TWI602932B - Hot press formed member - Google Patents

Description

Hot press forming member

The present invention relates to a hot press formed member.

In order to improve the fuel consumption, the components for automobiles such as the door guard, the front side member, the cross member, and the side members are reduced in weight. Regarding the means for weight reduction, it is conceivable to make the thickness of the material thin. However, the above automotive components are also required to have high strength. Therefore, in order to sufficiently ensure the collision safety and the like even when the thickness is reduced, the steel sheet which is a material of the members is further increased in strength. Specifically, attempts were made to increase the tensile buildup (product of ductility and tensile strength), the Rankford value, and the bending limit.

The automotive components exemplified above are often manufactured by hot pressing. The hot pressing technique is a technique in which the steel sheet is heated to a high temperature in the Vostian domain and then subjected to press forming, and the forming load is considerably smaller than the usual press working at room temperature. Further, since the hot pressing technique performs quenching treatment in the mold simultaneously with press forming, high strength can be imparted to the steel sheet. Therefore, the hot pressing technique is attracting attention as a technique that can achieve both shape freezeability and strength assurance (for example, refer to Patent Document 1).

However, the member after the steel sheet is processed by the hot pressing technique (hereinafter sometimes referred to simply as "hot press forming member") has excellent strength, but may not be sufficiently ductile. When the automobile collides, the surface portion of the hot press-formed member is subjected to the bending deformation violently due to the extreme plastic deformation of the member for the automobile. In the case where the ductility of the hot press-formed member is insufficient, there is a possibility that the hot press-formed member is cracked due to the severe bending deformation. That is, the conventional hot press formed member has a flaw in that it cannot exhibit excellent collision characteristics.

On the other hand, TRIP (Transformed Induced Plasiticity) steel which is excellent in ductility due to the metamorphosis of the vestibule of the remaining Worth field is also known (refer to Patent Documents 2 and 3).

In general, TRIP steel can contain a residual Worth field body which is stable even at room temperature by metamorphosis in heat treatment. However, if the strength is promoted, the metamorphic deformation will be delayed, so that it takes a long time to generate the remaining Worth. In this case, productivity is significantly impaired. In addition, when the retention time at the time of formation of the tough body is insufficient, the unstable and untransformed Worth field body becomes a hard mass of the field at room temperature, so that the ductility and the bendability of the member are lowered, and a sufficient collision cannot be obtained. The essence of the feature.

Regarding the technique for promoting metamorphosis of the tough body, the technique of annealing the steel in the single phase domain of the Worth field, followed by cooling to a temperature between the Ms point and the Mf point, and reheating to 350 ° C or more and 400 ° C or less, has been maintained. It is known (for example, refer to Non-Patent Document 1). According to this technique, it is possible to obtain a stable residual Worth field in a short time.

It is known that TRIP steel is used as a steel sheet for cold forming by utilizing its excellent ductility. However, in the case of manufacturing a member by cold forming, the residual ductility of the formed member affects the collision characteristics of the member. In the part subjected to strong processing during cold forming, the residual ductility becomes small, and there is a possibility of cracking at the time of collision. Then, in recent years, there has been proposed a method of ensuring the ductility of the member in the steel sheet including the residual Worth field even in the hot press forming method (for example, refer to Patent Documents 4 to 6).

Patent Document 4 discloses a technique in which the average cooling rate from steel (Ms point - 150) ° C to 40 ° C is 5 ° C / sec or less in the hot press forming method, and the member contains a residual Worth field. However, it has been known that it is difficult to ensure the residual volume of the Vostian which can greatly improve the ductility by the control of the cooling rate alone.

Patent Document 5 discloses a technique in which the steel is cooled to a temperature range of Ms or more and below the (metamorphic transformation starting temperature Bs - 100 ° C) in the hot press forming method, and then retained at this temperature for 10 s or more. However, in this technique, the metamorphic body is slow in metamorphosis, and it is highly likely that the Worstian body will become a hard mass in the field after cooling. When a hard granulated loose body is formed, the difference in hardness between the tissues becomes large, and there is a possibility that the excellent bending property cannot be exhibited.

Patent Document 6 discloses a technique in which, after the hot press forming method, the steel is kept at a temperature of 750 ° C or more and 1000 ° C or less, and then cooled to a first temperature range of 50 ° C or more and 350 ° C or less, and a part of the Ma Tian bulk body is metamorphosed, and then The deformed body is metamorphosed by heating to a second temperature range of 350 ° C or more and 490 ° C or less to obtain a stable residual Worth field. However, even with this technology, there is no way to perform excellent bending. Because there is no regulation on the organization of the steel plates before hot pressing. Advanced technical literature

CITATION LIST Patent Literature Patent Literature No. JP-A-2002-174004, Japanese Patent Application Laid-Open No. Hei No. Hei. Patent Document 5: Japanese Laid-Open Patent Publication No. 2013-14842

Non-Patent Literature Non-Patent Document 1: H. Kawata, K. Hayashi, N. Sugiura, N. Yoshinaga and M. Takahashi: Materials Science Forum, 638-642 (2010), p3307

Disclosure of the Invention Problems to be Solved by the Invention The present invention has been made in view of the above circumstances, and an object of the invention is to provide a high-strength hot-press molded member excellent in ductility and flexibility. Specifically, an object of the present invention is to provide a high-strength hot press-formed member having a tensile modulus of 26,000 (MPa‧%) or more, and a Rankorford value in the rolling direction and a direction perpendicular to the rolling direction ( Hereinafter, the Rankford value may be 0.80 or less, which is simply referred to as "rolling direction", and the bending limit in the rolling direction and the bending limit in the right angle direction are both 2.0 or less. Hereinafter, the Rankford value is sometimes referred to simply as "r value". Means to solve the problem

The gist of the present invention is as follows.

(1) The hot press formed member according to one aspect of the present invention contains, in terms of unit mass%, C: 0.100 to 0.600%, Si: 1.00 to 3.00%, Mn: 1.00 to 5.00%, and P: 0.040% or less. , S: 0.050% or less, Al: 0.001 to 2.000%, N: 0.0100% or less, O: 0.0100% or less, Mo: 0 to 1.00%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 ~2.00%Nb: 0~0.300%, Ti: 0~0.300%, V: 0~0.300%, B: 0~0.1000%, Ca: 0~0.0100%, Mg: 0~0.0100%, and REM: 0~ 0.0100%, the remainder is made of iron and impurities; the micro-structure of 1/4 of the thickness of the plate contains: tempering Ma Tian bulk: 20~90%, metamorphic body: 5~75%, and Residual Worth field: 5~25%, and the fat granules are limited to 10% or less; the extreme density of the {211}<011> azimuth of the 1/4 part of the plate thickness is 3.0 or more. (2) The hot press-molded member according to the above (1) may be selected from the group consisting of Mo: 0.01 to 1.00%, Cr: 0.05 to 2.00%, Ni: 0.05 to 2.00%, and Cu: 0.05. One or more of the groups of ~2.00%. (3) The hot press-molded member according to the above (1) or (2) may be selected from the group consisting of Nb: 0.005 to 0.300%, Ti: 0.005 to 0.300%, and V: 0.005 to 0.300%, per unit mass%. One or more types of the group are formed. (4) The hot press-molded member according to any one of the above (1) to (3) may contain B: 0.0001 to 0.1000% in terms of unit mass%. (5) The hot press formed member according to any one of the above (1) to (4) may be selected from the group consisting of Ca: 0.0005 to 0.0100%, Mg: 0.0005 to 0.0100%, and REM: One or more of the groups consisting of 0.0005 to 0.0100%. Effect of the invention

The high-strength hot press-formed member related to the above aspect of the present invention is adjusted in terms of the composition and structure of the steel, in particular, the structure of the steel is a composite structure, and the ratio of each structure constituting the composite structure is improved. Further, the high-strength hot press-formed member associated with the above aspect of the present invention is also suitably controlled for the extreme density of steel. Thereby, according to the high-strength hot press-formed member related to the above aspect of the present invention, not only the use of the granulated bulk of the above-mentioned composite structure can be obtained, but excellent strength can be obtained, and the excellent ductility caused by the Worth field can be ensured together. Excellent bending properties caused by the toughening body. As a result, in the high-strength hot-pressed formed member related to the above aspect of the present invention, the r value in the rolling direction and the r-value in the right-angle direction of the rolling can be set to 0.80 or less, and the bending limit and rolling of the rolling direction are obtained. The bending limit in the right angle direction is 2.0 or less.

(Embodiment for Carrying Out the Invention) Hereinafter, an embodiment of the high-strength hot press-molded member according to the present invention will be described in detail. Incidentally, the embodiments shown below are not intended to limit the invention. Further, the constituent elements of the embodiment include content that is replaceable and easy for the user, or substantially the same content. Further, various aspects included in the embodiments described below can be arbitrarily combined by the manufacturer within the scope of the self-explanation.

In the member according to the present embodiment, "the thickness of the member is 1/4" means a surface having a depth of about 1/8 of the thickness of the member from the rolling surface of the member and a depth of about 3/8. The field between the faces. The rolling surface of the member is the rolling surface of the billet plate (cold-rolled steel sheet or annealed steel sheet) for hot pressing of the material of the member. "1/4" of the thickness of the billet for hot pressing" means a surface having a depth of about 1/8 of the thickness of the billet for hot pressing from the rolling surface of the billet for hot pressing. The field between the faces of 3/8 depth. Incidentally, the thickness of the member related to the present embodiment is not uniform, and the thickness of the sheet is increased or decreased in the field of processing. The 1/4 portion of the thickness of the member to be processed is a field corresponding to a quarter of the thickness of the billet for hot pressing before processing, and can be identified based on the cross-sectional shape.

The inventors of the present invention have made an effort to review the above-mentioned objects. As a result, the following findings have been obtained: to improve the ductility and the bendability of the hot-pressed molded member, and to make the structure of the steel of the predetermined composition include the tempered granules and the residual Worth field. It is important to have a composite structure of the toughness and to set the proportion of these organizations. More specifically, the present inventors have obtained the following knowledge: in hot press forming, a steel sheet of a predetermined composition is formed at a high temperature, and after reheating after being temporarily cooled, a process of holding is performed, thereby not only because of the above-mentioned composite structure Ma Tian's loose body gains excellent strength, and also ensures the excellent ductility caused by the Worth field and the excellent bending property caused by the metamorphic body. As a result, the Rankford value (r value) of the rolling direction can be obtained. And the r value in the direction perpendicular to the rolling is 0.80 or less, and the bending limit in the rolling direction and the bending limit in the right angle direction are both 2.0 or less.

Lankford value (r value) is applied as prescribed in JISZ2254 width direction of the test piece of the uniaxial tensile stress is generated by a plate-shaped tensile test piece of the true strain ε _b the thickness direction of the ratio of the true strain ε _a ε _b /ε _a . The r value in the rolling direction is an r value obtained by applying a uniaxial tensile stress in a direction parallel to the rolling direction, and the r value in the direction perpendicular to the rolling is by uniaxial pulling in a direction perpendicular to the rolling direction The r value obtained by stretching the stress.

<High-Strength Hot Press Forming Member> Hereinafter, an embodiment of the high-strength hot press forming member according to the present embodiment will be described in detail.

[Component] First, the reason for limiting the components of the high-strength hot press-molded member (hereinafter sometimes referred to as a member) according to the present embodiment will be described. Incidentally, in the present specification, the unit "%" of the chemical composition means "% by mass".

(C: 0.100 to 0.600%) Carbon (C) is an element necessary for increasing the strength of the member and ensuring a predetermined amount or more of the remaining Worth. If the C content is less than 0.100%, it will be difficult to ensure the tensile strength and ductility of the member. On the other hand, when the C content exceeds 0.600%, it is difficult to ensure the spot weldability of the member, and the ductility of the member is lowered. For the above reasons, the C content is from 0.100 to 0.600%. Incidentally, the lower limit of the C content is preferably 0.150%, 0.180%, or 0.200%. The upper limit of the C content is preferably 0.500%, 0.480%, or 0.450%.

(Si: 1.00 to 3.00%) 矽 (Si) is a strengthening element and is effective for increasing the strength of the member. Further, Si suppresses the precipitation and coarsening of the smectite carbon in the granulated bulk, thereby contributing to the increase in strength and the improvement in flexibility of the member. Further, Si increases the concentration of C in the Worth field to help ensure a predetermined amount or more of the remaining Worth field body, that is, it helps to suppress the snowiness when the member is temporarily cooled and then reheated and held. The element of precipitation of carbon.

When the Si content is less than 1.00%, the above effects (intensification of steel and suppression of precipitation of smectite carbon, etc.) are not sufficiently obtained. On the other hand, when the Si content exceeds 3.00%, the workability of the member is lowered. For the above reasons, the Si content is 1.00 to 3.00%. Incidentally, the lower limit of the Si content is preferably 1.10%, 1.20%, or 1.30%. The upper limit of the Si content is preferably 2.50%, 2.40%, or 2.30%.

(Mn: 1.00 to 5.00%) Manganese (Mn) is a strengthening element and is effective for increasing the strength of the member. When the Mn content is less than 1.00%, the fertilizer body, the corrugated body, and the smectite carbon body are formed during cooling of the member, and it is difficult to increase the strength of the member. On the other hand, when the Mn content exceeds 5.00%, co-segregation of Mn, P, and S tends to occur, and the workability of the member is remarkably lowered. For the above reasons, the Mn content is 1.00 to 5.00%. Incidentally, the lower limit of the Mn content is preferably 1.80%, 2.00%, or 2.20%. The upper limit of the Mn content is preferably 4.50%, 4.00%, or 3.50%.

(P: 0.040% or less) Phosphorus (P) has a depth of about 3/8 of the thickness of the member from the rolling plane and a depth of about 5/8 in the central portion of the thickness of the steel sheet constituting the member. The tendency of segregation in the field between the faces is an element that causes the welded portion formed when the member is welded to be embrittled. When the P content exceeds 0.040%, the brittleness of the welded portion becomes remarkable, so the P content is 0.040% or less. Incidentally, the preferred upper limit of the P content is 0.010%, 0.009%, or 0.008%. Further, since the lower limit of the P content is not particularly required, the lower limit of the P content may be 0%. However, from the economic point of view, it is not good to make the P content less than 0.0001%, so the lower limit of the P content may also be 0.0001%.

(S: 0.050% or less) Sulfur (S) is an element which has an adverse effect on the weldability of the member, the casting of the steel sheet constituting the member, and the manufacturability at the time of hot rolling. Further, S is an element which forms coarse MnS and hinders the flexibility and hole expandability of the member. When the S content exceeds 0.0500%, the above-mentioned adverse effects and hindrance become remarkable, so the S content is 0.0500% or less. Incidentally, the preferred upper limit of the S content is 0.0100%, 0.0080%, or 0.0050%. In addition, since the lower limit of S is not particularly necessary, the lower limit of the S content may be 0%. However, from the economic point of view, it is not good to make the S content less than 0.0001%, so the lower limit of the S content may also be 0.0001%.

(Al: 0.001 to 2.000%) Like Si, aluminum (Al) is an effective element for suppressing precipitation and coarsening of sulphur carbon. Further, Al is an element which can be used as an oxygen scavenger. If the Al content is less than 0.001%, the above effects are not exhibited. On the other hand, when the Al content exceeds 2.000%, the number of coarse inclusions of the Al-based system increases, which causes deterioration of the bendability of the steel sheet and causes of scratches on the surface of the steel sheet. For the above reasons, the Al content is 0.001 to 2.000%. Incidentally, the lower limit of the Al content is preferably 0.010%, 0.020%, or 0.030%. The upper limit of the Al content is preferably 1.500%, 1.200%, 1.000%, 0.250%, or 0.050%.

(N: 0.0100% or less) Nitrogen (N) is an element which forms a coarse nitride and lowers the flexibility and hole expandability of the member. Furthermore, N is an element of the cause of the occurrence of pores when the member is welded. When the N content exceeds 0.0100%, not only the bending property and the hole expansibility of the member are markedly lowered, but also a large number of pores are generated when the member is welded, so the content of N is 0.0100% or less. Incidentally, the preferred upper limit of the N content is 0.0070%, 0.0050%, or 0.0030%. In addition, since the lower limit of the N content is not particularly necessary, it may be 0%. However, since the N content is less than 0.0005%, the manufacturing cost is greatly increased, so the lower limit of the N content may be 0.0005%.

(O: 0.0100% or less) Oxygen (O) is an element which forms an oxide and causes fracture elongation, bending property, hole expandability, and the like of the member. In particular, when the oxide is present as an inclusion on the punched end surface or the cut surface of the member, the oxide forms a notch-like flaw and a coarse pit, and the stress concentrates during the hole expansion and the strong processing. Cracking occurs, and the hole expandability and/or bendability are greatly reduced.

When the O content is more than 0.0100%, the decrease in elongation at break, bendability, hole expandability, and the like is remarkable, so the O content is 0.0100% or less. Incidentally, the preferred upper limit of the O content is 0.0050%, 0.0040%, or 0.0030%. In addition, since the lower limit of the O content is not particularly necessary, it may be 0%. However, since the O content is less than 0.0001%, the excessively high cost is caused, and the economic consideration is not good. Therefore, the lower limit of the O content may be 0.0001%.

Further, the high-strength hot press-molded member according to the present embodiment may include, in addition to the above components, Mo: 0.01 to 1.00%, Cr: 0.05 to 2.00%, and Ni: 0.05 to 2.00%, and Cu: 0.05 to 2.00% of the group selected is one or more selected. However, these elements are not essential components. Since the member according to the present embodiment can solve the problem even when the elements are not included, the lower limit of the content of the elements is 0%.

(Mo: 0 to 1.00%) Molybdenum (Mo) is a strengthening element and is an element which contributes to the improvement of the hardenability of the steel sheet constituting the member. In order to obtain this effect, the lower limit of the Mo content may also be 0.01%. On the other hand, when the Mo content exceeds 1.00%, the manufacturability at the time of production of the steel sheet and hot rolling may be inhibited. For the above reasons, the Mo content is preferably from 0.01% to 1.00%. Incidentally, the lower limit of the Mo content is 0.05%, 0.10%, or 0.15%. A better upper limit of the Mo content is 0.60%, 0.50%, or 0.40%.

(Cr: 0 to 2.00%) Chromium (Cr) is a strengthening element and is an element that contributes to the improvement of the hardenability of the steel sheet constituting the member. In order to obtain this effect, the lower limit of the Cr content may also be 0.05%. On the other hand, when the Cr content exceeds 2.00%, the manufacturability at the time of production of the steel sheet and hot rolling may be inhibited. For the above reasons, the Cr content is preferably 0.05% or more and 2.00% or less. Incidentally, the lower limit of the Cr content is 0.10%, 0.15%, or 0.20%. A better upper limit of the Cr content is 1.80%, 1.60%, or 1.40%.

(Ni: 0 to 2.00%) Nickel (Ni) is a strengthening element and is an element which contributes to the improvement of the hardenability of the steel sheet constituting the member. Further, Ni is an element which contributes to the improvement of the wettability of the steel sheet and the promotion of the alloying reaction. In order to obtain such effects, the lower limit of the Ni content may also be 0.05%. On the other hand, when the Ni content exceeds 2.00%, the manufacturability at the time of production of a steel sheet and hot rolling may be inhibited. For the above reasons, the Ni content is preferably 0.05% or more and 2.00% or less. Incidentally, the lower limit of the Ni content is 0.10%, 0.15%, or 0.20%. A better upper limit of the Ni content is 1.80%, 1.60%, or 1.40%.

(Cu: 0 to 2.00%) Copper (Cu) is a strengthening element and is an element which contributes to the improvement of the hardenability of the steel plate constituting the member. Further, Cu is an element which contributes to the improvement of the wettability of the steel sheet and the promotion of the alloying reaction. In order to obtain such effects, the lower limit of the Cu content may also be 0.05%. On the other hand, when the Cu content exceeds 2.00%, the manufacturability at the time of production of the steel sheet and hot rolling may be inhibited. For the above reasons, the Cu content is preferably 0.05% or more and 2.00% or less. Incidentally, the lower limit of the Cu content is 0.10%, 0.15%, or 0.20%. A better upper limit of the Cu content is 1.80%, 1.60%, or 1.40%.

Further, the high-strength hot press-molding member according to the embodiment may include at least one of Nb: 0.005 to 0.300%, Ti: 0.005 to 0.300%, and V: 0.005 to 0.300% in addition to the above components. Kind. However, these elements are not essential components. Since the member according to the present embodiment can solve the problem even when the elements are not included, the lower limit of the content of the elements is 0%.

(Nb: 0 to 0.300%) Niobium (Nb) is a strengthening element, which is a strengthening of precipitates, a strengthening of fine particles by inhibition of growth of crystal grains of fat and granules, and a strengthening of the difference caused by inhibition of recrystallization. An element that contributes to the strength of the component. In order to obtain such effects, the lower limit of the Nb content may also be 0.005%. On the other hand, when the Nb content exceeds 0.300%, carbonitride may be excessively precipitated to deteriorate the formability of the member. For the above reasons, the content of Nb should preferably be 0.005% or more and 0.300% or less. Incidentally, the lower limit of the Nb content is preferably 0.008%, 0.010%, or 0.012%. A better upper limit of the Nb content is 0.100%, 0.080%, or 0.060%.

(Ti: 0 to 0.300%) Titanium (Ti) is a strengthening element, which is enhanced by precipitates, fine grain strengthening by inhibition of growth of fat and grain crystal grains, and poor stretching by suppression of recrystallization. An element that contributes to the strength of the component. In order to obtain such effects, the lower limit of the Ti content may also be 0.005%. On the other hand, when the Ti content exceeds 0.300%, carbonitride may be excessively precipitated to deteriorate the formability of the member. For the above reasons, the Ti content is preferably 0.005% or more and 0.300% or less. Incidentally, the lower limit of the Ti content is 0.010%, 0.015%, or 0.020%. A better upper limit of the Ti content is 0.200%, 0.150%, or 0.100%.

(V: 0 to 0.300%) Vanadium (V) is a strengthening element, which is a strengthening of precipitates, a strengthening of fine particles by inhibition of growth of crystal grains of fertilizer grains, and a strengthening of the difference caused by inhibition of recrystallization. An element that contributes to the strength of the component. In order to obtain such effects, the lower limit of the V content may also be 0.005%. On the other hand, when the V content exceeds 0.300%, carbonitride may be excessively precipitated to deteriorate the formability of the member. For the above reasons, the V content is preferably 0.005% or more and 0.300% or less. Incidentally, the lower limit of the V content is 0.010%, 0.015%, or 0.020%. A better upper limit of the V content is 0.200%, 0.150%, or 0.100%.

Further, the high-strength hot press-molded member according to the present embodiment may contain B: 0.0001 to 0.1000% in addition to the above components. However, B is not an essential component. Since the member according to the present embodiment can solve the problem even when B is not included, the lower limit of the B content is 0%.

(B: 0 to 0.1000%) Boron (B) is an element which contributes to the improvement of the strength of the grain boundary and the strength of the steel. For the above effects, the lower limit of the B content may also be 0.0001%. On the other hand, when the B content exceeds 0.1000%, not only the above effects are saturated, but also the manufacturability at the time of hot rolling of the steel sheet may be hindered. For the above reasons, the B content should preferably be 0.0001% or more and 0.1000% or less. Incidentally, the lower limit of the B content is 0.0003%, 0.0005%, or 0.0007%. A better upper limit of the B content is 0.0100%, 0.0080%, or 0.0060%.

Further, the high-strength hot press-molding member according to the present embodiment may contain at least one of Ca: 0.0005 to 0.0100%, Mg: 0.0005 to 0.0100%, and REM: 0.0005 to 0.0100% in addition to the above components. Kind. However, these elements are not essential components. Since the member according to the present embodiment can solve the problem even when the elements are not included, the lower limit of the content of the elements is 0%.

(Ca: 0 to 0.0100%) (Mg: 0 to 0.0100%) (REM: 0 to 0.0100%) Ca, Mg, and REM (Rare Earth Metal) are effective elements for deoxidizing steel sheets. In order to obtain this effect, the member may include one or more selected from the group consisting of 0.0005% or more of Ca, 0.0005% or more of Mg, and 0.0005% or more of REM. On the other hand, when the content of each of Ca, Mg, and REM exceeds 0.0100%, the workability of the member is hindered. For the above reasons, the contents of Ca, Mg, and REM are preferably 0.0005% or more and 0.0100% or less, respectively. Incidentally, the lower limit of each of the Ca content, the Mg content, and the REM content is 0.0010%, 0.0020%, or 0.0030%. A more preferable upper limit of each of the Ca content, the Mg content, and the REM content is 0.0090%, 0.0080%, or 0.0070%. In addition, when the member includes two or more types selected from the group consisting of Ca, Mg, and REM, the total content of Ca, Mg, and REM is preferably 0.0010% or more and 0.0250% or less.

Incidentally, the term "REM" means a total of 17 elements composed of Sc, Y, and a lanthanoid element, and the "content of REM" refers to the total content of the 17 elements. The REM may be added in the form of a rare earth metal alloy (an alloy containing a plurality of rare earth elements). The rare earth metal alloy sometimes contains La, Ce, and other lanthanides. The high-strength hot press-molded member according to the present embodiment may contain lanthanoid elements other than La and Ce as impurities. Further, the high-strength hot press-molded member according to the present embodiment may contain La and Ce in a range that does not inhibit various characteristics (especially ductility and flexibility) of the member.

(Remaining portion: iron and impurities) The remainder of the chemical composition of the member related to the present embodiment contains iron and impurities. Impurity refers to a component contained in a raw material of a component, or a component which is mixed in a manufacturing process of the component, and which does not affect the characteristics of the component. Specific examples thereof include P, S, O, Sb, Sn, W, Co, As, Pb, Bi, and H as impurities. Among them, P, S and O need to be controlled as described above. Further, according to a usual production method, the steel material may be mixed with impurities in which Sb, Sn, W, Co, and As are 0.1% or less, Pb and Bi are 0.010% or less, and H is 0.0005% or less. Within the scope, there is no need to specifically control the content of such elements.

Further, Si, Al, Cr, Mo, V, and Ca, which are components of the high-strength cold-rolled steel sheet of the present embodiment, may be intentionally mixed as impurities. However, as long as the components are within the above range, the properties of the high-strength hot-press molded member according to the present embodiment are not adversely affected. Further, although N is generally regarded as an impurity in the steel sheet, it is preferable to control the member according to the embodiment of the present invention within the above range.

[Microstructure] Next, the reason for limiting the microstructure of the high-strength hot press-molded member according to the present embodiment will be described. Incidentally, in this specification, the unit "%" of the ratio of each organization means "volume fraction (% by volume)". Further, the microstructure of the member according to the present embodiment is defined as a quarter of the member. Because the 1/4 portion between the rolling surface and the center surface is a typical configuration with members. In the present specification, the description of the microstructure is based on the micro tissue of the 1/4 portion unless otherwise specified. Further, although the member according to the present embodiment has a portion to be processed and a portion not to be processed, the microstructures of the two are substantially the same.

(Returning Ma Tian Loose Body: 20 to 90%) The tempering Ma Tian Loose Body is a structure in which steel is strengthened, and is a structure included to secure the strength of the member related to the present embodiment. If the volume fraction of the tempered granules is less than 20%, the strength of the components will be insufficient. On the other hand, if the volume fraction of the tempered mass is more than 90%, the necessary toughness and Worthing experience are insufficient to ensure the ductility and flexibility of the member. For the above reasons, the volume fraction of the tempered granules is 20% or more and 90% or less. Incidentally, the preferred lower limit of the volume fraction of the tempered granules is 25%, 30%, or 35%. The preferred upper limit of the volume fraction of the tempered granules is 85%, 80%, or 75%.

(Toughened body: 5 to 75%) The toughened body is a structure that is important for the bending property of the lifting member. Usually, when the member is a structure made of a hard Ma Tian bulk body and a ductile residual Worth field body, because of the difference in hardness between the Ma Tian bulk body and the residual Worth field body, the Ma Tian bulk body is generated when the member is deformed. Stress concentration. Due to this stress concentration, pores are formed at the interface between the Ma Tian bulk body and the residual Worth field body, and as a result, the bending property of the member is lowered. However, when the member is a structure having a toughened body in addition to the Ma Tian bulk body and the residual Worth field body, the toughness body makes the difference in hardness between the tissues small, whereby the stress concentration on the Ma Tian bulk body is alleviated. The bending of the component is improved.

If the volume fraction of the tough body is less than 5%, the stress concentration on the Ma Tian bulk is not sufficiently alleviated, and the excellent bendability cannot be ensured. On the other hand, if the volume fraction of the toughener exceeds 75%, the Ma Tian bulk and the residual Worthing which are necessary for ensuring the strength and ductility of the member are insufficient. For the above reasons, the volume fraction of the tough body is 5% or more and 75% or less. Incidentally, the preferred lower limit of the volume fraction of the tough body is 10%, 15%, or 20%. The preferred upper limit of the volume fraction of the toughened body is 70%, 65%, or 60%.

(Residual Worth Field: 5~25%) Residual Worth Field is an important organization to ensure the ductility of components. The residual Worth field is metamorphosed to the Matian bulk when the steel sheet is pressed and formed, and the steel sheet is provided with excellent work hardening and a high degree of uniform extension. If the volume fraction of the residual Worth is less than 5%, sufficient uniform elongation cannot be obtained, and it is difficult to ensure excellent formability. On the other hand, if the volume fraction of the remaining Worthfield body exceeds 25%, the Ma Tian bulk and the toughened body which are necessary for ensuring the strength and hole expandability of the steel sheet will be insufficient. For the above reasons, the volume fraction of the residual Worth field body is 5% or more and 25% or less. Incidentally, the preferred lower limit of the volume fraction of the residual Worth field is 7%, 10%, or 12%. The preferred upper limit of the volume fraction of the residual Worth field is 22%, 20%, or 18%.

(Fat granules: 0~10%) Fertilizers are soft tissues, so the volume fraction should be as small as possible. Therefore, the lower limit of the volume fraction of the fat granules is 0%. If the volume fraction of the fat granules exceeds 10%, it becomes difficult to secure the strength of the steel sheet. Therefore, the volume fraction of the fat granules is limited to less than 10%. Incidentally, the upper limit of the volume fraction of the granules is 8%, 5%, or 3%.

Incidentally, the identification of the tempered granules, the tough body, the residual Worth field, and the identification of the fat granules, the determination of the existence position, and the determination of the volume fraction can be carried out by using the following method: Etchant and LePera solution, and an etching solution (pretreatment liquid) made of a mixed solution of picric acid, ethanol, sodium thiosulfate, citric acid, and nitric acid, and a mixture of nitric acid and ethanol 1000 times of the etching solution (post-treatment liquid) formed by the solution, and the cross section parallel to the rolling direction of the steel sheet and perpendicular to the rolling surface, or the cross section perpendicular to the rolling direction and the rolling surface of the steel sheet. An optical microscope and a scanning electron microscope of 1000 to 100,000 times and a transmission electron microscope were used to observe the cross section after corrosion.

The identification of the tempered granules is based on scanning electron microscopy and transmission electron microscopy to observe the cross section, and the granules containing the carbides (Fe-based carbides) containing a large amount of Fe in the interior of the carbides. It is considered to be a tempering Matian bulk, and the Matian bulk which does not contain the carbide is regarded as the usual 麻田散体 (primary 麻田散体) which is not tempered. Regarding the carbide containing a large amount of Fe, it may be a carbide of various crystal structures. However, the granulated bulk of the Fe-based carbide containing any of the crystal structures is included in the tempered granule of the present embodiment. Further, the tempered granules of the present embodiment are also included in the case where a plurality of types of Fe-based carbides are mixed due to heat treatment conditions.

In addition, the identification of the tempered Matian bulk, the tough body, the residual Worth field, and the fat body can also be utilized: by using an electric field radiation type scanning electron microscope (FE-SEM: Field Emission Scanning Electron) The hardness of the microscopic field such as the analysis of the crystal orientation by the crystal orientation analysis method (FE-SEM-EBSD method) of the EBSD (Electron Back-Scatter Diffraction) attached to the Microscope).

For example, when the volume fraction (%) of the residual Worth field in the metal structure is confirmed, the surface is at a depth of about 1/4 of the thickness of the plate parallel to the rolling surface of the member. The surface of the depth of about 1/4 of the thickness of the member from the rolling surface of the member was subjected to X-ray analysis as an observation surface. The area fraction of the residual Worth field obtained by this is taken as the volume fraction of the residual Worth field.

On the other hand, when confirming the volume fraction (%) of the tough body, the tempered mass, and the fat body in the metal structure, first, it is parallel to the rolling direction of the steel sheet and perpendicular to the rolling surface. The cross section (observation surface) is ground and etched with a nitric oxide. Next, the area fraction of each of the structures was measured by observing 1/4 of the thickness of the etched cross section by FE-SEM. Since the area fraction obtained in this case is a value substantially equal to the volume fraction, the area fraction is regarded as the volume fraction.

Incidentally, in the observation of FE-SEM, for example, each of the tissues on the observation surface of a square of 30 μm can be distinguished and recognized in the following manner. That is, the tempered granules are a collection of crystal grains of a strip shape (plate shape having a specific preferential growth direction), and can be recognized as a structure in which the above-described iron-based carbonization having a long diameter of 20 nm or more is contained inside the crystal grains. And the carbide belongs to a plurality of iron-based carbide groups elongated toward a plurality of positions (ie, different directions). The toughened body is a collection of strip-shaped crystal grains, and can be recognized as a structure in which iron-based carbide having a long diameter of 20 nm or more is not contained in the crystal grains, or a long diameter of 20 nm or more is contained in the inside of the crystal grains. Iron-based carbides, but the carbides belong to an iron-based carbide group that extends toward the single direction (in the same direction). Here, the iron-based carbide group elongated in the same direction means that the difference in the elongation direction of the iron-based carbide group is within 5°. The fat granules are block-shaped crystal grains, and are recognized as a structure in which iron-based carbide having a long diameter of 100 nm or more is not contained inside the crystal grains.

Incidentally, the iron-based carbide inside the strip-shaped crystal grains can be observed by using FE-SEM, and the direction of elongation can be investigated, and the tempered mass and the tough body can be easily distinguished.

[Polar Density of {211}<011> Azimuth in 1/4 of Plate Thickness] Next, the reason for limiting the extreme density of the high-strength hot press-molded member according to the present embodiment will be described. Incidentally, the extreme density of the member related to the present embodiment is defined as a quarter of the member (having a typical configuration of the member). In the present specification, the description of the polar density is about the extreme density of the 1/4 portion unless otherwise specified. Further, although the member according to the present embodiment has a portion to be processed and a portion not to be processed, the extreme densities of the two are substantially the same.

When the density of the {211}<011> orientation of the hot-pressed member at the 1/4 portion of the sheet thickness is less than 3.0, the r value in the rolling direction and the r value in the right angle direction of the rolling are not 0.80 or less. Therefore, the bending property is deteriorated. Therefore, the extreme density of the {211}<011> azimuth in the 1/4 portion of the plate thickness is 3.0 or more. The lower limit of the density of the {211}<011> azimuth of the 1/4 portion of the plate thickness is preferably 4.0 or 5.0. The upper limit of the extreme density of the {211}<011> azimuth of the 1/4 portion of the plate thickness is not particularly specified. However, when the density of the {211}<011> azimuth of the plate thickness is more than 15.0, there is a possibility that the workability of the member is lowered, so that it is also possible to make the thickness of the plate 1/4. The extreme density of the {211}<011> orientation is 15.0 or less, or 12.0 or less.

The extreme density is the ratio of the concentration of the test piece to a particular orientation and the standard sample that does not have a specific orientation. The pole density of the {211}<011> orientation of the member of the present embodiment in the 1/4 portion of the sheet thickness is measured by an EBSD (Electron Back Scattering Diffraction Pattern) method.

The polar density measurement using EBSD was carried out as follows. A cross section parallel to the rolling direction of the member and perpendicular to the rolling surface is used as the observation surface. For the rectangular area of the observation surface (the line of the 1/4 depth of the sheet thickness t from the surface of the member is centered, the rolling direction is 1000 μm and the normal direction of the rolling surface is 100 μm), and the measurement interval is 1 μm. Perform EBSD analysis to obtain the crystal orientation information of the rectangular field. The EBSD analysis is performed using a thermal field electric scanning electron microscope (for example, JSM-7001F manufactured by JEOL) and an EBSD detector (for example, a HIKARI detector manufactured by TSL), and the resolution is 200 to 300 points/second. Implementation. Using the EBSD analysis software "OIM Analysis" (registered trademark), the ODF (Orientation Distribution Function) of the rectangular region is calculated from the crystal orientation information of the rectangular region. Thereby, since the extreme density of each crystal orientation can be obtained, the pole density of the {211}<011> orientation of the member at the thickness of 1/4 can be obtained. Fig. 1 is a view showing the position of the main crystal orientation on the ODF (φ2 = 45° section). Generally, the crystal orientation perpendicular to the rolling surface is indicated by the mark of (hkl) or {hkl}, and the crystal orientation parallel to the rolling direction is indicated by the mark of [uvw] or <uvw>. {hkl} and <uvw> are the general names of the equivalent faces and directions. (hkl) and [uvw] are the crystal faces of each.

The crystal structure of the member of the present embodiment is mainly a body-centered cubic structure (bcc structure). Thus, for example, (111), (-111), (1-11), (11-1), (-1-11), (-11-1), (1-1-1), ( -1-1-1) is essentially equivalent and cannot be distinguished. In the present embodiment, the orientations are collectively referred to as {111}.

Incidentally, ODF is also used in the representation of the crystal orientation of a crystal structure with low symmetry. In general, φ1 = 0 to 360°, Φ = 0 to 180°, and φ2 = 0 to 360°, and the respective crystal orientations are represented by (hkl) [uvw]. However, the crystal structure of the hot-rolled steel sheet according to the present embodiment is a body-centered cubic structure having high symmetry. Therefore, Φ and φ2 can be expressed by 0 to 90°.

Φ1 will vary depending on whether the symmetry of the deformation is considered when calculating. In the present embodiment, calculation for performing orthotropic symmetry is expressed by φ1 = 0 to 90°. That is, the pole density measurement of the member of the present embodiment is a method of selecting an average value of the same orientation at φ1 = 0 to 360° on the ODF of 0 to 90°. In this case, (hkl)[uvw] is synonymous with {hkl}<uvw>. Therefore, the polar density of the (112) [1-10] azimuth (φ1 = 0°, Φ = 35°) of the ODF of the φ2 = 45° section shown in Fig. 1 is the extreme density of the {211} <011> orientation. Synonymous.

As described above, it is possible to achieve excellent ductility, that is, excellent fatigue resistance, by adjusting the composition, structure, and extreme density of the high-strength hot-pressed molded member so that the tensile modulus of the member is 26,000 (MPa‧%) or more. Component of sex and durability. In addition, by the above adjustment, the r value of the rolling direction of the member and the r value of the right angle direction of the rolling of the member are all 0.80 or less, and the bending limit of the rolling direction of the member and the bending of the right angle direction of the rolling of the member The limit is 2.0 to achieve a member with excellent flexibility.

Incidentally, when the punching is performed, the lower the r value, the more the deformation in the thickness direction is promoted, and the bending crack can be prevented. In general, when the r value in the direction perpendicular to the ridge line direction of the bending is 0.80 or less, the effect of preventing the bending crack can be exhibited at a high level. In the high-strength hot-pressed molded member according to the present embodiment, since both the r value in the rolling direction and the r value in the right-angle direction of the rolling are 0.80 or less, even if the member is subjected to large bending deformation during collision, the member may be Play excellent bending.

<Method for Producing High-Strength Hot Press-Formed Member> Next, a method of manufacturing the high-strength hot-press-molded member according to the present embodiment will be described in detail. The method for producing the high-strength hot-pressed formed member is carried out by sequentially performing the necessary steps of heating the billet for hot pressing (the cold-rolled steel sheet or the annealed steel sheet formed of the above chemical composition) to a maximum heating temperature of Ac _{3 .} The heating step above the point, the hot press forming of the billet plate for hot pressing, and simultaneous cooling to a temperature range of (Ms point - 250 ° C) above the Ms point of the hot press forming ‧ cooling step. Further, in the method for producing the high-strength hot-pressed molded member of the present embodiment, in addition to these steps, the member is reheated to a temperature range of 300 to 500 ° C after the hot press forming and the cooling step. The component is then held in the reheating temperature range for 10 to 1000 seconds, after which the component is cooled to room temperature for a reheating step. Hereinafter, each step will be described. Incidentally, the following is a preparation step of the billet for hot pressing which is carried out before the above heating step. In the description of the manufacturing method of the member according to the present embodiment, the "heating rate" and the "cooling rate" mean dT/dt (the instantaneous speed at time t) obtained by differentiating the temperature T by the time t. For example, "the heating rate in the temperature range from A ° C to B ° C is X ~ Y ° C / sec" means that dT / dt between the temperature T changes from A ° C to B ° C Always within the range of X~Y°C/sec.

(Preparation Step of Blank Sheet for Hot Pressing) This step is a preparation step of obtaining a billet for hot pressing (cold-rolled steel sheet or annealed steel sheet) used in a heating step to be described later. The manufacturing process prior to casting is not particularly limited. In other words, it is also possible to carry out various secondary smelting by continuing the melting of the blast furnace, the electric furnace, and the like. The cast flat embryo may be subjected to hot rolling after being cooled to a low temperature once, or may be continuously rolled (i.e., not cooled and reheated). Regarding the hot rolling, it is important that the total rolling reduction ratio in the temperature range of 920 ° C or lower is 25% or more. The reason is as follows. (1) Regarding the rolling of the temperature field exceeding 920 ° C, since recrystallization is performed in the rolling delay or in the idle time until the next rolling, it is difficult to accumulate strain in the steel, and as a result, there is insufficient contribution to the collection. The possibility of the formation of an organization. (2) When the total rolling reduction ratio in the temperature range of 920 ° C or less is less than 25%, since the crystal rotation effect by the rolling is not obtained sufficiently, there is a high possibility that the aggregate structure is not sufficiently formed.

For these reasons, it is important that the total reduction ratio in the temperature range below 920 ° C is 25% or more. The total rolling reduction ratio in the temperature range of 920 ° C or lower is preferably 30% or more, more preferably 40% or more. On the other hand, the upper limit of the total reduction ratio in the temperature range of 920 ° C or lower is preferably 80%. This is because the implementation of more than 80% of the rolling shrinkage results in an increase in the load on the rolling rolls and affects the durability of the rolling mill. Incidentally, scrap can also be used as a raw material for the billet for hot pressing.

Further, regarding the cooling conditions after the hot rolling is delayed, a cooling type for performing the structure control can be employed to make the effects (excellent ductility and flexibility) of the members related to the present embodiment effective.

The winding temperature is preferably 650 ° C or less. When the hot-rolled steel sheet is wound at a temperature exceeding 650 ° C, the thickness of the oxide formed on the surface of the hot-rolled steel sheet becomes too large, so the pickling property is inferior. Incidentally, the winding temperature is preferably 600 ° C or less. This is because, in the temperature range of 600 ° C or lower, it is easy to produce metamorphic metamorphosis. By making the hot-rolled sheet structure mainly a toughened body, the formation of the subsequent cold rolling delay can be sufficiently performed, that is, the r value of the object can be easily obtained.

The lower limit of the winding temperature is not particularly limited, and various effects (excellent ductility and flexibility) of the members according to the present embodiment can be exhibited. However, since it is technically difficult to wind the hot-rolled steel sheet at a temperature lower than room temperature, the room temperature is a substantial lower limit of the winding temperature. However, when the winding temperature is less than 350 ° C, the ratio of the loose mass of the hard ground is increased in the hot-rolled sheet structure, and the cold rolling becomes difficult. Therefore, the winding temperature is preferably 350 ° C or higher.

The hot-rolled steel sheet thus produced is subjected to pickling. The number of picklings is not specified.

The pickled hot-rolled steel sheet is cold rolled at a total rolling reduction ratio of 50 to 90%, and is used as a billet for hot pressing. In order to make the r value of the rolling direction and the r value of the rolling direction in the direction of the rolling of the high-strength hot-pressed forming member according to the present embodiment all of 0.80 or less, the billet plate for hot pressing is 1/4 of the sheet thickness. The extreme density of the 211}<011> orientation needs to be 3.0 or more. The density of the {211}<011> orientation of the billet plate for hot pressing in the 1/4 portion of the sheet thickness is preferably 4.0 or more, and more preferably 5.0 or more. When the total rolling reduction ratio of the cold rolling is less than 50%, the density of the {211}<011> orientation of the billet plate for hot pressing may be less than 3.0 in the 1/4 portion of the thickness of the sheet. The collection organization cannot be controlled as described above, so it is difficult to ensure the r value of the purpose. On the other hand, if the total rolling reduction ratio of the cold rolling is more than 90%, the driving force for recrystallization becomes too high, and the fat body recrystallizes at the heating step of the hot pressing described later. Although the hot pressing billet is heated to a temperature of Ac ₃ or higher in the heating step of the hot pressing described later, the unrecrystallized fat body must remain in the billet for hot pressing before reaching the Ac ₃ point. When the total rolling reduction of the cold rolling is more than 90%, this condition will not be achieved. Further, when the total rolling reduction ratio exceeds 90%, the cold rolling load becomes excessively large and cold rolling becomes difficult. Incidentally, the total rolling reduction ratio r of the cold rolling is obtained by bringing the thickness h1 (mm) after the end of the cold rolling and the thickness h2 (mm) before the start of the cold rolling to the following formula 1 Out. r=(h ₂ -h ₁ )/h ₂ (Expression 1)

For the above reasons, the total rolling reduction ratio of the cold rolled steel sheet of the pickled hot-rolled steel sheet is 50% or more and 90% or less. Incidentally, the preferred range of the total rolling reduction of the cold rolling is 60% or more and 80% or less. Further, the number of rolling passes and the rolling reduction ratio of each pass are not particularly limited.

Further, the cold-rolled steel sheet obtained by the above-described cold rolling may be subjected to heat treatment (annealing), and the annealed steel sheet may be used as a billet for hot pressing. The heat treatment is not particularly limited, and it may be carried out by passing it through a continuous annealing line, or by batch annealing. In the heat treatment, the heating rate is required to be 10 ° C / sec or more in a temperature range of 500 ° C or more and Ac ₁ or less. When the heating rate is less than 10 ° C / sec, the aggregate structure of the finally obtained formed body may not be suitably controlled. However, when the total content of the Ti content and the Nb content of the steel sheet is 0.005% by mass, the heating rate in the temperature range of 500 ° C or more and the Ac ₁ point or lower may be 3 ° C / sec or more.

The annealing temperature is preferably Ac ₁ point or more and Ac ₃ or less. This is because if the annealing temperature is less than Ac ₁ point, the recrystallization of the fat granules progresses. On the other hand, if the annealing temperature exceeds the Ac ₃ point, the steel sheet becomes a single phase structure of the Worth field, and it is difficult to leave the non-recrystallized fat body. In either case, in the heating step of the hot pressing, it becomes difficult for the hot pressing blank sheet to remain unrecrystallized fat granules in the hot pressing blank sheet before reaching the Ac ₃ point.

Although the annealing time in this temperature range (Ac ₁ point or more and Ac ₃ point or less) is not particularly limited, if the annealing time exceeds 600 seconds, the cost increases, which is not preferable from the viewpoint of economy. Incidentally, the annealing time is the length during which the steel sheet temperature is subjected to isothermal holding at the highest reaching temperature (annealing temperature). During this period, the steel sheet may be subjected to isothermal holding, or it may be immediately cooled after reaching the maximum heating temperature.

The cooling after annealing is 700 ° C or higher as the cooling start temperature, 400 ° C or lower is used as the cooling end temperature, and the cooling rate in the temperature range of 700 ° C to 400 ° C is preferably 10 ° C / sec or more. If the cooling rate in the temperature range of 700 ° C to 400 ° C is less than 10 ° C / sec, the recrystallization of the fat granules progresses. In this case, in the heating step of the hot pressing, it becomes difficult for the hot pressing blank sheet to remain unrecrystallized fat granules in the hot pressing blank sheet before reaching the Ac ₃ point.

(Heating Step) This step is a step of heating the cold-rolled steel sheet or the annealed steel sheet obtained by the above-described preparation step, that is, the billet for hot pressing, to a point of Ac ₃ or more. The maximum heating temperature of the billet for hot pressing is Ac ₃ or more. If the maximum heating temperature is less than ₃ points of Ac, a large amount of fat and granules are formed in the high-strength hot press-formed member, so that it is difficult to secure the strength of the high-strength hot press-formed member. Thus, the Ac ₃ point is used as the lower limit of the maximum heating temperature. On the other hand, excessive high-temperature heating causes an increase in cost, which is not preferable from the viewpoint of economy, and causes problems such as a decrease in the life of the press mold. Therefore, the maximum heating temperature is preferably Ac ₃ point + 50 ° C or less.

The heating rate in the temperature range from 500 ° C to Ac ₁ is preferably 10 ° C / sec or more in the heating up to the maximum heating temperature. However, when the total value of the Ti content and the Nb content of the hot pressed billet plate is 0.005% by mass, the heating rate can be made 3 ° C /sec or more. When the heating rate in the temperature range of 500 ° C to Ac ₁ is less than 10 ° C / sec, recrystallization of the fat granules occurs during heating, and it is difficult to retain the unrecrystallized fat granules before reaching the Ac ₃ point. Further, by heating at a heating rate of 10 ° C /sec or more, it is possible to suppress the coarsening of the Worthite body particles, and it is possible to improve the toughness and the delayed fracture resistance of the high-strength hot press-molded member.

Even if the heating rate in the temperature range of 500 ° C to Ac ₁ is increased, the unrecrystallized fertilizer granules remain before reaching the Ac ₃ point, and the productivity of the high-strength hot-pressed molded member can be improved. However, if the heating rate in the temperature range of 500 ° C to Ac ₁ exceeds 300 ° C / sec, these effects become saturated, and no other special effects are produced. Therefore, the upper limit of the heating rate is preferably 300 ° C / sec.

Incidentally, although the holding time at the highest heating temperature is not particularly limited, in order to melt the carbide, the holding time is preferably 20 seconds or longer. On the other hand, in order to retain a better aggregate structure to obtain the r value of the purpose, the hold time is preferably less than 100 seconds.

(Hot pressing step) The hot pressing step is a hot press forming of a hot pressing blank sheet subjected to the above heating step using a hot press forming means (for example, a mold), and at the same time, using a cooling means provided by a hot press forming means (for example, The refrigerant flows through the piping in the mold and the like, and is cooled to a temperature range of not less than Ms point (Ms point - 250 ° C). Hot press forming can also be carried out by any method known in the art.

In the hot pressing step, the member is cooled to a temperature range of not less than Ms point (Ms point - 250 ° C) at a cooling rate of 0.5 to 200 ° C / sec to form a granulated bulk. If the cooling stop temperature is not full (Ms point - 250 ° C), the Ma Tian bulk body is excessively formed, and the ductility and flexibility of the high-strength hot press-molded member are not sufficiently achieved. On the other hand, if the cooling stop temperature is higher than the Ms point, the Ma Tian bulk is not sufficiently formed, and the strength is not sufficiently ensured in the high-strength hot press-molded member. Therefore, the cooling stop temperature is taken at or below the Ms point (Ms point - 250 ° C). When the ambient gas temperature is low, even if the operation of the cooling means is stopped, the temperature decrease rate of the member is 0.5 ° C / sec or more, and the above-described cooling stop is not achieved. In this case, it is necessary to suitably use a heating means to suppress the temperature decrease rate of the member to less than 0.5 ° C / sec, and to achieve the above-described cooling stop. In addition, when the cooling stop temperature is (Ms point - 220 ° C) or more (Ms point - 50 ° C) or less, the above effects are effective at a high level, and therefore it is preferable.

The cooling rate from the highest heating temperature to the cooling stop temperature is not particularly limited, but is preferably 0.5 to 200 ° C / sec. If the cooling rate is less than 0.5 ° C / sec, in the cooling process, there will be a situation in which the Worth body is metamorphosed to the corrugated structure or a large amount of fat and granules are generated. Therefore, it is necessary to ensure sufficient Ma Tian bulk to ensure strength. And the volume ratio of the toughened body will become difficult.

On the other hand, although the increase in the cooling rate is not problematic for the material of the high-strength hot-pressed molded member, excessively increasing the cooling rate results in high manufacturing cost, so the upper limit of the above cooling rate is preferably 200 ° C / sec. .

(reheating step) The reheating step is to reheat the member subjected to the above hot press forming and cooling step to a temperature range of 300 to 500 ° C, and then to maintain the member in the reheating temperature range for 10 to 1000 seconds, after which the member is removed from The step of reheating the temperature domain to cool to room temperature. This reheating can be performed using electric heating and induction heating. The reheating step is an optional step, and the holding in the reheating step does not only mean isothermal holding, but also includes gradual cooling and heating in the above temperature range. Therefore, the retention time in the reheating step means the length of the member during the reheating temperature range.

If the reheating temperature (holding temperature) is less than 300 ° C, it takes a long time for the metamorphic body to be metamorphosed, so that excellent productivity cannot be achieved. On the other hand, if the reheating temperature (holding temperature) exceeds 500 ° C, the metamorphic deformation state is less likely to occur. Therefore, the reheating temperature is from 300 ° C to 500 ° C. Incidentally, the preferable range of the reheating temperature is 350 ° C or more and 450 ° C or less.

Further, if the holding time is less than 10 seconds, the progress of the metamorphic deformation is insufficient, and a deformable body sufficient for ensuring flexibility and a residual Worstian body sufficient for ensuring ductility are not obtained. On the other hand, when the holding time exceeds 1000 seconds, decomposition of the residual Worth field body occurs, and the residual Worth field body which is effective for ensuring ductility is not obtained, and productivity is lowered. Therefore, the holding time is 10 seconds or more and 1000 seconds or less. Incidentally, the preferred range of the holding time is 100 seconds or more and 900 seconds or less.

Further, the cooling state after the holding is not particularly limited, and it may be cooled to room temperature in the mold as it is. Incidentally, since this step is an optional step, when the step is not employed, the member may be taken out from the pressing mold after the hot pressing forming step is completed, and the charging is performed until the temperature is 300~. 500 ° C furnace. Incidentally, as long as the heat history is satisfied, it is possible to carry out heat treatment by what kind of equipment the steel plate is subjected to.

The method for producing the high-strength hot-pressed molded member of the present embodiment shown above is based on the steps of refining, steel-making, casting, hot-rolling, and cold-rolling in ordinary steel making, but the above-described steps are satisfied. According to the conditions, even if the design is appropriately changed, the effect of the high-strength hot press-molded member according to the present embodiment can be obtained. Example

Hereinafter, the effects of the present invention will be more specifically described by way of the invention. Incidentally, the present invention is not limited to the conditions used in the following invention examples.

The slabs A to R and a to d of the chemical compositions shown in Table 1 were sequentially subjected to the production steps and heating of the billet for hot pressing of the present invention under various conditions shown in Tables 2-1 to 3-3. The steps, the hot press forming step, the cooling step, and the reheating step are simulated to produce steel sheets A1 to d1, and then the steel sheet is cooled to room temperature. The steel sheets A1 to d1 obtained in each test example were not subjected to hot pressing of the mold. However, the mechanical properties of the obtained steel sheet are substantially the same as those of the hot pressed forming member having the same heat traverse. Therefore, the effects of the hot press formed member of the present invention can be confirmed by evaluating the obtained steel sheets A1 to d1.

Here, the steel grades A to R in Table 1 are steel grades of the components specified in the present invention, and the steel grades a to d are steel grades in which at least one of C, Si, and Mn is outside the range of the present invention. In addition, the letters included in the test symbols described in Table 2-1 and the like correspond to the steel types described in Table 1. In order to distinguish the test cases, a number is added next to the letter. For example, the chemical composition of the test symbols D1 to D18 in Table 2-1 is the chemical composition of the steel species D of Table 1. Further, in Table 1 and Tables 2-1 to 3-3, the numerical value with the bottom line is a value outside the prescribed range of the present invention. Incidentally, the "holding time at 300 to 500 ° C" of D7, D13, H6, K12, L6, L12, and L13 means the reheating temperature described in "Preservation temperature (°C) at 300 to 500 °C". The isothermal holding time, the "holding time at 300 to 500 ° C" of the other examples refers to the time when the steel sheet temperature is in the range of 300 to 500 ° C.

In addition, the Ac ₃ point and the Ms point of each test example are the values obtained by previously measuring the raw material for hot pressing of hot rolling and cold rolling in the laboratory. Further, the annealing temperature and the cooling temperature were set using the Ac ₃ point and the Ms point thus obtained.

【Table 1】

【table 2-1】

[Table 2-2]

[Table 2-3]

[Table 3-1]

[Table 3-2]

[Table 3-3]

Next, the identification of the microstructure of each of the steel sheets A1 to d1 and the analysis of the aggregate structure were carried out by the above method. Next, the mechanical properties of the respective steel sheets A1 to d1 were investigated by the following methods.

The tensile strength TS (MPa) and the elongation at break El (%) were measured by a tensile test. The tensile test piece was a JIS No. 5 test piece taken from a 1.2 mm thick plate in a right angle direction. A sample having a tensile strength of 1200 MPa or more is a sample which is judged to have a good tensile strength.

The r value in the rolling direction and the r value in the vertical direction of the rolling, and the bending limit (R/t) in the rolling direction and the bending limit (R/t) in the vertical direction of the rolling were measured by a bending test. The specific means are as follows.

The r value was determined by a test based on JIS Z 2 251 and based on a test based on JIS Z 2254. The r value in the rolling direction was measured by using a test piece having a rolling direction as a long direction, and the r value in the direction perpendicular to the rolling direction was measured by using a test piece having a longitudinal direction in the rolling direction as a long direction.

The bending limit R/t is obtained by performing a test based on the V-block method defined in JIS Z2248 on the No. 1 test piece specified in JIS Z2204. The bending limit in the rolling direction is measured by using a test piece in such a manner that the curved ridge line becomes the rolling direction, and the bending limit in the right angle direction is measured by using the bending ridge line in the direction perpendicular to the rolling direction. Determined by tablet. Incidentally, the test is repeated by using a plurality of pressing dies having different curvature radii R, and the bending of the bent portion is determined by an optical microscope or SEM after the bending test, and the bending limit R/t (R) in which cracking does not occur is calculated. : The bending radius of the test piece (that is, the radius of curvature of the pressing die) and t: the thickness of the test piece were evaluated.

The identification of the organization and the results of each performance are shown in Table 4-1 to Table 5-3. Incidentally, the numerical values with the bottom line in Tables 4-1 to 4-3 are values outside the scope of the present invention. In addition, in Tables 4-1 to 5-3, tM (%) means the volume fraction of the tempered granules in the microstructure, and B (%) means the volume of the tempering body in the microstructure. The fraction, γR (%) means the volume fraction of the residual Worth field in the microstructure, and F (%) means the volume fraction of the granules in the micro-tissue. TS (MPa) means Tensile strength, El (%) means elongation at break, and then TS × El means anti-tension.

[Table 4-1]

[Table 4-2]

[Table 4-3]

[Table 5-1]

[Table 5-2]

[Table 5-3]

As shown in Table 5-1 to Table 5-3, it can be seen that, in particular, in the invention examples in which the composition, the structure, and the aggregate structure of steel are improved, the tensile strength is 1200 MPa or more, and the tensile modulus is 26000 (MPa). ‧%) or more, the r value in the rolling direction and the r value in the right direction of the rolling are all 0.80 or less, and the bending limit in the rolling direction and the bending limit in the right angle direction are both 2.0 or less. Therefore, it can be said that each of the invention examples is high in strength and excellent in ductility and flexibility.

In contrast, as shown in Tables 5-1 to 5-3, it can be seen that the conventional examples in which the composition, the structure, and the aggregate structure of steel are not improved in the scope of the present invention are anti-sedimentation, At least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction, the bending limit in the rolling direction, and the bending limit in the right angle direction are in a poor range. Industrial utilization

According to the present invention, the high-strength hot press-formed member exhibits a high level of both ductility and flexibility. Therefore, the present invention is particularly useful in the field of structural members for automobiles.

(no)

Fig. 1 is a view showing the position of the main crystal orientation on the ODF (φ2 = 45° section).

Claims (9)

A hot press-formed member comprising C: 0.100 to 0.600%, Si: 1.00 to 3.00%, Mn: 1.00 to 5.00%, P: 0.040% or less, and S: 0.050% or less in terms of unit mass%. Al: 0.001 to 2.000%, N: 0.0100% or less, O: 0.0100% or less, Mo: 0 to 1.00%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Nb: 0~0.300%, Ti: 0~0.300%, V: 0~0.300%, B: 0~0.1000%, Ca: 0~0.0100%, Mg: 0~0.0100%, and REM: 0~0.0100%, the rest It is made of iron and impurities; the micro-structure of 1/4 of the thickness of the plate contains: tempering Ma Tian bulk: 20~90%, metamorphic body: 5~75%, and residual Worth field : 5 to 25%, and the fat granules are limited to 10% or less; the extreme density of the {211}<011> orientation of the 1/4 portion of the plate thickness is 3.0 or more. The hot press-formed member according to claim 1, wherein the content per unit mass% is selected from the group consisting of Mo: 0.01 to 1.00%, Cr: 0.05 to 2.00%, Ni: 0.05 to 2.00%, and Cu: 0.05 to 2.00%. One or more of the groups. The hot press-formed member according to claim 1 or 2, wherein the unit of mass % is one group selected from the group consisting of Nb: 0.005 to 0.300%, Ti: 0.005 to 0.300%, and V: 0.005 to 0.300%. the above. The hot press formed member according to claim 1 or 2, wherein B: 0.0001 to 0.1000% is contained in terms of unit mass%. The hot press formed member according to claim 3, which contains B: 0.0001 to 0.1000% in terms of unit mass%. The hot press-formed member according to claim 1 or 2, wherein the unit is contained in a unit selected from the group consisting of Ca: 0.0005 to 0.0100%, Mg: 0.0005 to 0.0100%, and REM: 0.0005 to 0.0100%. the above. The hot-pressed molded member according to claim 3, which contains one or more selected from the group consisting of Ca: 0.0005 to 0.0100%, Mg: 0.0005 to 0.0100%, and REM: 0.0005 to 0.0100%, per unit mass%. The hot-pressed molded member according to claim 4, which contains one or more selected from the group consisting of Ca: 0.0005 to 0.0100%, Mg: 0.0005 to 0.0100%, and REM: 0.0005 to 0.0100%, per unit mass%. The hot-press molded member according to claim 5, which is one or more selected from the group consisting of Ca: 0.0005 to 0.0100%, Mg: 0.0005 to 0.0100%, and REM: 0.0005 to 0.0100%, in terms of unit mass%.