WO2020189767A1 - ホットスタンプ成形体 - Google Patents
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- WO2020189767A1 WO2020189767A1 PCT/JP2020/012395 JP2020012395W WO2020189767A1 WO 2020189767 A1 WO2020189767 A1 WO 2020189767A1 JP 2020012395 W JP2020012395 W JP 2020012395W WO 2020189767 A1 WO2020189767 A1 WO 2020189767A1
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
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- B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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Definitions
- the present invention relates to a hot stamp molded article.
- the present application claims priority based on Japanese Patent Application No. 2019-052103 filed in Japan on March 20, 2019, the contents of which are incorporated herein by reference.
- Hot stamping is attracting attention as a technology that achieves both molding into automobile parts and ensuring strength by performing quenching in the mold at the same time as press working.
- Hot stamps are used as a processing method for deformation suppressing members and shock absorbing members of automobiles.
- the deformation suppressing member is required to be a member that is hardly deformed by a collision, and is required to have higher strength.
- the toughness decreases as the strength of the steel sheet increases, so that cracks are likely to occur during collision deformation. As a result, the proof stress and absorbed energy required for the automobile member may not be obtained.
- Patent Document 1 states that carbides are spheroidized by performing spheroidizing annealing at 650 to Ac 1 + 20 ° C. before quenching and tempering, and toughness is improved by reducing undissolved carbides during quenching and tempering heat treatment. Techniques that can be made to do so have been proposed.
- Patent Document 2 proposes a hot-rolled steel sheet in which tempered martensite and lower bainite are 90% or more in total and have a uniform microstructure to achieve both high strength and low temperature toughness.
- Patent Document 3 proposes an ultra-high-strength cold-rolled steel sheet in which the microstructure is tempered to form a single-phase martensite and the stretch flangeability is improved.
- Patent Document 4 proposes a method for producing a molded product capable of achieving both high strength and toughness by quenching twice.
- the microstructure of the steel material is made into martensite containing a large amount of fine carbides by the first quenching heat treatment (the number density of carbides is preferably 0.50 pieces / ⁇ m 2 or more). ing). After that, rapid heating is performed in the second quenching heat treatment, and the carbides are used as nucleation sites for reverse transformation to austenite to reduce the microstructure.
- annealing is performed by heating at less than 3 points of Ac for the purpose of spheroidizing carbides. Therefore, Mn is not sufficiently diffused, and a portion having a high Mn concentration exists in the steel material after annealing, and the toughness of the steel material deteriorates.
- spheroidizing annealing produces coarse carbides in the microstructure of the steel material. Since such carbides are likely to be a fracture starting point in a high-strength steel material of 2000 MPa or more, the toughness of the steel material may be significantly deteriorated.
- annealing is performed at 900 ° C. or lower in order not to coarsen the old austenite grains, but Mn may not be sufficiently diffused and Mn may be segregated in the microstructure. ..
- the portion having a locally high Mn concentration tends to be a fracture starting point in a high-strength steel material of 2000 MPa or more, so that the toughness of the steel material may deteriorate.
- the present invention has been made to solve the problems of the prior art, and an object of the present invention is to provide a hot stamp molded product having excellent strength and toughness.
- the present inventors control the average particle size of the former austenite grains to 5.0 ⁇ m or less as the microstructure of the hot stamped molded product, and may describe the grain boundaries of the former austenite grains (hereinafter referred to as the former austenite grain boundaries). ), It was found that the occurrence of cracks can be suppressed by setting the average Mn concentration to 1.0% by mass or less. In addition, as a result of diligent studies by the present inventors, it was found that the above microstructure can be obtained by the following method.
- the first heat treatment consists of a heating step of heating to a heating temperature of T1 of 3 points or more of Ac and 3 points of Ac + 200 ° C. or less, a holding step of holding at this heating temperature T1, and 10 ° C./s or more and 500 ° C./s or less. It is a heat treatment including a cooling step of cooling from a heating temperature T1 to a cooling stop temperature of "250 ° C. or higher and 400 ° C. or lower" at an average cooling rate.
- the heating step and the holding step of the first heat treatment have a role of re-dissolving the coarse carbide formed before the first heat treatment and a role of concentrating Mn in the former austenite grain boundaries. Further, since the cooling step of the first heat treatment controls the microstructure including martensite, tempered martensite, bainite and tempered bainite, a large amount of large tilt angle grain boundaries are formed in the old austenite grains.
- Second time heat treatment 10 ° C. / s or higher, the following average heating rate of 500 °C / s, Ac 3 'points or more, (Ac 3' point + 100 ° C.) a heating step of performing rapid heating to below the heating temperature T2, and
- This heat treatment includes a holding step of holding the heating temperature T2 for more than 10 seconds and 60 seconds or less.
- the difference (T2-cooling stop temperature) between the cooling stop temperature at the time of the first heat treatment and the heating temperature T2 at the time of the second heat treatment is less than 600 ° C.
- the steel material after the holding step of the second heat treatment is hot stamped and cooled.
- Ac 3 'point is the temperature determined by experiments. Details will be described later.
- Mn diffuses from the old austenite grain boundaries to the large inclination grain boundaries formed by the first heat treatment.
- Mn is concentrated in the fine retained austenite existing at the large tilt angle grain boundaries (between blocks).
- Ac 3 'point The reduced Ac 3 point, for convenience, referred to as "Ac 3 'point".
- Austenitizing proceeds in a temperature range of more than Ac 3 'point. However, since austenitization proceeds at a low temperature at this stage, the grain growth of austenite is suppressed. In addition, since fine austenite is maintained, Mn concentration from the old austenite grain boundary to the large inclination grain boundary continues.
- the steel material after the second heat treatment is hot stamped and cooled to room temperature. As a result, a hot stamp molded product is obtained.
- the average particle size of the old austenite grains of the hot stamped product is made into a fine grain structure of 5.0 ⁇ m or less, and the average Mn concentration of the grain boundaries of the old austenite grains is reduced to 1.0% by mass or less. Can be done.
- fracture (crack generation) at the time of collision is suppressed by reducing the high Mn concentration region of the former austenite grain boundary, and crack growth is also suppressed because the particle size of the former austenite grain is fine. As a result, it becomes possible to obtain a hot stamped molded product having excellent toughness.
- the gist of the present invention made based on the above findings is as follows.
- the hot stamp molded product according to one aspect of the present invention is based on mass%.
- Al 0.0010% or more, 0.500% or less, Ti: 0.010% or more, 0.100% or less, Cr: 0.010% or more, 0.80% or less, B: 0.0005% or more, 0.0100% or less, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, Nb: 0% or more, 0.100% or less, Mo: 0% or more, 1.00% or less, V: 0% or more, 0.100% or less, Ni: 0% or more, 0.50% or less, REM: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, Ca: 0% or more, 0.0100% or less, Co: 0% or more and 4.00% or less, and has a chemical composition in which the balance is Fe and impurities.
- the average particle size of the old austenite grains in the microstructure is 5.0 ⁇ m or less.
- the average Mn concentration at the grain boundaries of the former austenite grains is 1.0% by mass or less.
- Nb 0.010% or more, 0.100% or less, Mo: 0.01% or more, 1.00% or less, V: 0.001% or more, 0.100% or less, Ni: 0.001% or more, 0.50% or less, REM: 0.0010% or more, 0.0100% or less, Mg: 0.0010% or more, 0.0100% or less, It may contain one or more elements selected from Ca: 0.0010% or more, 0.0100% or less, and Co: 0.10% or more and 4.00% or less.
- the hot stamp molded product according to the above [1] or [2] may have a plating layer on its surface.
- the hot stamp molded product according to any one of the above [1] to [3] may have a softened region in a part thereof.
- the hot stamp molded product and the manufacturing method thereof according to the present embodiment will be described in detail.
- 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.
- the hot stamp molded product according to the present embodiment has C: 0.40% or more, 0.70% or less, Si: 0.010% or more, 1.30% or less, Mn: 0.40% or more in mass%. , 3.00% or less, sol. Al: 0.0010% or more, 0.500% or less, Ti: 0.010% or more, 0.100% or less, Cr: 0.010% or more, 0.80% or less, B: 0.0005% or more, It has a chemical composition of 0.0100% or less, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, and the balance is Fe and impurities.
- each element will be described in detail.
- C 0.40% or more, 0.70% or less
- C is an important element for obtaining a tensile strength of 2000 MPa or more in a hot stamp molded product. If the C content is less than 0.40%, martensite becomes soft and it is difficult to obtain a tensile strength of 2000 MPa or more. Therefore, the C content is set to 0.40% or more. The C content is preferably 0.43% or more and 0.45% or more. On the other hand, if the C content exceeds 0.70%, coarse carbides are generated and fracture is likely to occur, and the toughness of the hot stamped compact is lowered. Therefore, the C content is set to 0.70% or less. The C content is preferably 0.60% or less and 0.55% or less.
- Si: 0.010% or more, 1.30% or less Si has the effect of suppressing the formation of coarse cementite, and is an important element for ensuring the toughness of the hot stamped compact.
- Si has tempering and softening resistance, and has an effect of suppressing a decrease in strength due to self-quenching during hot stamp quenching. If the Si content is less than 0.010%, the above effect cannot be obtained, and the toughness of the hot stamped molded product may deteriorate. Therefore, the Si content is set to 0.010% or more. Preferably, it is 0.02% or more and 0.03% or more.
- the Si content is set to 1.30% or less.
- it is 1.20% or less and 1.00% or less.
- Mn 0.40% or more, 3.00% or less
- Mn is an element that contributes to the improvement of the strength of the hot stamped molded product by strengthening the solid solution. If the Mn content is less than 0.40%, the solid solution strengthening ability is poor and martensite becomes soft, and it is difficult to obtain a tensile strength of 2000 MPa or more in the hot stamped molded product. Therefore, the Mn content is set to 0.40% or more.
- the Mn content is preferably 0.50% or more and 0.60% or more.
- the Mn content is set to 3.00% or less. Preferably, it is 2.50% or less, 2.00% or less, and 1.50% or less.
- sol.Al 0.0010% or more, 0.500% or less
- Al is an element having an action of deoxidizing molten steel to make the steel sound (suppressing the occurrence of defects such as blow holes in the steel). sol. If the Al content is less than 0.0010%, deoxidation is not sufficiently performed. Therefore, sol. The Al content is 0.0010% or more. sol. The Al content is preferably 0.010% or more, 0.020% or more. On the other hand, sol. When the Al content exceeds 0.500%, coarse oxides are formed in the steel, and the toughness of the hot stamped compact is lowered. Therefore, sol. The Al content is 0.500% or less. Preferably, it is 0.400% or less and 0.350% or less. In addition, sol. Al means acid-soluble Al, and indicates solid solution Al existing in steel in a solid solution state.
- Ti 0.010% or more, 0.100% or less
- Ti is an element that forms a carbonitride and suppresses the grain growth of austenite during hot stamp heating (particularly during the second heat treatment). If the Ti content is less than 0.010%, the above effect cannot be obtained, the old austenite grains become coarse, and the toughness of the hot stamped body deteriorates. Therefore, the Ti content is set to 0.010% or more. The Ti content is preferably 0.020% or more and 0.025% or more. On the other hand, if Ti is contained in excess of 0.100%, coarse TiN is generated, so that the toughness of the hot stamped compact is deteriorated. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably 0.080% or less and 0.060% or less.
- Cr 0.010% or more, 0.80% or less
- Cr is a carbide-forming element and an element that refines carbides to improve the toughness of the hot stamped compact. If the Cr content is less than 0.010%, the above effect cannot be obtained. Therefore, the Cr content is set to 0.010% or more.
- the Cr content is preferably 0.10% or more and 0.15% or more. On the other hand, even if Cr of more than 0.80% is contained, the above effect is saturated. In addition, it fills the Mn segregation site of the old austenite grain boundary and inhibits the segregation of Mn to the old austenite grain boundary at the time of the first heat treatment.
- the Cr content is set to 0.80% or less.
- the Cr content is preferably 0.60% or less, 0.50% or less, and 0.40% or less.
- B 0.0005% or more, 0.0100% or less
- B is an element that segregates at the grain boundaries and enhances the hardenability of steel. If the B content is less than 0.0005%, the above effect cannot be obtained and ferrite may be formed. As a result, it may be difficult to obtain a tensile strength of 2000 MPa or more, or the toughness of the hot stamped molded product may deteriorate. Therefore, the B content is set to 0.0005% or more.
- the B content is preferably 0.0010% or more, 0.0015% or more, and 0.0020% or more.
- the B content is set to 0.0100% or less.
- the B content is preferably 0.0075% or less and 0.0050% or less.
- P 0.100% or less
- P is an element that segregates at the grain boundaries and reduces the strength of the grain boundaries.
- the P content is preferably 0.050% or less and 0.030% or less.
- the lower limit of the P content is not particularly limited, but if it is reduced to less than 0.0001%, the P removal cost will increase significantly, which is economically unfavorable. In actual operation, the P content may be 0.0001% or more.
- S 0.0100% or less
- S is an element that forms inclusions in steel.
- the S content is preferably 0.0040% or less.
- the lower limit of the S content is not particularly limited, but if it is reduced to less than 0.00015%, the cost of removing S is significantly increased, which is economically unfavorable. In actual operation, the S content may be 0.00015% or more and 0.0002% or more.
- N 0.0100% or less
- N is an impurity element, which is an element that forms a nitride in steel and deteriorates the toughness of the hot stamped compact.
- the N content exceeds 0.0100%, coarse nitrides are formed in the steel, and the toughness of the hot stamped compact is significantly reduced. Therefore, the N content is set to 0.0100% or less.
- the N content is preferably 0.0075% or less and 0.0050% or less.
- the lower limit of the N content is not particularly limited, but if it is reduced to less than 0.0001%, the N removal cost is significantly increased, which is economically unfavorable. In actual operation, the N content may be 0.0001% or more.
- the rest of the chemical composition of the hot stamped article according to this embodiment is Fe and impurities.
- Impurities are elements unavoidably mixed from steel raw materials or scrap, elements unavoidably mixed in the steelmaking process, and / or elements intentionally added in a small amount, and are hot stamped according to the present embodiment. Examples are examples of elements that are allowed as long as they do not interfere with the properties of the body.
- the hot stamp molded product according to the present embodiment may contain the following optional elements instead of a part of Fe.
- the lower limit of the content when the following optional elements are not contained is 0%.
- each arbitrary element will be described in detail.
- Nb 0% or more, 0.100% or less
- Nb is an element that improves the strength of the hot stamped compact by strengthening the solid solution and contributes to the refinement of the former austenite granules by forming a carbonitride. Therefore, Nb may be contained if necessary.
- the Nb content is preferably 0.010% or more in order to surely exert the above effect.
- the Nb content is more preferably 0.035% or more.
- the Nb content is preferably 0.100% or less.
- the Nb content is more preferably 0.080% or less.
- Mo 0% or more, 1.00% or less
- Mo is an element that improves the strength of the hot stamped compact by strengthening the solid solution, enhances the hardenability of steel, and suppresses the formation of ferrite that deteriorates toughness. Therefore, Mo may be contained if necessary.
- the Mo content is preferably 0.01% or more in order to ensure the above effect.
- the Mo content is more preferably 0.02% or more.
- the Mo content is preferably 1.00% or less.
- the Mo content is more preferably 0.80% or less.
- V 0% or more, 0.100% or less
- V is an element that improves the strength of the hot stamped molded product by strengthening the solid solution.
- the V content is preferably 0.001% or more. More preferably, the V content is 0.050% or more.
- the V content is preferably 0.100% or less.
- the V content is more preferably 0.090% or less.
- Ni 0% or more, 0.50% or less
- Ni is an element that dissolves in austenite, has the effect of enhancing the hardenability of steel, and improves the toughness of the hot stamped compact.
- the Ni content is preferably 0.001% or more. More preferably, it is 0.01% or more.
- the Ni content is preferably 0.50% or less. More preferably, it is 0.40% or less.
- REM 0% or more, 0.0100% or less
- REM is an element that has the effect of deoxidizing molten steel to make the steel sound, and is also an element that improves the toughness of the hot stamped compact. Therefore, REM may be contained if necessary.
- the REM content is preferably 0.0010% or more. More preferably, it is 0.0020% or more.
- the REM content is preferably 0.0100% or less. More preferably, it is 0.0080% or less.
- REM refers to a total of 17 elements composed of Sc, Y and lanthanoids.
- the REM content refers to the total content of these elements. In the case of lanthanoids, they are industrially added in the form of misch metal.
- Mg 0% or more, 0.0100% or less
- Mg is an element having an action of deoxidizing molten steel to make the steel sound, and improves the toughness of the hot stamped compact. Therefore, Mg may be contained if necessary.
- the Mg content is preferably 0.0010% or more. More preferably, it is 0.0020% or more.
- the Mg content is preferably 0.0100% or less. More preferably, it is 0.0080% or less.
- Ca 0% or more, 0.0100% or less
- Ca is an element having an action of deoxidizing molten steel to make the steel sound, and improves the toughness of the hot stamped compact. Therefore, Ca may be contained if necessary.
- the Ca content is preferably 0.0010% or more. More preferably, it is 0.0020% or more.
- the Ca content is preferably 0.0100% or less. More preferably, it is 0.0080% or less.
- Co 0% or more and 4.00% or less
- Co is an element having an action of raising the martensite start temperature (Ms point) and improves the toughness of the hot stamped molded product. Therefore, Co may be contained if necessary.
- the Co content is preferably 0.10% or more in order to surely exert the above effect. More preferably, it is 0.20% or more.
- the Co content is preferably 4.00% or less. More preferably, it is 3.00% or less.
- the chemical composition of the hot stamp molded product described above may be measured by a general analysis method.
- ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
- 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 microstructure of the hot stamp molded product is from the surface to the plate thickness t / 8 depth to the surface to the plate thickness 3 t / centered on the plate thickness t / 4 position (t is the plate thickness) from the surface. It means a microstructure in a region of 8 depths.
- the average particle size of the former austenite grains in the microstructure is 5.0 ⁇ m or less, and the average Mn concentration of the grain boundaries of the former austenite grains is 1.0% by mass or less. ..
- each regulation will be described.
- the average particle size of the former austenite grains is 5.0 ⁇ m or less, and the average Mn concentration at the grain boundaries of the former austenite grains is 1.0% by mass or less.”
- the microstructure is finer. The present inventors have found that in a high-strength hot stamped article having a tensile strength of more than 2000 MPa, the toughness deteriorates when the average particle size of the former austenite grains exceeds 5.0 ⁇ m. Therefore, the average particle size of the old austenite grains is 5.0 ⁇ m or less. More preferably, it is 4.5 ⁇ m or less, 4.0 ⁇ m or less, and 3.5 ⁇ m or less.
- the average particle size of the old austenite grains may be 1.0 ⁇ m or more, or 2.0 ⁇ m or more.
- the present inventors have also found that it is important to reduce the Mn concentration at the grain boundaries of the former austenite grains (former austenite grain boundaries) in order to obtain excellent toughness in the hot stamped body. If a large amount of Mn is unevenly distributed at the former austenite grain boundaries, the ductile fracture limit is significantly deteriorated and becomes a fracture starting point at the time of collision. As a result, the toughness of the hot stamp molded product deteriorates. When the average Mn concentration of the old austenite grain boundaries exceeds 1.0% by mass, the fracture sensitivity is increased and the toughness of the hot stamped molded product is significantly deteriorated. Therefore, the average Mn concentration of the old austenite grain boundaries is set to 1.0% by mass or less. Preferably, it is 0.8% by mass or less, 0.6% by mass or less, and 0.5% by mass or less. The average Mn concentration of the former austenite grain boundaries may be 0.1% by mass or more, or 0.2% by mass or more.
- the average particle size of the former austenite grains is measured by the following method.
- First, the hot stamp molded product is heat-treated at 540 ° C. for 24 hours. This promotes corrosion of the old austenite grain boundaries.
- the heat treatment may be performed by heating in a furnace or energizing, and the heating rate is 0.1 to 100 ° C./s and the cooling rate is 0.1 to 150 ° C./s.
- a plate thickness cross section perpendicular to the plate surface is cut out from the central portion (the portion avoiding the end portion) of the hot stamp molded body after the heat treatment.
- a diamond powder having a particle size of 1 to 6 ⁇ m is mirror-finished using a diluted solution such as alcohol or a liquid dispersed in pure water. .. This plate thickness cross section is used as an observation surface.
- the observation surface is immersed in a 3-4% sulfuric acid-alcohol (or water) solution (% is volume%) for 1 minute to reveal the old austenite grain boundaries.
- the immersion work is carried out in the exhaust treatment device, and the temperature of the work atmosphere is normal temperature (10 to 30 ° C., the same applies hereinafter).
- the observation surface on which the old austenite grain boundaries are exposed is washed with acetone or ethyl alcohol, dried, and then the observation surface is observed with a scanning electron microscope.
- the scanning electron microscope used shall be equipped with a secondary electron detector.
- the sample In a vacuum of 9.6 ⁇ 10-5 Pa or less, the sample is irradiated with an electron beam at an accelerating voltage of 15 kV and an irradiation current level of 13, and the plate thickness is t / 8 depth from the surface of the hot stamped compact to the plate thickness from the surface. A secondary electron image in the region of 3t / 8 depth is taken.
- the shooting magnification is 4000 times based on a screen having a width of 386 mm and a height of 290 mm, and the number of shooting fields of view is 10 or more.
- the old austenite grain boundaries are imaged as bright contrast.
- the particle size of the observed old austenite grains is obtained by measuring the shortest diameter and the longest diameter of the old austenite grains included in the field of view and calculating the average value of these. If the entire austenite grain, such as the edge of the field of view, is not included in the field of view, the particle size of the old austenite grain is not measured.
- the average particle size of the old austenite grains is obtained by calculating the particle size of all the old austenite grains in the entire field of view and calculating the average value thereof.
- the average particle size of the former austenite grains is a value obtained by dividing the sum of the calculated grain sizes of the former austenite grains by the total number of the measured grain sizes of the former austenite grains.
- a test piece having the dimensions shown in FIG. 1 is produced from the central portion (the portion avoiding the end portion) of the hot stamp molded product.
- the front and back surfaces of the test piece are removed by mechanical grinding in equal amounts so that the plate thickness (the length of the test piece in the direction perpendicular to the paper surface in FIG. 1) is 1.2 mm.
- a notch is provided in the central portion of the test piece in the length direction (left-right direction in FIG. 1). This notch is formed by inserting a wire cutter having a thickness of 1 mm.
- the distance between the bottom of the notch and the side surface where the notch is not provided is controlled to 100 to 200 ⁇ m.
- the test piece is then immersed in a 20% -ammonium thiocyanate solution (% is by volume) for 24-48 hr. Galvanize the front and back surfaces of the test piece within 0.5 hr after the immersion is completed. After galvanizing, it is subjected to Auger electron emission spectroscopic analysis within 1.5 hr.
- the type of apparatus for performing Auger electron emission spectroscopic analysis is not particularly limited.
- the test piece is set in the analyzer, and in a vacuum of 9.6 ⁇ 10-5 Pa or less, the test piece is broken from the notch portion of the test piece to expose the old austenite grain boundaries.
- the exposed former austenite grain boundaries are irradiated with an electron beam at an accelerating voltage of 1 to 30 kV, and the Mn concentration (mass%) at the former austenite grain boundaries is measured.
- the measurement is carried out for three or more former austenite grains at 10 or more positions at the grain boundaries of each former austenite. Measurements are completed within 30 minutes of destruction to prevent contamination of the old austenite grains.
- the average Mn concentration of the former austenite grain boundaries is obtained.
- the microstructure of the hot stamped product is not particularly limited, but may include martensite (including fresh martensite and tempered martensite), upper bainite, lower bainite and retained austenite, and iron carbides and / or alloy carbides.
- the microstructure has martensite (including fresh martensite and tempered martensite) as the main phase (90% or more in area ratio) and the residual structure (upper bainite, lower bainite and retained austenite, and iron carbides).
- / or alloy carbide has an area ratio of 10% or less.
- the area ratio of martensite is more preferably 95% or more, still more preferably 100%.
- the area ratio of the residual structure is more preferably 5% or less, still more preferably 0%, in relation to the area ratio of martensite.
- the area ratio of martensite is measured by the following method. A sample is taken from a position 50 mm or more away from the end face of the hot stamped body (or a position avoiding the end portion) so that the plate thickness cross section can be observed. After polishing the observation surface, nital corrosion is performed to clarify the contrast between carbides and grain boundaries. Next, using an electrolytic radiation scanning electron microscope (FE-SEM) equipped with a secondary electron detector, a region centered on the plate thickness t / 4 position of the sample (1/8 depth from the surface to the plate thickness). A secondary electron image is photographed at a magnification of 5000 times for a region (3/8 depth of the plate thickness from the surface).
- FE-SEM electrolytic radiation scanning electron microscope
- the phases other than martensite (ferrite, pearlite, upper bainite, lower bainite and retained austenite, etc.) and martensite (fresh martensite and tempered martensite) are distinguished.
- Upper bainite, lower bainite and tempered martensite can be distinguished by the presence or absence of iron carbide in the lath-shaped crystal grains and the elongation direction of the iron carbide.
- Fresh martensite is not sufficiently etched by nightal etching and is therefore distinguishable from other etched structures.
- retained austenite is not sufficiently etched like martensite, the area ratio of fresh martensite can be obtained by obtaining the difference from the area ratio of retained austenite obtained by the method described later.
- Upper bainite is a phase consisting of aggregates of lath-like crystal grains, accompanied by precipitation of carbides between laths.
- Lower bainite and tempered martensite are also phases consisting of aggregates of lath-like crystal grains, but are phases containing carbides inside the lath.
- Lower bainite and tempered martensite are distinguished by the direction of carbide elongation.
- the carbides of lower bainite have a single variant, the angular difference of the carbides present within one crystal grain is within 5 ° and have substantially a single direction.
- the carbide of tempered martensite has a plurality of variants, and the carbide existing in one crystal grain extends in a plurality of directions.
- the area ratio of retained austenite is measured in the same area as the observation area in which the photographed photograph is obtained.
- the observation surface is polished with # 600 to # 1500 silicon carbide paper, and then a mirror surface is finished using a diluted solution such as alcohol or a liquid in which diamond powder having a particle size of 1 to 6 ⁇ m is dispersed in pure water.
- the strain introduced into the surface layer of the observation surface is removed by polishing for 8 minutes with colloidal silica containing no alkaline solution at room temperature.
- the observation surface is measured by electron backscatter diffraction at measurement intervals of 0.1 ⁇ m to obtain crystal orientation information.
- an apparatus composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used.
- the degree of vacuum in the apparatus is 9.6 ⁇ 10 -5 Pa or less
- the acceleration voltage is 15 kv
- the irradiation current level is 13
- the electron beam irradiation level is 62.
- the structure is distinguished by the method described above, and the area ratio of martensite (fresh martensite and tempered martensite) is determined.
- the area ratio of the residual tissue is obtained by subtracting the area ratio of martensite from 100%.
- the number density of carbides with a circle-equivalent diameter of 0.20 ⁇ m or more is 0.5 pieces / ⁇ m 2 or less. If the microstructure of the hot stamped product contains a large amount of coarse carbides, the toughness of the hot stamped product may deteriorate. Therefore, it is desirable that the amount of coarse carbide is as small as possible.
- the number density of carbides having a circle-equivalent diameter of 0.20 ⁇ m or more is preferably 0.5 pieces / ⁇ m 2 or less. More preferably, it is 0.3 pieces / ⁇ m 2 or less and 0.2 pieces / ⁇ m 2 or less. Since it is preferable that the number density of carbides having a circle-equivalent diameter of 0.20 ⁇ m or more is smaller, the number may be 0 / ⁇ m 2 .
- the hot stamp molded product according to the present embodiment may have a tensile (maximum) strength of 2000 MPa or more. It is preferably 2200 MPa or more. The upper limit is not particularly limited, but may be 2600 MPa or less and 2500 MPa or less.
- the tensile (maximum) strength is determined according to the test method described in JIS Z 2241: 2011 by preparing the No. 5 test piece described in JIS Z 2241: 2011 from a position as flat as possible in the hot stamp molded product.
- the hot stamped molded product according to the present embodiment may have a value of 0.60 MPa / Hv or more, which is an index of early breaking characteristics, and a hardness variation ( ⁇ Hv) of 50 Hv or less.
- the value that is an index of the early breaking characteristics is the value obtained by dividing the tensile strength (unit: MPa) by the value obtained by multiplying the average hardness (unit: Hv) obtained by the method described later by 3.3 (tensile strength). / (Average hardness x 3.3)). This value is preferably 0.75 MPa / Hv or more and 0.80 MPa / Hv or more.
- the value obtained by multiplying the average hardness by 3.3 is the estimated tensile strength estimated from the hardness, and if the measured value of the tensile strength is 0.60 MPa / Hv times or more of the estimated tensile strength, it is early. Since it has excellent breaking characteristics, it can be judged that it has excellent toughness.
- the hardness variation ( ⁇ Hv) is 50 Hv or less, it is judged that the hot stamp molded body having a tensile strength of 2000 MPa or more is excellent in toughness because stress concentration is unlikely to occur when deformation (stress) occurs from the outside. be able to.
- the hardness variation ( ⁇ Hv) is preferably 40 Hv or less, 30 Hv or less, and 20 Hv or less.
- the average hardness used to calculate the index of early breaking characteristics is measured by the following method.
- a test piece is cut out so that a thick cross section perpendicular to the surface can be observed from an arbitrary position (a position avoiding the end portion) of the hot stamped body.
- the length of the test piece depends on the measuring device, but may be about 10 mm.
- a mirror surface using a diluted solution such as alcohol or a liquid in which diamond powder having a particle size of 1 to 6 ⁇ m is dispersed in pure water is used. Finish to. This plate thickness cross section is used as the measurement surface.
- a load of 1 kgf gives 3 indentations Measure Vickers hardness at intervals of 2 times or more.
- the hardness variation ( ⁇ Hv) is obtained by calculating the difference between the maximum value and the minimum value of the Vickers hardness at 20 points, which is obtained when the average hardness is obtained by the above method.
- the hot stamped molded product according to the present embodiment can be obtained by a manufacturing method in which a steel sheet for hot stamping is subjected to a first heat treatment and a second heat treatment.
- a large amount of large tilt angle grain boundaries are formed in the old austenite grains.
- Mn is diffused from the former austenite grain boundary to the large inclination grain boundary in the former austenite grain.
- the Mn concentration at the former austenite grain boundaries can be reduced in the microstructure of the hot stamped product. That is, it is preferable that a sufficient amount of large tilt angle grain boundaries are formed in the hot stamping steel sheet (steel sheet after the first heat treatment and before the second heat treatment) processed into the hot stamping compact according to the present embodiment. ..
- the hot stamping steel sheet processed into the hot stamping compact according to the present embodiment has a large plate thickness t / 4 position (region from surface to plate thickness t / 8 depth to surface to plate thickness 3 t / 8 depth).
- the ratio of tilted grain boundaries is preferably 40% or more. However, even if the ratio of the large tilt angle grain boundaries of the hot stamping steel sheet is less than 40%, the hot stamping molded product according to the present embodiment can be manufactured depending on the manufacturing conditions after the first heat treatment.
- the ratio of the large tilt angle grain boundaries of the stamp steel sheet is not particularly limited.
- a method of calculating the ratio of large tilt angle grain boundaries of the hot stamping steel sheet will be described.
- a test piece is cut out so that a cross section perpendicular to the surface (thick cross section) can be observed from an arbitrary position on the hot stamping steel plate.
- the length of the test piece depends on the measuring device, but may be about 10 mm.
- the diamond powder having a particle size of 1 to 6 ⁇ m is mirror-finished using a diluted solution such as alcohol or a liquid dispersed in pure water. .. This plate thickness cross section is used as an observation surface.
- the observation surface is polished for 8 minutes with colloidal silica containing no alkaline solution at room temperature to remove the strain introduced into the surface layer of the test piece.
- the plate thickness t / 4 position region from the surface to the plate thickness t / 8 depth to the surface to the plate thickness 3 t / 8 depth
- the crystal orientation information is obtained by measuring with the electron backscatter diffraction method.
- an apparatus composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used.
- the degree of vacuum in the apparatus is 9.6 ⁇ 10 -5 Pa or less
- the acceleration voltage is 15 kv
- the irradiation current level is 13
- the irradiation time of the electron beam is 0.01 seconds / point.
- the steel piece (steel material) to be subjected to hot rolling may be a steel piece manufactured by a conventional method, and may be, for example, a steel piece manufactured by a general method such as a continuously cast slab or a thin slab caster. It is preferable that the steel material having the above-mentioned chemical composition is subjected to hot rolling, heated to a temperature range of 1100 ° C. or higher in the hot rolling step, and held in this temperature range for 20 minutes or longer. When the heating temperature is less than 1100 ° C.
- the heating temperature is 1200 ° C. or higher, and the holding time is 25 minutes or longer.
- the heating temperature is preferably 1400 ° C. or lower, and the holding time is preferably 120 minutes or less.
- the finish rolling temperature is preferably Ar 3 points or more. More preferably, it is Ar 3 points + 10 ° C. or higher. Further, the finish rolling temperature is preferably Ar 3 points + 100 ° C. or less.
- Ar 3 points are represented by the following equation (1). Each element symbol in the formula (1) indicates the content (mass%) of each element. If the element is not contained, 0 is substituted.
- Ar 3 points 850 + 10 ⁇ (C + N) ⁇ Mn + 350 ⁇ Nb + 250 ⁇ Ti + 40 ⁇ B + 10 ⁇ Cr + 100 ⁇ Mo ⁇ ⁇ ⁇ Equation (1)
- Winding process The steel sheet after finish rolling is wound into a coil in a temperature range of 750 ° C. or lower.
- the winding temperature is preferably 750 ° C. or lower. More preferably, it is 600 ° C. or lower.
- the winding temperature is preferably 400 ° C. or higher.
- a hot-rolled steel sheet is obtained by the above method.
- the hot-rolled steel sheet obtained by the above method may be reheated for the purpose of softening, if necessary.
- a cold-rolled steel sheet may be obtained by cold-rolling a hot-rolled steel sheet, or a plated steel sheet may be obtained by applying plating. Moreover, you may perform continuous annealing.
- Cold rolling may be performed at a normal cumulative rolling reduction rate, for example, 30 to 90%.
- the hot-rolled steel sheet may be subjected to a hot stamping process without being cold-rolled.
- the hot-rolled steel sheet or cold-rolled steel sheet may have a plating layer on its surface.
- Various known hot-dip metal plating, electroplating, and the like may be applied depending on the purpose of suppressing scale formation in the hot stamping step and improving the corrosion resistance of the hot stamped molded product.
- hot-dip metal plating examples include hot-dip galvanizing, alloying hot-dip galvanizing, hot-dip aluminum plating, and hot-dip aluminum-zinc plating. If the molten metal plating layer is hard, cracks may occur during hot stamp molding and the corrosion resistance of the hot stamp molded product may deteriorate. Therefore, the hot-dip metal plating is preferably hot-dip galvanizing or alloyed hot-dip galvanizing in which the plating layer is soft.
- the amount of plating adhered to the surface of the hot-rolled steel sheet or cold-rolled steel sheet is preferably 3 to 800 g / m 2 per side. If the amount of plating adhered is less than 3 g / m 2 per side, the effect of improving corrosion resistance may not be reliably obtained. On the other hand, if the amount of plating adhered exceeds 800 g / m 2 per side, defects such as blow holes may easily occur during welding. From the viewpoint of improving corrosion resistance and suppressing cost increase, the amount of plating adhered is more preferably 10 to 200 g / m 2 .
- the plating is alloyed hot dip galvanizing.
- the degree of alloying of the alloyed hot dip galvanizing it is preferable that the Fe content in the plating layer is 3 to 25%. If the Fe content in the plating layer is less than 3%, evaporation of the plating layer during hot stamping may not be sufficiently suppressed. If the Fe content in the plating layer is more than 25%, the powdering property of the hot stamp molded product may deteriorate.
- the Fe content in the plating layer is more preferably 7 to 18%.
- the surface of the hot-dip galvanized layer or the alloyed hot-dip galvanized layer may be further coated with an organic or inorganic film.
- the hot stamping compact according to the present embodiment is manufactured by the following manufacturing method. As described above, in the present embodiment, two heat treatments are performed in order to obtain a desired microstructure in the hot stamped molded product.
- Heating temperature T1 Ac 3 points or more, Ac 3 + 200 ° C or less
- the hot stamping compact according to the present embodiment is subjected to the first heat treatment before the hot stamping steel sheet is subjected to the hot stamping step.
- Ac is heated to 3 points or more, Ac 3 points + 200 ° C. or less to a heating temperature T1, and the temperature is maintained at T1.
- Mn is concentrated at the old austenite grain boundaries.
- the heating temperature T1 is set to Ac 3 points or more.
- the heating temperature T1 is set to Ac 3 + 200 ° C. or lower.
- the average heating rate up to the heating temperature T1 may be 1 to 30 ° C./s.
- Ac 3 points can be obtained from the following equation (2).
- the hot stamping steel sheet heated to the heating temperature T1 is held at the heating temperature T1.
- the holding time is preferably 60 seconds to 20 minutes. If the holding time is less than 60 seconds, redissolution of carbides does not proceed, coarse carbides remain undissolved, and the number density of carbides becomes too high, so that a desired microstructure may not be obtained. If the retention time is more than 20 minutes, the old austenite grains may be excessively coarsened, the proportion of large tilt angle grain boundaries may be reduced, and a desired microstructure may not be obtained.
- the average cooling rate from the heating temperature T1 to the cooling stop temperature described later is less than 10 ° C./s, a soft phase such as ferrite may be formed, and the introduction of large tilt angle grain boundaries becomes insufficient. As a result, the reduction of the Mn concentration of the old austenite grain boundaries in the second heat treatment becomes insufficient, and the average Mn concentration of the old austenite grain boundaries may not be reduced to 1.0% by mass or less. Therefore, the average cooling rate is set to 10 ° C./s or more. It is preferably 20 ° C./s or higher.
- the cooling rate exceeds 500 ° C./s the internal stress due to martensitic transformation becomes large, and cracks may occur in the cooling process to room temperature. Therefore, the average cooling rate is set to 500 ° C./s or less. It is preferably 300 ° C./s or less.
- Cooling stop temperature 250 ° C. or higher, 400 ° C. or lower Cooling in the first heat treatment not only forms martensite, but also requires austenite to remain at the block interface of martensite. This is because, as described above, this remaining austenite serves as a diffusion path for Mn in the second heat treatment. In order to stabilize this austenite, it is necessary to promote C diffusion from martensite to untransformed austenite. Therefore, cooling is stopped in a temperature range of 250 ° C. or higher and 400 ° C. or lower. When the cooling stop temperature is less than 250 ° C., C diffusion from martensite to untransformed austenite does not proceed. Therefore, the cooling stop temperature is set to 250 ° C. or higher. It is preferably 260 ° C. or higher. When the cooling stop temperature exceeds 400 ° C., carbides are formed and the stabilization of retained austenite between blocks does not proceed. Therefore, the cooling stop temperature is set to 400 ° C. or lower.
- Average cooling rate below the cooling stop temperature Less than 10 ° C / s
- the cooling rate below the above cooling stop temperature is controlled to less than 10 ° C./s. It is preferably 8 ° C./s or less.
- the cooling rate below the cooling stop temperature is 10 ° C./s or higher, carbon diffusion from martensite to untransformed austenite does not proceed, the stability of austenite becomes low, and retained austenite cannot be left. However, it may become coarse in the heating process during the second heat treatment, and the Mn concentration of the old austenite grain boundaries may not be reduced.
- the average heating rate is set to 10 ° C./s or more. Preferably, it is 30 ° C./s or higher.
- the average heating rate exceeds 1000 ° C./s, it becomes difficult to control the heating temperature of the hot stamp molded product, and the average particle size of the old austenite grains may not be 5.0 ⁇ m or less depending on the site. As a result, the toughness of the hot stamped molded product may deteriorate. Therefore, the average heating rate is set to 1000 ° C./s or less. Preferably, it is 700 ° C./s or less.
- heating temperature T2 Ac 3 'or more points, Ac 3' residual austenite formed at point + 100 ° C. first time heat treatment below, Mn is concentrated. Since Mn is an austenite stabilizing element, the Ac 3 points are lower than those of the first heat treatment. This reduced Ac 3 point, and referred to as "Ac 3 'point", is referred to as the heating temperature at the time of the second time and heat treatment T2.
- the heating temperature T2 is Ac 3 'point + 100 ° C. greater proceeds grain growth of prior austenite grains, which may mean particle size of prior austenite grains exceeds 5.0 .mu.m. As a result, the toughness of the hot stamped molded product may deteriorate. Therefore, the heating temperature T2 is set to Ac 3 'point + 100 ° C. or less. Preferably, it is less Ac 3 'point + 80 ° C..
- the steel sheet for hot stamping after the first heat treatment was subjected to thermal expansion measurement, and the temperature at which the microstructure was completely austenitized was obtained from the change in the amount of thermal expansion during heating, and this temperature was set to Ac. 3, 'point.
- the device used for the thermal expansion measurement may be any device that can continuously measure the amount of thermal expansion during heating, and for example, a thin plate for master tester manufactured by Fuji Denpa Koki may be used.
- the holding time at the heating temperature T2 shall be more than 10 seconds and 60 seconds or less. If the holding time is 10 seconds or less, the Mn diffusion from the old austenite grain boundaries to the large inclination grain boundaries does not proceed sufficiently, so that the Mn amount of the old austenite grain boundaries may not be reduced. If the retention time is more than 60 seconds, the growth of the old austenite grains may proceed and the toughness may deteriorate.
- the preferable holding time in consideration of the balance between the miniaturization of the austenite grains and the Mn diffusion from the austenite grain boundaries to the large inclination grain boundaries is 20 seconds or more and 30 seconds or less.
- the difference (T2-cooling stop temperature) between the cooling stop temperature at the time of the first heat treatment and the heating temperature T2 at the time of the second heat treatment is less than 600 ° C.
- the T2-cooling stop temperature is 600 ° C. or higher, the grain growth of austenite progresses in the heating stage during the second heat treatment, and the average particle size of the austenite grains exceeds 5.0 ⁇ m and / or the austenite grain boundaries.
- the average Mn concentration of the above may be high.
- the difference (T2-cooling stop temperature) between the cooling stop temperature at the time of the first heat treatment and the heating temperature T2 at the time of the second heat treatment is 570 ° C. or less.
- FIG. 2 is a diagram showing the relationship between the T2-cooling stop temperature and the average Mn concentration of the grain boundaries of the former austenite grains in the examples.
- FIG. 3 is a diagram showing the relationship between the T2-cooling stop temperature and the average particle size of the former austenite grains in the examples. As shown in FIG. 2, it can be seen that by setting the T2-cooling stop temperature to less than 600 ° C., the average Mn concentration at the grain boundaries of the former austenite grains becomes 1.0% by mass or less. Further, as shown in FIG. 3, it can be seen that the average particle size of the old austenite grains is 5.0 ⁇ m or less by setting the T2-cooling stop temperature to less than 600 ° C.
- the invention examples and comparative examples of FIGS. 2 and 3 are obtained by extracting a part of all the invention examples and all the comparative examples in the examples.
- FIG. 4 is a diagram showing the relationship between the holding time at the heating temperature T2 and the average Mn concentration of the grain boundaries of the former austenite grains in the examples.
- FIG. 5 is a diagram showing the relationship between the holding time at the heating temperature T2 and the average particle size of the old austenite grains in the examples.
- the average Mn concentration of the grain boundaries of the old austenite grains is 1.0% by mass or less by setting the holding time at the heating temperature T2 to more than 10 seconds and 60 seconds or less.
- the average particle size of the old austenite grains is 5.0 ⁇ m or less by setting the holding time at the heating temperature T2 to more than 10 seconds and 60 seconds or less.
- the invention examples and comparative examples of FIGS. 4 and 5 are obtained by extracting a part of all the invention examples and all the comparative examples in the examples.
- the hot stamping steel sheet heated and held at the heating temperature T2 is made into a hot stamping molded body by hot stamping, and is cooled at the following cooling rate.
- Average cooling rate in the temperature range from hot stamp molding to 200 ° C 10 ° C / s or more, 500 ° C / s or less
- the average cooling rate in the temperature range from hot stamp molding to 200 ° C is 10 ° C.
- the microstructure of the hot stamped product becomes the main phase of martensite (including fresh martensite and tempered martensite). If the average cooling rate is less than 10 ° C./s, baking is not sufficiently performed, a soft phase such as ferrite is formed in the microstructure, and the toughness of the hot stamped compact is deteriorated. Therefore, the average cooling rate is set to 10 ° C./s or more.
- the average cooling rate is set to 500 ° C./s or less. Preferably, it is 300 ° C./s or less.
- tempering may be performed by heating to a temperature range of 100 ° C. to 600 ° C. and holding the temperature in that temperature range. Further, for the purpose of improving the deformability of the hot stamped molded product, a softened region may be formed in a part of the hot stamped molded product after hot stamping and cooling.
- the softened region referred to here means a region formed by irradiating only a part (for example, a flange portion) of the hot stamp molded product with a laser and tempering it.
- the conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is described in this one condition example. It is not limited. In the present invention, various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
- hot stamping steel sheet Each hot stamping steel sheet and hot stamping plated steel sheet (hereinafter collectively referred to as “hot stamping steel sheet”) are subjected to the first heat treatment (pre-heat treatment) and the second heat treatment shown in Tables 4 to 8 to perform hot stamping.
- pre-heat treatment pre-heat treatment
- Tables 4 to 8 to perform hot stamping.
- Cooling 1 indicates cooling from the heating temperature T1 to "cooling stop temperature of 250 ° C. or higher and 400 ° C. or lower”
- cooling 2 is in the temperature range below the cooling stop temperature.
- Indicates cooling, and "cooling 3" indicates the average cooling rate in the temperature range from after hot stamping to 200 ° C.
- some of the hot stamped compacts were tempered by heating and holding them in a temperature range of 100 to 600 ° C. for the purpose of adjusting the strength.
- “Yes” was described in the "Annealed” column in Tables 4 to 8.
- a part of the hot stamped molded product was heated to 200 ° C. by irradiating a part of the hot stamped molded product with a laser to form a partially softened region.
- “Yes” was described in the column of "Partially softened region” in Tables 9 to 13.
- the microstructure of the hot stamping steel sheet and the hot stamped compact was measured by the above-mentioned measuring method.
- the mechanical properties of the hot stamp molded product were measured.
- the results are shown in Tables 9 to 13.
- the mechanical properties of the hot stamped article were measured and evaluated by the following methods.
- the test numbers in Tables 6 and 11 are shown in Table 11. In No. 66, the cooling rate at the time of the first heat treatment was too fast and cracks were generated, so that the microstructure of the hot stamped molded product was not observed.
- the tensile strength of the hot stamped product was determined by preparing the No. 5 test piece described in JIS Z 2241: 2011 from a position as flat as possible in the hot stamped product and following the test method described in JIS Z 2241: 2011. It was. When the tensile strength was 2000 MPa or more, it was judged to be acceptable as having excellent strength. On the other hand, when the tensile strength was less than 2000 MPa, it was judged as rejected because it did not have excellent strength.
- the indentation Vickers hardness was measured at intervals of 3 times or more. A total of 20 points were measured, and the average value thereof was calculated to obtain an average value (average hardness) of Vickers hardness.
- the average hardness obtained by this method was used for the toughness evaluation described later. When the average hardness is 650 Hv or more, it can be determined that the hardness is sufficient.
- the toughness of the hot stamped body was evaluated by the early breaking characteristics and the hardness variation ( ⁇ Hv).
- the value obtained by dividing the tensile strength (unit: MPa) of the hot stamped product by the value obtained by multiplying the average hardness (unit: Hv) by 3.3 was used as an index of the early breaking characteristics.
- the tensile strength and the average hardness are values obtained by the above method.
- the value obtained by multiplying the average hardness by 3.3 is the tensile strength estimated from the hardness, and if the measured value of the tensile strength is 0.60 MPa / Hv times or more of the estimated tensile strength, early fracture occurs. It can be judged that the characteristics are excellent.
- Hardness variation ( ⁇ Hv) In a hot stamped body having a tensile strength of 2000 MPa or more, when deformation (stress) occurs from the outside, if the hardness variation ( ⁇ Hv) in the hot stamped body is large, stress concentration occurs and the toughness deteriorates. May be done. The toughness deteriorates when the hardness variation ( ⁇ Hv) exceeds 50 Hv.
- the hardness variation ( ⁇ Hv) was defined as the difference between the maximum value and the minimum value of Vickers hardness at 20 points, which was obtained when the average hardness was obtained by the above method.
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Abstract
Description
本願は、2019年3月20日に、日本に出願された特願2019-052103号に基づき優先権を主張し、その内容をここに援用する。
二回目熱処理の保持工程後の鋼材には、ホットスタンプ、および、冷却が実施される。
なお、Ac3’点は、実験によって求められる温度である。詳細は後述する。
[1]本発明の一態様に係るホットスタンプ成形体は、質量%で、
C :0.40%以上、0.70%以下、
Si:0.010%以上、1.30%以下、
Mn:0.40%以上、3.00%以下、
sol.Al:0.0010%以上、0.500%以下、
Ti:0.010%以上、0.100%以下、
Cr:0.010%以上、0.80%以下、
B :0.0005%以上、0.0100%以下、
P :0.100%以下、
S :0.0100%以下、
N :0.0100%以下、
Nb:0%以上、0.100%以下、
Mo:0%以上、1.00%以下、
V :0%以上、0.100%以下、
Ni:0%以上、0.50%以下、
REM:0%以上、0.0100%以下、
Mg:0%以上、0.0100%以下、
Ca:0%以上、0.0100%以下、
Co:0%以上、4.00%以下、並びに
残部がFe及び不純物である化学組成を有し、
ミクロ組織中の旧オーステナイト粒の平均粒径が5.0μm以下であり、
前記旧オーステナイト粒の粒界の平均Mn濃度が1.0質量%以下である。
[2]上記[1]に記載のホットスタンプ成形体は、質量%で、
Nb:0.010%以上、0.100%以下、
Mo:0.01%以上、1.00%以下、
V :0.001%以上、0.100%以下、
Ni:0.001%以上、0.50%以下、
REM:0.0010%以上、0.0100%以下、
Mg:0.0010%以上、0.0100%以下、
Ca:0.0010%以上、0.0100%以下、および
Co:0.10%以上、4.00%以下
から選択される1種または2種以上の元素を含有してもよい。
[3]上記[1]または[2]に記載のホットスタンプ成形体は、表面にめっき層を備えてもよい。
[4]上記[1]~[3]のいずれか1項に記載のホットスタンプ成形体は、一部に軟化領域を有してもよい。
まず、本実施形態に係るホットスタンプ成形体の化学組成の限定理由について説明する。以下、化学組成についての%は全て質量%を意味する。「以上」または「以下」と示す数値には、その値が数値範囲に含まれる。「未満」または「超」と示す数値には、その値が数値範囲に含まれない。
Cは、ホットスタンプ成形体において2000MPa以上の引張強さを得るために重要な元素である。C含有量が0.40%未満では、マルテンサイトが軟質となり、2000MPa以上の引張強さを得ることが困難である。そのため、C含有量は0.40%以上とする。C含有量は、好ましくは0.43%以上、0.45%以上である。一方、C含有量が0.70%超では、粗大な炭化物が生成して破壊が生じやすくなり、ホットスタンプ成形体の靭性が低下する。そのため、C含有量は0.70%以下とする。C含有量は、好ましくは0.60%以下、0.55%以下である。
Siは、粗大なセメンタイトの形成を抑制する効果があり、ホットスタンプ成形体の靭性を確保するために重要な元素である。また、Siは、焼戻し軟化抵抗を有しており、ホットスタンプ焼入れ時の自己焼戻しによる強度低下を抑える作用がある。Si含有量が0.010%未満では上記効果が得られず、ホットスタンプ成形体の靭性が劣化する場合がある。そのため、Si含有量は0.010%以上とする。好ましくは、0.02%以上、0.03%以上である。一方、1.30%超のSiを含有する場合、オーステナイトの安定度が低下し、二回目熱処理時に大傾角粒界へのMnの拡散が十分に進まないことから、ホットスタンプ成形体の靭性が劣化する。そのため、Si含有量は1.30%以下とする。好ましくは、1.20%以下、1.00%以下である。
Mnは、固溶強化によりホットスタンプ成形体の強度の向上に寄与する元素である。Mn含有量が0.40%未満では、固溶強化能が乏しくマルテンサイトが軟質となり、ホットスタンプ成形体において2000MPa以上の引張強さを得ることは困難である。そのため、Mn含有量は0.40%以上とする。Mn含有量は、好ましくは0.50%以上、0.60%以上である。一方、Mn含有量を3.00%超とすると、鋼中に粗大な介在物が生成して破壊が生じやすくなり、ホットスタンプ成形体の靭性が低下する。そのため、Mn含有量は3.00%以下とする。好ましくは、2.50%以下、2.00%以下、1.50%以下である。
Alは、溶鋼を脱酸して鋼を健全化する(鋼にブローホールなどの欠陥が生じることを抑制する)作用を有する元素である。sol.Al含有量が0.0010%未満では、脱酸が十分に行われない。そのため、sol.Al含有量は0.0010%以上とする。sol.Al含有量は、好ましくは0.010%以上、0.020%以上、である。一方、sol.Al含有量が0.500%を超えると、鋼中に粗大な酸化物が生成し、ホットスタンプ成形体の靱性が低下する。そのため、sol.Al含有量は0.500%以下とする。好ましくは、0.400%以下、0.350%以下である。
なお、sol.Alとは酸可溶性Alを意味し、固溶状態で鋼中に存在する固溶Alのことを示す。
Tiは、炭窒化物を形成し、ホットスタンプ加熱時(特に、二回目熱処理時)のオーステナイトの粒成長を抑制する元素である。Ti含有量が0.010%未満では上記効果が得られず、旧オーステナイト粒が粗大化し、ホットスタンプ成形体の靭性が劣化する。そのため、Ti含有量は0.010%以上とする。Ti含有量は、好ましくは0.020%以上、0.025%以上である。一方、0.100%を超えてTiを含有させると、粗大なTiNが生成するため、ホットスタンプ成形体の靭性が劣化する。そのため、Ti含有量は0.100%以下とする。Ti含有量は、好ましくは0.080%以下、0.060%以下である。
Crは、炭化物形成元素であるとともに炭化物を微細化させて、ホットスタンプ成形体の靭性を向上させる元素である。Cr含有量が0.010%未満では上記効果が得られない。そのため、Cr含有量は0.010%以上とする。Cr含有量は、好ましくは0.10%以上、0.15%以上である。一方、0.80%超のCrを含有させても上記効果は飽和する。また、旧オーステナイト粒界のMn偏析サイトを埋めてしまい、一回目熱処理時のMnの旧オーステナイト粒界への偏析を阻害する。その結果、旧オーステナイト粒内のMn量が増加し、ホットスタンプ成形体の靭性が劣化する場合がある。そのため、Cr含有量は0.80%以下とする。Cr含有量は、好ましくは0.60%以下、0.50%以下、0.40%以下である。
Bは、粒界に偏析して鋼の焼き入れ性を高める元素である。B含有量が0.0005%未満では上記効果が得られず、フェライトが形成される場合がある。その結果、2000MPa以上の引張強さを得ることが困難となる場合およびホットスタンプ成形体の靭性が劣化する場合がある。そのため、B含有量は0.0005%以上とする。B含有量は、好ましくは0.0010%以上、0.0015%以上、0.0020%以上である。一方、Bは旧オーステナイト粒界に偏析しやすいため、0.0100%を超えて含有させると、一回目熱処理時に、旧オーステナイト粒界へのMnの偏析を阻害する。その結果、旧オーステナイト粒内のMnが増加し、ホットスタンプ成形体の靭性が劣化する場合がある。よって、B含有量は0.0100%以下とする。B含有量は、好ましくは0.0075%以下、0.0050%以下である。
Pは、粒界に偏析し、粒界の強度を低下させる元素である。P含有量が0.100%を超えると、粒界の強度が著しく低下して、ホットスタンプ成形体の靱性が低下する。そのため、P含有量は0.100%以下とする。P含有量は、好ましくは0.050%以下、0.030%以下である。P含有量の下限は特に限定しないが、0.0001%未満に低減すると、脱Pコストが大幅に上昇し、経済的に好ましくない。実操業上、P含有量は0.0001%以上としてもよい。
Sは、鋼中に介在物を形成する元素である。S含有量が0.0100%を超えると、鋼中に多量の介在物が生成し、ホットスタンプ成形体の靱性が低下する。そのため、S含有量は0.0100%以下とする。S含有量は、好ましくは0.0040%以下である。S含有量の下限は特に限定しないが、0.00015%未満に低減すると、脱Sコストが大幅に上昇し、経済的に好ましくない。実操業上、S含有量は0.00015%以上、0.0002%以上としてもよい。
Nは、不純物元素であり、鋼中に窒化物を形成してホットスタンプ成形体の靱性を劣化させる元素である。N含有量が0.0100%を超えると、鋼中に粗大な窒化物が生成されて、ホットスタンプ成形体の靱性が著しく低下する。そのため、N含有量は0.0100%以下とする。N含有量は、好ましくは0.0075%以下、0.0050%以下である。N含有量の下限は特に限定しないが、0.0001%未満に低減すると、脱Nコストが大幅に上昇し、経済的に好ましくない。実操業上、N含有量は0.0001%以上としてもよい。
Nbは、固溶強化によりホットスタンプ成形体の強度を向上させるとともに炭窒化物を形成することにより旧オーステナイト粒の細粒化に寄与する元素である。そのため、必要に応じてNbを含有させても良い。Nbを含有させる場合、上記効果を確実に発揮させるために、Nb含有量は0.010%以上とすることが好ましい。Nb含有量は、より好ましくは0.035%以上である。一方、0.100%を超えてNbを含有させると、炭窒化物が過剰に生成し、ホットスタンプ成形体の靭性が低下する場合がある。そのため、Nb含有量は0.100%以下とすることが好ましい。Nb含有量は、より好ましくは0.080%以下である。
Moは、固溶強化によりホットスタンプ成形体の強度を向上させるとともに、鋼の焼き入れ性を高め、靭性を劣化させるフェライトの形成を抑制する元素である。そのため、必要に応じてMoを含有させても良い。Moを含有させる場合、上記効果を確実に発揮させるために、Mo含有量は0.01%以上とすることが好ましい。Mo含有量は、より好ましくは0.02%以上である。一方、1.00%を超えてMoを含有させても上記効果は飽和するばかりか、合金コストの上昇を引き起こす。そのため、Mo含有量は1.00%以下とすることが好ましい。Mo含有量は、より好ましくは0.80%以下である。
Vは、固溶強化によりホットスタンプ成形体の強度を向上させる元素である。この効果を確実に得るためには、V含有量は0.001%以上とすることが好ましい。より好ましくは、V含有量は0.050%以上である。一方、V含有量が0.100%を超えると、炭窒化物が過剰に生成し、ホットスタンプ成形体の靱性が低下する。そのため、V含有量は0.100%以下とすることが好ましい。V含有量は、より好ましくは0.090%以下である。
Niは、オーステナイトに固溶し、鋼の焼き入れ性を高める作用を有するとともに、ホットスタンプ成形体の靭性を向上させる元素である。上記効果を確実に得るためには、Ni含有量を0.001%以上とすることが好ましい。より好ましくは、0.01%以上である。一方、0.50%を超えてNiを含有させても上記効果は飽和するとともに合金コストの上昇を引き起こす。そのため、Ni含有量は0.50%以下とすることが好ましい。より好ましくは、0.40%以下である。
REMは、溶鋼を脱酸して鋼を健全化する作用を有する元素であるとともに、ホットスタンプ成形体の靭性を向上させる元素である。そのため、必要に応じてREMを含有させてもよい。上記効果を確実に得るためには、REM含有量は0.0010%以上とすることが好ましい。より好ましくは、0.0020%以上である。一方、0.0100%を超えてREMを含有させても上記効果が飽和して、コストの上昇を引き起こす。そのため、REM含有量は0.0100%以下とすることが好ましい。より好ましくは、0.0080%以下である。
なお、本実施形態においてREMとは、Sc、Y及びランタノイドからなる合計17元素を指す。本実施形態では、REMの含有量とはこれらの元素の合計含有量を指す。ランタノイドの場合、工業的にはミッシュメタルの形で添加される。
Mgは、溶鋼を脱酸して鋼を健全化する作用を有する元素であり、ホットスタンプ成形体の靭性を向上させる。そのため、必要に応じてMgを含有させてもよい。上記効果を確実に得るためには、Mg含有量は0.0010%以上とすることが好ましい。より好ましくは、0.0020%以上である。一方、0.0100%を超えてMgを含有させても、上記効果は飽和してコストの上昇を引き起こす。そのため、Mg含有量は0.0100%以下とすることが好ましい。より好ましくは、0.0080%以下である。
Caは、溶鋼を脱酸して鋼を健全化する作用を有する元素であり、ホットスタンプ成形体の靭性を向上させる。そのため、必要に応じてCaを含有させてもよい。上記効果を確実に得るためには、Ca含有量は0.0010%以上とすることが好ましい。より好ましくは、0.0020%以上である。一方、0.0100%を超えてCaを含有させても、上記効果は飽和してコストの上昇を引き起こす。そのため、Ca含有量は0.0100%以下とすることが好ましい。より好ましくは、0.0080%以下である。
Coは、マルテンサイト開始温度(Ms点)を上昇させる作用を有する元素であり、ホットスタンプ成形体の靭性を向上させる。そのため必要に応じてCoを含有させてもよい。Coを含有させる場合、上記効果を確実に発揮させるためには、Co含有量は0.10%以上とすることが好ましい。より好ましくは0.20%以上である。一方、Co含有量が4.00%超を超えると鋼の焼き入れ性が低下し、2000MPa以上の引張強さを得ることが困難となる。そのため、Co含有量は4.00%以下とすることが好ましい。より好ましくは3.00%以下である。
次に、本実施形態に係るホットスタンプ成形体のミクロ組織について説明する。本実施形態において、ホットスタンプ成形体のミクロ組織とは、表面から板厚t/4位置(tは板厚)を中心とした、表面から板厚t/8深さ~表面から板厚3t/8深さの領域におけるミクロ組織を意味する。
本実施形態に係るホットスタンプ成形体は、ミクロ組織中の旧オーステナイト粒の平均粒径が5.0μm以下であり、前記旧オーステナイト粒の粒界の平均Mn濃度が1.0質量%以下である。以下、各規定について説明する。
ホットスタンプ成形体において優れた靭性を得るためには、ミクロ組織は微細であるほど好ましい。本発明者らは、引張強さが2000MPaを超える高強度のホットスタンプ成形体では、旧オーステナイト粒の平均粒径が5.0μmを超えると靭性が劣化することを知見した。従って、旧オーステナイト粒の平均粒径は5.0μm以下とする。より好ましくは、4.5μm以下、4.0μm以下、3.5μm以下である。
旧オーステナイト粒の平均粒径は、1.0μm以上、または2.0μm以上としてもよい。
旧オーステナイト粒界の平均Mn濃度は、0.1質量%以上、または0.2質量%以上としてもよい。
旧オーステナイト粒の平均粒径は、次の方法により測定する。
まず、ホットスタンプ成形体を540℃で24hr熱処理する。これにより、旧オーステナイト粒界の腐食が促進される。熱処理は、炉加熱や通電加熱によって行えばよく、昇温速度は0.1~100℃/s、冷却速度は0.1~150℃/sとする。熱処理後のホットスタンプ成形体の中央部(端部を避けた部分)から板面に垂直な板厚断面を切り出す。この板厚断面を#600から#1500の炭化珪素ペーパーを使用して研磨した後、粒度1~6μmのダイヤモンドパウダーをアルコール等の希釈液や純水に分散させた液体を使用して鏡面に仕上げる。この板厚断面を観察面とする。
旧オーステナイト粒の粒界の平均Mn濃度の測定方法について説明する。
ホットスタンプ成形体の中央部(端部を避けた部分)から、図1に示す寸法の試験片を作製する。試験片は、板厚(図1中の紙面に垂直な方向の試験片長さ)が1.2mmとなるように、その表裏面を同量ずつ機械研削によって除去する。試験片の長さ方向(図1中の左右方向)の中央部には、切れ込みが設けられる。この切れ込みは、厚さ1mmのワイヤーカッターを挿入して形成する。試験片の幅方向(図1中の上下方向)において、切れ込み底と、切り込みが設けられていない側面との距離は、100~200μmに制御する。
好ましくは、ミクロ組織は、マルテンサイト(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を含む)が主相(面積率で90%以上)であり、残部組織(上部ベイナイト、下部ベイナイトおよび残留オーステナイト、並びに、鉄炭化物および/または合金炭化物)の面積率が10%以下である。マルテンサイトの面積率は、より好ましくは95%以上であり、更に好ましくは100%である。残部組織の面積率は、マルテンサイトの面積率との関係から、より好ましくは5%以下、更に好ましくは0%であることが好ましい。
マルテンサイトの面積率の測定は以下の方法により行う。
ホットスタンプ成形体の端面から50mm以上離れた位置(または端部を避けた位置)から、板厚断面が観察できるようにサンプルを採取する。観察面を研磨した後、ナイタール腐食を行い、炭化物および結晶粒界のコントラストを明瞭にする。次に、2次電子検出器を装備した電解放射型走査型電子顕微鏡(FE-SEM)を用い、サンプルの板厚t/4位置を中心とした領域(表面から板厚の1/8深さ~表面から板厚の3/8深さの領域)について、5000倍の撮影倍率で2次電子像を撮影する。
下部ベイナイトおよび焼戻しマルテンサイトもラス状結晶粒の集合からなる相であるが、ラス内部に炭化物を含む相である。下部ベイナイトと焼戻しマルテンサイトとは炭化物の伸長方向により区別する。下部ベイナイトの炭化物は単一のバリアントを有し、一つの結晶粒内に存在する炭化物の角度差は5°以内であり、実質的に単一の方向を有する。一方、焼戻しマルテンサイトの炭化物は複数のバリアントを有し、一つの結晶粒内に存在する炭化物は複数の方向に伸長している。これらの差異より、下部ベイナイトと焼戻しマルテンサイトとを区別する。
残部組織の面積率は、100%からマルテンサイトの面積率を差し引くことで得る。
ホットスタンプ成形体のミクロ組織において、粗大な炭化物が多く含まれると、ホットスタンプ成形体の靭性が劣化する場合がある。そのため、粗大な炭化物量は極力少ないことが望ましい。本実施形態では、円相当直径が0.20μm以上である炭化物の個数密度は、0.5個/μm2以下とすることが好ましい。より好ましくは、0.3個/μm2以下、0.2個/μm2以下である。円相当直径が0.20μm以上である炭化物の個数密度は少ない程好ましいため、0個/μm2としてもよい。
ホットスタンプ成形体の板厚断面が観察面となるように試料を採取し、観察面を電界研磨によって仕上げる。その後、表面から板厚t/8深さ~表面から板厚3t/8深さの領域を、倍率20000倍で10視野以上観察する。画像解析により、観察された各炭化物の面積から、各炭化物の円相当直径を求める。円相当直径が0.20μm以上である炭化物の個数密度を算出することで、円相当直径が0.20μm以上である炭化物の個数密度を得る。
なお、本実施形態では、マルテンサイト中のラス内あるいはラス状に存在する長径5nm以上の粒子を炭化物とみなす。
本実施形態に係るホットスタンプ成形体は、引張(最大)強さが2000MPa以上であってもよい。好ましくは、2200MPa以上である。上限は特に限定しないが、2600MPa以下、2500MPa以下としてもよい。
本実施形態に係るホットスタンプ成形体は、早期破断特性の指標である値が0.60MPa/Hv以上であり、且つ硬度ばらつき(ΔHv)が50Hv以下であってもよい。早期破断特性の指標である値は、引張強さ(単位:MPa)を、後述の方法により得られる平均硬さ(単位:Hv)に3.3を乗じた値で除した値(引張強さ/(平均硬さ×3.3))である。この値は、好ましくは、0.75MPa/Hv以上、0.80MPa/Hv以上である。平均硬さに3.3を乗じた値は、硬さから推定される推定引張強さであり、引張強さの実測値が推定引張強さの0.60MPa/Hv倍以上であれば、早期破断特性に優れるため、靭性に優れると判断することができる。
ホットスタンプ成形体の任意の位置(端部を避けた位置)から表面に垂直な板厚断面が観察できるように試験片を切り出す。試験片の長さは、測定装置にもよるが、10mm程度で良い。試験片の板厚断面を#600から#1500の炭化珪素ペーパーを使用して研磨した後、粒度1~6μmのダイヤモンドパウダーをアルコール等の希釈液や純水に分散させた液体を使用して鏡面に仕上げる。この板厚断面を測定面とする。マイクロビッカース硬さ試験機を用いて、測定面の板厚t/4位置(表面から板厚t/8深さ~表面から板厚3t/8深さの領域)において、荷重1kgfで圧痕の3倍以上の間隔でビッカース硬さを測定する。合計で20点測定し、それらの平均値を算出することで、ビッカース硬さの平均値(平均硬さ)を得る。
なお、硬度ばらつき(ΔHv)は、上述の方法で平均硬さを求めた際に得られた、20点におけるビッカース硬さの最大値と最小値との差を算出することで得る。
ホットスタンプ用鋼板の大傾角粒界の割合の算出方法について説明する。
ホットスタンプ用鋼板の任意の位置から表面に垂直な断面(板厚断面)が観察できるように試験片を切り出す。試験片の長さは、測定装置にもよるが、10mm程度で良い。試験片の断面を#600から#1500の炭化珪素ペーパーを使用して研磨した後、粒度1~6μmのダイヤモンドパウダーをアルコール等の希釈液や純水に分散させた液体を使用して鏡面に仕上げる。この板厚断面を観察面とする。
次に、本実施形態に係るホットスタンプ成形体の好ましい製造方法について説明する。まず、本実施形態に係るホットスタンプ成形体に適用されるホットスタンプ用鋼板の製造方法について説明する。
「加熱工程」
熱間圧延に供する鋼片(鋼材)は、常法で製造した鋼片であればよく、例えば、連続鋳造スラブ、薄スラブキャスターなどの一般的な方法で製造した鋼片であればよい。前述の化学組成を有する鋼材を熱間圧延に供し、熱間圧延工程で1100℃以上の温度域に加熱し、この温度域で20分以上の保持を行うことが好ましい。加熱温度が1100℃未満となる場合または保持時間が20分未満となる場合には、Ti等の粗大介在物の再溶解が進まず、破壊起点として残存するため、ホットスタンプ成形体の靭性が劣化する場合がある。より好ましくは、加熱温度は1200℃以上であり、保持時間は25分以上である。また、加熱温度は1400℃以下が好ましく、保持時間は120分以下が好ましい。
次に、仕上げ圧延の完了温度(仕上げ圧延温度)がAr3点以上の温度域となるように熱間圧延を行うことが好ましい。Ar3点未満の温度で仕上げ圧延を終了すると、二相域圧延となることから圧延での板形状が劣化する場合がある。このため、仕上げ圧延温度はAr3点以上とするのが好ましい。より好ましくはAr3点+10℃以上である。また、仕上げ圧延温度は、Ar3点+100℃以下とすることが好ましい。
Ar3点=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo ・・・式(1)
仕上げ圧延後の鋼板を750℃以下の温度域でコイル状に巻き取る。巻取り温度が750℃を超えるとスケールが多量に生成し、次工程の酸洗工程でのスケール除去が困難となる。そのため、巻取り温度は750℃以下とすることが好ましい。より好ましくは600℃以下である。また、巻取り温度は、400℃以上とすることが好ましい。
以上の方法により熱延鋼板を得る。
上記の方法で得られたホットスタンプ用鋼板を用いて、例えば、下記の製造方法により本実施形態に係るホットスタンプ成形体を製造する。上述の通り本実施形態では、ホットスタンプ成形体において所望のミクロ組織を得るため、二回の熱処理を行う。
本実施形態に係るホットスタンプ成形体は、上記ホットスタンプ用鋼板をホットスタンプ工程に供する前に、一回目熱処理を行う。一回目熱処理では、Ac3点以上、Ac3点+200℃以下加熱温度T1まで加熱し、この温度T1で保持を行う。この一回目熱処理の加熱において、旧オーステナイト粒界にMnを濃化させる。加熱温度T1がAc3点を下回る場合には、旧オーステナイト粒界へのMn濃化が十分に進まず、続く二回目熱処理でMn濃度の低減を十分に行えない。このため、加熱温度T1はAc3点以上とする。好ましくはAc3点+20℃以上である。一方、加熱温度T1がAc3点+200℃超となる場合、旧オーステナイト粒が粗大化し、旧オーステナイト粒の平均粒径を5.0μm以下とすることができない場合がある。このため、加熱温度T1は、Ac3+200℃以下とする。加熱温度T1までの平均加熱速度は、1~30℃/sとすればよい。
Ac3点は以下の式(2)より得られる。
上記式(2)中の各元素記号は、各元素の含有量(質量%)を示す。当該元素を含有しない場合は0を代入する。
上記の加熱温度T1から、後述の冷却停止温度までの平均冷却速度が10℃/s以上、500℃/s以下となるように冷却する。この冷却により、ミクロ組織をマルテンサイト主相とすることで、旧オーステナイト粒内に多量の大傾角粒界が導入される。この大傾角粒界であるブロック界面には、微細なオーステナイトが存在しており、二回目熱処理時のオーステナイトの微細化、および旧オーステナイト粒界のMn濃度の低減に強い影響を及ぼす。すなわち、この大傾角粒界は、二回目熱処理での旧オーステナイト粒界のMnの拡散経路となることから、旧オーステナイト粒界のMn濃度を低下させるのに重要な役割を担う。
一回目熱処理の冷却は、単に、マルテンサイトを形成するだけではなく、マルテンサイトのブロック界面にオーステナイトを残存させる必要がある。上述したように、この残存したオーステナイトは、二回目熱処理でのMnの拡散経路となるからである。このオーステナイトの安定化のためには、マルテンサイトから未変態オーステナイトへのC拡散を促進させる必要がある。このために、250℃以上、400℃以下の温度域で冷却を停止する。冷却停止温度が250℃未満の場合、マルテンサイトから未変態オーステナイトへのC拡散が進まない。従って、冷却停止温度は250℃以上とする。好ましくは260℃以上である。冷却停止温度が400℃超となる場合、炭化物が生成し、ブロック間の残留オーステナイトの安定化が進まない。従って、冷却停止温度は400℃以下とする。
二回目熱処理でのMnの拡散経路となるオーステナイトを残すため、上記冷却停止温度以下の冷却速度を制御し、マルテンサイトから未変態オーステナイトへの炭素拡散を促進させ、オーステナイトを安定化させる必要がある。この作用を発現させるために、冷却停止温度以下の平均冷却速度を10℃/s未満に制御する。好ましくは8℃/s以下である。冷却停止温度以下の冷却速度が10℃/s以上となる場合、マルテンサイトから未変態オーステナイトへの炭素拡散が進まず、オーステナイトの安定度が低くなり、残留オーステナイトを残すことができないため、オーステナイト粒が二回目熱処理時の加熱過程で粗大化するとともに、旧オーステナイト粒界のMn濃度を低減できない場合がある。
一回目熱処理を行ったホットスタンプ用鋼板について、旧オーステナイト粒の微細化、および旧オーステナイト粒界のMn濃度を低減するために、ホットスタンプ時の加熱(二回目熱処理)の平均加熱速度を制御する。二回目熱処理の平均加熱速度を10℃/s以上とすることで、旧オーステナイト粒の粒成長を抑制できる。また、一回目熱処理で導入した大傾角粒界を拡散経路として、旧オーステナイト粒界から大傾角粒界へのMnの拡散を進めることができる。それらの結果、旧オーステナイト粒を微細化すること、および旧オーステナイト粒界のMn濃度を低減することができる。これにより、ホットスタンプ成形体の靭性を向上することができる。このため、平均加熱速度を10℃/s以上とする。好ましくは、30℃/s以上である。一方、平均加熱速度が1000℃/s超となる場合、ホットスタンプ成形体の加熱温度制御が困難となり、部位によって旧オーステナイト粒の平均粒径を5.0μm以下とすることができない場合がある。その結果、ホットスタンプ成形体の靭性が劣化する場合がある。このため、平均加熱速度は1000℃/s以下とする。好ましくは、700℃/s以下である。
一回目熱処理で形成した残留オーステナイトは、Mnが濃化している。Mnがオーステナイト安定化元素であるために、Ac3点が一回目熱処理よりも低下する。この低下したAc3点を、「Ac3 ’点」と呼称し、二回目熱処理時の加熱温度をT2と呼称する。
図2に示す通り、T2-冷却停止温度を600℃未満とすることで、旧オーステナイト粒の粒界の平均Mn濃度が1.0質量%以下となることが分かる。また、図3に示す通り、T2-冷却停止温度を600℃未満とすることで、旧オーステナイト粒の平均粒径が5.0μm以下となることが分かる。
なお、図2および図3の発明例および比較例は、実施例における全発明例および全比較例のうち一部を抽出したものである。
図4に示す通り、加熱温度T2での保持時間を10秒超、60秒以下とすることで、旧オーステナイト粒の粒界の平均Mn濃度が1.0質量%以下となることが分かる。また、図5に示す通り、加熱温度T2での保持時間を10秒超、60秒以下とすることで、旧オーステナイト粒の平均粒径が5.0μm以下となることが分かる。
なお、図4および図5の発明例および比較例は、実施例における全発明例および全比較例のうち一部を抽出したものである。
ホットスタンプ成形後から200℃までの温度域の平均冷却速度を10℃/s以上、500℃/s以下に制御することにより、ホットスタンプ成形体のミクロ組織をマルテンサイト(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を含む)主相とする。平均冷却速度が10℃/s未満の場合、焼きが十分に入らず、ミクロ組織中にフェライト等の軟質相が形成され、ホットスタンプ成形体の靭性が劣化する。このため、平均冷却速度を10℃/s以上とする。好ましくは30℃/s以上である。一方、平均冷却速度が500℃/s超となる場合、マルテンサイトの自己焼戻しが十分に進まず、ミクロ組織中の内部応力が高くなり、ホットスタンプ成形体の靭性が劣化する場合がある。このため、平均冷却速度を500℃/s以下とする。好ましくは、300℃/s以下である。
更に、一部のホットスタンプ成形体には、ホットスタンプ成形体の一部にレーザーを照射することで200℃に加熱して、部分軟化領域を形成した。部分軟化領域を形成したホットスタンプ成形体については、表9~13中の「部分軟化領域」の欄に「有り」と記載した。
なお、表6および表11の試験No.66は、一回目熱処理時の冷却速度が速すぎて割れが発生したため、ホットスタンプ成形体のミクロ組織等について観察を行わなかった。
ホットスタンプ成形体の引張強さは、ホットスタンプ成形体の出来るだけ平坦な位置から、JIS Z 2241:2011に記載の5号試験片を作製し、JIS Z 2241:2011に記載の試験方法に従って求めた。引張強さが2000MPa以上であった場合、優れた強度を有するとして合格と判定した。一方、引張強さが2000MPa未満であった場合、優れた強度を有しないとして不合格と判定した。
ホットスタンプ成形体の任意の位置(端部を避けた位置)から表面に垂直な断面(板厚断面)が観察できるように試験片を切り出した。試験片の長さは、10mm程度とした。試験片の板厚断面を#600から#1500の炭化珪素ペーパーを使用して研磨した後、粒度1~6μmのダイヤモンドパウダーをアルコール等の希釈液や純水に分散させた液体を使用して鏡面に仕上げた。この板厚断面を測定面とした。マイクロビッカース硬さ試験機を用いて、測定面の板厚t/4位置(表面から板厚t/8深さ~表面から板厚3t/8深さの領域)において、荷重1kgfで、圧痕の3倍以上の間隔でビッカース硬さを測定した。合計で20点測定し、それらの平均値を算出することで、ビッカース硬さの平均値(平均硬さ)を得た。この方法で得た平均硬さは、後述の靭性評価に使用した。
なお、平均硬さが650Hv以上である場合に、十分な硬度を有していると判断できる。
ホットスタンプ成形体の靭性は早期破断特性および硬度ばらつき(ΔHv)により評価した。ホットスタンプ成形体の引張強さ(単位:MPa)を、平均硬さ(単位:Hv)に3.3を乗じた値で除した値を、早期破断特性の指標である値とした。なお、引張強さおよび平均硬さは、上述の方法により得た値である。
平均硬さに3.3を乗じた値は、硬さから推定される引張強さであり、引張強さの実測値が推定引張強さの0.60MPa/Hv倍以上であれば、早期破断特性に優れると判断できる。
2000MPa以上の引張強さを有するホットスタンプ成形体においては、外部から変形(応力)が生じる場合に、ホットスタンプ成形体内の硬度ばらつき(ΔHv)が大きいと、応力集中が生じることで、靭性が劣化する場合がある。硬度ばらつき(ΔHv)が50Hv超となる場合に靭性が劣化する。
なお、硬度ばらつき(ΔHv)は、上述の方法で平均硬さを求めた際に得られた、20点におけるビッカース硬さの最大値と最小値との差とした。
Claims (4)
- 質量%で、
C :0.40%以上、0.70%以下、
Si:0.010%以上、1.30%以下、
Mn:0.40%以上、3.00%以下、
sol.Al:0.0010%以上、0.500%以下、
Ti:0.010%以上、0.100%以下、
Cr:0.010%以上、0.80%以下、
B :0.0005%以上、0.0100%以下、
P :0.100%以下、
S :0.0100%以下、
N :0.0100%以下、
Nb:0%以上、0.100%以下、
Mo:0%以上、1.00%以下、
V :0%以上、0.100%以下、
Ni:0%以上、0.50%以下、
REM:0%以上、0.0100%以下、
Mg:0%以上、0.0100%以下、
Ca:0%以上、0.0100%以下、
Co:0%以上、4.00%以下、並びに
残部がFe及び不純物である化学組成を有し、
ミクロ組織中の旧オーステナイト粒の平均粒径が5.0μm以下であり、
前記旧オーステナイト粒の粒界の平均Mn濃度が1.0質量%以下である、
ホットスタンプ成形体。 - 前記化学組成が、質量%で、
Nb:0.010%以上、0.100%以下、
Mo:0.01%以上、1.00%以下、
V :0.001%以上、0.100%以下、
Ni:0.001%以上、0.50%以下、
REM:0.0010%以上、0.0100%以下、
Mg:0.0010%以上、0.0100%以下、
Ca:0.0010%以上、0.0100%以下、および
Co:0.10%以上、4.00%以下
から選択される1種または2種以上の元素を含有する、
請求項1に記載のホットスタンプ成形体。 - 表面にめっき層を備える、請求項1または2に記載のホットスタンプ成形体。
- ホットスタンプ成形体の一部に軟化領域を有する、請求項1~3のいずれか1項に記載のホットスタンプ成形体。
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