[0001]The present invention relates to a hot press forming member.
BACKGROUND
[0002]Door Guard, front side member, automobile members such as cross members and the side members, for improved fuel efficiency, weight reduction is demanded. As means for performing lighter, thinner materials can be considered. However, the automobile members are also required to have high strength. Therefore, as is crashworthiness by thinning or the like is sufficiently ensured, the steel sheet as a material of the member, the further higher strength have been developed. Specifically, the tensile product which is the product of the tensile strength and ductility, be improved Lankford value, and the limit bending has been attempted.
[0003]
　Automotive exemplified above members are often manufactured by hot pressing. Hot pressing technique is a technique of press molding after heating the steel to a high temperature of the austenite region, it is extremely small forming load than ordinary press working performed at room temperature. Further, in the hot press technique, since quenching in the same time the mold and press-molding is carried out, it is possible to impart high strength to the steel sheet. Thus, hot pressing technology has been receiving attention as a technique that can achieve both shape fixability and strength secured (e.g., see Patent Document 1).
[0004]
　However, member (hereinafter, simply referred to as "hot press forming member") that the steel sheet was processed by hot pressing technique, excellent although having strength, there is a case where ductility is not obtained sufficiently. When an automobile collision, by extreme plastic deformation occurs in automotive parts, it may undergo vigorous surface layer portion bending deformation of the hot press forming member. If ductility hot press forming member is insufficient, there is a possibility that cracking hot press forming member by the vigorous bending deformation occurs. That is, conventional hot press forming member, may not be exhibited excellent crashworthiness.
[0005]
　Meanwhile, TRIP (Transformed Induced Plasiticity) steel having excellent ductility by utilizing martensitic transformation of residual austenite is also known (see Patent Documents 2 and 3).
[0006]
　Generally, TRIP steel, by bainite transformation in the heat treatment, it is possible to include a stable retained austenite even at room temperature to that tissue. However, to promote high strength, since the bainite transformation is delayed, it takes a long time to generate the retained austenite. In this case, the productivity is significantly impaired. Further, when the holding time at bainite is insufficient, since unstable untransformed austenite becomes hard martensite at room temperature, and decreases the ductility and bendability members, thus sufficient collision characteristics obtained Never fear there is.
[0007]
　Known as the technique to promote bainite transformation, annealed steel in austenite single phase region and then cooled to a temperature between Ms point and Mf point, further reheated to 350 ° C. or higher 400 ° C. or less, holding a technique It is (for example, see non-Patent Document 1). According to this technique, it is possible to obtain a shorter period of time in a stable retained austenite.
[0008]
　Conventionally, TRIP steels, taking advantage of its excellent ductility, has been a cold formed steel plate. However, when producing member by cold forming, the residual ductility of the member after molding affects the collision properties of the member. Is a site that received high deformation during cold forming will residual ductility is reduced, there is a possibility that cracks are generated during a collision. Therefore, in recent years, even in the hot press forming method, by containing residual austenite in the steel sheet, a method of ensuring the ductility of the member has been proposed (e.g., see Patent Documents 4 6).
[0009]
　Patent Document 4, the hot press forming method, by the steel (Ms point -150) ° C. The average cooling rate to 40 ° C. and 5 ° C. / sec or less, the technique of including the retained austenite member It has been disclosed. However, only control of the cooling rate has been found to be ductile it is difficult to ensure a large can improve the amount of retained austenite.
[0010]
　Patent Document 5, the hot press forming method, after cooling to a (bainite transformation starting temperature Bs-100 ° C.) temperature range of more than Ms point or lower steel, technology to stay 10s or more at this temperature is disclosed . However, in this technique, slow bainite transformation rate is likely that residual austenite becomes hard martensite after cooling. When hard martensite is produced, the hardness difference between the tissue increases, it may not be exhibited excellent bendability.
[0011]
　Patent Document 6, in the hot press molding, after holding the steel temperatures below 1000 ° C. 750 ° C. or higher, cooled to a first temperature range of 50 ° C. or higher 350 ° C. or less, to partially martensitic transformation and then, by reheating to bainitic transformation to the second temperature range of 350 ° C. or higher 490 ° C. or less, a technique for obtaining a stable retained austenite is disclosed. However, even in this technique, it may be impossible to exhibit excellent bendability. It relates texture hot press before the steel sheet, because not defined in any way.
CITATION
Patent Document
[0012]
Patent Document 1: Japanese Patent 2002-18531 JP
Patent Document 2: Japanese Patent Laid-Open 1-230715 discloses
Patent Document 3: Japanese Patent Laid-Open 2-217425 discloses
Patent Document 4: Japanese Patent 2013-174004 JP
Patent Document 5: Japanese Patent 2013-14842 JP
Patent Document 6: Japanese Patent 2011-184758 JP
Non-patent literature
[0013]
Non-Patent Document 1: H. Kawata, K. Hayashi, N. Sugiura, N. Yoshinaga and M. Takahashi: Materials Science Forum, 638-642 (2010), p3307
Summary of the Invention
Problems that the Invention is to Solve
[0014]
　The present invention has been made in view of the above circumstances, and an object thereof is to provide a high strength hot press-forming member having excellent ductility and bendability. Specifically, the present invention is to provide a tensile product is 26000 (MPa ·%) or more, Lankford value in the rolling direction, and the direction perpendicular to the rolling direction (hereinafter, simply as "the direction perpendicular to the rolling direction" Lankford value when there is a) referred to is not less both 0.80 or less, and the critical bending and perpendicular to the rolling direction high strength hot press forming member limits bending which is 2.0 or less either in the rolling direction an object of the present invention is to provide. Below, there is a case in which simply the Lankford value is referred to as "r-value".
Means for Solving the Problems
[0015]
　The gist of the present invention is as follows.
[0016]
　(1) hot press forming member according to one embodiment of the present invention is a unit mass%, C: 0.100 ~ 0.600% , Si: 1.00 ~ 3.00%, Mn: 1.00 ~ 5.00%, P: 0.040% or less, S: 0.0500% or less, Al: 0.001 ~ 2.000%, N: 0.0100% or less, O: 0.0100% or less, Mo: 0 ~ 1.00%, Cr: 0 ~ 2.00%, Ni: 0 ~ 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 contained ~ 0.0100%, the balance being iron and impurities, the microstructure in the sheet thickness 1/4 parts of a unit volume%, tempered martensite: 20 to 90% bainite : 5 to 75%, and residual austenite: comprises 5-25%, and the ferrite is limited to 10% or less, {211} in the sheet thickness 1/4 parts <011> pole density of orientation of 3.0 or more it is.
(2) hot press forming member according to the above (1) is a unit mass%, Mo: 0.01 ~ 1.00% , Cr: 0.05 ~ 2.00%, Ni: 0.05 ~ 2.00%, and Cu: may contain one or more selected from the group consisting of 0.05 to 2.00%.
(3) hot press forming member according to the above (1) or (2) is a unit mass%, Nb: 0.005 ~ 0.300% , Ti: 0.005 ~ 0.300%, and V : it may contain one or more selected from the group consisting of 0.005 to 0.300%.
(4) above (1) hot press forming member according to any one of - (3) is a unit mass%, B: may contain 0.0001 to 0.1000 percent.
(5) above (1) hot press forming member according to any one of to (4), at unit mass%, Ca: 0.0005 ~ 0.0100%, Mg: 0.0005 ~ 0. 0100%, and REM: may contain one or more selected from the group consisting of 0.0005 to 0.0100 percent.
Effect of the invention
[0017]
　The high-strength hot press forming member according to the embodiment of the present invention, when adjusting the organization and components of the steel, particularly the structure of the steel and composite structure, and the improvement for the percentage of each tissue that make up the composite structure Is going. Furthermore, the high strength hot press forming member according to the embodiment of the present invention, it is preferably also controls pole density of steel. Thus, according to the high strength hot press forming member according to the embodiment of the present invention, not only the good strength by martensite in the composite structure is obtained, excellent bending by excellent ductility and bainite by austenite for even and sex, together it can be secured. As a result, the high-strength hot press forming member according to the embodiment of the present invention, r values ​​in the rolling direction and the r value in the rolling direction perpendicular to both a 0.80 or less, and, in the rolling direction critical bending and perpendicular to the rolling any direction limit bending can be 2.0 or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018]
FIG. 1 is a diagram illustrating the position of the major crystal orientation on ODF (φ2 = 45 ° cross section).
DESCRIPTION OF THE INVENTION
[0019]
　Hereinafter, an embodiment of a high-strength hot press forming member according to the present invention in detail. Note that the embodiments described below are not intended to limit the present invention. The constituent elements of the embodiments, those skilled in the art replaceable and easy ones, or substantially the same. Furthermore, various embodiments are encompassed by the embodiments of the following, a person skilled in the art can arbitrarily combined within obvious limits.
[0020]
　In members of this embodiment, "thickness 1/4 parts of the member", the surface of about 1/8 of the depth of the surface and about 3/8 of the depth from the rolling surface of the plate thickness of the members of the member It refers to the area between. The rolled surface of the member, a rolling surface of the hot press the material plate (cold-rolled steel sheet or annealed steel sheet) which is a material of the member. The "hot plate thickness 1/4 parts of the press element plate" hot press about 1/8 of the depth of the surface and about 3 of the plate thickness of the hot press raw sheets of rolled surface of the material plate It means the area between the depth of the surface of the / 8. The thickness of the member according to the present embodiment is not uniform in the regions that received the machining plate thickness is increased or decreased. Members, the plate thickness 1/4 parts of the region undergoing processing, a region corresponding to the plate thickness 1/4 parts of the hot press raw sheets before undergoing processing, certain possible on the basis of the cross-sectional shape is there.
[0021]
　The present inventors have made intensive studies to achieve the above object, in order to improve the ductility and bendability of hot press forming member, the steel structure of the predetermined component, and tempered martensite, retained austenite If, as a composite structure including a bainite, further, it is important to set the percentage of each tissue as appropriate to give the knowledge that. More specifically, the present inventors have found that, in the hot press forming, by forming a steel plate of a predetermined component at elevated temperature, by going through a process to hold reheated after one o'clock cooling, martensite in the composite tissue not only excellent strength can be obtained by the site, also the excellent bendability by excellent ductility and bainite by austenite, together is ensured, as a result, Lankford value in the rolling direction (r value) and the direction perpendicular to the rolling direction of the r value was either 0.80 or less, and can be a limitation of the rolling direction bending and the direction perpendicular to the rolling direction limit bending both 2.0 or less, to obtain a knowledge that.
[0022]
　Lankford value (r value), specified in JISZ2254, caused by the addition of uniaxial tensile stress in the plate-shaped tensile test pieces, strain true width direction of the test piece epsilon b and thickness direction true strain epsilon a the ratio epsilon of the b / epsilon a is. The r values in the rolling direction, a r value obtained by adding the uniaxial tensile stress in a direction parallel to the rolling direction, and the r value in the rolling direction perpendicular tensile uniaxial in a direction perpendicular to the rolling direction stress a r value obtained by adding.
[0023]
　
　Hereinafter, an embodiment of a high strength hot press forming member according to the present embodiment in detail.
[0024]
[Ingredients]
　First, the high strength hot press forming member according to the present embodiment (hereinafter, sometimes referred to as members) of, explaining the reasons for limitation of the ingredients. In this specification, the unit of chemical components "%" means "% by mass".
[0025]
　(C: 0.100 ~ 0.600%)
　carbon (C) raises the strength of the member, and is an essential element for ensuring the retained austenite less than a predetermined amount. When C content is less than 0.100%, it is difficult to ensure the tensile strength and ductility of the member. On the other hand, when the C content exceeds 0.600%, ensuring spot weldability member becomes difficult, a possibility that further ductility of members is reduced. For the above reasons, C content should be 0.100 to .600 percent. The lower limit of the C content is preferably 0.150%, 0.180%, or 0.200%. Upper limit of the C content is preferably 0.500%, the 0.480%, or 0.450%.
[0026]
　(Si: 1.00 ~ 3.00%)
　Silicon (Si) is strengthening element and is effective in increasing the strength of the member. Further, Si suppresses the precipitation and coarsening of cementite during martensite, thereby contributing to the improvement of high strength and bending resistance of the member. Furthermore, Si is to increase the C concentration in the austenite, it contributes to secure a predetermined amount or more of retained austenite, thus, in an element contributing to the suppression of the precipitation of cementite during reheating retention after temporarily cooling the member is there.
[0027]
　Si content is less than 1.00%, the above effect (strengthening of steel and cementite precipitation inhibition, etc.) can not be sufficiently obtained. On the other hand, when the Si content exceeds 3.00%, workability of the member is reduced. For the above reasons, Si content should be 1.00 to 3.00%. The lower limit of the Si content is preferably 1.10%, 1.20%, or 1.30%. Upper limit of the Si content is preferably 2.50%, 2.40%, or 2.30%.
[0028]
　(Mn: 1.00 ~ 5.00%)
　manganese (Mn) are strengthening element and is effective in increasing the strength of the member. The Mn content is less than 1.00%, during the cooling of the member, ferrite, pearlite and cementite will generate, it is difficult to increase the strength of the member. On the other hand, when the Mn content exceeds 5.00%, it tends to occur co segregation of Mn and P and S, the workability of the member is significantly reduced. For the above reasons, Mn content should be 1.00 to 5.00%. The lower limit of the Mn content is preferably 1.80%, the 2.00% or 2.20%. Upper limit of the Mn content is preferably 4.50% is 4.00%, or 3.50%.
[0029]
　(P: 0.040% or less)
　phosphorus (P) is the plate thickness center part (depth of the depth of the surface about the rolling plane of the plate thickness of members 3/8 and about 5/8 of the steel plate constituting the member tend to segregate to the region) between the surface, it is an element that embrittle the weld formed during the welding member. Since embrittlement of the welded portions P content exceeds 0.040% is significant, P content is 0.040% or less. A preferable upper limit of the P content is 0.010% 0.009% or 0.008%. Further, since it is not necessary to define particularly the lower limit of the P content, the lower limit of the P content may be 0%. However, since making the P content less than 0.0001% is economically disadvantageous, the lower limit of the P content may be 0.0001%.
[0030]
　(S: 0.0500% or less)
　sulfur (S) is a welding of member is adversely affect element and the casting of a steel sheet and hot rolling of manufacturability constituting the member. Further, S is, to form a coarse MnS, is an element that inhibits bending of the member and hole expandability and the like. When S content exceeds 0.0500%, the above adverse effects and inhibition becomes remarkable, S content is at most 0.0500%. A preferable upper limit of the S content is 0.0100%, 0.0080%, or 0.0050%. Further, since it is not necessary to define particularly the lower limit of S, the lower limit of the S content may be 0%. However, since making the S content is less than 0.0001% is economically disadvantageous, the lower limit of the S content may be 0.0001%.
[0031]
　(Al: 0.001 ~ 2.000%)
　Aluminum (Al), like Si, is an element effective in the precipitation and the suppression of coarsening like cementite. Further, Al is also possible utilization element as a deoxidizer. The Al content is less than 0.001%, the above effect is not exhibited. On the other hand, when the Al content exceeds 2.000%, the number of Al-based coarse inclusions increases, causing causes the steel sheet bending deterioration, and the scratches on the surface of the steel sheet occur. For the above reasons, Al content is set to 0.001 to 2.000%. The lower limit of the Al content is preferably 0.010% 0.020% or 0.030% or. Upper limit of the Al content is preferably 1.500%, the 1.200% 1.000% 0.250% or 0.050%.
[0032]
　(N: 0.0100% or less)
　nitrogen (N) forms a coarse nitride is an element that reduces the bendability and hole expandability of the member. Further N is an element that causes blowhole occurrence during member welding. When N content exceeds 0.0100% bendability and hole expandability reduction in member not only it becomes significant, since a large number of blowholes are generated during welding of the member, the N content 0. to 0100% or less. A preferable upper limit of the N content is 0.0070%, 0.0050%, or 0.0030%. The lower limit of the N content, especially since there is no need to determine may be 0%. However, the content of N to less than 0.0005%, so leads to a significant increase in production cost, the lower limit of the N content may be 0.0005%.
[0033]
　(O: 0.0100% or less)
　of oxygen (O) forms an oxide, elongation at break of the members is an element to reduce the bending resistance, and hole expandability, and the like. In particular, it invited the oxides are present as inclusions punched end face or cut surface of the member, oxides form a notch-like flaws and coarse dimples or the like, the stress concentration during hole expansion or strong working or the like There is generated the crack, greatly reduce the hole expandability and / or bendability.
[0034]
　When O content exceeds 0.0100% elongation at break, since lowering of such properties expandability bendability 及穴 it becomes remarkable, O content is 0.0100% or less. A preferable upper limit of the O content is 0.0050%, 0.0040%, or 0.0030%. Further, since the lower limit of the O content need not specifically defined, and may be 0%. However, to the O content is less than 0.0001% can lead to excessive cost, since economically undesirable, the lower limit of O content may be 0.0001%.
[0035]
　The high strength hot press forming member according to the present embodiment, in addition to the above components, Mo: 0.01 ~ 1.00%, Cr: 0.05 ~ 2.00%, Ni: 0.05 ~ 2 .00%, and Cu: may contain one or more selected from the group consisting of 0.05 to 2.00%. However, these elements are not an essential component. Since these elements members of the present embodiment, even if not contained can solve the problems, the lower limit of the content of these elements is 0%.
[0036]
　(Mo: 0 ~ 1.00%)
　molybdenum (Mo) is strengthening element, an element which contributes to improving hardenability of steel plates constituting the members. In order to obtain this effect, the lower limit of the Mo content may be 0.01%. On the other hand, if the Mo content is more than 1.00%, the productivity in manufacturing the steel sheet and hot rolling is inhibited. For the above reasons, Mo content is preferably not more than 1.00% 0.01%. Still further preferred lower limit of the Mo content is 0.05%, 0.10%, or 0.15%. A further preferred upper limit of the Mo content 0.60% 0.50%, or 0.40%.
[0037]
　(Cr: 0 ~ 2.00%)
　of chromium (Cr) is a strengthening element, an element which contributes to improving hardenability of steel plates constituting the members. In order to obtain this effect, the lower limit of the Cr content may be 0.05%. On the other hand, when the Cr content exceeds 2.00%, the productivity in manufacturing the steel sheet and hot rolling is inhibited. For the above reasons, Cr content is preferably not more than 2.00% 0.05% or more. Still further preferred lower limit of the Cr content is 0.10%, 0.15%, or 0.20%. A further preferred upper limit of the Cr content is 1.80%, 1.60%, or 1.40%.
[0038]
　(Ni: 0 ~ 2.00%)
　nickel (Ni) is strengthening element, an element which contributes to improving hardenability of steel plates constituting the members. Further, Ni is an element which contributes to the promotion of improvement in wettability of the steel sheet and the alloying reaction. In order to obtain these effects, the lower limit of the Ni content may be 0.05%. On the other hand, when the Ni content exceeds 2.00%, the productivity in manufacturing the steel sheet and hot rolling is inhibited. For the above reasons, Ni content is preferably not more than 2.00% 0.05% or more. Still further preferred lower limit of the Ni content is 0.10%, 0.15%, or 0.20%. A further preferred upper limit of the Ni content is 1.80%, 1.60%, or 1.40%.
[0039]
　(Cu: 0 ~ 2.00%)
　copper (Cu) is a strengthening element, an element which contributes to improving hardenability of steel plates constituting the members. Further, Cu is an element contributing to the promotion of improvement in wettability of the steel sheet and the alloying reaction. In order to obtain these effects, the lower limit of Cu content may be 0.05%. On the other hand, when the Cu content exceeds 2.00%, the productivity in manufacturing the steel sheet and hot rolling is inhibited. For the above reasons, Cu content is preferably set to 2.00% to 0.05%. Still further preferred lower limit of the Cu content is 0.10%, 0.15%, or 0.20%. A further preferred upper limit of the Cu content is 1.80%, 1.60%, or 1.40%.
[0040]
　Furthermore, the high strength hot press forming member according to the present embodiment, in addition to the above components, Nb: 0.005 ~ 0.300%, Ti: 0.005 ~ 0.300% and V: 0.005 ~ 0 .300% of may contain at least one. However, these elements are not an essential component. Since these elements members of the present embodiment, even if not contained can solve the problems, the lower limit of the content of these elements is 0%.
[0041]
　(Nb: 0 ~ 0.300%)
　niobium (Nb) are strengthening element, precipitation strengthening, fine grain strengthening by ferrite grain growth inhibition, and the dislocation strengthening by inhibiting recrystallization, increases the strength of the member an element which contributes to. In order to obtain these effects, the lower limit of the Nb content may be 0.005%. On the other hand, when the Nb content exceeds 0.300% carbonitrides are excessively precipitated moldability member may be reduced. For the above reasons, the content of Nb is preferably set to 0.300% or less than 0.005%. Still further preferred lower limit of the Nb content is 0.008% or 0.010%, or 0.012%. A further preferred upper limit of the Nb content is 0.100% 0.080% or 0.060%.
[0042]
　(Ti: 0 ~ 0.300%)
　titanium (Ti) is a strengthening element, precipitation strengthening, fine grain strengthening by ferrite grain growth inhibition, and the dislocation strengthening by inhibiting recrystallization, increases the strength of the member an element which contributes to. In order to obtain these effects, the lower limit of the Ti content may be 0.005%. On the other hand, if the Ti content exceeds 0.300%, the carbonitrides decreases the moldability of excessively precipitated members. For the above reasons, Ti content is preferably 0.300% or less than 0.005%. Still further preferred lower limit of the Ti content is 0.010%, 0.015%, or 0.020%. A further preferred upper limit of the Ti content is 0.200% 0.150% or 0.100%.
[0043]
　(V: 0 ~ 0.300%)
　vanadium (V) is a strengthening element, precipitation strengthening, fine grain strengthening by ferrite grain growth inhibition, and the dislocation strengthening by inhibiting recrystallization, increases the strength of the member an element which contributes to. In order to obtain these effects, the lower limit of the V content may be 0.005%. On the other hand, when the V content exceeds 0.300%, the carbonitrides decreases the moldability of excessively precipitated members. For the above reasons, V content is preferably 0.300% or less than 0.005%. Still further preferred lower limit of the V content is 0.010%, 0.015%, or 0.020%. A further preferred upper limit of the V content is 0.200% 0.150% or 0.100%.
[0044]
　Furthermore, the high strength hot press forming member according to the present embodiment, in addition to the above components, B: it may contain 0.0001 to 0.1000%. However, B is not an essential component. Since B is member of the present embodiment, even if not contained can solve the problems, the lower limit of the B content is 0%.
[0045]
　(B: 0 ~ 0.1000%)
　boron (B) is an element effective for improving the strength of the grain boundaries and high strengthening of the steel and the like. In order to obtain these effects, the lower limit of the B content may be 0.0001%. On the other hand, when the B content exceeds 0.1000%, the effect described above not only saturated, manufacturability of the hot rolling of the steel sheet is inhibited. For the above reasons, B content is preferably not more than 0.1000% 0.0001% or more. Still further preferred lower limit of the B content 0.0003% 0.0005%, or 0.0007%. A further preferred upper limit of the B content is 0.0100%, 0.0080%, or 0.0060%.
[0046]
　In addition, high-strength hot press forming member according to the present embodiment, in addition to the above components, Ca: 0.0005 ~ 0.0100%, Mg: 0.0005 ~ 0.0100%, and REM: 0.0005 ~ 0.0100% of may contain at least one. However, these elements are not an essential component. Since these elements members of the present embodiment, even if not contained can solve the problems, the lower limit of the content of these elements is 0%.
[0047]
　(Ca:
　0 ~
　0.0100%) (Mg: 0 ~ 0.0100%) (REM: 0 ~
　0.0100%) Ca, Mg, and REM (Rare Earth Metal) is effective for deoxidation of steel it is an element. To obtain this effect, 0.0005% or more Ca, 0.0005% or more Mg, and one or more selected from the group consisting of 0.0005% or more REM may be contained in the member. On the other hand, Ca, when Mg, and REM each content exceeds 0.0100% workability member is inhibited. For the above reasons, Ca, Mg, and REM content, respectively, it is preferably 0.0100% or less than 0.0005%. Incidentally, Ca content, Mg content, and more preferred lower limit of the REM content, respectively, 0.0010% 0.0020%, or 0.0030%. Ca content, Mg content, and more preferably the upper limit of the REM content, respectively, 0.0090%, 0.0080%, or 0.0070%. Further, Ca, Mg, and when two or more selected from the group consisting of REM is contained in member, Ca, Mg, and the total content of REM is 0.0010% or more 0.0250% or less there it is preferable.
[0048]
　Incidentally, the term "REM" refers to a total of 17 elements consisting of Sc, Y and lanthanoid, The "content of REM" means the total content of these 17 elements. REM may be added in misch form of metal (alloy contains more than one rare earth element). Misch metal, in addition to La and Ce, there is a case containing an element of the lanthanide series. High strength hot press forming member according to the present embodiment, as impurities, it may contain elements of lanthanoid series other than La and Ce. The high strength hot press forming member according to the present embodiment, within the range not inhibiting the various properties of the member (in particular, ductility and bendability) may contain La or Ce.
[0049]
　: (Balance iron and impurities)
　of the chemical components of the member according to the present embodiment the balance includes iron and impurities. The impurity components contained in the raw material member, or a component to be mixed in the manufacturing process of the member, means the ingredients which do not affect the properties of the member. Specifically, P, S, O, Sb , Sn, W, Co, As, Pb, Bi , and H, and the like as impurities. Of these, P, S and O, it is necessary to control as described above. Further, according to the conventional manufacturing method, Sb, Sn, W, Co, and As 0.1% is less, Pb and Bi 0.010% below, H is as an impurity in the range of 0.0005% It can be incorporated in the steel material, but if it is within this range, there is no particular need to control the content of these elements.
[0050]
　Further, a component of high strength cold rolled steel sheet of the present embodiment, Si, Al, Cr, Mo, V, Ca also, may be incorporated as impurities unintentionally. However, these components as long as it is within the range described above, does not in various properties of a high strength hot press forming member according to the present embodiment a negative impact. Further, N is generally, it is to be treated as an impurity in the steel, the member according to the present embodiment form, it is preferably controlled within the above range.
[0051]
　[Microstructure]
　Next, the high-strength hot press forming member according to the present embodiment, the reasons for limitation of the microstructure will be described. In this specification, the unit "%" percentage of each tissue means "volume fraction (vol%)". Further, the microstructure of the member according to the present embodiment are defined in the 1/4 parts of the member. 1/4 parts located between the rolling surface and the center plane is because with a typical configuration of a member. In this specification, unless otherwise specified, description of microstructure relates microstructure in 1/4 parts. Moreover, members of the present embodiment has a portion that does not receive a portion undergoing machining, both microstructure is substantially the same.
[0052]
　(Tempered martensite: 20 to 90%) of
　tempered martensite is a structure to strengthen the steel, a tissue to be included in order to ensure the strength of the member according to the present embodiment. The volume fraction of tempered martensite is less than 20%, the strength of the member is insufficient. On the other hand, if the volume fraction of tempered martensite is more than 90%, insufficient bainite and austenite necessary in order to ensure ductility and bendability members. For the above reasons, the volume fraction of tempered martensite is 20% or more 90% or less. A preferable lower limit of the volume fraction of tempered martensite is 25%, 30%, or 35%. Preferred upper limit of the volume fraction of tempered martensite is 85%, 80%, or 75%.
[0053]
　(Bainite: 5 to 75%)
　bainite is an important organization in order to improve the bending properties of the member. Usually, if the member has a structure consisting of residual austenite having excellent hard martensite and ductility due to the difference in hardness between the martensite and residual austenite, the stress concentration to martensite occurs during deformation of the member. This stress concentration, the void is formed at the interface between the martensite and residual austenite, As a result, there is a fear that bending of member is reduced. However, if the member has a tissue containing bainite in addition to martensite and retained austenite, bainite reduce the tissue between hardness difference, thereby, is alleviated stress concentration to martensite, the bending of the member There is improved.
[0054]
　The volume fraction of bainite is less than 5%, the stress concentration to martensite is not sufficiently relaxed, can not be realized ensuring excellent bendability. On the other hand, if the volume fraction of bainite is more than 75%, insufficient martensite and retained austenite required to secure the strength and ductility of the member. For the above reasons, the volume fraction of bainite is 75% or less than 5%. A preferable lower limit of the volume fraction of bainite is 10%, 15%, or 20%. Preferred upper limit of the volume fraction of bainite is 70%, 65%, or 60%.
[0055]
　(Residual austenite: 5-25%)
　retained austenite is an important organization in order to ensure the ductility of the member. It retained austenite, by transformation into martensite at the time of press molding of steel sheets, resulting in excellent work hardening and high uniform elongation in the steel sheet. Uniform elongation volume fraction less than 5% residual austenite can not be sufficiently obtained, is difficult to secure excellent moldability. On the other hand, the volume fraction of residual austenite is more than 25%, insufficient and martensite and bainite necessary to ensure strength and hole expandability of the steel sheet. For the above reasons, the volume fraction of residual austenite is 25% or less than 5%. A preferable lower limit of the volume fraction of residual austenite is 7%, 10%, or 12%. Preferred upper limit of the volume fraction of residual austenite is 22%, 20%, or 18%.
[0056]
　(Ferrite: 0-10%)
　ferrite are the soft tissue, it is preferably as small as possible the volume fraction. Therefore, the lower limit of the volume fraction of ferrite is 0%. When the volume fraction of ferrite exceeds 10%, it is difficult to secure the strength of the steel sheet. Therefore, the volume fraction of the ferrite is limited to 10% or less. A preferable upper limit of the volume fraction of the ferrite is 8%, 5%, or 3%.
[0057]
　Incidentally, tempered martensite, bainite, residual austenite, and the ferrite, identification, confirmation of the location and measurement of the volume fraction, nital reagent and Repera solution, and, picric acid, ethanol, sodium thiosulfate, citric acid , and the etching solution (pre-treatment liquid) of a mixed solution of nitric acid, and using an etching solution comprising a mixed solution of nitric acid and ethanol (post-treatment liquid), the cross section perpendicular to the parallel and rolled surface in the rolling direction of the steel sheet or corrode the cross section perpendicular to the rolling direction and rolling surfaces of the steel sheet, it is done by observing with an optical microscope and 1000-100000 fold scanning electron microscopy and transmission electron microscopy sectional 1,000 times after the corrosion can.
[0058]
　Tempering the identification of martensite performing sectional observation by a scanning electron microscope and transmission electron microscope, regarded as tempered martensite martensite containing carbides (Fe-based carbides) containing a large amount of Fe in the interior of the carbides, martensite that does not include the carbide was considered a normal martensite that has not been tempered (fresh martensite). The carbide containing a large amount of Fe, there are carbides of various crystal structures, even martensite containing Fe-based carbides of any crystal structure, and those falling under the tempered martensite in the present embodiment . Further, the tempered martensite in the present embodiment, due to the heat treatment conditions, Fe-based carbides multiple species also include those mixed.
[0059]
　Further, tempered martensite, bainite, residual austenite, and the identification of ferrite, field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope) to the supplied EBSD: using Electron Back-Scatter Diffraction crystals analysis and crystal orientation by orientation analysis method (FE-SEM-EBSD method), it is possible by the hardness measurement of micro regions such as a micro Vickers hardness measurement.
[0060]
　For example, the time of confirmation of the volume fraction of retained austenite metal structure (%) is parallel to the rolling surface of the member, of about 1/4 of the depth position of the sheet thickness surface (the rolling surface of the member of the member it may be subjected to X-ray analysis as a viewing surface of about 1/4 of the depth of the surface) thickness. The area fraction of retained austenite obtained by this, the volume fraction of retained austenite.
[0061]
　In contrast, bainite metal structure, tempered martensite, and upon confirmation of the volume fraction of ferrite (%) is a cross-section perpendicular to and parallel rolling surface to the rolling direction of the first steel sheet (observation surface), polished , etched with a nital solution. Then, the plate thickness 1/4 parts in etched cross section, by observing with FE-SEM, measuring the area fraction of each tissue. Area fraction obtained in this case, since a value substantially equal the volume fraction, deemed the area fraction and volume fraction.
[0062]
　In the observation with the FE-SEM, for example, one side of each tissue in the observation plane of 30μm square, distinguished as follows, can be recognized. That is, the tempered martensite is a set of crystal grains (plate shape having a certain preferential growth direction) lath comprises crystal grains inside the above-described long diameter 20nm or more iron-based carbides, and the carbides several variants (i.e., a different direction) can organize the recognition belonging to a plurality of iron-based carbides group that has been stretched. Bainite is a set of lath-like grains, which does not contain crystal grains inside diameter 20nm or more iron-based carbides, or inside of the crystal grains including major axis 20nm or more iron-based carbides, the carbides There can be recognized and organizations belonging to the iron-based carbides group that has been stretched in a single variant (same direction). Here, the iron-based carbides group that has been stretched in the same direction, means a direction of elongation of the difference in the iron-based carbides group is within 5 °. Ferrite is a grain bulk, can be recognized within the grain, and the tissue that does not contain more iron-based carbide diameter 100 nm.
[0063]
　Incidentally, by using the FE-SEM observation of the iron-based carbides within the lath-shaped crystal grains by examining the direction of elongation, it is possible to easily distinguish the tempered martensite and bainite.
[0064]
[{211} in the sheet thickness 1/4 parts <011> pole density orientation]
　Next, the high-strength hot press forming member according to the present embodiment will be described the reasons for limitation of the pole density. Incidentally pole density of member according to the present embodiment has an exemplary configuration of the member, it is defined in the 1/4 parts of the member. In this specification, unless otherwise specified, the description of the electrode density for electrode density at 1/4 parts. Moreover, members of the present embodiment has a portion that does not receive a portion undergoing machining, electrode density at both are substantially the same.
[0065]
　If the pole density of {211} <011> orientation in the thickness 1/4 parts of the hot press member is less than 3.0, the rolling direction of the r value, and both the r value in the rolling direction perpendicular 0. can not be 80 or less, the bending property is deteriorated. Therefore, the pole density of {211} <011> orientation of the sheet thickness 1/4 parts to 3.0 or more. The lower limit of {211} <011> orientation of the pole density of the sheet thickness 1/4 parts, preferably 4.0 or 5.0. The upper limit of the {211} <011> orientation of the pole density of the sheet thickness 1/4 parts is not particularly defined. However, if the electrode density of the {211} <011> orientation of the sheet thickness 1/4 parts exceeds 15.0, the workability of the member may be deteriorated, the plate thickness 1/4 parts {211 } <011> orientation of the pole density 15.0 or less, or may be 12.0 or less.
[0066]
　The electrode density, for a standard sample having no accumulation in specific orientation is the ratio of the integration to a specific orientation of the specimen. Pole density of {211} <011> orientation in the thickness 1/4 parts of the member according to the present embodiment, EBSD (electron back scattering pattern: Electron Back Scattering Diffraction pattern) is measured in method.
[0067]
　Measurements pole density using EBSD is conducted as follows. And observation surface a cross section perpendicular to the parallel and rolled surface in the rolling direction of the member. The observation plane, the center lines of 1/4 depth of thickness t from the surface of the member, with respect to the rectangular region of 100μm to 1000μm and the rolling surface normal direction to the rolling direction, EBSD analysis measurement interval 1μm and carrying out, to obtain the crystal orientation information of the rectangular region. EBSD analysis, using a thermal field emission scanning electron microscope (e.g., manufactured by JEOL JSM-7001F) and EBSD detector (e.g. TSL manufactured HIKARI detector) and constructed from device, analyzing the speed of 200 to 300 points / sec in implementing. From the crystal orientation information of the rectangular region, using EBSD analysis software "OIM Analysis" (registered trademark), and calculates the ODF (Orientation Distribution Function) of the rectangular region. Accordingly, since it is very density of each crystal orientation, it is possible to find the {211} <011> orientation pole density in a plate thickness 1/4 parts of the member.
　Figure 1 is a diagram illustrating the position of the major crystal orientation on ODF (φ2 = 45 ° cross section). Usually, displayed in notation perpendicular crystal orientation as (hkl) or {hkl} in the rolling surface, it displays a crystal orientation parallel to the rolling direction denoted with [uvw] or . {Hkl} and is a generic term for equivalent planes and orientation, (hkl) and [uvw] represents an individual crystal faces.
[0068]
　The crystal structure of members of this embodiment are mainly body-centered cubic structure (bcc structure). Therefore, for example, (111), (- 111), (1-11), (11-1), (- 1-11), (- 11-1), (1-1-1), (- 1 -1-1) is substantially equivalent, indistinguishable. In this embodiment, indicated as they are collectively orientation {111}.
[0069]
　Incidentally, ODF is also used to display the crystal orientation of the low symmetry crystal structure. Generally, φ1 = 0 ~ 360 °, Φ = 0 ~ 180 °, are displayed in φ2 = 0 ~ 360 °, each crystal orientation is displayed in (hkl) [uvw]. However, the crystal structure of the hot-rolled steel sheet of the present embodiment is a highly symmetrical body-centered cubic structure. Therefore, the Φ and φ2 can be displayed at 0 ~ 90 °.
[0070]
　φ1 is in its calculation, varies whether symmetry considerations due to deformation. In the present embodiment, and perform the calculations considering symmetry (ORTHOTROPIC), displays at φ1 = 0 ~ 90 °. That is, the measurement of very Density member according to the present embodiment, the average value of the same orientation at .phi.1 = 0 ~ 360 °, to select the method of displaying the 0-90 on ° the ODF. In this case, it is synonymous with the (hkl) [uvw] and {hkl} . Accordingly, it is shown in FIG. 1, the ODF in .phi.2 = 45 ° cross section (112) [1-10] electrode density orientation (φ1 = 0 °, Φ = 35 °), the {211} <011> orientation pole density of as synonymous.
[0071]
　As shown above, the components of the high-strength hot press forming member, tissue, and by adjusting the electrode density, as the tensile product of the member 26000 (MPa ·%) or more, excellent ductility, thus excellent it is possible to realize a member having a fatigue resistance and durability. Further, the above adjustment, both the r value in the rolling direction perpendicular rolling direction r value and the members of the members and 0.80 or less, and, in the direction perpendicular to the rolling direction of the rolling direction of the critical bending and members of the members limits bending both the 2.0 or less, it is possible to realize a member having excellent bendability.
[0072]
　Note that when impacted, the thickness direction of the deformation as r value is low is promoted, it is possible to prevent the bending cracks. Generally, when the vertical direction of the r values ​​in the ridge line direction of the bending is 0.80 or less, the effect of preventing the bending cracking is exhibited at a high level. The high-strength hot press forming member according to the present embodiment, since both the r value and the direction perpendicular to the rolling direction of the r values ​​in the rolling direction is 0.80 or less, even when subjected to members a large bending deformation at the time of collision, member it can exhibit an excellent bending property.
[0073]
　
　Next, a method for manufacturing a high strength hot press forming member according to the present embodiment in detail. Method for producing the high-strength hot press forming member, the hot-press element plate is cold-rolled steel sheet or annealed steel sheet made from the above chemical composition, the maximum heating temperature Ac 3 heating above points, a heating step , concurrently with providing hot press molding hot press raw sheets, cooled to a temperature range of (Ms point -250 ° C.) or higher Ms point, sequentially and hot press-forming and cooling process, as essential steps do. A method for manufacturing a high strength hot press forming member of the present embodiment, the these steps separately, after the hot press forming and cooling process, members reheated to a temperature range of 300 ~ 500 ° C., then, after the member and held for 10 to 1000 seconds in reheating temperature region, cooling the member to room temperature, reheating step optionally. Hereinafter, the respective steps will be described. In the following, takes place before the heating step, it referred together also preparation step of hot pressing raw sheets.
　The "heating rate" and "cooling rate" in the description of the manufacturing method of the member according to the present embodiment, means (instantaneous velocity at time t) dT / dt obtained by differentiating the temperature T at time t. For example, reference to "a heating rate in the temperature range from A ° C. to B ° C. to X ~ Y ° C. / sec" is always X ~ temperature T is dT / dt during which changes from A ° C. to B ° C. It means within a range of Y ° C. / sec.
[0074]
　(Preparation step of hot pressing raw sheets)
　This step is to obtain a hot-press workpieces to be subjected to a heating step to be described later (cold rolled steel plate or annealed steel sheet), a preparation step. Each manufacturing process preceding the casting is not particularly limited. That is, the blast furnace, subsequent to melting by electric furnace or the like, may be subjected to various secondary refining. Cast slab, after once cooled to a low temperature, it may be hot rolled after reheating, successively (i.e., without cooling and reheating) it may be hot rolled. In hot rolling, the total rolling reduction in the temperature range at 920 ° C. or less be 25% or more is important. The reason for this is as follows.
(1) 920 ° C. than rolling in the temperature region of, since recrystallization progresses in the fancy time until the rolling or during subsequent rolling, difficult to accumulate strain to the steel, as a result, sufficient for the formation of texture there is a possibility that does not contribute.
(2) When the total rolling reduction at 920 ° C. below the temperature region is less than 25%, the crystal rotation effect by rolling is not sufficiently obtained, it is likely that texture is not sufficiently formed.
[0075]
　For these reasons, the total rolling reduction in the temperature region at 920 ° C. or less be 25% or more is important. The total reduction ratio in the temperature region at 920 ° C. or less, preferably 30% or more, more preferably 40% or more. On the other hand, the upper limit of the total reduction ratio in a temperature region at 920 ° C. or less, it is desirable that 80%. Giving a reduction of 80% can lead to increased load on the rolling roll, is because the influence on the durability of the rolling mill. Incidentally, as a raw material for hot press material plate can also be used scrap.
[0076]
　Further, as the cooling conditions after hot rolling, so that achieve the respective effects of the member according to the present embodiment (excellent ductility and bendability), it may be employed a cooling pattern for performing tissue control.
[0077]
　Coiling temperature is preferably set to 650 ° C. or less. When winding the hot-rolled steel sheet at temperatures above 650 ° C., the thickness of the oxide formed on the hot-rolled steel sheet surface becomes excessively large, pickling is poor. Incidentally, the coiling temperature is more preferably set to 600 ° C. or less. The temperature range of 600 ° C. or less, the likely cause bainite transformation. By mainly bainite hot rolled sheet structure, texture formation during cold rolling followed by a sufficiently performed, it is easy to obtain and thus the r value of the objective.
[0078]
　Each effect of the member according to the present embodiment without limiting particularly also the lower limit of the coiling temperature (excellent ductility and bendability) is exerted. However, since it is technically difficult to wind the hot-rolled steel sheet at a temperature below room temperature, room temperature is a substantial lower limit of the coiling temperature. However, the coiling temperature is lower than 350 ° C., in hot rolled sheet structure, the proportion of hard martensite becomes large, since the cold rolling is difficult, the coiling temperature is preferably set to 350 ° C. or higher.
[0079]
　In this way, hot-rolled steel sheet was prepared, subjected to pickling. Pickling number of times is not particularly defined.
[0080]
　The pickled hot-rolled steel sheet, cold rolling at a total reduction of 50 to 90% and hot pressing raw sheets. To r value in the rolling direction of the high-strength hot press forming member according to the present embodiment and the r value in the rolling direction perpendicular both 0.80 or less, the thickness of the hot-press workpieces 1/4 must {211} <011> pole density of orientation is 3.0 or more in parts. Pole density of {211} <011> orientation in the thickness 1/4 parts of the hot press material plate is preferably 4.0 or more, more desirably 5.0 or more. When the total rolling reduction of cold rolling is less than 50%, since the pole density of {211} <011> orientation in the thickness 1/4 parts of the hot-press material plate is less than 3.0, member the texture can not be controlled as described above, it is difficult to secure the r value of the objective.
　On the other hand, when the total rolling reduction of cold rolling is more than 90%, too high driving force of recrystallization, ferrite resulting in recrystallization during the heating step of hot press which will be described later. In the heating step of hot press which will be described later, the hot-press workpiece is Ac 3 but is heated to a temperature above points, Ac 3 non-recrystallized ferrite to hot pressing raw sheets may remain until reaching the point it is necessary that is. When the total reduction ratio of cold rolling exceeds 90%, this condition can not be achieved. In addition, when the total rolling rate exceeds 90%, cold-rolled becomes difficult cold rolling load becomes too large. The total rolling reduction rate r of cold rolling, the sheet thickness after the end of the cold rolling h 1 and (mm), a plate thickness before the start of cold rolling h 2 substitutes and (mm)
　= R (H 2-H 1 ) / H 2 ...... (Formula 1)
[0081]
　For the above reasons, the total rolling reduction of cold rolling of pickled hot-rolled steel sheet is 90% or less than 50%. Incidentally, preferable range of the total rolling reduction of cold rolling is 80% or less than 60%. Moreover, not particularly limited for rolling reduction in the number of rolling passes, and each path.
[0082]
　Moreover, what was annealed steel sheet is subjected a heat treatment (annealing) in cold-rolled steel sheet obtained by cold rolling mentioned above, may be hot-press element plate. Heat treatment is not particularly limited, it is performed by a method of Tsuban the continuous annealing line may be performed by batch annealing. During the heat treatment, 500 ° C. or higher Ac 1 needs to be a heating rate above 10 ° C. / sec in the temperature range of points. If the heating rate is less than 10 ° C. / sec, texture of the finally obtained molded article may not be preferably controlled. However, if the sum of Ti content and the Nb content of the steel sheet is not less than 0.005 wt%, 500 ° C. or higher Ac 1 may be any heating rate in the temperature range of points always 3 ° C. / sec or more.
[0083]
　Annealing temperature Ac 1 Ac or point 3 is preferably not more than points. Annealing temperature Ac 1 is less than point, since recrystallization of ferrite will progress. On the other hand, the annealing temperature is Ac 3 exceeds point, steel sheet is an austenite single-phase structure, it is difficult to leave the non-recrystallized ferrite. In any case, the heating step of hot pressing, hot pressing raw sheets is Ac 3 to leaving the non-recrystallized ferrite to hot pressing raw sheets until reaching the point becomes difficult.
[0084]
　The temperature range (Ac 1 point or more Ac 3 but the annealing time is not particularly limited in the following points), the annealing time exceeds 600 seconds, economically undesirable because it leads to increase in cost. The time and the annealing, the length of the period during which the steel sheet temperature is kept isothermally at a maximum attained temperature (annealing temperature). In this period, the steel sheet may be kept isothermally, or after reaching the maximum heating temperature immediately may be cooled.
[0085]
　The cooling after the annealing, the cooling start temperature of 700 ° C. or higher, the cooling end temperature of 400 ° C. or less, it is preferable that the cooling rate in the temperature range of 700 ° C. ~ 400 ° C. and 10 ° C. / sec or more. When the cooling rate in the temperature range of 700 ° C. ~ 400 ° C. is less than 10 ° C. / sec, recrystallization of the ferrite will progress. In this case, the heating step of hot pressing, hot pressing raw sheets is Ac 3 to leaving the non-recrystallized ferrite to hot pressing raw sheets until reaching the point becomes difficult.
[0086]
　(Heating step)
　This step is a hot-press element plate is cold-rolled steel sheet or annealed steel sheet obtained through the above preparation step, Ac 3 is a step of heating above points. Maximum heating temperature of the hot press raw sheets are, Ac 3 and points or more. Maximum heating temperature Ac 3 is less than point, since a large amount of ferrite is formed in the high strength hot press forming member, difficult to ensure the strength of the high strength hot press forming member. Therefore, Ac 3 to the point and the lower limit of the maximum heating temperature. On the other hand, excessively high temperatures heating is not only economically undesirable because it leads to increase in cost, since it induces troubles such as lowering the press die life, maximum heating temperature Ac 3 point + 50 ℃ it is preferable that the following.
[0087]
　In heating up the heating temperature, 500 ° C. ~ Ac 1 is preferably set to 10 ° C. / sec or higher heating rate in the temperature range of points. However, if the sum of Ti content and the Nb content of hot pressing workpiece is not less than 0.005 wt%, it is possible to make the heating rate 3 ° C. / sec or more. 500 ° C. ~ Ac 1 when the heating rate in the temperature range of points is less than 10 ° C. / sec, occur recrystallization ferrite in the middle heating, Ac 3 difficult to leave the non-recrystallized ferrite to reach the point it is. Further, by heating at a heating rate of 10 ° C. / sec or more, it is possible to suppress the coarsening of austenite grains, toughness and delayed fracture resistance of high strength hot press forming member can be improved.
[0088]
　Thus, 500 ° C. ~ Ac 1 increasing the heating rate in the temperature range of points, Ac 3 enhanced while leaving the non-recrystallized ferrite to reach the point, thus the productivity of the high-strength hot press forming member Although it is possible to, 500 ° C. ~ Ac 1 when the heating rate in the temperature range of points is more than 300 ° C. / sec, these effects become saturated, it does not occur any particular effect on the other. Therefore, the upper limit of the heating rate is preferably 300 ° C. / sec.
[0089]
　Although the maximum heating temperature holding time in the not particularly limited, in order to dissolve the carbides, retention time is preferably 20 seconds or more. On the other hand, in order to leave the preferred texture in order to obtain the r-value of the object, it is preferable that the retention time less than 100 seconds.
[0090]
　(Hot press step)
　in the hot pressing process, the hot pressing raw sheets having undergone the heating step, concurrently with providing hot press molding using a hot press-forming means (for example, a mold), hot press using provided forming means cooling means (e.g., a refrigerant flowing through a conduit in the mold) and the like, is cooled to a temperature range of Ms point or higher (Ms point -250 ° C.). The hot press forming, it is also possible to use any known method.
[0091]
　The hot pressing process, the member (Ms point -250 ° C.) to a temperature range of more than Ms point, by cooling at a cooling rate 0.5 ~ 200 ° C. / sec to produce a martensite. Cooling stop temperature is martensite is excessively generated at (Ms point -250 ° C.) below, is not sufficiently accomplished secure ductility and bendability in a high strength hot press forming member. In contrast, when the cooling stop temperature is higher than the Ms point, martensite is not sufficiently generated, ensuring the strength is not sufficiently achieved in high strength hot press forming member. Therefore, the cooling stop temperature is set to Ms point or lower (Ms point -250 ° C.) or higher. If the ambient temperature is low, the temperature drop speed of the member also stops the operation of the cooling means becomes 0.5 ° C. / sec or more, cooling stop is not achieved, which is described above. In this case, by using a heating means appropriate, to suppress the temperature drop speed of the member to less than 0.5 ° C. / sec, it is necessary to achieve a cooling stop above. Further, when the cooling stop temperature (Ms point -220 ° C.) or higher (Ms point -50 ° C.) or less, because the effect is exhibited at a high level, respectively, preferred.
[0092]
　Cooling rate from the maximum heating temperature to the cooling stop temperature is not particularly limited, preferably in a 0.5 ~ 200 ° C. / sec. When the cooling rate is less than 0.5 ° C. / sec, austenite transforms into pearlite during cooling, or a large amount of ferrite is produced, sufficient martensite and bainite to ensure the strength it is difficult to ensure a volume fraction.
[0093]
　On the other hand, even if increasing the cooling rate, there is no any problem on the material of the high-strength hot press forming member, to excessively increase the cooling rate, since the may lead to high production cost, the cooling rate the upper limit is preferably set to 200 ° C. / sec.
[0094]
　(Reheating step)
　re-heating step, the member passing through the hot press-forming and cooling process, is reheated to a temperature range of 300 ~ 500 ° C., and then was maintained for 10 to 1000 seconds to reheat temperature range of member a step of cooling the reheating temperature range the member to room temperature. The reheating can be carried out using electric heating or induction heating. Reheating step is an optional step, the retention in the reheating step, not only the isothermal holding, including annealing heating at the above temperature range. Therefore, the retention time in the reheating step, which means the length of the period during which the member is in the reheat temperature range.
[0095]
　In less than reheating temperature (retention temperature) is 300 ° C., it takes a long time to bainite transformation, it is impossible to realize excellent productivity. On the other hand, when the reheating temperature (retention temperature) is higher than 500 ° C., less prone to bainitic transformation. Therefore, the reheating temperature shall be 300 ℃ ~ 500 ℃. Incidentally, preferable range of the reheating temperature is 350 ° C. or higher 450 ° C. or less.
[0096]
　Further, when the holding time is less than 10 seconds, the progress of bainite transformation is insufficient, the bending of sufficient bainite to secure, and sufficient residual austenite can not be obtained to ensure ductility. On the other hand, when the holding time exceeds 1,000 seconds, occur decomposition of the residual austenite is not effective residual austenite is obtained ductility secure, productivity is lowered. Therefore, the holding time is 1000 seconds or less 10 seconds or more. Incidentally, preferable range of the retention time is 900 seconds or less than 100 seconds.
[0097]
　In addition, for cooling mode after the holding it is not particularly limited, and may be cooled to room temperature while maintaining in the mold. Since this step is optional step, when not employing the present process, after the hot press forming step is completed, remove the member from the press mold, a furnace heated to 300 ~ 500 ° C. You may be charged. Incidentally, if the satisfaction of these thermal history, the steel sheet may be subjected to a heat treatment at any facility.
[0098]
　Shown above, the method of producing a high strength hot press forming member of the present embodiment, in the normal steel, refining, steelmaking, casting, hot rolling, but in principle that the respective steps of cold rolling, the above-described if satisfied the conditions of the respective steps, be suitably design changes, it is possible to obtain the effect of the high-strength hot press forming member according to the present embodiment.
Example
[0099]
　It will be specifically described by the inventors example the effect of the present invention. The present invention is not limited to the conditions used in the following invention examples.
[0100]
　Table 1 are shown the chemical composition slab A-R and a ~ d in, in conditions shown in Tables 2-1 to 3-3, the hot process production of press material plates of the present invention, the heating step, and heat during the press molding step, the cooling step, and the reheating step are sequentially performed steps simulating the manufacture steel A1 ~ d1, then cooled steel sheet to room temperature. Obtained steel sheet A1 ~ d1 in each test example, hot pressing with a die is not subjected. However, the mechanical properties of the resulting steel sheet is substantially the same as the unprocessed portion of the hot press forming member having the same thermal history. Therefore, by evaluating the steel sheet A1 ~ d1 obtained, it is possible to confirm the effect of the hot press forming member is present invention.
[0101]
　Here, the steel types A ~ R in Table 1, a steel type of component specified by the present invention, steels a ~ d is, C, at least one of the content of Si and Mn in steels outside the scope of the present invention is there. Further, the alphabet contained in the test code according to Table 2-1, etc. correspond to the steels listed in Table 1. To distinguish each test example are denoted by the numbers appended to the alphabet. For example, in Table 2-1, the chemical components of the test symbols D1 ~ D18 is a chemical component of steel type D shown in Table 1. Further, in Table 1, and Table 2-1 to 3-3, the values ​​underlined are value outside the specified range of the present invention. Incidentally, D7, D13, H6, K12, L6, L12, L13 "300-500 retention time at ° C." of the "holding temperature at 300-500 ° C. (° C.)" The reheating temperature that is described as isothermal holding time in the "300 ~ 500 ° C. the holding time at" the other embodiments, the time at which the steel sheet temperature was in the range of 300 ~ 500 ° C..
[0102]
　Also, Ac of each test example 3 -point and Ms point, the hot press element plate subjected to hot rolling-cold rolling is a value obtained by measuring in advance the laboratory. Then, thus obtained Ac 3 with point and Ms point, setting the annealing temperature and the cooling temperature.
[0103]
[Table 1]

[0104]
[table 2-1]

[0105]
[Table 2-2]

[0106]
[Table 2-3]

[0107]
[Table 3-1]

[0108]
[Table 3-2]

[0109]
[Table 3-3]

[0110]
　Then, the analysis of the identification and texture of the microstructure of the steel sheet A1 ~ d1 was carried out by the method described above. Then, the mechanical properties of the steel sheet A1 ~ d1 were investigated by the following method.
[0111]
　Tensile strength TS (MPa) and elongation at break El (%) was determined by tensile test. Tensile test piece was JIS5 No. specimen is perpendicular to the rolling direction taken from 1.2mm thick plate. Sample tensile strength is not less than 1200MPa, the tensile strength was determined to be good sample.
[0112]
　r value and the rolling vertical r values ​​in the rolling direction, as well as limitations of the rolling direction bending (R / t) and the rolling vertical critical bending (R / t) were measured by the bending test. Specific means are as follows.
[0113]
　r values ​​were taken test specimens conforming to JISZ2201, obtained by test according to the provisions of JISZ2254. r values ​​in the rolling direction is measured by a test piece in which the rolling direction as the longitudinal direction, r value perpendicular to the rolling direction were measured by a test piece was perpendicular to the rolling direction as the longitudinal direction.
[0114]
　Critical bending R / t is in the No. 1 test piece defined in JISZ2204, I was determined by performing a test based on the V-block method specified in JISZ2248. Limit of rolling direction bending, is measured by sampling specimens as bending ridge line is the rolling direction, perpendicular to the rolling direction of the critical bending is determined by taking specimens so bending ridge line is perpendicular to the rolling direction It has been. In the test, repeated bending with a curvature radius R different press metal plate member, bent after the cracking of the bent portion tested as judged by optical microscope or SEM, cracks are not generated critical bending R / t (R: bend radius of the test piece (i.e. the radius of curvature of the press metal plate member), t: was evaluated by calculating the thickness) of the test piece.

claims

　In unit
　mass%,
　C: 0.100
　~ 0.600%, Si: 1.00 ~ 3.00%, Mn: 1.00
　~ 5.00%, P: 0.040% or
　less, S: 0. 0500% or
　less, Al: 0.001
　~ 2.000%, N: 0.0100% or
　less, O: 0.0100% or
　less,
　Mo: 0 ~ 1.00%, Cr: 0 ~
　2.00%, Ni :
　0
　~ 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%
containing,
　made balance being iron and impurities,
　micro in the sheet thickness 1/4 parts organization, with unit volume%, tempered martensite: 20 to 90% bainite: 5 to 75%, and residual austenite Application: comprises 5-25%, and the ferrite is limited to 10% or less,
　Pole density of {211} <011> orientation in the thickness 1/4 part is 3.0 or more
hot press-forming member, characterized in that.
[Requested item 2]
　In unit
　mass%,
　Mo: 0.01 ~ 1.00%,
　Cr: 0.05 ~ 2.00%, Ni: 0.05 ~ 2.00%, and
　Cu: 0.05 ~ 2.00%
from contains one or more selected from the group consisting
hot press-forming member according to claim 1, characterized in that.
[Requested item 3]
　In unit
　mass%,
　Nb: 0.005 ~ 0.300%, Ti: 0.005 ~ 0.300%, and
　V: 0.005 ~ 0.300%
containing at least one member selected from the group consisting of to
hot press forming member according to claim 1 or 2, characterized in that.
[Requested item 4]
　In unit
　mass%, B: 0.0001 ~ 0.1000%
containing
hot press forming member according to any one of claims 1 to 3, characterized in that.
[Requested item 5]
　In unit
　mass%,
　Ca: 0.0005 ~ 0.0100%, Mg: 0.0005 ~ 0.0100%, and
　REM: 0.0005 ~ 0.0100%
containing at least one member selected from the group consisting of to
hot press forming member according to claim 1, any one of 4, characterized in that.