WO2015037059A1 - プレス成形品の製造方法およびプレス成形品 - Google Patents
プレス成形品の製造方法およびプレス成形品 Download PDFInfo
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Definitions
- the present invention relates to a press-formed product used when manufacturing a structural part of an automobile, and a method for manufacturing such a press-formed product.
- a press-formed product used when manufacturing a structural part of an automobile
- a method for manufacturing such a press-formed product In particular, when a preheated steel plate (blank) is formed into a predetermined shape, a press-molded product manufactured by applying a heat treatment to obtain a predetermined strength by applying heat treatment simultaneously with the shape formation, and such
- the present invention relates to a useful method for producing a pressed product.
- the steel sheet is heated to a predetermined temperature (for example, the temperature at which it becomes an austenite phase) to lower the strength, and then formed with a mold having a temperature lower than that of the steel sheet (for example, room temperature).
- a hot press molding method is employed in the production of parts that performs quenching heat treatment (quenching) using the temperature difference between the two to ensure the strength after molding.
- a hot press forming method is called by various names such as a hot forming method, a hot stamping method, a hot stamp method, and a die quench method in addition to the hot press method.
- FIG. 1 is a schematic explanatory diagram showing a mold configuration for carrying out hot press molding as described above.
- 1 is a punch
- 2 is a die
- 3 is a blank holder
- 4 is a steel plate (blank)
- BHF is a crease pressing force
- rp is a punch shoulder radius
- rd is a die shoulder radius
- CL is a punch / die clearance.
- the punch 1 and the die 2 have passages 1a and 2a through which a cooling medium (for example, water) can pass, and the cooling medium is allowed to pass through the passages.
- a cooling medium for example, water
- the steel plate (blank) 4 is subjected to the two-phase region temperature (Ac 1 transformation point to Ac 3 transformation point) or Ac 3 transformation. Molding is started in a state of being softened by heating to a single-phase temperature above the point. That is, in a state where the steel plate 4 in a high temperature state is sandwiched between the die 2 and the blank holder 3, the steel plate 4 is pushed into the hole of the die 2 (between 2 and 2 in FIG. 1) by the punch 1, and the outer diameter of the steel plate 4 is reduced. A shape corresponding to the outer shape of the punch 1 is formed while shrinking.
- a steel sheet for hot pressing that is widely used at present, a steel sheet made of 22MnB5 steel is known.
- This steel sheet has a tensile strength of 1500 MPa and an elongation of about 6 to 8%, and is applied to an impact resistant member (a member that is not deformed as much as possible and does not break).
- an impact resistant member a member that is not deformed as much as possible and does not break.
- Patent Documents 1 to 4 As hot-press steel sheets exhibiting good elongation, techniques such as Patent Documents 1 to 4 have been proposed. In these technologies, the basic strength class of each steel sheet is adjusted by setting the carbon content in the steel sheet to various ranges, and ferrite with high deformability is introduced, and the average of ferrite and martensite Elongation is improved by reducing the particle size. These techniques are effective for improving the elongation, but are still insufficient from the viewpoint of improving the elongation according to the strength of the steel sheet. For example, the tensile strength TS is 1270 MPa or more and the elongation EL is about 12.7% at the maximum, and further improvement is required.
- Non-Patent Document 1 Non-Patent Document 1
- the present invention has been made in view of the above circumstances, and its purpose is to obtain a press-molded product that can achieve a high balance between high strength and elongation, and has good softening prevention characteristics in HAZ. It is an object of the present invention to provide a method useful for obtaining a press-molded product and a press-molded product that exhibits the above characteristics.
- the method for producing a press-formed product of the present invention that has achieved the above-mentioned object is as follows: C: 0.15 to 0.5% (meaning mass%, hereinafter the same for chemical composition) Si: 0.2-3%, Mn: 0.5 to 3%, P: 0.05% or less (excluding 0%), S: 0.05% or less (excluding 0%), Al: 0.01 to 1%, B: 0.0002 to 0.01%, Ti: 3.4 [N] + 0.01% or more, 3.4 [N] + 0.1% or less (where [N] indicates the content (% by mass) of N), and N: 0.001 ⁇ 0.01%, Each of which contains iron and inevitable impurities, Among the Ti-containing precipitates contained in the steel sheet, the average equivalent circle diameter of those having an equivalent circle diameter of 30 nm or less is 6 nm or less, and the relationship between the precipitated Ti amount in the steel and the total Ti amount in the following formula (1)
- the steel sheet for hot pressing that satisfies the above conditions is heated to
- the “equivalent circle diameter” means the diameter when converted to a circle of the same area when focusing on the size (area) of the Ti-containing precipitate (eg, TiC) (the “average equivalent circle diameter” is its average Value).
- the steel sheet for hot pressing used in the production method of the present invention contains at least one of the following (a) to (c) as other elements as required. Depending on the type of element contained as required, the properties of the press-formed product are further improved.
- C 0.01% or less (excluding 0%) of at least one selected from the group consisting of Mg, Ca and REM
- the metal structure is bainitic ferrite: 60 to 97 area%, martensite: 37 area% or less, retained austenite: 3 to 20 area%, and remaining structure: 5 area%.
- the average equivalent circle diameter of those having an equivalent circle diameter of 30 nm or less is 10 nm or less, and satisfies the relationship of the formula (1), The balance between high strength and elongation can be achieved as a uniform characteristic at a high level in a press-formed product.
- the chemical component composition is strictly defined, the size of Ti-containing precipitates is controlled, and for Ti that does not form TiN, a steel plate with a controlled precipitation rate is used.
- a steel plate with a controlled precipitation rate is used.
- the inventors of the present invention when heating a steel plate to a predetermined temperature and then producing a press-formed product by hot press forming, show good ductility (elongation) while ensuring high strength after press forming. In order to realize a simple press-formed product, we examined it from various angles.
- C is important in reducing the bainite transformation start temperature Bs to make the bainitic ferrite produced in the cooling process finer and increasing the dislocation density in the bainitic ferrite to improve the strength. It is an element. Also, by increasing the amount of fine retained austenite formed between the laths of bainitic ferrite, a high level of balance between strength and elongation can be secured. If the C content is less than 0.15%, the bainite transformation start temperature Bs increases, the bainitic ferrite becomes coarse and low dislocation density, and the strength of the hot press-formed product cannot be ensured. On the other hand, if the C content is excessive and exceeds 0.5%, the strength becomes too high and good ductility cannot be obtained.
- the preferable lower limit of the C content is 0.18% or more (more preferably 0.20% or more), and the preferable upper limit is 0.45% or less (more preferably 0.40% or less).
- Si exhibits the effect of forming retained austenite by suppressing the decomposition of residual austenite formed between the laths of bainitic ferrite during the quenching of mold quenching and the formation of cementite.
- the Si content needs to be 0.2% or more. Further, if the Si content is excessive and exceeds 3%, ferrite tends to be formed, and it becomes difficult to make austenite single phase during heating, and the structure fraction other than bainitic ferrite and residual austenite in the steel sheet for hot pressing. Exceeds 5 area%.
- the preferable lower limit of the Si content is 0.5% or more (more preferably 1.0% or more), and the preferable upper limit is 2.5% or less (more preferably 2.0% or less).
- Mn is an element effective in enhancing hardenability and suppressing the formation of soft structures such as ferrite and pearlite during cooling of mold hardening. Further, by reducing the bainite transformation start temperature Bs, the bainitic ferrite generated in the cooling process is made finer, and the dislocation density in the bainitic ferrite is increased, which is important in improving the strength. It is an element. Furthermore, it is an element that stabilizes austenite and contributes to an increase in the amount of retained austenite. In order to exert these effects, it is necessary to contain Mn in an amount of 0.5% or more. Considering only the characteristics, it is preferable that the Mn content is large, but the alloy addition cost increases, so the content was made 3% or less. The minimum with preferable Mn content is 0.7% or more (more preferably 1.0% or more), and a preferable upper limit is 2.5% or less (more preferably 2.0% or less).
- P 0.05% or less (excluding 0%)
- P is an element inevitably contained in the steel, but it deteriorates ductility, so it is preferable to reduce P as much as possible.
- extreme reduction leads to an increase in steelmaking cost, and it is difficult to produce 0%, so 0.05% or less (excluding 0%) was set.
- the upper limit with preferable P content is 0.045% or less (more preferably 0.040% or less).
- S 0.05% or less (excluding 0%)
- S is an element inevitably contained in steel, and deteriorates ductility. Therefore, S is preferably reduced as much as possible.
- extreme reduction leads to an increase in steelmaking cost, and it is difficult to produce 0%, so 0.05% or less (excluding 0%) was set.
- the upper limit with preferable S content is 0.045% or less (more preferably 0.040% or less).
- Al 0.01-1%)
- Al is useful as a deoxidizing element, and also fixes solid solution N present in steel as AlN, which is useful for improving ductility.
- the Al content needs to be 0.01% or more.
- the minimum with preferable Al content is 0.02% or more (more preferably 0.03% or more), and a preferable upper limit is 0.8% or less (more preferably 0.6% or less).
- B has the effect of suppressing ferrite transformation and pearlite transformation, and therefore prevents the formation of ferrite and pearlite during cooling after heating to the two-phase region temperature of (Ac 1 transformation point to Ac 3 transformation point) It is an element that contributes to securing austenite. In order to exert such an effect, B needs to be contained in an amount of 0.0002% or more, but the effect is saturated even if it is contained in excess of 0.01%.
- a preferable lower limit of the B content is 0.0003% or more (more preferably 0.0005% or more), and a preferable upper limit is 0.008% or less (more preferably 0.005% or less).
- Ti 3.4 [N] + 0.01% or more, 3.4 [N] + 0.1% or less: [N] is N content (mass%)
- Ti fixes N and allows B to be maintained in a solid solution state, thereby exhibiting an effect of improving hardenability. In order to exert such an effect, it is important to contain 0.01% or more than the stoichiometric ratio of Ti and N (3.4 times the N content). Further, Ti added excessively to N is present in a solid solution state in the hot stamping molded product, and the precipitated compound is dispersed finely, so that the solid solution is dissolved when the hot stamping molded product is welded.
- Strength reduction in HAZ can be suppressed by effects such as precipitation strengthening due to the formation of Ti as TiC and an increase delay of dislocation density due to the effect of preventing dislocation movement due to TiC.
- the Ti content becomes excessive and exceeds 3.4 [N] + 0.1%, the Ti-containing precipitates formed (for example, TiN) are coarsened and the ductility of the steel sheet is lowered.
- the preferable lower limit of the Ti content is 3.4 [N] + 0.02% or more (more preferably 3.4 [N] + 0.05% or more), and the preferable upper limit is 3.4 [N] + 0.09%. Or less (more preferably 3.4 [N] + 0.08% or less).
- N (N: 0.001 to 0.01%) N is preferably reduced as much as possible in order to reduce the hardenability improvement effect by fixing B as BN.
- 0.001% is set as the lower limit.
- the N content is excessive, the Ti-containing precipitates formed (for example, TiN) are coarsened, and the precipitates act as a starting point for fracture and reduce the ductility of the steel sheet. It was.
- the upper limit with preferable N content is 0.008% or less (more preferably 0.006% or less).
- the basic chemical components in the steel sheet for hot pressing used in the present invention are as described above, and the balance is iron and inevitable impurities other than P, S, N (for example, O, H, etc.).
- the steel sheet for hot pressing used in the present invention contains at least one of the following (a) to (c) as other elements as required.
- the properties of the press-formed product are further improved.
- the preferable range when these elements are contained and the reason for limiting the range are as follows.
- A 0.1% or less in total of one or more selected from the group consisting of V, Nb and Zr (excluding 0%)
- B 1% or less in total of at least one selected from the group consisting of Cu, Ni, Cr and Mo (not including 0%)
- C 0.01% or less (excluding 0%) of at least one selected from the group consisting of Mg, Ca and REM
- V, Nb, and Zr have the effect of forming fine carbides and making the structure fine by the pinning effect. In order to exhibit such an effect, it is preferable to contain 0.001% or more in total. However, when the content of these elements is excessive, coarse carbides are formed, and the ductility is deteriorated by becoming the starting point of fracture. For these reasons, the total content of these elements is preferably 0.1% or less. The more preferable lower limit of the content of these elements is 0.005% or more (more preferably 0.008% or more) in total, and the more preferable upper limit is 0.08% or less (more preferably 0.06%) in total. The following).
- the more preferable lower limit of the content of these elements is 0.05% or more (more preferably 0.06% or more) in total, and the more preferable upper limit is 0.5% or less (more preferably 0.3% or less) in total. ).
- One or more selected from the group consisting of Mg, Ca, and REM is 0.01% or less in total (not including 0%)
- these elements refine the inclusions, they effectively work to improve ductility.
- the content is large, but since the effect is saturated, the total content is preferably 0.01% or less.
- the more preferable lower limit of the content of these elements is 0.0002% or more (more preferably 0.0005% or more) in total, and the more preferable upper limit is 0.005% or less (more preferably 0.003% or less) in total. ).
- the Ti-containing precipitates and the control of the formula (1) are for preventing the softening of the HAZ, and are essentially necessary control in the molded product, but the change of these values before and after hot press molding is small. Therefore, it is necessary to already control at the stage before forming (steel plate for hot pressing).
- a Ti-containing precipitate can be maintained in a solid solution state or a fine state during heating by hot pressing. It becomes like this.
- the amount of precipitated Ti in the press-formed product can be controlled to a predetermined amount or less, and joint characteristics can be improved by preventing softening in the HAZ.
- the average equivalent circle diameter of those having an equivalent circle diameter of 30 nm or less is 6 nm or less.
- the size (average equivalent circle diameter) of the Ti-containing precipitate is preferably 5 nm or less, and more preferably 3 nm or less.
- the Ti-containing precipitates targeted in the present invention include TiC and TiN, as well as precipitates containing Ti such as TiVC, TiNbC, TiVCN, and TiNbCN.
- the average equivalent circle diameter of Ti-containing precipitates in a press-formed product is specified to be 10 nm or less, whereas it is specified to be 6 nm or less before forming (hot-press steel plate). .
- the reason is that Ti is present in a fine precipitate or a solid solution state in the steel plate, but when heating for about 15 minutes or more near 800 ° C., the Ti-containing precipitate is slightly coarsened. Rather than the molded product, the size of the precipitate is defined larger.
- the average equivalent circle diameter of the Ti-containing precipitate is required to be 10 nm or less, and in order to realize the precipitation state with a hot stamped product, It is necessary that the average equivalent circle diameter of fine precipitates of 30 nm or less at the stage is 6 nm or less, and most of Ti is present in a solid solution state.
- the amount of Ti present as precipitates other than TiN is less than the remaining 0.5 times of the total Ti minus Ti forming TiN. (That is, less than 0.5 ⁇ [total Ti amount (%) ⁇ 3.4 [N]]) (requirement (B) above).
- the amount of precipitated Ti-3.4 [N] is preferably 0.4 ⁇ [total Ti amount (%)-3.4 [N]] or less, more preferably 0.3 ⁇ [total Ti amount (% ) -3.4 [N]] or less.
- a slab obtained by melting a steel material having the chemical composition as described above is heated at a temperature of 1100 ° C. or higher (preferably 1150 ° C. or higher).
- Hot rolling is performed at 1300 ° C. or lower (preferably 1250 ° C. or lower), the finish rolling temperature is 850 ° C. or higher (preferably 900 ° C. or higher), and 1000 ° C. or lower (preferably 950 ° C. or lower).
- 450 ° C or lower is cooled (rapidly cooled) at an average cooling rate of 20 ° C / second or higher (preferably 30 ° C / second or higher), 200 ° C or higher (preferably 250 ° C or higher), 500 ° C or lower (preferably It may be wound at 450 ° C. or lower).
- the steel sheet for hot pressing having the above chemical composition and Ti precipitation state may be used for the production of hot pressing as it is, and the reduction ratio after pickling: 10 to 80% (preferably 20 to 70%) ) May be used for manufacturing a hot press after cold rolling. Further, after heating the steel sheet for hot pressing or the cold rolled material thereof to 830 ° C. or higher (preferably 850 ° C. or higher, 900 ° C. or lower), the temperature reaches 500 ° C. or lower (preferably 450 ° C. or lower) at 20 ° C./second or higher ( After quenching at a cooling rate of preferably 30 ° C./second or higher), a heat treatment may be performed such that holding at 500 ° C.
- the steel plate for hot pressing according to the present invention may be plated on the surface (base steel plate surface) containing one or more of Al, Zn, Mg, and Si.
- the steel sheet for hot pressing as described above, after heating to a temperature of 900 ° C. or more and 1100 ° C. or less, press molding is started, and during molding and after the molding, an average of 20 ° C./second or more in the mold After cooling to a temperature 100 ° C. lower than the bainite transformation start temperature Bs (Bs ⁇ 100 ° C.) or lower and a martensite transformation start temperature Ms or higher while ensuring a cooling rate, the average cooling rate of less than 20 ° C./second is 200 ° C.
- a press-formed product having a single characteristic can be formed into an optimum structure (structure mainly composed of bainitic ferrite) having a predetermined strength and high ductility.
- the reasons for defining the requirements in this molding method are as follows.
- the heating temperature of the steel sheet is lower than 900 ° C., sufficient austenite cannot be obtained during heating, and the martensite fraction becomes excessive in the final structure (structure of the molded product). Further, when the heating temperature of the steel sheet exceeds 1100 ° C., the grain size of austenite increases during heating, the martensite transformation start temperature Ms and the martensite end temperature Mf increase, and residual austenite cannot be secured during quenching, which is good. Formability is not achieved.
- the heating temperature is preferably 950 ° C. or higher and 1050 ° C. or lower.
- the heating time at this time is too long, the Ti-containing precipitates in the steel sheet are difficult to be refined, and even if the amount is small, the Ti-containing precipitates are formed and coarsened during the heating, thereby improving the weldability. Since it becomes small, the one where a heating time is short is preferable.
- the preferable range of the heating time is 3600 seconds or less, more preferably 20 seconds or less.
- the average during and after forming It is necessary to appropriately control the cooling rate and the cooling end temperature. From this point of view, the average cooling rate during molding needs to be 20 ° C./second or more, and the cooling end temperature needs to be 100 ° C. lower than the bainite transformation start temperature Bs or less than the martensite transformation start temperature Ms.
- the average cooling rate during molding is preferably 30 ° C./second or more (more preferably 40 ° C./second or more). By setting the cooling end temperature to 100 ° C.
- the austenite existing during heating is transformed into bainite while preventing the formation of a structure such as ferrite or pearlite, and the amount of bainitic ferrite.
- a predetermined amount of retained austenite is secured by leaving fine austenite between the laths of bainitic ferrite.
- Rapid cooling is stopped when the temperature is below 100 ° C. below the bainite transformation start temperature Bs and above the martensite transformation start temperature Ms, and then cooled to 200 ° C. or less at an average cooling rate of less than 20 ° C./second. Do. By adding such a cooling step, the bainitic ferrite transformation is promoted. If the average cooling rate at this time is 20 ° C./second or more, martensite is formed and the strength is increased, but good elongation cannot be obtained. The average cooling rate at this time is preferably 15 ° C./second or less, more preferably 10 ° C./second or less. The reason for cooling to 200 ° C. or less by this cooling is that carbon is distributed from bainitic ferrite to untransformed austenite to increase the amount of residual austenite remaining at room temperature.
- the control of the average cooling rate during the press molding and after the molding is finished is (a) controlling the temperature of the molding die (cooling medium shown in FIG. 1), and (b) controlling the thermal conductivity of the die. This can be achieved by means such as
- the metal structure is bainitic ferrite: 60 to 97 area%, martensite: 37 area% or less, retained austenite: 3 to 20 area%, and remaining structure: 5 area%.
- the amount of carbon in the retained austenite becomes 0.50% or more, and the balance between high strength and elongation can be achieved as a high level and uniform characteristic in the molded product.
- the reasons for setting the ranges of the requirements (basic structure and carbon content in retained austenite) in such a hot press-formed product are as follows.
- the area fraction of bainitic ferrite needs to be 60 area% or more. However, when this fraction exceeds 97 area%, the fraction of retained austenite becomes insufficient and ductility (residual ductility) decreases.
- the preferable lower limit of the bainitic ferrite fraction is 65 area% or more (more preferably 70 area% or more), and the preferable upper limit is 95 area% or less (more preferably 90 area% or less).
- the area fraction of martensite needs to be 37 area% or less.
- a preferred lower limit of the martensite fraction is 5 area% or more (more preferably 10 area% or more), and a preferred upper limit is 30 area% or less (more preferably 25 area% or less).
- Residual austenite has the effect of increasing the work hardening rate (transformation-induced plasticity) and improving the ductility of the molded product by transforming into martensite during plastic deformation.
- the retained austenite fraction needs to be 3 area% or more.
- the higher the retained austenite fraction the better.
- the retained austenite that can be secured is limited, and the upper limit is about 20 area%.
- the preferable lower limit of retained austenite is 5 area% or more (more preferably 7 area% or more).
- ferrite, pearlite, and the like may be included as the remaining structure.
- these structures have a lower contribution to strength and ductility than other structures, and it is preferable that they do not basically contain (0 area). % Is acceptable). However, up to 5 area% is acceptable.
- the remaining structure is more preferably 4 area% or less, and still more preferably 3 area% or less.
- the average equivalent circle diameter of the equivalent circle diameter of 30 nm or less is 10 nm or less.
- the average equivalent circle diameter of the Ti-containing precipitate is preferably 8 nm or less, and more preferably 6 nm or less.
- the amount of Ti present as precipitates other than TiN is 0.5 times the remaining Ti after subtracting Ti forming TiN out of all Ti. (That is, less than 0.5 ⁇ [total Ti amount (%) ⁇ 3.4 [N]]).
- Ti that is solid-dissolved during welding is finely precipitated in HAZ, or the existing fine Ti-containing precipitates suppress the recovery of dislocation, etc., thereby preventing softening in HAZ, Good weldability.
- the amount of precipitated Ti-3.4 [N] is preferably 0.4 ⁇ [total Ti amount (%)-3.4 [N]] or less, more preferably 0.3 ⁇ [total Ti amount (% ) -3.4 [N]] or less.
- the press molding conditions heatating temperature and cooling rate
- properties such as strength and elongation of the molded product
- high ductility residual ductility
- a press-molded product can be obtained, it can be applied to parts that have been difficult to apply with conventional hot-pressed products (for example, energy absorbing members), which is extremely useful in expanding the range of application of hot-pressed products. It is.
- Steel materials (steel Nos. 1-31) having the chemical composition shown in Table 1 below are vacuum-melted to make slabs for experiment, then hot rolled into steel plates, and then cooled and wound up Simulated treatment was performed (plate thickness: 3.0 mm).
- the winding simulation processing method after cooling to the winding temperature, the sample was placed in a furnace heated to the winding temperature, held for 30 minutes, and then cooled in the furnace.
- the steel plate manufacturing conditions at this time are shown in Table 2 below.
- the Ac 3 transformation point, Ms point, and Bs point in Table 1 were obtained using the following formulas (2) to (4) (for example, “Leslie Steel Material Science” Maruzen, (1985 )reference). Further, the treatments (1) and (2) shown in the remarks column of Table 2 are obtained by performing the following treatments (rolling, cooling, and alloying).
- Treatment (1) After cold-rolling a hot-rolled steel sheet (sheet thickness: 1.6 mm), simulating continuous annealing with a heat treatment simulator, heating to 800 ° C., holding for 90 seconds, and average cooling at 20 ° C./second Cooled to 500 ° C. at a rate and held for 300 seconds.
- the obtained steel sheet (press forming steel sheet) was analyzed for Ti precipitation (precipitation Ti amount-3.4 [N], average circle equivalent diameter of Ti-containing precipitates) in the following manner.
- the results are shown in Table 3 below together with a calculated value of 0.5 ⁇ [total Ti amount-3.4 [N]].
- Precipitation Ti amount-3.4 [N] (Ti amount present as precipitates) was subjected to extraction residue analysis using a mesh with a mesh diameter of 0.1 ⁇ m (the precipitates aggregated during the extraction process). Thus, fine precipitates can be measured), and the precipitated Ti amount-3.4 [N] was determined. When the Ti-containing precipitate partially contains V or Nb, the content of these precipitates was also measured.
- the precipitation state of Ti was measured by the method described above.
- TS tensile strength
- EL elongation
- TS ⁇ EL strength-elongation balance
- Table 5 The observation results of the metal structure (fraction of each structure, Ti precipitation state, amount of precipitated Ti-3.4 [N]) are shown in Table 5 below.
- Table 6 shows the mechanical properties (tensile strength TS, elongation ELTS ⁇ EL, and hardness reduction amount ⁇ Hv) of the press-formed product.
- the value of the precipitated Ti amount-3.4 [N] in the press-formed product is slightly different from the value of the precipitated Ti amount-3.4 [N] in the press-formed steel sheet, but this is a measurement error.
- Steel No. Samples 3, 7, 11-14, 17, and 21 are comparative examples that do not satisfy any of the requirements defined in the present invention, and any of the characteristics is deteriorated. That is, Steel No. No. 3 uses a steel sheet with a low Si content, the retained austenite fraction in the press-formed product is not secured, only low elongation EL is obtained, and the strength-elongation balance (TS ⁇ EL) is also obtained. It has deteriorated. Steel No. No. 7 has a low finish rolling temperature during the production of the steel sheet and does not satisfy the relationship of the formula (1), and the Ti-containing precipitate is coarsened and the strength-elongation balance (TS ⁇ EL) is lowered. At the same time, the softening prevention characteristics are deteriorated.
- Steel No. No. 17 uses a steel sheet with an excessive C content, and the strength of the molded product is increased and only a low elongation EL is obtained.
- Steel No. No. 21 uses a steel plate with an excessive Ti content, and the press-formed product does not satisfy the relationship of the formula (1), and the Ti-containing precipitate in the molded product becomes coarse. The softening prevention property is deteriorated.
- the average equivalent circle diameter of those having an equivalent circle diameter of 30 nm or less is 6 nm or less, and the amount of precipitated Ti in the steel
- the steel sheet for hot pressing that satisfies the predetermined relationship between the total Ti content and the total Ti amount is heated to a temperature of 900 ° C. or higher and 1100 ° C. or lower, and then press forming is started. After cooling to a temperature not higher than 100 ° C.
Abstract
Description
C :0.15~0.5%(質量%の意味。以下、化学成分組成について同じ。)、
Si:0.2~3%、
Mn:0.5~3%、
P :0.05%以下(0%を含まない)、
S :0.05%以下(0%を含まない)、
Al:0.01~1%、
B :0.0002~0.01%、
Ti:3.4[N]+0.01%以上、3.4[N]+0.1%以下(但し、[N]はNの含有量(質量%)を示す)、および
N:0.001~0.01%、
を夫々含有し、残部が鉄および不可避不純物からなり、
鋼板中に含まれるTi含有析出物のうち、円相当直径が30nm以下のものの平均円相当直径が6nm以下であると共に、鋼中の析出Ti量と全Ti量とが下記(1)式の関係を満足する熱間プレス用鋼板を、900℃以上、1100℃以下の温度に加熱した後、プレス成形を開始し、成形中および成形終了後は金型内で20℃/秒以上の平均冷却速度を確保しつつベイナイト変態開始温度Bsより100℃低い温度以下、マルテンサイト変態開始温度Ms以上の温度まで冷却した後、20℃/秒未満の平均冷却速度で200℃以下まで冷却することを特徴とする。尚、「円相当直径」とは、Ti含有析出物(例えばTiC)の大きさ(面積)に着目したときに、同一面積の円に換算したときの直径(「平均円相当直径」はその平均値)である。
析出Ti量(質量%)-3.4[N]<0.5×[全Ti量(質量%)-3.4[N]] …(1)
((1)式中、[N]は鋼中のNの含有量(質量%)を示す)
(a)V,NbおよびZrよりなる群から選択される1種以上を合計で0.1%以下(0%を含まない)
(b)Cu,Ni,CrおよびMoよりなる群から選択される1種以上を合計で1%以下(0%を含まない)
(c)Mg,CaおよびREMよりなる群から選択される1種以上を合計で0.01%以下(0%を含まない)
Cは、ベイナイト変態開始温度Bsを低下させることによって、冷却過程で生成されるベイニティックフェライトを微細にし、且つベイニティックフェライト中の転位密度を上昇させることによって強度を向上させる上で重要な元素である。また、ベイニティックフェライトのラス間に形成される微細な残留オーステナイト量を増加させることで、高強度と伸びのバランスを高レベルで確保できる。C含有量が0.15%未満では、ベイナイト変態開始温度Bsが上昇し、ベイニティックフェライトが粗大・低転位密度となり、熱間プレス成形品の強度が確保できない。またC含有量が過剰になって0.5%を超えると、強度が高くなり過ぎ、良好な延性が得られない。C含有量の好ましい下限は0.18%以上(より好ましくは0.20%以上)であり、好ましい上限は0.45%以下(より好ましくは0.40%以下)である。
Siは、金型焼入れの冷却中にベイニティックフェライトのラス間に形成された残留オーステナイトが分解してセメンタイトが形成されるのを抑制することで、残留オーステナイトを形成させる効果を発揮する。こうした効果を発揮させるためには、Si含有量は0.2%以上とする必要がある。またSi含有量が過剰になって3%を超えると、フェライトが形成されやすくなり、加熱時にオーステナイト単相化が難しくなり、熱間プレス用鋼板においてベイニティックフェライトおよび残留オーステナイト以外の組織分率が5面積%を超えることになる。Si含有量の好ましい下限は0.5%以上(より好ましくは1.0%以上)であり、好ましい上限は2.5%以下(より好ましくは2.0%以下)である。
Mnは、焼入れ性を高め、金型焼入れの冷却中にフェライトやパーライト等の軟質な組織の形成を抑制するのに有効な元素である。また、ベイナイト変態開始温度Bsを低下させることによって、冷却過程で生成されるベイニティックフェライトを微細にし、且つベイニティックフェライト中の転位密度を上昇させることで、強度を向上させる上で重要な元素である。更に、オーステナイトを安定化させる元素であり、残留オーステナイト量の増加に寄与する元素である。これらの効果を発揮させるためには、Mnは0.5%以上含有させる必要がある。特性だけを考慮した場合は、Mn含有量は多い方が好ましいが、合金添加のコストが上昇することから、3%以下とした。Mn含有量の好ましい下限は0.7%以上(より好ましくは1.0%以上)であり、好ましい上限は2.5%以下(より好ましくは2.0%以下)である。
Pは、鋼中に不可避的に含まれる元素であるが延性を劣化させるので、Pは極力低減することが好ましい。しかしながら、極端な低減は製鋼コストの増大を招き、0%とすることは製造上困難であるので、0.05%以下(0%を含まない)とした。P含有量の好ましい上限は0.045%以下(より好ましくは0.040%以下)である。
SもPと同様に鋼中に不可避的に含まれる元素であり、延性を劣化させるので、Sは極力低減することが好ましい。しかしながら、極端な低減は製鋼コストの増大を招き、0%とすることは製造上困難であるので、0.05%以下(0%を含まない)とした。S含有量の好ましい上限は0.045%以下(より好ましくは0.040%以下)である。
Alは、脱酸元素として有用であると共に、鋼中に存在する固溶NをAlNとして固定し、延性の向上に有用である。こうした効果を有効に発揮させるためには、Al含有量は0.01%以上とする必要がある。しかしながら、Al含有量が過剰になって1%を超えると、Al2O3が過剰に生成し、延性を劣化させる。尚、Al含有量の好ましい下限は0.02%以上(より好ましくは0.03%以上)であり、好ましい上限は0.8%以下(より好ましくは0.6%以下)である。
Bは、フェライト変態やパーライト変態を抑制する作用を有するため、(Ac1変態点~Ac3変態点)の二相域温度に加熱後の冷却中に、フェライト、パーライトの形成を防止し、残留オーステナイトの確保に寄与する元素である。こうした効果を発揮させるためには、Bは0.0002%以上含有させる必要があるが、0.01%を超えて過剰に含有させても効果が飽和する。B含有量の好ましい下限は0.0003%以上(より好ましくは0.0005%以上)であり、好ましい上限は0.008%以下(より好ましくは0.005%以下)である。
Tiは、Nを固定し、Bを固溶状態で維持させることで焼入れ性の改善効果を発現させる。こうした効果を発揮させるためには、TiとNの化学量論比(Nの含有量の3.4倍)よりも0.01%以上多く含有させることが重要である。またNに対して過剰に添加されたTiをホットスタンプ成形品内に固溶状態で存在させ、且つ析出した化合物を微細に分散させておくことによって、ホットスタンプ成形品を溶接した際に固溶したTiがTiCとして形成されることによる析出強化や、TiCによる転位の移動防止効果による転位密度の増加遅延等の効果により、HAZにおける強度低下が抑制できる。但し、Ti含有量が過剰になって3.4[N]+0.1%よりも多くなると、形成されるTi含有析出物(例えばTiN)が粗大化され、鋼板の延性が低下する。Ti含有量の好ましい下限は3.4[N]+0.02%以上(より好ましくは3.4[N]+0.05%以上)であり、好ましい上限は3.4[N]+0.09%以下(より好ましくは3.4[N]+0.08%以下)である。
Nは、BをBNとして固定することで、焼入れ性改善効果を低下させるため、できるだけ低減することが好ましいが、実プロセスの中で低減するには限界があるため、0.001%を下限とした。また、N含有量が過剰になると、形成されるTi含有析出物(例えばTiN)が粗大化され、この析出物が破壊の起点として働き、鋼板の延性を低下させるため、上限を0.01%とした。N含有量の好ましい上限は0.008%以下(より好ましくは0.006%以下)である。
(a)V,NbおよびZrよりなる群から選択される1種以上を合計で0.1%以下(0%を含まない)
(b)Cu,Ni,CrおよびMoよりなる群から選択される1種以上を合計で1%以下(0%を含まない)
(c)Mg,CaおよびREMよりなる群から選択される1種以上を合計で0.01%以下(0%を含まない)
V,NbおよびZrは、微細な炭化物を形成し、ピン止め効果により組織を微細にする効果がある。こうした効果を発揮させるためには、合計で0.001%以上含有させることが好ましい。しかしながら、これらの元素の含有量が過剰になると、粗大な炭化物が形成され、破壊の起点になることで逆に延性を劣化させる。こうしたことから、これらの元素は合計で0.1%以下とすることが好ましい。これらの元素の含有量のより好ましい下限は合計で0.005%以上(更に好ましくは0.008%以上)であり、より好ましい上限は合計で0.08%以下(更に好ましくは0.06%以下)である。
Cu,Ni,CrおよびMoは、フェライト変態およびパーライト変態を抑制するため、加熱後の冷却中に、フェライト、パーライトの形成を防止し、残留オーステナイトの確保に有効に作用する。こうした効果を発揮させるためには、合計で0.01%以上含有させることが好ましい。特性だけを考慮すると含有量は多いほうが好ましいが、合金添加のコストが上昇することから、合計で1%以下とすることが好ましい。また、オーステナイトの強度を大幅に高める作用を有するため、熱間圧延の負荷が大きくなり、鋼板の製造が困難になるため、製造性の観点からも1%以下とすることが好ましい。これらの元素含有量のより好ましい下限は合計で0.05%以上(更に好ましくは0.06%以上)であり、より好ましい上限は合計で0.5%以下(更に好ましくは0.3%以下)である。
これらの元素は、介在物を微細化するため、延性向上に有効に作用する。こうした効果を発揮させるためには、合計で0.0001%以上含有させることが好ましい。特性だけを考慮すると含有量は多いほうが好ましいが、効果が飽和することから、合計で0.01%以下とすることが好ましい。これらの元素含有量のより好ましい下限は合計で0.0002%以上(更に好ましくは0.0005%以上)であり、より好ましい上限は合計で0.005%以下(更に好ましくは0.003%以下)である。
Ms点(℃)=550-361×[C]-39×[Mn]-10×[Cu]-17×[Ni]-20×[Cr]-5×[Mo]+30×[Al] …(3)
Bs点(℃)=830-270×[C]-90×[Mn]-37×[Ni]-70×[Cr]-83×[Mo] …(4)
但し、[C],[Si],[Mn],[P],[Al],[Ti],[V],[Cr],[Mo],[Cu]および[Ni]は、夫々C,Si,Mn,P,Al,Ti,V,Cr,Mo,CuおよびNiの含有量(質量%)を示す。また、上記(2)式~(4)式の各項に示された元素が含まれない場合は、その項がないものとして計算する。
処理(2):熱間圧延鋼板を冷間圧延後(板厚:1.6mm)、熱処理シミュレータで連続溶融亜鉛めっきラインを模擬するため860℃に加熱した後、30℃/秒の平均冷却速度で400℃まで冷却し、保持後、めっき浴への浸漬-合金化処理を模擬するために更に500℃×10秒保持後、20℃/秒の平均冷却速度で室温まで冷却した。
抽出レプリカサンプルを作製し、透過型電子顕微鏡(TEM)にてTi含有析出物の透過型電子顕微鏡像(倍率:10万倍)を撮影した。このとき、エネルギー分散型X線分光器(EDX)により析出物の組成分析をすることによって、Ti含有析出物(円相当直径で30nm以下のもの)を特定した。少なくとも100個以上のTi含有析出物の面積を画像解析により測定し、そこから円相当直径を求め、その平均値を析出物サイズ(Ti含有析出物の平均円相当直径)とした。また、析出Ti量-3.4[N](析出物として存在するTi量)は、メッシュ径:0.1μmのメッシュを用いて抽出残渣分析を行い(抽出処理の際に、析出物が凝集して微細な析出物も測定できる)、析出Ti量-3.4[N]を求めた。尚、Ti含有析出物がVやNbを一部含有している場合は、これら析出物の含有量についても測定した。
JIS5号試験片を用いて引張試験を行い、引張強度(TS)、伸び(EL)を測定した。このとき、引張試験の歪速度:10mm/秒とした。本発明では、引張強度(TS)が1180MPa以上で伸び(EL)が12.0%以上を満足し、強度-伸びバランス(TS×EL)が16000(MPa・%)以上のときに合格と評価した。
(1)成形品中の、ベイニティックフェライト、マルテンサイト、フェライトの組織については、鋼板をナイタールで腐食し、SEM(倍率:1000倍または2000倍)観察により、ベイニティックフェライト、マルテンサイト、フェライトを区別し、夫々の分率(面積率)を求めた。
(2)成形品中の残留オーステナイト分率は、鋼板の1/4の厚さまで研削した後、化学研磨してからX線回折法によって測定した(例えば、ISJJ Int.Vol.33.(1933),No.7,P.776)。
スポット溶接に準ずる熱履歴として、熱処理シミュレータで平均加熱速度50℃/秒で700℃に加熱後、平均冷却速度50℃/秒で冷却し、元の硬さ(ビッカース硬さ)に対する硬さ低下量(ΔHv)を測定した。硬さ低下量(ΔHv)が50Hv以下のときに、HAZでの軟化防止特性が良好であると判断した。
2 ダイ
3 ブランクホルダー
4 鋼板(ブランク)
Claims (3)
- C :0.15~0.5%(質量%の意味。以下、化学成分組成について同じ。)、
Si:0.2~3%、
Mn:0.5~3%、
P :0.05%以下(0%を含まない)、
S :0.05%以下(0%を含まない)、
Al:0.01~1%、
B :0.0002~0.01%、
Ti:3.4[N]+0.01%以上、3.4[N]+0.1%以下[但し、[N]はNの含有量(質量%)を示す]、および
N:0.001~0.01%、
を夫々含有し、残部が鉄および不可避不純物からなり、
鋼板中に含まれるTi含有析出物のうち、円相当直径が30nm以下のものの平均円相当直径が6nm以下であると共に、鋼中の析出Ti量と全Ti量とが下記(1)式の関係を満足する熱間プレス用鋼板を、900℃以上、1100℃以下の温度に加熱した後、プレス成形を開始し、成形中および成形終了後は金型内で20℃/秒以上の平均冷却速度を確保しつつベイナイト変態開始温度Bsより100℃低い温度以下、マルテンサイト変態開始温度Ms以上の温度まで冷却した後、20℃/秒未満の平均冷却速度で200℃以下まで冷却することを特徴とするプレス成形品の製造方法。
析出Ti量(質量%)-3.4[N]<0.5×[全Ti量(質量%)-3.4[N]] …(1)
((1)式中、[N]は鋼中のNの含有量(質量%)を示す) - 前記熱間プレス用鋼板は、更に他の元素として、下記(a)~(c)の少なくとも1つを含有するものである請求項1に記載のプレス成形品の製造方法。
(a)V,NbおよびZrよりなる群から選択される1種以上を合計で0.1%以下(0%を含まない)
(b)Cu,Ni,CrおよびMoよりなる群から選択される1種以上を合計で1%以下(0%を含まない)
(c)Mg,CaおよびREMよりなる群から選択される1種以上を合計で0.01%以下(0%を含まない) - 請求項1または2に記載の化学成分を有する鋼板のプレス成形品であって、前記プレス成形品の金属組織が、ベイニティックフェライト:60~97面積%、マルテンサイト:37面積%以下、残留オーステナイト:3~20面積%、残部組織:5面積%以下であり、プレス成形品中に含まれるTi含有析出物のうち、円相当直径が30nm以下のものの平均円相当直径が10nm以下であると共に、前記(1)式の関係を満足することを特徴とするプレス成形品。
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WO2019003447A1 (ja) * | 2017-06-30 | 2019-01-03 | Jfeスチール株式会社 | 熱間プレス部材およびその製造方法ならびに熱間プレス用冷延鋼板 |
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