WO2015102051A1 - 熱間成形部材およびその製造方法 - Google Patents
熱間成形部材およびその製造方法 Download PDFInfo
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- WO2015102051A1 WO2015102051A1 PCT/JP2014/050027 JP2014050027W WO2015102051A1 WO 2015102051 A1 WO2015102051 A1 WO 2015102051A1 JP 2014050027 W JP2014050027 W JP 2014050027W WO 2015102051 A1 WO2015102051 A1 WO 2015102051A1
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- hot
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- steel sheet
- formed member
- base steel
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Images
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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
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
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- B—PERFORMING OPERATIONS; TRANSPORTING
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- B21D37/00—Tools as parts of machines covered by this subclass
- B21D37/16—Heating or cooling
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- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
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- C—CHEMISTRY; METALLURGY
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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
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present invention relates to a hot-formed member used for machine structural parts such as body structural parts and underbody parts of automobiles, and a manufacturing method thereof.
- the present invention has an excellent ductility in which the total elongation in the tensile test is 15% or more and an impact value in the Charpy test at 0 ° C. of 20 J / cm while having a tensile strength of 900 MPa to 1300 MPa.
- the present invention relates to a hot-formed member having excellent impact characteristics of 2 or more, and a method for producing the same.
- Patent Document 1 in a method called hot press in which a heated steel plate is press-formed, a member having a complicated shape can be formed from a high-strength steel plate with high dimensional accuracy.
- the hot pressing process the steel sheet is processed in a state of being heated to a high temperature, so that the steel sheet at the time of processing is soft and has high ductility.
- the steel sheet is heated to an austenite single-phase region before pressing, and the steel sheet is rapidly cooled (quenched) in the mold after pressing to increase the strength of the member by martensitic transformation. Can also be achieved. Therefore, the hot pressing method is an excellent forming method that can simultaneously ensure the strength of the member and the formability of the steel sheet.
- Patent Document 2 a steel plate is formed into a predetermined shape in advance at room temperature, the member obtained thereby is heated to an austenite region, and further quenched in a mold to increase the strength of the member.
- a pre-press quench method to achieve is disclosed.
- the pre-press quench method which is an aspect of hot pressing, can restrain a member from being deformed due to thermal strain by restraining the member with a mold.
- the pre-press quench method is an excellent molding method capable of increasing the strength of a member and obtaining high dimensional accuracy.
- Patent Document 3 the steel sheet is heated in a two-phase temperature range of ferrite and austenite to press the steel sheet in a state in which the metal structure of the steel sheet has a ferrite-austenite two-phase structure.
- a member having high strength and excellent ductility is obtained by rapidly cooling the steel sheet to change the metal structure of the steel sheet to a ferrite-martensite two-phase structure.
- the elongation of the member obtained by the above technique is about 10% or less, the member disclosed in Patent Document 3 is not sufficiently excellent with respect to ductility.
- a member that requires excellent shock absorption characteristics such as a member required in the technical field related to automobiles, has a ductility superior to that of the above member, specifically, an elongation of 15% or more. Necessary, preferably 18% or more, more preferably 21% or more is required.
- Patent Document 4 a steel plate in which Si and Mn are positively added is preheated to a ferrite-austenite two-phase temperature range, and then forming and quenching are simultaneously performed on the steel plate using a deep drawing device, A technique for obtaining a member having high strength and excellent ductility by changing the metal structure of the obtained member to a multiphase structure containing ferrite, martensite, and austenite is disclosed.
- isothermal holding treatment at 300 ° C. to 400 ° C., that is, austempering treatment on the steel sheet. Therefore, the die of the deep drawing apparatus of Patent Document 4 must be controlled to be heated to 300 ° C. to 400 ° C.
- Patent Document 5 a steel plate to which Si and Mn are positively added is preheated to a two-phase temperature region or an austenite single-phase region, and then formed into a steel plate and rapidly cooled to reach a predetermined temperature.
- a technique for obtaining a member having high strength and excellent ductility by simultaneously reheating the obtained member and thereby making the metal structure of the member into a multiphase structure containing martensite and austenite. Is disclosed.
- the manufacturing method according to the above-described technique has a problem that the tensile strength of the member varies significantly depending on the rapid cooling conditions, specifically, the temperature at which cooling is stopped. Furthermore, the process problem that the control of the cooling stop temperature is extremely difficult is unavoidable in the above manufacturing method.
- the manufacturing method according to Patent Document 5 requires a further heat treatment step of reheating. Therefore, the manufacturing method according to Patent Document 5 is significantly less productive than the conventional method for manufacturing a hot-formed member.
- the second phase such as martensite is sparse in the metal structure of the member. It becomes easy to be distributed. This causes a problem that the impact characteristics of the member are significantly deteriorated.
- Non-Patent Document 1 contains tens of% retained austenite obtained by hot rolling a 0.1% C-5% Mn alloy and further reheating, has high strength, A steel material that is extremely excellent in ductility is disclosed.
- the chemical composition of the hot-formed member is optimized, and further, the heat treatment temperature in the hot-forming step is strictly controlled near the A 1 point, thereby reducing the retained austenite. It is possible to produce hot-formed members containing.
- the influence of heating time on tensile strength and elongation is extremely large. In order to suppress changes in the tensile strength and elongation obtained, heating for 30 minutes or more is required. Such structure control by heating for a long time cannot be applied to the production technology of a hot-formed member in consideration of productivity and the surface quality of the member.
- the method disclosed in Non-Patent Document 1 tends to cause insufficient cementite dissolution, it is easily expected that the impact characteristics of the hot-formed member obtained by this technique are not sufficient.
- An object of the present invention is to provide a hot-formed member having a tensile strength of 900 MPa or more, excellent in ductility and impact properties, and a method for producing the same, which have been impossible to mass-produce as described above. It is.
- the metal structure of the hot-formed member is a metal structure containing a predetermined amount of austenite and containing fine austenite and martensite as a whole.
- new findings have been obtained that the ductility and impact properties of hot-formed members are significantly improved.
- it has the same chemical composition as the chemical composition of the hot-formed member described above, contains one or two selected from bainite and the martensite, and is made of cementite. New knowledge that this can be achieved by using a base steel sheet with a metal structure with crystal grains at a predetermined number density as a raw material for hot forming members and by optimizing the heat treatment conditions during hot forming. Obtained.
- the hot-formed member according to one aspect of the present invention has a chemical composition of mass%, C: 0.05% to 0.40%, Si: 0.5% to 3.0%, Mn: 1.2% to 8.0%, P: 0.05% or less, S: 0.01% or less, sol.
- Al 0.001% to 2.0%, N: 0.01% or less, Ti: 0% to 1.0%, Nb: 0% to 1.0%, V: 0% to 1.0%, Cr: 0% to 1.0%, Mo: 0% to 1.0%, Cu: 0% to 1.0%, Ni: 0% to 1.0%, Ca: 0% to 0.01%, Mg: 0% to 0.01%, REM: 0% to 0.01%, Zr: 0% to 0.01%, B: 0% to 0.01%, Bi: 0% to 0.01%, And the balance: Fe and impurities, containing 10 area% to 40 area% austenite, and having a total number density of the austenite crystal grains and martensite crystal grains of 1.0 / ⁇ m 2 or more It has a structure and a tensile strength of 900 MPa to 1300 MPa.
- the hot-formed member according to the above (1) has the chemical composition of mass%, Ti: 0.003% to 1.0%, Nb: 0.003% to 1.0%, V : 0.003% to 1.0%, Cr: 0.003% to 1.0%, Mo: 0.003% to 1.0%, Cu: 0.003% to 1.0%, and Ni: One or more selected from the group consisting of 0.003% to 1.0% may be contained.
- the chemical composition is, by mass%, Ca: 0.0003% to 0.01%, Mg: 0.0003% to 0.00.
- One or more selected from the group consisting of 01%, REM: 0.0003% to 0.01%, and Zr: 0.0003% to 0.01% or less may be contained.
- a method for producing a hot-formed member according to another aspect of the present invention has the same chemical composition as the chemical composition of the hot-formed member according to any one of (1) to (5). And a Mn content of 2.4% by mass to 8.0% by mass, containing one or two types selected from bainite and martensite in a total of 70% by area or more, and the cementite crystal grains are
- the earth steel including the step of cooling under the condition average cooling rate is 5 ° C. / sec ⁇ 500 ° C. / sec at a temperature range of 600 °C ⁇ 150 °C.
- a method for producing a hot-formed member according to another aspect of the present invention has the same chemical composition as the chemical composition of the hot-formed member according to any one of (1) to (5) above. Further, the Mn content is 1.2% by mass or more and less than 2.4% by mass, and one or two kinds selected from bainite and martensite are contained in total of 70% by area or more, and cementite crystal grains there a heating step of heating to 1.0 pieces / [mu] m temperature range of less than 3 points 670 ° C. or higher 780 ° C. below and Ac the basis steel sheet having existing metal structure by two or more number density, next to the heating step, the A holding step of holding the temperature of the base steel plate in the temperature range of 670 ° C.
- the average cooling rate of the base steel sheet in the temperature range of 600 ° C. to 500 ° C. is 5 ° C./second to 500 ° C./second and in the temperature range of less than 500 ° C. and 150 ° C. or more. Cooling step of cooling under the condition of 5 ° C./second to 20 ° C./second.
- a technically valuable effect is achieved, that is, for the first time, a hot-formed member having a tensile strength of 900 MPa or more that is extremely excellent in ductility and excellent in impact characteristics can be put into practical use.
- hot forming will be described by taking a hot press as a specific embodiment as an example. However, if substantially the same manufacturing conditions as the manufacturing conditions disclosed in the following description are achieved, a molding method other than hot pressing, such as roll molding, may be adopted as the hot molding method. .
- C (C: 0.05% to 0.40%) C is a very important element that enhances the hardenability of steel and has the strongest influence on the strength of the hot-formed member after quenching. If the C content is less than 0.05%, it becomes difficult to ensure a tensile strength of 900 MPa or more after quenching. Therefore, the C content is 0.05% or more. On the other hand, when the C content exceeds 0.40%, the impact characteristics of the hot-formed member are significantly deteriorated. Therefore, the C content is set to 0.40% or less. In order to improve the weldability of the hot-formed member, the C content is preferably 0.25% or less. In order to stably secure the strength of the hot-formed member, the C content is preferably set to 0.08% or more.
- Si 0.5% to 3.0%
- Si is a very effective element in order to stably secure the strength of the steel after quenching. Furthermore, by adding Si, austenite in the metal structure is increased, and the ductility of the hot-formed member is improved. If the Si content is less than 0.5%, it is difficult to obtain the above effect. In particular, when austenite is insufficient in this embodiment, the required ductility cannot be obtained, which is extremely disadvantageous for industrial use. Therefore, the Si content is 0.5% or more. Note that when the Si content is 1.0% or more, the ductility is further improved. Therefore, the Si content is preferably 1.0% or more.
- the Si content is 3.0% or less.
- the Si content is preferably 2.5% or less.
- Mn is a very effective element in order to improve the hardenability of steel and to secure the strength after quenching stably. Furthermore, Mn also has the effect of increasing the ductility of the hot formed member after quenching. However, if the Mn content is less than 1.2%, these effects cannot be obtained sufficiently, and it becomes very difficult to ensure a tensile strength of 900 MPa or more after quenching. Therefore, the Mn content is 1.2% or more. When the Mn content is 2.4% or more, the ductility of the hot-formed member is further increased, and slow cooling after hot forming described later is not necessary in the manufacturing process, and the productivity is remarkably improved.
- Mn content shall be 2.4% or more.
- austenite is excessively generated in the hot-formed member, and delayed fracture tends to occur. Therefore, the Mn content is 8.0% or less.
- the Mn content is preferably 6.0% or less.
- P 0.05% or less
- P is an impurity inevitably contained in steel.
- P has an effect of increasing the strength of the steel by solid solution strengthening, and therefore P may be positively included.
- the P content is 0.05% or less.
- the P content is preferably set to 0.02% or less.
- the P content is preferably set to 0.003% or more.
- the P content is preferably set to 0.003% or more.
- S is an impurity contained in steel, and in order to improve weldability, the lower the S content, the better. If the S content is more than 0.01%, the weldability deteriorates to an unacceptable extent. Therefore, the S content is 0.01% or less. In order to further prevent deterioration of weldability, the S content is preferably 0.003% or less, and more preferably 0.0015% or less. The smaller the S content, the better. Therefore, it is not necessary to define the lower limit of the S content. That is, the lower limit of the S content is 0%.
- sol.Al 0.001% to 2.0% sol.
- Al refers to solid solution Al present in steel in a solid solution state.
- Al is an element having a function of deoxidizing steel, and is also an element having a function of preventing carbonitride forming elements such as Ti from being oxidized and promoting the formation of carbonitride. By these actions, generation of surface flaws in the steel material can be suppressed and the production yield of the steel material can be improved.
- sol. If the Al content is less than 0.001%, it is difficult to obtain the above effect. Therefore, sol.
- the Al content is 0.001% or more. In order to obtain the above action more reliably, sol.
- the Al content is preferably 0.01% or more. On the other hand, sol.
- the Al content is 2.0% or less. In order to more reliably avoid the above phenomenon, sol.
- the Al content is preferably 1.5% or less.
- N is an impurity inevitably contained in steel, and in order to improve weldability, it is preferable that the N content is low.
- the N content exceeds 0.01%, the decrease in weldability of the hot-formed member becomes significant to an unacceptable level. Therefore, the N content is 0.01% or less.
- the N content is preferably 0.006% or less. The smaller the N content, the better. Therefore, it is not necessary to define the lower limit of the N content. That is, the lower limit of the N content is 0%.
- the balance is Fe and impurities.
- Impurities are components that are mixed due to various factors in the manufacturing process, such as ore or scrap, when industrially manufacturing steel materials, and are characteristic of the hot-formed member according to the present embodiment. It means that the content is allowed within a range that does not adversely affect.
- the hot forming member according to the embodiment may further contain an element as described below as an optional component.
- limit the lower limit of arbitrary element content is 0%.
- any of these elements is an effective element for enhancing the hardenability of the hot-formed member and stably securing the strength of the hot-formed member after quenching. Therefore, you may contain 1 type, or 2 or more types among these elements. However, if Ti, Nb, and V are contained in amounts exceeding 1.0%, it is difficult to perform hot rolling and cold rolling in the manufacturing process.
- any of these elements contributes to inclusion control, in particular, fine dispersion of inclusions, and has an effect of increasing the low temperature toughness of the hot formed member. Therefore, you may contain 1 type, or 2 or more types among these elements. However, if any element is contained in excess of 0.01%, the surface properties of the hot-formed member may be deteriorated. Therefore, when each element is contained, the content of each element is as described above.
- REM refers to a total of 17 elements composed of Sc, Y, and a lanthanoid
- REM content means the total content of these 17 elements.
- B is an element having an effect of increasing the low temperature toughness of the hot formed member. Therefore, B may be contained in the hot-formed member. However, if B is contained in excess of 0.01%, the hot workability of the base steel sheet is deteriorated, making it difficult to perform hot rolling. Therefore, when B is contained in the hot-formed member, the B content is 0.01% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable to make B content 0.0003% or more.
- Bi 0% to 0.01%
- Bi is an element having an action of suppressing cracking during deformation of the hot-formed member. Therefore, Bi may be included in the hot-formed member. However, if Bi is included in an amount exceeding 0.01%, the hot workability of the base steel sheet is deteriorated, making it difficult to perform hot rolling. Therefore, when Bi is contained in the hot-formed member, the Bi content is 0.01% or less. In addition, in order to acquire the effect by the said action
- the structure of the metal structure described below is a structure at a position from about 1/2 t to about 1/4 t of the plate thickness and not the center segregation portion.
- the center segregation part may have a metal structure different from a typical metal structure of a steel material.
- the center segregation portion is a minute region with respect to the entire plate thickness, and hardly affects the characteristics of the steel material. That is, it cannot be said that the metal structure of the central segregation part represents the metal structure of the steel material.
- the definition of the metal structure of the hot-formed member according to the present embodiment is assumed to be at a position from about 1/2 t to about 1/4 t of the plate thickness and not at the center segregation portion.
- “1 / 2t position” indicates a position that is 1/2 the thickness of the member thickness t from the surface of the hot-formed member
- “1 ⁇ 4t position” indicates the hot-formed member. The position which is 1/4 of the member thickness t from the surface of is shown.
- the ductility of the hot-formed member is significantly improved. If the area ratio of austenite is less than 10%, it is difficult to ensure excellent ductility. Therefore, the area ratio of austenite is 10% or more. In addition, making the area ratio of austenite 18% or more contributes to making the elongation of the hot-formed member 21% or more and exhibiting excellent ductility in the hot-formed member. Therefore, the area ratio of austenite is preferably 18% or more. On the other hand, if the area ratio of austenite exceeds 40%, delayed fracture tends to occur in the hot-formed member. Therefore, the area ratio of austenite is 40% or less. In order to reliably prevent the occurrence of delayed fracture, the austenite area ratio is preferably set to 32% or less.
- the method for measuring the area ratio of austenite is well known to those skilled in the art, and can also be measured by a conventional method in this embodiment. In examples shown later, the area ratio of austenite was determined by X-ray diffraction.
- the metal structure of the hot-formed member is a total of 1.0 / ⁇ m 2 for austenite and martensite.
- Metal structure present at a number density of In order to obtain the above-mentioned impact property improvement effect more reliably, the lower limit of the total number density of austenite and martensite crystal grains is more preferably 1.3 / ⁇ m 2 .
- the total number density of austenite particles and martensite particles is preferably as large as possible. This is because as the total number density of the austenite particles and martensite particles is larger, the localization of deformation is suppressed and the impact characteristics are further improved.
- the number density of austenite particles and martensite particles can be determined by the following method. First, a test piece is sampled from the hot-formed member along the rolling direction of the base steel sheet that is the raw material of the hot-formed member and the direction perpendicular to the rolling direction.
- the number density of austenite particles and martensite particles is calculated by image analysis of the electron micrograph of the 800 ⁇ m square region obtained in this way.
- the austenite particles and martensite particles can be easily distinguished from surrounding structures by using an electron microscope. It is not necessary to define the average crystal grain size of austenite particles and martensite particles. Generally, when the average crystal grain size is large, the strength of steel may be adversely affected. However, if the number density described above is achieved, the particle sizes of the austenite particles and martensite particles will not be coarsened.
- one or more of ferrite, bainite, cementite and pearlite may be contained in the hot-formed member. If the contents of austenite and martensite are within the above specified range, the contents of ferrite, bainite, cementite and pearlite are not particularly specified.
- the tensile strength of the hot-formed member according to this embodiment is 900 MPa or more. By having such tensile strength, it is possible to achieve weight reduction of various members using the steel plate according to the present embodiment. However, if the tensile strength exceeds 1300 MPa, brittle fracture tends to occur in the steel sheet. Therefore, the upper limit of the tensile strength of the steel sheet is 1300 MPa. Such tensile strength is achieved by the above-described chemical components and the production method described later.
- the quenched structure contains 10 to 40 area% austenite as described above, and includes austenite and martensite. It is necessary to have a metal structure in which the total number density of site crystal grains is 1.0 / ⁇ m 2 or more.
- such a metal structure has the same chemical composition as the above-mentioned hot-formed member, and contains one or two selected from bainite and martensite in a total of 70 area% or more, Heating the base steel sheet having a metal structure having a number density of cementite crystal grains of 1.0 pieces / ⁇ m 2 or more to a temperature range of 670 ° C. or more and less than 780 ° C. and less than Ac 3 points in the heating step; Next, in the holding step, the temperature of the base steel plate is held in a temperature range of 670 ° C. or higher and lower than 780 ° C. and less than Ac 3 points for 2 to 20 minutes, and then in the hot forming step, the base steel plate is hot pressed.
- a temperature range of 670 ° C. or more and less than 780 ° C. and less than Ac 3 points means “a temperature range of 670 ° C. or more and less than 780 ° C.” if Ac 3 points is 780 ° C. or more, and Ac 3 points is less than 780 ° C. In this case, “a temperature range of 670 ° C. or more and less than Ac 3 points” is indicated.
- the base steel sheet is averaged in the temperature range of 600 ° C. to 150 ° C. in the cooling step after the hot forming step. Cooling is performed at a cooling rate of 5 ° C./second to 500 ° C./second.
- the average cooling rate is 5 in the temperature range of 600 ° C. to 500 ° C. in the cooling step after the hot forming step.
- the cooling is performed under the condition that the average cooling rate is 5 ° C./second to 20 ° C./second in a temperature range of from 500 ° C./second to 500 ° C./second and less than 500 ° C. and 150 ° C.
- the base steel sheet to be subjected to hot pressing has the same chemical composition as that of the hot-formed member described above, and a total of one or two selected from bainite and martensite is 70 area% or more.
- a base steel plate having a metal structure containing cementite crystal grains with a number density of 1.0 / ⁇ m 2 or more is used.
- This base steel plate is, for example, a hot-rolled steel plate, a cold-rolled steel plate, a hot-dip galvanized cold-rolled steel plate, or an alloyed hot-dip galvanized cold-rolled steel plate.
- the base steel sheet having the metal structure is hot-pressed under the heat treatment conditions described later, thereby having the above-described metal structure, a tensile strength of 900 MPa or more, and excellent ductility and impact characteristics.
- An intermediate formed member is obtained.
- the above-described definition of the metal structure of the base steel sheet is performed at a position from about 1/2 t to about 1/4 t of the plate thickness and not the center segregation portion.
- the reason why the metal structure of the base steel sheet is defined at this position is that the metal structure of the hot-formed member is located at a position of about 1/2 t to about 1/4 t of the plate thickness and is center segregated. This is the same reason as that specified at a position that is not a part.
- the total area ratio of bainite and martensite in the base steel sheet is 70% or more, in the heating process of the hot press described later, the metal structure of the hot-formed member described above is formed, and the strength after quenching is stabilized. It becomes easy to secure. Therefore, the total area ratio of bainite and martensite in the base steel plate is preferably 70% or more. Although it is not necessary to specify the upper limit of the total area ratio of bainite and martensite, in order to make the cementite crystal grains present at a number density of 1.0 particles / ⁇ m 2 or more, the upper limit of the substantial total area ratio is It becomes about 99.5 area%.
- a method for measuring the area ratio of each of bainite and martensite is well known to those skilled in the art, and can be measured by a conventional method also in this embodiment.
- the area ratios of bainite and martensite were determined by image analysis of an electron microscope image of the metal structure.
- the cementite crystal grains in the base steel sheet become precipitation nuclei for austenite and martensite during heating and cooling during hot pressing.
- the total number density of austenite and martensite needs to be 1.0 piece / ⁇ m 2 or more.
- the cementite crystal grains be present at a number density of 1.0 / ⁇ m 2 or more.
- the total number density of austenite and martensite in the hot-formed member may be less than 1.0 / ⁇ m 2 .
- the number density of cementite can be determined by the following method. First, a test piece is sampled from the base steel plate along the rolling direction of the base steel plate and the direction perpendicular to the rolling direction. Next, the cross section along the rolling direction of the test piece and the metal structure of the cross section perpendicular to the rolling direction are photographed with an electron microscope.
- the number density of cementite is calculated by image analysis of the electron micrograph of the 800 ⁇ m square region obtained in this way. Distinguishing the cementite particles from the surrounding tissue can be easily performed using an electron microscope. It is not necessary to define the average crystal grain size of cementite particles. If the above-described number density is achieved, coarse cementite will not precipitate to such an extent that it adversely affects the steel material.
- the hot-rolled steel sheet that satisfies the conditions required for the base steel sheet in the present embodiment is, for example, subjected to finish rolling in a temperature range of 900 ° C. or less on a slab having the same chemical composition as the chemical composition of the hot-formed member described above. Then, the steel sheet after finish rolling can be manufactured by rapidly cooling to a temperature range of 600 ° C. or lower at a cooling rate of 5 ° C./second or higher.
- the cold-rolled steel sheet satisfying the requirements for the base steel sheet in the present embodiment is, for example, annealing the hot-rolled steel sheet at Ac 3 points or higher and rapidly cooling to a temperature range of 600 ° C. or lower at an average cooling rate of 5 ° C./second or higher. Can be manufactured.
- the hot-dip galvanized cold-rolled steel sheet and alloyed hot-dip galvanized cold-rolled steel sheet satisfying the requirements for the base steel sheet in the present embodiment are obtained, for example, by subjecting the cold-rolled steel sheet to hot-dip galvanization and alloyed hot-dip galvanization, respectively. Can be manufactured.
- Heating temperature of the base steel sheet temperature range of 670 ° C. or more and less than 780 ° C. and less than Ac 3 points
- Heating temperature and holding time of the base steel plate 670 ° C. or higher and lower than 780 ° C. and less than 3 points of Ac for 2 minutes to 20 minutes
- the base steel sheet is heated to a temperature range of 670 ° C. or more and less than 780 ° C. and less than Ac 3 points (° C.).
- the temperature of the base steel plate is held in the above temperature range, that is, a temperature range of 670 ° C. or more and less than 780 ° C.
- Ac 3 point is a temperature defined by the following formula (i) obtained by experiment, and when the steel is heated to a temperature range of Ac 3 point or higher, the metal structure of the steel becomes an austenite single phase.
- the holding temperature in the holding step is set to 670 ° C. or higher.
- the holding temperature is 780 ° C. or higher or Ac 3 points or higher, a sufficient amount of austenite is not contained in the metal structure of the hot-formed member after quenching, and the ductility of the hot-formed member is significantly deteriorated. To do. Further, when the holding temperature is 780 ° C.
- the holding temperature is less than 780 ° C. and less than Ac 3 points.
- the holding temperature is preferably set to 680 ° C. to 760 ° C. If the holding time in the holding step is less than 2 minutes, it is difficult to stably ensure the strength of the hot-formed member after quenching. Accordingly, the holding time is 2 minutes or more.
- the holding time is 20 minutes or less.
- the holding time is preferably set to 3 minutes to 15 minutes.
- the heating rate up to a temperature range of 670 ° C. or higher and lower than 780 ° C. and lower than Ac 3 point need not be particularly limited. However, it is preferable to heat the steel sheet at an average heating rate of 0.2 ° C./second to 100 ° C./second. By setting the average heating rate to 0.2 ° C./second or more, higher productivity can be secured. In addition, when the average heating rate is 100 ° C./second or less, the heating temperature can be easily controlled in the case of heating using a normal furnace. However, if high-frequency heating or the like is used, the heating temperature can be accurately controlled even if heating is performed at a heating rate exceeding 100 ° C./second.
- the average cooling rate in the temperature range of 150 ° C. to 600 ° C. is less than 5 ° C./second, soft ferrite and pearlite are excessively generated in the hot-formed member, and it is difficult to secure a tensile strength of 900 MPa or more after quenching. It becomes. Therefore, the average cooling rate in the temperature range is set to 5 ° C./second or more.
- the upper limit value of the average cooling rate in the cooling step varies depending on the Mn content of the base steel sheet. When the Mn content of the base steel sheet is 2.4% by mass to 8.0% by mass, there is no need to particularly limit the upper limit value of the average cooling rate.
- the average cooling rate in the temperature range of 150 ° C. to 600 ° C. is 500 ° C./second or less.
- the average cooling rate in the temperature range is preferably 200 ° C./second or less.
- the Mn content of the base steel sheet is 1.2% or more and less than 2.4%, it is necessary to perform slow cooling in a temperature range of less than 500 ° C and 150 ° C or more in order to increase the ductility of the hot-formed member. is there.
- the Mn content of the base steel sheet is 1.2% or more and less than 2.4%, specifically, an average cooling rate of 5 ° C./second to 20 ° C./second in a temperature range of less than 500 ° C. and 150 ° C. or more. More specifically, it is preferable to control the cooling rate as described below.
- the heat capacity of the steel mold may be changed by changing the dimensions of the mold. If the mold dimensions cannot be changed, the cooling rate can also be changed by using a fluid cooling mold and changing the flow rate of the cooling medium.
- the cooling rate can also be changed by using a mold having grooves cut in advance and flowing a cooling medium (water or gas) through the grooves during pressing.
- the cooling rate can also be changed by operating the press machine during pressing to separate the mold and the hot forming member and flowing gas between them.
- the cooling rate can also be changed by changing the mold clearance and changing the contact area between the mold and the steel plate (hot forming member).
- the following means can be considered as means for changing the cooling rate around 500 ° C.
- the hot forming member is moved to a mold having a different heat capacity or a mold heated to over 100 ° C. to change the cooling rate;
- the cooling rate is changed by changing the flow rate of the cooling medium in the mold immediately after reaching 500 ° C .;
- the pressing machine is operated to separate the mold and the hot forming member, and a gas is flowed between them, and the flow rate of this gas is changed to change the cooling rate.
- the form of molding in the hot press method in this embodiment is not particularly limited.
- Exemplified molding forms are bending, drawing, stretch molding, hole expansion molding, and flange molding. What is necessary is just to select a preferable thing from the above-mentioned shaping
- molding forms suitably according to the kind and shape of the target hot forming member.
- Representative examples of hot forming members include door guard bars and bumper reinforcement, which are reinforcing parts for automobiles.
- the hot-formed member is a bumper reinforcement
- the above-mentioned hot-formed member that is an alloyed hot-dip galvanized steel sheet of a predetermined length is prepared, and the above-mentioned conditions are set in the mold. Processing such as bending may be performed sequentially.
- hot forming has been described by exemplifying hot pressing which is a specific aspect, but the manufacturing method according to the present embodiment is not limited to hot pressing.
- the manufacturing method according to the present embodiment can be applied to any hot forming including a means for cooling a steel sheet at the same time as forming or immediately after forming, similarly to hot pressing.
- An example of such hot forming is roll forming.
- the hot-formed member according to this embodiment is characterized by excellent ductility and impact characteristics.
- the hot-formed member according to the present embodiment preferably has ductility such that the total elongation in the tensile test is 15% or more. More preferably, the total elongation in the tensile test of the hot-formed member according to this embodiment is 18% or more. Most preferably, the total elongation in the tensile test of the hot-formed member according to this embodiment is 21% or more.
- the hot-formed member according to the present embodiment has an impact characteristic that an impact value of a Charpy test at 0 ° C. is 20 J / cm 2 or more. A hot-formed member having such characteristics is realized by satisfying the above-mentioned regulations concerning chemical composition and metal structure.
- shot blasting is usually applied to the hot formed member for scale removal.
- This shot blasting treatment has the effect of introducing compressive stress into the surface of the material to be treated. Therefore, subjecting the hot-formed member to the shot blasting treatment has the advantages of suppressing delayed fracture in the hot-formed member and improving the fatigue strength of the hot-formed member.
- a steel plate having the chemical composition shown in Table 1 and the thickness and metal structure shown in Table 2 was used as the base steel plate.
- These base steel sheets are obtained by hot rolling a slab melted in a laboratory (referred to as a hot rolled steel sheet in Table 2), or cold rolling and recrystallization annealing of a hot rolled steel sheet. It is a steel plate manufactured by (denoted as a cold-rolled steel plate in Table 2). In addition, by using a plating simulator, some steel plates were subjected to hot dip galvanizing treatment (plating adhesion amount per side was 60 g / m 2 ) or alloyed hot dip galvanizing treatment (plating adhesion amount per side was 60 g / m 2 ). m 2 , Fe content in the plating film was 15% by mass). In Table 2, each is described as a hot dip galvanized steel sheet and an alloyed hot dip galvanized steel sheet. Further, a steel sheet that was cold-rolled (denoted as “full hard” in Table 2) was also used.
- Specimens produced in the examples and comparative examples are not subjected to hot pressing with a mold, but receive the same thermal history as that of hot-formed members. Accordingly, the mechanical properties of the test material are substantially the same as those of a hot-formed member having the same thermal history.
- the heat-treated specimen was machined to produce a V-notch test piece having a thickness of 1.2 mm.
- V-notch test pieces were stacked and screwed, and then subjected to a Charpy impact test.
- the direction of the V notch was parallel to the rolling direction.
- the impact value at 0 ° C. was 20 J / cm 2 or more, it was determined that the impact characteristics were “good”.
- the heat-treated specimen was descaled, and then the presence or absence of scale residue on the specimen surface was confirmed. Those in which scale residue occurred were judged to be comparative examples having poor surface properties. Also, the heat-treated specimen was immersed in 0.1N normal hydrochloric acid to confirm whether or not delayed fracture occurred. Those in which delayed fracture occurred were judged to be comparative examples having poor delayed fracture resistance.
- Table 4 shows the results of tests simulating these hot presses.
- Specimen No. which is an example of the present invention in Table 4. 1 to 3, 8, 9, 11, 13, 15, 18, 20, 21, 25, 26, 30 and 32 had high tensile strength of 900 MPa or more and excellent ductility and impact characteristics. Furthermore, these sample materials of the present invention had no scale residue after descaling, that is, excellent surface properties, and the cut end surfaces did not crack during hydrochloric acid immersion, that is, excellent delayed fracture resistance.
- the test material No. No. 4 because the cooling rate was out of the range defined in the present invention, the target tensile strength could not be obtained.
- the test material No. 10 the metal structure of the base steel plate deviated from the range specified in the present invention, and thus the target tensile strength could not be obtained.
- Specimen No. 12 had a poor ductility because the cooling rate was out of the range defined in the present invention. Since the test materials No.
- Specimen No. 14 and 16 were out of the range specified in the present invention, the ductility and impact characteristics were poor.
- Specimen No. 17 had a poor ductility because the heating temperature was outside the range defined in the present invention.
- Sample No. No. 19 had a bad impact property because the chemical composition was outside the range defined in the present invention. Since test material No. 22 was outside the range defined in the present invention, the target tensile strength could not be obtained.
- Specimen No. 27 was poor in ductility because the chemical composition was outside the range defined in the present invention.
- Specimen No. No. 23 is an example in which the holding time is out of the range defined in the present invention. 28 and 31 are examples in which the chemical composition is outside the range defined in the present invention.
- the test material No. In 1-3, 7-9, 11, 13, 15, 17, 19 and 21 the Si content is in a preferable range, and the ductility is further improved.
- the test material No. 2, 8, 11, 17, 19, and 21 are in a preferable range of the area ratio of austenite, and the ductility is very good.
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Abstract
Description
(1)本発明の一態様に係る熱間成形部材は、化学組成が、質量%で、C:0.05%~0.40%、Si:0.5%~3.0%、Mn:1.2%~8.0%、P:0.05%以下、S:0.01%以下、sol.Al:0.001%~2.0%、N:0.01%以下、Ti:0%~1.0%、Nb:0%~1.0%、V:0%~1.0%、Cr:0%~1.0%、Mo:0%~1.0%、Cu:0%~1.0%、Ni:0%~1.0%、Ca:0%~0.01%、Mg:0%~0.01%、REM:0%~0.01%、Zr:0%~0.01%、B:0%~0.01%、Bi:0%~0.01%、および残部:Feおよび不純物であり、10面積%~40面積%のオーステナイトを含有するとともに、前記オーステナイトの結晶粒およびマルテンサイトの結晶粒の合計個数密度が1.0個/μm2以上である金属組織を有し、引張強度が900MPa~1300MPaである。
はじめに、本発明の一実施形態に係る熱間成形部材の化学組成について説明する。以下の説明において、各合金元素の含有量を表す「%」は、特に断りがない限り「質量%」を意味する。なお、鋼の化学組成は熱間成形が行われても変化しないので、熱間成形を受ける前の素地鋼板中の各元素の含有量と、熱間成形後の熱間成形部材中の各元素の含有量とはそれぞれ等しい。
Cは、鋼の焼入れ性を高め、かつ焼入れ後の熱間成形部材の強度に最も強く影響する、非常に重要な元素である。C含有量が0.05%未満では、焼入れ後に900MPa以上の引張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0.05%以上とする。一方、C含有量が0.40%超では、熱間成形部材の衝撃特性が顕著に劣化する。したがって、C含有量は0.40%以下とする。熱間成形部材の溶接性を向上させるためには、C含有量を0.25%以下とすることが好ましい。熱間成形部材の強度を安定して確保するためには、C含有量を0.08%以上とすることが好ましい。
Siは、焼入れ後の鋼の強度を安定して確保するために、非常に効果的な元素である。さらに、Siを添加することによって金属組織中のオーステナイトが増加し、熱間成形部材の延性が向上する。Si含有量が0.5%未満では、上記作用を得ることが困難である。特に、本実施形態においてオーステナイトが不足した場合、必要な延性が得られないので、産業利用上極めて不利となる。したがって、Si含有量は0.5%以上とする。なお、Si含有量を1.0%以上にすると、延性がさらに向上するようになる。したがって、Si含有量は1.0%以上とすることが好ましい。一方、Si含有量が3.0%超では、上記作用による効果は飽和して経済的に不利となるうえに、熱間成形部材の表面性状の劣化が著しくなる。したがって、Si含有量は3.0%以下とする。熱間成形部材の表面性状の劣化をさらに確実に防止するためには、Si含有量を2.5%以下とすることが好ましい。
Mnは、鋼の焼入れ性を高め、焼入れ後の強度を安定して確保するために、非常に効果的な元素である。さらに、Mnは、焼き入れ後の熱間成形部材の延性を高める効果をも有する。しかし、Mn含有量が1.2%未満では、それらの効果が十分に得られず、焼入れ後に900MPa以上の引張強度を確保することが非常に困難となる。したがって、Mn含有量は1.2%以上とする。なお、Mn含有量を2.4%以上にすると、熱間成形部材の延性がさらに高まり、後述する熱間成形後の緩冷却が製造工程において不要になり、生産性が著しく向上する。このため、Mn含有量は2.4%以上とすることが好ましい。一方、Mn含有量が8.0%超では、オーステナイトが熱間成形部材中に過剰に生成し、遅れ破壊が発生し易くなる。したがって、Mn含有量は8.0%以下とする。なお、熱間成形を適用する前の素地鋼板の引張強度を低くすると、後の熱間成形工程における生産性が向上する。この効果を得るためには、Mn含有量を6.0%以下とすることが好ましい。
Pは、一般的には鋼に不可避的に含有される不純物である。しかし本実施形態において、Pは固溶強化により鋼の強度を高める作用を有するので、Pを積極的に含有させてもよい。しかし、P含有量が0.05%超では、熱間成形部材の溶接性の劣化が著しくなる場合がある。したがって、P含有量は0.05%以下とする。熱間成形部材の溶接性の劣化をさらに確実に防止するためには、P含有量を0.02%以下とすることが好ましい。上記の強度向上作用をより確実に得るためには、P含有量を0.003%以上とすることが好ましい。しかしながら、P含有量が0%であったとしても、課題を解決するために必要な特性を得ることができるので、P含有量の下限値を制限する必要はない。即ち、P含有量の下限値は0%である。
Sは、鋼に含有される不純物であり、溶接性を向上させるためには、S含有量が低いほど好ましい。S含有量が0.01%超では、溶接性の低下が、許容できない程度に著しくなる。したがって、S含有量は0.01%以下とする。溶接性の低下をさらに確実に防ぐためには、S含有量は、0.003%以下にすることが好ましく、0.0015%以下にすることがさらに好ましい。S含有量は少なければ少ないほど好ましいので、S含有量の下限値を規定する必要はない。即ち、S含有量の下限値は0%である。
sol.Alとは、固溶状態で鋼中に存在する固溶Alのことを示す。Alは、鋼を脱酸して作用を有する元素であり、また、Ti等の炭窒化物形成元素が酸化することを防ぎ、炭窒化物の形成を促進する作用を有する元素でもある。これら作用によって、表面疵が鋼材に発生することを抑制し、鋼材の製造歩留まりを向上させることができる。sol.Al含有量が0.001%未満では、上記作用を得ることが困難となる。したがって、sol.Al含有量は0.001%以上とする。上記作用をさらに確実に得るためには、sol.Al含有量が0.01%以上であることが好ましい。一方、sol.Al含有量が2.0%超では、熱間成形部材の溶接性が著しく低下するとともに、酸化物系介在物が熱間成形部材中に増加し、熱間成形部材の表面性状が著しく劣化する。したがって、sol.Al含有量は2.0%以下とする。上記の現象をさらに確実に回避するためには、sol.Al含有量が1.5%以下であることが好ましい。
Nは、鋼に不可避的に含有される不純物であり、溶接性を向上させるためには、N含有量が低い方が好ましい。N含有量が0.01%超では、熱間成形部材の溶接性の低下が、許容できない程度に著しくなる。したがって、N含有量は0.01%以下とする。溶接性の低下をさらに確実に回避するために、N含有量は好ましくは0.006%以下である。N含有量は少なければ少ないほど好ましいので、N含有量の下限値を規定する必要はない。即ち、N含有量の下限値は0%である。
これらの元素は、いずれも熱間成形部材の焼入れ性を高め、かつ焼入れ後の熱間成形部材の強度を安定して確保するために効果的な元素である。したがって、これらの元素のうち1種または2種以上を含有させてもよい。しかし、Ti、NbおよびVについては、それぞれ1.0%を超えて含有させると、製造工程において熱間圧延および冷間圧延の実施が困難になる。また、Cr、Mo、CuおよびNiについては、1.0%を超えて含有させると、上記作用による効果が飽和して、経済的に不利となる。したがって、各元素を含有させる場合、各元素の含有量は、それぞれ上記の通りとする。なお、上記作用による効果をより確実に得るには、Ti:0.003%以上、Nb:0.003%以上、V:0.003%以上、Cr:0.003%以上、Mo:0.003%以上、Cu:0.003%以上およびNi:0.003%以上の少なくとも1種を満足させることが好ましい。
これらの元素は、いずれも介在物制御、特に介在物の微細分散化に寄与し、熱間成形部材の低温靭性を高める作用を有する元素である。したがって、これらの元素のうち1種または2種以上を含有させてもよい。しかし、いずれの元素も0.01%を超えて含有させると、熱間成形部材の表面性状を劣化させる場合がある。したがって、各元素を含有させる場合、各元素の含有量は、それぞれ上記の通りとする。なお、上記作用による効果をより確実に得るには、添加する上記各元素の含有量をそれぞれ0.0003%以上とすることが好ましい。
ここで、「REM」との用語は、Sc、Yおよびランタノイドからなる合計17元素を指し、「REMの含有量」とは、これら17元素の合計含有量を意味する。ランタノイドをREMとして用いる場合、工業的には、REMはミッシュメタルの形で添加される。
Bは、熱間成形部材の低温靭性を高める作用を有する元素である。したがって、熱間成形部材にBを含有させてもよい。しかし、0.01%を超えてBを含有させると、素地鋼板の熱間加工性が劣化して、熱間圧延の実施が困難になる。したがって、Bを熱間成形部材中に含有させる場合、B含有量は0.01%以下とする。なお、上記作用による効果をより確実に得るためには、B含有量を0.0003%以上とすることが好ましい。
Biは、熱間成形部材の変形時における割れを抑制する作用を有する元素である。したがって、Biを熱間成形部材に含有させてもよい。しかし、0.01%を超える量のBiを含有させると、素地鋼板の熱間加工性が劣化して、熱間圧延の実施が困難になる。したがって、Biを熱間成形部材中に含有させる場合、Bi含有量は0.01%以下とする。なお、上記作用による効果をより確実に得るためには、Bi含有量を0.0003%以上とすることが好ましい。
次に、本実施形態に係る熱間成形部材の金属組織について説明する。以下の説明において、各金属組織の含有量を表す「%」は、特に断りがない限り「面積%」を意味する。
以下で説明する金属組織の構成は、板厚の略1/2tの位置~略1/4tの位置であって、且つ中心偏析部ではない位置における構成である。中心偏析部は、鋼材の代表的な金属組織とは異なる金属組織を有する場合がある。しかしながら、中心偏析部は、板厚全体に対して微小な領域であり、鋼材の特性にほとんど影響を及ぼさない。すなわち、中心偏析部の金属組織は、鋼材の金属組織を代表していると言えない。従って、本実施形態に係る熱間成形部材の金属組織の規定は、板厚の略1/2tの位置~略1/4tの位置であって、且つ中心偏析部ではない位置におけるものとする。なお、「1/2tの位置」とは、熱間成形部材の表面から部材厚さtの1/2の深さである位置を示し、「1/4tの位置」とは、熱間成形部材の表面から部材厚さtの1/4の深さである位置を示す。
鋼中に適量のオーステナイトを含有させることにより、熱間成形部材の延性が著しく向上する。オーステナイトの面積率が10%未満では、優れた延性を確保することが困難である。したがって、オーステナイトの面積率は10%以上とする。なお、オーステナイトの面積率を18%以上にすることは、熱間成形部材の伸びを21%以上とし、極めて優れた延性を熱間成形部材に発現させることに寄与する。したがって、オーステナイトの面積率は18%以上とすることが好ましい。一方、オーステナイトの面積率が40%超では、遅れ破壊が熱間成形部材に発生し易くなる。したがって、オーステナイトの面積率は40%以下とする。遅れ破壊の発生を確実に防ぐためには、オーステナイトの面積率を32%以下とすることが好ましい。
微細な硬質組織を金属組織中に多く存在させること、すなわち、金属組織中のオーステナイトおよびマルテンサイトの個数密度を高めることにより、熱間成形時の熱間成形部材の塑性変形が微視的に局在化することを防ぐことができる。これにより、変形時に生じるオーステナイトおよびマルテンサイトの割れが抑制され、熱間成形部材の衝撃特性を向上させることができる。引張強度が900MPa以上であり、且つ優れた衝撃特性を有する熱間成形部材を達成するためには、熱間成形部材の金属組織を、オーステナイトおよびマルテンサイトが合計で1.0個/μm2以上の個数密度で存在する金属組織とする。なお、上述の衝撃特性向上効果をさらに確実に得るために、オーステナイトおよびマルテンサイトの結晶粒の合計個数密度の下限値を1.3個/μm2とすることがさらに好ましい。オーステナイト粒子およびマルテンサイト粒子の合計個数密度は、大きいほど好ましい。オーステナイト粒子およびマルテンサイト粒子の合計個数密度が大きいほど、変形の局在化が抑制され、衝撃特性がさらに向上するからである。従って、オーステナイト粒子およびマルテンサイト粒子の合計個数密度の上限値を規定する必要はない。しかしながら、製造設備の能力を考慮すると、3.0個/μm2程度が、オーステナイト粒子およびマルテンサイト粒子の合計個数密度の実質的な上限値となる。
オーステナイト粒子の個数とマルテンサイト粒子の個数との比を規定する必要はない。もし、金属組織中にマルテンサイト粒子が含まれなくても、上述の割れ抑制効果を得ることができる。
オーステナイト粒子およびマルテンサイト粒子の個数密度は、以下のような方法によって求めることができる。まず、熱間成形部材の原料である素地鋼板の圧延方向と圧延方向に対して垂直な方向とに沿って、熱間成形部材から試験片を採取する。次いで、試験片の、圧延方向に沿った断面および圧延方向に対して垂直な断面の金属組織を電子顕微鏡で撮影する。これにより得られた、800μm四方の領域の電子顕微鏡写真を画像解析することによって、オーステナイト粒子およびマルテンサイト粒子の個数密度を算出する。オーステナイト粒子およびマルテンサイト粒子を周囲の組織から区別することは、電子顕微鏡を用いれば、容易に行える。
なお、オーステナイト粒子およびマルテンサイト粒子の平均結晶粒径を規定する必要はない。一般的に、平均結晶粒径が大きい場合、鋼の強度に悪影響を及ぼす場合がある。しかし、上述した個数密度が達成されていれば、オーステナイト粒子およびマルテンサイト粒子の粒径が粗大化することはない。
前述したオーステナイトおよびマルテンサイト以外の金属組織として、フェライト、ベイナイト、セメンタイトおよびパーライトのうち1種または2種以上を熱間成形部材に含有させてもよい。オーステナイト及びマルテンサイトの含有量が上述の規定範囲内であれば、フェライト、ベイナイト、セメンタイトおよびパーライトの含有量は特に規定されない。
本実施形態に係る熱間成形部材の引張強度は900MPa以上である。このような引張強度を有することにより、本実施形態に係る鋼板を用いる各種部材の軽量化を達成することができる。しかし、引張強度が1300MPaを上回ると、鋼板に脆性破壊が生じやすくなる。従って、鋼板の引張強度の上限値を1300MPaとする。このような引張強度は、上述の化学成分、および後述する製造方法によって達成される。
次に、上記の特徴を有する本実施形態に係る熱間成形部材の好ましい製造方法について説明する。
そして、素地鋼板のMn含有量が2.4質量%~8.0質量%である場合、熱間成形工程に次いで、冷却工程にて、素地鋼板を600℃~150℃の温度域にて平均冷却速度が5℃/秒~500℃/秒である条件で冷却する。素地鋼板のMn含有量が1.2質量%以上2.4質量%未満である場合、熱間成形工程に次いで、冷却工程にて、600℃~500℃の温度域にて平均冷却速度が5℃/秒~500℃/秒であり、かつ、500℃未満150℃以上の温度域にて平均冷却速度が5℃/秒~20℃/秒である条件で冷却する。
上述した素地鋼板の金属組織の規定は、板厚の略1/2tの位置~略1/4tの位置であって、且つ中心偏析部ではない位置において行われるものとする。素地鋼板の金属組織の構成をこの位置にて規定する理由は、熱間成形部材の金属組織の構成を板厚の略1/2tの位置~略1/4tの位置であって、且つ中心偏析部ではない位置にて規定する理由と同じである。
素地鋼板におけるベイナイトおよびマルテンサイトの合計面積率が70%以上であれば、後述する熱間プレスの加熱工程において、上述した熱間成形部材の金属組織が形成され、焼入れ後の強度を安定して確保しやすくなる。したがって、素地鋼板におけるベイナイトおよびマルテンサイトの合計面積率は70%以上であることが好ましい。ベイナイトおよびマルテンサイトの合計面積率の上限を規定する必要はないが、セメンタイトの結晶粒を1.0個/μm2以上の個数密度で存在させるためには、実質的な合計面積率の上限は99.5面積%程度となる。
ベイナイト及びマルテンサイトそれぞれの面積率の測定法は当業者には周知であり、本実施形態においても常法により測定することができる。後述する実施例では、ベイナイト及びマルテンサイトそれぞれの面積率は、金属組織の電子顕微鏡像を画像解析することによって求められた。
素地鋼板中のセメンタイトの結晶粒は、熱間プレスの際の加熱および冷却の際に、オーステナイトおよびマルテンサイトの析出核となる。熱間成形部品の金属組織では、オーステナイトおよびマルテンサイトの合計個数密度が1.0個/μm2以上である必要があるが、このような金属組織を得るためには、素地鋼板の金属組織中には、セメンタイトの結晶粒が1.0個/μm2以上の個数密度で存在することが必要である。素地鋼板中のセメンタイトの個数密度が1.0個/μm2未満である場合、熱間成形部材中のオーステナイトおよびマルテンサイトの合計個数密度が1.0個/μm2を下回るおそれがある。素地鋼板中のセメンタイトの結晶粒の個数密度が大きいほど、得られる熱間成形部材中のオーステナイト粒子およびマルテンサイト粒子の合計個数密度が大きくなるので好ましい。しかし、設備能力の上限を考慮すると、セメンタイトの結晶粒の個数密度の実質的な上限は3.0個/μm2程度となる。
セメンタイトの個数密度は、以下のような方法によって求めることができる。まず、素地鋼板の圧延方向と圧延方向に対して垂直な方向とに沿って、素地鋼板から試験片を採取する。次いで、試験片の、圧延方向に沿った断面と圧延方向に対して垂直な断面の金属組織を電子顕微鏡で撮影する。これにより得られた、800μm四方の領域の電子顕微鏡写真を画像解析することによって、セメンタイトの個数密度を算出する。セメンタイト粒子を周囲の組織から区別することは、電子顕微鏡を用いれば、容易に行える。
なお、セメンタイト粒子の平均結晶粒径を規定する必要はない。上述した個数密度が達成されていれば、鋼材に悪影響を及ぼす程度に粗大なセメンタイトが析出することはない。
(素地鋼板の保持温度および保持時間:670℃以上780℃未満かつAc3点未満の温度域にて2分間~20分間保持)
熱間プレスに供する素地鋼板の加熱工程では、670℃以上780℃未満かつAc3点(℃)未満の温度域まで素地鋼板を加熱する。素地鋼板の保持工程では、素地鋼板の温度を上記温度域、即ち670℃以上780℃未満かつAc3点(℃)未満の温度域に2分間~20分間保持する。Ac3点は、実験により求められた下記式(i)により規定される温度であり、Ac3点以上の温度域に鋼を加熱した場合、鋼の金属組織はオーステナイト単相になる。
ここで、上記式中における元素記号は、前記鋼板の化学組成における各元素の含有量(単位:質量%)を示す。「sol.Al」は、固溶Alの濃度(単位:質量%)を示す。
保持工程における保持時間が2分間未満では、焼入れ後の熱間成形部材の強度を安定して確保することが困難となる。したがって、保持時間は2分間以上とする。一方、保持時間が20分間超では、生産性が低下するばかりか、スケールや亜鉛系酸化物の生成により、熱間成形部材の表面性状が劣化する。したがって、保持時間は20分間以下とする。上述した好ましくない現象をさらに確実に回避するためには、保持時間を3分間~15分間とすることが好ましい。
(素地鋼板のMn含有量が1.2質量%以上2.4質量%未満である場合の、冷却工程における平均冷却速度:600℃~500℃の温度域にて5℃/秒~500℃/秒、かつ500℃未満150℃以上の温度域にて5℃/秒~20℃/秒)
冷却工程では、150℃~600℃の温度域において、拡散型変態が熱間成形部材にて起きないように冷却する。150℃~600℃の温度域における平均冷却速度が5℃/秒未満では、軟質なフェライトおよびパーライトが熱間成形部材中に過度に生成し、焼入れ後に900MPa以上の引張強度を確保することが困難となる。したがって、上記温度域における平均冷却速度は5℃/秒以上とする。
冷却工程における平均冷却速度の上限値は、素地鋼板のMn含有量に応じて異なる。素地鋼板のMn含有量が2.4質量%~8.0質量%である場合、平均冷却速度の上限値を特に制限する必要はない。しかし、150℃~600℃の温度域における平均冷却速度を500℃/秒超とすることは、通常の設備においては困難である。したがって、素地鋼板のMn含有量が2.4質量%~8.0質量%である場合の、150℃~600℃の温度域における平均冷却速度は500℃/秒以下とする。平均冷却速度が過度に大きい場合、冷却に係るエネルギーによって生産コストが増大するので、素地鋼板のMn含有量が2.4質量%~8.0質量%である場合の、150℃~600℃の温度域における平均冷却速度は好ましくは200℃/秒以下である。
(2)流体冷却方式の金型の場合、500℃到達直後に金型中の冷却媒体の流量を変化させて、冷却速度を変える;
(3)500℃到達直後に、プレス機を操作して金型と熱間成形部材とを離間させ、両者の間にガスを流し、このガスの流量を変化させることで、冷却速度を変える。
表1に示す化学組成、および表2に示す板厚および金属組織を有する鋼板を素地鋼板とした。
素地鋼板の圧延方向と、素地鋼板の圧延方向に対して垂直な方向とに沿って、熱処理した供試材から試験片を採取した。次いで、試験片の、圧延方向に沿った断面および圧延方向に対して垂直な断面の金属組織を電子顕微鏡で撮影した。これにより得られた、合計0.01mm2の領域の電子顕微鏡像を画像解析することによって、金属組織を同定し、ベイナイトおよびマルテンサイトの合計面積率を測定した。また、上述の試料を電子顕微鏡で撮影することにより得られた800μm四方の領域の電子顕微鏡像を画像解析することによって、セメンタイト粒子の個数密度を算出した。
素地鋼板の圧延方向と、素地鋼板の圧延方向に対して垂直な方向とに沿って、熱処理した供試材から試験片を採取した。次いで、試験片の、圧延方向に沿った断面および圧延方向に対して垂直な断面の金属組織を電子顕微鏡で撮影した。これにより得られた、800μm四方の領域の電子顕微鏡像を画像解析することによって、オーステナイト粒子およびマルテンサイト粒子の個数密度を算出した。
熱処理した各供試材から幅25mm、および長さ25mmの試験片を切り出し、この試験片の表面に化学研磨を施して0.3mm減厚した。化学研磨後の試験片表面に対してX線回折を実施し、これにより得られたプロファイルを解析し、残留オーステナイトの面積率を得た。このX線回折を計三回繰り返し、得られた面積率を平均した値を「オーステナイトの面積率」として表に記載した。
熱処理した各供試材から、荷重軸が圧延方向に対して垂直となるように、JIS5号引張試験片を採取し、TS(引張強度)およびEL(全伸び)を測定した。引張強度が900MPa未満である供試材、および全伸びが15%未満である供試材は「不良」と判定した。
熱処理した供試材を機械加工して、厚さが1.2mmであるVノッチ試験片を作製した。そのVノッチ試験片を4枚積層してねじ止めした後、シャルピー衝撃試験に供した。Vノッチの方向は、圧延方向に平行とした。0℃での衝撃値が20J/cm2以上となる場合、衝撃特性が「良好」であると判定した。
熱処理した供試材をデスケーリングし、その後、供試材表面におけるスケール残りの有無を確認した。スケール残りが生じたものは、表面性状が不良である比較例であると判断した。また、熱処理した供試材を0.1N規定の塩酸に浸漬して、遅れ破壊が生じるか否かを確認した。遅れ破壊が生じたものは、耐遅れ破壊特性が不良である比較例であると判断した。
これらの熱間プレスを模擬した試験の結果を表4に示す。
供試材No.7および24は、化学組成が本発明で規定される範囲を外れたので、目標とする引張強度が得られなかった。
供試材No.10は、素地鋼板の金属組織が本発明で規定される範囲を外れたので、目標とする引張強度が得られなかった。
供試材No.12は、冷却速度が本発明で規定される範囲を外れたので、延性が悪かった。供試材No.14および16は、加熱温度が本発明で規定される範囲を外れたので、延性と衝撃特性が悪かった。
供試材No.17は、加熱温度が本発明で規定される範囲を外れたので、延性が悪かった。
試材No.19は、化学組成が本発明で規定される範囲を外れたので、衝撃特性が悪かった。
供試材No.22は、保持時間が本発明で規定される範囲を外れたので、目標とする引張強度が得られなかった。
供試材No.27は、化学組成が本発明で規定される範囲を外れたので、延性が悪かった。
供試材No.23は、保持時間が本発明で規定される範囲を外れた例であり、供試材No.28、および31は、化学組成が本発明で規定される範囲を外れた例である。これら供試材は、引張強度、全伸び、および衝撃特性は良好であったが、デスケーリングした後にスケール残りが生じ、表面性状が不良であった。供試材No.29は、化学組成が本発明で規定される範囲を外れたので、0.1N規定の塩酸に浸漬すると遅れ破壊が生じ、耐遅れ破壊特性が不良であると判断された。
Claims (7)
- 化学組成が、質量%で、
C:0.05%~0.40%、
Si:0.5%~3.0%、
Mn:1.2%~8.0%、
P:0.05%以下、
S:0.01%以下、
sol.Al:0.001%~2.0%、
N:0.01%以下、
Ti:0%~1.0%、
Nb:0%~1.0%、
V:0%~1.0%、
Cr:0%~1.0%、
Mo:0%~1.0%、
Cu:0%~1.0%、
Ni:0%~1.0%、
Ca:0%~0.01%、
Mg:0%~0.01%、
REM:0%~0.01%、
Zr:0%~0.01%、
B:0%~0.01%、
Bi:0%~0.01%、および
残部:Feおよび不純物であり、
10面積%~40面積%のオーステナイトを含有するとともに、前記オーステナイトの結晶粒およびマルテンサイトの結晶粒の合計個数密度が1.0個/μm2以上である金属組織を有し、
引張強度が900MPa~1300MPaであることを特徴とする熱間成形部材。 - 前記化学組成が、質量%で、
Ti:0.003%~1.0%、
Nb:0.003%~1.0%、
V:0.003%~1.0%、
Cr:0.003%~1.0%、
Mo:0.003%~1.0%、
Cu:0.003%~1.0%、および
Ni:0.003%~1.0%からなる群から選ばれた1種または2種以上を含有することを特徴とする請求項1に記載の熱間成形部材。 - 前記化学組成が、質量%で、
Ca:0.0003%~0.01%、
Mg:0.0003%~0.01%、
REM:0.0003%~0.01%、および
Zr:0.0003%~0.01%以下からなる群から選ばれた1種または2種以上を含有することを特徴とする請求項1または請求項2に記載の熱間成形部材。 - 前記化学組成が、質量%で、
B:0.0003%~0.01%を含有することを特徴とする請求項1から請求項3のいずれか1項に記載の熱間成形部材。 - 前記化学組成が、質量%で、
Bi:0.0003%~0.01%以下を含有することを特徴とする請求項1から請求項4のいずれか1項に記載の熱間成形部材。 - 請求項1から請求項5のいずれか1項に記載の熱間成形部材の前記化学組成と同一の化学組成を有し、さらにMn含有量が2.4質量%~8.0質量%であり、ベイナイトおよびマルテンサイトから選ばれた1種または2種を合計で70面積%以上含有し、セメンタイトの結晶粒が1.0個/μm2以上の個数密度で存在する金属組織を有する素地鋼板を670℃以上780℃未満かつAc3点未満の温度域に加熱する加熱工程と、
前記加熱工程に次いで、前記素地鋼板の温度を670℃以上780℃未満かつAc3点未満の温度域に2分間~20分間保持する保持工程と、
前記保持工程に次いで、前記素地鋼板に熱間成形を行う熱間成形工程と、
前記熱間成形工程に次いで、前記素地鋼板を、600℃~150℃の温度域にて平均冷却速度が5℃/秒~500℃/秒である条件で冷却する冷却工程と、
を含むことを特徴とする熱間成形部材の製造方法。 - 請求項1から請求項5のいずれか1項に記載の熱間成形部材の前記化学組成と同一の化学組成を有し、さらにMn含有量が1.2質量%以上2.4質量%未満であり、ベイナイトおよびマルテンサイトから選ばれた1種または2種を合計で70面積%以上含有し、セメンタイトの結晶粒が1.0個/μm2以上の個数密度で存在する金属組織を有する素地鋼板を670℃以上780℃未満かつAc3点未満の温度域に加熱する加熱工程と、
前記加熱工程に次いで、前記素地鋼板の温度を前記670℃以上780℃未満かつAc3点未満の温度域に2分間~20分間保持する保持工程と、
前記保持工程に次いで、前記素地鋼板に熱間成形を行う熱間成形工程と、
前記熱間成形工程に次いで、前記素地鋼板を、600℃~500℃の温度域にて平均冷却速度が5℃/秒~500℃/秒であり、かつ、500℃未満150℃以上の温度域にて前記平均冷却速度が5℃/秒~20℃/秒である条件で冷却する冷却工程と、
を含むことを特徴とする熱間成形部材の製造方法。
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Also Published As
Publication number | Publication date |
---|---|
CN105874091A (zh) | 2016-08-17 |
EP3093359A1 (en) | 2016-11-16 |
JP6098733B2 (ja) | 2017-03-22 |
KR20160097347A (ko) | 2016-08-17 |
JPWO2015102051A1 (ja) | 2017-03-23 |
CA2935308C (en) | 2018-09-25 |
US10266911B2 (en) | 2019-04-23 |
CN114438418A (zh) | 2022-05-06 |
CA2935308A1 (en) | 2015-07-09 |
EP3093359A4 (en) | 2017-08-23 |
MX2016008809A (es) | 2016-09-08 |
KR101831544B1 (ko) | 2018-02-22 |
RU2659549C2 (ru) | 2018-07-02 |
IN201617022707A (ja) | 2016-08-31 |
US20160319389A1 (en) | 2016-11-03 |
RU2016128754A (ru) | 2018-02-13 |
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