Patent Application
20250101557
The present invention relates to a hot-stamping formed body.
Priority is claimed on Japanese Patent Application No. 2022-067020, filed Apr. 14, 2022, the content of which is incorporated herein by reference.
In recent years, there has been a demand for a reduction in a weight of a vehicle body for a vehicle in terms of environmental protection and resource saving, and a high-strength steel sheet has been applied to vehicle members. Vehicle members are manufactured by press forming, but not only a forming load is increased but also the formability deteriorates as the strength of a steel sheet is increased. For this reason, the formability of a high-strength steel sheet into a member having a complicated shape becomes an issue.
In order to solve this issue, the application of a hot stamping technique in which press forming is performed after a steel sheet is heated up to a high temperature of an austenite range where the steel sheet softens is in progress. Hot stamping is attracting attention as a technique that achieves both the formability of a steel sheet into a vehicle member and strength of a vehicle member by performing hardening of the steel sheet in a die at the same time as press working.
For example, Patent Document 1 discloses an electrolytic zinc-based plated steel sheet having a high yield ratio and excellent bendability, in which the critical hydrogen amount in the steel is 0.20 mass ppm or less.
In order to make vehicle members lighter, it is effective to increase the strength of steel sheets. In order to increase the strength of the steel sheets, there is a method of increasing the Mn content in order to improve hardenability of the steel sheet. However, increasing the Mn content poses problems such as hydrogen embrittlement cracking and early fracture.
Hydrogen embrittlement cracking is a phenomenon in which a steel member, to which high stress is applied in use, suddenly fractures due to hydrogen which is irrupted into the steel from an external environment. This phenomenon is also called delayed fracture due to the mode of the occurrence of fracture. It is generally known that hydrogen embrittlement cracking is more likely to occur in the steel sheet as tensile strength of the steel sheet increases. It is considered that this is because the higher tensile strength of the steel sheet, the greater residual stress in the steel sheet after a component is formed. This susceptibility to hydrogen embrittlement cracking (delayed fracture) is called hydrogen embrittlement resistance.
Early fracture is a phenomenon in which fracture occurs at a stress lower than tensile strength estimated from the hardness of the steel member. This susceptibility to early fracture is called early fracture resistance:
In Patent Document 1, bendability is considered, but hydrogen embrittlement resistance and early fracture resistance are not considered.
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 hydrogen embrittlement resistance and early fracture resistance.
The gist of the present invention is as follows.
[1] A hot-stamping formed body according to an aspect of the present invention comprising, as a chemical composition, by mass %;
[2] The hot-stamping formed body according to [1] may comprise, as the chemical composition, by mass %, one or more selected from the group consisting of:
According to the above-described aspects of the present invention, it is possible to provide a hot-stamping formed body having high strength, excellent hydrogen embrittlement resistance and early fracture resistance.
The present inventors found that by reducing the standard deviation of grain sizes of prior austenite grains in the interior region, hydrogen embrittlement resistance and early fracture resistance of the hot-stamping formed body can be improved. In addition, the present inventors found that in the surface layer region, by generating a desired amount of bainite, by creating the texture with a desired crystal orientation, and by achieving a desired deboronization index, hydrogen embrittlement resistance can be further improved.
The present inventors found that in order to obtain a hot-stamping formed body having the above features, it is particularly effective to perform finish rolling and annealing under desired conditions during manufacturing of a steel sheet for hot stamping.
Hereinafter, the hot-stamping formed body according to the present embodiment will be described in detail. First, the reason the chemical composition of the hot-stamping formed body according to the present embodiment is limited will be described.
A limited numerical range described using “to” 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 hot-stamped formed body according to the present embodiment comprises, as a chemical composition, by mass %, C: more than 0.40% and 0.70% or less, Si: 0.010% to 3.00%, Mn: 0.60% to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010% to 0.5000%, Nb: 0.0010% to 0.100%, Ti: 0.010% to 0.200%, Cr: 0.01% to 0.80%, Mo: 0.0010% to 1.000%, B: 0.0005% to 0.0200%, and a remainder: Fe and impurities. Each element will be described below.
C: More than 0.40% and 0.70% or Less
C is an element that improves the strength of the hot-stamping formed body. When the C content is 0.40% or less, a desired strength of the hot-stamping formed body cannot be obtained. For this reason, the C content is set to more than 0.40%. The C content is preferably 0.42% or more or 0.44% or more.
Meanwhile, when the C content is more than 0.70%, toughness of martensite deteriorates and excellent hydrogen embrittlement resistance cannot be obtained. For this reason, the C content is set to 0.70% or less. The C content is preferably 0.65% or less or 0.60% or less.
Si is an element that improves strength of the hot-stamping formed body by solid-solution strengthening. When the Si content is less than 0.010%, a desired strength cannot be obtained. For this reason, the Si content is set to 0.010% or more. The Si content is preferably 0.05% or more, 0.10% or more or 0.15% or more.
Meanwhile, when the Si content is more than 3.00%, the amount of ferrite increases and a desired microstructure cannot be obtained. For this reason, the Si content is set to 3.00% or less. The Si content is preferably 2.00% or less, 1.00% or less or 0.70% or less.
Mn promotes the transformation from prior austenite to pearlite in a hot-rolled steel sheet having the chemical composition of the present embodiment, and contributes to control of grain size distribution of prior austenite of the hot-stamping formed body. In order to set the standard deviation of grain sizes of prior austenite grains, the Mn content is set to 0.60% or more. The Mn content is preferably 0.70% or more or 1.00% or more.
Meanwhile, when the Mn content is more than 3.00%, transformation from prior austenite to pearlite in a hot-rolled steel sheet having the chemical composition of the present embodiment is excessively promoted, and the standard deviation of grain sizes of prior austenite grains in the hot-stamping formed body cannot be set to a desired range. For this reason, the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less or 2.30% or less.
P is an impurity element, and by segregating in the grain boundaries, it becomes a starting point for fracture and deteriorates early fracture resistance. For this reason, the P content is set to 0.100% or less. The P content is preferably 0.050% or less or 0.010% or less.
The lower limit of the P content is not particularly limited, but 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, 0.001% or more or 0.005% or more.
S is an impurity element, and forms inclusions in steel. The inclusions become starting points for fracture and deteriorate early fracture resistance. For this reason, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0050% or less or 0.0030% or less.
The lower limit of the S content is not particularly limited, but 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, 0.0002% or more, 0.0003% or more or 0.0010% or more.
N is an impurity element, and forms nitrides in steel. The nitrides become starting points for fracture and deteriorate early fracture resistance. For this reason, the N content is set to 0.0200% or less. The N content is preferably 0.0150% or less, 0.0100% or less, 0.0060% or less or 0.0040% or less.
The lower limit of the N content is not particularly limited, but 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 or 0.0010% or more.
O forms a coarse oxide that becomes a starting point for fracture in steel when a large amount of O is comprised, and deteriorates early fracture resistance of the hot-stamping formed body. For this reason, the O content is set to 0.0200% or less. The O content is preferably 0.0100% or less, 0.0070% or less or 0.0040% or less.
The O content may be 0%, in order to disperse many oxides during deoxidizing of molten steel, the O content may be set to 0.0005% or more or 0.0010% or more.
Al is an element having an effect of deoxidizing molten steel and achieving soundness of the steel. When the Al content is less than 0.0010%, deoxidation is not sufficiently performed, and coarse oxides are generated and early fracture resistance deteriorates. For these reasons, the Al content is set to 0.0010% or more. The Al content is preferably 0.0050% or more, 0.0100% or more or 0.0300% or more.
Meanwhile, when the Al content is more than 0.5000%, coarse oxides are generated in steel, and early fracture resistance of the hot-stamping formed body deteriorates. For this reason, the Al content is set to 0.5000% or less. The Al content is preferably 0.4000% or less, 0.3000% or less, or 0.2000% or less or 0.1000% or less.
Nb is an element that forms carbonitride in steel and improves strength of the hot-stamping formed body by precipitation strengthening. When the Nb content is less than 0.0010%, a desired strength cannot be obtained. For this reason, the Nb content is set to 0.0010% or more. The Nb content is preferably 0.005% or more, 0.009% or more or 0.015% or more.
Meanwhile, when the Nb content is more than 0.100%, many carbonitrides are generated in steel, and early fracture resistance of the hot-stamping formed body deteriorates. For this reason, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less or 0.060% or less.
Ti is an element that forms carbonitride in steel and improves strength of the hot-stamping formed body by precipitation strengthening. When the Ti content is less than 0.010%, a desired strength cannot be obtained. For this reason, the Ti content is set to 0.010% or more. The Ti content is preferably 0.020% or more or 0.025% or more.
Meanwhile, when the Ti content is more than 0.200%, many carbonitrides are generated in steel, and early fracture resistance of the hot-stamping formed body deteriorates. For this reason, the Ti content is set to 0.200% or less. The Ti content is preferably 0.150% or less, 0.100% or less, 0.080% or less, 0.060% or less or 0.050% or less.
Cr is an element that increases strength of the hot-stamping formed body by dissolving in prior austenite grains during heating before hot stamping. When the Cr content is less than 0.01%, a desired strength cannot be obtained. For this reason, the Cr content is set to 0.01% or more. The Cr content is preferably 0.10% or more, 0.15% or more or 0.20% or more.
Meanwhile, when the Cr content is more than 0.80%, coarse intermetallic compounds are formed in the hot-stamping formed body and early fracture resistance deteriorates. For this reason, the Cr content is set to 0.80% or less. The Cr content is preferably 0.70% or less, 0.50% or less or 0.40% or less.
Mo is an element that increases strength of the hot-stamping formed body by dissolving in prior austenite grains during heating before hot stamping. When the Mo content is less than 0.0010%, a desired strength cannot be obtained. For this reason, the Mo content is set to 0.0010% or more. The Mo content is preferably 0.010% or more, 0.050% or more or 0.100% or more.
Meanwhile, when the Mo content is more than 1.000%, coarse intermetallic compounds are formed in the hot-stamping formed body and early fracture resistance deteriorates. For this reason, the Mo content is set to 1.000% or less. The Mo content is preferably 0.800% or less, 0.600% or less or 0.400% or less.
B is an element that improves the hardenability of steel. When the B content is less than 0.0005%, a desired strength cannot be obtained. For this reason, the B content is set to 0.0005% or more. The B content is preferably 0.0010% or more or 0.0015% or more.
Meanwhile, when the B content is more than 0.0200%, coarse intermetallic compounds are formed in the hot-stamping formed body and early fracture resistance deteriorates. For this reason, the B content is set to 0.0200% or less. The B content is preferably 0.0150% or less, 0.0100% or less, 0.0080% or less, 0.0040% or less or 0.0030% or less.
The remainder of the chemical composition of the hot-stamping formed body may be Fe and impurities. Elements which are unavoidably mixed from a steel raw material or scrap and/or during the manufacture of steel and are allowed in a range where the properties of the hot-stamping formed body according to the present embodiment do not deteriorate are exemplary examples of the impurities.
The hot-stamping formed body may comprise the following elements as: optional elements. The content of the following optional elements obtained in a case where the following optional elements are not contained is 0%.
Co is an element that improves strength of the hot-stamping formed body by solid-solution strengthening. In order to reliably obtain the effect, it is preferable that the Co content be set to 0.01% or more. The Co content is more preferably set to 0.05% or more.
Meanwhile, since the above effect will be saturated even if a large amount is comprised, the Co content is set to 4.00% or less. If necessary, the upper limit of Co content may be set to 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.
Ni: 0% to 3.00%
Ni has an effect of increasing strength of the hot-stamping formed body by dissolving in prior austenite grains during heating before hot stamping. In order to reliably obtain the effect, the Ni content is preferably set to 0.01% or more.
Meanwhile, since the above effect will be saturated even if a large amount is comprised, the Ni content is preferably set to 3.00% or less. If necessary, the upper limit of Ni content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.
Cu has an effect of increasing strength of the hot-stamping formed body by dissolving in prior austenite grains during heating before hot stamping. In order to reliably obtain the effect, the Cu content is preferably set to 0.01% or more. The Cu content is more preferably set to 0.05% or more.
Meanwhile, since the above effect will be saturated even if a large amount is comprised, the Cu content is preferably set to 3.00% or less. If necessary, the upper limit of Cu content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.
V has an effect of forming carbonitride in steel and improves strength of the hot-stamping formed body by precipitation strengthening. In order to reliably obtain the effect, the V content is preferably set to 0.01% or more. The V content is more preferably set to 0.05% or more.
Meanwhile, when the V content is more than 3.00%, many carbonitrides are generated in steel, and early fracture resistance of the hot-stamping formed body deteriorates. For this reason, the V content is set to 3.00% or less. If necessary, the upper limit of V content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.
W has an effect of improving strength of the hot-stamping formed body. In order to reliably obtain the effects, the W content is preferably set to 0.01% or more. The W content is preferably set to 0.05% or more.
Meanwhile, since the above effect will be saturated even if a large amount is comprised, the W content is preferably set to 3.00% or less. If necessary, the upper limit of W content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.
Ca is an element that suppresses generation of carbides that become starting points for fracture, and contributes to improvement of early fracture resistance. In order to reliably obtain the effect, the Ca content is preferably set to 0.001% or more.
Meanwhile, since the above effect will be saturated even if a large amount is comprised, the Ca content is set to 1.000% or less. If necessary, the upper limit of Ca content may be set to 0.100%, 0.010%, 0.005%, 0.001%, 0.0005% or 0.0002%.
Mg forms oxides and sulfides in molten steel, suppresses formation of a coarse MnS, disperses a lot of fine oxides, miniaturizes the microstructure, and contributes to improvement of early fracture resistance. In order to reliably obtain these effects, the Mg content is preferably set to 0.001% or more.
Meanwhile, since the above effect will be saturated even if a large amount is comprised, the Mg content is set to 1.000% or less. If necessary, the upper limit of Mg content may be set to 0.100%, 0.010%, 0.005%, 0.001%, 0.0005% or 0.0002%.
REM suppresses generation of oxides that become starting points of fracture and contributes to improvement of early fracture resistance. In order to reliably obtain the effect, the REM content is preferably set to 0.001% or more.
Meanwhile, since the above effect will be saturated even if a large amount is comprised, the REM content is set to 1.000% or less. If necessary, the upper limit of REM content may be set to 0.100%, 0.010%, 0.005%, 0.001%, 0.0005% or 0.0002%.
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 suppresses generation of oxides that become starting points of fracture and contributes to improvement of early fracture resistance. In order to reliably obtain the effect, the Sb content is preferably set to 0.001% or more.
Meanwhile, since the above effect will be saturated even if a large amount is comprised, the Sb content is set to 1.000% or less. If necessary, the upper limit of Sb content may be set to 0.100%, 0.050%, 0.020%, 0,010%, 0.005% or 0.002%,
Sn suppresses generation of oxides that become starting points of fracture and contributes to improvement of early fracture resistance. In order to reliably obtain the effect, the Sn content is preferably set to 0.001% or more.
Meanwhile, since the above effect will be saturated even if a large amount is comprised, the Sn content is set to 1.000% or less. If necessary, the upper limit of Sn content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005% or 0.002%.
Zr suppresses generation of oxides that become starting points of fracture and contributes to improvement of early fracture resistance. In order to reliably obtain the effect, the Zr content is preferably set to 0.001% or more.
Meanwhile, since the above effect will be saturated even if a large amount is comprised, the Zr content is set to 1.000% or less. If necessary, the upper limit of Zr content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005% or 0.002%.
As refines the prior austenite grains by lowering an austenite single-phase transformation temperature, and contributes to improvement of early fracture resistance. In order to reliably obtain the effect, the As content is preferably set to 0.001% or more.
Meanwhile, since the above effect will be saturated even if a large amount is comprised, the As content is set to 0.100% or less. If necessary, the upper limit of As content may be set to 0.100%, 0.050%, 0.020%, 0,010%, 0.005% or 0.002%.
The above-mentioned chemical composition of the hot-stamping formed body may be measured by a standard analysis method. For example, the chemical composition 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.
When a plating layer or a coating film is provided on the surface of the hot-stamping formed body, the chemical composition is analyzed after the plating layer or the coating film is removed by mechanical grinding.
Next, the microstructure of the hot-stamping formed body according to the present embodiment will be described.
In the hot-stamping formed body according to the present embodiment, in the interior region, which is a region between 4/16 depth of the sheet thickness (thickness of the hot-stamping formed body) from the surface of the hot-stamping formed body and 5/16 depth of the sheet thickness from the surface, the standard deviation of grain sizes of prior austenite grains is 5.0 μm or less; in the surface layer region, which is a region between the surface and 1/25 depth of the sheet thickness from the surface, the area ratio of bainite is more than 10%, the maximum value of pole density of the texture is 4.0 or less, and the deboronization index is 0.05 or more.
The interior region in the present embodiment refers to a region between 4/16 depth of the sheet thickness from the surface of the hot-stamping formed body and 5/16 depth of the sheet thickness from the surface.
In addition, the surface layer region refers to a region between the surface of the hot-stamping formed body and 1/25 depth of the sheet thickness from the surface.
When the hot-stamping formed body has the plating layer or the coating film on the surface thereof, the “surface” refers to the interface of the plating layer or the coating film and the base steel sheet, and for convenience, the plating layer or the coating film is excluded from the hot-stamping formed body. Specifically, when the hot-stamping formed body has the plating layer or the coating film on the surface thereof, as described below, for convenience, a region where the Fe concentration is less than 90% by mass in GD-OES measurement, that is, the plating layer or the coating film is excluded from the hot-stamping formed body, the measuring point where the Fe concentration is 90% by mass (the interface of the base steel sheet and the plating layer) is regarded as the surface of the hot-stamping formed body. As described above, the plating layer or the coating film is excluded from the hot-stamping formed body, when the thickness of the plating layer or the coating film is very small compared to the sheet thickness (thickness) of the hot-stamping formed body and can be ignored (however, when only the plating layer is formed, the thickness of the plating layer is often very small and can be ignored in most cases), when measuring the sheet thickness (thickness) of the hot-stamping formed body, the sheet thickness (thickness) of the hot-stamping formed body may be regarded as the sheet thickness (thickness) including the plating layer or the coating film.
By reducing the dispersion of grain sizes of prior austenite grains in the interior region, that is, by reducing the standard deviation, an increase of local residual stress can be suppressed. As a result, hydrogen embrittlement resistance and early fracture resistance of the hot-stamping formed body can be improved. When the standard deviation of grain sizes of prior austenite grains is more than 5.0 μm, hydrogen embrittlement resistance and early fracture resistance deteriorate. For this reason, the standard deviation of grain sizes of prior austenite grains is set to 5.0 μm or less, preferably 4.0 μm or less, 3.0 μm or less or 2.5 μm or less.
The lower limit of the standard deviation of grain sizes of prior austenite grains is not particularly limited, but may be set to 0.1 μm. 0.5 μm, 1.0 μm or 1.5 μm.
The standard deviation of grain sizes of prior austenite grains is obtained by the following method.
A sample is cut out from an arbitrary position away from an end surface of the hot-stamping formed body by a distance of 50 mm or more (a position that avoids an end portion in a case where the sample cannot be collected at this position) so that a sheet thickness cross section parallel to a rolling direction can be observed. The size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction.
After polishing the cross section of the sample using silicon carbide paper of #600 to #1500, the cross section of the sample is mirror-finished using liquid in which diamond powder having a grain size in the range of 1 μm to 6 μm is dispersed in a diluted solution of alcohol or the like or pure water. Next, the observation surface is finished by electrolytic polishing. At an arbitrary position on the cross section of the sample in a longitudinal direction, a region which has a length of 50 μm and is present between 4/16 depth of the sheet thickness from the surface and 5/16 depth of the sheet thickness from the surface is measured at a measurement interval of 0.1 μm by an electron backscatter diffraction method, and thus, crystal orientation information is obtained. An EBSD analyzer composed of a Schottky emission scanning electron microscope and an EBSD detector may be used for measurement, for example, an EBSD analyzer composed of JSM-7001F manufactured by JEOL Ltd. and DVC 5-type detector manufactured by TSL Solutions may be used for measurement. In this case, the degree of vacuum in the EBSD analyzer may be set to 9.6×10−5 Pa or less, an accelerating voltage may be set to 15 kV, and an irradiation current level may be set to 13.
By using the obtained crystal orientation information, the crystal orientation of prior austenite grains is calculated from the crystal orientation relationship between general prior austenite grains and grains having a body-centered structure after transformation, and after calculating the average grain size of prior austenite grains using the crystal orientation, the standard deviation is calculated.
The method for calculating the crystal orientation of prior austenite grains is the following method. First, the crystal orientation map of the prior austenite grains is created by the method described in Non-Patent Document 1. For one of prior austenite grain included in the observed visual field, an average value of a shortest diameter and a longest diameter is calculated, and the average value is regarded as the grain size of the prior austenite grain. The above operation is performed on all 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. By calculating the standard deviation from the obtained grain sizes of all austenite grains, the standard deviation of grain sizes of austenite grains is obtained.
In addition, in the present embodiment, the rolling direction of the hot-stamping formed body is determined by the following method.
First, a sample is cut out from an arbitrary 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. After finishing the cross section of the collected sample by mirror polishing, observations with an optical microscope at 100, 200, 500, and 1000 magnifications are performed respectively. Depending on the size of the inclusion, an observation result with an appropriate magnification that the size of the inclusion can be measured is selected. The observation area is width of 500 μm or more and full of the sheet thickness, and the areas with low brightness are determined to be inclusions. The observation may be performed at multiple fields when observing. Next, using the sheet thickness cross section initially observed by the above method as a reference, in the range of 0° to 180° with the sheet thickness direction as the axis, the cross-sectional observation of the plane parallel to the plane rotated in 5° increments is performed in the same way as the above method. The average values of the lengths of the long axes of the plurality of inclusions in each cross section are calculated respectively. The cross section in which the obtained average value of the length of the long axes of the inclusions is maximum is specified. A direction parallel to the longitudinal direction of the inclusion in the cross section is determined as the rolling direction.
The microstructure of the interior region is not particularly limited as long as the desired strength, hydrogen embrittlement resistance and early fracture resistance can be obtained, for example, in area %, the microstructure may consist of martensite and bainite of 90% to 100% (90% or more and 100% or less) in total, and ferrite and residual austenite of 0% to 10% (0% or more and 10% or less) in total. Martensite in the present embodiment includes untempered martensite (fresh martensite) and tempered martensite.
The microstructure of the hot-stamping formed body is measured by the following method.
A sample is cut out from an arbitrary position away from an end surface of the hot-stamping formed body by a distance of 50 mm or more (a position that avoids an end portion in a case where the sample cannot be collected at this position) so that a sheet thickness cross section parallel to the rolling direction can be observed. The size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction.
After polishing the cross section of the sample using silicon carbide paper of #600 to #1500, the cross section is mirror-finished using liquid in which diamond powder having a grain size in the range of 1 μm to 6 μm is dispersed in a diluted solution of alcohol or the like or pure water. Next, the observation surface is finished by electrolytic polishing. At an arbitrary position on the cross section of the sample in a longitudinal direction, a region which has a length of 50 μm and is present between 4/16 depth of the sheet thickness from the surface and 5/16 depth of the sheet thickness from the surface is measured at a measurement interval of 0.1 μm by an electron backscatter diffraction method, and thus, crystal orientation information is obtained. An EBSD analyzer composed of a Schottky emission scanning electron microscope and an EBSD detector may be used for measurement, for example, an EBSD analyzer composed of JSM-7001F manufactured by JEOL Ltd. and DVC 5-type detector manufactured by TSL Solutions may be used for measurement. In this case, the degree of vacuum in the EBSD analyzer may be set to 9.6×10:5 Pa or less, an accelerating voltage may be set to 15 kV, and an irradiation current level may be set to 13.
In the obtained crystal structure information, using “Phase Map” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer, a region where a crystal structure is fec is determined as residual austenite. The area ratio of the residual austenite is calculated, thereby the area ratio of the residual austenite is obtained. Next, regions where the crystal structure is bcc is determined as bainite, martensite, and ferrite. In these regions, under the condition that boundary with 5° is regarded as the grain boundary, using “Grain Average Misorientation” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer, regions where a grain average misorientation is 0.5° or lower are extracted as ferrite. By calculating the area ratio of the extracted ferrite, the area ratio of ferrite is obtained.
Subsequently, the area ratio of the remaining region (the region where “Grain Average Misorientation” is more than 0.5°) is calculated, and this area ratio is determined as the total area ratio of martensite and bainite.
Area Ratio of Bainite: More than 10%
By generating bainite in the surface layer region, dislocation density of the surface layer region can be decreased. As a result, irruption of hydrogen from the external environment can be suppressed, and hydrogen embrittlement resistance of the hot-stamping formed body can be improved. Furthermore, by generating bainite in the surface layer region, since excessive softening of the surface layer can be suppressed, hydrogen embrittlement resistance can be improved while maintaining a load bearing of the member. When the area ratio of bainite in the surface layer region is 10% or less, hydrogen embrittlement resistance deteriorates. For this reason, the area ratio of bainite is set to more than 10%, preferably 20% or more, 40% or more or 60% or more. The upper limit of the area ratio of bainite is not particularly limited, but may be set to 100%, 90% or 80%.
In the microstructure of the surface layer region, except for bainite, martensite of 0% to 90% (0% or more and 90% or less), ferrite and residual austenite of 0% to 65% (0% or more and 65% or less) may be included.
The area ratio of the microstructure is calculated for the surface layer region (the region between the surface and 1/25 depth of the sheet thickness from the surface) by the following method.
A sample is cut out from an arbitrary position away from an end surface of the hot-stamping formed body by a distance of 50 mm or more (a position that avoids an end portion in a case where a sample cannot be collected at this position) so that a sheet thickness cross section parallel to the rolling direction can be observed. The size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction.
After polishing the cross section of the sample using silicon carbide paper of #600 to #1500, the cross section is mirror-finished using liquid in which diamond powder having a grain size in the range of 1 μm to 6 μm is dispersed in a diluted solution of alcohol or the like or pure water, Next, the observation surface is finished by electrolytic polishing. At an arbitrary position on the cross section of the sample in a longitudinal direction, a region which has a length of 50 μm and is present between the surface of the hot-stamping formed body and 1/25 depth of the sheet thickness from the surface is measured at a measurement interval of 0.1 μm by an electron backscatter diffraction method, and thus, crystal orientation information is obtained. An EBSD analyzer composed of a Schottky emission scanning electron microscope and an EBSD detector may be used for measurement, for example, an EBSD analyzer composed of JSM-7001F manufactured by JEOL Ltd. and DVC 5-type detector manufactured by TSL Solutions may be used for measurement. In this case, the degree of vacuum in the EBSD analyzer may be set to 9.6×10−5 Pa or less, an accelerating voltage may be set to 15 kV, and an irradiation current level may be set to 13.
In the obtained crystal structure information, using the “Phase Map” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer, a region where a crystal structure is fcc is determined as residual austenite. The ratio of the residual austenite is calculated, thereby the area ratio of the residual austenite is obtained. Next, in the regions where the crystal structure is bcc, under the condition that boundary with 5° is regarded as the grain boundary, using the “Grain Average Misorientation” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer, regions where a grain average misorientation is more than 0.50° and 0.75° or lower are extracted as bainite. By calculating the area ratio of the extracted bainite, the area ratio of bainite is obtained.
Subsequently, the region where “Grain Average Misorientation” is 0.5° or lower is extracted as ferrite. By calculating the area ratio of the extracted ferrite, the area ratio of ferrite is obtained. The remaining region (the region where “Grain Average Misorientation” is more than) 0.75° is extracted as martensite, and the area ratio thereof is calculated, thereby the area ratio of martensite is obtained.
By controlling the texture in the surface layer region, irruption of hydrogen from the external environment can be suppressed, and hydrogen embrittlement resistance of the hot-stamping formed body can be improved. When the maximum value of pole density of the texture in the surface layer region is more than 4.0, hydrogen embrittlement resistance of the hot-stamping formed body deteriorates. For this reason, the maximum value of pole density of the texture in the surface layer region is set to 4.0 or less, preferably 3.5 or less, 3.0 or less or 2.5 or less.
The lower limit of the pole density of the texture in the surface layer region is not particularly limited, but may be set to 1.0 or 1.2.
In the surface layer region (the region between the surface and 1/25 depth of the sheet thickness from the surface), the texture in the surface layer region is obtained by the following method.
A sample is cut out from an arbitrary position away from an end surface of the hot-stamping formed body by a distance of 50 mm or more (a position that avoids an end portion in a case where the sample cannot be collected at this position) so that a sheet thickness cross section parallel to a rolling direction can be observed. The size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction.
After polishing the cross section of the sample using silicon carbide paper of #600 to #1500, the cross section of the sample is mirror-finished using liquid in which diamond powder having a grain size in the range of 1 μm to 6 μm is dispersed in a diluted solution of alcohol or the like or pure water. Next, the observation surface is finished by electrolytic polishing. At an arbitrary position on the cross section of the sample in a longitudinal direction, a region which has a length of 1000 μm and is present between the surface and 1/25 depth of the sheet thickness from the surface is measured at a measurement interval of 5.0 μm by an electron backscatter diffraction method, and thus, crystal orientation information is obtained. An EBSD analyzer composed of a Schottky emission scanning electron microscope and an EBSD detector may be used for measurement, for example, an EBSD analyzer composed of JSM-7001F manufactured by JEOL Ltd. and DVC 5-type detector manufactured by TSL. Solutions may be used for measurement. In this case, the degree of vacuum in the EBSD analyzer may be set to 9.6×10−5 Pa or less, an accelerating voltage may be set to 15 kV, and an irradiation current level may be set to 13.
By using the obtained crystal orientation information, using the “Texture” function installed in the software “OIM Analysis (registered trademark)” which is attached to the EBSD analyzer, intensity calculation is performed using a Harmonic Series Expansion for grains whose crystal structure is bcc. At this time, the expansion order is set to 16, and a half width when applied to a Gaussian distribution is set to 5°. Next, the “Texture Plot” function is used for the output file after the intensity calculation to output a φ2=45° cross section in the orientation distribution function (ODF). The maximum value of the pole density in the @2=45° cross section is regarded as the maximum value of pole density of the texture in the surface layer region.
The deboronization index is an index that quantitatively represents the amount of decrease of the B concentration in the surface layer region. By decreasing the B concentration in the surface layer region, deformability of prior austenite grain is improved by reducing strength of prior austenite before transformation, and the generation of grains having random orientation is facilitated in the surface layer region. When the deboronization index in the surface layer region is less than 0.05, grains having a desired texture cannot be obtained in the surface layer region. For this reason, the deboronization index is set to 0.05 or more, preferably 0.20 or more, 0.30 or more or 0.35 or more.
The upper limit of the deboronization index is not particularly limited, but may be set to 1.00, 0.80 or 0.60.
The deboronization index in the surface layer region is obtained by the following method.
An element concentration distribution in the sheet thickness direction in the hot-stamping formed body is measured using glow discharge optical emission spectrometry (GD-OES: Manufactured by Horiba, Ltd., Marcus type high-frequency glow discharge optical emission spectrometer, GD-PROFILER-HR). The measurement conditions are an analysis diameter of 4 mm, a sputtering rate of 4 μm/min, an argon pressure of 600 Pa, an RF output of 35 W, and a measurement interval of 0.02 μm or less. All elements that are comprised in the hot-stamping formed body are measured.
In a case where the hot-stamping formed body has the plating layer on the surface, the “surface” refers to the interface of the plating layer and the base steel sheet. In a case where the hot-stamping formed body has the plating layer or the coating film on the surface, GD-OES measurement is performed after removing a part or all of the plating layer or the coating film by mechanical polishing or chemical polishing such that measurement to 200 μm depth from the surface of the base steel sheet (the interface of the plating layer and the base steel sheet) can be performed. In the GD-OES measurement, a measuring point where the Fe concentration becomes 90 mass % is regarded as the surface of the hot-stamping formed body. In addition, in the following description, for ease of explanation, the hot-stamping formed body may be referred to as a base steel sheet.
Next, B concentrations from the surface of the hot-stamping formed body to at least 100 μm depth from the surface are measured. After measuring the B concentration at a position of 100 μm depth from the surface, in a case where the absolute value of the difference between the average value of the B concentration in a region from 80 μm to 100 μm and the maximum value of the measured value of the B concentration in the region from 80 μm to 100 μm is 0.0006% by mass or less, and, in a case where the absolute value of the difference between the average value of the B concentration in the region from 80 μm to 100 μm and the minimum value of the measured value of the B concentration in the region from 80 μm to 100 μm is 0.0006% by mass or less, the measurement in the depth direction of the B concentration is finished at the position of 100 μm depth from the surface.
In a case where the requirements for ending the measurement are not satisfied, the measurement of the B concentration in the depth direction is continued. Then, each time a new B concentration measurement value is obtained in the depth direction, the average value of the B concentration in the region between the deepest part and 20 μm from the deepest part to the surface side is calculated. In a case where the absolute value of the difference between the average value of the B concentration in the region between the deepest part and 20 μm from the deepest part to the surface side and the maximum value of the measured value of the B concentration in the region between the deepest part and 20 μm from the deepest part to the surface side is 0.0006 mass % or less, and, in a case where the absolute value of the difference between the average value of the B concentration in the region between the deepest part and 20 μm from the deepest part to the surface side and the minimum value of the measured value of the B concentration in the region between the deepest part and 20 μm from the deepest part to the surface side is 0.0006 mass % or less, the measurement of the B concentration in the depth direction is finished at the position. For example, when the measured value of the B concentration at 150 μm depth from the surface is obtained, in a case where the absolute value of the difference between the average value of the B concentration in the region between 130 μm depth from the surface and 150 μm depth from the surface and the maximum value of the measured value of the B concentration in the region between 130 μm depth from the surface and 150 μm depth from the surface is 0.0006 mass % or less, and, in a case where the absolute value of the difference between the average value of the B concentration in the region between 130 μm depth from the surface and 150 μm depth from the surface and the minimum value of the measured value of the B concentration in the region between 130 μm depth from the surface and 150 μm depth from the surface is 0.0006 mass % or less, the measurement of the B concentration in the depth direction is finished at the position of 150 μm depth from the surface.
Even if the requirements for ending the measurement described above are not satisfied and the measurement of the B concentration in the depth direction cannot be finished, the measurement of the B concentration in the depth direction is finished when the measurement of the B concentration at the position of 200 μm depth from the surface is completed. Then, at the time when the measurement of the B concentration in the depth direction is finished, the average value of the B concentration in the region between the deepest part (the deepest position where the B concentration used for calculating the deboronization index was obtained) and the position of 20 μm from the deepest part to the surface side is used for the below calculation of the deboronization index (hereinafter, the average value of the B concentration in the region will be referred to as the average B concentration at the deepest part of 20 μm).
For convenience of measurement, for example, after measuring the B concentration to 200 μm depth from the surface, in a region between 100 μm and 200 μm from the surface, the shallowest depth position that satisfies the ending condition for the B concentration measurement in the depth direction is searched for, and in a case where the depth position is found, the deboronization index may be calculated without using the measurement results of the B concentration at the position deeper than the shallowest depth position. For example, the B concentration may be measured from the surface to 200 μm depth from the surface, in this case, in a case where a shallowest depth position that satisfies the ending condition for B concentration measurement in the depth direction exists in a region of 100 μm or more depth from the surface, the measurement is regarded as ending at the depth position, and the deboronization index is calculated.
In the region between the deepest part and 20 μm from the deepest part to the surface side of the hot-stamping formed body, the amount of decrease in the B concentration per unit depth (the value obtained by subtracting the B concentration at each measurement point from the average B concentration at the deepest part of 20 μm) is calculated, the integrated value of the product of the unit depth and the amount of decrease in the B concentration is calculated and determined as the area of the B-depletion region (area of region A in
The hot-stamping formed body may have a plating layer on the surface. By having the plating layer on the surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, aluminum-galvanized layer, aluminum-silicon plating layer, hot-dip galvanized layer, electrogalvanized layer, galvannealed layer, zinc-nickel plating layer, aluminum-magnesium-zinc-based plating layer.
Next, a steel sheet for hot stamping for obtaining the hot-stamping formed body according to the present embodiment will be described.
The steel sheet for hot stamping has the above-described chemical composition. The microstructure of the steel sheet for hot stamping is not particularly limited as long as a desired strength, hydrogen embrittlement resistance and early fracture resistance are obtained after hot stamping, for example, in area %, the microstructure may consist of ferrite: 5% to 90%, bainite and martensite: 0% to 100%, pearlite: 10% to 95%, and residual austenite: 0% to 5%. In addition to these, iron carbides, alloy carbides, intermetallic compounds, and inclusions may be included.
Further, the steel sheet for hot stamping may have a plating layer on the surface. By having the plating layer on the surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, aluminum-galvanized layer, aluminum-silicon plating layer, hot-dip galvanized layer, electrogalvanized layer, galvannealed layer, zinc-nickel plating layer, aluminum-magnesium-zinc-based plating layer.
A manufacturing method to obtain the steel sheet for hot stamping for obtaining the hot-stamping formed body according to the present embodiment will be described. In order to obtain the above-described hot-stamping formed body, it is particularly effective to control the finish rolling condition and the annealing condition in the manufacturing method of the steel sheet for hot stamping.
In finish rolling, it is preferable to set the rolling reduction of the final pass (final rolling reduction) to 20% or more. The final rolling reduction can be expressed as {(t0−t1)/t0}×100(%), where to is the sheet thickness before rolling of the final pass, and t1 is the sheet thickness after rolling of the final pass. By increasing the final rolling reduction, pearlite is uniformly dispersed in the hot-rolled steel sheet after rolling. This pearlite becomes a reverse transformation site of prior austenite during heating of hot stamping. For this reason, when pearlite is uniformly dispersed, the standard deviation of grain sizes of prior austenite grains in the hot-stamping formed body becomes small. As a result, the early fracture resistance of the hot-stamping formed body can be improved. More preferably, the final rolling reduction is 30% or more, 40% or more or 45% or more.
In the chemical composition of the hot-stamping formed body according to the present embodiment, when the Mn content is 0.60% or more, in order to preferably control the texture of the surface layer region of the hot-stamping formed body, it is important to increase the final rolling reduction of final rolling as described above.
The casting method of molten steel, the conditions of heating before hot rolling, rough rolling, coiling, and cold rolling are not particularly limited, and may be standard conditions. The coiling temperature may be set to 750° C. or lower. By setting the coiling temperature to 750° C. or lower, it is possible to suppress ferrite from being connected and arranged in the hot-rolled steel sheet after rolling, and pearlite is uniformly dispersed. This pearlite becomes a reverse transformation site of prior austenite during heating of hot stamping. For this reason, when pearlite is uniformly dispersed, the standard deviation of the grain sizes of prior austenite grains in the hot-stamping formed body becomes small. As a result, early fracture resistance of the hot-stamping formed body can be improved.
Furthermore, for the purpose of softening the hot-rolled steel sheet, a softening heat treatment may be performed on the coil after coiling. The softening heat treatment method is not particularly limited, and standard conditions may be adopted.
After cold rolling, it is preferable to perform annealing to heat for 15 seconds or more in an oxidizing atmosphere. Generally, it is preferable to perform annealing in a reducing atmosphere in order to suppress formation of scale. However, in the present embodiment, formation of scale on the steel sheet surface is promoted by performing annealing in the oxidizing atmosphere. During heating of hot stamping, the scale formed on the steel sheet surface becomes an oxidation source, and C and B in the surface layer region are oxidized. Since oxidized C and B leave the surface layer of the steel sheet, the amounts of C and B are reduced in the surface layer region. As a result, the strength of the prior austenite grains decreases and they become easily deformed, and grains having random orientation are likely to be generated. Thereby, grains having a desired texture can be generated in the surface layer region.
The heating temperature during annealing may be set to a temperature range of 730° C. to 900° C., and by staying in this heating temperature range for 15 seconds or more, formation of scale can be promoted while suppressing peeling of scale. The time for annealing is preferably 100 seconds or more, more preferably 200 seconds or more, and even more preferably 300 seconds or more. On the other hand, annealing for more than 3600 seconds is not preferable since the prior austenite grain sizes become coarser, the grain boundary diffusion rate of B decreases, removal of B does not proceed, and the deboronization index cannot be 0.05 or more. For this reason, the annealing time is preferably 3600 seconds or less.
After annealing in the oxidizing atmosphere, the annealing step may be performed again in an oxidizing atmosphere or a non-oxidizing atmosphere unless a treatment for removing oxide scale (for example, pickling) is performed.
In the present embodiment, the oxidizing atmosphere may be any heating atmosphere that generates oxide scale on the surface layer of the steel sheet, and may be a standard condition. For example, in a gas combustion atmosphere, it is preferable to create an atmosphere in which the mixture ratio of air and fuel (air-fuel ratio) is controlled to 0.80 or more, and more preferably controlled to exceed 1.00. It is preferable to generate an oxide scale of 15 μm or more on the steel sheet surface by annealing in the oxidizing atmosphere.
It is preferable that the oxide scale on the steel sheet surface remain in subsequent processes. That is, it is preferable to perform hot stamping, which will be described later, with the oxide scale remaining. Oxide scale is removed by shot blasting after hot stamping.
Furthermore, even when the plating layer is formed on the surface of the steel sheet for hot stamping, oxide scale remains at the interface between the base steel sheet and the plating layer. When the plating layer is formed, the oxide scale disappears after hot stamping due to an alloying reaction during heating before hot stamping.
A hot-stamping formed body according to the present embodiment is obtained by hot stamping the steel sheet for hot stamping manufactured by the above-described method. The hot stamping conditions are not particularly limited. However, for example, it is preferable to heat the steel sheet for hot stamping to a temperature range of 800° C. to 1000° C. and hold in this temperature range for 60 to 600 seconds. When the heating temperature is lower than 800° C., austenitization becomes insufficient, a desired distribution of prior austenite grain sizes cannot be obtained, and early fracture resistance may deteriorate. On the other hand, when the heating temperature is higher than 1000° C., the grains of prior austenite grow excessively, a desired distribution of prior austenite grain sizes cannot be obtained, and early fracture resistance may deteriorate. When the holding time is shorter than 60 seconds, austenitization becomes insufficient, a desired distribution of prior austenite grain sizes cannot be obtained, and early fracture resistance may deteriorate. When the holding time is longer than 600 seconds, grains of prior austenite grow excessively, a desired distribution of prior austenite grain sizes cannot be obtained, and early fracture resistance may deteriorate.
A heating atmosphere is not particularly limited, and may be standard conditions, for example, such as the atmosphere, a gas combustion atmosphere with a controlled ratio of air and fuel, or a nitrogen atmosphere, and the dew point of these gases may be controlled.
After holding in the temperature range, hot stamping is performed. After hot stamping, cooling may be performed to a temperature range of 250° C. or lower at an average cooling rate of 20° C./s or faster.
Examples of heating methods before hot stamping include heating using an electric furnace and gas furnace, flame heating, electrical heating, high-frequency heating, and induction heating.
By the above methods, the hot-stamping formed body according to the present embodiment is obtained. A tempering treatment at 130° C. to 600° C. may be performed after hot stamping, or a baking hardening treatment after painting may be performed. In addition, a portion of the hot-stamping formed body may be tempered by laser irradiation or the like to provide a partially softened region.
Next, examples of the present invention will be described. Conditions in the examples are one example of conditions employed to confirm the feasibility and effects of the present invention, but the present invention is not limited to these examples. The present invention may employ various conditions to achieve the object of the present invention without departing from the scope of the present invention.
Slabs manufactured by casting molten steel having a chemical composition shown in Tables 1A to IT were heated, held in a temperature range of 1200° C. or higher for 20 minutes or longer, and then subjected to finish rolling, coiling, and annealing under conditions shown in Tables 2A to 2H. Except for some examples, annealing was performed in an oxidizing atmosphere. For examples not specifically described in the notes in the tables, in annealing in the oxidizing atmosphere, the mixture ratio of air and fuel (air-fuel ratio) was controlled to 1.05 in the gas combustion atmosphere. For some examples, as described in the tables, annealing was performed in the reducing atmosphere, and the coils after coiling were subjected to softening heat treatment.
The obtained steel sheets for hot stamping were heated to a temperature range of higher than 800° C. in a furnace continuously supplied with nitrogen gas (hot stamp heating), held in the temperature range, subjected to hot stamping, and then cooled to 250° C. or lower at an average cooling rate of 20° C./s or faster. As a result, the hot-stamping formed bodies shown in Tables 3A to 3H were obtained. In addition, for the examples not specifically described in the notes in the tables, a gas combustion atmosphere was used in which the mixture ratio of air and fuel (air-fuel ratio) was controlled to 0.85.
However, for some examples, as described in the tables, heating in a furnace adjusted to a different atmosphere, re-annealing, plating, tempering, heating of hot stamping, or the like were performed.
The underlines in the tables indicate that it is outside the scope of the present invention, falls outside the preferable manufacturing conditions, or the characteristic value is not preferable.
Measurements of the microstructure (including the standard deviation of the grain sizes of austenite grains), deboronization index, and pole density of the texture of the hot-stamping formed body were performed by the above-described methods. In addition, the mechanical properties of the hot-stamping formed body were evaluated by the following methods.
The tensile (maximum) strength TS of the hot-stamping formed body was obtained, in accordance with JIS Z 2241:2011, by preparing a No. 5 test piece from an arbitrary position of the hot-stamping formed body and conducting a tensile test. The crosshead speed was set to 1 mm/min. When the tensile strength TS was 2200 MPa or more, it was determined as having high strength and successful, and when the tensile strength TS was less than 2200 MPa, it was determined as not having high strength and not successful.
In addition, for examples in which early fracture resistance described below was determined as not successful, the value obtained by multiplying the Vickers hardness, which is measured by the method for early fracture resistance evaluation described below, by 3.3 (=Vickers hardness×3.3) was regarded as the tensile strength.
Hydrogen embrittlement resistance of the hot-stamping formed body was evaluated by the following method, A test piece with a length of 68 mm and a width of 6 mm was taken from an arbitrary position of the hot-stamping formed body, and the edges of the test piece were polished using silicon carbide paper of #200 to #1500, and then mirror finishing was performed using a liquid in which diamond powder with a particle size of 1 μm to 6 μm was dispersed in a diluent such as alcohol and pure water. Furthermore, the corners of the test piece were chamfered using silicon carbide paper of #200 to #1500. A stress of 800 MPa or more was applied to the test piece, the test piece was immersed in a liter of hydrochloric acid adjusted to pH 4 at room temperature for 48 hours, and the presence or absence of cracks was determined. When no crack occurred under the load stress of 800 MPa or more, it was determined as successful. When no crack occurred at 800 MPa, an evaluation of “Fair” was used in the tables, when no crack occurred at 900 MPa, an evaluation of “Good” was used in the tables, when no crack occurred at 1000 MPa, an evaluation of “Very Good” was used in the tables, and when no crack occurred at 1100 MPa or higher, an evaluation of “Excellent” was used in the tables. On the other hand, when a crack occurred at a load stress of 800 MPa, it was determined as not successful and “Bad” was described in the tables.
The early fracture resistance was evaluated by the value calculated by dividing the tensile strength of the hot-stamping formed body, which was obtained by the above method, by the value obtained by multiplying the Vickers hardness, which was obtained by the following method, by 3.3 (tensile strength/(Vickers hardness×3.3)). When the value was 0.60 or more, it was determined as having excellent early fracture resistance and successful, and when the value was less than 0.60, it was determined as not successful. The value obtained by multiplying the Vickers hardness by 3.3 is the tensile strength estimated from the hardness, and when the measured value of the tensile strength is 0.60 times or more of the estimated tensile strength, then it can be determined as having excellent early fracture resistance.
The Vickers hardness used for evaluation of early fracture resistance was obtained by the following method. First, from an arbitrary position 50 mm or more away from the end surface of the hot-stamping formed body, a sample was cut out so that a cross section perpendicular to the surface (sheet thickness cross section) could be observed. The size of the sample depended on the measuring device, but was set to a size that could be observed by 10 mm in the rolling direction. A cross section of the sample was polished using silicon carbide paper of #600 to #1500, and then mirror finishing was performed using a liquid in which diamond powder with a particle size of 1 μm to 6 μm was dispersed in a diluent such as alcohol and pure water. For a mirror-finished cross section, using a micro Vickers hardness tester at any position in the area between a position of 4/16 depth of the sheet thickness from the surface and a position of 5/16 depth of the sheet thickness from the surface, hardness was measured in a direction parallel to the sheet surface (rolling direction) under a load of 1 kgf at intervals of three times or more the indentations. The Vickers hardness was obtained by measuring a total of 20 points and calculating the average value.
0.36
0.72
3.10
0.56
3.12
0.130
0.0170
0.0240
0.0241
0.0008
0.5214
0.0007
0.121
0.008
0.209
0.009
0.84
0.0008
1.241
0.0002
0.0216
A1
A14
B1
B15
C1
C14
D9
E9
F9
G8
H1
H14
I1
I14
J1
J12
K1
K13
L1
L13
M1
M11
15
774
720
919
772
1021
623
3700
A1
2171
14
A14
0.47
15
B1
2173
29
B15
Bad
30
C1
5.8
0.46
43
C14
5.6
0.56
52
D9
0.39
61
E9
0.32
70
F9
0.31
78
G8
0.34
79
H1
0.31
92
H14
0.37
93
I1
106
I14
0.38
107
J1
1951
118
J12
0.53
119
K1
2034
131
K13
0.39
132
L1
2009
144
L13
0.36
145
M1
2095
155
M11
0.35
276
5.6
4.2
0.40
Bad
286
5.5
0.33
290
4.9
0.03
Bad
303
5.1
0.02
Bad
314
4.6
0.01
Bad
315
5.1
0.02
Bad
316
4.4
0.02
Bad
326
5.4
0.36
340
5.6
0.39
341
5.8
0.58
357
5.3
0.39
375
4.2
0.02
Bad
376
0.03
Bad
From Tables 3A to 3H, it can be seen that the hot-stamping formed bodies according to the present invention examples had high strength and excellent hydrogen embrittlement resistance and early fracture resistance. On the other hand, 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 hydrogen embrittlement resistance and early fracture resistance.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-067020 | Apr 2022 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2023/007855 | 3/2/2023 | WO |
