WO2014171062A1 - 高強度熱延鋼板およびその製造方法 - Google Patents
高強度熱延鋼板およびその製造方法 Download PDFInfo
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- WO2014171062A1 WO2014171062A1 PCT/JP2014/001508 JP2014001508W WO2014171062A1 WO 2014171062 A1 WO2014171062 A1 WO 2014171062A1 JP 2014001508 W JP2014001508 W JP 2014001508W WO 2014171062 A1 WO2014171062 A1 WO 2014171062A1
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- Prior art keywords
- steel sheet
- hot
- rolled steel
- less
- mass
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
- C21D1/20—Isothermal quenching, e.g. bainitic hardening
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
- B21D22/02—Stamping using rigid devices or tools
- B21D22/022—Stamping using rigid devices or tools by heating the blank or stamping associated with heat treatment
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B15/00—Layered products comprising a layer of metal
- B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
- B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
- C21D1/28—Normalising
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- C21D1/84—Controlled slow cooling
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- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C21D6/00—Heat treatment of ferrous alloys
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- 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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- C21D8/0226—Hot rolling
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- 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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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- 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
- C21D8/0426—Hot rolling
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- 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
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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- 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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- C22C—ALLOYS
- C22C18/00—Alloys based on zinc
- C22C18/04—Alloys based on zinc with aluminium as the next major constituent
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- 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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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
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
Definitions
- the present invention is, for example, a structural member such as a member of an automobile body or a frame, an underbody member such as a suspension, or a vehicle such as a truck frame part.
- the present invention relates to a high-strength hot-rolled steel sheet suitable for a member, and particularly relates to an improvement in punchability during mass production (hereinafter also referred to as mass production punchability).
- Patent Document 1 in mass%, C: 0.05 to 0.15%, Si: 1.50% or less, Mn: 0.5 to 2.5%, P: 0.035% or less, S: 0.01%
- a high-strength hot-rolled steel sheet excellent in expansion (formability) is described.
- Patent Document 1 After hot rolling, it is cooled to a temperature range of 400 to 550 ° C. at an average cooling rate of 30 ° C./s or more, wound on a coil, and then heated to 300 ° C. or less.
- a cooling rate of 400 ° C./h diffusion of P to the grain boundary can be prevented, the fracture surface transition temperature becomes 0 ° C. or less, toughness is improved, and hole expansion workability is improved.
- truck frame parts and undercarriage parts are used for connecting parts and reducing weight, and for subsequent burring (borring process) and hole expanding (bore expanding process) A lot of drilling is required.
- this type of drilling is performed by punching from the viewpoint of productivity, there is often a strong demand for improvement of punchability.
- Patent Document 1 merely prevents the P grain boundary segregation and improves the hole expansion processability.
- Patent Document 1 describes the punching processability. There is no mention, and prevention of segregation of P to the grain boundary does not necessarily immediately improve the properties of the punched end face and contribute to the improvement of the punching workability.
- Patent Document 2 in mass%, C: 0.01 to 0.07%, N: 0.005% or less, S: 0.005% or less, Ti: 0.03 to 0.2%, B: 0.0002 Punching workability with a composition containing up to 0.002% and a structure in which the main phase is ferrite or bainitic ferrite and the hard second phase and cementite are 3% or less in area ratio
- a high-strength hot-rolled steel sheet excellent in the above has been proposed.
- defects in the punched surface can be prevented by maintaining B in a solid solution state.
- ferrite or bainitic ferrite is used as the phase of the maximum area, and the hard second phase that adversely affects hole expansibility is limited to 3% or less.
- Patent Document 3 in mass%, C: 0.05 to 0.15%, Si: 0.1 to 1.5%, Mn: 1 to 2%, P: 0.03% or less, S: 0.003% or less, Al: 0.01 to 0.08 %, Ti: 0.05 to 0.15%, N: 0.005% or less, the bainite phase is more than 95% in area ratio, and the average grain size of the bainite structure at 1/4 position of the sheet thickness is the rolling direction ( Aspect ratio in the region that is 5 ⁇ m or less in the thickness section parallel to the rolling direction and 4 ⁇ m or less in the thickness section perpendicular to the rolling direction and is 1/10 of the thickness centering on the center position of the thickness.
- a high strength hot-rolled steel sheet having a structure in which the number of crystal grains expanded in the rolling direction of 5 or more is 7 or less and excellent in punchability having a tensile strength of 780 MPa or more has been proposed.
- the punchability is improved by reducing the average grain size of bainite and reducing the number of spreading grains in the central region of the plate thickness.
- Japanese Patent No. 3889766 Japanese Unexamined Patent Publication No. 2004-315857 Japanese Unexamined Patent Publication No. 2012-62562
- JFS T1001 Japan Iron Steel Federation Standards
- JFS T1001 Japan Iron Steel Federation Standards
- a blank sheet of about 100 mm ⁇ 100 mm is collected from a steel plate, and a clearance condition of 12% ⁇ 1% of the plate thickness (plate thickness of 2 mm or more) with respect to the blank plate.
- the punchability of the steel sheet is often evaluated.
- a clearance of 17 to 23% of the plate thickness, or 10 to 20% of the plate thickness, which is different from the clearance at the time of punching specified in JFS T1001, is 10 mm ⁇ . Holes are punched to evaluate the punchability of steel sheets.
- high-strength steel sheets manufactured as a steel sheet excellent in punchability by the techniques described in Patent Documents 2 and 3 often have poor punching due to punching during mass production, and are excellent in punchability during mass production. There was a problem that it was difficult to say that it was a steel plate, and further improvement of the material was necessary.
- the present invention aims to solve the problems of the prior art, and to provide a high-strength hot-rolled steel sheet that has high strength and has excellent punchability during mass production of parts and a method for producing the same.
- the present inventors have examined various factors affecting the mass production punchability of a high-strength hot-rolled steel sheet.
- the punching direction is not a vertical direction but an oblique direction, centering of the hole is difficult, and sheet pressing conditions (sheet ⁇ clamping conditions) are likely to be poor.
- sheet pressing conditions sheet ⁇ clamping conditions
- the punching process in mass production unlike the punching process in the laboratory, in addition to punching under extremely severe conditions, it will be subject to various process variations (process variability) described above. Even in steel sheets that have been evaluated as having excellent punchability in the punching evaluation performed in the laboratory as described above, there are many cases where punching by punching during mass production of parts is defective. I found out that
- the present inventors have further investigated a method for evaluating mass production punchability.
- the punched hole diameter and the plate pressing conditions are also improved in the punching end surface properties (appearances of punched surface). It was found for the first time that it had a significant effect.
- the punch diameter was set to 50 mm ⁇ flat-bottomed type, and the die diameter was determined so that the punching clearance would be 30%. It has been found that the method of placing a spacer on top of the substrate, placing a blank plate thereon and fixing it with a plate press from above and punching it is the best method for evaluating mass production punchability.
- the present inventors diligently examined the influence of the steel sheet structure on the mass production punchability using the above-described evaluation method.
- size control of the bainite phase which reduces the size of the bainite phase, is not sufficient to achieve the desired mass production punchability, and another type of further microstructure control (microstructure)
- microstructure another type of further microstructure control
- the steel structure is mainly composed of the bainite phase, the lower structure, the lath interval, is reduced, and iron based carbide is precipitated in the grains of the bainite lath. It was found that such adjustment is effective for remarkable improvement of mass production punchability of high-strength hot-rolled steel sheets.
- the present invention has been completed based on such knowledge and further investigation. That is, the gist of the present invention is as follows.
- High-strength heat excellent in mass-production punching characterized in that it has a structure that is less than ⁇ m and has a structure in which the number ratio of Fe-based carbides precipitated in the grains of bainite lath among all Fe-based carbides is 10% or more Rolled steel sheet.
- the composition further comprises one or two selected from Nb: 0.005 to 0.2% and B: 0.0002 to 0.0030% by mass%. High strength hot rolled steel sheet.
- a high-strength hot-rolled steel sheet comprising:
- a high-strength hot-rolled steel sheet comprising:
- a hot dip galvanizing layer or an alloyed hot dip galvanizing layer is formed on the surface of the high-strength hot-rolled steel sheet according to any one of (1) to (5). Hot dip galvanized steel sheet formed.
- the steel slab is, in mass%, C: more than 0.07% and 0.2% or less, Si: 2.0% or less, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.01% or less, Ti: 0.05 to 0.3%, V: 0.05 to 0.3%
- the finishing temperature of the finish rolling is in the temperature range of (A r3 transformation point) to (A r3 transformation point + 120 ° C.). After finishing the finish rolling, cooling is started within 2 s, and the average cooling rate is 40 ° C./s or more.
- High-strength hot-rolled steel sheet with excellent mass production punching characteristics characterized by rolling at a coiling temperature of 300 to 500 ° C after cooling to the coiling temperature Manufacturing method.
- the composition further contains one or two selected from Nb: 0.005 to 0.2% and B: 0.0002 to 0.0030% by mass%.
- Nb 0.005 to 0.2%
- B 0.0002 to 0.0030% by mass%.
- annealing and plating are performed to obtain a plated steel sheet.
- the annealing is performed at a soaking temperature of 730 ° C. or less, and after the annealing, a hot dip galvanizing bath is passed as the plating treatment to form a hot dip galvanized layer on the surface of the high strength hot rolled steel sheet.
- an alloying treatment for alloying the hot-dip galvanized layer is performed.
- the composition in addition to the above composition, the composition further contains one or two selected from Nb: 0.005 to 0.2% and B: 0.0002 to 0.0030% by mass%. High strength hot rolled steel sheet.
- the mass is further selected from Cu: 0.005 to 0.3%, Ni: 0.005 to 0.3%, Sn: 0.005 to 0.3%.
- a high-strength hot-rolled steel sheet characterized by containing seeds or two or more kinds.
- a high-strength hot-rolled steel sheet comprising:
- a high-strength hot-rolled steel sheet comprising:
- the steel slab is in mass%, C: 0.05 to 0.15%, Si: 1.5% Below, Mn: 1.0-2.0%, P: 0.05% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.01% or less, Ti: 0.05-0.2%, remaining Fe and unavoidable impurities
- the hot rolling heats the steel slab to 1100 ° C. or more, sets the total rolling reduction of the final two passes of the finish rolling to 30% or more, and sets the finish rolling temperature of the finish rolling to 30% or more.
- the composition in addition to the above composition, the composition further contains one or two selected from Nb: 0.005 to 0.2% and B: 0.0002 to 0.0030% by mass%.
- Nb 0.005 to 0.2%
- B 0.0002 to 0.0030% by mass%.
- the surface is plated by annealing and plating.
- the annealing is performed at a soaking temperature of 730 ° C. or less, and after the annealing, a hot dip galvanizing bath is passed as the plating treatment, and the hot-rolled steel sheet surface is hot-dip galvanized.
- a method for producing a hot-dip galvanized steel sheet, comprising forming a plating layer or further subjecting the hot-dip galvanized layer to an alloying treatment.
- the high-strength hot-rolled steel sheet according to the present invention is suitable for structural members such as truck frame parts and automobile body members and frames, and suspension members such as suspensions, and contributes effectively to reducing the weight of the members. There is also an effect of doing.
- Embodiment 1 The reason for limiting the composition of the high-strength hot-rolled steel sheet according to Embodiment 1 will be described. “%” Means “% by mass” unless otherwise specified.
- the “high strength” in the embodiment refers to a case where the tensile strength TS is 900 MPa or more.
- C more than 0.07% and 0.2% or less C is an element that contributes effectively to increasing the strength of the steel sheet, and is a useful element that promotes bainite transformation and contributes to bainite phase formation.
- an appropriate amount of C has the effect of increasing the amount of carbides in the grains of bainite lath and improving the mass production punchability. In order to exhibit such an effect, the content needs to exceed 0.07%. On the other hand, an excessive content exceeding 0.2% impairs workability and weldability.
- C is limited to the range of more than 0.07% and less than 0.2%.
- it is 0.079% or more, More preferably, it is 0.10% or more. Moreover, 0.19% or less is preferable.
- Si 2.0% or less
- Si is an element that increases the steel sheet strength by solute strengthening and contributes to the improvement of the ductility of the steel sheet. In order to exhibit such an effect, it is desirable to contain 0.05% or more.
- excessive Si content increases the transformation point and inhibits bainite phase formation.
- Si type complex oxide penetration into the grain boundary of the surface layer becomes significant during the heating stage of the steel slab, and descaling during hot rolling ( Even if a large amount of descaling) is used, it becomes difficult to remove, and the quality of the punching end face is lowered during mass production of steel sheets, and the mass production punchability is lowered. For this reason, Si was limited to 2.0% or less. In addition, Preferably it is 1.5% or less. More preferably, it is 1.0% or less.
- Mn 1.0-3.0%
- Mn is an effective element that contributes to increasing the strength of a steel sheet by solid solution strengthening and transformation strengthening. Further, Mn has an effect of reducing the transformation point and miniaturizing the bainite lath. In order to obtain such an effect, a content of 1.0% or more is required. On the other hand, if it exceeds 3.0% and excessively contained, center segregation becomes prominent, and workability is remarkably reduced. For this reason, Mn was limited to the range of 1.0 to 3.0%. It is preferably 1.4 to 2.6%.
- P 0.05% or less
- P is an element that has the effect of increasing the strength of the steel sheet by solid solution. Although it is desirable to reduce as much as possible, the content up to 0.05% is acceptable. In addition, Preferably, it is 0.03% or less.
- S 0.005% or less S forms sulfides, and particularly when coarse sulfides are formed, the ductility and workability of the steel sheet decrease. Therefore, it is desirable to reduce as much as possible, but 0.005% is acceptable. For this reason, S was limited to 0.005% or less. In addition, Preferably it is 0.003% or less, More preferably, it is 0.0015% or less.
- Al 0.1% or less
- Al is an important element that acts as a deoxidizing agent for steel. In order to exhibit such an effect, it is desirable to contain 0.01% or more. On the other hand, if the content exceeds 0.1%, castability deteriorates, and a large amount of inclusions (oxides) remain in the steel, leading to deterioration of surface quality and workability. For this reason, Al was limited to 0.1% or less. In addition, Preferably it is 0.06% or less.
- N 0.01% or less N is combined with a nitride-forming element and precipitates as a nitride, contributing to refinement of crystal grains.
- the N content exceeds 0.01%, a large amount of nitride is formed, which causes a decrease in hot ductility and a significant decrease in burring formability. Is preferably reduced as much as possible, but is acceptable up to 0.01%. For this reason, N was limited to 0.01% or less.
- Ti 0.05-0.3%
- Ti is one of the most important elements in the present invention, which easily forms carbonitrides and contributes to refinement of bainite lath spacing after transformation by refining austenite ( ⁇ ) grains before transformation. is there. Furthermore, Ti increases the carbide (carbonitride) in the grains of fine bainite lath and contributes to the increase in strength through precipitation strengthening, and also increases the void generation site (site) during punching. This contributes to improvement of mass production punchability. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if it exceeds 0.3% and excessively contained, the rolling force becomes very large, making the rolling operation difficult, and the size of the precipitates is too coarse to reduce the workability. Therefore, Ti is limited to the range of 0.05 to 0.3%. Note that. Preferably it is 0.07 to 0.25%, more preferably 0.07 to 0.23%.
- V has the effect of improving the strength-elongation balance and the strength-hole expansibility balance, and is one of the most important elements in the present invention.
- V also has an effect of reducing the bainite lath interval, which reduces the occurrence interval of micro voids at the time of punching and facilitates linking between voids, Improve mass production punchability.
- V also has the effect of suppressing the precipitation of coarse Fe-based carbides, thereby improving the edge face properties during punching. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, even if it contains excessively exceeding 0.3%, an effect will be saturated, manufacturing cost will rise, and it will become economically disadvantageous.
- V is limited to the range of 0.05 to 0.3%.
- it is 0.07% or more, More preferably, it is 0.22% or more.
- it is 0.28% or less, More preferably, it is 0.26% or less.
- the above-mentioned components are basic components.
- one or more elements selected from Nb: 0.005 to 0.2% and B: 0.0002 to 0.0030% are selected as the selective elements.
- Cr One or two selected from 0.002 to 0.3% and / or Ca: 0.0002 to 0.004%
- REM One or two selected from 0.0002 to 0.004% are required Can be selected according to the content.
- Nb One or two selected from 0.005 to 0.2%, B: 0.0002 to 0.0030% Nb and B are both elements that contribute to the improvement of mass production punchability. 1 type or 2 types can be contained.
- Nb contributes to the improvement of mass production punching by reducing the microvoid generation interval during punching by refining the structure through the formation of precipitates (carbonitrides) and further by finely dispersing the carbides. In order to acquire such an effect, it is preferable to contain 0.005% or more. On the other hand, if the content exceeds 0.2%, precipitates are coarsened and workability is lowered, and the production cost is increased. For this reason, when contained, Nb is preferably limited to a range of 0.005 to 0.2%. More preferably, it is 0.005 to 0.15%.
- B contributes to the improvement of mass production punching through refinement of the bainite lath interval. In order to acquire such an effect, it is preferable to contain 0.0002% or more. On the other hand, if it exceeds 0.0030% and contains excessively, workability will be reduced. Therefore, when contained, B is preferably limited to a range of 0.0002 to 0.0030%. More preferably, it is 0.0003 to 0.0020%.
- One or more selected from Cu: 0.005 to 0.3%, Ni: 0.005 to 0.3%, Sn: 0.005 to 0.3% Cu, Ni, and Sn all increase in strength through solid solution strengthening It is an element which contributes to, and can be selected as necessary and can contain one or more. In order to acquire such an effect, it is desirable to contain Cu: 0.005% or more, Ni: 0.005% or more, Sn: 0.005% or more. On the other hand, if it contains more than Cu: 0.3%, Ni: 0.3% and Sn: 0.3%, hot workability is lowered, and there is a risk of causing surface layer cracking during hot rolling.
- Cu 0.005 to 0.3%
- Ni 0.005 to 0.3%
- Sn 0.005 to 0.3%. More preferably, Cu is 0.005 to 0.2%, Ni is 0.005 to 0.2%, and Sn is 0.005 to 0.2%.
- Mo and Cr are elements that contribute to the improvement of hardenability, and the fineness of bainite lath through the reduction of the bainite transformation point. It is also an element that contributes to chemical conversion, and it can be selected as necessary and can contain one or two kinds. In order to obtain such an effect, it is desirable to contain Mo: 0.002% or more and Cr: 0.002% or more. On the other hand, an excessive content exceeding Mo: 0.3% and Cr: 0.3% causes an increase in manufacturing cost, which is economically disadvantageous. Therefore, when it is contained, it is preferable to limit it to the range of Mo: 0.002 to 0.3% and Cr: 0.002 to 0.3%. More preferably, Mo is 0.002 to 0.2% and Cr is 0.002 to 0.2%.
- Ca 0.0002 to 0.004% and REM: 0.0002 to 0.004%.
- Both Ca and REM are effective in improving processability through the morphology control of inclusions. It is an element which contributes to, and can be selected as necessary and can contain one or two kinds. In order to acquire such an effect, it is desirable to contain Ca: 0.0002% or more and REM: 0.0002% or more. On the other hand, if the content exceeds Ca: 0.004% and REM: 0.004%, the inclusions in the steel increase and the workability decreases. Therefore, when it is contained, it is preferable to limit the range to Ca: 0.0002 to 0.004% and REM: 0.0002 to 0.004%. More preferably, Ca is 0.0002 to 0.003% and REM is 0.0002 to 0.003%.
- the balance other than the above components is composed of Fe and inevitable impurities.
- the high-strength hot-rolled steel sheet of the present invention is a Fe-based steel in which the bainite phase is more than 90% by volume, the average interval of bainite lath is 0.45 ⁇ m or less, and precipitated in the grains of bainite lath among all Fe-based carbides. It has a structure with a carbide ratio of 10% or more.
- the steel sheet structure In order to ensure the desired mass production punchability, it is important to first make the steel sheet structure into a substantially bainite single-phase structure with a volume ratio exceeding 90% as described above. Note that it is preferably more than 92%, more preferably more than 94%.
- the bainite phase is a mixed structure of ferrite and Fe-based carbide. By making it a structure of a single bainite phase, the interface between the ferrite and Fe-based carbide becomes the starting point for microvoid generation at the time of punching. This is advantageous in terms of both sides of the void connection.
- the bainite phase is a bainite phase having a bainite lath interval of 0.45 ⁇ m or less, which is the substructure.
- the bainite lath interval was limited to 0.45 ⁇ m or less.
- it is 0.40 micrometer or less, More preferably, it is 0.35 micrometer or less.
- the second phase (remainder) other than the bainite phase is one or more of martensite, retained austenite, ferrite, and pearlite.
- the bainite phase is a bainite phase in which carbides are generated in the phase, and among all the precipitated Fe-based carbides, Fe-based carbides precipitated in ferrite grains are included.
- An organization with a number ratio of 10% or more If the number of Fe carbides precipitated in the ferrite grains is less than 10% of the total number of Fe carbides precipitated, the desired mass production punchability cannot be ensured. For this reason, the number of Fe-based carbides precipitated in the grains is limited to 10% or more of the total number of Fe-based carbides. In addition, Preferably it is 15% or more, More preferably, it is 20% or more.
- a steel slab having the above composition is heated and subjected to hot rolling consisting of rough rolling and finish rolling to obtain a hot rolled steel sheet.
- the method for producing the steel slab is not particularly limited, and the molten steel having the above composition is melted by a conventional melting method such as a converter, an electric furnace or an induction furnace, or further by a vacuum degassing apparatus or the like. Secondary refining is performed, and a steel slab having a predetermined size is formed by a conventional casting method such as continuous casting. It should be noted that there is no problem even if the ingot-bundling method is used.
- the steel slab may be a thin slab having a thickness of about 30 mm. If it is a thin slab, rough rolling can be omitted.
- electromagnetic-stirring EMS
- IBSR intentional-bulging-soft-reduction-casting
- Can do By applying an electromagnetic stirring treatment, equiaxed crystals can be formed at the center of the plate thickness, and segregation can be reduced.
- segregation at the central portion of the plate thickness can be reduced by preventing the flow of molten steel in the unsolidified portion of the continuous cast slab.
- the steel slab is heated to a heating temperature of 1100 ° C. or higher and subjected to hot rolling.
- Heating temperature of steel slab 1100 ° C. or higher
- the steel slab is heated to a heating temperature of 1100 ° C. or higher. If the heating temperature is less than 1100 ° C., the precipitate is not sufficiently re-dissolved, and the desired precipitate distribution cannot be secured in the subsequent steps.
- the heating temperature is 1150 degreeC or more.
- heating temperature becomes high too much, a crystal grain will coarsen and a bainite lath will coarsen finally. For this reason, it is desirable to limit the heating temperature of the steel slab to 1300 ° C. or less.
- the heated steel slab is subjected to hot rolling consisting of rough rolling and finish rolling to form a hot rolled steel sheet.
- Rough rolling is not particularly limited as long as a desired sheet bar size (sheet bar size) can be ensured.
- finish rolling followed by finish rolling.
- the finish rolling conditions are extremely important for obtaining a desired bainite structure.
- Total rolling reduction in the final two passes of finish rolling 30% or more
- austenite ( ⁇ ) in which sufficient strain is accumulated into bainite. Therefore, in the present invention, first, the total rolling reduction of the final two passes of finish rolling is limited. If the total rolling reduction in the final two passes of finish rolling is less than 30%, the accumulation of strain in ⁇ is insufficient, and a desired bainite structure cannot be secured after transformation. For this reason, the total rolling reduction in the final two passes of finish rolling is limited to 30% or more.
- it is 40% or more, More preferably, it is 50% or more.
- Finishing rolling end temperature (A r3 transformation point) to (A r3 transformation point + 120 ° C)
- the finishing temperature of finish rolling is less than the Ar3 transformation point, it becomes difficult to secure a desired bainite single-phase structure, which is a desired structure.
- the finishing temperature of finish rolling exceeds ( Ar 3 transformation point + 120 ° C.) and becomes high, it becomes difficult to obtain a fine bainite phase. For this reason, the finishing temperature of finish rolling is limited to a temperature in the range of (A r3 transformation point) to (A r3 transformation point + 120 ° C.).
- (A r3 transformation point) it is preferably (A r3 transformation point) to (A r3 transformation point + 80 ° C.).
- the finishing temperature of finish rolling is represented by the surface temperature.
- the thermal expansion curve obtained by cooling at a cooling rate 1 ° C. / s after the processing applied thermal expansion curve
- the transformation temperature obtained from the changing point is used.
- Cooling conditions are also extremely important to obtain the desired tissue.
- Cooling start within 2 s after finish rolling finish
- the cooling start time is within 2 s after finish rolling is finished. It is necessary to start cooling. If the start of cooling exceeds 2 s after finishing rolling, recovery of ⁇ and recrystallization proceed, the nuclei of bainite transformation decrease, and the desired bainite lath structure cannot be obtained. For this reason, cooling was started within 2 s after finishing rolling.
- it is less than 1.5 s, More preferably, it is less than 1 s.
- Average cooling rate 40 ° C / s or more If the average cooling rate from the finish rolling finish temperature to the cooling stop temperature is less than 40 ° C / s, pro-eutectoid ferrite precipitates and the volume ratio exceeds 90%. It is difficult to ensure a structure having a bainite phase and having a desired bainite lath interval. For this reason, the average cooling rate of cooling after finishing rolling was limited to 40 ° C./s or more. In addition, Preferably it is 50 degrees C / s or more, More preferably, it is 60 degrees C / s or more. The upper limit of the cooling rate is determined depending on the capacity of the cooling facilities, but is preferably about 150 ° C./s or less from the viewpoint of the steel plate shape. In the present invention, on the premise that the cooling after finish rolling is controlled to the above-described cooling rate, and to cool down to the cooling stop temperature, which will be described later, in order to obtain a microstructure characterized in the present invention This is a necessary requirement.
- Cooling stop temperature 300 ⁇ 500 °C
- the coil is wound immediately after the cooling is stopped. For this reason, the cooling stop temperature is taken up as the winding temperature.
- the cooling stop temperature (winding temperature) is less than 300 ° C. or more than 500 ° C.
- the cooling stop temperature was limited to a temperature in the range of 300 to 500 ° C.
- the temperature is preferably 350 to 500 ° C.
- pickling may be performed to remove the scale formed on the surface.
- the hot rolled steel sheet may be subjected to temper rolling.
- the steel sheet is further annealed at a soaking temperature of 730 ° C or less and passed through a hot dip galvanizing bath to form a galvanized layer on the surface. And it is good also as a hot-dip galvanized steel plate.
- the soaking temperature of the annealing process exceeds 730 ° C., the bainite is tempered, so that it becomes difficult to secure a structure having a bainite phase exceeding 90% by volume and having a desired bainite lath interval. .
- the soaking temperature in the annealing process is set to 730 ° C. or less.
- the lower limit of the annealing temperature is not particularly limited, but from the viewpoint of adhesion between the hot dip galvanized layer and the underlying steel plate, the soaking temperature of the annealing treatment is preferably 600 ° C or higher.
- it is good also as an alloying hot dip galvanized steel plate by giving the alloying process of this galvanized layer further.
- hot-dip galvanized steel sheets but also hot-rolled steel sheets obtained can be used as plated steel sheets such as electrogalvanized steel sheets.
- Example 1 The steel slab having the composition shown in Table 1 was subjected to heating, finish rolling, and cooling after rolling as shown in Table 2 to obtain a hot-rolled steel sheet. During the continuous casting, electromagnetic stirring (EMS) was performed for the segregation reduction treatment of the components other than the hot rolled steel plate No. 1 ′ of steel A1 in Tables 1 to 3 described later. Table 1 also shows the Ar3 transformation point of each steel slab obtained from the thermal expansion curve.
- EMS electromagnetic stirring
- hot-rolled steel sheets are pickled, passed through a continuous hot dip galvanizing line, annealed under the conditions shown in Table 2, then hot dip galvanized, and hot dip galvanized steel sheets (GI ).
- hot dip galvanizing treatment the hot-rolled steel sheet after annealing is immersed in a 480 ° C zinc plating bath (0.1 mass% Al-Zn), and a hot dip galvanized layer with an adhesion amount of 45 g / m 2 per side is applied to the steel sheet. It was set as the process formed in both surfaces.
- some hot-rolled steel sheets were subjected to galvanizing treatment and further subjected to alloying treatment to obtain alloyed galvanized steel sheets (GA).
- the alloying temperature was 520 ° C.
- Specimens were collected from the obtained hot-rolled steel sheets (partially including plated steel sheets) and subjected to structure observation, tensile tests, and mass production punchability tests.
- the test method was as follows.
- (1) Microstructure observation A specimen for microstructural observation was collected from the obtained hot-rolled steel sheet (plated steel sheet), and after polishing a plate thickness section (L section) parallel to the rolling direction, 3% nital solution Corroded and exposed the structure. Then, at the 1/4 thickness position of the L cross-section, the tissue is observed with a scanning electron microscope (magnification: 3000 times), the tissue is imaged with 10 fields of view, and image analysis processing (image analysis) is performed. Then, phases other than the bainite phase were separated, the structure fraction of the phase other than bainite was determined, and the area ratio of the bainite phase was calculated. The area ratio thus obtained was defined as the volume ratio of the bainite phase.
- a thin film sample was taken from the 1/4 position of the thickness of the obtained hot-rolled steel sheet (plated steel sheet), made into a thin film specimen by mechanical polishing and electrolytic polishing, and transmission electron microscope (transmission electron microscope) (magnification : About 30000 times), the structure was observed, the structure was photographed in 10 fields of view, the bainite lath interval was measured, the average value thereof was obtained, and the bainite lath interval of each hot-rolled steel sheet was obtained.
- a specimen for structure observation was collected from the obtained hot-rolled steel sheet (plated steel sheet), and after polishing a plate thickness section (L section) parallel to the rolling direction, the structure was corroded with 3% nital solution.
- a replica sample (replica-sample) was prepared at a thickness of 1/4 position.
- the tissue was observed with a transmission electron microscope (magnification: about 30000 times), and the tissue was photographed in 10 fields of view.
- the number of Fe-based precipitates was measured for each precipitation location (grain boundaries and within the grains), and the total Fe-based precipitation of Fe-based precipitates precipitated within the grains of bainite lath. The ratio to the number of objects was calculated.
- the Fe-based precipitates were identified by the form of the precipitates and EDX analysis (energy-dispersive-X-ray-analysis).
- Mass production stamping with cracks, chips, brittle fracture surfaces, secondary shear surfaces, and those with no rough cross-section as ⁇ (passed), those with only rough cross-section as ⁇ (passed), and others with x (failed) Sex was evaluated.
- All of the examples of the present invention are hot-rolled steel sheets (plated steel sheets) having high tensile strength TS: 900 MPa or more and excellent mass production punchability. On the other hand, in the comparative example that is out of the scope of the present invention, the mass production punchability is lowered.
- Embodiment 2 The reason for limiting the composition of the high-strength hot-rolled steel sheet according to Embodiment 2 will be described. “%” Means “% by mass” unless otherwise specified. In the second embodiment, “high strength” refers to the case where the tensile strength TS is 700 to 900 MPa.
- C 0.05-0.15%
- C is an element that contributes effectively to increasing the strength of the steel sheet, and is a useful element that promotes bainite transformation and contributes to bainite phase formation.
- an appropriate amount of C has the effect of increasing the amount of carbides in the grains of bainite lath and improving the mass production punchability. In order to exhibit such an effect, the content of 0.05% or more is required. On the other hand, an excessive content exceeding 0.15% impairs workability and weldability. Therefore, C is limited to the range of 0.05 to 0.15%. In addition, Preferably it is 0.071% or more, More preferably, it is 0.080% or more and 0.14% or less.
- Si 1.5% or less
- Si is an element that increases the strength of the steel sheet by solid solution strengthening and contributes to the improvement of the ductility of the steel sheet. In order to exhibit such an effect, it is desirable to contain 0.05% or more. On the other hand, excessive Si content exceeding 1.5% raises the transformation point and inhibits bainite phase formation. For this reason, Si was limited to 1.5% or less. In addition, Preferably it is 1.0% or less.
- Mn 1.0-2.0%
- Mn is an effective element that contributes to increasing the strength of the steel sheet by solid solution strengthening and transformation strengthening. Further, Mn has the effect of reducing the transformation point and miniaturizing the bainite lath. In order to obtain such an effect, a content of 1.0% or more is required. On the other hand, when the content exceeds 2.0%, the center segregation becomes remarkable and the workability is remarkably lowered. For this reason, Mn was limited to a range of 1.0 to 2.0%. It is preferably 1.2 to 1.9%.
- P 0.05% or less
- P is an element that has the effect of increasing the strength of the steel sheet by solid solution, but if contained in a large amount, it is likely to segregate at grain boundaries, etc. Although it is desirable to reduce, inclusion up to 0.05% is acceptable. In addition, Preferably, it is 0.03% or less.
- S 0.005% or less S forms sulfides, and particularly when coarse sulfides are formed, the ductility and workability of the steel sheet decrease. Therefore, it is desirable to reduce as much as possible, but 0.005% is acceptable. For this reason, S was limited to 0.005% or less. In addition, Preferably it is 0.003% or less, More preferably, it is 0.0015% or less.
- Al 0.1% or less
- Al is an important element that acts as a deoxidizer for steel. In order to exhibit such an effect, it is desirable to contain 0.01% or more. On the other hand, if the content exceeds 0.1%, castability deteriorates, and a large amount of inclusions (oxides) remain in the steel, leading to deterioration of surface properties and workability. For this reason, Al was limited to 0.1% or less. In addition, Preferably it is 0.06% or less.
- N 0.01% or less N combines with a nitride-forming element and precipitates as a nitride, contributing to refinement of crystal grains.
- the N content exceeds 0.01%, a large amount of nitride is generated, which causes a decrease in hot ductility and a significant decrease in burring workability. Therefore, it is desirable to reduce N as much as possible. Up to 0.01% is acceptable. For this reason, N was limited to 0.01% or less. In addition, Preferably it is 0.006% or less, More preferably, it is 0.004% or less.
- Ti 0.05-0.2%
- Ti is one of the most important elements in the present invention, which easily forms carbonitrides and contributes to refinement of bainite lath spacing after transformation through refinement of austenite ( ⁇ ) grains before transformation. It is.
- Ti increases the fine grained bainite lath carbide (carbonitride) and contributes to increased strength through precipitation strengthening, and becomes a void generation site during punching, increasing voids, and mass production punchability Contributes to improvement.
- 0.05% or more of content is required.
- the content exceeds 0.2%, the rolling load becomes very large and the rolling operation becomes difficult, or the precipitate size becomes too coarse and the workability is lowered. Therefore, Ti is limited to the range of 0.05 to 0.2%. Note that. Preferably it is 0.065 to 0.125%, more preferably 0.065 to 0.10%.
- the above-mentioned components are basic components.
- one or more elements selected from Nb: 0.005 to 0.2% and B: 0.0002 to 0.0030% are selected as the selective elements.
- Cr One or two selected from 0.002 to 0.3% and / or Ca: 0.0002 to 0.004%
- REM One or two selected from 0.0002 to 0.004% are required Can be selected according to the content.
- Nb One or two selected from 0.005 to 0.2%, B: 0.0002 to 0.0030% Nb and B are both elements that contribute to the improvement of mass production punchability. 1 type or 2 types can be contained.
- Nb contributes to the improvement of mass production punching by reducing the microvoid generation interval at the time of punching by refining the structure and finely dispersing the carbide through the formation of precipitates (carbonitrides). In order to acquire such an effect, it is preferable to contain 0.005% or more. On the other hand, if the content exceeds 0.2%, precipitates are coarsened and workability is lowered, and the production cost is increased. For this reason, when contained, Nb is preferably limited to a range of 0.005 to 0.2%. More preferably, it is 0.005 to 0.15%.
- B contributes to the improvement of mass production punching through refinement of the bainite lath interval. In order to acquire such an effect, it is preferable to contain 0.0002% or more. On the other hand, if it exceeds 0.0030% and contains excessively, workability will be reduced. Therefore, when contained, B is preferably limited to a range of 0.0002 to 0.0030%. More preferably, it is 0.0003 to 0.0020%.
- One or more selected from Cu: 0.005-0.3%, Ni: 0.005-0.3%, Sn: 0.005-0.3% Cu, Ni, and Sn all increase in strength through solid solution strengthening It is an element that contributes, and it can be selected as necessary and can contain one or more. In order to acquire such an effect, it is desirable to contain Cu: 0.005% or more, Ni: 0.005% or more, Sn: 0.005% or more. On the other hand, if it contains more than Cu: 0.3%, Ni: 0.3% and Sn: 0.3%, hot workability is lowered, and there is a risk of causing surface layer cracking during hot rolling.
- Cu 0.005 to 0.3%
- Ni 0.005 to 0.3%
- Sn 0.005 to 0.3%
- Cu is 0.005 to 0.2%
- Ni is 0.005 to 0.2%
- Sn is 0.005 to 0.2%
- Mo and Cr are easy to form carbides (precipitates), and mass production punching through the formation of precipitates Mo and Cr are both elements that contribute to improving hardenability and elements that contribute to refinement of bainite lath through lowering of bainite transformation point.
- Mo and Cr are both elements that contribute to improving hardenability and elements that contribute to refinement of bainite lath through lowering of bainite transformation point.
- it can contain one or two.
- an excessive content exceeding Mo: 0.3% and Cr: 0.3% causes an increase in manufacturing cost, which is economically disadvantageous. Therefore, when it is contained, it is preferable to limit it to the range of Mo: 0.002 to 0.3% and Cr: 0.002 to 0.3%. More preferably, Mo is 0.002 to 0.2% and Cr is 0.002 to 0.2%.
- Ca 0.0002 to 0.004% and REM: 0.0002 to 0.004%.
- Both Ca and REM are elements that contribute to improving workability effectively through the form control of inclusions. Yes, it can be selected as necessary and can contain one or two. In order to acquire such an effect, it is desirable to contain Ca: 0.0002% or more and REM: 0.0002% or more. On the other hand, if the content exceeds Ca: 0.004% and REM: 0.004%, the inclusions in the steel increase and the workability decreases. Therefore, when it is contained, it is preferable to limit the range to Ca: 0.0002 to 0.004% and REM: 0.0002 to 0.004%. More preferably, Ca is 0.0002 to 0.003% and REM is 0.0002 to 0.003%.
- the balance other than the above components is composed of Fe and inevitable impurities.
- the high-strength hot-rolled steel sheet according to Embodiment 2 has an Fe-based bainite phase with a volume fraction of more than 92%, an average interval between bainite laths of 0.60 ⁇ m or less, and precipitated in the grains of bainite lath among all Fe-based carbides. It has a structure in which the number ratio of carbides is 10% or more.
- the steel sheet structure In order to ensure the desired mass production punchability, it is important to first make the steel sheet structure into a substantially bainite single-phase structure with a volume ratio exceeding 92% as described above. Note that it is preferably more than 94%.
- the balance other than the bainite phase is one or more of a ferrite phase, a martensite phase, a retained austenite phase, and pearlite. Since the martensite phase and the retained austenite phase are harder and more brittle than the main phase bainite and lower the mass production punchability, it is preferable that these phases are combined to be less than 1% by volume. Residual austenite itself is not hard, but strain-induced transformation occurs during punching to become martensite, which has an adverse effect on punchability like martensite.
- the bainite phase is a mixed structure of ferrite and Fe-based carbides.
- the bainite phase is a bainite phase having a bainite lath interval of 0.60 ⁇ m or less, which is the substructure.
- the structure factor that governs mass production punchability is not the size of the bainite phase itself, but its substructure, bainite lath, and making the bainite lath interval fine is important for improving mass production punchability. Based on finding something. If the bainite lath interval exceeds 0.60 ⁇ m, the desired mass production punchability cannot be ensured. For this reason, the bainite lath interval was limited to 0.60 ⁇ m or less. In addition, Preferably it is 0.50 micrometer or less, More preferably, it is 0.45 micrometer or less.
- the hot-rolled steel sheet of the present invention is a bainite phase single phase, a bainite phase in which carbides (Fe-based carbides) are precipitated in the phase, and, among all the precipitated Fe-based carbides, Fe-based precipitates in the grains of bainite lath.
- Carbide has a structure whose number ratio is 10% or more. In order to improve mass production punchability, it is important to control the precipitation site of carbide (Fe-based carbide). If the number of Fe-based carbides precipitated in the grains of bainite lath is less than 10% of the total number of Fe-based carbides precipitated, the desired excellent mass production punchability cannot be ensured.
- the number of Fe-based carbides precipitated in the grains of bainite lath was limited to 10% or more of the total number of Fe-based carbides precipitated. In addition, Preferably it is 15% or more, More preferably, it is 20% or more.
- a steel slab having the above composition is heated and subjected to hot rolling consisting of rough rolling and finish rolling to obtain a hot rolled steel sheet.
- the method for producing the steel slab is not particularly limited, and the molten steel having the above composition is melted by a conventional melting method such as a converter, an electric furnace or an induction furnace, or further by a vacuum degassing apparatus or the like. Secondary refining is performed, and a steel slab having a predetermined size is formed by a conventional casting method such as continuous casting. It should be noted that there is no problem even if the ingot-bundling method is used.
- the steel slab may be a thin slab having a thickness of about 30 mm. If it is a thin slab, rough rolling can be omitted.
- electromagnetic-stirring EMS
- IBSR intentional-bulging-soft-reduction-casting
- EMS electromagnetic-stirring
- IBSR intentional-bulging-soft-reduction-casting
- Can do By applying an electromagnetic stirring treatment, equiaxed crystals can be formed at the center of the plate thickness, and segregation can be reduced.
- segregation at the central portion of the plate thickness can be reduced by preventing the flow of molten steel in the unsolidified portion of the continuous cast slab.
- the steel slab having the above composition is heated to a heating temperature of 1100 ° C. or higher and subjected to hot rolling.
- Heating temperature of steel slab 1100 ° C. or higher
- the steel slab is heated to a heating temperature of 1100 ° C. or higher. If the heating temperature is less than 1100 ° C., the precipitate is not sufficiently re-dissolved, and the desired precipitate distribution cannot be secured in the subsequent steps. It is preferably 1150 ° C. or higher.
- the heating temperature exceeds 1300 ° C. and becomes excessively high, the crystal grains become coarse and eventually the bainite lath becomes coarse. For this reason, it is desirable to limit the heating temperature of the steel slab to 1300 ° C. or less.
- the heated steel slab is subjected to hot rolling consisting of rough rolling and finish rolling to form a hot rolled steel sheet.
- Rough rolling is not particularly limited as long as a desired sheet bar dimension can be ensured.
- finish rolling followed by finish rolling.
- the finish rolling conditions are extremely important for obtaining a desired bainite structure.
- Total rolling reduction in the final two passes of finish rolling 30% or more
- austenite ( ⁇ ) in which sufficient strain is accumulated into bainite. Therefore, in the present invention, first, the total rolling reduction of the final two passes of finish rolling is limited to 30% or more. If the total rolling reduction in the final two passes of finish rolling is less than 30%, the accumulation of strain in ⁇ is insufficient, and a desired bainite structure cannot be secured after transformation. For this reason, the total rolling reduction in the final two passes of finish rolling is limited to 30% or more. In addition, Preferably it is 40% or more, More preferably, it is 50% or more.
- Finishing rolling end temperature (A r3 transformation point) to (A r3 transformation point + 120 ° C)
- the finishing temperature of finish rolling is less than the Ar3 transformation point, it becomes difficult to secure a desired bainite single-phase structure, which is a desired structure.
- the finishing temperature of finish rolling exceeds ( Ar 3 transformation point + 120 ° C.) and becomes high, it becomes difficult to obtain a fine bainite phase. For this reason, the finishing temperature of finish rolling is limited to a temperature in the range of (A r3 transformation point) to (A r3 transformation point + 120 ° C.).
- the “ Ar3 transformation point” referred to here is the transformation temperature obtained from the thermal expansion curve obtained by cooling at a cooling rate of 1 ° C./s after machining by the machining for master test machine. To do.
- Cooling conditions are also extremely important to obtain the desired tissue.
- Cooling start within 2 s after finishing rolling
- Average cooling rate 50 ° C / s or more
- the average cooling rate from the finish rolling finish temperature to the cooling stop temperature is less than 50 ° C / s
- pro-eutectoid ferrite precipitates and has a bainite phase of more than 92% by volume.
- the average cooling rate of cooling after finishing rolling was limited to 50 ° C./s or more.
- it is 60 degrees C / s or more, More preferably, it is 70 degrees C / s or more.
- the upper limit of the cooling rate is limited depending on the capacity of the cooling facility, but is preferably limited to about 150 ° C./s from the viewpoint of the steel plate shape.
- the microstructure characterized in the present invention is obtained by cooling in one stage to the cooling stop temperature described later. This is a necessary requirement.
- Cooling stop temperature 300 ⁇ 500 °C
- the coil is wound immediately after the cooling is stopped. For this reason, the cooling stop temperature is taken up as the winding temperature.
- the cooling stop temperature (winding temperature) is less than 300 ° C. or more than 500 ° C.
- the cooling stop temperature was limited to a temperature in the range of 300 to 500 ° C.
- the temperature is preferably 350 to 500 ° C, more preferably 400 to 500 ° C.
- electromagnetic stirring EMS
- IBSR light pressure casting
- EMS electromagnetic stirring
- IBSR light pressure casting
- the like can be applied to reduce segregation of steel components during continuous casting.
- EMS electromagnetic stirring
- IBSR light pressure casting
- equiaxed crystals can be formed in the center portion of the plate thickness, and segregation can be reduced.
- segregation at the central portion of the plate thickness can be reduced by preventing the flow of molten steel in the unsolidified portion of the continuous cast slab.
- the scale formed on the surface may be removed by pickling according to a conventional method. Further, after the pickling treatment, temper rolling may be performed. Further, after the pickling treatment or temper rolling, the steel sheet may be further annealed at a soaking temperature of 730 ° C. or lower using a conventional hot dip galvanizing line, and further subjected to a plating treatment.
- the plating process may be a process of passing a hot dip galvanizing bath to form a galvanized layer on the surface. Furthermore, it is good also as an alloying hot-dip galvanized steel plate by performing the alloying process which performs the alloying process of this galvanized layer.
- the soaking temperature in the annealing process is set to 730 ° C. or less.
- the lower limit of the soaking temperature of the annealing treatment is not particularly limited, but the soaking temperature of the annealing treatment is preferably 590 ° C. or higher from the viewpoint of adhesion between the hot dip galvanized layer and the base steel plate.
- hot-dip galvanized steel sheets but also hot-rolled steel sheets obtained can be used as plated steel sheets such as electrogalvanized steel sheets.
- Example 2 The steel slab having the composition shown in Table 4 was subjected to heating, finish rolling, and cooling after rolling as shown in Table 5 to obtain a hot-rolled steel sheet.
- Table 4 also shows the Ar3 transformation point of each steel slab obtained from the thermal expansion curve.
- electromagnetic stirring (EMS) was performed for the segregation reduction treatment of the components other than the hot-rolled steel plate No. 1 ′ of steel A2 in Tables 4 to 6 described later.
- hot-rolled steel sheets are pickled, passed through a continuous hot dip galvanizing line, annealed under the conditions shown in Table 5, then hot dip galvanized, and hot dip galvanized steel sheets (GI ).
- the hot-rolled steel sheet after the annealing treatment is immersed in a 480 ° C galvanizing bath (0.1% Al-Zn), and a hot dip galvanized layer with an adhesion amount of 45 g / m 2 per side is applied to the steel sheet. It was set as the process formed in both surfaces.
- some hot-rolled steel sheets were further subjected to alloying treatment after the hot dip galvanizing treatment to obtain alloyed hot dip galvanized steel plates (GA).
- the alloying treatment temperature was 520 ° C.
- Test pieces were sampled from the obtained hot-rolled steel sheets (partially including plated steel sheets) and subjected to structure observation, tensile tests, and mass production punchability tests.
- the test method was as follows.
- (1) Microstructure observation A specimen for microstructural observation is collected from the obtained hot-rolled steel sheet (plated steel sheet), and after polishing a plate thickness section (L section) parallel to the rolling direction, it is corroded with 3% nital liquid. Appeared the organization. Then, at the 1/4 thickness position of the L cross section, the structure is observed with a scanning electron microscope (magnification: 3000 times), the structure is photographed with 10 fields of view, and phases other than the bainite phase are separated by image analysis processing. After determining the structural fraction of the phase other than bainite, the area ratio of the bainite phase was calculated. The area ratio thus obtained was defined as the volume ratio of the bainite phase.
- a thin film sample was taken from the position of 1/4 thickness of the obtained hot-rolled steel sheet (plated steel sheet), made into a thin film specimen by mechanical polishing and electrolytic polishing, and a transmission electron microscope (magnification: about 30000 times) The tissue was observed using, and the tissue was photographed in 10 fields of view. From the obtained structure photograph, the bainite lath interval was measured, the average value thereof was obtained, and the bainite lath interval of each hot-rolled steel sheet was obtained.
- a specimen for structure observation was collected from the obtained hot-rolled steel sheet (plated steel sheet), and after polishing a plate thickness section (L section) parallel to the rolling direction, the structure was corroded with 3% nital solution.
- a replica sample was prepared at a 1/4 thickness position.
- the tissue was observed with a transmission electron microscope (magnification: about 30000 times), and the tissue was photographed in 10 fields of view.
- the number of Fe-based carbides was measured at each precipitation location (grain boundary, within the grain), and the number of Fe-based precipitates deposited on the bainite lath relative to the number of all Fe-based precipitates. The ratio was calculated.
- the discrimination of the Fe-based carbide (precipitate) was performed by the form of the precipitate and EDX analysis.
- Mass production stamping with cracks, chips, brittle fracture surfaces, secondary shear surfaces, and those with no rough cross-section as ⁇ (passed), those with only rough cross-section as ⁇ (passed), and others with x (failed) Sex was evaluated.
- Each of the inventive examples is a hot-rolled steel sheet (plated steel sheet) having a high tensile strength TS: 700 MPa or more and excellent mass production punchability.
- TS tensile strength
- the mass production punchability is lowered.
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Abstract
Description
実施の形態1の高強度熱延鋼板の組成限定理由について説明する。なお、「%」はとくに断わらないかぎり「質量%」を意味する。実施の形態でいう「高強度」とは、引張強さTS:900MPa以上である場合をいうものとする。
Cは、鋼板の高強度化に有効に寄与する元素であり、また、ベイナイト変態を促進し、ベイナイト相形成に寄与する有用な元素である。また、適正量のC含有は、ベイナイトラスの粒内の炭化物を増加させ、量産打抜き性を向上させる作用を有する。このような効果を発現させるためには0.07%超の含有を必要とする。一方、0.2%を超える過剰な含有は、加工性、溶接性を損なう。このようなことから、Cは0.07%超0.2%以下の範囲に限定した。なお、好ましくは0.079%以上、さらに好ましくは、0.10%以上である。
また、0.19%以下が好ましい。
Siは、固溶強化(solute strengthening)により鋼板強度を増加させるとともに、鋼板の延性向上にも寄与する元素である。このような効果を発現させるためには、0.05%以上含有することが望ましい。一方、過剰なSi含有は変態点を上昇させ、ベイナイト相形成を阻害する。また、2.0%を超えてSiを含有すると、鋼スラブの加熱段階で、表層の結晶粒界へのSi系複合酸化物(Si type complex oxide)の侵入が顕著となり、熱間圧延時にデスケーリング(descaling)を多用しても除去することが困難となり、鋼板の量産打抜き加工時に打抜き端面性状を低下させ、量産打抜き性が低下する。このため、Siは2.0%以下に限定した。なお、好ましくは1.5%以下である。さらに好ましくは1.0%以下である。
Mnは、固溶強化および変態強化(transformation strengthening)により、鋼板の高強度化に寄与する有効な元素である。さらに、Mnは、変態点を低下させて、ベイナイトラスを微細化する作用を有する。このような効果を得るためには1.0%以上の含有を必要とする。一方、3.0%を超えて過剰に含有すると、中心偏析(center segregation)が顕著になり、加工性が著しく低下する。このため、Mnは1.0~3.0%の範囲に限定した。なお、好ましくは1.4~2.6%である。
Pは、固溶して鋼板の強度を増加させる作用を有する元素であるが、多量に含有すると粒界等に偏析しやすく、加工性等の低下を招く悪影響が懸念され、できるだけ低減することが望ましいが、0.05%までの含有は許容できる。なお、好ましくは、0.03%以下である。
Sは、硫化物を形成し、とくに粗大な硫化物を形成すると、鋼板の延性および加工性が低下するため、できるだけ低減することが望ましいが、0.005%までは許容できる。このため、Sは0.005%以下に限定した。なお、好ましくは0.003%以下、より好ましくは0.0015%以下である。
Alは、鋼の脱酸剤(deoxidizing agent)として作用する重要な元素である。このような効果を発現させるためには、0.01%以上含有することが望ましい。一方、0.1%を超えて含有すると、鋳造性が低下したり、鋼中に多量の介在物(酸化物)が残存して、表面性状(surface quality)や加工性の低下を招く。このため、Alは0.1%以下に限定した。なお、好ましくは0.06%以下である。
Nは、窒化物形成元素(nitride-forming element)と結合し窒化物として析出して、結晶粒の微細化に寄与する。しかし、0.01%を超えてN含有量が多くなると、多量の窒化物を生成し、熱間延性(hot ductility)の低下や、バーリング加工性(burring formability)の著しい低下の原因となるため、Nはできるだけ低減することが望ましいが0.01%までは許容できる。このため、Nは0.01%以下に限定した。なお、好ましくは0.006%以下、より好ましくは0.004%以下である。
Tiは、炭窒化物を形成しやすく、変態前のオーステナイト(γ)粒を微細化することにより、変態後のベイナイトラス間隔の微細化に寄与する、本発明で最も重要な元素の一つである。さらに、Tiは、微細なベイナイトラスの粒内の炭化物(炭窒化物)を増加させ、析出強化を介して強度増加に寄与するとともに、打抜き加工に際しボイド(void)生成サイト(site)を増加させて、量産打抜き性向上に寄与する。このような効果を得るためには、0.05%以上の含有を必要とする。一方、0.3%を超えて過剰に含有すると、圧延荷重(rolling force)が非常に大きくなり圧延操業(rolling operation)を難しくしたり、また析出物のサイズを粗大にしすぎて加工性を低下させる。このため、Tiは0.05~0.3%の範囲に限定した。なお。好ましくは0.07~0.25%、より好ましくは0.07~0.23%である。
Vは、強度-伸びバランス、強度-穴拡げ性バランスを向上させる作用を有し、本発明で最も重要な元素の一つである。また、Vは、ベイナイトラス間隔を小さくする作用も有し、これにより、打抜き時のマイクロボイド(micro void)の発生間隔(occurrence interval)が小さくなり、ボイド間の連結(linking)が起こりやすく、量産打抜き性を向上させる。また、Vは、粗大なFe系炭化物の析出を抑制する作用も有し、これにより、打抜き時の端面性状(edge face properties)を向上させる。このような効果を得るためには、0.05%以上の含有を必要とする。一方、0.3%を超えて過剰に含有しても、効果が飽和し、製造コストの高騰を招き、経済的に不利となる。このため、Vは0.05~0.3%の範囲に限定した。なお、好ましくは0.07%以上、さらに好ましくは、0.22%以上である。また、0.28%以下、より好ましくは0.26%以下である。
Nb、Bはいずれも、量産打抜き性の向上に寄与する元素であり、必要に応じて選択して1種または2種を含有できる。
Cu、Ni、Snはいずれも、固溶強化を介して、強度増加に寄与する元素であり、必要に応じて選択して1種または2種以上を含有できる。このような効果を得るためには、Cu:0.005%以上、Ni:0.005%以上、Sn:0.005%以上、含有することが望ましい。一方、Cu:0.3%、Ni:0.3%、Sn:0.3%をそれぞれ超えて含有すると、熱間加工性が低下し、熱間圧延中に表層割れを起こす恐れがある。このため、含有する場合には、Cu:0.005~0.3%、Ni:0.005~0.3%、Sn:0.005~0.3%の範囲に限定することが好ましい。なお、より好ましくはCu:0.005~0.2%、Ni:0.005~0.2%、Sn:0.005~0.2%である。
Mo、Crはいずれも、炭化物(析出物)を形成しやすく、析出物の形成を介して量産打抜き性の向上に寄与する元素であり、また、Mo、Crはいずれも、焼入れ性(hardenability)の向上に寄与する元素であり、ベイナイト変態点(bainite transformation point)の低下を介してベイナイトラスの微細化に寄与する元素でもあり、必要に応じて選択して1種または2種を含有できる。このような効果を得るためには、Mo:0.002%以上、Cr:0.002%以上、含有することが望ましい。一方、Mo:0.3%、Cr:0.3%を超える過剰の含有は、製造コストの高騰を招き、経済的に不利となる。このため、含有する場合には、Mo:0.002~0.3%、Cr:0.002~0.3%の範囲に限定することが好ましい。なお、より好ましくはMo:0.002~0.2%、Cr:0.002~0.2%である。
Ca、REMは、いずれも、介在物の形態制御(morphology control)を介して加工性の向上に有効に寄与する元素であり、必要に応じて選択して1種または2種を含有できる。このような効果を得るためには、Ca:0.0002%以上、REM:0.0002%以上含有することが望ましい。一方、Ca:0.004%、REM:0.004%を超えて含有すると、鋼中介在物の増加を招き、加工性が低下する。このため、含有する場合には、Ca:0.0002~0.004%、REM:0.0002~0.004%の範囲に限定することが好ましい。なお、より好ましくはCa:0.0002~0.003%、REM:0.0002~0.003%である。
本発明では、スラブ段階で析出している析出物を再固溶される必要がある。そのために、鋼スラブを1100℃以上の加熱温度に加熱する。加熱温度が1100℃未満では、析出物の再固溶が十分でなく、その後の工程で所望の析出物分布を確保できなくなる。なお、好ましくは1150℃以上である。また、加熱温度が過剰に高くなると、結晶粒が粗大化し、最終的にベイナイトラスが粗大化する。このため、鋼スラブの加熱温度は1300℃以下に限定することが望ましい。
所望のベイナイトラス組織を得るには、十分に歪が蓄積されたオーステナイト(γ)をベイナイト変態させることが必要である。そのために、本発明では、まず、仕上圧延の最終2パスの合計圧下率を限定する。仕上圧延の最終2パスの合計圧下率が30%未満では、γへの歪蓄積が不十分で、変態後に所望のベイナイトラス組織を確保できなくなる。このため、仕上圧延の最終2パスの合計圧下率を30%以上に限定した。なお、好ましくは40%以上、さらに好ましくは50%以上である。
十分に歪が蓄積されたオーステナイト(γ)からベイナイト変態させるために、仕上圧延の圧延終了温度の調整も重要となる。仕上圧延の圧延終了温度がAr3変態点未満では、所望の組織である、ほぼベイナイト単相の組織を確保することが難しくなる。一方、仕上圧延の圧延終了温度が(Ar3変態点+120℃)を超えて高温となると、微細なベイナイト相を得ることが難しくなる。このため、仕上圧延の圧延終了温度は(Ar3変態点)~(Ar3変態点+120℃)の範囲の温度に限定した。なお、好ましくは(Ar3変態点)~(Ar3変態点+80℃)である。ここで、仕上圧延の圧延終了温度は表面温度で表すものとする。また、ここでいう「Ar3変態点」は、加工フォーマスタ試験機(Thermecmastor-Z)で、加工付与後に冷却速度1℃/sで冷却して得られた熱膨張曲線(thermal expansion curve)から、その変化点(changing point)により求めた変態温度とする。
十分に歪が蓄積されたγ(austenite)からベイナイト変態させて、所望のベイナイトラス組織を得るためには、冷却開始時間を、仕上圧延終了後、2s以内に冷却を開始する必要がある。冷却開始が、仕上圧延終了後、2sを超えると、γの回復および、再結晶が進行し、ベイナイト変態の核が減少し、所望のベイナイトラス組織を得ることができなくなる。このようなことから、冷却は、仕上圧延終了後、2s以内に開始することにした。なお、好ましくは1.5s以内、より好ましくは1s以内である。
仕上圧延終了温度から冷却停止温度までの平均冷却速度が40℃/s未満では、初析フェライト(pro-eutectoid ferrite)が析出して、体積率で90%超のベイナイト相を有し、かつ所望のベイナイトラス間隔を有する組織を確保することが困難となる。このため、仕上圧延終了後の冷却の平均冷却速度は40℃/s以上に限定した。なお、好ましくは50℃/s以上、より好ましくは60℃/s以上である。冷却速度の上限は、冷却設備(cooling facilities)の能力に依存して決定されるが、鋼板形状の観点から150℃/s以下程度にすることが好ましい。本発明においては、仕上げ圧延後の冷却は前記した冷却速度に制御することを前提として、かつ後述する、冷却停止温度まで1段で冷却することが、本発明で特徴とするミクロ組織を得るために必要な要件である。
本発明では冷却停止後、直ちに巻き取る。このため、冷却停止温度を巻取り温度として巻き取る。冷却停止温度(巻取り温度)が、300℃未満、あるいは500℃超となると、ベイナイトラス間隔とFe系炭化物の分布状態をともに所望の最適範囲に調整することができなくなる。このようなことから、冷却停止温度(巻取り温度)を300~500℃の範囲の温度に限定した。なお、好ましくは350~500℃である。
[実施例1]
表1に示す組成を有する鋼スラブに、表2に示す、加熱、仕上圧延、圧延後冷却を施し、熱延鋼板とした。連続鋳造の際には、後述する表1~3中の鋼A1の熱延鋼板No.1’以外のものについては、成分の偏析低減処理のため、電磁撹拌(EMS)を行った。なお、熱膨張曲線から求めた、各鋼スラブのAr3変態点を表1に併記した。なお、一部の熱延鋼板では、酸洗後、連続溶融亜鉛めっきラインに通板し、表2に示す条件で焼鈍処理を施したのち、溶融亜鉛めっき処理を施し、溶融亜鉛めっき鋼板(GI)とした。なお、溶融亜鉛めっき処理は、焼鈍処理後の熱延鋼板を480℃の亜鉛めっき浴(0.1質量%Al-Zn)中に浸漬し、片面当たり付着量45g/m2の溶融亜鉛めっき層を鋼板両面に形成する処理とした。また、一部の熱延鋼板については溶融亜鉛めっき処理ののち、さらに合金化処理を施し、合金化溶融亜鉛めっき鋼板(GA)とした。なお、合金化処理温度は520℃とした。
(1)組織観察
得られた熱延鋼板(めっき鋼板)から組織観察用試験片を採取し、圧延方向に平行な板厚断面(L断面)を研磨した後、3%ナイタール液(nital solution)で腐食して組織を現出した。そして、L断面の板厚1/4位置において、走査型電子顕微鏡(scanning electron microscope)(倍率:3000倍)で組織を観察し、10視野で組織を撮影し、画像解析処理(image analysis)で、ベイナイト相以外の相を分離して、ベイナイト以外の相の組織分率を決定し、ベイナイト相の面積率を算出した。このようにして得られた面積率をベイナイト相の体積率とした。
(2)引張試験
得られた熱延鋼板(めっき鋼板)から、引張方向が圧延方向に直角方向となるように、JIS 5号引張試験片を各3本採取し、JIS Z 2241の規定に準拠して引張試験を実施した。なお、引張速度は10mm/minとした。なお、得られた引張特性(引張強さTS、伸びEl)の平均値を、その鋼板の引張特性とした。
(3)量産打抜き性試験
得られた熱延鋼板(めっき鋼板)から、ブランク板(大きさ:150×150mm)を採取した。そして、打抜きポンチを50mmφの平底型として、打抜きクリアランスが30%となるように、ダイ側の穴径を決定し、さらに打抜きダイの上にスペーサーを置き、その上にブランク板を置いて上から板押さえで固定してポンチ穴を打ち抜いた。打ち抜き後、ポンチ穴の全周に亘り、打抜き端面の破面状況を走査型電子顕微鏡(倍率:100倍)で、割れ、欠け、脆性破面、2次せん断面(secondary shear surface)、および断面の荒れの有無を観察した。割れ、欠け、脆性破面、2次せん断面、および断面の荒れのないものを○(合格)、断面の荒れのみあるものを△(合格)、それ以外を×(不合格)として、量産打抜き性を評価した。
[実施の形態2]
実施の形態2に係る高強度熱延鋼板の組成限定理由について説明する。「%」はとくに断わらないかぎり「質量%」を意味する。実施の形態2において、「高強度」とは、引張強さTS:700~900MPaである場合をいうものとする。
Cは、鋼板の高強度化に有効に寄与する元素であり、また、ベイナイト変態を促進し、ベイナイト相形成に寄与する有用な元素である。また、適正量のC含有は、ベイナイトラスの粒内の炭化物を増加させ、量産打抜き性を向上させる作用を有する。このような効果を発現させるためには0.05%以上の含有を必要とする。一方、0.15%を超える過剰な含有は、加工性、溶接性を損なう。このようなことから、Cは0.05~0.15%の範囲に限定した。なお、好ましくは0.071%以上、さらに好ましくは、0.080%以上で、0.14%以下である。
Siは、固溶強化により鋼板強度を増加させるとともに、鋼板の延性向上にも寄与する元素である。このような効果を発現させるためには、0.05%以上含有することが望ましい。一方、1.5%を超える過剰なSi含有は変態点を上昇させ、ベイナイト相形成を阻害する。このため、Siは1.5%以下に限定した。なお、好ましくは1.0%以下である。
Mnは、固溶強化および変態強化により、鋼板の高強度化に寄与する有効な元素である。さらに、Mnは、変態点を低下させて、ベイナイトラスを微細化する効果を有する。このような効果を得るためには1.0%以上の含有を必要とする。一方、2.0%を超えて含有すると、中心偏析が顕著になり、加工性が著しく低下する。このため、Mnは1.0~2.0%の範囲に限定した。なお、好ましくは1.2~1.9%である。
Pは、固溶して鋼板の強度を増加させる作用を有する元素であるが、多量に含有すると粒界等に偏析しやすく、加工性の低下を招く悪影響が懸念され、できるだけ低減することが望ましいが、0.05%までの含有は許容できる。なお、好ましくは、0.03%以下である。
Sは、硫化物を形成し、とくに粗大な硫化物を形成すると、鋼板の延性、加工性が低下するため、できるだけ低減することが望ましいが、0.005%までは許容できる。このため、Sは0.005%以下に限定した。なお、好ましくは0.003%以下、より好ましくは0.0015%以下である。
Alは、鋼の脱酸剤として作用する重要な元素である。このような効果を発現させるためには、0.01%以上含有することが望ましい。一方、0.1%を超えて含有すると、鋳造性が低下したり、鋼中に多量の介在物(酸化物)が残存して、表面性状や加工性の低下を招く。このため、Alは0.1%以下に限定した。なお、好ましくは0.06%以下である。
Nは、窒化物形成元素と結合し窒化物として析出して、結晶粒の微細化に寄与する。しかし、0.01%を超えてN含有量が多くなると、多量の窒化物を生成し、熱間延性の低下や、バーリング加工性の著しい低下の原因となるため、Nはできるだけ低減することが望ましいが0.01%までは許容できる。このため、Nは0.01%以下に限定した。なお、好ましくは0.006%以下、より好ましくは0.004%以下である。
Tiは、炭窒化物を形成しやすく、変態前のオーステナイト(γ)粒を微細化することを介し、変態後のベイナイトラス間隔の微細化に寄与する、本発明で最も重要な元素の一つである。さらに、Tiは、微細なベイナイトラスの粒内炭化物(炭窒化物)を増加させ、析出強化を介して強度増加に寄与するとともに、打抜き加工に際しボイド生成サイトとなりボイドを増加させて、量産打抜き性向上に寄与する。このような効果を得るためには、0.05%以上の含有を必要とする。一方、0.2%を超えて過剰に含有すると、圧延荷重が非常に大きくなり圧延操業を難しくしたり、また析出物サイズを粗大にしすぎて加工性を低下させる。このため、Tiは0.05~0.2%の範囲に限定した。なお。好ましくは0.065~0.125%、より好ましくは0.065~0.10%である。
Nb、Bはいずれも、量産打抜き性の向上に寄与する元素であり、必要に応じて選択して1種または2種を含有できる。
Cu、Ni、Snはいずれも、固溶強化を介して強度増加に寄与する元素であり、必要に応じて選択して1種または2種以上を含有できる。このような効果を得るためには、Cu:0.005%以上、Ni:0.005%以上、Sn:0.005%以上、含有することが望ましい。一方、Cu:0.3%、Ni:0.3%、Sn:0.3%をそれぞれ超えて含有すると、熱間加工性が低下し、熱間圧延中に表層割れを起こす恐れがある。このため、含有する場合には、それぞれ、Cu:0.005~0.3%、Ni:0.005~0.3%、Sn:0.005~0.3%の範囲に限定することが好ましい。なお、より好ましくはCu:0.005~0.2%、Ni:0.005~0.2%、Sn:0.005~0.2%である。
Mo、Crはいずれも、炭化物(析出物)を形成しやすく、析出物形成を介して量産打抜き性の向上に寄与する元素であり、また、Mo、Crはいずれも、焼入れ性向上に寄与する元素であり、ベイナイト変態点の低下を介してベイナイトラスの微細化に寄与する元素でもあり、必要に応じて選択して1種または2種を含有できる。このような効果を得るためには、Mo:0.002%以上、Cr:0.002%以上、含有することが望ましい。一方、Mo:0.3%、Cr:0.3%を超える過剰の含有は、製造コストの高騰を招き、経済的に不利となる。このため、含有する場合には、Mo:0.002~0.3%、Cr:0.002~0.3%の範囲に限定することが好ましい。なお、より好ましくはMo:0.002~0.2%、Cr:0.002~0.2%である。
Ca、REMは、いずれも、介在物の形態制御を介して加工性向上に有効に寄与する元素であり、必要に応じて選択して1種または2種を含有できる。このような効果を得るためには、Ca:0.0002%以上、REM:0.0002%以上含有することが望ましい。一方、Ca:0.004%、REM:0.004%を超えて含有すると、鋼中介在物の増加を招き、加工性が低下する。このため、含有する場合には、Ca:0.0002~0.004%、REM:0.0002~0.004%の範囲に限定することが好ましい。なお、より好ましくはCa:0.0002~0.003%、REM:0.0002~0.003%である。
本発明では、スラブ段階で析出している析出物を再固溶される必要がある。そのために、鋼スラブを1100℃以上の加熱温度に加熱する。加熱温度が1100℃未満では、析出物の再固溶が十分でなく、その後の工程で所望の析出物分布を確保できなくなる。なお好ましくは1150℃以上である。また、加熱温度が1300℃を超えて過剰に高くなると、結晶粒が粗大化し、最終的にベイナイトラスが粗大化する。このため鋼スラブの加熱温度は1300℃以下に限定することが望ましい。
所望のベイナイトラス組織を得るには、十分に歪が蓄積されたオーステナイト(γ)をベイナイト変態させることが必要である。そのために、本発明では、まず、仕上圧延の最終2パスの合計圧下率を30%以上に限定する。仕上圧延の最終2パスの合計圧下率が30%未満では、γへの歪蓄積が不十分で、変態後に所望のベイナイトラス組織を確保できなくなる。このため、仕上圧延の最終2パスの合計圧下率を30%以上に限定した。なお、好ましくは40%以上、さらに好ましくは50%以上である。
十分に歪が蓄積されたオーステナイト(γ)からベイナイト変態させるために、仕上圧延の圧延終了温度の調整も重要となる。仕上圧延の圧延終了温度がAr3変態点未満では、所望の組織である、ほぼベイナイト単相の組織を確保することが難しくなる。一方、仕上圧延の圧延終了温度が(Ar3変態点+120℃)を超えて高温となると、微細なベイナイト相を得ることが難しくなる。このため、仕上圧延の圧延終了温度は(Ar3変態点)~(Ar3変態点+120℃)の範囲の温度に限定した。なお、好ましくは(Ar3変態点)~(Ar3変態点+80℃)である。ここで、仕上圧延の圧延終了温度は表面温度で表すものとする。また、ここでいう「Ar3変態点」は、加工フォーマスタ試験機で、加工付与後に冷却速度1℃/sで冷却して得られた熱膨張曲線から、その変化点により求めた変態温度とする。
十分に歪が蓄積されたγからベイナイト変態させて、所望のベイナイトラス組織を得るためには、仕上圧延終了後、2s以内に冷却を開始する必要がある。冷却開始が、仕上圧延終了後、2sを超えると、γの回復、再結晶が進行し、ベイナイト変態の核が減少し、所望のベイナイトラス間隔を得ることができなくなる。このようなことから、冷却は、仕上圧延終了後、2s以内に開始することにした。なお、好ましくは1.5s以内、より好ましくは1s以内である。
仕上圧延終了温度から冷却停止温度までの平均冷却速度が50℃/s未満では、初析フェライトが析出して、体積率で92%超のベイナイト相を有し、かつ所望のベイナイトラス間隔を有する組織を確保することが困難となる。このため、仕上圧延終了後の冷却の平均冷却速度は50℃/s以上に限定した。なお、好ましくは60℃/s以上、より好ましくは70℃/s以上である。冷却速度の上限は、冷却設備の能力に依存して限定されるが、鋼板形状の観点から150℃/s程度に限定することが好ましい。本発明においては、仕上げ圧延後の冷却は前記した冷却速度に制御することを前提として、かつ、後述する、冷却停止温度まで1段で冷却することが、本発明で特徴とするミクロ組織を得るために必要な要件である。
本発明では冷却停止後、直ちに巻き取る。このため、冷却停止温度を巻取り温度として巻き取る。冷却停止温度(巻取り温度)が、300℃未満、あるいは500℃超となると、ベイナイトラス間隔とFe系炭化物の分布状態をともに所望の最適範囲に調整することができなくなる。このようなことから、冷却停止温度(巻取り温度)を300~500℃の範囲の温度に限定した。なお、好ましくは350~500℃、さらに好ましくは400~500℃である。
[実施例2]
表4に示す組成を有する鋼スラブに、表5に示す、加熱、仕上圧延、圧延後冷却を施し、熱延鋼板とした。なお、熱膨張曲線から求めた、各鋼スラブのAr3変態点を表4に併記した。連続鋳造の際には、後述する表4~6中の鋼A2の熱延鋼板No.1’以外のものについては、成分の偏析低減処理のため、電磁撹拌(EMS)を行った。なお、一部の熱延鋼板では、酸洗後、連続溶融亜鉛めっきラインに通板し、表5に示す条件で焼鈍処理を施したのち、溶融亜鉛めっき処理を施し、溶融亜鉛めっき鋼板(GI)とした。なお、溶融亜鉛めっき処理は、焼鈍処理後の熱延鋼板を480℃の亜鉛めっき浴(0.1%Al-Zn)中に浸漬し、片面当たり付着量45g/m2の溶融亜鉛めっき層を鋼板の両面に形成する処理とした。また、一部の熱延鋼板については溶融亜鉛めっき処理の後、さらに合金化処理を施し、合金化溶融亜鉛めっき鋼板(GA)とした。合金化処理温度は520℃とした。
(1)組織観察
得られた熱延鋼板(めっき鋼板)から組織観察用試験片を採取し、圧延方向に平行な板厚断面(L断面)を研磨した後、3%ナイタール液で腐食して組織を現出した。そして、L断面の板厚1/4位置において、走査型電子顕微鏡(倍率:3000倍)で組織を観察し、10視野で組織を撮影し、画像解析処理で、ベイナイト相以外の相を分離して、ベイナイト以外の相の組織分率を決定したのち、ベイナイト相の面積率を算出した。このようにして得られた面積率をベイナイト相の体積率とした。
(2)引張試験
得られた熱延鋼板(めっき鋼板)から、引張方向が圧延方向に直角方向となるように、JIS 5号引張試験片を各3本採取し、JIS Z 2241の規定に準拠して引張試験を実施した。なお、引張速度は10mm/minとした。なお、得られた引張特性(引張強さTS、伸びEl)の平均値を、その鋼板の引張特性とした。
(3)量産打抜き性試験
得られた熱延鋼板(めっき鋼板)から、ブランク板(大きさ:150×150mm)を採取した。そして、打抜きポンチを50mmφの平底型として、打抜きクリアランスが30%となるように、ダイ側の穴径を決定し、さらに打抜きダイの上にスペーサーを置き、その上にブランク板を置いて上から板押さえで固定してポンチ穴を打ち抜いた。打ち抜き後、ポンチ穴の全周に亘り、打抜き端面の破面状況を走査型電子顕微鏡(倍率:100倍)で、割れ、欠け、脆性破面、2次せん断面、および断面の荒れの有無を観察した。割れ、欠け、脆性破面、2次せん断面、および断面の荒れのないものを○(合格)、断面の荒れのみあるものを△(合格)、それ以外を×(不合格)として、量産打抜き性を評価した。
Claims (24)
- 質量%で、
C :0.07%超0.2%以下、 Si:2.0%以下、
Mn:1.0~3.0%、 P :0.05%以下、
S :0.005%以下、 Al:0.1%以下、
N :0.01%以下、 Ti:0.05~0.3%、
V :0.05~0.3%
を含有し、残部Feおよび不可避的不純物からなる組成と、ベイナイト相が体積率で90%超で、かつベイナイトラスの平均間隔が0.45μm以下であり、かつ全Fe系炭化物のうちベイナイトラスの粒内に析出したFe系炭化物の個数比率が10%以上である組織を有する高強度熱延鋼板。 - 前記組成に加えてさらに、質量%で、Nb:0.005~0.2%、B:0.0002~0.0030%のうちから選ばれた1種または2種を含有する請求項1に記載の高強度熱延鋼板。
- 前記組成に加えてさらに、質量%で、Cu:0.005~0.3%、Ni:0.005~0.3%、 Sn:0.005~0.3%のうちから選ばれた1種または2種以上を含有する請求項1または2に記載の高強度熱延鋼板。
- 前記組成に加えてさらに、質量%で、Mo:0.002~0.3%、Cr:0.002~0.3%のうちから選ばれた1種または2種を含有する請求項1ないし3のいずれかに記載の高強度熱延鋼板。
- 前記組成に加えてさらに、質量%で、Ca:0.0002~0.004%、REM:0.0002~0.004%のうちから選ばれた1種または2種を含有する請求項1ないし4のいずれかに記載の高強度熱延鋼板。
- 請求項1ないし5のいずれかに記載の高強度熱延鋼板の表面に溶融亜鉛めっき層または合金化溶融亜鉛めっき層を形成してなる溶融亜鉛めっき鋼板。
- 鋼スラブを、加熱し粗圧延と仕上圧延とからなる熱間圧延を施して、熱延鋼板とするに当たり、
前記鋼スラブを、質量%で、
C :0.07%超0.2%以下、 Si:2.0%以下、
Mn:1.0~3.0%、 P :0.05%以下、
S :0.005%以下、 Al:0.1%以下、
N :0.01%以下、 Ti:0.05~0.3%、
V :0.05~0.3%
を含有し、残部Feおよび不可避的不純物からなる組成を有する鋼スラブとし、
前記熱間圧延を、前記鋼スラブを1100℃以上に加熱し、前記仕上圧延の最終2パスの合計圧下率を30%以上とし、該仕上圧延の圧延終了温度を(Ar3変態点)~(Ar3変態点+120℃)の温度範囲とし、前記仕上圧延終了後、2s以内に冷却を開始し、平均冷却速度40℃/s以上で巻取り温度まで冷却した後、巻取り温度:300~500℃で巻き取る圧延とする高強度熱延鋼板の製造方法。 - 前記組成に加えてさらに、質量%で、Nb:0.005~0.2%、B:0.0002~0.0030%のうちから選ばれた1種または2種を含有する請求項7に記載の高強度熱延鋼板の製造方法。
- 前記組成に加えてさらに、質量%で、Cu:0.005~0.3%、Ni:0.005~0.3%、Sn:0.005~0.3%のうちから選ばれた1種または2種以上を含有する請求項7または8に記載の高強度熱延鋼板の製造方法。
- 前記組成に加えてさらに、質量%で、Mo:0.002~0.3%、Cr:0.002~0.3%のうちから選ばれた1種または2種を含有する請求項7ないし9のいずれかに記載の高強度熱延鋼板の製造方法。
- 前記組成に加えてさらに、質量%で、Ca:0.0002~0.004%、REM:0.0002~0.004%のうちから選ばれた1種または2種を含有する請求項7ないし10のいずれかに記載の高強度熱延鋼板の製造方法。
- 請求項7ないし11のいずれかに記載の高強度熱延鋼板の製造方法により製造された高強度熱延鋼板を酸洗したのち、焼鈍とめっき処理を施してめっき鋼板とするに当たり、
前記焼鈍を均熱温度:730℃以下とする焼鈍とし、該焼鈍終了後に、前記めっき処理として溶融亜鉛めっき浴を通過させて、前記高強度熱延鋼板の表面に溶融亜鉛めっき層を形成し、あるいはさらに該溶融亜鉛めっき層を合金化する合金化処理を施す溶融亜鉛めっき鋼板の製造方法。 - 質量%で、
C :0.05~0.15%、 Si:1.5%以下、
Mn:1.0~2.0%、 P :0.05%以下、
S :0.005%以下、 Al:0.1%以下、
N :0.01%以下、 Ti:0.05~0.2%
を含有し、残部Feおよび不可避的不純物からなる組成を有し、ベイナイト相が体積率で92%超、ベイナイトラスの平均間隔が0.60μm以下、かつ全Fe系炭化物のうちベイナイトラスの粒内に析出したFe系炭化物の個数比率が10%以上である組織を有する高強度熱延鋼板。 - 前記組成に加えてさらに、質量%で、Nb:0.005~0.2%、B:0.0002~0.0030%のうちから選ばれた1種または2種を含有する請求項13に記載の高強度熱延鋼板。
- 前記組成に加えてさらに、質量%で、Cu:0.005~0.3%、Ni:0.005~0.3%、Sn:0.005~0.3%のうちから選ばれた1種または2種以上を含有する請求項13または14に記載の高強度熱延鋼板。
- 前記組成に加えてさらに、質量%で、Mo:0.002~0.3%、Cr:0.002~0.3%のうちから選ばれた1種または2種を含有する請求項13ないし15のいずれかに記載の高強度熱延鋼板。
- 前記組成に加えてさらに、質量%で、Ca:0.0002~0.004%、REM:0.0002~0.004%のうちから選ばれた1種または2種を含有する請求項13ないし16のいずれかに記載の高強度熱延鋼板。
- 請求項13ないし17のいずれかに記載の高強度熱延鋼板の表面に、溶融亜鉛めっき層または合金化溶融亜鉛めっき層を形成してなる溶融亜鉛めっき鋼板。
- 鋼スラブを、加熱し粗圧延と仕上圧延とからなる熱間圧延を施し、熱延鋼板とするに当たり、
前記鋼スラブが、質量%で、
C :0.05~0.15%、 Si:1.5%以下、
Mn:1.0~2.0%、 P :0.05%以下、
S :0.005%以下、 Al:0.1%以下、
N :0.01%以下、 Ti:0.05~0.2%
を含有し、残部Feおよび不可避的不純物からなる組成を有する鋼スラブとし、
前記熱間圧延が、前記鋼スラブを1100℃以上に加熱し、前記仕上圧延の最終2パスの合計圧下率を30%以上とし、該仕上圧延の圧延終了温度を(Ar3変態点)~( Ar3変態点+120℃)の温度範囲とし、前記仕上圧延終了後、2s以内に冷却を開始し、平均冷却速度50℃/s以上で巻取り温度まで冷却した後、巻取り温度:300~500℃で巻き取る圧延である高強度熱延鋼板の製造方法。 - 前記組成に加えてさらに、質量%で、Nb:0.005~0.2%、B:0.0002~0.0030%のうちから選ばれた1種または2種を含有する請求項19に記載の高強度熱延鋼板の製造方法。
- 前記組成に加えてさらに、質量%で、Cu:0.005~0.3%、Ni:0.005~0.3%、Sn:0.005~0.3%のうちから選ばれた1種または2種以上を含有する請求項19または20に記載の高強度熱延鋼板の製造方法。
- 前記組成に加えてさらに、質量%で、Mo:0.002~0.3%、Cr:0.002~0.3%のうちから選ばれた1種または2種を含有する請求項19ないし21のいずれかに記載の高強度熱延鋼板の製造方法。
- 前記組成に加えてさらに、質量%で、Ca:0.0002~0.004%、REM:0.0002~0.004%のうちから選ばれた1種または2種を含有する請求項19ないし22のいずれかに記載の高強度熱延鋼板の製造方法。
- 請求項19ないし23のいずれかに記載の高強度熱延鋼板の製造方法で製造された高強度熱延鋼板を酸洗したのち、焼鈍とめっき処理を施して、表面にめっき層を有するめっき鋼板とするに当たり、
前記焼鈍を、均熱温度:730℃以下とする焼鈍とし、該焼鈍終了後、前記めっき処理として溶融亜鉛めっき浴を通過させて、前記熱延鋼板表面に溶融亜鉛めっき層を形成し、あるいはさらに該溶融亜鉛めっき層を合金化する合金化処理を施す溶融亜鉛めっき鋼板の製造方法。
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US15/143,965 Division US10301693B2 (en) | 2013-04-15 | 2016-05-02 | High-strength hot-rolled steel sheet and method for producing the same |
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EP3395997A4 (en) * | 2015-12-24 | 2018-11-07 | Posco | Low-yield-ratio type high-strength steel, and manufacturing method therefor |
JP2020042004A (ja) * | 2018-09-10 | 2020-03-19 | 日本製鉄株式会社 | 析出物識別方法、析出物情報取得方法およびプログラム |
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Also Published As
Publication number | Publication date |
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MX2015014436A (es) | 2016-02-03 |
EP3358033A1 (en) | 2018-08-08 |
EP2987883A1 (en) | 2016-02-24 |
EP2987883B1 (en) | 2019-05-08 |
MX2020003880A (es) | 2020-08-17 |
KR101749948B1 (ko) | 2017-06-22 |
US20160068937A1 (en) | 2016-03-10 |
EP2987883A4 (en) | 2016-06-01 |
KR20160012126A (ko) | 2016-02-02 |
EP3358033B1 (en) | 2020-07-15 |
US20160258032A1 (en) | 2016-09-08 |
CN105143485B (zh) | 2017-08-15 |
US10301693B2 (en) | 2019-05-28 |
CN105143485A (zh) | 2015-12-09 |
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