WO2020204037A1 - ホットスタンプ成形品およびホットスタンプ用鋼板、並びにそれらの製造方法 - Google Patents
ホットスタンプ成形品およびホットスタンプ用鋼板、並びにそれらの製造方法 Download PDFInfo
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- WO2020204037A1 WO2020204037A1 PCT/JP2020/014841 JP2020014841W WO2020204037A1 WO 2020204037 A1 WO2020204037 A1 WO 2020204037A1 JP 2020014841 W JP2020014841 W JP 2020014841W WO 2020204037 A1 WO2020204037 A1 WO 2020204037A1
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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
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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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
- 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
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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
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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
- C21D8/0273—Final recrystallisation annealing
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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
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
-
- 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
- C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—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
- 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
Definitions
- the present invention relates to hot stamped articles and steel sheets for hot stamping, and methods for producing them.
- the present application claims priority based on Japanese Patent Application No. 2019-070211 filed in Japan on April 1, 2019, the contents of which are incorporated herein by reference.
- press molding which is the process of forming automobile body members
- the thinner the steel sheet used the more likely it is that cracks and wrinkles will occur. Therefore, steel sheets for automobiles are also required to have excellent press formability.
- Patent Document 1 a technique of press-forming a heated steel sheet using a low-temperature press die has been proposed. .. This technology is called hot stamping or hot pressing, and since steel sheets that have become soft when heated to a high temperature are press-formed, it is possible to manufacture members with complex shapes with high dimensional accuracy. it can. In addition, since the steel sheet is rapidly cooled by contact with the mold, it is possible to significantly increase the strength at the same time as press molding by quenching. For example, Patent Document 1 describes that a member having a tensile strength of 1400 MPa or more can be obtained by hot stamping a steel sheet having a tensile strength of 500 to 600 MPa.
- skeletal structural parts such as center pillars and side members are often provided with a hard part and a soft part in the member in order to control the deformation state of the member when an automobile collides.
- Patent Document 2 discloses a method in which the heating temperature of a steel sheet is partially changed by induction heating or infrared heating to soften the portion heated to a low temperature.
- Patent Document 3 discloses a method in which a heat insulating material is attached to a part of a steel sheet when the steel sheet is heated in a furnace, and the heating temperature is partially lowered to soften the steel sheet.
- Patent Document 4 and Patent Document 5 disclose a method of partially changing the cooling rate of a steel sheet by changing the contact area between the steel sheet and the mold during molding to soften the portion having a low cooling rate.
- Patent Document 6 discloses a technique of hot stamping using a so-called tailored blank material in which two base plates are welded and connected.
- the strength of martensite is increased by forming a single structure of martensite by heating the steel sheet to the austenite region and then cooling it at a cooling rate higher than the critical cooling rate.
- the heating temperature or cooling rate of the steel sheet is partially lowered, and a structure other than martensite is partially generated to soften the steel sheet.
- the methods of Patent Documents 2 to 5 have a problem that the strength of the soft portion is not stable.
- Patent Document 6 a soft portion can be formed under certain heating and cooling conditions by using a steel plate having a low hardenability for one of the base plates.
- the metallographic structure and strength characteristics of the soft portion largely depend on the composition of the steel sheet, Patent Document 6 does not give any consideration to the composition of the steel sheet having low hardenability.
- Patent Documents 7 and 8 disclose a method for stabilizing the strength of a soft portion in a hot stamp member composed of a hard portion and a soft portion, or an overall soft hot stamp member. Has been done.
- Patent Document 7 describes a height for 600 to 1200 MPa class automobiles in which the C content is limited to a low level and the quenching element is contained in a certain amount or more to suppress the formation of ferrite, pearlite and martensite during cooling.
- a strength member and a method for manufacturing the same are disclosed.
- Patent Document 8 discloses a hot stamping member having a tensile strength of 500 MPa or more and a method for producing the same, in which the C content is limited to a low level and Ti is contained to control the amount of martensite produced. ..
- Patent Documents 7 and 8 it is possible to improve the uniformity of strength and elongation in the member.
- the metal structure contains a hard structure such as bainite and martensite, the thermal stability is low and the member is coated and baked. It was found that the strength may decrease when the treatment is applied. Since paint baking treatment is often performed on automobile parts, there is room for improvement in the techniques described in Patent Documents 7 and 8.
- Japanese Patent Application Laid-Open No. 2002-102980 Japanese Patent Application Laid-Open No. 2005-193287 Japanese Patent Application Laid-Open No. 2009-61473 Japanese Patent Application Laid-Open No. 2003-328031 International Publication No. 2006/38868 Japanese Patent Application Laid-Open No. 2004-58082 Japanese Patent Application Laid-Open No. 2005-248320 International Publication No. 2008/132303
- the present invention solves the above-mentioned problems and is excellent in thermal stability. More specifically, the variation in strength (tensile strength) before and after the coating baking treatment accompanying the coating baking treatment is small, and the tensile strength is 700 MPa. It is an object of the present invention to provide a hot stamped product having a portion less than that, a steel sheet for hot stamping suitable as a material thereof, and a method for producing the same.
- the present invention has been made to solve the above problems, and the gist of the following hot stamp molded products and hot stamping steel sheets and their manufacturing methods.
- a hot stamped product in which all or part of the hot stamped product is C: 0.001% or more and less than 0.090%, Si: 2.50% or less, Mn in mass%. : 0.01% or more, less than 0.50%, P: 0.200% or less, S: 0.0200% or less, sol.
- the metal structure contains, by area%, ferrite: more than 60.0%, maltensite: 0% or more and less than 20.0%, bainite: 0% or more and less than 20.0%.
- the tensile strength is less than 700 MPa
- ⁇ TS which is the amount of decrease in the tensile strength when heat-treated at 170 ° C. for 20 minutes, is 100 MPa or less.
- the chemical composition is Ti: 0.001 to 0.300%, Nb: 0.001 to 0.300%, V: 0.001 to 0.300%, Zr: 0.001 in mass%.
- the hot stamped product according to (1) above which contains one or more of the following types.
- Chemical composition is C: 0.001% or more, less than 0.090%, Si: 2.50% or less, Mn: 0.01% or more, less than 0.50%, P: 0 in mass%. .200% or less, S: 0.0200% or less, sol.
- a steel plate for hot stamping wherein the metal structure contains iron carbide, and the Mn content and Cr content in the iron carbide satisfy the following formula (i).
- the meaning of each symbol in the above formula is as follows.
- [Cr] ⁇ In the iron carbide Cr content in the iron carbide in atomic% when the total content of Fe, Mn and Cr contained is 100 atomic% (6)
- the chemical composition is Ti: 0.001 to% by mass.
- the steel sheet for hot stamping according to any one of (5) to (7) above which has a plating layer on its surface.
- a method for producing a hot stamped product comprising a hot stamping step of starting hot stamping on a steel sheet at a temperature of (T-80) ° C. or higher. (11) The method for producing the hot stamped molded product according to (4) above, which comprises a heating step of heating the hot stamping steel sheet according to (8) to a heating temperature of T ° C. exceeding 3 points of Ac.
- a method for producing a hot stamped product comprising a hot stamping step of starting hot stamping on the hot stamping steel sheet after the heating step at a temperature of (T-80) ° C. or higher. (12) The method for producing the hot stamped molded product according to (4) above, wherein the hot stamping steel sheet according to (8) is joined to the joining steel sheet to form a joined steel sheet, and the above-mentioned.
- a heating step of heating the joined steel sheet after the joining step to a heating temperature of T ° C. exceeding three Ac points of the hot stamping steel sheet, and a temperature of (T-80) ° C. or higher with respect to the joined steel sheet after the heating step.
- a method for manufacturing a hot stamped product which comprises a hot stamping process for starting hot stamping in.
- a hot-rolling process in which a slab is hot-rolled and then wound in a temperature range of 800 ° C.
- Manufacture of a steel sheet for hot stamping which comprises a hot-rolled sheet ablation step of subjecting the sheet to anneading to obtain a hot-rolled and annealed steel sheet, and a cold-rolling step of cold-rolling the hot-rolled and annealed steel sheet to obtain a cold-rolled steel sheet.
- Method. (14) The method for producing a hot stamping steel sheet according to (13) above, further comprising a plating step of optionally performing continuous annealing and then plating on the cold-rolled steel sheet after the cold rolling step. ..
- a hot stamped product having a portion having a portion having a tensile strength of less than 700 MPa and having a small fluctuation in strength due to a coating baking process (excellent in thermal stability).
- FIG. 1 It is a schematic diagram which shows the shape of the hot stamp molded article manufactured in Example 1.
- FIG. 2 It is a schematic diagram which shows the shape of the hot stamp molded article manufactured in Example 2.
- FIG. 1 It is a schematic diagram which shows the shape of the hot stamp molded article manufactured in Example 1.
- the present inventors have diligently studied a method for suppressing a decrease in strength during coating baking for a hot stamped product having a tensile strength of less than 700 MPa. As a result, the following findings were obtained.
- the present inventors limit the Mn content to a low level and contain a desired amount of Cr to reduce the Mn content and Cr content in the iron carbide to the desired amount.
- a desired temperature in the hot stamping process using a controlled steel sheet for hot stamping, it has a metal structure mainly composed of ferrite, has excellent thermal stability, and has a decrease in strength due to coating baking treatment.
- a hot stamped product according to an embodiment of the present invention (a hot stamped product according to the present embodiment), a steel plate for hot stamping suitable as a material thereof (a steel plate for hot stamping according to the present embodiment), and the like.
- a hot stamped product according to the present embodiment a hot stamped product according to the present embodiment
- a steel plate for hot stamping suitable as a material thereof a steel plate for hot stamping according to the present embodiment
- the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention.
- All or part of the hot stamped article according to this embodiment has the following chemical composition.
- the reasons for limiting each element are as follows. In the following description, all "%" for the content of the chemical composition mean "mass%".
- the hot stamped product includes a portion having a tensile strength of less than 700 MPa and a portion having a tensile strength of 700 MPa or more, at least the portion having a tensile strength of less than 700 MPa has the following chemical composition. You just have to have it.
- C 0.001% or more and less than 0.090%
- C is an element having an effect of increasing the tensile strength of the steel sheet after hot stamping (the steel sheet provided in the hot stamped product). If the C content is less than 0.001%, the tensile strength cannot be expected to increase due to hot stamping. Therefore, the C content is set to 0.001% or more.
- the preferred C content is 0.010% or higher, 0.020% or higher, or 0.030% or higher.
- the C content is 0.090% or more, the area ratio of martensite and / or bainite increases in the metal structure after hot stamping, and the tensile strength of the hot stamped product becomes 700 MPa or more.
- the C content is set to less than 0.090%.
- Preferred C content is less than 0.085%, less than 0.080%, less than 0.070%, or less than 0.060%.
- Si 2.50% or less Si is an element contained as an impurity in steel. If the Si content exceeds 2.50%, the weldability deteriorates and the transformation point becomes too high, making it difficult to heat the steel sheet to a temperature equal to or higher than the transformation point in the heating process of the hot stamp. Therefore, the Si content is set to 2.50% or less.
- the preferred Si content is 2.00% or less, 1.50% or less, 1.00% or less, or 0.50% or less.
- the Si content is preferably less than 0.50%, more preferably less than 0.40%, and 0.30% in order to ensure the plating property. It is more preferably less than.
- the lower limit of the Si content is not particularly limited, but an excessive decrease in the Si content causes an increase in steelmaking cost. Therefore, the Si content is preferably 0.001%. Further, since Si has an effect of increasing the tensile strength of the steel sheet after hot stamping, it may be positively contained. From the viewpoint of increasing the strength, the preferable Si content is 0.10% or more, 0.20% or more, or 0.30% or more.
- Mn 0.01% or more and less than 0.50%
- Mn is an element that deteriorates the thermal stability of the hot stamped product.
- the Mn content is preferably less than 0.40%, less than 0.35%, less than 0.30%, less than 0.25%, or less than 0.20%.
- Mn is an element that combines with S, which is an impurity, to form MnS, and has an effect of suppressing embrittlement of steel due to the inclusion of S. In order to obtain this effect, the Mn content is set to 0.01% or more.
- the Mn content is preferably 0.05% or more, 0.10% or more, or 0.15% or more.
- P 0.200% or less
- P is an element contained as an impurity in steel. If the P content exceeds 0.200%, the weldability and toughness after hot stamping are significantly deteriorated, so the P content is set to 0.200% or less.
- the preferred P content is 0.100% or less, 0.050% or less, or 0.020% or less.
- the lower limit of the P content is not particularly limited, but excessively lowering the P content causes an increase in steelmaking cost. Therefore, the P content is preferably 0.001% or more.
- the preferable P content is 0.010% or more, 0.020% or more, or 0.030% or more.
- the P content is preferably 0.050% or less, and more preferably 0.040% or less in order to ensure the plating property.
- S 0.0200% or less
- S is an element contained in steel as an impurity and embrittles the steel. Therefore, the smaller the S content, the more preferable, but when the S content exceeds 0.0200%, the embrittlement of the steel becomes remarkable. Therefore, the S content is set to 0.0200% or less.
- the preferred S content is 0.0100% or less, 0.0050% or less, or 0.0030% or less.
- the lower limit of the S content is not particularly limited, but an excessive decrease in the S content causes an increase in steelmaking cost. Therefore, the S content is preferably 0.0001% or more.
- Al 0.001 to 2.500%
- Al is an element having an action of deoxidizing molten steel. sol. If the Al content (acid-soluble Al content) is less than 0.001%, deoxidation becomes insufficient. Therefore, sol. The Al content is 0.001% or more. sol. The Al content is preferably 0.010% or more, 0.020% or more, or 0.040% or more. On the other hand, sol. If the Al content is too high, the transformation point rises, and it becomes difficult to heat the steel sheet to a temperature equal to or higher than the transformation point in the heating process of the hot stamp. Therefore, sol. The Al content is 2.500% or less. sol. The Al content is preferably 1.000% or less, 0.500% or less, 0.100% or less, or 0.060% or less.
- N 0.0200% or less
- N is an element contained as an impurity in steel and forming a nitride during continuous casting of steel. Since this nitride deteriorates the toughness after hot stamping, it is preferable that the N content is low. When the N content is more than 0.0200%, the deterioration of toughness becomes remarkable. Therefore, the N content is set to 0.0200% or less.
- the N content is preferably less than 0.0100%, less than 0.0080%, or less than 0.0050%.
- the lower limit of the N content is not particularly limited, but it is preferable that the N content is 0.0010% or more because excessively lowering the N content causes an increase in steelmaking cost.
- Cr 0.01% or more and less than 2.00% Cr is an element having an action of improving the thermal stability of a hot stamped molded product (steel plate after hot stamping) having a metal structure mainly composed of ferrite. ..
- the Cr content is set to 0.01% or more.
- the Cr content is preferably 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.
- the Cr content is set to less than 2.00%.
- the Cr content is preferably less than 0.30%, more preferably less than 0.25%.
- Ti, Nb, V and Zr are elements that have the effect of increasing the tensile strength of the hot stamped product by refining the metal structure.
- one or more selected from Ti, Nb, V and Zr may be contained as required. Since these elements do not have to be contained, the lower limit of the content of these elements is 0%.
- the Ti content is more preferably 0.010% or more, and particularly preferably 0.020% or more.
- Nb is contained, the Nb content is more preferably 0.020% or more, and particularly preferably 0.030% or more.
- V it is more preferable that the V content is 0.020% or more.
- Zr it is more preferable that the Zr content is 0.010% or more.
- the Ti content is preferably less than 0.060%, more preferably less than 0.040%.
- the Nb content is preferably less than 0.060%, more preferably less than 0.040%.
- the V content is preferably less than 0.200%, more preferably less than 0.100%.
- the Zr content is preferably less than 0.200%, more preferably less than 0.100%.
- Mo 0 to 2.00%
- Cu 0 to 2.00%
- Ni 0 to 2.00%
- Mo, Cu and Ni may be contained as required. Since these elements do not have to be contained, the lower limit of the content of these elements is 0%.
- the more preferable Mo content is 0.05% or more, the more preferable Cu content is 0.10% or more, and the more preferable Ni content is 0.10% or more.
- the contents of Mo, Cu and Ni each exceed 2.00%, the area ratio of martensite and / or bainite contained in the metal structure of the hot stamped product becomes excessive, and the heat of the hot stamped product becomes heat. Deterioration of stability. Therefore, even when the above elements are contained, the contents of Mo, Cu, and Ni are set to 2.00% or less, respectively.
- the preferred Mo content is 0.50% or less, the preferred Cu content is 1.00% or less, and the preferred Ni content is 1.00% or less.
- B 0 to 0.0200% B is an element having an action of segregating at grain boundaries to improve the toughness of the steel sheet after hot stamping. In order to obtain this effect, B may be contained if necessary. Since B does not have to be contained, the lower limit of the B content is 0%.
- the B content is preferably 0.0001% or more.
- the B content is more preferably 0.0006% or more, still more preferably 0.0010% or more.
- the B content exceeds 0.0200%, the area ratio of martensite and / or bainite contained in the metal structure of the hot stamped product becomes excessive, and the thermal stability of the hot stamped product deteriorates. Therefore, even when B is contained, the B content is 0.0200% or less.
- the B content is preferably 0.0050% or less, and more preferably 0.0030% or less.
- Ca, Mg and REM are elements having an action of improving the toughness of the steel sheet (hot stamped product) after hot stamping by adjusting the shape of inclusions. Therefore, one or more selected from Ca, Mg and REM may be contained as required. Since these elements do not have to be contained, the lower limit of the content of these elements is 0%.
- the above effect is desired, it is preferable to contain 0.0001% or more of each of one or more selected from Ca, Mg and REM.
- the content of Ca or Mg is more than 0.0100%, or when the content of REM is more than 0.1000%, the above effect is saturated and the manufacturing cost of the steel sheet is increased. Therefore, even when the above elements are contained, the Ca and Mg contents are 0.0100% or less, and the REM content is 0.1000% or less.
- REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the REM content means the total content of these elements.
- Lanthanoids are industrially added in the form of misch metal.
- Bi 0-0.0500% Bi is an element having an action of improving the toughness of a steel sheet (hot stamped product) after hot stamping by refining the solidified structure. Therefore, Bi may be contained if necessary. Since Bi does not have to be contained, the lower limit of the Bi content is 0%.
- the Bi content is preferably 0.0001% or more.
- the Bi content is more preferably 0.0003% or more, still more preferably 0.0005% or more.
- the Bi content exceeds 0.0500%, the above effect is saturated and the manufacturing cost of the steel sheet increases. Therefore, even when Bi is contained, the Bi content is 0.0500% or less.
- the Bi content is preferably 0.0100% or less, more preferably 0.0050% or less.
- the balance is Fe and impurities.
- impurity is a component mixed by various factors of raw materials such as ore and scrap, and various factors in the manufacturing process when the steel sheet is industrially manufactured, and has an adverse effect on the hot stamped product according to the present embodiment. Means what is allowed within the range that does not give.
- the chemical composition of the hot stamped product described above may be measured by a general analysis method.
- ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrum
- sol. Al may be measured by ICP-AES using a filtrate obtained by heat-decomposing the sample with an acid.
- C and S may be measured by using the combustion-infrared absorption method, and N may be measured by using the inert gas melting-thermal conductivity method.
- the metal structure (microstructure) of the hot stamped product according to this embodiment will be described. All or part of the hot stamped article according to this embodiment has a metallographic structure containing the following amounts of ferrite, martensite and bainite. In the following description of the metallographic structure, "%" means “area%”.
- the area ratio of ferrite is set to more than 60.0%.
- the area ratio of ferrite is preferably more than 70.0%, more preferably more than 80.0%.
- the upper limit of the area ratio of ferrite does not need to be particularly set, but in order to increase the strength of the hot stamped product, it is preferably less than 98.0%, more preferably less than 96.0%, 94. More preferably, it is less than 0.0%.
- the ferrite includes, in addition to the polygonal ferrite, a pseudo-polygonal ferrite having a higher dislocation density than the polygonal ferrite, a granular bainitic ferrite, and an acylular ferrite having a serrated grain boundary.
- the ratio of polygonal ferrite to the entire ferrite is preferably 5.0% or more in terms of area ratio.
- Martensite 0% or more and less than 20.0% Bainite: 0% or more and less than 20.0% If the metal structure contains a large amount of martensite and bainite, the thermal stability of the hot stamped product deteriorates. Therefore, the area ratio of martensite and bainite is set to less than 20.0%.
- the area ratios of martensite and bainite are both preferably less than 10.0%, more preferably less than 5.0%, and even more preferably less than 2.0%.
- the lower limit of the area ratio of martensite and bainite is 0%.
- martensite and bainite have an effect of increasing the strength of the hot stamped product, they may be contained in the metal structure as long as they are within the above range. If the area ratios of martensite and bainite are both less than 0.1%, the effect of the above action cannot be sufficiently obtained. Therefore, when increasing the strength, the lower limit of the area ratio of martensite and bainite is preferably 0.1% or more, and more preferably 0.5% or more.
- martensite includes fresh martensite and tempered martensite.
- Fresh martensite is untempered martensite, and tempered martensite is self-tempering and / or tempered martensite.
- the rest of the metallographic structure may contain pearlite or retained austenite, and may further contain precipitates such as cementite. Since it is not necessary to positively contain pearlite, retained austenite and precipitates, the lower limit of the area ratio of pearlite, retained austenite and precipitates is 0%.
- the area ratio of pearlite is preferably 1.0% or more, preferably 2.0% or more. More preferably, it is more preferably 5.0% or more.
- the area ratio of pearlite is preferably 20.0% or less, and more preferably 10.0% or less.
- Residual austenite has the effect of improving the shock absorption of hot stamped articles. Therefore, when this effect is obtained, the area ratio of retained austenite is preferably 0.5% or more, and more preferably 1.0% or more. On the other hand, if the retained austenite is excessively contained, the toughness after hot stamping decreases. Therefore, the area ratio of retained austenite is preferably 5.0% or less, and more preferably 3.0% or less.
- the area ratio of each metal structure is calculated as follows. First, a test piece is collected from a hot stamped product, and the sheet thickness section (vertical section of the steel sheet) is polished. Then, in the case of a non-plated steel sheet, the depth position is 1/4 of the sheet thickness of the steel sheet (steel plate). From the surface to the depth of 1/8 of the plate thickness to the region of the steel plate surface to the depth of 3/8 of the plate thickness), in the case of a plated steel plate, from the boundary between the base steel plate and the plating layer, the base steel plate Structure at 1/4 depth position of plate thickness (region from 1/8 depth of the plate thickness of the base steel plate from the boundary to 3/8 depth of the plate thickness of the base steel plate from the boundary) Observe.
- the hot stamped product has a portion having a tensile strength of less than 700 MPa and a portion having a tensile strength of 700 MPa or more, a test piece is collected from the portion where the tensile strength is less than 700 MPa and observed. I do.
- the structure is observed using an optical microscope and a scanning electron microscope (SEM), and the obtained tissue photograph is image-analyzed. , Ferrite, pearlite, baynite, and tempered martensite, respectively. Then, after repeller corrosion at the same observation position, tissue observation is performed using an optical microscope and a scanning electron microscope (SEM), and image analysis is performed on the obtained tissue photograph to obtain retained austenite and Calculate the total area ratio of fresh martensite.
- SEM scanning electron microscope
- the area ratio of retained austenite is measured using an SEM equipped with an electron backscatter pattern analyzer (EBSP).
- EBSP electron backscatter pattern analyzer
- Tempering martensite can be distinguished from fresh martensite by the presence of iron carbide inside, and can be distinguished from bainite by the fact that iron carbide existing inside extends in multiple directions. be able to.
- the area ratio of martensite is obtained by calculating the total area ratio of fresh martensite and tempered martensite.
- All or part of the hot stamped article according to this embodiment has a tensile strength of less than 700 MPa. That is, the tensile strength of all or part of the base steel plate of the hot stamped product according to the present embodiment is less than 700 MPa. This is because if the tensile strength of all the hot stamped products is 700 MPa or more, the thermal stability of the hot stamped products cannot be ensured.
- the tensile strength of all or part of the hot stamped article is less than 600 MPa or less than 560 MPa.
- the tensile strength of the hot stamped product is 300 MPa or more, 340 MPa or more, 390 MPa or more, 440 MPa or more, 460 MPa or more, or 490 MPa or more.
- the yield ratio (yield ratio of the steel sheet after hot stamping provided in the hot stamped product) in the portion where the tensile strength of the hot stamped product is less than 700 MPa. Is preferably 0.65 or more, and more preferably 0.67 or more. The yield ratio is obtained by dividing the yield stress by the tensile strength (yield stress / tensile strength). The yield stress is set to the upper yield point when the steel sheet after hot stamping yields discontinuously, and 0.2% proof stress when the steel sheet yields continuously.
- a soft portion having a tensile strength of less than 700 MPa and a hard portion having a tensile strength of 700 MPa or more may coexist in the hot stamp molded product.
- portions having different strengths it is possible to control the deformed state of the hot stamped molded product at the time of collision, and it is possible to improve the impact absorption of the hot stamped molded product.
- a hot stamped product having parts having different strengths can be produced by joining two or more types of steel sheets having different chemical compositions and then hot stamping.
- the hot stamp molded product according to the present embodiment has a decrease in tensile strength ( ⁇ TS) of 100 MPa or less with respect to the tensile strength before the heat treatment when the heat treatment is performed at 170 ° C. for 20 minutes.
- ⁇ TS is preferably 60 MPa or less, and more preferably 30 MPa or less.
- the lower limit of ⁇ TS is not particularly limited, but in order to significantly reduce ⁇ TS, it is necessary to excessively increase the Mn content and Cr content in the iron carbide in the steel sheet for hot stamping described later, and the steel sheet can be manufactured easily. It is impaired. Therefore, ⁇ TS is preferably 1 MPa or more, 5 MPa or more, or 10 MPa or more.
- the reason why the strength of hot stamped products having a structure mainly composed of ferrite (more than 60.0% in area ratio) decreases during coating baking is that the fine iron carbides or fine particles present in ferrite due to the coating baking treatment. It is considered that this is due to the fact that the iron carbon clusters are changed to coarse iron carbides and that the solid-dissolved carbon in ferrite is precipitated as coarse iron carbides. Although it is not easy to directly and quantitatively evaluate the existence state of such iron carbides, iron carbon clusters, and solid solution carbon, the tensile strength when heat-treated at 170 ° C. for 20 minutes It can be evaluated indirectly by the amount of decrease ( ⁇ TS). When ⁇ TS is 100 MPa or less, the formation of fine iron carbides or fine iron carbon clusters in ferrite and the increase of solid solution carbon are suppressed, and it can be judged that the thermal stability is excellent.
- Tensile strength is obtained by collecting a JIS13B tensile test piece and performing a tensile test at a tensile speed of 10 mm / min.
- the hot stamp molded product according to the present embodiment may have a plating layer on the surface.
- a plating layer By providing a plating layer on the surface, it is possible to prevent scale formation during hot stamping and further improve the corrosion resistance of the hot stamped molded product.
- the type of plating is not particularly limited as long as it is suitable for the above purpose.
- a hot stamped product having a plated layer can be obtained by hot stamping using a plated steel sheet, as will be described later.
- Examples of hot stamped products having a plating layer include zinc-based galvanized steel sheets or aluminum-based plated steel sheets, specifically, for example, hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, hot-dip aluminum-plated steel sheets, and hot-dip Zn-Al alloy plating.
- An example is a hot stamped product having an aluminum-based plating layer.
- the plating layer may be formed on one side or both sides.
- the hot stamping steel sheet Since the chemical composition is not substantially changed by hot stamping, the hot stamping steel sheet has the same chemical composition as the hot stamping molded product described above.
- the metal structure of the steel plate for hot stamping according to the present embodiment contains iron carbide, and the chemical composition of the iron carbide (Mn content and Cr content in the iron carbide) satisfies the following formula (i).
- the meaning of each symbol in the above formula is as follows.
- the chemical composition of the iron carbide contained in the metal structure of the steel sheet for hot stamping satisfies the above formula (i)
- the lvalue of the above equation (i) is 0.8 or less, the thermal stability of the hot stamped product cannot be ensured even if the manufacturing conditions of the hot stamped product are adjusted as described later. ..
- the lvalue of the above equation (i) is preferably more than 1.0, more preferably more than 1.2, and even more preferably more than 1.4.
- the lvalue of the above equation (i) is preferably less than 5.0, more preferably less than 4.0.
- the chemical composition of iron carbide is measured by the following procedure. First, a test piece is taken from an arbitrary position of the steel plate, and a plate thickness cross section (longitudinal cross section) parallel to the rolling direction of the steel plate is polished, and then a 1/4 depth position from the steel plate surface (from the steel plate surface to the plate). Precipitates are extracted by the replica method at a depth of 1/8 of the thickness to a region of 3/8 of the thickness from the surface of the steel sheet). This precipitate is observed using a transmission electron microscope (TEM), and the precipitate is identified and composed by electron diffraction and energy dispersive X-ray analysis (EDS).
- TEM transmission electron microscope
- EDS energy dispersive X-ray analysis
- the iron carbide includes cementite that exists isolated in the metal structure in addition to cementite that constitutes pearlite.
- a depth position of 1/4 of the plate thickness from the surface of the steel plate (1/8 depth from the surface of the steel plate to 3/8 depth from the surface of the steel plate to the plate thickness).
- the depth position is 1/4 of the plate thickness of the base steel plate from the boundary between the base steel plate and the plating layer (1 of the plate thickness of the base steel plate from the above boundary).
- the above-mentioned metal structure is defined in the region (from / 8 depth to 3/4 depth of the thickness of the steel plate as the base material from the boundary).
- the area ratio of iron carbide does not need to be specified, but the area ratio of iron carbide is preferably 1% or more in order to finely granulate the metal structure after hot stamping and increase the tensile strength. Is more preferable. On the other hand, if the area ratio of the iron carbide becomes excessive, the tensile strength of the steel sheet after hot stamping becomes too high and the thermal stability is impaired. Therefore, the area ratio of iron carbide is preferably 20% or less, and more preferably 15% or less.
- the metallographic structure of the steel plate for hot stamping according to the present embodiment is mainly ferrite, but may contain martensite (including fresh martensite and tempered martensite), bainite and retained austenite as a balance, and further. , Precipitates other than iron carbide may be contained. However, martensite, bainite and retained austenite deteriorate the toughness after hot stamping, so a smaller area ratio of these tissues is preferable.
- the area ratios of martensite (including fresh martensite and tempered martensite), bainite and retained austenite are all preferably less than 1.0%, more preferably less than 0.5%.
- the area ratio of the hot stamped steel sheet in the metal structure can be obtained by the same method as in the case of the hot stamped product.
- the tensile strength of the steel sheet for hot stamping is not particularly limited, but it is preferably 300 MPa or more or 340 MPa or more from the viewpoint of steel sheet manufacturability, and 650 MPa or less or less than 590 MPa from the viewpoint of steel sheet cutability. preferable.
- the method for producing a hot stamping molded product according to the present embodiment includes a heating step of heating a hot stamping steel sheet having the above-mentioned chemical composition and metal structure, and a hot stamping process of hot stamping the heated hot stamping steel sheet. Including the process.
- cooling and molding are performed by a mold to obtain a hot stamped product.
- the heating temperature T (° C.) is set to more than 3 points of Ac.
- the Ac 3 points are the temperatures at which ferrite disappears in the metal structure when the material steel sheet is heated, and can be obtained from the thermal expansion change of the steel sheet in the heating step.
- the heating temperature T (° C.) is 3 points or less, the formation of martensite or bainite is promoted in the metal structure of the hot stamped product, and the amount of solid solution carbon in ferrite increases, so that the hot stamped product has a heating temperature of 3 points or less. Thermal stability deteriorates.
- the heating temperature T (° C.) is preferably (Ac 3 points + 50) ° C. or higher, and more preferably (Ac 3 points + 100) ° C. or higher.
- the steel sheet for hot stamping to be heated preferably has the above-mentioned structure.
- the upper limit of the heating temperature T (° C.) is not particularly limited, but if the heating temperature T (° C.) is too high, the austenite becomes coarse and the strength of the hot stamped product decreases. Therefore, the heating temperature T (° C.) is preferably 1100 ° C. or lower, more preferably 1000 ° C. or lower, and even more preferably 950 ° C. or lower.
- the holding time of the preferable hot stamp at the heating temperature T (° C.) is 1 to 5 minutes.
- the start temperature of hot stamping is set to (T-80) ° C. or higher.
- T is the heating temperature T (° C.). If the starting temperature of the hot stamp is less than (T-80) ° C., the amount of solid solution carbon in the ferrite increases in the metal structure of the hot stamped product, and the thermal stability of the molded product deteriorates.
- the starting temperature of the hot stamp is preferably (T-50) ° C. or higher.
- the starting temperature of the hot stamp is (T-80) ° C. or higher and It is preferable that Ar is more than 3 points.
- the Ar 3 points are the temperatures at which ferrite begins to form in the metal structure when the material steel sheet is cooled, and are obtained from the thermal expansion change when the steel sheet is cooled after the heating step.
- another method for producing the hot stamped product according to the present embodiment includes a joining step of joining a steel sheet having the above-mentioned chemical composition and metal structure (hot stamping steel sheet) to a joining steel sheet to form a joined steel sheet. , A heating step of heating the above-mentioned joined steel sheet, and then a hot stamping step of hot-stamping the above-mentioned heated joined steel sheet.
- the joining method include a method in which a hot stamping steel plate and a joining steel plate are butted or overlapped and joined by welding.
- the heating temperature T (° C.) of the bonded steel sheet is set to more than 3 points of Ac of the hot stamping steel sheet, and the starting temperature of hot stamping is set to (T-80) ° C. or higher.
- the preferable heating temperature T (° C.) is (Ac 3 points + 50) ° C. or higher of the hot stamping steel sheet
- the more preferable heating temperature T (° C.) is (Ac 3 points + 100) ° C. of the hot stamping steel sheet. That is all.
- the heating temperature T (° C.) is preferably 1100 ° C. or lower, more preferably 1000 ° C. or lower, and even more preferably 950 ° C. or lower.
- the preferred starting temperature for hot stamping is (T-50) ° C. or higher, and the more preferred starting temperature is (T-80) ° C. or higher and Ar 3 points or higher.
- the holding time of the preferable hot stamp at the heating temperature T (° C.) is 1 to 5 minutes.
- the chemical composition and mechanical properties of the steel sheet for joining are not particularly limited.
- the bonding steel sheet preferably has a tensile strength of 700 MPa or more after hot stamping.
- a more preferable tensile strength of the steel sheet for joining after hot stamping is more than 1000 MPa, more than 1200 MPa, or more than 1500 MPa.
- the C content of the joining steel sheet is preferably 0.090% or more. More preferably, it is 0.100% or more, 0.120% or more, or 0.200% or more.
- the Mn content of the joining steel sheet is preferably 0.50% or more. More preferably, it is 0.80% or more, 1.00% or more, or 1.20% or more.
- the steel sheet used as the above material is subjected to hot-rolled sheet annealing as described later, and after hot-rolled sheet annealing, further cold rolling is performed. After the cold rolling, optionally further continuous annealing may be performed.
- the steel sheet for joining a hot-rolled steel sheet, a cold-rolled steel sheet obtained by cold-rolling a hot-rolled steel sheet, a hot-rolled annealed steel sheet obtained by annealing a hot-rolled steel sheet, and a cold-rolled steel sheet obtained by quenching a cold-rolled steel sheet. Any of the hardened steel sheets may be used.
- a plated steel plate having a plated surface may be used as the hot stamping steel plate and the joining steel plate.
- the type of the plated steel sheet is not particularly limited, but is a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, a hot-dip aluminum-plated steel sheet, a hot-dip Zn-Al alloy plated steel sheet, a hot-dip Zn-Al-Mg alloy-plated steel sheet, and a hot-dip Zn-Al-. Examples thereof include Mg—Si alloy plated steel sheets, electrogalvanized steel sheets, and electric Ni—Zn alloy plated steel sheets.
- a slab having the above-mentioned chemical composition is hot-rolled and then wound in a temperature range of 800 ° C. or lower to obtain a hot-rolled steel sheet.
- a rolling step, a hot-rolled sheet annealing step of heating the hot-rolled steel sheet to a temperature range exceeding 650 ° C. to obtain a hot-rolled annealed steel sheet, and a cold-rolling of the hot-rolled steel sheet. Includes a cold rolling process.
- the take-up temperature after hot rolling is set to 800 ° C. or lower.
- the take-up temperature is preferably less than 650 ° C, more preferably less than 600 ° C, still more preferably less than 550 ° C. Further, if the winding temperature is too low, the hot-rolled steel sheet becomes hard and cold rolling becomes difficult. Therefore, the winding temperature is preferably 400 ° C. or higher.
- the slab manufacturing method used in the hot stamping steel sheet manufacturing method according to the present embodiment is not particularly limited.
- the steel having the above-mentioned chemical composition (component composition) is melted by a known means and then made into a steel ingot by a continuous casting method, or an arbitrary casting method. It is made into a steel piece by a method such as ingot rolling after making it into a steel ingot.
- the continuous casting step it is preferable to generate an external additional flow such as electromagnetic agitation in the molten steel in the mold in order to suppress the occurrence of surface defects caused by inclusions.
- the ingot or piece of steel may be reheated once cooled and subjected to hot rolling, and the ingot in a high temperature state after continuous casting or the piece of steel in a high temperature state after ingot rolling may be used as it is. Alternatively, it may be kept warm or supplementarily heated for hot rolling. In the present embodiment, such ingots and steel pieces are collectively referred to as "slabs" as materials for hot rolling.
- the temperature of the slab to be subjected to hot rolling is preferably less than 1250 ° C., more preferably less than 1200 ° C. in order to prevent coarsening of austenite.
- the hot rolling is preferably completed in a temperature range of Ar 3 points or more in order to miniaturize the metal structure of the hot-rolled steel sheet by transforming austenite after the rolling is completed.
- the rough rolled material may be heated between rough rolling and finish rolling in order to complete the finish rolling at the above temperature. At this time, it is desirable to suppress the temperature fluctuation over the entire length of the rough-rolled material at the start of finish rolling to 140 ° C. or lower by heating the rear end of the rough-rolled material so that the temperature is higher than that of the tip. As a result, the uniformity of product characteristics in the coil after the winding process is improved.
- the method for heating the rough-rolled material may be performed by using a known means.
- a solenoid type induction heating device is provided between the rough rolling mill and the finish rolling mill, and the heating temperature is controlled based on the temperature distribution in the longitudinal direction of the rough rolled material on the upstream side of the induction heating device. You may.
- the hot-rolled and wound steel sheet is annealed after being degreased or the like according to a known method, if necessary.
- the annealing applied to the hot-rolled steel sheet is called hot-rolled sheet annealing, and the steel sheet after hot-rolled sheet annealing is called hot-rolled annealed steel sheet.
- descaling may be performed by pickling or the like.
- the heating temperature in the hot-rolled plate annealing step is over 650 ° C. This is to increase the Mn content and Cr content in the iron carbide in the metal structure of the hot-rolled annealed steel sheet. When the heating temperature is 650 ° C. or lower, the Mn content and Cr content in the iron carbide do not satisfy the above formula (i), and the thermal stability of the hot stamped product is not ensured.
- the heating temperature in the hot-rolled plate annealing step is preferably more than 680 ° C, more preferably more than 700 ° C.
- the heating temperature in the hot-rolled sheet annealing step is preferably less than 750 ° C, more preferably less than 720 ° C.
- the holding time at the heating temperature in the hot-rolled plate annealing step is preferably less than 10 hours, more preferably less than 5 hours, and even more preferably less than 2 hours.
- the hot-rolled annealed steel sheet is cold-rolled to obtain a cold-rolled steel sheet. This is to refine the metal structure after hot stamping and increase the tensile strength.
- the thickness of the cold-rolled steel sheet is preferably 2.8 mm or less, more preferably 2.0 mm or less, further preferably 1.8 mm or less, and 1 It is particularly preferably 1.6 mm or less. Further, from the viewpoint of steel sheet manufacturability, the thickness of the cold-rolled steel sheet is preferably 0.6 mm or more.
- Cold rolling may be performed according to a conventional method, and descaling may be performed by pickling or the like before cold rolling.
- the cold rolling ratio (cumulative rolling reduction in cold rolling) is preferably 30% or more, and more preferably 40% or more. If the cold pressure ratio is too high, the toughness after hot stamping deteriorates. Therefore, the cold pressure ratio is preferably 65% or less, and more preferably 60% or less. As will be described later, when continuous annealing is performed after cold rolling, the cold pressure ratio is preferably 60% or more, more preferably 70% or more in order to refine the metal structure of the annealed steel sheet. ..
- the cold-rolled steel sheet may be continuously annealed to obtain an annealed steel sheet.
- the continuous annealing may be carried out according to a conventional method, and a treatment such as degreasing may be performed by a known method before the continuous annealing.
- the soaking temperature in continuous annealing is preferably 600 ° C. or higher, 650 ° C. or higher, or 700 ° C. or higher.
- the average heating rate to the soaking temperature in continuous annealing is preferably 1 ° C./sec or more, the soaking temperature is preferably 800 ° C. or lower, or 760 ° C. or lower, and the soaking time is 300 seconds. It is preferably less than, or less than 120 seconds.
- the cold-rolled steel sheet and the annealed steel sheet thus obtained may be temper-rolled according to a conventional method.
- the steel sheet for hot stamping may be provided with a plating layer on the surface layer for the purpose of preventing scale formation during hot stamping and improving the corrosion resistance of the steel sheet after hot stamping.
- the type of plating is not particularly limited as long as it meets the above-mentioned purpose, but is not particularly limited, but is limited to hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, hot-dip aluminum-plated steel sheet, hot-dip Zn-Al alloy-plated steel sheet, and hot-dip Zn-Al-. Examples thereof include Mg alloy plated steel sheets, molten Zn—Al—Mg—Si alloy plated steel sheets, electrogalvanized steel sheets, and electric Ni—Zn alloy plated steel sheets.
- the cold-rolled steel sheet or annealed steel sheet produced by the above method may be used as a material steel sheet and plated according to a conventional method.
- the soaking temperature in the annealing process of continuous hot-dip galvanizing is set to 600 ° C. or higher, 650 ° C. or higher, or 700 ° C. or higher in order to refine the metal structure of the plated steel sheet by recrystallization. Is preferable.
- the soaking temperature in the annealing process of continuous hot dip galvanizing is 800 ° C. or lower or 760 ° C. The following is preferable. After hot dip galvanizing, the steel sheet may be reheated for alloying treatment.
- a cold-rolled steel sheet or an annealed steel sheet manufactured by the above-mentioned method is used as a material steel sheet, and if necessary, a well-known pretreatment for surface cleaning and adjustment is performed, and then the steel sheet is usually manufactured. Electroplating may be performed according to the method.
- the plated steel sheet thus obtained may be temper-rolled according to a conventional method.
- Example 1 Molten steel was cast using a vacuum melting furnace to produce steels A to O having the chemical compositions shown in Table 1.
- the points Ac 1 and Ac 3 in Table 1 were determined from the changes in thermal expansion when the cold-rolled steel sheets of steels A to O were heated at 2 ° C./sec.
- the Ar 3 points in Table 1 were obtained from the change in thermal expansion when the cold-rolled steel sheets of steels A to O were heated to 920 ° C. and then cooled at 10 ° C./sec.
- Steels A to O were heated to 1200 ° C. and held for 60 minutes, and then hot rolled under the hot rolling conditions shown in Table 2. Specifically, steels A to O were rolled for 10 passes in a temperature range of 3 points or more in Ar to obtain a hot-rolled steel sheet having a thickness of 3.6 mm. After hot rolling, the hot-rolled steel sheet is cooled to 540 to 580 ° C. with a water spray, the cooling end temperature is set as the winding temperature, and the hot-rolled steel sheet is charged into an electric heating furnace maintained at this winding temperature. After holding for 60 minutes, the hot-rolled steel sheet was furnace-cooled to room temperature at an average cooling rate of 20 ° C./hour to simulate slow cooling after winding.
- hot-rolled steel sheets were annealed. Specifically, the hot-rolled steel sheet is heated to 620 to 710 ° C. at an average heating rate of 50 ° C./hour using an electric heating furnace, held for 1 to 12 hours, and then cooled at an average temperature of 20 ° C./hour. It was cooled at a rate to obtain a hot-spread annealed steel sheet.
- the hot-rolled steel sheet and the hot-rolled annealed steel sheet were pickled to form a base material for cold rolling, and cold-rolled at a cold pressure ratio of 61% to obtain a cold-rolled steel sheet having a thickness of 1.4 mm.
- Some cold-rolled steel sheets were heated to the annealing temperature of table 2 at an average heating rate of 10 ° C./sec using a continuous annealing simulator, and the heat was equalized for 60 seconds. Subsequently, it was cooled to 400 ° C. and held for 180 seconds, and then cooled to room temperature to obtain an annealed steel sheet.
- the obtained annealed steel sheet was described as "ACR” in the "Steel type” column and "-” in the "Plating type” column in Table 3.
- For cold-rolled steel sheets "CR” was described in the "Steel type” column and "-” was described in the "Plating type” column in Table 3.
- the structure is obtained from the cold-rolled steel sheet, the tempered steel sheet, the hot-dip galvanized steel sheet, the alloyed hot-dip galvanized steel sheet, and the hot-dip aluminum-plated steel sheet (these steel sheets are collectively called the hot stamping steel sheet) obtained in this manner.
- a test piece for observation was collected and the tissue was observed.
- the precipitate was extracted and the iron carbide was identified using TEM.
- a base plate for hot stamping having a width of 240 mm and a length of 170 mm was collected, and a hat member having a shape shown in FIG. 1 was manufactured by hot stamping.
- the base plate was heated at the heating temperature shown in Table 4 for 4 minutes using a gas heating furnace, then taken out from the heating furnace and allowed to cool, and a cooling device was provided at the starting temperature shown in Table 4. It was sandwiched between molds and hat-molded.
- Some hat members were heat-treated using an electric heating furnace at 170 ° C. for 20 minutes, which is equivalent to a paint baking process.
- a test piece for SEM observation is taken from the bottom of the punch of the hat member before heat treatment, and a sheet thickness section parallel to the rolling direction of the steel plate of this test piece is polished, and then nightal corrosion and repeller corrosion are performed on this sheet thickness cross section.
- the position from the surface of the steel sheet to the depth of 1/4 of the thickness of the steel sheet (the region from the surface of the steel sheet to the depth of 1/8 to the surface of the steel sheet to the depth of 3/8 of the thickness).
- the depth position is 1/4 of the thickness of the base steel plate from the boundary between the base steel plate and the plating layer (from the above boundary to 1/8 depth of the base steel plate thickness).
- a JIS13B tensile test piece is collected from the punch bottom of the hat member before heat treatment along the longitudinal direction of the member, and a tensile test is performed at a tensile speed of 10 mm / min to determine the tensile strength, yield stress and yield ratio. I asked.
- the yield stress was set to the upper yield point in the case of discontinuous yield, and 0.2% proof stress in the case of continuous yield.
- a JIS13B tensile test piece was similarly collected from the punch bottom of the hat member after the heat treatment, and a tensile test was conducted in the same manner to determine the tensile strength.
- the difference ( ⁇ TS) between the tensile strength of the hat member that has not been heat-treated and the tensile strength of the hat member that has been heat-treated is determined. If ⁇ TS is 100 MPa or less, the thermal stability of the hat member is good. It was judged.
- Table 4 shows the results of observing the metallographic structure of the hat member and the results of evaluating the mechanical properties of the hat member.
- underlined values mean outside the scope of the present invention.
- the tensile strength of the hot stamped product is less than 700 MPa.
- ⁇ TS is 100 MPa or less, showing good thermal stability.
- Test numbers 1, 3, 4, 6, 9, 11, 14, 16, 22 to 27, 31 and 33 which have a Cr content of less than 0.30% in the chemical composition of the hot stamped article, are all hot.
- the yield ratio of the stamped product was 0.65 or more, and the strength characteristics were particularly good.
- test numbers 1, 3, 4, 9, 11, 14, 16, 22 to 27, 31 and 33 which have a holding time at the heating temperature of less than 10 hours, are all hot.
- the tensile strength of the stamped product was 440 MPa or more, and the strength characteristics were particularly good.
- the tensile strength of the hot stamped product is 700 MPa or more and the ⁇ TS is 100 MPa or more, or ⁇ TS was 100 MPa or more, and the thermal stability was inferior.
- ⁇ TS was large because the Mn content of the steel was too high.
- test number 28 using steel M since the Cr content of the steel was too high, the ferrite area ratio was insufficient in the metallographic structure of the hot stamped product and the martensite area ratio was excessive, resulting in (heat treatment) of the hot stamped product.
- the tensile strength (previous) was 700 MPa or more and ⁇ TS was large.
- the chemical composition is within the scope of the present invention, but the method for producing the metal structure of the hot stamping steel sheet or the hot stamped product is outside the scope of the present invention.
- Test Nos. 2, 5, 7, 8, 10, 12 of Comparative Examples , 13, 15, 17, 18, 29, 30 and 32 had a ⁇ TS of 100 MPa or more in the hot stamped product and were inferior in thermal stability.
- test number 8 using steel A test number 13 using steel B
- test number 18 using steel C test number 29 using steel A
- test number 29 using steel B test number using steel B.
- the hot-rolled sheet was not annealed, the sum of the Mn content and the Cr content in the iron carbide was low in the metal structure of the hot stamping steel sheet, and ⁇ TS was large in the hot stamping molded product.
- Test number 2 using steel A, test number 10 using steel B, test number 15 using steel C, and test number 32 using steel N have low hot stamping start temperatures in the hot stamping process. Because it was too much, ⁇ TS was large in the hot stamped product.
- Test No. 5 using steel A, test number 12 using steel B, and test number 17 using steel C had a large ⁇ TS in the hot stamped product because the heating temperature in the heating step was too low. ..
- Example 2 Molten steel was cast using a vacuum melting furnace to produce steels A to C having the chemical compositions shown in Table 1 in Example 1. Using steels A to C, hot rolling, hot rolling plate annealing, cold rolling, and annealing are performed under the conditions shown in Table 5 in the same manner as in Example 1, and then plating treatment is performed to obtain a hot-dip zinc-plated steel sheet. , An alloyed hot-dip zinc-plated steel sheet and a hot-stamped steel sheet (steel for hot stamping) were manufactured.
- Table 6 shows the results of observing the metallographic structure of the hot stamping steel sheet and the results of investigating the mechanical properties of the hot stamping steel sheet.
- hot stamping base plates having a thickness of 1.4 mm, a width of 240 mm, and a length of 170 mm were collected.
- This base plate was joined to a steel plate for joining having the same dimensions by laser welding to prepare a joined steel plate having a thickness of 1.4 mm, a width of 240 mm, and a length of 340 mm.
- the steel sheet for joining has a chemical composition of 0.21% C-0.13% Si-1.31% Mn-0.012% P-0.0018% S-0.043% sol.
- a cold-rolled steel sheet having Al-0.0030% N-0.21% Cr-0.0018% B was used.
- the joined steel plate was hot stamped under the conditions shown in Table 7 in the same manner as in Example 1, and a hat member having the shape shown in FIG. 2 was manufactured. Then, some of the hat members were heat-treated using an electric heating furnace at 170 ° C. for 20 minutes, which corresponds to a coating baking treatment.
- Example 7 shows the results of observing the metallographic structure of the hat member (hot stamp molded product) and the results of evaluating the mechanical properties of the hat member.
- the TS (before heat treatment) of the hot stamped product was less than 700 MPa, and the ⁇ TS was 100 MPa or less, showing good thermal stability.
- the metal structure of the steel plate portion for joining the hat member was a single structure of martensite, and the tensile strengths were 1580 MPa, 1583 MPa, and 1575 MPa, respectively, with respect to test numbers 34 to 36.
- a hot stamped product having a small variation in strength due to a coating baking process, having a portion having a tensile strength of less than 700 MPa, and having excellent thermal stability, and a hot stamping product suitable as a material thereof. It becomes possible to obtain steel sheets and their manufacturing methods.
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Abstract
Description
本願は、2019年4月1日に、日本に出願された特願2019-070211号に基づき優先権を主張し、その内容をここに援用する。
特許文献3には、鋼板を炉加熱する際に鋼板の一部に断熱材を装着し、部分的に加熱温度を低下させて軟質化する方法が開示されている。
特許文献6には、二枚の素板を溶接して連結させた、いわゆるテーラードブランク材を用いてホットスタンプする技術が開示されている。
また、特許文献8には、C含有量を低めに制限するとともにTiを含有させ、マルテンサイトの生成量を制御した、引張強さが500MPa以上のホットスタンプ部材およびその製造方法が開示されている。
(2)前記化学組成が、質量%で、Ti:0.001~0.300%、Nb:0.001~0.300%、V:0.001~0.300%、Zr:0.001~0.300%、Mo:0.001~2.00%、Cu:0.001~2.00%、Ni:0.001~2.00%、B:0.0001~0.0200%、Ca:0.0001~0.0100%、Mg:0.0001~0.0100%、REM:0.0001~0.1000%、および、Bi:0.0001~0.0500%からなる群から選択される1種または2種以上を含有する上記(1)に記載のホットスタンプ成形品。
(3)前記化学組成が、質量%で、Mn:0.01%以上、0.30%未満、を含有する上記(1)または(2)に記載のホットスタンプ成形品。
(4)表面にめっき層を有する、上記(1)~(3)のいずれかに記載のホットスタンプ成形品。
(5)化学組成が、質量%で、C:0.001%以上、0.090%未満、Si:2.50%以下、Mn:0.01%以上、0.50%未満、P:0.200%以下、S:0.0200%以下、sol.Al:0.001~2.500%、N:0.0200%以下、Cr:0.01%以上、2.00%未満、Ti:0~0.300%、Nb:0~0.300%、V:0~0.300%、Zr:0~0.300%、Mo:0~2.00%、Cu:0~2.00%、Ni:0~2.00%、B:0~0.0200%、Ca:0~0.0100%、Mg:0~0.0100%、REM:0~0.1000%、Bi:0~0.0500%、並びに、残部:Feおよび不純物であり、金属組織が、鉄炭化物を含み、前記鉄炭化物中のMn含有量及びCr含有量が下記(i)式を満足する、ホットスタンプ用鋼板。
[Mn]θ+[Cr]θ>0.8 ・・・(i)
但し、上記式中の各記号の意味は以下のとおりである。
[Mn]θ:前記鉄炭化物に含まれるFe、MnおよびCrの合計含有量を100原子%としたときの、原子%での前記鉄炭化物中のMn含有量
[Cr]θ:前記鉄炭化物に含まれるFe、MnおよびCrの合計含有量を100原子%としたときの、原子%での前記鉄炭化物中のCr含有量
(6)前記化学組成が、質量%で、Ti:0.001~0.300%、Nb:0.001~0.300%、V:0.001~0.300%、Zr:0.001~0.300%、Mo:0.001~2.00%、Cu:0.001~2.00%、Ni:0.001~2.00%、B:0.0001~0.0200%、Ca:0.0001~0.0100%、Mg:0.0001~0.0100%、REM:0.0001~0.1000%、および、Bi:0.0001~0.0500%からなる群から選択される1種または2種以上を含有する、上記(5)に記載のホットスタンプ用鋼板。
(7)前記化学組成が、質量%で、Mn:0.01%以上、0.30%未満、を含有する上記(5)または(6)に記載のホットスタンプ用鋼板。
(8)表面にめっき層を有する、上記(5)~(7)のいずれかに記載のホットスタンプ用鋼板。
(9)上記(1)~(3)のいずれかに記載のホットスタンプ成形品を製造する方法であって、(5)~(7)のいずれかに記載のホットスタンプ用鋼板を、Ac3点を超える加熱温度T℃まで加熱する加熱工程と、前記加熱工程後の前記ホットスタンプ用鋼板に対して、(T-80)℃以上の温度でホットスタンプを開始するホットスタンプ工程と、を備える、ホットスタンプ成形品の製造方法。
(10)上記(1)~(3)のいずれかに記載のホットスタンプ成形品を製造する方法であって、(5)~(7)のいずれかに記載のホットスタンプ用鋼板を、接合用鋼板と接合して接合鋼板とする接合工程と、前記接合工程後の接合鋼板を前記ホットスタンプ用鋼板のAc3点を超える加熱温度T℃まで加熱する加熱工程と、前記加熱工程後の前記接合鋼板に対して、(T-80)℃以上の温度でホットスタンプを開始するホットスタンプ工程と、を備える、ホットスタンプ成形品の製造方法。
(11)上記(4)に記載のホットスタンプ成形品を製造する方法であって、(8)に記載のホットスタンプ用鋼板を、Ac3点を超える加熱温度T℃まで加熱する加熱工程と、前記加熱工程後の前記ホットスタンプ用鋼板に対して、(T-80)℃以上の温度でホットスタンプを開始するホットスタンプ工程と、を備える、ホットスタンプ成形品の製造方法。
(12)上記(4)に記載のホットスタンプ成形品を製造する方法であって、(8)に記載のホットスタンプ用鋼板を、接合用鋼板と接合して接合鋼板とする接合工程と、前記接合工程後の接合鋼板を前記ホットスタンプ用鋼板のAc3点を超える加熱温度T℃まで加熱する加熱工程と、前記加熱工程後の前記接合鋼板に対して、(T-80)℃以上の温度でホットスタンプを開始するホットスタンプ工程と、を備える、ホットスタンプ成形品の製造方法。
(13)上記(5)~(8)のいずれかに記載のホットスタンプ用鋼板を製造する方法であって、化学組成が、質量%で、C:0.001%以上、0.090%未満、Si:2.50%以下、Mn:0.01%以上、0.50%未満、P:0.200%以下、S:0.0200%以下、sol.Al:0.001~2.500%、N:0.0200%以下、Cr:0.01%以上、2.00%未満、Ti:0~0.300%、Nb:0~0.300%、V:0~0.300%、Zr:0~0.300%、Mo:0~2.00%、Cu:0~2.00%、Ni:0~2.00%、B:0~0.0200%、Ca:0~0.0100%、Mg:0~0.0100%、REM:0~0.1000%、Bi:0~0.0500%、並びに、残部:Feおよび不純物であるスラブに対して、熱間圧延を施した後、800℃以下の温度域で巻取って熱延鋼板とする熱間圧延工程と、前記熱延鋼板に650℃を超える温度域まで加熱する熱延板焼鈍を施して熱延焼鈍鋼板とする熱延板焼鈍工程と、前記熱延焼鈍鋼板に冷間圧延を施して冷延鋼板とする冷間圧延工程と、を備える、ホットスタンプ用鋼板の製造方法。
(14)さらに、前記冷間圧延工程後の前記冷延鋼板に、任意に、連続焼鈍を行った後、めっきを行うめっき工程を備える、上記(13)に記載のホットスタンプ用鋼板の製造方法。
この理由は明らかではないが、本発明者らは、次の理由に起因すると推測する。(a)ホットスタンプの開始温度が高いと、ホットスタンプ成形品においてフェライト中に含まれる固溶状態の炭素量が低下する。(b)フェライト中の固溶炭素は、塗装焼付時の熱処理により粗大な鉄炭化物として析出し、フェライトの強度が低下する。
本実施形態に係るホットスタンプ成形品の全部または一部は、以下に示す化学組成を有する。各元素の限定理由は下記のとおりである。以下の説明において、化学組成の含有量についての「%」は全て、「質量%」を意味する。ホットスタンプ成形品が、700MPa未満の引張強さを有する部分と、700MPa以上の引張強さを有する部分とを備えている場合、少なくとも引張強さが700MPa未満となる部分が、以下の化学組成を有していればよい。
Cは、ホットスタンプ後の鋼板(ホットスタンプ成形品が備える鋼板)の引張強さを上昇させる効果を有する元素である。C含有量が0.001%未満では、ホットスタンプによる引張強さの上昇が望めない。そのため、C含有量を0.001%以上とする。好ましいC含有量は0.010%以上、0.020%以上、または0.030%以上である。
一方、C含有量が0.090%以上であると、ホットスタンプ後の金属組織においてマルテンサイトおよび/またはベイナイトの面積率が増加して、ホットスタンプ成形品の引張強さが700MPa以上となる。この場合、後述するようにMn含有量およびCr含有量を調整しても、ホットスタンプ成形品の熱的安定性を確保することができなくなる。したがって、C含有量は0.090%未満とする。好ましいC含有量は0.085%未満、0.080%未満、0.070%未満、または0.060%未満である。
Siは、鋼中に不純物として含有される元素である。Si含有量が2.50%を超えると、溶接性が劣化するとともに、変態点が高くなりすぎ、ホットスタンプの加熱過程で変態点以上の温度に鋼板を加熱することが困難となる。そのため、Si含有量は2.50%以下とする。好ましいSi含有量は2.00%以下、1.50%以下、1.00%以下、または0.50%以下である。ホットスタンプ用鋼板としてめっき鋼板を用いる場合は、めっき性を確保するためにSi含有量を0.50%未満とすることが好ましく、0.40%未満とすることがより好ましく、0.30%未満とすることがさらに好ましい。
Si含有量の下限は特に限定しないが、Si含有量を過度に低下させることは製鋼コストの上昇を引き起こす。そのため、Si含有量を0.001%とすることが好ましい。また、Siは、ホットスタンプ後の鋼板の引張強さを高める作用を有するので、積極的に含有させてもよい。高強度化の観点からは、好ましいSi含有量は0.10%以上、0.20%以上、または0.30%以上である。
Mnは、ホットスタンプ成形品の熱的安定性を劣化させる元素である。特に、Mn含有量が0.50%以上であると、ホットスタンプ後の成形品の熱的安定性が著しく劣化する。したがって、Mn含有量は0.50%未満とする。Mn含有量は好ましくは0.40%未満、0.35%未満、0.30%未満、0.25%未満、または0.20%未満である。
一方、Mnは、不純物であるSと結合してMnSを形成し、Sを含有することによる鋼の脆化を抑制する作用を有する元素である。この効果を得るため、Mn含有量は0.01%以上とする。Mn含有量は好ましくは0.05%以上、0.10%以上、または0.15%以上である。
Pは、鋼中に不純物として含有される元素である。P含有量が0.200%を超えると、溶接性およびホットスタンプ後の靭性が著しく劣化するので、P含有量は0.200%以下とする。好ましいP含有量は0.100%以下、0.050%以下、または0.020%以下である。
P含有量の下限は特に限定しないが、P含有量を過度に低下させることは製鋼コストの上昇を引き起こす。そのため、P含有量を0.001%以上とすることが好ましい。また、Pは、ホットスタンプ後の成形品の引張強さを高める作用を有するので、積極的に含有させてもよい。高強度化の観点からは、好ましいP含有量は0.010%以上、0.020%以上、または0.030%以上である。ホットスタンプ用鋼板としてめっき鋼板を用いる場合は、めっき性を確保するためにP含有量を0.050%以下とすることが好ましく、0.040%以下とすることがより好ましい。
Sは、鋼中に不純物として含有され、鋼を脆化させる元素である。そのため、S含有量は少ないほど好ましいが、S含有量が0.0200%を超えると鋼の脆化が顕著となる。そのため、S含有量は0.0200%以下とする。好ましいS含有量は0.0100%以下、0.0050%以下、または0.0030%以下である。
S含有量の下限は特に限定しないが、S含有量を過度に低下させることは製鋼コストの上昇を引き起こす。そのため、S含有量を0.0001%以上とすることが好ましい。
Alは、溶鋼を脱酸する作用を有する元素である。sol.Al含有量(酸可溶Al含有量)が0.001%未満であると脱酸が不十分となる。そのため、sol.Al含有量を0.001%以上とする。sol.Al含有量は好ましくは、0.010%以上、0.020%以上、または0.040%以上である。
一方、sol.Al含有量が高すぎる場合、変態点が上昇し、ホットスタンプの加熱過程で変態点以上の温度に鋼板を加熱することが困難となる。そのため、sol.Al含有量は2.500%以下とする。sol.Al含有量は好ましくは1.000%以下、0.500%以下、0.100%以下、または0.060%以下である。
Nは、鋼中に不純物として含有され、鋼の連続鋳造中に窒化物を形成する元素である。この窒化物はホットスタンプ後の靭性を劣化させるので、N含有量は低い方が好ましい。N含有量が0.0200%超であると、靭性の劣化が顕著となる。そのため、N含有量は0.0200%以下とする。N含有量は好ましくは0.0100%未満、0.0080%未満、または0.0050%未満である。
N含有量の下限は特に限定しないが、N含有量を過度に低下させることは製鋼コストの上昇を引き起こすので、N含有量を0.0010%以上とすることが好ましい。
Crは、フェライトを主体とする金属組織を有するホットスタンプ成形品(ホットスタンプ後の鋼板)の熱的安定性を向上させる作用を有する元素である。Cr含有量が0.01%未満である場合、後述するようにホットスタンプ工程におけるホットスタンプ開始温度を調整しても上記効果が十分に得られない。したがって、Cr含有量は0.01%以上とする。Cr含有量は、好ましくは0.05%以上、0.10%以上、0.15%以上、または0.20%以上である。
一方、Cr含有量が2.00%以上であると、ホットスタンプ成形品の金属組織においてマルテンサイトおよび/またはベイナイトの面積率が過剰となり、ホットスタンプ成形品の熱的安定性が劣化する。したがって、Cr含有量は2.00%未満とする。ホットスタンプ成形品の降伏比を上昇させ、衝撃吸収性を向上させるためには、Cr含有量を0.30%未満とすることが好ましく、0.25%未満とすることがより好ましい。
Nb:0~0.300%
V:0~0.300%
Zr:0~0.300%
Ti、Nb、VおよびZrは金属組織を微細化することで、ホットスタンプ成形品の引張強さを上昇させる作用を有する元素である。この効果を得るために、Ti、Nb、VおよびZrから選択される1種以上を必要に応じて含有させてもよい。これらの元素は含有させなくてもよいため、これらの元素の含有量の下限は0%である。
Nbを含有させる場合には、Nb含有量を0.020%以上とすることがさらに好ましく、0.030%以上とすることが特に好ましい。
Vを含有させる場合には、V含有量を0.020%以上とすることがさらに好ましい。
Zrを含有させる場合には、Zr含有量を0.010%以上とすることがさらに好ましい。
したがって、Ti含有量は好ましくは0.060%未満、さらに好ましくは0.040%未満である。
Nb含有量は好ましくは0.060%未満、さらに好ましくは0.040%未満である。
V含有量は好ましくは0.200%未満、さらに好ましくは0.100%未満である。
Zr含有量は好ましくは0.200%未満、さらに好ましくは0.100%未満である。
Cu:0~2.00%
Ni:0~2.00%
Mo、CuおよびNiは、ホットスタンプ成形品(ホットスタンプ後の鋼板)の引張強さを高める作用を有する。したがって、Mo、CuおよびNiから選択される1種以上を必要に応じて含有させてもよい。これらの元素は含有させなくてもよいため、これらの元素の含有量の下限は0%である。
そのため、上記元素を含有させる場合でも、Mo、CuおよびNiの含有量は、それぞれ2.00%以下とする。好ましいMo含有量は0.50%以下であり、好ましいCu含有量は1.00%以下であり、好ましいNi含有量は1.00%以下である。
Bは、粒界に偏析してホットスタンプ後の鋼板の靭性を向上させる作用を有する元素である。この効果を得るため、必要に応じてBを含有させてもよい。Bは含有させなくてもよいため、B含有量の下限は0%である。
Mg:0~0.0100%
REM:0~0.1000%
Ca、MgおよびREMは、介在物の形状を調整することによりホットスタンプ後の鋼板(ホットスタンプ成形品)の靭性を向上させる作用を有する元素である。そのため、Ca、MgおよびREMから選択される1種以上を必要に応じて含有させてもよい。これらの元素は含有させなくてもよいため、これらの元素の含有量の下限は0%である。
一方、CaもしくはMgの含有量が0.0100%超である場合、またはREMの含有量が0.1000%超である場合、上記効果が飽和して、鋼板の製造コストが上昇する。したがって、上記元素を含有させる場合でも、CaおよびMgの含有量はそれぞれ0.0100%以下とし、REM含有量は0.1000%以下とする。
Biは、凝固組織を微細化することにより、ホットスタンプ後の鋼板(ホットスタンプ成形品)の靭性を向上させる作用を有する元素である。そのため、必要に応じてBiを含有させてもよい。Biは含有させなくてもよいため、Bi含有量の下限は0%である。
一方、Bi含有量が0.0500%を超える場合、上記効果が飽和して、鋼板の製造コストが上昇する。したがって、Biを含有させる場合でも、Bi含有量は0.0500%以下とする。Bi含有量は、好ましくは0.0100%以下であり、より好ましくは0.0050%以下である。
本実施形態に係るホットスタンプ成形品の金属組織(ミクロ組織)について説明する。本実施形態に係るホットスタンプ成形品の全部または一部は、以下に示す量のフェライト、マルテンサイトおよびベイナイトを含む金属組織を有する。金属組織に関する以下の説明において、「%」は、「面積%」を意味する。
フェライトの面積率が60.0%以下であると、ホットスタンプ後の成形品(ホットスタンプ成形品)の引張強さが700MPa以上となり、熱的安定性を確保することができない。そのため、フェライトの面積率を60.0%超とする。フェライトの面積率は、好ましくは70.0%超、さらに好ましくは80.0%超である。
フェライトの面積率の上限は特に定める必要がないが、ホットスタンプ成形品の強度を上昇させるために、98.0%未満とすることが好ましく、96.0%未満とすることがより好ましく、94.0%未満とすることがさらに好ましい。
熱的安定性の観点から、フェライト全体に対するポリゴナルフェライトの割合が、面積率で5.0%以上であることが好ましい。
ベイナイト:0%以上、20.0%未満
金属組織が多量のマルテンサイトおよびベイナイトを含むと、ホットスタンプ成形品の熱的安定性が劣化する。そのため、マルテンサイトおよびベイナイトの面積率はいずれも20.0%未満とする。マルテンサイトおよびベイナイトの面積率は、いずれも10.0%未満とすることが好ましく、5.0%未満とすることがより好ましく、2.0%未満とすることがさらに好ましい。
しかしながら、マルテンサイトおよびベイナイトは、ホットスタンプ成形品の強度を上昇させる作用を有するので、上記範囲内であれば金属組織に含まれていてもよい。マルテンサイトおよびベイナイトの面積率がいずれも0.1%未満であると、上記作用による効果が十分に得られない。そのため、強度を上昇させる場合には、マルテンサイトおよびベイナイトの面積率の下限値を、いずれも0.1%以上とすることが好ましく、0.5%以上とすることがより好ましい。
一方、パーライトを過剰に含有する場合、ホットスタンプ後の靭性が劣化する。そのため、パーライトの面積率を20.0%以下とすることが好ましく、10.0%以下とすることがより好ましい。
一方、残留オーステナイトを過剰に含有すると、ホットスタンプ後の靱性が低下する。そのため、残留オーステナイトの面積率を5.0%以下とすることが好ましく、3.0%以下とすることがより好ましい。
まず、ホットスタンプ成形品から試験片を採取し、板厚断面(鋼板の縦断面)を研磨した後、非めっき鋼板の場合は、鋼板表面から鋼板の板厚の1/4深さ位置(鋼板表面から板厚の1/8深さ~鋼板表面から板厚の3/8深さの領域)、めっき鋼板の場合は、基材の鋼板とめっき層との境界から、基材である鋼板の板厚の1/4深さ位置(上記境界から基材である鋼板の板厚の1/8深さ~上記境界から基材である鋼板の板厚の3/8深さの領域)において組織観察する。ホットスタンプ成形品が、700MPa未満の引張強さを有する部分と、700MPa以上の引張強さを有する部分とを備えている場合、引張強さが700MPa未満となる部分から試験片を採取して観察を行う。
なお、焼戻しマルテンサイトは、内部に鉄炭化物が存在する点でフレッシュマルテンサイトと区別することができ、また、内部に存在する鉄炭化物が複数の方向に伸長している点で、ベイナイトと区別することができる。本実施形態では、フレッシュマルテンサイトおよび焼戻しマルテンサイトの合計の面積率を算出することで、マルテンサイトの面積率を得る。
本実施形態に係るホットスタンプ成形品の全部または一部は、引張強さで700MPa未満である。すなわち、本実施形態に係るホットスタンプ成形品の母材鋼板の全部または一部の引張強さが700MPa未満である。ホットスタンプ成形品の全部において引張強さが700MPa以上であると、ホットスタンプ成形品の熱的安定性を確保することができなくなるためである。好ましくは、ホットスタンプ成形品の全部または一部において、引張強さが、600MPa未満、または560MPa未満である。一方、ホットスタンプ成形品の衝撃吸収性を向上させるためには、ホットスタンプ成形品の引張強さを300MPa以上、340MPa以上、390MPa以上、440MPa以上、460MPa以上、または490MPa以上とすることが好ましい。
本実施形態に係るホットスタンプ成形品は、170℃で20分間の熱処理を施した際の、熱処理前の引張強さに対する引張強さの低下量(ΔTS)が100MPa以下である。ΔTSは、60MPa以下であることが好ましく、30MPa以下であることがより好ましい。ΔTSの下限は特に限定しないが、ΔTSを大きく低下させるためには、後述するホットスタンプ用鋼板において、鉄炭化物中のMn含有量およびCr含有量を過度に高める必要があり、鋼板の製造性が損なわれる。そのため、ΔTSは、1MPa以上、5MPa以上、または10MPa以上であることが好ましい。
本実施形態に係るホットスタンプ成形品は、表面にめっき層を有していてもよい。表面にめっき層を備えることで、ホットスタンプ時におけるスケールの生成を防止し、さらにホットスタンプ成形品の耐食性を向上させることが可能になる。めっきの種類は、前記目的に適うものであればよく、特に限定されない。めっき層を有するホットスタンプ成形品は、後述するように、めっき鋼板を用いてホットスタンプすることにより得ることができる。めっき層を有するホットスタンプ成形品として、亜鉛系めっき鋼板またはアルミニウム系めっき鋼板、具体的には、例えば、溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板、溶融アルミニウムめっき鋼板、溶融Zn-Al合金めっき鋼板、溶融Zn-Al-Mg合金めっき鋼板、溶融Zn-Al-Mg-Si合金めっき鋼板、電気亜鉛めっき鋼板、および電気Ni-Zn合金めっき鋼板等を用いてホットスタンプした、亜鉛系めっき層またはアルミニウム系めっき層を有するホットスタンプ成形品が例示される。めっき層は片面に形成されていてもよく、両面に形成されていてもよい。
ホットスタンプによって化学組成は実質的に変化しないので、ホットスタンプ用鋼板は、上述したホットスタンプ成形品と同じ化学組成を有する。
本実施形態に係るホットスタンプ用鋼板の金属組織は、鉄炭化物を含み、鉄炭化物の化学組成(鉄炭化物中のMn含有量およびCr含有量)が下記(i)式を満足する。
但し、上記式中の各記号の意味は以下のとおりである。
[Mn]θ:鉄炭化物に含まれるFe、MnおよびCrの合計含有量を100原子%としたときの、鉄炭化物中のMn含有量(原子%)
[Cr]θ:鉄炭化物に含まれるFe、MnおよびCrの合計含有量を100原子%としたときの、鉄炭化物中のCr含有量(原子%)
まず、鋼板の任意の位置から試験片を採取し、鋼板の圧延方向に平行な板厚断面(縦断面)を研磨した後、鋼板表面から板厚の1/4深さ位置(鋼板表面から板厚の1/8深さ~鋼板表面から板厚の3/8深さの領域)においてレプリカ法により析出物を抽出する。この析出物を、透過型電子顕微鏡(TEM)を用いて観察し、電子線回折およびエネルギー分散型X線分析(EDS)により析出物の同定および組成分析を行う。
一方、鉄炭化物の面積率が過剰となると、ホットスタンプ後の鋼板の引張強さが高くなりすぎるとともに、熱的安定性が損なわれる。したがって、鉄炭化物の面積率は20%以下とすることが好ましく、15%以下とすることがより好ましい。
ホットスタンプ用鋼板の引張強さは特に限定しないが、鋼板の製造性の観点からは300MPa以上または340MPa以上とすることが好ましく、鋼板の切断性の観点からは650MPa以下または590MPa未満とすることが好ましい。
本実施形態に係るホットスタンプ成形品および本実施形態に係るホットスタンプ用鋼板の好ましい製造方法について説明する。
本実施形態に係るホットスタンプ成形品の製造方法は、上述の化学組成および金属組織を有するホットスタンプ用鋼板を加熱する加熱工程と、加熱されたホットスタンプ用鋼板に対してホットスタンプを行うホットスタンプ工程と、を含む。ホットスタンプ工程では、金型による冷却及び成形が行われ、ホットスタンプ成形品が得られる。
また、加熱に供するホットスタンプ用鋼板は、上記の組織を有していることが好ましい。
また、好ましいホットスタンプの加熱温度T(℃)での保持時間は、1~5分間である。
Ar3点とは、素材鋼板を冷却した際に金属組織中にフェライトが生成し始める温度であり、加熱工程の後、鋼板を冷却する際の熱膨張変化から求められる。
好ましいホットスタンプの開始温度は(T-50)℃以上であり、より好ましい開始温度は(T-80)℃以上かつAr3点超である。
また、好ましいホットスタンプの加熱温度T(℃)での保持時間は、1~5分間である。
これらの理由は、上述した、接合工程を含まないホットスタンプ成形品の製造方法と同じである。
本実施形態に係るホットスタンプ用鋼板の製造方法は、上述の化学組成を有するスラブに対して、熱間圧延を施した後、800℃以下の温度域で巻取って熱延鋼板とする熱間圧延工程と、上記熱延鋼板を、650℃を超える温度域まで加熱する熱延板焼鈍を施して熱延焼鈍鋼板とする熱延板焼鈍工程と、上記熱延焼鈍鋼板に冷間圧延を施す冷間圧延工程と、を含む。
また、巻取温度が低すぎると、熱延鋼板が硬質化し冷間圧延が困難となるため、巻取温度は400℃以上であることが好ましい。
一方、熱延板焼鈍工程における加熱温度が高くなりすぎると、熱延焼鈍鋼板の金属組織が粗大化し、ホットスタンプ後の引張強さが低下する。したがって、熱延板焼鈍工程における加熱温度は750℃未満であることが好ましく、720℃未満であることがより好ましい。
真空溶解炉を用いて溶鋼を鋳造し、表1に示す化学組成を有する鋼A~Oを製造した。表1中のAc1点およびAc3点は、鋼A~Oの冷延鋼板を2℃/秒で加熱した際の熱膨張変化から求めた。また、表1中のAr3点は、鋼A~Oの冷延鋼板を920℃に加熱した後10℃/秒で冷却した際の熱膨張変化から求めた。
また、熱延板焼鈍工程において、加熱温度での保持時間が10時間未満である試験番号1、3、4、9、11、14、16、22~27、31および33は、いずれも、ホットスタンプ成形品の引張強さが440MPa以上であり、強度特性が特に良好であった。
真空溶解炉を用いて溶鋼を鋳造し、実施例1において、表1に示した化学組成を有する鋼A~Cを製造した。鋼A~Cを用い、実施例1と同様に、表5に示す条件で、熱間圧延、熱延板焼鈍、冷間圧延、および、焼鈍を行い、次いでめっき処理を行い、溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板、および、溶融アルミニウムめっき鋼板(ホットスタンプ用鋼板)を製造した。
Claims (14)
- ホットスタンプ成形品であって、
前記ホットスタンプ成形品の全部または一部が、
質量%で、
C :0.001%以上、0.090%未満、
Si:2.50%以下、
Mn:0.01%以上、0.50%未満、
P :0.200%以下、
S :0.0200%以下、
sol.Al:0.001~2.500%、
N :0.0200%以下、
Cr:0.01%以上、2.00%未満、
Ti:0~0.300%、
Nb:0~0.300%、
V :0~0.300%、
Zr:0~0.300%、
Mo:0~2.00%、
Cu:0~2.00%、
Ni:0~2.00%、
B :0~0.0200%、
Ca:0~0.0100%、
Mg:0~0.0100%、
REM:0~0.1000%、
Bi:0~0.0500%、並びに
残部:Feおよび不純物である化学組成を有し、
金属組織が、面積%で、
フェライト:60.0%超、
マルテンサイト:0%以上、20.0%未満、
ベイナイト:0%以上、20.0%未満、を含み、
引張強さが、700MPa未満であり、
170℃で20分間の熱処理を施した際の、前記引張強さの低下量であるΔTSが100MPa以下である、
ホットスタンプ成形品。 - 前記化学組成が、質量%で、
Ti:0.001~0.300%、
Nb:0.001~0.300%、
V :0.001~0.300%、
Zr:0.001~0.300%、
Mo:0.001~2.00%、
Cu:0.001~2.00%、
Ni:0.001~2.00%、
B :0.0001~0.0200%、
Ca:0.0001~0.0100%、
Mg:0.0001~0.0100%、
REM:0.0001~0.1000%、および
Bi:0.0001~0.0500%
からなる群から選択される1種または2種以上を含有する、
請求項1に記載のホットスタンプ成形品。 - 前記化学組成が、質量%で、
Mn:0.01%以上、0.30%未満、
を含有する、請求項1または2に記載のホットスタンプ成形品。 - 表面にめっき層を有する、
請求項1~3のいずれか一項に記載のホットスタンプ成形品。 - 化学組成が、質量%で、
C :0.001%以上、0.090%未満、
Si:2.50%以下、
Mn:0.01%以上、0.50%未満、
P :0.200%以下、
S :0.0200%以下、
sol.Al:0.001~2.500%、
N :0.0200%以下、
Cr:0.01%以上、2.00%未満、
Ti:0~0.300%、
Nb:0~0.300%、
V :0~0.300%、
Zr:0~0.300%、
Mo:0~2.00%、
Cu:0~2.00%、
Ni:0~2.00%、
B :0~0.0200%、
Ca:0~0.0100%、
Mg:0~0.0100%、
REM:0~0.1000%、
Bi:0~0.0500%、並びに
残部:Feおよび不純物であり、
金属組織が、鉄炭化物を含み、前記鉄炭化物中のMn含有量及びCr含有量が下記(i)式を満足する、
ホットスタンプ用鋼板。
[Mn]θ+[Cr]θ>0.8 ・・・(i)
但し、上記式中の各記号の意味は以下のとおりである。
[Mn]θ:前記鉄炭化物に含まれるFe、MnおよびCrの合計含有量を100原子%としたときの、原子%での前記鉄炭化物中のMn含有量
[Cr]θ:前記鉄炭化物に含まれるFe、MnおよびCrの合計含有量を100原子%としたときの、原子%での前記鉄炭化物中のCr含有量 - 前記化学組成が、質量%で、
Ti:0.001~0.300%、
Nb:0.001~0.300%、
V :0.001~0.300%、
Zr:0.001~0.300%、
Mo:0.001~2.00%、
Cu:0.001~2.00%、
Ni:0.001~2.00%、
B :0.0001~0.0200%、
Ca:0.0001~0.0100%、
Mg:0.0001~0.0100%、
REM:0.0001~0.1000%、および
Bi:0.0001~0.0500%
からなる群から選択される1種または2種以上を含有する、
請求項5に記載のホットスタンプ用鋼板。 - 前記化学組成が、質量%で、
Mn:0.01%以上、0.30%未満、
を含有する、請求項5または6に記載のホットスタンプ用鋼板。 - 表面にめっき層を有する、
請求項5~7のいずれか一項に記載のホットスタンプ用鋼板。 - 請求項1~3のいずれか一項に記載のホットスタンプ成形品を製造する方法であって、
請求項5~7のいずれか一項に記載のホットスタンプ用鋼板を、Ac3点を超える加熱温度T℃まで加熱する加熱工程と、
前記加熱工程後の前記ホットスタンプ用鋼板に対して、(T-80)℃以上の温度でホットスタンプを開始するホットスタンプ工程と、を備える、
ホットスタンプ成形品の製造方法。 - 請求項1~3のいずれか一項に記載のホットスタンプ成形品を製造する方法であって、
請求項5~7のいずれか一項に記載のホットスタンプ用鋼板を、接合用鋼板と接合して接合鋼板とする接合工程と、
前記接合工程後の接合鋼板を前記ホットスタンプ用鋼板のAc3点を超える加熱温度T℃まで加熱する加熱工程と、
前記加熱工程後の前記接合鋼板に対して、(T-80)℃以上の温度でホットスタンプを開始するホットスタンプ工程と、を備える、
ホットスタンプ成形品の製造方法。 - 請求項4に記載のホットスタンプ成形品を製造する方法であって、
請求項8に記載のホットスタンプ用鋼板を、Ac3点を超える加熱温度T℃まで加熱する加熱工程と、
前記加熱工程後の前記ホットスタンプ用鋼板に対して、(T-80)℃以上の温度でホットスタンプを開始するホットスタンプ工程と、を備える、
ホットスタンプ成形品の製造方法。 - 請求項4に記載のホットスタンプ成形品を製造する方法であって、
請求項8に記載のホットスタンプ用鋼板を、接合用鋼板と接合して接合鋼板とする接合工程と、
前記接合工程後の接合鋼板を前記ホットスタンプ用鋼板のAc3点を超える加熱温度T℃まで加熱する加熱工程と、
前記加熱工程後の前記接合鋼板に対して、(T-80)℃以上の温度でホットスタンプを開始するホットスタンプ工程と、を備える、
ホットスタンプ成形品の製造方法。 - 請求項5~8のいずれか一項に記載のホットスタンプ用鋼板を製造する方法であって、
化学組成が、質量%で、C:0.001%以上、0.090%未満、Si:2.50%以下、Mn:0.01%以上、0.50%未満、P:0.200%以下、S:0.0200%以下、sol.Al:0.001~2.500%、N:0.0200%以下、Cr:0.01%以上、2.00%未満、Ti:0~0.300%、Nb:0~0.300%、V:0~0.300%、Zr:0~0.300%、Mo:0~2.00%、Cu:0~2.00%、Ni:0~2.00%、B:0~0.0200%、Ca:0~0.0100%、Mg:0~0.0100%、REM:0~0.1000%、Bi:0~0.0500%、並びに、残部:Feおよび不純物であるスラブに対して、熱間圧延を施した後、800℃以下の温度域で巻取って熱延鋼板とする熱間圧延工程と、
前記熱延鋼板に650℃を超える温度域まで加熱する熱延板焼鈍を施して熱延焼鈍鋼板とする熱延板焼鈍工程と、
前記熱延焼鈍鋼板に冷間圧延を施して冷延鋼板とする冷間圧延工程と、
を備える、
ホットスタンプ用鋼板の製造方法。 - 前記冷間圧延工程後の前記冷延鋼板に、
任意に、連続焼鈍を行った後、めっきを行うめっき工程を備える、
請求項13に記載のホットスタンプ用鋼板の製造方法。
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