The present invention relates to a hot-rolled steel sheet and a method for manufacturing the same. More specifically, the present invention relates to a hot-rolled steel sheet having a tensile strength of 1470 MPa or more, which is excellent in toughness and hole-expanding property and is suitable as a material for transportation machines such as automobiles and mechanical structural members, and a method for manufacturing the same.
Background technology
[0002]
From the viewpoint of environmental regulations, steel sheets are becoming thinner in order to reduce the weight of transportation equipment such as automobiles, and steel sheets are required to have high strength (for example, tensile strength) that can withstand the thinning. Further, in general, not only the thinning of the steel sheet but also the high toughness of the steel sheet is required from the viewpoint of impact resistance. Therefore, as a steel sheet used for the above-mentioned applications, mechanical properties having excellent strength, toughness, and workability (for example, hole expandability) are required. In addition, strength and toughness are in a trade-off relationship, and it is necessary to control the hard phase, which is the starting point of fracture, in order to comprehensively increase strength and toughness (impact characteristics).
[0003]
Many proposals have been made so far for achieving and improving the strength, toughness (impact characteristics), and hole-expandability of steel sheets.
[0004]
Patent Document 1 describes in terms of mass%, C: 0.08% or more and less than 0.16%, Si: 0.01 to 1.0%, Mn: 0.8 to 2.0%, P: 0.025. % Or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.002 to 0.006%, and further contains Nb, Ti, Cr, B, and the balance is Fe. A steel material having a composition consisting of unavoidable impurities is heated to a temperature of 1100 to 1250 ° C., and the heated steel material is subjected to rough rolling with a rough rolling output side temperature RDT of 900 to 1100 ° C. and a finish rolling inlet temperature. FET: 900 to 1100 ° C., finish rolling output side temperature FDT: 800 to 900 ° C., and finish rolling with a cumulative rolling reduction of 20 to 90% in the temperature range below 930 ° C. Described is a method for producing a high-strength hot-rolled steel sheet having excellent low-temperature toughness, which is cooled to a cooling stop temperature of 300 ° C. or lower at an average cooling rate of / sec or more and wound at a temperature of 300 ° C. or lower. Further, in Patent Document 1, according to the production method, 90% by volume or more of martensite phase or tempered martensite phase is the main phase, and the average particle size of the old austenite grains is 20 μm in the L cross section parallel to the rolling direction. Hereinafter, it is described that a high-strength hot-rolled steel sheet having an aspect ratio of 18 or less, a yield strength of YS: 960 MPa or more, and excellent low-temperature toughness can be obtained.
[0005]
In Patent Document 2, C: 0.15% by mass to 0.35% by mass, total of Si and Al: 0.5% by mass to 3.0% by mass, Mn: 1.0% by mass to 4.0% by mass. %, P: 0.05% by mass or less, S: 0.01% by mass or less, the balance is composed of Fe and unavoidable impurities, the steel structure has a ferrite content of 5% or less, and the tempered martensite content. The rate is 60% or more, the amount of retained austenite is 10% or more, the average size of MA (complex of martensite and austenite) is 1.0 μm or less, and the q value in X-ray small angle scattering is 1 nm-. A high-strength steel sheet having a scattering intensity of 1.0 cm -1 or less at 1 is described, and the high-strength steel sheet has tensile strength (TS), yield ratio (YR), (TS) and total elongation (EL). It is taught that the product with (TS × EL), the hole expansion ratio (λ), and the impact resistance characteristics are all at high levels.
Prior art literature
Patent documents
[0006]
Patent Document 1: Japanese Unexamined Patent Publication No. 2016-211073
Patent Document 2: Japanese Unexamined Patent Publication No. 2017-214647
Outline of the invention
Problems to be solved by the invention
[0007]
Patent Document 1 describes that a high-strength hot-rolled steel sheet has a tempered martensite phase or a martensite phase as a main phase and is excellent in bendability and low-temperature toughness, but mentions hole-expandability. No. Generally, when rolling in an unrecrystallized region such as less than 930 ° C., the texture develops and the aspect ratio becomes relatively large, resulting in a decrease in hole expandability. Therefore, the technique described in Patent Document 1 does not always obtain a hot-rolled steel sheet having excellent hole-expanding properties. Further, the invention described in Patent Document 2 relates to a high-strength steel plate having tempered martensite as a main phase and having retained austenite dispersed therein. In general, retained austenite is transformed and hardened during punching, so when drilling is performed after punching, stress concentrates on the interface between the transformed martensite and the matrix, and the hole expanding property is reduced. Therefore, the technique described in Patent Document 2 cannot obtain a hot-rolled steel sheet having a desired hole-expanding property.
[0008]
From the above, it is difficult for the high-strength steel sheets disclosed in Patent Documents 1 and 2 to satisfy sufficient toughness and hole expandability while having a tensile strength (TS) of 1470 MPa class, and these requirements can be satisfied. Hot-rolled steel sheets are required.
[0009]
The present invention provides a high-strength hot-rolled steel sheet having excellent hole-expanding properties and good toughness, and a method for manufacturing the same, which has not been substantially studied so far in view of the prior art.
Means to solve problems
[0010]
In order to obtain a high-strength hot-rolled steel sheet with excellent toughness and hole-spreading property, the present inventors control the chemical composition of the steel sheet, especially B (boron) is essential, and the metal structure is tempered martensite. It was found that it is effective to suppress the coarsening of the old austenite grains, to refine the carbides in the tempered martensite, and to reduce the toughness of the structure.
[0011]
The gist of the present invention is as follows.
(1)
By mass%,
C: 0.12% or more, 0.25% or less,
Si: 0.01% or more, 2.0% or less,
Mn: 0.5% or more, 3.0% or less,
P: 0.020% or less,
S: 0.010% or less,
Al: 0.001% or more, 0.10% or less,
B: 0.0005% or more, 0.0050% or less,
Cu: 0% or more, 0.50% or less,
Ni: 0% or more, 0.50% or less,
Cr: 0% or more, 0.50% or less,
Mo: 0% or more, 0.50% or less,
V: 0% or more, 0.05% or less,
Ca: 0% or more, 0.05% or less, and
REM: 0% or more, 0.01% or less
Contains,
Nb: 0.001% or more, 0.020% or less, and
Ti: 0.001% or more, 0.20% or less
It contains one or two of them, and the balance has a chemical component consisting of Fe and impurities.
The metallographic structure at the position of 1/4 plate thickness from the surface contains tempered martensite with an area ratio of more than 90%.
The average particle size of carbides in tempered martensite is 10 nm or less,
The average particle size of the old austenite grains is less than 40 μm,
The aspect ratio of the old austenite grains is 3.5 or less,
A hot-rolled steel sheet characterized in that the X-ray random intensity ratio in the {112} <110> direction at a position 1/2 plate thickness from the surface is 4.0 or less.
(2)
By mass%,
Cu: 0.01% or more, 0.50% or less,
Ni: 0.01% or more, 0.50% or less,
Cr: 0.001% or more, 0.50% or less,
Mo: 0.001% or more, 0.50% or less,
V: 0.001% or more, 0.05% or less,
Ca: 0.0005% or more, 0.05% or less, and
REM: 0.001% or more, 0.01% or less
The hot-rolled steel sheet according to (1), which contains one or more of the above.
(3)
The hot-rolled steel sheet according to (1) or (2), wherein the metallographic structure contains tempered martensite in an area ratio of more than 95%.
(4)
The residual structure of the metal structure at a position 1/4 of the plate thickness from the surface is characterized by consisting of at least one of retained austenite, fresh martensite, bainite, ferrite, and pearlite, (1) to (3). ). The hot-rolled steel sheet according to any one of.
(5)
The residual structure of the metal structure at the position of 1/4 plate thickness from the surface is characterized by containing 0% or more and 5% or less of retained austenite and 0% or more and 5% or less of ferrite, (1) to (4). ). The hot-rolled steel sheet according to any one of.
(6)
The hot-rolled steel sheet according to any one of (1) to (5), wherein the aspect ratio of the old austenite grains is 3.0 or less.
(7)
A heating step of heating a slab containing the chemical component according to (1) or (2) at 1250 ° C. or lower, and
A hot rolling process including finishing rolling of a heated slab at a rolling reduction of 10% or more and 40% or less in the final stage, wherein the finishing temperature of the finishing rolling is 900 ° C. or higher and 1050 ° C. or lower. ,
Cooling is started within 2.0 seconds after the end of the hot spreading process, the average cooling rate from the cooling start temperature to 700 ° C is 20 ° C / sec or more and 200 ° C / sec or less, and further, winding from the cooling start temperature. A cooling process that continuously cools the hot-rolled steel plate at a cooling rate with an average cooling rate of 40 ° C / sec or more up to the taking temperature.
The winding process of winding the cooled hot-rolled steel sheet at 20 ° C or higher and 100 ° C or lower,
The hot-rolled steel sheet is characterized by including a tempering step in which the wound hot-rolled steel sheet is air-cooled to room temperature and then tempered at a low temperature under the condition that the integrated tempering parameter ST is 13.0 or more and 27.0 or less. Manufacturing method.
(8)
The method for manufacturing a hot-rolled steel sheet according to (7), wherein the integrated tempering parameter ST is 20.0 or more and 25.0 or less.
The invention's effect
[0012]
According to the present invention, it is possible to obtain a hot-rolled steel sheet which is suitable for obtaining automobile members, particularly undercarriage parts for automobiles, by improving strength, hole expandability and toughness. Specifically, according to the present invention, a hot-rolled steel sheet having characteristics of tensile strength (TS) of 1470 MPa or more, hole expansion ratio (λ) of 60% or more, and brittle ductility transition temperature (vTrs) of −40 ° C. or less is obtained. be able to.
Embodiment for carrying out the invention
[0013]
[Hot-rolled steel plate]
Hereinafter, the hot-rolled steel sheet according to the present invention will be described in detail. First, the reason for limiting the chemical composition of the hot-rolled steel sheet according to the present invention will be described. Hereinafter,% for the chemical composition means mass%.
[0014]
(C: 0.12% or more, 0.25% or less)
C is an essential element for obtaining a desired structure and ensuring a property with a tensile strength of 1470 MPa or more, and it is necessary to add 0.12% or more in order to effectively exert such an action. However, if the C content exceeds 0.25%, fresh martensite remains and a desired metal structure cannot be obtained, and the workability and weldability are further deteriorated, so that the cost at the time of manufacturing the steel sheet is increased. Therefore, the C content is set to 0.25% or less. The C content is preferably 0.14% or more, more preferably 0.16% or more. The C content is preferably 0.23% or less, more preferably 0.20% or less.
[0015]
(Si: 0.01% or more, 2.0% or less)
Si has a function of suppressing cementite precipitation during tempering. In order to effectively exert such an action, it is necessary to add 0.01% or more of Si. The Si content is preferably 0.1% or more, more preferably 0.2% or more. Further, since Si has a function of promoting the formation of retained austenite, if it exceeds 2.0%, it tends to remain after winding and the toughness decreases. Therefore, the Si content is 2.0% or less, preferably 1.5% or less, and more preferably 1.3% or less.
[0016]
(Mn: 0.5% or more, 3.0% or less)
Mn has the function of suppressing the formation of ferrite and improving hardenability. In order to effectively exert such an action, it is necessary to add Mn content of 0.5% or more. However, when the Mn content exceeds 3.0%, macrosegregation of Mn becomes remarkable, and a difference in hardness occurs between the segregated portion and the unsegregated portion, resulting in an inhomogeneous structure. Therefore, the Mn content is set to 3.0% or less. The Mn content is preferably 0.8% or more, more preferably 1.0% or more. The Mn content is preferably 2.8% or less, more preferably 2.5% or less.
[0017]
(P: 0.020% or less)
P is inevitably present as an impurity element. The presence of P in excess of 0.020% reduces drilling and toughness. Therefore, the P content is 0.020% or less. Preferably, the P content is 0.015% or less. The lower limit of P content is particularly limited Although not determined, it may be, for example, 0.001%.
[0018]
(S: 0.010% or less)
S is inevitably present as an impurity element. If S exceeding 0.010% is present, sulfide-based inclusions such as MnS are formed, which becomes the starting point of cracking and reduces the hole-spreading property. Therefore, the S content is 0.010% or less. Preferably, the S content is 0.005% or less. The lower limit of the S content is not particularly limited, but may be, for example, 0.001%.
[0019]
(Al: 0.001% or more, 0.10% or less)
Al has the effect of deoxidizing steel to make the steel sheet sound. In order to effectively exert such an action, it is necessary to add 0.001% or more of Al. However, if the Al content exceeds 0.10%, the effect of the above action is saturated, which is disadvantageous in terms of cost. Therefore, the Al content is set to 0.10% or less. Preferably, the Al content is 0.05% or less. In order to obtain the deoxidizing effect more reliably, the Al content is preferably 0.01% or more.
[0020]
(Nb: 0.001% or more, 0.020% or less and Ti: 0.001% or more, 0.20% or less, one or two types)
Nb and Ti have the effect of suppressing the coarsening of old austenite grains. If the content of Nb is less than 0.001% and the content of Ti is less than 0.001%, it is difficult to obtain the effect of the above action. Therefore, one or two of Nb: 0.001% or more and Ti: 0.001% or more are contained. The Nb content is preferably 0.010% or more, and the Ti content is preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.020% or the Ti content exceeds 0.20%, not only the cost becomes high, but also it becomes difficult to obtain shock absorption characteristics. Therefore, the Nb content is 0.020% or less, preferably 0.015% or less, and the Ti content is 0.20% or less, preferably 0.18% or less, more preferably 0.15% or less. ..
[0021]
(B: 0.0005% or more, 0.0050% or less)
B is effective for improving the hardenability of the steel sheet and further enhancing the effect of ensuring stable strength after quenching. B is also an important element in that it segregates at the grain boundaries, detoxifies P of the grain boundary embrittlement element, increases the grain boundary strength, and improves toughness. The effect of B on suppressing embrittlement of P is due to the fact that the diffusion rate of B at high temperature is faster than the diffusion rate of P, and the grain boundary segregation of B preferentially fills the site where P segregates. This is because P existing at the grain boundaries decreases. Therefore, in the present invention, the addition of B is indispensable for improving toughness. In order to obtain the above effect of B, the B content needs to be 0.0005% or more, preferably 0.0010% or more, and more preferably 0.0015% or more. However, when the B content exceeds 0.0050%, the effect is saturated and not only causes an increase in cost, but also B inclusions (BN) are generated, which becomes a fracture starting point at the time of drilling and the drilling property is lowered. do. Therefore, the B content is 0.0050% or less, preferably 0.0040% or less, more preferably 0.0030% or less, still more preferably 0.0020% or less.
[0022]
The above-mentioned elements are the basic chemical components, but in addition to the basic chemical components, they were further selected from Cu, Ni, Cr, Mo, V, Ca, and REM as selection elements as needed. It can contain one kind or two or more kinds.
[0023]
(Cu: 0% or more, 0.50% or less)
Cu is an element that dissolves in steel and contributes to increasing strength and also improves corrosion resistance. In order to obtain such an effect, it is preferable to contain Cu in an amount of 0.01% or more, and more preferably 0.05% or more. On the other hand, if Cu is contained in an amount of more than 0.50%, the surface texture of the steel sheet is deteriorated. Therefore, when it is contained, the Cu content is preferably 0.50% or less, more preferably 0.45% or less.
[0024]
(Ni: 0% or more, 0.50% or less)
Ni is an element that dissolves in steel to contribute to increasing strength and improve toughness. In order to obtain such an effect, it is preferable to contain 0.01% or more of Ni, more preferably 0.03% or more, and further preferably 0.05% or more. On the other hand, if Ni is contained in excess of 0.50%, the material cost will rise. Therefore, when it is contained, the Ni content is preferably 0.50% or less, more preferably 0.45% or less, and further preferably 0.40% or less.
[0025]
(Cr: 0% or more, 0.50% or less)
Cr is an element that contributes to the increase in strength of the steel sheet by being solidly dissolved in the steel, and also precipitates in the steel sheet as carbides, nitrides or carbonitrides and contributes to the increase in strength by precipitation strengthening. In order to obtain such an effect, Cr is preferably contained in an amount of 0.001% or more, more preferably 0.01% or more, and further preferably 0.05% or more. On the other hand, if Cr is contained in an amount of more than 0.50%, the toughness is lowered. Therefore, when it is contained, the Cr content is preferably 0.50% or less, more preferably 0.45% or less, and further preferably 0.40% or less.
[0026]
(Mo: 0% or more, 0.50% or less)
Mo is an element that contributes to the increase in strength of the steel sheet by being solidly dissolved in steel, and also precipitates in the steel sheet as carbide, nitride or carbonitride, and contributes to the increase in strength by precipitation strengthening. In order to obtain such an effect, Mo is preferably contained in an amount of 0.001% or more, more preferably 0.01% or more, and further preferably 0.05% or more. On the other hand, if Mo is contained in an amount of more than 0.50%, the toughness is lowered. Therefore, when it is contained, the Mo content is preferably 0.50% or less, more preferably 0.45% or less, and further preferably 0.40% or less.
[0027]
(V: 0% or more, 0.05% or less)
V is an element that contributes to the increase in strength of the steel sheet by being solidly dissolved in steel, and also precipitates in the steel sheet as carbides, nitrides or carbonitrides and contributes to the increase in strength by precipitation strengthening. In order to obtain such an effect, V is preferably contained in an amount of 0.001% or more, more preferably 0.01% or more. On the other hand, if V is contained in an amount of more than 0.05%, the toughness is lowered. Therefore, when it is contained, the V content is preferably 0.05% or less, more preferably 0.04% or less.
[0028]
(Ca: 0% or more, 0.05% or less)
Ca has the effect of fixing S as CaS, spheroidizing sulfide-based inclusions, and controlling the morphology of inclusions. Further, it is an element having an action of reducing the lattice strain of the matrix around the inclusions and lowering the ability to trap hydrogen, and can be contained as needed. In order to obtain such an effect, it is preferable to contain Ca in an amount of 0.0005% or more, more preferably 0.001% or more, and further preferably 0.005% or more. On the other hand, if Ca is contained in an amount of more than 0.05%, CaO is increased and corrosion resistance and toughness are lowered. Therefore, when it is contained, the Ca content is preferably 0.05% or less. More preferably, the Ca content is 0.03% or less.
[0029]
(REM: 0% or more, 0.01% or less)
REM contributes to the improvement of strength-ductility balance and hole expandability by finely dispersing inclusions typified by MnS. Here, examples of the REM (rare earth element) used in the present invention include Y, lanthanoids, and the like. In order to obtain such an effect, it is preferable to contain 0.001% or more of REM. However, even if these elements are excessively contained, the above-mentioned effects are saturated and it is economically unfavorable. Therefore, it is preferable that each of these elements is 0.01% or less.
[0030]
In the hot-rolled steel sheet according to the present invention, the balance other than the above elements consists of Fe and impurities. Here, the "impurity" is a component that is mixed by various factors in the manufacturing process, including raw materials such as ore and scrap, when the hot-rolled steel sheet is industrially manufactured, and relates to the present invention. It includes those that are not intentionally added to the hot-rolled steel sheet. Impurities also include elements other than the components described above, which are contained in the base steel sheet at a level at which the action and effect peculiar to the element do not affect the characteristics of the hot-rolled steel sheet according to the present invention. It is something to do.
[0031]
(Tissue fraction of metal structure)
The hot-rolled steel sheet according to the present invention has a tempered martensite phase as the main phase. The "main phase" means a case where the phase has an area ratio of more than 90%, preferably more than 95% at a position of 1/4 plate thickness from the surface of the steel sheet. Therefore, in the hot-rolled steel sheet according to the present invention, the metallographic structure at a position 1/4 thickness from the surface of the steel sheet contains a tempered martensite phase of more than 90% in area ratio, preferably more than 95%. Includes tempered martensite phase. By using the tempered martensite phase as the main phase, the desired high strength can be obtained. The residual structure other than the main phase may be composed of any structure, and for example, a retained austenite (γ) phase, a fresh martensite (fM) phase, a bainite (B) phase, a ferrite (α) phase, and the like. And may consist of at least one of the pearlite phases. Here, the fresh martensite phase refers to a martensite phase that has not been quenched (that is, has not been tempered). When the total tissue fraction of the residual structure is high, at least one of the properties of strength, openness and toughness is reduced, and the desired properties cannot be obtained. Therefore, the remaining structure has an area ratio of less than 10%, preferably less than 5%. The area ratio of each phase constituting the remaining structure may be less than 10% in total. For example, in the hot-rolled steel sheet according to the present invention, the metal structure at a position 1/4 thickness from the surface of the steel sheet has an area. Even if it contains a residual austenite phase of 0% or more and 5% or less, a fresh martensite phase of 0% or more and 5% or less, a bainite phase of 0% or more and 5% or less, and / or a ferrite phase of 0% or more and 5% or less. good. Further, among these residual structures, the metal structure may contain a residual austenite phase of 0% or more and 5% or less and a ferrite phase of 0% or more and 5% or less.
[0032]
The microstructure fraction of the metal structure of the hot-rolled steel sheet according to the present invention is measured after the L cross section (cross section parallel to the rolling direction) of the steel piece collected from the position of 1/4 plate thickness from the surface of the steel sheet is mirror-polished. It is carried out by corroding with a nital solution and observing with an optical microscope. Specifically, the corroded steel pieces are observed with an optical microscope (magnification: 500 times), an image is taken, and the type of metal structure and the structure fraction of each phase are determined using an image analyzer. This operation is performed in five consecutive adjacent visual fields with a field of view of 200 μm × 200 μm, and the structural fractions of each of the obtained five phases are averaged to determine the type and microstructural fraction of the metal structure.
[0033]
(Average particle size of old austenite grains)
Further, the hot-rolled steel sheet according to the present invention has a metal structure in which the average particle size of the old austenite grains in the L cross section is less than 40 μm at a position 1/4 thickness from the surface of the steel sheet. The average particle size of the old austenite grains is preferably 39 μm or less, more preferably 38 μm or less, still more preferably 35 μm or less, and most preferably 25 μm or less. By adopting such a metal structure, the brittle ductile transition temperature vTrs can be set to −40 ° C. or lower, and the hot-rolled steel sheet having high toughness and excellent hole expandability can be obtained. If the average particle size of the old austenite grains is coarsened to 40 μm or more in the L cross section, sufficient toughness and hole-expanding property cannot be obtained.
[0034]
(Aspect ratio of old austenite grains)
In addition, the hot-rolled steel sheet according to the present invention Then, the aspect ratio of the old austenite grains in the L cross section is set to 3.5 or less. The aspect ratio of the old austenite grain is the ratio of the length in the rolling direction of the old austenite grain to the length in the plate thickness direction measured in the L cross section at the position of 1/4 plate thickness from the surface of the steel plate, that is, (the rolling direction of the old austenite grain). Length / length in the plate thickness direction of the old austenite grain). The aspect ratio of the old austenite grains is an index showing the anisotropy of the metal structure, and when the aspect ratio of the old austenite grains exceeds 3.5, the hole-spreading property is lowered. Therefore, the aspect ratio of the old austenite grains was limited to the range of 3.5 or less. The aspect ratio of the old austenite grains is preferably 3.0 or less, more preferably 2.5 or less. The closer the aspect ratio is to 1.0, the better the hole expandability. Therefore, the lower limit of the aspect ratio may be 1.0. However, since the hot-rolled steel plate according to the present invention is rolled, the old austenite grains are at least in the rolling direction. Since it is difficult for the old austenite grains to have an aspect ratio of less than 1.2 under the rolling conditions of the present invention, the lower limit of the aspect ratio is 1.2. You may.
[0035]
The average particle size and aspect ratio of the old austenite grains of the hot-rolled steel sheet according to the present invention are SEM / EBSD (SEM / EBSD) in a region of 200 μm × 200 μm of the L cross section of the steel piece collected from the position of 1/4 plate thickness from the surface of the steel sheet. It is measured by analysis with a scanning electron microscope / backscattered electron diffraction). Specifically, the martensite structure obtained by SEM / EBSD is subjected to a predetermined crystal orientation conversion to obtain an image obtained by reconstructing the old austenite grains. From the old austenite grains in the image, a circle having the same area, that is, a circle-equivalent diameter is obtained, and the circle-equivalent diameter is defined as the particle size of the old austenite grains. This operation is performed for a total of 10 austenite grains, and the average particle size of the austenite grains is calculated by averaging the obtained 10 values. For the aspect ratio of the old austenite grains, the lengths in the rolling direction and the lengths in the plate thickness direction of the 10 austenite grains obtained by the above crystal orientation conversion were measured, and the aspect ratios of the respective austenite grains were obtained. Is calculated by averaging.
[0036]
({112} <110> Orientation X-ray random intensity ratio)
In the hot-rolled steel sheet according to the present invention, the X-ray random intensity ratio in the {112} <110> direction at a position 1/2 plate thickness from the surface is 4.0 or less. The X-ray random intensity ratio of the crystal orientation indicates the degree of anisotropy of the developed texture. When the X-ray random intensity ratio exceeds 4.0, the anisotropy of the metal structure increases, so that the hole expandability decreases. Therefore, in the hot-rolled steel sheet according to the present invention, the X-ray random intensity ratio in the {112} <110> orientation is 4.0 or less, preferably 3.5 or less, and more preferably 3.0 or less. The lower limit of the X-ray random intensity ratio is not particularly limited, but may be, for example, 1.0, 1.5 or 2.0.
[0037]
The X-ray random intensity ratio of the hot-rolled steel plate according to the present invention in the {112} <110> orientation is the X-ray diffraction (XRD) method of the X-ray intensity ratio of the steel plate at a position 1/2 plate thickness from the surface of the steel plate. The crystal orientation distribution function (ODF) is obtained based on the measured X-ray intensity ratio, and the intensity ratio of the {112} <110> orientation is calculated.
[0038]
(Average particle size of carbides in tempered martensite)
The hot-rolled steel sheet according to the present invention has a structure in which fine carbides having an average particle size of 10 nm or less are deposited in a lath of a tempered martensite phase at a position 1/4 of the thickness of the steel sheet. When the carbides in the tempered martensite become coarse, they become the starting point of fracture and the toughness and hole-expanding property are deteriorated. Therefore, in the present invention, the average particle size of the carbides precipitated in the tempered martensite is set to 10 nm or less. The lower limit of the average particle size of the carbide in the tempered martensite is not particularly limited, but may be, for example, 1 nm, 2 nm or 3 nm. Carbide in martensite refers to cementite present in martensite.
[0039]
The average particle size of carbides in the tempered martensite of the hot-rolled steel sheet according to the present invention is determined by observing the L cross section of the steel sheet at a position 1/4 thickness from the surface of the steel sheet with a transmission electron microscope. It is measured by measuring and obtaining the equivalent circle diameter based on the area. This measurement is performed in five consecutive adjacent visual fields, and the average particle size of the carbides in the tempered martensite is calculated by averaging the circle-equivalent diameters of the five carbides obtained in each visual field.
[0040]
(Mechanical characteristics)
The strength of the hot-rolled steel sheet according to the present invention is evaluated by a tensile test according to JIS Z 2241: 2011. The tensile strength (TS) of the hot-rolled steel sheet according to the present invention is 1470 MPa or more. The higher the TS, the more preferable, and for example, it may be 1500 MPa or more, 1550 MPa or more, or 1600 MPa or more. The higher the TS, the lower the thickness of the steel sheet by increasing the strength when the steel sheet is used as a member of an automobile, and the lighter the weight can be achieved. The upper limit of TS is not particularly limited, but may be, for example, 2500 MPa or 2000 MPa.
[0041]
The toughness of the hot-rolled steel sheet according to the present invention is evaluated by a Charpy impact test according to JIS Z 2242: 2005. More specifically, it is evaluated by determining the brittle ductile transition temperature vTrs (° C.). The vTrs of the hot-rolled steel sheet according to the present invention is −40 ° C. or lower. The lower the vTrs, the more preferable, and for example, it may be −45 ° C. or lower or −50 ° C. or lower. The lower limit of vTrs is not particularly limited, but may be, for example, −100 ° C., −90 ° C. or −80 ° C.
[0042]
The hole expandability of the hot-rolled steel sheet according to the present invention is evaluated by the hole expansion ratio λ according to JIS Z 2256: 2010. For λ, a punch hole having a diameter d0 is made in the test piece, a punch having a tip angle of 60 ° is pushed into the punch hole, and the diameter d of the punch hole when the generated crack penetrates the plate thickness of the test piece is measured. Therefore, it is calculated from the following formula.
Λ (%) = {(d-d0) / d0} x 100
The λ of the hot-rolled steel sheet according to the present invention is 60% or more. The higher the λ, the more preferable, and for example, it may be 65% or more, 70% or more, or 75% or more. The upper limit of λ is not particularly limited, but may be, for example, 100%, 95%, or 90%.
[0043]
Next, the method for manufacturing the hot-rolled steel sheet of the present invention described above will be described.
[0044]
[Manufacturing method of hot-rolled steel sheet]
In the production method of the present invention, a heating step of heating a slab having the above-mentioned chemical components, a hot-rolling step of hot-rolling the heated slab consisting of rough rolling and finish rolling, and cooling of a hot-rolled steel sheet are performed. The cooling step, the winding step of winding the cooled hot-rolled steel sheet, and the tempering step of air-cooling the wound hot-rolled steel sheet and then rewinding at a low temperature are sequentially carried out.
[0045]
(Heating process)
First, the heating process will be explained. In the heating step, the slab of the above-mentioned chemical composition is heated to a temperature of 1250 ° C. or lower, preferably 1240 ° C. or lower. Further, when the heating temperature is less than 1100 ° C., the low temperature toughness of the hot-rolled steel sheet obtained due to insufficient dissolution of carbides and nitrides may decrease. Therefore, the heating temperature is preferably 1100 ° C. or higher, preferably 1150 ° C. The above is more preferable. On the other hand, when the heating temperature exceeds 1250 ° C. and becomes high, the crystal grains are coarsened, the low temperature toughness of the obtained hot-rolled steel sheet is lowered, the scale generation amount is increased, and the yield is lowered. Therefore, the heating temperature of the slab was limited to a temperature of 1250 ° C. or lower.
[0046]
(Hot spreading process)
Next, a hot rolling process consisting of rough rolling using the heated slab as a rough bar and finish rolling using the rough bar as a hot rolled steel sheet is carried out. In rough rolling, the slab is made into a rough bar having a desired size and shape, and the rough rolling output side temperature RDT is set so that the reduction rate in the temperature range of 900 ° C. or higher in finish rolling can be adjusted within a desired range. The temperature is preferably 950 ° C. or higher and 1150 ° C. or lower.
[0047]
If the rough rolling output side temperature is less than 950 ° C, it may be difficult to obtain a final rolling temperature of 900 ° C or higher in the finish rolling following the rough rolling. Further, when the rough-rolled output side temperature exceeds 1150 ° C., the crystal grains may become coarse and the low-temperature toughness of the obtained hot-rolled steel sheet may decrease.
[0048]
Further, in the finish rolling following the rough rolling, it is preferable that the finish rolling inlet side temperature is 1000 ° C. or higher and 1150 ° C. or lower. Further, in the manufacturing method according to the present invention, the finish rolling output side temperature (that is, the finish rolling end temperature) is set to a temperature of 900 ° C. or higher and 1050 ° C. or lower, and the rolling reduction of the final stage (finish rolling) is set to 10% or higher and 40% or lower. And.
[0049]
In the chemical composition within the scope of the present invention, the temperature range of 900 ° C. or lower corresponds to the unrecrystallized austenite range. In the unrecrystallized austenite region, the austenite crystal grains are stretched in the rolling direction by rolling, crystal grains having a high aspect ratio are formed, specific crystal orientations are accumulated, and the hole expandability is deteriorated. Therefore, in the present invention, the finish rolling output side temperature, that is, the finish rolling end temperature is set to 900 ° C. or higher. Further, when the temperature on the finished rolling output side is more than 1050 ° C., the old austenite grains are coarsened, and the toughness and the hole expanding property are deteriorated. Therefore, in the present invention, the finish rolling output side temperature, that is, the finish rolling end temperature is set to 1050 ° C. or lower. The lower limit of the finish rolling end temperature is preferably 920 ° C, more preferably 950 ° C. The upper limit of the finish rolling end temperature is preferably 1020 ° C, more preferably 1000 ° C.
[0050]
If the finish rolling inlet temperature is less than 1000 ° C, it may be difficult to adjust the finish rolling exit temperature to 900 ° C or higher. On the other hand, if the inlet temperature of the finish rolling exceeds 1150 ° C., the old austenite grains may become coarse and the desired particle size may not be obtained. Therefore, it is preferable that the finish rolling inlet temperature is 1000 ° C. or higher and 1150 ° C. or lower.
[0051]
Further, if the reduction rate of the final stage is less than 10%, the old austenite grains become coarse and the desired hole-expanding property and toughness cannot be obtained. Further, when the reduction rate of the final stage is more than 40%, austenite grains are developed, crystal grains having a high aspect ratio are formed, specific crystal orientations are accumulated, and the hole expanding property is deteriorated. The lower limit of the reduction rate in the final stage is preferably 12%, more preferably 15%. The upper limit of the reduction rate in the final stage is preferably 35%, more preferably 30%.
[0052]
By setting the rolling conditions described above, it is possible to obtain a structure in which the average particle size of the old austenite grains is less than 40 μm and the aspect ratio of the old austenite grains is 3.5 or less. Next, immediately after the hot rolling process (completion of hot rolling), cooling is started by a cooling device installed on a run-out table (ROT), and the cooling process is carried out.
[0053]
(Cooling process)
In the cooling step, the average cooling rate at the initial stage of cooling from the cooling start temperature to 700 ° C is 20 ° C / sec or more and 200 ° C / sec or less, and the average cooling rate from the cooling start temperature to the winding temperature is 40 ° C / sec or more. Is. The lower limit of the average cooling rate from the cooling start temperature to 700 ° C. is preferably 30 ° C./sec, more preferably 40 ° C./sec, and even more preferably 50 ° C./sec. The upper limit of the average cooling rate from the cooling start temperature to 700 ° C. is preferably 180 ° C./sec, more preferably 150 ° C./sec, and even more preferably 100 ° C./sec. The lower limit of the average cooling rate from the cooling start temperature to the winding temperature is preferably 50 ° C./sec, more preferably 60 ° C./sec.
[0054]
Further, in order to obtain the desired average particle size of the old austenite grains, it is necessary that the time from the end of the hot spreading process to the start of cooling is within 2.0 seconds. If the residence time until the start of cooling is long, the crystal grains may become coarse in the recrystallization temperature range, and it becomes difficult to adjust the grain size to the desired old austenite particle size. The time from the end of the hot spreading step to the start of cooling is preferably 1.0 second or less, more preferably 0.5 second or less.
[0055]
Also When the average cooling rate from the cooling start temperature to 700 ° C, which is a feature of the present invention, exceeds 200 ° C / sec, B segregation to the old austenite grain boundaries is suppressed, the hardenability is lowered, and the temperature is 500 to 600 ° C. A part of austenite undergoes bainite transformation in the region, and the desired metallographic structure cannot be obtained, and the desired impact characteristics, that is, toughness and hole-expanding property cannot be obtained. On the other hand, if the average cooling rate is less than 20 ° C./sec, martensite is not sufficiently generated during cooling, and a desired structure having the tempered martensite phase as the main phase cannot be obtained after tempering. Further, if the average cooling rate from the cooling start temperature to the take-up temperature is less than 40 ° C./sec, martensite is not sufficiently generated during cooling, and it is desirable to use the tempered martensite phase as the main phase after tempering. You can't get the tissue. Therefore, the cooling rate in the cooling process (from the end of the hot spreading process to the winding) is set to 40 ° C./sec or more. The upper limit of the average cooling rate from the cooling start temperature to the take-up temperature is determined depending on the capacity of the cooling device used, but is a cooling rate that does not cause deterioration of the steel plate shape such as warpage, 150 ° C. It is preferably / sec. A more preferable cooling rate is 50 ° C./sec or more and 100 ° C./sec or less.
[0056]
If the cooling stop temperature exceeds 100 ° C, the carbides in the tempered martensite may become coarse and the desired toughness may not be obtained. Therefore, it is preferable that the cooling stop temperature is 100 ° C. or lower. The more preferable cooling stop temperature is 90 ° C. or lower. On the other hand, if the cooling stop temperature is less than 20 ° C, the cooling cost increases, so it is preferable to set the cooling stop temperature to 20 ° C or higher.
[0057]
(Winling process)
After completing the cooling process, the winding process of winding into a coil at a winding temperature of 20 ° C or higher and 100 ° C or lower is then carried out. When the winding temperature for winding the hot-rolled steel sheet exceeds 100 ° C, the proportion of fresh martensite increases, and it becomes impossible to obtain a metal structure containing tempered martensite as the main phase, and further, in the tempered martensite. Since the carbide becomes coarse, the desired toughness and hole-expanding property cannot be obtained. On the other hand, if the winding temperature is less than 20 ° C, the cooling cost increases, so the winding temperature is set to 20 ° C or higher. The lower limit of the winding temperature is preferably 25 ° C, more preferably 30 ° C. The upper limit of the winding temperature is preferably 90 ° C, more preferably 80 ° C.
[0058]
(Tempering process)
Further, a tempering process of low temperature tempering is carried out under the condition that the integrated tempering parameter ST is 13.0 or more and 27.0 or less. When the integrated tempering parameter ST is within the above range, a desired tempered martensite area ratio and an average particle size of carbides in the martensite can be obtained. When the integrated tempering parameter ST is less than 13.0, the desired metallographic structure cannot be obtained because fresh martensite remains, and sufficient toughness and hole-expanding property cannot be obtained. On the other hand, when the integrated tempering parameter ST exceeds 27.0, the carbides in the tempered martensite become coarse and sufficient hole expandability and toughness cannot be obtained. Further, ferrite may be precipitated to obtain a desired structure, and the strength may be lowered. The lower limit of the integrated tempering parameter ST is preferably 15.0, more preferably 18.0, and even more preferably 20.0. The upper limit of the tempering parameter ST is preferably 25.0, more preferably 23.0.
[0059]
The integrated tempering parameter ST can be calculated by the following simple method. First, the temperature history at the time of tempering is expressed by temperature T (° C.) and time t (seconds). Next, the time is divided into minute intervals Δt, the tempering parameter ΔST for each minute interval Δt is obtained from Eq. (1), and the tempering parameter ΔST is integrated using (2) for the region where T ≧ 100 ° C. By doing so, the integrated tempering parameter ST is calculated.
ΔST = (logΔt) -16740 / (T + 273) +50 (1)
ST = Log (Σ10 ΔST) (2)
In order to set the integrated tempering parameter ST to 13.0 or more and 27.0 or less, it is preferable to temper at a low temperature (for example, 100 ° C or more and 300 ° C or less) for 1 second or more and 6 hours or less. It is preferable to temper at a temperature of 150 ° C. or higher and 300 ° C. or lower for 1 minute or more and 60 minutes or less. By tempering at a low temperature, a structure having tempered martensite as the main phase can be obtained, and the hole-spreading property and toughness are improved. If the tempering temperature exceeds 300 ° C., the carbides in martensite may become coarse.
[0060]
By manufacturing the hot-rolled steel sheet as described above, the hole-expanding has the characteristics of tensile strength (TS) of 1470 MPa or more, hole expansion ratio (λ) of 60% or more, and brittle ductility transition temperature (vTrs) of -40 ° C or less. It is possible to obtain a high-strength hot-rolled steel sheet having excellent properties and toughness. The thickness of the hot-rolled steel sheet according to the present invention is not particularly limited, but may be, for example, 0.5 to 8.0 mm. The upper limit of the plate thickness may be 7.0 mm, 6.0 mm or 5.0 mm.
Example
[0061]
(Sample preparation of hot-rolled steel sheet)
The slabs of the chemical components shown in Table 1 are subjected to a heating process and a hot rolling process under the conditions shown in Table 2, and after the hot rolling is completed, a cooling process, a winding process, and a tempering process are sequentially performed under the conditions shown in Table 2. It was applied to make a hot-rolled steel plate (steel strip) with a plate thickness of 2.3 mm. The integrated parameter ST was calculated by the above equations (1) and (2). In this embodiment, Δt = 1 (seconds). Sample No. 9 to 11 are examples in which tempering was not performed after the winding step, and in Table 2, the tempering parameter ST is shown as "-".
[0062]
(Measurement of metallographic structure)
From the obtained hot-rolled steel plate sample, the microstructure fraction of tempered martensite and residual structure, the average grain size of carbides in the tempered martensite, the average grain size and aspect ratio of the former austenite grains, and {112} < The X-ray random intensity ratio of 110> orientation was determined. These values ​​for each sample are shown in Table 3. In addition, a tensile test for strength evaluation, an impact test for toughness evaluation, and a hole expansion test were carried out.
[0063]
A test piece for structure observation was collected from the obtained hot-rolled steel sheet, the cross section parallel to the rolling direction (L cross section) was polished, corroded with a nital solution, and the structure was observed with an optical microscope (magnification: 500 times). The observation position was 1/4 t (here, t: plate thickness) from the surface of the steel plate, and five consecutive adjacent visual fields were observed in a visual field of 200 μm × 200 μm. The tissue image was imaged in each field, the type of metal structure was discriminated using an image analysis device, and the tissue fraction of each phase was obtained. The value of the tissue fraction obtained in five visual fields for each phase. Was averaged to determine the microstructural fraction of the metallographic structure. In Table 3, fM means a fresh martensite phase, γ means a retained austenite phase, α means a ferrite phase, and B means a bainite phase.
[0064]
The average particle size and aspect ratio of the old austenite grains were determined by the following method. The average grain size of the old austenite grains was analyzed by SEM / EBSD in a region of 200 μm × 200 μm in the L cross section of the steel piece collected from the surface of the steel sheet at a plate thickness of 1/4. The martensite structure obtained by SEM / EBSD was subjected to a predetermined crystal orientation conversion to obtain an image in which old austenite grains were reconstructed. A circle having the same area, that is, a corresponding circle diameter was obtained from the old austenite grains in the image, and the diameter corresponding to the circle was taken as the particle size of the old austenite grains. This was performed for a total of 10 austenite grains, and the average particle size of the austenite grains was obtained by averaging them. The aspect ratio of the old austenite grains was obtained by calculating and averaging the ratios of the rolling direction length and the plate thickness direction length of the 10 old austenite grains reconstructed as described above.
[0065]
The average particle size of the carbide in the tempered martensite is based on the area of ​​the carbide measured by observing the L cross section of the steel piece collected from the surface of the steel sheet at a plate thickness of 1/4 with a transmission electron microscope. It was measured by finding the equivalent diameter of the circle. This measurement was performed in five consecutive adjacent fields of view, and the average particle size of the carbides was calculated by averaging the circle-equivalent diameters of the five carbides.
[0066]
To measure the texture of the steel plate, the X-ray intensity ratio of the 1 / 2t surface of the plate thickness is measured by XRD, and the crystal orientation distribution function (ODF) is obtained based on the measured X-ray intensity ratio. The X-ray random intensity ratio was calculated.
[0067]
(Measurement of mechanical properties of hot-rolled steel sheet sample)
A plate-shaped test piece so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction from a predetermined position (the end in the longitudinal direction of the coil, the position of 1/4 in the width direction) of the obtained hot-rolled steel plate. (Parallel width: 25 mm, distance between gauge points: 50 mm) was sampled, and a tensile test was conducted at room temperature in accordance with the provisions of JIS Z 2241: 2011. Tensile strength TS (MPa) and yield stress (MPa) Asked.
[0068]
V so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction from the center of the plate thickness at a predetermined position (the end in the longitudinal direction of the coil, the position of 1/4 in the width direction) of the obtained hot-rolled steel sheet. A notch test piece was collected and a Charpy impact test was carried out in accordance with the provisions of JIS Z 2242: 2005 to determine the brittle ductile transition temperature vTrs (° C.).
[0069]
The hole expansion rate λ was determined according to JIS Z 2256: 2010. Specifically, a punched hole having a diameter of d0 = 10 mm is made in the test piece, a punch having a tip angle of 60 ° is pushed into the punched hole, and the diameter of the punched hole at the time when the generated crack penetrates the plate thickness of the test piece. d was measured and calculated from the following formula.
Λ (%) = {(d-d0) / d0} x 100
[0070]
In the examples, TS was 1470 MPa or more, λ was 60% or more, and vTrs was −40 ° C. or less, and the desired strength, toughness, and hole-expanding properties were obtained. Those whose characteristics do not meet the above range are underlined.
[0071]
[table 1]

[0072]
[Table 2]

[0073]
[Table 3]

[0074]
Sample No. In No. 2, the finish rolling end temperature is low, the aspect ratio of the old austenite grains is high, and the texture is developed, so that the hole expanding property is not good. On the other hand, sample No. In No. 3, the finish rolling end temperature was high, and the old austenite grains were coarsened, so that the perforation property and the transition temperature were not good.
[0075]
Sample No. In No. 4, the reduction rate of the final stage was low, the old austenite was not sufficiently crushed, the particle size was coarsened, and the hole expandability and the transition temperature were not good. On the other hand, sample No. In No. 5, the reduction rate is too high, so that the old austenite grains are crushed flat, the aspect ratio becomes high, the anisotropy becomes stronger, and the hole expanding property is not good.
[0076]
Sample No. In No. 6, it took time to start cooling after the rolling was completed, so that the old austenite grains were coarsened, and the hole-spreading property and the transition temperature were not good.
[0077]
Sample No. In No. 7, the average cooling rate from the cooling start temperature to 700 ° C. was too fast, and the B segregation on the old austenite grains was not sufficient. It is probable that the hole widening property and the transition temperature were not good.
[0078]
Sample No. In No. 8, the average cooling rate from the cooling start temperature to winding was fast in the first half, but the cooling rate was slow in the second half, so that bainite transformation occurred and the required tempered martensite structure was not obtained, so the strength decreased. Furthermore, the hole-spreading property and the transition temperature also decreased.
[0079]
Sample No. In Nos. 9 to 11, the take-up temperature was over 100 ° C., tempering was not performed, fresh martensite remained, the desired structure could not be obtained, and the carbides in the martensite became coarse. The drilling property and transition temperature were not good.
[0080] [0080]
Sample No. In Nos. 12 to 14, the tempering parameter was more than 27.0, and the carbides in the tempered martensite were coarsened, so that the perforation property and the transition temperature were not good. In addition, sample No. In No. 12, the strength was also lowered because a part of the crystal was recrystallized to generate ferrite. On the other hand, sample No. 15-18 are tempered parameters Since the tar was less than 13.0 and fresh martensite remained, the perforation property and the transition temperature were not good.
[0081]
Sample No. In No. 35, since B was not contained as a chemical component, B segregation on the old austenite grains was not performed, so that the transition temperature was not favorable. Sample No. In No. 36, the B content was excessive, so that the hole-spreading property was not good. Sample No. In No. 37, the desired strength could not be obtained because the C content was insufficient. Sample No. In No. 38, since the C content was excessive, fresh martensite remained, and the desired perforation property and transition temperature could not be obtained. Sample No. In No. 39, since the Si content was excessive, the transition temperature was increased by the solid solution Si, and the desired transition temperature could not be obtained.
[0082]
Sample No. In No. 51, since both Nb and Ti were not contained as chemical components, the old austenite grains were coarsened and the transition temperature was not good.
[0083]
Sample No. 1, No19. ~ 34, No. 40-50 and No. Nos. 52 to 60 were within the range of the present invention, and the characteristics of strength, hole expandability and transition temperature were good.
The scope of the claims
[Claim 1]
By mass%,
C: 0.12% or more, 0.25% or less,
Si: 0.01% or more, 2.0% or less,
Mn: 0.5% or more, 3.0% or less,
P: 0.020% or less,
S: 0.010% or less,
Al: 0.001% or more, 0.10% or less,
B: 0.0005% or more, 0.0050% or less,
Cu: 0% or more, 0.50% or less,
Ni: 0% or more, 0.50% or less,
Cr: 0% or more, 0.50% or less,
Mo: 0% or more, 0.50% or less,
V: 0% or more, 0.05% or less,
Ca: 0% or more, 0.05% or less, and
REM: 0% or more, 0.01% or less
Contains,
Nb: 0.001% or more, 0.020% or less, and
Ti: 0.001% or more, 0.20% or less
It contains one or two of them, and the balance has a chemical component consisting of Fe and impurities.
The metallographic structure at the position of 1/4 plate thickness from the surface contains tempered martensite with an area ratio of more than 90%.
The average particle size of carbides in tempered martensite is 10 nm or less,
The average particle size of the old austenite grains is less than 40 μm,
The aspect ratio of the old austenite grains is 3.5 or less,
A hot-rolled steel sheet characterized in that the X-ray random intensity ratio in the {112} <110> direction at a position 1/2 plate thickness from the surface is 4.0 or less.
[Claim 2]
By mass%,
Cu: 0.01% or more, 0.50% or less,
Ni: 0.01% or more, 0.50% or less,
Cr: 0.001% or more, 0.50% or less,
Mo: 0.001% or more, 0.50% or less,
V: 0.001% or more, 0.05% or less,
Ca: 0.0005% or more, 0.05% or less, and
REM: 0.001% or more, 0.01% or less
The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet contains one or more of the two or more.
[Claim 3]
The hot-rolled steel sheet according to claim 1 or 2, wherein the metallographic structure contains tempered martensite in an area ratio of more than 95%.
[Claim 4]
13. Claims 1-3, wherein the remnant structure of the metal structure at a position 1/4 of the plate thickness from the surface is composed of at least one of retained austenite, fresh martensite, bainite, ferrite, and pearlite. The hot-rolled steel sheet according to any one item.
[Claim 5]
Claims 1 to 4, wherein the residual structure of the metal structure at a position of 1/4 plate thickness from the surface contains residual austenite of 0% or more and 5% or less, and ferrite of 0% or more and 5% or less. The hot-rolled steel sheet according to any one item.
[Claim 6]
The hot-rolled steel sheet according to any one of claims 1 to 5, wherein the aspect ratio of the old austenite grains is 3.0 or less.
[Claim 7]
A heating step of heating the slab containing the chemical component according to claim 1 or 2 at 1250 ° C. or lower, and
A hot rolling process including finishing rolling of a heated slab at a rolling reduction of 10% or more and 40% or less in the final stage, wherein the finishing temperature of the finishing rolling is 900 ° C. or higher and 1050 ° C. or lower. ,
Cooling is started within 2.0 seconds after the end of the hot spreading process, the average cooling rate from the cooling start temperature to 700 ° C is 20 ° C / sec or more and 200 ° C / sec or less, and further, winding from the cooling start temperature. A cooling process that continuously cools the hot-rolled steel plate at a cooling rate with an average cooling rate of 40 ° C / sec or more up to the taking temperature.
The winding process of winding the cooled hot-rolled steel sheet at 20 ° C or higher and 100 ° C or lower,
The hot-rolled steel sheet is characterized by including a tempering step in which the wound hot-rolled steel sheet is air-cooled to room temperature and then tempered at a low temperature under the condition that the integrated tempering parameter ST is 13.0 or more and 27.0 or less. Manufacturing method.
[Claim 8]
The method for manufacturing a hot-rolled steel sheet according to claim 7, wherein the integrated tempering parameter ST is 20.0 or more and 25.0 or less.