法 - Google Patents
鋼板およびその製造方法 Download PDFInfo
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- WO2011148490A1 WO2011148490A1 PCT/JP2010/059013 JP2010059013W WO2011148490A1 WO 2011148490 A1 WO2011148490 A1 WO 2011148490A1 JP 2010059013 W JP2010059013 W JP 2010059013W WO 2011148490 A1 WO2011148490 A1 WO 2011148490A1
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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
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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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
- C21D8/0221—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
- C21D8/0421—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 working steps
- C21D8/0436—Cold 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
- C21D8/0473—Final recrystallisation annealing
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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
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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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
- C22C38/06—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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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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
- C21D2211/002—Bainite
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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
- C21D2211/005—Ferrite
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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
- C21D2211/008—Martensite
Definitions
- the present invention relates to a steel plate having a tensile strength of 750 MPa or more, excellent press formability and dynamic deformation characteristics, and a method for producing the same.
- collision resistant members materials for members that bear the load at the time of collision
- the strength of a steel sheet is affected by the deformation speed. As the strain rate of the steel sheet during deformation increases, the deformation stress of the steel sheet increases.
- a steel plate having a remarkably high tensile strength during high-speed deformation is suitable as a material for the impact-resistant member.
- Patent Document 1 discloses a cold-rolled steel sheet having a two-phase structure of ferrite and 10 to 50% martensite in a volume ratio and excellent in impact resistance.
- This cold rolled steel sheet has improved dynamic deformation characteristics (strength difference between tensile strength at high tensile strain rate and tensile strength at low tensile strain rate) by reducing the amount of solid solution element in ferrite, Thereby, it has high yield strength at the time of high-speed tensile deformation.
- the tensile strength of the steel plate having the chemical components and characteristics disclosed in Patent Document 1 is not described in Patent Document 1, it is considered to be about 590 MPa.
- Patent Document 2 A method for producing a fine-structure high-strength steel sheet is disclosed in Patent Document 2. However, this method is extremely low in productivity because it is necessary to repeatedly roll a plurality of laminated steel plates a plurality of times.
- Cold-rolled annealing with an ultrafine ferrite structure is performed on hot-rolled steel sheets having a martensite phase of 90% or more by performing cold rolling at a total rolling reduction of 20% or more and less than 80% and low-temperature annealing at 500 to 600 ° C.
- a method of manufacturing a plate is disclosed in Patent Document 3. However, since this method uses a hot-rolled steel sheet having a martensite phase as a raw material, the material to be rolled is strengthened and hardened during the cold rolling, so that the cold rolling property is remarkably lowered and the productivity is low. .
- Non-Patent Document 1 it is known that the uniform elongation of the steel sheet is significantly reduced as the crystal grains become finer.
- the conventional technology cannot provide a steel sheet having both a tensile strength of 750 MPa or more, excellent press formability and dynamic deformation characteristics.
- the steel sheet having both a tensile strength of 750 MPa or more, excellent press formability and dynamic deformation characteristics has a fine ferrite structure as the main phase of the metal structure of the steel sheet, and the type and dispersion of the second phase.
- the precipitation of strengthening elements contained in the steel sheet is suppressed, the ferrite crystal grains are refined, and the second phase is uniformly and finely dispersed, whereby the dynamic deformation characteristics of the steel sheet are improved. improves.
- (B) The effect described in the above item (a) is obtained not only for the steel sheet that has been hot-rolled but also for the steel sheet that has been cold-rolled and annealed after being hot-rolled. It is done.
- the present invention is a steel sheet having the following chemical composition and the following metal structure.
- Chemical composition C: 0.05 to 0.20% (in this specification, “%” means “mass%” unless otherwise specified), Si: 0.02 to 3.0%, Mn: 0.5 to 3.0%, P: 0.5% or less, S: 0.05% or less, Cr: 0.05 to 1.0%, sol. Al: 0.01 to 1.0%, one or more selected from the group consisting of Ti, Nb, Mo, V and W: 0.002 to 0.03% or less in total, as required
- the average crystal grain size of ferrite is 3.0 ⁇ m or less in a region of 100 to 200 ⁇ m in the thickness direction from the surface of the steel plate, and the region is composed of ferrite with an area ratio of 30 to 80% and the remaining structure. And the average sheet thickness direction interval of the remaining structure in the region is 3.0 ⁇ m or less.
- the present invention relates to a product of tensile strength and elongation at break, which has a tensile strength of 750 MP or more by cooling and winding a steel slab having the above-described chemical composition in multiple passes.
- Steel sheet having mechanical properties such that the difference between the dynamic tensile strength at a tensile strain rate of 10 3 / sec and the static tensile strength at a tensile strain rate of 0.01 / sec is 80 MPa or more.
- Condition 1 The rolling temperature of the final rolling pass of the hot rolling finish rolling is Ar 3 points or more.
- Condition 2 The total of the passage time of three consecutive passes including the final rolling pass and the cooling time from the temperature at the end of finish rolling to 720 ° C. is within 4.0 seconds.
- Condition 3 Cooling is started within 0.5 seconds from the end of finish rolling.
- Condition 4 winding is performed at a temperature of 630 ° C. or lower.
- cold rolling is performed at a rolling rate of 40 to 80%, and then annealing is performed for 10 to 300 seconds in a temperature range of Ac 1 to (Ac 3 + 10 ° C.). You may go.
- the steel plate according to the present invention or the steel plate produced by the method according to the present invention has the following mechanical properties.
- Mechanical properties Tensile strength is 750 MP or more, product of tensile strength and breaking elongation is 13000 MPa ⁇ % or more, and dynamic tensile strength and tensile strain rate at a tensile strain rate of 10 3 / sec are 0.01 The difference from the static tensile strength at / sec is 80 MPa or more.
- a steel plate having a tensile strength of 750 MPa or more, excellent press formability and dynamic deformation characteristics is provided without impairing productivity.
- Chemical composition [C: 0.05-0.20%] C lowers the transformation temperature from austenite to ferrite and lowers the finishing temperature of hot rolling, and therefore effectively promotes refinement of ferrite crystal grains.
- C secures the strength of the steel sheet.
- the C content is 0.05% or more, and preferably 0.08% or more in order to further promote the refinement of ferrite crystal grains.
- the C content is 0.20% or less, and preferably 0.17% or less in order to improve the workability of the welded portion.
- Si improves the strength of the steel sheet.
- Si content is 0.02% or more, Preferably it is 0.1% or more, More preferably, it is 0.3% or more.
- Si content is 3.0% or less, Preferably it is 2.0% or less, More preferably, it is 1.8% or less.
- Si and sol. The total content of Al is preferably 1.0% or more.
- Mn ensures the strength of the steel sheet. Moreover, since Mn lowers both the transformation temperature from austenite to ferrite and the finishing temperature of hot rolling, it promotes refinement of ferrite crystal grains. For this reason, Mn content is 0.5% or more, Preferably it is 1.0% or more, More preferably, it is 1.5% or more. On the other hand, if the Mn content exceeds 3.0%, the ferrite transformation after hot rolling is delayed and the ferrite volume fraction is lowered. For this reason, Mn content is 3.0% or less, Preferably it is 2.5% or less.
- P 0.5% or less
- P is contained as an inevitable impurity. If the P content exceeds 0.5%, P segregates at the grain boundaries and the stretch flangeability of the steel sheet deteriorates. For this reason, P content is 0.5% or less, Preferably it is 0.2% or less, More preferably, it is 0.05% or less.
- S 0.05% or less
- S is contained as an inevitable impurity. If the S content exceeds 0.05%, sulfide inclusions are formed, and the workability of the steel sheet decreases. The lower the S content, the better the workability of the steel sheet. For this reason, S content is 0.05% or less, Preferably it is 0.008% or less, More preferably, it is 0.003% or less.
- Cr 0.05 to 1.0%
- Cr strengthens the ferrite and increases the hardenability of the steel sheet, generating martensite and bainite in the ferrite.
- Cr suppresses the formation of coarse pearlite, contributes to the fine dispersion of the structure, and improves the dynamic strength.
- Cr content is 0.05% or more, More preferably, it is 0.1% or more.
- Cr content is 1.0% or less, Preferably it is 0.8% or less.
- sol. Al 0.01 to 1.0%
- Al improves the ductility of the steel sheet. For this reason, sol. Al content is 0.01% or more.
- the Al content is 1.0% or less, preferably 0.5% or less.
- Ti, Nb, Mo, V and W all form carbonitrides, or some elements exist in the steel as a solid solution state, thereby suppressing grain coarsening and refinement. It is effective for. For this reason, 0.002% or more in total of 1 type, or 2 or more types of Ti, Nb, Mo, V, and W is contained.
- the content of one or more of Ti, Nb, Mo, V and W exceeds 0.03% in total, movable dislocations easily occur in the ferrite, and the dynamic deformation characteristics of the steel sheet descend. For this reason, the content of one or more selected from the group consisting of Ti, Nb, Mo, V and W is 0.03% or less in total.
- Ca, Mg and REM may be contained as required as optional elements in order to refine oxides and nitrides precipitated during solidification of the slab and maintain the soundness of the slab.
- REM rare earth elements
- the content of one or more of these elements exceeds 0.0050% in total, inclusions are generated, the formability of the steel sheet deteriorates, and the manufacturing cost of the steel sheet increases. Therefore, the content of one or more of these elements is 0.0050% or less in total.
- the content of one or more of these elements is preferably 0.0005% or more in total in order to reliably obtain the above-described effects.
- N is illustrated as an impurity other than the above. N exists as an inevitable impurity. When N content exceeds 0.01%, the workability of a steel plate will fall. For this reason, it is preferable that N content is 0.01% or less, and it is more preferable that it is 0.006% or less.
- decarburization and concentration of easily oxidizable elements are caused by the influence of the atmosphere of the heating furnace and the coiling temperature of the hot-rolled steel sheet at the time of manufacturing the steel sheet.
- tissue and mechanical characteristic of the outermost layer part of a steel plate are easy to change with the position of the plate
- the structure and mechanical properties at a position where a minute distance (100 to 200 ⁇ m) has entered from the outermost layer portion of the steel plate in the thickness direction are stabilized.
- the average crystal grain size of ferrite in the region of 100 to 200 ⁇ m from the surface of the steel sheet in the thickness direction is 3.0 ⁇ m or less, so that not only hot-rolled steel sheets but also cold-rolled steel sheets subjected to cold rolling and annealing are used. In order to have sufficient dynamic deformation properties.
- This average crystal grain size is preferably 2.5 ⁇ m or less, more preferably 2.0 ⁇ m or less, and most preferably 1.5 ⁇ m or less.
- the average crystal grain size of ferrite is preferably 0.3 ⁇ m or more, and more preferably 0.5 ⁇ m or more in consideration of productivity.
- the average spacing in the thickness direction of the remaining structure excluding ferrite in the region is 3.0 ⁇ m or less.
- the average thickness direction interval is preferably 2.5 ⁇ m or less, more preferably 2.0 ⁇ m or less, and most preferably 1.6 ⁇ m or less.
- the type of the remaining structure is not particularly limited, and is, for example, bainite, martensite, retained austenite, or granular cementite depending on the static tensile strength required for the steel sheet.
- the balance structure is preferably bainite, tempered martensite, bainite, tempered martensite, or granular cementite.
- the “average thickness direction interval” means that the cross section in the rolling longitudinal direction of the steel sheet is mirror-polished and then subjected to nital corrosion, and a scanning electron microscope is used to shoot a digital image of 1000 to 2000 times in an area of 100 to 200 ⁇ m from the surface layer. Then, drawing a line having a length of 40 to 80 ⁇ m in the plate thickness direction of this digital image and measuring the interval of the remaining tissue relative to the plate thickness direction is repeated five times at an arbitrary position, and an average value thereof is obtained. .
- the average interval in the plate thickness direction of the remaining structure excluding ferrite is 3.0 ⁇ m. If it exceeds, the remaining structure will be present in a band shape and partially, and the remaining structure as the second phase will not be uniformly and finely dispersed. For this reason, the press formability and dynamic strength of the steel sheet are reduced.
- the remaining structure of the steel sheet exists in a band shape and partially as described above, the remaining structure of the cold-rolled annealed steel sheet manufactured by cold rolling and annealing the steel sheet becomes a band shape.
- the dynamic strength of the annealed steel sheet becomes insufficient.
- the average spacing in the thickness direction of the remaining structure excluding ferrite is 3.0 ⁇ m or less, preferably 2.5 ⁇ m or less, more preferably 2.5 ⁇ m or less in the region of 100 to 200 ⁇ m from the steel plate surface to the plate thickness direction. It is 2.0 ⁇ m or less, and most preferably 1.6 ⁇ m or less.
- the lower limit of the sheet thickness direction average interval is preferably 0.3 ⁇ m or more, and more preferably 0.5 ⁇ m or more, in view of the average ferrite particle size effective for dynamic deformation characteristics.
- the average interval in the rolling direction of the remaining structure is preferably 3.0 ⁇ m or less in the region of 100 to 200 ⁇ m in the plate thickness direction from the steel plate surface. Thereby, the flatness of the ferrite structure of the parent phase is lowered, and more equiaxed ferrite grains are finely dispersed. Therefore, more uniform strain is imparted to the ferrite during dynamic deformation as well as during static deformation, and as a result, static elongation and dynamic strength are further improved.
- the average interval in the rolling direction is preferably 2.5 ⁇ m or less, more preferably 2.0 ⁇ m or less, and most preferably 1.6 ⁇ m or less.
- the press formability of the steel sheet after hot rolling is significantly improved.
- the area ratio of retained austenite is less than 5%, the press formability is not sufficiently improved.
- the area ratio of retained austenite exceeds 30%, the austenite is unstable, and thus the effect of improving the press formability. Is killed. Therefore, the area ratio of retained austenite in the remaining structure is preferably 5 to 30% in order to improve the press formability of the steel sheet.
- the product of tensile strength and breaking elongation is 13000 MPa ⁇ % or more. Thereby, the outstanding press moldability is obtained.
- the product of the tensile strength and the breaking elongation is preferably 14000 MPa ⁇ % or more.
- the product of tensile strength and elongation at break is preferably 16000 MPa ⁇ % or more, More preferably, it is 17000 MPa ⁇ % or more.
- FIG. 1 is an explanatory view showing the shape of a high-speed tensile test piece. This strength difference is obtained by taking a micro test piece having the shape shown in FIG. 1 and using a test block type high-speed tensile tester and a dynamic tensile strength at a tensile strain rate of 10 3 / sec and a tensile strain rate of 0. Defined as the difference ⁇ TS from the static tensile strength at 0.01 / second.
- the excellent dynamic deformation characteristic means that the strength difference ⁇ TS is 80 MPa or more, preferably 100 MPa or more, and most preferably 120 MPa or more.
- Hot rolling is preferably performed in the austenite temperature range from a temperature exceeding 1000 ° C. using a lever mill or tandem mill. At least the final several stages of rolling are preferably performed using a tandem mill from the viewpoint of industrial productivity.
- a slab obtained by continuous casting or casting / splitting, a steel plate obtained by strip casting, and if necessary, hot-worked or cold-worked once, etc. are used as a steel piece. If the temperature of the billet is low, hot rolling is started after reheating the billet to a temperature above 1000 ° C.
- the starting temperature of hot rolling is 1000 ° C. or less, not only the rolling load becomes excessive and it becomes difficult to obtain a sufficient rolling rate, but hot rolling with a sufficient rolling rate is not less than Ar 3 points. It cannot be terminated at temperature, and the desired mechanical properties and thermal stability cannot be obtained.
- the starting temperature of hot rolling is more preferably 1025 ° C. or more, and further preferably 1050 ° C. or more.
- the hot rolling start temperature is preferably 1350 ° C. or less, and more preferably 1280 ° C. or less, in order to suppress coarsening of austenite grains and suppress equipment costs and heating fuel costs.
- the starting temperature of hot rolling is a relatively low temperature (for example, 1050 to 1250 ° C.) in the temperature range in the case of a steel type that does not need to sufficiently dissolve precipitates such as TiC and NbC in austenite. It is preferable. Thereby, the initial austenite crystal grains are refined, and the ferrite crystal grains of the obtained steel sheet are easily refined.
- the finishing temperature of hot rolling is Ar 3 or higher in order to transform from austenite to ferrite after hot rolling, and it is preferable to further satisfy a temperature condition of 780 ° C. or higher from the viewpoint of avoiding an increase in rolling load.
- the temperature at which the hot rolling is finished is not less than Ar 3 points, but is preferably as low as possible. This is because, as the temperature at which hot rolling is finished is lower, the effect of accumulating work strain introduced into austenite by hot rolling is increased and the refinement of ferrite crystal grains is promoted.
- Ar 3 point of the steel type used in the present invention is 950 ° C. from approximately 730 ° C..
- Hot rolling is continuous multi-pass rolling.
- the amount of reduction per pass is preferably 15 to 60%.
- the larger the amount of reduction per pass the more the strain to austenite accumulates, and the ferrite crystal grains generated by transformation become finer. For this reason, it is preferable that the reduction amount per pass is 20% or more particularly in three consecutive passes including the final rolling pass of the hot rolling finish rolling.
- the rolling amount per pass is less than 50% in order to avoid an increase in rolling equipment due to an increase in rolling load and to ensure controllability of the shape of the steel sheet.
- the rolling reduction of each of the three passes is 40% / pass or less.
- the steel plate that has finished hot rolling is cooled. This cooling transforms the deformation band from austenite to ferrite as a ferrite nucleation site without releasing the deformation band (working strain) introduced into the austenite.
- the steel sheet has a metal structure in which fine ferrite and the remaining structure are uniformly dispersed.
- the hot rolling is performed so that the total time of the passing time of the three passes and the cooling time from the temperature at the end of finish rolling to 720 ° C. is within 4.0 seconds. After that, cooling is started within 0.5 seconds from the end of finish rolling.
- the total time of the sheet passing time and the cooling time to 720 ° C. is determined by measuring the timing at which the tip of the steel sheet reaches the first roll of 3 passes with the sensor, and the temperature sensor installed in the cooling zone. Can be calculated from the relationship between these measured values and the plate feed speed. Moreover, the cooling start time from the end of finish rolling can be calculated from the sheet passing speed and the distance between the final roll and the cooling zone.
- the passage time of the three passes affects the rate at which the deformation zone introduced by hot rolling, that is, the nucleation site disappears.
- the cooling time also affects the rate at which the deformation zone disappears during cooling. Therefore, hot rolling and subsequent cooling are performed with the total time being within 4.0 seconds in order to sufficiently preserve the deformation zone introduced by hot rolling.
- the reason for controlling the passing time of the three passes is that these passes are rolling passes near the lower limit of the recrystallization temperature, so that austenite is not recrystallized, and hot rolling is performed by processing heat. This is because hot rolling is performed at approximately isothermal temperatures of approximately 800 to 950 ° C., and therefore rolling time is a main factor for preserving the deformation zone.
- cooling time affects the rate at which the deformation band disappears, that is, the formation of fine ferrite crystal grains. For this reason, cooling is started as soon as possible after finish rolling, specifically within 0.5 seconds from the end of finish rolling. Cooling is preferably initiated within 0.3 seconds, more preferably within 0.1 seconds, and most preferably within 0.05 seconds.
- the temperature range of 720 ° C. or lower is a transformation temperature range in which transformation from austenite to ferrite is activated. Further, the ferrite transformation temperature range in which the desired fine ferrite structure can be obtained is a temperature range of 720 to 600 ° C. For this reason, after the temperature of the steel sheet reaches 720 ° C. or lower, the steel sheet may be retained in the temperature range of 720 to 600 ° C. for 1 to 10 seconds by temporarily stopping cooling or reducing the cooling rate. .
- Winding process The steel plate that has undergone the hot rolling process and the cooling process is wound at 630 ° C. or less by the winding process. Thereby, the remainder structure other than the ferrite of a steel plate is controlled.
- the coiling temperature is higher than 630 ° C.
- a large amount of pearlite is generated to reduce the elongation of the steel sheet, and a static tensile strength of 750 MPa or more is not ensured.
- the remaining structure is martensite, it is preferable to cool a temperature range of 600 ° C. or lower at a cooling rate of 40 ° C./second or more and wind up at a temperature range of room temperature to 200 ° C.
- the coiling temperature is higher than 200 ° C.
- the desired steel plate strength may not be obtained due to tempering of martensite, and the balance between strength and ductility is lowered.
- it is more preferable to set the coiling temperature to 100 ° C. to 150 ° C.
- the remaining structure is bainite
- it is preferably wound at a temperature of 400 ° C. or higher and lower than 600 ° C.
- the remaining structure contains residual austenite together with bainite, it is more preferable to wind at a temperature of 400 to 450 ° C.
- the remaining structure is granular cementite, it is preferably wound at 600 ° C. or more and 630 ° C. or less. In order to make the remaining structure finer, it is more preferable to set the coiling temperature to 620 ° C. or lower.
- the steel sheet that has undergone the winding process may be further subjected to cold rolling and annealing. At this time, the surface layer scale of the steel sheet may be removed by pickling treatment before cold rolling.
- Cold rolling is performed at a rolling rate of 40 to 80%.
- the rolling rate is defined as ⁇ (steel plate thickness before cold rolling ⁇ steel plate thickness after cold rolling) / steel plate thickness before cold rolling) ⁇ ⁇ 100%.
- the rolling ratio is less than 40%, sufficient strain is not imparted to the ferrite, and the static elongation of the steel sheet after annealing is lowered.
- the rolling rate is preferably 50% or more.
- a high rolling rate exceeding 80% a great load is applied to the rolling mill, and the productivity of the steel sheet is lowered.
- the remaining structure excluding ferrite of the steel sheet that is subjected to cold rolling through the hot rolling and cooling is a structure containing martensite or bainite, which can more efficiently impart strain to the ferrite. This is preferable because it is possible. For example, by cooling at 600 ° C. or less at a cooling rate of 40 ° C./second and winding in a temperature range of room temperature to 200 ° C. or winding in a temperature range of 400 ° C. to less than 600 ° C., The remaining structure may be a structure containing martensite or bainite.
- the holding temperature is Ac 1 to (Ac 3 + 10 ° C.) in the steel sheet.
- the second phase that originally contributes to the static tensile strength is only cementite, and sufficient static tensile strength cannot be obtained. Even if static tensile strength is obtained, recovery to normal structure and recrystallization do not proceed sufficiently in some cases, static tensile elongation decreases, and dynamic tensile strength is caused by the presence of work strain remaining in ferrite. Strength decreases.
- the lower limit of the holding temperature is preferably 750 ° C. from the viewpoint of productivity.
- the upper limit of the holding temperature is preferably Ac 3 temperature.
- Holding time is 10 to 300 seconds.
- the holding time is less than 10 seconds, it is difficult to carry out in the existing manufacturing process, the metal structure tends to be banded due to segregation of substitutional elements, and the holding temperature is relatively low within the above range. In such a case, the removal of processing strain by cold rolling becomes insufficient, and the elongation of the steel sheet is lowered.
- the holding time is longer than 300 seconds, austenite becomes coarse during holding, ferrite grains precipitated in the subsequent cooling process become coarse, and both static tensile strength and dynamic tensile strength decrease.
- Cooling after holding affects the metal structure of the steel sheet.
- the remaining structure excluding ferrite becomes martensite by cooling to the Ms point or less without passing through the bainite nose in the CCT curve. If the bainite nose is passed or the cooling is stopped in the bainite region, the remaining structure becomes bainite.
- the cooling rate of 700 ° C. or less is preferably 20 ° C./second or more.
- the tensile strength is 750 MPa or more
- a steel sheet having a mechanical property in which the product of the elongation at break is 13000 MPa ⁇ % or more and the difference between the dynamic tensile strength and the static tensile strength is 80 MPa or more is produced.
- Hot-rolled steel sheets were manufactured from the steel pieces having chemical compositions A to L shown in Table 1 under the conditions shown in Table 2.
- Chemical compositions F to I in Table 1 do not satisfy the chemical composition defined in the present invention, and trial numbers 13 and 14 in Table 2 do not satisfy the production conditions defined in the present invention.
- F1 to F3 in Table 2 indicate the reduction ratios in each stand, ⁇ t indicates the elapsed time from the end of finish rolling to the start of cooling, and (F1 to 720 ° C. time) is a continuous 3 including the final rolling pass. The total time of the sheet passing time of passes F1 to F3 and the cooling time from the temperature at the end of finish rolling to 720 ° C. is shown.
- the metal structure and mechanical properties of the hot-rolled steel sheets Nos. 1 to 15 were measured according to the procedure described below.
- Method After mirror-polishing the cross section in the longitudinal direction of the rolling, the metal structure that had undergone the night erosion was measured in a region of 100 to 200 ⁇ m from the surface layer based on a digital image taken at 1000 to 2000 times with a scanning electron microscope.
- the average ferrite grain size was determined as the average ferrite grain size by a cutting method.
- the average spacing in the thickness direction of the remaining tissue is drawn 5 times in the thickness direction, measuring the spacing of the second phase relative to the thickness direction 5 times at an arbitrary position, and the average value thereof. As calculated.
- the ferrite fraction was obtained by calculating the area ratio of ferrite by binarizing by image processing using the fact that phases such as martensite and bainite are shown darker than ferrite in the SEM image.
- Formability was determined by taking a JIS No. 5 tensile test piece and conducting a tensile test, and obtaining a good strength and formability balance in which the product of tensile strength and elongation at break was 13000 MPa ⁇ % or more.
- the dynamic deformation characteristics were obtained by collecting the minute TPs shown in FIG. 1 and using a force-block type high-speed tensile tester and tensile strength and tensile strain rate at a tensile strain rate of 10 3 / sec.
- Steel having a high static difference of 80 MPa or more considering that the strength difference ⁇ TS with respect to the tensile strength at 0.01 / sec is about 60 MPa in the conventional steel of 780 to 980 MPa class. was passed.
- trial numbers 1 to 5, 10 to 12 and 15 use slabs having chemical compositions A to E and J to L that satisfy the chemical composition of the present invention, and satisfy the manufacturing conditions of the present invention. It can be seen that it has high strength of 750 MPa, excellent moldability of TS ⁇ EL ⁇ 13000 (MPa ⁇ %), and high dynamic deformation characteristics (static difference) of 80 MPa or more.
- trial number 6 uses a slab having a chemical composition F that does not satisfy the chemical composition of the present invention, and thus shows a relatively high static TS and static EL, but a large amount of Ti as a precipitation strengthening element. Since it contains, the static difference is as low as the conventional material.
- Test Nos. 7 to 9 use slabs having chemical compositions G to I that do not satisfy the chemical composition of the present invention, so that high static strength and ductility are not compatible with high static difference.
- Trial No. 13 uses a slab having a chemical composition A that satisfies the composition of the present invention, but the rolling conditions do not satisfy the production conditions of the present invention, so that the precipitation of fine ferrite is not sufficient, the static elongation is low, Also, the static difference is low.
- Example 1 The steel plates Nos. 1 to 4, 6, 7, 13 and 15 that had been subjected to hot rolling and cooling in Example 1 were subjected to cold rolling and annealing under the conditions shown in Table 4, and then the windings shown in Table 2 Cold-rolled steel sheets of trial numbers 18 to 31 in Table 4 were produced by winding at the take-up temperature.
- Table 5 shows the measurement results of the metal structure and mechanical properties of the cold-rolled steel plates of trial numbers 18 to 31.
- the measuring method of a metal structure and a mechanical characteristic is the same as the measuring method of Example 1.
- the cold-rolled steel plates of trial numbers 18, 19, 24, 25, 27, 28, and 30 are hot-rolled steel plates manufactured under the conditions in the scope of the present invention, within the scope of the present invention. Manufactured by cold rolling and annealing under conditions.
- the cold-rolled steel sheets of trial numbers 18, 19, 24, 25, 27, 28, and 30 have high strength of 750 MPa, excellent formability of TS ⁇ EL ⁇ 13000 (MPa ⁇ %), and high dynamic of 80 MPa or more. It can be seen that it also has deformation characteristics (static difference).
- the trial number 20 uses the slab of the composition F that does not satisfy the chemical composition of the present invention, it exhibits a relatively high static TS and static EL, but, like the trial number 6 in Table 3, precipitation occurs. Since it contains a large amount of Ti, which is a strengthening element, the static difference is as low as that of conventional materials.
- Test No. 21 uses a coarse hot-rolled steel sheet (hot-rolled sheet number 13) having a ferrite grain size of more than 3.0 ⁇ m as a base material, so that the ferrite after cold rolling and annealing also becomes coarse grains, As low as conventional materials.
- the trial number 22 since the specimen No. 15 satisfying the present invention as a base material was annealed at a low annealing temperature, the work structure by cold rolling remained, and the average ferrite grain size and the average thickness direction interval of the remaining structure were measured. could not. Moreover, the trial number 22 has remarkably low elongation and a low static motion difference.
- the trial number 23 Since the trial number 23 annealed at the high annealing temperature about the trial number 1 which satisfies this invention as a base material, since a ferrite also becomes coarse, dynamic tensile strength is low. Since the trial number 26 uses the slab of the composition G which does not satisfy the composition of the present invention, the ferrite fraction is high and the static strength is low even after cold rolling.
- Test No. 29 was annealed for a test No. 1 satisfying the present invention as a base material with a long annealing time (holding time) exceeding 300 seconds, so that the ferrite was coarsened. Further, the average sheet thickness direction interval of the remaining structure is wide. For this reason, a static difference is low.
- the steel sheet of the present invention has ultrafine ferrite crystal grains and a uniform dispersion structure of the remaining structure (second phase), and not only as a hot-rolled steel sheet, but also by formability and dynamics by cold rolling and annealing. It is also suitable as a base material for cold-rolled steel sheets that have both deformation characteristics.
- a steel sheet having a tensile strength of 750 MPa or more, excellent press formability, and dynamic deformation characteristics is easily manufactured without impairing productivity by the manufacturing method of the present invention.
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Abstract
Description
化学組成;C:0.05~0.20%(本明細書では特に断りがない限り化学組成に関する「%」は「質量%」を意味する)、Si:0.02~3.0%、Mn:0.5~3.0%、P:0.5%以下、S:0.05%以下、Cr:0.05~1.0%、sol.Al:0.01~1.0%、Ti、Nb、Mo、V及びWからなる群から選ばれた1種または2種以上:合計で0.002~0.03%以下、必要に応じて、Ca、MgおよびREMの1種または2種以上:合計で0.0050%以下、残部Feおよび不純物からなること。
条件2;最終圧延パスを含む連続する3パスの通板時間と、仕上げ圧延終了時の温度から720℃までの冷却時間との合計が4.0秒間以内であること。
条件4;巻取りが、630℃以下の温度で行われること。
本発明に係る製造方法では、巻取りを行った後に、圧延率40~80%の冷間圧延を行ってからAc1~(Ac3+10℃)の温度域に10~300秒間保持する焼鈍を行ってもよい。
機械特性;引張強度が750MP以上であること、引張強度と破断伸びの積が13000MPa・%以上であること、及び、引張歪み速度103/秒での動的引張強度と引張歪み速度0.01/秒での静的引張強度との差が80MPa以上であること。
[C:0.05~0.20%]
Cは、オーステナイトからフェライトへの変態温度を低下し、熱間圧延の仕上げ温度を低下するので、フェライト結晶粒の微細化を効果的に促進する。また、Cは、鋼板の強度を確保する。このため、C含有量は、0.05%以上であり、フェライト結晶粒の微細化をより促進するために0.08%以上であることが好ましい。しかし、C含有量が0.20%超であると、熱間圧延後のフェライト変態が遅延し、フェライト体積率が低下するとともに、溶接性が劣化する。このため、C含有量は、0.20%以下であり、溶接部の加工性を向上するために0.17%以下であることが好ましい。
Siは鋼板の強度を向上する。このため、Si含有量は、0.02%以上であり、好ましくは0.1%以上であり、より好ましくは0.3%以上である。一方、Si含有量が3.0%を超えると、鋼板の延性が著しく劣化するとともに、熱間圧延時に被圧延材の表面酸化が発生する。このため、Si含有量は、3.0%以下であり、好ましくは2.0%以下であり、より好ましくは1.8%以下である。なお、フェライト組織中に残留オーステナイトを生成させる場合には、Si及びsol.Alの合計含有量が1.0%以上であることが好ましい。
Mnは、鋼板の強度を確保する。また、Mnは、オーステナイトからフェライトへの変態温度、及び、熱間圧延の仕上温度をいずれも低下するため、フェライト結晶粒の微細化を促進する。このため、Mn含有量は、0.5%以上であり、好ましくは1.0%以上であり、より好ましくは1.5%以上である。一方、Mn含有量が3.0%を超えると、熱間圧延後のフェライト変態が遅延し、フェライト体積率が低下する。このため、Mn含有量は、3.0%以下であり、好ましくは2.5%以下である。
Pは、不可避的不純物として含有される。P含有量が0.5%を超えると、Pが結晶粒界に偏析し、鋼板の伸びフランジ性が劣化する。このため、P含有量は、0.5%以下であり、好ましくは0.2%以下であり、より好ましくは0.05%以下である。
Sは、不可避的不純物として含有される。S含有量が0.05%を超えると、硫化物系介在物が形成され、鋼板の加工性が低下する。S含有量が低いほど鋼板の加工性が向上する。このため、S含有量は、0.05%以下であり、好ましくは0.008%以下であり、より好ましくは0.003%以下である。
Crは、フェライトを強化するとともに、鋼板の焼入れ性を増加させ、フェライト中にマルテンサイトやベイナイトを生成する。また、Crは、粗大なパーライトの形成を抑制し、組織の微細分散化に寄与し、動的強度を向上する。このため、Cr含有量は、0.05%以上であり、より好ましくは0.1%以上である。一方、Cr含有量が1.0%を超えると、鋼板の表面性状および延性が劣化する。このため、Cr含有量は、1.0%以下であり、好ましくは0.8%以下である。
Alは、鋼板の延性を向上する。このため、sol.Al含有量は0.01%以上である。一方、sol.Al含有量が1.0%を超えると、高温でのオーステナイトが不安定となり、熱間圧延における仕上温度を過度に高める必要を生じるとともに、連続鋳造を安定して行うことができなくなる。このため、sol.Al含有量は、1.0%以下であり、好ましくは0.5%以下である。
[Ti、Nb、Mo、V及びWからなる群から選ばれた1種または2種以上:合計で0.002~0.03%]
Ti、Nb、Mo、V及びWは、いずれも、炭窒化物を形成すること、または一部の元素については固溶状態として鋼中に存在することによって、結晶粒の粗大化抑制および微細化に有効である。このため、Ti、Nb、Mo、VおよびWの1種または2種以上を合計で0.002%以上含有する。一方、Ti、Nb、Mo、VおよびWの1種または2種以上の含有量が合計で0.03%を超えると、フェライト中に可動転位が容易に発生し、鋼板の動的変形特性が低下する。このため、Ti、Nb、Mo、V及びWからなる群から選ばれた1種または2種以上の含有量は、合計で0.03%以下である。
Ca、Mgおよび希土類元素(REM)は、鋳片の凝固時に析出する酸化物や窒化物を微細化して鋳片の健全性を保つため、任意元素として必要に応じて含有してもよい。これらの元素の1種または2種以上の含有量が合計で0.0050%を超えると、介在物が生成して鋼板の成形性が劣化するとともに、鋼板の製造コストが嵩む。そこで、これらの元素の1種または2種以上の含有量は、合計で0.0050%以下である。なお、これらの元素の1種または2種以上の含有量は、上述した効果を確実に得るために、合計で0.0005%以上であることが好ましい。
2.金属組織
自動車の耐衝突部材による衝突エネルギーの吸収は、一般的に、ハット型の閉断面又はこれに類する多角形の閉断面を有する耐衝突部材に、軸方向あるいは横方向からの外力を負荷して、複数の曲げ変形を部分的に発生することによって、行われることが多い。したがって、この耐衝突部材の素材である鋼板の板厚方向の表面近傍における機械特性は、耐衝突部材が優れた衝突エネルギー吸収性を発揮するために、重要である。
少なくとも鋼板表面から板厚方向に100~200μmの領域におけるフェライトの平均結晶粒径が3.0μm以下であることが、鋼板(熱延鋼板のみならず冷間圧延及び焼鈍を施した冷延鋼板も含む)が充分な動的変形特性を具備するために、必要である。この平均結晶粒径は、好ましくは2.5μm以下であり、より好ましくは2.0μm以下であり、最も好ましく1.5μm以下である。
上記領域のフェライトの面積分率が30%未満であると、鋼板が十分な動的変形特性を有さない。一方、この面積分率が80%を超えると、鋼板の動的変形特性はさらに向上するものの、鋼板の静的引張強度が低下する。そこで、この面積分率は30~80%である。この面積分率は、40%以上であることが好ましく、50%以上であることがより好ましい。また、この面積分率は、75%以下であることが好ましく、70%以下であることがより好ましい。
上記領域におけるフェライトを除く残部組織の板厚方向平均間隔は、3.0μm以下である。板厚方向平均間隔は、好ましくは2.5μm以下であり、さらに好ましくは2.0μm以下であり、最も好ましくは1.6μm以下である。
さらに、残部組織の圧延方向平均間隔は、鋼板表面から板厚方向に100~200μmの領域において3.0μm以下であることが好ましい。これにより、母相のフェライト組織の扁平度が低下し、より等軸なフェライト粒が微細に分散して存在する。したがって、より均一な歪みが、静的変形時のみならず動的変形時のフェライトに対して付与され、結果として、静的延びや動的強度がより向上する。圧延方向平均間隔はこの領域において2.5μm以下であることが好ましく、2.0μm以下であることがより好ましく、1.6μm以下であることが最も好ましい。
3.機械特性
[引張強度:750MPa以上]
引張強度は、750MPa以上である。引張強度は、JIS5号引張試験片を採取して引張試験を行うことによって測定される。
引張強度と破断伸びとの積が13000MPa・%以上である。これにより、優れたプレス成形性が得られる。耐衝突部材がより複雑な形状である場合、引張強度と破断伸びとの積が14000MPa・%以上であることが好ましい。耐衝突部材が他の構造部材と一体的に成型されるため等の理由によって、より高い成形性を要求される場合、引張強度と破断伸びとの積が16000MPa・%以上であることが好ましく、17000MPa・%以上であることがより好ましい。
図1は、高速引張試験片の形状を示す説明図である。この強度差は、図1に示す形状を有する微小試験片を採取し、検力ブロック式高速引張試験機を用いて、引張歪み速度103/秒での動的引張強度と、引張歪み速度0.01/秒での静的引張強度との差ΔTSとして規定される。優れた動的変形特性とは、この強度差ΔTSが80MPa以上であること、好ましくは100MPa以上であること、最も好ましくは120MPa以上であることを意味する。
4.製造方法
[熱間圧延工程]
上述した化学組成を有する鋼片を複数パスで熱間圧延する。そして、熱間圧延の仕上げ圧延の最終圧延パスの圧延温度は、Ar3点以上である。
熱間圧延を終了した鋼板は冷却される。この冷却により、オーステナイトに導入された変形帯(加工歪み)を解放することなく、この変形帯をフェライトの核生成サイトとしてオーステナイトからフェライトへ変態させる。鋼板は、微細なフェライトと残部組織とが均一に分散した金属組織を有する。
熱間圧延工程及び冷却工程を経た鋼板は、巻取工程によって630℃以下で巻取られる。これにより、鋼板のフェライト以外の残部組織が制御される。
残部組織をマルテンサイトとする場合には、600℃以下の温度域を40℃/秒以上の冷却速度で冷却し、室温から200℃以下の温度域で巻取ることが好ましい。巻取り温度が200℃超であると、マルテンサイトの焼戻しにより、所望の鋼板強度が得られない場合があり、強度および延性のバランスが低下する。鋼板の錆の発生が、残存する冷却水によって懸念される場合には、巻取り温度を100℃~150℃とすることがより好ましい。
前記巻取工程を経た鋼板に、その後さらに冷間圧延および焼鈍を行ってもよい。このとき冷間圧延を施す前に、鋼板の表層スケールを酸洗処理により除去してもよい。
40%未満の低い圧延率であると、充分な歪みがフェライトに付与されず、焼鈍後の鋼板の静的伸びが低下する。このとき圧延率は50%以上であることが好ましい。一方、80%を超える高い圧延率では、多大な負荷が圧延機に与えられ、また鋼板の生産性が低下する。
冷間圧延工程により歪エネルギーを蓄えた組織を有する鋼板を、焼鈍すること、すなわち加熱後一定の温度で保持してから冷却することによって、高い動的引張強度を有する高強度鋼板が得られる。
このようにして、引張強度が750MPa以上であり、引張強度と破断伸びとの積が13000MPa・%以上であるとともに、動的引張強度と静的引張強度との差が80MPa以上である機械特性を有する鋼板が製造される。
[金属組織]
圧延長手方向断面を鏡面研磨した後、表層から100~200μmの領域に関して、ナイタル腐食した金属組織を、走査電子顕微鏡により1000~2000倍で撮影したデジタル画像に基づいて、測定した。
残部組織の平均板厚方向間隔は、板厚方向に長さ80μmの線を描き、板厚方向に相対する第2相の間隔を測定することを任意の位置で5回繰り返し、それらの平均値として、算出した。
成形性は、JIS5号引張試験片を採取して引張試験を行い、引張強度と破断伸びの積が13000MPa・%以上の良好な強度および成形性バランスを得られるものを合格とした。
試番13は、本発明の組成を満足する化学組成Aを有するスラブを用いるものの圧延条件が本発明の製造条件を満足しないため、微細なフェライトの析出が十分でなく、静的伸びが低く、また静動差も低い。
試番26は、本発明の組成を満足しない組成Gのスラブを用いるため、冷間圧延後もフェライト分率が高く、静的強度が低い。
Claims (4)
- 下記化学組成及び下記金属組織を有することを特徴とする鋼板:
化学組成;質量%で、C:0.05~0.20%、Si:0.02~3.0%、Mn:0.5~3.0%、P:0.5%以下、S:0.05%以下、Cr:0.05~1.0%、sol.Al:0.01~1.0%、Ti、Nb、Mo、V及びWからなる群から選ばれた1種または2種以上:合計で0.002~0.03%以下、残部Feおよび不純物からなること、及び
金属組織;少なくとも鋼板表面から板厚方向に100~200μmの領域においてフェライトの平均結晶粒径が3.0μm以下であること、前記領域は30~80%の面積率のフェライトと残部組織とからなること、及び、前記領域において前記残部組織の平均板厚方向間隔が3.0μm以下であること。 - さらに、Ca、MgおよびREMの1種または2種以上を合計で0.0050%以下含有する請求項1に記載された鋼板。
- 請求項1または請求項2に記載された化学組成を有する鋼片に複数パスで熱間圧延を行った後に冷却を行い、巻取りを行うことによって鋼板を製造する方法であって、前記熱間圧延、前記冷却及び巻取りを、下記条件1~条件4を満足する条件で行うことを特徴とする鋼板の製造方法:
条件1;前記熱間圧延の仕上げ圧延の最終圧延パス出側の圧延温度がAr3点以上であること、
条件2;前記最終圧延パスを含む連続する3パスの通板時間と、前記仕上げ圧延終了時の温度から720℃までの冷却時間との合計が4.0秒間以内であること、
条件3;前記冷却が、前記仕上げ圧延の終了時から0.5秒間以内に開始されること、及び
条件4;前記巻取りが、630℃以下の温度で行われること。 - 前記巻取りが終了した後、圧延率40~80%の冷間圧延を行ってからAc1~(Ac3+10℃)の温度域に10~300秒間保持する焼鈍を行うことを特徴とする請求項3に記載された鋼板の製造方法。
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JP2016050334A (ja) * | 2014-08-29 | 2016-04-11 | 新日鐵住金株式会社 | 熱延鋼板の製造方法 |
JP2016194136A (ja) * | 2015-03-31 | 2016-11-17 | 株式会社神戸製鋼所 | 製造安定性に優れた高強度高延性鋼板、及びその製造方法、並びに高強度高延性鋼板の製造に用いられる冷延原板 |
JP2016191106A (ja) * | 2015-03-31 | 2016-11-10 | 株式会社神戸製鋼所 | 切断端面の耐遅れ破壊性に優れた高強度鋼板およびその製造方法 |
JP2017066505A (ja) * | 2015-10-02 | 2017-04-06 | 新日鐵住金株式会社 | 疲労特性と成形性に優れた高強度鋼板およびその製造方法 |
TWI598158B (zh) * | 2016-09-02 | 2017-09-11 | China Steel Corp | Wire production method that dynamically adjusts cooling rate |
JP6264515B1 (ja) * | 2017-03-31 | 2018-01-24 | 新日鐵住金株式会社 | 熱間圧延鋼板 |
WO2018179387A1 (ja) * | 2017-03-31 | 2018-10-04 | 新日鐵住金株式会社 | 熱間圧延鋼板 |
US10894996B2 (en) | 2017-03-31 | 2021-01-19 | Nippon Steel Corporation | Hot rolled steel sheet |
Also Published As
Publication number | Publication date |
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JPWO2011148490A1 (ja) | 2013-07-25 |
CN103038381A (zh) | 2013-04-10 |
KR101456772B1 (ko) | 2014-10-31 |
EP2578711A4 (en) | 2017-05-24 |
EP2578711A1 (en) | 2013-04-10 |
CN103038381B (zh) | 2015-11-25 |
KR20130034027A (ko) | 2013-04-04 |
EP2578711B1 (en) | 2019-10-09 |
JP5252128B2 (ja) | 2013-07-31 |
PL2578711T3 (pl) | 2020-05-18 |
US20130192724A1 (en) | 2013-08-01 |
US10538823B2 (en) | 2020-01-21 |
ES2756584T3 (es) | 2020-04-27 |
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