WO2023032225A1 - Hot-rolled steel sheet - Google Patents

Abstract

This hot-rolled steel sheet has a specific chemical composition; and the metal structure of the inner region of this hot-rolled steel sheet contains, in area ratios, a total of 40% to 80% of martensite and/or bainite, and 20% to 60% of ferrite, with the remainder structure being less than 5% by area. The ratio αs/αc of the ferrite area ratio αs in the surface region to the ferrite area ratio αc in the inner region is from 1.15 to 2.50; the hardness difference ratio (1 – Hvs/Hvc) of the Vickers hardness Hvs in the surface region to the Vickers hardness Hvc in the inner region is 0.20 or less; and the tensile strength is 980 MPa or more.

Description

hot rolled steel

The present invention relates to hot-rolled steel sheets. Specifically, it relates to a hot-rolled steel sheet having high strength and excellent fatigue strength, toughness and ductility.

In recent years, the application of high-strength steel sheets to automobile parts has been actively studied with the aim of improving the durability and collision safety of automobiles. However, increasing the strength of a steel sheet generally degrades its toughness. Therefore, in the development of high-strength steel sheets, it is an important issue to increase the strength without deteriorating the material properties. In particular, in high-strength steel sheets applied to automobile members, it is important to ensure the fatigue durability of the parts. When it is processed into parts, cracks develop from the punched surface or the like, and even if a high-strength steel plate is used, the fatigue durability of the parts does not necessarily improve.

On the other hand, in Patent Document 1, the metal structure has a surface layer region having a ferrite phase as a main phase and an inner region having a bainite phase as a main phase, and the ratio of the surface layer region in the thickness direction of the steel sheet is A high-strength hot-rolled steel sheet having excellent bending workability has been proposed in which each of the front and back surfaces of the steel sheet has a total thickness of 1.0 to 5.0%.

In Patent Document 2, there is a central portion mainly composed of bainite and a surface layer portion mainly composed of polygonal ferrite, and the surface layer portion is formed at least in a region from both surfaces of the steel plate to a depth of 0.2 mm. A high-strength hot-rolled steel sheet with excellent toughness has been proposed.

Patent Document 3 proposes a high-strength steel sheet with excellent bendability in which the average Vickers hardness from the surface layer to the half thickness position and the standard deviation of hardness are kept low.

Patent Document 4 proposes a hot-rolled steel sheet with improved fatigue properties and surface machinability by controlling the area fraction of martensite and Vickers hardness within a predetermined range for each thickness direction. It is

WO2014/171057 Japanese Patent Application Laid-Open No. 2001-279378 WO2018/151331 Japanese Patent Application Laid-Open No. 2017-186634

However, in the hot-rolled steel sheets described in Patent Documents 1 to 3, the surface layer has ferrite as the main phase and is softened, leaving room for further improvement in fatigue properties.

In addition, in the invention described in Patent Document 4, the surface layer is softened, and there is room for further improvement in fatigue strength. Furthermore, since precipitation strengthening is carried out inside the plate thickness, dislocation movement in ferrite is hindered, and from this point of view, there is room for further improvement in toughness.

In recent years, against the background of demand for further weight reduction of automobiles, complicated shapes of parts, and the like, there is a demand for high-strength hot-rolled steel sheets having higher fatigue strength and toughness.
The present invention has been made in view of the above problems, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent fatigue strength and toughness. Another object of the present invention is to provide a hot-rolled steel sheet having the above properties and excellent ductility, which is generally required for hot-rolled steel sheets applied to automotive parts. do.

The precipitation-strengthened structure inhibits dislocation movement, so it has excellent fatigue strength. Therefore, precipitation-strengthened structures are often used in automotive underbody parts. On the other hand, when dislocation motion is suppressed, plastic deformation is less likely to occur, resulting in deterioration of impact properties (particularly toughness). Therefore, it is presumed that the fatigue strength and impact properties are in a conflicting relationship. In order to improve both fatigue strength and toughness, the present inventors analyzed the deformation mechanisms of fatigue strength and impact strength in detail. As a result, the present inventors found that the metallographic structure and hardness of the surface layer region of the hot-rolled steel sheet have a great effect on the fatigue strength, and the metallographic structure and hardness of the internal region of the hot-rolled steel plate have a great effect on the propagation of cracks. I thought it would work.

The gist of the present invention made based on the above knowledge is as follows.
(1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition, in mass%,
C: 0.02 to 0.30%,
Si: 0.10 to 2.00%,
Mn: 0.5-3.0%,
sol. Al: 0.10 to 1.00%,
Ti: 0.06-0.20%,
P: 0.1000% or less,
S: 0.0100% or less,
N: 0.0100% or less,
Nb: 0 to 0.100%,
Ca: 0 to 0.0060%,
Mo: 0-0.50%,
Cr: 0 to 1.00%,
V: 0 to 0.40%,
Ni: 0 to 0.40%,
Cu: 0-0.40%,
B: 0 to 0.0020%, and Sn: 0 to 0.20%,
The balance consists of Fe and impurities,
The metal structure of the internal region contains, in terms of area ratio, 40 to 80% in total of one or two of martensite and bainite, and 20 to 60% ferrite, and the area ratio of the remaining structure is less than 5%. ,
αs/αc, which is the ratio of the ferrite area ratio αs of the surface layer region to the ferrite area ratio αc of the internal region, is 1.15 to 2.50;
A hardness difference ratio (1-Hvs/Hvc) between the Vickers hardness Hvs of the surface region and the Vickers hardness Hvc of the inner region is 0.20 or less,
Tensile strength is 980 MPa or more.
(2) In the hot-rolled steel sheet described in (1) above, the chemical composition is, in mass%,
Nb: 0.010 to 0.100%,
Ca: 0.0005 to 0.0060%,
Mo: 0.02-0.50%,
Cr: 0.02 to 1.00%,
V: 0.01 to 0.40%,
Ni: 0.01 to 0.40%,
Cu: 0.01 to 0.40%,
B: 0.0001-0.0020%, and Sn: 0.01-0.20%
It may contain one or more selected from the group consisting of.

According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent fatigue strength, toughness and ductility. This hot-rolled steel sheet has a high industrial value because it can reduce the weight and improve the durability of the body of an automobile or the like.

A hot-rolled steel sheet according to an embodiment of the present invention (hereinafter sometimes referred to as a hot-rolled steel sheet according to this embodiment) will be described. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the gist of the present invention.

Each constituent element of the present invention will be described in detail below. First, reasons for limiting the chemical composition of the hot-rolled steel sheet according to the present embodiment will be described.
The numerical limits described below with "-" in between include the lower limit and the upper limit. Any numerical value indicated as "less than" or "greater than" excludes that value from the numerical range. In the following description, % regarding chemical composition is mass % unless otherwise specified.

The hot-rolled steel sheet according to the present embodiment has a chemical composition in mass% of C: 0.02 to 0.30%, Si: 0.10 to 2.00%, Mn: 0.5 to 3.0%. , sol. Al: 0.10 to 1.00%, Ti: 0.06 to 0.20%, P: 0.1000% or less, S: 0.0100% or less, N: 0.0100% or less, and the balance: Contains Fe and impurities. Each element will be described in detail below.

<C: 0.02 to 0.30%>
C is an important element for improving the strength of hot-rolled steel sheets. In order to obtain the desired strength, the C content should be 0.02% or more. Preferably it is 0.04% or more.
On the other hand, if the C content exceeds 0.30%, the toughness of the hot-rolled steel sheet deteriorates. Therefore, the C content is made 0.30% or less. Preferably, it is 0.20% or less.

<Si: 0.10 to 2.00%>
Si is an element that has the effect of suppressing the formation of carbides during ferrite transformation and improving the toughness of the hot-rolled steel sheet. In order to obtain this effect, the Si content is set to 0.10% or more. Preferably, it is 0.20% or more or 0.50% or more.
On the other hand, if the Si content exceeds 2.00%, the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 2.00% or less. Preferably, it is 1.50% or less.

<Mn: 0.5 to 3.0%>
Mn is an element effective in improving the strength of hot-rolled steel sheets by improving hardenability and solid-solution strengthening. To obtain this effect, the Mn content is set to 0.5% or more. Preferably it is 1.0% or more.
On the other hand, when the Mn content exceeds 3.0%, MnS is generated which is harmful to toughness and fatigue strength. Therefore, the Mn content is set to 3.0% or less. Preferably, it is 2.5% or less or 2.0% or less.

<sol. Al: 0.10 to 1.00%>
Al is an important element for controlling ferrite transformation. To obtain this effect, sol. The Al content is set to 0.10% or more. Preferably, it is 0.15% or more or 0.20% or more.
On the other hand, sol. If the Al content exceeds 1.00%, alumina precipitates in clusters and the toughness of the hot-rolled steel sheet deteriorates. Therefore, sol. The Al content is set to 1.00% or less. Preferably, it is 0.80% or less or 0.50% or less.
In addition, sol. Al means acid-soluble Al, and indicates solid-solution Al present in steel in a solid-solution state.

<Ti: 0.06 to 0.20%>
Ti is an element that strengthens ferrite by precipitation and is an important element for controlling ferrite transformation to obtain a desired amount of ferrite. In order to obtain excellent fatigue strength by controlling precipitation strengthening and ferrite transformation, the Ti content is made 0.06% or more. Preferably it is 0.08% or more.
On the other hand, when the Ti content exceeds 0.20%, inclusions caused by TiN are formed, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Ti content is set to 0.20% or less. Preferably, it is 0.16% or less or 0.13% or less.

<P: 0.1000% or less>
P is an impurity, and the lower the P content, the better. In particular, when the P content exceeds 0.1000%, the workability and weldability of the hot-rolled steel sheet are remarkably lowered, and the fatigue strength is also lowered. Therefore, the P content is made 0.1000% or less. Preferably, it is 0.0500% or less or 0.0200% or less.
Although the lower limit of the P content does not have to be specified, it is preferably 0.0010% or more from the viewpoint of refining cost.

<S: 0.0100% or less>
S is an impurity, and the lower the S content, the better. In particular, when the S content exceeds 0.0100%, a large amount of inclusions such as MnS, which is harmful to the isotropy of toughness, is produced. Therefore, the S content is made 0.0100% or less. If higher toughness is required, the S content is preferably 0.0060% or less. More preferably, it is 0.0050% or less.
Although the lower limit of the S content does not have to be specified, it is preferably 0.0001% or more from the viewpoint of refining cost.

<N: 0.0100% or less>
N is an impurity. If the N content exceeds 0.0100%, coarse Ti nitrides are formed in the high temperature range, which deteriorates the toughness of the hot-rolled steel sheet. Therefore, the N content is set to 0.0100% or less. Preferably, it is 0.0060% or less or 0.0050% or less.
Although the lower limit of the N content does not have to be specified, it is preferably 0.0001% or more from the viewpoint of refining cost.

The hot-rolled steel sheet according to the present embodiment may contain the chemical components described above, with the balance being Fe and impurities. In the present embodiment, the term “impurities” refers to ores used as raw materials, scraps, or impurities that are mixed from the manufacturing environment, etc., and/or those that are allowed within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment. do.

Although not essential for providing the desired properties, the following optional elements may be included in order to reduce manufacturing variations and further improve the strength of the hot-rolled steel sheet. However, since it is not essential to contain these elements, the lower limit of the content of these elements is 0%. In addition, if the content of each arbitrary element is less than the lower limit of the content explained below, it can be regarded as an impurity.

<Nb: 0.010 to 0.100%>
Nb is an element that has the effect of increasing the strength of the hot-rolled steel sheet by refining the crystal grain size of the hot-rolled steel sheet and strengthening the precipitation of NbC. To reliably obtain this effect, the Nb content is preferably 0.010% or more.
On the other hand, if the Nb content exceeds 0.100%, the above effect is saturated. Therefore, even when Nb is contained, the Nb content is set to 0.100% or less. Preferably, it is 0.060% or less.

<Ca: 0.0005 to 0.0060%>
Ca is an element that has the effect of dispersing a large number of fine oxides during deoxidation of molten steel and refining the structure of the hot-rolled steel sheet. In addition, Ca is an element that fixes S in steel as spherical CaS, suppresses the formation of elongated inclusions such as MnS, and improves the hole expandability of hot-rolled steel sheets. To reliably obtain these effects, the Ca content is preferably 0.0005% or more.
On the other hand, even if the Ca content exceeds 0.0060%, the above effect is saturated. Therefore, even when Ca is contained, the Ca content is made 0.0060% or less. Preferably, it is 0.0040% or less.

<Mo: 0.02 to 0.50%>
Mo is an element effective for precipitation strengthening of ferrite. In order to reliably obtain this effect, the Mo content is preferably 0.02% or more. More preferably, it is 0.10% or more.
On the other hand, if the Mo content is excessive, the slab becomes more susceptible to cracking and becomes difficult to handle. Therefore, even when Mo is contained, the Mo content is made 0.50% or less. Preferably, it is 0.30% or less.

<Cr: 0.02 to 1.00%>
Cr is an effective element for improving the strength of the hot-rolled steel sheet. To reliably obtain this effect, the Cr content is preferably 0.02% or more. More preferably, it is 0.10% or more.
On the other hand, if the Cr content is excessive, the ductility of the hot-rolled steel sheet is lowered. Therefore, even when Cr is contained, the Cr content is set to 1.00% or less. Preferably, it is 0.80% or less.

<V: 0.01 to 0.40%>
V improves the strength of hot-rolled steel sheets through strengthening by precipitates, grain refinement strengthening by suppressing the growth of ferrite grains, and dislocation strengthening by suppressing recrystallization. In order to reliably obtain these effects, the V content is preferably 0.01% or more.
On the other hand, if the V content is excessive, a large amount of carbonitrides precipitate and the formability of the hot-rolled steel sheet deteriorates. Therefore, the V content is set to 0.40% or less. Preferably, it is 0.20% or less.

<Ni: 0.01 to 0.40%>
Ni suppresses phase transformation at high temperatures and improves the strength of the hot-rolled steel sheet. In order to reliably obtain this effect, the Ni content is preferably 0.01% or more.
On the other hand, when the Ni content is excessive, the weldability of the hot-rolled steel sheet deteriorates. Therefore, the Ni content is set to 0.40% or less. Preferably, it is 0.20% or less.

<Cu: 0.01 to 0.40%>
Cu exists in steel in the form of fine particles and improves the strength of hot-rolled steel sheets. In order to reliably obtain this effect, the Cu content is preferably 0.01% or more.
On the other hand, if the Cu content is excessive, the weldability of the hot-rolled steel sheet deteriorates. Therefore, the Cu content is set to 0.40% or less. Preferably, it is 0.20% or less.

<B: 0.0001 to 0.0020%>
B suppresses phase transformation at high temperatures and improves the strength of the hot-rolled steel sheet. To reliably obtain this effect, the B content is preferably 0.0001% or more.
On the other hand, when the B content is excessive, B precipitates are formed and the strength of the hot-rolled steel sheet is lowered. Therefore, the B content is set to 0.0020% or less. Preferably, it is 0.0005% or less.

<Sn: 0.01 to 0.20%>
Sn is an element that suppresses the coarsening of crystal grains and improves the strength of the hot-rolled steel sheet. In order to reliably obtain this effect, the Sn content is preferably 0.01% or more.
On the other hand, if the Sn content becomes excessive, the steel becomes embrittled and easily broken during rolling. Therefore, the Sn content is set to 0.20% or less. Preferably, it is 0.10% or less.

The chemical composition of the hot-rolled steel sheet mentioned above can be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In addition, sol. Al can be measured by ICP-AES using the filtrate obtained by thermally decomposing the sample with acid. C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method.

Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described.
In the hot-rolled steel sheet according to the present embodiment, the metal structure of the inner region contains 40 to 80% in total of one or two of martensite and bainite, 20 to 60% ferrite, and the balance is The area ratio of the structure is less than 5%, the ratio αs/αc of the ferrite area ratio αs of the surface layer region to the ferrite area ratio αc of the internal region is 1.15 to 2.50, and the surface layer region and the Vickers hardness Hvc of the inner region (1-Hvs/Hvc) is 0.20 or less.

The internal region is defined as a depth of 1/8 of the plate thickness from the surface of the hot-rolled steel plate to 3/ of the plate thickness from the surface, centering on the 1/4 depth position of the plate thickness from the surface of the hot-rolled steel plate. It refers to an area of 8 depths. Further, the surface layer region means a region from the surface of the hot-rolled steel sheet to a depth of 20 μm from the surface.

The structure mainly composed of martensite and bainite has a fine structure and excellent toughness. In addition, although there are many unclear points about the mechanism, steel having a structure mainly composed of martensite and bainite exhibits fatigue resistance compared to precipitation-strengthened steel and composite structure (DP) steel of ferrite and martensite. It is known to be inferior in strength. On the other hand, precipitation-strengthened steel and DP steel are inferior in fatigue strength and toughness because high-speed dislocation movement in ferrite is inhibited. Conventionally, in automotive parts, a steel plate structure has been created according to the required properties, but as the strength is further increased, it is becoming difficult to obtain both high fatigue strength and toughness. Therefore, unlike the conventional technology, the hot-rolled steel sheet according to the present embodiment utilizes a composite structure of ferrite and martensite with excellent fatigue strength and precipitation strengthening by increasing the amount of ferrite in the surface layer region. On the other hand, in the inner region, a metal structure mainly composed of one or both of martensite and bainite, which are excellent in toughness, is utilized. As a result, high strength of 980 MPa or more and excellent fatigue strength, toughness and ductility can be obtained.

Metallographic Structure of Internal Region The metallographic structure of the internal region of the hot-rolled steel sheet has a great effect on the toughness of the hot-rolled steel sheet. Therefore, the metal structure of the inner region is mainly composed of a low-temperature transformation structure. The low temperature transformation structure is martensite and bainite. If the total area ratio of these structures is less than 40%, the toughness of the hot-rolled steel sheet is inferior. Therefore, the total area ratio of martensite and bainite is set to 40% or more. It is preferably 45% or more, more preferably 50% or more.

On the other hand, if the total area ratio of martensite and bainite exceeds 80%, the fatigue strength of the hot-rolled steel sheet is inferior due to the large difference in hardness from the metal structure of the surface layer region. Therefore, the total area ratio of martensite and bainite is set to 80% or less. It is preferably 75% or less, more preferably 70% or less.

In the present embodiment, when the metal structure of the inner region contains only one of martensite and bainite, the content of only one of martensite and bainite may be within the range described above. When both site and bainite are included, the total content of both martensite and bainite should be within the range described above.

If the area ratio of ferrite is less than 20% in the metal structure of the inner region, the fatigue strength of the hot-rolled steel sheet is inferior due to the large difference in hardness from the metal structure of the surface layer region. Therefore, the area ratio of ferrite is set to 20% or more. It is preferably 25% or more, more preferably 30% or more.

On the other hand, if the ferrite area ratio exceeds 60%, the precipitation-strengthened ferrite grains may not alleviate the strain and may not ensure workability, resulting in deterioration of the toughness of the hot-rolled steel sheet. Therefore, the area ratio of ferrite is set to 60% or less. It is preferably 55% or less, more preferably 50% or less.

The area ratio of the metal structure in the internal region is less than 5% of the residual structure. The residual structure is one or more of pearlite and retained austenite. The residual tissue is preferably less than 3%, more preferably 2.5% or less, even more preferably 2% or less.

Metallographic structure of surface region In the metallographic structure of the surface region of the hot-rolled steel sheet, if αs/αc, which is the ratio of the ferrite area ratio αs of the surface layer region to the ferrite area ratio αc of the internal region, is less than 1.15, dislocations in ferrite Suppression of movement becomes insufficient, and the fatigue strength of the hot-rolled steel sheet becomes inferior. Therefore, αs/αc is set to 1.15 or more. It is preferably 1.20 or more or 1.30 or more, more preferably 1.50 or more.

On the other hand, when αs/αc is more than 2.50, carbon is concentrated inside the plate thickness during ferrite transformation, and the difference in hardness from the metal structure in the inner region increases, so that the toughness and / or Fatigue strength becomes inferior. Therefore, αs/αc is set to 2.50 or less. It is preferably 2.20 or less, more preferably 2.00 or less.

In the metallographic structure of the surface region of the hot-rolled steel sheet, βs/βc, which is the ratio of the total area ratio βs of martensite and bainite in the surface region to the total area ratio βc of martensite and bainite in the inner region, is 0.0. It is preferably between 30 and 0.90. When βs/βc is 0.90 or less, dislocation motion in martensite and bainite is sufficiently suppressed, and the fatigue strength of the hot-rolled steel sheet is increased. βs/βc is more preferably 0.85 or less, and even more preferably 0.80 or less.

On the other hand, when βs/βc is 0.30 or more, it is suppressed that carbon is concentrated inside the sheet thickness during the transformation of martensite and bainite, and that the difference in hardness from the metal structure in the inner region becomes large. The toughness and fatigue strength of the rolled steel sheet are enhanced. βs/βc is more preferably 0.40 or more, still more preferably 0.45 or more, and still more preferably 0.50 or more.

The metallographic structure of the surface layer region may contain 30 to 80% ferrite in area ratio. In addition, the metal structure of the surface layer region may contain one or more of bainite, martensite, pearlite and retained austenite in an area ratio of 20 to 70% in total as a residual structure other than ferrite.

Metal structure measurement method A sample is cut from a hot-rolled steel sheet so that a thickness cross-section perpendicular to the surface can be observed. After polishing the plate thickness cross section of this sample using #600 to #1500 silicon carbide paper, a diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water to make a mirror surface. Finish and apply nital etching. Next, photographs of multiple fields of view are taken at arbitrary positions in the longitudinal direction of the cross section of the sample using a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL). An equidistant grid is drawn on the photograph to identify the tissue at the grid points. The area ratio of each tissue is obtained by calculating the number of grid points corresponding to each tissue and dividing it by the total number of grid points. The larger the total number of grid points, the more accurately the area ratio can be obtained. In this embodiment, the grid spacing is 2 μm×2 μm, and the total number of grid points is 1,500.

The area where cementite is precipitated in lamellar form inside the grain is judged to be pearlite. A region with low brightness and no substructure is judged to be ferrite. Regions with high brightness and in which the substructure is not revealed by etching are judged to be martensite and retained austenite. A region that does not correspond to any of the above is determined to be bainite. The area ratio of martensite is obtained by subtracting the area ratio of retained austenite obtained by EBSD analysis, which will be described later, from the area ratio of martensite and retained austenite obtained from photographed photographs.

A sample is cut from the same position as the above measurement so that a thickness cross-section perpendicular to the surface can be observed. After polishing the plate thickness cross section of this sample using #600 to #1500 silicon carbide paper, a diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water to make a mirror surface. to finish. Next, the sample is polished for 8 minutes with colloidal silica containing no alkaline solution at room temperature to remove strain introduced into the surface layer of the sample. Crystallographic orientation information is obtained by electron backscatter diffraction measurements at arbitrary positions in the longitudinal direction of the sample cross section at intervals of 0.1 μm. For the measurement, an EBSD apparatus composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD apparatus is 9.6×10 ⁻⁵ Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. The obtained crystal orientation information is used to calculate the area ratio of retained austenite using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. It should be noted that a crystal structure of fcc is determined to be retained austenite.

By performing each of the above measurements on a region from the surface to a depth of 1/8 of the plate thickness to a depth of 3/8 of the plate thickness from the surface, and a region from the surface of the hot-rolled steel sheet to a depth of 20 μm from the surface, Obtain the area ratio of the metallographic structure in each of the internal region and the surface layer region.

The hardness difference ratio between the Vickers hardness of the surface region and the Vickers hardness of the internal region: 0.20 or less The hardness difference ratio between the Vickers hardness Hvs of the surface region and the Vickers hardness Hvc of the internal region (1-Hvs/Hvc) ) is more than 0.20, the surface region is softened and the fatigue strength of the hot-rolled steel sheet is inferior. Therefore, the hardness difference ratio between Hvs and Hvc (1-Hvs/Hvc) is set to 0.20 or less. It is preferably 0.15 or less, more preferably 0.10 or less.
The hardness difference ratio between Hvs and Hvc (1-Hvs/Hvc) is preferably as small as possible, but from the viewpoint of manufacturing, it may be -0.10 or more, 0.00 or more, or 0.01 or more.

Measurement method of Vickers hardness A test piece is cut out from a hot-rolled steel sheet so that a thickness cross-section perpendicular to the surface can be observed. After polishing the plate thickness cross section of the test piece using #600 to #1500 silicon carbide paper, a mirror surface is obtained by using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. to finish. Let this plate thickness section be the measurement surface. Using a micro Vickers hardness tester, on the measurement surface, in the area from the surface to the depth of 1/8 of the plate thickness to the depth of 3/8 of the plate thickness from the surface, at a load of 1 kgf at intervals of 3 times or more of the indentation. Measure the Vickers hardness. By measuring 20 points in total and calculating their average value, the Vickers hardness Hvc of the metal structure of the internal region is obtained. Similarly, the Vickers hardness is measured in a region from the surface to a depth of 20 μm from the surface of the measurement surface, and the average value of 20 points is calculated to obtain the Vickers hardness Hvs of the metal structure of the surface layer region. Using the obtained Hvs and Hvc, (1−Hvs/Hvc) is calculated to obtain the Vickers hardness height difference ratio.

The hot-rolled steel sheet according to this embodiment has a tensile (maximum) strength of 980 MPa or more. It is preferably 1000 MPa or more. If the tensile strength is less than 980 MPa, the applicable parts are limited and the contribution to vehicle weight reduction is small. Although the upper limit is not particularly limited, it may be 1500 MPa or less or 1300 MPa or less from the viewpoint of mold wear suppression.
In addition, the hot-rolled steel sheet according to the present embodiment may have a total elongation of 10% or more, an absorbed energy at −20° C. of 80 J/cm ² or more, and a fatigue limit ratio (fatigue strength/ tensile strength) may be 0.48 or more.

Tensile strength and total elongation are evaluated by performing a tensile test according to JIS Z 2241:2011. The test piece shall be JIS Z 2241:2011 No. 5 test piece. A tensile test piece is taken from a quarter portion from the edge in the width direction of the sheet, and the direction perpendicular to the rolling direction is taken as the longitudinal direction.
For toughness, first, a 2.5 mm sub-sized V-notch test piece specified in JIS Z 2242:2018 is sampled from a position close to the sampling position of the test piece used in the tensile test. Absorbed energy is measured by performing a C-direction notch Charpy impact test at −20° C. using this test piece. If the thickness of the hot-rolled steel sheet is less than 2.5 mm, the test is performed on the full thickness.
Fatigue strength is measured using a Schenk plane bending fatigue tester in accordance with JIS Z 2275:1978. The stress load at the time of measurement is set at a test speed of 30 Hz in both swings, and the fatigue strength is measured at 107 cycles. Then, the fatigue limit ratio (fatigue strength/tensile strength) is calculated by dividing the fatigue strength at 107 cycles by the tensile strength measured by the tensile test described above.

The thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but may be 1.2 to 8.0 mm. If the thickness of the hot-rolled steel sheet is less than 1.2 mm, it may become difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the hot-rolled steel sheet according to this embodiment may be 1.2 mm or more. Preferably, it is 1.4 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to obtain the metal structure described above after hot rolling. Therefore, the plate thickness may be 8.0 mm or less. Preferably, it is 6.0 mm or less.

The hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and metallographic structure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance, etc., and may be used as a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing and electroplating of Zn—Ni alloy. Examples of hot-dip coating layers include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn--Al alloy plating, hot-dip Zn--Al--Mg alloy plating, and hot-dip Zn--Al--Mg--Si alloy plating. be. The amount of plating deposited is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by applying an appropriate chemical conversion treatment (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying) after plating.

The hot-rolled steel sheet according to the present embodiment has the above-described chemical composition and metallographic structure, regardless of the manufacturing method. However, according to the manufacturing method described below, the hot-rolled steel sheet according to the present embodiment can be stably obtained, which is preferable.

In the preferred method of manufacturing the hot-rolled steel sheet according to the present embodiment, strain is imparted to the surface layer region by performing bending during finish rolling of hot rolling to promote ferrite transformation in the surface layer region. By crystallizing precipitation-strengthened ferrite in the surface layer region and then quenching, martensite and bainite are generated in addition to ferrite in the inner region. Therefore, it is possible to reduce the difference in hardness between the precipitation-strengthened surface layer region and the internal region where the low temperature transformation structure is generated without precipitation strengthening.

The heating temperature of hot-rolled slabs has a great effect on the elimination of solution and elemental segregation. By setting the heating temperature of the slab to 1100° C. or higher, it is possible to prevent insufficient elimination of solution treatment and elemental segregation, and as a result, it is possible to prevent deterioration of the tensile properties and toughness of the product. Further, by setting the heating temperature of the slab to 1350° C. or less, the effects of solutionization and elimination of elemental segregation can be saturated. Therefore, it is preferable to set the heating temperature of the slab to 1100 to 1350.degree. More preferably, it is 1150 to 1300°C.
The temperature of the slab and the temperature of the steel plate in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate.

In the finish rolling, the slab is continuously passed through a rolling stand for finish rolling a plurality of times. In the finish rolling, it is preferable to set the temperature of the hot-rolled steel sheet after the final pass (finishing temperature) to Ar ₃ point or higher and the rolling reduction of the final pass to 12 to 45%.
The temperature of the hot-rolled steel sheet after the final pass is the lowest temperature in finish rolling performed by a plurality of stands. The rolling reduction after the final pass is _{ (t ₀ −t ₁ )/t ₀ }× ₁₀₀ ( %). Also, the Ar ₃ point is represented by the following formula (1).

Ar ₃ points=901−325×C+33×Si−92×Mn+287×P+40×sol. Al... Formula (1)
Each element symbol in the above formula (1) indicates the content (% by mass) of each element. If the element is not contained, 0 is substituted.

By setting the temperature (finishing temperature) of the hot-rolled steel sheet after the final pass of finish rolling to Ar ₃ or more, the formation of ferrite during finish rolling can be suppressed, and as a result, the desired metal structure and properties can be obtained. can be done.

By setting the rolling reduction in the final pass of the finish rolling to 12% or more, recrystallization can be promoted in the finish rolling, the metal structure of the inner region and the surface layer region can be preferably controlled, and excellent fatigue strength can be obtained. can. Further, by setting the rolling reduction of the final pass to 45% or less, it is possible to suppress an increase in the load on the rolling stand and deterioration in the shape of the hot-rolled steel sheet after finish rolling. Therefore, it is preferable that the draft of the final pass in the finish rolling is 12 to 45%. More preferably, it is 15-45%.

By performing bending between the final pass of finish rolling and the pass one stage before that, the surface layer region of the hot-rolled steel sheet (area from surface to 20 μm deep from the surface) has a thickness of 0.002 to 0.020. Straining is preferred. By setting the strain during bending to 0.002 or more, it is possible to create a desired metal structure in the surface layer region. Therefore, the strain during bending is preferably 0.002 or more. It is more preferably 0.003 or more or 0.004 or more.
In addition, by setting the strain during bending to 0.020 or less, it is possible to suppress the loss of production stability due to the tendency of buckling to occur during finish rolling. Also, by setting the strain during bending to 0.020 or less, it is possible to preferably control the metal structure of the surface layer region and the inner region. Therefore, the strain during bending is preferably 0.020 or less. It is more preferably 0.015 or less or 0.010 or less.

Bending is performed by a method such as pushing up the steel sheet from below with rolls between stands, and the strain during bending can be controlled by adjusting the bending angle by adjusting the amount of pushing up and the diameter of the rolls.
For example, when bending is performed by pushing up a steel plate from below between stands with rolls, the amount of strain during bending can be obtained from the following equation (2).

Strain amount = 1.5 x (plate thickness) x (push-up amount) / (diameter at the tip of the push-up device) ² Equation (2)

The elapsed time from the end of finish rolling to the start of cooling is preferably 1.6 seconds or less. By setting the elapsed time from the completion of finish rolling to the start of cooling to 1.6 seconds or less, it is possible to suppress recovery from bending and rolling strains, and to preferably control the metal structure of the surface layer region.

After the finish rolling is completed, it is preferable to cool the steel sheet to a temperature range of 600 to 750° C. at an average cooling rate of 40° C./second or more as primary cooling, and then air-cool for 2 to 6 seconds. In general, the cooling rate during air cooling is 2 to 10° C./sec.
By setting the cooling stop temperature at an average cooling rate of 40° C./second or more to a temperature range of 600 to 750° C. and then performing air cooling, ferrite transformation can be promoted and a desired amount of ferrite can be obtained. .

After air cooling, as secondary cooling, it is preferable to cool to a temperature range of 200°C or less at an average cooling rate of 60°C/second or more, and then coil it. By setting the average cooling rate to a temperature range of 200° C. or lower at 60° C./second or more, martensite transformation can be promoted, and desired amounts of martensite and bainite can be obtained.

Here, the average cooling rate is a value obtained by dividing the temperature drop width of the steel sheet from the start of cooling to the end of cooling by the time required from the start of cooling to the end of cooling.

In addition, some cooling facilities have no air-cooling section on the way, and some have one or more air-cooling sections on the way. In this embodiment, any cooling equipment may be used. Even in the case of using a cooling facility having an air-cooling section, the average cooling rate from the start of cooling to the end of cooling should be within the range described above.

Since the hot-rolled steel sheet is coiled immediately after secondary cooling, the coiling temperature is almost equal to the secondary cooling stop temperature. By setting the coiling temperature to 200° C. or less, it is possible to suppress the formation of a large amount of polygonal ferrite or bainite, and to obtain the desired metal structure and properties.

After the coiling, the hot-rolled steel sheet may be temper-rolled according to a conventional method, or may be pickled to remove scales formed on the surface. Alternatively, plating such as the hot-dip galvanizing or electro-galvanizing described above may be formed, and further chemical conversion treatment may be performed.

According to the manufacturing method described above, it is possible to stably manufacture a hot-rolled steel sheet having the metal structure described above. Therefore, it is possible to stably produce a hot-rolled steel sheet having high strength and excellent fatigue strength and toughness.

Next, the effects of one aspect of the present invention will be described in more detail with reference to examples. The present invention is not limited to this one conditional example. Various conditions can be adopted in the present invention as long as the objects of the present invention are achieved without departing from the gist of the present invention.

A steel having the chemical composition shown in Table 1 was melted, and a slab having a thickness of 240 to 300 mm was produced by continuous casting. Using the obtained slabs, hot-rolled steel sheets shown in Tables 4 and 5 were obtained under the manufacturing conditions shown in Tables 2 and 3.
The bending process was performed by pushing up the steel plate from below between stands with a roll. The amount of strain during bending was controlled by adjusting the bending angle with the amount of pushing up and the diameter of the rolls. At this time, the amount of strain during bending was determined by the above formula (2).

For the obtained hot-rolled steel sheet, the area fraction, Vickers hardness, tensile strength, total elongation, absorbed energy at -20 ° C. and fatigue limit ratio of the metal structure of the inner region and the surface layer region were measured by the above-described method. asked. The measurement results obtained are shown in Tables 4 and 5.

Evaluation Method of Properties of Hot-Rolled Steel Sheet When the tensile strength TS was 980 MPa or more, the hot-rolled steel sheet was determined to be excellent in strength and judged to pass. On the other hand, when the tensile strength TS was less than 980 MPa, it was determined that the hot-rolled steel sheet did not have excellent strength and was rejected.

When the total elongation was 10% or more, the hot-rolled steel sheet was judged to have excellent ductility and was judged as acceptable. On the other hand, when the total elongation was less than 10%, it was determined that the hot-rolled steel sheet did not have excellent ductility and was rejected.
If the absorbed energy at −20° C. was 80 J/cm ² or more, the hot-rolled steel sheet was determined to be excellent in toughness and judged to be acceptable. On the other hand, when the absorbed energy at −20° C. was less than 80 J/cm ² , it was determined that the hot-rolled steel sheet did not have excellent toughness and was rejected.
When the fatigue limit ratio was 0.48 or more, the hot-rolled steel sheet was judged to have excellent fatigue strength and was judged to be acceptable. On the other hand, when the fatigue limit ratio was less than 0.48, it was determined that the hot-rolled steel sheet did not have excellent fatigue strength and was rejected.

From Tables 4 and 5, it can be seen that the hot-rolled steel sheets according to the examples of the present invention have high strength and excellent toughness, fatigue strength and ductility.
On the other hand, it can be seen that the hot-rolled steel sheets according to the comparative examples are inferior in at least one of strength, toughness and fatigue strength.

According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent fatigue strength, toughness and ductility. According to this hot-rolled steel sheet, it is possible to reduce the weight of the body of an automobile, etc., integrally mold parts, shorten the processing process, etc., and improve fuel efficiency and reduce manufacturing costs. Good value.

Claims (2)

The chemical composition, in mass %,
C: 0.02 to 0.30%,
Si: 0.10 to 2.00%,
Mn: 0.5-3.0%,
sol. Al: 0.10 to 1.00%,
Ti: 0.06-0.20%,
P: 0.1000% or less,
S: 0.0100% or less,
N: 0.0100% or less,
Nb: 0 to 0.100%,
Ca: 0 to 0.0060%,
Mo: 0-0.50%,
Cr: 0 to 1.00%,
V: 0 to 0.40%,
Ni: 0 to 0.40%,
Cu: 0-0.40%,
B: 0 to 0.0020%, and Sn: 0 to 0.20%,
The balance consists of Fe and impurities,
The metal structure of the internal region contains, in terms of area ratio, 40 to 80% in total of one or two of martensite and bainite, and 20 to 60% ferrite, and the area ratio of the remaining structure is less than 5%. ,
αs/αc, which is the ratio of the ferrite area ratio αs of the surface layer region to the ferrite area ratio αc of the internal region, is 1.15 to 2.50;
A hardness difference ratio (1-Hvs/Hvc) between the Vickers hardness Hvs of the surface region and the Vickers hardness Hvc of the inner region is 0.20 or less,
A hot-rolled steel sheet having a tensile strength of 980 MPa or more. The chemical composition, in mass %,
Nb: 0.010 to 0.100%,
Ca: 0.0005 to 0.0060%,
Mo: 0.02-0.50%,
Cr: 0.02 to 1.00%,
V: 0.01 to 0.40%,
Ni: 0.01 to 0.40%,
Cu: 0.01 to 0.40%,
B: 0.0001 to 0.0020%, and Sn: 0.01 to 0.20%, characterized by containing one or more selected from the group consisting of 0.01 to 0.20% Heat according to claim 1 Rolled steel plate.