WO2019208556A1 - 鋼部材およびその製造方法 - Google Patents
鋼部材およびその製造方法 Download PDFInfo
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- WO2019208556A1 WO2019208556A1 PCT/JP2019/017177 JP2019017177W WO2019208556A1 WO 2019208556 A1 WO2019208556 A1 WO 2019208556A1 JP 2019017177 W JP2019017177 W JP 2019017177W WO 2019208556 A1 WO2019208556 A1 WO 2019208556A1
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- 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
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
-
- 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
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0236—Cold rolling
Definitions
- the present invention relates to a steel member and a manufacturing method thereof.
- This application claims priority based on Japanese Patent Application No. 2018-082625 filed in Japan on April 23, 2018, the contents of which are incorporated herein by reference.
- hot stamping technology has been adopted as a technology for press-forming materials that are difficult to form, such as high-strength steel plates.
- the hot stamping technique is a hot forming technique in which a material used for forming is heated and then formed.
- the steel material is soft and has good formability at the time of forming. Thereby, even a high-strength steel material can be accurately formed into a complicated shape.
- quenching is performed at the same time as molding with a press die, and thus the steel material after molding has sufficient strength.
- Patent Document 1 it is possible to impart a tensile strength of 1400 MPa or more to a steel material after forming by hot stamping technology.
- Patent Document 2 discloses a press-molded product that is excellent in toughness and hot press-molded with a tensile strength of 1.8 GPa or more.
- Patent Document 3 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more and further having good toughness and ductility.
- Patent Document 4 discloses a steel material having a tensile strength of 1.4 GPa or more and excellent ductility.
- Patent Document 5 discloses a hot press-formed product having excellent ductility.
- Patent Document 6 discloses a press-formed member having a tensile strength of 980 MPa or more and excellent ductility.
- Patent Document 7 discloses a molded member having a tensile strength of 1000 MPa or more and excellent ductility.
- Japanese Unexamined Patent Publication No. 2002-102980 Japanese Unexamined Patent Publication No. 2012-180594 Japanese Unexamined Patent Publication No. 2012-1802 International Publication No. 2016/163468 International Publication No. 2012/169638 International Publication No. 2011-111333 International Publication No. 2012/091328
- ⁇ Automotive steel plates applied to the car body are required to have not only the above-described formability but also crash safety after forming.
- the crash safety of automobiles is evaluated by the crushing strength and absorbed energy in the crash test of the entire vehicle body or steel members.
- the crushing strength greatly depends on the material strength, the demand for ultra-high strength steel sheets is dramatically increasing.
- automobile members have fracture toughness and deformability that decrease with the increase in strength of the steel sheet material. Therefore, the automobile members break at an early stage when the automobile member collides, or break at a site where deformation is concentrated.
- the crushing strength commensurate with the material strength is not exhibited, and the absorbed energy decreases. Therefore, in order to improve the collision safety, it is important to improve not only the material strength but also the fracture toughness and deformability of the automobile member, that is, the toughness and ductility of the steel plate material.
- Patent Documents 1 and 2 Although tensile strength and toughness are described, ductility is not considered. Further, according to the techniques described in Patent Documents 3 and 4, it is possible to improve the tensile strength, toughness, and ductility. However, the methods described in Patent Documents 3 and 4 are not sufficient to eliminate the fracture starting point and to control the highly ductile structure, and may not be able to further improve toughness and ductility. Further, in the techniques of Patent Documents 5, 6 and 7, although tensile properties and ductility are described, no consideration is given to toughness.
- the present invention has been made to solve the above-described problems, and an object of the present invention is to provide a steel member having high tensile strength and excellent ductility and a method for producing the same. It is an object of the present invention to provide a steel member having the above characteristics and excellent toughness, and a method for producing the same.
- the hot-formed steel member is not a flat plate but a formed body.
- the steel member is also referred to as a “steel member” including a formed body.
- the steel plate used as the raw material before heat processing of a steel member is also called "material steel plate.”
- the steel member according to one embodiment of the present invention has a chemical composition of mass%, C: 0.10 to 0.60% Si: 0.40 to 3.00%, Mn: 0.30 to 3.00%, P: 0.050% or less, S: 0.0500% or less, N: 0.010% or less, Ti: 0.0010 to 0.1000%, B: 0.0005 to 0.0100%, Cr: 0 to 1.00%, Ni: 0 to 2.0%, Cu: 0 to 1.0%, Mo: 0 to 1.0%, V: 0 to 1.0%, Ca: 0 to 0.010%, Al: 0 to 1.00%, Nb: 0 to 0.100%, Sn: 0 to 1.00%, W: 0 to 1.00%, REM: 0 to 0.30% And the balance is Fe and impurities,
- the metal structure in terms of volume fraction, martensite is 60.0-85.0%, bainite is 10.0-30.0%, retained austenite is 5.0-15.0%, and the remaining structure is 0-4.
- the maximum minor axis length of the retained austenite is 30 nm or more;
- the number density of carbides having an equivalent circle diameter of 0.1 ⁇ m or more and an aspect ratio of 2.5 or less is 4.0 ⁇ 10 3 pieces / mm 2 or less.
- the chemical composition is mass%, Cr: 0.01 to 1.00%, Ni: 0.01 to 2.0%, Cu: 0.01 to 1.0%, Mo: 0.01 to 1.0%, V: 0.01 to 1.0%, Ca: 0.001 to 0.010%, Al: 0.01 to 1.00%, Nb: 0.010 to 0.100%, Sn: 0.01 to 1.00%, One or more of W: 0.01 to 1.00% and REM: 0.001 to 0.30% may be contained.
- the value of the strain-induced transformation parameter k represented by the following formula (1) may be less than 18.0.
- f [gamma] 0 volume fraction of retained austenite present in the steel member before the true strain imparted f gamma (0.02): a true strain of 0.02 was assigned to the steel member, the steel member after unloading
- the volume fraction of retained austenite present therein [4] In the steel member according to any one of the above [1] to [3], the tensile strength is 1400 MPa or more and the total elongation is 10.0% or more. Also good.
- the steel member according to any one of [1] to [4] may have a local elongation of 3.0% or more.
- an impact value at ⁇ 80 ° C. may be 25.0 J / cm 2 or more.
- the cleanliness value of steel defined in JIS G 0555: 2003 may be 0.100% or less.
- a method for producing a steel member according to another aspect of the present invention is the method for producing a steel member according to any one of the above [1] to [7],
- Chemical composition is mass%, C: 0.10 to 0.60% Si: 0.40 to 3.00%, Mn: 0.30 to 3.00%, P: 0.050% or less, S: 0.0500% or less, N: 0.010% or less, Ti: 0.0010 to 0.1000%, B: 0.0005 to 0.0100%, Cr: 0 to 1.00%, Ni: 0 to 2.0%, Cu: 0 to 1.0%, Mo: 0 to 1.0%, V: 0 to 1.0%, Ca: 0 to 0.010%, Al: 0 to 1.00%, Nb: 0 to 0.100%, Sn: 0 to 1.00%, W: 0 to 1.00%, REM: 0 to 0.30% And the balance is Fe and impurities, and the number density of carbides having an equivalent circle diameter of 0.1 ⁇ m or more and an aspect ratio of 2.5 or
- a holding step of holding for up to 200 seconds may be provided.
- a holding step of holding for ⁇ 60 seconds may be provided.
- the raw steel plate is hot-formed between the heating step and the first cooling step. Also good.
- the steel plate is heated at the same time as cooling is performed at the first cooling rate. Inter-molding may be performed.
- C 0.10 to 0.60% C is an element that enhances the hardenability of steel and improves the strength of the steel member after quenching.
- the C content is 0.10% or more.
- the C content is preferably 0.15% or more, or 0.20% or more.
- the C content is 0.60% or less.
- the C content is preferably 0.50% or less, or 0.45% or less.
- Si 0.40 to 3.00% Si is an element that improves the hardenability of the steel and improves the strength of the steel member by solid solution strengthening. Furthermore, since Si hardly dissolves in the carbide, it suppresses the precipitation of the carbide during hot forming and promotes the C concentration to untransformed austenite. As a result, the Ms point is remarkably lowered and a large amount of austenite strengthened by solid solution can be left. In order to acquire this effect, it is necessary to contain 0.40% or more of Si. If the Si content is 0.40% or more, the residual carbide tends to decrease.
- the Si content is set to 0.40% or more.
- the Si content is preferably 0.50% or more, or 0.60% or more.
- the Si content in the steel exceeds 3.00%, the heating temperature required for the austenite transformation during the heat treatment becomes extremely high. As a result, the cost required for the heat treatment may increase, and ferrite may remain without being sufficiently austenitic, and a desired metal structure and strength may not be obtained. Therefore, the Si content is 3.00% or less.
- the Si content is preferably 2.50% or less, or 2.00% or less.
- Mn 0.30 to 3.00%
- Mn is an element that is very effective for enhancing the hardenability of the steel sheet and ensuring the strength after quenching stably. Further, Mn is an element that lowers the Ac 3 point and promotes lowering of the quenching temperature. However, if the Mn content is less than 0.30%, the above effect cannot be obtained sufficiently. Therefore, the Mn content is 0.30% or more. The Mn content is preferably 0.40% or more. On the other hand, when the Mn content exceeds 3.00%, the above effect is saturated, and further, the toughness of the quenched portion is deteriorated. Therefore, the Mn content is 3.00% or less. The Mn content is preferably 2.80% or less, and more preferably 2.50% or less.
- P 0.050% or less
- P is an element that deteriorates the toughness of the steel member after quenching.
- the P content is limited to 0.050% or less.
- the P content is preferably limited to 0.030% or less, 0.020% or less, or 0.005% or less.
- P is mixed as an impurity, there is no need to limit the lower limit in particular, and in order to obtain the toughness of the steel member, the content of P is preferably low. However, if the P content is excessively reduced, the manufacturing cost increases. From the viewpoint of manufacturing cost, the P content may be 0.001% or more.
- S 0.0500% or less
- S is an element that deteriorates the toughness of the steel member after quenching.
- the S content is limited to 0.0500% or less.
- the S content is preferably limited to 0.0030% or less, 0.0020% or less, or 0.0015% or less.
- S is mixed as an impurity, there is no need to limit the lower limit in particular, and in order to obtain the toughness of the steel member, the content of S is preferably low. However, if the S content is excessively reduced, the manufacturing cost increases. From the viewpoint of manufacturing cost, the S content may be 0.0001% or more.
- N 0.010% or less
- N is an element that deteriorates the toughness of the steel member after quenching.
- the N content exceeds 0.010%, coarse nitrides are formed in the steel, and the local deformability and toughness of the steel member are significantly deteriorated. Therefore, the N content is 0.010% or less.
- the lower limit of the N content is not particularly limited, but it is not economically preferable to make the N content less than 0.0002% because it causes an increase in steelmaking cost. Therefore, the N content is preferably 0.0002% or more, and more preferably 0.0008% or more.
- Ti 0.0010 to 0.1000% Ti suppresses recrystallization when the steel plate is heated to a temperature of Ac 3 point or higher and heat treatment is performed, and fine austenite grains are formed by forming fine carbides and suppressing grain growth. It is an element having an action. For this reason, the effect of greatly improving the toughness of the steel member is obtained by containing Ti. Further, Ti preferentially bonds with N in the steel to suppress the consumption of B due to the precipitation of BN, and promote the effect of improving the hardenability by B described later. When the Ti content is less than 0.0010%, the above effects cannot be obtained sufficiently. Therefore, the Ti content is set to 0.0010% or more. The Ti content is preferably 0.0100% or more, or 0.0200% or more.
- the Ti content is 0.1000% or less.
- the Ti content is preferably 0.0800% or less, or 0.0600% or less.
- B 0.0005 to 0.0100%
- B is a very important element in this embodiment because it has the effect of dramatically increasing the hardenability of steel even in a small amount. Moreover, B segregates at the grain boundary, thereby strengthening the grain boundary and increasing the toughness of the steel member. Furthermore, B suppresses the grain growth of austenite when the material steel plate is heated. If the B content is less than 0.0005%, the above effects may not be sufficiently obtained. Therefore, the B content is 0.0005% or more.
- the B content is preferably 0.0010% or more, 0.0015% or more, or 0.0020% or more.
- the B content exceeds 0.0100%, a large amount of coarse compounds are precipitated, and the toughness of the steel member deteriorates. Therefore, the B content is 0.0100% or less.
- the B content is preferably 0.0080% or less, or 0.0060% or less.
- the balance is Fe and impurities.
- impurities are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when industrially manufacturing steel sheets, and have an adverse effect on the steel member according to the present embodiment. It means what is allowed in the range.
- the above arbitrary elements may be contained. However, since the steel member according to the present embodiment can solve the problem without containing any of the following optional elements, the lower limit of the content when the optional element is not contained is 0%.
- Cr 0 to 1.00% Cr is an element that enhances the hardenability of the steel and makes it possible to stably secure the strength of the steel member after quenching, and thus may be contained.
- the Cr content is preferably 0.01% or more, and more preferably 0.05% or more.
- the Cr content exceeds 1.00%, the above effect is saturated, and the cost is increased unnecessarily.
- Cr has the effect
- when Cr content exceeds 1.00% coarse iron carbide will remain undissolved at the time of heating of a raw steel plate, and the toughness of a steel member will deteriorate. Therefore, when Cr is contained, the Cr content is 1.00% or less.
- the Cr content is preferably 0.80% or less.
- Ni 0 to 2.0%
- Ni is an element that enhances the hardenability of the steel and makes it possible to stably secure the strength of the steel member after quenching, so Ni may be contained.
- the Ni content is preferably 0.01% or more, and more preferably 0.1% or more.
- the Ni content exceeds 2.0%, the above effect is saturated and the cost is increased. Therefore, when Ni is contained, the Ni content is 2.0% or less.
- Cu 0 to 1.0%
- Cu is an element that enhances the hardenability of steel and makes it possible to stably secure the strength of the steel member after quenching, and thus may be contained. Moreover, Cu improves the corrosion resistance of the steel member in a corrosive environment.
- the Cu content is preferably 0.01%, more preferably 0.1% or more. However, if the Cu content exceeds 1.0%, the above effect is saturated and the cost is increased. Therefore, when Cu is contained, the Cu content is 1.0% or less.
- Mo 0 to 1.0%
- Mo is an element that enhances the hardenability of steel and makes it possible to stably secure the strength of the steel member after quenching, and thus may be contained.
- the Mo content is preferably 0.01% or more, and more preferably 0.1% or more.
- the Mo content exceeds 1.0%, the above effect is saturated and the cost is increased.
- Mo has the effect
- V 0 to 1.0%
- V is an element that forms fine carbides and makes it possible to increase the toughness of the steel member due to its fine graining effect.
- the V content is preferably 0.01% or more, and more preferably 0.1% or more. However, if the V content exceeds 1.0%, the above effect is saturated and the cost is increased. Therefore, when V is contained, the V content is 1.0% or less.
- Ca 0 to 0.010%
- Ca is an element having an effect of refining inclusions in the steel and improving the toughness and ductility of the steel member after quenching, and therefore may be contained.
- the Ca content is preferably 0.001% or more, and more preferably 0.002% or more.
- the Ca content is set to 0.010% or less.
- the Ca content is preferably 0.005% or less, and more preferably 0.004% or less.
- Al 0 to 1.00% Since Al is generally used as a deoxidizer for steel, it may be contained. In order to sufficiently deoxidize with Al, the Al content is preferably 0.01% or more. However, if the Al content exceeds 1.00%, the above effect is saturated and the cost is increased. Therefore, the Al content when Al is contained is 1.00% or less.
- Nb 0 to 0.100%
- Nb is an element that forms fine carbides and makes it possible to increase the toughness of the steel member due to the refinement effect thereof, and thus Nb may be contained.
- the Nb content is preferably 0.010% or more.
- the Nb content when Nb is contained is 0.100% or less.
- Sn 0 to 1.00% Sn may be contained in order to improve the corrosion resistance of the steel member in a corrosive environment.
- the Sn content is preferably 0.01% or more.
- the Sn content when Sn is contained is 1.00% or less.
- W 0 to 1.00%
- W is an element that enhances the hardenability of the steel and makes it possible to stably secure the strength of the steel member after quenching, and thus may be contained. Moreover, W improves the corrosion resistance of the steel member in a corrosive environment. In order to reliably obtain these effects, the W content is preferably 0.01% or more. However, if the W content exceeds 1.00%, the above effect is saturated and the cost is increased. Therefore, when W is contained, the W content is 1.00% or less.
- REM 0 to 0.30% Since REM is an element that has the effect of refining inclusions in steel and improving the toughness and ductility of the steel member after quenching in the same manner as Ca, it may be contained. In order to obtain this effect with certainty, the REM content is preferably 0.001% or more, and more preferably 0.002% or more. However, when the REM content exceeds 0.30%, the effect is saturated, and the cost is increased unnecessarily. Therefore, the REM content when REM is contained is 0.30% or less. The REM content is preferably 0.20% or less.
- REM refers to a total of 17 elements composed of lanthanoids such as Sc, Y, La and Nd, and the content of REM means the total content of these elements.
- REM is added to the molten steel using, for example, an Fe—Si—REM alloy, which includes, for example, Ce, La, Nd, Pr.
- (B) Metal structure of steel member The steel member according to this embodiment has a volume fraction of 60.0 to 85.0% martensite, 10.0 to 30.0% bainite, and 5.3% residual austenite. It has a metal structure of 0 to 15.0% and the balance structure of 0 to 4.0%. Further, the maximum minor axis length of retained austenite is 30 nm or more.
- the martensite present in the steel member according to this embodiment includes automatic tempered martensite.
- Automatic tempered martensite is tempered martensite generated during cooling during quenching without performing heat treatment for tempering, and the generated martensite is tempered by the heat generated by the martensitic transformation. Is to be generated.
- Tempered martensite can be distinguished from as-quenched martensite by the presence or absence of fine cementite precipitated in the lath.
- Martensite 60.0-85.0% Martensite is a hard phase and is a structure necessary for increasing the strength of steel members. If the martensite volume fraction is less than 60.0%, sufficient tensile strength of the steel member cannot be secured. Therefore, the volume fraction of martensite is 60.0% or more. Preferably, it is 65.0% or more. On the other hand, when the volume fraction of martensite exceeds 85.0%, other structures such as bainite and retained austenite described later cannot be sufficiently secured. Therefore, the volume fraction of martensite is 85.0% or less. Preferably, it is 80.0% or less.
- Bainite 10.0-30.0% Bainite has a higher hardness than retained austenite and a lower hardness than martensite.
- the presence of bainite relaxes the hardness gap between retained austenite and martensite, prevents cracking at the boundary between retained austenite and martensite when stress is applied, and improves the toughness and ductility of steel members. Improve. If the bainite volume fraction is less than 10.0%, the above effect cannot be obtained. Therefore, the bainite volume fraction is set to 10.0% or more.
- a preferable volume fraction of bainite is 15.0% or more. Moreover, since the intensity
- the preferred volume fraction of bainite is 25.0% or less, more preferably 20.0% or less.
- Residual austenite 5.0 to 15.0% Residual austenite has the effect of preventing necking and promoting work hardening and improving ductility (TRIP effect) by martensitic transformation (work-induced transformation) during plastic deformation. Furthermore, the stress concentration at the crack tip is relaxed by the transformation of retained austenite, and there is an effect of improving not only the ductility of the steel member but also the toughness. In particular, if the volume fraction of residual austenates is less than 5.0%, the ductility of the steel member is significantly reduced, the risk of fracture of the steel member is increased, and the collision safety is lowered. Therefore, the volume fraction of retained austenite is 5.0% or more.
- the volume fraction of retained austenite is 6.0% or more, More preferably, it is 7.0% or more.
- the volume fraction of retained austenite is 15.0% or less.
- it is 12.0% or less, or 10.0% or less.
- the retained austenite present in the steel member according to the present embodiment is present between the martensite lath, the bainite bainitic ferrite, or the former austenite grain boundary (old ⁇ grain boundary).
- the retained austenite is preferably present between laths of the martensite or bainitic ferrite of the bainite. Since the retained austenite present at these positions is flat, it has the effect of promoting deformation near these positions and improving the ductility and toughness of the steel member.
- ferrite and pearlite may be mixed as the remaining structure.
- the total volume fraction of martensite, bainite and retained austenite needs to be 96.0% or more. That is, in this embodiment, the remaining structure other than martensite, bainite, and retained austenite is limited to 4.0% or less in volume fraction. Since the remaining tissue may be 0%, the volume fraction of the remaining tissue is set to 0 to 4.0%.
- Maximum minor axis of retained austenite 30 nm or more
- the maximum minor axis of retained austenite is 30 nm or more. Residual austenite having a maximum minor axis of less than 30 nm is not stable in deformation, that is, martensitic transformation occurs in a low strain region in the early stage of plastic deformation, and thus cannot sufficiently contribute to the improvement of ductility and collision safety of a steel member. Therefore, the maximum minor axis of retained austenite is 30 nm or more.
- the upper limit of the maximum minor axis of retained austenite is not particularly limited, but if it is excessively stable in deformation, the TRIP effect is not sufficiently exhibited, so it may be 600 nm or less, 100 nm or less, or 60 nm or less.
- a method for measuring the volume fraction of martensite, bainite, and retained austenite, the location of retained austenite, and the maximum minor axis of retained austenite will be described.
- the volume fraction of retained austenite is measured using an X-ray diffraction method.
- a test piece is collected from a position 100 mm away from the end of the steel member. If the test piece cannot be collected from a position 100 mm away from the end due to the shape of the steel member, the test piece may be taken from a soaking part that avoids the end. This is because the end portion of the steel member is not sufficiently heat-treated and may not have the metal structure of the steel member according to the present embodiment.
- Chemical polishing is performed from the surface of the test piece to a depth of 1 ⁇ 4 of the plate thickness using hydrofluoric acid and hydrogen peroxide.
- the measurement conditions are a Co tube and a range of 45 ° to 105 ° at 2 ⁇ .
- the diffraction X-ray intensity of the face-centered cubic lattice (residual austenite) contained in the steel member is measured, and the volume fraction of retained austenite is calculated from the area ratio of the diffraction curve. Thereby, the volume fraction of retained austenite is obtained.
- the volume fraction of retained austenite in the steel member can be measured with high accuracy.
- the volume fraction of martensite and the volume fraction of bainite are measured by a transmission electron microscope (TEM) and an electron diffraction device attached to the TEM.
- a measurement sample is cut out from a position at a distance of 100 mm from the end of the steel member and at a thickness of 1 ⁇ 4 depth to obtain a thin film sample for TEM observation. If the measurement sample cannot be collected from a position 100 mm away from the end due to the shape of the steel member, the measurement sample may be collected from a soaking part that avoids the end.
- the range of TEM observation is 50 ⁇ m 2 or more in area, and the magnification is 1 to 50,000 times.
- iron carbide in martensite and bainite by diffraction pattern, observing its precipitation form, distinguishing martensite and bainite, and measuring martensite area fraction and bainite area fraction . If the precipitation form of iron carbide is three-directional precipitation, it is determined as martensite, and if it is limited precipitation in one direction, it is determined as bainite. Although the martensite and bainite fractions measured by TEM are measured as area fractions, the steel member according to the present embodiment has an isotropic metal structure. Can be replaced by rate. In addition, although iron carbide is observed for discrimination between martensite and bainite, in this embodiment, iron carbide is not included in the volume fraction of the metal structure.
- ferrite or pearlite is present as the remaining structure is confirmed by an optical microscope or a scanning electron microscope.
- these area fractions are obtained, and the values are converted into volume fractions as they are to obtain the volume fraction of the remaining tissue.
- the remaining structure is often hardly observed.
- a measurement sample is cut out from a cross section at a position 100 mm away from the end of the steel member, and used as a measurement sample for observing the remaining tissue. If the measurement sample cannot be collected from a position 100 mm away from the end due to the shape of the steel member, the measurement sample may be collected from a soaking part that avoids the end.
- the observation range by the optical microscope or the scanning electron microscope is an area of 40000 ⁇ m 2 or more, the magnification is 500 to 1000 times, and the observation position is 1/4 part of the plate thickness.
- the cut measurement sample is mechanically polished and then mirror-finished.
- etching is performed with a nital etchant (mixed solution of nitric acid and ethyl or methyl alcohol) to reveal ferrite and pearlite, and the presence of ferrite or pearlite is confirmed by observing this under a microscope.
- a structure in which ferrite and cementant are alternately arranged in layers is determined as pearlite, and a structure in which cementite is precipitated in a granular form is determined as bainite.
- the total area fraction of the observed ferrite and pearlite is obtained, and the value is directly converted into the volume fraction to obtain the volume fraction of the remaining tissue.
- the sum of the three volume fractions is 100. It may not be 0%.
- the three volume fractions may be adjusted so that the total becomes 100.0%.
- a value obtained by multiplying the obtained volume fraction of each tissue by 100.0 / 101.0 may be used as the volume fraction of each tissue. If the sum of the volume fraction of martensite and bainite, the retained austenite volume fraction, and the volume fraction of the remaining structure is less than 95.0%, or more than 105.0%, the volume is again Measure the fraction.
- the location of residual austenite is confirmed using TEM.
- the martensite in the metal structure of the steel member according to the present embodiment has a plurality of packets in the prior austenite grains, and inside each packet, there are blocks that are parallel strip structures, and each block further includes: There is a set of laths that are martensite crystals of almost the same crystal orientation.
- the limited-field diffraction pattern measurement is performed near the boundary between the laths to confirm the electron diffraction pattern near the boundary between the laths, and when the electron beam diffraction pattern of the face-centered cubic lattice is detected, It is determined that there is residual austenite between the laths. Since the lath is a body-centered cubic lattice and the retained austenite is a face-centered cubic lattice, it can be easily identified by electron diffraction.
- bainite in the metal structure of the steel member according to the present embodiment exists in a state where a plurality of bainitic ferrite crystal grains are aggregated.
- the grain of bainitic ferrite is confirmed by TEM, and the limited-field diffraction pattern is measured in the vicinity of the grain boundary of bainitic ferrite to obtain the electron beam diffraction pattern in the vicinity of the grain boundary of bainitic ferrite.
- an electron diffraction pattern of the face-centered cubic lattice it is determined that residual austenite exists between bainitics. Since bainitic ferrite is a body-centered cubic lattice and retained austenite is a face-centered cubic lattice, it can be easily distinguished by electron diffraction.
- the prior austenite grain boundaries exist in the metal structure of the steel member according to the present embodiment.
- a limited-field diffraction pattern measurement is performed to confirm the electron diffraction pattern in the vicinity of the prior austenite grain boundary. It is determined that austenite is present. Since martensite or bainite having a body-centered cubic lattice exists in the vicinity of the prior austenite grain boundary, the retained austenite of the face-centered cubic lattice can be easily determined by electron diffraction.
- the maximum minor axis of retained austenite is measured by the following method. First, a thin film sample is sampled from a position 100 mm away from the end of the steel member (if the test piece cannot be sampled from this position, a soaking part avoiding the end) and a position at a depth of 1/4 of the plate thickness. This thin film sample is magnified 50000 times with a transmission electron microscope, randomly observed in 10 fields (one field is 1.0 ⁇ m ⁇ 0.8 ⁇ m), and residual austenite is identified using an electron beam diffraction pattern. .
- the “maximum retained austenite” is a measurement of the cross-sectional area of the residual austenite crystal grains identified in each field of view, obtaining the equivalent circle diameter of a circle having the cross-sectional area, and indicating the largest equivalent austenite diameter.
- the “minor axis” of retained austenite is the distance between the parallel lines when assuming two parallel lines in contact with the outline of the crystal grain and sandwiching the crystal grain with respect to the crystal grain of retained austenite identified in each field of view. It is defined as the shortest distance (minimum ferret diameter) between parallel lines when parallel lines are drawn so as to have the shortest distance.
- a raw steel plate when heat treatment is performed on the raw steel plate.
- Sufficient hardenability can be ensured by re-dissolving the carbide.
- coarse carbides exist in the raw steel plate and the carbides are not sufficiently re-dissolved, sufficient hardenability cannot be ensured, and ferrite having low strength is precipitated. Therefore, as the coarse carbide in the raw steel plate is smaller, the hardenability is improved, and high strength can be obtained in the steel member after the heat treatment.
- the number density of carbides having an equivalent circle diameter of 0.1 ⁇ m or more in the steel member exceeds 4.0 ⁇ 10 3 pieces / mm 2 , the toughness and ductility of the steel member deteriorate. Therefore, the number density of carbide having an equivalent circle diameter of 0.1 ⁇ m or more present in the steel member is 4.0 ⁇ 10 3 pieces / mm 2 or less. Preferably, it is 3.5 ⁇ 10 3 pieces / mm 2 or less.
- the number density of carbides having an equivalent circle diameter of 0.1 ⁇ m or more present in the material steel plate is 8.0 ⁇ 10 3 pieces / mm 2 or less.
- the carbide carbonized_material in a steel member and a raw material steel plate points out a granular thing, and specifically targets what has an aspect ratio of 2.5 or less.
- the composition of the carbide is not particularly limited. Examples of the carbide include iron-based carbide, Nb-based carbide, and Ti-based carbide.
- carbides having a size of less than 0.1 ⁇ m do not have a significant effect on ductility, particularly local elongation, the size of the carbide whose number is limited is set to 0.1 ⁇ m or more in this embodiment.
- the number density of carbides is determined by the following method.
- the test piece is cut out from a position 100 mm away from the end of the steel member (if the test piece cannot be sampled from the position, a soaking part avoiding the end) or from a 1/4 width of the steel plate. After mirror-finishing the observation surface of the test piece, it was corroded with a picral solution, magnified 10,000 times with a scanning electron microscope, and randomly 10 fields (1 field is 10 ⁇ m ⁇ 8 ⁇ m) ).
- the equivalent circle diameter is 0.1 ⁇ m or more and A number density of carbides having an aspect ratio of 2.5 or less is obtained.
- (D) Mechanical property of steel member The steel member which concerns on this embodiment can obtain high ductility by the TRIP effect using the process induction transformation of a retained austenite. However, if the retained austenite is transformed with a low strain, high ductility due to the TRIP effect cannot be expected. That is, in order to further increase the ductility, it is preferable to control not only the amount and size of retained austenite but also its properties.
- the value of the strain-induced transformation parameter k represented by the following formula (1) is increased, the retained austenite is transformed at a low strain. For this reason, the value of the strain-induced transformation parameter k is preferably less than 18.0.
- f [gamma] 0 volume fraction of retained austenite present in the steel member before the true strain imparted f ⁇ (0.02): 0.02 true strain of grants against steel member, the steel member after dividing pressurized Volume fraction of retained austenite present therein
- Log in the above formula (1) is a logarithm having a base of 10, that is, a common logarithm.
- the volume fraction of retained austenite present in the steel member with respect to f ⁇ 0 and f ⁇ (0.02) is measured by the X-ray diffraction method described above. Note that it is considered that the amount of solute C in the retained austenite dominates whether or not transformation is likely to occur when strain is applied to the retained austenite, and the range of the Mn content in the steel member according to the present embodiment Then, there is a positive correlation between the volume fraction of retained austenite and the amount of dissolved C in retained austenite. For example, if the amount of dissolved C in the retained austenite is about 0.8%, the value of k is about 15 and shows excellent ductility, but the amount of dissolved C in the retained austenite is about 0.2%. If so, the value of k is about 53, so that all of the retained austenite is transformed with low strain, the ductility is lowered, and as a result, the collision safety is deteriorated.
- the steel member according to this embodiment preferably has a tensile strength of 1400 MPa or more and a total elongation of 10.0% or more. Furthermore, it is more preferable that the impact value at ⁇ 80 ° C. is 25.0 J / cm 2 or more while having these characteristics.
- the impact value at ⁇ 80 ° C. is 25.0 J / cm 2 or more while having these characteristics.
- the total elongation is an elongation obtained by adding a uniform elongation (a uniform elongation) until a constriction occurs and a local elongation until the subsequent breakage when a tensile test is performed.
- a uniform elongation a uniform elongation
- the local elongation is preferably set to 3.0% or more.
- ASTM E8-69 (ANNUAL BOOK OF ASTM STANDARD, PART 10, AMERICA SOCIETY FOR TESTING AND MATERIALS is used to measure the mechanical properties including the strain-induced transformation parameter k, tensile strength, total elongation, and local elongation. , P120-140), use the half-size plate-shaped test piece specified. Specifically, the tensile test is performed in accordance with the provisions of ASTM E8-69. A plate-shaped test piece having a thickness of 1.2 mm, a parallel part length of 32 mm, and a parallel part plate width of 6.25 mm is used.
- a room temperature tensile test is performed at a strain rate of 3 mm / min, and the maximum strength (tensile strength) is measured. Further, a 25 mm ruled line is put in advance in the parallel part of the tensile test, and the elongation rate (total elongation) is measured by attaching the broken samples. Then, the local elongation is obtained by subtracting the plastic strain (uniform elongation) at the maximum strength from the total elongation.
- the Charpy impact test for measuring the impact value is performed in accordance with the provisions of JIS Z 2242: 2005.
- the steel member is ground to a thickness of 1.2 mm, and a test piece having a length of 55 mm and a width of 10 mm is cut out in parallel with the rolling direction, and three of these are laminated to produce a test piece having a V notch.
- the V notch has an angle of 45 °, a depth of 2 mm, and a notch bottom radius of 0.25 mm.
- a Charpy impact test at a test temperature of -80 ° C. is performed to determine the impact value.
- Mn segregation degree of steel member Mn segregation degree ⁇ : 1.6 or less
- Mn is concentrated due to center segregation.
- MnS concentrates in the center of the plate thickness as inclusions, making it easy to form hard martensite, resulting in a difference in hardness from the surroundings and deterioration of the toughness of the steel member There is a case.
- the value of the Mn segregation degree ⁇ represented by the following formula (2) exceeds 1.6, the toughness of the steel member may deteriorate.
- the value of the Mn segregation degree ⁇ of the steel member may be 1.6 or less. In order to further improve the toughness, the value of the Mn segregation degree ⁇ may be 1.2 or less.
- the lower limit need not be specified, but the lower limit may be 1.0.
- Mn segregation degree ⁇ [maximum Mn concentration (mass%) at 1/2 part of plate thickness] / [average Mn concentration (mass%) at 1/4 part of plate thickness] Equation (2)
- the Mn segregation degree ⁇ is mainly controlled by the chemical composition, particularly the impurity content, and the conditions for continuous casting, and the Mn segregation degree ⁇ value does not change greatly due to heat treatment or hot forming.
- the value of Mn segregation degree ⁇ of the steel sheet By setting the value of Mn segregation degree ⁇ of the steel sheet to 1.6 or less, the value of Mn segregation degree ⁇ of the steel member after heat treatment can also be made 1.6 or less, that is, the toughness of the steel member is further increased. It becomes possible to improve.
- the maximum Mn concentration at 1/2 part of the plate thickness and the average Mn concentration at 1/4 part of the plate thickness are determined by the following methods.
- the observation surface is parallel to the rolling direction from a position 100 mm away from the end of the steel member (if the test piece cannot be sampled from that position, a soaking part avoiding the end) or from a half width of the steel plate And a sample is cut out so that it may become parallel to a plate thickness direction.
- EMA electronic probe microanalyzer
- 10 line analysis (1 ⁇ m) is randomly performed in the rolling direction at 1/2 part thickness of the sample, and three measured values are selected in descending order of Mn concentration from the analysis results.
- the average Mn concentration at 1/4 part of the plate thickness is also analyzed by using EPMA, analyzing 10 points at 1/4 part of the plate thickness of the sample, and calculating the average value.
- the average Mn concentration at can be determined.
- the cleanliness value is more preferably 0.060% or less.
- the value of the cleanliness of steel is obtained by calculating the area percentage occupied by the above-described A-based inclusions, B-based inclusions, and C-based inclusions.
- the cleanliness value of the steel member is also 0.100 by setting the cleanliness value of the material steel plate to 0.100% or less. % Or less.
- the cleanliness value of the raw steel plate or steel member is obtained by the point calculation method described in Annex 1 of JIS G 0555: 2003.
- the sample is cut out from a position where the width of the steel sheet is 1 ⁇ 4 part or 100 mm away from the end of the steel member (if the test piece cannot be sampled from the position, the soaking part avoiding the end).
- the plate thickness 1 ⁇ 4 part of the observation surface is magnified 400 times with an optical microscope, the A-type inclusions, the B-type inclusions and the C-type inclusions are observed, and the area percentages are calculated by the point calculation method. Observation is performed randomly in 10 fields (one field is 200 ⁇ m ⁇ 200 ⁇ m), and out of all fields, the value with the highest cleanliness value (lowest cleanliness) is set as the cleanliness of the material steel plate or steel member. Value.
- a hot-formed steel member is a molded body in many cases, and in this embodiment, the steel member is referred to as a “steel member” including a molded body.
- the steel member according to this embodiment has the above-described chemical composition, and the number density of carbides having an equivalent circle diameter of 0.1 ⁇ m or more and an aspect ratio of 2.5 or less is 8.0 ⁇ 10 3 pieces / mm. 2 or less, (Nb, Ti) average value of equivalent-circle diameters of C Whereas steel sheet is less than 5.0 .mu.m, it can be produced by heat treatment to be described later.
- the carbide precipitation form is limited as described above in the raw steel sheet to be subjected to the heat treatment is as follows. Although it is as above-mentioned to reduce the precipitation of the coarse carbide
- (Nb, Ti) C refers to Nb carbide and Ti carbide.
- the average value of the equivalent circle diameter of (Nb, Ti) C present in the raw steel plate exceeds 5.0 ⁇ m, the ductility of the steel member after heat treatment is deteriorated. Therefore, the average value of the equivalent circle diameter of (Nb, Ti) C existing in the raw steel plate is 5.0 ⁇ m or less.
- requiring the average value of a circle equivalent diameter of (Nb, Ti) C is as follows. After cutting out the cross section from the 1/4 width part of the material steel plate and mirror-polishing the observation surface of the sample, it was magnified 3000 times with a scanning electron microscope, and randomly 10 fields of view (one field is 40 ⁇ m ⁇ 30 ⁇ m) Make observations. For all the observed (Nb, Ti) C, the area of each (Nb, Ti) C is calculated, and the diameter of a circle having the same area as this area is defined as the equivalent circle diameter of each (Nb, Ti) C. . By calculating the average value of the equivalent circle diameters, the average value of the equivalent circle diameters of (Nb, Ti) C is obtained.
- a slab After melting steel having the above chemical composition in a furnace, a slab is produced by casting. At this time, in order to suppress the concentrated precipitation of MnS, which is the starting point of delayed fracture, it is desirable to perform a center segregation reduction process that reduces the center segregation of Mn.
- the center segregation reduction treatment include a method of discharging molten steel enriched in Mn in an unsolidified layer before the slab is completely solidified. Specifically, the molten steel in which Mn before complete solidification is concentrated can be discharged by performing a treatment such as electromagnetic stirring and unsolidified layer pressure reduction.
- the superheat temperature of the molten steel (molten steel superheat temperature) is set to a temperature 5 ° C. or more higher than the liquidus temperature of the steel, And it is desirable to suppress the amount of molten steel casting per unit time to 6 t / min or less.
- the molten steel superheating temperature is less than 5 ° C higher than the liquidus temperature at the time of continuous casting, the viscosity of the molten steel becomes high, and inclusions hardly float in the continuous casting machine. As a result, the inclusions in the slab It cannot increase and cleanliness cannot be reduced sufficiently. Further, when the casting amount of molten steel per unit time exceeds 6 t / min, the molten steel flow in the mold is fast, so that inclusions are easily trapped in the solidified shell, and inclusions in the slab increase and cleanliness. Is likely to get worse. On the other hand, by making the molten steel superheated temperature 5 ° C.
- the molten steel superheating temperature is preferably 8 ° C. or more higher than the liquidus temperature, and the molten steel casting amount per unit time is preferably 5 t / min or less.
- the cleanliness of the raw steel sheet may be 0.060% or less. Since it becomes easy, it is preferable.
- ⁇ ⁇ Soaking (soaking) treatment may be performed on the slab obtained by the above method as necessary.
- a preferable soaking temperature is 1150 to 1300 ° C.
- a preferable soaking time is 15 to 50 hours.
- Hot rolling is performed on the slab obtained by the above-described method.
- the slab In order to dissolve coarse (Nb, Ti) C, the slab is heated at 1200 ° C. or higher and subjected to hot rolling. From the viewpoint of more uniformly generating carbides, it is preferable that the hot rolling start temperature is 1000 to 1300 ° C. and the hot rolling completion temperature is 950 ° C. or higher.
- the coiling temperature after hot rolling is preferably higher from the viewpoint of workability, but if it is too high, the yield decreases due to scale formation, so it is preferably 450 to 700 ° C. Further, when the coiling temperature is lowered, the carbide is easily finely dispersed, and coarsening of the carbide can be suppressed.
- the form of carbide can be controlled by adjusting the subsequent annealing conditions in addition to the conditions in hot rolling.
- the annealing temperature is set to a high temperature, the carbide is once dissolved in the annealing stage, and then transformed at a low temperature. Since carbide is hard, its form does not change in cold rolling, and the existence form after hot rolling is maintained even after cold rolling.
- the material steel plate according to the present embodiment may be a hot-rolled steel plate or a hot-rolled annealed steel plate, a cold-rolled steel plate or a cold-rolled annealed steel plate, or a surface-treated steel plate such as a plated steel plate. What is necessary is just to select a process process suitably according to the required level etc. of the plate
- the hot-rolled steel sheet that has been descaled is annealed as necessary to obtain a hot-rolled annealed steel sheet.
- the hot-rolled steel sheet or hot-rolled annealed steel sheet is subjected to cold rolling as necessary to obtain a cold-rolled steel sheet, and the cold-rolled steel sheet is subjected to annealing as necessary to obtain a cold-rolled annealed steel sheet.
- the steel plate to be used for cold rolling is hard, it is preferable to increase the workability of the steel plate to be used for cold rolling by annealing before cold rolling.
- Cold rolling may be performed using a normal method. From the viewpoint of ensuring good flatness, the cumulative rolling reduction in cold rolling is preferably 30% or more. On the other hand, in order to avoid an excessive load, the cumulative rolling reduction in cold rolling is preferably 80% or less.
- the hot-rolled steel plate or the cold-rolled steel plate is annealed.
- annealing for example, a hot-rolled steel sheet or a cold-rolled steel sheet is held in a temperature range of 550 to 950 ° C.
- the temperature maintained by annealing is preferably 550 ° C. or higher.
- the temperature maintained by annealing exceeds 950 ° C.
- the structure may become coarse. The coarsening of the structure may reduce the toughness after quenching.
- the temperature maintained by annealing exceeds 950 ° C.
- the effect of increasing the temperature cannot be obtained, the cost increases, and the productivity only decreases. Therefore, even when producing either a hot-rolled annealed steel plate or a cold-rolled annealed steel plate, the temperature maintained by annealing is preferably 950 ° C. or lower.
- the average cooling rate at the time of annealing is a value obtained by dividing the temperature drop width of the steel sheet from the end of annealing holding to 550 ° C. by the required time from the end of annealing holding to 550 ° C.
- the plating layer may be an electroplating layer, or a hot dipping layer or an alloyed hot dipping layer.
- the electroplating layer include an electrogalvanizing layer and an electro Zn—Ni alloy plating layer.
- the molten plating layer include a molten aluminum plated layer, a molten Al—Si plated layer, a molten Al—Si—Mg plated layer, a hot dip galvanized layer, and a molten Zn—Mg plated layer.
- Alloyed hot-dip plating layers include alloyed hot-dip aluminum plating layers, alloyed hot-dip Al-Si plating layers, alloyed hot-melt Al-Si-Mg plating layers, alloyed hot-dip galvanizing layers, alloyed hot-dip Zn-Mg plating layers Etc. are exemplified.
- the plating layer may contain Mn, Cr, Cu, Mo, Ni, Sb, Sn, Ti, or the like.
- the adhesion amount of the plating layer is not particularly limited, and may be a general adhesion amount, for example.
- a plated layer or an alloyed plated layer may be provided on the steel member after the heat treatment.
- a steel plate having a tensile strength of 1400 MPa or more cannot be used as a raw steel plate. This is because when such a steel plate is used as a material steel plate, the strength is high, and cracks occur during the manufacture of the steel member.
- the average rate of temperature increase described below is a value obtained by dividing the temperature rise of the steel sheet from the start of heating to the end of heating by the required time from the start of heating to the end of heating.
- the first average cooling rate is a value obtained by dividing the temperature drop width of the steel plate from the start of cooling (when taken out of the heating furnace) to the Ms point by the time required for cooling from the start of cooling to the Ms point.
- the second average cooling rate is a value obtained by dividing the temperature drop width of the steel sheet from the Ms point to the end of cooling by the time from the Ms point to the end of cooling.
- the third average cooling rate is the temperature drop width of the steel sheet from the start of cooling (when taken out of the heating furnace) after the reheating step after the second cooling step to the end of cooling. The value divided by the time required until the hour.
- Heating process The material steel plate is heated to a temperature range of Ac 3 points to (Ac 3 points + 200) ° C. at an average temperature increase rate of 5 to 300 ° C./s (heating step). By this heating step, the structure of the material steel plate is made into an austenite single phase. In addition, if an average temperature increase rate is in the said range, even if it heats the raw material steel plate of room temperature, you may heat the raw material steel plate cooled to 550 degrees C or less by the cooling after the said annealing.
- the average heating rate is less than 5 ° C./s in the heating process, or when the temperature reached in the heating process exceeds (Ac 3 points + 200) ° C., the ⁇ grains become coarse and the strength of the steel member after heat treatment deteriorates. There is a fear. Moreover, in the 1st cooling process and 2nd cooling process mentioned later, austenite does not fully remain
- the strength of the steel member deteriorates.
- the ultimate temperature is less than Ac 3 points, ferrite remains in the metal structure of the material steel plate after the heating step, and it cannot be made an austenite single phase, and the strength of the steel member after heat treatment deteriorates. There is.
- deterioration of the strength, ductility and toughness of the steel member can be prevented by performing a heating process that satisfies the above conditions.
- First cooling process From the temperature range of Ac 3 point to (Ac 3 point +200) ° C., the Ms point (martensitic transformation) is applied to the steel plate that has undergone the above heating process so that diffusion transformation does not occur, in other words, ferrite and pearlite do not precipitate. Cooling at a first average cooling rate equal to or higher than the upper critical cooling rate until the start point) (first cooling step).
- the upper critical cooling rate is the minimum cooling rate at which austenite is supercooled to produce martensite without precipitation of ferrite or pearlite in the metal structure. When cooled below the upper critical cooling rate, ferrite is generated and the strength of the steel member is insufficient.
- the Ac 3 point, Ms point, and upper critical cooling rate are measured by the following methods.
- a test piece having a width of 30 mm and a length of 200 mm is cut out from the material steel plate having the above chemical components.
- the test piece is heated to 1000 ° C. at a temperature increase rate of 10 ° C./second in a nitrogen atmosphere, held at that temperature for 5 minutes, and then cooled to room temperature at various cooling rates.
- the cooling rate is set from 1 ° C / second to 100 ° C / second at intervals of 10 ° C / second.
- the Ac 3 point and Ms point are measured by measuring the thermal expansion change of the test piece during heating and cooling.
- the upper critical cooling rate is defined as the lowest critical cooling rate at which the ferrite phase does not precipitate among the test pieces cooled at the various cooling rates described above.
- the temperature range of (Ms-30) to (Ms-70 ° C) is 5 ° C / s or more, 150 ° C / s. Cooling is performed at a second average cooling rate that is less than the first average cooling rate (second cooling step).
- cooling is performed at a second average cooling rate that is 5 ° C./s or more and less than 150 ° C./s and is slower than the first average cooling rate, and cooling It is important that the stop temperature is in the temperature range of (Ms-30) to (Ms-70) ° C.
- residual austenite having a maximum minor axis of 30 nm or more that greatly contributes to the improvement of the ductility and toughness of the steel member is formed between martensite laths, bainitic ferrites, or old ⁇ grain boundaries. be able to.
- the second cooling step diffuses and concentrates supersaturated solute carbon from a part of the generated martensite into untransformed austenite in a temperature range below the Ms point, and is difficult to transform against plastic deformation.
- Stable retained austenite having a value of less than 18 can be produced.
- the second average cooling rate when the second average cooling rate is less than 5 ° C./s, carbon is excessively concentrated to the untransformed austenite around the martensite generated just below the Ms point, and is precipitated as a carbide. As a result, carbon does not diffuse sufficiently throughout the untransformed austenite, and retained austenite cannot be secured between the laths of martensite, between bainitic ferrite, or the prior ⁇ grain boundaries, and the amount is not sufficient. The ductility and toughness of the steel member are insufficient. When the second average cooling rate is 150 ° C./s or more, the time for carbon to diffuse into untransformed austenite is not sufficient, and martensite is formed adjacent to each other. As a result, the width of retained austenite between martensites is reduced (the maximum minor axis of retained austenite is less than 30 nm), and the amount is not sufficient, so that the ductility and toughness of the steel member are insufficient.
- the cooling stop temperature is less than (Ms-70) ° C.
- a large amount of martensite is generated, resulting in a shortage of retained austenite and a decrease in the maximum minor axis of retained austenite. Insufficient ductility.
- the cooling stop temperature is more than 250 ° C, more preferably 300 ° C or more.
- the cooling stop temperature is higher than (Ms-30) ° C., only a very small amount of martensite is generated, so that the amount of C concentrated from martensite to untransformed austenite is insufficient.
- Reheating process and "3rd cooling process” After the second cooling step (cooling to a temperature range of (Ms-30) to (Ms-70) ° C. at the second average cooling rate), an average temperature increase of 5 ° C./s or more to a temperature range of Ms to (Ms + 200) ° C. Reheating is performed at a speed (reheating process), and then cooling is performed at a third average cooling rate of 5 ° C./s or more (third cooling process).
- the reheating process promotes the diffusion and concentration of carbon into the untransformed austenite, and can increase the stability of the retained austenite.
- the ultimate temperature in the reheating step is lower than the Ms point, carbon diffusion and concentration into untransformed austenite are not sufficient, the stability of retained austenite is lowered, and the ductility and toughness of the steel member are insufficient.
- the ultimate temperature in the reheating step exceeds (Ms + 200) ° C., ferrite and pearlite are generated or bainite is excessively generated, so that the strength of the steel member is insufficient.
- the third average cooling rate when the third average cooling rate is less than 5 ° C./s, the carbon concentrated in the untransformed austenite precipitates as carbides, and the stability of the retained austenite becomes insufficient. Insufficient ductility and toughness.
- the first cooling step may be performed after holding in the temperature range of Ac 3 points to (Ac 3 points + 200) ° C. for 5 to 200 seconds.
- the material steel plate is made of Ac 3 It is preferable to hold for 5 s or more in the temperature range from point to (Ac 3 points + 200) ° C.
- the holding time is preferably set to 200 s or less from the viewpoint of productivity.
- a holding step may be performed between the reheating step and the third cooling step. That is, after the reheating step, the third cooling step may be performed after holding in the temperature range of Ms to (Ms + 200) ° C. for 3 to 60 seconds.
- the steel plate temperature may be varied in the temperature range of Ms to (Ms + 200) ° C., or the steel plate temperature may be kept constant in the temperature range of Ms to (Ms + 200) ° C.
- the steel sheet is held in the temperature range of Ms to (Ms + 200) ° C. for 3 seconds or more from the viewpoint of diffusing carbon and increasing the stability of retained austenite. It is preferable.
- the holding time is preferably 60 s or less from the viewpoint of productivity.
- the retained austenite By performing the holding step between the reheating step and the third cooling step, the retained austenite can be further stabilized, the k value can be reduced, and the TRIP effect can be further increased.
- the holding step it is presumed that the release of carbon from martensite and the concentration of carbon in the retained austenite are further promoted, and the retained austenite is further stabilized. If the temperature range of the holding step is less than the Ms point, the concentration of carbon to retained austenite is not promoted.
- the holding temperature in the holding process before the first cooling process and before the third cooling process may not be constant, and may vary as long as it is within a predetermined temperature range.
- Such hot forming may be performed.
- hot forming include bending, draw forming, stretch forming, hole expansion forming, and flange forming.
- a means for cooling the raw steel plate is provided at the same time as or immediately after forming, a forming method other than press forming, for example, roll forming may be performed.
- press forming for example, roll forming
- Hot forming may be performed simultaneously with the first cooling step, that is, the material steel plate may be hot formed simultaneously with the first cooling step of cooling at a cooling rate equal to or higher than the upper critical cooling rate.
- the forming since the forming is performed hot, since the raw steel plate is in a soft state, it is possible to obtain a steel member with high dimensional accuracy, which is preferable.
- the series of heat treatments described above can be performed by any method, and may be performed, for example, by induction heating, electric heating, or furnace heating.
- the present invention will be described more specifically by way of examples. However, the present invention is not limited to these examples.
- the present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
- a heat-treated steel plate as a raw steel plate was produced as follows.
- the cooling rate of the slab was controlled by changing the amount of water in the secondary cooling spray zone. Further, the center segregation reduction treatment was performed by performing a light reduction at a gradient of 1 mm / m using a roll in the final solidification portion and discharging the concentrated molten steel in the final solidification portion. Some slabs were then soaked at 1250 ° C. for 24 hours.
- the obtained slab was hot rolled by a hot rolling tester to obtain a hot rolled steel sheet having a thickness of 3.0 mm.
- descaling was performed after rough rolling, and finally finish rolling was performed.
- finish rolling was performed.
- the hot-rolled steel sheet was pickled in a laboratory. Furthermore, it cold-rolled by the cold rolling test machine, and it was set as the cold-rolled steel plate of thickness 1.4mm, and obtained the raw material steel plate.
- ⁇ Number density of carbides> When obtaining the number density of carbides with an equivalent circle diameter of 0.1 ⁇ m or more, a sample is cut out from the 1/4 width of the steel plate, the observation surface is mirror-finished, and then corroded and scanned with Picral solution. The image was magnified 10,000 times with a scanning electron microscope, and 10 fields of view (one field of view is 10 ⁇ m ⁇ 8 ⁇ m) and a thickness of 1/4 part were observed.
- the equivalent circle diameter is 0.1 ⁇ m or more and A number density of carbides having an aspect ratio of 2.5 or less was obtained.
- Mn segregation degree was measured by the following procedure. A sample was cut out from the plate width 1 ⁇ 2 part of the raw steel plate so that the observation surface was parallel to the rolling direction, and the electronic probe microanalyzer (EPMA) was used to measure the rolling direction and thickness at the plate thickness 1 ⁇ 2 part of the steel plate. Line analysis (1 ⁇ m) was performed at 10 locations parallel to the direction. After selecting three measured values in descending order from the analysis results, the average value was calculated to determine the maximum Mn concentration at the center of the plate thickness. In addition, at the 1/4 depth position of the plate thickness from the surface of the material steel plate (plate thickness 1 ⁇ 4 part), similarly, analysis is performed at 10 locations using EPMA, and the average value is calculated.
- EPMA electronic probe microanalyzer
- the average Mn concentration at the 1/4 depth position was determined. Then, by dividing the maximum Mn concentration at the center of the plate thickness by the average Mn concentration at the 1/4 depth position of the plate thickness from the surface, the Mn segregation degree ⁇ ([plate thickness at 1/2 part) Maximum Mn concentration (mass%)] / [average Mn concentration (mass%) at 1/4 part of plate thickness] was determined.
- ⁇ Ac 3 points, Ms point and upper critical cooling rate> The three Ac points and the upper critical cooling rate of each steel type were measured by the following method. A strip test piece having a width of 30 mm and a length of 200 mm was cut out from the obtained material steel plate, and the test piece was heated to 1000 ° C. at a heating rate of 10 ° C./second in a nitrogen atmosphere and held at that temperature for 5 minutes. After that, it was cooled to room temperature at various cooling rates. The cooling rate was set at an interval of 10 ° C./second from 1 ° C./second to 100 ° C./second. The Ac 3 point and Ms point were measured by measuring the thermal expansion change of the test piece during heating and cooling at that time. The upper critical cooling rate was defined as the lowest critical cooling rate at which the ferrite phase did not precipitate out of the test pieces cooled at the above cooling rate.
- the average value of (Nb, Ti) C circle equivalent diameter, Mn segregation degree ⁇ and cleanliness value of steel sheet is the average value of (Nb, Ti) C circle equivalent diameter, Mn segregation degree ⁇ and cleanness value of steel members.
- Example 1 A sample having a thickness of 1.4 mm, a width of 30 mm, and a length of 200 mm was taken from each of the above-described raw steel plates. The sample was collected so that the longitudinal direction was parallel to the rolling direction. Next, the collected sample is heated to a temperature range of (Ac 3 points + 50) ° C. at an average temperature increase rate of 10 ° C./s and held for 120 seconds, and then at a first average cooling rate equal to or higher than the upper critical cooling rate to the Ms point. Then, it is cooled to (Ms-50) ° C. at an average cooling rate (10 ° C./s) slower than the first average cooling rate, and then heated to (Ms + 75) ° C.
- ⁇ Tensile test> The tensile test was performed with an Instron tensile tester in accordance with ASTM standard E8-69. After the steel member sample was ground to a thickness of 1.2 mm, a half-size plate test piece (parallel portion length: 32 mm, parallel portion plate width: 6.25 mm) defined in ASTM standard E8-69 was collected. In addition, in the current heating apparatus cooling device used in the heat treatment of this example, the soaking part obtained from the sample having a length of about 200 mm is limited, so the ASTM standard E8-69 half size plate test piece should be adopted. It was.
- a strain gauge (KFGS-5 manufactured by Kyowa Denki Co., Ltd., gauge length: 5 mm) was attached to each test piece, a room temperature tensile test was performed at a strain rate of 3 mm / min, and the maximum strength (tensile strength) was measured. Further, a 25 mm ruled line was put in advance in the parallel part of the tensile test, and the elongation percentage (total elongation) was measured by attaching the broken sample. Then, the local elongation was obtained by subtracting the plastic strain (uniform elongation) at the maximum strength from the total elongation.
- the product of tensile strength and total elongation (tensile strength TS ⁇ total elongation EL) is obtained, and when TS ⁇ EL is 14000 MPa ⁇ % or more, it is determined that the strength-ductility balance is excellent, and the case of less than 14000 MPa ⁇ % It was determined that the strength-ductility balance was poor. Further, when TS ⁇ EL was 16000 MPa ⁇ % or more, it was evaluated that the strength-ductility balance was excellent, and when TS ⁇ EL was 18000 MPa ⁇ % or more, it was evaluated that the strength-ductility balance was further excellent.
- the Charpy impact test was performed in accordance with the provisions of JIS Z 2242: 2005.
- the steel member was ground to a thickness of 1.2 mm, a test piece having a length of 55 mm and a width of 10 mm was cut out, and three of these were laminated to prepare a test piece with a V notch.
- the V notch was set at an angle of 45 °, a depth of 2 mm, and a notch bottom radius of 0.25 mm.
- a Charpy impact test at a test temperature of ⁇ 80 ° C. was performed to determine the impact value. In this example, it was evaluated that the case having an impact value of 25.0 J / cm 2 or more was excellent in toughness.
- ⁇ X-ray diffraction> In the X-ray diffraction, first, a test piece was collected from the soaking part of the steel member, and chemically polished from the surface to a depth of 1 ⁇ 4 part by using hydrofluoric acid and hydrogen peroxide solution. With respect to the test piece after chemical polishing, the diffraction X-ray intensity of the face-centered cubic lattice (residual austenite) was measured by performing measurement at 2 ⁇ in the range of 45 ° to 105 ° using a Co tube. The volume fraction of retained austenite ( f ⁇ 0 ) was obtained by calculating the volume fraction of retained austenite from the area ratio of the obtained diffraction curve.
- the volume fraction of retained austenite (f ⁇ (0.02)) was determined by the same method as in the X-ray diffraction described above. From these, the strain-induced transformation parameter k represented by the following formula (i) was calculated and used as an index for increasing ductility due to the TRIP effect. As k increases, the retained austenite transforms at a lower strain, so that it is not possible to prevent squeezing at a high strain, that is, a high ductility due to the TRIP effect.
- f [gamma] 0 volume fraction of retained austenite present in the steel member before the true strain imparted f ⁇ (0.02): 0.02 true strain of grants against steel member, the steel member after dividing pressurized Volume fraction of residual austenite present in water
- ⁇ Maximum minor axis of residual ⁇ > A thin film sample was collected by thin film processing from the soaking part of the steel member and the position of the plate thickness 1 ⁇ 4 depth. Next, the image was magnified 50000 times using a transmission electron microscope, and 10 visual fields were randomly observed (1 visual field is 1.0 ⁇ m ⁇ 0.8 ⁇ m). At this time, retained austenite was identified using the electron diffraction pattern. Measure the shortest diameter of “maximum retained austenite” in each field of view, select the three “shortest diameters” from the largest in 10 fields of view, and calculate the average value of the “residual austenite” of the steel member. Of the maximum minor axis ”.
- the “maximum retained austenite” is a measurement of the cross-sectional area of the residual austenite crystal grains identified in each field of view, obtaining the equivalent circle diameter of a circle having the cross-sectional area, and indicating the largest equivalent austenite diameter. It was.
- the “minor axis” of retained austenite is the distance between the parallel lines when assuming two parallel lines in contact with the outline of the crystal grain and sandwiching the crystal grain with respect to the crystal grain of retained austenite identified in each field of view. It was set as the shortest space
- the measurement method of the martensite and bainite structure fraction (volume fraction) and the location of retained austenite was as follows. Each volume fraction of martensite and bainite was measured by an electron beam diffractometer attached to the TEM. A measurement sample was cut out from the position of the soaking part of the steel member and the plate thickness 1 ⁇ 4 depth, and was used as a thin film sample for TEM observation. The TEM observation range was 400 ⁇ m 2 in terms of area, and the magnification was 50000 times.
- the iron carbide (Fe 3 C) in martensite and bainite is found by the diffraction pattern of the electron beam irradiated on the thin film sample, and by observing the precipitation form, martensite and bainite are distinguished.
- the area fraction and the area fraction of bainite were measured. If the precipitation form of iron carbide was three-directional precipitation, it was determined to be martensite, and if it was limited precipitation in one direction, it was determined to be bainite. Although the fraction of martensite and bainite measured by electron beam diffraction of TEM is measured as an area fraction, the steel member of this example has an isotropic metal structure. The volume fraction was replaced as it was. In addition, although iron carbide was observed for discrimination between martensite and bainite, iron carbide was not included in the volume fraction of the metal structure.
- a measurement sample was cut out from the soaking part of the steel member and used as a measurement sample for observation of the remaining tissue.
- the observation range with a scanning electron microscope was an area of 40000 ⁇ m 2 , the magnification was 1000 times, and the measurement position was 1/4 part of the plate thickness.
- the cut out measurement sample was mechanically polished and then mirror-finished.
- etching was performed with a nital corrosion solution (mixed solution of nitric acid and ethyl or methyl alcohol) to reveal ferrite and pearlite, and the presence of ferrite or pearlite was confirmed by microscopic observation.
- the structure in which ferrite and cementant were alternately arranged in layers was determined as pearlite, and the cementite precipitated in a granular form was determined as bainite.
- the total area fraction of the observed ferrite and pearlite was obtained, and the value was converted into the volume fraction as it was to obtain the volume fraction of the remaining tissue.
- the location of residual austenite was confirmed using an electron diffraction pattern obtained by TEM.
- martensite of steel members there are multiple packets in the prior austenite grains, and there are blocks that are parallel strip structures inside each packet, and each block has martensite with almost the same crystal orientation.
- the electron diffraction pattern of the face-centered cubic lattice was detected, it was determined that residual austenite was present between the laths.
- the grain structure of bainitic ferrite is confirmed by TEM, and the limited-field diffraction pattern measurement is performed in the vicinity of the grain boundary of bainitic ferrite crystal grain. Confirmed the pattern.
- an electron diffraction pattern of a face-centered cubic lattice was detected, it was determined that residual austenite was present between bainitic ferrites.
- the limited-field diffraction pattern measurement was performed in the vicinity of the prior austenite grain boundary to confirm the electron beam diffraction pattern in the vicinity of the prior austenite grain boundary.
- the electron diffraction pattern of the face-centered cubic lattice was detected, it was determined that residual austenite was present at the prior austenite grain boundaries.
- Invention Examples B1 to B28 satisfying the scope of the present invention have good results in both metal structure and mechanical properties.
- Comparative Examples b1 to b16 that do not satisfy the scope of the present invention in Table 2B did not satisfy at least one of the metal structure and mechanical properties.
- Inventive Examples B1 to B28 in Table 2A all had good Mn segregation degrees of 1.6 or less and cleanliness of 0.100% or less.
- residual austenite was present between the martensite laths, between the bainitic ferrites of bainite, and at the prior austenite grain boundaries.
- the superheating temperature, the casting speed (casting amount), and the slab cooling rate were changed to change the Mn segregation degree and cleanliness of the slab. Thereafter, the slab was subjected to the same hot rolling, pickling, and cold rolling as described above, and then subjected to heat treatment under the same conditions as in Example 1 to produce a steel member.
- Table 3 shows the evaluation results of the obtained steel members C1 to C10. The evaluation method of each characteristic was carried out in the same manner as in Example 1.
- Inventive examples C1, C3 and C5 having a Mn segregation degree of 1.6 or less and a cleanliness of 0.100% or less are higher in impact value and local elongation than Inventive examples C2 and C4 manufactured from the same steel. Is even better.
- Inventive examples C6, C8 and C10 having a Mn segregation degree of 1.6 or less and a cleanliness of 0.100% or less are more effective than the inventive examples C7 and C9 manufactured from the same steel. The elongation is even better.
- Invention Example C2 having a slightly larger Mn segregation degree has slightly lower impact value and local elongation than Invention Examples C1, C3 and C5 produced from the same steel.
- Inventive example C7 having a slightly larger Mn segregation degree has a slightly lower impact value and local elongation than inventive examples C6, C8 and C10 manufactured from the same steel.
- Inventive Example C4 which has a slightly higher cleanliness, has a slightly lower impact value and local elongation than Inventive Examples C1, C3, and C5 manufactured from the same steel.
- the inventive example C9 which has a slightly higher cleanliness, has a slightly lower impact value and local elongation than C6, C8 and C10 manufactured from the same steel.
- retained austenite was present between the martensite laths, between the bainitic ferrites of bainite, and at the prior austenite grain boundaries.
- the steel plate was manufactured by performing the heat treatment shown in Table 4A and Table 4B on the material steel plate having the chemical composition of A26 and A27.
- the evaluation results of the metal structure and mechanical properties of the obtained steel members are shown in Table 5A and Table 5B.
- Invention Examples D1 to D28 satisfying the scope of the present invention are good results in both metal structure and mechanical properties, but Comparative Examples d1 to d34 not satisfying the scope of the present invention are: As a result, at least one of the metallographic structure and mechanical properties was not satisfied.
- Inventive Examples D1 to D28 all had good Mn segregation degrees of 1.6 or less and cleanliness of 0.100% or less.
- Invention Examples D1 to D28 retained austenite was present between martensite laths, between bainite bainitic ferrite, and at prior austenite grain boundaries.
- a steel member having a tensile strength of 1400 MPa or more and excellent ductility can be obtained.
- the steel member according to the present invention is particularly suitable for use as a collision-resistant component for automobiles.
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Abstract
Description
本願は、2018年4月23日に、日本に出願された特願2018-082625号に基づき優先権を主張し、その内容をここに援用する。
なお、熱間成形された鋼部材は、多くの場合、平板ではなく成形体であるが、本発明では、成形体である場合も含めて「鋼部材」という。また、鋼部材の熱処理前の素材となる鋼板を「素材鋼板」ともいう。
C:0.10~0.60%、
Si:0.40~3.00%、
Mn:0.30~3.00%、
P:0.050%以下、
S:0.0500%以下、
N:0.010%以下、
Ti:0.0010~0.1000%、
B:0.0005~0.0100%、
Cr:0~1.00%、
Ni:0~2.0%、
Cu:0~1.0%、
Mo:0~1.0%、
V:0~1.0%、
Ca:0~0.010%、
Al:0~1.00%、
Nb:0~0.100%、
Sn:0~1.00%、
W:0~1.00%、
REM:0~0.30%、
を含み、残部がFeおよび不純物であり、
金属組織が、体積分率で、マルテンサイトが60.0~85.0%、ベイナイトが10.0~30.0%、残留オーステナイトが5.0~15.0%および残部組織が0~4.0%であり、
前記残留オーステナイトの最大短径の長さが30nm以上であり、
円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の数密度が4.0×103個/mm2以下である。
[2]上記[1]に記載の鋼部材では、前記化学組成が、質量%で、
Cr:0.01~1.00%、
Ni:0.01~2.0%、
Cu:0.01~1.0%、
Mo:0.01~1.0%、
V:0.01~1.0%、
Ca:0.001~0.010%、
Al:0.01~1.00%、
Nb:0.010~0.100%、
Sn:0.01~1.00%、
W:0.01~1.00%、および
REM:0.001~0.30%の1種以上を含有してもよい。
[3]上記[1]または[2]に記載の鋼部材では、下記式(1)で表されるひずみ誘起変態パラメータkの値が18.0未満であってもよい。
k=(logfγ0-logfγ(0.02))/0.02 ・・・ 式(1)
但し、上記式(1)中の各記号の意味は以下の通りである。
fγ0:真ひずみ付与前の鋼部材中に存在する残留オーステナイトの体積分率
fγ(0.02):鋼部材に対して0.02の真ひずみを付与し、除荷した後の鋼部材中に存在する残留オーステナイトの体積分率
[4]上記[1]~[3]のいずれか一項に記載の鋼部材では、引張強度が1400MPa以上および全伸びが10.0%以上であってもよい。
[5]上記[1]~[4]のいずれか一項に記載の鋼部材では、局部伸びが3.0%以上であってもよい。
[6]上記[1]~[5]のいずれか一項に記載の鋼部材では、-80℃における衝撃値が25.0J/cm2以上であってもよい。
[7]上記[1]~[6]のいずれか一項に記載の鋼部材では、JIS G 0555:2003で規定される鋼の清浄度の値が0.100%以下であってもよい。
[8]本発明の別の態様に係る鋼部材の製造方法は、上記[1]~[7]の何れか1項に記載の鋼部材の製造方法であって、
化学組成が、質量%で、
C:0.10~0.60%、
Si:0.40~3.00%、
Mn:0.30~3.00%、
P:0.050%以下、
S:0.0500%以下、
N:0.010%以下、
Ti:0.0010~0.1000%、
B:0.0005~0.0100%、
Cr:0~1.00%、
Ni:0~2.0%、
Cu:0~1.0%、
Mo:0~1.0%、
V:0~1.0%、
Ca:0~0.010%、
Al:0~1.00%、
Nb:0~0.100%、
Sn:0~1.00%、
W:0~1.00%、
REM:0~0.30%、
を含み、残部がFeおよび不純物であり、かつ円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の数密度が8.0×103個/mm2以下であり、(Nb,Ti)Cの円相当直径の平均値が5.0μm以下である素材鋼板を、
Ac3点~(Ac3点+200)℃の温度域まで平均昇温速度5~300℃/sで加熱する加熱工程と、
前記加熱工程後、Ms点まで上部臨界冷却速度以上の第1平均冷却速度で冷却する第1冷却工程と、
前記第1冷却工程後、(Ms-30)~(Ms-70)℃の温度域まで、5℃/s以上、150℃/s未満であって前記第1平均冷却速度よりも遅い第2平均冷却速度で冷却する第2冷却工程と、
前記第2冷却工程後、Ms~(Ms+200)℃の温度域まで平均昇温速度5℃/s以上で加熱する再加熱工程と、
前記再加熱工程後、5℃/s以上の第3平均冷却速度で冷却する第3冷却工程と、を備える。
[9]上記[8]に記載の鋼部材の製造方法では、前記加熱工程と前記第1冷却工程との間に、前記Ac3点~(Ac3点+200)℃の前記温度域にて5~200秒間保持する保持工程を備えてもよい。
[10]上記[8]または[9]に記載の鋼部材の製造方法では、前記再加熱工程と前記第3冷却工程との間に、前記Ms~(Ms+200)℃の前記温度域にて3~60秒間保持する保持工程を備えてもよい。
[11]上記[8]~[10]のいずれか一項に記載の鋼部材の製造方法では、前記加熱工程と前記第1冷却工程との間において、前記素材鋼板に熱間成形を施してもよい。
[12]上記[8]~[10]のいずれか一項に記載の鋼部材の製造方法では、前記第1冷却工程において、前記第1冷却速度で冷却を行うと同時に、前記素材鋼板に熱間成形を施してもよい。
本実施形態に係る鋼部材の各元素の限定理由は下記のとおりである。なお、以下の説明において含有量についての「%」は、「質量%」を意味する。以下に記載する数値限定範囲には、下限値および上限値がその範囲に含まれる。「超」、「未満」と示す数値には、その値が数値範囲に含まれない。化学組成についての%は全て質量%を示す。
Cは、鋼の焼入れ性を高め、かつ焼入れ後の鋼部材の強度を向上させる元素である。しかし、C含有量が0.10%未満では、焼入れ後の鋼部材において十分な強度を確保することが困難となる。したがって、C含有量は0.10%以上とする。C含有量は、0.15%以上、または0.20%以上であることが好ましい。一方、C含有量が0.60%を超えると、焼入れ後の鋼部材の強度が高くなり過ぎて、靱性の劣化が著しくなる。したがって、C含有量は0.60%以下とする。C含有量は0.50%以下、または0.45%以下であることが好ましい。
Siは、鋼の焼入れ性を高め、かつ固溶強化により鋼部材の強度を向上させる元素である。さらに、Siは炭化物中にほとんど固溶しないため、熱間成形時に炭化物の析出を抑え、未変態オーステナイトへのC濃化を助長する。その結果、Ms点が著しく低下し、かつ固溶強化されたオーステナイトを多く残留させることができる。この効果を得るためには、Siを0.40%以上含有させる必要がある。なお、Si含有量が0.40%以上であると、残留炭化物は少なくなる傾向にある。後述するが、熱処理前の素材鋼板中に析出する炭化物が多いと、それらが熱処理時に溶け残り、十分な焼入れ性を確保できず、低強度なフェライトが析出し、鋼部材において強度が不足する場合がある。そのため、この意味でもSi含有量は0.40%以上とする。Si含有量は、0.50%以上、または0.60%以上であることが好ましい。
ただし、鋼中のSi含有量が3.00%を超えると、熱処理に際して、オーステナイト変態のために必要となる加熱温度が著しく高くなる。これにより、熱処理に要するコストの上昇を引き起こす場合および十分にオーステナイト化せずにフェライトが残留し、所望の金属組織および強度が得られない場合がある。したがって、Si含有量は3.00%以下とする。Si含有量は2.50%以下、または2.00%以下であることが好ましい。
Mnは、素材鋼板の焼入れ性を高め、かつ焼入れ後の強度を安定して確保するために、非常に効果のある元素である。さらに、Mnは、Ac3点を下げ、焼入れ処理温度の低温化を促進する元素である。しかし、Mn含有量が0.30%未満では上記効果が十分に得られない。そのため、Mn含有量は0.30%以上とする。Mn含有量は0.40%以上であることが好ましい。一方、Mn含有量が3.00%を超えると上記の効果は飽和し、さらに焼入れ部の靱性劣化を引き起こす。そのため、Mn含有量は3.00%以下とする。Mn含有量は2.80%以下であることが好ましく、2.50%以下であることがより好ましい。
Pは、焼入れ後の鋼部材の靱性を劣化させる元素である。特に、P含有量が0.050%を超えると、鋼部材の靱性が著しく劣化する。したがって、P含有量は0.050%以下に制限する。P含有量は、0.030%以下、0.020%以下、または0.005%以下に制限することが好ましい。Pは不純物として混入するが、その下限を特に制限する必要はなく、鋼部材の靭性を得るためには、Pの含有量は低い方が好ましい。ただし、P含有量を過剰に低減すると、製造コストが増加する。製造コストの観点からは、P含有量は0.001%以上としてもよい。
Sは、焼入れ後の鋼部材の靱性を劣化させる元素である。特に、S含有量が0.0500%を超えると、鋼部材の靱性が著しく劣化する。したがって、S含有量は0.0500%以下に制限する。S含有量は、0.0030%以下、0.0020%以下、または0.0015%以下に制限することが好ましい。Sは不純物として混入するが、その下限を特に制限する必要はなく、鋼部材の靭性を得るためには、Sの含有量は低い方が好ましい。ただし、S含有量を過剰に低減すると、製造コストが増加する。製造コストの観点からは、S含有量は0.0001%以上としてもよい。
Nは、焼入れ後の鋼部材の靱性を劣化させる元素である。特に、N含有量が0.010%を超えると、鋼中に粗大な窒化物が形成され、鋼部材の局部変形能および靱性が著しく劣化する。したがって、N含有量は0.010%以下とする。N含有量の下限は特に限定する必要はないが、N含有量を0.0002%未満とすることは製鋼コストの増大を引き起こすため、経済的に好ましくない。そのため、N含有量は0.0002%以上とすることが好ましく、0.0008%以上とすることがより好ましい。
Tiは、素材鋼板をAc3点以上の温度に加熱して熱処理を施す際に再結晶を抑制するとともに、微細な炭化物を形成して粒成長を抑制することで、オーステナイト粒を細粒にする作用を有する元素である。このため、Tiを含有させることによって、鋼部材の靱性が大きく向上する効果が得られる。また、Tiは、鋼中のNと優先的に結合することによってBNの析出によるBの消費を抑制し、後述するBによる焼入れ性向上の効果を促進する。Ti含有量が0.0010%未満では、上記の効果を十分に得られない。したがって、Ti含有量は0.0010%以上とする。Ti含有量は0.0100%以上、または0.0200%以上であることが好ましい。一方、Ti含有量が0.1000%を超えると、TiCの析出量が増加してCが消費されるため、焼入れ後の鋼部材の強度が低下する。したがって、Ti含有量は0.1000%以下とする。Ti含有量は0.0800%以下、または0.0600%以下であることが好ましい。
Bは、微量でも鋼の焼入れ性を劇的に高める作用を有するので、本実施形態において非常に重要な元素である。また、Bは粒界に偏析することで、粒界を強化して鋼部材の靱性を高める。さらに、Bは、素材鋼板の加熱時にオーステナイトの粒成長を抑制する。B含有量が0.0005%未満では、上記の効果を十分に得られない場合がある。したがって、B含有量は0.0005%以上とする。B含有量は0.0010%以上、0.0015%以上、または0.0020%以上であることが好ましい。一方、B含有量が0.0100%を超えると、粗大な化合物が多く析出し、鋼部材の靱性が劣化する。したがってB含有量は0.0100%以下とする。B含有量は0.0080%以下、または0.0060%以下であることが好ましい。
本実施形態に係る鋼部材には、残部のFeの一部に代えて、下記に示すCr、Ni、Cu、Mo、V、Ca、Al、Nb、Sn、WおよびREMから選択される1種以上の任意元素を含有させてもよい。ただし、下記に示す任意元素を含有させなくても本実施形態に係る鋼部材はその課題を解決することができるので、任意元素を含有させない場合の含有量の下限は0%である。
Crは、鋼の焼入れ性を高め、かつ焼入れ後の鋼部材の強度を安定して確保することを可能にする元素であるため、含有させてもよい。この効果を確実に得るためには、Cr含有量は0.01%以上であることが好ましく、0.05%以上であることがより好ましい。しかし、Cr含有量が1.00%を超えると上記の効果は飽和し、いたずらにコストの増加を引き起こす。また、Crは鉄炭化物を安定化させる作用を有するため、Cr含有量が1.00%を超えると素材鋼板の加熱時に粗大な鉄炭化物が溶け残り、鋼部材の靱性が劣化する。したがって、Crを含有させる場合のCr含有量は1.00%以下とする。Cr含有量は0.80%以下であることが好ましい。
Niは、鋼の焼入れ性を高め、かつ焼入れ後の鋼部材の強度を安定して確保することを可能にする元素であるため、含有させてもよい。この効果を確実に得るためには、Ni含有量は0.01%以上であることが好ましく、0.1%以上であることがより好ましい。しかし、Ni含有量が2.0%を超えると、上記の効果が飽和してコストの増加を引き起こす。したがって、Niを含有させる場合のNi含有量は2.0%以下とする。
Cuは、鋼の焼入れ性を高め、かつ焼入れ後の鋼部材の強度を安定して確保することを可能にする元素であるため、含有させてもよい。また、Cuは、腐食環境において鋼部材の耐食性を向上させる。これらの効果を確実に得るためには、Cu含有量は0.01%であることが好ましく、0.1%以上であることがより好ましい。しかし、Cu含有量が1.0%を超えると、上記の効果が飽和してコストの増加を引き起こす。したがって、Cuを含有させる場合のCu含有量は1.0%以下とする。
Moは、鋼の焼入れ性を高め、かつ焼入れ後の鋼部材の強度を安定して確保することを可能にする元素であるため、含有させてもよい。この効果を確実に得るためには、Mo含有量は0.01%以上であることが好ましく、0.1%以上であることがより好ましい。しかし、Mo含有量が1.0%を超えると、上記の効果が飽和してコストの増加を引き起こす。またMoは鉄炭化物を安定化させる作用を有するため、Mo含有量が1.00%を超えると素材鋼板の加熱時に粗大な鉄炭化物が溶け残り、鋼部材の靱性が劣化する。したがって、Moを含有させる場合のMo含有量は1.0%以下とする。
Vは、微細な炭化物を形成し、その細粒化効果により鋼部材の靱性を高めることを可能とする元素であるため、含有させてもよい。この効果を確実に得るためには、V含有量は0.01%以上であることが好ましく、0.1%以上であることがより好ましい。しかし、V含有量が1.0%を超えると、上記の効果が飽和してコストの増加を引き起こす。したがって、Vを含有させる場合のV含有量は1.0%以下とする。
Caは、鋼中の介在物を微細化し、焼入れ後の鋼部材の靱性および延性を向上させる効果を有する元素であるため、含有させてもよい。この効果を確実に得る場合は、Ca含有量は0.001%以上であることが好ましく、0.002%以上であることがより好ましい。しかし、Ca含有量が0.010%を超えると上記効果は飽和して、いたずらにコストの増加を引き起こす。したがって、Caを含有する場合のCa含有量は0.010%以下とする。Ca含有量は0.005%以下であることが好ましく、0.004%以下であることがより好ましい。
Alは、鋼の脱酸剤として一般的に用いられるため、含有させてもよい。Alによって十分に脱酸させるためには、Al含有量は0.01%以上であることが好ましい。しかし、Al含有量が1.00%を超えると、上記の効果が飽和してコストの増加を引き起こす。したがって、Alを含有させる場合のAl含有量は1.00%以下とする。
Nbは、微細な炭化物を形成し、その細粒化効果により鋼部材の靱性を高めることを可能とする元素であるため、含有させてもよい。この効果を確実に得るためには、Nb含有量は0.010%以上であることが好ましい。しかし、Nb含有量が0.100%を超えると、上記の効果が飽和してコストの増加を引き起こす。したがって、Nbを含有させる場合のNb含有量は0.100%以下とする。
Snは腐食環境において鋼部材の耐食性を向上させるため、含有させてもよい。この効果を確実に得るためには、Sn含有量は0.01%以上であることが好ましい。しかし、Sn含有量が1.00%を超えると粒界強度が低下し、鋼部材の靭性が劣化する。したがって、Snを含有させる場合のSn含有量は1.00%以下とする。
Wは鋼の焼入れ性を高め、かつ焼入れ後の鋼部材の強度を安定して確保することを可能にする元素であるため、含有させてもよい。また、Wは、腐食環境において鋼部材の耐食性を向上させる。これらの効果を確実に得るためには、W含有量は0.01%以上であることが好ましい。しかし、W含有量が1.00%を超えると、上記の効果が飽和してコストの増加を引き起こす。したがって、Wを含有させる場合のW含有量は1.00%以下とする。
REMは、Caと同様に鋼中の介在物を微細化し、焼入れ後の鋼部材の靱性および延性を向上させる効果を有する元素であるため、含有させてもよい。この効果を確実に得る場合は、REM含有量を0.001%以上とすることが好ましく、0.002%以上とすることがより好ましい。しかし、REM含有量が0.30%を超えるとその効果は飽和して、いたずらにコストの増加を引き起こす。したがって、REMを含有させる場合のREM含有量は0.30%以下とする。REM含有量は0.20%以下であることが好ましい。
本実施形態に係る鋼部材は、体積分率で、マルテンサイトが60.0~85.0%、ベイナイトが10.0~30.0%、残留オーステナイトが5.0~15.0%および残部組織が0~4.0%である金属組織を有する。
また、残留オーステナイトの最大短径の長さは30nm以上である。
マルテンサイトは硬質な相であり、鋼部材の高強度化を図る上で必要な組織である。マルテンサイトの体積分率が60.0%未満では、鋼部材の引張強度を十分に確保できない。そのため、マルテンサイトの体積分率は60.0%以上とする。好ましくは、65.0%以上である。一方、マルテンサイトの体積分率が85.0%を超えると、後述するベイナイトや残留オーステナイト等の他の組織を十分に確保できない。したがって、マルテンサイトの体積分率は、85.0%以下とする。好ましくは、80.0%以下である。
ベイナイトは、残留オーステナイトよりも高硬度で、マルテンサイトよりも低硬度な組織である。ベイナイトが存在することで残留オーステナイトとマルテンサイトとの間の硬度のギャップを緩和し、応力印加時に残留オーステナイトとマルテンサイトとの境界でのき裂の発生を予防し、鋼部材の靭性および延性を向上させる。ベイナイトの体積分率が10.0%未満では上記の効果が得られないため、ベイナイトの体積分率は10.0%以上とする。ベイナイトの好ましい体積分率は15.0%以上である。また、ベイナイトの体積分率が30.0%を超えると鋼部材の強度が低下するため、ベイナイトの体積分率は30.0%以下とする。ベイナイトの好ましい体積分率は25.0%以下であり、より好ましくは20.0%以下である。
残留オーステナイトは、塑性変形時にマルテンサイト変態(加工誘起変態)することによって、くびれを防止して加工硬化を助長し、延性を向上させる効果(TRIP効果)がある。さらに、残留オーステナイトの変態によってき裂先端の応力集中が緩和され、鋼部材の延性だけでなく靱性も向上させる効果がある。特に、残留オーステイトの体積分率が5.0%未満であると、鋼部材の延性が顕著に低下し、鋼部材の破断リスクが高まり、衝突安全性が低下する。したがって、残留オーステナイトの体積分率は5.0%以上とする。好ましくは6.0%以上であり、さらに好ましくは7.0%以上である。一方、残留オーステナイトの体積分率が過剰であると強度が低下してしまう場合があるため、残留オーステナイトの体積分率は15.0%以下とする。好ましくは、12.0%以下、または10.0%以下である。
本実施形態に係る鋼部材中には、残部組織として、フェライトおよびパーライトが混在する場合もある。本実施形態では、マルテンサイト、ベイナイト及び残留オーステナイトの合計の体積分率を96.0%以上とする必要がある。すなわち、本実施形態では、マルテンサイト、ベイナイト及び残留オーステナイト以外の残部組織が、体積分率で、4.0%以下に制限される。残部組織は0%でもよいため、残部組織の体積分率は0~4.0%とする。
本実施形態では、残留オーステナイトの最大短径を30nm以上とする。最大短径が30nm未満の残留オーステナイトは、変形において安定でない、つまり塑性変形初期の低ひずみ領域にてマルテンサイト変態してしまうため、鋼部材の延性および衝突安全性の向上へ十分に寄与できない。したがって、残留オーステナイトの最大短径は30nm以上とする。なお残留オーステナイトの最大短径の上限は特に限定されないが、変形において過度に安定であるとTRIP効果が十分に発現されないことから、600nm以下、100nm以下、または60nm以下としてもよい。
残留オーステナイトの体積分率は、X線回折法を用いて測定する。まず、鋼部材の端部から100mm離れた位置から試験片を採取する。鋼部材の形状により端部から100mm離れた位置から試験片を採取できない場合は、端部を避けた均熱部位から試験片を採取すればよい。鋼部材の端部は熱処理が十分に行われず、本実施形態に係る鋼部材の金属組織を有しない場合があるためである。
フッ化水素酸と過酸化水素水とを用いて、試験片の表面から板厚1/4の深さまで化学研磨する。測定条件は、Co管球を用い、2θで45°から105°の範囲とする。鋼部材に含まれる面心立方格子(残留オーステナイト)の回折X線強度を測定し、その回折曲線の面積比から残留オーステナイトの体積分率を算出する。これにより、残留オーステナイトの体積分率を得る。X線回折法によれば、鋼部材中の残留オーステナイトの体積分率を高精度で測定可能である。
残部組織の体積分率は、鋼部材の端部から100mm離れた位置の断面から測定試料を切り出し、残部組織の観察用の測定試料とする。鋼部材の形状により端部から100mm離れた位置から測定試料を採取できない場合は、端部を避けた均熱部位から測定試料を採取すればよい。また、光学顕微鏡または走査型電子顕微鏡による観察範囲は面積で40000μm2以上、倍率は500~1000倍、観察位置は板厚1/4部とする。切り出した測定試料を機械研磨し、続いて鏡面仕上げする。次いで、ナイタール腐食液(硝酸とエチルまたはメチルアルコールとの混合液)によりエッチングを行ってフェライト及びパーライトを現出させ、これを顕微鏡観察することで、フェライトまたはパーライトの存在を確認する。フェライトとセメンタントとが交互に層状に並んだ組織をパーライトと判別し、セメンタイトが粒状に析出した組織をベイナイトと判別する。観察されたフェライトおよびパーライトの面積分率の合計を求め、その値をそのまま体積分率に変換することで、残部組織の体積分率を得る。
マルテンサイトおよびベイナイトの体積分率と、残留オーステナイト体積分率と、残部組織の体積分率との合計が95.0%未満である場合、または105.0%超である場合は、再度、体積分率の測定を行う。
本実施形態に係る鋼部材の金属組織におけるマルテンサイトは、旧オーステナイト粒内にパケットが複数存在し、それぞれのパケットの内部に、平行な帯状組織であるブロックが存在し、更にそれぞれのブロックに、ほぼ同じ結晶方位のマルテンサイトの結晶であるラスの集合が存在している。TEMによってラスを確認し、ラス同士の境界近傍において制限視野回折パターン測定を行ってラス同士の境界近傍の電子線回折パターンを確認し、面心立方格子の電子線回折パターンを検出した場合に、ラス間に残留オーステナイトが存在すると判別する。ラスは体心立方格子であり、残留オーステナイトは面心立方格子であるため、電子線回折によって容易に判別できる。
まず、鋼部材の端部から100mm離れた位置(当該位置から試験片を採取できない場合は、端部を避けた均熱部位)かつ板厚1/4深さの位置から薄膜試料を採取する。この薄膜試料について、透過型電子顕微鏡にて50000倍に拡大し、ランダムに10視野の観察(1視野は1.0μm×0.8μm)を行い、電子線回折パターンを用いて残留オーステナイトを同定する。各視野において同定した残留オーステナイトのうち、「最大となる残留オーステナイト」の短径を測定し、10視野の内、大きい順から3つの「短径」を選択し、それらの平均値を算出することで「残留オーステナイトの最大短径」を得る。ここで、「最大となる残留オーステナイト」は、各視野において同定した残留オーステナイト結晶粒の断面積を測定し、当該断面積を有する円の円相当直径を求め、最大の円相当直径を示す残留オーステナイトと定義する。また、残留オーステナイトの「短径」は、各視野において同定した残留オーステナイトの結晶粒に対し、結晶粒の輪郭に接して結晶粒を挟む二本の平行線を想定したとき、平行線の間隔が最短距離になるように平行線を描いた場合の平行線の最短間隔(最小フェレ径)と定義する。
円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物:4.0×103個/mm2以下
素材鋼板に熱処理を行う場合、素材鋼板中に一般に存在する炭化物が再固溶することにより十分な焼入れ性を確保することができる。しかしながら、素材鋼板中に粗大な炭化物が存在し、この炭化物が十分に再固溶されない場合は、十分な焼入れ性を確保できず、低強度であるフェライトが析出する。したがって、素材鋼板中の粗大な炭化物が少ないほど、焼入れ性が向上し、熱処理後の鋼部材において高強度を得ることができる。
素材鋼板中に粗大な炭化物が多く存在すると、焼入れ性が低下するだけでなく、鋼部材においても炭化物が多く残留する(残留炭化物)。この残留炭化物は旧γ粒界に多く堆積するため、旧γ粒界を脆化させる。さらに、残留炭化物の量が過剰であると、変形時に残留炭化物がボイド起点となり、連結が容易となるため、鋼部材の延性、特に局部伸びが低下し、結果的に衝突安全性が劣化する。
また、0.1μm未満の炭化物については、延性、特に局部伸びに大きな影響を及ぼさないため、本実施形態では、個数制限の対象となる炭化物のサイズを0.1μm以上とした。
鋼部材の端部から100mm離れた位置(当該位置から試験片を採取できない場合は、端部を避けた均熱部位)または素材鋼板の板幅1/4部から試験片を切り出す。その試験片の観察面を鏡面加工した後、ピクラール液を使って腐食し、走査型電子顕微鏡で10000倍に拡大し、板厚1/4部にてランダムに10視野(1視野は10μm×8μm)の観察を行う。このときに、円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の個数を全て数え、全視野面積に対する数密度を算出することで、円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の数密度を得る。
本実施形態に係る鋼部材は、残留オーステナイトの加工誘起変態を利用したTRIP効果によって高い延性を得ることができる。しかしながら、低いひずみで残留オーステナイトが変態してしまうと、TRIP効果による高延性化は期待できない。すなわち、さらなる高延性化のためには、残留オーステナイトの量やサイズだけでなく、その性質を制御することが好ましい。
但し、上記式(1)中の各記号の意味は以下の通りである。
fγ0:真ひずみ付与前の鋼部材中に存在する残留オーステナイトの体積分率
fγ(0.02):鋼部材に対して0.02の真ひずみを付与し、除加した後の鋼部材中に存在する残留オーステナイトの体積分率
なお、上記式(1)中のlogは、底が10である対数、すなわち常用対数である。
なお、残留オーステナイトにひずみが付与された際に変態しやすいかどうかを支配するのは、残留オーステナイト中の固溶C量であると考えられ、本実施形態に係る鋼部材におけるMn含有量の範囲では、残留オーステナイトの体積分率と残留オーステナイト中の固溶C量との間には正の相関関係がある。そして、例えば残留オーステナイト中の固溶C量が0.8%程度であると上記kの値は15程度となり優れた延性を示すが、残留オーステナイト中の固溶C量が0.2%程度であると上記kの値は53程度となるため低ひずみで残留オーステナイトが全て変態してしまい、延性が低下し、結果的に衝突安全性が悪化する。
Mn偏析度α:1.6以下
鋼部材の板厚断面中心部(板厚1/2部)では、中心偏析が起きることでMnが濃化する。板厚中心部にMnが濃化すると、MnSが介在物として板厚中心部に集中し、硬質なマルテンサイトができやすくなるため、周囲との硬さに差が生じ、鋼部材の靱性が劣化する場合がある。特に、下記式(2)で表されるMn偏析度αの値が1.6を超えると、鋼部材の靱性が劣化する場合がある。したがって、鋼部材の靱性をより改善するために、鋼部材のMn偏析度αの値を1.6以下としてもよい。靱性をより一層改善するために、Mn偏析度αの値を1.2以下としてもよい。下限は特に規定する必要は無いが、下限は1.0としてもよい。
鋼部材の端部から100mm離れた位置(当該位置から試験片を採取できない場合は、端部を避けた均熱部位)または素材鋼板の板幅1/2部から、観察面が圧延方向と平行かつ板厚方向と平行となるように試料を切り出す。電子プローブマイクロアナライザ(EPMA)を用いて試料の板厚1/2部において圧延方向にランダムに10ヶ所のライン分析(1μm)を行い、分析結果からMn濃度が高い順に3つの測定値を選択し、その平均値を算出することで板厚1/2部での最大Mn濃度を求めることができる。また、板厚1/4部での平均Mn濃度も同じくEPMAを用いて、試料の板厚1/4部において10ヶ所の分析を行い、その平均値を算出することで板厚1/4部での平均Mn濃度を求めることができる。
清浄度:0.100%以下
鋼部材中にJIS G 0555:2003に記載のA系介在物、B系介在物およびC系介在物が多く存在すると、鋼部材の靱性が劣化する場合がある。これらの介在物の量が増加すると、亀裂伝播が容易に起こるためである。特に、1400MPa以上の引張強度を有するような鋼部材の場合、これらの介在物の存在割合を低く抑えることが好ましい。JIS G 0555:2003で規定される鋼の清浄度の値が0.100%を超えると、介在物の量が多いため、実用上十分な靱性を確保することが困難となる場合がある。そのため、鋼部材の清浄度の値は0.100%以下とすることが好ましい。鋼部材の靱性をより一層改善するためには、清浄度の値を0.060%以下とすることがより好ましい。なお、鋼の清浄度の値は、上記のA系介在物、B系介在物およびC系介在物の占める面積百分率を算出したものである。
本実施形態に係る鋼部材は、上述した化学組成を有し、かつ円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の数密度が8.0×103個/mm2以下であり、(Nb,Ti)Cの円相当直径の平均値が5.0μm以下である素材鋼板に対し、後述する熱処理を施すことで製造することができる。
鋼部材の延性の低下を抑制すべく、鋼部材における粗大な炭化物の析出を低減することは上記の通りだが、熱処理前の素材鋼板においても、粗大な炭化物は少ない方が好ましい。そのため、本実施形態では、素材鋼板中に存在する円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の数密度は8.0×103個/mm2以下とする。素材鋼板の炭化物の数密度は、素材鋼板の幅方向端部から1/4部から試験片を切り出し、鋼部材と同様の方法により測定すればよい。
特に、素材鋼板中に存在する(Nb,Ti)Cの円相当直径の平均値が5.0μmを超えると、熱処理後の鋼部材の延性が悪化する。そのため、素材鋼板中に存在する(Nb,Ti)Cの円相当直径の平均値は5.0μm以下とする。
なお、(Nb,Ti)Cの円相当直径の平均値を求める方法は次の通りである。素材鋼板の板幅1/4部から、断面を切り出し、その試料の観察面を鏡面研磨した後、走査型電子顕微鏡で3000倍に拡大し、ランダムに10視野(1視野は40μm×30μm)の観察を行う。観察された全ての(Nb,Ti)Cについて、各(Nb,Ti)Cの面積を算出し、この面積と同じ面積を持つ円の直径を各(Nb,Ti)Cの円相当直径とする。それらの円相当直径の平均値を算出することで、(Nb,Ti)Cの円相当直径の平均値を得る。
(H)素材鋼板の製造方法
本実施形態に係る鋼部材の熱処理前の鋼板である、素材鋼板の製造条件について特に制限はない。しかし、以下に示す製造方法を用いることにより、上述のように炭化物の析出形態が制御された素材鋼板を製造することができる。以下の製造方法では、例えば、連続鋳造、熱間圧延、酸洗、冷間圧延および焼鈍処理を行う。
具体的には、電磁攪拌、未凝固層圧下等の処理を施すことで、完全凝固前のMnが濃化した溶鋼を排出させることができる。
一方、溶鋼過熱温度を、液相線温度から5℃以上高い温度とし、かつ単位時間当たりの溶鋼鋳込み量を6t/min以下として鋳造することにより、介在物がスラブ内に持ち込まれにくくなる。その結果、スラブを作製する段階での介在物の量を効果的に減少させることができ、0.100%以下という素材鋼板の清浄度を容易に達成できるようになる。
粗大な(Nb,Ti)Cを溶解させるためにスラブを1200℃以上で加熱し、熱間圧延に供する。また、炭化物をより均一に生成させる観点から、熱間圧延開始温度を1000~1300℃とし、熱間圧延完了温度を950℃以上とすることが好ましい。
なお、焼鈍時の平均冷却速度とは、焼鈍保持の終了時から550℃までの鋼板の温度降下幅を、焼鈍保持の終了時から550℃までの所要時間で除した値とする。
次に、鋼部材の製造方法について説明する。
上記の素材鋼板に対して、図1に示すような温度履歴を経る熱処理を施すことによって、体積分率で、マルテンサイトが60.0~85.0%、ベイナイトが10.0~30.0%および残留オーステナイトが5.0~15.0%であり前記残留オーステナイトの最大短径の長さが30nm以上であり、円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の数密度が4.0×103個/mm2以下である金属組織を有し、高い強度を有するとともに延性に優れる鋼部材を得ることが可能となる。
また、第1平均冷却速度は、冷却開始時(加熱炉から取り出した時)からMs点までの鋼板の温度降下幅を、冷却開始時からMs点まで冷却した時の所要時間で除した値とする。第2平均冷却速度は、Ms点から冷却終了時までの鋼板の温度降下幅を、Ms点から冷却終了時までの時間で除した値とする。第3平均冷却速度は、第2冷却工程後に再加熱工程を行った後の冷却開始時(加熱炉から取り出した時)から冷却終了時までの鋼板の温度降下幅を、冷却開始時から冷却終了時までの所要時間で除した値とする。
5~300℃/sの平均昇温速度で、Ac3点~(Ac3点+200)℃の温度域まで上記の素材鋼板を加熱する(加熱工程)。この加熱工程によって、素材鋼板の組織をオーステナイト単相にする。なお、平均昇温速度が上記範囲内であれば、室温の素材鋼板を加熱しても、上記焼鈍後の冷却により550℃以下まで冷却された素材鋼板を加熱してもよい。
加熱工程において平均昇温速度が5℃/s未満の場合、または加熱工程における到達温度が(Ac3点+200)℃超の場合、γ粒が粗大化し、熱処理後の鋼部材の強度が劣化するおそれがある。また、後述する第1冷却工程および第2冷却工程においてオーステナイトが十分に残留せず、鋼部材の延性および靭性が劣化する場合がある。一方、加熱工程において平均昇温速度が300℃/sを超える場合、炭化物の溶解が十分に進まず焼入れ性が低下し、後述する第1冷却工程および第2冷却工程においてフェライトおよびパーライトが析出し、鋼部材の強度が劣化する。なお、到達温度がAc3点未満である場合、加熱工程後の素材鋼板の金属組織に、フェライトが残留し、オーステナイト単相とすることができず、熱処理後の鋼部材の強度が劣化する場合がある。
本実施形態では、上記の条件を満たした加熱工程を実施することによって、鋼部材の強度、延性および靭性の劣化を防止できる。
上記加熱工程を経た素材鋼板を、拡散変態が起きないように、言い換えると、フェライトやパーライトが析出しないように、Ac3点~(Ac3点+200)℃の温度域からMs点(マルテンサイト変態開始点)まで上部臨界冷却速度以上の第1平均冷却速度で冷却する(第1冷却工程)。
上部臨界冷却速度とは、金属組織にフェライトやパーライトを析出させず、オーステナイトを過冷してマルテンサイトを生成させる最小の冷却速度のことである。上部臨界冷却速度未満で冷却するとフェライトが生成し、鋼部材の強度が不足する。また、上部臨界冷却速度未満で冷却すると、パーライトが生成し、炭素が炭化物として析出してしまうため、後工程の第2冷却工程および再加熱工程において未変態オーステナイト中へ炭素を濃化させることができず、鋼部材の延性および靭性が不足する。
上述の化学成分を有する素材鋼板から、幅30mm、長さ200mmの試験片を切り出す。この試験片を窒素雰囲気中で1000℃まで10℃/秒の昇温速度で加熱し、その温度にて5分間保持したのち、種々の冷却速度で室温まで冷却する。冷却速度の設定は、1℃/秒から100℃/秒まで、10℃/秒の間隔で設定する。加熱中、冷却中の試験片の熱膨張変化を測定することにより、Ac3点およびMs点を測定する。
また、上部臨界冷却速度は、上記の種々の冷却速度で冷却したそれぞれの試験片のうち、フェライト相の析出が起きなかった最低の冷却速度を、上部臨界冷却速度とする。
第1冷却工程(上部臨界冷却速度以上の第1平均冷却速度でMs点まで冷却)後、(Ms-30)~(Ms-70℃)の温度域まで5℃/s以上、150℃/s未満であって第1平均冷却速度よりも遅い第2平均冷却速度で冷却する(第2冷却工程)。
第2平均冷却速度が150℃/s以上の場合、未変態オーステナイトへ炭素が拡散する時間が十分でなく、マルテンサイトが次々と隣り合って生成する。その結果、マルテンサイト間の残留オーステナイトの幅が小さくなり(残留オーステナイトの最大短径が30nm未満となり)、またその量が十分でないため鋼部材の延性および靭性が不足する。
冷却停止温度が(Ms-30)℃超の場合、微量のマルテンサイトしか生成しないため、マルテンサイトから未変態オーステナイトへ濃化するC量が不足する。その結果、後工程である再加熱工程においても同様に、マルテンサイトから未変態オーステナイトへ濃化するC量が不足するため、安定な残留オーステナイトを確保できず、後述する第3冷却過程において再びマルテンサイトが生成するため、鋼部材の延性および靭性が不足する。
第2冷却工程(第2平均冷却速度で(Ms-30)~(Ms-70)℃の温度域まで冷却)後、Ms~(Ms+200)℃の温度域まで5℃/s以上の平均昇温速度で再加熱し(再加熱工程)、その後5℃/s以上の第3平均冷却速度で冷却する(第3冷却工程)。
再加熱工程において、Ms~(Ms+200)℃の温度域までの平均昇温速度が5℃/s未満の場合、未変態オーステナイト中に炭素が過度に濃化し、Ms~(Ms+200)℃の温度域におけるベイナイト生成を抑制し、ベイナイトの体積分率が少なくなるため、鋼部材の延性および靭性が不足する。
具体的には、Ac3点~(Ac3点+200)℃の温度域に加熱した後において、オーステナイト変態を進めて炭化物を溶解させることによって鋼の焼入れ性を高める観点から、素材鋼板をAc3点~(Ac3点+200)℃の温度域で5s以上保持することが好ましい。また、上記保持時間は、生産性の観点からは、200s以下とすることが好ましい。
具体的には、Ms~(Ms+200)℃の温度域に再加熱した後、炭素を拡散させ残留オーステナイトの安定度を高める観点から、鋼板をMs~(Ms+200)℃の温度域に3s以上保持することが好ましい。また、この保持時間は、生産性の観点から60s以下とすることが好ましい。
また、熱間成形を第1冷却工程と同時に行ってもよい。熱間成形を第1冷却工程と同時行う、つまり、上部臨界冷却速度以上の冷却速度で冷却する第1冷却工程を施すと同時に素材鋼板に熱間成形を施してもよい。この場合、熱間で成形を施すことになるので、素材鋼板が軟質な状態であることから、寸法精度の高い鋼部材を得ることが可能となり好ましい。
表1Aおよび表1Bに示す化学成分を有する鋼を試験転炉で溶製し、連続鋳造試験機にて連続鋳造を実施し、幅1000mm、厚さ250mmのスラブを作製した。この際、素材鋼板の清浄度を制御すべく、溶鋼の過熱温度および単位時間当たりの溶鋼鋳込み量の調整を行った。
また、表4Aおよび表4B中に示すAc3点、Ms点および上部臨界冷却速度は、以下の実験によって求めた。
円相当直径が0.1μm以上の炭化物の数密度を求めるに際しては、素材鋼板の板幅1/4部から試料を切り出し、その観察面を鏡面加工した後、ピクラール液を使って腐食し、走査型電子顕微鏡で10000倍に拡大し、ランダムに10視野(1視野は10μm×8μm)、板厚1/4部の観察を行った。このときに、円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の個数を全て数え、全視野面積に対する数密度を算出することで、円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の数密度を得た。
(Nb,Ti)Cの円相当直径の平均値を求めるに際しては、素材鋼板の板幅1/4部から試料を切り出し、その観察面を鏡面加工した後、走査型電子顕微鏡で3000倍に拡大し、10視野(1視野は40μm×30μm)、板厚1/4部の観察を行った。観察された全ての(Nb,Ti)Cの面積を算出し、この面積と同じ面積を持つ円の直径を各(Nb,Ti)Cの円相当直径とし、それらの平均値を算出することで、(Nb,Ti)Cの円相当直径の平均値を得た。
Mn偏析度の測定は以下の手順により行った。素材鋼板の板幅1/2部から、観察面が圧延方向と平行となるように試料を切り出し、電子プローブマイクロアナライザ(EPMA)を用いて鋼板の板厚1/2部において圧延方向かつ板厚方向と平行に10ヶ所のライン分析(1μm)を行った。分析結果から高い順に3つの測定値を選択した後、その平均値を算出し、板厚中心部での最大Mn濃度を求めた。また、素材鋼板の表面から板厚の1/4深さ位置(板厚1/4部)において、同様にEPMAを用いて10ヶ所の分析を行い、その平均値を算出し、表面から板厚の1/4深さ位置での平均Mn濃度を求めた。そして、上記の板厚中心部での最大Mn濃度を、表面から板厚の1/4深さ位置での平均Mn濃度で割ることによって、Mn偏析度α([板厚1/2部での最大Mn濃度(質量%)]/[板厚1/4部での平均Mn濃度(質量%)])を求めた。
清浄度は、素材鋼板の板幅1/4部から試料を切り出し、観察面の板厚1/4部を光学顕微鏡で400倍に拡大し10視野(1視野は200μm×200μm)の観察を行った。そしてJIS G 0555:2003の附属書1に記載の点算法によって、A系介在物、B系介在物およびC系介在物の面積百分率を点算法により算出した。複数視野における清浄度の値が最も大きい(清浄性が最も低い)数値を、その素材鋼板の清浄度の値とした。
各鋼種のAc3点および上部臨界冷却速度は、次の方法にて測定した。
得られた素材鋼板から、幅30mm、長さ200mmの短冊試験片を切り出し、この試験片を窒素雰囲気中で1000℃まで10℃/秒の昇温速度で加熱し、その温度に5分間保持したのち、種々の冷却速度で室温まで冷却した。冷却速度の設定は、1℃/秒から100℃/秒まで、10℃/秒の間隔で設定した。そのときの加熱、冷却中の試験片の熱膨張変化を測定することにより、Ac3点、Ms点を測定した。
上部臨界冷却速度は、上記の冷却速度で冷却したそれぞれの試験片のうち、フェライト相の析出が起きなかった最低の冷却速度を、上部臨界冷却速度とした。
上記の各素材鋼板から、厚さ:1.4mm、幅:30mm、および長さ:200mmのサンプルを採取した。なおサンプルの長手方向が圧延方向と平行になるように採取した。
次に、採取したサンプルを(Ac3点+50)℃の温度域まで平均昇温速度10℃/sで加熱し120秒保持した後、Ms点まで上部臨界冷却速度以上の第1平均冷却速度で冷却し、その後(Ms-50)℃まで、第1平均冷却速度よりも遅い平均冷却速度(10℃/s)で冷却し、その後(Ms+75)℃まで平均昇温速度10℃/sで加熱し、その後平均冷却速度8℃/sで冷却する熱処理を施すことで、鋼部材を得た。
その後、得られた鋼部材の均熱部位から試験片を切り出し、引張試験、シャルピー衝撃試験、X線回折、光学顕微鏡観察、透過型電子顕微鏡観察を以下の方法で行い、機械特性および金属組織を評価した。評価結果を表2Aおよび表2Bに示す。
引張試験は、ASTM規格E8―69の規定に準拠して、インストロン社製引張試験機で実施した。上記鋼部材のサンプルを1.2mm厚まで研削した後、ASTM規格E8-69に規定のハーフサイズ板状試験片(平行部長さ:32mm、平行部板幅:6.25mm)を採取した。なお、本実施例の熱処理で用いた通電加熱装置冷却装置では、長さ200mm程度のサンプルから得られる均熱部位は限られるため、ASTM規格E8-69のハーフサイズ板状試験片を採用することとした。
そして、各試験片にひずみゲージ(共和電業製KFGS-5、ゲージ長:5mm)を貼付け、3mm/minのひずみ速度で室温引張試験を行い、最大強度(引張強度)を測定した。また、引張試験の平行部には予め25mmの罫書きを入れておき、破断サンプルをつき合わせ伸び率(全伸び)を測定した。そして、全伸びから最大強度時の塑性ひずみ(均一伸び)を差し引くことで、局部伸びを得た。
本実施例では、引張強度が1400MPa以上の場合、強度に優れるとして合格と判定し、1400MPa未満の場合、強度に劣るとして不合格と判定した。
また、全伸びが10.0%以上の場合、延性に優れるとして合格と判定し、全伸びが10.0%未満の場合、延性に劣るとして不合格と判定した。
更に、引張強度と全伸びとの積(引張強度TS×全伸びEL)を求め、TS×ELが14000MPa・%以上の場合を強度-延性バランスに優れると判定し、14000MPa・%未満の場合を強度-延性バランスに劣ると判定した。また、TS×ELが16000MPa・%以上の場合、強度-延性バランスにより優れると評価し、18000MPa・%以上の場合、強度-延性バランスにより一層優れると評価した。
シャルピー衝撃試験はJIS Z 2242:2005の規定に準拠して実施した。上記鋼部材を厚さが1.2mmとなるまで研削し、長さ55mm、幅10mmの試験片を切り出し、これを3枚積層しVノッチを入れた試験片を作製した。なお、Vノッチは、角度45°、深さ2mmおよびノッチ底半径0.25mmとした。試験温度-80℃におけるシャルピー衝撃試験を行い、衝撃値を求めた。なお、本実施例においては、25.0J/cm2以上の衝撃値を有する場合を靱性に優れると評価した。
X線回折では、まず、上記鋼部材の均熱部位から試験片を採取し、フッ化水素酸と過酸化水素水とを用いて表面から板厚1/4部の深さまで化学研磨した。化学研磨後の試験片について、Co管球を用いて、2θで45°から105°の範囲で測定を行うことで、面心立方格子(残留オーステナイト)の回折X線強度を測定した。得られた回折曲線の面積比から残留オーステナイトの体積分率を算出することで、残留オーステナイトの体積分率(fγ0)を得た。
上記鋼部材のサンプルを上記引張試験片と同様の形状に加工し、一定塑性ひずみ(真歪み:ε=0.02)を付与し、除加した引張試験片から上記X線回折用試験片を作製し、上述のX線回折と同様の方法により残留オーステナイトの体積分率(fγ(0.02))を求めた。これらより下記(i)式で示されるひずみ誘起変態パラメータkを計算し、TRIP効果による高延性化の指標とした。kが大きいほど低ひずみで残留オーステナイトが変態するため、高ひずみにおける括れ防止、つまりTRIP効果による高延性化は期待できない。
但し、上記式中の各記号の意味は以下のとおりである。
fγ0:真ひずみ付与前の鋼部材中に存在する残留オーステナイトの体積分率
fγ(0.02):鋼部材に対して0.02の真ひずみを付与し、除加した後の鋼部材中に存在する残留オーステナイトの体積分率
上記鋼部材の均熱部位から断面を切り出し、断面を鏡面加工した後、ピクラール液を使って腐食し、走査型電子顕微鏡で板厚1/4部を10000倍に拡大し、10視野(1視野は10μm×8μm)の観察を行った。このときに、円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の個数を全て数えて、全視野面積に対する個数(数密度)を算出することで、円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の数密度を得た。
上記鋼部材の均熱部位かつ板厚1/4深さの位置から、薄膜加工により薄膜試料を採取した。次に、透過型電子顕微鏡を用いて50000倍に拡大し、ランダムに10視野の観察(1視野は1.0μm×0.8μm)を行った。このとき、電子線回折パターンを用いて残留オーステナイトを同定した。各視野において「最大となる残留オーステナイト」の短径を測定し、10視野の内、大きい順から3つの「短径」を選択し、それらの平均値を算出することで鋼部材の「残留オーステナイトの最大短径」を得た。ここで、「最大となる残留オーステナイト」は、各視野において同定した残留オーステナイト結晶粒の断面積を測定し、当該断面積を有する円の円相当直径を求め、最大の円相当直径を示す残留オーステナイトとした。また、残留オーステナイトの「短径」は、各視野において同定した残留オーステナイトの結晶粒に対し、結晶粒の輪郭に接して結晶粒を挟む二本の平行線を想定したとき、平行線の間隔が最短距離になるように平行線を描いた場合の平行線の最短間隔(最小フェレ径)とした。
マルテンサイトおよびベイナイトの組織分率(体積分率)、並びに、残留オーステナイトの存在位置の測定方法は以下の通りとした。
マルテンサイトおよびベイナイトのそれぞれの体積分率は、TEMに付属する電子線回折装置によって測定した。鋼部材の均熱部位かつ板厚1/4深さの位置から測定試料を切り出し、TEM観察用の薄膜試料とした。また、TEM観察の範囲は面積で400μm2の範囲とし、倍率は50000倍とした。マルテンサイトおよびベイナイト中の鉄炭化物(Fe3C)を、薄片膜試料に照射した電子線の回折パターンにより見出し、その析出形態を観察することで、マルテンサイトとベイナイトとを判別し、マルテンサイトの面積分率およびベイナイトの面積分率を測定した。鉄炭化物の析出形態が3方向析出ならマルテンサイトと判断し、1方向の限定析出ならベイナイトと判断した。TEMの電子線回折によって測定されるマルテンサイトおよびベイナイトの分率は面積分率として測定されるが、本実施例の鋼部材は、金属組織が等方性であるため、面積分率の値をそのまま体積分率に置き換えた。なお、マルテンサイトとベイナイトとの判別のために鉄炭化物を観察したが、鉄炭化物は金属組織の体積分率に含めなかった。
鋼部材の均熱部位から測定試料を切り出し、残部組織の観察用の測定試料とした。走査型電子顕微鏡による観察範囲は面積で40000μm2、倍率は1000倍、測定位置は板厚1/4部とした。切り出した測定試料を機械研磨し、続いて鏡面仕上げした。次いで、ナイタール腐食液(硝酸とエチルまたはメチルアルコールとの混合液)によりエッチングを行ってフェライト及びパーライトを現出させ、これを顕微鏡観察することで、フェライトまたはパーライトの存在を確認した。フェライトとセメンタントとが交互に層状に並んだ組織をパーライトと判別し、セメンタイトが粒状に析出したものをベイナイトと判別した。観察されたフェライトおよびパーライトの面積分率の合計を求め、その値をそのまま体積分率に変換することで、残部組織の体積分率を得た。
また、TEMによってベイニティックフェライトの結晶粒組織を確認し、ベイニティックフェライト結晶粒の粒界近傍において制限視野回折パターン測定を行って、ベイニティックフェライト結晶粒の粒界近傍の電子線回折パターンを確認した。面心立方格子の電子線回折パターンを検出した場合に、ベイニティックフェライト間に残留オーステナイトが存在すると判別した。
更に、旧オーステナイト粒界近傍において制限視野回折パターン測定を行って旧オーステナイト粒界近傍の電子線回折パターンを確認した。面心立方格子の電子線回折パターンを検出した場合に、旧オーステナイト粒界に残留オーステナイトが存在すると判別した。
なお、表2Aの発明例B1~B28は全て、Mn偏析度が1.6以下、清浄度が0.100%以下と良好であった。また、発明例B1~B28では、残留オーステナイトが、マルテンサイトのラス間、ベイナイトのベイニティックフェライト間及び旧オーステナイト粒界に存在していた。
表1Aに示す鋼種のうち、鋼No.A26およびA27の化学組成を有するスラブの鋳造時に、過熱温度、鋳造速度(鋳込量)、スラブ冷却速度を変化させて、スラブのMn偏析度、清浄度を変化させた。その後、スラブに、上記と同様の熱間圧延、酸洗、冷間圧延を施した後、実施例1と同じ条件にて熱処理を施して、鋼部材を製造した。
得られた鋼部材C1~C10の評価結果を表3に示す。各特性の評価方法は実施例1と同様に実施した。
一方、Mn偏析度がやや大きい発明例C2は、同じ鋼から製造された発明例C1、C3およびC5と比較して衝撃値および局部伸びがやや低くなっている。Mn偏析度がやや大きい発明例C7は、同じ鋼から製造された発明例C6、C8およびC10と比較して衝撃値および局部伸びがやや低くなっている。清浄度がやや高い発明例C4は、同じ鋼から製造された発明例C1、C3およびC5と比較して衝撃値および局部伸びがやや低くなっている。清浄度がやや高い発明例C9は、同じ鋼から製造されたC6、C8およびC10と比較して衝撃値および局部伸びがやや低くなっている。
なお、発明例C1~C10では、残留オーステナイトが、マルテンサイトのラス間、ベイナイトのベイニティックフェライト間、及び旧オーステナイト粒界に存在していた。
表1Aに示す鋼種のうち、鋼No.A26およびA27の化学組成を有する素材鋼板に、表4Aおよび表4Bに示す熱処理を施して、鋼部材を製造した。
得られた鋼部材の金属組織および機械特性の評価結果を表5Aおよび表5Bに示す。
表4A~表5Bを見ると、本発明範囲を満足する発明例D1~D28は、金属組織および機械特性ともに良好な結果であるが、本発明範囲を満足していない比較例d1~d34は、金属組織および機械特性の少なくとも1つを満足しない結果となった。
なお、発明例D1~D28は全て、Mn偏析度が1.6以下、清浄度が0.100%以下と良好であった。また、発明例D1~D28では、残留オーステナイトが、マルテンサイトのラス間、ベイナイトのベイニティックフェライト間、及び旧オーステナイト粒界に存在していた。
Claims (12)
- 化学組成が、質量%で、
C:0.10~0.60%、
Si:0.40~3.00%、
Mn:0.30~3.00%、
P:0.050%以下、
S:0.0500%以下、
N:0.010%以下、
Ti:0.0010~0.1000%、
B:0.0005~0.0100%、
Cr:0~1.00%、
Ni:0~2.0%、
Cu:0~1.0%、
Mo:0~1.0%、
V:0~1.0%、
Ca:0~0.010%、
Al:0~1.00%、
Nb:0~0.100%、
Sn:0~1.00%、
W:0~1.00%、
REM:0~0.30%、
を含み、残部がFeおよび不純物であり、
金属組織が、体積分率で、マルテンサイトが60.0~85.0%、ベイナイトが10.0~30.0%、残留オーステナイトが5.0~15.0%および残部組織が0~4.0%であり、
前記残留オーステナイトの最大短径の長さが30nm以上であり、
円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の数密度が4.0×103個/mm2以下である
ことを特徴とする鋼部材。 - 前記化学組成が、質量%で、
Cr:0.01~1.00%、
Ni:0.01~2.0%、
Cu:0.01~1.0%、
Mo:0.01~1.0%、
V:0.01~1.0%、
Ca:0.001~0.010%、
Al:0.01~1.00%、
Nb:0.010~0.100%、
Sn:0.01~1.00%、
W:0.01~1.00%、および
REM:0.001~0.30%の1種以上を含有する
ことを特徴とする請求項1に記載の鋼部材。 - 下記式(1)で表されるひずみ誘起変態パラメータkの値が18.0未満であることを特徴とする請求項1または2に記載の鋼部材。
k=(logfγ0-logfγ(0.02))/0.02 ・・・ 式(1)
但し、上記式(1)中の各記号の意味は以下の通りである。
fγ0:真ひずみ付与前の鋼部材中に存在する残留オーステナイトの体積分率
fγ(0.02):鋼部材に対して0.02の真ひずみを付与し、除荷した後の鋼部材中に存在する残留オーステナイトの体積分率 - 引張強度が1400MPa以上および全伸びが10.0%以上であることを特徴とする請求項1~3の何れか一項に記載の鋼部材。
- 局部伸びが3.0%以上であることを特徴とする請求項1~4の何れか1項に記載の鋼部材。
- -80℃における衝撃値が25.0J/cm2以上であることを特徴とする請求項1~5の何れか一項に記載の鋼部材。
- JIS G 0555:2003で規定される鋼の清浄度の値が0.100%以下であることを特徴とする請求項1~6のいずれか1項に記載の鋼部材。
- 請求項1~7の何れか1項に記載の鋼部材の製造方法であって、
化学組成が、質量%で、
C:0.10~0.60%、
Si:0.40~3.00%、
Mn:0.30~3.00%、
P:0.050%以下、
S:0.0500%以下、
N:0.010%以下、
Ti:0.0010~0.1000%、
B:0.0005~0.0100%、
Cr:0~1.00%、
Ni:0~2.0%、
Cu:0~1.0%、
Mo:0~1.0%、
V:0~1.0%、
Ca:0~0.010%、
Al:0~1.00%、
Nb:0~0.100%、
Sn:0~1.00%、
W:0~1.00%、
REM:0~0.30%、
を含み、残部がFeおよび不純物であり、かつ円相当直径が0.1μm以上かつアスペクト比が2.5以下である炭化物の数密度が8.0×103個/mm2以下であり、(Nb,Ti)Cの円相当直径の平均値が5.0μm以下である素材鋼板を、
Ac3点~(Ac3点+200)℃の温度域まで平均昇温速度5~300℃/sで加熱する加熱工程と、
前記加熱工程後、Ms点まで上部臨界冷却速度以上の第1平均冷却速度で冷却する第1冷却工程と、
前記第1冷却工程後、(Ms-30)~(Ms-70)℃の温度域まで、5℃/s以上、150℃/s未満であって前記第1平均冷却速度よりも遅い第2平均冷却速度で冷却する第2冷却工程と、
前記第2冷却工程後、Ms~(Ms+200)℃の温度域まで平均昇温速度5℃/s以上で加熱する再加熱工程と、
前記再加熱工程後、5℃/s以上の第3平均冷却速度で冷却する第3冷却工程と、
を備えることを特徴とする鋼部材の製造方法。 - 前記加熱工程と前記第1冷却工程との間に、前記Ac3点~(Ac3点+200)℃の前記温度域にて5~200秒間保持する保持工程を備えること特徴とする請求項8に記載の鋼部材の製造方法。
- 前記再加熱工程と前記第3冷却工程との間に、前記Ms~(Ms+200)℃の前記温度域にて3~60秒間保持する保持工程を備えること特徴とする請求項8または9に記載の鋼部材の製造方法。
- 前記加熱工程と前記第1冷却工程との間において、前記素材鋼板に熱間成形を施すことを特徴とする請求項8~10の何れか1項に記載の鋼部材の製造方法。
- 前記第1冷却工程において、前記第1冷却速度で冷却を行うと同時に、前記素材鋼板に熱間成形を施すことを特徴とする請求項8~10の何れか1項に記載に鋼部材の製造方法。
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