Patent Application
20240183015
The present invention relates to a steel sheet for hot stamping and a hot-stamping formed body.
Priority is claimed on Japanese Patent Application No. 2021-081620, filed May 13, 2021, the content of which is incorporated herein by reference.
In the related art, from the viewpoint of global environmental problems and collision safety performance, thinning and high-strengthening of vehicle members have been required. In order to meet these demands, the number of vehicle members made of a high strength steel sheet as a material is increasing. In addition, as a forming method of a high strength steel sheet, a method called hot stamping is known. In the hot stamping, a high strength steel sheet is press-formed in a high temperature range of 700° C. or higher and quenched inside or outside a press die. According to the hot stamping, since forming is performed in a high temperature range in which the strength of the steel sheet decreases, it is possible to suppress forming defects that occur in cold pressing. In addition, since a structure having martensite as a primary phase is obtained by quenching after forming, high strength can be obtained. For this reason, hot-stamping formed bodies having a tensile strength of about 1,500 MPa are widely used worldwide.
In order to obtain a higher effect of reducing the weight of a vehicle body from a vehicle member into which a high strength steel sheet is formed by hot stamping, it is necessary to obtain a member that has high strength and is also excellent in collision characteristics. In order to improve the collision characteristics of vehicle members, particularly, vehicle members are required to have excellent bendability.
Patent Document 1 discloses a hot-stamping formed body having a tensile strength of 1,900 MPa or more and capable of suppressing low-stress fracture and a manufacturing method thereof.
The present inventors found that, in a vehicle member having an improved tensile strength, it is necessary to further improve the bendability in order to obtain a higher effect of reducing the weight of a vehicle body.
[Patent Document 1] PCT International Publication No. WO2018/134874
Non-Patent Document 1: Acta Materialia, 58 (2010), 6393-6403
The present invention has been made in view of the above-mentioned problem. An object of the present invention is to provide a hot-stamping formed body having high strength and excellent bendability, and a steel sheet for hot stamping capable of manufacturing this hot-stamping formed body.
The gist of the present invention is as follows.
[1] A steel sheet for hot stamping according to an aspect of the present invention includes, as a chemical composition, by mass %:
[2] The steel sheet for hot stamping according to [1], in which the steel sheet for hot stamping may contain, as the chemical composition, by mass %, one or more selected from the group consisting of:
[3] A hot-stamping formed body according to another aspect of the present
[4] The hot-stamping formed body according to [3], in which the hot-stamping formed body may contain, as the chemical composition, by mass %, one or more selected from the group consisting of:
[5] In the hot-stamping formed body according to [3] or [4], an area ratio of the prior austenite grains having an average grain size of 0.5 to 3.0 μm may be 60% or less.
According to the above-described aspects of the present invention, it is possible to provide a hot-stamping formed body having high strength and excellent bendability, and a steel sheet for hot stamping capable of manufacturing this hot-stamping formed body.
The present inventors examined bendability of a hot-stamping formed body. As a result, the present inventors found that in a microstructure of the hot-stamping formed body, the bendability deteriorates when a large amount of fine prior austenite grains are present. In addition, the present inventors found that, in the microstructure of the hot-stamping formed body, when prior austenite grains are set to a desired size and unevenness in the size of the prior austenite grains is suppressed, that is, the prior austenite grains are grain-sized, the bendability of the hot-stamping formed body can be further improved.
Next, the present inventors examined a method for obtaining the above-described hot-stamping formed body. As a result, the present inventors found that the above-described hot-stamping formed body can be obtained by controlling an area ratio of ferrite and an area ratio of granular bainite so as to form a desired amount of ferrite and granular bainite in a microstructure of a steel sheet for hot stamping and to have a desired relationship.
Hereinafter, the steel sheet for hot stamping and the hot-stamping formed body according to the present embodiment made based on the above-described findings will be described. First, the reason why the chemical composition of the steel sheet for hot stamping according to the present embodiment is to be limited will be described.
A limited numerical range described using “to” to be described below includes a lower limit and an upper limit. Numerical values represented using “less than” or “more than” are not included in a numerical range. All percentages (%) related to the chemical composition mean mass %.
The steel sheet for hot stamping according to the present embodiment includes, as a chemical composition, by mass %, C: more than 0.40% and 0.70% or less, Si: 0.010% to 1.30%, Mn: more than 0.60% and 3.00% or less, P: 0.100% or less, S: 0.0100% or less, N: 0.0130% or less, O: 0.0200% or less, Al: 0.0010% to 0.500%, Cr: 0.010% to 0.80%, and a remainder including Fe and impurities. Each element will be described below.
C: more than 0.40% and 0.70% or less
C greatly contributes to improvement in the strength of the hot-stamping formed body. When the C content is 0.40% or less, it becomes difficult to obtain sufficient strength in the hot-stamping formed body. For this reason, the C content is set to more than 0.40%. The C content is preferably 0.42% or more, more preferably 0.45% or more, and still more preferably 0.47% or more.
Meanwhile, when the C content is more than 0.70%, coarse carbides are generated and the bendability of the hot-stamping formed body deteriorates. Therefore, the C content is set to 0.70% or less. The C content is preferably 0.65% or less and more preferably 0.60% or less.
Si: 0.010% to 1.30%
Si is an element that improves distortion capability of the hot-stamping formed body by suppressing the formation of an oxide which is combined with oxygen and becomes an origin of fracture. When the Si content is less than 0.010%, a coarse oxide is formed in the hot-stamping formed body, and desired bendability cannot be obtained. Therefore, the Si content is set to 0.010% or more. The Si content is preferably 0.05% or more and more preferably 0.10% or more.
Meanwhile, when the Si content is more than 1.30%, a coarse oxide is formed, and the bendability of the hot-stamping formed body deteriorates. For this reason, the Si content is set to 1.30% or less. The Si content is preferably less than 1.00% and more preferably 0.50% or less.
Mn: more than 0.60% and 3.00% or less
Mn stabilizes austenite and improves the hardenability of the steel sheet. When the Mn content is 0.60% or less, sufficient hardenability cannot be obtained. Therefore, the Mn content is set to more than 0.60%. The Mn content is preferably 0.80% or more and more preferably 1.20% or more.
Meanwhile, when the Mn content is more than 3.00%, coarse inclusions are generated and the bendability of the hot-stamping formed body deteriorates. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.20% or less and more preferably 1.80% or less.
P: 0.100% or less
P segregates in the grain boundaries of the steel sheet and deteriorates the bendability of the hot-stamping formed body. Therefore, the lower P content is more preferable. In particular, when the P content is more than 0.100%, the workability of the steel sheet and the bendability of the hot-stamping formed body significantly deteriorate. For this reason, the P content is set to 0.100% or less. The P content is preferably 0.080% or less and more preferably 0.020% or less.
The lower limit of the P content is not particularly limited and may be 0%. However, when the P content is reduced to less than 0.0001%, the dephosphorization cost increases significantly, which is not preferable economically. For this reason, the P content may be set to 0.0001% or more.
S: 0.0100% or less
S forms coarse inclusions and deteriorates the bendability of the hot-stamping formed body. Accordingly, the lower S content is more preferable. In particular, when the S content is more than 0.0100%, the formability of the steel sheet and the bendability of the hot-stamping formed body significantly deteriorate. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0050% or less and more preferably 0.0010% or less.
The lower limit of the S content is not particularly limited and may be 0%. However, when the S content is reduced to less than 0.0001%, the desulfurization cost increases significantly, which is not preferable economically. For this reason, the S content may be set to 0.0001% or more.
N: 0.0130% or less
N forms a coarse nitride and deteriorates the bendability of the hot-stamping formed body. Therefore, the lower N content is more preferable. In particular, when the N content is more than 0.0130%, the formability of the steel sheet significantly deteriorates. Therefore, the N content is set to 0.0130% or less. The C content is preferably 0.0100% or less or 0.0070% or less and more preferably 0.0040% or less.
The lower limit of the N content is not particularly limited and may be 0%. However, when the N content is reduced to less than 0.0001%, the denitrification cost increases significantly, which is not preferable economically. For this reason, the N content may be set to 0.0001% or more.
O: 0.0200% or less
O forms a coarse oxide in steel and deteriorates the bendability of the hot-stamping formed body. Therefore, the lower O content is more preferable. In particular, when the O content is more than 0.0200%, the bendability of the hot-stamping formed body significantly deteriorates. Therefore, the O content is set to 0.0200% or less. The O content is preferably 0.0100% or less and more preferably 0.0060% or less.
The lower limit of the O content is not particularly limited and may be 0%. However, when the O content is reduced to less than 0.0001%, the manufacturing cost increases significantly, which is not preferable economically. Therefore, the O content may be set to 0.0001% or more.
Al: 0.0010% to 0.500%”
Al is an element that improves the distortion capability by deoxidizing molten steel to suppress the formation of oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body. In a case where the Al content is less than 0.0010%, deoxidation is not sufficiently performed and a coarse oxide is generated. As a result, the above-mentioned effects cannot be obtained. For this reason, the Al content is set to 0.0010% or more. The Al content is preferably 0.010% or more and more preferably 0.030% or more.
Meanwhile, when the Al content is more than 0.500%, a coarse oxide is formed in steel, and the bendability of the hot-stamping formed body deteriorates. Therefore, the Al content is set to 0.500% or less. The Al content is preferably 0.450% or less and more preferably 0.350% or less.
Cr: 0.010% to 0.80%
Cr increases the strength of the hot-stamping formed body by dissolving in prior austenite grains during heating at the time of hot stamping. When the Cr content is less than 0.010%, this effect cannot be obtained. Therefore, the Cr content is set to 0.010% or more. The Cr content is preferably 0.10% or more and more preferably 0.20% or more.
Meanwhile, when the Cr content is more than 0.80%, a coarse carbide is formed and the bendability of the hot-stamping formed body deteriorates. Therefore, the Cr content is set to 0.80% or less. The Cr content is preferably 0.60% or less and more preferably 0.40% or less.
The remainder of the chemical composition of the steel sheet for hot stamping according to the present embodiment may be Fe and impurities. An example of the impurities includes an element that is unavoidably incorporated from a steel raw material or scrap and/or during a steelmaking process and is allowed in a range in which properties of the hot-stamping formed body according to the present embodiment are not inhibited.
The steel sheet for hot stamping according to the present embodiment may contain the following elements as arbitrary elements instead of a part of Fe. The contents of the following arbitrary elements, which are obtained in a case where the following arbitrary elements are not contained, are 0%.
Nb: 0% to 0.100%
Nb forms carbonitride in steel to improve the strength of the hot-stamping formed body by precipitation hardening. In order to obtain this effect, the Nb content is preferably set to 0.001% or more.
Meanwhile, when the Nb content is more than 0.100%, a large amount of carbonitride is formed in steel, and the bendability of the hot-stamping formed body deteriorates. Therefore, the Nb content is set to 0.100% or less.
Ti: 0% to 0.100%
Similar to Nb, Ti forms carbonitride in steel to improve the strength of the hot-stamping formed body by precipitation hardening. In order to obtain the effects, a Ti content is preferably set to 0.010% or more.
Meanwhile, when the Ti content is more than 0.100%, a large amount of carbonitride is formed in steel, and the bendability of the hot-stamping formed body deteriorates. For this reason, the Ti content is set to 0.100% or less.
B: 0% to 0.0100%
B improves the hardenability of the steel and improves the strength of the hot-stamping formed body. In order to obtain the effects, the B content is preferably set to 0.0015% or more.
Meanwhile, when the B content is more than 0.0100%, a coarse carbide is generated and the bendability of the hot-stamping formed body deteriorates. Therefore, the B content is set to 0.0100% or less.
Mo: 0% to 1.00%
Mo improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body. In order to obtain the effects, the Mo content is preferably set to 0.05% or more.
Meanwhile, when the Mo content is more than 1.00%, a coarse carbide is generated and the bendability of the hot-stamping formed body deteriorates. Therefore, the Mo content is set to be 1.00% or less.
Co: 0% to 2.00%
Co improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body. In order to reliably exert the effects, it is preferable that the Co content is set to 0.05% or more.
Meanwhile, when the Co content is more than 2.00%, a coarse carbide is generated and the bendability of the hot-stamping formed body deteriorates. For this reason, the Co content is set to 2.00% or less.
Ni: 0% or more and less than 3.00%
Ni improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body. In order to obtain the effects, the Ni content is preferably set to 0.01% or more.
Meanwhile, when the Ni content is 3.00% or more, segregation is promoted and the bendability of the hot-stamping formed body deteriorates. Therefore, the Ni content is set to less than 3.00%.
Cu: 0% to 1.00%
Similar to Ni, Cu improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body. In order to obtain the effects, the Cu content is preferably set to 0.01% or more.
Meanwhile, when the Cu content is more than 1.00%, segregation is promoted and the bendability of the hot-stamping formed body deteriorates. Therefore, the Cu content is set to 1.00% or less.
V: 0% to 1.00%
V improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body. In order to obtain the effects, the V content is preferably set to 0.01% or more.
Meanwhile, when the V content is more than 1.00%, coarse carbides are generated and the bendability of the hot-stamping formed body deteriorates. Therefore, the V content is set to 1.00% or less.
W: 0% to 1.000%
W improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body. In order to obtain the effects, the W content is preferably set to 0.001% or more.
Meanwhile, when the W content is more than 1.000%, segregation is promoted and the bendability of the hot-stamping formed body deteriorates. Therefore, the W content is set to 1.000% or less.
Ca: 0% to 0.010%
Ca improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improve the bendability of the hot-stamping formed body. In order to reliably obtain the effects, the Ca content is preferably set to 0.001% or more.
Meanwhile, when the Ca content is more than 0.010%, a coarse oxide is formed, and the bendability of the hot-stamping formed body deteriorates. Therefore, the Ca content is set to 0.010% or less.
Mg: 0% to 1.000%
Mg improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body. In order to obtain the effects, the Mg content is preferably set to 0.001% or more.
Meanwhile, when the Mg content is more than 1.000%, a coarse oxide is generated, and the bendability of the hot-stamping formed body deteriorates. Therefore, the Mg content is set to 1.000% or less.
REM: 0% to 1.000%
REM improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body. In order to obtain the effects, the REM content is preferably set to 0.001% or more.
Meanwhile, when the REM content is more than 1.000%, a coarse oxide is generated, and the bendability of the hot-stamping formed body deteriorates. Therefore, the REM content is set to 1.000% or less.
In the present embodiment, REM refers to a total of 17 elements that are composed of Sc, Y, and lanthanoid and the REM content refers to the total content of these elements.
Sb: 0% to 1.000%
Sb improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body. In order to obtain the effects, the Sb content is preferably set to 0.005% or more.
Meanwhile, when the Sb content is more than 1.000%, a coarse oxide is generated, and the bendability of the hot-stamping formed body deteriorates. Therefore, the Sb content is set to 1.000% or less.
Zr: 0% to 1.000%
Zr improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body. In order to obtain the effects, the Zr content is preferably set to 0.001% or more.
Meanwhile, when the Zr content is more than 1.000%, a coarse oxide is generated, and the bendability of the hot-stamping formed body deteriorates. Therefore, the Zr content is set to 1.000% or less.
Sn: 0% to 1.000%
Sn improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body. In the case of reliably obtaining the effects, the Sn content is preferably set to 0.001% or more.
Meanwhile, since the above effects are saturated even when a large amount of Sn is contained, the Sn content is set to 1.000% or less.
As: 0% to 0.100%; and
As refines the prior austenite grains by lowering an austenite single-phase formation temperature and improve the bendability of the hot-stamping formed body. In the case of reliably obtaining the effects, the As content is preferably set to 0.001% or more.
Meanwhile, since the above effects are saturated even when a large amount of As is contained, the As content is set to 0.100% or less.
The above-mentioned chemical composition of the steel sheet for hot stamping may be measured by an ordinary analysis method. For example, the chemical composition of the steel sheet for hot stamping may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-nondispersive infrared absorption method. In a case where a plating layer is provided on the surface of the steel sheet for hot stamping, the chemical composition may be analyzed after the plating layer is removed by mechanical grinding.
Next, the microstructure of the steel sheet for hot stamping according to the present embodiment will be described.
The steel sheet for hot stamping according to the present embodiment has a microstructure in which Sα+SGB, which is a total of an area ratio Sα of ferrite and an area ratio SGB of granular bainite, is 10% or more and less than 50%, and SGB/Sα which is a ratio between the area ratio SGB of the granular bainite and the area ratio Sα of the ferrite is 0.30 to 0.70. Hereinafter, each specification will be described.
In addition, in the present embodiment, it should be noted that, in a sheet thickness cross section parallel to a rolling direction, the microstructure is specified at a ¼ depth position of the sheet thickness from the surface (in a region from a ⅛ depth of the sheet thickness from the surface to a ⅜ depth of the sheet thickness from the surface). The reason therefor is that the microstructure at this position indicates a typical microstructure of the steel sheet.
“Sα+SGB, which is total of area ratio Sα of ferrite and area ratio SGB of a granular bainite, is 10% or more and less than 50%”
When Sα+SGB, which is the total of the area ratio Sα of the ferrite and the area ratio SGB of the granular bainite, is less than 10%, the prior austenite grains cannot be grain-sized in the hot-stamping formed body, and as a result, it is not possible to obtain a hot-stamping formed body having excellent bendability. Since the solid solubility limits of carbon in ferrite and granular bainite are low, by setting Sα+SGB to 10% or more and setting SGB/Sα to be described below within a desired range, carbon diffuses into ferrite grain boundaries, and a segregation region of carbon is formed at ferrite grain boundaries. During hot stamping, the segregation region of carbon becomes the origin of the prior austenite grains, so that the prior austenite grains are uniformly dispersed and formed. As a result, it is presumed that prior austenite grains can be grain-sized in the hot-stamping formed body. Sα+SGB is preferably 20% or more and more preferably 30% or more.
Meanwhile, when Sα+SGB is 50% or more, segregation of carbon into ferrite grain boundaries is excessively promoted, the generation density of carbides at ferrite grain boundaries increases, and the prior austenite grains cannot be uniformly dispersed and generated after the hot stamping. Sα+SGB is preferably 40% or less.
“SGB/Sα which is a ratio between area ratio SGB of granular bainite and area ratio Sα of ferrite is 0.30 to 0.70”
SGB/Sα is set to 0.30 to 0.70. Since ferrite does not include subgrain boundaries, carbon is less likely to be segregated in the grains than granular bainite. Therefore, by controlling the area ratio of the ferrite and granular bainite to the above range, the amount of segregation of carbon at ferrite grain boundaries can increase. The subgrain boundaries contained in the grains of granular bainite can serve as the segregation origins of carbon and thus function as the origins of prior austenite during hot stamping heating. Accordingly, the average grain size of the prior austenite grains in the hot-stamping formed body can be controlled to 25 μm or less. SGB/Sα is preferably 0.40% or more.
Meanwhile, when SGB/Sα is more than 0.70, the segregation of carbon to subgrain boundaries is excessively promoted, and the distance between the austenite grains becomes short during hot stamping heating. Therefore, the average grain size of the prior austenite grains cannot be controlled to 5 μm or more. Therefore, SGB/Sα is set to 0.70 or less. SGB/Sα is preferably 0.50 or less.
In the microstructure of the steel sheet for hot stamping according to the present embodiment, the remainder in microstructure is one or more of pearlite, martensite, lower bainite, residual austenite, and tempered martensite. The area ratio of the remainder in the microstructure may be set to more than 50% and 90% or less in consideration of the relationship with Sα+SGB.
A sample is cut out from an arbitrary position (a position that avoids an end portion in a case where the sample cannot be collected at this position) away from an end surface of the steel sheet for hot stamping by a distance of 50 mm or more so that a sheet thickness cross section parallel to a rolling direction can be observed. The size of the sample also depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction.
The cross section of the sample is polished using silicon carbide paper having a grit of #600 to #1500, then, is finished as a mirror surface using liquid in which diamond powder having a grain size of 1 to 6 μm is dispersed in diluted solution of alcohol or the like or pure water and finish-polished by electrolytic polishing. Next, in a region that has a length of 100 μm and between the ⅛ depth of the sheet thickness from the surface and the ⅜ depth of the sheet thickness from the surface at an arbitrary position on the cross section of the sample in a longitudinal direction so that the ¼ depth position of the sheet thickness from the surface can be observed, the structure is observed using a device including a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5-type detector manufactured by TSL Solutions). The scanning electron microscope used is equipped with a secondary electron detector. In a vacuum of 9.6×10−5 Pa or less, the sample is irradiated with an electron beam at an acceleration voltage of 15 kV and an irradiation current level of 13, and a secondary electron image is photographed with the scanning electron microscope.
In the obtained photographed photograph, a region where cementite is precipitated in a lamellar shape in the grains is determined as pearlite. Lath-shaped grains are determined as lower bainite, martensite, and tempered martensite. Next, EBSD analysis is performed on the same visual field at an analysis speed of 200 to 300 points/sec using an EBSD analyzer. The area ratio Sα of the ferrite and the area ratio SGB of the granular bainite are calculated using the “Grain Average Misorientation” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer. With this function, for grains having a body-centered structure, it is possible to calculate an orientation difference between adjacent measurement points and then obtain an average value of all measurement points in the grains. For the crystal orientation information obtained by the EBSD analysis, a region surrounded by grain boundaries having an average crystal orientation difference of 5° or more is defined as a grain, and a map is drawn by the “Grain Average Misorientation” function. In a region where regions determined to be pearlite, lower bainite, martensite, and tempered martensite are excluded from the map, a region where an average crystal orientation difference in grains is less than 0.4° is determined as ferrite, and a region where the average crystal orientation difference in grains is 0.4° or more and 3.0° or less is determined as granular bainite. An area ratio of the region determined as ferrite is calculated, so that the area ratio of ferrite is obtained. The area ratio of the granular bainite is obtained by calculating the area ratio of the region determined to be the granular bainite.
The steel sheet for hot stamping according to the present embodiment may have a plating layer formed on the surface for the purpose of improving corrosion resistance after hot stamping. The plating layer may be any of an electroplating layer and a hot-dip plating layer. The electroplating layer includes, for example, an electrogalvanized layer, an electrolytic Zn—Ni alloy plating layer, and the like. The hot-dip plating layer includes, for example, a hot-dip galvanized layer, a hot-dip galvannealed layer, a hot-dip aluminum plating layer, a hot-dip Zn—Al alloy plating layer, a hot-dip Zn—Al—Mg alloy plating layer, a hot-dip Zn—Al—Mg—Si alloy plating layer, and the like. An adhesion amount of a plating layer is not particularly limited and may be a general adhesion amount.
The sheet thickness of the steel sheet for hot stamping according to the present embodiment is not particularly limited, but is preferably 0.5 to 3.5 mm from the viewpoint of a reduction in the weight of the vehicle body or the like.
Next, a hot-stamping formed body according to the present embodiment that is obtained by hot-stamping the above-described steel sheet for hot stamping will be described. The hot-stamping formed body according to the present embodiment has the same chemical composition as the above-described steel sheet for hot stamping. A measurement method of the chemical composition may be the same as that for the steel sheet for hot stamping. In addition, in the hot-stamping formed body according to the present embodiment, the prior austenite grains are grain-sized in the microstructure. That is, the hot-stamping formed body according to the present embodiment has a microstructure in which the average grain size of the prior austenite grains is 5 to 25 μm and the standard deviation of the grain sizes of the prior austenite grains is 0.1 to 2.0 μm.
In addition, in the present embodiment, the microstructure is specified at the ¼ depth position (the region from the ⅛ depth of the sheet thickness from the surface to the ⅜ depth of the sheet thickness from the surface) of the sheet thickness from the surface of the cross section perpendicular to the sheet surface. The reason therefor is that the microstructure at this position indicates a typical microstructure of the hot-stamping formed body. Hereinafter, the microstructure will be described.
“Average grain size of prior austenite grains is 5 to 25 μm”
“Standard deviation of grain size of prior austenite grains is 0.1 to 2.0 μm”
In the microstructure of the hot-stamping formed body, by setting the average grain size of the prior austenite grains to be 5 to 25 μm and setting the standard deviation of the grain sizes of the prior austenite grains to 0.1 to 2.0 μm, the bendability of the hot-stamping formed body can be improved. When the average grain size of the prior austenite grains or the standard deviation of the grain sizes of the prior austenite grains is outside the above range, it is not possible to obtain excellent bendability in the hot-stamping formed body.
The average grain size of the prior austenite grains is preferably 10 μm or more and more preferably 15 μm or more. The average grain size of the prior austenite grains is preferably 20 μm or less.
By setting the standard deviation of the grain sizes of the prior austenite grains to 2.0 μm or less, excellent bendability in the hot-stamping formed body can be obtained. Therefore, the standard deviation of the grain sizes of the prior austenite grains is set to 2.0 μm or less. The standard deviation is preferably 1.2 μm or less, more preferably 1.1 μm or less, and still more preferably 0.4 μm or less.
In an actual operation, since it is difficult to set the standard deviation of the grain sizes of the prior austenite grains to less than 0.1 μm, the substantial lower limit is set to 0.1 μm or more.
When the area ratio of the prior austenite grains having the average grain size of 0.5 to 3.0 μm is 60% or less, more excellent bendability can be obtained in the hot-stamping formed body. Therefore, the area ratio of the prior austenite grains having the average grain size of 0.5 to 3.0 μm may be set to 60% or less. The area ratio is more preferably 50% or less and still more preferably 40% or less.
Next, the measurement method of the average grain size of the prior austenite grains will be described. A sample is cut out from an arbitrary position (a position that avoids an end portion in a case where the sample cannot be collected at this position) away from an end surface of the hot-stamping formed body by a distance of 50 mm or more so that a sheet thickness cross section parallel to a rolling direction can be observed. The size of the sample also depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction. The cross section of the sample is polished using silicon carbide paper having a grit of #600 to #1500, then, is finished as a mirror surface using liquid in which diamond powder having a grain size of 1 to 6 μm is dispersed in diluted solution of alcohol or the like or pure water and finish-polished by electrolytic polishing.
Next, in a region from the ⅛ depth of the sheet thickness from the surface to the ⅜ depth of the sheet thickness from the surface at an arbitrary position of the sample cross section in the longitudinal direction so that the ¼ depth position of the sheet thickness from the surface can be observed and in a region having 100 μm in the length and 100 μm in the sheet thickness direction, a sample is irradiated with an electron beam at an acceleration voltage of 15 kV and an irradiation current level of 13 in a vacuum of 9.6×10−5 Pa or less using the device including a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5-type detector manufactured by TSL Solutions), and the EBSD analysis is performed at an analysis speed of 200 to 300 points/sec. Using the obtained crystal orientation information, the crystal orientation of the prior austenite grains is calculated from a crystal orientation relationship between the general prior austenite grains and grains having a body-centered structure after transformation, and the average grain size of the prior austenite grains is calculated using the calculated crystal orientation.
The method for calculating the crystal orientation of the prior austenite grains is not particularly limited, and for example, the calculation may be performed using the following method. First, the crystal orientation of the prior austenite grains is calculated by the method described in Non-Patent Document 1, and the crystal orientation of the prior austenite in each coordinate of the EBSD-measured region is specified. Next, a crystal orientation map of the prior austenite grain is created using the “Inverse Pole Figure” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer. For one of the prior austenite grains included in the observed visual field, an average value of a shortest diameter and a longest diameter is calculated, and the average value is used as the grain size of the prior austenite grains. The above operation is performed on all the prior austenite grains except for the prior austenite grains which are not entirely included in the photographed visual fields, such as grains in an end portion of the photographed visual field, and the grain sizes of all the prior austenite grains in the photographed visual fields are obtained. The average grain size of the prior austenite grains in the photographed visual fields is obtained by calculating a value obtained by dividing the sum of the obtained grain sizes of the prior austenite grains by the total number of prior austenite grains of which grain sizes are measured. This operation is performed on all the photographed visual fields, and the average grain size of the prior austenite grains of all the photographed visual fields is calculated, thereby obtaining the average grain size of the prior austenite grains.
By calculating the standard deviation from the grain sizes of the prior austenite grains, the standard deviation of the grain sizes of the prior austenite grains is obtained. At this time, in order to eliminate the influence of locally generated fine grains or coarse grains, the standard deviation is calculated by excluding the minimum value and the maximum value of the prior austenite grain sizes.
By calculating a value obtained by dividing the area of the prior austenite grains having an average grain size of 0.5 to 3.0 μm by the area of the entire measurement visual field, the area ratio of the prior austenite grains having an average grain size of 0.5 to 3.0 μm is obtained.
The microstructure of the hot-stamping formed body is not particularly limited as long as desired strength and desired bendability can be obtained after hot stamping. However, the microstructure may include, for example, by area%, ferrite: 0% to 50%, bainite and martensite: 0% to 100%, pearlite: 0% to 30%, and residual austenite: 0% to 5%. The microstructure of the hot-stamping formed body may be measured by the following method.
A sample is cut out from an arbitrary position (a position that avoids an end portion in a case where the sample cannot be collected at this position) away from an end surface of the hot-stamping formed body by a distance of 50 mm or more so that the cross section perpendicular to the sheet surface can be observed. The cross section of the sample is polished using silicon carbide paper having a grit of #600 to #1500, then, is finished as a mirror surface using liquid in which diamond powder having a grain size of 1 to 6 μm is dispersed in diluted solution of alcohol or the like or pure water and is performed on Nital etching. In a region that has a length of 50 μm and between the ⅛ depth of the sheet thickness from the surface and the ⅜ depth of the sheet thickness from the surface at an arbitrary position on the cross section of the sample in a longitudinal direction so that the ¼ depth position of the sheet thickness from the surface can be observed, photographs having a plurality of visual fields are taken using a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.). Evenly spaced grids are drawn in the taken photographs, and structures at grid points are identified. The number of grid points corresponding to each structure is obtained and is divided by the total number of grid points, so that the area ratio of each structure is obtained. The area ratio can be more accurately obtained as the total number of grid points is larger. In the present embodiment, grid spacings are set to 2 μm×2 μm and the total number of grid points is set to 1500.
A region where cementite is precipitated in a lamellar shape in the grains is determined as pearlite. A region in which brightness is low and no sub-microstructure is observed is determined as ferrite. A region in which the brightness is high and the sub-microstructure is not exposed by etching is determined as “martensite or residual austenite”. A region that does not correspond to any of the above-described microstructures is determined as bainite.
The area ratio of martensite is obtained by subtracting the area ratio of residual austenite obtained by EBSD analysis described later from the area ratio of martensite and residual austenite obtained from the taken photographs.
The area ratio of residual austenite is measured using an electron backscatter diffraction method (EBSD). In the analysis by EBSD, a sample collected at the same sample collection position as in the measurement using the above-described taken photograph is used, and the analysis is performed on the region between the ⅛ depth of the sheet thickness from the surface and the ⅜ depth of the sheet thickness from the surface. The sample is polished using silicon carbide paper having a grit of #600 to #1500, then, finished into a mirror surface using liquid in which diamond powder having a grain size of 1 to 6 μm is dispersed in diluted solution of alcohol or the like or pure water, and then finished by electrolytic polishing for the purpose of sufficiently removing strain in a cross section to be measured. In the electrolytic polishing, in order to remove mechanical polishing strain on the observed section, the sample may be polished a minimum of 20 μm and polished a maximum of 50 μm. The sample is preferably polished 30 μm or less in consideration of rollover at the end portion.
With regard to the measurement in EBSD, an acceleration voltage is set to 15 to 25 kV, the measurement is performed at intervals of at least 0.25 μm or less, and the crystal orientation information about each measurement point in a range of 150 μm or more in the sheet thickness direction and 250 μm or more in the rolling direction is obtained. In the obtained crystal structure, a measurement point at which a crystal structure is fcc is determined as residual austenite using “Phase Map” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer. The ratio of measurement points determined as the residual austenite is obtained, thereby obtaining the area ratio of the residual austenite. Here, the larger the number of the measurement points, the more preferable, and thus it is preferable that the measurement intervals are narrow and the measurement range is wide. However, in a case where the measurement intervals are less than 0.01 μm, adjacent points interfere with the expansion width of an electron beam. For this reason, the measurement interval is set to 0.01 μm or more. In addition, the measurement range may be set to 200 μm in the sheet thickness direction and 400 μm in the sheet width direction at a maximum. An EBSD device including a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5-type detector manufactured by TSL Solutions) is used for measurement. In this case, a degree of vacuum in the device is set to 9.6×10−5 Pa or less, the irradiation current level is set to 13, and the irradiation level of the electron beam is set to 62.
The hot-stamping formed body according to the present embodiment may have a plating layer formed on the surface for the purpose of improving corrosion resistance after the hot stamping or the like. The plating layer may be any of an electroplating layer and a hot-dip plating layer. The electroplating layer includes, for example, an electrogalvanized layer, an electrolytic Zn—Ni alloy plating layer, and the like. The hot-dip plating layer includes, for example, a hot-dip galvanized layer, a hot-dip galvannealed layer, a hot-dip aluminum plating layer, a hot-dip Zn—Al alloy plating layer, a hot-dip Zn—Al—Mg alloy plating layer, a hot-dip Zn—Al—Mg—Si alloy plating layer, and the like. An adhesion amount of a plating layer is not particularly limited and may be a general adhesion amount.
The sheet thickness of the hot-stamping formed body according to the present embodiment is not particularly limited. However, in terms of reducing the weight of a vehicle body or the like, it is preferable that the sheet thickness of the hot-stamping formed body according to the present embodiment is set to 0.5 to 3.5 mm.
The hot-stamping formed body according to the present embodiment has a tensile (maximum) strength of 2,200 MPa or more. The tensile strength is preferably 2,400 MPa or more and more preferably 2,550 MPa or more. The tensile strength is obtained according to the test method described in JIS Z 2241:2011 by producing a No. 5 test piece described in JIS Z 2241:2011 from a position as flat as possible in the hot-stamping formed body.
In addition, in the hot-stamping formed body according to the present embodiment, the maximum bending angle that is obtained by a bending test based on the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry is preferably 20° or more. The maximum bending angle is more preferably 30° or more or 40° or more. The conditions in the bending test were as described below.
Next, a manufacturing method of the steel sheet for hot stamping according to the present embodiment will be described.
In the manufacturing method of the steel sheet for hot stamping according to the present embodiment, in order to obtain the steel sheet for hot stamping having the above-described microstructure, the final rolling reduction of the finish rolling in the hot rolling is preferably set to 40% to 80%. Normally, the final rolling reduction of the finish rolling is less than 10%, but in the present embodiment, it is preferable to set the final rolling reduction to be higher than a normal final rolling reduction.
A steel piece (steel material) to be subjected to hot rolling may be a steel piece manufactured by an ordinary method, and may be, for example, a steel piece manufactured by a general method such as a continuous cast slab or a thin slab caster. In addition, in a casting step, the steel piece after solidification may be rolled at a rolling reduction of 30% to 70% in a temperature range in which a center temperature of a slab is 1,200° C. or higher and equal to or lower than a solidus temperature. As a result, the segregation of Mn is relaxed, which makes it possible to improve the bendability of the hot-stamping formed body. The solidus temperature can be obtained from Expression (1).
Solidus temperature (° C.)=1536−(415.5×% C+12.3×% Si+6.8×% Mn+124.5×% P+183.9×% S+4.3×% Ni+1.4×% Cr+4.1×% Al) (1)
In Expression (1), % C, % Si, % Mn, % P, % S, % Ni, % Cr, and % Al mean the content (mass %) of each element.
In the hot rolling, rough rolling and finish rolling are performed. In the finish rolling, the slab after the rough rolling is rolled by a plurality of finishing mills. In the present embodiment, the finish rolling is preferably performed so that the rolling reduction (final rolling reduction) in the final pass of the finish rolling becomes 40% or more. When the sheet thickness before the final pass of the finish rolling is to and the sheet thickness after the final pass of the finish rolling is ti, the final rolling reduction can be represented by {(t0−t1)/t0}×100 (%).
By setting the final rolling reduction of the finish rolling to 40% to 80%, the prior austenite grains are refined, and the origins of ferrite and granular bainite increase. As a result, in the microstructure of the steel sheet for hot stamping, Sα+SGB and SGB/Sα can be set within desired ranges. When the final rolling reduction of the finish rolling is less than 40%, in the microstructure of the steel sheet for hot stamping, Sα+SGB and SGB/Sα cannot be set within desired ranges. Therefore, the final rolling reduction of the finish rolling is preferably set to 40% or more. The final rolling reduction of the finish rolling is preferably 50% or more. Meanwhile, when the final rolling reduction of the finish rolling is more than 80%, SGB/Sα cannot be controlled to 0.70 or less. Therefore, the final rolling reduction of the finish rolling is preferably set to 80% or less. The final rolling reduction is more preferably less than 70%.
The heating temperature and holding time of the steel piece before hot rolling are not particularly limited, but it is preferable that the steel piece is held in a temperature range of 1,200° C. or higher for 20 minutes or longer.
After the finish rolling, the steel sheet is preferably coiled in the temperature range of 400° C. to 750° C. When the coiling temperature is higher than 750° C., ferritic transformation is excessively promoted, and SGB+Sα becomes 50% or more and SGB/Sα becomes less than 0.30. The coiling temperature is preferably 700° C. or lower and more preferably 660° C. or lower.
In addition, the coiling temperature is preferably 400° C. or higher. When the coiling temperature is lower than 400° C., the formation of granular bainite is suppressed, and SGB/Sα becomes less than 0.30. The coiling temperature is preferably 450° C. or higher and more preferably 530° C. or higher.
In addition, after the finish rolling (after the completion of hot rolling), the cooling is preferably performed after 2.5 seconds or longer elapses. The cooling mentioned here is cooling that does not include air cooling and has an average cooling rate of 50 to 200° C./s. When the time from the finish rolling to the start of cooling is shorter than 2.5 seconds, a desired amount of Sα+SGB may not be obtained.
After the coiling, cold rolling may be performed as necessary. In addition, the above-mentioned plating may be formed after finish rolling or after cold rolling. Pickling may be performed between the hot rolling and the cold rolling. In the cold rolling, a normal cumulative rolling reduction, for example, 30% to 90% may be set. In addition, temper rolling may be performed under normal conditions. In addition, for the purpose of softening the hot-rolled steel sheet, hot-rolled sheet annealing in which the hot-rolled steel sheet is heated to a temperature range of 730° C. or lower may be performed.
The steel sheet for hot stamping according to the present embodiment can be manufactured by the above method. Next, a manufacturing method of the hot-stamping formed body according to the present embodiment that can be manufactured using the above-described steel sheet for hot stamping will be described. The manufacturing method of the hot-stamping formed body according to the present embodiment is not particularly limited, and for example, the following manufacturing method may be used.
First, the above-mentioned steel sheet for hot stamping is heated in a temperature range of 800° C. or higher. When the heating temperature is lower than 800° C., there are cases where coarse carbides that are being heated remain and the bendability of the hot-stamping formed body decreases. The heating temperature is preferably 820° C. or higher and more preferably 860° C. or higher.
The upper limit of the heating temperature is not particularly limited. However, when the heating temperature is too high, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot-stamping formed body decreases. Therefore, the heating temperature is preferably 1,000° C. or lower, more preferably 960° C. or lower, and even more preferably 930° C. or lower.
The holding time at the heating temperature is preferably 1.0 to 10.0 minutes. When the holding time is shorter than 1.0 minutes, there are cases where coarse carbides remain and the bendability of the hot-stamping formed body decreases. Meanwhile, when the holding time is more than 10.0 minutes, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot-stamping formed body may decrease.
In addition, the average heating rate up to the heating temperature is preferably set to 1.0° C./s or faster. When the average heating rate is slower than 1.0° C./s, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot-stamping formed body decreases. Although the upper limit of the average heating rate is not particularly determined, since it is difficult to set the upper limit to faster than 1,000° C./s in actual operation, the actual upper limit is 1,000° C./s or slower.
Hot stamping is performed after the heating and the holding described above. After the hot stamping, it is preferable to perform cooling to a temperature range of, for example, 300° C. or lower at an average cooling rate of 10° C./s or faster. When the average cooling rate is slower than 10° C./s, the strength may be insufficient. Although the upper limit of the average heating rate is not particularly determined, since it is difficult to set the upper limit to faster than 1,000° C./s in actual operation, the actual upper limit is 1,000° C./s or slower.
In the heating during hot stamping, it is not preferable to perform preheating, that is, to perform two-stage heating. The segregation region of carbon in the grain boundaries created in the stage of the steel sheet for hot stamping is eliminated, it is not possible to uniformly disperse and form the prior austenite grains, and as a result, the standard deviation of the prior austenite grains cannot be controlled within a desired range.
The hot-stamping formed body according to the present embodiment can be obtained by the preferable manufacturing method described above. After the hot stamping, a tempering treatment may be performed at 150° C. to 600° C. In addition, a part of the hot-stamping formed body may be tempered by laser irradiation or the like to partially provide a softened region. Weldability improves in the softened region. For example, when spot welding is performed after softening the end portion of the hot-stamping formed body, it is possible to reduce a difference in strength between the softened end portion and the spot-welding portion of the end portion, and thus, the fracture from the interface between the end portion and the spot-welding portion can be suppressed. In addition, for example, in a case where the hot-stamping formed body is applied to a high strength member of an automobile, it is possible to control a fracture or deformation mode of the high strength member in the time of a collision by providing a softened region in a part of the high strength member.
Next, examples of the present invention will be described. Conditions in the examples are one condition example that is employed to confirm the feasibility and effects of the present invention, and the present invention is not limited to this condition example. The present invention may employ various conditions to achieve the object of the present invention without departing from the scope of the present invention.
A steel piece manufactured by casting molten steel having a chemical composition shown in Tables 1A to 1D was heated, held in a temperature range of 1,200° C. or higher and lower than 1,350° C. for 20 minutes or longer, and then subjected to hot rolling, cooling, and coiling under conditions shown in Tables 2A to 2F, and subjected to cold rolling, hot-rolled sheet annealing, pickling, and plating as necessary. Therefore, steel sheets for hot stamping shown in Table 2A to Table 2F were obtained. The average cooling rate of cooling after the finish rolling to coiling was set to 50 to 200° C./s. In addition, cooling was performed at the above-described average cooling rate after a lapse of 2.5 seconds or longer after the finish rolling. Note that, for Steel sheet No. 172 marked with “*”, after the finish rolling, cooling was performed after 2.0 seconds elapsed.
In addition, for Steel sheet No. 107, in the casting step, the steel piece after solidification was rolled with a rolling reduction of 30% to 70% in a temperature range in which the center temperature of a slab was the solidus temperature or lower.
For Steel sheet No. 108, the heating temperature before the hot rolling was set to 1,350° C.
Steel sheet No. 125 was subjected to hot-rolled sheet annealing of heating and holding in a temperature range of 730° C. or lower.
For Steel sheet No. 126, the cold rolling was not performed.
An electrogalvanized layer was formed on the surface of Steel sheet No. 127.
An electrolytic Zn—Ni alloy plating layer was formed on the surface of Steel sheet No. 128.
A hot-dip galvanized layer was formed on the surface of Steel sheet No. 129.
A hot-dip galvannealed layer was formed on the surface of Steel sheet No. 130.
A hot-dip aluminum plating layer was formed on the surface of Steel sheet No. 131.
A hot-dip Zn—Al alloy plating layer was formed on the surface of Steel sheet No. 132.
A hot-dip Zn—Al—Mg alloy plating layer was formed on the surface of Steel sheet No. 133.
A hot-dip Zn—Al—Mg—Si alloy plating layer was formed on the surface of Steel sheet No. 134.
The obtained steel sheets for hot stamping were subjected to hot stamping under the conditions shown in Tables 3A to 3F to obtain hot-stamping formed bodies shown in Tables 3A to 3F.
For Manufacturing No. 161, a tempering treatment was performed at 150° C. to 600° C. after hot stamping.
For Manufacturing No. 162, a partially softened region was formed by irradiating a portion of the hot-stamping formed body with a laser and tempering the portion.
After Manufacturing No. 163 was heated to a heating temperature shown in Table 3F, Manufacturing No. 163 was cooled to a temperature range of 250° C. or lower. Thereafter, Manufacturing No. 163 was heated to 900° C. and hot-stamped, and then cooled at the average cooling rate in Table 3D.
In the examples of the present invention shown in Tables 2A to 2F, the remainder in the microstructure was one or more of pearlite, martensite, lower bainite, residual austenite, and tempered martensite, and the total area ratio of these was more than 50% and 90% or less. In addition, in the examples of the present invention shown in Tables 3A to 3F, the microstructures included, by area %, ferrite: 0% to 50%, bainite and martensite: 0% to 100%, pearlite: 0% to 30%, and residual austenite: 0% to 5%.
In addition, a method for measuring the microstructure of the steel sheet for hot stamping and a method for measuring the microstructure and mechanical properties of the hot-stamping formed body were as described above. In a case where the tensile strength of the hot-stamping formed body was 2,200 MPa or more, the hot-stamping formed body was determined to be acceptable for having high strength, and, in a case where the tensile strength of the hot-stamping formed body was less than 2,200 MPa, the hot-stamping formed body was determined to be unacceptable for not having high strength.
In addition, in a case where the maximum bending angle was 20° or more, it was determined to be acceptable for having excellent bendability, and, in a case where the maximum bending angle was less than 20°, it was determined to be unacceptable for not having excellent bendability.
0.36
0.73
1.40
0.40
3.20
0.150
0.0150
0.0140
0.0250
0.510
0.85
0.130
0.150
0.0170
1.20
2.20
3.20
1.30
1.10
1.300
0.016
1.300
1.300
1.300
1.200
13
14
21
29
30
34
38
42
46
53
54
61
62
66
70
74
77
80
83
86
89
92
95
98
101
104
84
0.94
35
0.21
770
60
0.29
376
0.20
22
0.18
10
0.21
2149
13
13
16
14
14
12
21
21
17
22
22
2005
29
29
15
30
30
16
34
34
16
12
42
42
14
46
46
15
53
53
18
54
54
2188
61
61
12
62
62
17
66
66
13
70
70
15
74
74
13
77
77
80
80
16
83
83
19
86
86
15
89
15
92
13
95
18
98
17
101
101
17
104
104
15
109
13
116
36
14
117
2.3
12
124
29
14
2099
1022
2009
11.0
2038
2188
3.3
18
171
33
13
34
14
From Table 3A to Table 3F, it is found that the hot-stamping formed bodies according to the examples of the present invention have high strength and excellent bendability. Meanwhile, it can be seen that in the hot-stamping formed bodies according to comparative examples, one of the properties deteriorated.
According to the above-described aspects of the present invention, it is possible to provide a hot-stamping formed body having high strength and excellent bendability, and a steel sheet for hot stamping capable of manufacturing this hot-stamping formed body.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-081620 | May 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/019656 | 5/9/2022 | WO |
