WO2015037060A1 - Hot-pressing steel plate, press-molded article, and method for manufacturing press-molded article - Google Patents

Abstract

　Provided is a hot-pressing steel plate useful for obtaining a press-molded article having excellent anti-softening characteristics in heat-affected zones (HAZ) while attaining a press-molded article that can achieve a high level of balance between high strength and stretchability if uniform characteristics within the molded article are required, and a high level of balance between high strength and stretchability in respective regions if regions corresponding to shock-resistant portions and energy-absorbing portions are required within one molded article; molding and processing prior to hot-pressing being facilitated by the hot-pressing steel plate having a prescribed chemical composition, having the equivalent circular diameter of Ti-containing deposits included in the steel plate be 30 nm or less with the average equivalent circular diameter of the Ti-containing deposits being 6 nm or less, having the deposited Ti amount and the total Ti amount within the steel satisfy a prescribed relation, and having a metal structure with a proportion of ferrite of 30% by area or greater.

Description

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

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. It relates to a manufacturing method. In particular, when a preheated steel plate (blank) is formed into a predetermined shape, the hot press steel plate is useful for application to a hot press forming method in which a heat treatment is performed simultaneously with shape formation to obtain a predetermined strength. And 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 car body has been reduced, and it is necessary to increase the strength of steel sheets 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 used for manufacturing a part (press-molded product) that performs quenching heat treatment (quenching) using the temperature difference between the two to ensure 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 diagram showing a mold configuration for carrying out hot press molding as described above. In FIG. 1, 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. Each shows. 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 parts are configured to be cooled.

In 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, 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. 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 (steel plate) is quenched by 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 sheets exhibiting good elongation, for example, the techniques of 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.

Especially in recent years, the development of technology that gives a difference in strength within one part has been underway. 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, impact resistance is included in the parts of the B pillar and rear side member in consideration of compatibility (a function to protect the other party when a small car collides) in a side collision or a rear collision. 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.

By the way, although it is necessary to join automobile parts mainly by spot welding, it is known that the strength reduction in the welding heat affected zone (HAZ) is remarkable and the strength of the welded joint is reduced (softened) ( For example, Non-Patent Document 1).

JP 2010-65292 A JP 2010-65293 A JP 2010-65294 A JP 2010-65295 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. Steel sheet for hot pressing that can achieve a high balance between high strength and elongation at a high level, and that is useful in obtaining a press-formed product with good softening prevention characteristics in HAZ, and a press-formed product that exhibits the above characteristics And providing a useful method for producing such a press-formed product.

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 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: 0.0010 ~ 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) 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, it is also useful to contain 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.
(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

The method for producing a press-formed product of the present invention that has achieved the above-mentioned object includes the steel sheet for hot pressing of the present invention as described above 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 of the press-formed product is as follows: retained austenite: 3 to 20 area%, ferrite: 30 to 80 area%, bainitic ferrite: less than 30 area% (excluding 0 area%) Martensite: 31 area% or less (excluding 0 area%), and among the Ti-containing precipitates contained in the press-formed product, the average equivalent circle diameter of the equivalent circle diameter of 30 nm or less is 10 nm or less. The amount of precipitated Ti in the steel and the total amount of Ti satisfy the relationship of the following formula (1), and the balance between high strength and elongation can be achieved as a uniform characteristic at a high level in the molded product.
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)

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. region of Ac ₃ transformation point or more, while heating to a temperature of 950 ° C. or less, the other one region Ac ₁ transformation point + 20 ° C. or higher, then heated to Ac ₃ transformation point -20 ° C. or less of the temperature, both 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 press-formed product has a metal structure of retained austenite: 3 to 20 area%, martensite: The first region is 80 area% or more, 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 31 area% or less (not including 0 area%), and among the Ti-containing precipitates contained in the steel of this second region, the equivalent circle diameter is The average equivalent circle diameter of those having a thickness of 30 nm or less is 10 nm or less, and the amount of precipitated Ti in the steel and the total amount of Ti satisfy the relationship of the following formula (1). 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 impact-resistant sites and energy-absorbing sites in a single molded product. The softening prevention property of the HAZ when spot welding is performed in the second region is good.
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)

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 a steel sheet with adjusted thickness is used, the strength-elongation balance of the press-formed product can be raised to a high level by hot pressing this under predetermined conditions. In addition, when hot pressing is performed under different conditions in a plurality of regions, 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 each part at a high level. The anti-softening property of is improved.

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 component composition of the steel sheet for hot pressing is strictly defined, the size of Ti-containing precipitates and the amount of precipitated Ti are controlled, the metal structure is made appropriate, and the steel sheet is subjected to predetermined conditions. It has been found that by performing hot press molding, a predetermined amount of retained austenite can be secured after press molding, and a press-molded product having increased inherent ductility (residual ductility) can be obtained, and the present invention has been completed.

In the steel sheet for hot pressing of 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 corresponds 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 properties are required in a press molded product. It is an important element for securing retained austenite when a region is required, particularly in a low strength / high ductility region. 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. Further, if the Si content is excessive and exceeds 3%, ferrite transformation is promoted during cooling after hot rolling, so that TiC in the ferrite formed at that time is easily formed coarsely. Therefore, the HAZ softening preventing property cannot be obtained. 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: 0.5-3%)
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. When only the characteristics are taken into consideration, it is preferable that the Mn content is high, but the upper limit is made 3% or less because the cost of alloy addition increases. 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 make it 0%, so the upper limit was made 0.05% or less (excluding 0%). 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 make it 0%, so the upper limit was made 0.05% or less (excluding 0%). 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. 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. The preferable lower limit of the Al content is 0.02% or more (more preferably 0.03% or more), and the 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 portion side, so that during the 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). 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. However, when 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 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 an element inevitably mixed in and is preferably reduced as much as possible. However, since there is a limit to reducing it in the actual process, 0.001% was set as the lower limit. Further, when 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 more 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 (for example, O, H, etc.) other than P, S, and N. In addition, it is useful that the steel sheet for hot pressing according to the present invention further contains at least one of the following (a) to (c) if necessary. Depending on the type of element contained as necessary, the properties of the steel sheet for hot pressing (ie, 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

(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. However, 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 one or more selected from the group consisting of Mg, Ca and REM (rare earth elements) 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 of the present invention, (A) of Ti-containing precipitates contained in the steel sheet, the average equivalent circle diameter of the equivalent circle diameter of 30 nm or less is 6 nm or less, and (B) the amount of precipitated Ti (Mass%) − 3.4 [N] <0.5 × [total Ti amount (mass%) − 3.4 [N]] (satisfaction of the 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 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). By allowing excessive Ti to exist in a solid solution state or a fine state in the steel sheet before forming, 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. As a result, 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.

From such a viewpoint, it is necessary to finely disperse the Ti-containing precipitates. For that purpose, 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. (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 less, and more preferably 3 nm or less. In addition, the Ti-containing precipitates that are the subject of the present invention include TiC and TiN as well as precipitates containing Ti such as TiVC, TiNbC, TiVCN, and TiNbCN.

As will be described later, 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. In order to secure the properties as a molded product, 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 μm or less is 6 nm or less at the stage, and most of Ti is present in a solid solution state.

Further, in a steel sheet for hot pressing, it is necessary to make most of Ti other than Ti used for precipitation fixing of Ti in a solid solution state or a fine state. For this purpose, the amount of Ti present as precipitates other than TiN (ie, the amount of precipitated Ti (mass%)-3.4 [N]) is the remaining amount of 0.1% after subtracting the Ti forming TiN out of the total Ti. The amount needs to be less than 5 times (that is, 0.5 × [total Ti amount (mass%) − 3.4 [N]]) (requirement (B) above). The amount of precipitated Ti (mass%)-3.4 [N] is preferably 0.4 × [total Ti amount (mass%)-3.4 [N]] or less, more preferably 0.3 × [ The total Ti amount (% by mass) is -3.4 [N] or less.

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 (C) above). The ferrite fraction is preferably 50 area% or more, more preferably 70 area% or more.

In addition, in the steel sheet for hot pressing, the balance 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 above chemical composition 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), and the finish rolling temperature is 850 ° C. or higher (preferably 900 ° C. or higher), or 1050 ° C. or lower (preferably 1000 ° C. or lower). (Preferably 625 ° C. or less) is cooled (rapidly cooled) at an average cooling rate of 20 ° C./second or more (preferably 30 ° C./second or more), and 620 ° C. to 580 ° C. is 10 ° C./second or less (preferably 5 ° C. After cooling at an average cooling rate of 10 ° C./second or higher, then 350 ° C. or higher (preferably 380 ° C. or higher), 450 ° C. or lower (preferably 4 0 ℃ should be as winding below).

In the above method, (1) rolling is terminated in a temperature range where dislocations introduced into austenite by hot rolling remain, and (2) Ti-containing precipitates such as TiC on the dislocations by quenching immediately after that. (3) Further, two-stage cooling and winding are performed to control the ferrite transformation while securing the amount of Ti-containing precipitates.

The steel sheet for hot pressing having the chemical composition, the metal structure and the Ti precipitation state as described above may be used for the production of the hot press 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. In addition, the steel sheet for hot pressing of the present invention may have a microstructure when the hot-rolled material is heat-treated in a continuous annealing furnace or a continuous hot dip galvanizing line. In short, as long as the required characteristics such as the metal structure and the Ti precipitation state are satisfied, it is included in the steel sheet for hot pressing of the present invention.

Using the steel sheet for hot pressing as described above, after heating to a temperature not lower than Ac ₁ transformation point + 20 ° C. (Ac ₁ + 20 ° C.) and not higher than Ac ₃ transformation point −20 ° C. (Ac ₃ -20 ° C.), press forming is performed. By cooling to a temperature (Bs-100 ° C.) 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 molding. A press-molded product having a single characteristic (hereinafter sometimes referred to as a single-region molded product) can be formed into an optimum structure as a low-strength and high-ductility product. 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). If the heating temperature of the steel sheet exceeds the Ac ₃ transformation point of −20 ° C., the amount of transformation to austenite increases too much 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: 31 area% or less (excluding 0 area%), and a high level and uniform balance between high strength and elongation in the molded product It can be achieved. 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 preferred lower limit of the ferrite fraction is 35 area% or more (more preferably 40 area% or more), and a preferred upper limit is 75 area% or less (more preferably 70 area% or less).

Bainitic ferrite is an effective structure for improving the strength of a molded product, but it is a structure that is slightly poor in ductility. From such a viewpoint, the fraction of bainitic ferrite is less than 30 area%. A preferable upper limit of the fraction of bainitic ferrite 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 31 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%).

In the above-mentioned press-formed product (single-region product), among the Ti-containing precipitates contained in the press-formed product (that is, in the steel plate constituting the press-formed product), the equivalent of the average circle having an equivalent circle diameter of 30 nm or less The diameter is 10 nm or less. By satisfying these requirements, a press-formed product that can achieve a high balance between high strength and elongation can be obtained. The average equivalent circle diameter of the Ti-containing precipitate is preferably 8 nm or less, and more preferably 6 nm or less.

In the press-molded product (single-region molded product), the amount of Ti present as precipitates other than TiN (precipitated Ti amount-3.4 [N]) is the remainder after subtracting Ti that forms TiN out of all Ti. The Ti content is less than 0.5 times that of Ti (that is, less than 0.5 × [total Ti amount (%) − 3.4 [N]]). By satisfying these requirements, 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 deposited Ti amount-3.4 [N] is preferably 0.4 × [total Ti amount (mass%) − 3.4 [N]] or less, more preferably 0.3 × [total Ti amount ( % By mass) -3.4 [N]] or less.

By using the steel sheet for hot pressing according to the present invention, the properties such as strength and elongation of the press-formed product can be controlled by appropriately adjusting the press forming 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). Useful. 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. By adjusting the structure of each region, a press-formed 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 the other region (hereinafter referred to as second region) is not lower than Ac ₁ transformation point + 20 ° C. and temperature not higher 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 is obtained. A press-formed product exhibiting 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 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-molded product obtained by such a method, the metal structure and precipitates are 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 and Ti state (average equivalent circle diameter of Ti-containing precipitates, amount of precipitated Ti (mass%) -3.4 [N], etc.) as the single region molded product are satisfied. .

By making the main structure of the first region high martensite containing a predetermined amount of retained austenite, it is possible to ensure the ductility and high strength of the specific region in the press-formed product. 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 remaining 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. 1-16, 18-32) having the chemical composition shown in Tables 1 and 2 below are vacuum-melted to form experimental slabs, followed by hot rolling to obtain steel plates, and then The process which cooled and simulated winding was performed (plate | board 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. Further, when the element shown in each term of the above formulas (2) to (5) is not included, the calculation is made assuming that the term is not present.

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 The sample was cooled to 400 ° C., held, and then held at 500 ° C. for 10 seconds to simulate immersion-alloying treatment in a plating bath, and then cooled to room temperature at an average cooling rate of 20 ° C./second.

The obtained steel plate (press forming steel plate) was analyzed for the precipitation state of Ti and the observation of the metal structure (fraction of each structure) in the following manner. 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 state of 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) was 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 contains V or Nb, the content of these precipitates was also measured.

(Observation of metal structure (fraction of each structure))
(1) Regarding the structure of ferrite, bainitic ferrite, pearlite, and martensite in the steel plate, the steel plate is corroded with nital, and the ferrite, bainitic ferrite, pearlite are observed by SEM (magnification: 1000 times or 2000 times) observation. The martensite was distinguished and the respective fractions (area ratios) were 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).

Above for each steel sheet (1.6mm ^t × 150 mm × 200 mm) (the processing (1), the other than (2), adjusting the thickness t to 1.6mm by hot rolling), a predetermined in a heating furnace After heating to temperature, 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).

About the obtained press-formed product, while measuring the tensile strength (TS), elongation (total elongation EL), observation of metal structure (fraction of each structure), and hardness reduction after heat treatment by the following methods, The Ti precipitation state was analyzed by the method described above.

(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 16% or more, and the strength-elongation balance (TS × EL) was 16000 (MPa ·%) or more, it was evaluated as passing.

(Observation of metal structure (fraction of each structure))
(1) Regarding the structure of ferrite, bainitic ferrite and pearlite in the steel sheet, the steel sheet is corroded with nital, and ferrite, bainitic ferrite and pearlite are distinguished by SEM (magnification: 1000 times or 2000 times) observation. (Including the distinction between ferrite and acicular ferrite), each fraction (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.

(Hardness reduction after heat treatment)
As a heat history equivalent to spot welding, after heating to 700 ° C at an average heating rate of 50 ° C / sec with a heat treatment simulator, cooling is performed at an average cooling rate of 50 ° C / sec to reduce the hardness against the original hardness (Vickers hardness) (ΔHv) was measured. When the hardness reduction amount (ΔHv) was 50 Hv or less, it was judged that the anti-softening property in HAZ was good.

The observation results of the metal structure (Ti precipitation state, fraction of each structure, amount of precipitated Ti-3.4 [N]) are shown in Tables 8 and 9 below. Table 10 below shows the mechanical properties (tensile strength TS, elongation EL, TS × EL, and hardness reduction amount ΔHv) of the molded product. The value of the precipitated Ti amount-3.4 [N] in the 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.

From these results, it can be considered as follows. Steel No. Examples 1, 2, 4 to 6, 10, 11, 15, 16, 19 to 21, and 23 to 32 are examples that satisfy the requirements defined in the present invention, have a good strength-ductility balance, and are softened. It can be seen that parts having good prevention characteristics are obtained.

In contrast, Steel No. Those of 3, 7 to 9, 12 to 14, 18, and 22 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 for press forming with a low Si content, and the retained austenite fraction in the press-formed product is not ensured and elongation is not achieved, so that the strength-elongation balance deteriorates. ing. Steel No. No. 7 has a low finish rolling temperature at the time of manufacturing the steel sheet, the Ti-containing precipitates in the press forming steel sheet become coarse, and does not satisfy the relationship of formula (1) at any stage of the press forming steel sheet or the press formed product. The anti-softening property is deteriorated.

Steel No. No. 8 has a high cooling rate of 620 ° C. to 580 ° C. at the time of manufacturing the steel sheet, the ferrite transformation does not proceed sufficiently, the ferrite fraction in the press forming steel sheet cannot be secured, and the strength becomes high. It is expected that molding and processing before press molding will be difficult. Steel No. In No. 9, the coiling temperature at the time of manufacturing the steel sheet is high, and the Ti-containing precipitates in the steel sheet for press forming are coarsened. When press forming using such a steel sheet, the forming conditions are appropriate and the strength- Even if the ductility balance is good, the anti-softening property is deteriorated.

Steel No. In No. 12, the heating temperature during press molding is high, martensite is generated, ferrite is not generated, elongation is lowered, and strength-elongation balance (TS × EL) is deteriorated. Steel No. In No. 13, the cooling rate at the time of press molding was slow, the ferrite fraction increased at the stage of the press molded product, and the strength decreased.

Steel No. No. 14 has a high cooling end temperature during press molding, pearlite is generated, retained austenite cannot be secured, strength and elongation are lowered, and strength-elongation balance (TS × EL) is deteriorated. ing. Steel No. No. 18 uses a steel sheet for press forming with an excessive C content, the ferrite fraction of the steel sheet cannot be secured, the ferrite fraction in the press-formed product cannot be secured, and only a low elongation EL is obtained. The strength-elongation balance (TS × EL) is also deteriorated. Steel No. No. 22 uses a steel sheet for press forming with an excessive Ti content, and does not satisfy the relationship of the formula (1) at any stage of the steel sheet for press forming and the press-formed product. Ti-containing precipitates are coarsened, and the softening prevention properties are deteriorated.

[Example 2]
Steel materials (steel Nos. 33 to 37) having the chemical composition shown in Table 11 below were melted in vacuum to form experimental slabs, followed by hot rolling, followed by cooling and winding (sheet thickness) : 3.0 mm). The steel plate production conditions at this time are shown in Table 12 below.

For the obtained steel plate (press forming steel plate), analysis of the precipitation state of Ti-containing precipitates, observation of the metal structure (fraction of each structure), and measurement of tensile strength were carried out in the same manner as in Example 1. 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 sheet is placed in an infrared furnace, and the portion (the steel plate portion corresponding to the first region) to be strengthened is directly irradiated with infrared rays so that the portion can be heated at a high temperature. The steel plate portion corresponding to the region of (1) 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).

About the obtained press-formed product, tensile strength (TS), elongation (total elongation EL), observation of metal structure (fraction of each structure), and hardness reduction amount (ΔHv) in each region are the same as in Example 1. I asked for it.

Table 15 below shows the observation results of metal structures (fraction of each structure) and the analysis results of the precipitation state of Ti. In addition, the mechanical properties (tensile strength TS, elongation EL, TS × EL, and hardness reduction amount ΔHv) of the press-formed product are shown in Table 16 below. 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. Nos. 33 to 36 are examples that satisfy the requirements defined in the present invention, and it can be seen that press-formed products having a good strength-ductility balance in each region are obtained. On the other hand, Steel No. In No. 37, the heating temperature at the time of press molding is low, the martensite fraction on the high strength side is insufficient, and the strength on the high strength side is reduced (the strength difference from the low strength side is Less than 300 MPa).

In the present invention, among the Ti-containing precipitates having a predetermined chemical composition and 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 amount of precipitated Ti in the steel And the total Ti content satisfy a predetermined relationship, and the metal structure has a ferrite fraction of 30% by area or more, so that molding and processing can be easily performed before hot pressing, and in the molded product. When uniform characteristics are required, a press-molded product that can achieve a high level of balance between strength and elongation can be obtained, and the area corresponding to the impact-resistant and energy-absorbing sites in a single molded product. Is required, it is possible to achieve a high level of balance between high strength and elongation according to each region, and hot press useful for obtaining a press-molded product with good softening prevention properties in HAZ. Steel sheet can be realized.

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

Claims (6)

1. 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) And the metal structure has 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 steel sheet for hot pressing according to claim 1, further comprising at least one of the following (a) to (c) as another element.
   (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
3. The steel sheet for hot pressing according to claim 1 or 2 is heated to a temperature of Ac ₁ transformation point + 20 ° C or higher and Ac ₃ transformation point -20 ° C or lower, and then press forming of the steel sheet is started. A method for producing a press-molded product, characterized in that after molding is completed, cooling is performed 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 in the mold.
4. 3. A press-formed product of a steel sheet having the chemical composition according to claim 1 or 2, wherein the metal structure in the press-formed product is a retained austenite: 3 to 20 area%, ferrite: 30 to 80 area%, bay Nitic ferrite: less than 30 area% (excluding 0 area%), martensite: 31 area% or less (not including 0 area%), and among the Ti-containing precipitates contained in the press-formed product, A press-formed product characterized in that the average equivalent-circle diameter of an equivalent diameter of 30 nm or less is 10 nm or less, and the amount of precipitated Ti in the steel and the total Ti amount satisfy the relationship of the following formula (1).
   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)
5. The steel sheet for hot pressing according to claim 1 or 2 is used, 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 Ac ₃ transformation point and not higher than 950 ° C. After heating one region to a temperature of Ac ₁ transformation point + 20 ° C. or higher and Ac ₃ transformation point −20 ° C. or lower, press molding was started for both regions, and either region during or after molding However, the method for producing a press-molded product is characterized by cooling to a temperature below the martensite transformation start temperature Ms while securing an average cooling rate of 20 ° C./second or more in the mold.
6. 3. A press-formed product of a steel sheet having the chemical composition according to claim 1 or 2, wherein the press-formed product has a metal structure of residual austenite: 3 to 20 area%, martensite: 80 area% or more. The first region and the metal structure are retained austenite: 3 to 20 area%, ferrite: 30 to 80 area%, bainitic ferrite: less than 30 area% (excluding 0 area%), martensite: 31 area % Of the Ti-containing precipitates contained in the steel of the second region having an equivalent circle diameter of 30 nm or less. A press-formed product characterized in that the equivalent diameter is 10 nm or less, and the amount of precipitated Ti in the steel and the total amount of Ti satisfy the relationship of the following formula (1).
   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)

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