Patent Application
20250230515
The present invention relates to a hot-rolled steel sheet.
Priority is claimed on Japanese Patent Application No. 2022-109509, filed Jul. 7, 2022, the content of which is incorporated herein by reference.
In recent years, a reduction in weight of vehicles and mechanical components has proceeded. The reduction in weight of vehicles and mechanical components can be achieved by designing a component shape into an optimum shape and securing stiffness. Furthermore, a reduction in weight of a blank formed component such as a press-formed component can be achieved by reducing a sheet thickness of a component material.
However, in a case of securing strength properties of a component such as static fracture strength and yield strength while reducing the sheet thickness, it is necessary to use a high strength material. In particular, for vehicle suspension components such as lower arms, trailing links, and knuckles, studies have begun about the application of higher than 780 MPa-grade steel sheets. These vehicle suspension components are manufactured by performing burring, stretch flanging, bending forming, or the like on steel sheets. Therefore, the steel sheet applied to these vehicle suspension components is required to have excellent formability, particularly, ductility and hole expansibility.
For example, Patent Document 1 discloses a high strength steel sheet having a strength of 700 MPa or more, in which C/(Mo+Ti), which is a ratio of an amount of C to a total amount of Mo and Ti in terms of atomic %, is 0.5 to 3.0, a microstructure is a dual phase structure of ferrite and bainite, and carbides containing Ti and Mo are dispersed and precipitated in ferrite.
Patent Document 2 discloses a high strength hot-rolled steel sheet having a small anisotropy in stretch flangeability, in which 88% or more of a steel structure is a bainitic structure.
In order to further increase strength for the purpose of thinning components, there are cases where a hot-rolled steel sheet having a high C content and containing residual austenite is adopted. In such a hot-rolled steel sheet, hydrogen embrittlement cracking is likely to occur in a cut end surface, so that there are cases where hydrogen embrittlement resistance deteriorates.
In Patent Documents 1 and 2, it is necessary to further increase the strength, and the hydrogen embrittlement resistance of the cut end surface is not considered.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a hot-rolled steel sheet having high strength and excellent ductility, hole expansibility, and hydrogen embrittlement resistance.
The present inventors have intensively studied a method for improving hydrogen embrittlement resistance of a cut end surface of a hot-rolled steel sheet. As a result, it has been found that the hydrogen embrittlement resistance of the cut end surface of the hot-rolled steel sheet can be improved by reducing an area ratio of aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more.
The present inventors have found that in order to manufacture the hot-rolled steel sheet, it is effective to control a grain size and a shape of prior austenite grains in a rough rolling step and a finish rolling step, and to strictly control a temperature history after a coiling step.
The gist of the present invention made on the basis of the above-mentioned findings is as follows.
(1) A hot-rolled steel sheet according to an aspect of the present invention includes, as a chemical composition, by mass %:
(2) The hot-rolled steel sheet according to (1) may contain pearlite of less than 3% in the microstructure.
(3) In the hot-rolled steel sheet according to (1) or (2), a region of bainite in which an average value of nanoindentation hardnesses at a load of 5,000 μN and a loading rate of 500 gN/s is 4.7 GPa or less may be 30% or less.
(4) In the hot-rolled steel sheet according to any one of (1) to (3), the chemical composition may comprise, by mass %, one or two or more selected from the group consisting of
According to the above aspect according to the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent ductility, hole expansibility, and hydrogen embrittlement resistance.
Hereinafter, a hot-rolled steel sheet according to the present embodiment will be described in detail. However, the present invention is not limited to configurations disclosed in the present embodiment, and various modifications can be made without departing from the gist of the present invention.
A limited numerical range described using “to” described below includes a lower limit and an upper limit. Numerical values indicated as “less than” or “more than” do not fall within the numerical range. All “%” with respect to a chemical composition refer to “mass %”.
A hot-rolled steel sheet according to the present embodiment includes, as a chemical composition, by mass %: C: 0.13% to 0.23%; Si: 0.70% to 1.79%; Mn: 1.79% to 3.00%; P: 0.060% or less; S: 0.005% or less; N: 0.0070% or less; O: 0.010% or less; Al: 0.010% to 0.430%; Ti: 0.006% to 0.055%; Nb: 0.005% to 0.040%; B: 0.0001% to 0.0030%; and a remainder: Fe and impurities.
Hereinafter, each element will be described in detail.
C is an element that improves tensile strength of the hot-rolled steel sheet. When a C content is less than 0.13%, an area ratio of ferrite becomes too high, and a desired tensile strength cannot be obtained in the hot-rolled steel sheet. Therefore, the C content is set to 0.13% or more. The C content is preferably 0.14% or more, and more preferably 0.16% or more.
On the other hand, when the C content is more than 0.23%, the area ratio of fresh martensite becomes too high, and hole expansibility and hydrogen embrittlement resistance of the hot-rolled steel sheet deteriorate. Therefore, the C content is set to 0.23% or less. The C content is preferably 0.21% or less, and more preferably 0.20% or less.
Si is an element that stabilizes residual austenite. When a Si content is less than 0.70%, a desired amount of residual austenite cannot be obtained, and ductility of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 0.70% or more. The Si content is preferably 0.90% or more, and more preferably 1.00% or more.
On the other hand, when the Si content is more than 1.79%, an area ratio of residual austenite becomes too high, and the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 1.79% or less. The Si content is preferably 1.60% or less, more preferably 1.40% or less, and even more preferably 1.30% or less,
Mn is an element necessary to improve strength of the hot-rolled steel sheet. When a Mn content is less than 1.79%, the area ratio of ferrite becomes too high, and a desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.79% or more. The Mn content is preferably 2.00% or more, and more preferably 2.20% or more.
On the other hand, when the Mn content is more than 3.00%, the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.60% or less, and more preferably 2.40% or less.
P is an element that segregates to a sheet thickness center portion of the hot-rolled steel sheet. In addition, P is also an element that embrittles a welded part. When a P content is more than 0.060%, slab cracking is likely to occur, and it becomes difficult to perform casting. Therefore, the P content is set to 0.060% or less. The P content is preferably 0.020% or less, and more preferably 0.015% or less.
The P content is preferably as low as possible and is preferably 0%. However, when the P content is excessively reduced, a dephosphorization cost significantly increases. Therefore, the P content may be set to 0.001% or more.
S is an element that embrittles a slab by being present in steel as a sulfide. In addition, S is also an element that deteriorates formability of the hot-rolled steel sheet. When a S content is more than 0.005%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the S content is set to be 0.005% or less. The S content is preferably 0.004% or less, and more preferably 0.003% or less.
The S content is preferably as low as possible and is preferably 0%. However, when the S content is excessively reduced, a desulfurization cost significantly increases. Therefore, the S content may be set to 0.001% or more.
N is an element that forms coarse nitrides in steel and embrittles the slab. When a N content is more than 0.0070%, a risk of slab cracking significantly increases. Therefore, the N content is set to 0.0070% or less. The N content is preferably 0.0050% or less, and more preferably 0.0035% or less.
The N content is preferably as low as possible and is preferably 0%. However, when the N content is excessively reduced, a denitriding cost significantly increases. Therefore, the N content may be set to 0.0005% or more.
When O is contained in a large amount in steel, O forms a coarse oxide which serves as an origin of fracture and deteriorates the hydrogen embrittlement resistance of the hot-rolled steel sheet. Therefore, an O content is set to 0.010% or less. The O content is preferably 0.008% or less, and more preferably 0.006% or less.
In order to disperse a large number of fine oxides during deoxidation of molten steel, the 0 content may be set to 0.001% or more.
Al is an element that acts as a deoxidizing agent and improves cleanliness of steel. When an Al content is less than 0.010%, a sufficient deoxidizing effect cannot be obtained, and a large amount of inclusions (oxides) are formed in the steel sheet. Such inclusions deteriorate the hole expansibility of the hot-rolled steel sheet. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.040% or more, and more preferably 0.100% or more.
On the other hand, when the Al content is more than 0.430%, slab cracking is likely to occur, and it becomes difficult to perform casting. Therefore, the Al content is set to 0.430% or less. The Al content is preferably 0.400% or less, 0.350% or less, and more preferably 0.200% or less.
Ti is an element that increases the strength of the hot-rolled steel sheet by forming fine nitrides in steel. When a Ti content is less than 0.006%, a desired tensile strength cannot be obtained in the hot-rolled steel sheet. Therefore, the Ti content is set to 0.006% or more. The Ti content is preferably 0.010% or more, and more preferably 0.020% or more.
On the other hand, when the Ti content is more than 0.055%, the risk of slab cracking significantly increases. Therefore, the Ti content is set to 0.055% or less. The Ti content is preferably 0.040% or less, and more preferably 0.030% or less.
Nb is an element that suppresses abnormal grain growth of austenite grains in hot rolling. In addition, Nb is also an element that increases the strength of the hot-rolled steel sheet by forming fine carbides. When a Nb content is less than 0.005%, prior austenite grains cannot be refined, an area ratio of aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more increases, and the hole expansibility and hydrogen embrittlement resistance of the hot-rolled steel sheet deteriorate. Therefore, the Nb content is set to 0.005% or more. The Nb content is preferably 0.010% or more, and more preferably 0.020% or more.
On the other hand, when the Nb content is more than 0.040%, an aspect ratio of the prior austenite grains increases, the area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more increases, and the hole expansibility and the hydrogen embrittlement resistance of the hot-rolled steel sheet deteriorate. Therefore, the Nb content is set to 0.040% or less. The Nb content is preferably 0.030% or less.
B is an element that increases the strength of the hot-rolled steel sheet by suppressing the generation of ferrite in a cooling step. When a B content is less than 0.0001%, a desired tensile strength cannot be obtained in the hot-rolled steel sheet. Therefore, the B content is set to 0.0001% or more. The B content is preferably 0.0005% or more, and more preferably 0.0010% or more.
On the other hand, when the B content is more than 0.0030%, hot deformation resistance increases, and it becomes difficult to perform hot rolling. Therefore, the B content is set to 0.0030% or less. The B content is preferably 0.0025% or less.
The remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities. In the present embodiment, the impurities mean substances that are incorporated from ore as a raw material, scrap, manufacturing environments, or the like or substances that are permitted to an extent that characteristics of the hot-rolled steel sheet according to the present embodiment are not adversely affected.
The hot-rolled steel sheet according to the present embodiment may contain the following optional elements instead of a portion of Fe. A lower limit of amounts of the optional elements in a case where the following optional elements are not included is 0%. Hereinafter, each optional element will be described.
Cr is an element that reduces the amount of ferrite and increases the strength of the hot-rolled steel sheet. In order to reliably obtain this effect, it is preferable that a Cr content is set to 0.020% v or more. The Cr content is more preferably 0.050% or more, 0.100% or more, and even more preferably 0.200% or more.
On the other hand, when the Cr content is more than 0.660%, the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Cr content is set to 0.660% or less. The Cr content is preferably 0.500% or less, and more preferably 0.400% or less.
Mo is an element that increases the strength of the hot-rolled steel sheet by forming fine carbides in steel. In order to reliably obtain this effect, it is preferable that a Mo content is set to 0.001% or more.
On the other hand, when the Mo content is more than 0.300%, the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Mo content is set to 0.300% or less.
Cu has an action of enhancing hardenability of steel and an action of being precipitated as a carbide in steel at a low temperature to increase the strength of the hot-rolled steel sheet. In order to more reliably obtain the effects of the actions, it is preferable that a Cu content is set to 0.001% or greater.
However, when the Cu content is more than 1.000%, there are cases where intergranular cracking may occur in a slab. Therefore, the Cu content is set to 1.000% or less.
Ni has an action of increasing the hardenability of the steel sheet and increasing the strength of the hot-rolled steel sheet. In addition, in a case where Cu is contained, Ni acts to effectively suppress the intergranular cracking of a slab caused by Cu. In order to more reliably obtain the effect of the action, it is preferable that a Ni content is set to 0.001% or more.
Since Ni is an expensive element, it is not economically preferable to contain a large amount of Ni. Therefore, the Ni content is set to 1.000% or less.
Sn increases the strength of the hot-rolled steel sheet by being solid-solubilized in steel and also increases the ductility and hole expansibility. In order to reliably exhibit this effect, the Sn content may be set to 0.001% or more.
On the other hand, when the Sn content is more than 0.100%, intergranular embrittlement cracking occurs during hot working. Therefore, the Sn content is set to 0.100% or less.
Ca has an action of enhancing the formability of the hot-rolled steel sheet by controlling the shape of the inclusions to a preferable shape. In order to more reliably obtain the effects of the actions, it is preferable that a Ca content is set to 0.0005% or more.
On the other hand, when the Ca content is more than 0.0200%, an excessive amount of inclusions is generated in steel, and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Ca content is set to 0.0200% or less.
As lowers an austenitizing temperature and thus refines the prior austenite grains, thereby contributing to an improvement of the hydrogen embrittlement resistance of the hot-rolled steel sheet. In order to reliably obtain the effects, it is preferable that an As content is set to 0.001% or more.
On the other hand, since the above effects are saturated even in a case where a large amount of As is contained, the As content is set to 0.100% or less.
Bi has an action of enhancing the formability of the hot-rolled steel sheet by refining a solidification structure. In order to more reliably obtain the effect, it is preferable that a Bi content is set to 0.001% or more.
In addition, even when the Bi content is more than 0.020%, the effect of the action is saturated, which is not economically preferable. Therefore, the Bi content is set to 0.020% or less.
Mg has an action of enhancing the formability of the hot-rolled steel sheet by controlling the shape of the inclusions to a preferable shape. In order to more reliably obtain the effect of the action, it is preferable that a Mg content is set to 0.0005% or more.
On the other hand, when the Mg content is more than 0.0200%, an excessive amount of inclusions is generated in steel, and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Mg content is set to 0.0200% or less.
Zr is an element that contributes to the control of inclusions, particularly to fine dispersion of inclusions, and increases the strength of the hot-rolled steel sheet. In order to reliably obtain this effect, it is preferable that a Zr content is set to 0.001% or more.
On the other hand, when a large amount of Zr is contained, there are cases where surface properties of the hot-rolled steel sheet significantly deteriorate. Therefore, the Zr content is set to 0.400% or less.
V is an element that increases the strength of the hot-rolled steel sheet by forming fine carbides in steel. In order to reliably obtain the effect, it is preferable that a V content is set to 0.001% or more.
On the other hand, when the V content is more than 0.200%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the V content is set to 0.200% or less.
REM has an action of enhancing the formability of the hot-rolled steel sheet by controlling the shape of the inclusions to a preferable shape. In order to more reliably obtain the effect of the action, it is preferable that a REM content is set to 0.0005% or more.
On the other hand, when the REM content is more than 0.1000%, an excessive amount of inclusions is generated in the steel, and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the REM content is set to 0.1000% or less.
Here, REM refers to a total of 17 kinds of rare earth elements including Sc, Y, and lanthanoids, and the REM content refers to the total amount of these elements. Lanthanoids are added in the form of mischmetal in industry.
Co is an element that contributes to the control of inclusions, particularly to the fine dispersion of inclusions, and increases the strength of the hot-rolled steel sheet. In order to reliably obtain this effect, it is preferable that a Co content is set to 0.0005% or more.
On the other hand, when a large amount of Co is contained, there are cases where the surface properties of the hot-rolled steel sheet significantly deteriorate. Therefore, the Co content is set to 0.2000% or less.
W is an element that contributes to the control of inclusions, particularly to the fine dispersion of inclusions, and increases the strength of the hot-rolled steel sheet. In order to reliably obtain this effect, it is preferable that a W content is set to 0.0005% or more.
On the other hand, when a large amount of W is contained, there are cases where the surface properties of the hot-rolled steel sheet significantly deteriorate. Therefore, the W content is set to 0.2000% or less.
Zn is an element that contributes to the control of inclusions, particularly to the fine dispersion of inclusions, and increases the strength of the hot-rolled steel sheet. In order to reliably obtain this effect, it is preferable that a Zn content is set to 0.0005% or more.
On the other hand, when a large amount of Zn is contained, there are cases where the surface properties of the hot-rolled steel sheet significantly deteriorate. Therefore, the Zn content is set to 0.2000% or less.
The above-described chemical composition of the hot-rolled steel sheet may be analyzed using a spark discharge emission spectrophotometer or the like. For C and S, values identified by burning the hot-rolled steel sheet in an oxygen stream using a gas component analyzer or the like and measuring C and S by an infrared absorption method are adopted. In addition, for N, a value identified by melting a test piece collected from the hot-rolled steel sheet in a helium stream and measuring N by a thermal conductivity method is adopted.
Next, a microstructure of the hot-rolled steel sheet according to the present embodiment will be described.
The hot-rolled steel sheet according to the present embodiment has, in the microstructure, by area %, bainite: 10% or more, tempered martensite: 10% or more, a total of bainite and tempered martensite: 70% to 96%, fresh martensite: 20% or less, residual austenite: 4% to 12%, ferrite: 5% or less, and pearlite: 5% or less, an average grain size of the prior austenite grains: 20.0 μm or less, an average aspect ratio of the prior austenite grains: 3.00 or less, and an area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more: 5% or less.
In the present embodiment, the microstructure at a thickness ¼ position of the hot-rolled steel sheet (a region from a thickness ⅛ depth from a surface to a thickness ⅜ depth from the surface) is specified. The reason for this is that the microstructure at this position represents a typical microstructure of the hot-rolled steel sheet.
Hereinafter, each specification will be described.
Bainite is a structure that increases the ductility of the hot-rolled steel sheet. When an area ratio of bainite is less than 10%, a desired ductility cannot be obtained in the hot-rolled steel sheet. Therefore, the area ratio of the bainite is set to 10% or more. The area ratio of bainite is preferably 20% or more or 30% or more, and more preferably 40% or more.
An upper limit of the area ratio of bainite is not particularly limited, but may be set to 96% or less based on a relationship with the area ratio of residual austenite. The area ratio of bainite may be set to 80% or less or 75% or less.
Tempered martensite is a structure effective for obtaining desired ductility, hole expansibility, and hydrogen embrittlement resistance in the hot-rolled steel sheet. When an area ratio of tempered martensite is less than 10%, desired ductility, hole expansibility, and hydrogen embrittlement resistance cannot be obtained in the hot-rolled steel sheet. Therefore, the area ratio of the tempered martensite is set to 10% or more. The area ratio of tempered martensite is preferably 15% or more or 20% or more, and more preferably 30% or more.
An upper limit of the area ratio of the tempered martensite is not particularly limited, but may be set to 96% or less based on a relationship with the area ratio of the residual austenite. The area ratio of tempered martensite may be set to 80% or less or 60% or less, or may be set to 50% or less.
When an area ratio of bainite and tempered martensite is less than 70% in total, desired ductility, hole expansibility, and/or hydrogen embrittlement resistance cannot be obtained in the hot-rolled steel sheet. Therefore, the area ratio of bainite and tempered martensite is set to 70% or more in total. The area ratio of bainite and tempered martensite is preferably 75% or more, and more preferably 80% or more.
An upper limit of the total area ratio of bainite and tempered martensite is not particularly limited, but is set to 96% or less based on a relationship with the area ratio of residual austenite. The total area ratio of bainite and tempered martensite may be set to 90% or less.
Fresh martensite is a structure that increases the strength of the hot-rolled steel sheet, but is also a structure that deteriorates the hole expansibility and hydrogen embrittlement resistance. When an area ratio of fresh martensite is more than 20%, the hole expansibility and hydrogen embrittlement resistance of the hot-rolled steel sheet deteriorate. Therefore, the area ratio of the fresh martensite is set to 20% or less. The area ratio of fresh martensite is preferably 15% or less, more preferably 10% or less, and even more preferably 7% or less.
A lower limit of the area ratio of fresh martensite is not particularly limited, and may be set to 0%.
Residual austenite is a structure that increases the ductility of the hot-rolled steel sheet. When the area ratio of residual austenite is less than 4%, a desired ductility cannot be obtained in the hot-rolled steel sheet. Therefore, the area ratio of the residual austenite is set to 4% or more. The area ratio of residual austenite is preferably 6% or more, and more preferably 7% or more.
On the other hand, when the area ratio of residual austenite is more than 12%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of residual austenite is set to 12% or less. The area ratio of residual austenite is preferably 10% or less.
When the area ratio of ferrite is more than 5%, the strength of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of the ferrite is set to 5% or less. The area ratio of ferrite is preferably 3% or less, and more preferably 2% or less. The area ratio of ferrite may be 0%.
When an area ratio of pearlite is more than 5%, the strength of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of the pearlite is set to 5% or less.
The area ratio of pearlite is preferably 3% or less. By setting the area ratio of pearlite to 3% or less, a TS-EI balance in the hot-rolled steel sheet can be further increased. That is, it is possible to realize high strength and excellent ductility at an even higher level. Since the area ratio of pearlite is preferably small, the area ratio of pearlite may be set to 0%.
Hereinafter, a method of measuring the area ratio of each structure will be described.
A test piece is collected from a cross section parallel to a rolling direction of the hot-rolled steel sheet so that a microstructure at a thickness ¼ position from the surface (a region from a thickness ⅛ depth from the surface to a thickness ⅜ depth from the surface) and at a center position in a sheet width direction can be observed.
The cross section of the test piece is polished using #600 to #1500 silicon carbide paper and is thereafter mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 μm in a diluted solution such as alcohol or in pure water. Next, the cross section of the test piece is polished at room temperature using colloidal silica containing no alkaline solution to remove strain introduced into a surface layer of the sample. At a certain position in a longitudinal direction of the cross section of the sample, a region with a length of 100 μm from the thickness ⅛ depth from the surface to the thickness ⅜ depth from the surface is set as an observed visual field (hereinafter, referred to as an EBSD observed visual field), and is measured at a measurement interval of 0.1 μm by an electron backscattering diffraction method, thereby obtaining crystal orientation information.
For the measurement, an EBSD apparatus including a thermal field-emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5 type detector manufactured by TSL solutions) is used. In this case, a degree of vacuum in the EBSD apparatus is set to 9.6×10−5 Pa or less, an accelerating voltage is set to 15 kV, an irradiation current level is set to 13, and an irradiation level of an electron beam is set to 62. From the obtained crystal orientation information, a region where a crystal structure is fcc is specified using a “Phase Map” function installed in software “OIM Analysis (registered trademark)” attached to an EBSD analyzer, and an area ratio of this region is calculated. Accordingly, the area ratio of the residual austenite is obtained.
Next, those having a bcc crystal structure are determined to be bainite, ferrite, pearlite, fresh martensite, and tempered martensite. For these regions, using a “Grain Orientation Spread” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer, a region having a “Grain Orientation Spread” of 1° or less is extracted as ferrite under a condition in which a 15° grain boundary is regarded as a grain boundary. By calculating an area ratio of the extracted ferrite, the area ratio of ferrite is obtained.
Subsequently, under a condition in which a 15° grain boundary is defined as a grain boundary in the remaining region (a region having a “Grain Orientation Spread” of more than 1°), when a maximum value of “Grain Average IQ” of the ferrite region is indicated as Iα, a region of more than Iα/2 is extracted as bainite, and a region of IW2 or less is extracted as “pearlite, fresh martensite, and tempered martensite”. By calculating the area ratio of the extracted bainite, the area ratio of bainite is obtained.
For the extracted “pearlite, fresh martensite and tempered martensite”, pearlite, fresh martensite and tempered martensite are distinguished by the following method.
Note that IQ may change depending on a state of the surface of the sample to be observed and accuracy at the time of measurement. In order to eliminate these effects, those with a confidence index (CI) value of 0.8 or more, which indicates reliability of the crystal orientation information, may be used. In a case where the CI value is 0.8 or less, electrolytic polishing in the method described above is performed again, and by adjusting an operation distance between the sample and an EBSD pattern detector, by adjusting the accelerating voltage or the irradiation current level, or by adjusting a gain or an exposure of the detector, data is acquired so that the CI value becomes 0.8 or more.
In order to observe the same region as the EBSD measurement region by SEM, a Vickers indentation is imprinted in the vicinity of an observation position. Thereafter, contamination on the surface layer is removed by polishing while leaving a structure of an observed section, and nital etching is performed. Next, the same visual field (EBSD observed visual field) as the EBSD observed section is observed by the SEM at a magnification of 3,000-fold. Among the regions determined to be “pearlite, fresh martensite, and tempered martensite” in the EBSD measurement, a region having a substructure in the grains and having cementite precipitated with a plurality of variants is determined to be tempered martensite. Aregion having cementite precipitated in a lamellar form is determined to be pearlite. A region having high brightness and having no substructure exposed by etching is determined as fresh martensite. By calculating the area ratio of each structure, the area ratios of tempered martensite, pearlite, and fresh martensite are obtained.
For the removal of the contamination on the surface layer of the observed section, a method such as buffing using alumina particles having a particle size of 0.1 pin or less or Ar ion sputtering may be used.
The rolling direction of the hot-rolled steel sheet can be determined by the following method.
First, a test piece is collected so that a sheet thickness cross section of the hot-rolled steel sheet can be observed. The sheet thickness cross section of the collected test piece is finished by mirror polishing and then observed using an optical microscope. An observation range is set to an overall thickness of the sheet thickness, and a region with dark brightness is determined to be an inclusion. Among inclusions, in inclusions having a major axis length of 5 μm or more, a direction parallel to a direction in which the inclusion extends is determined to be the rolling direction.
When a grain size of the prior austenite grains is large, local elongation deteriorates, and as a result, the hole expansibility of the hot-rolled steel sheet deteriorates. When the average grain size of the prior austenite grains is more than 20.0 μm, the deterioration of the hole expansibility of the hot-rolled steel sheet becomes significant. Therefore, the average grain size of prior austenite grains is set to 20.0 μm or less. The average grain size of the prior austenite grains is preferably 15.0 μm or less, and more preferably 13.0 μm or less.
The smaller the average grain size of the prior austenite grains is, the greater the improvement of the hole expansibility of the hot-rolled steel sheet is. However, the effect is saturated even when the average grain size is less than 7.0 μm. Therefore, the average grain size of the prior austenite grains may be set to 7.0 μm or more.
When the average aspect ratio of the prior austenite grains is more than 3.00, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the average aspect ratio of the prior austenite grains is set to 3.00 or less. The average aspect ratio of the prior austenite grains is preferably 2.50 or less, more preferably 2.00 or less, and even more preferably 1.50 or less.
An aspect ratio of the prior austenite grain is a value obtained by dividing a major axis of the prior austenite grain by a minor axis and takes a value of 1.00 or more. As the aspect ratio is smaller, the grains are more equiaxed, and as the aspect ratio is larger, the grains become more flat.
A sample is collected from a cross section parallel to the rolling direction of the hot-rolled steel sheet so that a thickness ¼ position from the surface (a region from a thickness ⅛ depth to a thickness ⅜ depth from the surface) can be observed. A structure of the sheet thickness cross section is revealed by a picric acid saturated aqueous solution and a sodium dodecylbenzene sulfonate corrosive solution. The grain sizes of prior austenite grains are measured from photographs of microstructures taken of at least three EBSD observed visual fields at a magnification of 500-fold at the thickness ¼ depth position from the surface of the sample (a region from the thickness ⅛ depth to the thickness ⅜ depth from the surface). A circle equivalent diameter is calculated for one of the prior austenite grains included in each of the observed visual fields. The above operation is performed on all the prior austenite grains included in each of the observed visual fields except for the prior austenite grains which are not entirely included in the photographed visual fields, such as prior austenite grains in an end portion of the photographed visual field, and the circle equivalent diameters of all the prior austenite grains in each of the photographed visual fields are obtained. The average grain size of the prior austenite grains is obtained by calculating an average value of the circle equivalent diameters of the prior austenite grains obtained in each of the photographed visual fields.
In a case where prior austenite grains having a circle equivalent diameter of less than 2.0 μm are included, the prior austenite grains are excluded and the above-described measurement is performed.
In addition, the major axis and the minor axis of at least 20 prior austenite grains having a circle equivalent diameter of 2.0 μm or more included in each of the photographed visual fields are measured. Average values of the major axes and the minor axes obtained by measuring each prior austenite grain are calculated to obtain an average major axis and an average minor axis of the prior austenite grains. The average aspect ratio of the prior austenite grains is obtained by calculating a ratio therebetween (average major axis/average minor axis).
When the area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more is more than 5%, the hydrogen embrittlement resistance in the hot-rolled steel sheet, particularly in the cut end surface deteriorates. Therefore, the area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more is set to 5% or less. In order to further enhance the hydrogen embrittlement resistance, the area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more is preferably set to 2% or less.
The area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more is preferably as low as possible. Therefore, the area ratio of the aggregates of fresh marten site and residual austenite having a long diameter of 30 μm or more may be set to 0%.
The aggregates of fresh martensite and residual austenite will be described.
A grain of fresh martensite or residual austenite is approximated as an ellipsoid (ellipsoid 1), and the fresh martensite or residual austenite adjacent to the ellipsoid 1 is also approximated as an ellipsoid (ellipsoid 2). Next, a distance l between the ellipsoid 1 and the ellipsoid 2 is obtained. In a case where a long diameter of the ellipsoid 1 is indicated as a and the distance l between the ellipsoid 1 and the ellipsoid 2 is smaller than 1.5×a, the ellipsoid 1 and the ellipsoid 2 adjacent to the ellipsoid 1 are regarded as a single aggregate. Fresh martensite and residual austenite in the microstructure are approximated as ellipsoids, the long diameter a of the ellipsoid and the distance 1 between the ellipsoids adjacent to each other are obtained, and an aggregate of fresh martensite and residual austenite is specified based on the above-described criteria. The long diameter of the aggregate of fresh martensite and residual austenite that has been specified is obtained, and an area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more is calculated, whereby the area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more is obtained.
In addition, fresh martensite and residual austenite are specified by the same method as the method of measuring the area ratios of the above-described structures.
A method of approximating fresh martensite and residual austenite as ellipsoids will be described.
As shown in
By approximating fresh martensite and residual austenite as the ellipsoids in this manner, x0, y0, a, and b are obtained.
Region of Bainite in Which Average Value of Nanoindentation Hardness at Load of 5,000 μN and Loading Rate of 500 μN/s Is 4.7 GPa or Less: 30% or Less
A TS-EI balance in the hot-rolled steel sheet can be further increased by reducing an area ratio of a region of bainite having a low hardness. That is, it is possible to realize high strength and excellent ductility at an even higher level. Therefore, a region of bainite in which an average value of nanoindentation hardness at a load of 5,000 μN and a loading rate of 500 μN/s is 4.7 GPa or less may be set to 30% or less.
A method of measuring a proportion (percentage) of the region of bainite in which the average value of the nanoindentation hardness at a load of 5,000 μN and a loading rate of 500 μN/s is 4.7 GPa or less will be described.
Bainite is specified by the same method as the method of measuring the area ratios of the above-described structures. The nanoindentation hardness is measured on all the regions specified as bainite at a load of 5,000 μN and a loading rate of 500 μN/s. The nanoindentation hardness is measured at 10 or more points in one region specified as bainite, and an average value of the nanoindentation hardness of the region is calculated. Next, an area ratio of regions in which the average value of the nanoindentation hardness is 4.7 GPa or less is calculated. The area ratio of the regions in which the average value of the nanoindentation hardness is 4.7 GPa or less is divided by the area ratio of bainite and multiplied by 100, thereby obtaining the proportion (percentage) of the region of bainite in which the average value of the nanoindentation hardness at a load of 5,000 μN and a loading rate of 500 μN/s is 4.7 GPa or less.
For the measurement, TriboScope/Tribolndenter manufactured by Hysitron is used.
The hot-rolled steel sheet according to the present embodiment may have a tensile strength of 1,100 MPa or more. By setting the tensile strength to 1,100 MPa or more, the hot-rolled steel sheet can be suitably applied to various vehicle suspension components. The tensile strength may be set to 1,150 MPa or more, 1,200 MPa or more, or 1,300 MPa or more.
The higher the tensile strength is, the more preferable it is, but the tensile strength may be set to 1,450 MPa or less.
The hot-rolled steel sheet according to the present embodiment may have a uniform elongation of 5.5% or more. By setting the uniform elongation to 5.5% or more, the hot-rolled steel sheet can be suitably applied to a vehicle suspension component. The uniform elongation is preferably 6.0% or more or 7.0% or more.
An upper limit of the uniform elongation is not particularly limited, but may be set to 20.0% or less.
The tensile strength and the uniform elongation are measured by performing a tensile test in accordance with JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011. A collecting position of the tensile test piece is set to a center position in the sheet width direction, and a direction perpendicular to the rolling direction is set as a longitudinal direction.
The uniform elongation is a “percentage total extension at maximum force” defined in JIS Z 2241:2011.
The hot-rolled steel sheet according to the present embodiment may have a hole expansion ratio of 30% or more. By setting the hole expansion ratio to 30% or more, the hot-rolled steel sheet can be suitably applied to a vehicle suspension component. The hole expansion ratio is preferably 35% or more, 40% or more, or 45% or more. An upper limit of the hole expansion ratio is not particularly specified and may be set to 70% or less.
The hole expansion ratio is measured by performing a hole expanding test in accordance with JIS Z 2256:2020.
The hydrogen embrittlement resistance of the hot-rolled steel sheet is evaluated by the following method.
A test piece of 50 mm×50 mm is collected from the hot-rolled steel sheet. A punch of φ10 mm and a die of φ10 mm and 0.2× sheet thickness are used to form a punched hole in a center portion of the test piece. Next, the test piece is immersed in hydrochloric acid having a pH of 2, and the hydrogen embrittlement resistance is evaluated based on the presence or absence of the occurrence of cracking in the cut end surface. In a case where no cracking occurs in the cut end surface even after the test piece is immersed in hydrochloric acid for 72 hours or longer, it can be determined that the test piece has excellent hydrogen embrittlement resistance. Furthermore, in a case where no cracking occurs in the cut end surface even after the test piece is immersed in hydrochloric acid under the same conditions, the hydrochloric acid is replaced with new hydrochloric acid after 36 hours, and the test piece is immersed for another 36 hours (a total of 72 hours of immersion), it is determined that the test piece has superior hydrogen embrittlement resistance.
In a case where a crack having a length of more than 100 μm is observed by observing the cut end surface with an optical microscope, it is determined that cracking has occurred.
The hot-rolled steel sheet according to the present embodiment may be a surface-treated steel sheet provided with a plating layer on the surface for the purpose of improving corrosion resistance or the like. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrogalvanizing, and electro Zn—Ni alloy plating. Examples of the hot-dip plating layer include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, and hot-dip Zn—Al—Mg—Si alloy plating. A plating adhesion amount is not particularly limited and may be the same as in the related art. In addition, it is also possible to further enhance the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion liquid) after plating.
Next, a preferred manufacturing method of the hot-rolled steel sheet according to the present embodiment will be described.
Temperatures described below refer to surface temperatures of a slab or a steel sheet unless otherwise specified.
The preferred manufacturing method of the hot-rolled steel sheet according to the present embodiment includes:
In the finish rolling step, it is more preferable to perform the finish rolling so that a finish rolling start temperature becomes a temperature range of 1,060° C. to 1,080° C.
In the coiling step, it is more preferable to perform the coiling so that a highest temperature after the start of coiling is lower than 500° C.
In addition, in the cooling step and the coiling step, it is more preferable that the cooling and the coiling are performed so that a total stay time in a temperature range of 450° C. to 500° C. is shorter than 2,000 seconds.
Hereinafter, each step will be described.
When a heating temperature is lower than 1,220° C., solutionizing does not proceed, the amount of ferrite increases, and the strength of the hot-rolled steel sheet deteriorates. Therefore, the heating temperature is preferably set to 1,220° C. or higher. The heating temperature is more preferably 1,240° C. or higher.
On the other hand, when the heating temperature is higher than 1,300° C., austenite grains are coarsened during heating, and as a result, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the heating temperature is preferably set to 1,300° C. or lower. From the viewpoint of reducing energy costs, the heating temperature is preferably set to 1,280° C. or lower.
When a holding time in a temperature range of 1,220° C. to 1,300° C. is shorter than 40 minutes, the solutionizing does not proceed, the amount of ferrite increases, and the strength of the hot-rolled steel sheet deteriorates. Therefore, the holding time in the temperature range is preferably set to 40 minutes or longer. The holding time is more preferably 60 minutes or longer, and even more preferably 80 minutes or longer.
An upper limit of the holding time is not particularly limited, and may be set to 200 minutes or shorter.
The slab to be heated is not particularly limited except that the slab has the above-described chemical composition. For example, a slab manufactured by melting molten steel having the above chemical composition using a converter or an electric furnace and performing a continuous casting method thereon can be used. Instead of the continuous casting method, an ingot-making method, a thin slab casting method, or the like may also be adopted.
When rolling with a rolling reduction of less than 10% is performed in the first to third passes or rolling with a rolling reduction of less than 15% is performed in the fourth and subsequent passes, the prior austenite grains are coarsened. Therefore, it is preferable that the rolling reduction in each of the first to third passes is set to 10% or more, and the rolling reduction in each of the fourth and subsequent passes is set to 15% or more. More preferably, the rolling reduction in each of the first to third passes is 15% or more or 20% or more, and the rolling reduction in each of the fourth and subsequent passes is 20% or more or 25% or more.
In addition, when rolling with a rolling reduction of more than 30% is performed in the first to third passes or rolling with a rolling reduction of more than 50% is performed in the fourth and subsequent passes, the finish rolling is performed in a state where the prior austenite grains are non-uniform, and the prior austenite grains stretched in a specific direction are likely to be formed after the finish rolling. As a result, the average aspect ratio of the prior austenite grains increases, and the area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more increases. Therefore, it is preferable that the rolling reduction in each of the first to third passes is set to 30% or less, and the rolling reduction in each of the fourth and subsequent passes is set to 50% or less. More preferably, the rolling reduction in each of the first to third passes is 25% or less, and the rolling reduction in each of the fourth and subsequent passes is 40c or less.
The rolling reduction of each pass can be represented by {1−(t1/t0)}×100(%) when an inlet sheet thickness t0 of each pass and an outlet sheet thickness t1 of each pass are used.
A rough rolling completion temperature (an exit-side temperature of a final pass of the rough rolling) is not particularly limited, but is preferably 1,070° C. or higher from the viewpoint of hot deformation resistance. In addition, from the viewpoint of reducing defects due to scale entrapment, the rough rolling completion temperature is preferably 1,200° C. or lower.
In the finish rolling step, in a case where a rolling temperature or a rolling reduction in the final pass is too low, recrystallization does not proceed sufficiently, and the average grain size of the prior austenite grains, the average aspect ratio of the prior austenite grains, and/or the area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more cannot be preferably controlled. Therefore, in the finish rolling step, it is preferable that the rolling of the final pass is performed in a temperature range of 940° C. or higher at a rolling reduction of 25% or more.
The rolling of the final pass is performed more preferably at 960° C. or higher, more preferably performed at 1,010° C. or lower, and more preferably performed at a rolling reduction of 30% or more. The rolling reduction of the final pass may be set to 50% or less.
In addition, in the finish rolling step, it is more preferable that the finish rolling is performed so that the finish rolling start temperature (an entry-side temperature of a first stage of the finish rolling) is in a temperature range of 1,060° C. to 1,080° C. By setting the finish rolling start temperature to a temperature range of 1,060° C. to 1,080° C., the area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more can be further reduced. As a result, it is possible to manufacture a hot-rolled steel sheet having superior hydrogen embrittlement resistance.
When a time from the completion of the finish rolling to the start of the cooling is longer than 2.0 seconds, grain growth of austenite recrystallized grains proceeds, and as a result, the prior austenite grains are coarsened. Therefore, it is preferable to start the cooling within 2.0 seconds after the completion of the finish rolling.
The cooling mentioned here does not include air cooling, and the time until the start of cooling refers to a time until water cooling is started after the rolling in the final pass of the finish rolling.
The cooling may be started immediately after the completion of the finish rolling, but this requires spraying of cooling water directly under a finish rolling mill and results in excessive cooling of rolls. Therefore, it becomes difficult to control a final rolling temperature. Therefore, it is more preferable that the time until the start of cooling is set to 1.0 second or longer.
When a cooling time from the completion of the finish rolling to the temperature range of 300° C. to (Ms-10)° C. is longer than 16.0 seconds, ferrite is excessively generated. Therefore, after the start of cooling, it is preferable to cool the hot-rolled steel sheet to a temperature range of 300° C. to (Ms-10)° C. within 16.0 seconds after the completion of the finish rolling. The shorter the cooling time is, the more preferable it is. However, in order to cool the hot-rolled steel sheet to a desired temperature range within a short period of time, it is necessary to increase a water density, which increases a load on a cooling nozzle and impairs an economic efficiency. Therefore, the cooling time to the temperature range of 300° C. to (Ms-10)° C. is more preferably set to 5.0 seconds or longer or 7.0 seconds or longer from the completion of the finish rolling.
After cooling the hot-rolled steel sheet to a temperature range of 300° C. to (Ms-10)° C., the cooling is stopped. When a cooling stop temperature is lower than 300° C., bainite is excessively generated. Therefore, the cooling stop temperature is set to 300° C. or higher. The cooling stop temperature is preferably 320° C. or higher.
On the other hand, when the cooling stop temperature is higher than Ms-10° C., the amounts of bainite and tempered martensite decrease. Therefore, the cooling stop temperature is preferably set to Ms-10° C. or lower. The cooling stop temperature is more preferably 400° C. or lower.
After the cooling is stopped, the hot-rolled steel sheet is immediately coiled. That is, a coiling temperature and the cooling stop temperature are the same temperature.
In addition, Ms (° C.) can be obtained by the following formula.
Ms=496×(1−0.62×C)×(1−0.0092×Mn)×(1−0.033×Si)×(1−0.045×Ni)×(1−0.07×Cr)×(1−0.029×Mo)×(1−0.018×W)×(1+0.012×Co)
After the start of coiling, the temperature of the steel sheet increases due to heat generation caused by transformation as the hot-rolled steel sheet is coiled into a coil shape. In particular, the temperature is likely to rise in a center portion of a coil or a center portion of an outermost peripheral surface of the coil in the sheet width direction. By securing a sufficient stay time in a temperature range of 300° C. to 450° C. from the start of coiling, the area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more can be reduced. When the stay time in the temperature range of 300° C. to 450° C. after the start of coiling is shorter than 100 seconds, the area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more cannot be reduced. Therefore, the stay time in the temperature range of 300° C. to 450° C. after the start of coiling is preferably set to 100 seconds or longer.
The temperature mentioned here is a surface temperature of the center portion of the outermost peripheral surface of the coil in the sheet width direction.
The stay time in the temperature range of 300° C. to 450° C. refers to a time from when the cooling to a temperature range of 300° C. to (Ms-10)° C. is performed and the coiling is started to when the temperature is lowered and reaches 300° C., or a time from when the temperature rises due to heat generation caused by transformation to when the temperature reaches 450° C.
Even in a case where the temperature rises to higher than 450° C. due to heat generation caused by transformation after the start of coiling and then the temperature is lowered to a temperature range of 300° C. to 450° C., a time in this case is not added to the stay time at a time when the temperature rises to higher than 450° C.
In the coiling step, it is more preferable to perform coiling so that the highest temperature after the start of coiling is lower than 500° C. After the start of coiling, the temperature rise due to reheating can be controlled so that the surface temperature of the center portion of the outermost peripheral surface of the coil in the sheet width direction is lower than 500° C., whereby the amount of pearlite can be further reduced. As a result, the TS-EI balance in the hot-rolled steel sheet can be further increased. That is, it is possible to realize high strength and excellent ductility at an even higher level.
In the cooling step and the coiling step, it is more preferable to perform cooling and coiling so that the total stay time in a temperature range of 450° C. to 500° C. is shorter than 2,000 seconds. By performing cooling and coiling so that the total stay time in the temperature range of 450° C. to 500° C. is shorter than 2,000 seconds, the area ratio of a region of bainite having a low hardness can be reduced. Accordingly, the TS-EI balance in the hot-rolled steel sheet can be further increased. That is, it is possible to realize high strength and excellent ductility at an even higher level.
The total stay time mentioned here is a sum of the stay time in the temperature range of 450° C. to 500° C. during the cooling and the stay time in the temperature range of 450° C. to 500° C. when the temperature rises due to heat generation caused by transformation after the start of coiling.
The hot-rolled steel sheet according to the present embodiment can be stably manufactured by the manufacturing method including the above-described steps.
Next, examples of the present invention will be described. However, conditions in the examples are examples of conditions that are adopted to confirm the feasibility and effect of the present invention. The present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
By performing continuous casting, slabs having the chemical compositions shown in Tables 1 Ato 2B were obtained. Steel Nos. I, L, U, and Z were found to have cracks in the slab during casting, so that manufacturing was stopped at that time.
Next, a coil made of the hot-rolled steel sheet was manufactured under the conditions shown in Tables 3A to 4C.
In Test No. 5, hot deformation resistance was high, so that manufacturing after the finish rolling was stopped.
From the outermost peripheral portion of the obtained coil, a test piece for microstructure observation, a test piece for a tensile test, a test piece for a hole expanding test, and a test piece for evaluating the hydrogen embrittlement resistance were cut out. Each test method was the same method as the above-described method.
In addition, the microstructure observation was performed using the obtained test piece by the above-described method. The results obtained are shown in Tables 5A to 5D.
Each item in Tables 5A to 5D shows the following.
In addition, regarding “Area ratio of aggregates of FM and γr” in the tables, a case where the area ratio of the aggregates of fresh martensite and residual austenite having a long diameter of 30 μm or more was more than 2% and 5% or less was marked as “B”, a case of 2% or less was marked as “A”, and a case of more than 5% was marked as “C”. Here, “A” and “B” were determined to be acceptable, and “C” was determined to be unacceptable.
In addition, a case where the region of bainite in which the average value of the nanoindentation hardnesses at a load of 5,000 μN and a loading rate of 500 μN/s was 4.7 GPa or less was 30% or less was marked as “OK” in “Region of 4.7 GPa or less is 30% or less”, and a case where this condition was not satisfied was marked as “NG”.
In a case where the tensile strength was 1,100 MPa or more, the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having high strength and determined to be acceptable. On the other hand, in a case where the tensile strength was less than 1,100 MPa, the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having no high strength and determined to be unacceptable.
In a case where the uniform elongation was 5.5% or more, the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having excellent ductility and determined to be acceptable. On the other hand, in a case where the uniform elongation was less than 5.5%, the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having no excellent ductility and determined to be unacceptable.
In a case where the hole expansion ratio was 30% or more, the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having excellent hole expansibility and determined to be acceptable. On the other hand, in a case where the hole expansion ratio was less than 30%, the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having no excellent hole expansibility and determined to be unacceptable.
In the evaluation of the hydrogen embrittlement resistance, in a case where no cracking occurred in the cut end surface even after the test piece was immersed in hydrochloric acid for 72 hours or longer, the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having excellent hydrogen embrittlement resistance, determined to be acceptable, and marked as “B” in the tables. On the other hand, in a case where the test piece was immersed in hydrochloric acid for 72 hours or longer and cracking occurred in the cut end surface, the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having no excellent hydrogen embrittlement resistance, determined to be unacceptable, and marked as “C” in the tables.
Furthermore, in a case where no cracking occurred in the cut end surface even after the test piece was immersed in hydrochloric acid under the same conditions, the hydrochloric acid was replaced with new hydrochloric acid after 36 hours, and the test piece was immersed for another 36 hours (a total of 72 hours of immersion), the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having superior hydrogen embrittlement resistance, and marked as “A” in the tables.
0.12
0.24
0.60
1.80
1.70
3.10
0.098
0.009
0.003
0.510
0.007
0.003
0.051
0.670
0.407
0.0098
0.069
0.0036
0.0000
0.0461
0.0812
I
L
U
Z
AC
A
B
E
F
G
H
J
K
M
N
P
R
T
1190
1315
37
49
930
925
1040
22
AD
13
56
I
L
U
Z
AC
A
B
NG
E
F
G
H
J
K
M
N
P
R
T
19
20
NG
21
NG
22
23
24
25
26
27
2.6
18.0
293
422
NG
NG
AD
56
57
I
L
U
Z
AC
A
12
B
64
25
C
E
F
16
G
11
H
J
K
M
N
26.0
C
P
3.10
C
R
T
19
6
1068
21
C
4.0
29
4.6
25
1098
29
22
C
28
C
5.0
4.0
19
1092
20
67
21
21
10
22
21.0
23
3.10
C
24
68
21
25
3.50
C
26
21.0
C
27
3.10
C
28
22.0
29
11
6
30
31
24
65
C
32
53
37
C
20
28
21
1071
22
25
23
28
C
24
26
25
29
C
26
21
C
27
5.0
26
C
28
21
29
1072
30
4.0
31
4.0
21
C
32
25
C
AD
8
56
21.0
57
3.15
C
1090
56
29
57
C
As shown in Tables 5A to 51D, it can be seen that the hot-rolled steel sheets according to the present invention examples have high strength, and excellent ductility, hole expansibility, and hydrogen embrittlement resistance.
On the other hand, it can be seen that the hot-rolled steel sheets according to the comparative examples are inferior in any one or more of the above properties.
According to the above aspect according to the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent ductility, hole expansibility, and hydrogen embrittlement resistance.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-109509 | Jul 2022 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2023/023384 | 6/23/2023 | WO |
