WO2013137453A1 - 熱間プレス成形品およびその製造方法 - Google Patents
熱間プレス成形品およびその製造方法 Download PDFInfo
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- WO2013137453A1 WO2013137453A1 PCT/JP2013/057468 JP2013057468W WO2013137453A1 WO 2013137453 A1 WO2013137453 A1 WO 2013137453A1 JP 2013057468 W JP2013057468 W JP 2013057468W WO 2013137453 A1 WO2013137453 A1 WO 2013137453A1
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
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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
- B21D22/20—Deep-drawing
- B21D22/208—Deep-drawing by heating the blank or deep-drawing associated with heat treatment
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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
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—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
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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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
- C21D2211/001—Austenite
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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
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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
- C21D2221/00—Treating localised areas of an article
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/002—Heat treatment of ferrous alloys containing Cr
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
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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
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0236—Cold rolling
Definitions
- the present invention relates to a hot press-formed product used for a structural member of an automobile part and capable of adjusting strength and ductility in accordance with different regions in the molded product, and a manufacturing method thereof.
- a hot press-molded product capable of obtaining strength and ductility corresponding to different regions by applying heat treatment simultaneously with the shape formation, and such a hot press-molded product It relates to a useful method for manufacturing.
- the steel sheet is heated to a predetermined temperature (for example, a temperature at which it becomes an austenite phase) to reduce the strength (that is, to facilitate forming), and then at a lower temperature than the thin steel sheet (
- a predetermined temperature for example, a temperature at which it becomes an austenite phase
- a hot press molding method that secures the strength after molding by forming a mold with a room temperature mold and performing a quenching heat treatment (quenching) using the temperature difference between the two at the same time as giving the shape. It has been adopted.
- Such a hot press forming method since the material is formed in a low strength state, the spring back is also small (the shape freezing property is good), and a material with good hardenability to which alloy elements such as Mn and B are added is used. By doing so, a strength of 1500 MPa class is obtained as a tensile strength by rapid cooling.
- a hot press forming method is called by various names such as a hot forming method, a hot stamping method, a hot stamp method, and a die quench method in addition to the hot press method.
- FIG. 1 is a schematic explanatory view showing a mold configuration for carrying out the above hot press molding (hereinafter may be represented by “hot stamp”).
- 3 is a blank holder
- 4 is a steel plate (blank)
- BHF is a crease pressing force
- rp is a punch shoulder radius
- rd is a die shoulder radius
- CL is a punch / die clearance.
- the punch 1 and the die 2 have passages 1a and 2a through which a cooling medium (for example, water) can pass, and the cooling medium is allowed to pass through the passages.
- a cooling medium for example, water
- a steel sheet for hot stamping As a steel sheet for hot stamping that is currently widely used, a steel sheet made of 22MnB5 steel is known. This steel sheet has a tensile strength of 1500 MPa and an elongation of about 6 to 8%, and is applied to an impact resistant member (a member that is not deformed as much as possible at the time of collision and does not break).
- an impact resistant member a member that is not deformed as much as possible at the time of collision and does not break.
- the development of increasing the C content and further increasing the strength (1500 MPa or higher, 1800 MPa class) based on 22MnB5 steel is also in progress.
- Non-Patent Document 1 proposes a method of hot stamping 22MnB5 steel for hot stamping and a material that does not become high strength even when quenched with a mold and laser welding (tailored weld blank: TWB).
- the tensile strength is 1500 MPa (elongation 6-8%) on the high strength side (impact resistant site side), and the tensile strength is 440 MPa (elongation 12%) on the low strength side (energy absorption site side).
- a technique such as Non-Patent Document 2 has also been proposed.
- the tensile strength is 600 MPa or less and the elongation is about 12 to 18% on the energy absorption site side, but it is necessary to perform laser welding (tailored weld blank: TWB) in advance, As the number increases, the cost increases. In addition, an energy absorbing portion that originally does not need to be quenched is heated, which is not preferable from the viewpoint of heat consumption.
- Non-Patent Documents 3 and 4 have also been proposed as techniques for creating different strengths within the parts.
- the technique of Non-Patent Document 3 is to make a difference by giving a temperature difference (distribution) to the blank in the heating furnace, but it is based on 22MnB5 steel.
- the robustness of the strength after quenching is poor with respect to the heating at the region temperature, the strength control on the energy absorption site side is difficult, and the elongation is only about 15%.
- Non-Patent Document 4 the process is performed by changing the cooling rate within the mold (a part of the mold is heated by a heater or a material having a different thermal conductivity is used).
- 22MnB5 steel which is not rational in terms of controlling the 22MnB5 steel with good hardenability so as not to be quenched (die cooling control).
- the present invention has been made in view of the above circumstances, and the object thereof is to have an area corresponding to an impact resistant part and an energy absorbing part in a single molded article without applying a welding method, It is an object of the present invention to provide a hot press-formed product that can achieve a high balance between high strength and elongation depending on the region, and a useful method for producing such a hot press-formed product.
- the hot press-formed product of the present invention that has achieved the above object is a hot press-formed product obtained by forming a thin steel plate by a hot press forming method, and the metal structure has a martensite: 80 to 97 area. %, Retained austenite: 3 to 20 area%, the remaining structure: 1st region consisting of 5 area% or less, and the metal structure is ferrite: 30 to 80 area%, bainitic ferrite: less than 30 area% It is characterized by having a second region consisting of (not including 0 area%), martensite: 30 area% or less (not including 0 area%), and retained austenite: 3 to 20 area%.
- the chemical component composition is not limited, but as a typical example, C: 0.1 to 0.3% (meaning mass%. The same applies to the chemical component composition hereinafter).
- C 0.1 to 0.3%
- Mn 0.5 to 2%
- P 0.05% or less (not including 0%)
- S 0.05% or less
- Al 0.01 to 0.1%
- N 0.001 to 0.01%
- the hot press-formed product of the present invention if necessary, as other elements, (a) B: 0.01% or less (excluding 0%) and Ti: 0.1% or less (0%) (B) one or more selected from the group consisting of Cu, Ni, Cr and Mo: 1% or less in total (not including 0%), (c) V and / or Nb: in total It is also useful to contain 0.1% or less (not including 0%) and the like, and the properties of the hot press-formed product are further improved depending on the type of element contained.
- the method of the present invention is a method of manufacturing a hot press-formed product as described above by dividing a thin steel plate into a plurality of regions including at least first and second, wherein the thin steel plate includes a ferrite Using a hot-rolled steel sheet having a metal structure of 50% by area or more, or a cold-rolled steel sheet having a cold rolling rate of 30% or more, and heating the first forming region to a temperature of Ac 3 transformation point or higher and 1000 ° C.
- Another method of the present invention is a method of manufacturing a hot press-formed product as described above by dividing a thin steel plate into a plurality of regions including at least first and second, The first molding region and the second molding region are heated to a temperature not lower than the Ac 3 transformation point and not higher than 1000 ° C., and thereafter the first molding region maintains the heating temperature until the molding starts, The molding region is cooled to a temperature of 700 ° C. or less and 500 ° C. or more at an average cooling rate of 10 ° C./second or less, and at least the first molding region and the second molding region are both pressed with a mold. Thus, cooling and molding at an average cooling rate of 20 ° C./second or more are started, and in the first and second molding regions, molding is finished at a temperature lower than 50 ° C. below the martensite transformation start temperature.
- the metal structure of each region can be adjusted while the appropriate amount of retained austenite is present. It is possible to realize a hot press-formed product having a higher ductility (residual ductility) inherent in the molded product than when using conventional 22MnB5 steel, and the heat treatment conditions and the structure (initial structure) of the steel sheet before forming. By combining these, strength and elongation can be appropriately controlled according to each region.
- the inventors of the present invention after heating a thin steel plate to a predetermined temperature, when producing a molded product by hot press forming, after forming, good strength is ensured according to the required characteristics of each different region In order to realize a hot press-formed product that also exhibits ductility (elongation), examination was performed from various angles.
- the reason for setting the range of each structure (basic structure) in each region of the hot press-formed product of the present invention is as follows.
- the area fraction of martensite needs to be 80 area% or more. However, when this fraction exceeds 97 area%, the fraction of retained austenite becomes insufficient and ductility (residual ductility) decreases.
- a preferred lower limit of the martensite fraction is 83 area% or more (more preferably 85 area% or more), and a preferred upper limit is 95 area% or less (more preferably 93 area% or less).
- Residual austenite has the effect of increasing the work hardening rate (transformation-induced plasticity) and improving the ductility of the molded product by transforming into martensite during plastic deformation.
- the fraction of retained austenite needs to be 3 area% or more.
- the ductility the higher the retained austenite fraction, the better.
- the retained austenite that can be secured is limited, and the upper limit is about 20 area%.
- the preferable lower limit of retained austenite is 5 area% or more (more preferably 7 area% or more).
- ferrite, pearlite, bainite and the like may be included as the remaining structure.
- these structures are softer than martensite and contribute less to the strength than other structures, and are preferably as small as possible. However, up to 5 area% is acceptable.
- the remaining structure is more preferably 3 area% or less, and still more preferably 0 area%.
- a portion having a strength (tensile strength TS) of 1470 MPa or more and an elongation (total elongation EL) of 10% or more can be formed.
- the area fraction of ferrite needs to be 30 area% or more. However, when the area fraction exceeds 80 area%, the predetermined strength cannot be secured.
- a preferred lower limit of the ferrite fraction is 40 area% or more (more preferably 45 area% or more), and a preferred upper limit is 70 area% or less (more preferably 65 area% or less).
- Bainitic ferrite is effective in improving the strength, but the ductility is slightly reduced, so the upper limit of the fraction must be less than 30 area%.
- the preferable lower limit of the bainitic ferrite fraction is 5 area% or more (more preferably 10 area% or more), and the preferable upper limit is 25 area% or less (more preferably 20 area% or less).
- Martensite is effective in improving strength, but in order to significantly reduce ductility, the upper limit of the fraction needs to be 30 area% or less.
- a preferred lower limit of the martensite fraction is 5 area% or more (more preferably 10 area% or more), and a preferred upper limit is 25 area% or less (more preferably 20 area% or less).
- the fraction of retained austenite is 3 area% or more and 20 area% or less.
- the preferable lower limit of retained austenite is also the same.
- a portion for example, an energy absorbing portion of an automobile part having a strength (tensile strength TS) of 800 MPa or more and an elongation (total elongation EL) of 15% or more. Can be formed.
- the molded product of the present invention has at least a first molding region and a second molding region, but is not necessarily limited to two molding regions, and may have a third or fourth molding region. Good. In forming such a molding region, it is possible to make it according to the manufacturing method described later.
- a hot-rolled steel sheet having a metal structure of 50% by area or more of ferrite or a cold-rolled steel sheet having a cold rolling rate of 30% or more is used, and the first forming region is used.
- the Ac 3 transformation point or more a first heating process of heating to a temperature of 1000 ° C.
- the second forming region Ac 1 transformation point or more, (Ac 1 transformation point ⁇ 0.3 + Ac 3 transformation point ⁇ 0.7)
- a heating process in which a plurality of heat treatments including a second heat treatment that is heated to a temperature equal to or lower than that is performed in parallel, at least for the first forming region and the second forming region Starts cooling and forming at an average cooling rate of 20 ° C./second or more by pressing both in the mold, and the first and second forming regions are 50 ° C. lower than the martensitic transformation start temperature (Ms point). Temperature (hereinafter “Ms point –50 ° C”) Notation is) it may be terminated molded below by.
- finishing the molding basically means a state where the bottom dead center of the molding (the time when the punch tip is located at the deepest part: the state shown in FIG. 1) has been reached.
- the mold is released after the mold is cooled and held.
- the heating region of the steel sheet is divided into at least two regions (for example, a high-strength side region and a low-strength side region), and the manufacturing conditions are controlled in accordance with each region, whereby the strength corresponding to each region.
- -A molded product exhibiting a ductile balance can be obtained.
- Manufacturing conditions for forming each region will be described. In carrying out this manufacturing method, it is necessary to form regions with different heating temperatures with a single steel plate, but by using an existing heating furnace (for example, a far-infrared furnace, electric furnace + shield), It is possible to control the temperature boundary portion while keeping it at 50 mm or less.
- an existing heating furnace for example, a far-infrared furnace, electric furnace + shield
- a hot-rolled steel sheet having a metal structure with ferrite of 50 area% or more, or a cold-rolled steel sheet with a cold rolling rate of 30% or more In order to obtain a ferrite structure that greatly contributes to ductility when heated to a two-phase region temperature, it is necessary to appropriately select the type of steel plate (forming steel plate).
- a hot-rolled steel sheet is used as the forming steel sheet, it is important that the ferrite fraction is high and the ferrite remains at the two-phase temperature when heated. From such a viewpoint, it is preferable that the hot-rolled steel sheet to be used has a ferrite metal structure of 50 area% or more.
- the preferred lower limit of this ferrite fraction is 60 area% or more (more preferably 70 area% or more), but if the ferrite fraction becomes too high, the ferrite fraction in the molded product becomes too large, so 95 area% The following is preferable. More preferably, it is 90 area% or less.
- Cold rolling ratio is a value obtained by the following equation (1).
- Cold rolling ratio (%) [(steel plate thickness before cold rolling ⁇ steel plate thickness after cold rolling) / steel plate thickness before cold rolling] ⁇ 100 (1)
- the heating temperature of the thin steel plate exceeds 1000 ° C.
- the grain size of austenite increases during heating, the martensite transformation start temperature (Ms point) and the martensite transformation end temperature (Mf point) rise, and remain during quenching. Austenite cannot be secured and good moldability cannot be achieved.
- the heating temperature is preferably (Ac 3 transformation point + 50 ° C.) or more and 950 ° C. or less.
- ⁇ Cooling conditions during molding and molding end temperature need to be controlled appropriately for each region.
- first steel plate region a steel plate region corresponding to the first forming region of the formed product (this region may be referred to as “first steel plate region”)
- an average cooling rate of 20 ° C./second or more is secured in the mold. It is necessary to finish the molding at a temperature equal to or lower than (Ms point ⁇ 50 ° C.).
- the average cooling rate during molding needs to be 20 ° C./second or more, and the molding end temperature needs to be Ms point ⁇ 50 ° C. or less.
- the average cooling rate during molding is preferably 30 ° C./second or more (more preferably 40 ° C./second or more).
- the molding end temperature may be finished while cooling to room temperature at the above average cooling rate, but after cooling to the Ms point ⁇ 50 ° C. or lower, the cooling is stopped, and then the molding is finished. good. The molding end temperature at this time will be described in detail later.
- a thin steel plate is used (chemical composition is the same as that of the formed product), and at least the first forming region and the first forming region are Ac 3 transformed.
- the first molding region is maintained at the heating temperature until the temperature is not lower than the point and not higher than 1000 ° C. and thereafter molding is started, and the second molding region is 700 ° C. at an average cooling rate of 10 ° C./second or less.
- the first molding region and the second molding region are both cooled and molded at an average cooling rate of 20 ° C./second or more by pressing with a mold.
- the molding may be started and finished at (Ms point ⁇ 50 ° C.) or less in the first and second molding regions.
- the heating temperature In order to appropriately adjust the structure of the hot press-formed product, it is necessary to control the heating temperature within a predetermined range.
- the heating temperature of the thin steel sheet is less than the Ac 3 transformation point, a sufficient amount of austenite cannot be obtained during heating, and a predetermined amount of retained austenite cannot be ensured in the final structure (structure of the molded product).
- the heating temperature of the thin steel plate exceeds 1000 ° C.
- the particle size of austenite increases during heating, and (a) the martensite transformation start temperature (Ms point) and the martensite transformation end temperature (Mf point) increase. Residual austenite cannot be secured during quenching, and good formability is not achieved (first molding region), or (b) ferrite cannot be generated by subsequent cooling (second molding region).
- ⁇ Cooling conditions during molding and molding end temperature need to be controlled appropriately for each region.
- the average cooling rate during molding is 20 ° C./second or more, and the molding end temperature is (Ms point ⁇ 50 ° C.) or less.
- the martensite can be made into a mixed structure of residual austenite by cooling under such conditions.
- the average cooling rate during molding is preferably 30 ° C./second or more (more preferably 40 ° C./second or more).
- the forming end temperature in the first steel plate region may be finished while cooling to room temperature at the above average cooling rate, but is not higher than (Ms point ⁇ 50 ° C.) (preferably up to Ms point ⁇ 50 ° C.). ) After cooling, cooling to 200 ° C. or less may be performed at an average cooling rate of 20 ° C./second or less (two-stage cooling). By adding such a cooling step, carbon in martensite is concentrated to untransformed austenite, whereby the amount of retained austenite can be increased. When performing such two-stage cooling, the average cooling rate during the second stage cooling is preferably 10 ° C./second or less (more preferably 5 ° C./second or less).
- an average cooling rate of 10 ° C./second or less is 700 ° C. or less and 500 ° C. It is preferable to cool to the above temperature and then start molding.
- This cooling step is an important step in forming ferrite during cooling. If the average cooling rate at this time exceeds 10 ° C./second, a predetermined amount of ferrite cannot be secured.
- This average cooling rate is preferably 7 ° C./second or less, more preferably 5 ° C./second or less.
- the cooling stop temperature in this cooling step needs to be 700 ° C. or lower and 500 ° C. or higher.
- a preferable upper limit of the cooling stop temperature is 680 ° C. or lower (more preferably 660 ° C. or lower), and a preferable lower limit is 520 ° C. or higher (more preferably 550 ° C. or higher).
- the first steel plate region is maintained in a heated state without being cooled.
- cooling and forming at an average cooling rate of 20 ° C./second or more may be started by pressing in a mold, and forming may be terminated at a temperature of Ms point ⁇ 50 ° C. or less.
- the bainite transformation start temperature Bs point may be finished at a temperature of ⁇ 100 ° C. or lower.
- the average cooling rate during forming in the second steel plate region is set to 20 ° C./second or more, and the forming end temperature is (bainite transformation start temperature Bs point ⁇ 100 ° C .: hereinafter abbreviated as “Bs-100 ° C.”). (It may be the same in the previous manufacturing method).
- the average cooling rate at this time is preferably 30 ° C./second or more (more preferably 40 ° C./second or more).
- the molding end temperature may be finished while cooling to room temperature at the above average cooling rate, but cooling may be stopped after cooling to Bs-100 ° C. or lower, and then the molding may be finished. .
- the forming end temperature of the second steel plate region is preferably a temperature range equal to or higher than the martensite transformation start temperature Ms point, and is preferably maintained for 10 seconds or more in that temperature range.
- the holding time at this time is preferably 50 seconds or more (more preferably 100 seconds or more). However, if the holding time becomes too long, the austenite starts to decompose, and the retained austenite fraction cannot be secured. Or less (more preferably 800 seconds or less).
- the above-described holding may be any of isothermal holding, monotonous cooling, and reheating step as long as it is within the above temperature range.
- the above-described holding may be added at the stage of completion of molding, but a holding step may be added within the above temperature range in the middle of finishing molding. . After the molding is completed in this manner, it may be cooled to room temperature (25 ° C.) by cooling or at an appropriate cooling rate.
- Control of the average cooling rate during molding can be achieved by means such as (a) controlling the temperature of the molding die (cooling medium shown in FIG. 1), (b) controlling the thermal conductivity of the die. (The same applies to cooling in the following method).
- the cooling conditions during molding may differ depending on each region, but the control means such as (a) and (b) above are separately formed in a single mold, Cooling control corresponding to each region may be performed in a single mold.
- the present invention can also be applied to the case of manufacturing a molded product having a relatively complicated shape.
- a method of performing cold press forming in a pre-process of hot press forming (this method is called “indirect method”) can be employed.
- This method is a method in which a portion that is difficult to be molded is preliminarily molded to an approximate shape by cold working, and the other portions are hot press molded. If such a method is adopted, for example, when a part having three uneven portions (peaks) of a molded product is formed, the two parts are formed by cold press molding, and then the third part is formed. Will be hot pressed.
- the present invention is made assuming a hot press-formed product made of a high-strength steel plate, and its steel type may be of a normal chemical composition as a high-strength steel plate, but C, Si, About Mn, P, S, Al, and N, it is good to adjust to an appropriate range. From such a viewpoint, the preferable ranges of these chemical components and the reasons for limiting the ranges are as follows.
- C 0.1-0.3%) C is an important element in securing retained austenite. At the time of heating at a single phase region temperature equal to or higher than the Ac 3 transformation point, the austenite is concentrated to form retained austenite after quenching. It is also an important element in controlling the increase in martensite amount and the strength of martensite (first region). When the C content is less than 0.1%, a predetermined retained austenite amount cannot be secured, and good ductility cannot be obtained. In addition, the strength of martensite is insufficient. On the other hand, if the C content is excessive and exceeds 0.3%, the strength becomes too high. A more preferable lower limit of the C content is 0.15% or more (more preferably 0.20% or more), and a more preferable upper limit is 0.27% or less (more preferably 0.25% or less).
- Si suppresses the formation of cementite from austenite after heating to a single-phase region temperature equal to or higher than the Ac 3 transformation point, and exerts an effect of increasing and forming retained austenite during quenching.
- the solid solution strengthening also exerts the effect of increasing the strength without significantly degrading the ductility. If the Si content is less than 0.5%, a predetermined retained austenite amount cannot be secured, and good ductility cannot be obtained. On the other hand, if the Si content is excessive and exceeds 3%, the solid solution strengthening amount becomes too large, and the ductility is greatly deteriorated.
- the more preferable lower limit of the Si content is 1.15% or more (more preferably 1.20% or more), and the more preferable upper limit is 2.7% or less (more preferably 2.5% or less).
- Mn is an element that stabilizes austenite and contributes to an increase in retained austenite. Moreover, it is an element effective in improving hardenability, suppressing the formation of ferrite, pearlite, and bainite during cooling after heating and ensuring retained austenite (first region). In order to exhibit such an effect, it is preferable to contain 0.5% or more of Mn. However, if the Mn content is excessive, it is impossible to secure a predetermined amount of ferrite by preventing the formation of ferrite (second region). Further, since the strength of austenite is significantly improved, the hot rolling load becomes large and the production of the steel sheet becomes difficult. Therefore, it is not preferable to contain more than 2% from the viewpoint of productivity. A more preferable lower limit of the Mn content is 0.7% or more (more preferably 0.9% or more), and a more preferable upper limit is 1.8% or less (more preferably 1.6% or less).
- P 0.05% or less (excluding 0%)
- P is an element inevitably contained in the steel, but it deteriorates ductility, so it is preferable to reduce P as much as possible.
- extreme reduction leads to an increase in steelmaking cost, and since it is difficult to make it 0%, it is preferable to make it 0.05% or less (not including 0%).
- a more preferable upper limit of the P content is 0.045% or less (more preferably 0.040% or less).
- S 0.05% or less (excluding 0%)
- S is an element inevitably contained in steel, and deteriorates ductility. Therefore, S is preferably reduced as much as possible.
- extreme reduction leads to an increase in steelmaking cost, and since it is difficult to make it 0%, it is preferable to make it 0.05% or less (not including 0%).
- a more preferable upper limit of the S content is 0.045% or less (more preferably 0.040% or less).
- Al 0.01-0.1%
- Al is useful as a deoxidizing element, and also fixes solid solution N present in steel as AlN, which is useful for improving ductility.
- the Al content is preferably 0.01% or more.
- a more preferable lower limit of the Al content is 0.013% or more (more preferably 0.015% or more), and a more preferable upper limit is 0.08% or less (more preferably 0.06% or less).
- N (N: 0.001 to 0.01%) N is an element inevitably mixed in, and is preferably reduced. However, since there is a limit to reducing it in the actual process, 0.001% was set as the lower limit. If the N content is excessive, the ductility deteriorates due to strain aging, or when B is added, it precipitates as BN and lowers the effect of improving hardenability by solute B, so the upper limit is 0.01. %. The upper limit with more preferable N content is 0.008% or less (more preferably 0.006% or less).
- substantially iron means a trace component that does not inhibit the properties of the steel material of the present invention other than iron (for example, Mg, Ca, Sr, Ba, REM such as La, and Zr, Hf). , Ta, W, Mo and other carbide-forming elements) are acceptable, and inevitable impurities other than P, S, N (for example, O, H, etc.) can also be included.
- B is an element that prevents formation of cementite during cooling after heating and contributes to securing retained austenite.
- B is preferably contained in an amount of 0.0001% or more, but the effect is saturated even if it is contained in excess of 0.01%.
- a more preferable lower limit of the B content is 0.0002% or more (more preferably 0.0005% or more), and a more preferable upper limit is 0.008% or less (more preferably 0.005% or less).
- Ti fixes N and causes B to be maintained in a solid solution state, thereby improving the hardenability.
- the Ti content is excessive and exceeds 0.1%, a large amount of TiC is formed, The strength increases by precipitation strengthening, but the ductility deteriorates.
- a more preferable lower limit of the Ti content is 0.05% or more (more preferably 0.06% or more), and a more preferable upper limit is 0.09% or less (more preferably 0.08% or less).
- Cu, Ni, Cr and Mo prevent the formation of cementite during cooling after heating and effectively act to secure retained austenite.
- the content is large, but since the cost of alloy addition increases, the total content is preferably 1% or less.
- strength of austenite significantly since the load of hot rolling becomes large and manufacture of a steel plate becomes difficult, it is preferable to set it as 1% or less also from a viewpoint of productivity.
- the more preferable lower limit of the content of these elements is 0.05% or more (more preferably 0.06% or more) in total, and the more preferable upper limit is 0.9% or less (more preferably 0.8% or less) in total. ).
- V and Nb 0.1% or less in total (excluding 0%)
- V and Nb have the effect of forming fine carbides and making the structure fine by the pinning effect. In order to exhibit such an effect, it is preferable to contain 0.001% or more in total. However, if the content of these elements is excessive, coarse carbides are formed and the ductility is deteriorated by becoming the starting point of destruction, so the total content is preferably 0.1% or less.
- the more preferable lower limit of the content of these elements is 0.005% or more (more preferably 0.008% or more) in total, and the more preferable upper limit is 0.08% or less (more preferably 0.06% or less) in total. ).
- the present invention by appropriately adjusting the press molding conditions (heating temperature and cooling rate corresponding to each region), it is possible to control properties such as strength and elongation in each region in the molded product, and Since a hot-pressed product with ductility (residual ductility) can be obtained, it is also difficult to apply it to conventional hot-pressed products (for example, members that require both impact resistance and energy absorption suppression). It can be applied and is extremely useful for expanding the application range of hot press-formed products.
- the molded product obtained by the present invention has a larger residual ductility than a molded product whose structure is adjusted by performing normal annealing after cold press molding.
- a steel material having the chemical composition shown in Table 1 below was vacuum-melted to obtain a slab for experiment, then hot rolled, and then cooled and wound up. Furthermore, it cold-rolled and made it the thin steel plate.
- the Ac 1 transformation point, Ac 3 transformation point, Ms point, and (Bs-100 ° C.) in Table 1 were determined using the following formulas (2) to (5) (for example, (See “Leslie Steel Materials Science” Maruzen, (1985)).
- Table 1 also shows the calculated value (hereinafter referred to as “A value”) of (Ac 1 transformation point ⁇ 0.3 + Ac 3 transformation point ⁇ 0.7).
- the obtained steel sheet was subjected to forming / cooling treatment by changing the heating temperature in each steel sheet region.
- press molding was performed using a HAT (hat channel) -shaped bending mold shown in FIG.
- the heating temperature and average cooling rate in each steel plate region are shown in the following Table 2 (the forming end temperature (mold release temperature) is 200 ° C. in all regions).
- the steel plate size at the time of forming and cooling was 220 mm ⁇ 500 mm (plate thickness: 1.4 mm) (the area ratio of the first steel plate region and the second steel plate region was 1: 1).
- the shape of the molded press-molded product is shown in FIG. 3 [FIG. 3 (a) is a perspective view and FIG. 3 (b) is a cross-sectional view].
- Test No. Examples 1, 3, and 4 are examples that satisfy the requirements defined in the present invention, and it can be seen that a molded article in which the strength-ductility balance in each region is achieved with high performance is obtained.
- test no. Examples 2 and 5 are comparative examples that do not satisfy any of the requirements defined in the present invention, and any of the characteristics is deteriorated. That is, test no. Since No. 2 was heated below the Ac 1 transformation point in the second region, it had a structure mainly composed of ferrite, no martensite was generated, and the strength was not ensured.
- Test No. No. 5 is a conventional 22MnB5-equivalent steel (steel type B in Table 1), and although strength is obtained, residual austenite is not secured, and low elongation is not observed in any region ( EL) is only obtained.
- the metal structure includes martensite: 80 to 97 area%, retained austenite: 3 to 20 area%, and the remaining structure: a first region consisting of 5 area% or less, Metal structure: ferrite: 30 to 80 area%, bainitic ferrite: less than 30 area% (excluding 0 area%), martensite: 30 area% or less (excluding 0 area%), residual austenite: 3
- a single molded product has regions corresponding to impact-resistant sites and energy-absorbing sites, and has high strength and elongation according to each region. Can be achieved at a high level.
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Abstract
Description
第1の領域の主要組織を、高強度のマルテンサイトにすることで、熱間プレス成形品における特定領域の高強度を確保することができる。こうした観点から、マルテンサイトの面積分率は、80面積%以上とする必要がある。しかしながら、この分率が97面積%を超えると、残留オーステナイトの分率が不足し、延性(残存延性)が低下する。マルテンサイト分率の好ましい下限は83面積%以上(より好ましくは85面積%以上)であり、好ましい上限は95面積%以下(より好ましくは93面積%以下)である。
第2の領域の主要組織を、微細で且つ延性の高いフェライトにすることで、熱間プレス成形品における特定領域の高延性を実現することができる。こうした観点から、フェライトの面積分率は、30面積%以上とする必要がある。しかしながら、この面積分率が80面積%を超えると、所定強度が確保できなくなる。フェライト分率の好ましい下限は40面積%以上(より好ましくは45面積%以上)であり、好ましい上限は70面積%以下(より好ましくは65面積%以下)である。
二相域温度に加熱時に、延性への寄与の大きいフェライト組織を得るためには、鋼板(成形用鋼板)の種類を適切に選ぶ必要がある。成形用鋼板として熱延鋼板を用いる場合には、フェライト分率が高く、二相域温度に加熱時にフェライトが残存するようにすることが重要である。こうした観点から、用いる熱延鋼板はフェライトが50面積%以上の金属組織を有するものであることが好ましい。このフェライト分率の好ましい下限は、60面積%以上(より好ましくは70面積%以上)であるが、フェライト分率があまり高くなると、成形品中のフェライト分率が多くなり過ぎるので、95面積%以下であることが好ましい。より好ましくは90面積%以下である。
冷延率(%)=[(冷間圧延前の鋼板厚さ-冷間圧延後の鋼板厚さ)/冷間圧延前の鋼板厚さ]×100…(1)
熱間プレス成形品の組織を適切に調整するためには、加熱温度は所定の範囲に制御する必要がある。この加熱温度を適切に制御することによって(第1の加熱処理)、その後の冷却過程で、所定量の残留オーステナイトを確保しつつ、第1の成形領域をマルテンサイト主体とする組織に変態させ、最終的な熱間プレス成形品で所望の組織に作り込むことができる。薄鋼板の加熱温度がAc3変態点未満であると、加熱時に十分な量のオーステナイトが得られず、最終組織(成形品の組織)で所定量の残留オーステナイトを確保できない。また、薄鋼板の加熱温度が1000℃を超えると、加熱時にオーステナイトの粒径が大きくなり、マルテンサイト変態開始温度(Ms点)およびマルテンサイト変態終了温度(Mf点)が上昇し、焼入れ時に残留オーステナイトを確保できず、良好な成形性が達成されない。加熱温度は好ましくは(Ac3変態点+50℃)以上、950℃以下である。
鋼板中に含まれるフェライトを残存させつつ、部分的にオーステナイトにさせるためには、加熱温度は所定の範囲に制御する必要がある。この加熱温度を適切に制御することによって、その後の冷却過程で、残留オーステナイト若しくはマルテンサイトに変態させ、最終的な熱間プレス成形品で所望の組織に作り込むことができる。鋼板の加熱温度がAc1変態点未満であると、加熱時に十分な量のオーステナイトが得られず、最終組織(成形品の組織)で所定量の残留オーステナイトを確保できない。また、薄鋼板の加熱温度が(Ac1変態点×0.3+Ac3変態点×0.7)を超えると、加熱時にオーステナイトへの変態量が増加し過ぎて、最終組織(成形品の組織)で所定量のフェライトを確保できない。
Cは、残留オーステナイトを確保する上で重要な元素である。Ac3変態点以上の単相域温度の加熱時に、オーステナイトに濃化することで、焼入れ後に残留オーステナイトを形成させる。また、マルテンサイト量の増加や、マルテンサイトの強度を支配する上でも(第1の領域)重要な元素である。C含有量が0.1%未満では、所定の残留オーステナイト量が確保できず、良好な延性が得られない。またマルテンサイトの強度が不足することにもなる。一方、C含有量が過剰になって0.3%を超えると、強度が高くなり過ぎることになる。C含有量のより好ましい下限は0.15%以上(更に好ましくは0.20%以上)であり、より好ましい上限は0.27%以下(更に好ましくは0.25%以下)である。
Siは、Ac3変態点以上の単相域温度に加熱後のオーステナイトがセメンタイトに形成されることを抑制し、焼入れ時に残留オーステナイトを増加・形成させる作用を発揮する。また、固溶強化によって、延性をあまり劣化させずに強度を高める作用も発揮する。Si含有量が0.5%未満では、所定の残留オーステナイト量が確保できず、良好な延性が得られない。またSi含有量が過剰になって3%を超えると、固溶強化量が大きくなり過ぎ、延性が大幅に劣化することになる。Si含有量のより好ましい下限は1.15%以上(更に好ましくは1.20%以上)であり、より好ましい上限は2.7%以下(更に好ましくは2.5%以下)である。
Mnは、オーステナイトを安定化させる元素であり、残留オーステナイトの増加に寄与する。また、焼入れ性を高め、加熱後の冷却中にフェライト、パーライト、ベイナイトの形成を抑制し、残留オーステナイトを確保する上でも有効な元素である(第1の領域)。こうした効果を発揮させるためには、Mnは0.5%以上含有させることが好ましい。しかしながら、Mn含有量が過剰になると、フェライトの形成を妨げることで、所定量のフェライトを確保できなくなるので(第2の領域)、2%以下とすることが好ましい。また、オーステナイトの強度を大幅に向上させるため、熱間圧延の負荷が大きくなり、鋼板の製造が困難になるため、生産性の上からも、2%を超えて含有させることは好ましくない。Mn含有量のより好ましい下限は0.7%以上(更に好ましくは0.9%以上)であり、より好ましい上限は1.8%以下(更に好ましくは1.6%以下)である。
Pは、鋼中に不可避的に含まれる元素であるが延性を劣化させるので、Pは極力低減することが好ましい。しかしながら、極端な低減は製鋼コストの増大を招き、0%とすることは製造上困難であるので、0.05%以下(0%を含まない)とすることが好ましい。P含有量のより好ましい上限は0.045%以下(更に好ましくは0.040%以下)である。
SもPと同様に鋼中に不可避的に含まれる元素であり、延性を劣化させるので、Sは極力低減することが好ましい。しかしながら、極端な低減は製鋼コストの増大を招き、0%とすることは製造上困難であるので、0.05%以下(0%を含まない)とすることが好ましい。S含有量のより好ましい上限は0.045%以下(更に好ましくは0.040%以下)である。
Alは、脱酸元素として有用であると共に、鋼中に存在する固溶NをAlNとして固定し、延性の向上に有用である。こうした効果を有効に発揮させるためには、Al含有量は0.01%以上とすることが好ましい。しかしながら、Al含有量が過剰になって0.1%を超えると、Al2O3が過剰に生成し、延性を劣化させる。尚、Al含有量のより好ましい下限は0.013%以上(更に好ましくは0.015%以上)であり、より好ましい上限は0.08%以下(更に好ましくは0.06%以下)である。
Nは、不可避的に混入する元素であり、低減することが好ましいが、実プロセスの中で低減するには限界があるため、0.001%を下限とした。また、N含有量が過剰になると、歪み時効により延性が劣化したり、Bを添加している場合はBNとして析出し、固溶Bによる焼入れ性改善効果を低下させるため、上限を0.01%とした。N含有量のより好ましい上限は0.008%以下(更に好ましくは0.006%以下)である。
Bは、加熱後の冷却中に、セメンタイトの形成を防止し、残留オーステナイトの確保に寄与する元素である。こうした効果を発揮させるためには、Bは0.0001%以上含有させることが好ましいが、0.01%を超えて過剰に含有させても効果が飽和する。B含有量のより好ましい下限は0.0002%以上(更に好ましくは0.0005%以上)であり、より好ましい上限は0.008%以下(更に好ましくは0.005%以下)である。
Cu,Ni,CrおよびMoは、加熱後の冷却中に、セメンタイトの形成を防止し、残留オーステナイトの確保に有効に作用する。こうした効果を発揮させるためには、合計で0.01%以上含有させることが好ましい。特性だけを考慮すると含有量は多いほうが好ましいが、合金添加のコストが上昇することから、合計で1%以下とすることが好ましい。また、オーステナイトの強度を大幅に高める作用を有するため、熱間圧延の負荷が大きくなり、鋼板の製造が困難になるため、製造性の観点からも1%以下とすることが好ましい。これらの元素含有量のより好ましい下限は合計で0.05%以上(更に好ましくは0.06%以上)であり、より好ましい上限は合計で0.9%以下(更に好ましくは0.8%以下)である。
VおよびNbは、微細な炭化物を形成し、ピン止め効果により組織を微細にする効果がある。こうした効果を発揮させるためには、合計で0.001%以上含有させることが好ましい。しかしながら、これらの元素の含有量が過剰になると、粗大な炭化物が形成され、破壊の起点になることで逆に延性を劣化させるので、合計で0.1%以下とすることが好ましい。これらの元素含有量のより好ましい下限は合計で0.005%以上(更に好ましくは0.008%以上)であり、より好ましい上限は合計で0.08%以下(更に好ましくは0.06%以下)である。
Ac3変態点(℃)=910-203×[C]1/2+44.7×[Si]-30×[Mn]+700×[P]+400×[Al]+400×[Ti]+104×[V]-11×[Cr]+31.5×[Mo]-20×[Cu]-15.2×[Ni] …(3)
Ms点(℃)=550-361×[C]-39×[Mn]-10×[Cu]-17×[Ni]-20×[Cr]-5×[Mo]+30×[Al] …(4)
Bs点(℃)=830-270×[C]-90×[Mn]-37×[Ni]-70×[Cr]-83×[Mo] …(5)
但し、[C],[Si],[Mn],[P],[Al],[Ti],[V],[Cr],[Mo],[Cu]および[Ni]は、夫々C,Si,Mn,P,Al,Ti,V,Cr,Mo,CuおよびNiの含有量(質量%)を示す。また、上記(2)式~(5)式の各項に示された元素が含まれない場合は、その項がないものとして計算する。
JIS5号試験片を用いて引張試験を行い、引張強度(TS)、伸び(EL)を測定した。このとき、引張試験の歪速度:10mm/秒とした。本発明では、(a)第1の領域において、引張強度(TS)が1470MPa以上で伸び(EL)が10%以上を満足し、且つ(b)第2の領域において引張強度(TS)が800MPa以上で伸び(EL)が15%以上を満足するときに合格と評価した。
(1)鋼板中のフェライト、ベイニティックフェライトの組織については、鋼板をナイタールで腐食し、SEM(倍率:1000倍または2000倍)観察により、フェライト、ベイニティックフェライトを区別し、夫々の分率(面積率)を求めた。
(2)鋼板中の残留オーステナイト分率(面積率)は、鋼板の1/4の厚さまで研削した後、化学研磨してからX線回折法によって測定した(例えば、ISJJ Int.Vol.33.(1933),No.7,P.776)。
(3)マルテンサイト(焼入れままマルテンサイト)の面積率については、鋼板をレペラ腐食し、SEM観察により白いコントラストを、マルテンサイト(焼入れままマルテンサイト)と残留オーステナイトの混合組織として面積率を測定し、そこからX線回折により求めた残留オーステナイト分率を差し引いて、焼入れままマルテンサイト分率を計算した。
2 ダイ
3 ブランクホルダー
4 鋼板(ブランク)
Claims (7)
- 熱間プレス成形法によって薄鋼板を成形した熱間プレス成形品であって、金属組織が、マルテンサイト:80~97面積%、残留オーステナイト:3~20面積%を夫々含み、残部組織:5面積%以下からなる第1の領域と、金属組織が、フェライト:30~80面積%、ベイニティックフェライト:30面積%未満(0面積%を含まない)、マルテンサイト:30面積%以下(0面積%を含まない)、残留オーステナイト:3~20面積%からなる第2の領域を有するものであることを特徴とする熱間プレス成形品。
- 化学成分組成が、
C :0.1~0.3%(質量%の意味。以下、化学成分組成について同じ。)、
Si:0.5~3%、
Mn:0.5~2%、
P :0.05%以下(0%を含まない)、
S :0.05%以下(0%を含まない)、
Al:0.01~0.1%、および
N :0.001~0.01%、
を夫々含有し、残部が鉄および不可避不純物からなる請求項1に記載の熱間プレス成形品。 - 更に他の元素として、B:0.01%以下(0%を含まない)およびTi:0.1%以下(0%を含まない)を含有するものである請求項2に記載の熱間プレス成形品。
- 更に他の元素として、Cu,Ni,CrおよびMoよりなる群から選択される1種以上:合計で1%以下(0%を含まない)含有するものである請求項2または3に記載の熱間プレス成形品。
- 更に他の元素として、Vおよび/またはNb:合計で0.1%以下(0%を含まない)含有するものである請求項2または3に記載の熱間プレス成形品。
- 薄鋼板を少なくとも第1および第2を含む複数の領域に分けて成形することによって、請求項1~3のいずれかに記載の熱間プレス成形品を製造する方法であって、
前記薄鋼板として、フェライトが50面積%以上の金属組織を有する熱延鋼板、または冷延率30%以上を施した冷延鋼板を用い、
第1の成形領域をAc3変態点以上、1000℃以下の温度に加熱する第1加熱処理と、第2の成形領域をAc1変態点以上、(Ac1変態点×0.3+Ac3変態点×0.7)に相当する温度以下に加熱する第2加熱処理とを含む複数の加熱処理を並行して行う加熱工程によって前記薄鋼板を加熱した後、
少なくとも第1の成形領域および第2の成形領域に対しては、共に金型でプレスすることによって平均冷却速度20℃/秒以上の冷却と成形を開始し、
第1および第2の成形領域ではマルテンサイト変態開始温度より50℃低い温度以下で成形を終了することを特徴とする熱間プレス成形品の製造方法。 - 薄鋼板を少なくとも第1および第2を含む複数の領域に分けて成形することによって、請求項1~3のいずれかに記載の熱間プレス成形品を製造する方法であって、
少なくとも第1の成形領域および第2の成形領域をAc3変態点以上、1000℃以下の温度に加熱し、
その後成形開始するまでの間、第1の成形領域は加熱温度を維持し、第2の成形領域は10℃/秒以下の平均冷却速度で700℃以下、500℃以上の温度まで冷却した後、
少なくとも第1の成形領域および第2の成形領域に対しては、共に金型でプレスすることによって平均冷却速度20℃/秒以上の冷却と成形を開始し、
第1および第2の成形領域ではマルテンサイト変態開始温度より50℃低い温度以下で成形を終了することを特徴とする熱間プレス成形品の製造方法。
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