JP5894470B2 - Steel sheet for hot pressing, press-formed product, and method for producing press-formed product - Google Patents

Description

  The present invention is used when manufacturing a structural part of an automobile, and is a hot-press steel sheet suitable for hot press forming, a press-formed product obtained from such a hot-press steel plate, and a press-formed product. With regard to the manufacturing method, especially when pre-heated steel plate (blank) is formed into a predetermined shape, heat useful for applying to a hot press forming method in which a heat treatment is performed simultaneously with shape formation to obtain a predetermined strength. The present invention relates to an inter-press steel sheet, a press-formed product, and a useful method for producing such a press-formed product.

  As one of the measures to improve the fuel efficiency of automobiles that originated from global environmental problems, the weight of the vehicle body has been reduced, and it is necessary to increase the strength of steel plates used in automobiles as much as possible. On the other hand, when the strength of the steel plate is increased, the shape accuracy at the time of press forming is lowered.

  From this, 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). At the same time, 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. Such 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 view showing a mold configuration for carrying out hot press molding as described above, in which 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, and CL is a punch / die clearance. Of these components, 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. These members are configured to be cooled.

When hot press forming (for example, hot deep drawing) using such a mold, 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, the steel plate 4 in a high temperature state is sandwiched between the die 2 and the blank holder 3, and the steel plate 4 is pushed into the hole of the die 2 by the punch 1 to correspond to the outer shape of the punch 1 while reducing the outer diameter of the steel plate 4. Mold into shape. Further, by cooling the punch and die in parallel with the forming, heat is removed from the steel plate 4 to the mold (punch 1 and die 2) and the bottom dead center of the forming (when the punch tip is located at the deepest part) : The material is quenched by further holding and cooling in the state shown in FIG. By carrying out such a molding method, it is possible to obtain a 1500 MPa class molded product with good dimensional accuracy and to reduce the molding load compared to the case of molding parts of the same strength class in the cold. The capacity of the can be small.

  As 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). However, it is difficult to apply to parts that require deformation, such as an energy absorbing member, because the elongation (ductility) is low.

  As hot-press steel plates that exhibit good elongation, for example, 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 1470 MPa or more and the elongation EL is about 10.2% at the maximum, and further improvement is required.

  On the other hand, compared to hot stamped molded products that have been studied so far, even for molded products with a lower strength class, such as tensile strength TS of 980 MPa class and 1180 MPa class, cold pressing has a problem in molding accuracy and its improvement As a countermeasure, there is a need for a low-strength hot press. At that time, it is necessary to greatly improve the energy absorption characteristics of the molded product.

  In particular, in recent years, development of a technique for providing a strength difference in one component has been promoted. As such a technique, the part where deformation should be prevented is high strength (high strength side: impact resistant part side), and the part requiring energy absorption is low strength and high ductility (low strength side: energy absorbing part side). Technology has been proposed. For example, in medium-sized and larger passenger cars, in consideration of compatibility at the time of side collision or rear collision (function to protect the other party when a small car collides), there is impact resistance in the parts of the B pillar and rear side member. In some cases, it has both functional parts of energy absorption. In order to produce such a part, (a) a method of joining a steel plate that is low in strength even when heated and mold-quenched to a normal hot-press steel plate (tailored weld blank: TWB), (b) There have been proposed a method of giving a difference in the strength of each steel plate region with a difference in the cooling rate in the mold, and a method of giving a strength difference by giving a difference in the heating temperature for each region of the steel plate. .

  In these techniques, a tensile strength of 1500 MPa is achieved on the high strength side (impact resistant site side), but the maximum tensile strength is 700 MPa and the elongation EL is about 17% on the low strength side (energy absorption site side). In order to further improve the energy absorption characteristics, it is required to realize higher strength and higher ductility.

  In addition, in order to realize complex shapes with hot stamping, it is required to apply in the direction of hot stamping after forming to some extent by press forming at room temperature, cutting out steel plates for hot stamping press forming, etc. Therefore, it is also required that the strength of the hot stamping steel plate does not become too high.

JP 2010-6292 A JP 2010-6293 A JP 2010-6294 A JP 2010-6295 A

  The present invention has been made in view of the above circumstances, and its purpose is to facilitate molding and processing before hot pressing, and when a uniform characteristic is required in a molded product, it has high strength. If a region corresponding to an impact resistant region and an energy absorbing region is required in a single molded product, it is possible to obtain a press molded product that can achieve a high balance between elongation and elongation. , A steel sheet for hot pressing that is useful in obtaining a hot press-formed product that can achieve a high balance between high strength and elongation, a press-formed product that exhibits the above characteristics, and such a press-formed product. It is to provide a useful method for manufacturing.

The steel sheet for hot pressing of the present invention that was able to achieve the above object,
C: 0.15 to 0.5% (meaning mass%, hereinafter the same for the chemical composition)
Si: 0.2-3%,
Mn: 0.5-3%,
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 3 nm or more, and the relationship between the precipitated Ti amount in the steel and the total Ti amount is expressed by the following formula (1): And the metal structure is characterized in that the ferrite fraction is 30 area% or more. 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).
Precipitated Ti amount (mass%)-3.4 [N] ≧ 0.5 × [Total Ti amount (mass%)-3.4 [N]] (1)
(In the formula (1), [N] indicates the content (% by mass) of N in the steel)

  In the hot-press forming steel sheet of the present invention, if necessary, as another element, (a) one or more selected from the group consisting of V, Nb and Zr is 0.1% or less in total ( (B) 0% is not included), (b) one or more selected from the group consisting of Cu, Ni, Cr and Mo is 1% or less in total (not including 0%), (c) from Mg, Ca and REM It is also useful to contain one or more selected from the group consisting of 0.01% or less (excluding 0%), etc., and depending on the type of element contained, the properties of the press-formed product Is further improved.

The method for producing a press-formed product of the present invention that has achieved the above-mentioned object includes the above-described hot-press steel sheet of the present invention having an Ac ₁ transformation point of + 20 ° C. or higher and an Ac ₃ transformation point of −20 ° C. or lower. After heating to the above temperature, press forming of the steel sheet is started, and during the forming and after the forming is completed, a temperature lower by 100 ° C. than the bainite transformation start temperature Bs while ensuring an average cooling rate of 20 ° C./second or more in the mold. It is characterized by cooling to the following.

  In the press-formed product of the present invention, the metal structure in the pressed steel is retained austenite: 3 to 20 area%, ferrite: 30 to 80 area%, bainitic ferrite: less than 30 area% (excluding 0 area%) Martensite: 30 area% or less (excluding 0 area%), and among the Ti-containing precipitates contained in the pressed steel, the average equivalent circle diameter of the equivalent circle diameter of 30 nm or less is 3 nm or more, The amount of carbon in the retained austenite is 0.50% or more, and the balance between high strength and elongation can be achieved as a uniform characteristic at a high level in the press-formed product.

On the other hand, another method for producing a press-formed product of the present invention that has achieved the above object is to use a steel sheet for hot pressing as described above, and divide the heating area of the steel sheet into at least two areas. The region is heated to a temperature not lower than the Ac ₃ transformation point and not higher than 950 ° C., and the other region is heated to a temperature not lower than the Ac ₁ transformation point + 20 ° C. and not higher than the Ac ₃ transformation point −20 ° C. Press molding is started for the region, and during molding and after completion of molding, the mold is cooled to a temperature below the martensite transformation start temperature Ms while ensuring an average cooling rate of 20 ° C./second or more in the mold. It is characterized by that.

  Another press-formed product of the present invention is a steel plate press-formed product having the chemical composition as described above, and the pressed steel has a metal structure of retained austenite: 3 to 20 area%, martensite: 80. The first region is not less than area% and the metal structure is retained austenite: 3 to 20 area%, ferrite: 30 to 80 area%, bainitic ferrite: less than 30 area% (excluding 0 area%), Martensite: having a second region that is 30 area% or less (not including 0 area%), and that the amount of carbon in the retained austenite in this second region is 0.50% or more Features. In such a press-molded product, a balance between high strength and elongation can be achieved at a high level according to each region, and there are regions corresponding to an impact resistant site and an energy absorption site in a single molded product.

  According to the present invention, the chemical component composition is strictly defined, the size of Ti-containing precipitates is controlled, the precipitation rate is controlled for Ti that does not form TiN, and the ferrite ratio for the metal structure. Since the steel plate adjusted is used, the strength-elongation balance of the press-formed product can be set to a high level by hot pressing this under predetermined conditions. Further, when hot pressing is performed in a plurality of regions under different conditions, an impact resistant part and an energy absorbing part can be formed in a single molded product, and a high strength and elongation balance can be achieved at a high level in each part.

It is a schematic explanatory drawing which shows the metal mold | die structure for implementing hot press molding.

  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 hot-press steel sheet that can provide a simple press-formed product, studies were made from various angles.

  As a result, the chemical composition of the steel sheet for hot pressing is strictly defined, the size of the Ti-containing precipitates and the amount of precipitated Ti are controlled, and the metal structure is made appropriate. Thus, the present inventors completed the present invention by finding that a press-formed product having a predetermined amount of retained austenite after molding and having a high inherent ductility (residual ductility) can be obtained by hot press molding.

  In the steel sheet for hot pressing according to the present invention, it is necessary to strictly define the chemical composition, but the reasons for limiting the range of each chemical composition are as follows.

[C: 0.15-0.5%]
C is a region corresponding to an impact resistant site and an energy absorbing site in a single molded product in order to achieve a high balance between high strength and elongation when uniform characteristics are required in the molded product. Is an important element for securing retained austenite particularly in the low strength and high ductility regions. Moreover, at the time of heating by hot press molding, C concentrates to austenite, so that residual austenite can be formed after quenching. Furthermore, it contributes to an increase in the amount of martensite and raises the strength. In order to exert these effects, the C content needs to be 0.15% or more.

  However, if the C content becomes excessive and exceeds 0.5%, the two-phase region heating region becomes narrow, and the balance between high strength and elongation is high when uniform properties are required in the molded product. When not achieved, or when a region corresponding to an impact resistant part and an energy absorbing part is required in a single molded article, the target metal structure (ferrite, bainitic ferrite, especially in low strength and high ductility parts) , It is difficult to adjust to a structure in which a predetermined amount of martensite is secured. The preferable lower limit of the C content is 0.17% or more (more preferably 0.20% or more), and the more preferable upper limit is 0.45% or less (more preferably 0.40% or less).

[Si: 0.2-3%]
Si exhibits the effect of forming retained austenite by suppressing martensite from tempering to form cementite and decomposition of untransformed austenite during cooling of mold quenching. In order to exert such effects, the Si content needs to be 0.2% or more. On the other hand, when the Si content is excessive and exceeds 3%, the solid solution strengthening amount becomes excessively large, and the ductility is significantly lowered. 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: 0.5 to 3%]
Mn is an element effective in enhancing hardenability and suppressing the formation of structures (ferrite, pearlite, bainite, etc.) other than martensite and retained austenite during cooling of mold hardening. Further, it is an element that stabilizes austenite and contributes to an increase in the amount of retained austenite. In order to exhibit such an effect, it is necessary to contain 0.5% or more of Mn. 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 P is preferably reduced as much as possible. However, 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%)]
Similarly to P, S is an element inevitably contained in steel, and deteriorates ductility. Therefore, S is preferably reduced as much as possible. However, 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 to 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. In order to exhibit such an effect effectively, the Al content needs to be 0.01% or more. However, when the Al content becomes excessive and exceeds 1%, Al ₂ O ₃ is excessively generated and ductility is deteriorated. In addition, 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: 0.0002 to 0.01%]
B has an action of suppressing ferrite transformation, pearlite transformation, and bainite transformation on the high-strength part side, so that during cooling after heating to the two-phase region temperature (Ac ₁ transformation point to Ac ₃ transformation point), It is an element that prevents the formation of pearlite and bainite and contributes to securing retained 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]. However, when the Ti content becomes excessive and exceeds 3.4 [N] + 0.1%, the Ti-containing precipitates (eg, TiN) that are formed are finely dispersed and during cooling after heating in the austenite region The growth in the longitudinal direction of martensite formed in a lath shape is inhibited, and a lath structure having a small aspect ratio is obtained. Conversely, if the precipitates are made sufficiently large, a martensitic structure with a large aspect ratio is obtained, and stable retained austenite can be obtained even if the amount of C in the retained austenite is equal, and the characteristics (elongation) are improved. The more 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 more preferable upper limit is 3.4 [N] +0. 09% or less (more preferably 3.4 [N] + 0.08% or less).

[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. However, since there is a limit to reducing it in the actual process, 0.001% is set as the lower limit. did. Further, when the N content is excessive, the ductility deteriorates due to strain aging, or precipitates as BN, and the effect of improving the hardenability by the solid solution B is lowered, so the upper limit was made 0.01%. 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 of the present invention are as described above, and the balance is iron and inevitable impurities other than P and S (for example, O, H, etc.). Further, in the steel sheet for hot pressing of the present invention, if necessary, (a) at least one selected from the group consisting of V, Nb and Zr is 0.1% or less in total (not including 0%), (B) 1 or more selected from the group consisting of Cu, Ni, Cr and Mo in total 1% or less (not including 0%), (c) 1 selected from the group consisting of Mg, Ca and REM It is also useful to contain a total of 0.01% or less (not including 0%) of the seeds or more, and the characteristics of the steel sheet for hot pressing are further improved depending on the type of element contained. . The preferable range when these elements are contained and the reason for limiting the range are as follows.

[A total of one or more selected from the group consisting of V, Nb and Zr is 0.1% or less (excluding 0%)]
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).

[One or more selected from the group consisting of Cu, Ni, Cr and Mo: 1% or less in total (excluding 0%)]
Cu, Ni, Cr, and Mo suppress ferrite transformation, pearlite transformation, and bainite transformation, and thus prevent formation of ferrite, pearlite, and bainite during cooling after heating, and effectively act to secure retained austenite. In order to exhibit such an effect, it is preferable to contain 0.01% or more in total. Considering only the characteristics, it is preferable that the content is large, but since the cost of alloy addition increases, the total content is preferably 1% or less. Moreover, since it has the effect | action which raises the intensity | strength of austenite significantly, since the load of hot rolling becomes large and manufacture of a steel plate becomes difficult, it is preferable to set it as 1% or less also from a viewpoint of productivity. 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. ).

[A total of at least one selected from the group consisting of Mg, Ca and REM is 0.01% or less (excluding 0%)]
Since these elements refine the inclusions, they effectively work to improve ductility. In order to exhibit these effects, it is preferable to contain 0.0001% or more in total. Considering only the characteristics, it is preferable that 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. ).

  In the steel sheet for hot pressing according to the present invention, among the (A) 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 3 nm or more, and (B) the amount of precipitated Ti (Mass%) − 3.4 [N] ≧ 0.5 × [total Ti amount (mass%) − 3.4 [N]] [Relationship of formula (1)] is satisfied, (C ) It is also an important requirement that the metal structure has a ferrite fraction of 30 area% or more.

  The presence state of Ti-containing precipitates in the molded product and the condition of the formula (1) itself have little effect on the strength and elongation of the steel sheet, but by affecting the structure formed when hot pressing the steel sheet, This is intended to improve the elongation of the final characteristic molded product. Therefore, it is necessary to already control at the stage before forming (steel plate for hot pressing). Excess Ti with respect to N in the steel plate before forming is finely dispersed in the steel plate before hot pressing, or most of it exists in a solid solution state. Will do. Then, in the martensitic transformation that occurs during the rapid cooling in the mold after the heating, the growth in the longitudinal direction of the martensite lath is inhibited, the growth in the width direction is promoted, and the aspect ratio becomes small. As a result, carbon discharge from the martensite lath to the surrounding residual austenite is delayed, the amount of carbon in the residual austenite is reduced, and the stability of the residual austenite is lowered, so that the effect of improving the elongation cannot be sufficiently obtained.

  From this point of view, it is necessary to finely disperse Ti-containing precipitates. To that end, 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 3 nm or more. [Requirement (A) above]. Note that the equivalent circle diameter of the target Ti-containing precipitate is defined as 30 nm or less, except for TiN, which is coarsely formed in the melting stage and does not affect the structure change or properties thereafter. This is because it is necessary to control the Ti-containing precipitates. The size (average equivalent circle diameter) of the Ti-containing precipitate is preferably 5 nm or more, and more preferably 10 nm or more. In addition, the Ti-containing precipitates targeted by the present invention are intended to include precipitates containing Ti such as TiVC and TiNbC in addition to TiC.

  Moreover, in the steel sheet for hot pressing, it is necessary to make most of Ti except for being used for precipitation fixing of Ti out of Ti in a precipitation state. For this purpose, the amount of Ti existing as a precipitate other than TiN (ie, the amount of precipitated Ti—3.4 [N]) is 0.5 times or more of the remaining Ti minus Ti forming TiN (ie, the Ti content) 0.5 × [total Ti amount (mass%) − 3.4 [N] or more] [Requirement (B) above] Precipitated Ti amount (mass%) − 3.4 [N] Is preferably 0.6 × [total Ti amount (% by mass) −3.4 [N]] or more, and more preferably 0.7 × [total Ti amount (% by mass) −3.4 [N]. That is all.

  In addition, it is necessary to always process the steel before hot stamping, and press forming may be performed. In such a case, it is necessary to secure a predetermined amount of ferrite which is a soft structure. From this point of view, the ferrite fraction in the hot-press steel sheet needs to be 30 area% or more [requirement for (C) above]. The ferrite fraction is preferably 50 area% or more, more preferably 70 area% or more.

  In the steel sheet for hot pressing, the remainder of the metal structure is not particularly limited, and examples thereof include at least one of pearlite, bainite, martensite, and retained austenite.

  In order to produce the steel plate (hot press steel plate) of the present invention as described above, 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.). Above) 1300 ° C. or lower (preferably 1250 ° C. or lower), the finish rolling temperature is 750 ° C. or higher (preferably 780 ° C. or higher), and 850 ° C. or lower (preferably 830 ° C. or lower). After staying in a temperature range of 650 ° C. for 10 seconds or longer, winding may be performed at 450 ° C. or higher (preferably 480 ° C. or higher) and 650 ° C. or lower (preferably 630 ° C. or lower).

  The above-described method makes the Ti-containing precipitates such as TiC formed during ferrite transformation coarse by sufficient ferrite transformation at a high temperature. Further, by increasing the winding temperature, the formed Ti-containing precipitate such as TiC is grown and coarsened.

  The steel sheet for hot pressing having the chemical composition, metal structure and Ti precipitation state as described above may be used for the production of hot pressing as it is, and the reduction ratio after pickling: 60% or less (preferably 40% The following may be applied to the production of a hot press after cold rolling. Moreover, you may heat-process in the temperature range (for example, 1000 degrees C or less) in which the Ti containing precipitates do not completely dissolve in the hot-pressed steel sheet or its cold rolled material. Moreover, 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.

Using the steel sheet for hot press as described above, after heating to a temperature of Ac ₁ transformation point + 20 ° C. or higher and Ac ₃ transformation point −20 ° C. or lower, press molding is started, and during molding and after completion of molding In this case, by cooling to a temperature lower than 100 ° C. below the bainite transformation start temperature Bs while securing an average cooling rate of 20 ° C./second or more, a press-formed product having a single characteristic (hereinafter referred to as a single region molded product) Therefore, it can be formed into an optimum structure as a material having low strength and high ductility. The reasons for defining the requirements in this molding method are as follows.

In a steel sheet containing a predetermined amount of ferrite, it is necessary to control the heating temperature within a predetermined range in order to partially transform the ferrite into austenite while partially retaining the ferrite. When the heating temperature of the steel sheet is less than the Ac ₁ transformation point + 20 ° C., a sufficient amount of austenite cannot be obtained during heating, and a predetermined amount of retained austenite cannot be secured in the final structure (structure of the molded product). On the other hand, if the heating temperature of the steel sheet exceeds the Ac ₃ transformation point of −20 ° C., the amount of transformation to austenite increases excessively during heating, and a predetermined amount of ferrite cannot be secured in the final structure (structure of the molded product).

  In order to make the austenite formed in the heating process into a desired structure while preventing the formation of a structure such as ferrite or pearlite, the average cooling rate and the cooling end temperature during and after molding are appropriately controlled. There is a need. From such a viewpoint, the average cooling rate during molding must be 20 ° C./second or more, and the cooling end temperature must be 100 ° C. or lower than the bainite transformation start temperature Bs. The average cooling rate during molding is preferably 30 ° C./second or more (more preferably 40 ° C./second or more). By making the cooling end temperature 100 ° C. or less lower than the bainite transformation start temperature Bs, the austenite existing during heating is transformed into bainite or martensite while preventing the formation of a structure such as ferrite or pearlite. In addition, a predetermined amount of retained austenite is secured by allowing fine austenite to remain between bainite and martensite lath while securing martensite.

  When the cooling end temperature is higher than the temperature lower by 100 ° C. than the bainite transformation start temperature Bs or the average cooling rate is less than 20 ° C./second, a structure such as ferrite or pearlite is formed, and a predetermined amount of retained austenite is secured. This is not possible, and the elongation (ductility) of the molded product is deteriorated. The cooling end temperature is not particularly limited as long as it is 100 ° C. or lower than Bs, and may be, for example, martensitic transformation start temperature Ms or lower.

  Although control of the average cooling rate is basically unnecessary when the temperature is lower than the bainite transformation start temperature Bs by 100 ° C. or less, for example, at an average cooling rate of 1 ° C./second or more and 100 ° C./second or less to room temperature. It may be cooled. Control of the average cooling rate during molding and after molding is completed by controlling (a) the temperature of the molding die (cooling medium shown in FIG. 1) and (b) controlling the thermal conductivity of the die. It can be achieved by such means.

  In a press-molded product (single-region molded product) manufactured by hot pressing as described above, the metal structure is retained austenite: 3 to 20 area%, ferrite: 30 to 80 area%, bainitic ferrite: 30 Less than area% (excluding 0 area%), martensite: 30 area% or less (not including 0 area%), and the amount of carbon in the retained austenite is 0.50% or more. A balance between high strength and elongation can be achieved as a uniform characteristic at a high level. The reason for setting the range of each requirement (basic structure) in such a hot press-formed product is as follows.

  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. In order to exert such an effect, the retained austenite fraction needs to be 3 area% or more. For ductility, the higher the retained austenite fraction, the better. In the composition used for the steel sheet for automobiles, 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).

  The ductility (elongation) of a press-formed product can be increased by making the main structure fine and highly ductile ferrite. From this point of view, the ferrite fraction is 30% by area or more. However, if this fraction exceeds 80 area%, the strength of the molded product cannot be ensured. A preferable lower limit of the ferrite fraction is 35 area% or more (more preferably 40 area% or more), and a more preferable upper limit is 75 area% or less (more preferably 70 area% or less).

  Although bainitic ferrite is an effective structure for improving the strength of a molded product, it is a structure that is slightly poor in ductility. From this point of view, the fraction of bainitic ferrule is less than 30% by area. A preferable upper limit of the fraction of bainitic ferrule is 25 area% or less (more preferably 20 area% or less).

  Martensite (an as-quenched martensite) is an effective structure for improving the strength of a molded product, but is a structure having poor ductility, and therefore, when present in a large amount, it deteriorates elongation. From such a viewpoint, the martensite fraction is 30% by area or less. A preferred upper limit of the martensite fraction is 25 area% or less (more preferably 20 area% or less).

  Other than the above structure, it is not particularly limited, and pearlite may be included as the remaining structure. However, these structures have a lower contribution to strength and ductility than other structures, and it is preferable not to basically contain them. (It may be 0 area%).

  The amount of carbon in retained austenite affects the timing at which retained austenite undergoes work-induced transformation to martensite during deformation in tensile tests, etc., and transformation-induced plasticity (TRIP) is caused by processing-induced transformation in the higher strain region as the carbon content increases. Increase the effect. In the process of the present invention, carbon is expelled from the formed martensite lath to the surrounding austenite during cooling. At that time, when Ti carbide or carbonitride dispersed in the steel is coarsely dispersed, the growth in the longitudinal direction of the martensite lath proceeds without being hindered, so the aspect ratio is narrow and long. Big martensite lath. As a result, carbon is easily discharged from the martensite lath in the width direction, the amount of carbon in the retained austenite is increased, and ductility is improved. From such a viewpoint, in the press-formed product of the present invention, the carbon content in the retained austenite in the steel is defined as 0.60% or more. The carbon content in the retained austenite can be concentrated to about 0.70%, but the limit is about 1.0%.

  If the steel sheet for hot pressing of the present invention is used, properties such as strength and elongation of the molded product can be controlled by appropriately adjusting press molding conditions (heating temperature and cooling rate), and high ductility ( (Residual ductility) press-molded products can be obtained, so it can be applied to parts that have been difficult to apply with conventional press-molded products (for example, energy absorbing members), and is extremely useful for expanding the range of application of press-molded products. It is. In addition to the above-mentioned single-region molded product, when manufacturing a press-molded product by press-molding a steel sheet using a press-molding die, the heating temperature and the conditions of each region at the time of molding are appropriately controlled. If the structure of each region is adjusted, a press-molded product that exhibits a strength-ductility balance corresponding to each region (hereinafter sometimes referred to as a multi-region molded product) can be obtained.

In producing a multi-region molded product as described above using the hot-press steel plate of the present invention, the heating region of the steel plate is divided into at least two regions, one of which is hereinafter referred to as the first region. Is heated to a temperature not lower than Ac ₃ transformation point and not higher than 950 ° C., and another region (hereinafter referred to as second region) is temperature not lower than Ac ₁ transformation point + 20 ° C. and lower than Ac ₃ transformation point −20 ° C. Then, press molding is started for both the first and second regions, and during molding and after completion of molding, both the first and second regions have a temperature of 20 ° C./second or more in the mold. What is necessary is just to cool to the temperature below the martensitic transformation start temperature Ms, ensuring an average cooling rate.

  In the above method, the heating region of the steel sheet is divided into at least two regions (high-strength side region and low-strength side region), and the manufacturing conditions are controlled according to each region, whereby the strength-ductility balance corresponding to each region. A molded product that exhibits the above can be obtained. Of the two regions, the second region corresponds to the low-strength side region, and the manufacturing conditions, structure, and characteristics in this region are basically the same as those of the single-region molded product described above. Hereinafter, manufacturing conditions for forming the other first region (corresponding to the high-strength side region) will be described. In carrying out this manufacturing method, it is necessary to form regions with different heating temperatures with a single steel plate, but by using an existing heating furnace (for example, a far-infrared furnace, electric furnace + shield), It is possible to control the temperature boundary portion while keeping it at 50 mm or less.

(Production conditions for the first region and the high-strength side region)
In order to appropriately adjust the structure of the hot press-formed product, it is necessary to control the heating temperature within a predetermined range. By appropriately controlling this heating temperature, in the subsequent cooling process, while maintaining a predetermined amount of retained austenite, it is transformed into a structure mainly composed of martensite, and within the region of the final hot press-formed product. It can be built into the desired tissue. If the steel sheet heating temperature in this region is less than the Ac ₃ transformation point, a sufficient amount of austenite cannot be obtained during heating, and a predetermined amount of retained austenite cannot be secured in the final structure (structure of the molded product). If the heating temperature of the steel sheet exceeds 950 ° C., the grain size of austenite increases during heating, the martensite transformation start temperature (Ms point) and the martensite transformation end temperature (Mf point) rise, and residual austenite during quenching. Cannot be secured, and good moldability is not achieved. The heating temperature of the steel sheet is preferably Ac ₃ transformation point + 50 ° C. or higher and 930 ° C. or lower.

  In order to make the austenite formed in the heating process into a desired structure while preventing the formation of a structure such as ferrite or pearlite, the average cooling rate and the cooling end temperature during and after molding are appropriately controlled. There is a need. 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 lower than the martensite transformation start temperature (Ms point). The average cooling rate during molding is preferably 30 ° C./second or more (more preferably 40 ° C./second or more). By ensuring that the cooling end temperature is lower than the martensite transformation start temperature (Ms point), the formation of ferrite or pearlite is prevented, and austenite existing during heating is transformed into martensite, thereby securing martensite. To do. The cooling end temperature is specifically 400 ° C. or lower, preferably 300 ° C. or lower.

  In the press-formed product obtained by such a method, the metal structure is different between the first region and the second region. In the first region, the metal structures are retained austenite: 3 to 20 area% (the effect of retained austenite is the same as described above), and martensite: 80 area% or more. In the second region, the same metal structure as that of the single region molded product and the carbon content in the retained austenite are satisfied.

  By making the main structure of the first region a high-strength martensite containing a predetermined amount of retained austenite, the ductility and high strength of the specific region in the press-formed product can be ensured. From such a viewpoint, the area fraction of martensite needs to be 80 area% or more. The fraction of martensite is preferably 85 area% or more (more preferably 90 area% or more). The structure in the first region may partially include ferrite, pearlite, bainite, and the like.

  Hereinafter, the effects of the present invention will be described more specifically by way of examples. However, the following examples are not intended to limit the present invention, and any design changes in accordance with the gist of the preceding and following descriptions are technical aspects of the present invention. It is included in the range.

[Example 1]
Steel materials (steel Nos. 1 to 31) having the chemical composition shown in Tables 1 and 2 below are vacuum-melted to obtain experimental slabs, then hot-rolled into steel plates, and then cooled and wound. A treatment simulating removal was performed (plate thickness: 1.6 mm or 3.0 mm). In 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 sheet manufacturing conditions at this time are shown in Tables 3 and 4 below. In Tables 1 and 2, the Ac ₁ transformation point, Ac ₃ transformation point, Ms point, and Bs point were determined using the following formulas (2) to (5) (for example, “Leslie Steel) Materials Science, Maruzen, (1985)). Further, the treatments (1) and (2) shown in the remarks column of Table 3 are obtained by performing the following treatments (rolling, cooling, and alloying).

Ac ₁ transformation point (° C.) = 723 + 29.1 × [Si] −10.7 × [Mn] + 16.9 × [Cr] −16.9 [Ni] (2)
Ac ₃ transformation point (° C.) = 910−203 × [C] ^1/2 + 44.7 × [Si] −30 × [Mn] + 700 × [P] + 400 × [Al] + 400 × [Ti] + 104 × [V ] -11 × [Cr] + 31.5 × [Mo] −20 × [Cu] −15.2 × [Ni] (3)
Ms point (° C.) = 550−361 × [C] −39 × [Mn] −10 × [Cu] −17 × [Ni] −20 × [Cr] −5 × [Mo] + 30 × [Al] ( 4)
Bs point (° C.) = 830−270 × [C] −90 × [Mn] −37 × [Ni] −70 × [Cr] −83 × [Mo] (5)
However, [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu] and [Ni] are C, The contents (mass%) of Si, Mn, P, Al, Ti, V, Cr, Mo, Cu and Ni are shown. Moreover, when the element shown by each term of said Formula (2)-Formula (5) is not included, it calculates as the thing without the term.

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.
Treatment (2): After cold rolling a hot-rolled steel sheet (sheet thickness: 1.6 mm), after heating to 860 ° C. to simulate a continuous hot-dip galvanizing line with a heat treatment simulator, an average cooling rate of 30 ° C./second In order to simulate the immersion-alloying treatment in the plating bath, the sample was further held at 500 ° C. for 10 seconds, and then cooled to room temperature at an average cooling rate of 20 ° C./second.

About the obtained steel plate, the precipitation state of Ti and the observation of metal structure (fraction of each structure) were performed as follows. Moreover, the tensile strength (TS) of each steel plate was measured by the method mentioned later.
The results are shown in Table 5 below together with a calculated value of 0.5 × [total Ti amount (mass%) − 3.4 [N]] [displayed as 0.5 × [total Ti amount−3.4 [N]]. , 6.

[Analysis of Ti precipitation on steel sheet]
An extraction replica sample was prepared, and a transmission electron microscope image (magnification: 100,000 times) of the Ti-containing precipitate was taken with a transmission electron microscope (TEM). At this time, Ti-containing precipitates (those with an equivalent circle diameter of 30 nm or less) were identified by analyzing the composition of the precipitates using an energy dispersive X-ray spectrometer (EDX). The area of at least 100 Ti-containing precipitates was measured by image analysis, the equivalent circle diameter was determined therefrom, and the average value was defined as the precipitate size (average equivalent circle diameter of the Ti-containing precipitate). Further, the amount of precipitated Ti (mass%)-3.4 [N] (the amount of Ti present as a precipitate) is subjected to extraction residue analysis using a mesh having a mesh diameter of 0.1 μm (in the extraction process, Precipitates aggregate and fine precipitates can be measured), and the amount of precipitated Ti (mass%)-3.4 [N] (in Tables 5 and 6, indicated as precipitated Ti amount-3.4 [N]) is obtained. It was. When the Ti-containing precipitate partially contained V or Nb, the content thereof was also measured.

[Observation of metal structure (fraction of each structure)]
(1) About the structure of martensite and bainite in the steel sheet, the steel sheet is corroded with nital, and martensite and bainite are distinguished by SEM (magnification: 1000 times or 2000 times) observation. )
(2) The retained austenite fraction in the steel sheet was measured by X-ray diffraction after being ground to ¼ thickness of the steel sheet and then chemically polished (for example, ISJJ Int. Vol. 33. (1933), No. 7, P.776). At this time, the amount of carbon in the retained austenite was also measured.

Above for each steel sheet (1.6mm ^t × 150 mm × 200 mm) (the processing (1), the other than (2) adjust the thickness to 1.6mm by hot rolling), a predetermined temperature in a heating furnace Then, press molding and cooling treatment were performed with a hat-shaped mold (FIG. 1) to obtain a molded product. Table 7 below shows the press molding conditions (heating temperature, average cooling rate, rapid cooling end temperature during press molding).

  For the obtained molded product, the tensile strength (TS), elongation (total elongation EL), observation of metal structure (fraction of each structure) was measured by the following method, and the amount of carbon in the retained austenite was described above. Measured with

[Measurement of tensile strength (TS) and elongation (total elongation EL)]
A tensile test was performed using a JIS No. 5 test piece, and tensile strength (TS) and elongation (EL) were measured. At this time, the strain rate of the tensile test was set to 10 mm / second. In the present invention, when the tensile strength (TS) was 980 MPa or more and the elongation (EL) was 18% or more, and the strength-elongation balance (TS × EL) was 20000 (MPa ·%) or more, it was evaluated as passing.

[Observation of metal structure (fraction of each structure)]
(1) Regarding the structure of ferrite and bainitic ferrite in the steel sheet, the steel sheet is corroded with nital, and the ferrite and bainitic ferrite are distinguished by SEM (magnification: 1000 times or 2000 times) observation. The rate (area ratio) was determined.
(2) The retained austenite fraction in the steel sheet was measured by X-ray diffraction after being ground to ¼ thickness of the steel sheet and then chemically polished (for example, ISJJ Int. Vol. 33. (1933), No. 7, P.776).
(3) The martensite (as-quenched martensite) fraction was determined by X-ray diffraction from the area ratio of the martensite and retained austenite as it was quenched by repeller corrosion of the steel sheet. The martensite fraction was calculated by subtracting the residual austenite fraction.

  The observation results of the metal structure (fraction of each structure, amount of carbon in retained austenite) are shown in Tables 8 and 9 below. The mechanical properties (tensile strength TS, elongation EL and TS × EL) of the molded product are shown in Table 10 below.

  From these results, it can be considered as follows. Steel No. 1, 2, 4, 5, 8 to 10, 14 to 16, 18 to 20, 22 to 31 are examples that satisfy the requirements defined in the present invention, and parts having a good strength-ductility balance. You can see that it is obtained.

  On the other hand, Steel No. Those of 3, 6, 7, 11-13, 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, and the retained austenite fraction in the molded product is not ensured and elongation is not achieved, and the strength-elongation balance is deteriorated. Steel No. In the case of No. 6, the cooling time from 700 ° C. to 650 ° C. at the time of manufacturing the steel plate is insufficient, the ferrite transformation does not proceed sufficiently, the ferrite fraction in the steel plate cannot be secured, and the strength is increased and press forming is performed. Pre-molding and processing are expected to be difficult.

  Steel No. No. 7 has a high finish rolling temperature during the production of the steel sheet, and does not satisfy the relationship of the formula (1) of the steel sheet for hot pressing, and the strength-elongation balance is deteriorated. Steel No. In No. 11, the heating temperature at the time of press molding is high, a large amount of martensite is generated, the strength is increased, and only a low elongation EL is obtained (the strength-elongation balance (TS × EL) is also deteriorated). ).

  Steel No. In No. 12, the cooling rate during press molding is slow, a large amount of phalite is generated at the stage of the molded product, the strength is lowered, and the strength-elongation balance (TS × EL) is degraded. Steel No. In the case of No. 13, the rapid cooling end temperature at the time of press molding is high, a large amount of pearlite is generated at the stage of the molded product, and the retained austenite fraction cannot be secured, and the amount of carbon in the retained austenite is insufficient. Strength and elongation are reduced, and strength-elongation balance (TS × EL) is deteriorated.

  Steel No. No. 17 uses a steel sheet with an excessive C content, the ferrite fraction of the steel sheet is low, the ferrite fraction in the molded product cannot be secured, the strength increases, and the elongation EL is low. Only obtained (strength-elongation balance (TS × EL) is also deteriorated). Steel No. No. 21 uses a steel sheet with an excessive Ti content, and the strength-elongation balance (TS × EL) is deteriorated.

[Example 2]
Steel materials (steel Nos. 32-36) having the chemical composition shown in Table 11 below were vacuum-melted to obtain slabs for experiment, then hot-rolled, and then cooled and wound up (sheet thickness) : 3.0 mm). The steel plate production conditions at this time are shown in Table 12 below.

  The obtained steel sheet was analyzed in the same manner as in Example 1 for the analysis of the precipitation state of the Ti-containing precipitate, the observation of the metal structure (fraction of each structure), and the tensile strength. The results are shown in Table 13 below.

Above for each steel sheet ^{(3.0mm t × 150mm × 200mm)} , was heated to a predetermined temperature in a heating furnace, performing a press-forming and cooling process in a mold of hat-shaped (FIG. 1), and a molded article . At this time, the steel plate is put in an infrared furnace, and the portion (steel plate portion corresponding to the first region) to be strengthened is directly irradiated with infrared rays so that it can be heated at a high temperature. The steel plate portion corresponding to the second region was covered with a cover so as to block a part of infrared rays so that it could be heated at a low temperature, thereby giving a heating temperature difference. Therefore, the molded product has regions having different strengths within a single part. Table 14 shows the press molding conditions (heating temperature, average cooling rate, rapid cooling end temperature in each region during press molding).

  For the obtained molded product, the tensile strength (TS), elongation (total elongation EL) in each region, observation of metal structure (fraction of each structure), and measurement of carbon content in retained austenite were the same as in Example 1. I asked for it.

  Table 15 below shows the observation results of metal structure (fraction of each structure) and the amount of carbon in retained austenite. Table 16 below shows the mechanical properties (tensile strength TS, elongation EL, and TS × EL) of the press-formed product. The tensile strength (TS) on the high strength side is 1470 MPa or higher, the elongation (EL) satisfies 8% or higher, and the strength-elongation balance (TS × EL) is 14000 (MPa ·%) or higher. (Evaluation criteria on the low strength side are the same as in Example 1).

  From this result, it can be considered as follows. Steel No. 32 to 35 are examples that satisfy the requirements defined in the present invention, and it can be seen that parts having a good balance between strength and ductility in each region are obtained. On the other hand, Steel No. In the case of 36, the heating temperature at the time of press molding is low, and the strength on the high strength side is reduced (the strength difference from the low strength side is less than 300 MPa).

1 Punch 2 Die 3 Blank holder 4 Steel plate (blank)

Claims (8)

C: 0.15 to 0.5% (meaning mass%, hereinafter the same for the chemical composition)
Si: 0.2-3%,
Mn: 0.5-3%,
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 the equivalent circle diameter of 30 nm or less is 10 nm or more, and the precipitated Ti amount and the total Ti amount in the steel are expressed by the following formula (1): A steel sheet for hot pressing characterized by satisfying the relationship and having a metal structure with a ferrite fraction of 30% by area or more.
Precipitated Ti amount (mass%)-3.4 [N] ≧ 0.5 × [Total Ti amount (mass%)-3.4 [N]] (1)
(In the formula (1), [N] indicates the content (% by mass) of N in the steel)   2. The hot press according to claim 1, further comprising, as another element, one or more selected from the group consisting of V, Nb and Zr in a total of 0.1% or less (excluding 0%). Steel plate.   The heat according to claim 1 or 2, further comprising at least 1% (not including 0%) selected from the group consisting of Cu, Ni, Cr and Mo as other elements. Steel sheet for hot pressing.   Furthermore, as another element, 1 or more types selected from the group which consists of Mg, Ca, and REM are contained in total 0.01% or less (0% is not included) in any one of Claims 1-3 The steel plate for hot press as described. After heating the steel sheet for hot press according to any one of claims 1 to 4 to a temperature of Ac ₁ transformation point + 20 ° C or higher and Ac ₃ transformation point -20 ° C or lower, press forming of the steel sheet is started, By cooling to a temperature below 100 ° C. lower than the bainite transformation start temperature Bs while securing an average cooling rate of 20 ° C./second or more in the mold during and after the molding ,
When the total microstructure of retained austenite, ferrite, bainitic ferrite and martensite is 100% by area, the retained austenite: 3 to 20% by area, ferrite: 30 to 80% by area, bainitic ferrite: 30 areas % (Not including 0 area%), martensite: 30 area% or less (not including 0 area%) . A pulp-less molded product having a chemical composition according to claim 1, the metal structure in the press molded article, residual austenite, ferrite, the sum of bainitic ferrite and martensite When 100 area%, residual austenite: 3 to 20 area%, ferrite: 30 to 80 area%, bainitic ferrite: less than 30 area% (excluding 0 area%), martensite: 30 area% or less ( 0% by area), and among the Ti-containing precipitates contained in the press- molded product , the average equivalent circle diameter of the equivalent circle diameter of 30 nm or less is 3 nm or more, and the amount of carbon in the retained austenite Is a press-molded product characterized in that it is 0.50% or more. Using the steel sheet for hot pressing according to any one of claims 1 to 4, the heating region of the steel plate is divided into at least two regions, and one region is heated to a temperature not lower than the Ac ₃ transformation point and not higher than 950 ° C. At the same time, the other region is heated to a temperature of Ac ₁ transformation point + 20 ° C. or more and Ac ₃ transformation point −20 ° C. or less, and then press molding is started for both regions. By cooling to a temperature below the martensitic transformation start temperature Ms while ensuring an average cooling rate of 20 ° C./second or more in the mold in any region ,
A first region in which the metal structure is retained austenite: 3 to 10 area%, martensite: 90 area% or more;
When the total microstructure of retained austenite, ferrite, bainitic ferrite and martensite is 100% by area, the retained austenite: 3 to 20% by area, ferrite: 30 to 80% by area, bainitic ferrite: 30 areas % (Not including 0 area%) and martensite: 30 area% or less (not including 0 area%) . A pulp-less molded product having a chemical composition according to claim 1, wherein the press-molded article, the metal structure, retained austenite: 3-10 area%, martensite: 90 area A first region that is greater than or equal to
When the total microstructure of retained austenite, ferrite, bainitic ferrite and martensite is 100% by area, the retained austenite: 3 to 20% by area, ferrite: 30 to 80% by area, bainitic ferrite: 30 areas % (Not including 0 area%), martensite: having a second area of 30 area% or less (not including 0 area%), and carbon in the retained austenite in the second area A press-molded product characterized in that the amount is 0.50% or more.

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