WO2020203934A1

WO2020203934A1 - High-strength hot-rolled steel sheet

Info

Publication number: WO2020203934A1
Application number: PCT/JP2020/014396
Authority: WO
Inventors: 伊藤　大輔; 健悟竹田; 武豊田
Original assignee: 日本製鉄株式会社
Priority date: 2019-03-29
Filing date: 2020-03-27
Publication date: 2020-10-08
Also published as: JP7070794B2; JPWO2020203934A1

Abstract

The present invention provides a high-strength steel sheet having excellent bending workability and fatigue characteristics. The high-strength steel sheet is characterized in that the high-strength steel sheet is provided with a base section and a surface layer section, the average Vickers hardness of the surface layer section is 50-80% of the average Vickers hardness at the 1/2 thickness position of the high-strength hot-rolled steel sheet, the arithmetic average roughness Ra of the surface of the surface layer section is 3.0 µm or less, the effective crystal grain size of the surface layer section is 50.0 µm or less, the difference ∆Si = Si_B-Sis between Si-content Si_B of the base section and Si-content Si_S of the surface layer section is 0.60% by mass or greater, and the base section has a metallographic structure comprising 90% or more in area ratio of tempered martensite.

Description

High-strength hot-rolled steel sheet

The present invention relates to a high-strength hot-rolled steel sheet.

In recent years, from the viewpoint of improving the fuel efficiency of automobiles, which leads to environmental protection, it has been required to increase the strength and thinning of steel sheets for automobiles and reduce the weight of the vehicle body. Further, since a steel sheet for automobiles is processed into a complicated shape, bending workability is also required.

As a conventional steel sheet having excellent bendability, for example, Patent Document 1 has a surface soft portion arranged on one side or both sides of a central portion of the plate thickness, and the standard deviation of the nanohardness of the surface soft portion is 0. High-strength steel sheets with a deviation of 0.8 or less are disclosed.

Further, in Patent Document 2, ferrite is 50% or more in area ratio, and the average particle size at a position of 50 μm in the plate thickness (thickness) depth direction from the steel plate surface is 3000 × [tensile strength TS (MPa). )] -0.85 μm or less, the amount of C in the precipitate having a particle size of less than 20 nm precipitated in steel is 0.010% by mass or more, the amount of precipitated Fe is 0.03 to 1.0% by mass, and the arithmetic mean roughness. A high-strength steel plate having an excellent bending workability having a Ra of 3.0 μm or less is disclosed.

Further, in Patent Document 3, bainite is more than 50% in area ratio, and the average particle size at a position of 50 μm from the surface of the steel sheet in the depth direction of the sheet thickness is 2500 × [tensile strength TS (MPa)] −0. A high-strength steel sheet with excellent bending workability having a C content of 0.005% by mass or more and an arithmetic mean roughness Ra of 3.0 μm or less in a precipitate having a particle size of less than 20 nm and having a particle size of .85 μm or less. It is disclosed.

International Publication No. 2018/151322 JP-A-2017-115191 Japanese Unexamined Patent Publication No. 2017-150051

By the way, steel sheets for automobiles are required to have fatigue characteristics in addition to strength and bendability.

Therefore, an object of the present invention is to provide a high-strength hot-rolled steel sheet having bending workability and fatigue characteristics.

The present inventors investigated the fatigue characteristics and bending workability of high-strength hot-rolled steel sheets. First, the present inventors manufactured a steel sheet having a soft portion on the surface layer with reference to the conventional knowledge, and investigated the bending workability and fatigue characteristics. Improvements in bending workability were observed in all steel sheets having a soft portion on the surface layer. Regarding fatigue characteristics, it was found that by adjusting the average hardness and thickness of the surface layer portion, for example, the residual stress inside the bend generated during bending molding is reduced, and fatigue fracture from the inside of the bending is suppressed. However, as a result of continuing a more detailed investigation, the present inventors have found that there is room for further improvement of the fatigue characteristics of the steel sheet by means other than adjusting the average hardness and thickness of the surface layer portion.

Therefore, the present inventors conducted a more detailed study. The mode of fatigue fracture in the region subjected to bending deformation is roughly divided into a case where fatigue cracks are generated from the outside of the bending and a case where fatigue cracks are generated from the inside of the bending. Generally, a tensile residual stress is generated inside the bend, and the residual stress increases according to the strength of the steel sheet. Therefore, the higher the strength of the steel sheet, the more the influence of the residual stress on the fatigue characteristics cannot be ignored. Therefore, as the strength of the steel sheet increases, the reduction of the residual stress inside the bending contributes to the improvement of the fatigue characteristics. Therefore, by using a soft layer on the outside of the bend and a hard layer on the inside of the bend, the residual stress inside the bend is reduced, the fatigue characteristics inside the bend are maximized, and the fatigue characteristics of the steel sheet as a whole can be improved. all right. However, if the outside of the bend is softened unnecessarily, fatigue fracture may occur outside the bend, so a lower limit must be set for the hardness outside the bend.

As described above, a guideline for improving fatigue characteristics was obtained by adjusting the hardness distribution in the thickness direction of the steel sheet (hereinafter, the "thickness direction of the steel sheet" is referred to as the "thickness direction"). As a result of further studies, it was found that by controlling the surface roughness and the effective crystal grain size of the surface layer in addition to controlling the hardness distribution, the fatigue characteristics are further improved and the variation is further reduced. It is considered that the reason why the fatigue characteristics are improved by controlling the surface roughness of the surface layer portion is that the occurrence of cracks is suppressed by controlling the surface roughness of the surface layer portion to a specific roughness or less. The reason why the fatigue characteristics are improved by controlling the effective crystal grain size of the surface layer is that by controlling the effective crystal grain size of the surface layer to a specific size or less, minute wrinkles generated outside the bending after bending deformation are generated. This is thought to be because it tends to be suppressed. It is considered that the synergistic effect of these two points further suppresses the occurrence of fatigue cracks outside the bending and facilitates the transition to the fracture mode inside the bending. As a result, it is considered that the occurrence of fatigue cracks outside the bending is further suppressed, the fatigue characteristics inside the bending are easily utilized, and the fatigue characteristics of the steel sheet as a whole are further improved.

Further, the present inventors have stated that it is important to control the content of Si, which is one of the additive elements, and the rolling conditions from the viewpoint of controlling the hardness difference in the plate thickness direction and the surface roughness. I found it. Si is an element that exhibits temper softening resistance. By making a difference in the Si content in the plate thickness direction and baking it under specific heat treatment conditions after water cooling, the hardness difference in the plate thickness direction can be controlled according to the Si content. Further, it is considered that when the Si content of the surface layer is reduced, the hot deformation resistance of the surface layer is reduced during hot rolling, so that the surface layer is preferentially stretched and the surface is easily formed flat.

The gist of the present invention obtained in this way is as follows.

[1] A high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and a yield ratio of 0.75 or more, comprising a base portion and a surface layer portion arranged on the surface of one side or both sides of the base portion. The thickness of the portion is 30% or less of the thickness of the high-strength hot-rolled steel plate and exceeds 10 μm, and the average Vickers hardness of the surface layer portion is 1 of the thickness of the high-strength hot-rolled steel plate. The average Vickers hardness at the position / 2 is 50 to 80%, the arithmetic average roughness Ra of the surface of the surface layer portion is 3.0 μm or less, and the effective crystal grain size of the surface layer portion is 50.0 μm. The chemical composition of the base is as follows, in terms of mass%, C: 0.05 to 0.50%, Si: 1.00 to 3.00%, Mn: 0.5 to 3.0%, P: 0.10% or less, S: 0.050% or less, Cu: 0 to 1.0%, Ni: 0 to 1.0%, Cr: 0 to 3.0%, Mo: 0 to 1.0%, Nb: 0 to 0.02%, V: 0 to 0.2%, Ti: 0 to 0.20%, B: 0 to 0.0100%, N: 0.010% or less, Ca: 0 to 0. 0100%, REM: 0 to 0.0100%, Al: 0 to 1.00%, and the balance: Fe and impurities, which are the Si content Si _{B of the} base and the Si content Si _S of the surface layer. High-intensity heat characterized in that the difference ΔSi = Si _{B −} Sis is 0.60% or more in mass%, and the metallographic structure of the base is an area ratio of tempered martensite: 90% or more. Inter-rolled steel plate. However, the surface layer portion refers to a region where the Vickers hardness is 80% or less of the average Vickers hardness at a position of 1/2 the thickness from the surface of the high-strength hot-rolled steel sheet.

[2] The above-mentioned [1], wherein the average Vickers hardness of the surface layer portion is 61% or more of the average Vickers hardness at a position halved of the thickness of the high-strength hot-rolled steel sheet. High-strength hot-rolled steel sheet.

[3] The high-strength hot-rolled steel sheet of the above [1] or [2], wherein the effective crystal grain size of the surface layer portion is 10.0 μm or less.

[4] The content of the element in the surface layer portion is C: 0.60 to 1.10 times, Mn: 1.0 times or less, Cu: 1.0 times or less, with respect to the content of the element in the base layer. However, only when the Cu content of the base is 0.10% or more, Ni: 1.0 times or less, but only when the Ni content of the base is 0.10% or more, Cr: 1.0 times. Hereinafter, however, only when the Cr content of the base is 0.10% or more, Mo: 1.0 times or less, but only when the Mo content of the base is 0.10% or more, Nb: 1. 0 times or less, but only when the Nb content of the base is 0.02% or more, V: 1.0 times or less, but only when the V content of the base is 0.02% or more, Ti: 1.0 times or less, but only when the Ti content of the base is 0.05% or more, B: 1.0 times or less, but only when the B content of the base is 0.001% or more. The high-strength hot-rolled steel sheet according to any one of the above [1] to [3].

[5] The chemical composition of the surface layer portion is C: 0.03 to 0.55%, Si: 0 to 2.40%, Mn: 0.5 to 3.0%, P: 0.10 in mass%. % Or less, S: 0.050% or less, Cu: 0 to 1.0%, Ni: 0 to 1.0%, Cr: 0 to 3.0%, Mo: 0 to 1.0%, Nb: 0 ~ 0.05%, V: 0 to 0.2%, Ti: 0 to 0.20%, B: 0 to 0.0100%, N: 0.010% or less, Ca: 0 to 0.0100%, The high-strength hot-rolled steel sheet according to the above [4], which comprises REM: 0 to 0.0100%, Al: 0 to 1.00%, and the balance: Fe and impurities.

[6] The high-strength hot rolling of the above [4] or [5], wherein the C content of the surface layer portion is 0.91 to 1.10 times the C content of the base portion. Steel plate.

[7] The high-strength hot-rolled steel sheet according to any one of [1] to [6], wherein the metal structure of the surface layer portion has an area ratio of tempered martensite: 90% or more.

According to the present invention, a high-strength hot-rolled steel sheet having excellent bending workability and fatigue characteristics can be obtained. Such a high-strength hot-rolled steel sheet is suitable as, for example, a material for automobile parts.

It is a figure explaining the region to observe the presence or absence of peeling of a base part and a surface layer part after a fatigue test. It is a figure explaining the evaluation criteria of the presence or absence of peeling.

Hereinafter, embodiments of the present invention will be described.

The high-strength hot-rolled steel sheet of the present invention has a tensile strength of 980 MPa or more, preferably 1000 to 1500 MPa, a yield ratio of 0.75 or more, preferably 0.80 to 0.90, and has a base portion and a surface layer portion. Be prepared. In the high-strength hot-rolled steel sheet of the present invention, the surface layer portion is a region where the Vickers hardness is 80% or less as compared with the average Vickers hardness at a position of 1/2 of the thickness of the steel sheet from the surface of the steel sheet ( Part).

The yield ratio is the value obtained by dividing the yield stress by the tensile strength. The tensile strength and yield stress of the high-strength hot-rolled steel sheet of the present invention are subjected to a tensile test using a JIS No. 5 test piece collected so as to be perpendicular to the rolling direction of the steel sheet in accordance with JIS Z2241 (2011). Required by doing.

The metallographic structure of the base contains tempered martensite having an area ratio of 90% or more, preferably 95% or more. The metal structure of the surface layer portion is not particularly limited as long as the surface layer portion satisfies the characteristics described later.

By using the tempered martensite phase as the main phase as described above, it is easy to set the yield ratio of the steel sheet within the desired range. The residual structure other than the main phase of the base is composed of at least one of a retained austenite (γ) phase, a fresh martensite (fM) phase, a bainite (B) phase, a ferrite (α) phase, and a pearlite phase. .. When the microstructure fraction of the residual structure is high, at least one of tensile strength, bending workability, and fatigue characteristics is lowered, and it becomes difficult to obtain desired characteristics. Therefore, the area ratio of the remaining structure is less than 10%, preferably less than 5%. For example, in the steel sheet according to the present invention, the metal structure at the base may contain at least one of a retained austenite phase of 0% or more and 5% or less and a ferrite phase of 0% or more and 5% or less in terms of area ratio.

In the present invention, the identification of each metal structure at the base and the calculation of the area ratio are performed as follows.

"Ferrite"
First, a sample having a cross section parallel to the rolling direction and the thickness direction of the steel sheet is collected, and the cross section is used as an observation surface. Of this observation surface, a region of 200 μm × 200 μm centered on a position halved from the surface of the steel plate to the thickness of the steel plate (hereinafter, “thickness of steel plate” is referred to as “plate thickness”) is defined as an observation region. .. The electron channeling contrast image that can be seen by observing this observation area at a magnification of 1000 to 5000 times with a scanning electron microscope is an image that displays the crystal orientation difference of the crystal grains as a contrast difference. In this electron channeling contrast image, the portion having a uniform contrast is ferrite. Then, the area ratio of the ferrite identified in this way is calculated by the point counting method (based on ASTM E562-83 (1988)).

"Perlite"
First, the observation surface is corroded with a nital reagent. Of the corroded observation surface, a region of 200 μm × 200 μm centered on the position of 1/2 of the thickness from the surface of the steel sheet is defined as the observation region. This observation area is observed with an optical microscope at a magnification of 100 to 500, and the dark contrast area in the observation image is defined as pearlite. Then, the area ratio of the pearlite identified in this way is calculated by the point counting method.

"Bainite and tempered martensite"
Field-emission scanning electron microscope (FE-SEM: Field Emission Scanning) covers the observation area corroded by the nital reagent as described above (a region of 200 μm × 200 μm centered on the position of 1/2 of the thickness from the surface of the steel plate). Observe by 1000 to 5000 times by Electron Microscope). In this observation region, bainite and tempered martensite are identified as follows from the position of cementite contained inside the tissue and the arrangement of cementite.

As the state of existence of bainite, there are cases where a precipitate or retained austenite is present at the interface of the lath-shaped bainite ferrite, and there is a case where a precipitate is present inside the lath-shaped bainite ferrite. .. If a precipitate or retained austenite is present at the interface of the lath-shaped bainite ferrite, the bainite can be identified because the interface of the bainite ferrite is known. In addition, when a precipitate is present inside the lath-shaped bainitic ferrite, the crystal orientation relationship between the bainitic ferrite and the precipitate is one type, and the precipitate has the same variant. Can be identified. The area ratio of bainite identified in this way is calculated by the point counting method. If necessary, elemental analysis of the precipitate may be performed by a method such as SEM-EDS to confirm that the precipitate is cementite.

In tempered martensite, precipitates are present inside the martensite, but the tempered martensite is identified because there are two or more crystal orientations of the martensite and the precipitate, and the precipitate has multiple variants. be able to. The area ratio of tempered martensite identified in this way is calculated by the point counting method. When it is necessary to confirm that the above-mentioned precipitates are cementite, if necessary, several precipitates are analyzed by SEM-EDS or the like to confirm that the precipitates are cementite. You may.

"Quenched martensite"
First, an observation surface similar to the observation surface used for identifying the ferrite is etched with a repera solution, and a region similar to the identification of the ferrite is set as an observation region. Corrosion by the repera solution does not corrode martensite and retained austenite. Therefore, the observation area corroded by the repera solution is observed by FE-SEM, and the non-corroded area is designated as martensite and retained austenite. Then, the total area ratio of martensite and retained austenite identified in this way is calculated by the point counting method. Next, the volume fraction of retained austenite calculated as follows is regarded as the area ratio of retained austenite, and the area ratio of martensite can be calculated as it is quenched by subtracting the area ratio from the total area ratio. it can.

"Residual austenite"
The volume fraction of retained austenite can be determined by X-ray diffraction. First, of the samples collected as described above, the surface of the steel sheet to the position of 1/2 of the plate thickness is removed by mechanical polishing and chemical polishing to expose the surface of the steel plate at the position of 1/2 of the plate thickness. Let me. Then, the exposed surfaces are irradiated with MoKα rays, and the bcc phase (200) plane, (211) plane, and fcc phase (200) plane, (220) plane, and (311) plane are diffracted. Find the integrated intensity ratio of the peaks. The volume fraction of retained austenite can be calculated from the integrated intensity ratio of this diffraction peak. As this calculation method, a general 5-peak method can be used. The volume fraction of retained austenite thus obtained is regarded as the area ratio of retained austenite.

Next, the chemical composition of the base will be described. Hereinafter, "%" regarding the content of the element means "mass%" unless otherwise specified. In the present invention, the chemical composition of the base means a chemical composition measured at a position of 1/2 of the thickness.

(C: 0.05 to 0.50%)
C is an element effective for strengthening steel. In order to secure the fatigue characteristics of the steel sheet by heat treatment such as quenching and tempering, the C content is preferably 0.05% or more, more preferably 0.08% or more or 0.10% or more. , 0.12% or more, 0.14% or more, or 0.16% or more is more preferable. However, when the C content is high, cracks are likely to occur during cold molding. Therefore, the C content is preferably 0.50% or less, and more preferably 0.45% or less or 0.40% or less.

(Si: 1.00 to 3.00%)
Si is an element that suppresses softening during tempering of steel sheets. In order to effectively exert such an action, the Si content is preferably 1.00% or more. Further, depending on the Si content, there is an effect that the deformation resistance increases during hot rolling of the steel sheet. Therefore, the Si content is preferably 1.10% or more or 1.20% or more, more preferably 1.30% or more, 1.40% or more or 1.50% or more. In addition, Si has a function of promoting the formation of retained austenite. In order to suppress the residual austenite after winding and suppress the decrease in the toughness of the steel sheet, the Si content is preferably 3.00% or less, and 2.90% or less or 2.80. More preferably, it is% or less.

(ΔSi is 0.60% or more)
In the high-strength hot-rolled steel sheet of the present invention, ΔSi = Si _B −Si _S, which represents the difference between the Si content Si _B [mass%] at the base and the Si content Si _S [mass%] at the surface layer, is It is 0.60% or more. As described above, Si is an element that suppresses softening during tempering of a steel sheet. By setting ΔSi to 0.60% or more, a difference is likely to occur between the softening behavior of the base and the softening behavior of the surface layer, so that it is easy to make a difference in hardness between the base and the surface layer in the steel sheet manufacturing process. become. Further, during hot rolling in the production of a steel sheet, a difference in deformation resistance occurs between the base portion and the surface layer portion due to the difference in the Si content, and the larger ΔSi, the more the strain is given priority over the surface layer portion. It will be easier to introduce. By introducing strain into the surface layer portion, it becomes easy to reduce the effective crystal grain size of the surface layer portion. Therefore, ΔSi is preferably 0.80% or more or 1.00% or more, more preferably 1.20% or more or 1.50% or more. ΔSi has a maximum value of 3.00% when Si _B = 3.00% and Si _S = 0%. It is not necessary to further limit the upper limit of ΔSi, but it may be 2.80% or less or 2.60% or less.

The amount of Si in the base and surface layer is measured using an emission spectroscopic analyzer (Kantback). The amount of Si in the base portion is measured at a position of 6/10 t from the side provided with the surface layer portion, where t is the plate thickness. When the surface layer portion is provided on both sides, it may be 6 / 10t from either side. The amount of Si in the surface layer portion is measured in the outermost layer of the surface layer portion.

(Mn: 0.5 to 3.0%)
Mn has a function of suppressing the formation of ferrite and improving hardenability. In order to effectively exert such an action, the Mn content is preferably 0.5% or more, more preferably 0.8% or more, and preferably 1.0% or more. More preferred. However, in order to prevent the macrosegregation of Mn from becoming remarkable and the hardness difference between the segregated portion and the unsegregated portion becoming large, it is preferable that the Mn content is 3.0% or less. It is more preferably 8% or less, and further preferably 2.5% or less.

(P: 0.10% or less)
P is not an essential element but is contained in steel as, for example, an impurity. P may segregate at the old austenite grain boundaries and reduce the formability of the steel sheet due to grain boundary embrittlement. Therefore, the smaller the P content, the better. Specifically, the content of P is preferably 0.10% or less, and more preferably 0.02% or less. It is not necessary to set a lower limit for the content of P, and the lower limit is 0%. However, in order to reduce the P content to less than 0.001% in the refining step, the time required for refining becomes long and the cost increases significantly. Therefore, the content of P may be 0.001% or more in consideration of cost.

(S: 0.050% or less)
S is not an essential element but is contained in steel as an impurity, for example. S may form non-metallic inclusions such as MnS in the steel, leading to an increase in the hardness of the steel sheet and a decrease in ductility. Therefore, the smaller the S content, the better. Specifically, the content of S is preferably 0.050% or less, more preferably 0.020% or less, and further preferably 0.015% or less. It is not necessary to set a lower limit for the content of S, and the lower limit is 0%. However, in order to reduce the S content to less than 0.001% in the refining step, the time required for refining becomes long and the cost increases significantly. Therefore, the S content may be 0.001% or more in consideration of cost.

The following elements are not essential elements, but are optional elements that may be appropriately contained in the steel plate and steel up to a predetermined amount, and the content of each element may be 0%.

(Cu: 0 to 1.0%)
Cu is an element effective in increasing the strength of the steel sheet, and is added as needed. In order to increase the strength of the steel sheet by Cu, the Cu content is preferably 0.1% or more, and more preferably 0.2% or more. However, if the Cu content is high, reddish brittleness may occur and the productivity in hot rolling may decrease. Therefore, the Cu content is preferably 1.0% or less, more preferably 0.5% or less, and even more preferably 0.4% or less.

(Ni: 0-1.0%)
Ni is an element effective for improving the fatigue characteristics of the steel sheet, and is added as needed. In order to effectively exert the effect of containing Ni, the content of Ni is preferably 0.1% or more, and more preferably 0.2% or more. However, if the Ni content is high, the ductility of the steel sheet may decrease and the cold formability may decrease. Therefore, the Ni content is preferably 1.0% or less, more preferably 0.5% or less, and even more preferably 0.4% or less.

(Cr: 0 to 3.0%)
Cr, like Mn, is an element that suppresses pearlite transformation and is effective in increasing the strength of steel, and is added as necessary. In order to obtain the effect of containing Cr, the content of Cr is preferably 0.1% or more, and more preferably 0.2% or more. However, if the Cr content is high, coarse Cr carbides may be formed and the cold formability may be lowered. Therefore, the Cr content is preferably 3.0% or less, more preferably 2.0% or less, and even more preferably 1.0% or less.

(Mo: 0-1.0%)
Mo is an element effective for strengthening steel like Mn and Cr, and is added as needed. In order to obtain the effect of containing Mo, the content of Mo is preferably 0.1% or more, and more preferably 0.2% or more. However, when the Mo content is high, coarse Mo carbides may be formed and the cold workability may be lowered. Therefore, the Mo content is preferably 1.0% or less, and more preferably 0.7% or less.

(Nb: 0 to 0.02%)
Like Ti, Nb is an element that is effective in controlling the morphology of carbides, and since its addition makes the structure finer, it is also an element that is also effective in improving the toughness of steel sheets. In order to obtain the effect of containing Nb, the content of Nb is preferably 0.01% or more. However, when the Nb content is high, a large number of fine and hard Nb carbides may be deposited, the ductility may decrease as the strength of the steel sheet increases, and the workability may decrease. Therefore, the Nb content is preferably 0.02% or less.

(V: 0 to 0.2%)
Like Nb, V is also an element effective in controlling the morphology of carbides, and since the structure is miniaturized by its addition, it is also an element effective in improving the toughness of the steel sheet. In order to obtain the effect of containing V, the content of V is preferably 0.1% or more. However, when the V content is high, a large number of fine V carbides may be deposited, the ductility may decrease as the strength of the steel sheet increases, and the processability may decrease. Therefore, the V content is preferably 0.2% or less.

(Ti: 0 to 0.20%)
Ti is an element that controls the morphology of carbides and increases the strength of ferrite. In order to obtain such an effect, the Ti content is preferably 0.01% or more, more preferably 0.02% or more. However, if the Ti content is high, coarse Ti oxides or TiNs may be formed and the cold formability may decrease. Therefore, the Ti content is preferably 0.20% or less.

(B: 0 to 0.0100%)
B is an element that suppresses the formation of ferrite and pearlite in the cooling process from austenite and promotes the formation of a low temperature metamorphic structure such as bainite or martensite. Further, B is an element useful for increasing the strength of the steel sheet, and is added as needed. In order to increase the strength of the steel sheet or improve the fatigue characteristics by containing B, the content of B is preferably 0.0001% or more, more preferably 0.0005% or more. .. However, when the B content is high, a coarse B oxide may be generated, which may become a starting point for voids during molding of the steel sheet, resulting in a decrease in cold formability. Therefore, the content of B is preferably 0.0100% or less, and more preferably 0.0050% or less.

(N: 0.010% or less)
N is not an essential element. Like C, N is an element effective for increasing the strength of steel, but it is also an element that affects the occurrence of cross-slip of dislocations during cold forming. If the N content is high, the concentration of strain cannot be suppressed during the molding of the steel sheet, causing voids, and thus the cold formability is lowered. From the viewpoint of ensuring cold formability, the smaller the N content, the better. Specifically, the N content is preferably 0.010% or less, more preferably 0.008% or less, and even more preferably 0.006% or less. However, in order to reduce the N content to less than 0.001% in the refining step, the time required for refining becomes long and the cost increases significantly. Therefore, the content of N may be 0.001% or more in consideration of cost.

(Ca: 0 to 0.0100%)
Ca is an element that affects the strength and contributes to the improvement of the material, and is added as needed. In order to obtain the effect of improving the material by containing Ca, the Ca content is preferably 0.0003% or more. It is more preferably 0.0005% or more. When the Ca content is high, the workability in hot water may decrease. Therefore, the Ca content is preferably 0.0100% or less, and more preferably 0.0050% or less.

(REM: 0 to 0.0100%)
Like Ca, REM is an element that contributes to the improvement of the material, and is added as needed. In order to obtain the effect of improving the material by containing REM, the content of REM is preferably 0.0003% or more. It is more preferably 0.0005% or more. If the content of REM is large, the workability in hot water may decrease. Therefore, the content of REM is preferably 0.0100% or less, and more preferably 0.0050% or less.

(Al: 0 to 1.00%)
Al is an element used as a deoxidizer in the steelmaking process. Al does not have to be contained in the steel sheet of the final product, but Al used for deoxidation may remain. However, when the Al content is high, alumina (non-metal inclusions) whose hardness is significantly different from that of steel may be deposited in clusters, resulting in a large variation in hardness within the steel sheet and a decrease in toughness of the steel sheet. is there. Therefore, the Al content is preferably 1.00% or less, more preferably 0.50% or less, and further preferably 0.45% or less. The Al content specified here is the total amount of Al including Al as a precipitate and solid solution Al.

The balance of the chemical composition of the high-strength hot-rolled steel sheet according to this embodiment is Fe and impurities. Impurities include elements unavoidably mixed from steel raw materials or scrap, and elements unavoidably mixed in the steelmaking process that are allowed as long as they do not impair the characteristics of the high-strength steel sheet according to the present embodiment. Is exemplified.

Next, the surface layer portion will be described.

The thickness of the surface layer of the high-strength hot-rolled steel sheet of the present invention is from more than 10 μm per side to 30% or less of the total thickness of the steel sheet. By setting the thickness of the surface layer portion per one surface to more than 10 μm, the bendability of the steel sheet can be improved. The thickness of the surface layer portion is preferably 100 μm or 200 μm or more, more preferably 300 μm or more or 400 μm or more, and further preferably 600 μm or more. Further, it is preferably 5% or more, more preferably 8% or more, 10% or more, or 15% or more of the plate thickness. On the other hand, when the thickness of the surface layer portion per one side is 30% or less of the plate thickness, the shortage of the tensile strength of the steel sheet can be suppressed. The thickness of the surface layer portion is preferably 28% or less, more preferably 25% or less, and further preferably 20% or less of the plate thickness.

Further, the average Vickers hardness of the surface layer portion is 50 to 80% of the average Vickers hardness at the position of 1/2 of the plate thickness. The bending workability of the steel sheet can be improved by setting the average Vickers hardness of the surface layer portion to 80% or less of the average Vickers hardness at the position of 1/2 of the plate thickness. The average Vickers hardness of the surface layer portion is preferably 78% or less with respect to the average Vickers hardness at the position of 1/2 of the plate thickness. Further, by setting the average Vickers hardness of the surface layer portion to 50% or more of the average Vickers hardness at the position of 1/2 of the plate thickness, the shortage of the tensile strength of the steel sheet can be suppressed. When the average Vickers hardness of the surface layer portion is 61% or more, 62% or more, 63% or more or 65% or more of the average Vickers hardness at the position of 1/2 of the plate thickness, the fatigue characteristics are improved, which is preferable, 70%. The above is more preferable. Since the tensile strength of the steel sheet is determined by the thickness and material of each of the base portion and the surface layer portion, it is not determined only by the ratio of the average Vickers hardness of the base portion and the average Vickers hardness of the surface layer portion.

In the present invention, the average Vickers hardness at the position of 1/2 of the plate thickness and the average Vickers hardness of the surface layer portion conform to JIS Z 2244 (2009) with a pushing load of 100 g weight (0.98 N) as follows. It is determined by measuring with a Vickers hardness tester according to the above method. First, a sample having a cross section perpendicular to the surface of the steel sheet is prepared, and the cross section is smoothly polished. In the cross section, at a position of 1/2 of the plate thickness, the Vickers hardness at 5 points in the direction perpendicular to the plate thickness direction is measured, the average value is obtained, and this is taken as the average hardness of the base. Next, the Vickers hardness is set at an appropriate interval parallel to the plate thickness direction (however, the interval is preferably 2% or less of the plate thickness, more preferably 1% or less). Is measured, a position that becomes 80% of the average hardness of the base is measured, and the operation of measuring the distance between the position and the surface of the steel plate (that is, the thickness of the surface layer portion) is repeated 5 times. The average thickness of the surface layer portion obtained in the five operations is taken as the thickness of the surface layer portion. At the position of 1/2 of the thickness of the surface layer from the surface of the steel sheet, that is, at the position of 1/2 of the thickness of the surface layer, the Vickers hardness of 5 points in the direction perpendicular to the plate thickness direction was measured, and the average thereof. Let the value be the average Vickers hardness of the surface layer. However, if the thickness of the surface layer is less than 0.20 mm (200 μm) in the Vickers hardness test under a load of 100 g (0.98 N), the thickness of the surface layer is measured (80 of the average hardness of the base). %) And the average Vickers hardness of the surface layer, the load at the Vickers hardness test was 10 g weight (0.098N), and the same measurement work as above was performed 10 times, 10 times. Is the average of. Even in this case, as the average Vickers hardness of the base, the measured value (average value of 5 times) with a load of 100 g weight (0.98 N) is used as it is.

The arithmetic mean roughness Ra of the surface of the surface layer is 3.0 μm or less. The arithmetic mean roughness Ra is an average value of values measured in accordance with JIS B 0601 (2013) by randomly selecting 10 locations on the surface of the surface layer portion. By setting the arithmetic mean roughness Ra of the surface of the surface layer portion to 3.0 μm or less, the occurrence of cracks can be suppressed when the steel sheet is bent.

Further, in the metal structure of the surface layer portion, the effective crystal grain size is 50.0 μm or less. The effective crystal grain size is preferably 10.0 μm or less, more preferably 8.0 μm or less, and further preferably 5.0 μm or less. By reducing the effective grain size of the surface layer portion in this way, the degree of surface unevenness when the high-strength hot-rolled steel sheet of the present invention undergoes bending deformation is reduced, and the fatigue characteristics after bending deformation are improved. obtain. The effective crystal grain size is measured by analyzing the L cross section of the sample collected from the surface layer portion by SEM-EBSD. With respect to the metallographic image obtained by SEM-EBSD, the region surrounded by the boundary with an orientation difference of 15 ° or more is defined as effective crystal grains, and the total size of 100 effective crystal grains is the diameter when equivalent to a circle. The average value of their diameters is used as the effective crystal grain size.

Subsequently, the chemical composition of the surface layer portion desirable for obtaining the effect of the present invention will be described.

(C: 0.60 to 1.10 times the C content of the base)
C is an element that enhances the strength of the steel sheet. If the difference between the C content of the surface layer portion and the C content of the base portion is small, the martensite structure of the surface layer portion and the base portion obtained at the time of cooling becomes a structure close to that of the single-phase material, and the fatigue characteristics are improved. When the difference between the C content of the surface layer and the C content of the base becomes large, the morphology of martensite changes according to the amount of C, so that the structure changes discontinuously at the boundary between the surface layer and the base, and as a result. , The boundary becomes a stress concentration part, and the fatigue strength may decrease. Therefore, the content of C in the surface layer portion is 0.60 to 1.10 times the content of C in the base layer portion. The C content of the surface layer portion is more preferably 1.08 times or less, and further preferably 1.00 times or less, the C content of the base layer portion. Further, the C content of the surface layer portion is more preferably 0.70 times or more, more preferably 0.80 times or more, 0.91 times or more, or 0.92 times or more the content of C in the base layer portion. Is more preferable.

The specific content of C in the surface layer portion is preferably 0.03% or more, more preferably 0.04% or more, and further preferably 0.05% or more. The C content is preferably 0.55% or less, more preferably 0.50% or less, and even more preferably 0.45% or less.

(Si, Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, and B)
The content of Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, and B in the surface layer is 1.1 times or less or 1.0 times or less, respectively, with respect to the content of the same component in the base. Is preferable. In particular, by setting the content of the above elements to 1.0 times or less the content of the same component in the central part of the surface layer, the surface roughness utilizing the difference in hardness ratio and hot deformation resistance between the surface layer part and the base part is utilized. Is easy to set in the above desired range.

However, when the Cu content of the base is less than 0.10%, the Ni content of the base is less than 0.10%, the Cr content of the base is less than 0.10%, the Mo content of the base is If it is less than 0.10%, if the Nb content of the base is less than 0.02%, if the V content of the base is less than 0.02%, if the Ti content of the base is less than 0.05%, When the B content of the base is less than 0.001%, the influence of these elements on the hardness is small, so that there is no problem even if the content in the surface layer exceeds 1.0 times the content of the base.

The preferable specific content of each element other than C may be the same as the content at the base described above, except for Si in which ΔSi is 0.60% or more. Since the Si content at the base is 3.00% at the maximum, the Si content at the surface layer is 0 to 2.40%. The Si content of the surface layer portion is preferably lower than that of the base portion, and the upper limit thereof may be 2.00%, 1.50%, 1.00% or 0.80%, more preferably 0.60% or 0. .40%, 0.30% or 0.20%. It is not necessary to set a lower limit of the Si content of the surface layer portion, and it is 0%. If necessary, the lower limit may be 0.01% or 0.05%.

The metal structure of the surface layer is not particularly limited, but if the area ratio of 90% or more is tempered martensite, it is preferable because the yield ratio of the steel sheet can be easily set in a desired range and the fatigue characteristics can be improved. More preferably, the area ratio is 95% or more. The method for measuring the area ratio of tempered martensite is the same as for the base. The observation region is a region of 200 μm × 200 μm centered on a position halved in the thickness of the surface layer portion.

(Production method)
Next, a manufacturing method for obtaining a high-strength hot-rolled steel sheet of the present invention will be described. The following description is intended merely as an example of a manufacturing method for obtaining a high-strength hot-rolled steel sheet of the present invention, and limits the manufacturing method of the high-strength hot-rolled steel sheet of the present invention to the following methods. Not intended.

First, prepare the base steel plate as the base and the surface steel plate as the surface layer. Then, the surface of the base steel sheet is degreased, and the surface steel sheet is laminated on the surface. In this way, a slab in which the surface steel plate is laminated on one side or both sides of the base steel plate is obtained.

Next, heat the above slab. In order to suppress the anisotropy of the crystal orientation caused by casting, the heating temperature of the slab is preferably 1100 ° C. or higher, preferably more than 1150 ° C. On the other hand, from the viewpoint of suppressing a significant increase in manufacturing cost, the heating temperature of the slab in the process is preferably 1350 ° C. or lower. Further, in order to diffuse the elements between the base steel plate and the surface steel plate and sufficiently join the base steel plate and the surface steel plate, it is preferable to heat the slab in the above temperature range for 2 hours or more. ..

Next, the slab heated as described above is subjected to hot rolling. This hot rolling step includes a rough rolling step and a multi-step finish rolling step.

Path control in the rough rolling process is important from the viewpoint of more sufficiently joining the base steel sheet and the surface layer steel sheet after adjusting the heating temperature of the slab as described above. In the rough rolling step, it is preferable that the plate thickness reduction rate per pass is 5% or more and less than 50%, the time between passes is 3 seconds or more, and the number of rolling times is 2 times or more. These conditions are for promoting element diffusion by the strain introduced in rough rolling and for sufficiently joining the base steel sheet and the surface layer steel sheet.

Next, the finish rolling process will be described.

In the finish rolling process, the time t1 in which the upper surface of the steel sheet in the vertical direction is covered with the water film is excluded from the time t2 from the start to the completion of the finish rolling process, and the total time t3 in which the steel sheet is in contact with the rolling roll is excluded. When the ratio divided by the total time is F, it is preferable that the condition of the following equation (1) is satisfied.

F = t1 / (t2-t3) ≧ 0.5 (1)

The scale that grows during finish rolling can also cause the formation of dents in the steel sheet. Since the growth of scale is suppressed by covering the surface of the steel sheet with the water film, it is desirable that the time for covering the surface of the steel sheet with the water film is longer. From this viewpoint, F is more preferably 0.6 or more, and further preferably 0.7 or more. By controlling the range of the ratio F in this way, the surface roughness of the steel sheet can be reduced.

Examples of the method of covering the surface of the steel sheet with a water film include spraying water on the steel sheet by spraying between rolling rolls. In this method, it is easy to cover the upper surface of the steel sheet in the vertical direction with water. Therefore, from the viewpoint of reducing the surface roughness of the surface layer portion, it is preferable that the surface on the surface layer portion side is the upper side in the vertical direction in the finish rolling step. Further, in this method, the ratio F is controlled, for example, by controlling the number of spray nozzles used.

Regarding the time when the upper surface is covered with the water film, the above equation (1) is taken into consideration in the upper surface of rolling in automobile wheels, lower arms, etc. to which the high-strength hot-rolled steel sheet of the present invention is mainly applied. This is because it is common for the side to be the front side of the product after pressing, and it is particularly required to improve the scale adhesion on the upper surface side of rolling.

Further, in finish rolling, the completion temperature (finishing temperature) of hot rolling is preferably 850 to 950 ° C. By setting the finishing temperature to 850 ° C. or higher, it is possible to suppress an increase in the difference in hot deformation resistance between the base portion and the surface layer portion, and it becomes easy to suppress breakage of the surface layer portion. Further, by setting the finishing temperature to 950 ° C. or lower, it is possible to prevent the effective crystal grain size of the surface layer portion from becoming coarse. Further, when the finishing temperature is 850 ° C. or higher and 950 ° C. or lower, the ratio of the hot deformation resistance between the base portion and the surface layer portion becomes appropriate, and the surface layer portion is preferentially deformed to cause surface irregularities on the surface layer portion side. It is stretched and the surface on the surface layer side can be smoothed.

Further, in the finish rolling, in order to suppress the effective crystal grain size of the surface layer portion to 50.0 μm or less, the rolling reduction ratio of the final stage is preferably 10% or more. Further, the reduction rate of the final stage is preferably 40% or more. By setting the reduction rate of the final stage to 40% or more, the effective crystal grain size of the surface layer portion can be suppressed to 10.0 μm or less.

Next, the hot-rolled steel sheet obtained by hot-rolling the slab as described above is cooled. In this cooling step, heat is directed so that the surface layer side faces upward in the vertical direction and the average cooling rate until the surface temperature of the surface layer portion reaches a predetermined winding temperature is 30 ° C./s or more. Water-cool the rolled steel sheet. The average cooling rate is preferably 50 ° C./s or higher, and more preferably 80 ° C./s or higher. This cooling step is a step necessary to reduce the surface roughness Ra of the surface layer portion to 3 μm or less and to obtain a martensite main phase structure which is a pre-structure of the tempered martensite main phase structure in the subsequent heat treatment step. .. By setting the surface layer side to the upper side in the vertical direction in this cooling step, a water film is formed on the surface of the surface layer side and scale generation is suppressed. Therefore, deterioration of the surface roughness of the surface layer portion due to scale is suppressed. Further, by setting the average cooling rate as described above, it becomes easy to obtain the martensite main phase structure while maintaining stable and homogeneous quenching in the entire steel sheet.

Next, the hot-rolled steel sheet cooled as described above is wound at a predetermined winding temperature. The winding temperature is preferably 100 ° C. or lower. By setting the winding temperature to 100 ° C. or lower, it is possible to suppress the occurrence of self-annealing (autotemper) according to the staying time at a temperature of 100 ° C. or higher and Ms point or lower, and it is easy to obtain a desired structure before heat treatment. Become. The desired structure before heat treatment referred to here is a martensite main phase structure as hardened, and preferably a martensite single phase structure as hardened. Martensitic transformation occurs continuously below the martensitic transformation point. Tempering progresses as the martensite is transformed at a higher temperature, and if the residence time at a temperature of 100 ° C. or higher and Ms point or lower is long, the structure may be a mixture of tempered martensite having different hardness. If the structure is mixed in this way, the hardness will vary after the subsequent heat treatment, which will lead to a decrease in bending workability and a decrease in fatigue characteristics of the steel sheet. Therefore, the winding temperature is preferably 80 ° C. or lower, more preferably 50 ° C. or lower. By setting the take-up temperature in this way, the metal structure of the entire steel sheet can be easily tempered to 90% or more of martensite, and the tensile strength and yield ratio of the steel sheet can be improved.

Next, the steel sheet wound as described above is rewound, and the steel sheet is heat-treated under the following conditions.

In order to set the yield ratio of the steel sheet to 0.75 or more, in the heat treatment step after the cooling step, the heating temperature is set to 590 ° C or lower, and the tempering parameter LMP defined by the following equation (2) is 8000 or higher. It is preferable to perform heat treatment. By performing a heat treatment having an LMP of 8000 or more, it can be sufficiently tempered to obtain a desired yield ratio. Further, when the LMP exceeds 17,000, the tensile strength of the steel sheet can be less than 980 MPa, so the LMP is preferably 17,000 or less. The LMP is more preferably 10,000 or more and 16,000 or less.

LMP = (T + 273.15) × (20 + log (t / 3600)) (2)
Here, T is the heating temperature [° C.] and t is the holding time [seconds].

Further, by performing heat treatment under the condition that the heat treatment parameter H ^* (ΔSi, LMP) consisting of the function of ΔSi and LMP obtained by the following equation (3) is 1.0 or less, tempering softening utilizing the difference in Si concentration is performed. The difference in resistance can be adjusted appropriately. Therefore, the hardness ratio between the base portion and the surface layer portion can be made appropriate.

^{H * (ΔSi, LMP) =} {(ΔSi-1.8) /0.9} 2 + {(LMP-16500) / 7500} 2 -1 (3)

The thickness of the high-strength hot-rolled steel sheet of the present invention is not particularly limited, but 1.8 to 3.0 mm is suitable and preferable for a steel sheet for automobiles.

Further, the high-strength hot-rolled steel sheet of the present invention is intended to soften the surface layer on the side outside the bending in order to improve bending workability and fatigue characteristics, and the surface layer portion on one side or both sides of the base portion. Is characterized by being arranged. Here, the fact that the surface layer portion is arranged on one side does not mean that there is no layer different from the base on the surface of the base portion on the other side. A steel sheet in which the above-mentioned surface layer portion is arranged only on one side of the base portion and the surface layer portion or the like is not provided on the surface of the base portion on the other side is naturally included in the present invention. Further, a steel plate in which the above-mentioned surface layer portion is arranged on one side of the base portion and a soft portion, a hard portion or the like having different properties from the above-mentioned surface layer portion is formed on the other side is naturally included in the present invention. Even in a steel sheet in which soft portions (including the surface layer portion of the present invention) are arranged on both sides, the effect of the present invention can be obtained by removing part or all of the soft portions as necessary during bending. Can be obtained.

The high-strength hot-rolled steel sheet according to the present invention will be described in more detail below with reference to Examples. However, the following examples are examples of the high-strength hot-rolled steel sheet of the present invention, and the high-strength hot-rolled steel sheet of the present invention is not limited to the following aspects. The conditions in the examples described below are one-condition examples adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to these one-condition examples. In the present invention, various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

First, a base steel plate as a base and a surface steel plate as a surface layer were prepared, and surface steel plates were laminated on one side of the base steel plate to form a laminate. The chemical composition of the base portion of the steel sheet obtained by subjecting such a laminate to the treatment described later is shown in Tables 1-1 and 1-2, and the chemical composition of the surface layer portion is shown in Tables 2-1 and 2-2. Further, the difference between the Si content Si _B [mass%] at the base and the Si content Si _S [mass%] at the surface layer ΔSi, and C, Mn, Cu, Ni, Cr, Mo, Nb, V, Tables 3-1 and 3-2 show the content ratio of the surface layer portion to the content of the base portion of Ti and B. Regarding the content ratio, "-" indicates that the element is not contained in the base.

The produced laminate was hot-rolled, cooled, wound, and heat-treated under the conditions shown in Tables 4-1 and 4-2. The underlines in Tables 4-1 and 4-2 indicate that they are outside the range of the above-mentioned preferable manufacturing conditions.

Then, the obtained steel sheet was evaluated as follows.

(Evaluation method of steel sheet characteristics)

The arithmetic mean roughness (μm) of the surface layer was measured in accordance with JIS B 0601 (2013). That is, 10 places are randomly selected on the surface on the surface layer side, the surface profile is measured at each place by a contact type surface roughness meter, and the arithmetic average roughness Ra obtained by arithmetically averaging the surface roughness at those places is obtained. The evaluation index is used, and the results are shown in Tables 5-1 and 5-2. The underlines in Tables 5-1 and 5-2 indicate that the characteristics required for the high-strength hot-rolled steel sheet of the present invention were not satisfied.

Tensile strength TS (MPa) and yield stress YS (MPa) were determined by conducting a tensile test in accordance with the provisions of JIS Z 2241 (2011). A JIS No. 5 test piece is taken from a position 1/4 of the width direction of the steel sheet so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction, and the maximum tensile strength obtained by a tensile test using the test piece. Was defined as the tensile strength TS (MPa) of the steel sheet. The results are shown in Tables 5-1 and 5-2. Further, a steel sheet having a tensile strength TS (MPa) ≧ 980 MPa and a yield ratio YR = YS / TS ≧ 0.75 was determined to be a high-strength hot-rolled steel sheet.

The bending workability was evaluated as follows. First, a strip-shaped test piece of 100 mm × 30 mm was cut out from the 1/2 position in the width direction of the steel plate. JIS Z for both bending in which the bending ridge is parallel to the rolling direction (L direction) (L-axis bending) and bending in which the bending ridge is parallel to the direction perpendicular to the rolling direction (C direction) (C-axis bending). The bendability of the steel sheet was evaluated by the V-block method specified in 2248 (2014). The apex angle of the groove of the V block was 90 °. The minimum bending radius at which cracks do not occur is obtained for each of the L-axis bending and the C-axis bending, and the value R / t obtained by dividing the average value R of these minimum bending radii by the plate thickness t is used as the index value of bending workability. The results are shown in Tables 5-1 and 5-2. When R / t ≦ 4.0, it was judged that the steel sheet had excellent bending workability. In this evaluation, the presence or absence of cracks was judged as follows. That is, the test piece subjected to the V block method is cut on a surface parallel to the bending direction and perpendicular to the plate surface, and the cut surface is mirror-polished and then observed with an optical microscope to observe the inside and outside of the bending of the test piece. When the length of the crack observed in at least one of them exceeds 30 μm, it is judged that there is a crack.

The fatigue characteristics after bending deformation were evaluated as follows. First, the test pieces described in JIS Z 2275 (1978) were collected from a position 1/4 of the width direction of the steel sheet so that the rolling direction and the vertical direction (C direction) were the longitudinal directions. The test piece was bent and deformed so that the radius of curvature was 15 mm and the angle was 90 ° about the center of the test piece. A test was conducted in which bending deformation was repeatedly applied to the test piece 200,000 times under a stress load of 0.5 times the tensile strength measured as described above. The steel sheet that broke during this test was designated as C, the steel sheet that was visually confirmed to have microcracks after the test was designated as B, and the steel sheet that was not visually confirmed to have microcracks after the test was designated as A. It is shown in 5-2. In this test, the unbroken steel sheet was judged to be a high-strength hot-rolled steel sheet with excellent fatigue characteristics.

Furthermore, the cross-section was observed after the above test to evaluate the presence or absence of peeling at the interface between the surface layer and the base. As shown in FIG. 1, the presence or absence of peeling and the length of peeling were observed using a profile projector with the cross section in the plate thickness direction passing through the inner and outer bent ridges as an observation region. As shown in FIG. 2, the region of 20 mm in the center is set as the observation region, the steel plate in which peeling is not confirmed is A, and the steel plate in which a minute peeling region of less than 0.5 mm is confirmed is B, 0.5 to 1. C is a steel sheet in which a minute peeling region of 0 mm is confirmed, and D is a steel sheet in which peeling is confirmed over the entire surface, and the results are shown in Tables 5-1 and 5-2. In this test, a steel sheet in which peeling was not confirmed or only a minute peeling region of 100 μm or less was confirmed was judged to be a high-strength hot-rolled steel sheet having excellent fatigue characteristics.

Tables 5-1 and 5-2 show the results of the above evaluation. Further, the thickness (total thickness) of the steel plate [mm], the thickness of the surface layer portion (surface layer thickness) [mm], the thickness of the steel sheet the thickness ratio of the surface layer portion to the (total thickness) (ItaAtsuhi V _t) [%], Multiplier of average Vickers hardness of surface layer (hardness ratio) to average Vickers hardness at 1/2 of plate thickness, area ratio of tempered martensite at base (fraction _VMB ) [%] The area ratio (fraction _VMS ) [%] of the tempered martensite in the surface layer portion and the effective crystal grain size D [μm] in the surface layer portion are shown in Tables 5-1 and 5-2. As shown in Tables 5-1 and 5-2, it was confirmed that according to the present invention, a steel sheet having good bending workability and fatigue characteristics can be obtained.

According to the present invention, a high-strength hot-rolled steel sheet having bending workability and fatigue characteristics suitable as a material for automobile parts and the like can be obtained.

Claims

A high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and a yield ratio of 0.75 or more.
It comprises a base and a surface layer located on the surface of one or both sides of the base.
The thickness of the surface layer portion is 30% or less of the thickness of the high-strength hot-rolled steel sheet and exceeds 10 μm.
The average Vickers hardness of the surface layer portion is 50 to 80% of the average Vickers hardness at a position of 1/2 the thickness of the high-strength hot-rolled steel sheet.
The arithmetic mean roughness Ra of the surface of the surface layer portion is 3.0 μm or less.
The effective crystal grain size of the surface layer portion is 50.0 μm or less.
The chemical composition of the base is mass%.
C: 0.05 to 0.50%,
Si: 1.00 to 3.00%,
Mn: 0.5-3.0%,
P: 0.10% or less,
S: 0.050% or less,
Cu: 0-1.0%,
Ni: 0-1.0%,
Cr: 0-3.0%,
Mo: 0-1.0%,
Nb: 0 to 0.02%,
V: 0 to 0.2%,
Ti: 0 to 0.20%,
B: 0 to 0.0100%,
N: 0.010% or less,
Ca: 0-0.0100%,
REM: 0-0.0100%,
Al: 0 to 1.00%, and the balance: Fe and impurities.
The difference ΔSi = Si B − Sis between the Si content Si B of the base portion and the Si content Si S of the surface layer portion is 0.60% or more in mass%.
The metallographic structure of the base is, by area ratio,
Tempered martensite: A high-strength hot-rolled steel sheet characterized by being 90% or more.
However, the surface layer portion refers to a region where the Vickers hardness is 80% or less of the average Vickers hardness at a position of 1/2 of the plate thickness from the surface of the high-strength hot-rolled steel sheet.
The high strength according to claim 1, wherein the average Vickers hardness of the surface layer portion is 61% or more of the average Vickers hardness at a position halved of the thickness of the high-strength hot-rolled steel sheet. Hot rolled steel sheet.
The high-strength hot-rolled steel sheet according to claim 1 or 2, wherein the effective crystal grain size of the surface layer portion is 10.0 μm or less.
The content of the element in the surface layer is relative to the content of the element in the base.
C: 0.60 to 1.10 times,
Mn: 1.0 times or less,
Cu: 1.0 times or less, but only when the Cu content at the base is 0.10% or more.
Ni: 1.0 times or less, but only when the Ni content of the base is 0.10% or more.
Cr: 1.0 times or less, but only when the Cr content at the base is 0.10% or more.
Mo: 1.0 times or less, but only when the Mo content of the base is 0.10% or more.
Nb: 1.0 times or less, but only when the Nb content of the base is 0.02% or more.
V: 1.0 times or less, but only when the V content of the base is 0.02% or more.
Ti: 1.0 times or less, but only when the Ti content of the base is 0.05% or more.
B: 1.0 times or less, but only when the B content of the base is 0.001% or more.
The high-strength hot-rolled steel sheet according to any one of claims 1 to 3, wherein the high-strength hot-rolled steel sheet is characterized by the above.
The chemical composition of the surface layer is C: 0.03 to 0.55% by mass.
Si: 0-2.40%,
Mn: 0.5-3.0%,
P: 0.10% or less,
S: 0.050% or less,
Cu: 0-1.0%,
Ni: 0-1.0%
Cr: 0-3.0%,
Mo: 0-1.0%,
Nb: 0 to 0.05%,
V: 0 to 0.2%,
Ti: 0 to 0.20%,
B: 0 to 0.0100%,
N: 0.010% or less,
Ca: 0-0.0100%,
REM: 0-0.0100%,
The high-strength hot-rolled steel sheet according to claim 4, wherein Al: 0 to 1.00%, and the balance: Fe and impurities.
The high-strength hot-rolled steel sheet according to claim 4 or 5, wherein the C content of the surface layer portion is 0.91 to 1.10 times the C content of the base portion.
The high-strength hot-rolled steel sheet according to any one of claims 1 to 6, wherein the metal structure of the surface layer portion has an area ratio of tempered martensite: 90% or more.