Hot press-molded body

Abstract

A hot press formed body having a prescribed chemical composition satisfying the following formula (1) and formula (2), a metal structure containing 90 area% or more of martensite, and among the martensite, a soft region being 5 to 25 area%. 561-474xC-33xMn-17xCr-17xNi-7.5xSi-21xMo+10xCo>440... (1) 0.265xTi+0.140xZr>N... (2).

Description

TECHNICAL FIELD

[0001] The present application relates to a hot press-formed body.

[0002] This application is based on patent application No. 2021-022063 filed in Japan on February 15, 2021, the contents of which are hereby incorporated by reference. BACKGROUND

[0003] In recent years, from the viewpoint of environmental protection and resource saving, the application of high-strength steel sheets to vehicle body parts is accelerating in order to reduce the weight of the vehicle body. However, vehicle body parts are manufactured by press forming. With the high-strength of the steel sheet constituting the vehicle body parts, not only the forming load at the time of press forming increases, but also the formability decreases. Therefore, in the case of press forming a high-strength steel sheet, the formability of a member formed into a complex shape becomes a problem. In order to solve such a problem, the application of a hot press technology that performs press forming after heating to a high temperature in the austenite region in which the steel sheet is softened is being carried out. Hot stamping is attracting attention as a technology that balances the forming and strength assurance to vehicle body parts by performing quenching treatment in the mold at the same time as press forming.

[0004] Among the vehicle body parts, for parts for impact absorption and deformation control of the skeleton, it is required that it is difficult to cause a fracture due to deformation at the time of collision. In order to suppress the occurrence of a fracture due to deformation at the time of collision, it is required that the bendability of the vehicle body part is excellent. In addition, in order to be able to suppress the occurrence of a fracture even in the case where deformation in a variety of deformation modes occurs at the time of collision, it is required that the anisotropy of the bendability is small.

[0005] The bendability of a material has a correlation with the tensile strength, and if the tensile strength is reduced, the bendability improves. It is known that the main phase of the microstructure of a hot press material is martensite, and the tensile strength of the martensite is greatly affected by C among the steel components.

[0006] For example, in Patent Literature 1, a high-strength steel sheet is disclosed, characterized in that the area ratio of martensite in the entire structure is 95% or more, and the solid solution C of the martensite is 0.05 mass% or less, and the density of carbides having a length diameter of 200 nm or more is 50 / μm 3 Hereinafter, the tensile strength is 1270 MPa or more.

[0007] In Patent Literature 2, a non-tempered high-strength thick steel sheet having a yield strength of 885 MPa or more is disclosed, characterized in that the microstructure is a mixed structure of martensite and lower bainite, and the total area ratio of the two structures is 95% or more.

[0008] A high-strength steel having excellent delayed fracture resistance is disclosed in Patent Literature 3, which is characterized in that 70% or more by volume of the metal structure is a martensite phase or a tempered martensite phase, and 50% or more by volume of the martensite phase or the tempered martensite phase is a martensite phase or a tempered martensite phase generated from a non-recrystallized austenite phase.

[0009] In the technologies of Patent Literatures 1 to 3 described above, various characteristics are improved by specifying the fraction of martensite and controlling the microstructure. However, there is no mention of controlling the behavior of auto tempering of the martensite, and there is also no mention of improving the bendability and reducing the anisotropy of the bendability.

[0010] Prior Art Documents

[0011] Patent Literature

[0012] Patent Literature 1: Japanese Patent Application Laid-Open No. 2018-109222

[0013] Patent Literature 2: Japanese Patent Application Laid-Open No. 2011-12315

[0014] Patent Literature 3: Japanese Patent Application Laid-Open No. H11-229075 SUMMARY

[0015] In view of the above problems, an object of the present application is to provide a hot press-formed body having high strength and excellent bendability, and small anisotropy of the bendability.

[0016] The present inventors have studied a method of obtaining the above hot press-formed body, and as a result, have obtained the following insights.

[0017] In order to obtain excellent bendability in a high-strength hot press-formed body, and to reduce the anisotropy of the bendability, it is important to have a desired amount of soft martensite present in the metal structure. Soft martensite has a low dislocation density, and is generated by auto tempering of the martensite.

[0018] In order to cause the martensite to sufficiently auto temper, it is important to use a steel sheet having a high martensite transformation start temperature (Ms point). This is because, by hot pressing a steel sheet having a high Ms point, auto tempering begins in a high-temperature region.

[0019] Furthermore, in order to cause the martensite to sufficiently auto temper, it is also effective to control the hot press conditions. The slower the cooling rate after hot pressing, the more likely auto tempering is to occur. In order to control the cooling rate after hot pressing, it is effective to control the surface pressure of the mold during hot pressing, and to control the holding time at the bottom dead center.

[0020] The gist of the present application completed based on the above insights is as follows.

[0021] [1] One embodiment of the present application relates to a hot press formed body containing, in mass%, C: 0.050 to 0.150%,

[0022] C: 0.050 to 0.150%,

[0023] Si: 0.010 to 1.000%,

[0024] Mn: 1.00 to 2.00%,

[0025] Al: 0.001 to 0.500%,

[0026] P: 0.100% or less,

[0027] S: 0.0100% or less,

[0028] N: 0.0100% or less,

[0029] B: 0.0005 to 0.0050%,

[0030] Cr: 0 to 0.50%,

[0031] Mo: 0 to 0.500%,

[0032] Ni: 0 to 3.00%,

[0033] Cu: 0 to 3.00%,

[0034] Co: 0 to 0.50%,

[0035] Sn: 0 to 0.500%,

[0036] Ca: 0 to 0.0050%,

[0037] Mg: 0 to 0.0050%,

[0038] REM: 0 to 0.0050%, and

[0039] Sb: 0 to 0.0200%, and

[0040] one or both of Ti: 0.005 to 0.100% and Zr: 0.005 to 0.100% are contained,

[0041] one or both of Nb: 0.015 to 0.100% and V: 0.005 to 0.100% are contained,

[0042] the balance including Fe and impurities,

[0043] satisfy the following formula (1) and formula (2),

[0044] The metal structure contains 90 area% or more of martensite, of which the soft region is 5 to 25 area%.

[0045] 561 - 474 x C - 33 x Mn - 17 x Cr - 17 x Ni - 7.5 x Si - 21 x Mo + 10 x Co > 440... (1)

[0046] 0.265 x Ti + 0.140 x Zr > N... (2)

[0047] wherein the element symbols in the above formula (1) and formula (2) represent the content of each element in mass%, and 0 is substituted in the case where the element is not contained.

[0048] [2] The hot press-molded body according to the above [1], the chemical composition can contain, in mass%,

[0049] Mo: 0.005 to 0.500%,

[0050] Ni: 0.005 to 3.00%,

[0051] Cu: 0.005 to 3.00%,

[0052] Co: 0.005 to 0.50%,

[0053] Sn: 0.005 to 0.500%,

[0054] Ca: 0.0005 to 0.0050%,

[0055] Mg: 0.0005 to 0.0050%,

[0056] REM: 0.0005 to 0.0050%, and

[0057] Sb: 0.0005 to 0.0200%

[0058] one or two or more kinds thereof.

[0059] [3] The hot press-molded body according to the above [1], the chemical composition can contain, in mass%,

[0060] Sn: 0.005 to 0.500%,

[0061] Sb: 0.0005 to 0.0200%

[0062] one or two or more kinds thereof.

[0063] [4] The hot press-molded body according to any one of the above [1] to [3], wherein the number density of the carbide having an average particle diameter of 20 to 500 nm and containing one or two or more of Nb, Ti, Zr, and V in the metal structure can be 0.3 to 10.0 pieces / μm 2 .

[0064] [5] The hot press-molded body according to any one of the above [1] to [4], wherein a plating layer can be provided on the surface.

[0065] According to the above-described aspect of the present application, it is possible to provide a hot press-molded body having high strength and excellent bendability, and small anisotropy of bendability. DETAILED DESCRIPTION

[0066] The hot press-molded body according to the present embodiment will be described in detail below. First, the reasons for the limitations of the chemical composition of the hot press-molded body according to the present embodiment will be described.

[0067] Further, in the numerical limitation range described with "to" described below, the lower limit value and the upper limit value are included in the range. For the values described as "less than" and "more than", the values are not included in the numerical range. The % of the chemical composition all indicates mass %.

[0068] The hot press-molded body according to the present embodiment contains, in mass %, C: 0.050 to 0.150 %, Si: 0.010 to 1.000 %, Mn: 1.00 to 2.00 %, Al: 0.001 to 0.500 %, P: 0.100 % or less, S: 0.0100 % or less, N: 0.0100 % or less, B: 0.0005 to 0.0050 %, Cr: 0 to 0.50 %, Mo: 0 to 0.500 %, Ni: 0 to 3.00 %, Cu: 0 to 3.00 %, Co: 0 to 0.50 %, Sn: 0 to 0.500 %, Ca: 0 to 0.0050 %, Mg: 0 to 0.0050 %, REM: 0 to 0.0050 %, and Sb: 0 to 0.0200 %, and contains one or two of Ti: 0.005 to 0.100 % and Zr: 0.005 to 0.100 %, one or two of Nb: 0.015 to 0.100 % and V: 0.005 to 0.100 %, and the balance contains Fe and impurities.

[0069] Each element will be described in detail below.

[0070] C: 0.050 to 0.150 %

[0071] C is an element that greatly affects the strength of the hot press-formed body. If the C content is less than 0.050%, the strength of the hot press-formed body becomes low. Therefore, the C content is set to 0.050% or more. It is preferable to be 0.070% or more, 0.080% or more, or 0.090% or more.

[0072] On the other hand, if the C content exceeds 0.150%, the strength of the hot press-formed body becomes too high, and the bendability deteriorates. Therefore, the C content is set to 0.150% or less. It is preferable to be 0.140% or less, 0.130% or less, 0.120% or less, 0.110% or less, or 0.100% or less.

[0073] Si: 0.010 to 1.000%

[0074] Si has temper softening resistance, and has an effect of suppressing a decrease in strength due to spontaneous tempering at the time of hot press quenching. When the Si content is less than 0.010%, the above effect cannot be obtained, and there are cases where the desired strength cannot be obtained, and cases where the bendability deteriorates. Therefore, the Si content is set to 0.010% or more. It is preferable to be 0.020% or more, 0.030% or more.

[0075] On the other hand, if the Si content exceeds 1.000%, a problem of surface scale generation occurs. That is, after pickling of the scale generated at the time of hot rolling, a pattern due to surface unevenness occurs, and the surface appearance deteriorates. In addition, in the case where plating treatment is performed on the surface of the steel sheet, the platability deteriorates. Therefore, the Si content is set to 1.000% or less. It is preferable to be 0.700% or less, 0.500% or less, 0.300% or less.

[0076] Mn: 1.00 to 2.00%

[0077] Mn is an element that improves the strength of the hot press-formed body and the quenchability of the steel. When the Mn content is less than 1.00%, the hot press-formed body cannot obtain sufficient strength. Therefore, the Mn content is set to 1.00% or more. It is preferable to be 1.20% or more, 1.40% or more.

[0078] On the other hand, even if Mn is contained in excess of 2.00%, the above effect saturates, and the bendability decreases. Therefore, the Mn content is set to 2.00% or less. It is preferable to be 1.80% or less, 1.60% or less.

[0079] Al: 0.001 to 0.500%

[0080] Al is an element that is used as a deoxidizing material for molten steel. If deoxidization is insufficient, the bendability of the hot press-formed body decreases due to excess generation of oxides. In order to sufficiently deoxidize the molten steel, the Al content is set to 0.001% or more. It is preferable to be 0.010% or more, 0.030% or more.

[0081] On the other hand, if the Al content exceeds 0.500%, non-metallic inclusions are formed in a large amount, and surface flaws are easily generated in the hot press-formed body. Therefore, the Al content is set to 0.500% or less. It is preferable that the Al content be 0.300% or less, 0.200% or less, or 0.100% or less.

[0082] P: 0.100% or less

[0083] P is an element that segregates at the grain boundaries and reduces the strength of the grain boundaries. If the P content exceeds 0.100%, the strength of the grain boundaries is significantly reduced, and the toughness and bendability of the hot press-formed body are reduced. Therefore, the P content is set to 0.100% or less. It is preferable that the P content be 0.080% or less, or 0.050% or less.

[0084] The lower limit of the P content is not particularly limited, but if the P content is excessively reduced, the refining cost increases, and therefore the P content can be set to 0.001% or more.

[0085] S: 0.0100% or less

[0086] S deteriorates the bendability of the hot press-formed body by affecting non-metallic inclusions in the steel. Therefore, the S content is set to 0.0100% or less. It is preferable that the S content be 0.0080% or less, or 0.0050% or less.

[0087] The lower limit of the S content is not particularly limited, but if the S content is excessively reduced, the manufacturing cost of the desulfurization process increases, and therefore the S content can be set to 0.0001% or more.

[0088] N: 0.0100% or less

[0089] N is an impurity element that deteriorates the bendability of the hot press-formed body by forming nitrides that become the starting points of bend cracks in the steel. When the N content exceeds 0.0100%, coarse nitrides are generated in the steel, and the bendability of the hot press-formed body is significantly reduced. Therefore, the N content is set to 0.0100% or less. It is preferable that the N content be 0.0080% or less, or 0.0060% or less.

[0090] The lower limit of the N content is not particularly limited, but if the N content is reduced to less than 0.0001%, the cost of removing N greatly increases, and is not preferable in terms of economy. In actual operations, the N content can be set to 0.0001% or more, or 0.0005% or more.

[0091] B: 0.0005 to 0.0050%

[0092] B has an effect of improving the quenching property in the hot press or in the cooling after the hot press, thereby improving the strength of the hot press formed body. When the B content is less than 0.0005%, the above effect cannot be obtained. Therefore, the B content is set to 0.0005% or more. It is preferable to be 0.0007% or more, 0.0010% or more.

[0093] On the other hand, when the B content exceeds 0.0050%, there are cases where cracks are generated at the time of hot rolling, cases where the above effect is saturated, and cases where the bendability is reduced due to borides. Therefore, the B content is set to 0.0050% or less. It is preferable to be 0.0030% or less.

[0094] one or both of Ti: 0.005 to 0.100% and Zr: 0.005 to 0.100%

[0095] Ti and Zr have an effect of forming carbonitride in the steel, improving the strength of the hot press formed body by precipitation strengthening. Also, they have an effect of fixing N as nitride to suppress the generation of BN, and an effect of improving the quenching property of B. In order to obtain these effects, one or more of Ti: 0.005% or more and Zr: 0.005% or more is contained. It is not necessary to contain both Ti and Zr, and it is sufficient to contain either one of them at the above content. If either one of Ti and Zr is contained at the above content, the other element can be contained at a content lower than the above content, or can not be contained. It is preferable that the content of the above elements be one or more of Ti: 0.010% or more and Zr: 0.010% or more.

[0096] On the other hand, even one of these elements, when its content exceeds 0.100%, carbonitride is generated in a large amount, and the bendability of the hot press formed body is reduced. Therefore, the content of Ti and Zr is set to 0.100% or less, respectively. It is preferable to be 0.080% or less, respectively.

[0097] one or both of Nb: 0.015 to 0.100% and V: 0.005 to 0.100%

[0098] Nb and V have an effect of forming carbonitride in the steel, improving the strength of the hot press formed body by precipitation strengthening. Also, they have an effect of improving the strength and bendability of the hot press formed body by also making the structure fine as solid solution elements. In order to obtain these effects, one or more of Nb: 0.015% or more and V: 0.005% or more is contained. It is not necessary to contain both of Nb and V, and it is sufficient to contain either one of them in the above-mentioned content. If either one of Nb and V is contained in the above-mentioned content, the other element can be contained in a content lower than the above-mentioned content, or can not be contained. The content of the above-mentioned elements is preferably one or more of Nb: 0.020% or more and V: 0.010% or more.

[0099] On the other hand, even one of these elements, if its content exceeds 0.100%, carbonitride is generated in a large amount, and the bendability of the hot press formed body is reduced. Therefore, the content of Nb and V is set to 0.100% or less, respectively. It is preferably 0.080% or less, respectively.

[0100] 561 - 474 x C - 33 x Mn - 17 x Cr - 17 x Ni - 7.5 x Si - 21 x Mo + 10 x Co > 440... (1)

[0101] 0.265 x Ti + 0.140 x Zr > N... (2)

[0102] The chemical composition of the hot press formed body according to the present embodiment satisfies formula (1) and formula (2). Further, the element symbols in formula (1) and formula (2) indicate the content of each element in mass%, and 0 is substituted in the case where the element is not contained.

[0103] The left side of formula (1) is a formula for calculating Ms point (°C). When the left side of formula (1) is 440°C or less, the Ms point is low, and even if hot pressing is performed under ideal conditions, the desired amount of soft region cannot be obtained. Therefore, the left side of formula (1) (Ms point) is set to exceed 440°C. The left side of formula (1) is preferably 450°C or more, and more preferably 460°C or more. The upper limit of the left side of formula (1) is not particularly specified, but can be set to 600°C or less, 550°C or less, 500°C or less.

[0104] The left side of formula (2) is a formula for calculating the amount of nitrogen (mass%) fixed to nitride containing Ti and Zr. When the left side of formula (2) is the N content or less, BN is generated, and the effect of improving the quenching property of B cannot be sufficiently obtained. Therefore, the left side of formula (2) is set to exceed the N content.

[0105] The upper limit of the left side of formula (2) is not particularly specified, but can be 0.150% or less.

[0106] The remainder of the chemical composition of the hot press formed body according to the present embodiment can be Fe and impurities. As the impurities, elements that are inevitably mixed from a steel raw material or scrap and / or are inevitably mixed during steelmaking and are allowed within a range that does not impair the characteristics of the hot press formed body according to the present embodiment can be exemplified.

[0107] The hot press formed body according to the present embodiment can also contain the following elements as arbitrary elements instead of a part of Fe. The content in the case where the hot press formed body does not contain the following arbitrary elements is 0%.

[0108] Cr: 0.005 to 0.50%, Mo: 0.005 to 0.500%, Ni: 0.005 to 3.00%, and Cu: 0.005 to 3.00%

[0109] Cr, Mo, Ni, and Cu are elements that increase the hardenability of steel and have an effect of increasing the strength of the hot press formed body. Therefore, one or two or more of these elements can be contained as needed. In order to reliably exert this effect, the content of at least one of Cr, Mo, Ni, and Cu is preferably 0.005% or more.

[0110] On the other hand, when the content of Cr exceeds 0.50%, when the content of Mo exceeds 0.500%, or when the content of Ni or Cu exceeds 3.00%, the carbide existing after hot rolling, after cold rolling, or after annealing (including after plating treatment) is stabilized, and there is a case where the dissolution of the carbide during heating at the time of hot pressing is delayed, and the hardenability is reduced. Therefore, the content of Cr is set to 0.50% or less, the content of Mo is set to 0.500% or less, and the content of Ni and Cu is set to 3.00% or less, respectively.

[0111] Co: 0.005 to 0.50%

[0112] Co is an element that has an effect of increasing the Ms point, and increases the bendability of the hot press formed body. Therefore, Co can be contained as needed. In order to reliably exert the above effect, the content of Co is preferably 0.005% or more.

[0113] On the other hand, when the content of Co exceeds 0.50%, the hardenability of the steel is reduced. Therefore, the content of Co is set to 0.50% or less.

[0114] Sn: 0.005 to 0.500%

[0115] Sn has an effect of increasing the corrosion resistance of the hot press formed body, and therefore can be contained as needed. In order to reliably exert this effect, the content of Sn is preferably 0.005% or more, more preferably 0.010% or more, 0.020% or more.

[0116] On the other hand, even if Sn is contained in excess of 0.500%, the above effects are saturated, and therefore the Sn content is set to 0.500% or less. It is preferably 0.300% or less, 0.150% or less.

[0117] Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, REM: 0.0005 to 0.0050%

[0118] Ca, Mg, and REM have effects of refining inclusions in the steel and preventing generation of cracks at the time of hot pressing due to inclusions. Therefore, one or two or more of these elements can be contained as needed. In order to reliably exert the above effects, it is preferable to set the content of at least one of Ca, Mg, and REM to 0.0005% or more.

[0119] On the other hand, if the content of Ca, Mg, or REM exceeds 0.0050%, the effect of refining inclusions in the steel is saturated, and the alloy cost increases. Therefore, the content of Ca, Mg, and REM is set to 0.0050% or less, respectively.

[0120] In the present embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoid elements, and the content of REM refers to the total content of these elements.

[0121] Sb: 0.0005 to 0.0200%

[0122] In order to suppress decarburization in the hot state, Sb can be contained as needed. By containing Sb, decarburization can be suppressed in hot rolling and hot pressing in the case of a cold-rolled steel sheet without plating. In order to reliably exert this effect, the Sb content is preferably set to 0.0005% or more.

[0123] On the other hand, even if the Sb content exceeds 0.0200%, the above effects are saturated, and therefore the Sb content is set to 0.0200% or less.

[0124] In the chemical composition of the hot-pressed formed body to which the present embodiment relates, in order to improve the properties of the hot-pressed formed body, among the above-mentioned arbitrary elements, it is particularly preferable to contain one or two or more of Co, Sn, and Sb in the above-mentioned contents.

[0125] The chemical composition of the hot press-formed body described above can be measured using a general analysis method. For example, the measurement can be performed using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Further, C and S can be measured using a combustion-infrared absorption method, and N can be measured using an inert gas fusion-thermal conductivity method. In the case where the hot press-formed body has a plated layer on the surface thereof, the plated layer on the surface can be removed by mechanical grinding, and then the chemical composition can be analyzed.

[0126] Next, the metal structure of the hot press-formed body according to the present embodiment will be described.

[0127] The metal structure of the hot press-formed body according to the present embodiment contains 90 area% or more of martensite, and among the martensite, the soft region is 5 to 25 area%.

[0128] Martensite: 90 area% or more

[0129] Martensite is a structure required to obtain a desired strength. If the martensite is less than 90 area%, a desired strength cannot be obtained. Therefore, the martensite is set to 90 area% or more. It is preferable that the martensite be 93 area% or more, 95 area% or more, or 97 area% or more. The martensite can also be 100 area%.

[0130] In the present embodiment, as the remaining structure other than the martensite, ferrite, pearlite, upper bainite, lower bainite, and residual austenite can be contained. From the viewpoint of the area ratio with respect to the martensite, the total of these remaining structures is preferably set to 10 area% or less. The total of the remaining structures is preferably 7 area% or less, 5 area% or less, 3 area% or less, and can also be 0 area%.

[0131] Soft region: 5 to 25 area% among the martensite

[0132] The soft region is a region in which the dislocation density is low and the strength is relatively low among the martensite. In the present embodiment, the soft region means a region in which the average GAIQ value (Grain Average Image Quality value) is 123000 to 200000.

[0133] When the soft region among the martensite is less than 5 area%, a desired bendability cannot be obtained, and in addition, anisotropy of the bendability cannot be reduced. Therefore, the soft region among the martensite is set to 5 area% or more. It is preferable that the soft region be 10 area% or more or 15 area% or more.

[0134] On the other hand, when the soft region in the martensite exceeds 25 area%, it is not possible to obtain the desired strength, and in addition, it is not possible to reduce the anisotropy of the bendability. Therefore, the soft region in the martensite is set to 25 area% or less. It is preferable to be 23 area% or less, 20 area% or less.

[0135] The mechanism by which the soft martensite phase increases the bendability and reduces the anisotropy of the bendability is not clear, but the present inventors have the following conjecture.

[0136] It is considered that since the soft region in the martensite is softer than the surrounding soft regions, plastic deformation easily occurs at the time of bend deformation, but since the fracture limit is high, the occurrence of cracks is suppressed. Furthermore, it is considered that there is an effect of suppressing the progress of cracks even in the propagation of cracks. Therefore, it is conjectured that the bendability is increased. In addition, the anisotropy of the bendability is generally affected by inclusions extending in the rolling direction. It is considered that in the hot press-formed body according to the present embodiment, since the soft regions are uniformly dispersed, the effect of the inclusions is moderated. As a result, it is considered that the anisotropy of the bendability can be reduced.

[0137] The area ratio of the martensite was measured using the following method.

[0138] A sample was cut out from an arbitrary position (a position avoiding the end portion in the case where it is not possible to take the sample from the position) of 50 mm or more from the end surface of the hot press-formed body in a manner that enables observation of the cross section of the plate thickness. The size of the sample also depends on the measuring device, but was set to a size that enables observation of about 10 mm in the rolling direction.

[0139] The cross section of the above sample was etched with Lepara reagent. The cross section etched with Lepara reagent was observed at 10 fields of view at a magnification of 500 times at the 1 / 4 position of the plate thickness (a region from a depth of 1 / 8 of the plate thickness from the surface to a depth of 3 / 8 of the plate thickness from the surface), and an optical microscope photograph was obtained. The optical microscope photograph obtained was subjected to image analysis using the "Photoshop CS5" image analysis software manufactured by Adobe, and the area ratio of the martensite was calculated.

[0140] As the image analysis method, the maximum lightness value L max and the minimum lightness value L min of the image were obtained from the image. max A portion having pixels with lightness from L max - 0.3 (L min - L max ) to L min + 0.3 (L min - L max ) was defined as a white region, and a portion having pixels with lightness from L minThe portion of the pixel of the image in which the area ratio of the martensite is 0% to 10% of the area ratio of the entire image is defined as a black region, the portion other than this is defined as a gray region, and the area ratio of the martensite as a white region is calculated. The area ratio of the martensite is calculated in the same manner as described above for the total of 10 fields of view, and the average value of the area ratios thus obtained is regarded as the area ratio of the martensite. Thus, the area ratio of the martensite is obtained.

[0141] The area ratio of the remaining structure is obtained by subtracting the area ratio of the martensite from 100%.

[0142] The area ratio of the soft region is measured by the following method.

[0143] A sample is cut out from a position at a distance of 50 mm or more from the end surface of the hot press-formed body (in a position avoiding the end portion in the case where the sample cannot be taken from this position) in a manner that enables observation of the cross section of the plate thickness. After the cross section of the plate thickness of the sample is ground using silicon carbide paper of #600 to #1500, the surface is finished to a mirror surface using a liquid obtained by dispersing diamond powder having a particle size of 1 to 6 μm in a diluent such as alcohol or pure water. Next, the sample is polished using colloidal silica not containing an alkaline solution at room temperature for 8 minutes to remove strain introduced into the surface layer of the sample.

[0144] At an arbitrary position in the length direction of the cross section of the plate thickness of the sample, the crystal orientation information is measured at a measurement interval of 0.1 μm for a length of 50 μm and a position of 1 / 4 of the plate thickness (a region from a depth of 1 / 8 of the plate thickness from the surface to a depth of 3 / 8 of the plate thickness from the surface) using an electron backscatter diffraction method. In the measurement, an EBSD device composed of a hot field emission type scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5-type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD device is set to 9.6 x 10 -5 The acceleration voltage is set to 15 kV, the irradiation current level is set to 13, and the irradiation level of the electron rays is set to 62.

[0145] For the obtained crystal orientation information, using the software "OIM Data Collection" function attached to the EBSD analysis device and the "Grain Average Misorientation" function mounted in "OIM Analysis (registered trademark)", a Grain Average Image Quality (GAIQ) map was obtained. In the obtained GAIQ map, a region surrounded by an interface having a crystal orientation difference of 5° or more was defined as a grain, and the area ratio of a region in which the average GAIQ value in the unit grain was 123000 to 200000 was calculated. For the observation field of view of 10 in total, the area ratio of a region in which the average GAIQ value in the unit grain was 123000 to 200000 was calculated. The average value of the obtained area ratios was calculated, and this average value was regarded as the area ratio of the soft region. The area ratio of the soft region in the martensite was obtained by dividing the obtained area ratio of the soft region by the area ratio of the martensite obtained by the above method (area ratio of soft region / area ratio of martensite x 100).

[0146] In the metal structure according to the present embodiment, the number density of the carbide having an average particle diameter of 20 to 500 nm and containing one or two or more of Nb, Ti, Zr, and V can be 0.3 to 5.0 pieces per μm 2 . By making the number density of the carbide having an average particle diameter of 20 to 500 nm and containing one or two or more of Nb, Ti, Zr, and V 0.3 to 5.0 pieces per μm 2 , the bendability of the hot press-formed body can be further improved. The precipitation state of the above-described carbide contributes to the propagation of cracks at the time of bending deformation. It is considered that the larger the size of the above-described carbide and the lower the number density, the lower the bendability.

[0147] The number density of the above-described carbide is measured by the following method.

[0148] A sample is cut out from an arbitrary position of 50 mm or more from the end surface of the hot press-formed body (in a position avoiding the end portion in the case where the sample cannot be taken from the position) in a manner that enables observation of the plate thickness cross section. After the observation surface is finished by mirror polishing, the precipitates are made to appear by dissolving the steel matrix using a constant potential electrolytic etching method. The conditions of the constant potential electrolytic etching method are set as follows: as the electrolyte, a mixed solution of acetylacetone at 10% by volume, tetramethylammonium chloride at 1% by volume, and the remaining portion being methanol is used, the potential is set to -200 mV, and the coulomb amount is 10 c / cm 2 .

[0149] Then, for the 1 / 4-thickness position (a region from a depth of 1 / 8 of the thickness from the surface to a depth of 3 / 8 of the thickness from the surface), observation was performed at a magnification of 20,000 times for 10 or more fields of view. The composition of the precipitates can be determined using an EDS (energy dispersive X-ray spectrometer). From the observation photograph, the length and the short diameter of each carbide containing one or two or more of Nb, Ti, Zr, and V among the precipitates were determined. The average of the length and the short diameter was calculated, and the average was regarded as the average particle diameter of the carbide. The number density of the carbide having an average particle diameter of 20 to 500 nm and containing one or two or more of Nb, Ti, Zr, and V was obtained by counting the number of carbides having an average particle diameter of 20 to 500 nm and dividing the number by the measurement area.

[0150] Further, in a case where EDS analysis of the precipitates detects one or two or more of Nb, Ti, Zr, and V and C, the precipitates are regarded as the carbide containing one or two or more of Nb, Ti, Zr, and V.

[0151] plating layer

[0152] For the purpose of further improving corrosion resistance, the hot press-formed body according to the present embodiment can have a plating layer on the surface. The plating layer can include, for example, an Al-based plating layer such as a hot-dip aluminum layer and an aluminum-zinc plating layer, a hot-dip zinc layer, an alloyed hot-dip zinc layer, an electroplated zinc layer, a zinc-nickel plating layer, and the like.

[0153] The plating layer can be provided on any one surface of the hot press-formed body, or on both surfaces. The attachment amount is not particularly limited, but is preferably: 15 to 120 g / m 2 for the Al-based plating layer, 30 to 120 g / m 2 for the hot-dip zinc layer, 30 to 120 g / m 2 for the alloyed hot-dip zinc layer, and 5 to 100 g / m 2 for the electroplated zinc layer and the zinc-nickel plating layer.

[0154] Further, in the present embodiment, the Al-based plating layer means a plating layer containing 50% by mass or more of Al. As elements other than Al, Si: 0.1 to 20% by mass, Fe: 0.1 to 10% by mass, and Zn: 0.1 to 45% by mass, and the remainder (Cu, Na, K, Co, Ni, Mg, and the like): less than 0.5% by mass can be contained.

[0155] In addition, in the present embodiment, the so-called Zn-based plated layer means a plated layer containing 50 mass% or more of Zn. As elements other than Zn, Si: 0.01 to 20 mass%, Fe: 0.1 to 10 mass%, Al: 0.01 to 45 mass%, and the remainder (Cu, Na, K, Co, Ni, Mg, etc.): less than 0.5 mass% can be contained.

[0156] The composition analysis of the plated layer was performed using the following method.

[0157] A sample was cut out from any position of 50 mm or more from the end face of the hot press-formed body (in a position avoiding the end portion in the case where it was not possible to take the sample from the position) in a manner that enabled observation of the cross section of the sheet thickness. The size of the sample also depended on the measuring device, but was set to a size that enabled observation of about 10 mm in the rolling direction.

[0158] After embedding the above sample in resin and performing polishing, the layer structure of the cross section of the sheet thickness was observed using a scanning electron microscope (SEM). Specifically, the magnification at which the steel sheet and the plated layer entered the field of view was observed using the SEM. For example, if observation was performed using a backscattered electron composition image (COMPO image), it was possible to infer from the cross-sectional structure how many layers were present.

[0159] Next, using an electron probe micro-analyzer (EPMA), the range of 50 μm in the sheet surface direction and the thickness of the plated layer + 30 μm in the sheet thickness direction was analyzed by mapping. In the case where the plated layer was an Al-based plated layer, the respective average values of the Fe concentration and the Al concentration in the sheet surface direction were calculated. Next, the relationship between the sheet thickness position and the Al concentration, and the relationship between the sheet thickness position and the Fe concentration were calculated. The sheet thickness position at which the Al concentration and the Fe concentration became the same concentration as the Al concentration and the Fe concentration of the steel sheet was judged to be the interface between the steel sheet and the Al-based plated layer. The Al concentration and the Fe concentration of the steel sheet referred to here were obtained by measurement using the EPMA.

[0160] In addition, in the case where the plated layer was a Zn-based plated layer, the respective average values of the Fe concentration and the Zn concentration in the sheet surface direction were calculated. Next, the relationship between the sheet thickness position and the Zn concentration, and the relationship between the sheet thickness position and the Fe concentration were calculated. The sheet thickness position at which the Zn concentration and the Fe concentration became the same concentration as the Zn concentration and the Fe concentration of the steel sheet was judged to be the interface between the steel sheet and the Zn-based plated layer. The Zn concentration and the Fe concentration of the steel sheet referred to here were obtained by measurement using the EPMA.

[0161] Sheet thickness

[0162] The sheet thickness of the hot press-formed body according to the present embodiment is not particularly limited, but can be set to 0.5 to 3.5 mm from the viewpoint of lightweighting of the vehicle body.

[0163] Tensile strength

[0164] In order to improve the effect of lightweighting of the vehicle body, the tensile strength of the hot press-formed body according to the present embodiment is preferably 980 MPa or greater. On the other hand, if the tensile strength is too high, the bendability decreases, and thus the tensile strength is preferably 1380 MPa or less.

[0165] With respect to the tensile strength, a No. 5 test piece described in JIS Z 2241:2011 is produced, and the test method described in JIS Z 2241:2011 is followed to calculate the tensile strength.

[0166] Next, the manufacturing method of the hot press-formed body according to the present embodiment will be described. First, the manufacturing method of the hot press steel sheet for hot pressing will be described.

[0167] First, a hot-rolled steel sheet is obtained by hot rolling. The billet (steel material) for hot rolling can be a billet manufactured by a conventional method, for example, a billet manufactured by a general method such as continuous casting slab casting, thin slab casting, or the like. The billet having the above-described chemical composition is subjected to hot rolling. In order to desirably control the number density of the carbide containing one or two or more of Nb, Ti, V, and Zr, it is preferable to set the heating temperature before hot rolling to 1200°C or higher, set the coiling temperature to 600°C or lower, and set the time from the end of finish rolling to the start of coiling to 5 seconds or greater.

[0168] By setting the heating temperature to 1200°C or higher, the carbide containing one or two or more of Nb, Ti, V, and Zr can be dissolved, and the above-described carbide can be finely precipitated during rolling. The upper limit of the heating temperature is not particularly limited, but can be set to 1400°C or lower from the viewpoint of productivity.

[0169] By setting the coiling temperature to 600°C or lower, the number density and the average particle diameter of the carbide containing one or two or more of Nb, Ti, V, and Zr can be desirably controlled. The lower limit of the coiling temperature is not particularly limited, but can be set to 400°C or greater from the viewpoint of productivity.

[0170] If the time from the end of finish rolling to the start of coiling is short, the temperature rises due to the heat of phase transition at the time of becoming a coiled material after coiling, and sometimes the number density and average particle diameter of the carbides containing one or more of Nb, Ti, V, and Zr cannot be controlled desirably. Therefore, it is preferable to set the time from the end of finish rolling to the start of coiling to 5 seconds or more. The upper limit is not particularly specified, and can be set in a manner that enables coiling at the above-mentioned coiling temperature, taking into account the passage speed and the cooling speed.

[0171] Next, for the obtained hot-rolled steel sheet, the coiled material is uncoiled, and after pickling, cold rolling is performed. The cumulative reduction rate at the time of cold rolling can be set to, for example, 30 to 80%, as long as it is within a range that does not impair productivity. Thus, a cold-rolled steel sheet is obtained.

[0172] For the obtained cold-rolled steel sheet, annealing can also be performed in order to soften it. After annealing, it is preferable to perform quenching and tempering rolling. The reduction rate in the quenching and tempering rolling of the steel sheet can be set to 2% or less, as long as it is within a range that does not impair productivity. In order to correct the shape, a tension leveller can also be used.

[0173] For the cold-rolled steel sheet, an Al-based plating layer such as an aluminum plating layer and an aluminum-zinc plating layer, or a Zn-based plating layer can be applied as needed. The composition of the plating layer is mainly aluminum and / or zinc, but in order to improve corrosion resistance, elements such as Ni can also be added. In addition, elements such as iron can be contained as impurities in the plating layer.

[0174] The plating layer can be imparted using usual plating conditions. If it is an aluminum plating layer, it is appropriate for the Si concentration in the bath to be 5 to 12% by mass, with the remainder being aluminum and less than 0.5% of impurities. When it is an aluminum-zinc plating layer, it is appropriate for the Zn concentration in the bath to be 40 to 50% by mass, with the remainder being aluminum and less than 0.5% of impurities. In addition, even if Mg and Zn are mixed in the aluminum plating layer, or even if Mg is mixed in the aluminum-zinc plating layer, there is no particular problem. The atmosphere at the time of imparting the plating layer is not particularly specified, and can be set to usual plating conditions, regardless of whether a continuous plating apparatus with an oxidation-free furnace is used or a continuous plating apparatus without an oxidation-free furnace is used. For zinc plating, methods such as hot-dip galvanizing, electro-galvanizing, and alloyed hot-dip galvanizing can be used.

[0175] Metal pre-plating can also be performed on the surface of the steel sheet before plating. As metal pre-plating, pre-plating of Ni, pre-plating of Fe, and other metal pre-plating that improves platability can be exemplified. In addition, there is no particular problem even if a different metal plating layer, an inorganic or organic compound film, or the like is imparted to the surface of the plating layer.

[0176] A hot-pressed steel sheet is obtained using the above method.

[0177] Next, the hot press forming body according to the present embodiment is manufactured by applying, for example, the following hot press conditions to the steel sheet for hot press obtained by the above method.

[0178] The steel sheet for hot press is heated to a temperature region of Ac3 transformation point to 1000°C, and after being held in the temperature region for 0.1 to 30.0 minutes, is rapidly transported to a mold, and hot press is performed. Then, the steel sheet is pressurized, and the steel sheet is cooled in the mold by heat transfer between the steel sheet and the mold.

[0179] Further, in the temperature region of Ac3 transformation point to 1000°C, the steel sheet temperature can be varied, or the steel sheet temperature can be constant. The Ac3 transformation point can be calculated by the following equation.

[0180] Ac3 transformation point (°C) = exp(X) + 31.5 x Mo - 28

[0181] X = 6.8165 - 0.47132 x C - 0.057321 x Mn + 0.0660261 x Si - 0.050211 x Cr + 0.10593 x Ti + 2.0272 x N + 1.0536 x S - 0.12024 x Si x C + 0.11629 x Cr x C + 0.29225 x C 2 + 0.01566 x Mn 2 + 0.017315 x Cr 2

[0182] Further, the element symbols in the above equation are the contents of the elements in mass%, and 0 is substituted in the case of not containing.

[0183] By setting the surface pressure of the mold at the time of hot press to a value (P a (MPa)) or less, the cooling speed after hot press can be desirably controlled. As a result, a desired amount of soft region can be obtained. max

[0184] The lower limit of the surface pressure of the mold at the time of hot press is not particularly limited, but can be set to 0.1 MPa or more. This is because, in the case of small surface pressure, the heat transfer between the mold and the steel sheet becomes insufficient, and the cooling speed required for martensitic transformation cannot be obtained.

[0185] ​Further, the surface pressure of the mold at the time of hot pressing can be controlled by changing the pressing load during the holding at the bottom dead center. Since it is difficult to calculate the accurate surface pressure in the actual part shape, the numerical simulation of the press forming can be effectively used to determine the pressing load during the holding at the bottom dead center. In the case where the shape of the part is simple and the normal direction load of the part surface can be calculated from the pressing load, the surface pressure can be obtained by dividing the normal direction load by the surface area of the part.

[0186] P a = 8 x 10 -177 Ms 67.08 … (3)

[0187] Ms in the above formula (3) is the Ms point (martensitic transformation starting temperature) represented by the left side of the above formula (1).

[0188] Further, by controlling the holding time at the bottom dead center at the time of hot pressing, the cooling speed after the hot pressing can be desirably controlled. As a result, the desired amount of soft region can be obtained. In the holding at the bottom dead center at the time of hot pressing, the steel sheet is rapidly cooled by the heat transfer between the steel sheet and the mold, and the martensitic transformation occurs. The temperature at the time of taking out the hot press-formed body from the mold (taking-out temperature) is preferably below the temperature at which the martensitic transformation ends. Therefore, the temperature at the time of taking out the hot press-formed body from the mold is preferably set to 250°C or lower. Further, the taking-out temperature referred to herein means the surface temperature of the hot press-formed body at the time of taking out from the mold.

[0189] Even if the holding time at the bottom dead center is short, in the case where the taking-out temperature is set to a low temperature, the cooling speed of the steel sheet held in the mold becomes high, and sometimes it is difficult to obtain the soft region. Therefore, it is desirable to set the holding time at the bottom dead center to t (seconds) or more obtained from the following formula (5) or (6) with respect to the taking-out temperature.

[0190] In the case where the taking-out temperature is less than 150°C, t = 3.15 x 10 87 Ms s -32.7 … (5);

[0191] In the case where the taking-out temperature is 150 to 250°C, t = 2.20 x 10 87 Ms s -32.7 … (6).

[0192] Ms in the above formula (5) and (6) is the Ms point (martensitic transformation starting temperature) represented by the left side of the above formula (1), and it is considered that the higher the Ms point, the easier it is to obtain the soft region, and therefore the holding at the bottom dead center can be shorter.

[0193] Using the above method, the hot-pressed molded body involved in this embodiment can be manufactured.

[0194] Furthermore, it is desirable to avoid tempering after hot pressing. Specifically, it is desirable not to heat for more than 15 minutes in a temperature range above 250°C. This is because if such tempering is performed, the soft regions in the martensite become excessive, and the desired strength cannot be obtained.

[0195] Example

[0196] Next, embodiments of the present invention will be described. However, the conditions in the embodiments are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to these specific examples. Various conditions can be adopted by the present invention without departing from the spirit of the invention and to achieve the purpose of the invention.

[0197] Using steel billets with the chemical compositions shown in Tables 1A to 1C, hot rolling, pickling, and cold rolling are performed, followed by continuous annealing or continuous hot-dip galvanizing as needed, thereby producing cold-rolled steel sheets and coated steel sheets with thicknesses of 0.5 to 3.5 mm. The heating temperatures before hot rolling are recorded in Tables 2A to 2C, and the cumulative reduction rate during cold rolling is 30% to 80%.

[0198] The types of coatings listed in Tables 3A to 3C are as follows.

[0199] CR: No coating

[0200] GI: Hot-dip galvanized coating (target unit area mass: 60g / m² per side) 2 Double-sided plating)

[0201] GA: Alloyed hot-dip galvanized layer (target unit area mass: 45g / m² on one side) 2 Double-sided plating)

[0202] AL: Al-based coating (target unit area mass: 80g / m² on one side) 2 Double-sided plating)

[0203] EG: Electroplated zinc layer (target unit area mass: 40g / m² on one side) 2 Double-sided plating)

[0204] Using the manufactured cold-rolled steel sheet and coated steel sheet, hot pressing was performed under the conditions shown in Tables 2A to 2C. Hot pressing involved clamping the flat steel sheet with a water-cooled mold and applying pressure to facilitate the fabrication of specimens for tensile testing and observation of the metal structure. This resulted in the hot-pressed bodies shown in Tables 3A to 3C. Furthermore, for manufacture No. 77, tempering at 300°C for 20 minutes was performed after hot pressing.

[0205] Next, the metal structure observation and the tensile strength measurement were performed by the above-described method.

[0206] In the case where the obtained tensile strength was 980 MPa or more, it was considered to have high strength, and was determined to be acceptable. On the other hand, in the case where the obtained tensile strength was less than 980 MPa, it was considered not to have high strength, and was determined to be unacceptable. In addition, in the case where the obtained tensile strength exceeded 1380 MPa, it was considered to have excessively high strength, and was determined to be unacceptable.

[0207] The bending property of the hot press-formed body was evaluated based on the VDA standard (VDA 238-100) prescribed by the German Automotive Industry Association, using the following method. In the present embodiment, the displacement at the time of the maximum load obtained in the bending test was converted to an angle according to the VDA standard, and the bending angle a (°) was calculated.

[0208] The conditions in the bending test were as follows.

[0209] Sample size: 60 mm (rolling direction) x 30 mm (direction parallel to the plate width direction)

[0210] Sample plate thickness: 1.6 mm

[0211] Bending edge: direction parallel to the rolling direction, direction forming 45° with the rolling direction, direction perpendicular to the rolling direction

[0212] Test method: roll support, punch press-in

[0213] Roll diameter:

[0214] Punch shape: tip R = 0.4 mm

[0215] Roll distance: 2.0 x plate thickness (mm) + 0.5 mm

[0216] Press-in speed: 20 mm / min

[0217] Testing machine: SHIMADZU AUTOGRAPH 20 kN

[0218] From the bending test in three directions, the average bending angle a (°) and the anisotropy of the bending property Da (%) were calculated as follows. m

[0219] a m = (a L + 2 x a D + a C ) / 4

[0220] Da = (a m - a min ) / a m x 100​

[0221] α L : α when bent with an axis parallel to the rolling direction

[0222] α D : α when bent with an axis making 45° with the rolling direction

[0223] α C : α when bent with an axis perpendicular to the rolling direction

[0224] α min : the minimum value among α L , α D , and α C

[0225] Further, in the case where the rolling direction is not clear, a test can be performed in five directions at intervals of 22.5°, and the direction giving the minimum value of the bending angle α is regarded as the rolling direction to find α L , α D , and α C . In the case where the rolling direction is not clear even if this method is employed, a test can be performed in ten directions at intervals of 11.25°, and the direction giving the minimum value of the bending angle α is regarded as the rolling direction to find α L , α D , and α C .

[0226] α m is affected by the sheet thickness and the tensile strength. In addition, α m is also affected by the plating layer of the hot press-formed body. In the case where an aluminum plating layer is used, since a hard Fe-Al alloy layer is formed on the surface in heating in hot pressing, α m is lower than in the case of GA, GI, CR (a test piece on which a shot blasting treatment is performed after hot pressing), or EG.

[0227] In the present embodiment, in the case where α m is equal to or higher than the lower limit value shown in Table 4, it is regarded as excellent in bendability, and is determined to be acceptable. On the other hand, in the case where α m is lower than the lower limit value shown in Table 4, it is regarded as poor in bendability, and is determined to be unacceptable.

[0228] In addition, in the case where Δα is 15% or less, it is regarded as small in anisotropy of bendability, and is determined to be acceptable. On the other hand, in the case where Δα exceeds 15%, it is regarded as large in anisotropy of bendability, and is determined to be unacceptable.

[0229] In addition, the corrosion resistance was evaluated by the method prescribed in JASO M609-91 established by the Japan Automobile Technology Council. Specifically, the evaluation was performed by the following method. ​

[0230] A sample was taken from the hot press-formed body, a linear scratch of 70 mm in length was formed in the planar portion of the sample after an electrodeposited coating film was applied to the sample at a thickness of 15 μm using a knife, and the sample was subjected to a cyclic corrosion test. The sample after 120 cycles was immersed in a commercially available coating film peeling agent for 30 minutes, and then the coating film was peeled off using a brush. Subsequently, the sample was immersed in a 5% by volume aqueous ammonium citrate solution containing a corrosion inhibitor for a steel sheet, and rust generated in the corroded portion was removed using a brush. Using a digital microscope VHX-7000 manufactured by Keyence Corporation, the thickness reduction from the reference surface was measured for each of 35 mm from both ends of the 70 mm scratch, and the maximum value was obtained. The reference surface was the surface of the uncorroded portion after peeling of the coating film, regardless of the presence or absence of the plated layer. The average of the maximum values of the two thickness reductions obtained was calculated.

[0231] The average of the maximum values of the thickness reductions obtained was evaluated according to the following criteria. In the case of the AL plated steel sheet, in the cases where the evaluation was El, E2, VI, it was judged to be a hot press-formed body having particularly excellent corrosion resistance. In the cases where the plated layer was GA, GI, or EG, in the cases where the evaluation was V2 or more, it was judged to be a hot press-formed body having particularly excellent corrosion resistance. In the case of the cold-rolled steel sheet CR, the cases where the evaluation was G or more were judged to be hot press-formed bodies having particularly excellent corrosion resistance.

[0232] Further, the following evaluation shows the corrosion resistance in the order of B, G, V2, VI, E2, El from poor to more excellent.

[0233] El (Excellent-1): less than 0.03 mm

[0234] E2 (Excellent-2): 0.03 mm or more and less than 0.05 mm

[0235] VI (Very Good-1): 0.05 mm or more and less than 0.07 mm

[0236] V2 (Very Good-2): 0.07 mm or more and less than 0.10 mm

[0237] G (Good): 0.10 mm or more and less than 0.15 mm

[0238] B (Bad): 0.15 mm or more

[0239] The results above are shown in Tables 3A to 3C. As seen from Tables 3A to 3C, in the examples of the present application, a hot press-molded body having high strength and excellent bendability, and small anisotropy of bendability was obtained. On the other hand, it was found that in the comparative examples, one or more of the above-mentioned characteristics did not satisfy the qualification criteria.

[0240]

[0241]

[0242]

[0243]

[0244]

[0245]

[0246] Table 3A

[0247]

[0248] Table 3B

[0249]

[0250] Underlined indicates outside the scope of the present application, or the characteristics are not ideal.

[0251] Table 3C

[0252]

[0253] Underlined indicates outside the scope of the present application, or the characteristics are not ideal. Tempering at 300°C for 20 minutes was performed after hot pressing.

[0254]

[0255] Industrial applicability

[0256] According to the above-described mode relating to the present application, a hot press-molded body having high strength and excellent bendability, and small anisotropy of bendability can be provided.

Claims

1. A hot-pressed molded body, characterized in that, Chemical composition, expressed in mass percent, contains C：0.050～0.150％、 Si: 0.010~1.000% Mn: 1.00~2.00% Al：0.001～0.500％、 P: below 0.100% S: below 0.0100% N: below 0.0100% B：0.0005～0.0050％、 Cr：0～0.50％、 Mo: 0–0.500% Ni: 0-3.00% Cu: 0–3.00% Co: 0-0.50% Sn: 0~0.500% Ca: 0–0.0050% Mg: 0–0.0050% REM: 0–0.0050%, and Sb: 0~0.0200%, and, Contains one or both of the following: Ti: 0.005–0.100% and Zr: 0.005–0.100%. Contains one or two of the following: Nb: 0.015–0.100% and V: 0.005–0.100%. The balance includes Fe and impurities. Satisfying the following equations (1) and (2), The metallic microstructure contains over 90% martensite by area, of which a soft region comprises 5-25% by area. This soft region is defined as an area with an average grain image quality value of 123,000-200,000. 561-474×C-33×Mn-17×Cr-17×Ni-7.5×Si-21×Mo+10×Co>440…(1)0.265×Ti+0.140×Zr>N…(2) In the above formulas (1) and (2), the element symbols represent the content of each element in terms of mass % and are substituted with 0 if the element is not present.

2. The hot-pressed molded body according to claim 1, characterized in that, The chemical composition contains, in mass percent: Cr：0.005～0.50％、 Mo: 0.005~0.500% Ni: 0.005~3.00% Cu: 0.005–3.00% Co: 0.005~0.50% Sn: 0.005~0.500% Ca: 0.0005~0.0050% Mg: 0.0005~0.0050% REM: 0.0005~0.0050%, and Sb: 0.0005~0.0200% One or more of them.

3. The hot-pressed molded body according to claim 1, characterized in that, The chemical composition contains, in mass percent: Co: 0.005~0.50% Sn: 0.005–0.500%, and Sb: 0.0005~0.0200% One or more of them.

4. The hot-pressed molded article according to any one of claims 1 to 3, characterized in that, In the aforementioned metallic microstructure, the number density of carbides with an average particle size of 20–500 nm and containing one or more of Nb, Ti, Zr, and V is 0.3–10.0 particles / μm. 2 .

5. The hot-pressed molded article according to any one of claims 1 to 3, characterized in that, It has a coating on the surface.

6. The hot-pressed molded body according to claim 4, characterized in that, It has a coating on the surface.

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